The synthesis of water-soluble analogues of Pt₂(μ₂-S)₂(PPh₃)₄

Abstract

Platinum has a very rich chemistry with sulfur, and this field has been extensively studied. Of particular interest are complexes containing the {Pt₂(μ₂-S)₂} core, comprising two platinum atoms with μ₂-sulfurs bridging through a flexible hinge. These complexes can act as potent metalloligands (through the two sulfurs) toward a wide range of transition metal and other centres, allowing for the synthesis of complexes with trinuclear mixed-metal cores {Pt₂(μ₃-S)₂M} (M = heterometal). The {Pt₂(μ₂-S)₂} core is also highly nucleophilic and alkylated by the mildest of reagents. The ancillary ligands coordinated to the {Pt₂(μ₂-S)₂} core are primarily triphenylphosphine (PPh₃), resulting in the complexes being significantly insoluble in water, preventing any forms of bioactivity or biphasic catalysis from being assessed. This research project has widened the scope of {Pt₂(μ₂-S)₂} complexes via the synthesis of the new water-soluble complexes {Pt₂(μ₂-S)₂}(PTA)₄, [Pt₂(μ₃- S)₂(PTA)₄Rh(cod)]Cl, and [Pt₃(μ3-S)₂(PTA)6]²⁺ (PTA = 1,3,5-triaza-7- phosphaadamantane, cod = 1,5-cyclooctadiene) that were characterised by electrospray ionisation mass spectrometry (ESI-MS) and phosphorus-31 nuclear magnetic resonance spectroscopy (³¹P{¹H} NMR). The complex [Pt₃(μ₃- S)₂(PTA)₆]²⁺ was synthesised during the research into the use of a sulfide exchange column as an alternative source of sulfide for the synthesis of Pt₂(μ₂- S)₂(PTA)₄. The water-soluble phosphine ligands tris(2-carboxyethyl)phosphine (TCEP) and monosulfonated triphenylphosphine [TPPMS]⁻ were determined to be unsuitable ligands for complexes of the general form {Pt₂(μ₂-S)₂}(L)₄ (L = TCEP, [TPPMS]⁻). During these attempted syntheses the complex {Pt₂(μ₂-S)₂} (cod)₂ was successfully produced and characterised with ESI-MS, ¹H NMR, ¹H-¹H COSY, and ¹³C-¹H HSQC (Proton nuclear magnetic resonance spectroscopy, Correlation Spectroscopy, Heteronuclear Single Quantum Coherence). The use of thioacetamide (CH₂C(S)NH₂) as an alternative source of sulfide in the synthesis of {Pt₂(μ₂-S)₂}(PPh₃)₄ was found to be a poor substitute for the well established Na₂S·9H₂O. The reaction does result in [Pt(CH₃C(S)NH)(PPh₃)₂]⁺+, subsequently isolated as the BPh₄- salt, [Pt(CH₃C(S)NH)(PPh₃)₂](BPh₄) and was characterised by ESI-MS and ³¹P{1H} NMR. The X-ray crystal structure of the by-product trans-Pt(SC(O)CH₃)₂(PPh₃)₂ was obtained.