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Multiple Bonds Between Metal Atoms / 09-Ruthenium Compounds

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9

Ruthenium Compounds

Panagiotis Angaridis,

Texas A&M University

9.1Introduction

In the previous edition of this book the chapter on Ru2 compounds was rather short, covering in only a few pages the small number of compounds known at that time. However, since then a variety of new Ru2 compounds have been synthesized, structurally characterized, and studied using theoretical and spectroscopic methods providing a better understanding of the Ru–Ru bond and its reactivity.

The Ru2 complexes have been found to adopt either the paddlewheel (9.1a), or the face-shar- ing bioctahedral structures (9.1b). The lack of the tetragonal prismatic structure (9.1c) is surprising, since it is a common structural motif for dimetal compounds of Mo, W, Tc, Re, and Os. The majority of Ru2 complexes adopt the paddlewheel structure, in which four monoanionic, three-atom donor ligands are bridging two multiply-bonded Ru atoms. Typical examples of bridging ligands with O,O'-, N,O- and N,N'-donor atoms are shown in Fig. 9.1.

9.1

Complexes of the paddlewheel framework have been isolated in three different formal oxidation states: Ru24+, Ru25+, and Ru26+. Those with the Ru25+ core are the most common, while those having the Ru24+ and Ru26+ cores represent the most recent additions to the Ru2 family. Recent electrochemical experiments have given support for the existence of Ru27+ complexes; however, attempts to synthesize such complexes have been unsuccessful. This chapter describes the syntheses, properties and electronic structures of paddlewheel Ru2 compounds classified according to the oxidation state of the dimetal unit and the ligand type.

377

378Multiple Bonds Between Metal Atoms Chapter 9

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R2

 

 

 

 

 

 

R

 

 

 

R

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

O

 

 

O

 

 

O

 

NH

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Y N

 

O

 

 

R2 N

 

 

 

NR1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

a

 

 

 

b

 

 

c

 

 

 

d

 

 

 

Y

 

 

 

 

 

H

 

 

 

 

Y

Y

 

 

 

 

 

 

 

 

 

 

 

 

 

Y

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

N

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

N

 

 

N

 

 

 

 

 

 

 

 

 

 

N

 

 

N

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

e

 

 

 

 

 

 

 

 

 

 

 

f

 

 

 

Y1

N N Y2

R

 

 

R

 

 

N

 

 

N

 

 

 

 

 

g

 

h

Fig. 9.1. Generic examples of bridging ligands with O,O'-, N,O- and N,N'-do- nor atoms used in Ru2 complexes of the paddlewheel framework: (a) carboxylate,

(b) amidate, (c) oxopyridinate, (d) aminopyridinate, (e) formamidinate, (f) triazenate,

(g) naphthyridine and (h) benzamidinate.

9.2Ru25+ Compounds

Complexes of the Ru25+ core are the most common and stable Ru2 complexes. They have been often designated as mixed-valent Ru2+Ru3+ or Ru2(II,III); however, since the two Ru atoms are equivalent, they should be referred to as having Ru25+ cores. The stability of this oxidation state can be attributed to the half-filled highest occupied molecular orbitals, the nearly degenerate /* and β* molecular orbitals, which give rise to an electronic configuration with three unpaired electrons, (/*β*)3, as discussed later. Structurally characterized compounds of this type together with their corresponding Ru–Ru bond lengths are given in Table 9.1.

Table 9.1. Structurally characterized paddlewheel Ru25+ compounds

Compound

r(Ru–Ru) (Å)

ref.

O,O'-donor bridging ligands

 

 

carboxylate ligands

 

 

Ru2(O2CPrn)4Cl

2.281(4)

6

[Ru2(O2CMe)4(H2O)2]BF4

2.248(1)

7

Cs[Ru2(O2CMe)4Cl2]

2.286(2)

7

Ru2(O2CMe)4Cl·2H2O

2.267(1)

7

K[Ru2(O2CH)4Cl2]

2.290(1)

7

Ru2(O2CEt)4Cl

2.292(7)

7

Ru2(O2CC(Me)=CHEt)4Cl

2.281(1)

8

Ru2(O2CMe)4Cl

2.281(3)

9

Ru2(O2CC6H4-p-OMe)4Cl·0.25H2O

2.286[2]

10

Ru2(O2CPh)4Cl

2.290(1)

11

Ru2(O2C(Me)C=CH2)4Cl

2.289[2]

12

Ru2(O2CH)4Br

2.290[2]

13

Ruthenium Compounds 379

Angaridis

Compound

r(Ru–Ru) (Å)

ref.

