University of Groningen Exploring coordination chemistry and reactivity of formazanate ligands Travieso Puente, Raquel
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(6) Exploring coordination chemistry and reactivity of formazanate ligands . . . . . . . . . to obtain the degree of PhD at the University of Groningen on the authority of the Rector Magnificus Prof. E. Sterken and in accordance with the decision by the College of Deans. This thesis will be defended in public on Friday 13 October 2017 at 11:00 hours . . . .
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(20) Esta tesis va dedicada a mi guía y mi ejemplo de vida. A la sonrisa infinita, luchadora insaciable. Esta tesis va dedicada a ti, Mamá..
(21) Queda prohibido llorar sin aprender, levantarte un día sin saber qué hacer, tener miedo a tus recuerdos. Queda prohibido no sonreír a los problemas, no luchar por lo que quieres, abandonarlo todo por miedo, no convertir en realidad tus sueños. [...] Queda prohibido no intentar comprender a las personas, pensar que sus vidas valen más que la tuya, no saber que cada uno tiene su camino y su dicha. Queda prohibido no crear tu historia, no tener un momento para la gente que te necesita, no comprender que lo que la vida te da también te lo quita. [...] Queda prohibido no buscar tu felicidad, no vivir tu vida con una actitud positiva, no pensar en que podemos ser mejores, no sentir que sin ti, este mundo no sería igual. ~~~~~~ It is forbidden to cry without learning, to wake up one day not knowing what to do, to be afraid of your memories. It is forbidden not to smile at problems, not to fight for what you want, to abandon everything because of fear, not to transform your deams into reality. [...] It is forbidden not to try to understand people, to think that their lives are more valuable than yours, not to know that each person has his own ways and his own joy. It is forbidden not to create your history, not to have a moment for the people who need you, not to understand that whatever life gives you, it takes it away as well. [...] It is forbidden not to search for your happiness, not to live your life with a positive attitude, not to think that we can be better, not to feel that, without you, this world wouldn’t be the same.. ~ Pablo Neruda ~.
(22) Contents List of compounds. 1. Chapter 1. 5. Introduction 1.1 General Introduction. -----------------------------------------. 6. 1.2 Redox-active ligands. -----------------------------------------. 7. 1.3 β-diketimine ligands. -----------------------------------------. 9. 1.3.1 β-diketiminate as supporting ligand. ---------------------------. 9. --------------------------. 10. -------------------------------------------. 11. 1.3.2 β-diketiminate as redox-active ligand 1.4 Formazan ligands. 1.4.1 General features of formazan ligands. --------------------------. 11. -----------------------------------. 13. ------------------------------------------. 16. -----------------------------------------------. 18. Alkali metal salts of formazanate ligands: diverse coordination modes as a result. 21. 1.4.2 Redox-active properties 1.5 Overview of thesis 1.6 References. Chapter 2. of the nitrogen-rich [NNCNN] ligand backbone 2.1 Introduction. -----------------------------------------------. 2.2 Formazanate alkali metal salts 2.2.1 Solid-state structures. ----------------------------------. -------------------------------------. 2.2.2 Solution-state structures: NMR studies. 23 23 28. 2.3 Conclusions. -----------------------------------------------. 30. 2.4 Experimental. ----------------------------------------------. 31. 2.4.1 General considerations. -----------------------------------. 2.4.2 Synthesis of formazanate salts. ------------------------------. 2.4.3 Crystal structure determination 2.5 References. Chapter 3. ------------------------. 22. ------------------------------. -----------------------------------------------. 31 31 34 36. Bis(formazanate)calcium and magnesium complexes. 37. 3.1 Introduction - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -. 38. 3.2 Synthesis and characterization of bis(formazanate) complexes. -------------. 39. -----------------------. 39. --------------------------------. 40. ------------------------------. 42. 3.2.1 Synthesis of bis(formazanate) complexes 3.2.2 NMR spectroscopy analysis 3.2.3 X-Ray crystallography studies.
(23) 3.3 UV/Vis spectroscopy of bis(formazanate) complexes. -------------------. 3.4 Cyclic voltammetry studies of bis(formazanate) complexes. ---------------. 49. 3.5 Conclusions. -----------------------------------------------. 49. 3.6 Experimental. ----------------------------------------------. 50. 3.6.1 General considerations. -----------------------------------. 3.6.2 Synthesis and characterization. ------------------------------. 3.6.2.1 Bis(formazanate)magnesium complexes 3.6.2.2 Bis(formazanate)calcium complexes 3.7 References. Chapter 4. 48. ------------------. ---------------------. -----------------------------------------------. Facile access to fluorinated [1,2,4,5]tetrazepine and indazole derivatives via. 50 51 51 56 62. 63. nucleophilic aromatic substitution 4.1 Introduction. -----------------------------------------------. 4.2 Tetrazepines derivatives. --------------------------------------. 4.2.1 Synthesis and characterization of tetrazepine derivatives. 66. ------------. 66. ---------------------. 68. -----------------------------------------. 71. 4.2.2 Functionalization of tetrazepine derivatives 4.3 Indazole derivatives. 64. 4.3.1 Synthesis and characterization of indazole derivatives. --------------. 71. ------------------------------. 74. 4.3.3 Proposed mechanism. ------------------------------------. 77. 4.4 Cyclic voltammetry studies. ------------------------------------. 81. -----------------------------------. 81. -------------------------------------. 83. 4.3.2 Functionalization of indazoles. 4.4.1 Tetrazepine derivatives 4.4.2 Indazole derivatives 4.5 Conclusions. -----------------------------------------------. 83. 4.6 Experimental. ----------------------------------------------. 84. 4.6.1 General considerations. -----------------------------------. 84. 4.6.2 Tetrazepine derivatives. -----------------------------------. 85. -------------------------------------. 87. -----------------------------------------. 90. 4.6.3 Indazole derivatives 4.6.4 Kinetic studies. 4.6.4.1 Cyclization of 8-K. ---------------------------------. 90. 4.6.4.2 Cyclization of 8-Zn. --------------------------------. 91. -----------------------------------------------. 95. 4.7 References.
(24) Chapter 5. Arylazoindazole photoswitches: facile synthesis and functionalization via SNAr. 97. substitution 5.1 Introduction. -----------------------------------------------. 5.2 Photoswitching studies of arylazoindazoles 5.2.1 Arylazoindazoles 4.3 and 4.4. -------------------------. 99. -------------------------------. 99. 5.2.2 Arylazoindazoles 4.3OMe and 4.3SC8H17. -------------------------. 101. ----------------------------------------. 101. 5.4 Conclusions. -----------------------------------------------. 107. 5.5 Experimental. ----------------------------------------------. 107. 5.3 Computational studies. 5.5.1 General methods. ---------------------------------------. 5.5.2 Photoisomerization quantum yields 5.5.2.1 Actinometry. 108. -------------------------------------. 108. 5.5.2.3 Quantum yield measurements Extinction coefficients. Quantum yield. 5.6 References. --------------. 108. -------------------------. 109. ------------------------------. Determination of constant K. 5.5.3 Computational studies. 107. ---------------------------. 5.5.2.2 Equations used for quantum yield calculations. Chapter 6. 98. 109. --------------------------. 110. -----------------------------------. 112. ------------------------------------. 112. -----------------------------------------------. 118. Formazanate complexes as potential catalysts for lactide polymerization. 121. 6.1 Introduction. 122. -----------------------------------------------. 6.2 Formazanate zinc methyl complexes (LZnMe). -----------------------. 6.3 Synthesis of alkoxide complexes [LMOR]n (M = Zn, Mg). 128. ---------------. 133. 6.3.1 Synthesis of alkoxide complexes [LZnOR]n. ---------------------. 133. 6.3.2 Synthesis of alkoxide complexes [LMgOR]n. ---------------------. 135. ------------------------------------------. 138. 6.4 DOSY experiments. 6.5 Cyclic voltammetry experiments 6.6 Reactivity towards lactide. ---------------------------------. -------------------------------------. 6.6.1 Conversion of lactide by LZnMe. -----------------------------. 6.6.2 Conversion of lactide by [LMOR]n (M = Zn, Mg). 141 144 144. -----------------. 148. 6.7 Conclusions. -----------------------------------------------. 149. 6.8 Experimental. ----------------------------------------------. 150.
(25) 6.8.1 General considerations 6.8.2 Synthesis of LZnMe. -----------------------------------. 150. -------------------------------------. 151. 6.8.3 Synthesis of [LMOR]n (M = Zn, Mg) 6.9 References. Chapter 7. --------------------------. -----------------------------------------------. 158. Spin-crossover in a pseudo-tetrahedral bis(formazanate) iron complex. 161. 7.1 Introduction. -----------------------------------------------. 162. -----------------------------. 162. 7.2 Synthesis and characterization of FeL2 1. 7.2.1 Synthesis and H NMR spectroscopy of FeL2 7.2.2 Solid state characterization of FeL2. --------------------. 162. ---------------------------. 165. ----------------------. 167. 7.2.3 SQUID, UV/Vis and DSC studies on FeL2 7.2.4 Mössbauer studies on FeL2. --------------------------------. 7.2.5 Cyclic Voltammetry studies on FeL2. --------------------------. 7.3 Synthesis and characterization of [FeL2][NBu4] 7.4 Experimental. ----------------------------------------------. 174. ------------------------------------------------------. 7.4.3 Analysis of temperature dependence of 1H NMR shifts in 1-Fe 7.4.4 UV/Vis absorption spectra 7.4.5 Magnetic measurements. 175 175. ---------------------------------. 176. ----------------------------------. 178. ------------. 179. ----------------------------------. 180. ------------------------------------. 180. ----------------------------------------. 185. 7.4.7 Mössbauer spectroscopy 7.4.8 Computational studies ----. 174. --------. 7.4.6 Differential Scanning Calorimetry (DSC) measurements. . 169 170. 7.4.2 Synthesis of bis(formazanate)iron complexes. 7.5 References. 168. -----------------------. 7.4.1 General considerations. . 152.
