Synthesis and structural characterization of tris(pyrazolyl)hydroaluminate and tris(pyrazolyl)hydrogallate lithium compounds

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The tris(pyrazolyl)hydroaluminate derivatives, [HAlTpBut,Me]Li and {[HAlTpMe2]Li}2, have been synthesized via the reactions of LiAlH4 with 3-tert-butyl-5-methylpyrazole and 3,5-dimethylpyrazole, respectively, while the tris(pyrazolyl)hydrogallate counterparts, [HGaTpBut,Me]Li and {[HGaTpMe2]Li}2, are obtained from the corresponding reactions with LiGaH4. X-ray diffraction studies demonstrate that the 3-tert-butyl-5-methylpyrazolyl derivatives, [HAlTpBut,Me]Li and [HGaTpBut,Me]Li, are monomeric with lithium in a trigonal pyramidal coordination environment, whereas the less sterically demanding 3,5-dimethylpyrazolyl derivatives, {[HAlTpMe2]Li}2 and {[HGaTpMe2]Li}2, are dimeric. In addition to the tris(pyrazolyl)hydroaluminate derivative, [HAlTpBut,Me]Li, the tetrakis derivative, [(pzBut,Me)AlTpBut,Me]Li, has also been isolated. X-ray diffraction studies demonstrate that [(pzBut,Me)AlTpBut,Me]Li, as obtained from solutions in benzene, exists as a monomer in the solid state with tridentate coordination, whereas crystals obtained from THF exhibit a bidentate coordination mode, which is supplemented by coordination of THF, i.e.[(pzBut,Me)2AlBpBut,Me]Li(THF). NMR spectroscopic studies, however, indicate that [(pzBut,Me)AlTpBut,Me]Li is fluxional in both benzene and THF solutions.