Influence of chain length on mono- versus di-alkylation in the reactivity of [Pt2(μ-S)2(PPh3)4] towards α,ω-dihalo-n-alkanes; a synthetic route to platinum(II) ω-haloalkylthiolate complexes

The reactions of [Pt2(μ-S)2(PPh3)4] with α,ω-dibromoalkanes Br(CH2)nBr (n = 4, 5, 6, 8, 12) gave mono-alkylated [Pt2(μ-S){μ-S(CH2)nBr}(PPh3)4]+ and/or di-alkylated [Pt2(μ-S(CH2)nS}(PPh3)4]2+ products, depending on the alkyl chain length and the reaction conditions. With longer chains (n = 8, 12), intramolecular di-alkylation does not proceed in refluxing methanol, with the mono-alkylated products [Pt2(μ-S){μ-S(CH2)nBr}(PPh3)4]+ being the dominant products when excess alkylating agent is used. The bridged complex [{Pt2(μ-S)2(PPh3)4}2{μ-(CH2)12}]2+ was accessible from the reaction of [Pt2(μ-S)2(PPh3)4] with 0.5 mol equivalents of Br(CH2)12Br. [Pt2(μ-S){μ-S(CH2)4Br}(PPh3)4]+ can be cleanly isolated as its BPh4− salt, but undergoes facile intramolecular di-alkylation at −18 °C, giving the known species [Pt2(μ-S(CH2)4S}(PPh3)4]2+. The reaction of I(CH2)6I with [Pt2(μ-S)2(PPh3)4] similarly gives [Pt2(μ-S){μ-S(CH2)6I}(PPh3)4]+, which is fairly stable towards intramolecular di-alkylation once isolated. These reactions provide a facile route to ω-haloalkylthiolate complexes which are poorly defined in the literature. X-ray crystal structures of [Pt2(μ-S){μ-S(CH2)5Br}(PPh3)4]BPh4 and [Pt2(μ-S(CH2)5S}(PPh3)4](BPh4)2 are reported, together with a study of these complexes by electrospray ionisation mass spectrometry. All complexes fragment by dissociation of PPh3 ligands, and the bromoalkylthiolate complexes show additional fragment ions [Pt2(μ-S){μ-S(CH2)n−2CHCH2}(PPh3)m]+ (m = 2 or 3; m ≠ 4), most significant for n = 4, formed by elimination of HBr.