Mononuclear and dinuclear complexes of rhodium and iridium: pyrazole complexes and pyrazolyl bridged dimers

Supervisor: Professor Stephen R. Stobart.

ABSTRACT

A series of mononuclear complexes of general formula [M(r|5 - C 5M e 5 ) C l 3_n (pzH) n ] (n _ 1 )+ (n = 1,2) has been prepared as a result of an investigation of the reactivity of

pyrazole w i t h rhodium and iridium cyclopentadienyl and pentamethyl-cyclopentadienyl precursors. These complexes are discussed in terms of the dynamic processes that are exhibited in the -^H NMR experiment and in terms of their use as precursors to dimeric species. Dinuclear complexes of formula [M(r|5-C5R 5 )Cl(|x-pz) ]2 containing pyrazolyl bridges have been prepared from the mononu c l e a r compounds and from the chloro-bridged dimers of formula [M(r|5 - C 5R 5 ) C l 2 ] 2 by treatment w i t h triethylamine, but not from the dipyrazole

iridium cation [Ir (r|5-C5M e 5 ) Cl (pzH) 2 ] + 26. w h i c h has been found to be unreactive to this type of symmetrical dimer formation: the low reactivity is attributed to a relative non-lability of the pyrazole groups. The d i m eric complexes have been shown to undergo a core conformational change upon chemical reduction or halide abstraction. The chair

conformation of the pyrazolyl b r idged complex [Rh(ri5- CgMes) Cl (|i~pz) ] 2 M has been proven c r y s t a l l o g r a p h i c a l l y . Chloride abstraction from 38 yields the b i n u c l e a r product

[ (Rh(ri5-C5M e 5 ) ((x-pz) }2 (M--C1) )BF4 46 which is b r idged by two pyrazolyl and one chloride ligand and has b e e n structurally characterized by X-ray diffraction to contain a boat

conformation for the pyrazolyl framework. Reduction of

either 38. or the C 5H 5 analogue 40 results in the metal-metal b onded dinuclear complexes [Rh(ri5 - C 5R 5 ) (p,-pz) ] 2 48 and 49.. The CgHg complex 49. has been c r y s t a llographically determined

metal-meta.l bonded products has been investigated: one and two fragment addition is discussed and a number of oxidative addition products have been structurally characterized. The mononuclear dipyrazole iridium cation 26. which contains non- labile pyrazole groups is utilized to prepare mixed-metal and mixed-oxidation state dimers with the formula [Ir(Ti5- C 5M e 5 ) Cl (ii-pz).MLn ] . The synthesis and potential for further investigation of these complexes is discussed.

Examiners :/\

/ D r . S.R. Stobart, Sup.rvisor, Department of Chemistry

Dr. A. McAuley, Departmental^ Member, Department of Chemistry

Dr. G.W. Bushnell, Departmental Member, Department of Chemistry

Dr. T.J. Trust, Oxxtsj^dJe Memb^r^ Department of Biochemistry

Dr. M. Cowie, E x t ernal Examiner, University of Alberta, Department of Chemistry

TABLE OF CONTENTS

Page

Abstract ii

Table of Contents iv

List of Tables viii

List of Figures x

List of Schemes xii

List of Compounds xiv

List of Abbreviations xvii

Acknowledgements xix

Chapter 1: Introduction 1

1.1 Metal-Metal Bonding 1

1.2 Cyclopentadienyl and Derivatives 12

1.3 Pyrazole 13

1.4 Objective 21

Chapter 2: Mononuclear Pyrazole Complexes: Precursors

to Dimers; Stereochemical Non-rigidity 23

2.1 Introduction 23

2.1.1 Metal Pyrazole Complexes. 23

2.1.2 Metal Pyrazolyl Complexes. 2 6 2.1.3 Fluxional Pyrazole Complexes. 27

2.2 Results and Discussion 29

2.2.1 Synthesis of Mononuclear Pyrazole

2.2.2 Crystallographic Structure

Determination. 37

2.2.3 S p ectroscopy of Mononuclear Pyrazole

C o m p l e x e s . 4q

2.2.4 Variable Temperature NMR Studies. 55

2.3 Conclusion 61

Chapter 3: Pyrazolyl Bridged Dimers of Rh(IIl) and

I r ( I I I ) : Synthesis and Structure 65

3.1 Introduction 65

3.1.1 Structural Characteristics. 66

3.2 Results and Discussion 71

3.2.1 Synthesis of Pyrazolyl Bridged

D i m e r s . 71

3.2.2 Spectroscopy of Pyrazolyl Bridged

Dimers. 77

3.2.3 Structure of Pyrazolyl Bridged Dimers. 80 3.2.4 R eactivity of the Double Pyrazolyl

B r i d g e d Dimers. 98

a. Core Conformational Change. 98

b. Chloride Abstraction from Complex

4j5. 110

3.3 Conclusion 12 3

Chapter 4: Reactivity of the Pyrazolyl Bridged

Rhodium(II) Dimers 125

4.1 Introduction 125

4.2 Results and Discussion 128

4.2.1 Two Fragment Addition. 128

4.3 Conclusion 147

Chapter 5: Mixed Metal and Mixed Oxidation State

Pyrazolyl Bridged Dimers 152

5.1 Introduction 152

5.2 Results and Discussion 154

5.2.1 Synthesis o f Mixed Metal Complexes. 155 5.2.2 Synthesis o f M i x e d Oxidation State

Iridium Dimers. 161

5.3 Conclusion 171

Chapter 6: Experimental 173

6.1 General Techniques 173

6.1.1 Synthetic Methods. 173

6.1.2 Analytical Methods and Equipment. 173 6.1.3 Crystallographic Structure

Determination. 174

6.2 Synthesis of Compounds 176

References 194

Appendix 1: Crystallographic Data 202

1.1 Rh (ri5-C5M e 5 ) C l2 (pzH) 2 2 202 1.2 [Rh(ri5-C5M e5)Cl(pzH)2]BF4 2 4 208 1.3 [Ir(r|5-C5M e 5 ) C l] 2 (|l-pz) (n-Cl) 43 213 1.4 [Rh(7l5- C5M e5)Cl(|l-pz) ] 2 38 221 1.5 [ {Rh (rj5“C5H 5 ) } 2 (ll-pz) 3 ] Cl • [Rh (t| C5H5) 2 ] Cl • 2 H 20 4 4 227 1.6 [ {Rh(tl5- C5M e5) (H-pz) }2 (n-ci) ]BF4 4 6 235 1.7 [Rh(tl5-C5H 5 ) (H-pz) ]2 49 242

1.9 E{Rh(Ti5- C5H5) (|i-pz) }2(H-NO) ]BF4 56 258 1.10 [{Rh(ti5- C5H 5 ) (n-pz) }2 (l^-OMe) ]CF3C 02 M 266

Table 2. Table 2.: Table 2.: Table 2. ‘ Table 2.1 Table 2.< Table 2.’ Table 2.1 Table 3. Table 3. Table 3. Table 3. Table 3. Table 3. Table 3. Table 3. LIST OF TABLES

: Elemental analyses for pyrazole mononuclear complexes.

