Citation
Quesada Vilar, M. (2007, March 29). Spin-transition frameworks based on bistetrazole and triazine ligands. Retrieved from https://hdl.handle.net/1887/11463
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2
Physical Techniques and General
Syntheses
2.1 Introduction
This chapter deals with a description of the various physical techniques used during the (thesis) research to characterise the compounds synthesised. In the first part some standard techniques used for a general characterisation of the different materials are presented. A more specific description of the single crystal X-ray structure determinations carried out on the different compounds is then presented. The descriptions of the structures are presented in the order in which the corresponding compounds appear in the thesis. This is then followed by a description of some more specific techniques used to study some of the compounds. An explanation of the syntheses and characterisations of all the materials used during the thesis is then presented. This part is subdivided in two sections, ligands and complexes, which in turn are divided into two sections, bistetrazole-based compound and triazine-based compounds.
2.2 Physical Techniques
2.2.1 General Laboratory Techniques
Elemental Analysis (C, H, N, S) were performed on a Perkin-Elmer 2400 series II at the Gorlaeus Laboratories by Mr Jos van Brussel. The iron content was determined by ICP (Inductively Coupled Plasma), using a Varian Vista-MPX apparatus.
UV-Visible-NIR. UV-Vis spectra were obtained on a Perkin-Elmer Lambda 900 spectrophotometer using the diffuse reflectance technique, with MgO as a reference. A sample holder mounted on a Dewar and in thermal contact with the refrigerant through a copper rod was used to perform measurements at temperatures around 100 K. The spectral range used was 200-1200 nm. All samples, crystalline or powder were powdered when placed in the sample holder.
FTIR. Spectra were obtained on a Perkin Elmer Paragon 1000 FTIR spectrophotometer equipped with a Golden Gate ATR device, using the diffuse reflectance technique (4000-300 cm-1, resolution of 4 cm-1).
NMR. Spectra were recorded on a DPX300 (300 MHz) apparatus. The used deuterated solvents are specified there were the characterisation of the corresponding material is described (section 2.3).
Thermogravimetric Measurements. These were carried out on a Setaram TAG 24 apparatus in the temperature range of RT to 573 K. The experimental uncertainty was ± 0.5 K.
Surface Area Measurements. The specific surface area was determined (single–point method) using a ThermoQuest Analyzer.
2.2.2 Single Crystal X-ray Diffraction
X-ray diffraction measurements are presented in this section. This information is given in the order in which the studied compounds appear in this thesis.
[FeII(btzpol)1.8(btzpol-OBF3)1.2](BF4)0.8(H2O)0.8(CH3CN) (Chapter 3)
Measurements were made using Si(111) monochromated synchrotron radiation (λ = 0.6894 Å) and a Bruker APEX II CCD diffractometer using standard procedures and programs for Station 9.8 of Daresbury SRS.1 Data were collected on a Bruker APEX II CCD diffractometer using the APEX 2 software and processed using SAINT v7.06a.2 The crystal was mounted onto the diffractometer at low temperature under nitrogen at ca. 150 K. The structure was solved using direct methods with the SHELXTL program package.3 All non- hydrogens were refined anisotropically except the partial water oxygen atoms O1 and O1'.
Displacement parameter restraints were used in modeling the BF4 anion. Geometrical restraints were used in modeling the OH and OBF3 disorders. Hydrogens were placed geometrically where possible (Uij = 1.2 Ueq for the atom to which they are bonded (1.5 for methyl)). It proved impossible to place or find the OH, water and acetonitrile hydrogens and so they were omitted from the refinement. The function minimised was [w(Fo2-Fc2)] with reflection weights w-1= [ 2Fo2+ ( g1P)2+ ( g2P)] where P= [max Fo2+ 2 Fc2]/3.
Btzmp; 1,2-bis(tetrazol-1-yl)-1-methylpropane (Chapter 3)
X-ray data were collected at 150 K on a Nonius KappaCCD diffractometer on rotating anode with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The structure was solved with the Direct Methods program SHELXS864 and refined on F2 with SHELXL-97. 5 H atoms were included in the refinement at calculated positions and refined riding on the atoms to which they are attached with standard geometry and isotropic displacement parameter constraints from SHELXL-97.5
[Fe(µ-btzmp)2(btzmp)2](ClO4)2 (Chapter 3)
X-ray data were collected at 200 K and 100 K, respectively, on a Nonius KappaCCD diffractometer on rotating anode with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) and corrected for absorption using the PLATON/MULABS program 6. The structures were solved with the DIRDIF99 package7 and refined on F2 with SHELXL-97.5 H atoms were included in the refinement at calculated positions and refined riding on the atoms to which they are attached with standard SHELXL-97 geometry and isotropic displacement parameter constraints.5 The perchlorate anion was included with a two-site disorder model for the 200 K structure.
[Fe(btzx)3](PF6)2·MeOH (Chapter 4)
X-ray data were collected at 200 K and 100 K, respectively, on a Nonius KappaCCD diffractometer on rotating anode with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The structures were solved with the Direct Methods program SHELXS864 and refined on F2 with SHELXL-97.5 H atoms were included in the refinement at calculated positions and refined riding on the atoms to which they are attached with standard SHELXL-97 geometry and isotropic displacement parameter constraints.5 The methanol inclusion solvent molecule
was included with a disorder model. Both the carbon and the oxygen atom of the methanol molecule are disordered in two positions with an occupational factor of 0.5.
[Fe(btzx)3](CF3SO3)2·CH3CN (Chapter 4)
X-ray data were collected at 170 K and 100 K respectively on a Nonius KappaCCD diffractometer on rotating anode with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The structures were solved with the Direct Methods program SHELXS864 and refined on F2 with SHELXL-97.5 H atoms were included in the refinement at calculated positions and refined riding on the atoms to which they are attached with standard SHELXL-97 geometry and isotropic displacement parameter constraints.5 Crystals are merohedrally twinned (matrix:
[–1 0 0 / 0 –1 0 / 0 0 1]) with twin fraction ~ 50:50. Merohedral twinning is a special case of crystallographic twinning where the lattices of twin (different) domains (in a single crystal) overlap in three dimensions. For instance, two domains in which one of them is rotated 180º with respect to the other.