Ru2(O2CCF3)4(O2CCF3)

2.278(1)

14

Ru2(O2CEt)4(O2CEt)

2.273(1)

14

[Ru2(O2CMe)4(O2CMe)2]H·0.7H2O

2.265(1)

14

[Ru2(O2CPh)4(O2CPh)]·PhCO2H

2.278[2]

15

[Ru2(O2CMe)4(PhPO3H)2]H·H2O

2.267(2)

16

[Ru2(O2CMe)4(HPhPO2)2]H

2.272(1)

16

[Ru2(O2CEt)4(phz)]BF4

2.276(1)

17

[Ru2(O2CBut)4(nitph)]BF4·2benzene

2.266(1)

18

Ru2(O2CC4H4N)4Cl(THF)·THF·H2O

2.268(1)

19

Ru2(O2CBut)4Cl(H2O)

2.274(2)

20

Ru2(O2CPri)4Cl(THF)

2.272(2)

20

Ru2(O2CPri)4Cl(OPPh3)

2.279(1)

21

[Ru2(O2CCH2CH2OPh)4I2][Ru2(O2CCH2CH2OPh)4(H2O)2]·H2O

2.310(2)

22

 

2.265(2)

 

 

Ru2(O2CCH2CH2OPh)4Cl(H2O)·2MeOH

2.279(3)

22

Ru2(O2CCMePh2)4Cl(H2O)

2.284(1)

22

[Ru2(O2CMe)4(H2O)2]PF6·3H2O

2.265(1)

23

[Ru2(O2CMe)4(DMF)2]PF6

2.262(3)

23

[Ru2(O2CMe)4(DMF)2]PF6·DMF

2.265(2)

23

[Ru2(O2CMe)4(DMSO)2]PF6

2.271(2)

23

[Ru2(O2CMe)4Cl(OPPh3)2]PF6·CH2ClCH2Cl

2.267(1)

24

[Ru2(O2CCH2OEt)4Cl][Ru2(O2CCH2OEt)4Cl2]-

2.277(1)

25

[Ru2(O2CCH2OEt)4(H2O)2]·3H2O

2.294(2)

 

 

 

 

 

2.261(1)

 

[Ru2(O2CMe)4(PCy3)2]PF6·CH2Cl2

2.427(1)

26

[RuCl(MeCN)4(PPh3)]2[Ru2(O2CC6H4-p-OMe)4Cl2]·0.5Et2O·0.75H2O

2.299(1)

30

[Ru2(O2CC6H4-p-But)4(THF)2]OH

2.260(1)

35

[Ru2(O2CC4H3S)4(OPPh3)2]BF4·2H2O

2.275(1)

36

[Ru2(O2CMe)4(urea)2][Ru2(O2CMe)4(PrnOH)2](PF6)2

2.264(1)

38

 

2.259(1)

 

 

 

 

 

[Ru2(O2CMe)4(tmu)2]PF6

2.275(1)

38

[Ru2(O2CMe)4(tmtu)2]PF6·2CH2ClCH2Cl

2.301(2)

38

[Ru2(O2CMe)4(tht)2]PF6

2.285(4)

39

[Ru2(O2CMe)4(quinoline)2]PF6·quinoline

2.282(2)

40

[Ru2(O2CMe)4(quin)2]PF6

2.292(1)

41

[Ru2(O2CMe)4(4-Mepy)2]PF6

2.279[2]

41

[Ru2(O2CMe)4(py)2]PF6·0.5H2O

2.281[10]

41

[Ru2(O2CMe)4(4-CNpy)2]PF6·CH2ClCH2Cl

2.274(1)

41

[Ru2(O2CMe)4(4-Phpy)2]PF6·H2O

2.279[7]

41

[Ru2(O2CMe)4](N(CN)2)·MeCN

2.279(1)

42

[Ru2(O2CMe)4](C(CN)3)

2.276(1)

42

[Ru2(O2CBut)4]3(H2O)[Fe(CN)6]·4H2O

2.273(5)

45

 

2.292(3)

 

 

 

 

 

{[Ru2(O2CBut)4(H2O)](µ-TCNQ)[Ru2(O2CBut)4(H2O)]}(BF4)2

2.263(1)

46

380 Multiple Bonds Between Metal Atoms

Chapter 9

Compound

r(Ru–Ru) (Å)

ref.