(26) Appendix English Summary. ---------------------------------------------. Nederlandse Samenvatting Acknowledgements. 187. ---------------------------------------. 195. --------------------------------------------. 203.
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(28) List of compounds ♦ Formazan ligands previously reported1: 1-H:PhNHNC(p-tol)NNPh 2-H: MesNHNC(CN)NNMes 3-H:PhNHNC(tBu)NNPh 4-H: MesNHNC(p-tol)NNPh 5-H: MesNHNC(p-tol)NNMes 6-H: C6F5NHNC(p-tol)NNMes 8-H: PhNHNC(C6F5)NNMes 9-H: C6F5NHNC(C6F5)NNMes ♦ Compounds presented in Chapter 2: p-tol. p-tol N Ph. N N. N. Ph. N Ph. N N. K O. N. Ph. 1-Bu 4N O. 1-K (dimer):K[PhNNC(p-tol)NNPh]·2THF . [Bu 4N]. 1-Bu4N: [Bu4N][PhNNC(p-tol)NNPh]. N N. C. C. N Mes. N N. N. Mes. N Mes. K. 2-K: K[MesNNC(CN)NNMes]. N N. N. O Na. O. 2-Na (polymer): Na[MesNNC(CN)NNMes]·2THF. tBu. tBu. N Ph. N. N N. N. Ph. Na O. 3-NaTHFNa[PhNNC(tBu)NNPh]·THF. . Mes. Ph. N. N Na. N Ph. 3-Na (hexamer): Na[PhNNC(tBu)NNPh]. .
(29) ♦ Compounds presented in Chapter 3: 1-Mg: [PhNNC(p-tol)NNPh]2Mg 1-MgTHF: [PhNNC(p-tol)NNPh]2Mg(THF) 3-Mg: [PhNNC(tBu)NNPh]2Mg 3-MgTHF: [PhNNC(tBu)NNPh]2Mg(THF) 4-Mg: [MesNNC(p-tol)NNPh]2Mg 4-MgTHF: [MesNNC(p-tol)NNPh]2Mg(THF) 6-Mg: [C6F5NNC(p-tol)NNMes]2Mg 6-MgTHF: [C6F5NNC(p-tol)NNMes]2Mg(THF) 8-Mg: [PhNNC(C6F5)NNMes]2Mg 9-Mg: [C6F5NNC(C6F5)NNMes]2Mg 1-Ca: [PhNNC(p-tol)NNPh]2Ca(THF)2 3-Ca: [PhNNC(tBu)NNPh]2Ca(THF)2 4-Ca: [MesNNC(p-tol)NNPh]2Ca(THF) 5-Ca: [MesNNC(p-tol)NNMes]2Ca(THF) Other compounds reported previously used for comparison1: 1-Zn: [PhNNC(p-tol)NNPh]2Zn 3-Zn: [PhNNC(tBu)NNPh]2Zn 4-Zn: [MesNNC(p-tol)NNPh]2Zn 6-Zn: [C6F5NNC(p-tol)NNMes]2Zn . ♦ Compounds presented in Chapter 4 and 5 p-tol N N. p-tol. N. N. N Mes. F. F F. N. N. N Ph. F. F. F F. Tetrazepine 4.1. F. Tetrazepine 4.2 p-tol. p-tol N N MeO. N. N. OMe. Tetrazepine 4.1. . MeO. F F. N. N Mes. OMe. N N Ph F. F. OMe. Tetrazepine 4.2OMe.
(30) F. F F. F. B. N. Mes. F. F. A. N. A. N. N N. Mes. Azoindazole 4.3. Azoindazole 4.4 F. F. N. A 7. 3. F 4. OMe 6. B. N. Mes. 5. 4. F. N. 1. S 6. A 7. 3. F. N N 2. Ph. Azoindazole 4.3OMe. 5. B. N. Mes. N N 2. F. N N. Ph. F. F. B. N. Ph. 1. Ph. Azoindazole 4.3SC8H17. ♦ Compounds presented in Chapter 6: 1-ZnOiPr: {[PhNNC(p-tol)NNPh]ZnOiPr}2 1-ZnOtBu:{[PhNNC(p-tol)NNPh]ZnOtBu}2 1-ZnOPh:{[PhNNC(p-tol)NNPh]ZnOPh}]2 5-ZnOiPr:{[MesNNC(p-tol)NNMes]ZnOiPr}2 1-MgOC(CH3Ph2):{[PhNNC(p-tol)NNPh]MgOCCH3(Ph)2}2 Other compounds reported previously1: 1-ZnMe: [PhNNC(p-tol)NNPh]ZnMe 3-ZnMe: [PhNNC(tBu)NNPh]2ZnMe 4-ZnMe: [MesNNC(p-tol)NNPh]2ZnMe 5-ZnMe: [MesNNC(p-tol)NNMes]ZnMe . ♦ Compounds presented in Chapter 7: 1-Fe:[PhNN(p-tol)NNPh]2Fe [1-Fe][NBu4]: [Bu4N][(PhNN(p-tol)NNPh)2Fe]. Chang, M.-C. Thesis: Formazanate as a Redox-Active, Structurally Versatile Ligand Platform; 2016.. (1). . .
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(32) . Chapter 1 . Introduction. .
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(34) . 1.1 General introduction Most chemical processes in industry rely on catalytic transformations. Especially in the area of fine chemicals, many of those are based on homogeneous catalysis, which often rely on two-electron redox changes at the metal center (oxidative addition and reductive elimination). These two oxidation states should be similar in energy for turnover. However, there are few metals which possess two easily accessible oxidation states n and n+2, most of them belonging to the platinum group metals (Ru, Rh, Pd, Os, Ir, Pt and Au, metals in purple in Figure 1.1). Even though they are often used, the platinum group metals are not sustainable because they are scarce, expensive and often toxic.1,2 In Figure 1.1 are shown in red the major industrial metals, from which Al, Mg, Ti, Fe and Mn are the most abundant in Earth’s upper continental crust (green region in Figure 1.1). However, most of the most abundant (therefore cheaper) transition metals undergo only one-electron transformation, limiting their applicability in reactions that require two-electron transfer.. Figure 1.1 Abundance expressed in atom fraction of the chemical elements in Earth’s upper continental crust as a function of atomic number. Figure by courtesy of the U.S. Geological Survey (https://pubs.usgs.gov/fs/2002/fs087-02). One example of the value of noble transition metals is the used of Pd in C-C bond formation by Heck, Negishi, and Suzuki (Nobel Prize for Chemistry in 2010).3 However, efforts have been made in the past decades for developing ligands that act as electron reservoirs (redox-active ligands) to emulate noble-metal properties with first-row transition metals.. .
(35) . . 1.2 Redox-active ligands Changes in oxidation states of a metal complex usually involve changes in oxidation state of the metal center. In contrast, when the ligand is responsible for the electron transfer and the oxidation state of the ligand-metal is clearly defined, the ligand acts as a ‘redox-active ligand’. However, in some cases this is not clearly defined due to an extensive mixing which leads to ambiguity in terms of assigning oxidation states of ligand-metal, in these cases the ligand is so-called ‘redox non-innocent’.4–6 In this section and throughout this thesis we will focus on these ligands. The main feature for a ligand to be redox-active/non-innocent7 is to possess orbitals that lie close, in both energetic and spatial proximity, to the ‘d’ orbitals of the coordinated metal, often leading to extensive mixing of the metal and ligand frontier orbitals.7–13 Electron loss could occur at the ligand rather than the metal when redox-active ligands are used (z to z+2) (Scheme 1.1) where the metal center is not involved in the process and therefore the oxidation state remains unchanged (n). This potentially avoids going through unstable metal oxidation states during chemical redox transformations, and enables new reaction pathways. Traditional oxidative addition LzxMn +. A-B. LxzMn+2. B. A Oxidative addition involving a redox-active ligand LxzMn + . A-B. n Lz+2 x M. B. A. Scheme 1.1 Metal complex oxidation: mediated by metal center (top) and mediated by redoxactive ligands (bottom). Oxidative addition is a fundamental transformation in organometallic chemistry that is often a key bond activation step responsible for introducing substrates into the metal coordination sphere during catalytic turnover. To avoid the generation of unstable oxidation states of the metal center, many efforts have been made towards the synthesis and study of new metal complexes involving redox-active ligands,4,14–19 due to their interesting electronic structures, role in metalloenzymes,20–23 the .
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(37) . ability to promote unusual group transfer reactivity,24–27 and their importance in base metal catalysis.8,28–32 Nature also makes use of redox-active ligands as storage of redox equivalents for chemical transformations and catalysis.23 One example is cytochrome P450, the active site contains a heme cofactor (Fe2+ and porphyrin as ligand).33 These species are found in all domains of life.34 In 2010, a highly reactive intermediate (P450 compound I, P450-I, Scheme 1.2) was isolated, which is an important intermediate in the catalytic cycle for biological functions through the controlled activation of C-H bonds.15 P450-I was found to contain an iron(IV)-oxo (ferryl) metal center and a singly oxidized porphyrin radical ligand (delocalized over the porphyrin and thiolate ligand), which is able to oxidize hydrocarbons to alcohols via a 2-electron process, one of which is provided by reduction of the metal center (Fe(IV) to Fe(III)) and the other by the ligand (singly oxidized porphyrin radical to porphyrin ligand).. N N. FeIII N. N. S HOOC Cys HOOC. . Cytochrome P450. NO N FeIV N N S HOOC Cys HOOC P450-I. Scheme 1.2 Active site of Cytochrome P450 (left) and P450 compound I (right). Another example is galactose oxidase35, which makes use of an organic ligand that can be oxidized. In this oxidized form, the copper oxidation state remains unaltered while the ligand is the one bearing an unpaired electron (one-electron oxidized tyrosyl radical that is stabilized by a nearby tryptophan), generating an active form of the enzyme towards selective oxidation (Scheme 1.3).. Scheme 1.3 Activation of galactose oxidase by oxidation of the redox-active ligand..
