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 electrochemical reduction of Rh(η 5 -C 5 Ph 5 )(η 4 -C 8 H 8 ), 1, has been studied in THF/0.1 M [NBu 4 ]A, where A ) [PF 6 ]or [CF 3 SO 3 ] -. Cyclic, differential pulse, and square-wave voltammetry establish that 1 exists as two isomers having tub-shaped (1,5bonded) or chair-shaped (1,3-bonded) cyclooctatetraene (COT) ligands. The former is the major isomer at room temperature. One-electron reduction of 1,5-1 (E pc ≈ -2.7 V vs ferrocene) results in rapid isomerization to the chair isomer. Electrolysis produces [1,3-1] -(E 1/2 ) -2.37 V), which has ESR spectra consistent with a SOMO that is largely (COT) ligand-based. Equilibrium and rate constants are estimated for a square scheme describing the combined electron-transfer/isomerization sequence. The tub isomer is more highly favored at room temperature for Rh(η 5 -C 5 Ph 5 )(COT) than for the previously studied Co(η 5 -C 5 H 5 )(COT). Because the redox behavior of 1 closely parallels that of the cobalt analogues, the present result strengthens evidence for the decidedly different electronic structures of nominally isoelectronic Co-group versus Ni-group η 4 -COT compounds. Formally d 9 complexes of the former are classified as highly delocalized "18 + δ" complexes, whereas the latter are more traditional 19-electron systems.
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