Atropisomerization, C−H Activation, and Dissociative Substitution at Some Biphenyl Platinum(II) Complexes

The reaction of 2,2‘-dilithiumbiphenyl with cis-[PtCl2(SEt2)2] at −10 °C in diethyl ether not only leads to the main product [Pt2(μ-SEt2)2(bph)2], containing the planar 2,2‘-biphenyl dianion (bph2-), but also forms a new dinuclear platinum(II) compound of formula [Pt2(μ-SEt2)2(Hbph)4], 1a (Hbph- = η1-biphenyl monoanion), in which each metal is in a square-planar environment. NMR spectroscopy and molecular mechanics (MMFF) calculations were used to characterize 1a. The results suggest that the favored conformation for the four Hbph biphenyl groups is αββα. In chloroform solution, 1a undergoes atropisomerization to 1b (αβαβ) (k is = 1.03 × 10-4 s-1, at 298 K) that subsequently cyclometalates (k obs = 4.48 × 10-6 s-1, at 298 K) to yield [Pt2(μ-SEt2)2(bph)2] and biphenyl. Both processes, atropisomerization and C−H activation, presumably involve preliminary thioether bridge splitting. The dinuclear complex 1a has been shown to be a versatile and useful precursor to a variety of mononuclear η1-biphenyl platinum(II) complexes. By reaction with diethyl sulfide, dimethyl sulfoxide, or with rigid dinitrogen containing ligands, such as 2,2‘-bipyridine or 1,10-phenanthroline, complexes cis-[Pt(Hbph)2(dmso)2] 3, cis-[Pt(Hbph)2(SEt2)2] 4, [Pt(Hbph)2(bpy)] 5, and [Pt(Hbph)2(phen)] 6 were obtained, respectively. The crystal structures of compounds 5 and 6 were determined. Only the head-to-tail isomer of these compounds was recognized in the solid state and in solution, where restricted rotation around the Pt−C bond prevents interconversion to the head-to-head form. A detailed kinetic study of ligand (dmso) exchange and substitution (by 2,2‘-bipyridine and 1,10-phenanthroline) has been performed on complex 3 in CDCl3 and toluene-d 8 by 1H NMR magnetization transfer experiments, and in toluene by UV/vis spectroscopy, respectively. The rates of both processes show no dependence on ligand concentration, the rate of ligand substitution being in reasonable agreement with that of ligand exchange at the same temperature. The kinetics are characterized by largely positive entropies of activation. The results are consistent with a dissociative mode of activation analogous to the pattern already found for compounds with a similar [Pt(C,C)(S,S)] set of coordinating ligands. The role of ML3 d8 T-shaped 14-electron species, as elusive reaction intermediates or structurally characterized compounds, is discussed.