Synthesis and coordination chemistry of chelating verdazyl radicals

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by

Martin Trent Lemaire B.Sc., Brandon University. 1996

A Dissertation Submitted in Partial Fulfillment o f the Requirements for the Degree o f

DOCTOR OF PHILOSOPHY

in the Department o f Chemistry

We accept this dissertation as conforming to the required standard

Dr. EL-G. Hicks. Supervisor (Department o f Chemistty)

Dr. D? J. Berg. D e p a jt^ h ta l M ember (Department o f Chemistry)

Dr. T. M. Pyles. Departmdhtal M ember (Department o f Chemistry)

Dr. J. T. Buckfey, Outside MembCfXDepartment ofBiochem istry & Microbiology)

__________________________________________________________

Dr. R. C. Thompson. Kxtemal Examiner (Department o f Chemistry. University o f British Columbia)

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Supervisor: Dr. Robin G. Hicks

Ab s t r a c t

A series o f new 1,5-dimethyI-6-oxo-verdazyl radicals was prepared bearing substituents such as 2-pyridyl. 2.2'-bipyridyi. and 2,2'-bipyrimidine, among others, in the 3 position o f the verdazyl ring, generating a viable family o f new open-shell chelating ligands. All verdazyl radical precursors (including aldehydes and tetrazanes) were characterized using elemental analysis, MS, 'H /'^C NM R and FT-IR spectroscopies. The radicals were obtained by oxidation o f the tetrazane precursors with benzoquinone. NalOa, or K3Fe(CN)6, and were isolated in pure crystalline form (in two cases as 1:1 molecular complexes with the hydroquinone reaction by-product) or as powders. Crystalline radicals exhibited indefinite stability to ambient conditions, but the radicals obtained as powders were immediately characterized and used in subsequent coordination reactions as a result o f decomposition. All verdazyl radicals were characterized by EPR, UV-visible, FT-IR spectroscopies, and high-resolution MS. In selected cases, variable temperature magnetic susceptibility data were obtained and fairly strong intermolecular anti ferromagnetic interactions were observed. Based on structural and magnetic data, as well as DFT calculations, we postulate a new intermolecular exchange mechanism for organic solids analogous to metal-oxide-based superexchange.

A wide-variety o f homoleptic and heteroleptic mono- and bimetallic coordination complexes with diamagnetic and paramagnetic transition metal salts were prepared with the verdazyl ligands described above. Metal-verdazyl complexes were structurally characterized, in many cases using X-ray crystallography, and in all cases by elemental analysis, and by MS, FT-IR, and UV-visible spectroscopies. Variable temperature magnetic susceptibility data indicated very strong ferromagnetic intramolecular nickel(II)-verdazyl exchange coupling in a wide-range o f complexes (Jnî-vii ranging from +35 to > +240 c m ') . Anti ferromagnetic manganese( 11)-verdazyl exchange was observed in a number o f complexes, generally around -5 0 cm ' in magnitude. Exchange couplings between verdazyls and nickel(II) or manganese(II) centres was rationalized with established molecular orbital symmetry arguments. Cobalt(II)-verdazyl exchange coupling was tentatively assigned as ferromagnetic, however, no quantitative conclusions were drawn owing to other complications associated with cobalt(II) magnetochemistry.

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verdazyls was weakly anti ferromagnetic in all cases.

Ruthenium(II)-verdazyl complexes were prepared as mimics o f well known Ru(bipy)3~^ com plexes and the electronic properties (including absorption and emission spectroscopy, EPR, and CV) o f these complexes were investigated. Interesting absorption features, including CT transitions between ruthenium(Il) centres and verdazyl ligands, and w eak emission upon excitation into the ruthenium(II)-bipy CT band, were observed from these complexes at room temperature. Complicated EPR spectra were obtained from selected ruthenium(II)-verdazyl complexes, suggesting some interaction with the ruthenium (ll) centre, but no quantitative conclusions were drawn. CV data suggest facile verdazyl reductions and oxidative stability o f the verdazyl ligand in selected complexes.

Examiners:

Dr. R. G. Hicks, S u p e rv i^ r (Department o f Chemistry)

Dr. D. J. Berg, Deparfmental Mefhber (Department o f Chemistry)

Dr. T. M. Fyles. Departmental Member (Department o f Chemistry)

DprfTT. Buckley, Outside M em Ber(bepartm ent o f Biochemistry & Microbiology)

Dr. R. C. Thomffson, External Examiner (Department o f Chemistry. University o f British Columbia)

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Ta b l e o f Co n t e n t s

P R E L IM IN A R Y PA G E S

Abstract...ii

Table o f Contents...iv

List o f Figures...ix

List o f Schemes... xvi

List o f Tables... xviii

List o f Numbered Com pounds... xx

List o f Abbreviations...xxix

Acknowledgements...xxxvi

Ch a p t e r I In t r o d u c t i o n a n dc o n t e x t... I 1.1 Molecular m agnetism ... 1

1.1.1 What is m agnetism ?... 2

1.1.2 Magnetic behaviour o f paramagnets: Van Vleck formula, the Curie law. and m agnetization... 4

1.1.3 Long-range magnetic ordering...7

1.1.4 Intramolecular magnetic exchange interactions...9

1.1.5 The magnetic properties of two-spin molecular system s...11

1.2 Current efforts towards m olecule-based magnetic m aterials... 12

1.2 .1 What are molecule-based magnetic m aterials?...12

1.2.2 The organic molecule/polymer strategy to molecular magnetic m aterials...13

1.2.2.1 High-spin molecules and polyradicals...14

1.2.2.2 Organic molecular crystal ferromagnets... 18

1.2.2.3 Charge-transfer ferromagnets...21

1.2.3 Coordination complexes with diamagnetic ligands...22

1.2.3.1 Hexacyanometallates...23

1.2.3.2 Two-dimensional layered oxalate complexes...25

1.2.4 Coordination complexes with paramagnetic ligands...26

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1.2.4.3 N itroxide and nitrony! nitroxide ra d ic a ls ... 30

1.2.4.4 C helating nitroxide ra d ic a ls...34

1.3 Aims and objectives o f the thesis...37

Ch a p t e r 2 Sy n t h e s i s a n d p r o p e r t i e s o f c h e l a t i n g v e r d a z y l r a d i c a l s... 40

2.1 Verdazyl radicals...40

2.1.1 Synthesis and physical properties o f 6-oxoverdazyl radicals...40

2.1.2 Properties o f 6-oxoverdazyl ra d ica ls...42

2.2 Synthesis and properties of chelating verdazyl radieal precursors...44

2.2.1 General methodology... 44

2.2.2 Carbonic acid bis( 1 -methylhydrazide) synthesis... 45

2.2.3 Aldehyde synthesis... 45

2.2.4 Synthesis of the radical precursor tetrazanes... 48

2.3 Synthesis and properties of chelating verdazyl radicals... 53

2.3.1 General methodology...53

2.3.2 Benzoquinone oxidations...54

2.3.2.1 Syntheses and structures o f 1.5-dim ethyl-3-(pyridin-2'-yl)-6-oxoverdazyl and 1.5- d im ethyl-3-{6-m ethylpyridin-2'-yl)-6-o.\overdazyl...54

2.3.2.2 Syntheses and structures o fl.5 -d im e th y l-3 -(m e th an o ic acid)-6-oxoverdazyl and 1.5- dim ethyl-3-(sodium m ethanoate)-6-oxoverdazyl*6.5H 20... 61

2.3.3 NalOa and K3Fe(CN)6 oxidations... 66

2 .3 .3 .1 Syntheses and structures o f 1.5-dim ethyl-3-(4.6-dim ethylpyrim idin-2'-yl)-6-o.xoverdazyl. l.5-dim ethyl-3-{2'.2"-bipyridm -6'-yl)-6-oxoverdazyl. 1.5-dim ethyl-3-(im idazol-2'-yl)-6- oxoverdazyl. and l.5-dim ethyl-3-{4.6-bis(pyridin-2"-yl)pyrim idin-2'-yl}-6-oxoverdazyl... 66

2.3.3.2 Syntheses and structures o f chelating verdazyl biradicals: 2.6-pyridinebis( 1.5-dim ethyl- 6-oxoverdazyl-3-yl) and 2 ,2 '-b ip y rid in e-6 ,6 '-b is(l,5 -d im eth y l-6 -o x o v erd azy l-3 -y l)... 71

