Poly(pyrazolyl)hydroborato and poly(pyrazolyl)methane aluminium alkyl derivatives

Bis- and tris(pyrazolyl)hydroborato aluminium methyl derivatives, {η3-HB(3,5- Me2pyz)3}Al(CH3)2, {η2 -HB(3-Butpyz)3}Al(CH3)2 and {η2-H2B(3-Butpyz)2}Al(CH3)2 have been prepared by metathesis of Al(CH3)3 with either K{HB(3,5-Me2pyz)3}, Tl{HB(3- Butpyz)3} or K{H2B(3-Butpyz)2}, respectively. In contrast to the formation of these 1 : 1 complexes, the isostructural but neutral ligand HC(pyz)3 readily coordinates three equivalents of (CH3)3Al giving {HC(pyz)3}[Al(CH3)3]3. 1H NMR studies provide evidence for the formation of the mono- and bis-complexes {HC(pyz)3}[Al(CH3)3] and {HC(pyz)3} [Al(CH3)3]2 as products of the reaction of HC(pyz)3 with less than three equivalents of Al(CH3)3. The aluminium-carbon bonds of {η3 -HB(3,5-Me2pyz)3}Al(CH3)2 are readily cleaved by H2O to give {η3-HB(3,5-Me2pyz)3}Al(OH)2 and CH4. However, the reaction of {η3-HB(3,5-Me2pyz)3}Al(CH3)2 with D2O gives CH4 in addition to CH3D and thus demonstrates that a secondary process involving coupling of the aluminium-methyl ligand with one of the hydrogen atoms of the pyrazolyl methyl group must be competitive with the direct reaction of the aluminium-carbon bond with D2O. The complex {η2-H2B(3- Butpyz)2}Al(CH3)2 undergoes a unimolecular rearrangement of the bis(3-t-butylpyrazolyl)hydroborato ligand to give the more stable isomer, {η2 -H2B(3-Butpyz)(5-Butpyz)} Al(CH3)2. The kinetics of this rearrangement are first-order with the activation parameters Δh‡ = 34.5(8) kcal mol−1 and Δs‡ = 6(2) e.u. and the proposed mechanism consists of steps involving a 1,2-shift of the t-butylpyrazolyl group.