C–H Bond Activation and Isoprene Polymerization by Lutetium Alkylaluminate/gallate Complexes Bearing a Peripheral Boryl and a Bulky Hydrotris(pyrazolyl)borate Ligand

Abstract

The bimetallic lutetium alkylaluminate complex [(TptBu,Me)Lu(Me)(AlMe3{B(NDippCH)2})] {TptBu,Me = hydrotris(3‐Me‐5‐tBu‐pyrazolyl)borate} formed readily upon addition of 0.5 equivalents of the organoaluminum boryl compound [Me2Al{B(NDippCH)2}]2 (Dipp = C6H3iPr2‐2,6) to [(TptBu,Me)LuMe2] in n‐hexane solution. Thermally induced cyclometalative C–H bond activation of this compound led to the formation of [(Tp(tBu–H)₂/tBu,Me)Lu(AlMe2{B(NDippCH)2})] {Tp(tBu–H)₂/tBu,Me = hydro(3‐Me‐5‐tBu‐pyrazolyl)bis[µ‐(3‐Me‐5‐Me2CCH2‐pyrazolyl)]borate} in quantitative yields. The latter complex bears an unusual κ5 coordination mode of the activated tris(pyrazolyl)borate ligand. Additionally, one Lu–CH2(R) moiety is coordinated by a “neutral” [Me2Al{B(NDippCH)2}] unit, which can be separated off by addition of THF or trimethylaluminum. The gallium analogue of complex [(TptBu,Me)Lu(Me)(AlMe3{B(NDippCH)2})] could not be isolated from the respective reaction of monomeric [Me2Ga{B(NDippCH)2}] with [(TptBu,Me)LuMe2], but the C–H bond activated gallate complex [(Tp(tBu–H)₂/tBu,Me)Lu(GaMe2{B(NDippCH)2})] was obtained instead following a prolonged mild thermal treatment. The performance of the new lutetium complexes as precatalysts in the polymerization of isoprene was assessed and possible activation and deactivation scenarios were studied by NMR spectroscopy.