Hydride-Bridged Pt₂M₂Pt₂ Hexanuclear Metal Strings (M = Pt, Pd) Derived from Reductive Coupling of Pt₂M Building Blocks Supported by Triphosphine Ligands

Linear Pt₂M₂Pt₂ hexanuclear clusters [Pt₄M₂(μ-H)­(μ-dpmp)₄(XylNC)₂]­(PF₆)₃ (M = Pt (<b>2a</b>), Pd (<b>3a</b>); dpmp = bis­(diphenylphosphinomethyl)­phenylphosphine) were synthesized by site-selective reductive coupling of trinuclear building blocks, [Pt₂M­(μ-dpmp)₂(XylNC)₂]­(PF₆)₂ (M = Pt (<b>1a</b>), Pd (<b>1b</b>)), and were revealed as the first example of low-oxidation-state metal strings bridged by a hydride with M–H–M linear structure. The characteristic intense absorption bands around 583 nm (<b>2a</b>) and 674 nm (<b>3a</b>) were assigned to the HOMO–LUMO transition on the basis of a net three-center/two-electron (3c/2e) bonding interaction within the central M₂(μ-H) part. The terminal ligands of <b>2a</b> were replaced by H^–, I^–, and CO to afford [Pt₆(μ-H)­(H)₂(μ-dpmp)₄]⁺ (<b>4</b>), [Pt₆(μ-H)­I₂(μ-dpmp)₄]­(PF₆) (<b>5</b>), and [Pt₆(μ-H)­(μ-dpmp)₄(CO)₂]­(PF₆)₃ (<b>6</b>). The electronic structures of these hexaplatinum cores, {Pt₆(μ-H)­(μ-dpmp)₄}³⁺, are varied depending on the σ-donating ability of axial ligands; the characteristic HOMO–LUMO transition bands interestingly red-shifted in the order of CO < XylNC < I^– < H^–, which was in agreement with calculated HOMO–LUMO gaps derived from DFT optimizations of <b>2a</b>, <b>4</b>, <b>5</b>, and <b>6</b>. The nature of the axial ligands influences the redox activities of the hexanuclear complexes; <b>2a</b>, <b>3a</b>, and <b>5</b> were proven to be redox-active by the cyclic voltammograms and underwent two-electron oxidation by potentiostatic electrolysis to afford [Pt₄M₂(μ-dpmp)₄(XylNC)₂]­(PF₆)₄ (M = Pt (<b>7a</b>), Pd (<b>8a</b>)). The present results are important in developing bottom-up synthetic methodology to create nanostructured metal strings by utilizing fine-tunable metallic building blocks

Hydride-Bridged Pt₂M₂Pt₂ Hexanuclear Metal Strings (M = Pt, Pd) Derived from Reductive Coupling of Pt₂M Building Blocks Supported by Triphosphine Ligands

Linear Pt₂M₂Pt₂ hexanuclear clusters [Pt₄M₂(μ-H)­(μ-dpmp)₄(XylNC)₂]­(PF₆)₃ (M = Pt (<b>2a</b>), Pd (<b>3a</b>); dpmp = bis­(diphenylphosphinomethyl)­phenylphosphine) were synthesized by site-selective reductive coupling of trinuclear building blocks, [Pt₂M­(μ-dpmp)₂(XylNC)₂]­(PF₆)₂ (M = Pt (<b>1a</b>), Pd (<b>1b</b>)), and were revealed as the first example of low-oxidation-state metal strings bridged by a hydride with M–H–M linear structure. The characteristic intense absorption bands around 583 nm (<b>2a</b>) and 674 nm (<b>3a</b>) were assigned to the HOMO–LUMO transition on the basis of a net three-center/two-electron (3c/2e) bonding interaction within the central M₂(μ-H) part. The terminal ligands of <b>2a</b> were replaced by H^–, I^–, and CO to afford [Pt₆(μ-H)­(H)₂(μ-dpmp)₄]⁺ (<b>4</b>), [Pt₆(μ-H)­I₂(μ-dpmp)₄]­(PF₆) (<b>5</b>), and [Pt₆(μ-H)­(μ-dpmp)₄(CO)₂]­(PF₆)₃ (<b>6</b>). The electronic structures of these hexaplatinum cores, {Pt₆(μ-H)­(μ-dpmp)₄}³⁺, are varied depending on the σ-donating ability of axial ligands; the characteristic HOMO–LUMO transition bands interestingly red-shifted in the order of CO < XylNC < I^– < H^–, which was in agreement with calculated HOMO–LUMO gaps derived from DFT optimizations of <b>2a</b>, <b>4</b>, <b>5</b>, and <b>6</b>. The nature of the axial ligands influences the redox activities of the hexanuclear complexes; <b>2a</b>, <b>3a</b>, and <b>5</b> were proven to be redox-active by the cyclic voltammograms and underwent two-electron oxidation by potentiostatic electrolysis to afford [Pt₄M₂(μ-dpmp)₄(XylNC)₂]­(PF₆)₄ (M = Pt (<b>7a</b>), Pd (<b>8a</b>)). The present results are important in developing bottom-up synthetic methodology to create nanostructured metal strings by utilizing fine-tunable metallic building blocks

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