Infrared spectroelectrochemistry of [Rh(TM4)4M(CO)5]22+ (TM4=2,5-diisocyano-2,5-dimethylhexane; M=Re, Mn): electrontransfer-promoted metal-metal bond cleavage and formation

Michael G. Hill, John P. Bullock, Timothy Wilson, Paul Bacon, Christine A. Blaine, Kent R. Mann

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Abstract

Two tetranuclear complexes of the form Rh2(TM4)4(M(CO)5)22+ (TM4=2,5-diisocyano-2,5-dimethylhexane; M=Mn or Re) have been studied by electrochemical (CV) and IR spectroelectrochemical methods. In 0.1 M TBA+PF6-/CH3CN (TBA+= tetrabutylammonium), 1e- oxidation of Rh2(TM4)4(Re(CO)5)22+ produces a stable radical (Rh2(TM4)4(Re(CO)5)23+) that has been characterized by EPR and IR spectroscopy. Upon further oxidation, one RhRe bond is cleaved, resulting in the formation of Rh2(TM4)4(Re(CO)5)3+ and Re(CO)5(CH3CN)+. The trimetallic fragment Rh2(TM4)4(Re(CO)5)3+ is reduced by a net 2e- process to yield free Rh2(TM4)42+ and Re(CO)5-. These successive 2e-oxidation/reduction reactions of Rh2(TM4)4(Re(CO)5)22+ ultimately yield a solution containing Rh2(TM4)42+, Re(CO)5(CH3CN)+ and Re(CO)5-. For Rh2(TM4)4(Mn(CO)5)22+, oxidation occurs via a net 2e- process to form the tetranuclear tetracation, Rh2(TM4)4(Mn(CO)5)24+ Further oxidation of this species results in RhMn bond cleavage, giving Rh2(TM4)44+ and two equivalents of Mn(CO)5(CH3CN)+. The initially formed, 1e-- oxidized radical, Rh2(TM4)4(Mn(CO)5)23+, has been observed transiently via fast-scan cyclic voltammetry. These ET-induced bond cleavage reactions are paralleled in the reduction chemistry of Rh2(TM4)4(Mn(CO)5)22+ and Rh2(TM4)4(Re(CO)5)22+. Reduction of both complexes occurs by a net 2e- process, yielding free Rh2(TM4)42+ and M(CO)5-. Interestingly, upon oxidation at the coupled return for this irreversible process, the tetranuclear starting materials are regenerated in quantitative yield (i.e. no M2(CO)10 is produced). This chemistry stands in sharp contrast to the oxidations of M(CO)5- anions, which produce M2(CO)10 in the absence of Rh2(TM4)42+. We suggest that oxidation of Rh2(TM4)42+, M(CO)5- tight ion aggregates produce M(CO)5 radicals that are entirely scavenged by Rh2(TM4)42+ to form the tetranuclear species.

Original languageEnglish (US)
Pages (from-to)61-68
Number of pages8
JournalInorganica Chimica Acta
Volume226
Issue number1-2
DOIs
StatePublished - Nov 1994

Keywords

  • Electron transfer
  • Isocyanide complexes
  • Rhodium complexes
  • Spectroelectrochemistry

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