2.4 Electronic and magnetic properties o f chelating verdazyl radicals... 72

2.4.1 EPR spectroscopy o f verdazyl radicals... 72

2.4.2 Cyclic voltammetry o f verdazyl radicals...74

2.4.3 Magnetic properties o f verdazyl radicals... 75

2 .4 .3 .1 M agnetic properties o f 2.41 :h q ...75

2.4.3.2 M agnetic properties o f 2.44»6.5H 20...80

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2.5 S u m m ary ... 82

2.6 E x p erim ental s e c tio n ... 84

2.6.1 General synthetic procedures... 84

2.6.2 Magnetic measurements... 84

2.6.3 Synthesis...85

Ch a p t e r s Th e c o o r d i n a t i o n c h e m i s t r y o f c h e l a t i n g v e r d a z y l l i g a n d s ... 101

3.1 In tro d u ctio n : Verdazyls as lig a n d s ...101

3.2 C oo rd in atio n chem istry o f py rid in e-su b stitu ted verdazyl 2 .4 1 ...104

3.2.1 Syntheses and structures o f (hlac)zM"(2.41) complexes (M = Ni. M n) 104 3.2.2 Magnetic properties o f (hfac)2M”(2.41) complexes (M = Ni. M n ) 109 3.2.3 Attempted preparation o f homoleptic complexes o f 2.41... 113

3.3 C o o rdination chem istry o f verdazyl 2.42... 114

3.3.1 Synthesis and structure o f [Cu(2.42):]PF6...114

3.3.2 Magnetic properties o f [Cu(2.42)z]PF6... 118

3.4 C o o rd in atio n chem istry o f verdazyl 2.45...120

3.4.1 Syntheses and structures o f [(hlac)2M "]:(2.4 5) complexes (M = Ni. Mn) ...120

3.4.2 Magnetic properties o f [(hfac)2M"]2(2.4 5) complexes (M = Ni. M n )...125

3.4.3 Attempted preparation o f extended coordination architectures with 2.45 129 3.5 C o o rd in atio n chem istry o f verdazyl 2.47... 129

3.5.1 Syntheses and structures o f [M“(2.47)2](X)2 complexes (M = Mn. Ni. Cu. Zn: X = PFô'. C104*)...130

3.5.1.1 X -ray crystal structure o f [Mn(2.47)2](C10^)2*H20 (3 .1 9 ) ... 131

3.5.1.2 X -ray crystal structure o f [N i(2.47)JCPFôIi'CjHôO (3 .2 0 ) ... 133

3.5.1.3 X -ray crystal structures o f [Cu(2.47) J ( C 104)2 (3.21) and [Zn(2.4 7 )2](C 104)2 (3 .2 2 )... 135

3.5.2 Magnetic properties o f [M”(2.47)2](X)2complexes (M = Mn, Ni. Cu. Zn; X = PFô', CIO4 ) ... 138

3.5.2.1 M agnetic properties o f [Mn(2.47)2](PF5)2 (3 .1 8 ) and [M n(2.47)2](C 104) :'H 2 0 (3 .1 9 ). 138 3.5.2.2 M agnetic properties o f [Ni(2.47)2](PF6)z"CsHeO ( 3 .2 0 ) ...139

3.5JZ.3 M agnetic properties o f [Cu(2.4 7 )2](C 104)2 (3 .2 1 )...140

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3.5.3 Discussion o f complexes 3.18-3.22...142

3.6 C o o rd in atio n chem istry of verdazyl 2.49... 144

3.6.1 Syntheses and structures o f [M”(2.4 9)2](C104)2 complexes (M = Mn, Ni) ...144

3.6.2 Magnetic properties o f [M“(2.4 9)2](C104)2 (M = Mn, Ni) com plexes... 146

3.7 C o o rd in atio n chem istry of verdazyl 2.50...148

3.7.1 Syntheses and structures of [M "(2.5 0)(H2 0)2](C104)2*xSolvent (M = Ni, Z n )... 149

3.7.2 Magnetic properties o f [M"(2.5 0)(H2 0)2](C104)2*xSolvent (M = Ni. Zn) com plexes... 152

3.7.3 Discussion... 155

3.7.4 Attempted preparation o f one-dimensional coordination polymers based on complexes 3.27 and 3.28 as synthons...156

3.8 C oo rd in atio n chem istry o f verdazyl 2.46... 157

3.8.1 Synthesis and structure o f di-p-Cl-[Co(2.4 6)(Cl)]2*2CH3 0H ...157

3.8.2 Magnetic properties o f di-p-Cl-[Co(2.4 6)(Cl)]2*2CH3 0H ... 159

3.8.3 Attempted preparation o f polymeric metal complexes o f 2.46...161

3.9 C oo rd in atio n chem istry of verdazyl 2.43...163

3.9.1 Syntheses and structures of [M”(2.4 4)2(H2 0)2]*2H2 0 (M = Co, N i) ...163

3.9.2 Magnetic properties o f [M"(2.4 4)2(H2 0)2]*2H2 0 complexes (M = Co. Ni) ... 167

3.9.3 Discussion...169

3.10 S u m m a ry ...171

3.11 E xperim ental s e c tio n ...173

3.11.1 Synthesis...173

C h a p t e r 4 R u(bipy)2*^ c o m p le x e s o f v e r d a z y l l i g a n d s : E l e c t r o n i c a n d L U M IN E SC E N T P R O P E R T IE S...183

4.1 In tro d u ctio n : M olecule-based m agneto-optical m a te ria ls...183

4.1.1 Luminescent investigations o f metal-radical com plexes... 187

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4.2 Syntheses and characterization o f Ru(II)-verdazyi complexes...191

4.3 Electronic properties o f Ru(Il)-verdazyl com plexes... 197

4.3.1 Absorption spectra o f Ru(II)-verdazyl com plexes... 197

4.3.2 Luminescence spectra o f Ru(II)-verdazyl com plexes...200

4.3.3 EPR spectra o f Ru(II)-verdazyl com plexes...202

4.3.4 CV o f Ru(II)-verdazyl com plexes... 203

4.4 Summary and future directions...207

Experimental sectio n ...209 4.4.1 Synthesis... 210 Ch a p t e r 5 Su m m a r y a n d g e n e r a l c o n c l u s i o n s... 2 14 R e f e r e n c e s ...219 Ap p e n d ix A Mo l e c u l a r St r u c t u r e s o f 3 .5 a n d 3 .2 7 ...232 Ap p e n d ix B Cr v s t a l l o g r a p h i c Pa r a m e t e r s... 2 34 Ap p e n d ix C Co m p l e t e Lis t in g s o f Bo n d Le n g t h s a n d An g l e s... 239

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Lis t o f Fig u r e s

F igure 1.1. Relation between electronic magnetic moment n and spin state ±ms...1

Figure 1.2. Magnetization M versus ^H /kT plots for different values o f S (g = 2.00)...7

Figure 1.3. % versus T for ferromagnetic, antiferromagnetic, and paramagnetic materials...8

F igure 1.4. A hysteresis loop...9

Figure 1.5. Parallel spin alignment in a carbon atom... 11

Figure 1.6. versus T for different values o f J ...12

Figure 1.7. /we/r/-substituted benzene di- and triradicals... 14

Figure 1.8. Stacked allyl free radicals...19

Figure 1.9. A typical superexchange interaction...22

Figure 1.10. Structure o f a hexacyanometallate...23

Figure 1.11. Schematic o f a unit of the structure o f the [M "m ’"(C20 4)3]' two- dimensional layer...26

F igure 1.12. Oxidation states o f o/v/io-quinones... 28

Figure 1.13. n* orbital o f a nitroxide radical...30

Figure 1.14. Magnetic coupling between nitroxides and two metal d-orbitals... 31

Figure 1.15. One-dimensional nitronyl nitroxide chains with Mn(hfac)z... 32

F igure 1.16. Schematic o f the two-dimensional structure o f polymer 1.33...33

Figure 1.17. Schematic o f a fragment o f the one-dimensional chain 1.43...36

Figure 1.18. Schematic o f a unit o f the two-dimensional layered structure o f 1.44 37 F igure 1.19. General structure o f a verdazyl radical... 37

F igure 1.20. Coordination o f a pyridine-substituted verdazyl and its structural relationship to that o f 2,2'-bipyridine... 38