[Ru2(O2CPh)4(EtOH)2][Ru2(O2CPh)4(HSO4)2]

2.265(2)

49

 

2.272(2)

 

 

[Ru2(O2CFc)4(PrnOH)2]PF6·PrnOH

2.260(4)

50

[Ru2(Fcpe)4(PrnOH)2]PF6·3PrnOH

2.262(2)

51

[Ru2(O2CRc)4(PrnOH)2]PF6·PrnOH

2.260(10)

51

Ru2(O2CMe)4Cl

2.287(2)

62

Ru2(O2CCMePh2)4Cl

2.289(1)

67

Ru2(O2CBun)4Cl

2.290(1)

68

Ru2(O2CCH=CHCH=CHMe)4Cl

2.286(1)

72

Ru2(O2CCH2OMe)4Cl

2.290(1)

72

[Ru2(O2CBut)4(tempo)2][Ru2(O2CBut)4(H2O)2](BF4)2

2.273(1)

73

 

2.260(1)

 

 

[Ru2(O2CBut)4(nitme)2]BF4·2CH2Cl2

2.272(1)

74

[Ru2(O2CBut)4(nitme)2][Ru2(O2CBut)4(H2O)2](BF4)2·2CH2Cl2

2.275(1)

74

 

2.262(1)

 

 

 

 

 

[Ru2(O2CBut)4(nitet)2][Ru2(O2CBut)4(H2O)2](BF4)2·2CH2Cl2

2.273(2)

74

 

2.256(1)

 

 

[Ru2(O2CBut)4(nitme)2][Ru2(O2CBut)4(H2O)2](BF4)2·2CH2Cl2

2.275(1)

75

 

2.262(1)

 

 

[Ru2(O2CBut)4(nitph)]BF4·benzene

2.266(1)

76

[Ru2(O2CBut)4(nitph)(H2O)]BF4·2CH2Cl2

2.265(1)

77

[Ru2(O2CBut)4(p-nitpy)]BF4·1.5CH2Cl2

2.272(1)

78

[Ru2(O2CBut)4(m-nitpy)2]BF4

2.276(1)

79

[Ru2(O2CBut)4(p-nitpy)2]BPh4·0.5CH2Cl2

2.282(1)

79

{Ru2(O2CMe)4[NCRu(PPh3)2(δ5-C5H5)]SbF6·CHCl3

2.296(1)

119

Ru2(O2CC10H15)3(O2CO)(MeOH)2·2MeOH

2.254(1)

147

[Ru2(O2CC6H4-p-Me)4(THF)2]BF4

2.262(2)

152

O,O'-donor bridging ligands other than carboxylates

 

Na3[Ru2(O2CO)4]·6H2O

2.254[7]

80

Na3[Ru2(O2CO)4]·6H2O

2.251[2]

81

K3[Ru2(O2CO)4]·4H2O

2.251(1)

81

K4[Ru2(HPO4)3(PO4)(H2O)2]

2.305(1)

82

K2H[Ru2(SO4)4(H2O)2]·4H2O

2.303(1)

83

(NH4)3[Ru2(hedp)2]·2H2O

2.347(1)

85

N,O-donor bridging ligands

 

 

amidate ligands

 

 

trans-(2,2)-Ru2(ONHCPh)4Cl·MeOH

2.293(2)

87

trans-(2,2)-Ru2(ONHCC6H4-p-Cl)4Cl·MeOH

2.296[1]

88

[trans-(2,2)-Ru2(ONHCC6H4-p-But)4(OPPh3)2]BF4

2.281[3]

89

[trans-(2,2)-Ru2(ONHCC4H3S)4(THF)2]SbF6·0.5cyclohexane

2.286(2)

90

oxopyridinate ligands

 

 

(4,0)-Ru2(hp)4Cl(Hhp)

2.286(1)

96

Ru2(O2CMe)(chp)3Cl·CH2Cl2

2.282(4)

97

[(4,0)-Ru2(chp)4]2(BF4)2·4CH2Cl2

2.254(1)

98

(4,0)-Ru2(fhp)4Cl

2.284(1)

99

Ruthenium Compounds 381

Angaridis

Compound

r(Ru–Ru) (Å)

ref.