[Fe2(dpyatriz)2(H2O)2(CH3CN)2](ClO4)4 (Chapter 6)
Intensity data and cell parameters were recorded at 293(2) K on a Nonius Kappa CCD diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). DENZO- SMN was used for data integration and SCALEPACK corrected data for Lorentz-polarisation effects. Absorption corrections were applied for all data using the DIFABS program.8 The SIR92 software package9 and SHELXL 975 included in the WinGX package were, respectively, used for phase determination and structure refinement. Direct methods of phase determination followed by some subsequent different Fourier maps led to an electron density map from which most of the non-hydrogen atoms were identified in the asymmetric unit of the unit cell. With subsequent isotropic refinement and Fourier difference synthesis, all of the non-hydrogen atoms were identified. Atomic coordinates, isotropic and anisotropic displacement parameters of all non-hydrogen atoms were refined by means of a full matrix least-squares procedure on F2. H atoms were included in the refinement at calculated positions, riding on the carbons atoms with an isotropic thermal parameter fixed 20% and 50%, respectively, higher than Csp2 and Csp3 atoms to which there were attached.
[Fe2(dpyatriz)2(H2O)2(CH3OH)2](BF4)4 (Chapter 6)
X-ray data were collected at 150 K on a Nonius KappaCCD diffractometer on rotating anode with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The structure was solved with direct methods10 and refined with SHELXL-975 against F2 of all reflections. All hydrogen atoms were located in the difference Fourier map. The OH hydrogen atoms were kept fixed at their located position; the other hydrogen atoms were refined as rigid groups.
Drawings, geometry calculations and checking for higher symmetry were performed with the program PLATON.6
[Fe2(dpyatriz)2Cl2](CF3SO3)2 (Chapter 6)
Intensity data and cell parameters were recorded at room temperature (293 K) on a Bruker AXS Smart 1000 single-crystal diffractometer (Mo-Kα radiation) equipped with a CCD area detector. The data reductions were performed using the SAINT2 and SADABS11 programs. The structure was solved by direct methods using the SIR97 program10 and refined on F02 by full-matrix least-squares procedures, using the SHELXL-97 program.5 The non- hydrogen atoms were refined with anisotropic atomic displacement parameters with the exception of the disordered triflate molecule. The hydrogen atoms were included in the refinement at idealised geometries (C–H 0.95 Å) and refined “riding” on the corresponding parent atoms. The weighting scheme used in the last cycle of refinement was w = 1/[σ2(F02)+(0.1687P)2], where P = (F02 + 2Fc2)/3. Molecular geometry calculations were carried out using the PARST97 program.12 Drawings were obtained by ORTEP3 in the WinGX suite.13 Calculations were carried out on a DIGITAL Alpha Station 255 computer.
[Co2(dpyatriz)2Cl2](CF3SO3)2 and [Ni2(dpyatriz)2Cl2](CF3SO3)2 (Chapter 6)
X-ray diffraction data for both compounds were recorded on a Nonius Kappa CCD diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). In all cases, the crystals were mounted onto the diffractometer at low temperature under nitrogen at ca.
150 K. DENZO-SMN was used for data integration,14 and SCALEPACK corrected the data for Lorentz-polarisation effects.14 The structures were solved using direct methods within the SHELXTS program.3 Further refinements were done using the SHELXTL package.3, 5 All non-hydrogens were refined anisotropically, while hydrogens were fixed at calculated positions on their riding atom.
[Fe(dpyatriz)2(SCN)2] (Chapter 7)
Data were collected at 150, 200 and 250 K using a Bruker APEX II CCD diffractometer on station 9.8 of the Synchrotron Radiation Source at CCLRC Daresbury Laboratory, using λ = 0.6897 Å from a Silicon 111 monochromator. The structure was solved by direct methods using the programme SHELXS-97.3 The refinement and all further calculations were carried out using SHELXL-97.5
[Fe3(dpyatriz)2(BF4)2(CH3CH2CN)4](BF4)4·3(CH3CH2CN)·H2O (Chapter 8)
The crystal structure was determined using a preliminary data set collected under a cold nitrogen stream (100 K) using a Cu-Kα wavelength (1.5418 Å) on a FR591 rotating-anode X- ray generator equipped with KappaCCD area detector. In this in-house experiment, the small crystal size (0.1 × 0.1 × 0.1 mm3) and the crystallographic disorder of the counter ions and solvent molecules produced a weak diffraction pattern (with this conventional source only about 1800 observable unique reflections were collected). Using this data set, the isotropic refinement of the structure let to an unsatisfactory R-factora (0.165) and imprecise bond
a R(F) ) Σ║Fo│–│Fc║/ Σ │Fo│for Fo > 4σ(Fo), Rw(F2) ) [Σw(Fo2 -Fc2)2/ ΣwFo2]1/2, w ) 1/( σ2(Fo2) + 0.035F2), F
= (Fo + 2Fc)/3.
lengths (estimated standard error on Fe-N bond length > 0.01 Å). Therefore, the final data collection was performed using synchrotron radiation (XRD1 diffraction beam-line of Elettra, Trieste) with a monochromatic X-ray beam (λ = 1.0000 Å) and MarCCD detector. The solution containing the crystals was pipetted into a drop of paratone-N oil. One crystal was picked directly from the drop using a thin nylon loop and flash-cooled in a cold (100 K) nitrogen stream. The crystal to detector distance was 40 mm and a total of 360° of data were collected with 3° oscillation per frame. The diffraction data were indexed (triclinic unit cell) and integrated using DENZO 14 and scaled with SCALEPACK14. The structure of the complex was solved by direct methods using SHELXS15 and anisotropically refined (H atoms at the calculated positions) by full-matrix least-squares methods on F2 (SHELXL-97).5 The disordered counter ions (3 BF4-) and solvent molecules (4 CH3CH2CN) were refined using both thermal and geometrical restrains. The electron density maps and the refinement of the structure revealed that the asymmetric unit is constituted by a dpyatriz ligand coordinated to two hexacoordinated FeII ions, one of which (Fe1) lies on a crystallographic centre of symmetry (1/2 occupation factor). The three positive charges (1.5 FeII ions) of the Fe ions are balanced by three BF4– anions, one of which is F-coordinated to Fe1. Four propionitrile molecules complete the asymmetric unit, two of which are N-coordinated in cis position to Fe2. The terminal atoms of both coordinated BF4- and CH3CH2CN molecules were disordered in two positions. The other remaining counter ions and solvent molecules were found severely disordered in two and three different close sites.