: S e l ected bond lengths for compound 22. : Selected bond angles for compound 2 2 . : Selec t e d bond lengths for compound 2 4 . : Selec t e d bond angles for compound 24. : Ranges o f distances of r h o dium to carbon

atoms in v^-CsMe^ rings and distances of rhodium to nitrogen atoms of terminal n itrogen containing ligands.

: I R data for mononuclear pyrazole complexes. : 1H NMR data for mononuclear pyrazole

c o m p l e x e s .

: Elemental analyses for dinuclear pyrazolyl bridged complexes.

: 1H NMR data for dinuclear pyrazolyl b r i d g e d complexes.

: Ranges o f distances of r h o dium to rhodium atoms a n d distances of r h odium to

nitro g e n atoms of bridging pyrazolyl groups.

: Selected bond lengths for compound 4 3 . : Selec t e d bond angles for compound 4 3 . : S e l ected bond lengths f o r compound 38. : S e l ected bond angles for compound 3 8 . : S e l ected bond lengths for compound 44.

Page 32 39 40 42 43 46 49 50 74 78 81 85 86 91 92 96

Table 3.9: S e l ected bond angles for compound 44. 97 Table 3 . 1 0 : Selected bond lengths for compound 4je>* 1 0 5 Table 3.11: Selected bond angles for compound 46. 105 Table 3.12: Selected bond lengths for compound 49. 108 Table 3.13: Selected bond angles for compound 49.. 109 Table 3.14: S e l ected bond lengths for compound 50a

and f o r compound JLQ. from reference 67. 1 1 5 Table 3.15: S e l ected bond angles for compound 50a and

for compound 50. from reference 67. 117 Table 4.1: Elemental analyses for products of

reaction with compounds 48 and 49. 132 Table 4.2: 1H NMR data for products of reaction with

compounds 48 and 49.. 132

Table 4.3: Selected bond lengths for compound 56.. 138 Table 4.4: Se l ected bond angles for compound 56. 139 Table 4.5: Selected bond lengths for compound 58. 143 Table 4.6: Se l ected bond angles for compound 58. 144 Table 4.7: Ot h e r reactions attempted with compound

49. 146

Table 4.8 Values of a and (3 for pyrazolyl bridged

dimers. 1 5 0

Table 5.1: Elemental Analyses for products of

reaction of compound 2J3. 1 57

Table 5.2: 1H NMR data for products of reaction from

LIST OF FIGURES

Page Figure 1.1: Orbital Diagrams f o r a) a d 6 Octahedral

Complex and b) a d 7 Octahedral Complex. 4 Figure 1.2: Orbital Diagram F o r Bimetallic

Complexes; a) Two d 3 Centers, by One d 3 and One d 7 center, and c) Two d 7

centers. 9

Figure 1.3: M i x i n g stabilisation of d z2o* and p zo. 10 Figure 1.4: Drawings of cyclopentadienyl derivatives

9 and 10. 12

Figure 1.5: Drawing of pyrazole with n u m b ering

scheme. 14

Figure 1.6: Tautomerization in pyrazole. 14

Figure 1.7: Dynamic site exchange in pyrazole

complexes. 16

Figure 2.1: Two isomers of compound 18. 25

Figure 2.2: ORTEP drawing of m o l e c u l a r structure of

compound 22.. 38

Figure 2.3: O RTEP drawing of cation structure of

compound 24.. 41

Figure 2.4: Homonuclear decoupling of compound 24- 53 Figure 2.5: Variable temperature 3H NMR of compound

22. 57

Figure 2.6: Variable temperature 1H N M R of compound

24 • 59

Figure 2.7: Variable temperature 1 3 C { 1H} NMR of

compound 24.. 60

Figure 2.8: Plot of ln(k/T) vs. (1/T) for V T 13C NMR

Core Conformations o f Pyrazolyl Bridged

Co m p l e x e s . 67

Figure 3.2: 1H NMR spectrum of compound 38. 79

Figure 3.3: ORTEP drawing o f m o l e cular structure of

compound 43. 84

Figure 3.4: ORTEP drawing o f m o l e cular structure of

compound 38. 90

Figure 3.5: ORTEP drawing o f cation structure of

compound 44. 95

Figure 3.6: ORTEP drawing o f cation structure of

compound 46. 104

Figure 3.7: O RTEP drawing o f molecular structure of

compound 49. 107

Figure 3.8: ORTEP drawing o f h a l f cation structure

of compound 50a. 114

Figure 3.9: Drawing of deltaphane. 1 2 0

Figure 4.1: ORTEP drawing o f cation geometry of

compound 56. 3.37

Figure 4.2: O RTEP drawing o f cation geometry of

compound 58. 142

Figure 4.3: Drawing showing angles a and (3 in

pyrazolyl bridged dimers. 149

Figure 5.1: 1H NMR spectrum of compound 57. 158

Figure 5.2: 1H NMR spectrum of compound 65. 165

Figure 5.3: 1H NMR spectrum of compound 6 8. 166

Scheme Scheme Scheme Scheme Scheme Scheme Scheme Scheme Scheme Scheme Scheme Scheme Scheme Scheme LIST OF SCHEMES Page L.l: Oxidative addition to a single metal

center. 3

1.2: Oxidative addition to the di-iridium(I)

compound 5. 6

L.3: a) Intermolecular oxidative addition to the di-iridium (II) compound 6.. b)

Intramolecular oxidative rearrangement

of the di-iridium(II) compound 7. 8

L.4: Preparation of the pyrazolyl anion. 15

L.5: Intermolecular pyrazole site exchange in

ruthenium porphyrins. 18

L.6: Intramolecular pyrazole site exchange in

silanes. 19

L.7: Intermolecular pyrazole site exchange involving a pyrazolyl-bridged

intermediate. 19

2.1: Formation of unsymmetrical dimers from

compound 20. 27

2.2: Synthesis o f Mononuclear Pyrazole

Adducts. 31

2.3: Synthesis o f Substituted Pyrazole

Mononuclear Complexes. 35

3.1: I2 oxidation of compound 36. 70

3.2: Formation o f pyrazolyl bridged compound

37. 71

3.3: Synthesis o f double pyrarolyl bridged

dimers,, 73

3.4: Synthesis of single pyrazolyl bridged

Scheme 3.5: Synthesis of triple pyrazolyl bLodged

dimer. 76

Scheme 3.6: R eactions of compounds 38, 39. and 40. 1 0 0

Scheme 3.7: Formation of compounds 50 and 50a. 11 2

Scheme 4.1: Reaction of compound 48 with 1 2 - 130

Scheme 4.2: Reactions of compound 49. 131

Scheme 5.1: Synthesis of dimeric iridium(III) and

mixed-metal complexes from compound 26. 156 Scheme 5.2: Synthesis

complexes of of

mixed-oxidation state

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 LIST OF COMPOUNDS ir(pph3 )2 (CO)cl Fe(n5-C5H 5 ) (T]6- C6M e 6 ) [Co(Y15-C5M e 5 ) (r16- C6M e 6 ) ]BF4 [Ir (cod) (p.-pz) ]2 [Ir(CO) (PPh3 ) (M--PZ) ]2 [Ir (cod) I (|i-pz) ] 2

