Rare, hypodentate L-κS coordination mode of N,N-dialkyl-N′-aroylthioureas leads to unprecedented mixed-ligand [Pt(phen)(L-κS)2] complexes

Abstract

Novel, uncharged mixed-ligand [Pt(1,10-phenanthroline)(Lⁿ-κS)₂] (n = 1, 2) complexes have been isolated, in which two usually chelating N,N-dibutyl-N′-1-naphthoyl-thioureato (L¹)⁻, or N,N-diethyl-N′-benzoyl-thioureato (L²)⁻ anions coordinate to Pt(phen)Cl₂ in an unprecedented κS-thio/amido mode of coordination. These previously unidentified compounds, are undesirable by-products in the synthesis of the biologically active mixed-ligand [Pt(diimine)(L-κO,S)]⁺X⁻ complex salts, in which the well-documented tendency of such HLⁿ ligands to coordinate in a bidentate (L-κO,S) mode of coordination is exploited. In the case of the N,N-dibutyl-N′-1-naphthoyl-thioureato, the [Pt(phen)(L¹-κS)₂] complex is the dominant product obtained in highest yield. Single-crystal X-ray diffraction analysis of [Pt(phen)(L¹-κS)₂] indicates significant intramolecular π–cation type stacking interactions that appear to stabilize these hypo-dentate (L-κS)⁻ modes of coordination. Significantly, 1D-NOE NMR spectroscopy shows that for the [Pt(phen)(L¹-κS)₂] complex, the intra-molecular π-stacking interactions between the naphthyl and Pt(phen) groups certainly persists in methanol solutions, at room temperature. By contrast, in the N,N-diethyl-N′-benzoyl-thioureato analogue of [Pt(phen)(L²-κS)₂], inter-molecular head-to-head, stacking interactions between the Pt(phen) moieties of adjacent independent molecules dominate the in the solid-state structure of this complex. Significantly, use of a non-aromatic N,N-diethyl-N′-pivaloylthiourea (HL³) results in the formation of only low yields (<10%) of the corresponding [Pt(phen)(L³-κS)₂] using identical preparative methods; moreover, [Pt(phen)(L³-κS)₂] could not be isolated as it is unstable on workup.