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(39) . . Scientists have been focussing in the past decades to mimic nature and expand the number of redox-active ligands. Although some examples of redox active ligands have been elucidated in the last decades,28,36–39 such as catechol17,40–42 o-aminophenol43, o-phenylenediamine44 and o-dithiol45, represented in Figure 1.2 as a) cat, b) o-O,N, c) o-N,N and d) o-S,S, respectively, still the diversity of redox-active ligands is very limited. Therefore, there is a need to develop new redox-active ligand scaffolds that are easily accessible, tunable in their steric and electronic properties, and can provide high chemical stability in various oxidation states.. O-. O-. O-. NR. NR c) o-N ,N. b) o-O,N. a) cat. XXL 2-. R N-. b) o -O,N. X-. - e+. e-. X. SSd) o-S,S. X. - e+. L-. e-. X L-. X = O, N, S. Figure 1.2 Skeleton of catecholate derivatives (top) and their three possible oxidation states (bottom).. 1.3 β-diketimine ligands 1.3.1 β-diketiminate as supporting ligand β-diketimines (nacnac, 1,5-diazapentadienyl) are an important class of monoanionic bidentate ligands which contain two nitrogens in the ligand framework R1-N=C(R2)-C(R3)=C(R4)NH-R5. The generated anionic charge, upon deprotonation of the ligand, is delocalized throughout the conjugated π-system over the –N–C–C–C–N– moiety (Figure 1.3, section 1.3.2). Structural, electronic and magnetic properties, and reactivity of the metal centers can be controlled mainly by the substituents introduced either in the nitrogen atoms (R1 and R5) or in the carbon ones (R2–R4). In most cases, the steric control on the coordination sphere is mainly due to the nitrogen substituents (R1 and R5). On the other hand, the carbon substituents (R2–R4) are also important in tuning the electron donor ability of the supporting ligands. .
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(41) . This ligand has been highly explored with a wide variety of metals in different oxidation states.46–48 Due to the wide variety of complexes that have been studied, there are many reactions involving β-diketiminates as supporting ligands for application in catalysis49. However, little attention has been focused on the redox activity (redox non-innocence) of the supporting ligand itself.. 1.3.2 β-diketiminate as redox-active ligand Diketiminate can be reduced or oxidized acting as non-innocent supporting ligand.50 The first ligand-based reduction was mentioned and characterized by Lappert et. al in 2002 involving Sm and Yb as metal centers.48 Crystallographic analysis suggests a dianionic ligand framework based on elongation of the N-C bond lengths with the interaction of one Li+ per ligand [SmL2]- (L' in Scheme 1.4). A year later, a ligand-based 2-electron reduction was reported within the same group with Yb and Li as metal centers.51,52 In this radical dianion (L'' in Scheme 1.4) as well as in the radical anions L', the elongation of the N-C bond lengths is due to population of orbitals that have antibonding character between C and N (Scheme 1.4). R2. R4. R2. N N R1 R5 β-diketiminate. R1. R4. R2. R5. R1. LUMO. R4 R5 HOMO. Figure 1.3 Ligand framework upon deprotonation (β-diketiminate, nacnac-): general structure (left), LUMO (center), HOMO (right). Ph R1. N. N. Ph. Ph. R5. R1 L. Ph N. N. R5. e-. Ph R1. Ph N. N L'. R5. e-. Ph R1. Ph. Ph N. N. R1. R5. N. N. R5. L''. Scheme 1.4 Consecutive two-electron reduction of a β-diketiminate ligand. The first diketiminate ligand-based oxidation was reported for a bis(β-diketiminate)Ni(II) complex in 2011 by Khusniyarov, Wieghardt et. al.,53 almost a decade later than the first example of reduction of a β-diketiminate complex was reported. Four years later the first crystallographic study of a singly oxidized β-diketiminate complex, also involving a bis(βdiketiminate)Ni(II).54 However, there were no substantial changes in the metrical parameters of the ligand framework comparing with its neutral form. This could be explained based on the non-bonding character of the HOMO of the β-diketiminate ligand (Figure 1.3). .
(42) . . Although in some complexes β-diketiminate could act as redox-active ligand, there are not many examples where the corresponding reduced complexes have been synthesized and characterized. This is due to their low reduction potential, some examples that could be synthesized show reduction potentials of -1.75, -2.21, -2.31 V when the metal centers Yb2+, Li+, Ca2+ are present, (respectively, potentials vs. SCE).55 Thus, strong reducing agents as Li metal or Yb-naphtalene complex are required.. 1.4 Formazan ligands 1.4.1 General features of formazans Formazans closely resemble the structure of β-diketimines, but contain two extra nitrogen atoms in the backbone R1-N=N-C(R3)=N-NH-R5 than their β-diketiminate analogues. Although extensive studies have been performed on metal complexes containing βdiketiminate ligands (see above) formazanates have been largely neglected. The two extra nitrogens present in the formazanate framework provide it an increased flexibility in their coordination chemistry. This allows not only the 6 membered ring coordination to the metal center through the two terminal nitrogens as shown in all the cases for β-diketiminate, but also a five membered ring coordination through coordination of one terminal and one internal nitrogen to the metal center (Scheme 1.5).56 Mes N p-tol N. Mes. C6F 5. N Zn N C6F 5. N N N N Mes. C6F 5. N N p-tol. p-tol. Zn N N. N N N N. p-tol. C6F 5 Mes. Scheme 1.5 Interconversion between two formazanate isomers (left: 5:6-chelated member ring, right: 6:6-chelated member ring)56. Formazans have been known as dye for more than hundred years.57–59 The reduction of tetrazolium salts by dehydrogenases and reductases lead to the formation of colorful formazan compounds, ranging from orange, deep-red to violet. MTT (oxidized form of formazans) are used in assays to evaluate the cell metabolic activity by applying the chromogenic properties of the formazan.60 . .
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(44) . Formazans are the precursors of verdazyls, a class of stable organic radicals first reported in 1963 by Trischmann and Kuhn.61 Verdazyl radicals are resonance stabilized six-membered heterocyclic radicals with the structure shown in Figure 1.4. Verdazyls and 6-oxoverdazyls (seven π electrons, left and center in Figure 1.4, respectively), benefit from significant delocalization and stabilized frontier molecular orbitals due to the presence of four nitrogen atoms in their heterocyclic backbones. They can be reversibly oxidized to cations (six π electrons) and anions (eight π electrons). Their versatile applications, ranging from molecular magnets62, as ESR spin labels63, polymerization inhibitor64 and mediators in living radical polymerizations65 amongst others, is due to their unusual stability to air and moisture. Verdazyl radicals remain monomeric in both, solution and solid state, without relying on bulky substituents. Closely related Kuhn-type verdazyls and 6-oxoverdazyls (seven π electrons), which also benefit from significant delocalization and stabilized frontier molecular orbitals due to the presence of four nitrogen atoms in their heterocyclic backbones, exhibit similar redox behavior and can be reversibly oxidized to cations (six π electrons) and anions (eight π electrons).. Figure 1.4 Verdazyl structure with possible substitution patterns (left) and SOMO orbital (right). The exceptional stability of verdazyl radicals is attributed to the delocalization of its unpaired electron across all four nitrogen atoms in the ring with the unpaired electron residing in a low energy SOMO.66,67 In the next section 1.4.2, we will introduce how by changing the methylene, carbonyl or phosphine oxide depicted in Figure 1.4 by a metal center the resulting formazanate can still act as an electron reservoir.. .
(45) . . 1.4.2 Redox-active properties The use of β-diketiminate as redox-active ligands are limited due to the poor accessibility of reduced forms (very negative potentials)51,52 and low stability of the oxidized forms53 (see section 1.3.2). In contrast, formazanate complexes show more accessible reduction potentials (-0.86 to -1.86 V vs. Fc0/+)56,68–74 than their analogous β-diketiminates (-1.75 to -2.31 V vs. SCE)55. The combination of the accessibility and stability of the reduced formazanate complexes make the formazan an interesting ligand platform, not only as a supporting but also as a potential redox-active ligand. The first synthesis of a reduced formazanate complex was reported in 2007 by Hicks and coworkers70 based on boratatetrazines (formazanate)B(OAc)2. Cyclic voltammetry studies on this neutral complex indicate an accessible reversible redox wave at -0.86 V vs. Fc0/+. Although the corresponding ligand-based reduction product [(formazanate)B(OAc)2][CoCp2) could not be isolated in pure form it was characterized by EPR and UV/Vis spectra (Figure 1.5). CoCp 2. R3 CoCp 2 R1. . N N AcO. B. N N. R5 OAc. . Figure 1.5 Synthesis of the first reduced formazanate complex (borataverdazyl radical anion L' [LB(OAc)2]−. In the following years, Gilroy and co-workers made several contributions to the synthesis of various (formazanate)boron difluoride complexes71–74 and they studied the substituent dependency in the optical and electrochemical properties. In general, substitution on the 1and 5-position of the formazanate backbone (N-R1, N-R5) has a higher effect than the 3substitutions on those properties. This behaviour is similar to the one observed previously by Kuhn-type verdazyls and 6-oxoverdazyls due to a Singly Occupied Molecular Orbital (πSOMO) with a nodal plane, and therefore little electron density in the 3-position (right in Figure 1.4).75,76 Most of the boron complexes studied by Gilroy and co-workers upon increasing the π conjugation of the system increase the fluorescence quantum yields and the reduction potentials (being easier to reduce) and lead to a red-shift of the maximum absorption and . .
