The redox chemistry of a variety of organometallic dinitrosyl complexes of Cr, Mo and W
Creators: name: Wassink, Berend
Publisher: name: University of British Columbia
Year: 2010

description: An understanding of the electrochemical properties of organometallic nitrosyl compounds provides a good understanding of their chemistry and a more rational way to approach synthetic investigations. Cyclic voltammetry studies of [(ƞ)⁵-C₅H₅)Cr(NO)₂]₂ in CH₂Cl₂ and CH₃CN reveal that the dimer undergoes a single two-electron oxidation to form [(ƞ⁵⁻C₅H₅)Cr(NO)₂]⁺ which is reduced to [(ƞ⁵-C₅H₅)Cr(NO)₂]• (or [(ƞ⁵-C₅H₅)Cr(NO)₂(CH₃CN)]• in the presence of CH₃CN) in a subsequent reduction step. The radical couples to form [(ƞ⁵-C₅H₅)Cr(NO)₂]₂ or decomposes. The dimer also is reversibly reduced in a one-electron step in CH₂Cl₂ and quasi-reversibly in CH₃CN. These inferences are supported by cyclic voltammograms of (ƞ)⁵-CH₅)Cr(NO)₂BF₄ and [(ƞ⁵-C₅H₅)Cr(NO)₂(CH₃CN)]PF₆. In contrast, the isoelectronic dimer [(ƞ⁵-C₅H₅)Fe(CO)₂]₂ oxidizes in two one-electron steps (the first of which is reversible and negative of the oxidation of [(ƞ⁵-C₅H₅)Cr(NO)₂]₂) and reduces to form [(ƞ⁵-C₅H₅)Fe(CO)₂]⁻. The differing oxidation behaviours of these dimers suggest that their reactions with HBF₄•OMe₂ [(ƞ⁵-C₅H₅)Cr(NO)₂]₂ cleaves into [(ƞ⁵-C₅H₅)Cr(NO)₂]⁺ and [ƞ⁵-C₅H₅)Fe(CO)₂]₂ forms [ {(ƞ⁵-C₅H₅)Fe(CO)₂}₂H]⁺) do not occur by initial electron transfer. The new radical anion complex [(ƞ⁵-C₅H₅)Fe(ƞ⁶-C₆Me₆)] [ {(ƞ₅-C₅H₅)Cr(NO)₂}₂] can be isolated by reaction of the neutral dimer with (ƞ⁵-C₅H₅)Fe(ƞ⁶-C₆Me₆) in Et₂0. Its spectroscopic properties are consistent with derealization of the extra electron onto the NO ligands, particularly the bridging nitrosyl groups. These observations provide a better understanding of the reactivity of [(ƞ⁵-C₅H₅)Cr(NO)₂]₂ with- nucleophiles. A comparative electrochemical study of the oxidations of [(ƞ⁵-C₅H₅)M(NO)₂R (M = Cr, R = CH₃; M = Mo, W, R = CH₃, C₂H₅),(ƞ⁵-C₅H₅)Fe(CO)₂CH₃ and (ƞ⁵-C₅H₅)M(CO) ₃R (M = Cr, R = CH₃; M = Mo, W, R = CH₃, C₂H₅) in CH₂Cl₂ reveals that the dinitrosyl complexes are harder to oxidize than their related carbonyl compounds. Electrophilic cleavage reactions of M-R bonds in these complexes, which proceed differently for the nitrosyl and carbonyl complexes are proposed to involve different mechanisms, with the nitrosyl-alkyl complexes reacting with electrophiles by direct attack at the metal-alkyl bonds, rather than by prior oxidation. Interestingly, (ƞ⁵-C₅H₅)Cr(NO) ₂CH₃ reacts with NOPF₆ to form the NO-insertion product [(ƞ⁵-C₅H₅)Cr(NO)₂(CH₂NOH)]PF₆ which has been structurally and spectroscopically characterized. The reactions of (ƞ⁵-C₅H₅)M(NO)₂CH₃ (M = Mo, W) with electrophiles and oxidants result in cleavage of the M-CH₃ bonds. The complexes (ƞ⁵-C₅H₅)M(NO)₂Y (M = Cr, Y = CH₃; M = Mo, Y = CH₃, C₂H₅, Cl; M = W, Y = CH₃, C₂H₅, H, Cl), [(ƞ⁵-C₅H₅)M(NO)₂L] BF₄ (M = Mo, L = PPh₃; M = W, L = PPh₃, P(OMe)₃, ƞ²-C₈H₁₄) and W(NO)₂Cl₂L₂ (L = P(OMe)₃, PMePh₂) exhibit quite reversible, one-electron reductions in CH₂Cl₂ and the new radical complexes [(ƞ⁵-C₅H₅) ₂Co] [(ƞ⁵-C₅H₅)M(NO)₂Y] (M = Mo, Y = CH₃, C₂H₅, Cl; M = W, Y = CH₃, H, Cl) are isolable by reactions of (ƞ⁵-C₅H₅)₂Co with the neutral precursor. Spectroscopic characterization of these and an X-ray crystallogrpahic analysis of [(ƞ⁵-C₅H₅)₂Co] [(ƞ⁵-C₅H₅)Mo(NO)₂C₂H₅] suggest that the anions possess monomeric, "three-legged piano stool" geometries with delocalization of the extra electron onto the NO ligands. In light of these observations the chemistry of (ƞ⁵-C₅H₅)M(N0)₂Y complexes becomes more understandable.