Experimental and Quantum Chemical Study of the Mechanism of an Unexpected Intramolecular Reductive Coupling of a Bridging Phosphido Ligand and a C₆F₅ Group and the Reversible Oxidative Addition of PPh₂C₆F₅^#,‖

The two-electron oxidation reactions of the [NBu₄]₂[(C₆F₅)₂M(μ-PPh₂)₂M‘(C₆F₅)₂] (M = M‘ = Pt, 1a; M = M‘ = Pd, 1b; M = Pt, M‘ = Pd, 1c) complexes using I₂ as oxidant have been investigated by experimental and electronic structure calculation methods. The geometric and energetic reaction profiles in a vacuum have been investigated at the B3LYP/LANL2DZ level. The first step of the oxidation processes involves the oxidative addition of I₂ to one of the four-coordinate M(II) metal centers yielding a five-coordinate intermediate with square pyramidal stereochemistry. The oxidized [(I)L₂M(μ-PH₂)₂M‘L₂]^- (L = CN) complex converts to a more stable intermediate involving a planar dimetallacycle five-membered ring, which through a reactant-like transition state surmounting a relatively low barrier of 18.5, 8.7, and 8.9 kcal/mol for Pt₂, PtPd, and Pd₂ compounds, respectively, yields the final product. At this stage an unusual intramolecular reductive coupling of one of the ancillary ligands with a bridging phosphido ligand takes place, affording the “M(II)(μ-PH₂)(μ-I)M‘(II)” framework. This process is exoergic by 11.4, 18.2, and 18.8 kcal/mol for Pt₂, PtPd, and Pd₂ compounds, respectively. For the Pt₂ and PtPd complexes removal of the iodide ligand from the oxidized iodo complexes yields the very unusual [L₂M^III(μ-PH₂)₂M‘^IIIL₂] (L = CN or C₆F₅) complexes. In contrast, [L₂Pd^III(μ-PH₂)₂Pd^IIIL₂] does not exist as local minima in the PES, but the [L₂Pd^II(μ-PH₂)Pd^II(PH₂L)L] species was identified, resulting from the intramolecular reductive coupling promoted by the iodide ligand abstraction. This species could be considered as an intermediate for the reverse intramolecular oxidative addition of the PH₂L ligand promoted by iodide abstraction with Ag⁺ ions. Finally, the molecular structure of the [(C₆F₅)₂Pt(μ-PPh₂)(μ-I)Pd(C₆F₅){PPh₂(C₆F₅)}] complex has been established by X-ray crystallography.