[Ir (CO) (PPh3 ) ((l-pz) ]2 (I) (CH2I) [ {Ir (cod) (M.-Me2pz) }2]BF4

C5H5" CsMes-pzH [Rh(Ti5-C5M e 5 )pz(lx-pz)]2 [Rh(r)5-C5M e 5 ) C l 2 ]2 [Rh(Ti5- C 5H 5 ) C l 2 ]2 [Ir(T15-C5M e 5 ) C l 2 ]2 [ Ir (i^-C5H 5 ) C l 2 ] 2 [Ni(pzH)6](N03 ) 2 [Ru{p-(ipr)(Me)CgH4) (acac)(3-MepzH)]BF4 Ir(PPh3)2C O (3,5 -Me2pz) Pt(PEt3)2 (Pz ) 2 [PdCl(PEt3)2(4-Br-3,5 - M e2p z H ) ] (C104 ) Rh(T15-C5M e5)Cl2 (pzH) [Rh (ti5- C5M e 5 ) Cl (pzH) 2 ] Cl [Rh (Ti5-C5M e 5 ) Cl (pzH) 2 ] B F4

25 Ir(r)5-C5Me5)Cl2(pzH) 26 [ir(n5-C5M e5)Cl(pzH)2 ]Cl 27 [Ir(n5-C5M e5)Cl(pzH)2]BF4 28 Rh(r|5- C5M e5)Cl2 (dmpzH) 29 Ir(ti5- C5M e5)Cl2(dinpzH) 30 [Ir(cod)Cl] 2 31 [Ir(Ti5-C5M e5)Cl(5-MepzH) 2J B F4 32 [{Rh(tl5-C5M e 5 ) (|i-Cl) )2(M.-pz) jBF4 33 [{Ir(Tl5-c5M e 5 ) (H-Cl) }2 (P.-PZ) ]BF4 34 [Ru(n6-C6H6)Cl([X-pz) ]2 35 _{[{Pt(Ti2-C2H4)Cl}2(M--pz) (M--C1) )} 36 [(Ir(cod)(n-pz)}2 (NO)]BF4 37 [Pd(n3-(CH2)2C (C6H 5 )) (p.pz)]2 38 [Rh(n5- C5M e5)Cl(VJL-pz) ] 2 39 [Ir(n5- C5M e5)Cl((x-pz) ]2 40 [Rh(n5- C5H5)Cl((JL-pz) ]2 41 [Ir (r)5- C5H 5 ) Cl (p,-pz) ] 2 42 [Rh(n5- C5M e5)Cl]2(M.-pz) (ji-Cl) 43 [Ir (r]5-C5M e 5 ) Cl] 2 (H-pz) (H-Cl) 44 [ (Rh(ti^-C5H 5 ) )2(M.-PZ)3]C1* [Rh(n5- C5H5)2]C1-2H2 0 45 [Ti(n5- C5H5)2 (^-pz) ] 2 46 [{Rh(r|5-C5M e 5 ) (p.-pz) )2 (|i-Cl) ]BF4 47 [ (Ir (r|5-C5M e 5 ) (M--pz) >2 (M--C1) ] B F4 48 [Rh(Ti5-C5M e 5 ) (|i-pz) ] 2 49 [Rh(Ti5- C5H 5 ) (M.-pz) ] 2

50 [{Rh(n5-C5M e 5 ) (ix-pz) }2(M.-OH) ] +

50a [ {Rh(n5-C5M e 5 ) (n-pz) }2(M.~OH) ]BI"4*|AgBF4

51 R h ( P P h3)2(CO)Cl 52 [Rh (CO) Cl2 ([1-dppm) ] 2 53 [Rh(CO)Cl4 (ji-dppm) ] 2 54 [Rh(n5-C5M e5)I(|i-I) ] 2 55 [Rh(ti5-C5H5)I(n-pz) ] 2 56. [{Rh(r)5-C5H5) (M.-PZ) }2(jt-NO) ] BF4 57 [Co(Tl5-C5H 5 ) (M.-NO) ] 2 SB [{Rh(n5-C5H 5 ) (n-pz) }2 (M*“OMe) ]CF3C 02 59 R h2Ir ( P A s P )2Cl3(CO) 2 60 Ir (r)5- C5M e 5 ) Cl(lJL-pz) 2Rh(Ti5-C5M e5) Cl 61 [Ru(C6H 6 )Cl2 ] 2 62 [Ru(C6M e6)C l2 ]2 63 [Ru(p-(ipr)(Me)C6H 4 )Cl2 ]2 64 Ti(Tl5-C5H 5 )2Cl2 65 Ir (ti5- C5M e 5 ) Cl (|j,“pz) 2 Ir (cod)

6 6 Ir (ri5-C5M e5) Cl (p,-pz) 2Ir (coe) 67 [Ir(coe)Cl] 2

68 Ir(ri5- C5M e5)Cl(lx-pz)2l r ( C O) 2

69 [Ir(cod) (M.-3-Mepz) ] 2

70 Ir (ri5- C5M e5) Cl ((i,-3-Mepz) (n,-Cl) Ir (cod) 71 [Ir (cod) {(1-3 , 5- (CF3pz) pz } ] 2BF4

LIST OF ABBREVIATIONS acac acetylacetonate = C H3C (0)C H C (0)C H3 ar aryl Bu butyl = C4H9 cod 1,5-cyclooctadiene = C g H i2 coe cyclooctene = C g H ^

Cp g e n eric cyclopentadienyl derivative dmpzH 3 , 5-dimethylpyrazole = (CH3)2C3H2N2

dppm bis(diphenylphosphino)met hane = (Ph2P)2C H2

Et ethyl = C2H5

FW formula weight

HMPT hexamethylphosphoric triamide = (Me2N )3PO Hz hertz = s**1

IR infra-red

kJ kilojoule

LUMO lowest u noccupied molecular orbital Me methyl = C H3

NMR n u c l e a r magnetic resonance N.O.E. n u c l e a r Overhauser effect

PAsP b i s [ (diphenylphosino)methyl]phenylarsine = (P h2P C H 2 )AsPh Ph phenyl = CgHs pm p i c o meter = 10""!2 m ppm part p e r million Pr propyl = C3H7

pz pyrazolyl = C3H3N2

pzH pyrazole = C3H4N2

THF tetrahydrofuran = C4H8O

ACKNOWLEDGEMENT

I w ould like to thank my supervisor, Professor S.R. Stobart for help and encouragement and Mrs. K. Beveridge and Mr. R. Brost for their h e l p with crystallography. I w o u l d also like to thank the University of Victoria and N S E R C for their support in the form of graduate

scholarships.

