Dynamic Titanium Phosphinoamides as Unique Bidentate Phosphorus Ligands for Platinum

Abstract

Treatment of lithium phosphinoamides, Ph2PN(Li)R [R = tBu, iPr], with TiCl4 results in formation of titanium phosphinoamides, (Ph2PNR)2TiCl2 [R = tBu (1a), iPr (1b)]. Crystallographic studies show that there are covalent bonds between the titanium and two nitrogen atoms, whereas two phosphorus atoms are coordinated to the metal center intramolecularly. Variable-temperature NMR studies suggest reversible dissociation of the phosphorus moieties from the titanium in solution. The dissociated phosphorus moieties are effectively captured by Pt(II) species; reactions of 1a with either (η4-COD)PtCl2, (η4-COD)Pt(R)(Cl) (R = Me, p-Tol), or [Me2Pt(μ-SMe2)]2 afford the corresponding Ti−Pt heterobimetallic complexes. The molecular structures of these complexes reveal that they have a six-membered dimetallacycle, in which a titanium and a platinum are connected by two bridging phosphinoamide ligands; the Pt−Ti distances indicate the existence of a Pt→Ti dative bond. The conformation of the dimetallacycle is a boat form, with two metals at the bow and the stern in the crystal; however, dynamic conformational change involving cleavage and re-formation of the Pt→Ti dative bond is indicated from variable-temperature NMR studies.