[Fe(Cl-dpyatriz)2(H2O)4](ClO4)2·4H2O (Chapter 9)
X-ray diffraction data for [Fe(Cl-dpyatriz)2(H2O)4](ClO4)2 4·H2O was recorded on a Nonius Kappa CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). DENZO-SMN was used for data integration,14 and SCALEPACK corrected the data for Lorentz-polarisation effects.14 The crystal was mounted onto the diffractometer at low temperature under nitrogen at ca. 150 K. The structure was solved using direct methods within the SHELXTL program. Further refinements were done using the SHELXTL package.3, 5 All non-hydrogens were refined anisotropically, while hydrogens were fixed at calculated positions on their riding atom.
Geometric calculations and molecular graphics for all compounds were carried out either with PLATON 6, ORTEP 6 or Mercury.16 A table with the relevant crystallographic information of each compound is given in each chapter.
2.2.3 X-ray powder diffraction measurements
X-ray powder diffraction measurements mentioned in chapter 8 were performed on a Bragg-Brentano θ/2θ Siemens D-500 diffractometer (radius = 215.5 mm) and an A PAAR TTK temperature camera. Cu Kα radiation (1.54 Å), selected by means of a secondary graphite monochromator. The divergence slit was of 1º and the receiving slit of 0.15º. The starting and the final 2θ angles were 4 and 32º, respectively. The step size was 0.05º 2θ and a variable measuring time (of the order seconds) per step. An inert atmosphere was achieved by
passing a flow of N2 gas through the sample holder chamber. To create the N2-solvent currents, the nitrogen gas was passed through a trap full of the corresponding solvent (propionitrile, acetonitrile and 1-propanol), before entering the chamber. In all cases, the compound was placed in the sample holder together with a small excess of mother liquor to assure that no change, due to desorption of the solvent, had occurred prior to the measurement. The compound was then immediately cooled to -20 ºC, and the current was subsequently connected.
2.2.4 Magnetic measurements
Magnetic susceptibility measurements were carried out using a Quantum Design MPMS-5S SQUID magnetometer. The SQUID probes the total magnetisation of the sample by measuring the induced currents in a Josephson junction when moving the sample in between coils. The accessible field is up to 5 T, and the temperature ranges from 1.8 to 400 K.
Data were corrected for magnetisation of the sample holder and for the diamagnetic contributions, which were estimated from the Pascal’s tables.17 Samples made out of single crystals were gently crushed prior to measurement.
2.2.5 LIESST and LITH experiments
LIESST (Light-Induced Excited Spin State Trapping) experiments were performed within the SQUID cavity with the use of a 110 W halogen lamp and a green-blue filter (300- 600 nm). The light was driven through a Y-shaped optical fibre that replaced the usual insert commonly used.18, 19
2.2.6 Mössbauerb
Mössbauer spectroscopy is a characterisation technique involving the nucleus of certain metals. If a metal is in its excited state, the transition to the ground state will occur with the emission of a gamma quantum. This quantum gamma ray may be absorbed by another nucleus of the same kind (same number of protons (Z) and neutrons (N)), whereby the transition to the excited state occurs (Figure 2.1). This phenomenon is called nuclear resonance absorption of gamma rays. In an ideal case, the absorption and emission energy is the same, and the resonance will thus be in its maximum. Nuclear resonance absorption in isolated atoms and/or molecules never occurs, due to the recoil effect.
b For more information on Mössbauer Spectroscopy the reader is referred to reference 20.
Figure 2.1. Nuclear resonant absorption of γ-radiation. The γ ray emitted by one nucleus is then absorbed by another nucleus. The resonant fluorescence competes with this process that occurs when the energy is transmitted from the nucleus to the electron shell. Figure taken from Gütlich et al.20
As the nucleus in its excited state emits a gamma ray, it moves with a velocity v in opposite direction to the emission of the gamma ray. This movement of the nucleus is known as the recoil effect. The kinetic recoil energy is expressed by:
ER = Mv2/2 (2.1)
or ER = Eo2/2Mc2 = 5.37 × 10-4 Eo2/A eV (2.2)
ER is the recoil energy, M is the mass of the molecule, v is the velocity, Eo is the quantum energy of the gamma ray, c is the velocity of light and A is the atomic number of the nucleus.
ER is in the 10-3 order of energies, six orders bigger than the natural line broadening (10-9).
As shown in Figure 2.2, the recoil energy causes the displacement of the absorption and emission band. The energy of the emission band will now be Eγ = Eo – ER. The absorption band will need to compensate the energy of the recoil and thus will now be Eγ = Eo + ER. The emitting and absorption bands are thus separated by 2ER, which is 106 times larger than the natural line width. Consequently there is no overlap between the two bands and thus no nuclear resonance occurs. In the gas phase, due to the movement of the atoms the Doppler energy that needs to be considered. This energy, which is in the order of the recoil energy, adds to Eγ,and in a low probability may cause the absorption and emission bands to overlap.
In the solid state the situation is different. The recoil energy in this case is expressed as:
ER = Etr + Evib (2.3)
The translational energy (Etr) is calculated with 2.1, but now M stands for the mass of the whole crystal. This energy turns out to be small due to the large mass of the crystal and is therefore neglected. Thus, ER is converted into lattice vibrational energy which, if bigger than the characteristic lattice vibration (phonon) but smaller than the displacement energy, will be dissipated by heating the near lattice surroundings of the Mössbauer atom. In cases where the energy is smaller than the phonon energy, the Evib causes a change of the energy of the
oscillators, which is now quantised by multiples of ħωE (ωE = Einstein frequency). This model tells us that there is a certain probability f, that the emission or absorption of the gamma rays occurs without recoil. The recoil-free fraction is thus represented by f. The discovery and explanation of this effect by R. Mössbauer permits the use of this spectroscopic technique as a characterisation method. The recoil-free fraction value increases with:
- decreasing transition energy Eγ, - decreasing temperature
- decreasing Debye temperature (ΘD), a measure for the strength of the bonds.
a) b)
Figure 2.2. Energy shift of the emission and absorption line due to the recoil energy.
