Further studies on the dialkylation chemistry of [Pt2(μ-S) 2(PPh3)4] with activated alkyl halides RC(O)CH2X (X = Cl, Br)

Abstract

Further studies have been carried out into the reactivity of [Pt 2(μ-S)2(PPh3)4] towards a range of activated alkylating agents of the type RC(O)CH2X (R = organic moiety, e.g. phenyl, pyrenyl; X = Cl, Br). Alkylation of both sulfide centers is observed for PhC(O)CH2Br, 3-(bromoacetyl)coumarin [CouC(O)CH 2Br], and 1-(bromoacetyl)pyrene [PyrC(O)CH2Br], giving dications [Pt2{μ-SCH2C(O)R}2(PPh 3)4]2+, isolated as their PF6 - salts. The X-ray structure of [Pt2{μ-SCH 2C(O)Ph}2(PPh3)4](PF 6)2 shows the presence of short Pt⋯O contacts. In contrast, the corresponding chloro compounds [typified by PhC(O)CH 2Cl] and imino analogues [e.g. PhC(NOH)CH2Br] do not dialkylate [Pt2(μ-S)2(PPh3)4]. The ability of PhC(O)CH2Br to dialkylate [Pt2(μ-S) 2(PPh3)4] allows the synthesis of new mixed-alkyl dithiolate derivatives of the type [Pt2{μ-SCH 2C(O)Ph}(μ-SR)(PPh3)4]2+ (R = Et or n-Bu), through alkylation of in situ-generated monoalkylated compounds [Pt2(μ-S)(μ-SR)(PPh3)4]+ (from [Pt2(μ-S)2(PPh3)4] and excess RBr). In these heterodialkylated systems ligand replacement of PPh3 occurs by the bromide ions in the reaction mixture forming monocations [Pt 2{μ-SCH2C(O)Ph}(μ-SR)(PPh3) 3Br]+. This ligand substitution can be easily suppressed by addition of PPh3 to the reaction mixture. The complex [Pt 2{μ-SCH2C(O)Ph}(μ-SBu)(PPh3) 4]2+ was crystallographically characterized. X-ray crystal structures of the bromide-containing complexes [Pt2{μ-SCH 2C(O)Ph}(μ-SR)(PPh3)3Br]+ (R = Et, Bu) are also reported. In both structures the coordinated bromide is trans to the SCH2C(O)Ph ligand, which adopts an axial position, while the ethyl and butyl substituents adopt equatorial positions, in contrast to the structures of the dialkylated complexes [Pt2{μ-SCH 2C(O)Ph}2(PPh3)4]2+ and [Pt2{μ-SCH2C(O)Ph}(μ-SBu)(PPh3) 4]2+ (and many other known analogues) where both alkyl groups adopt axial positions. © 2011 Elsevier B.V. All rights reserved.