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Figure 2.1. Type I and II verdazyl radicals, with the verdazyl numbering scheme

(at left)...40

Figure 2.2. Com parison o f the ring structures o f Type I (a) and II (b) verdazyl

radicals... 42

Figure 2.3. n-SO M O o f a Type II verdazyl radical... 43

Figure 2.4. Aldehydes used in the syntheses o f verdazyl radical ligands... 44

Figure 2.5. Potential bisimine (a) or polyimine (b) side products in the preparation o f 2 .9... 48

Figure 2.6. ORTEP diagram o f the molecular structure o f 2.30 (a), together with

a side-on view o f molecule (b), with hydrogen atoms removed for clarity (30% probability thermal ellipsoids)...49

Figure 2.7. 'H NM R spectrum o f 2.30 (starred peak represents CDCI3)... 51

Figure 2.8. '^C NM R spectrum of 2.30 (starred peak represents CDCI3)... 52

Figure 2.9. FT-IR spectrum of 2.30 (KBr disc, y-axis is in units o f percent

transmittance)... 53

Figure 2.10. ORTEP diagram of the molecular structure of 2.40 (a), together

with an edge-on view o f the tetrazine ring (b) viewed down the C = 0 axis, with hydrogen atoms (except H5n) removed for clarity (30% probability thermal ellipsoids)...55

Figure 2.11. ORTEP drawing of the molecular structure o f 2.41 as found in the

structure o f its hydroquinone complex, with hydroquinone molecules omitted for clarity (30% probability thermal ellipsoids)... 57

Figure 2.12. View o f the head-over-tail tt-stacking o f radicals in the structure o f

2.41 :hq in the xz plane, with hydroquinone molecules omitted for clarity... 58

Figure 2.13. Packing diagram o f 2.41:hq {yz plane). Hydrogen bonds are represented by dashed lines... 59

Figure 2.14. UV-visible spectrum o f 2.41 :hq in CH2CI2 ("A" refers to absorbance)... 60

Figure 2.15. ORTEP drawing o f the asymmetric unit o f 2.44»6.5H2Û, with water molecules om itted for clarity (30% probability thermal ellipsoids)... 63

Figure 2.16. View o f the packing o f 2.4 4»6.5H2 0 showing alternating chains consisting o f stacked verdazyls, sodium cations, and water molecules; coordinate

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bonds between sodium cations and ail other hydrogen bonds are indicated as

dashed lines... 65 Figure 2.17. View o f the stacking o f 2.44-6.5H20 showing alternating layers o f water molecules and radicals with coordinated sodium cations and other hydrogen-bonded water molecules (dashed lines indicate coordinate bonds to

sodium cations)...66

Figure 2.18. ORTEP drawing o f the asymmetric unit o f 2.45»2H20 (water molecules omitted for clarity; 30% probability thermal ellipsoids)... 67 Figure 2.19. View o f the two-dimensional molecular packing o f 2.45*2H20. with water molecules removed for clarity...69

Figure 2.20. FT-IR spectrum o f 2.47 (K Br disc, y-axis is in units o f percent

transmittance)...7 1 Figure 2.21. EPR spectrum o f 2.41 at 298 K in CH2CI2 (a). The total spectral

width is 100 G. Simulated EPR spectrum o f 2.41 (b). Units are in Gauss (x-axis). and intensity (y-axis)... 73

Figure 2.22. Temperature dependence o f % (o) and %T (□) 2.41;hq. Solid lines represent modeled fits to the observed data... 76

Figure 2.23. (a) Second LUMO o f 2.41. (b). SOMO o f 2.41. (c) Illustration o f the proposed superexchange between verdazyl SOMO s mediated by the second LUMO... 79

Figure 2.24. Temperature dependence o f % (o) and %T (□) 2.44*6.5H2O. Solid lines represent fits on % and to a one-dimensional anti ferromagnetic chain model...81

Figure 2.25. Temperature dependence o f % (o) and %T (□) 2.45*2H20. Solid lines represent modeled fits on % and %T to a one-dimensional anti ferromagnetic

chain... 82 Figure 3.1. ORTEP diagram o f the molecular structure o f 3.6 with fluorine atoms

removed for clarity (30% probability thermal ellipsoids)... 106 Figure 3.2. ORTEP diagram o f 3.6 illustrating the planar chelated verdazyl

radical... 106 Figure 3.3. FT-IR spectra (KBr discs, units o f y-axis are in percent

transmittance) o f 3.6 (a) and 3.5 (b)... 108

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Figure 3.5. Plot o f %T vs T for 3.5 (□) and 3.6 (o). Solid lines represent

modeled fits to the experimental data...110

F igure 3.6. Magnetization (M) versus field (H) data for 3.5 (□) at 3 K. The solid line represents a theoretical plot for an 5 = 3/2 ground state. The dashed line represents a theoretical plot for an S = 1 ground state...11 1

Figure 3.7. Magnetic orbital interactions in 3.5...112

F igure 3.8. dn-pTt magnetic orbital interactions in 3.6...112 Figure 3.9. ORTEP diagram o f the cation o f 3.11 (30% probability thermal

ellipsoids)...116

F igure 3.10. Ball and stick representation o f the cation o f 3.11 (0 is the

interligand dihedral angle)... 117 F igure 3.11. UV-visible spectrum o f 3.11 in C H iC b... 118

F igure 3.12. Plot of vs T for 3.11 (□). The solid line represents a modeled fit

to the experimental data...119 Figure 3.13. ORTEP diagram o f the molecular structure o f 3.14 with fluorine

atoms omitted for clarity (30% probability thermal ellipsoids)... 121 Figure 3.14. FT-IR spectra (KBr discs, units o f y-axis are in percent

transmittance) o f 3.13 (a) and 3.14 (b)...124

Figure 3.15. UV-visible spectra o f 2.45 (— ). 3.13 (— ), and 3.14 (...)... 124

Figure 3.16. Plot o f vs T for 3.13 (□) and 3.14 (o). Solid lines represent

modeled fits to the experimental data... 126 Figure 3.17. Magnetization (M) versus field (H) data for 3.13 (□) and 3.14 (o ) at 2 K. Solid lines represent theoretical plots for S = 9/2 and 5 = 5/2 ground states

for 3.14 and 3.13, respectively... 127 Figure 3.18. Irregular spin state structure o f the low-lying states in 3.14 (see

reference 4, p. 220)... 128 F igure 3.19. Structural relationship between terpyridine and 2.47 ligands... 130

Figure 3.20. UV-visible spectra o f 2.47(— ), 3.19 (•••), 3.20 (-••-••), 3.21 (-•-•),

and 3.22 (— ) ...131 Figure 3.21. ORTEP diagram o f the dication o f 3.19 with water molecule

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F ig u re 3.22. ORTEP diagram o f the dication o f 3.20 with acetone molecule

omitted for clarity (30% probability thermal ellipsoids)...134

F igure 3.23. ORTEP diagram o f the dication o f 3.21 (30% probability thermal ellipsoids)... 136

F igure 3.24. ORTEP diagram o f the dication o f 3.22 (30% probability thermal ellipsoids)... 137

F igure 3.25. Plot o f vs T for 3.18 (o), 3.19 (x), 3.20 (□). 3.21 (0), and 3.22 (+). Solid lines represent modeled fits to the experimental data... 138

Figure 3.26. Magnetization (M) versus field (H) data for 3.20 (□) at 2 K. The solid line represents a theoretical plot for an 5 = 2 ground state... 140

Figure 3.27. Magnetization (M) versus field (H) data for 3.22 (□) at 3 K. The solid line represents a theoretical plot for one-third o f an 5 = 1/2 ground state...142

F igure 3.28. Structural relationship between terpyridine and 2.49...144

F igure 3.29. UV-visible spectra o f 2.49 (— ). 3.23 (.«). and 3.24 (— )... 146

F igure 3.30. Plot o f vs T for 3.23 (o) and 3.24 (□)... 147

Figure 3.31. ORTEP diagram o f the asymmetric unit o f 3.28 with counterions and lattice water molecules omitted for clarity (30% probability thermal ellipsoids)...150

F igure 3.32. Plot o f '/iT vs T for 3.27 (□) and 3.28 (o). Solid lines represent modeled fits to the observed data... 153