Schematic representation of a Mössbauer spectrum. Figure taken from Gütlich.20
Mössbauer measurements are plotted as the relative transmission (T) versus the Doppler velocity (see Figure 2.2 b). The relative transmission is the proportion of the count rate during resonance (Tv0) to the count rate without absorption (Tv∞). The Doppler velocity (speed in mm/s) is used to control the energy of the source emission. If the source is moved the energy of the emission changes while that of the absorption (receiver) remains the same.
Consequently the overlap is not complete anymore and the absorbance decreases (see Figure 2.2 b). The nature of the Mössbauer spectrum will be determined by the Mössbauer nucleus.
Apart from the intensity and width of the signal, two key parameters, the isomer shift (δ) and the quadrupole splitting (∆EQ), reveal information over the chemical and physical properties of the compound (see below). These parameters arise from the interaction of the nuclear energy levels with the electrons around the nucleus. For the case of an FeII spin-transition compound, the HS and LS state have characteristic isomer shift and quadrupole splitting values due to their different distribution of the electrons in the d orbitals.
Isomer Shift (δ)
The isomer shift δ of a resonance line or multiplet, is defined as the shift of the resonance maximum, or the centre of the multiplet, compared to the relative velocity v = 0.
This is effect is due to a monopole exchange interaction between the nucleus and the s electrons. It helps determine i.e. the oxidation states of the nucleus as it is very sensitive to the distribution of the valence electrons. Spin states, bond properties, covalency or electronegativity can also be obtained from the isomer shift. For a spin-transition compound the LS state has δ values of around 0.2 mms-1, while for the HS state the value is around 1 mms-1
Quadrupole Splitting (∆EQ)
The quadrupole is the splitting of the resonance signal due to the electric quadrupole interaction between the electric quadrupole moment of the nucleus and an inhomogeneous electric field near the nucleus. From the quadrupole splitting one may obtain information of the oxidation state, spin state or symmetry of the molecule. For the case of an FeII spin- transition compound, the LS state has its electrons symmetrically distributed in the t2g orbitals and therefore shows quadrupole splittings close to 0 mms-1. In contrast, the HS state has an unsymmetrical distribution of the electrons resulting in quadrupole splitting values of around 1 mms-1.
In the present study, Mössbauer spectra were obtained using a constant acceleration spectrometer with a 57Co/Rh source, which was moved via triangular velocity wave, and the γ-counts were collected in a 512 multi-channel analyzer. Data collection is only allowed at the target temperature, controlled with an Oxford Instruments cryostat. The data were folded, plotted and fitted by a computer procedure. Velocity calibration was done using a 25 µm thick metallic iron foil and the Mössbauer spectral parameters are given relative to this standard at room temperature (see Chapter 6).
2.2.7 FTIR
Variable temperature infrared (IR) spectra were recorded on a FTIR 750 Nicolet spectrometer equipped with an Oxford Optistat CFV continuous flow cryostat connected to an ITC 601 temperature controller. IR spectra were recorded at temperatures between 40 and 360 K through two successive cycles in a KBr pellet (Chapter 7).
2.2.8 Differential Scanning Calorimetry
Differential Scanning Calorimetry is a thermoanalytical technique in which the differences in the amount of heat required to increase the temperature of a sample and that of a reference are measured as a function of temperature. When for instance the sample undergoes a transition, the amount of heat needed to increase its temperature will be different from that of the reference. In this way, one can measure the amount of heat absorbed (endothermic process) or released (exothermic process) during the transition.
Differential Scanning Calorimetry measurements for compounds reported in Chapter 7 were done with a Perkin Elmer Pyris apparatus using aluminium pans. The response of the empty aluminium pan was measured prior to filling it with the sample. The sample was then measured following the same procedure. The thermal equilibrium was checked at the initial
and final temperatures of each scan by 4 min isotherms. The heat capacity of the sample was then calculated from both measurements. Temperature and enthalpy calibrations were made with a standard sample of cyclohexane using its melting (279.69 K, 2678 J⋅mol-1) and crystal- to-crystal transitions (186.10 K, 6740 J⋅mol-1).
Differential Scanning Calorimetric measurements described in chapter 3 and 4 were performed on a Mettler Toledo DSC 821. The sample was introduced in an aluminium pan and was then placed in the oven, together with an empty pan that was used as a reference. The temperature and enthalpy calibration were done with a standard sample of Indium that has a phase transition at 429.6(0.2) K and 28.45(2) Jg-1.
2.3 Syntheses of the ligands 2.3.1 Tetrazole-based ligands
Tetrazoles should be handled with care, as they are explosive!
General experimental procedure for the synthesis of bistetrazoles.
All bistetrazole–based ligands were synthesised according to the procedure reported by Kamiya and Saito.21 0.056 mmol of the corresponding diamine, 0.113 mmol of sodium azide and 0.57 mmol of triethylorthoformate were dissolved in 100 ml of acetic acid, and stirred at 90 ºC for at least 12 hours. The solvent was removed under reduced pressure and a white solid, i.e. the crude (general name = 1α,β-bis(tetrazol-1-yl)alkane) precipitated. The material was then washed with water and the product was air-dried. As the purification steps may differ, for each bistetrazole specified these will be specifically described (see below).
1,2-bis(tetrazol-1-yl)-1-methylpropane (btzmp)
The synthesis of this ligand follows exactly the general procedure described above. After removal under reduced pressure of about half of the solvent used during the reaction, water was added and the target crystalline material precipitated from the solution. The compound was then filtered and washed with distilled water. The compound was dried in contact with air. The pure product was obtained in a yield of 65 %. 1H NMR (300 MHz, MeOH): δ 1.8 (s, 6H, (CH3)2-C), 5 (s, 2H, (C-CH2-ttz), 8.9 (s, 1H, ttz), 9.1 (s, 1H, ttz) ppm. IR (ν, cm-1) = 3132.5 (νCttz-H). Anal. Calcd. (found) for C6H10N8: C, 37.11 (36.60) %; H, 5.19 (5.64) %; N 57.7 (57.19) %.