(4,0)-Ru2(chp)4Cl·CH2Cl2

2.281(1)

100

(4,0)-Ru2(chp)4(OMe)

2.256(1)

101

trans-Ru2(O2CMe)2(mhp)2Cl·0.5CH2Cl2

2.278(2)

102

[(4,0)-Ru2(chp)4(THF)]BF4·2THF

2.266(1)

103

[(4,0)-Ru2(chp)4(py)]BF4·hexane·py

2.270(1)

104

{[(4,0)-Ru2(chp)4](µ-pyz)[(4,0)-Ru2(chp)4]}(BF4)2·4CH2Cl2

2.267(1)

104

N,N'-donor bridging ligands

 

 

aminopyridinate ligands

 

 

(4,0)-Ru2(ap)4Cl

2.275(3)

96

trans-Ru2(O2CMe)2(ap)2Cl(Hap)·CH2Cl2

2.308(1)

97

Ru2(O2CMe)3(admp)Cl·3CH2Cl2

2.277[2]

108

trans-Ru2(O2CMe)2(admp)2Cl·2.5CH2Cl2

2.274(1)

108

Ru2(O2CMe)(admp)3Cl·Hadmp·benzene

2.283(1)

108

(4,0)-Ru2(2,4,6-F3ap)4Cl

2.296*

109

(3,1)-Ru2(2,4,6-F3ap)4Cl

2.284*

109

(4,0)-Ru2(2,5-F2ap)4Cl

2.284*

109

(3,1)-Ru2(2,6-F2ap)4Cl

2.286*

109

(4,0)-Ru2(2-Meap)4Cl

2.279*

109

[(4,0)-Ru2(ap)4(H2O)]SbF6·Et2O

2.288(1)

111

(4,0)-Ru2(ap)4(C>CPh)·2CH2Cl2

2.319(2)

112

(4,0)-Ru2(ap)4(C>CSiMe3)

2.316(1)

113

(4,0)-Ru2(ap)4(C>CCH2OMe)·hexane

2.323(1)

113

(4,0)-Ru2(ap)4(C>CC>CSiMe3)·MeC(O)OEt

2.330(1)

114

[(4,0)-Ru2(ap)4](µ-C>CC>C)[(4,0)-Ru2(ap)4]·8H2O

2.332[3]

116

[(4,0)-Ru2(ap)4](µ-C>CC>CC>CC>C)[(4,0)-Ru2(ap)4]·THF·MeOH

2.329(1)

117

(4,0)-Ru2(ap)4(CN)·2THF ·H2O

2.336(2)

118

(4,0)-Ru2(2-Meap)4(CN)·2CH2Cl2

2.304(5)

118

{(4,0)-Ru2(ap)4[NCFe(dppe)(δ5-C5H5)]}SbF6·CH2Cl2

2.280(2)

119

{(4,0)-Ru2(ap)4[NCRu(PPh3)2(δ5-C5H5)]}SbF6·CH2ClCH2Cl

2.287(2)

119

{(3,1)-Ru2(2-Fap)4[NCFe(dppe)(δ5-C5H5)]}SbF6·2CH2ClCH2Cl

2.284(2)

119

(3,1)-Ru2(2-Fap)4Cl·CH2Cl2

2.286(1)

120

(3,1)-Ru2(2-Fap)4Cl(NO)

2.420(1)

120

Ru2(O2CMe)(HNC5H3NMe)3Cl

2.287(2)

123

formamidinate ligands

 

 

Ru2(DTolF)4Cl·hexane

2.370(2)

125

trans-Ru2(O2CMe)2(DAnioF)(AnioPho-OF)

2.311(1)

127

Ru2(O2CMe)(DAnioF)2(AnioPho-OF)

2.312(1)

127

cis-Ru2(DAniF)2(O2CMe)2Cl·H2O

2.319(1)