1.1 METAL-METAL BONDING

Oxidative addition of small molecules to diiridium(I) 1-14

complexes has been shown to occur readily to produce metal-metal bonded dimeric iridium(II) species. The

investigation of oxidative addition and reductive

elimination at the rhodium(I) and iridium(I) centers in dimeric species has revealed an extensive amount of

chemistry relating the + 1 t o + 2 oxidation states of these . . 15

systems. Oxidative addition refers to the reaction m which a substrate A-B adds to a metal complex (equation

1.1), while reductive elimination is the reverse of this

M L + A - B --- > A - M L - B 1.1

n n

process; these two steps are essential to the operation of many homogeneous c a t a l y s t s. 1 5 '16 Oxidative addition results

in the formal t r a nsfer of electrons from the metal center to the substrate, and decreasing the electron count at M from dn to dn - 1 or d n ~ 2 . Addition of an oxidizing substrate to a single metal center (equation 1.2) results in a two electron

M Z + A-B --- > A - M Z+2-B 1.2

oxidation at that center, and is a reaction that has been extensively s t udied17 for compounds like Vaska's complex,

(PPh3)2Ir(CO)Cl (l). In this case, the square planar d8

iridium(I) center is oxidized to an octahedral d6

♦ . 1 ft

i n d i u m (III) product as a result of this type of addition (scheme 1.1). For bimetallic complexes, parallel two center processes have b e e n characterized in which one electron

changes at each metal are accompanied by either (a) formation of or (b) cleavage of a metal-metal bond

(equations 1.3a,b).

A-B + M Z --- > A —M Z+1— — _M Z+:L-B 1.3. a

A-B + M Z— M Z --- > A - H* * 1 M Z + 1 -B 1.3.b

Metal-metal bond formation corresponds to spin pairing between formally paramagnetic centers and allows each metal center to achieve an eighteen electron configuration. A description of t h e interaction between metal centers can be constructed by examining the symmetry of the metal valence orbitals and the appropriate ligand orbitals. Isolated d6

octahedral complexes may use the d 2, d 2 2, p , p , p and Z X “ j X y Z

2 3 s orbitals to form six hybrid acceptor orbitals (d sp )

Scheme l.l: Oxidative addition to a single metal center. Pl.3 r-co I -Ir— I Cl

R X

which are occupied by electron pairs donated from the six coordinated ligands (figure 1.1). The six d electrons from the metal then fill the remaining d orbitals (d ) to

xy tx z ,yz give a total of eighteen electrons around the metal. Also, the d „ orbitals are of appropriate symmetry to

xy,xz,yz c J J

overlap with acceptor orbitals on the ligands (ir or d

symmetry) which may offer further stabilization of the metal centered electrons. When the metal center is formally

7

reduced by adding one electron to generate a d confi g u r ­ ation, the electron will occupy an orbital with the lowest available energy (previously L U M O ) . In octahedral complexes, this orbital is of antibonding character and is associated with one of the metal-ligand bonds. The single electron reduction results in a nineteen electron metal center. This state may be energetically the most favorable and some

19 2 0 21 examples of nineteen electron complexes exist, most ' '

Figure 1.1: Orbital Diagrams for a) a d Octahedral Complex and b) a d 7 Octahedral Complex.

pa* \ scr pa* \ sa* P s d \ \ da*\\

A

W

p

of which are formed by elements of the first transition

c /r

series. The mixed sandwich compound Fe(ri - C5H 5 ) (ti -CgMeg) 20

(2) has been structurally characterized and does show two planar aromatic rings bonded to the iron atom which formally

7

adds twelve electrons to the d center. The related cobalt s p ecies2 1 [Co (r|5- C5M e 5 ) (r|6- C6M e 6 ) ] B F4 (3) also has a

nineteen electron configuration. A more common alternative to the this state consists of loss of one of the ligands from the octahedral complex to give a seventeen electron

I 2L

metal center which may subsequently dimerize. The

3 . . .

rehybridization to dsp required for bonding to five ligands will allow the d 2 orbital to accommodate the unpaired

7

electron. When two d species are brought together along the z axis, then the dz 2 orbitals of each may mix as shown

in figure 1.2. The two electrons from the d z2 orbitals will occupy the lower energy da orbital which is bonding along the metal-metal direction, i.e. forming a metal-metal bond. If the dimerization results in an overall lowering in energy relative to the isolated octahedral monomers, then the dimer

22 . . .

will form. Many of the metal-metal bonded dimeric species are supported by some form of bridging ligands which will facilitate bond formation, but a range of examples

23 demonstrate that such bridging is not necessary.

The rhodium a n d iridium systems that have been studied

oxidative addition in w h i c h a metal-metal bond is formed (scheme 1.2). Substr a t e s including dihalides and alkyl halides from m ethyl iodide to those with very long carbon chains (Cl g ) will add to pyrazolyl bridged complexes of the formulae [Ir (LL') (jx-pz) ] 2 (pz = pyrazolyl = C3H3N 2 , LL' = cod = 1,5-cyclooctadiene 4; L = CO, L' = pph3 £)> it has been shown that increased steric hindrance at the metal

3 4 14 .

leads ' ' either to r e d uced reactivity or to one electron oxidation w i t h o u t addition. Related to this, the dimeric iridium(II) species [Ir (cod) I (|i-pz) ] 2 (6) which is formed by

•i *j n 2 ^

oxidative addition of I2 to 4., reacts ' with NO (scheme

Scheme 1.2: O x i d ative addition to the di-iridium(I) 3 6 8

previously in this laboratory ' 1 are highly susceptible to

compound 5.

pi.3p CO

1.3 a) to produce a triply b r i d g e d complex in which the substrate has oxidatively added to the complex resulting in cleavage of the metal-metal bond. Also, oxidative addition of CH I to 5 gives the unsymmetrical dimer [Ir (CO) (PPh ) (p.-

Z Z J

p z ) ]2(I)(CH2I) (7) which undergoes an intramolecular

13 .

rearrangement that is accompanied by formal oxidation at each metal atom; in this reaction, the ligand fragment C H2

from a terminal C H 2I group is inserted into a bridging

position (scheme 1.3 b) . In both cases the two consecutive oxidative steps add up to a formal four electron process, with the addition of three fragments to a single bimetallic c o r e .

The first two electron oxidation that converts a bimetallic iridium(I) complex into a metal-metal bonded

24

i n d i u m ( I I ) dimer has been observed to proceed as two one electron steps both chemically and e l e c t r o chemically. The

Scheme 1.3: a) Intermolecular oxidative addition to the di-iridium(II) compound 6 . b) Intramolecular oxidative rearrangement o f the d i ­

iridium (II) compound 7.

