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 (<b>7</b>), Pd (<b>8</b>); 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 (<b>2</b>), Pd (<b>3</b>)) 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 <b>7</b> and <b>8</b> are significantly reduced upon the apparent loss of a hydride unit from the M–H–M central part of <b>2</b> and <b>3</b>, 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 (<b>9</b>), was synthesized from [Pt₆(μ-H)­I₂(μ-dpmp)₄]­(PF₆) (<b>5</b>) by treatment with [Cp₂Fe]­[PF₆]. Complexes <b>7</b> and <b>8</b> 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 (<b>1a</b>), CO (<b>10</b>), PPh₃ (<b>11</b>); M = Pd, L = XylNC (<b>1b</b>)) through cleavage of the electron-deficient central M–M bond. When complex <b>7</b> was reacted with the diphosphines (<b>PP</b>) <i>trans</i>-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(<b>PP</b>)­Pt₃(μ-dpmp)₂(XylNC)]­(PF₆)₄ (<b>12</b>), which was transformed by treatment with another 1 equiv of diphosphine into the asymmetric trinuclear complexes [Pt₃(μ-dpmp)₂(XylNC)­(<b>PP</b>)]­(PF₆)₂ (<b>13</b>). A further ligand exchange reaction of <b>13a</b> (<b>PP</b> = <i>trans</i>-dppen) provided the diphosphine-terminated symmetrical Pt₃ complex [Pt₃(μ-dpmp)₂(L)₂]­(PF₆)₂ (L = <i>trans</i>-dppen (<b>14a</b>)). Complexes <b>7</b> and <b>8</b> were also reacted with [AuCl­(PPh₃)] to yield the Pt₂MAu heterotetranuclear complexes [Pt₂MAuCl­(μ-dpmp)₂(PPh₃)­(XylNC)]­(PF₆)₂ (M = Pt (<b>15</b>), Pd (<b>16</b>)), 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