128

{[cis-Ru2(DAniF)2Cl(H2O)](µ-O2CCO2)}4·MeCN·2hexane·12H2O

2.332[2]

128

{[cis-Ru2(DAniF)2Cl(4-Butpy)](µ-O2CC6H4CO2)}4·17CH2ClCH2Cl·hexane

2.332[2]

128

{[Ru2(DAniF)3Cl]2(µ-O2CC6H4CO2)·4.5benzene

2.329[2]

129

Ru2(O2CMe)(DPhF)3Cl·HDPhF

2.320(1)

130

Ru2(O2CMe)(DPhF)3Cl·2THF

2.325(2)

130

Ru2(O2CC6H4-p-OC10H21)(DPhF)3Cl

2.325(1)

130

Ru2(O2CMe)3(DXyl2,6F)Cl(THF)·0.5THF

2.305(1)

131

382 Multiple Bonds Between Metal Atoms

Chapter 9

Compound

r(Ru–Ru) (Å)

ref.

Ru2(O2CMe)3(DXyl2,6F)Cl(HDXyl2,6F)·toluene

2.333(1)

131

trans-Ru2(O2CMe)2(DXyl2,6F)2Cl(THF)

2.326(1)

131

trans-Ru2(O2CMe)2(DXyl2,6F)2Cl·2toluene

2.316(1)

131

Ru2(DAniF)4Cl·0.5CH2Cl2

2.396(1)

131

Ru2(DPhF)4Cl·pentane

2.339(1)

132

Ru2(DPhF)4(C>CPh)

2.400[2]

132

Ru2(DPhm-ClF)4(C>CPh)·hexane

2.387(1)

133

Ru2(DPh3,5-diClF)4(C>CPh)·4CH2Cl2

2.429(1)

133

Ru2(DAnimF)4(C>CC>CSiMe3)

2.506(1)

135

naphthyridine ligands

 

 

Ru2(O2CMe)3(bcnp)·2H2O

2.265[5]

136

trans-Ru2(O2CMe)2(mephonp)2Cl·3CHCl3

2.285(1)

137

other N,N'-donor bridging ligands

 

 

Ru2(O2CMe)3(admpym)Cl(MeOH)

2.290(1)

138

[Na(THF)2][Ru2(O2CMe)2(5-Clsalpy)2]·THF

2.295(1)

139

[K(18-crown-6)][Ru2(O2CMe)2(salpy)2]·toluene

2.300(1)

140

[Na(18-crown-6)(OC4H8)(H2O)][Ru2(O2CMe)2(5-Mesalpy)2]·0.5THF

2.297[2]

140

[Na(18-crown-6)(THF)(H2O)][Ru2(O2CMe)2(5-Clsalpy)2]·0.5THF

2.288[2]

140

[Na(18-crown-6)(THF)(H2O)][Ru2(O2CMe)2(5-Brsalpy)2]·0.5THF

2.291[2]

140

[K(18-crown-6)][Ru2(O2CMe)2(5-NO2salpy)2]·2toluene

2.283(1)

140

[Li2(THF)4Cl][Ru2(5-Clsalpy)3]·THF

2.313(1)

141

Ru2(dmat)4Cl·CH2Cl2

2.432(1)

142

[Ru2(DTolTA)4(MeCN)]BF4

2.373(1)

169

* no esds reported.

9.2.1 Ru25+ compounds with O,O'-donor bridging ligands

Carboxylate ligands

Paddlewheel Ru25+ compounds with carboxylate bridges were the first to be discovered and represent the majority of the Ru25+ complexes. The first syntheses were reported by Wilkinson and Stephenson in 1966.1 By refluxing RuCl3·xH2O in a mixture of a carboxylic acid and its anhydride, compounds of the general type Ru2(O2CR)4Cl (R = Me, Et, Prn) were synthesized. The synthesis of Ru25+ tetraformate was achieved the following year using a similar method.2 Two other synthetic procedures appeared later: one involved reaction of ‘ruthenium[(III),(IV)] chloride’ with acetic acid in a sealed stainless steel container to form the product in low yield,3 while the other was a modification of the original synthesis in which LiCl and O2 were added to the reaction mixture improving the yield to >80%.4 Other Ru25+ tetracarboxylates have been obtained via metathesis reactions by refluxing a solution of a Ru2(O2CR)4Cl complex in the presence of another carboxylic acid R'CO2H or its salt.5

The first insight into the structures of Ru25+ tetracarboxylates was gained in 1969, when the crystal structure of Ru2(O2CPrn)4Cl (Fig. 9.2) was published by Cotton and coworkers.6 This provided the first evidence of the existence of a strong Ru–Ru bond, with a bond order of 2.5 and a short distance of 2.281(4) Å. The compound exhibits a polymeric structure in which Ru25+ units bridged by four butyrate ligands, [Ru2(O2CPrn)4]+, are linked by Cl- ions into an infinite zig-zag chain.

