90064-72-5Relevant academic research and scientific papers
Redox reactions of chromium tetracarbonyl and tricarbonyl complexes: Thermodynamic, kinetic, and catalytic aspects of isomerization in the fac/mer-tricarbonyltris(trimethyl phosphite)chromium(1+/0) system
Bond,Colton,Kevekordes
, p. 749 - 756 (2008/10/08)
Electrochemical studies have been undertaken on mixtures of cis- and trans-[Cr(CO)4P2]+/0 systems (where P = P(m-tol)3, P(p-tol)3, P(OMe)3, P(OEt)3, P(OPh)3). In accordance with theoretical predictions, redox potentials for both cis- and trans-[Cr(CO)4P2]+/0 are similar and oxidation involves the formation of trans-[Cr(CO)4P2]+ via either direct electron transfer or cis-[Cr(CO)4P2]+ to trans-[Cr(CO)4P2]+ isomerization after electron transfer. The trans-[Cr(CO)4P2]+ species have all been identified by infrared spectroscopy. trans-[Cr(CO)4(P(OPh)3)2]+ is a very strong oxidant and can be used to oxidize all the other [Cr(CO)4P2] complexes: trans-[Cr(CO)4(P(OPh)3)2]+ + cis/trans-[Cr(CO)4P2] → cis/trans-[Cr(CO)4(P(OPh)3)2] + trans-[Cr(CO)4P2]+. Tricarbonyltris(phosphorus ligand)chromium complexes, Cr(CO)3P3, can exist in fac or mer isomeric forms. In contrast to those of the tetracarbonyl complexes, the oxidation potentials of the fac and mer isomers of the tricarbonyls occur at considerably different potentials, enabling detailed electrochemical studies to be made. In this work it is shown that isomerization of fac-[Cr(CO)3(P(OMe)3)3] → mer-[Cr(CO)3(P(OMe)3)3] occurs either slowly by an intramolecular twist mechanism or via art alternative redox-catalyzed pathway. Variable-temperature electrochemical studies at platinum electrodes (cyclic voltammetry and differential-pulse voltammetry at a stationary electrode, rotating-disk voltammetry, and controlled-potential electrolysis) of the redox properties of both fac- and mer-[Cr(CO)3(P(OMe)3)3] provide a complete thermodynamic and kinetic description of the catalytic scheme, which involves the following reactions: fac-[Cr(CO)3(P(OMe)3)3] →oxidant fac-[Cr(CO)3(P(OMe)3)3]+ (i) where oxidant = e-, NOPF6, or trans-[Cr(CO)4(P(OPh)3)2]+ fac-[Cr(CO)3(P(OMe)3)3]+ ? mer-[Cr(CO)3(P(OMe)3)3]+ (ii) mer-[Cr(CO)3(P(OMe)3)3]+ + fac-[Cr(CO)3(P(OMe)3)3] →fast fac-[Cr(CO)3(P(OMe)3)3]+ + mer-[Cr(CO)3(P(OMe)3)3] (iii) where fac/mer-[Cr(CO)3(P(OMe)3)3]+ is the catalyst. Data at 22°C in dichloromethane: fac-[Cr(CO)3(P(OMe)3)3] ?k-1k1 mer-[Cr(CO)3(P(OMe)3)3] k1 = (1.8 ± 0.3) × 10-4 s-1 k-1 = (4.5 ± 1.3) × 10-5 s-1 K1 = 4 ± 0.5 fac-[Cr(CO)3(P(OMe)3)3]+ →k-2k2 mer-[Cr(CO)3(P(OMe)3)3]+ k2 = 0.11 ± 0.01 k-2 = (1.7 ± 0.8) × 10-4 s-1 K2 = 640 ± 130 The activation parameters for fac-[Cr(CO)3(P(OMe)3)3]+ ? mer-[Cr(CO)3(P(OMe)3)3]+ are consistent with those expected for an intramolecular twist mechanism (EA = 15.9 ± 0.7 kJ/mol; ΔS* = -17.0 ± 1.0 J/(K mol)). The equilibrium constant, K3, for the redox cross-reaction fac-[Cr(CO)3(P(OMe)3)3]+ + mer-[Cr(CO)3(P(OMe)3)3] ?K3 fac-[Cr(CO)3(P(OMe)3)3] + mer-[Cr(CO)3(P(OMe)3)3]+ has a value of 160 ± 13. The electrochemical properties of fac/mer-[Cr(CO)3(P(OEt)3)3] are similar.
