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

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 A). 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 dπ 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 MnII to RhI on the geometry of the ancillary ligands bound to manganese.