| Abstract
| - The reaction of 2,2‘-dilithiumbiphenyl with cis-[PtCl2(SEt2)2] at −10 °C in diethyl ether not onlyleads to the main product [Pt2(μ-SEt2)2(bph)2], containing the planar 2,2‘-biphenyl dianion (bph2-), but alsoforms a new dinuclear platinum(II) compound of formula [Pt2(μ-SEt2)2(Hbph)4], 1a (Hbph- = η1-biphenylmonoanion), in which each metal is in a square-planar environment. NMR spectroscopy and molecularmechanics (MMFF) calculations were used to characterize 1a. The results suggest that the favoredconformation for the four Hbph biphenyl groups is αββα. In chloroform solution, 1a undergoes atropisomerization to 1b (αβαβ) (kis = 1.03 × 10-4 s-1, at 298 K) that subsequently cyclometalates (kobs = 4.48× 10-6 s-1, at 298 K) to yield [Pt2(μ-SEt2)2(bph)2] and biphenyl. Both processes, atropisomerization andC−H activation, presumably involve preliminary thioether bridge splitting. The dinuclear complex 1a hasbeen 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 ofcompounds 5 and 6 were determined. Only the head-to-tail isomer of these compounds was recognizedin the solid state and in solution, where restricted rotation around the Pt−C bond prevents interconversionto 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-d8 by 1HNMR magnetization transfer experiments, and in toluene by UV/vis spectroscopy, respectively. The ratesof both processes show no dependence on ligand concentration, the rate of ligand substitution being inreasonable agreement with that of ligand exchange at the same temperature. The kinetics are characterizedby largely positive entropies of activation. The results are consistent with a dissociative mode of activationanalogous to the pattern already found for compounds with a similar [Pt(C,C)(S,S)] set of coordinatingligands. The role of ML3 d8 T-shaped 14-electron species, as elusive reaction intermediates or structurallycharacterized compounds, is discussed.
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