F igure 3.33. Magnetization (M) versus field (H) data for 3.27 (□) at 2 K. The solid line represents a theoretical plot for an 5 = 2 ground state... 154

F igure 3.34. Plot o f 1/% versus T for 3.28 (o). The solid line represents the best-fit calculated values... 155

Figure 3.35. Potential -C N - linked coordination polymers based on [M‘*(2.50)]‘^ complexes...156

F igure 3.36. ORTEP diagram o f the molecular structure o f 3.29 with methanol solvate molecules omitted for clarity (30% probability them al ellipsoids)... 158

Figure 3.37. Plot o f %Tvs T for 3.29 (□)... 161

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F ig u re 3.39. UV-visible spectra o f 2.44*6.5H?0(— ) in CH3CN, 3.30 (•••), and

3 .3 1 (~ ) in DM SO... 164 F ig u re 3.40. ORTEP diagram o f the molecular structure o f 3.30 with lattice

water molecules omitted for clarity (30% probability thermal ellipsoids)...164 F ig u re 3.41. ORTEP diagram o f the molecular structure o f 3.31 with lattice

water molecules omitted for clarity (30% probability thermal ellipsoids)...165

F ig u re 3.42. Plot o f vs T for 3.30 (o) and 3.31 (□). The solid lines

represent modeled fits to the observed data... 167 F ig u re 3.43. M agnetization (M) versus field (H) for 3.30 (o ) and 3.31 (□) at 2 K 169 F ig u re 4.1 Photoinduced electron transfer reaction in lron(Il)-cobalt(lII)

cyanides... 184 F ig u re 4.2. Spin-crossover in octahedrall> coordinated d^ metal complexes... 185 F ig u re 4.3. Simplified depiction o f the orbital structure o f Ru(bipy)3~^

with electronic transitions denoted as arrows...189 F igure 4.4. Absorption (left) and emission (right) spectra o f Ru(bipy)3"^ in water (298 K)...190

F ig u re 4.5. Ru(bipy)3"^ and a structural mimic containing verdazyl ligand 2.41 (4.4)... 191 F ig u re 4.6. LSIMS o f 4.4 (w-NBA m atrix)...192 F ig u re 4.7. Calculated and experimental isotopic distribution for the m /z 763 peak in the LSIMS o f 4 .4 ... 193 F ig u re 4.8. FT-IR spectrum o f 4.4 (KBr disc, y-axis is in units o f percent transmittance)...194 F ig u re 4.9. LSIMS (m-NBA matrix) o f 4.8... 197 F ig u re 4.10. UV-visible absorption spectra o f 4.4 (— ) and 4.3 (•••) in acetone, and free 2.41:hq (— ) in C H iC L... 198 F ig u re 4.11. UV-visible spectrum o f 4.6 in acetone... 199 F ig u re 4.12. UV-visible spectra o f 4.8 (— ) and 4.9 (—) in acetone, and

2.4 4«6.5H2 0 (•••) in w ater...2 0 0

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Figure 4.14. Solution (CH2CI2) EPR spectra o f 4.4 (at left) and 4.9 (at right) recorded at 298 K.. Units are in Gauss (x-axis), and intensity (y-axis)... 203

Figure 4.15. Solution (CH2CI2) EPR spectrum o f 4.8 recorded at 298 K. Units

are in Gauss (x-axis), and intensity (y-axis)... 203

Figure 4.16. CV o f 4.4 in CH3CN (BU4NBF4 supporting electrolyte)... 205

Figure 4.17. CV o f 4.8 in CH3CN (B114NBF4 supporting electrolyte)... 206

Figure 4.18. Simplified schematic o f the orbital structure o f

Ru(bipy)2( verdazyl)"^ complexes...208

Figure 5.1. Structural comparison between a verdazyl radical with a hypothetical

dioxadiazinyl analogue... 216

Figure A-1. Ball and stick representation o f the molecular structure o f 3.5... 232

Figure A-2. Ball and stick representation o f the cation o f 3.27, with perchlorates

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Li s t o f Sc h e m e s

Schem e 2.1. Preparation o f verdazyls by the attempted alkylation o f formazans...40

Schem e 2.2. Preparation o f 6-oxo(or thioxo)verdazyl radicals...41

Schem e 2.3. Alternate preparation o f 6-oxoverdazyl radicals... 41

Schem e 2.4. Synthesis o f carbonic acid bis(l-m ethylhydrazide) 2.8... 45

Schem e 2.5. Synthesis o f 2,2'-bipyridine-6,6'-dicarboxaldehyde 2.22... 45

Schem e 2.6. Synthesis of 4,6-dimethylpyrimidine-2-carboxaldehyde 2.16...46

Schem e 2.7. Synthesis of 2,6-pyridinedicarboxaldehyde 2.21...46

Schem e 2.8. Synthesis o f 2.2'-bipyridine-6-carboxaldehyde 2.17... 47

Schem e 2.9. Synthesis o f 4,6-bis(pyridin-2'-yl)pyrimidine-2-carboxaldehyde 2.20 47 Schem e 2.10. General preparation o f precursor tetrazanes...48

Schem e 2.11. Reaction ofN alO ^ with 2.31...54

Schem e 2.12. Preparation o f 2.41 as a 1:1 m olecular complex with hydroquinone by oxidation o f 2.31 with benzoquinone... 56

Schem e 2.13. Preparation o f 2.42 as a 1:1 molecular complex with hydroquinone by oxidation o f 2.30 with benzoquinone... 61

Schem e 2.14. Preparation o f 2.43 by oxidation o f 2.38 with benzoquinone. and preparation o f 2.44»6.5H20 by deprotonation o f 2.43... 62

Schem e 2.15. Preparation o f 2.45... 67

Schem e 3.1. Preparation o f one-dimensional polymers o f 2.54 with copper halides... 101

Schem e 3.2. Reaction o f 2.31 with group 12 metal halides, and subsequent oxidation with tetraphenylhydrazine to generate metal-radical complexes... 103

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Scheme 3.3. Preparation o f verdazyl complex 3.4 by reaction o f CuBr with

tetrazane 2.31... 103

Scheme 3.4. Preparation o f (hfac)zM(2.41) complexes (M = Ni, M n)... 105

Scheme 3.5. Preparation o f [Cu(2.42)2]PF6 (3.11)...115

Scheme 3.6. Preparation o f [(hfac)2M]i(2.4S) complexes (M = Ni, Mn)...121

Scheme 3.7. Preparation o f [M"(2.47)2](X)2 complexes (M=Mn. Ni, Cu, Zn; X = PP6, CIO41... 130

Scheme 3.8. Preparation o f [M"(2.4 9)2](C104)2 complexes (M = Mn, Ni)... 145

Scheme 3.9. Preparation o f [M(2.50)(H20)2](C104)2*xSolvent complexes (M = Ni, Zn)... 149

Scheme 3.10. Preparation o f verdazyl complex 3.29 by reaction o f tetrazane 2.34 with CoCl2*6H20... 157

Scheme 3.11. Preparation o f [M(2.44)2(H20)2]*2H20 complexes (M = Co. Ni)...163

Scheme 3.12. Attempted use o f complexes 3.30 and 3.31 as precursors to potential one-dimensional chains... 170

Scheme 4.1. Photoinduced equilibrium o f spiropyran isomers... 186

Scheme 4.2. Photoinduced equilibrium o f nitronyl nitroxide-substituted diarylethene isomers...186

Scheme 4.3. Preparation o f 4.4... 194

Scheme 4.4. Preparation o f 4.6 and 4.7 from pyrimidine-substituted verdazyl 2.45...195

Scheme 4.5. Preparation o f 4.8 and 4.9 by reaction with carboxylic acid- substituted verdazyl 2.43...196

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Li s t o f Ta b l e s

Table 2.1. Selected bond distances and angles for the structure o f tetrazane 2.30

(estim ated standard deviations in parentheses)... 49

T able 2.2. Selected bond distances and angles for the structure o f 2.40 (estimated

standard deviations in parentheses)... 55

Table 2.3. Selected bond distances and angles for the structure o f 2.41 :hq

Table 2.4. Selected bond distances and angles for the structure o f 2.44*6.5HzO

(estimated standard deviations in parentheses)... 63

Table 2.5. Selected bond distances and angles for the structure o f 2.4 5»2H2 0

Table 2.6. EPR parameters o f verdazyl m onoradicals... 73

T able 2.7. Redox potentials o f selected verdazyl radicals with all values in V vs

saturated calomel electrode (SCE)... 74

Table 2.8. DFT-calculated spin populations for 2.41 and the structure 2.52...77

Table 3.1. Selected bond distances and angles for the structure o f 3.6 (estim ated standard deviations in parentheses)...107