Ruthenium Compounds 383

Angaridis

Fig. 9.2. Part of the polymeric zig-zag chain structure of Ru2(O2CPrn)4Cl.

Subsequent structural characterization of other Ru2(O2CR)4X compounds (X = halide) showed that in the solid state they form similar polymeric chain structures which can be either linear (Fig. 9.3a) as in Ru2(O2CEt)4Cl7 and Ru2(O2CC(Me)=CHEt)4Cl,8 or bent (Fig. 9.3b) as in Ru2(O2CMe)4Cl,9 Ru2(O2CC6H4-p-OMe)4Cl,10 Ru2(O2CPh)4Cl,11 Ru2(O2C(Me)C=CH2)4Cl12 and Ru2(O2CH)4Br.13 The last of the above series of polymers exhibits an extremely bent structure with Ru–Br–Ru ~110º. The type of polymeric structure (linear or bent) a particular Ru25+ tetracarboxylate adopts does not depend on the nature of the substituents R of the carboxylate bridges. Similar polymeric structures are also observed in Ru2(O2CR)4X compounds when X is not a halide, but a bifunctional linker. Examples of this type of polymers are Ru2(O2CEt)4(O2CEt),14 [Ru2(O2CPh)4(O2CPh)]·(HO2CPh),15and[Ru2(O2CMe)4(HPhPO2)2]H.16 In the first two compounds the chains result from the direct bonding between the Ru25+ units and the linkers EtCO2-, and PhCO2-, respectively, while in the latter the chain is supported by H-bonding. Neutral bifunctional molecules can also be linkers between Ru25+ tetracarboxylate units, like phz in [Ru2(O2CEt)4(phz)]BF4,17 and nitph in [Ru2(O2CBut)4(nitph)]BF4.18

Some Ru25+ tetracarboxylates exhibit non-polymeric structures in the solid state. These are of the general formula Ru2(O2CR)4XL, in which the ligands X and L (X = halide, L = neutral ligand, or solvent molecule) are axially coordinated to the Ru25+ unit (Fig. 9.3c). Examples of this type of compound include Ru2(O2CC4H4N)4Cl(THF),19 Ru2(O2CCHMe2)4Cl(THF),20 and Ru2(O2CCHMe2)4Cl(OPPh3).21 The factors which determine whether a particular compound will adopt the polymeric or non-polymeric structure remain unclear. However, it has been proposed that they are related to the presence of branched chains in the substituents of the carboxylate bridges and to the type of axial ligand.20-22

Another type of non-polymeric structure includes the diadducts of the formula [Ru2(O2CR)4L2]Y (Fig. 9.3d). In these complexes the axial ligands L can either be anions while Y is a positively charged ion, as in K[Ru2(O2CH)4Cl2],7 or neutral donor molecules with Y being a negatively charged ion, as in [Ru2(O2CMe)4(DMF)2]PF623 and [Ru2(O2CMe)4(OPPh3)2]PF6.24 There are also reports of Ru25+ tetracarboxylates that exist as pairs of discrete anionic and cationic units of the type [Ru2(O2CR)4X2][Ru2(O2CR)4L2] (Fig. 9.3e), where X is an anion and L is a neutral ligand, as in [Ru2(O2CH2CH2OPh)4I2][Ru2(O2CH2CH2OPh)4(H2O)2].22 Of interest is the compound Ru2(O2CCH2OEt)4Cl in which both the discrete anionic-cationic dinuclear

units and the polymeric chain coexist.25

384 Multiple Bonds Between Metal Atoms

Chapter 9

Fig. 9.3. Structural types of Ru25+ tetracarboxylates: (a) polymeric linear chain

(X = anionic ligand), (b) polymeric zig-zag chain (X = anionic ligand), (c) non-polymeric monoadduct (X = anionic ligand, L = neutral ligand), (d) nonpolymeric diadduct

(L = neutral ligand and Y = counter-anion, or L = anionic ligand and Y = countercation). (e) anion-cation pair (X = anionic ligand, L = neutral ligand).

