Electron-Deficient Pt2M2Pt2 Hexanuclear Metal Strings (M = Pt, Pd) Supported by Triphosphine Ligands

Electron-deficient Pt2M2Pt2 hexanuclear clusters, [Pt4M2(μ-dpmp)4(XylNC)2](PF6)4 (M = Pt (7), Pd (8); dpmp = bis((diphenylphosphino)methyl)phenylphosphine), were synthesized by oxidation of hydride-bridged hexanuclear clusters [Pt4M2(μ-H)(μ-dpmp)4(XylNC)2](PF6)3 (M = Pt (2), Pd (3)) and were revealed to involve a linearly ordered Pt2M2Pt2 array joined by delocalized bonding interactions with 84 cluster valence electrons, which are discussed on the basis of DFT calculations. The central M–M distances of 7 and 8 are significantly reduced upon the apparent loss of a hydride unit from the M–H–M central part of 2 and 3, indicating that the bonding electrons in the adjacent M–Pt bonds migrate into the central M–M bond to result in a dynamic structural change during two-electron oxidation of the hexanuclear metal strings. A similar Pt6 complex terminated by two iodide anions, [Pt6I2(μ-dpmp)4](PF6)2 (9), was synthesized from [Pt6(μ-H)I2(μ-dpmp)4](PF6) (5) by treatment with [Cp2Fe][PF6]. Complexes 7 and 8 were readily reacted with the neutral two-electron donors XylNC, CO, and phosphines to afford the trinuclear complexes [Pt2M(μ-dpmp)2(XylNC)L](PF6)2 (M = Pt, L = XylNC (1a), CO (10), PPh3 (11); M = Pd, L = XylNC (1b)) through cleavage of the electron-deficient central M–M bond. When complex 7 was reacted with the diphosphines (PP) trans-Ph2PCHCHPPh2 (dppen) and Ph2P(CH2)2PPh2 (dppe), the diphosphine was inserted into the central M–M bond to afford [(XylNC)Pt3(μ-dpmp)2(PP)Pt3(μ-dpmp)2(XylNC)](PF6)4 (12), which was transformed by treatment with another 1 equiv of diphosphine into the asymmetric trinuclear complexes [Pt3(μ-dpmp)2(XylNC)(PP)](PF6)2 (13). A further ligand exchange reaction of 13a (PP = trans-dppen) provided the diphosphine-terminated symmetrical Pt3 complex [Pt3(μ-dpmp)2(L)2](PF6)2 (L = trans-dppen (14a)). Complexes 7 and 8 were also reacted with [AuCl(PPh3)] to yield the Pt2MAu heterotetranuclear complexes [Pt2MAuCl(μ-dpmp)2(PPh3)(XylNC)](PF6)2 (M = Pt (15), Pd (16)), in which the Pt2M trinuclear fragment is inserted into the Au–Cl bond in a 1,1-fashion on the central M atoms of the Pt2M2Pt2 string.