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(47) . emission. Although changes in the symmetry of the complexes did not change significantly the absorption or electrochemical properties, the quantum yields increase upon breaking their symmetry.77 This is in line with previous studies on BODIPYs78–83 and (N-donor)BF2 complexes84–87. Enhancement of the emission quantum yields can be also achieved by substitution on 3-postition of the formazanate framework with substituents that increase the conjugated π system (3-cyano- and 3-nitro).77 (Formazanate)boron difluoride complexes (LBF2, six π electrons)71–74 show in all the cases two reversible one-electron reduction waves, first leading to the formation of the corresponding ligand-centered radical anions (seven π electrons) and the second to the dianions (eight π electrons). Several of LBF2 show additional irreversible oxidation events.73 This stabilization is due to the 4 nitrogens present which allows the delocalization of the unpaired electron similarly to the verdazyl discussed in section 1.4.1. In general, modification of the N-aryl electron donating/with-drawing character lead to lower/higher reduction potentials, being more difficult/easier to reduce the corresponding complexes, respectively.72– 74. In 2014, the synthesis and isolation of the one- and two-electron reduction products of bis(formazanate)zinc complexes was reported by our group (Figure 1.6).68 The stability of these complexes is due to the “metallaverdazyl”-type structures obtained. The first electron transfer process (-1.52 V vs. Fc0/+, I/I’, ZnL'L, radical anion) corresponds to the reduction of one of the formazanate ligands, and a subsequent reduction is observed at a somewhat more negative potential corresponding to reduction of the other formazanate (-1.84 V vs. Fc0/+, II/II’, ZnL'L', radical dianion). The lower reduction potential for the latter is evidence for electronic communication between the two ligands. The third and fourth redox couples correspond to the 2-electron reduction of each formazanate ligand (L'', reductions III and IV).. .
(48) . . R3. R1. R3. N. N. N. N. eR5. L. N R1. N. R3 e-. N 2 L'. N. R5. N R1. N. N. N. R5. L'' . . . .
(49) . . . Figure 1.6 Cyclic voltammogram of bis(formazanate)zinc (R1 = R5 = Ph, R3 = p-tol). showing the presence of four reduction waves. Within the same year, also studies on mono(formazanate)boron difluoride complexes were performed by our group.88 These show two accessible and reversible redox couples, suggesting the viability of two-electron reduction in a single formazanate ligand ([LBF2]2–, L''). The single electron reduced product could be isolated by reduction with cobaltocene (0.98 V vs. Fc0/+, [LBF2]–, L' ). However, the lower reduction potential of the second redox couple requires Na as reducing agent (-2.06 V vs. Fc0/+), leading to the releasing of NaF and the formation of N-heterocyclic boron carbenoid, present in the isolated unusual BN heterocycles.89 A two-electron reduced formazanate boron compound has been isolated recently.90 The use of phenyl, instead of fluoride, as B-substituents avoids the release of NaF, allowing the isolation of compounds containing the formazanate ligand in the two-electron reduced form (L''). In 2016, Sundermeyer and co-workers reported a series of formazanate complexes involving heavier group 13 elements (LMMe2, M = Al, Ga and In). Aluminium and gallium mononuclear complexes present a tetrahedral coordination around the metal center. In contrast, the indium analogue present an oligomeric structure in non-coordinating solvents in which they propose dimeric species with a 5-coordinated indium by N-bridging. In coordinating solvents the oligomer is cleaved, maintaining the 5-coordinated indium by the coordination of pyridine or 4-dimethylaminopyridine, according to NMR and X-ray crystallographic studies. Electrochemistry studies of these complexes reveal irreversible electron transfer in all the cases, tentatively attributed due to labile radical metal alkyls.91 Despite the expanded number of formazanate complexes synthesized in the last decade (Pd92 Co,93 Ni94,95, Cu96, Pt69 among others97) and their proven redox-activity (vide supra), the role . .
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(51) . of the formazanate in the most of the reactions described to date is as a supporting ligand without involving their potential redox-activity (see section 1.4.3).. One of the few examples where the observed reactivity involves the redox-active nature of the formazanate was reported by Tolman and co-workers in 2009.96 Electron deficient formazanate copper(I) [LCu(CH3CN)] was reacted with oxygen leading to the formation of bis(hydroxo)dicopper(II) at room temperature and bis(μ-oxo)dicopper at − 80 °C. Studies on this last compound and its intermediates suggest the formation of a metallaverdazyl radical complex intermediate due to the 'non-innocent' potential of the formazanate.. Although some complexes containing formazanate ligands have been reported to be active catalysts for chemical transformations including H2O2 decomposition98 and ethylene oligomerization92,99,100 is worth to mention that in all these cases the formazanate act as ‘innocent’ supporting ligand without exhibiting redox-activity. Thus, there is a need for a better understanding of the ways in which the distinctive electronic properties of formazanates (including their reduced froms) influence the properties and reactivity of their metal complexes in order to make full use of the potential of these ligands.. 1.5 Overview of the thesis In this thesis we expand the study on formazanate complexes by employing formazanate metal complexes that have not been explored till date such as alkali (Na and K), alkaline earth (Mg and Ca) and first row transition (Fe) metal complexes.. In Chapter 2, we present the synthesis and characterization of alkali metal salts of formazanate ligands (ML, M = Na, K), which open up a new synthetic pathway towards formazanate complexes via salt metathesis. The salts discussed in this chapter show the high coordination flexibility of the formazanate framework that results from the presence of 4 nitrogen atoms in the ligand backbone.. In Chapter 3, we discuss the synthesis of alkaline earth formazanate complexes employing magnesium and calcium (ML2(THF)n, M = Mg, Ca). This can be achieved through two different pathways: (a) direct synthesis from the formazan ligand and a metal alkyl/amide or (b) through metathesis of alkali metal salts discussed in Chapter 2 and a metal halide source. All complexes are characterized using different spectroscopic techniques and they were .
(52) . . compared in terms of steric and electronic effect of the ligand and the Lewis acidity of the metal center.. In Chapter 4, we investigate the decomposition reactions that occur for formazanate Mg/Ca complexes containing a C6F5-substituent. The organic products are identified as tetrazepine and arylazoindazole derivatives, which are formed as a result of nucleophilic aromatic substitution. The electrochemical properties of these products are also presented.. In Chapter 5, the photochemical switching behaviour of a new class of azoheteroarenes (arylazoindazoles and functionalized derivatives) are investigated.. Photostationary states, quantum yields, fatigue resistance, kinetic and theoretical studies are discussed and compared for the different compounds.. In Chapter 6, the synthesis and characterization of a series of mono- and bis(formazanate)zinc complexes are covered (LZnMe and ZnL2, respectively). In this chapter we describe preliminary studies on the activity of those and some representative magnesium and calcium complexes (from Chapter 3) towards lactide oligomerization/polymerization. To extend these studies, a series of alkoxide zinc and magnesium formazanate complexes were synthesized and characterized ([LMOR]n, M = Mg, Zn) Redox properties of (formazanate) zinc alkoxides are also presented.. In Chapter 7, the synthesis and characterization of a bis(formazanate)iron complex is presented (FeL2). The π-acceptor properties of the formazanate ligand lead to unusual properties and as a result a rare example of spin-crossover in a pseudo-tetrahedral is presented in detail. Cyclic voltammetry studies reveal reversible and accessible formation of the singly reduced complex, and isolation and characterization of this species is addressed ([FeL2][NBu4]). The properties of both complexes are investigated through several spectroscopic, magnetic and computational techniques to obtain insight in the factors that contribute to their unusual electronic structure.. . .
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(54) . 1.6 References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40) (41) (42) (43) (44) (45) (46) (47) (48) (49).
