“…In terms of a simple bonding model, the η 4 -hapticity of the COT ring is unaltered by electron transfer, but the tub-to-chair isomerization essentially relieves the metal of its excess electron density. This was the first finding of the redox-controlled geometric isomerization of a metal−polyolefin bond and was the origin of a number of papers on the general subject of metal−COT reductions. − 14 Two successive CV scans of an isomeric mixture of CoCp(η 4 -1,3-COT) (minor isomer A) and CoCp(η 4 -1,5-COT) (major isomer B) at ν = 100 V s -1 . The dashed line is the second scan, recorded without pause.…”
The evolution of organotransition metal electrochemistry from its origin with ferrocene to its promise in future applications is presented. Examples are given of key findings on the relationships of electron transfer to molecular structures and to the reactions of organotransition metal complexes.
“…In terms of a simple bonding model, the η 4 -hapticity of the COT ring is unaltered by electron transfer, but the tub-to-chair isomerization essentially relieves the metal of its excess electron density. This was the first finding of the redox-controlled geometric isomerization of a metal−polyolefin bond and was the origin of a number of papers on the general subject of metal−COT reductions. − 14 Two successive CV scans of an isomeric mixture of CoCp(η 4 -1,3-COT) (minor isomer A) and CoCp(η 4 -1,5-COT) (major isomer B) at ν = 100 V s -1 . The dashed line is the second scan, recorded without pause.…”
The evolution of organotransition metal electrochemistry from its origin with ferrocene to its promise in future applications is presented. Examples are given of key findings on the relationships of electron transfer to molecular structures and to the reactions of organotransition metal complexes.
“…The fact that various types of organometallic reactions are accelerated in odd-electron complexes suggests the possibility that metal−olefin isomerization processes may be profitably enhanced by oxidation or reduction of 18-electron complexes. Electron-transfer-induced isomerizations have been studied in depth for cyclooctatetraene (COT) complexes of Co , and, to a lesser extent, Ni and Pd . Cyclooctadiene (COD) complexes of the later transition metals have also received some attention, , but the metal−1,5-COD bond has generally been found to be unisomerized in the electron-transfer products.…”
The redox properties of Cp‡Rh (1,5-COD) (1) and Cp‡Rh (1,3-COD) (2) (Cp‡ = η5-C5Ph5,
COD = η4-cyclooctadiene) have been investigated by cyclic voltammetry and bulk coulometry.
Both compounds display a four-membered electron-transfer series involving complexes of
overall 2+, 1+, 0, and 1− charges. The 19-electron complexes [Cp‡Rh(COD)]- are short-lived, rapidly releasing [C5Ph5]-. The persistent 17-electron complexes [Cp‡Rh(COD)]+ have
been characterized by ESR and optical spectroscopies. Complex 1
+ displays an axially
symmetric g-tensor, demonstrating that the 17-electon system retains the approximate C
2
v
symmetry of its 18-electron precursor. The oxidized forms of 2
n
+ (n = 1 or 2) undergo slow
isomerization to those of 1
n
+; that is, isomerization of the diolefin ligand from the 1,3- to the
1,5-isomer occurs in the higher oxidation states of [Cp‡Rh(COD)]
n
+. The 1,3-COD ligand
imparts an unexpected thermodynamic stabilization of the higher Rh oxidation states.
Formation of the 17-electron and 16-electron complexes occurs at potentials ca 0.25 and
0.62 V, respectively, lower than expected. The increasing stabilization of the Rh(II) and
Rh(III) complexes is ascribed to progressive formation of an agostic interaction between the
metal atom and the hydrogen on carbon 5 of the 1,3-cyclooctadiene ring.
“…The electronic factors which govern the favoured mode of bonding have not been identified with confidence. 15 In only one system, namely [(η-C 5 R 5 )Co(C 8 H 8 )] 11a (R = H), 16 11b (R = Me), 17 has a measurable equilibrium between the two isomers been identified in solution. In the crystalline solid, 11a has the 1,2,5,6-η 4 -ligand; 18 this is also the more abundant isomer in solution.…”
Section: Discussion (A) Syntheses and Spectroscopic Propertiesmentioning
Molecular tetrarhodium cluster complexes with facial (µ 3 -) cyclooctatetraene ligands are synthesized from [Rh 4 (CO) 12 ] 2 and 1,3,5,7-cyclooctatetraene (cot) or 1,4-(SiMe 3 ) 2 C 8 H 6 , respectively. The reactions proceed in a cooperative fashion, with all four metal atoms of the cluster being involved. Reaction of 2 with cot at low temperature (boiling n-pentane) only gives the mono-cot complex [Rh 4 (CO) 8 (µ 3 -C 8 H 8 )] 4, whereas at higher temperature (boiling n-heptane) the bis-cot cluster complex [Rh 4 (CO) 6 (µ 3 -C 8 H 8 )(η 4 -C 8 H 8 )] 5 is formed exclusively. Complex 4 is also prepared from 5 and [Fe(CO) 5 ]. From 2 and 1,4-(SiMe 3 ) 2 C 8 H 6 in boiling n-pentane only the complex [Rh 4 (CO) 6 {µ 3 -C 8 H 6 (SiMe 3 ) 2 }{η 4 -C 8 H 6 (SiMe 3 ) 2 }] 6 is obtained in quantitative yield. Substitution of the apical ligands in 4 with 1,4-(SiMe 3 ) 2 C 8 H 6 , and in 5 with 1,3-cyclohexadiene or 1,5-cyclooctadiene gives the complexes [Rh 4 (CO) 6 (µ 3 -C 8 H 8 ){η 4 -C 8 H 6 (SiMe 3 ) 2 }] 7, [Rh 4 (CO) 6 (µ 3 -C 8 H 8 )(η 4 -C 6 H 8 )] 8 and [Rh 4 (CO) 6 (µ 3 -C 8 H 8 )(η 4 -C 8 H 12 )] 9, respectively. The crystal and molecular structures of 4, 5, 6, 8 and 9 were determined. The apical C 8 H 6 R 2 ligands in 5, 6 and 7 attain the 1,2,5,6-η 4 -coordination mode through non-adjacent double bonds.
DALTON
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