T able 3.2. Selected bond distances and angles for the structure o f 3.11

Table 3.3. Selected bond distances and angles for the structure o f 3.14

Table 3.6. Selected bond distances and angles for the structure o f 3.21

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T able 3.8. Selected bond distances and angles for the structure o f 3.28

(estimated standard deviations in parentheses)...150

Table 3.9. Selected bond distances and angles for the structure o f 3.29 (estimated standard deviations in parentheses)... 158

Table 4.1. Photophysical data for ruthenium( 11)-verdazyl com plexes... 202

Table 4.2. Electrochemical data (in V) for complexes 4.3, 4.4, 4.8, and 4.9 (peak separation in mV is indicated in parentheses after the peak potential for the complexes studied in this w o rk )... 207

Table B-1. Crvstallographic parameters... 234

Table C-1. Bond lengths [Â] and angles [deg] for 2.30...239

Table C 2. Bond lengths [Â] and angles [deg] for 2.40... 240

T able C 3. Bond lengths [À] and angles [deg] for 2.41 :hq...242

Table C-4. Bond lengths [À] and angles [deg] for 2.44*6.5 H :0 ... 245

Table C-5. Bond lengths [Â] and angles [deg] for 2.45*2H20... 247

Table C-6. Bond lengths [A] and angles [deg] for 3 .6 ...249

Table C-1. Bond lengths [A] and angles [deg] for 3.11... 252

Table C 8. Bond lengths [A] and angles [deg] for 3.14... 254

Table C-9. Bond lengths [A] and angles [deg] for 3.19... 257

Table C -15. Bond lengths [A] and angles [deg] for 3.30... 284

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List o f Nu m b e r e d Co m p o u n d s Ar 'Ar Ar Ar Ar Ar Ar Ar= *Bu-Ar Ar-Ar Ar Ar _An 1.1 Ar _Ar Ar Ar Ar R" 1.4 'O 'N'' N'P 1.5 1.6 1.7

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•Ar A r= ‘Bi 1.8 CH3Q « ^CHa H3C' 'N ' 'CH3 P 1.10 N=C F F F F 1.11 [C p :* F e ]'[T C N E ]- 1 .1 2 lC p 2 * M n '" l'[T C N Q r 1.1 3 MeoN NMe, MegN NMej 1.14

F e"tF e‘"(CN)6]6 1 1 5 C s'N i" [C r" '(C N )6l‘ 2 H 2 0 1 .1 6 [C r"3C r" '2(C N ),2]*10H2O 1 .1 7

C s'M n"[V '"(C N )6] 1 .1 8 (E tiN k s M n " , 25[V"(CN)5|*2H20 1 .1 9

N H 4 V [C r(C N )6 ]o 8 6 * 2 .8 H 2 0 1 .2 0 [M n'‘‘T P P llT C N E ]'* 1 .21

V (T C N E V y C H 2CU 1 .2 2 (x ~ 1 ; y - 0.5)

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o -z

I

m X a

I

% K n-Z O - Z o -z o -z CD CD c / CD c Z - O A 2 3 3" g z H w e 2 3 3" g' y z m o •o Z - O g CD %

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n % h-00 foeo II a. u £ H a: z o z z // z-z>-n: 0 U Z E P

1

E P 6 e 2 N g C S q: // z-z f z-z % \N Z Z z-z X=< z-z ■ : z I x=< y-": z-z / z ôc i- z 'k o : 3

I

z-z e o=<( k “ 5 Z - Z I z-z z o=< y-= o=< z-z z z-z « CM I o=<^" k ^ 3 z-z I o k ^ ' k ^ ; z-z / I z-z ° k _z-zk = s / I

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2.15 N ^N

X

Me M e ^ ^ ^ '^ M e 2.16 O ^ H HN ''N 2.18 N H0"^0 2.20 2.21 2.22 2.23 Br N ^N 2.24 HOCHz^ N CH2OH 2.25 2.26 2.27 2.28 2.29 2.30 2.31 Me.. J L ,Me N N HN^NH .Me 2-32 Me Me N ^ N 2.33 Me Me^ j L . , . M e N N HN^NH M e ^ .,J L .,.M e N N HN_ _NH Me^ JL_.Me N N HN_ _NH Me. J L . . . M e M e .^ ,A ^ ,-M e N N HN NH HN ^ N 2.37 I I " HN^NH HO^O 2.38

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o M e ... A..,Me 2.40 2.41 2.42 i I •N^N CO2H 2.43 2.47 "N^N . 6.5H2O C02Na 2.44 M e. A . , - M e N _I N_I •N.^N N^NH 2.48 M e.

A.,

-Me N N I I •N^N N ^N Me^^"^=^Me 2.45 N“ Me N-Me M e .

A.,

-Me N N 2.46 Me .- N _ * 0 2.49 A -N, N Me : ^ N ^ N .N - M e • 2.50 Me Hz I I •N.-^N 2.51 2.52 0==<^ Me' N— N -N Md^ 2.53 M e .|^ A |^ -M e I I •N^N 'I' I ‘ O 2.54

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Me^ JL N N o Me^ -Me N N NL^N X" n'^ 'n: 'n^ 'n'- X Me-'^y'^'Me Me"'^";"'^'Me O 3.1 O Me^ J L . ,, M e N N HN^^NH .ÆI Cl Me NHz O M e . . , J L .Me N N I I •N^^N CE: Me, Me N N".., M = NI (3.5) Mn (3.6) CF CFa NL^N Ni(hfac)2 O ' Me Me 3.9 3.8 Me, O

A

N^N,; PFs Me Me. 3.10 Me O ' Cu, Me CIO, PFg 3.12 ÇF: CFj FjC- Me N N'... ,.>o CF FjC OF; CF; M = Ni 3.13 Mn 3.14 . n (hfac)2Mn^ ^Mn(hfac)z u 3.15

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N ^N N ^N 3.16 M = Mn, X = PFg- (3.18) Mn, X = C IO / (3.19) [M(2.47)2p2X* Ni, X = P F s ' (3.20) Cu, X ^ C I O / (3.21) Zn, X = C IO / (3.22) (M(2.49)2l^"^2CIO/. xSolvent Mn (3.23) M = Ni (3.24) 3.26 H2O 12+ C N/,, I ^ (0 1 0 4)2- xSolvent OH2 M = Ni (3.27) Zn (3.28) N-M e 3.29 OH2 o I I . O H , •2H2O M = Co (3.30) NI (3.31) Y 4.2

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Me ;Ru; _{R u:} 4.3 4.4 _4.5 4+ ,M e I Me 4.6 4.7 Me-N-... ---3+ (P Ps)3 Me :Ru. 4.9 PFc NH 4.10

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Lis t o f Ab b r e v i a t i o n s 1-D 2-D 3-D a A Â acac AF. or AFM Ag/AgCl AgiO Ar B bipy br Bs(y) Bu B U 4 N B F 4 Bz C CaH2 Côo °C CD2CI2 CDCI3 CH2CI2 CHCI3 C H3CN CH3NO2 Cl one-dimensional two-dimensional three-dimensional

hyperfme coupling constant absorbance

angstrom (10‘‘“ m) acetylacetonate anti ferromagnetic

silver/silver chloride reference electrode silver(l) oxide

aryl substituent magnetic induction

2.2'-bipyridine

broad (IR and NM R peak descriptor) Brillouin function butyl tetrabutylammonium tetrafluoroborate benzo Curie constant calcium hydride buckminsterfullerene degree(s) Celsius deuterated dichloromethane deuterated chloroform dichloromethane chloroform acetonitrile nitromethane chemical ionization

