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

Abstract

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-(Bu^t₂CH)N(C₆H₄)PPh₂L¹ are described. This ligand was synthesised using a Schiff-base/Bu^tLi 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 η⁶:η⁶-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⋯π(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)₂6 was observed. Access to mixed-metal complexes was achieved using an early-metal-first approach. Reaction between the metalloligand TiCl₂(L¹)₂ and labile Group 10 and group 9 compounds resulted in the formation of TiCl₂(μ-L¹)₂PtCl₂8, TiCl₂(μ-L¹)₂PtMe₂9, TiCl₂(μ-L¹)₂PdCl₂10, TiCl₂(μ-L¹)₂NiBr₂11, and {TiCl₂(μ-L¹)₂RhCl(CO)}₂12. In the solid state, the Group 4/10 compounds 8, 9 and 10 adopt similar structures that exhibit both intramolecular Bu^t₂C–H⋯Cl–Ti hydrogen bonding and arylNP π-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 π-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 80 °C. Unlike with TiCl₂(L¹)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(L¹)₂ and TiCl(μ-L¹)₂PtCl₂. The reactivity of the metallocage TiCl(μ-L³)₃Pt was also investigated. While reduction reactions were unsuccessful, the metallocage reacted with CO to form the Ti–Pt carbonyl, TiCl(μ-L³)₃Pt(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(μ-L³)₃Pt with excess MeI resulted in the formation of the Ti(IV)–Pt(II) complex trans-TiCl₂(μ-L³)₂(κ¹-L³MeI)Pt(Me)I.