(55) . Spargo, P. L. Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2004. Diederich, F.; Stang, P. J. Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2008. Colacot, T. J. Platin. Met. Rev. 2011, 55, 84–90. Chirik, P. J. Inorg. Chem. 2011, 50 (20), 9737–9740. Chaudhuri, P.; Verani, C. N.; Bill, E.; Bothe, E.; Weyhermüller, T.; Wieghardt, K. J. Am. Chem. Soc. 2001, 123 (10), 2213–2223. Herebian, D.; Bothe, E.; Bill, E.; Weyhermüller, T.; Wieghardt, K. J. Am. Chem. Soc. 2001, 123 (41), 10012–10023. J. A. McCleverty. Prog. Inorg. Chem. 1968, 10, 49. Chirik, P. J.; Wieghardt, K. Science 2010, 327 (5967), 794–795. Gray, H. B.; Williams, R.; Bernal, I.; Billig, E. J. Am. Chem. Soc. 1962, 84 (18), 3596–3597. Smolinsky, G.; Wasserman, E.; Yager, W. A. J. Am. Chem. Soc. 1962, 84 (16), 3220–3221. Gray, H. B.; Billig, E. J. Am. Chem. Soc. 1963, 85 (13), 2019–2020. Schrauzer, G. N.; Mayweg, V. P. J. Am. Chem. Soc. 1965, 87 (16), 3585–3592. Davison, A.; Edelstein, N.; Holm, R. H.; Maki, A. H. Inorg. Chem. 1963, 2 (6), 1227–1232. Evangelio, E.; Ruiz-Molina, D. Eur. J. Inorg. Chem. 2005, 2005, 2957–2971. Rittle, J.; Green, M. T. Science (80-. ). 2010, 330 (6006), 933–937. Jørgensen, C. K. Coord. Chem. Rev. 1966, 1 (1–2), 164–178. Pierpont, C. G. Coord. Chem. Rev. 2001, 216 (1–2), 99–125. Ray, K.; Petrenko, T.; Wieghardt, K.; Neese, F. Dalt. Trans. 2007, No. 16, 1552. De Bruin, B.; Hetterscheid, D. G. H.; Koekkoek, A. J. J.; Grutzmacher, H. Prog. Inorg. Chem. 2007, 55, 247−354. Jazdzewski, B. A.; Tolman, W. B. Coord. Chem. Rev. 2000, 200–202, 633–685. Stubbe, J.; van der Donk, W. A. Chem. Rev. 1998, 98 (2), 705–762. Lewis, E. A.; Tolman, W. B. Chem. Rev. 2004, 104 (2), 1047–1076. Kaim, W.; Schwederski, B. Coord. Chem. Rev. 2010, 254 (13–14), 1580–1588. Heyduk, A. F.; Zarkesh, R. A.; Nguyen, A. I. Inorg. Chem. 2011, 50 (20), 9849–9863. Zarkesh, R. A.; Ziller, J. W.; Heyduk, A. F. Angew. Chemie Int. Ed. 2008, 47 (25), 4715–4718. Nguyen, A. I.; Zarkesh, R. A.; Lacy, D. C.; Thorson, M. K.; Heyduk, A. F. Chem. Sci. 2011, 2 (1), 166– 169. King, E. R.; Hennessy, E. T.; Betley, T. A. J. Am. Chem. Soc. 2011, 133 (13), 4917–4923. Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012, 2 (2), 270–279. van der Vlugt, J. I. Eur. J. Inorg. Chem. 2012, 2012 (3), 363–375. Munhá, R. F.; Zarkesh, R. a; Heyduk, A. F. Dalton Trans. 2013, 42, 3751–3766. Blanchard, S.; Derat, E.; Desage-El Murr, M.; Fensterbank, L.; Malacria, M.; Mouriès-Mansuy, V. Eur. J. Inorg. Chem. 2012, 2012, 376–389. Dzik, W. I.; van der Vlugt, J. I.; Reek, J. N. H.; de Bruin, B. Angew. Chemie Int. Ed. 2011, 50 (15), 3356–3358. Poulos, T. L.; Finzel, B. C.; Howard, A. J. J. Mol. Biol. 1987, 195, 687–700. Lamb, D. C.; Lei, L.; Warrilow, A. G. S.; Lepesheva, G. I.; Mullins, J. G. L.; Waterman, M. R.; Kelly, S. L. J. Virol. 2009, 83, 8266–8269. Que, L.; Tolman, W. B. Nature 2008, 455 (7211), 333–340. Broere, D. L. J.; Plessius, R.; van der Vlugt, J. I. Chem. Soc. Rev. 2015, 44 (19), 6886–6915. Luca, O. R.; Crabtree, R. H. Chem. Soc. Rev. 2013, 42 (4), 1440–1459. Moxey, G. J.; Ortu, F.; Goldney Sidley, L.; Strandberg, H. N.; Blake, A. J.; Lewis, W.; Kays, D. L. Dalton Trans. 2014, 43 (43), 4838. Blackmore, K. J.; Lal, N.; Ziller, J. W.; Heyduk, A. F. J. Am. Chem. Soc. 2008, 130 (9), 2728–2729. Pierpont, C. Coord. Chem. Rev. 2001, 219–221, 415–433. Pierpont, C. G. Inorg. Chem. 2011, 50, 9766–9772. Lippert, C. A.; Arnstein, S. A.; Sherrill, C. D.; Soper, J. D. J. Am. Chem. Soc. 2010, 132 (11), 3879– 3892. Poddel’sky, A. I.; Cherkasov, V. K.; Abakumov, G. A. Coord. Chem. Rev. 2009, 253 (3–4), 291–324. Mederos, A.; Domı́nguez, S.; Hernández-Molina, R.; Sanchiz, J.; Brito, F. Coord. Chem. Rev. 1999, 193–195, 913–939. Petrenko, T.; Ray, K.; Wieghardt, K. E.; Neese, F. J. Am. Chem. Soc. 2006, 128 (13), 4422–4436. Tsai, Y.-C. Coord. Chem. Rev. 2012, 256, 722–758. Asay, M.; Jones, C.; Driess, M. Chem. Rev. 2011, 111, 354–396. Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. Rev. 2002, 102, 3031–3066. Webster, R. L. Dalt. Trans. 2017, 46 (14), 4483–4498..
(56) . . Itoh, S.; Morimoto, Y. In Chemical Science of π-Electron Systems; 2015; pp 715–730. Eisenstein, O.; Hitchcock, P. B.; Khvostov, A. V.; Lappert, M. F.; Maron, L.; Perrin, L.; Protchenko, A. V. J. Am. Chem. Soc. 2003, 125, 10790–10791. Avent, A. G.; Hitchcock, P. B.; Khvostov, A. V.; Lappert, M. F.; Protchenko, A. V. Dalt. Trans. 2004, 33, 2272–2280. Khusniyarov, M. M.; Bill, E.; Weyhermueller, T.; Bothe, E.; Wieghardt, K. Angew. Chem. Int. Ed. 2011, 50, 1652–1655. Takaichi, J.; Morimoto, Y.; Ohkubo, K.; Shimokawa, C.; Hojo, T.; Mori, S.; Asahara, H.; Sugimoto, H.; Fujieda, N.; Nishiwaki, N.; Fukuzumi, S.; Itoh, S. Inorg. Chem. 2014, 53, 6159–6169. Ibrahim, S. K.; Khvostov, A. V.; Lappert, M. F.; Maron, L.; Perrin, L.; Pickett, C. J.; Protchenko, A. V. Dalt. Trans. 2006, No. 21, 2591–2596. Chang, M.-C.; Roewen, P.; Travieso-Puente, R.; Lutz, M.; Otten, E. Inorg. Chem. 2015, 54 (1), 379– 388. Bamberger, E.; Wheelwright, E. Ber. Dtsch. Chem. Ges. 1892, 25, 3201–3213. Friese, P. Ber. Dtsch. Chem. Ges. 1875, 8, 1078–1080. Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2000. Berridge, M. V.; *; , Patries M. Herst, and A. S. T. Biotechnol. Annu. Rev. VOLUME 11 (ISSN: 13872656), 127–152. Kuhn, R.; Trischmann, H. Angew. Chem. 1963, 6, 294–295. Koivisto, B. D.; Hicks, R. G. Coord. Chem. Rev. 2005, 249, 2612−2630. Spin Labeling: Theory and Applications; Berliner, L. J., Reuben, J., Eds.; Biological Magnetic Resonance; Springer US: Boston, MA, 1989; Vol. 8. Kinoshita, K.; Yozo, M. Makromol. Chem. 1969, 124, 211−221. Chen, E. K. Y.; Chan-Seng, D.; Otieno, P. O.; Hicks, R. G.; Georges, M. K. Macromolecules 2007, 40, 8609−8616. Chung, G.; Lee, D. Chem. Phys. Lett. 2001, 350, 339. Ciofini, I.; Daul, C. A. Coord. Chem. Rev. 2003, 238, 187–209. Chang, M. C.; Dann, T.; Day, D. P.; Lutz, M.; Wildgoose, G. G.; Otten, E. Angew. Chemie - Int. Ed. 2014, 53, 4118–4122. Kabir, E.; Wu, C.-H.; Wu, J. I.-C.; Teets, T. S. Inorg. Chem. 2016, 55 (2), 956–963. Gilroy, J. B.; Ferguson, M. J.; McDonald, R.; Patrick, B. O.; Hicks, R. G. Chem. Commun. (Camb). 2007, 126–128. Hesari, M.; Barbon, S. M.; Staroverov, V. N.; Ding, Z.; Gilroy, J. B. Chem. Commun. 2015, 51, 3766– 3769. Barbon, S. M.; Reinkeluers, P. A.; Price, J. T.; Staroverov, V. N.; Gilroy, J. B. Chem. - A Eur. J. 2014, 11340. Barbon, S. M.; Price, J. T.; Reinkeluers, P. A.; Gilroy, J. B. Inorg. Chem. 2014, 53, 10585–10593. Barbon, S. M.; Staroverov, V. N.; Gilroy, J. B. J. Org. Chem. 2015, 80, 5226–5235. Gilroy, J. B.; McKinnon, S. D. J.; Koivisto, B. D.; Hicks, R. G. Org. Lett. 2007, 9 (23), 4837–4840. Gilroy, J. B.; McKinnon, S. D. J.; Kennepohl, P.; Zsombor, M. S.; Ferguson, M. J.; Thompson, L. K.; Hicks, R. G. J. Org. Chem. 2007, 72, 8062−8069. Gilroy, J. B.; Patrick, B. O.; McDonald, R.; Hicks, R. G. Inorg. Chem. 2008, 47 (4), 1287–1294. Ziessel, R.; Retailleau, P.; Elliott, K. J.; Harriman, A. Chem. Eur. J. 2009, 15, 10369−10374. Didier, P.; Ulrich, G.; Mély, Y.; Ziessel, R. Org. Biomol. Chem. 2009, 7, 3639− 3642. Collado, D.; Casado, J.; Rodríguez González, S.; López Navarrete, J. T.; Suau, R.; Perez-Inestrosa, E.; Pappenfus, T. M.; Raposo, M. M. M. Chem. Eur. J. 2011, 17, 498−507. Niu, S.; Ulrich, G.; Retailleau, P.; Ziessel, R. Tetrahedron Lett. 2011, 52, 4848− 4853. Shi, W.-J.; Lo, P.-C.; Singh, A.; Ledoux-Rak, I.; Ng, D. K. P. Tetrahedron Lett. 2012, 68, 8712−8718. Ulrich, G.; Barsella, A.; Boeglin, A.; Niu, S.; Ziessel, R. Chem. Phys. Chem. 2014, 15, 2693−2700. Zhou, Y.; Xiao, Y.; Chi, S.; Qian, X. Org. Lett. 2008, 10, 633−636. Wu, Y.-Y.; Chen, Y.; Gou, G.-Z.; Mu, W.-H.; Lv, X.-J.; Du, M.-L.; Fu, W.-F. Org. Lett. 2012, 14, 5226−5229. D’Aléo, A.; Gachet, D.; Heresanu, V.; Giorgi, M.; Fages, F. Chem. Eur. J. 2012, 18, 12764−12772. D’Aléo, A.; Fages, F. Photochem. Photobiol. Sci. 2013, 12, 500−510. Chang, M.-C.; Otten, E. Chem. Commun. (Camb). 2014, 50, 7431–7433. Chang, M. C.; Otten, E. Inorg. Chem. 2015, 54, 8656–8664. Mondol, R.; Snoeken, D. A.; Chang, M.-C.; Otten, E. Chem. Commun. 2017, 53 (3), 513–516. Schorn, W.; Grosse-Hagenbrock, D.; Oelkers, B.; Sundermeyer, J. Dalt. Trans. 2016, 45 (3), 1201–1207. Siedle, A. R.; Pignolet, L. H. Inorg. Chem. 1980, 19, 2052–2056. Protasenko, N. A.; Poddel’sky, A. I.; Bogomyakov, A. S.; Fukin, G. K.; Cherkasov, V. K. Inorg. Chem.. (50) (51) (52) (53) (54) (55) (56) (57) (58) (59) (60) (61) (62) (63) (64) (65) (66) (67) (68) (69) (70) (71) (72) (73) (74) (75) (76) (77) (78) (79) (80) (81) (82) (83) (84) (85) (86) (87) (88) (89) (90) (91) (92) (93). . .
