Chalcogen bond stabilisation of platinum(II) sulfonyl-substituted thiourea complexes with ancillary phosphine ligands

A series of sulfonyl-substituted thiourea platinum(II) complexes bearing ancillary phosphine ligands were prepared by the reaction between cis-[PtCl2(PR3)2] (PR3 = triphenylphosphine, tris(2-cyanoethyl)phosphine and 1,3,5-triaza-7-phosphaadamantane) and either p-TolSO2NHC(S)NHPh or EtSO2NHC(S)NHPh, in a refluxing Et3N/methanol mixture. The mixed ligand complexes, bearing a single phosphine and pyridine, were also prepared by the reactions between cis-[PtCl2(COD)] (COD = cyclooctadiene), pyridine and p-TolSO2NHC(S)NHPh or EtSO2NHC(S)NHPh. For each complex, the thiourea ligand binds as a dianion through the S and NPh groups in the distal isomer with a remote NSO2R group. Only in the case of the reaction between the complex [PtCl2(PPh3)2] and TolSO2NHC(S)NHCH(CH3)3) was the proximal isomer formed on account of the bulky t-butyl substituent on the thiourea. X-ray structures and computational analysis revealed another reason for this preferential distal isomer formation is most likely due to the presence of a stabilising chalcogen bond, which is otherwise unavailable in the proximal isomer.