Application of molybdenum bis(imido) complexes in ethylene dimerisation catalysis

Abstract

In combination with EtAlCl₂ (Mo : Al = 1 : 15) the imido complexes [MoCl₂(NR)(NR′)(dme)] (R = R′ = 2,6-Prⁱ₂–C₆H₃ (1); R = 2,6-Prⁱ₂–C₆H₃, R′ = Bu^t (3); R = R′ = Bu^t (4); dme = 1,2-dimethoxyethane) and [Mo(NHBu^t)₂(NR)₂] (R = 2,6-Prⁱ₂–C₆H₃ (5); R = Bu^t (6)) each show moderate TON, activity, and selectivity for the catalytic dimerisation of ethylene, which is influenced by the nature of the imido substituents. In contrast, the productivity of [MoCl₂(NPh)₂(dme)] (2) is low and polymerisation is favoured over dimerisation. Catalysis initiated by complexes 1–4 in combination with MeAlCl₂ (Mo : Al = 1 : 15) exhibits a significantly lower productivity. Reaction of complex 5 with EtAlCl₂ (2 equiv.) gives rise to a mixture of products, while addition of MeAlCl₂ affords [MoMe₂(N–2,6-Prⁱ₂–C₆H₃)₂]. Treatment of 6 with RAlCl₂ (2 equiv.) (R = Me, Et) yields [Mo({μ-N–Bu^t}AlCl₂)₂] (7) in both cases. Imido derivatives 1 and 3 react with Me₃Al and MeAlCl₂ to form the bimetallic complexes [MoMe₂(N{R}AlMe₂{μ-Cl})(NR′)] (R = R′ = 2,6-Prⁱ₂–C₆H₃ (8); R = 2,6-Prⁱ₂–C₆H₃, R′ = Bu^t (10)) and [MoMe₂(N{R}AlCl₂{μ-Cl})(NR′)] (R = R′ = 2,6-Prⁱ₂–C₆H₃ (9); R = 2,6-Prⁱ₂–C₆H₃, R′ = Bu^t (11)), respectively. Exposure of complex 8 to five equivalents of thf or PMe₃ affords the adducts [MoMe₂(N–2,6-Prⁱ₂–C₆H₃)₂(L)] (L = thf (12); L = PMe₃ (13)), while reaction with NEt₃ (5 equiv.) yields [MoMe₂(N–2,6-Prⁱ₂–C₆H₃)₂]. The molecular structures of complexes 5, 9 and 11 have been determined.