Study of Intramolecular Competition between Carboxylate and Phosphonate for PtII with the Aid of a Novel Tridentate Carboxylato-Thioether-Phosphonato Ligand

The tridentate dianionic ligand 2-[2-(hydroxyisopropoxyphosphoryl)phenylsulfanyl]benzoate (L2-) reacts with cis-[Pt(NH3)2(H2O)2]2+ to form an S,O-chelate in which the O-coordinated group is either carboxylate or phosphonate, depending on the degree of protonation of the complex. Carboxylate appears to be the stronger ligand, and the stoichiometric reaction between cis-[Pt(NH3)2(H2O)2]2+ and L2- yields the neutral species [Pt(L)(NH3)2], with L bound by sulfanyl and carboxylate groups, both in solution and in the solid state. Upon protonation of [Pt(L)(NH3)2], the stronger basicity of the carboxylate causes the Pt coordination to switch from carboxylate to phosphonate, and the uncoordinated carboxylate group becomes protonated. In methanolic solution, the first-order kinetics of this rearrangement could be observed by 31P NMR spectroscopy. Both complexes - the carboxylate-bound neutral complex [Pt(L)(NH3)2]H2O (triclinic, P (no. 2), a=9.529(6), b=9.766(6), c=12.299(7) Å, =106.91(2), =101.71(2), =102.05(2)°, Z=2) and the perchlorate salt of the phosphonate-bound complex [Pt(LH)(NH3)2]ClO4H2O (monoclinic, P21/c (no. 14), a=12.095(2), b=14.046(2), c=14.448(2) Å, =95.55(2)°, Z=4) - were characterized by X-ray crystallography.