1,3-bis(tetrazol-1-yl)-2-propanol (btzpol)
25 g (0.278 mol) of 1,3-diamino-2-propanol, 354 g (2.38 mol) of triethylorthoformate, and 41.5 g (0.639 mol) of sodium azide were dissolved in 400 ml of acetic acid and heated at 90 °C for 2 days. After cooling to room temperature, HCl (conc.) was added to the solution and a first crop of compound, i.e. the monotetrazole (1-amino-3-(1H-tetrazol-1-yl)propan-2- ol) derivative was isolated by filtration and discarded. The filtrate was then dried and
columned (Eluent: MeOH 10 / CH2Cl2 90). The pure product was obtained in 10% yield (m = 5.45 g). 1H NMR (300 MHz, d6-DMSO) δ 4.30 (m,1H, tz-CH2-CH), 4.43 (dd, 2H, tz-CH2), 4,70 (d, 2H, tz-CH2), 9,32 (s, 2H, ttz-H5) ppm. IR (ν, cm-1): 3131.5 (νCttz-H), 3322.5 (νO-H).
Anal. Calcd (found) for C5H8N8O: C, 30.61 (29.79) %; N, 57.12 (57.15) %; H, 4.11 (4.37) %.
m-xylylene(bis)tetrazole (btzx)
5 g (0.037 mol) of m-xylylenediamine, 54 g (0.363 mol) of triethylorthoformate, and 4.79 g (0.074 mol) of sodium azide were dissolved in 90 ml of acetic acid and heated at 90 °C for 2 days. After cooling down to room temperature, the solvent was evaporated under reduced pressure. The remaining yellow solid was washed with methanol and water yielding the ligand as a white powder. Yield = 64 %. 1H NMR (300 MHz, d6-DMSO): δ 5.7 (s, 4H, ttz- CH2-ph), 7.4 (m, 4H, ph), 9.5 (s, 2H, ttz) ppm. IR (ν, cm-1): 3116.5 (νCttz-H), Anal. Calcd.
(found) for C10H10N8: C, 49.58 (49.09) %; N, 46.26 (45.59) %; H, 4.16 (4.13) %.
2.3.2 Triazine-based ligands
General synthetic procedure for the preparation of the triazine-based ligands.
The syntheses of the triazine-based ligands have been performed following the procedure developed by De Hoog et al.22 The substitution of each chloride atom of the triazine ring can be selectively achieved through careful control of the temperature of the reaction. The first chloride atom can be substituted at 0 ºC, the second at room temperature and the last under reflux of the corresponding solvent of the reaction (THF, acetonitrile, 1,4-dioxane, etc).
2,4,6-(dipyridin-2-ylamino)-[1,3,5]-triazine (Dpyatriz)
2,4,6-trichloro-[1,3,5]-triazine (5.00 g, 27.11 mmol) was dissolved in tetrahydrofuran (50 ml) in a two-necked round-bottom flask. Three equivalents (10.51 g, 81.34 mmol) of N- ethyldiisopropylamine (dipea) were subsequently added and the reaction vessel was cooled to 0 ºC. 2,2’–Dipyridylamine (13.93 g, 81.34 mmol) was then added portionwise. After completion of the addition, the reaction mixture was warmed to room temperature, and refluxed for 48 hours. The solid obtained was isolated on a glass filter, and washed with ethanol (3 × 50 ml). Yield = 67 %. 1H NMR (300 MHz, d6-DMSO): δ 7.1 (d, 6H, 3-py-H), 7.43 (dd, 6H, 5-py-H), 7.66 (dd, 6H, 4-py-H), 8.21 (d, 6H, 6-py-H) ppm. Anal. Calcd. (found) for C33H24N12: C, 67.34 (66.67) %; N, 28.55 (28.51) %; H, 4.11 (4.67) %. ESI-MS (m/z):
[M+H+] = 589.
2-chloro[4,6-(dipyridin-2-ylamino)]-[1,3,5]-triazine (Cl-bdpat)
2,4,6-trichloro-[1,3,5]-triazine (5 g, 27.11 mmol) was dissolved in tetrahydrofuran (50 ml) in a two-necked round-bottom flask. Two equivalents (7.01 g, 54.22 mmol) of N- ethyldiisopropylamine (dipea) were added and the two-necked round-bottom flask was cooled
to 0 ºC. 2,2’–Dipyridylamine was then added portionwise (9.3 g, 54.3 mmol). After completion of the addition, the reaction mixture was warmed to room temperature and stirred for 48 hours. The light-yellow solid obtained was isolated on a glass filter, and washed with ethanol (3 × 50 ml). Yield = 50 %. 1H NMR (300 MHz, d6-DMSO): δ 7.1 (d, 4H, 3-py-H), 7.43 (dd, 4H, 5-py-H), 7.66 (dd, 4H, 4-py-H), 8.21 (d, 4H, 6-py-H) ppm. Anal. Calcd. (found) for C23H16ClN9: C, 60.86 (60.12) %; N, 27.77 (27.43) %; H, 3.55 (3.21) %.
2-aminopyrene-[4,6-(dipyridin-2-ylamino)]-[1,3,5]-triazine (pe-bdpat)
One equivalent of Cl-bdpat (1.00 g, 2.2 mmol) and one equivalent of 1-aminopyrene (0.48 g, 2.2 mmol) were dissolved in 5 ml of dry THF in a special microwave tube. The tube was sealed and placed in a microwave Emrys synthesiser machine for 2 hours at 80 W (130 ºC).
The resulting solid material was filtered and washed with THF. The light-yellow solid was then dissolved in CH2Cl2 containing one equivalent of dipea. After a few hours stirring, the compound was filtered through silica on a glass filter to remove the ammonium salt produced.
The solvent was removed under reduced pressure, and the target material was obtained as a green crystalline powder. Yield = 73 %. 1H NMR (300 MHz, d6-DMSO) for ligand + 3·MeOH: δ 7.1 (d, 4H, 3-py-H), 7.43 (dd, 4H, 5-py-H), 7.66 (dd, 4H, 4-py-H), 8.21 (d, 4H, 6- py-H) ppm. Anal. Calcd. (found) for C42H40N10O3: C, 69.03 (69.10) %; N, 19.17 (19.22) %;
H, 5.24 (5.20) %. ESI-MS (m/z): [M+H+] = 636.25.