Ruthenium Compounds 385

Angaridis

As listed in Table 9.1, the Ru–Ru bond lengths of Ru25+ tetracarboxylates lie in the narrow range of 2.248-2.310 Å (an exception is discussed in the following paragraph). They show a very small dependence on the nature of the carboxylate bridge and the axially coordinated ligands. However, the diadducts exhibit slightly shorter Ru–Ru bond lengths than the corresponding polymeric compounds, as shown in [Ru2(O2CMe)4(H2O)2]BF4 and Ru2(O2CMe)4Cl in which the Ru–Ru bond lengths are 2.248(1) and 2.267(1) Å, respectively.7

The only compound exhibiting a Ru–Ru bond distance outside the aforementioned range is [Ru2(O2CMe)4(PCy3)2]PF6.26 The remarkably long distance of 2.427(1) Å is attributed to the strong electron donating nature of the axial PCy3 ligands, which increases the anti-bonding μ* electron density between the two metals that weakens the Ru–Ru bond. Typically, reactions of Ru2(O2CR)4Cl compounds with phosphines do not result in the formation of diadducts. This is because the phosphines prefer to coordinate to the equatorial instead of the axial positions of the dimetal core to maximize their /-back bonding. As a result they displace the equatorial ligands causing the disintegration of the paddlewheel structure and giving a number of decomposition products, such as oxo-centered trimers, oxo-bridged dimers and other mononuclear compounds, depending on the reaction conditions.27-30 However, for [Ru2(O2CMe)4(PCy3)2]+ steric factors (cone angle of PCy3 ~170º) force the PCy3 ligands to coordinate axially to the Ru25+ unit minimizing in this way their /-accepting ability.

Similarly to the reactions with phosphines, Ru25+ tetracarboxylate compounds react with diphosphines (P–P), Grignard reagents (or other Lewis bases which are also /-acceptors) resulting in the disintegration of the dimetal core and the formation of mononuclear complexes such as Ru(O2CR)2(P–P)2, RuCl2(P–P)2,31,32 and Ru(c-C6H11)4,33 respectively. In contrast, reactions with Lewis bases which are not /-acceptors result in axial monoor diadducts. The enthalpies of formation of such adducts of Ru2(O2CPrn)4Cl with various Lewis bases, such as py, DMSO, acetone and MeCN, were determined in a calorimetric study conducted by Drago et al. from which it was concluded that Ru2(O2CR)4Cl compounds are stronger Lewis acids than Rh2(O2CR)4 and Mo2(O2CR)4 compounds.34

The axial halide X in polymeric and non-polymeric Ru2(O2CR)4X compounds can be easily removed as AgX upon reaction with AgBF4, or AgPF6. This leaves both of the axial positions of the dimetal unit available for coordination by solvent molecules, L, resulting in diadducts of the general type [Ru2(O2CR)4L2]+.35,36 The axially coordinating solvent molecules can be exchanged with neutral O-, N- or S-donor ligands forming new diadducts. Examples of such ligands include DMSO,23 Ph3PO,37 urea,38 THT,39 quinoline,40 and py.41 With bifunctional ligands, such as phz, nitph, N(CN)2-, C(CN)3- and 9,10-anthraquinone, the same exchange reactions take place to form cationic, or neutral one-dimensional polymeric chains,17,18,42,43 while with polyfunctional ligands, like [Fe(CN)6]3-, [Cr(CN)6]3-, and [Co(CN)6]3-, three-dimensional coordination polymers are obtained.44,45 Unexpectedly, the reaction of [Ru2(O2CBut)4(H2O)2]BF4 with the polyfunctional ligand TCNQ results in the complex {[Ru2(O2CBut)4(H2O)](µ-TCNQ)- [Ru2(O2CBut)4(H2O)]}(BF4)2 instead of a twoor three-dimensional polymer.46