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CIO; perchlorate

cm centimeter

-CN cyanide

Cp2* decamethyldicyclopentadienyl

CT charge transfer

CTH 5.7,7,12,14.14-hexamethy I-1,4,8.11 -tetraazacyc lotetradecane

CuBr copper(I) bromide

CV cyclic voltammetry

d doublet (NMR peak descriptor)

dec decompose

DFT density functional theory

DMF dimethylformamide

dft-DMSO deuterated dimethylsulfoxide

DMSG dimethy Isul foxide

DPPH diphenylpicrylhydrazyl

DTBSQ 3.5-di-/m -butylsem iquinone

E energy

E f energy in zero magnetic field

E„'" coefficient o f the first order Zeeman effect

E,r-> coefficient o f the second order Zeeman effect

El electron impact

emu electromagnetic units (cm^)

en ethylenediamine

ENDOR electron nuclear double resonance

Eok oxidation potential

EPR electron paramagnetic resonance

Ered reduction potential

ES electrospray

Et ethyl

Et]N triethylamine

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EtOH EtiO eV F FeCb FM FT-IR fs g G GHz h h H H He HCl hfac HMO HOMO hq Hz I ImH Im I-Pr J k K KBr KCN ethanol diethyl ether electron volt

area o f emission spectrum iron(lll) chloride

ferromagnetic

Fourier transform infrared femtosecond(s) (10'*^ s) g-factor

gauss gigahertz

Planck’s constant (6.6260755 x lO’^"* J»s) hour(s) magnetic field spin Hamiltonian coercive field hydrochloric acid 1,1,1,5,5,5-hexafluoroacetylacetonate Hiickel molecular orbital

highest occupied molecular orbital hydroquinone hertz intensity imidazole imidazolate anion isopropyl

coupling constant (nmr), or magnetic exchange coupling constant Boltzmann constant (1.3806580 x 10*"^ J K '')

exchange integral, degree Kelvin potassium bromide

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K3Fe(CN)6 potassium ferricyanide

kJ/mol kilojoules per mole (83.5934612 cm'*)

L litre

LC ligand centered

LL bidentate ligand

LMCT ligand-to-metal charge transfer

LSIMS liquid secondary ion mass spectrometry

LUMO lowest unoccupied molecular orbital

m multiplet or medium (NMR or IR peak descriptor)

M magnetization M molarity M ' molecular ion M '‘ inverse molarity MC metal centered Me methyl MeOH methanol mg milligram MgS04 magnesium sulfate MHz megahertz min minute(s) mL millilitre

MLCT metal-to-ligand charge transfer

mmol millimole

mNBA wera-nitrobenzylalcohol

Mp melting point

Mr remnant magnetization

electron "spin up'’ state

-ms electron "spin down” state

Ms saturation magnetization

MS mass spectrometry

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miz N NaCI NaiCO] NaI04 NaOH NaiSOa «-BuLi NIT nm NMR Oac ORTEP ox Ox P PbO: Ph Ph^Nz PFa ppm py q R R rad rbf Rtrz s S 5 mass/charge ratio

Avogadro’s number (6.0221367 x 10“^ mol ') sodium chloride sodium carbonate sodium periodate sodium hydroxide sodium sulphate normal-h\x\y\ lithium nitronyl nitroxide nanometer (10 *^ m)

nuclear magnetic resonance acetate

Oakridge thermal ellipsoid program oxalate oxidizing agent para lead(lV) oxide phenyl tetraphenylhydrazine hexafl uorophosphate parts per million pyridine

quartet (NMR peak descriptor) generic functional group agreement factor

radical

round-bottomed flask 4-substituted-1.2,4-triazole

singlet, strong, (NMR, IR peak descriptor) or standard overlap integral

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sh SCE SeOi SOMO SQUID t T /-Bu /-BuLi Tc TCNE TCNQ TDAE terpy TIP THE TLC Tmax TMEN T s TPP u UV V vs V s ' vw w W X P -I shoulder

saturated calomel electrode selenium (IV) dioxide

singly occupied molecular orbital

superconducting quantum interference device triplet (NMR peak descriptor)

temperature, or Tesla tertiary butyl tertiary-hxxiyX lithium critical temperature tetracyanoethylene 7.7.8.8-tetracyano-p-quinodimethane tetrakis(dimethylamino)ethylene 2.2' :6'.2"-terpyridine temperature-independent paramagnetism tetrahydrofuran

thin layer chromatography

temperature at the maximum o f % tetramethylethylenediamine Neel temperature wcAo-tetraphenylporphinato unknown ultraviolet volt

very strong (IR peak descriptor), or versus volt per second

very weak (IR peak descriptor) weak (IR peak descriptor) watt

O or S atom

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6 parts per million

A heat

G molar absorptivity (M'* cm*')

<j>F quantum yield o f fluorescence

p refractive index

Xcm emission wavelength

Xcx excitation wavelength

Xmax wavelength o f lowest energy electronic absorption

p magnetic moment or denoting bridging ligand

/4n effective magnetic moment

p„ microscopic magnetization

p frequency

0 Weiss constant, or interligand dihedral angle

p paramagnetic impurity factor

Xg gram m agnetic susceptibility

Xm molar magnetic susceptibility

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Ac k n o w l e d g e m e n t s

I wish to express my sincere gratitude to the crystallographers w ho solved the X- ray structures that were presented in this thesis, especially Dr. Tosha Barclay who put a great deal o f time and energy into these efforts. Prof. Laurence Thompson and colleagues are gratefully acknowledged for perfoming all o f the variable temperature magnetic susceptibility and magnetization measurements, in addition to numerous helpful discussions aiding in data interpretation. Characterizing paramagnetic compounds can very often be a daunting task in the absense o f X-ray data, and Dr. David McGillivray is sincerely thanked for his efforts in obtaining all of the invaluable mass spectra, which is often one o f the few handles available to us. Bob Dean and Terry Wiley are thanked for keeping a number o f instruments and computers online and offering immediate assistance

whenever it was needed.

Learning how to carry out synthetic chemistry efficiently and effectively was one o f the reasons why I decided to pursue graduate studies, and Dr s. Richard Hooper and Greg Patenaude are acknowledged for their excellent synthetic skills— I have learned a great deal in this regard from them both. Other group members and graduate students, including Matt, Bryan, Dan, Steve. Dr. Mhamed Chahma, Miguel, Todd and Chi Wei are acknowledged for their friendship over the years.

Lastly, I wish to acknowledge the efforts and support o f my Ph.D. super\isor Prof. Robin Hicks, whose patience, insight, and clarity o f thought have both inspired and taught me a great deal about carrying out, interpreting, and communicating science.

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1.1 Molecular magnetism

Magnetic phenomena are bom from the motion o f the electrons that comprise matter. Electrons are negatively charged subatomic particles and generate magnetic dipole moments by virtue o f their intrinsic angular momentum, also known as "spin”. Electrons can assume one o f two spins states, ±1/2, commonly referred to as "spin up” or

I I n

F ig u r e 1 .1. R elation betw een electronic m agnetic m om ent n and spin state ±m,.

"spin down”, and each spin state is related to a magnetic moment that is equal in magnitude but opposite in sign to the other (Figure 1.1). In many molecules, all o f the electrons are paired in molecular orbitals, and any two electrons per orbital must assume opposing spin states {vide infra). Thus, the magnetic moment generated by one electron is canceled out by the magnetic moment generated by its orbital counterpart. This explains why magnetic molecules (molecules exhibiting magnetic moments) are not as common as their non-magnetic counterparts— Nature prefers electrons to pair up in orbitals and form chemical bonds, and this provides us with the most important requirement to observe magnetic behaviour— unpaired electrons.

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behaviour and so must contain unpaired electrons. These are typically transition metal or lanthanide complexes where the uncompensated spins are found in metal-centered d- or f- orbitals. Magnetic molecules can also be found in the realm o f organic chemistry, although less commonly than in transition metal chemistry, where the spins are in s- or p- type orbitals. Examples include free radicals that may be transient in nature, like the methyl radical, or stable to ambient conditions, like diphenylpicrylhydrazyl (dpph). a common standard used in electron paramagnetic resonance spectroscopy (EPR). W hether in the form o f a transition metal complex, or as a stable free radical, these magnetic species are molecular in nature, and the magnetic properties o f molecules are substantially different from those observed in traditional atom-based magnets, such as iron, nickel, and cobalt. Molecular magnetism is a branch o f magnetochemistry that is concerned with the study and elucidation o f the magnetic interactions present in paramagnetic molecular species, and is a relatively new area o f research compared to the study o f traditional magnetic materials that began hundreds, or perhaps thousands o f years ago. Fundamental differences between molecule-based magnetic materials and atom-based congeners will be explored in more detail later in this chapter.

