Oxidant-Induced Hydride Abstraction from [Pt(μ-PBut2)(H)(PBut2H)]2Yielding [Pt2(μ-PBut2)2(H)(PBut2H)2]C3(CN)5. Spectroscopic, Crystallographic, and Theoretical Comparison of the Structures of Two “Tautomers”

Abstract

The platinum(II) phosphido-bridged dihydride [Pt(μ-PBut2)(H)(PBut2H)]2, 1, reacts with tetracyanoethylene (TCNE) in toluene to give the charge-transfer complex 1·TCNE, which is stable only in the solid state. By dissolving 1·TCNE in polar solvents, electron transfer occurs rapidly, followed by loss of one of the hydrido ligands and formation of the unsaturated Pt(II)Pt(II) derivative [Pt2(μ-PBut2)2(H)(PBut2H)2]+TCNE-, 2a. Recrystallization of 2a in the air gives crystals of [Pt2(μ-PBut2)2(H)(PBut2H)2]+ [C3(CN)5]-, 2b, whose structure was solved by X-ray diffraction. The same type of reaction was observed when 1 was reacted with tetracyanoquinodimethane (TCNQ), forming the analogous, structurally characterized, TCNQ- salt of cation 2+. What happened to the hydrido ligand displaced from 1 was better seen when 1 was reacted with an equimolar amount of [Cp2Fe]PF6 as the oxidant (Cp = cyclopentadienyl). In this case we isolated a 1:1 mixture of (2)PF6, 2d, and [Pt2(μ-PBut2)(μ-H)(PBut2H)3(H)]PF6, 3d, the latter also known to form by the reaction of complex 1 with strong acids. According to this observation, the reaction of 1 with two equivalents of [Cp2Fe]PF6 and one of triethylamine (NEt3) gives quantitatively [NEt3H]PF6 and 2d. The electronic structure of cation 2+ was also investigated by means of ab initio and density functional calculations. Ignoring the different metals, cation 2+ can be considered a “tautomer” of the known phosphine-bridged dipalladium cation [Pd2(μ-PBut2)(μ-PBut2H)(PBut2H)2]+, 4+. Differences and analogies between the structures of the two cations were investigated by means of spectroscopic, crystallographic, and theoretical (ab initio and density functional) analyses.