, < N ^ > > * / .n-n

N 0 +

g

electron configuration of two square planar d iridium(I) centers held in close proximity by a bridging system can be described in a similar manner to that used earlier in which the d 2 metal v a lence orbitals overlap to form a delocalized

z

dado pair (see figure 1.2). The four electrons (two from each a iridium) fill both the bonding and antibonding

2 *2

orbitals (da da ); i.e. no net bonding. An electron

Figure 1.2: Orbital Diagram For Bimetallic Complexes;

8 8 7

a) Two d Centers, b) One d and One d 7

center, and c) Two d centers.

j da* d z2 d z2 \ / d z2 U— da (b) da* / d *2 da dz2 V _ U _ / da (a) d z2 (C)

Figure 1.3: M ixing stabilization of d 2a and p a.

z z

Pz

(1,2

25

spectroscopic study of compound 4. (which has this

c o n f i g u r a t i o n ) , has produced data that suggests a bonding interaction exists between t h e iridium centers. This contradiction is explained u sing orbital mixing

stabilization (see figure 1.3) of the (n)d 2a orbital with z

an empty (n+l)pza orbital t h a t possesses the same symmetry. This interaction lowers the da energy and stabilizes the

configuration. There are examples of molecules with these

g

interactions between d square planar centers which have 26 2 V

been structurally characterized; ' intermetal distances of less than 3 00 pm have b e e n observed for the complexes

[Rh(NCPh)4]BPh4 and [ R h(CO)(mac)] (mac = macrocycle, 2 * ? C 15H 23N 0 3S 2^ * Sin<3le electron oxidation of the do 'do

per per

Pz

Pz

2 * 1

configuration will lead to a da da configuration or a formal metal-metal bond order of 0.5, with a further one

2 *0

electron oxidation yielding a da da state and a bond order 2 8 of one (i.e. metal-metal single b o n d ) . It has been shown that oxidative addition reactions may proceed by either nucleophilic attack by the metal on the substrate or v i a a radical chain mechanism; the bimetallic radical cation

(single electron oxidation) [Ir (cod) (|i-Me2pz) ]2B F4 (8) has 9

been isolated and characterized. The physical properties 22

associated with metal-metal b onded species include short metal-metal separations in the crystal structures (< 3 00 pm) required to allow for overlap of the d 2 orbitals, and the

z

appearance of new low energy absorptions (> 350 nm) in the electronic spectra due to the da — > da transition.

In contrast to utie amount of data compiled for the two electron oxidation described above, there is a second

oxidative process involving two electrons which has been 29 30

less thoroughly studied. ' Dimeric M(II) species which have a metal-metal bond m a y undergo reaction in which the two bonding electrons are sequentially (or simultaneously)

0 * 0

removed from the a orbital to give a da da state (no b o n d ) . The research discussed in this thesis has examined this second process, specifically by approaching the

1.2 CYCLOPENTADIENYL AND DERIVATIVES

Cyclopentadienyl (CgHg , £) and the permethylated analogue pentaxnethylcyclopentadienyl (CgMe,. , .1 0) (figure

1.4) are both aromatic six electron anionic carbocycles that are suitable for stabilizing metal complexes in higher

• * b

oxidation states. Metal compounds containing a u “C5 H 5

31 . .

group are frequently of lower solubility and stabil \ty 5

than the t| -C5M e5 analogues. Removal of the bound C5H5

group is accomplished by many reagents and synthesis of starting materials of the type [M(ti5- C5H 5 ) C 1 2 ]x (M = Rh, Ir)

32

results m low yields of the metal complex. The

Figure 1.4: Drawings of cyclopentadienyl derivatives 9 and 1 0. CII CH CH CH CH H H H

pentamethyl analogue, C5Me,_H, however reacts cleanly with 33

the metal salts, M C 13*xH20 (M = Rh, I r ) , to produce high yields of soluble species that are stable to acidic, basic,

1

oxidizing, and reducing conditions. The H N M R signals are 5 singlets for both ligands when they are bonded in an ri manner. These ligands formally occupy three cofacial

coordination sites and the steric constraints imposed on the approach of substrates are potentially important,

particularly when it is noted that the reactivity of the 3 4 14

i n d i u m (I) dimers is lessened ' ' by steric crowding.

1.3 PYRAZOLE

34

Pyrazole (pzH, 11) is a colourless crystalline solid which in solution is a weak acid (pK = 2.53). The IUPAC

cl

numbering scheme for the pyrazole molecule is shown in figure 1.5. In solution, pyrazole undergoes extremely

35 35 37

facile tautomerization ' ' by exchange of the proton between the two adjacent nitrogen atoms (figure 1.6). The

38 39

heterocyclic ring has been determined ' to possess some delocalization of the double bonds and inclusion of the nitrogen lone p a i r in the tt system leads to a Huckel 4n + 2

(n = 1) aromatic ring. The molecular structure of pyrazole 39

Figure 1.5: Drawing of pyrazole with numbering scheme.

4

3

II

Figure 1.6: Tautomerization in pyrazole.

N * N

the C-C and C-N bond lengths are all between 13 3 and 14 2 pm. Pyrazole and its derivatives can act as Lewis bases by

ccmplexation through the unprotonated nitrogen at the 2

position in a terminal ri1 mode and many examples4 0 have been found in which pyrazole has formed metal complexes in this way. The extent of pyrazole complexation is dependent on pyrazole substitution as well as on the character of the

metal ion and counterion. Electronic and steric effects can be studied by substitution of the ring protons at the 3, 4, and 5 positions of the pyrazole molecule.

Deprotonation of pyrazole is accomplished with a strong base such as NaH and preparation of the anion salt occurs readily by reaction with a group 1 metal (see scheme 1.4). The pyrazolyl anion forms metal complexes in the same manner as neutral pyrazole, but more notably, it is capable of

forming dimeric species by complexation of metal ions at 41

both nitrogen atoms and forming a bridge between two metal

5 42

centers. The complex [Rh(r| -CgMe,.)pz (|i-pz) ] 2 (12) has

1

.

pyrazolyl groups in both r| terminal and bridging positions in the same molecule. Also, the pyrazolyl anion may act as a chelate ligand by bonding b o t h nitrogen atoms to one metal

43 •

center, although this is rare, occurring only in complexes of uranium in which the large size of U may be essential to

this form of bonding. In d i m eric complexes, the pyrazolyl bridged core is extremely flexible, and allows a large range

1-14 42

of xnter-metal distances ' (250 to 450 p m ) .

Complexes incorporating p y r a z o l e as a terminal rj1 bound

. 44 . .

ligand exhibit stereochemical non-rigid behaviour in the NMR experiment (figure 1.7). The pyrazole group has

available two possible b i n d i n g sites (allowing for proton exchange as w e l l ) , and, as t h e N-H proton is relatively

45

labile (e.g. it is readily e x c h anged with d e u t e r i u m ) , there exists an exchange p a thway b e tween the two equivalent configurations arising from such complexation. When the rate of exchange is fast, an average of the two

configurations is observed.