(57)
(58) . (94) (95) (96) (97) (98) (99) (100). . 2015, 54 (13), 6078–6080. Zaidman, A. V.; Pervova, I. G.; Vilms, A. I.; Belov, G. P.; Kayumov, R. R.; Slepukhin, P. A.; Lipunov, I. N. Inorganica Chim. Acta 2011, 367 (1), 29–34. Zaidman, A. V.; Khasbiullin, I. I.; Belov, G. P.; Pervova, I. G.; Lipunov, I. N. Pet. Chem. 2012, 52 (1), 28–34. Hong, S.; Hill, L. M. R.; Gupta, A. K.; Naab, B. D.; Gilroy, J. B.; Hicks, R. G.; Cramer, C. J.; Tolman, W. B. Inorg. Chem. 2009, 48 (10), 4514–4523. Sigeikin, G. I.; Lipunova, G. N.; Pervova, I. G. Russ. Chem. Rev. 2006, 75 (10), 885–900. Gorbatenko, Y. A.; Pervova, I. G.; Lipunova, G. N.; Maslakova, T. I.; Lipunov, I. N.; Sigeikin, G. I. Russ. J. Appl. Chem. 2005, 78, 936–939. Rezinskikh, Z. G.; Pervova, I. G.; Lipunova, G. N.; Maslakova, T. I.; Gorbatenko, Y. A.; Lipunov, I. N.; Sigeikin, G. I. Russ. J. Coord. Chem. 2008, 34, 659–663. Pavlova, I. S.; Pervova, I. G.; Belov, G. P.; Khasbiullin, I. I. Pet. Chem. 2013, 53, 127–133..
(59) . Chapter 2 Alkali metal salts of formazanate ligands: diverse coordination modes as a result of the nitrogen-rich [NNCNN] ligand backbone Alkali metal salts of redox-active formazanate ligands were prepared, and their structures in the solid-state and solution determined. The nitrogen-rich [NNCNN] backbone of formazanates results in a varied coordination chemistry, with both the internal and terminal nitrogen atoms available for bonding with the alkali metal. The potassium salt K[PhNNC(ptol)NNPh]·2THF (1-K) is dimeric in the solid state and even in THF solution, as a result of the K atom bridging via interaction with a terminal N atom and the aromatic ring of a second unit. Conversely, for the compounds Na[MesNNC(CN)NNMes]·2THF (2-Na) and Na[PhNNC(tBu)NNPh] (3-Na) polymeric and hexameric structures are found in the solid state respectively. The preference for binding the alkali metal through internal N atoms (1-K and 2-Na) to give a 4-membered chelate, or via internal/external N atoms (5-membered chelate in 3-Na), contrasts the 6-membered chelate mode observed in our recently reported formazanate zinc complexes.. This chapter has been published: R. Travieso-Puente, M.-C. Chang and E. Otten*, Dalton Trans., 2014, 43, 18035–18041. DOI: 10.1039/C4DT02578D..
(60) . 2.1 Introduction Oxidative addition and reductive elimination are elementary reaction steps that are key to myriad (catalytic) transformations in organic and organometallic chemistry.1 Such reactions typically occur at transition metal centres, which possess two different oxidation states that can be reversibly addressed. Especially useful are transition metals for which there are two relatively stable oxidation states that differ by 2 electrons, so that odd-electron (radical) chemistry is avoided (e.g., the Pd(0)/Pd(II) redox couple in cross-coupling chemistry).2 Despite the vast success of this type of chemistry, it mostly relies on relatively expensive and noble late transition elements such as those in the platinum group. Therefore, it is desirable to develop similar reactivity for cheap, earth-abundant first-row transition elements. A serious impediment in this regard is the tendency of the first-row transition elements to engage in (unselective) 1-electron redox-reactions. A potential strategy to mitigate this undesirable behaviour could be the use of an organic ligand scaffold that engages in redox-reactions. These so-called ‘redox-active’ ligands have gained prominence recently and several examples are known where such ligands play an important role in allowing unusual redox-reactivity to occur.3–7 Our efforts in this area have recently focussed on developing formazanates as redox-active ligands.8,9 The Gilroy group has recently reported formazanate boron complexes and studied their redox-properties by cyclic voltammetry.10,11 Monoanionic formazanate ligands (derived from the parent formazans) are close structural analogues of β-diketiminates. In contrast to βdiketiminates, which are a well-established and useful class of ligands in a wide range of coordination complexes and catalysts,12 the transition metal chemistry of formazanate ligands remains largely unexplored. Considering possible synthesis routes to a variety of transition metal complexes containing formazanate ligands, we concluded that salt metathesis strategies would constitute a promising approach. Lithium, sodium and potassium salts are used most frequently in such reactions since the high lattice energy of the resulting alkali metal halides provides a large thermodynamic driving force. Here we describe the synthesis and structural features of several formazanate salts, which show a large structural diversity as a result of the nitrogen-rich [NNCNN] formazanate backbone. Formazanate ions are shown to bind to alkali metal cations to give 4- and 5-membered chelate rings. Further interactions with additional Natoms from the NNCNN ligand backbone or a cyano substituent, or with an aromatic moiety leads to unusual dimeric, hexameric or polymeric aggregates.13–16 . .
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(66) . . 2.2 Formazanate alkali metal salts 2.2.1 Solid-state structures Three different formazans (1-H–3-H) were synthesized using published procedures.17–19 Subsequent treatment with sodium or potassium hydride under anhydrous conditions allowed for the synthesis and structural characterization of the corresponding formazanate salts, which were obtained in reasonable to good yields as intensely coloured solid products (Scheme 2.1). Deprotonation of the neutral formazan PhNNC(p-tol)NNHPh (1-H) is readily achieved by stirring a THF solution of 1-H with potassium hydride as evidenced by a rapid colour change from dark red to violet and evolution of H2 gas. Crystallization of the resulting potassium salt was achieved by slow diffusion of hexane into a THF solution of the product to yield K[PhNNC(p-tol)NNPh]·2THF (1-K). While single crystal X-ray analysis confirmed the formation of the desired potassium formazanate salt, the structure shows some surprising features (Figure 2.1, pertinent bond distances and angles in Table 2.1). In contrast to alkali metal salts of the structurally related β-diketiminates (which bind the alkali metal through the NCCCN atoms to give 6-membered chelate rings), the formazanate anion in compound 1-K is bound through the internal nitrogen atoms (NNCNN) of the ligand backbone to give rise to a 4-membered chelate ring with K(1)-N(2) and K(1)-N(3) bond distances of 2.8723(16) and 2.9270(17) Å, respectively. One of the terminal N atoms (N4) interacts with a potassium atom of another (formazanate)K fragment to result in a dimeric structure. In addition, two carbon atoms of the p-tol ring show a close approach to this K atom with K-C(8) and K-C(9) distances of 3.2448(19) and 3.292(2) Å,respectively. p-tol. [Bu 4N] p-tol. [Bu 4N][Br]. Ph. N. N. N Ph. N. N. N. N. N. Ph. K. Ph O. KH. 1-Bu 4N. O. 1-H R tBu. 3-H. N NaH Ph. N. N Na. N. Ar. 1-K (dimer). N. N. NH. N. Mes Ar. Ph. NaH. 3-Na (hexamer). N. 2-H O. N C. Na O 1-H Ar = Ph; R = p-tol 2-H Ar = Mes; R = CN 3-H Ar = Ph; R = tBu. N. N N Mes. 2-Na (polymer). Scheme 2.1 Synthesis of alkali metal formazanate salts. . .
