Zinc, cadmium and mercury complexes of redox-active cyanomanganese carbonyl ligands: intramolecular electron transfer through tetrahedral d10 metal centres

Abstract

The redox-active cyanomanganese carbonyl ligands cis- and trans-[Mn(CN)(CO)₂{P(OR)₃}(dppm)] (R = Ph or Et, dppm = Ph₂PCH₂PPh₂) reacted with ZnBr₂, CdI₂ and Hg(NO₃)₂ to give the tetrahedral (at M) complexes [X₂M{(µ-NC)MnL_x}₂] [MX₂ = ZnBr₂, CdI₂ or Hg(NO₃)₂; L_x = cis- or trans-(CO)₂{P(OR)₃}(dppm); R = Ph or Et]; similarly [Mn(CN)(NO)(PPh₃)(η-C₅H₄Me)] gives [X₂M{(µ-NC)MnL_x}₂] {MX₂ = ZnBr₂ or CdI₂, L_x = (NO)(PPh₃)(η-C₅H₄Me)}. Treatment of [Br₂Zn{(µ-NC)MnL_x}₂] [L_x = trans-(CO)₂{P(OEt)₃}(dppm)] 4 with TlPF₆ in the presence of 1 or 2 equivalents of trans-[Mn(CN)(CO)₂{P(OEt)₃}(dppm)] gave the tetra- and penta-metallic complexes [BrZn{(µ-NC)MnL_x}₃][PF₆] 13 and [Zn{(µ-NC)MnL_x}₄][PF₆]₂ 14 [L_x = trans-(CO)₂{P(OEt)₃}(dppm)] respectively. Differential pulse volammetry showed that 4, 13 and 14 are oxidised to weakly interacting mixed-valence complexes. The reaction of trans-[Mn(CN)(CO)(dppm)₂] with ZnBr₂ or CdX₂ (X = Br or I) in thf gave the bimetallic species [X₂(thf)M(µ-NC)Mn(CO)(dppm)₂] (MX₂ = ZnBr₂ or CdI₂) and [Br₂Cd(µ-NC)Mn(CO)(dppm)₂] which are oxidised by [Fe(η-C₅H₅)₂][PF₆] to the Mn^II complexes [X₂(thf)M(µ-NC)Mn(CO)(dppm)₂][PF₆] and [Br₂Cd(µ-NC)Mn(CO)(dppm)₂][PF₆]. The crystal structures of the tetrahedral polynuclear complexes [I₂Cd{(µ-NC)MnL_x}₂] [L_x = trans-(CO)₂{P(OEt)₃}(dppm)] 9 and [Br₂(thf)Zn(µ-NC)Mn(CO)(dppm)₂] 15 are reported, and the importance of steric effects (as quantified by cone angles) in the behaviour of cyanomanganese carbonyl ligands is noted.