Early-late, mixed-metal compounds supported by amidophosphine ligands

The sequential syntheses, structural characterisation and reactivity studies of a series of discrete early-late mixed-metal complexes supported by the unique amidophosphine ligand m-((Bu2CH)-C-t)N(C6H4)PPh2 L-1 are described. This ligand was synthesised using a Schiff-base/(BuLi)-Li-t protocol and the resultant lithium salt LiL1 found to adopt a tetrameric structure in the solid state in which both two-coordinate N-Li-N and eta(6):eta(6)-arylLi metallocene bonding motifs are present. Reaction between HL1 and labile Pt(II) and Pd(II) chlorides formed MCl2(HL1)(2) complexes 4 (M = Pt) and 5 (M = Pd) in which a weak N-H...pi(aryl) hydrogen bonding interaction was identified in the solid-state structure of 4. These compounds were found to be inert to transamination and protonolysis reactions with Ti amides and alkyls; instead, stepwise alkyl transfer from Ti to Pt, resulting in Pt(CH2SiMe3)(2)(HL1)(2) 6 was observed. Access to mixed-metal complexes was achieved using an early-metal-first approach. Reaction between the metalloligand TiCl2(L-1)(2) and labile Group 10 and group 9 compounds resulted in the formation of TiCl2(mu-L-1)(2)PtCl2 8, TiCl2(mu-L-1)(2)PtMe2 9, TiCl2(mu-L-1)(2)PdCl2 10, TiCl2(mu-L-1)(2)NiBr2 11, and {TiCl2(mu-L-1)(2)RhCl(CO)}(2) 12. In the solid state, the Group 4/10 compounds 8, 9 and 10 adopt similar structures that exhibit both intramolecular (Bu2C)-C-t-H...Cl-Ti hydrogen bonding and arylNP pi-stacking interactions; this hydrogen-bonding interaction is conserved in solution. Unlike the above Group 4/10 complexes, the Ti-Rh complex 12 adopts a tetranuclear structure in the solid state that is stabilised by similar hydrogen-bonding and p-stacking interactions. The Group 4/10 complexes were assessed as catalysts for olefin polymerisation and cross-coupling reactions. In combination with MAO, the mixed-metal complexes 8 and 10 were poor ethylene polymerisation catalysts and resulted in polymers of both high molecular weight and polydispersity. The Ti-Ni complex 11 formed oligomeric material only, while the mononuclear Ti metalloligand TiCl2( L1) 2 gave the best results, showing low activity (6.14 kg mol(-1) bar(-1) h(-1)) and moderate polydispersity (12). The Ti-Pd complex 10 was assessed in arylamination and Suzuki-Miyaura reactions. While little or no catalytic activity was observed in arylamination reactions, 10 was found to effect Suzuki coupling between activated aryl bromides and phenylboronic acid at 80degreesC. Unlike with TiCl2(L-1)(2), reactions between 8 and the reducing agents C8K or Mg led to intractable mixtures. However, the cyclic voltammetry of both compounds indicated that a reversible one-electron reduction process occurs at a similar potential (ca. -0.7 V) and was assigned to the formation of the monohalides TiCl(L-1)(2) and TiCl(mu-L-1)(2)PtCl2. The reactivity of the metallocage TiCl(mu-L-3)(3)Pt was also investigated. While reduction reactions were unsuccessful, the metallocage reacted with CO to form the Ti-Pt carbonyl, TiCl(mu-L-3)(3)Pt(CO) 13.

The X-ray crystal structure of 13 revealed that accommodation of CO at the Pt centre has caused the cage expansion and loss of agostic aryl-H...Pt interactions. Furthermore, reaction of TiCl(mu-L-3)(3)Pt with excess MeI resulted in the formation of the Ti(IV)-Pt(II) complex trans-TiCl2(mu-L-3)(2)(kappa(1)-(LMeI)-Me-3)Pt(Me)I.