The effect of µ-CN linkage isomerism and ancillary ligand set on directional metal–metal charge-transfer in cyanide-bridged dimanganese complexes

Abstract

The reaction of [Mn(CN)L′(NO)(η⁵-C₅R₄Me)] with cis- or trans-[MnBrL(CO)₂(dppm)], in the presence of Tl[PF₆], gives homobinuclear cyanomanganese(I) complexes cis- or trans-[(dppm)(CO)₂LMn(µ-NC)MnL′(NO)(η⁵-C₅R₄Me)]⁺, linkage isomers of which, cis- or trans-[(dppm)(CO)₂LMn(µ-CN)MnL′(NO)(η⁵-C₅R₄Me)]⁺, are synthesised by reacting cis- or trans-[Mn(CN)L(CO)₂(dppm)] with [MnIL′(NO)(η⁵-C₅R₄Me)] in the presence of Tl[PF₆]. X-Ray structural studies on the isomers trans-[(dppm)(CO)₂{(EtO)₃P}Mn(µ-NC)Mn(CNBu^t)(NO)(η⁵-C₅H₄Me)]⁺ and trans-[(dppm)(CO)₂{(EtO)₃P}Mn(µ-CN)Mn(CNBu^t)(NO)(η⁵-C₅H₄Me)]⁺ show nearly identical molecular structures whereas cis-[(dppm)(CO)₂{(PhO)₃P}Mn(µ-NC)Mn{P(OPh)₃}(NO)(η⁵-C₅H₄Me)]⁺ and cis-[(dppm)(CO)₂{(PhO)₃P}Mn(µ-CN)Mn{P(OPh)₃}(NO)(η⁵-C₅H₄Me)]⁺ differ, effectively in the N- and C-coordination respectively of two different optical isomers of the pseudo-tetrahedral units (NC)Mn{P(OPh)₃}(NO)(η⁵-C₅H₄Me) and (CN)Mn{P(OPh)₃}(NO)(η⁵-C₅H₄Me) to the octahedral manganese centre. Electrochemical and spectroscopic studies on [(dppm)(CO)₂LMn(µ-XY)MnL′(NO)(η⁵-C₅R₄Me)]⁺ show that systematic variation of the ligands L and L′, of the cyclopentadienyl ring substituents R, and of the µ-CN orientation (XY = CN or NC) allows control of the order of oxidation of the two metal centres and hence the direction and energy of metal–metal charge-transfer (MMCT) through the cyanide bridge in the mixed-valence dications. Chemical one-electron oxidation of cis- or trans-[(dppm)(CO)₂LMn(µ-NC)MnL′(NO)(η⁵-C₅R₄Me)]⁺ with [NO][PF₆] gives the mixed-valence dications trans-[(dppm)(CO)₂LMn^II(µ-NC)Mn^IL′(NO)(η⁵-C₅R₄Me)]²⁺ which show solvatochromic absorptions in the electronic spectrum, assigned to optically induced Mn(I)-to-Mn(II) electron transfer via the cyanide bridge.