Dynamic titanium phosphinoamides as unique bidentate phosphorus ligands for platinum

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 stem 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.