Figure 1.7: Dynamic site exchange in pyrazole complexes.

M H

— N M

Dynamic processes have been observed and studied since the advent of v a r i a b l e temperature NMR spectroscopy; this technique has generated a wealth of thermodynamic data

regarding these stereochemical site exchanges. Although the tautomerization of pyrazole (scheme 1.6) is extremely fast, the addition of an aprotic base such as dmso or HMPT [HMPT =

(Me2N )3PO] to a solution of pyrazole will slow the rate of exchange sufficiently so that the two tautomeric forms can

35 36 . . . . 45

be distinguished. ' Also, studies have identified a slower rate of pyrazole tautomerization in the solid state. From these experiments, it has been suggested that the

tautomerization in pyrazole proceeds by formation of trimers or oligomers; this association provides a low energy pathway for the hydrogen nuclei to move between adjacent pyrazole molecules. The slowed rates noted above occur because this pathway is inhibited by hydrogen bonding of pyrazole to the base, therefore blocking pyrazole association, and, in the solid state structure, by less than optimum orientation for the proton transfer between the pyrazole molecules. The mechanism of the pyrazole site exchange has also been the

44 46 47

subject of intensive investigation, ' ' but the steps involved have not yet been identified with certainty. The process requires two successive steps for which there are two general mechanistic pathways: intermolecular and

dissociation of pyrazole followed by pyrazole

tautomerization has been suggested for ruthenium porphyrin 46

complexes (scheme 1.5), and an intramolecular 1,2

m igration of metal-nitrogen bonds has been proposed for some 47

main group pyrazolyl complexes (scheme 1.6); however,

Scheme 1.5; Intermolecular pyrazole site exchange in ruthenium porphyrins.

Scheme 1.6; Intramolecular pyrazole site exchange in silanes. N- N*

//

Scheme 1.7: I n t e rmolecular pyrazole site exchange

there is little evidence to support the intramolecular mechanism for pyrazole derived complexes, while there is some evidence to support an intermolecular process. A study by Bushnell et. a l.4 8 , 4 9 , 5 0 of related dinitrogen donors has shown that the relative rates of intramolecular versus

intermolecular site exchange correlates well with the

orientation of the nitrogen lone pairs. Ligands with lone pairs that point towards each other (e.g. bipyridine) favour

intramolecular rearrangement, w hile lone pairs that point away from each other favour an intermolecular exchange. In the pyrazole molecule, the nitrogen lone pairs are

orientated away from each other so that intramolecular exchange is not favoured. Also, in pyrazole complexes, an intermolecular process may occur following the deprotonation wh i c h proceeds via a dimerization step and forms a bridged species. Cleavage of this dimeric species will allow the pyrazole ligands to exchange between metal atoms, the result of which is a local site exchange (scheme 1.7). This

m e c h anism requires an expansion of the coordination sphere or a dissociation of another ligand (or ring s l i p p a g e ) .

1.4 OBJECTIVE

A primary goal of this work was to synthesize pyrazolyl bridged dimers of iridium(III) and rhodium(III)

cyclopentadienyl derivatives which would be suitable

precursors to metal-metal bonded complexes. This approach is designed to follow a reductive pathway from the dimeric species to give the iridium(II) and rhodium(II) complexes, as opposed to the. oxidative synthetic strategy u s e d when preparing the metal(II) compounds from dimeric precursors which have the metal atoms in the +1 oxidation state. An

investigation of the reactivity of pyrazole w i t h the complexes [M(i‘)5- C5R [.) C l 2 ]x (M = Rh, Ir; R = H, Me) was

initiated prior to the synthesis of any dimeric compounds; the study concentrated on the formation arid structural

properties of the mononuclear pyrazole complexes of iridium and rhodium, and uxtimately on the formation of pyrazolyl bridged dimers. Formation of metal-metal bonded complexes by oxidative or reductive synthesis requires that the metal centers can approach to within 3 00 pm of each other. The pyrazolyl group has b e e n shown to accommodate this

intermetal distance, and the intramolecular rearrangement in bimetallic iridium(II) complexes suggests that the pyrazolyl bridged core can also undergo extreme changes in

The reductive synthesis of the rhodium (II) dimers and some other reactions of the dinuclear rhodium(III) compounds studied here have provided data that give insight into the mechanism of the rearrangement. W e have also investigated the two electron oxidation step from dimeric M(II) to M(III) complexes and attempted to isolate a single electron

oxidation product, M ( I I ) M ( I I I ) , for the cyclopentadienyl systems. To this end, a number of mixed metal and mixed oxidation state dimers have been synthesized and are reported here.

CHAPTER 2

MONONUCLEAR PYRAZOLE COMPLEXES:

PRECURSORS TO DIMERS; STEREOCHEMICAL NON-RIGIDITY

2.1 INTRODUCTION

In the previous chapter, a synthetic strategy which focussed on a reductive route to dimeric iridium(II) and

5

rhodium (II) pyrazolyl complexes starting from ri - C5M e5 and 5

T) -Cj-Hj. metal complex fragments w a s presented. This

approach requires that the syntheses proceed via addition of 5

a pyrazolyl group to t h e precursors [M(r| -C5R,-)C12 ] 2 (M = Rh, R = M e (13), R = H (14) r M = Ir, R = Me (15), R = H

(16)); therefore, it w a s necessary to launch an

investigation of the reactivity of neutral pyrazole with the binuclear chloro-bridged species.

2.1.1 Metal Pyrazole C o m p l e x e s .

Pyrazole is known to form complexes with transition

metals and other heavy metal atoms in which a bond is formed between the metal and the ligand b y donation of the nitrogen lone pair at the 2 position (see figure 1.5). The

metals is similar to that of other Lewis bases such as

amines and phosphines. Trofimenko has reviewed4 0 this area of chemistry, focussing on the formation of simple metal salt adducts and organometallic complexes of pyrazole for the period up to 1984. Complexes can form with up to six pyrazole ligands coordinated to the metal center, as shown

in the structure of the cation of [Ni(pzH) g ] (NC>3 ) (17) 51

w h i c h has been determined crysta l l o g r a p h i c a l l y . The

number of pyrazole molecules that are bonded to the metal is dependent on the donor properties of the counterion; i.e. a w e a k ligand (e.g. nitrate) as a counterion will lead to

ligation of a larger number of pyrazole groups. Octahedral and tetrahedral complexes w i t h from one to four pyrazole ligands have been characterized in which there is a single metal to nitrogen bond linking each pyrazole to the complex. Inclusion of four pyrazole ligands in the coordination

sphere is common with octahedral dihalogeno derivatives, giving, for example, the stoichiometry M ( p z H )4X2 (M = Ni,

52

Co, Fe) while mono- and di-pyrazoles are more abundant for 40 53

organometallic complexes. ' Substituted pyrazoles also form Lewis adducts w i t h transition metals, but steric

interactions of the substituent groups with the remainder of the ligand sphere effect the products obtained. For

example, the number of pyrazole ligands coordinated may be restricted by substitution at the 3 and 5 p o s i t i o n s.40

Also, 3-methylpyrazole may form complexes in which the 5 substituted tautomeT is the preferred isomer. The complex Mn(5-MepzH)4C 1 2 , which has been structurally

54

characterized, adopts this configuration. However, the complexation of the 5 substituted tautomer is not always regiospecific; the complex [Ru{p-(1Pr)(Me)CgH4}

(acac)(3-55 1

M e p z H ) ]BF4 , 18 (acac = acety l a c e t o n a t e ) , is observed by H NMR spectroscopy to exist as both isomers (see figure 2.1), with a selectivity to the 5 substituted tautomer resulting

in a 75-25% mixture.