2-phthalimide-4-yl-amino [4,6-(dipyridin-2-yl-amino)]-[1,3,5]-triazine (ph-bdpat)
One equivalent of Cl-bdpat (1.00 g, 2.2 mmol) and one equivalent of 4-aminophthalimide (0.38 g, 2.2 mmol) were dissolved in 5 ml of dry THF in a special microwave tube. The tube was then sealed and placed in a microwave Emrys synthesiser machine for 2 hours at 130 ºC.
The resulting light-brown solid material was filtered and washed with THF. The ph-bdpat compound was used without any further purification. Yield = 42 %. 1H NMR (DMSO-d6, 300 MHz): δ 7.55 (4H, 3-py-H), 7.86 (4H, 5-py-H), 7.22 (4H, 4-py-H), 8.33 (4H, 6-py-H), 7.76 (H, phtal), 7.73 (d, H, phtal), 7.39 (d, H, phtal) ppm. Anal. Calcd. (found) for C31H21N11O2: C, 64.24 (58.29) %; H, 3.65 (3.32) %; N, 26.58 (24.20) %.
2.4 Preparation of the coordination compounds
Numbers in bold correspond to the numbers that the compounds will have in the thesis.
Caution, perchlorate derivatives may be explosive!!
2.4.1 Tetrazole-based complexes [Fe(btzmp)4](ClO4)2 (1)
One equivalent of btzmp (0.10 g, 0.5 mmol) was dissolved in 10 ml of methanol in an Erlenmeyer. One equivalent of Fe(ClO4)2·4H2O (0.13 g, 0.5 mmol) was dissolved in 5 ml of methanol containing approximately 20 mg of ascorbic acid. The iron(II) solution was added to
the ligand solution, and the resulting mixture was heated to 50 ºC for 2 hours. Afterwards, the complex solution was left unperturbed for one week at room temperature while slow evaporation of the solvent took place. When the compound started to crystallise (almost dry!), the Erlenmeyer was closed, and the solution was left unperturbed for another two days. The title compound was then filtered, and was washed with 20 ml of methanol. Yield = 10 %. IR (ν, cm-1): 3130 (νCttz-H), 1084 (νCl-O) Anal. Calcd. (found) for C24H40Cl2FeN32O8: C, 27.94 (28.22) %; N, 43.45 (43.66) %; H, 3.91 (3.80) %.
[FeII(btzpol)1.8(btzpol-OBF3)1.2](BF4)0.8(H2O)(CH3CN) (2)
50 mg (0.26 mmol) of btzpol dissolved in 5 ml of acetonitrile were added to 29 mg (0.09 mmol) of Fe(BF4)2·6H2O dissolved in 5 ml of acetonitrile. The solution was heated for an hour at 50 °C, after which the solution was allowed to stand at room temperature. Colourless single crystals appeared after two days, as a result of the slow evaporation of the solvent at room temperature. The white crystals were then washed with acetonitrile. Yield = 10 % Compound 2 loses the solvent molecules, resulting in [(Compound 2)-(H2O)-0.5·(CH3CN)]
(2a). In all batches crystallised, the loss of solvent molecules led to the same composition.
Anal. Calcd. (found) for 2a, [FeII(btzpol)1.8(btzpol-OBF3)1.2](BF4)0.80.5·(CH3CN), C17H28.2B2F6.8FeN25O4.2: C, 23.59 (23.25) %; N, 42.13 (42.35) %; H, 3.01 (2.96) %.
[Fe(btzx)3](PF6)2·MeOH (3)
One equivalent of btzx (50 mg, 0.21 mmol) and two equivalents of NH4PF6 (68.5 mg, 0.42 mmol) dissolved in 10 ml of dry methanol were added to one equivalent of FeCl2·4H2O (41.5 mg, 0.21 mmol) dissolved in 5 ml of dry methanol, containing about 20 mg of ascorbic acid and 3 ml of triethylorthoformate. The resulting solution was heated for an hour at 50 °C. A white crystalline solid appeared after approximately 4 days, via the slow evaporation of the solvent at room temperature. The crystals were washed with methanol. Yield = 30 %. IR (ν, cm-1) = 3160 (νCttz-H), 833 (νP-F). Anal. Calcd. (found) for C31H34F12FeN24OP2: C, 33.71 (33.81) %; N, 30.43 (30.11) %; H, 3.10 (3.34) %.
[Fe(btzx)3](CF3SO3)2·CH3CN (4)
Three equivalents of btzx (50 mg, 0.21 mmol) dissolved in 5 ml of acetonitrile were added to one equivalent of Fe(CF3SO3)2·6H2O (32.3 mg, 0.07 mmol) dissolved in 5 ml of acetonitrile containing about 20 mg of ascorbic acid. The solution was heated for an hour at 50 °C. The solution was then placed in a tube and ether was added until a light colourless precipitate had formed. Next, the tube was capped and left unperturbed for 2–3 hours. Transparent single crystals appeared on the walls of the tube. The crystals were then washed with acetonitrile.
Yield = 56 %. IR (ν, cm-1): 3112 (νCttz-H), 1265 and 1233 (νC-F), 1145 and 1035 (νS-O). Anal.
Calcd. (found) for C34H33F6FeN25O6S2: C, 36.4 (35.94) %; N, 31.22 (30.67) %; H, 2.97 (3.03)
%.
[Fe(btzx)3](ClO4)2·CH3CN (5)
Three equivalents of btzx (100 mg, 0.41 mmol) were dissolved in 15 ml of acetonitrile. A solution of one equivalent of Fe(ClO4)2 (35 mg, 0.14 mmol) and around 20 mg in 15 ml of acetonitrile was added to the ligand solution. The resulting transparent solution was heated for two hours and filtered. The filtrate was left unperturbed to allow the slow evaporation of the solvent. A white crystalline powder appeared after a few days. This solid material was filtered and washed with acetonitrile. The compound may be formulated as:
[Fe(btzx)3](ClO4)2·CH3CN. Yield = 40 %. IR (ν, cm-1): 1080.3, 1354.7, 1435.5, 1505.4, 3136.3 Anal. Calcd. (found) for C32H33Cl2FeN25O8: C, 37.59 (37.29) %; N, 34.25 (33.95) %;
H, 3.25 (3.01) %.