1.1.1 What is magnetism?

The subject o f magnetism is both vast and complex, and a number o f texts are entirely devoted to a detailed discussion o f this phenomenon.' ^ The discussion that follow s is a brief overview o f a num ber o f topics relevant to an understanding o f the work that is described in later chapters o f this thesis.

Magnetism is a solid state property and is detected by a m aterial's attraction (or repulsion) to an external magnetic field. The varieties o f magnetic behaviour arise from the intrinsic spin and orbital angular momentum o f an electron and depend upon how this angular momentum on adjacent atoms or molecules interact with each other. This information is determined for a particular material by placing it in a magnetic field H and measuring its magnetic induction B (Equation 1.1). Magnetic induction relates directly to

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Magnetization can be thought o f in simple terms as the vector sum o f the individual magnetic moments o f the molecules that comprise the sample. A magnetic molecule in the absence o f an external field has its individual moments randomized by the available thermal energy, and these moments effectively cancel each other out affording no net magnetic moment in the bulk. In the presence o f a magnetic field, however, the behaviour of the individual spin moments is different, as they will tend to align with the field’s magnetic lines o f force, inducing a net magnetic moment. The ease or ability o f the individual moments to align with the external field is. qualitatively speaking, the

magnetic susceptibility %, arguably the most important quantity in magnetochemistry; %

is mathematically related to M as shown in Equation 1.2.

X = f (1.2)

The molar magnetic susceptibility xm is obtained by multiplying the gram susceptibility o f the substance under study (the experimentally determined value) by its molecular weight and it is an indicator o f the type o f magnetic behaviour exhibited by a material. In the absence o f other interactions between spins, there are two fundamental responses to an external magnetic field: attraction and repulsion, and each o f these phenomena is associated with the particular sign o f the magnetic susceptibility. A negative value o f xm is referred to as diamagnetic behaviour, and this implies that the substance is repelling the magnetic field. Diamagnetism results from the interaction o f the external magnetic field with the field generated by the motion o f the electrons in their orbitals, the direction o f which opposes the external field. This is the weakest variety o f magnetic behaviour (on the order o f - 10'^ emu, where emu = cm^/mol), it is temperature and field independent and is exhibited by all materials.

On the other hand, a positive xm is a signature o f paramagnetic behaviour, which results from the spin and orbital angular momenta o f unpaired electrons interacting with an external magnetic field. The spin and orbital angular momenta generated by unpaired electrons give rise to a magnetic moment and so unpaired electrons tend to align their moments with the external field (they are attracted into the field). Paramagnetism is temperature dependent, field independent and its magnitude is typically on the order o f

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molecule is the sum o f its diamagnetic and paramagnetic susceptibilities.

1.1.2 Magnetic behaviour o f paramagnets: Van Vleck formula, the Curie law, and magnetization

According to quantum mechanics, a molecule in the presence o f a magnetic field

H has a number o f discrete energy levels that depend upon how the m olecule's angular

momenta interacts with H (The Zeeman effect). The population o f each level is determined by its energy and the energy o f the surroundings (the temperature). For each particular energy level, a microscopic magnetization p» can be calculated (Equation 1.3), and this represents the change in energy o f the level with changing H. This microscopic

p„ = - a E „ / a / / (1.3)

magnetization is related to the observable macroscopic molar magnetization through the Boltzmann distribution to give what has been referred to as "the fundamental equation o f molecular magnetism" (Equation 1.4).^*'’ One o f the difficulties with applying Equation 1.4. however, is that it requires calculation o f derivatives a E „ /6H for each o f the thermally

( - ôE /d H ) expf - E„ kT )

“ Y . ‘V ( - E „ i k T )

n

populated states o f a molecule. Van Vleck demonstrated in 1932 that Equation 1.4 can be simplified by incorporating two approximations. First, the magnetic susceptibility o f a molecule may be calculated if the energies o f the thermally populated states o f that molecule in an external magnetic field H are known, and this can be accomplished by expanding the energies E„ o f the molecule according to increasing powers o f H (Equation

1.5), where E^"» is

E„ = E^' + EH'„^> + (1.5)

the energy in zero magnetic field, and E !‘\ is the coefficient o f the first order Zeeman effect. is the coefficient o f the second order Zeeman effect, which is normally neglected because it is in most cases insignificant with respect to the first order Zeeman term. Thus p„ in Equation 1.3 can be treated as a power series expansion, and this

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that H is not too large and T is not too small, in other words, to ignore saturation effects

(vide infra). The magnetization (Equation 1.4) can then be approximated by Equation 1.6

and the

N H ' £ ( E l , ' I k T - 2 E l ~ > ) e x p ( - 'kT)

° ° E a » / k T )

n

susceptibility is obtained by dividing through by H. which gives the Van Vleck formula (Equation 1.7). Equation 1.7 affords a very practical means for calculating % because it only requires knowledge o f the energies o f the molecule in zero field (E'''*„) and in the presence o f a magnetic field (first Zeeman perturbation E^“n)- These quantities are generally readily calculable for a molecular species.

fk T - 2 E l ~ ' ) e x p ( - E l l " k T )

n

In 1910 it was demonstrated that the temperature dependence o f a paramagnetic substance obeys Curie's Law— the magnetic susceptibility o f a paramagnetic material is inversely proportional to temperature (Equation 1.8). The proportionality constant C, the Curie constant, is directly related to S, the spin

= ^ = + U (1.8)

multiplicity o f the ground state, which is itself directly related to the number o f unpaired electrons present in the material (in the absence o f appreciable amounts o f orbital angular momentum and/or spin-orbit coupling). Equation 1.8 is also readily generated by applying Van V leck's equation to the case o f a paramagnetic species having a ground state energy without orbital angular momentum and a large separation between the ground state and any excited states with orbital angular momentum (no spin-orbit coupling). Materials whose susceptibilities exhibit a temperature dependence based on Equation 1.8 are said to display ideal Curie behaviour, which is rare and generally only occurs in the complete absence o f magnetic interactions between neighboring paramagnetic units.

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which result in deviations from Equation 1.8. There are a variety o f ways to account for these interactions, the simplest o f which is the Curie-W eiss law (Equation 1.9). The sign o f the Weiss constant, 6, depends upon the

<'•’ >

nature o f the interaction. For example, if all o f the spins tend to align parallel to each other. 6 is positive and the interaction is referred to as ferrom agnetic. On the other hand, if the spins tend to align antiparallel. 0is negative and an antiferromagnetic interaction is said to result. In general, these interactions are very weak and typically persist only in the presence o f an external magnetic field and disappear when the field is removed. The Curie-Weiss law. while providing an empirical basis for understanding the magnetic behaviour o f a molecular system, says nothing about the specific nature o f the interactions and is used only as a qualitative estimate o f the magnitude and sign o f these interactions.

Because o f the approximations inherent in the Van Vleck equation, the Curie law is only valid when H/kT is small and then the magnetization is linear in H. At high field strengths or very low temperatures {H/kT is large). M must be calculated using Equation

1.4. and the m olar magnetization M is then given by Equation 1 . 10. where Bsiy) is the

iVI = N g ^ S B f y )

D . . 25 + 1 .

B s ( y ) = ^ ^ c o t h 2 5 + 1 — — coth

25 2 5 y

2 5

Brillouin function. When H/kT is very large. Bs(y) goes to unity and the magnetization (Equation I . I I ) is said to be saturated {Ms). Measuring the saturation magnetization is a useful method for characterizing the ground state magnetic properties o f magnetic materials because Ms is directly proportional to 5. as shown in Figure 1.2 for a variety o f values o f 5.

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(p mol’) 0 0.4 0.8 1.2 1.6 2 - * - S = 3/2 -#-8 = 2 - « - 8 = 5/2 iH/kT

F ig u r e 1.2. M agnetization versus ^ H/ kTplots for different values o f 5 (g = 2.00).