Apart from the first row transition metal complexes mentioned above, there are also examples of heavier metals

forming complexes with pyrazole. Compounds of 40 55

ruthenium ' m several oxidation states have been

Figure 2.1: Two isomers of compound 18.

(C^-j R u - 0

— R u- 0

N — H

Me

investigated, while examples of pyrazole complexes of iridium, palladium, platinum and rhodium have also been

- 42,56,57,58 prepared. ' ' '

2.1.2 Metal Pyrazolyl Complexes.

The pyrazolyl anion (pz) is commonly observed as a bridging ligand in multinuclear species (see chapter 3); however, there are also examples of complexes in which the pyrazolyl ligands bond in a terminal position. The complex

5

[Rh(,n - C5M e5)pz(|x-pz) ] 2 (12) has been structurally 42

characterized and shown to have two bridging pyrazolyl groups and two terminal pyrazolyl ligands in each dimeric unit. Also, the complex Ir(PPh3)2C 0 (3,5-Me2pz) (19) has a

57

monodentate pyrazolyl group, and there are two terminal 59 .

pyrazolyl groups m the platinum complex P t ( P E t3)2 (pz) 2

57 59 (20^ . Both complexes 19 and 2jD have been used to form ' dinuclear bridged complexes by reacting the mononuclear species with an appropriate metal precursor to produce unsymmetrical dimers (scheme 2.1). This approach, which uses the m e t a l-centered poly-pyrazolyl complex as a multi- dentate ligand, presents the potential for great synthetic utility towards m ixed metal and unsymmetrical complexes. This m ethod of complex formation has been used with poly-pyrazolyl b o r a t e s4 0 and poly-pyrazolyl methane l i g a n d s.60

Scheme 2.1: Formation of unsymmetrical dimers from compound 2 0. ^ N: I / Et3P— Pt— N

Cr(CO)6

2.1.3 Fluxional Pyrazole Complexes.

Free pyrazole in a neutral solvent tautomerizes 37

extremely rapidly, and it has been estimated that the process has an activation energy of only 15 kJ m o l - 1 ; however, research has been unable to determine any

concentration dependence on the rate so that the mechanism by which this site exchange proceeds is still unknown. The tautomerization in pyrazole can be s l o w e d3 6 '3 7 '4 5 by

addition of an aprotic base (dmso or HMPT) which gives A g ^ «

-1

also determined A g ^ « 60 kJ mol 1 . It has been suggested45

that an intermolecular mechanism for the tautomerization involving oligomers or particularly trimers of H-bonded pyrazoles best accounts for these slower rates; i.e. the bonding of the base to the nitrogen proton effectively

blocks the approach of a second pyrazole, and similarly, the solid state does not allow the best orientation of pyrazole molecules for the proton m o v e m e n t that results in a local

site exchange.

As was introduced in Chapter 1, it is common for

pyrazole complexes to undergo a related process in which a dynamic stereochemical site exchange of the mei-al to

nitrogen bond between the nitrogen atoms at the 1 and 2

positions takes place (see figure 1.7). This process often proceeds at a rate that is observable by N M R and has been studied extehsively by that technique. The mechanism of exchange has been difficult to determine, but kinetic and thermodynamic data have enabled some speculation in this area. The species [PdCl(PEt3)2(4-Br-3,5 - M e2p z H ) ] (C104 ) (21) was shown4 4 '5 9 to have an activation energy for the pyrazole

_n

site exchange of only 15 kJ mol and the exchange was inferred to proceed by deprotonation followed by an

intramolecular mechanism as any mechanism involving bond breaking should require a higher activation energy; i.e., a non-dissociative process that included a 1 , 2 shift between

the two nitrogen atoms was suggested in this case. However, 46

a study of ruthenium porphyrin complexes showed a pyrazole

. • “ 1

site exchange with an activation energy of over 80 kJ mol which was attributed to a dissociative intermolecular

exchange. Comparison of the two processes is not directly possible because complex 2JL is only four coordinate while the ruthenium porphyrin complexes are six coordinate. Octahedral complexes are more likely to react via a

dissociative mechanism than are square complexes because a d octahedron is both coordinatively (six ligands) and electronically (18 electron) saturated and will oppose expansion of the coordination sphere.

The metal nitrogen site exchange is also observed in complexes of the deprotonated pyrazolyl anion. Complexes of

47 the formula M e3E(pz) (E = Si, Ge, Sn) were determined to have a

AG*

2.2 RESULTS AND DISCUSSION

Cleavage of halide bridged cyclopentadienyl rhodium(III) and iridium(III) complexes has been shown to occur with many classes of Lewis bases to form m o nonuclear species; for

example amines or phosphines react3 1 in this manner. The cleavage reaction proceeds according to equation 2.1.

[M(Ti5- C5M e5)Cl2 ]2 + 2L --- > 2 M(T)5-C5M e5)Cl2L 2.1

2.2.1 Synthesis of Mononuclear Pyrazole Complexes.

The w o r k reported in this section includes the synthesis of mononuclear complexes of rhodium and iridium by

complexation with pyrazole and derivatives of pyrazole. It has been found that reaction of pyrazole with the starting complexes [M(ri5- C5M e5)Cl2 ]2 (13, M = Rh; 15, M = Ir) in the absence of base results in the formation of one of two types of complexes depending on the reaction stoichiometry and on any substituents on the pyrazole ligand (see scheme 2.2). Elemental analyses of each compound prepared in this w o r k are reported in table 2.1. The spectroscopic and structural data are d i s c ussed in t h e following sections.

The addition of one equivalent of pyrazole (pzH) to a solution of 13 in dichloromethane resulted in an immediate colour change to give a pale orange solution. The product was isolated from this solution by precipitation caused by

. . 1

addition of diethylether and identified by H NMR, IR, elemental analysis, and X-ray crystallography to be the

Scheme 2.2: Synthesis o f Mononuclear Pyrazole Complexes.