2.4.2 Triazine-based complexes
[Fe2(dpyatriz)2(CH3CN)2(H2O)2](ClO4)4 (6)
To a solution of Fe(ClO4)2·6H2O (188 mg, 0.5 mmol) in acetonitrile (75 ml) was added dpyatriz (150 mg, 0.413 mmol), and the mixture was stirred. Slow diffusion of diethyl ether into this solution yielded pinkish parallelepiped crystals of [Fe2(dpyatriz)2(CH3CN)2(H2O)2](ClO4)4·2CH3CN in a yield of 50%. The complex [Fe2(dpyatriz)2(CH3CN)2(H2O)2](ClO4)4 immediately loses the coordinated acetonitrile molecules upon exposure to air, which are presumably replaced by water. The resulting species may be formulated as [Fe2(dpyatriz)2(H2O)4](ClO4)4·4H2O (6a) (see Chapter 5). Anal.
Calcd. (found) for C66H64Cl4Fe2N24O24: C, 43.30 (42.94) %; N, 18.40 (17.95) %; H, 3.52 (3.05) %.
[Fe2(dpyatriz)2(H2O)2(CH3OH)2](BF4)4 (7)
A solution of ligand dpyatriz (200 mg, 0.34 mmol) in methanol (20 ml) was added to a stirred solution of Fe(BF4)2·6H2O (344 mg, 1.01mmol) and excess ascorbic acid (30 mg, 0.01 mmol) in methanol (10 mL), resulting in a green solution. After slow evaporation of the solvent (3-4 days) at room temperature, green crystals (suitable for X-ray diffraction analysis) of [Fe2(dpyatriz)2(H2O)2(CH3OH)2](BF4)4·6H2O were formed in a yield of 30%, These crystals were collected by filtration, washed with methanol and dried in air. IR (ν, cm-1): 1001.5, 1053.9, 1373.3, 3430, 3631.1. Anal. Calcd. (found) for C74H84B4F16Fe2N24O10: C, 46.09 (45.75) %; N, 17.43 (17.99) %; H, 4.39 (4.06) %.
[Fe2(dpyatriz)2Cl2](CF3SO3)2·2C8H7N (8)
A solution of ligand dpyatriz (230 mg, 0.39 mmol) in benzyl cyanide (99 %, 25 mL) was added to a stirred solution of Fe(CF3SO3)2·6H2O (230 mg, 0.50 mmol) in benzyl cyanide (99%, 10 ml), followed by the addition of an excess of ascorbic acid (30 mg, 0.01 mmol). To this mixture, a solution of NBu4Cl (54 mg, 0.194 mmol) in benzyl cyanide (5 ml) was added and the resulting reaction mixture was stirred for several minutes. The resulting solution was
filtered, and the filtrate was placed into a pressure-sealed tube, which was tightly closed and introduced in an oven at 100 ºC. After two days at this temperature, yellow single crystals of [Fe2(dpyatriz)2Cl2](CF3SO3)2·2C8H7N, suitable for X-ray diffraction, were obtained. These crystals were collected and washed with methanol and diethyl ether. Yield = 15 %. IR (ν, cm-1): 995.8, 1263.3, and 1374.0. Anal. Calcd. (found) for C84H62Cl2F6Fe2N26O6S2: C, 53.32 (53.03) %; N, 19.25 (19.55) %; H, 3.30 (3.54) %.
[Ni2(dpyatriz)2Cl2](CF3SO3)2·2C8H7N (9)
Similarly to [Fe2(dpyatriz)2Cl2](CF3SO3)2, the title compound was prepared from Ni(CF3SO3)2·6H2O and dpyatriz. Blue single crystals were obtained after two days, which were washed with methanol and diethylether. Yield = 30 %. IR (ν, cm-1): 995.8, 1263.5, and 1374.3. Anal. Calcd. (found) for C84H62Cl2F6N26Ni2O6S2: C, 53.16 (53.31) %; N, 19.19 (19.34) %; H, 3.29 (3.15) %.
[Co2(dpyatriz)2Cl2](CF3SO3)2·2C8H7N (10)
The synthetic procedure used to prepare [Fe2(dpyatriz)2Cl2](CF3SO3)2 was applied to obtain the title compound from Co(CF3SO3)2·6H2O (as the starting metal salt). Purple crystals were obtained after two days, which were washed with methanol and diethylether. Yield = 34 %.
IR (ν, cm-1): 995.8, 1263.5, and 1374.3. Anal. Calcd. (found) for C84H62Cl2Co2F6N26O6S2: C, 53.14 (53.33) %; N, 19.25 (19.25) %; H, 3.29 (3.54) %.
[Fe(dpyatriz)2(NCS)2] (11)
A solution of NH4SCN (3 mg, 0.04 mmol) in methanol (10 ml) was added to a solution of [Fe2(dpyatriz)2Cl2](CF3SO3)2·2C8H7N (40 mg, 0.02 mmol) in methanol (50 ml). The resulting reaction mixture was left unperturbed to allow a slow evaporation of the solvent. After one day, small yellow single microcrystals of [Fe(dpyatriz)2(SCN)2], suitable for X-ray diffraction analysis were obtained, which were collected by filtration, and washed with methanol. Yield
= 50 % (14 mg). IR (ν, cm-1): 995.8, 1263.5, and 1374.3. Anal. Calcd. (found) for C68H48FeN26S2 (M = 1349 g mol-1): C, 60.53 (58.61) %; H, 3.59 (3.82) %; N, 26.99 (26.23)
%. The lower experimental carbon value might be due to the violent decomposition of the ligand dpyatriz above 250 °C.22
[Fe(dpyatriz)2(NCBH3)2)2] (12)
A solution of NaNCBH3 (2 mg, 0.03 mmol) in acetonitrile (10 ml) was added to a solution of [Fe2(dpyatriz)2Cl2](CF3SO3)2·2C8H7N (27 mg, 0.02 mmol) in acetonitrile (20 ml). The resulting reaction mixture was left unperturbed and the solvent was allowed to slowly evaporate. After one day, small brown single microcrystals appeared which were collected by filtration, and washed with methanol. Yield = 50% (14 mg). Anal. Calcd. (found) for C68H54B2FeN26: C, 62.21 (58.94) %; H, 4.15 (4.12) %; N, 27.74 (27.23) %. The lower
experimental carbon value might be due to the violent decomposition of the ligand dpyatriz above 250 °C.22
[Fe4(dpyatriz)2(N(CN)2)6](CF3SO3)2·7MeOH (13)
A solution of NaN(CN)2 (1.6 mg, 0.02 mmol) in methanol (10 ml) was added to a solution of [Fe2(dpyatriz)2Cl2](CF3SO3)2·2C8H7N (152 mg, 0.1 mmol) in methanol (50 ml). The resulting reaction mixture was left unperturbed and the solvent was allowed to slowly evaporate. After one day, small yellow single microcrystals appeared, which were collected by filtration and washed with methanol. Yield = 50 % (14 mg). Anal. Calcd. (found) for C87H76F6Fe4N42O13S2: (tetranuclear entity) C, 45.05 (45.01) %; H, 3.30 (3.45) %; N, 25.36 (25.20) %; Fe, 9.63 (10.96) %.
