Intramolecular electron transfer in linear trinuclear complexes of copper(I), silver(I) and gold(I) bound to redox-active cyanomanganese ligands
Abstract
The reaction of [Cu(NCMe)4][PF6] with 2 equivalents of [Mn(CN)Lx]{Lx=(CO)(dppm)2, cis-or trans-(CO)2[P(OR)3](dppm)(R = Ph or Et, dppm = Ph2PCH2PPh2)} in CH2Cl2 gave [Cu{µ-NC)MnLx}2][PF6]. With 2 equivalents of [Mn(CN)Lx] in toluene, AgPF6 gave [Ag{(µ-NC)MnLx}2]+{Lx=cis- or trans-(CO)2[P(OR)3](dppm)(R = Ph or Et)} but in CH2Cl2cis-[Mn(CN)(CO)2(PEt3)(dppe)](dppe = Ph2PCH2CH2PPh2) or trans-[Mn(CN)(CO)(dppm)2] and AgX (X = BF4–, PF6– or SbF6–) gave the tricationic manganese(II) complexes [Ag{(µ-NC)Mn(CO)(dppm)2}2][PF6]3 and [Ag{(µ-NC)MnLx}2]X3{Lx=trans-(CO)2(PEt3)(dppe)]; the complexes [Ag{(µ-NC)MnLx}2][PF6]3{Lx=trans-(CO)2(P(OR)3](dppm)(R = Ph or Et)} were prepared directly from Ag[PF6] and trans-[Mn(CN)(CO)2{P(OR)3}(dppm)][PF6](R = Ph or Et) in CH2Cl2. Treatment of [AuCl(tht)](tht = tetrahydrothiophene) with [Mn(CN)Lx] in CH2Cl2 in the presence of Tl[PF6] yielded [Au{(µ-NC)MnLx}2][PF6]{Lx=(CO)(dppm)2, cis- or trans-(CO)2[P(OR)3](dppm)(R = Ph or Et)}. X-Ray structural studies on [Ag{(µ-NC)MnLx}2][PF6]{Lx=trans-(CO)2[P(OPh)3](dppm)}, [Au{(µ-NC)MnLx}2][PF6]{Lx=trans-(CO)2[P(OEt)3](dppm)}, and [Ag{(µ-NC)MnLx}2][PF6]3[Lx=(CO)(dppm)2] showed, in each case, near linear Mn–CN–M′–NC–Mn skeletons (M′= Ag or Au); the Mn–P and P–substituent bond lengths are consistent with octahedral MnI and MnII centres in the monocations and trication respectively. Each of the complexes [M′{(µ-NC)MnLx}2][PF6]{M′= Cu or Au, Lx=(CO)(dppm)2; M′= Cu or Ag, Lx=trans-(CO)2[P(OR)3](dppm)(R = Ph or Et)} showed one reversible two-electron oxidation wave at a platinum electrode in CH2Cl2; the trication [Cu{(µ-NC)Mn(CO)(dppm)2}2]3+ was generated in solution by controlled potential electrolysis of [Cu{(µ-NC)Mn(CO)(dppm)2}2]+, and [Au{(µ-NC)Mn(CO)(dppm)2}2][PF6]3 was prepared by chemical oxidation of [Au{(µ-NC)Mn(CO)(dppm)2}2][PF6] with [Fe(cp)2][PF6](cp =η-C5H5) in CH2Cl2. Magnetic and ESR spectroscopic studies provided further evidence for the presence of two isolated low-spin MnII centres in the trications [Ag{(µ-NC)MnLx}2]3+{Lx=(CO)(dppm)2, trans-(CO)2[P(OR)3](dppm)(R = Ph or Et) or trans-(CO)2(PEt3)(dppe)}. By contrast, [Au{(µ-NC)MnLx}2]+{Lx=trans-(CO)2[P(OR)3(dppm)(R = Et or Ph)} showed two reversible one-electron oxidation waves corresponding to the stepwise formation of di- and tri-cations. Electrolytic oxidation of [Au{(µ-NC)MnLx}2]+ in tetrahydrofuran, or chemical oxidation with [N(C6H4Br-p)3]+ or [Fe(η-C5H4COMe)(cp)]+ in CH2Cl2, gave solutions of [Au{(µ-NC)MnLx}2]2+{Lx=trans-(CO)2[P(OEt)3](dppm)}, IR spectroscopic and voltammetric studies on which are compatible with weak interaction between the two manganese centres in the mixed-valence dication.
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