Experimental and quantum chemical study of the mechanism of an unexpected intramolecular reductive coupling of a bridging phosphido ligand and a C 6F5 group and the reversible oxidative addition of PPh2C6F5

Abstract

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 [L2MIII(μ-PH2)2M‘IIIL2] (L = CN or C6F5) complexes. In contrast, [L2PdIII(μ-PH2)2PdIIIL2] does not exist as local minima in the PES, but the [L2PdII(μ-PH2)PdII(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.