[Fe3(dpyatriz)2(BF4)2(CH3CH2CN)4](BF4)4·4(CH3CH2CN) (14)
25 µl of water were added to a solution of 52 mg (0.154 mmoles) of Fe(BF4)2·6H2O and 35 mg (0.198 mmol) of ascorbic acid in propionitrile (3 ml). After only a few seconds of stirring, a floculate appeared. A solution of 60 mg (0.10 mmol) of dpyatriz ligand in propionitrile (3 ml) was added to this turbid mixture. After complete reaction of the dpyatriz ligand (clear solution), the solution was filtered and the filtrate was introduced in a tube that was capped but not sealed. This tube was then placed in an oven at a temperature of 95 ºC. After one night, purple (white in the oven) single crystals of [Fe3(dpyatriz)2(BF4)2(CH3CH2CN)4](BF4)4·4(CH3CH2CN). 14 immediately loses the CH3CH2CN molecules upon exposure to air, which are replaced by water (see below).
[Fe3(dpyatriz)2(BF4)2(H2O)4](BF4)4·4H2O (15)
Single crystals of 14 were taken from the mother liquor and left in a vial in contact with air for one day. Within a few minutes, the purple crystals (14) converted to a yellow crystalline powder, named as 15. The rate of substitution of propionitrile by atmospheric water molecules was controlled by the size and number of hole(s) punched in the cap of the vial. In all cases, the same resulting [Fe3(dpyatriz)2(H2O)4(BF4)2](BF4)4·4H2O species was obtained, but with different crystallinity. IR (ν, cm-1): 3447.2 (νO–H), 1374.3 (triazine vibration), 1056, 1017 (νB–F). Anal. Calcd. (found) for C66H64B6F24Fe3N24O8: C, 39.44 (40.30) %; H, 3.21 (3.50) %; N, 16.73 (16.44) %.
[Fe3(dpyatriz)2(amyl)6](BF4)6 (14·amyl)
50 mg (0.025 mmol) of 15, obtained from 14, are mixed with 5 ml toluene (not soluble). 21 mg of amyltriazole (4-pentyl-1,2,4-triazole) ligand (0.150 mmol) is dissolved separately in another 5 ml of toluene. This solution is then added to the solution of 15. The heterogeneous mixture is then stirred for 12 hours, filtered and washed with toluene. The resulting compound may be formulated as [Fe3(dpyatriz)2(amyl)6](BF4)6. IR (ν, cm-1): 3129.2 (νC-H(triazole ring)),
2932.3 (aliphatic chain), 1379.9 (triazine), 1093 (νB–F) Anal. Calcd. (found) for C108H126B6F24Fe3N42: calcd. C, 48.03 (47.83) %; H, 4.70 (4.88) %; N, 21.78 (22.74) %.
[Fe3(dpyatriz)2(ethyl)6](BF4)6 (14·ethyl)
50 mg (0.025 mmol) of 15, obtained from 14, are mixed with 5 ml toluene (not soluble).
14.67 mg of ethylltriazole (4-ethyl-1,2,4-triazole) (0.150 mmol) is dissolved separately in another 5 ml of toluene. This solution is then added to the solution of 15. The heterogeneous mixture is then stirred for 12 hours, filtered and washed with toluene. The resulting compound may be formulated as [Fe3(dpyatriz)2(ethyl)6](BF4)6. IR (ν, cm-1): 3116.4 (νC-H(triazole ring)), 1380.4 (triazine), 1026 (νB–F) Anal. Calcd. (found) for C108H126B6F24Fe3N42: calcd. C, 44.15 (45.13) %; H, 3.71 (4.42) %; N, 24.03 (24.84) %.
* No detailed information is reported on the synthesis of the triazole ligands, which were obtained as reported in Bayer et al.23 1974
[Fe(Cl-bdpat)2(H2O)4](ClO4)2 4·H2O (16)
100 mg (0.22 mmoles) of 2-chloro[4,6-(dipyridin-2-ylamino)]-[1,3,5]-triazine (Cl-bdpat) were dissolved in 15 ml of acetonitrile. A solution of 168 mg (0.66 mmoles) of Fe(ClO4)2 in 5 ml of acetonitrile was added, and the resulting reaction mixture was stirred for 10 minutes.
Yellow/green single crystals were obtained within one day by ether diffusion into the corresponding acetonitrile solution. Yield = 40 %. The resulting compound may be formulated as [Fe(Cl-bdpat)2(H2O)4](ClO4)2·4H2O. IR (ν, cm-1): 1062.2, 1386.1 and 3402.2.
Anal. Calcd. (found) for C46H48Cl6Fe2N18O24: calcd. C, 35.38 (34.96) %; H, 3.10 (3.32) %; N, 16.15 (16.13) %.
2.5 References
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20. Gütlich, P.; Link, R.; Trautwein, A. X., Mössbauer Spectroscopy and Transition Metal Chemistry. Springer-Verlag: New York, 1978; Vol. 3.
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22. de Hoog, P.; Gamez, P.; Driessen, W. L.; Reedijk, J., Tetrahedron Lett. 2002, 43, 6783-6786.
23. Bayer, H. O.; Cook, R. S.; Von Meyer, W. C. US Patent, 1974 3,674,810.