(67) . The interaction between (metal) cations and electron-rich π-systems (cation-π interactions) constitutes an important class of non-covalent interaction that has widespread application, including in biological systems.20 Also in the coordination chemistry of main group metals to organic ligands, this type of interactions can greatly affect the structures, stability and reactivity of the resulting complexes.21–25 In the case of 1-K, the interaction between the potassium cation and the p-tol moiety leads to a dramatic stabilization of the dimeric structure, such that it remains intact even in THF solution (vide infra).. Figure 2.1 X-ray structure of 1-K: asymmetric unit (left) and core of the dimeric structure (right) showing 50% probability ellipsoids. Hydrogen atoms are omitted for clarity. MesNNC(CN)NNHMes (2-H) with NaH or KH allows isolation of a the formazanate products M[MesNNC(CN)NNMes]·xTHF (M = Na; x = 2 (2-Na), M = K; x = 0 (2-K)) in good yield as orange powders that are insoluble in aromatic and aliphatic solvents. The number of THF molecules bound to the alkali metal center was determined by analysis of the 1. H NMR spectra: in case of 2-K, no THF was present, while for 2-Na the NMR suggests two. molecules of THF are coordinated. We were unable to obtain crystals of 2-K, so its solid-state structure is unknown. However, based on the observation of K+-arene interactions in the structure of 1-K, it is tempting to conclude that the coordination sphere around the potassium atom in 2-K is complemented by interactions with the Mes ring(s). The X-ray crystal structure of a related base-free β-diketiminate potassium salt has been reported in the literature.21 In the case of 2-Na, crystalline material was obtained by recrystallization from THF/hexane. X-ray analysis shows that 2-Na is a coordination polymer in which the cyano group is bridging to a second (formazanate)Na unit, resulting in a linear polymer chain in which the C(102)-C(112)-N(32)-Na(1) atoms reside on a crystallographic C2 axis (Figure 2.2, pertinent bond distances and angles in Table 2.1). Each sodium atom is coordinated by the internal nitrogens of the formazanate fragment to give a 4-membered chelate ring, as is observed in 1-K. .
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(73) Table 2.1 Selected bond distances and angles in compounds 1-K, 2-Na, 3-Na, and 1-Bu4N 1-K. 2-Naa. M(1) – N(1). 3-Na. 1-Bu4N. 2.4006(11). M(1) – N(2). 2.9270(17) 2.4563(11) 2.4840(10). M(1) – N(3). 2.8723(16). 2.4192(11). M(1) – N(4_a) 2.9342(17). 2.5754(11). N(1) – N(2). 1.285(2). 1.2809(14) 1.2810(13) 1.3033(14) 1.2987(19). N(3) – N(4). 1.306(2). 1.3170(14) 1.3036(18). N(2) – C. b. 1.357(3). 1.3632(12) 1.3633(12) 1.3549(15) 1.363(2). N(3) – C. b. 1.347(3). 1.3465(15) 1.357(2). a. The labelling scheme for 2-Na includes a suffix (1 or 2) for the C and N atoms due to the presence of two independent formazanate(Na) (e.g., Na(1)-N(21) and Na(2)-N(22)). b The central carbon in the ligand backbone: C(101) and C(102) in case of 2-Na, C(7) for all others.. In contrast to 1-K, the terminal N-atoms in 2-Na do not engage in further M-N interactions. The Na-N(formazanate) distances are 2.4563(11) and 2.4840(10) Å for Na(1)-N(21) and Na(2)-N(22), respectively, and the bonding within each formazanate ligand is fully symmetric as imposed by the crystallographic twofold symmetry. Two THF molecules and the cyano group complete the coordination sphere around each Na atom to give 5-coordinate geometry. The Na-N(cyano) distances are 2.3612(15) and 2.3732(15) Å, while Na-O(THF) distances vary between 2.3412(8) and 2.3878(9) Å. Unlike the situation in 1-K, there are no additional interactions between the aromatic moieties and the metal found in the solid state structure of 2-Na, which reflects the smaller size of Na compared to K, its lower π-philicity26 and the increased steric demands of the mesityl rings. The equivalent N-N and N-C bond lengths within the ligand backbone indicate full delocalization of the negative charge of the formazanate anion.. . .
(74) . Figure 2.2 X-ray structure of 2-Na: molecular structure (left) showing 50% probability ellipsoids, and packing diagram (view along a-axis, right). Hydrogen atoms and THF molecules (except O atoms) are omitted for clarity. Deprotonation of 3-H with NaH in THF afforded the product 3-NaTHF upon removal of the solvent. According to NMR, one molecule of THF is coordinated to this material. In contrast to compounds 1-K and 2-Na/K, which are very insoluble in aliphatic/aromatic solvents, the product 3-NaTHF is quite soluble in toluene and crystals suitable for X-ray analysis were obtained from toluene/hexane. The compound Na[PhNNC(tBu)NNPh] (3-Na) crystallizes from this solvent mixture without any THF coordinated to the Na centre. The resulting structure is a hexameric aggregate of (formazanate)Na units (Figure 2.3, pertinent bond distances and angles in Table 2.1). The Na cation is surrounded by the N(1) (terminal) and N(3) (internal) nitrogen atoms of a formazanate fragment to result in a 5-membered chelate ring. Its coordination sphere is complemented by N(4) of a second formazanate ligand. Additional weak interactions between Na and the ortho-C of the phenyl ring are observed with a Na(1)-C(6) distance of 3.0080(13) Å. Interactions between hexameric units are observed in the {Na(formazanate)}6 plane, resulting in a 2D network (see Figure 2.3) that is held together by cation-π interactions.. .
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(80) . . . Figure 2.3 X-ray structure of 3-Na: molecular structure (asymmetric unit, top left), hexameric aggregate (top right) showing 50% probability ellipsoids. Packing diagram showing the cation-π interactions as cyan dotted lines (view along c, bottom left; along b, bottom right). Hydrogen atoms are omitted for clarity. With the structural data shown above, it was established that both 4- and 5-membered chelate rings are possible upon coordination of the formazanate NNCNN-framework to alkali metal cations. The recent work by Hicks and co-workers18,27,28 and our group8,9 and some older examples of metal formazanate complexes29-34 show that in most of the cases a 6-membered chelation mode analogous to that observed for β-diketiminate complexes is preferred, but this apparently is not favourable for very ionic compounds as described here. We became interested in the structure of formazanate anions devoid of a coordinating metal cation. A salt metathesis reaction between 1-K and [Bu4N][Br] cleanly affords the ionic compound [Bu4N][PhNNC(p-tol)NNPh] (1-Bu4N), which may be obtained in crystalline form by diffusion of hexane into a THF solution. The solid state structure of 1-Bu4N (Figure 2.4, pertinent bond distances and angles in Table 2.1) shows no interaction between the tetrabutyl ammonium cation and formazanate anion. The bond distances within the formazanate backbone are indicative of a fully delocalized structure with N-N and N-C bond lengths that are statistically indistinguishable. The observation that a ‘linear’ conformation is formed in . .
(81) . the absence of a bound metal center suggests that electrostatic repulsions are minimized in this situation.. . Figure 2.4 X-ray structure of 1-Bu4N showing 50% probability ellipsoids, hydrogen atoms and the [Bu4N]+ fragment omitted for clarity.. 2.2.2 Solution-state structures: NMR studies Having established the solid-state structure of a series of formazanate salts, we now turn to the characterization of these compounds in solution. The potassium salt 1-K is insoluble in C6D6 but dissolves reasonably well in THF-d8. 1H NMR analysis of a THF-d8 solution of 1-K shows broadened resonances at room temperature. A series of (2D) NMR spectra was recorded between -60 and 80 °C in order to elucidate the structure of 1-K in solution (see Figure 2.5 for representative 1H NMR spectra). At 80 °C, the 1H NMR spectrum is consistent with a C2v symmetric structure with equivalent N-Ph moieties and 2 sets of resonances for the p-tol CH groups. Even at this temperature, one of these (presumably the p-tol o-CH) is broadened due to dynamic exchange. Upon cooling the NMR tube in the spectrometer to 0 °C or below, the number of resonances in the aromatic region is doubled (to a total of 10), which results from splitting of each resonance into 2 components with a 1:3 ratio. A series of 2D NMR experiments at -25 °C allowed assignment of all peaks and indicates that 1-K retains a dimeric structure in THF solution. Based on the observation of a 1:3 ratio for all aromatic signals (Ph and p-tol groups) even down to -60 °C it is concluded that in solution only one potassium atom is bridging the two formazanate ligands to give an asymmetric dimeric compound (Figure 2.6). In this structure the hydrogen atom sites that correspond to the smaller fraction are those that are in proximity of the coordinated potassium atom (shown in red in Figure 2.6), i.e. 2 of the p-tol CH groups (out of 8 total) and the N-Ph group (1 of the 4).