Substitution reactions of the bridging carboxylate ligands are of special interest, since they offer a synthetic route to Ru25+ paddlewheel complexes with different types of bridging ligands. As mentioned, reactions of Ru2(O2CR)4X compounds with an excess of other carboxylic acids, R'CO2H, or their salts (e.g., NaO2CR') result in new Ru2(O2CR')4X compounds. Analogous reactions with other three-atom bridging ligands (e.g., amidates, oxopyridinates, aminopyridinates, formamidinates, triazinates) can take place and under appropriate conditions some or all of the RCO2- groups can be substituted resulting in new types of complexes. Such reactions will be considered in more detail in the following sections, where the syntheses of Ru25+ compounds with bridging ligands other than carboxylates will be discussed.

386Multiple Bonds Between Metal Atoms Chapter 9

The first electrochemical study on Ru25+ tetracarboxylates was reported in 1972 for Ru2(O2CMe)4Cl and showed a single redox wave at a potential of +0.06 V vs SCE which was assigned to the reduction Ru25+ + e- Α Ru24+.3 This process was later described as quasi-revers- ible.47 A more extensive electrochemical study of Ru2(O2CPrn)4Cl showed that the potential of this one-electron reduction process varies between 0.0 and -0.4 V, depending on the electrolyte and the solvent.48 For example, while in CH2Cl2 with Bun4NClO4 as electrolyte the compound exhibits a two-step reduction, in a coordinating solvent or using Bun4NCl as electrolyte a onestep reduction is observed for which the potential is shifted cathodically. This behavior (shown in 9.2) is attributed to the association equilibria between [Ru2(O2CPrn)4]+, Cl- ions and solvent molecules. Compounds of the type Ru2(O2CR)4L, where L = an anionic ligand other than halide, and diadducts of the type [Ru2(O2CR)4L2]+, where L = a neutral ligand, exhibit similar electrochemical behavior to the Ru2(O2CR)4X compounds, where X = halide. Cyclic voltammetry measurements showed a quasi-reversible (or sometimes reversible) reduction wave at potentials between 0.0 and -0.8 V vs SCE.38-40,49-52 Ru25+ compounds with a mixed set of bridging carboxylates have also been studied.53

9.2

The chemical reduction of Ru25+ tetracarboxylates has been the subject of a series of kinetic studies. The one-electron reduction of [Ru2(O2CMe)4]+ with Ti3+ in 1.0 M LiCF3SO3/CF3SO3H shows that the reaction follows a two-term, pH-dependent rate law, suggesting that both Ti3+ and Ti(OH)2+species are effective reducing agents; however, the reduction is faster for Ti(OH)2+.54 Analogous results are obtained from the study of the reduction of [Ru2(O2CMe)4]+ with oxalato complexes of Ti3+.55 There is a similar study in which the Ti3+ ion is complexed with N-(2-hydroxyethyl)-ethylenediaminetriacetic acid.56

Kinetic studies have also been employed to monitor the substitution of the axial ligands and the equatorial carboxylate ligands. For the former type of reactions it has been shown that in [Ru2(O2CMe)4(H2O)2]+, the H2O molecules are rapidly displaced by Cl- ions to give the complexes Ru2(O2CMe)4Cl(H2O) and [Ru2(O2CMe)4Cl2]-, with equilibrium constants for the first and second substitutions being 15 and 3.7 M-1, respectively.57 For the latter, the substitution reaction of [Ru2(O2CEt)4]+ with oxalate anions was studied which gives complexes with mixed EtCO2-/oxalate ligand sets. In this case the replacement of the EtCO2- groups by the oxalate anions takes place in a stepwise fashion, followed by a slow decomposition process.58

The determination of the electronic structure of this type of compounds has been rather challenging. Magnetic susceptibility measurements for the Ru2(O2CR)4Cl compounds (R = Me, Et, Prn), which showed magnetic moments of 3.6 to 4.4 BM per Ru25+ unit,1 and the EPR spectrum of Ru2(O2CPrn)4Cl,48 which suggested a quartet ground state, were consistent with the presence of three unpaired electrons delocalized over the Ru25+ unit. However, early attempts to correlate these data with the electronic spectra of Ru25+ tetracarboxylate compounds by constructing a qualitative molecular orbital diagram based on the Re2Cl82- model were unsuccessful.3,6