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 (-S) 2 (PPh 3) 4 ] towards a range of activated alkylating agents of the type RC(O)CH 2 X (R = organic moiety e.g. phenyl, pyrenyl; X = Cl, Br). Alkylation of both sulfide centres is observed for PhC(O)CH 2 Br, 3-(bromoacetyl)coumarin [CouC(O)CH 2 Br], and 1-(bromoacetyl)pyrene [PyrC(O)CH 2 Br], giving dications [Pt 2 {-SCH 2 C(O)R} 2 (PPh 3) 4 ] 2+ , isolated as their PF 6 salts. The X-ray structure of [Pt 2 {-SCH 2 C(O)Ph} 2 (PPh 3) 4 ](PF 6) 2 shows the presence of short Pt•••O contacts. In contrast, the corresponding chloro compounds [typified by PhC(O)CH 2 Cl] and imino analogues [e.g. PhC(NOH)CH 2 Br] do not dialkylate [Pt 2 (-S) 2 (PPh 3) 4 ]. The ability of PhC(O)CH 2 Br to dialkylate [Pt 2 (-S) 2 (PPh 3) 4 ] allows the synthesis of new mixed-alkyl dithiolate derivatives of the type [Pt 2 {-SCH 2 C(O)Ph}(-SR)(PPh 3) 4 ] 2+ (R = Et or n-Bu), through alkylation of in situ-generated monoalkylated compounds [Pt 2 (-S)(-SR)(PPh 3) 4 ] + (from [Pt 2 (-S) 2 (PPh 3) 4 ] and excess RBr). In these heterodialkylated systems ligand replacement of PPh 3 occurs by the bromide ions in the reaction mixture forming monocations [Pt 2 {-SCH 2 C(O)Ph}(-SR)(PPh 3) 3 Br] +. This ligand substitution can be easily suppressed by addition of PPh 3 to the reaction mixture. The complex [Pt 2 {-SCH 2 C(O)Ph}(-SBu)(PPh 3) 4 ] 2+ was crystallographically characterised. X-ray crystal structures of the bromide-containing complexes [Pt 2 {-SCH 2 C(O)Ph}(-SR)(PPh 3) 3 Br] + (R = Et, Bu) are also reported. In both structures the coordinated bromide is trans to the SCH 2 C(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 [Pt 2 {-SCH 2 C(O)Ph} 2 (PPh 3) 4 ] 2+ and [Pt 2 {-SCH 2 C(O)Ph}(-SBu)(PPh 3) 4 ] 2+ (and many other known analogues) where both alkyl groups adopt axial positions.