1.1.3 Long-range m agnetic ordering

Magnetic ordering is a different phenomenon from discrete nearest neighbour spin-spin interactions. It must be understood that the existence o f ferromagnetic interactions in a material does not imply that the material will exhibit long-range ferromagnetic ordering. In fact, most substances that display ferromagnetic interactions (as indicated by the magnitude o f their room temperature susceptibilities) are Curie paramagnets having a random arrangement o f spin magnetic moments. Ferromagnets

and antiferromagnets sustain long-range three-dimensional order. An important parameter for any m agnetically ordered material is its critical temperature the temperature below which the spins align and the formerly paramagnetic species exhibits a ferromagnetic ordering. Alternatively, for antiferromagnetic ordering, the temperature at which the neighboring spins order antiparallel is referred to as the Néel temperature T\. The temperature dependence o f the magnetic susceptibility for ferromagnetic, antiferromagnetic, and paramagnetic materials is depicted graphically in Figure 1.3. The magnitude o f Tc is also an indicator o f the strength o f the interaction; a high Tc implies

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that more thermal energy is required to disrupt the magnetic ordering. Iron, for example, is a robust ferromagnetic with a very high Tc o f 700°C.

X

F e r r o m a g n e t P a r a m a g n e t A ntiferrom agnet

F ig u r e 1 .3 . % versus T tb r ferrom agnetic, antiferrom agnetic, and param ag n etic m aterials.

An examination o f the magnetic behaviour o f an ordered ferromagnet below Tc is useful because at these temperatures ferromagnets exhibit characteristic magnetic phenomena. Inspection o f Figure 1.3 shows a divergence in % as the temperature o f the ferromagnetic material reaches its critical temperature, where a spontaneous

magnetization should appear. This means that a ferromagnet exhibits magnetization in

the absence o f a magnetic field. As the temperature is cooled below 71: this magnetization saturates to a value dictated by S. In practice, however, at zero magnetic field strength many ferromagnets exhibit zero magnetization due to the effects o f magnetic domain formation. Below T^ a ferromagnetic material is broken into a collection o f spin- containing regions called domains, each having a net magnetization in zero field. The net magnetic moment exhibited by each domain, however, is randomly oriented within the sample such that the resulting net magnetization is zero. This results in other interesting magnetic features exhibited by ferromagnets below Tc, namely, remnance and hysteresis.

Remnance occurs below Tc when the external magnetic field is switched o ff and the field-induced magnetization o f the material remains (this remnant magnetization may or may not equal the field-induced value). The field is then switched back on and slowly reversed in sign until the remnant magnetization (M ) decreases. A negative external

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The interplay between remnance and coercive field for a ferromagnetic material can be depicted graphically by a plot o f the magnetization M versus the applied field H. defining what is referred to as a hysteresis loop (Figure 1.4). Hysteresis is a key indicator o f bulk ferromagnetic behaviour.

F ig u re 1.4. A hysteresis loop.

1.1.4 Intramolecular magnetic exchange interactions

The varieties o f magnetic behaviour discussed thus far have been limited to simple paramagnetic systems containing only one spin-bearing centre exhibiting weak short-range interactions embodied in the Curie-Weiss law. or to more complex magnetic materials such as ferromagnets that exhibit long-range magnetic ordering. Magnetic interactions within a paramagnetic molecule may occur if the molecule contains more than one site bearing spin. This situation is commonly encountered in polymetallic transition metal complexes, and with organic di- or polyradical species.

Consider a bimetallic transition metal complex in which each metal centre has one unpaired electron. There are two intramolecular magnetic interactions possible in this example: If the electrons on each metal centre prefer to align parallel to one another, the interaction is ferromagnetic and the high-spin state is the ground state with the spin- paired state higher in energy. Conversely, if the electrons prefer to align antiparallel to one another the interaction is antiferromagnetic and the spin-paired state is the ground

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State. The strength o f the spin-spin interaction is symbolized by J and is referred to as the

exchange interaction. The exchange interaction is a strictly quantum-mechanical precept,

has no known classical analogue, and exists because o f the restrictions imposed on electron wave functions by the Pauli exclusion principle.^ In a ferromagnetic interaction

J is positive, and is negative in an anti ferromagnetic interaction.

Heisenberg first showed that intramolecular magnetic coupling has its origin in this quantum-mechanical exchange effect, and he described the interaction in the form o f his famous phenomenological Heisenberg spin Hamiltonian (Equation 1.12). where J is the exchange coupling constant, S/ is the spin angular momentum operator o f electron one, and S2 is the spin angular momentum operator o f electron two. Later, Kahn and

Briat

/ / = J , 2 S , S 2 ( 1 . 1 2 )

interpreted this scheme in the context o f molecular orbital theory, demonstrating that J can be reasonably described as the sum o f two competing terms— one anti ferromagnetic and another ferromagnetic in nature (Equation 1.13).^ The anti ferromagnetic term {ipS) is the product o f the quantum mechanical resonance integral ( ^ with the overlap integral (5) and is always negative (antiferromagnetic) because o f the Pauli exclusion principle— when electrons in orbitals overlap (the extent o f the overlap is governed by the magnitude o f S), they must assume different spin states and therefore the interaction is necessarily anti ferromagnetic. The ferromagnetic term (AT) is the two-electron exchange integral which is necessarily positive because o f Hund 's first rule, and is the reason why atomic carbon, for example, having the electronic configuration 1 s"2s‘2p" is a ground state triplet species.^

y = 2 p 5 + ^ (1 .1 3 )

The quantum mechanical details behind the parallel spin alignment in atomic carbon provide an explanation for magnetic exchange that can be applied to more complex molecular systems. In a carbon atom, two electrons are housed in two orthogonal p- orbitals. It so happens that the ferromagnetic triplet arrangement is the most stable because o f the exchange stabilization (Figure 1.5). In the triplet state the mutual repulsion o f the unpaired spins keeps them apart from one another and closer to the positive nucleus, thus lowering their energies. The basis for the ferromagnetic

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stabilization o f two unpaired electrons is twofold: The electrons must be housed in orbitals with zero quantum mechanical overlap (S = 0), and the orbitals must share common space, that is, the two-electron exchange integral m ust be large (K > 0). These conditions are met in atomic carbon— in more elaborate molecular species the mechanisms o f spin exchange may be more complex, but they all boil down to the basic exchange stabilization mechanism described above.

S — 0 K > 0

F ig u r e 1 .5 . Parallel spin alignm ent in a carb o n atom .

1.1.5 The magnetic properties of two-spin molecular systems

The temperature dependence o f the magnetic susceptibility for the two-spin bimetallic molecular system described in the previous section can be calculated using

kT\}>-¥exp{rJikT)]

Van Vleck's equation and is shown in Equation 1.14. This expression is named after Bleaney and Bowers who first derived it in 1952. In order to determine the type (antiferromagnetic or ferromagnetic) and magnitude (value o f J) o f the magnetic exchange, a plot o f the product o f % with temperature (%T) versus temperature is very useful, and is the most common way magnetic data is presented in the literature. The temperature dependence o f for a variety J values is presented in Figure 1.6. When J is positive the %T product characteristically increases with decreasing temperature because o f thermal depopulation o f the diamagnetic singlet excited state and tends to a value o f

2N g'/^/3k (%r = 0.9935). Antiferromagnetic behaviour is characterized by a gradual

decrease in the product with decreasing temperature, and tends to a value o f zero corresponding to complete population o f the diamagnetic singlet state. When J is zero, there is no variation o f with temperature as would be expected for a Curie paramagnet

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(% r = 0.7505). Figure 1.6 is shown for modest values o f 7 but in the case o f very strongly ferromagnetically or antiferromagnetically coupled systems, there may be no noticeable temperature dependence o f (and in the limit o f very strongly antiferromagnetically coupled spins, % r is zero, corresponding to bond formation) because depopulation o f the triplet or singlet manifolds may not occur. In general, the J value is obtained experimentally by measuring the temperature dependence o f % and fitting the observed data to the susceptibility equation derived for the particular spin system under investigation. Equation 1.14 is only valid for two-spin systems, but the same general procedure is used to calculate % for other spin systems o f interest.^

J = +100 cm

T

X

J = -100 c m ’

T

F ig u r e 1.6. 'fT versus T for different values o f J.

1.2 Current efforts towards molecule—based magnetic materials

1.2.1 W hat are molecule-based magnetic materials?

Molecule-based magnetic materials are defined herein as magnets composed o f molecular or macromolecular species (including both organic and coordination polymers) prepared by established low-temperature synthetic procedures o f organic, inorganic, organometallic, and polymer chemistry.* ’ The molecular nature o f these new magnetic materials may endow them with a range o f advantages over current magnets, including