1 pzH

2 pzH

xtall"

M = Rh

El,N

AgBF4

/ \

El,N

M = Rh

M = Rh

2 A ,

TABLE 2.1: Elemental analysesa for pyrazole mononuclear c o m p lexes. No FW C H N Cl 22 377.1 41.05(41.40) 5.05(5.08) 7.17 (7.43) 19.50(18.80) 23 445.2 42.97(43.17) 5.11(5.21) 12.13(12.58) 15.88(15.93) 24 496.5 39.20(38.70) 4.73(4.67) 11.20(11.28) 8.23(7.14) 25 466.4 33.35(33.48) 4.03(4.11) 5.89(6.01) 16.76(15.20) 26 562.5 38.41(38.43) 4.88(4.83) 9.97(10.48) -27 585.8 32.70(32.80) 3.80(3.96) 9.52(9.56) 6.00(6.05) 28 465.2 44.00(44.47) 5.62(5.72) 6.80(5.67) 17.84 (17.50) 29 554.5 35.06(36.43) 4.51(4.69) 5.43(5.67) 15.03(14.34) 30 534.5 35.85(35.95) 4.32(4.34) 10.48(10.48) 13.35(13.30) a

5

mononuclear mono-pyrazole complex [Rh(r) - C5M e 5 ) C l2 (pzH) ] (22). The addition of an excess of pyrazole to a

dichloromethane solution of 13 also caused an immediate colour change. With a pyrazole concentration of at least 4 times that of the rhodium atoms, precipitation from this solution by addition of diethylether yielded an orange solid

1

that was proved by H NMR, IR, elemental analysis and X-ray diffraction to be the cationic dipyrazole complex

5

[Rh (t) - C5M e 5 ) Cl (pzH) 2]C1 (23.). Attempted crystallization of complex 23. w i t hout an excess of pyrazole resulted in

precipitation of the mono-pyrazole complex 2 2 in appreciable yield. Treatment of the di-pyrazole cation 23. with silver tetrafluoroborate in acetone caused immediate precipitation of a white solid (AgCl). Isolation of the reaction product was accomplished by filtration followed by precipitation with diethylether. This procedure gave the compound

5

[Rh(n - C g M e 5 )Cl(pzH)2 ]BF4 (24), representing a quantitative exchange of the chloride ion for the tetrafluoroborate ion.

Because the pyrazole ligands in the monon u c l e a r rhodium cation 23. are labile, recrystallization m u s t be performed in a solution of excess pyrazole in order to optimize the

yield. This lability of the rhodium species 23. is reflected in its ability to dimerize upon addition of base with

subsequent loss of one pyrazole ligand. W h e n any one of the complexes 22, 23., or 24. was reacted with triethylamine in a

suitable solvent, the major rhodium containing product was t he symmetrical binuclear rhodium dimer [Rh(ri5-C5M e 5 ) Cl (p.— p z ) ] 2< These reactions will be described in detail in Chapter 3. The dipyrazole cation may also be prepared by reaction of the monopyrazole complex 22 with an excess of pyrazole.

A nearly identical set of reactions was carried out using the analogous iridium precursor 15 which produced the monopy r a z o l e (compound 25) and dipyrazole (compounds 26, and 2 7 ) analogues w i t h the important exception that the lability of the pyrazole ligands in the iridium complexes 26 or 27 is greatly reduced; i.e. an excess of pyrazole is not required for recrystallizations of the dipyrazole species, and the h omonuclear iridium dimer is not formed upon base addition to either complex 26 or 27,. This reduced reac tivity has allowed access to a route to the unsymmetrical dimers reported in Chapter 5.

Reaction of either 13 or 15 in d i d1 romethane with an excess of the more sterically hindered <,5-dimethylpyrazole

(dmpzH) resulted exclusively in t h e formation of the

mono(dmpzH) complexes M(ri5- C5Me5 ) C l2 (dmpzH) (28., M = Rh; 29., M = Ir) (scheme 2.3) (no bis(dmpzH) was detected). Further, no reaction was observed w i t h either 13 or 15 when 3,5- di(tertbutyl)pyrazole was employed as the reagent, so that t h e relative steric requirements of these three ligands is

Scheme 2.3: Synthesis of Substituted Pyrazole Mononuclear Complexes.

11,11

.

.

^ ■ ^ ^ 3 , 5 - B u 2p z H

N.R.

readily apparent through this series (H vs. Me vs. t B u ) . This lowered reactivity to complex formation of substituted pyrazoles has been observed in other systems, e.g.

[M(cod) (jjl-CI) ] 2 (M = Rh, Ir (30.); cod = 1,5-cyclooctadiene) reacts readily with many pyrazole derivatives except those

24 incorporating t-butyl substituents.

Substitution at only the 3 position on the pyrazole ring can lead to selectivity towards a single tautomeric form

(usually the 5 tautoxner) of the pyrazole adduct. Reaction of an excess of 3-methylpyrazole with compound 15 in

dichloromethane resulted in formation of the complex with two of the substituted pyrazole ligands, [Ir(r| -C5M e5 )Cl(5-M e p z H )2]BF4 (31); this species was formulated as the 5-tautomer based on comparisons with the data and assignments for ruthenium pyrazole complexes outlined by Elguero et.

55

al. (see discussion of NMR spectra later in this chapter) with no spectroscopic or other evidence indicating the existence of the 3-tautomeric form of complex 31. No attempt was made to prepare or isolate a m o n o (3-MepzH) complex.

61

Oro et. al. have independently reported the

preparation of 22., 24., and 25., but, the synthetic routes are 5

different; reaction of either Rh(ri -C,-Me5 ) Cl (acac) (acac = acetylacetonate) w i t h pzH and H B F ^ , or reaction of

5

[{Rh(ri -C5M e 5 ) ((JL-pz) ) 2 (|X“ C1) ] B F4 w i t h HC1 results m

formation of 24. The mono(pyrazole) adducts 22 and 25 can also be prepared by reaction of HC1 with the dimeric species

[(M(T|5- C5M e 5 ) (M.-C1) )2 ((JL-pz) ]BF4 (32 M = Rh, 33 Ir) . The 200 MHz H N M R of these complexes were all reported to show

unresolved broad singlets attributed to the pyrazole proton resonances; however, the spectra described here do show resolved peak multiplicity whicn will be discussed later in this chapter.

N — N

3 2, 3 3 .

2.2.2 Crystallographic Structure Determination.

The mononuclear pyrazole complexes crystallize as large orange (rhodium) or yellow (iridium) blocks from chloroform by carefully layering diethylether on top of the chloroform solution such that diffusion of the ether will slowly lower the solubility of the complex. The X-ray crystal structures of both of the rhodium mono- and di-pyrazole complexes (22

and 2A) have been determined, and the molecular or cation geometries are shown in figures 2.2 and 2.3 respectively. Crystals of the mono-pyrazole species 22. are monoclinic, space group P 2 1/n and gave a solution refinement to R = 0.0358. Selected bond lengths and angles are given in