Spin delocalisation and the geometry of redox-active cyanomanganesecarbonyl ligands in heteropolynuclear complexes of rhodium(I)

Abstract

The reactions of trans-[Mn(CN)(CO)(dppm)₂](dppm = Ph₂PCH₂PPh₂) and cis- or trans-[Mn(CN)(CO)₂(PR₃)(L–L)][R = OEt or OPh, L–L = dppm; R = Et, L–L = dppe (Ph₂PCH₂CH₂PPh₂)] with [{Rh(µ-Cl)(CO)₂}₂] give the heterobinuclear complexes [trans-(dppm)₂(OC)Mn(µ-CN)Rh(CO)₂Cl] and [(L–L)(R₃P)(OC)₂Mn(µ-CN)Rh(CO)₂Cl] respectively. Cyclic voltammetry shows that each complex undergoes oxidation at the manganese centre; chemical oxidation of [trans-(dppm)₂(OC)Mn(µ-CN)Rh(CO)₂Cl] gives [trans-(dppm)₂(OC)Mn(µ-CN)Rh(CO)₂Cl]⁺ which may also be prepared from trans-[Mn(CN)(CO)(dppm)₂]⁺ and [{Rh(µ-Cl)(CO)₂}₂]. The crystal structures of the redox pair [trans-(dppm)₂(OC)Mn(µ-CN)Rh(CO)₂Cl]^z(Z= 0 or +1) show that substantial changes in geometry resulting from oxidation are limited to the vicinity of the manganese atoms (e.g. mean Mn–P increases from 2.284 to 2.352 Å). These changes are similar to those observed for the free cyanomanganese complexes trans-[Mn(CN)(CO)(dppm)₂]^z(Z= 0 or +1) and indicate that the singly occupied molecular orbital in the radical cation is largely composed of the Mn d_π orbital in the MnP₄ plane. Changes in geometry in the Mn(µ-CN)Rh(CO)₂Cl unit are very small. Treatment of [cis-(L–L)(R₃P)(OC)₂Mn(µ-CN)Rh(CO)₂Cl] or [trans-(L–L)(R₃P)(OC)₂Mn(µ-CN)Rh(CO)₂Cl] with TIPF₆ in the presence of [Mn(CN)(CO)₂(PR₃)(L–L)] gives the heterotrinuclear cations [{(L–L)(R₃P)(OC)₂Mn(µ-CN)}₂Rh(CO)₂]⁺; the crystal structure of one, [{trans-(dppm)[(EtO)₃P](OC)₂Mn(µ-CN)}₂Rh(CO)₂], shows the two cyanomanganese ligands cis-co-ordinated to the rhodium centre. Cyclic, differential-pulse, and square-wave voltammetry of [{trans-(dppm)(R₃P)(OC)₂Mn(µ-CN)}₂Rh(CO)₂]⁺(R = OEt or OPh) show two closely spaced oxidation waves; the small separation (ca. 80–90 mV) suggests weak interaction between the two cyanomanganese ligands in the mixed-valence dication [{trans-(dppm)(R₃P)(OC)₂Mn(µ-CN)}₂Rh(CO)₂]²⁺(R = OEt or OPh). The results provide evidence for the dependence of spin delocalisation from Mn^II to Rh^I on the geometry of the ancillary ligands bound to manganese.