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(88) where K is bound. The 'free' N-Ph and p-tol groups are apparently not sufficiently different to be observed separately.. Figure 2.5 Variable-temperature 1H NMR spectra of 1-K.. . Figure 2.6 Schematic representation of the solution structure of 1-K (THF molecules omitted for clarity), highlighting the 1:3 ratio of resonances observed (red:black). A dynamic equilibrium between the sites of K-coordination is evidenced by exchange crosspeaks in the EXSY spectrum at -25 °C; all resonances that are affected by coordination of the potassium ion (see Figure 2.6) show exchange with those that are ‘free’. Upon increasing the temperature, broadening and eventually coalescence of the two sets of resonances is observed to obtain (averaged) C2v symmetry. NMR characterization of 1-Bu4N in THF-d8 shows that the appearance of the 1H spectrum is quite different compared to 1-K: the resonances are much sharper at room temperature, and chemicals shifts in the aromatic region are different. Most notably, the 13C NMR shift of the central NCN carbon atom appears at 153.17 ppm for 1-Bu4N, which is shifted downfield by ~ . .
(89) . 4.3 ppm relative to that in 1-K in line with what would be expected by removal of the coordinating cation. The alkali-metal salts of formazanates 2 and 3 are much more soluble in THF-d8, which indicates that the aggregates observed in the solid state are easily broken by the donor solvent. For 2-Na and 2-K, the NMR spectra are virtually identical suggesting that in THF solution the compounds exist primarily as solvent-separated ion-pairs: there is no influence of the alkali metal cation on the chemical shifts of the formazanate anion. For compound 3-Na, crystals of the hexameric aggregate that do not contain THF are sparingly soluble in C6D6. Its 1H NMR spectrum is consistent with a symmetric structure in which both Ph groups are equivalent. It is not clear from these data what the aggregation state of 3-Na is in this case, but it seems likely based on its solubility that the hexameric aggregate observed in the solid state is broken up in solution.. 2.3 Conclusions We have shown that formazanate alkali metal salts are accessible in a straightforward manner by deprotonation of the parent formazans with NaH or KH. The solid-state structure of several derivatives was determined and shown to favour coordination of the alkali metal cation to the internal nitrogen atoms of the formazanate [NNCNN] ligand backbone in most cases. The sodium salt of [PhNNC(tBu)NNPh]- forms an unusual hexameric structure in which the Na atom is bound in an asymmetric fashion through an internal and terminal N atom. These finding contrast the 6-membered chelate rings that have been previously observed from coordination of the terminal nitrogen atoms to (transition) metal centres. We anticipate that the versatile coordination behaviour due to the nitrogen-rich backbone of formazanates will lead to transition metal complexes with flexible coordination environment that can adapt their steric profile to (for example) accommodate binding of substrates. In combination with the established redox-active nature of formazanates, these features could be useful in transition metal catalysis. The synthesis of alkali metal formazanate salts described herein presents an opportunity for their use in salt metathesis with transition metal salts, the products of which could find subsequent application in (redox-)catalysis.. .
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(95) . . 2.4 Experimental 2.4.1 General considerations All manipulations were carried out under nitrogen atmosphere using standard glovebox, Schlenk, and vacuum-line techniques. Toluene, pentane and hexane (Aldrich, anhydrous, 99.8%) were passed over columns of Al2O3 (Fluka), BASF R3-11-supported Cu oxygen scavenger, and molecular sieves (Aldrich, 4 Å). THF (Aldrich, anhydrous, 99.8%) was dried by percolation over columns of Al2O3 (Fluka). All solvents were degassed prior to use and stored under nitrogen. C6D6 and THF-d8 (Aldrich) were vacuum transferred from Na/K alloy and stored under nitrogen. [Bu4N]Br (Sigma-Aldrich) was used as received, NaH (SigmaAldrich, 60% dispersion in mineral oil) and KH (Sigma-Aldrich, 30 wt% dispersion in mineral oil) were washed several times with hexane to free from mineral oil and subsequently dried in vacuo to obtain a fine powder. The compounds PhNNC(p-tol)NNHPh (1-H),18 MesNNC(CN)NNHMes (2-H)19 and PhNNCH(tBu)NNPh (3-H)17 were prepared via literature procedures.. 2.4.2 Synthesis of formazanate salts Synthesis of K[PhNNC(p-tol)NNPh]·2THF (1-K). 1 To a mixture of 1-H (2.00 g, 6.36 mmol) and KH (306 mg, 7.63 mmol) was added 40. p-tol N Ph. N N. N. Ph. K. mL of THF. Gas evolution was observed immediately and the color O. O. changed from dark red to violet. After stirring at room temperature for 16 h, the solution was separated from solid material by centrifugation. 1-K (dimer). and the clear supernatant was transferred to a clean flask. Removal of all volatiles and subsequent washing with hexane afforded 2.76 g of 1-K as a dark purple powder (5.56 mmol, 87%). Crystals suitable for X-ray analysis were obtained by slow diffusion of hexane into a THF solution of 1-K. 1H NMR (THF-d8, 80 ºC, 500 MHz): δ 7.65 (br, 2H, p-tol o-H), 7.33 (d, 4H, J = 7.8, Ph o-H), 7.15 (t, 4H, J = 7.6, Ph m-H), 7.02 (d, 2H, J = 7.8, p-tol m-H), 6.82 (t, 2H, J = 7.2, Ph p-H), 3.62 (m, 4H, THF), 2.29 (s, 3H, CH3), 1.77 (m, 4H, THF). 13C NMR (THF-d8, 80 ºC, 126 MHz): δ 158.54 (s, Ph ipso-C), 134.68 (s, p-tol p-C), 130.78 (br d, p-tol o-CH), 129.14 (d, Ph m-CH), 127.95 (d, p-tol m-CH), 122.64 (d, Ph p-CH), 120.39 (Ph oCH), 67.45 (THF), 25.37 (THF), 21.45 (q, CH3). The NCN and p-tol ipso-C were not observed. 1H NMR (THF-d8, -20 ºC, 500 MHz): δ 8.25 (br d, 1H, J = 7.6, p-tol o-H), 7.52 (br, 2H, Ph o-H), 7.33 (d, 3H, J = 7.5, p-tol o-H), 7.29 (d, 6H, J = 7.7, Ph o-H), 7.22 (t, 2H, J = 7.4, Ph m-H), 7.16 (t, 6H, J = 7.4, Ph m-H), 7.05 (d, 4H, J = 7.4, p-tol m-H), 6.90 (t, 1H, J . .
(96) . = 7.2, Ph p-CH), 6.82 (t, 3H, J = 7.1, Ph p-CH).. 13. C NMR (THF-d8, -20 ºC, 126 MHz): δ. 158.05 (Ph ipso-C), 157.42 (NCN), 135.25 (p-tol p-C), 133.92 (p-tol ipso-C), 132.21 (p-tol oCH), 129.42 (Ph m-CH), 129.26 (Ph m-CH), 128.25 (p-tol m-H), 128.03 (p-tol m-H), 123.40 (Ph p-CH), 122.50 (Ph p-CH), 120.78 (Ph o-H), 120.21 (Ph o-H), 68.39 (THF), 26.54 (THF), 21.84 (CH3). Anal. calcd. for C28H33N4KO2: C 67.71, H 6.70, N 11.28; found: C 67.74, H 6.62, N 11.31. N. Synthesis of K[MesNNC(CN)NNMes] (2-K). To a mixture of 2-H. C. (1.86 g, 5.58 mmol) and KH (265 mg, 6.63 mmol) was added 30 mL. N. N. Mes. N. N. of THF. After stirring at room temperature overnight, the supernatant. K. was separated by centrifugation and evaporated to dryness. The. 2-K. Mes. residue was washed with toluene (10 mL) and hexane (10 mL) and subsequently dried in vacuo to give 2-K as a dark orange powder (1.95 g, 5.25 mmol, 94%). Attempts to obtain crystalline material were unsuccessful. 1H NMR (THF-d8, 25 ºC, 500 MHz): δ 6.72 (s, 2H, Mes m-H), 2.24 (s, 6H, Mes o-CH3), 2.20 (s, 3H, Mes p-CH3). 13C NMR (THF-d8, 25 ºC, 126 MHz): δ 152.27 (Mes ipso-C), 132.71 (Mes p-C), 130.32 (Mes o-C), 129.88 (Mes m-CH), 129.47 (NCN), 116.56 (CN), 21.17 (Mes p-CH3), 19.71 (Mes o-CH3). A satisfactory elemental analysis could not be obtained. Synthesis of Na[MesNNC(CN)NNMes]·2THF (2-Na). A mixture of 2N. H (85 mg, 0.26 mmol) and NaH (10 mg, 0.41 mmol) was dissolved in THF (20 mL), and H2 gas evolution was observed immediately. The reaction mixture was stirred overnight. After the solvent was removed, a. C N Mes. N N. N. O Na. O. Mes. new portion of THF was added and the solution was filtered to separate 2-Na (polymer). from excess NaH. The solvents was removed from the extract to give 2Na as a pure orange powder (107 mg, 0.22 mmol, 85%). Recrystallization from THF/hexane gave crystals suitable for X-ray analysis. 1H NMR (THF-d8, 25 ºC, 500 MHz): δ 6.71 (s, 4H, Mes m-CH), 3.62 (m, 4H, THF), 2.25 (s, 12H, Mes o-CH3), 2.20 (s, 6H, Mes p-CH3), 1.78 (m, 4H, THF). 13C NMR (THF-d8, 25 ºC, 126 MHz): δ 152.12 (Mes ipso-C), 132.68 (Mes pC), 130.43 (Mes o-C), 129.86 (Mes m-CH), 128.94 (NCN), 116.99 (C≡N), 68.39 (THF), 25.56 (THF), 21.16 (Mes p-CH3), 19.88 (Mes o-CH3). Anal. calcd. for C28H38N5NaO2: C, 67.31; H, 7.67; N, 14.02; found: C 67.02, H 7.61, N 14.05.. .
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