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

Abstract

Bis- and tris(pyrazolyl)hydroborato aluminium methyl derivatives, {η 3-HB(3,5- Me 2pyz) 3}Al(CH 3) 2, {η 2 -HB(3-Bu tpyz) 3}Al(CH 3) 2 and {η 2-H 2B(3-Bu tpyz) 2}Al(CH 3) 2 have been prepared by metathesis of Al(CH 3) 3 with either K{HB(3,5-Me 2pyz) 3}, Tl{HB(3- Bu tpyz) 3} or K{H 2B(3-Bu tpyz) 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 (CH 3) 3Al giving {HC(pyz) 3}[Al(CH 3) 3] 3. 1H NMR studies provide evidence for the formation of the mono- and bis-complexes {HC(pyz) 3}[Al(CH 3) 3] and {HC(pyz) 3} [Al(CH 3) 3] 2 as products of the reaction of HC(pyz) 3 with less than three equivalents of Al(CH 3) 3. The aluminium-carbon bonds of {η 3 -HB(3,5-Me 2pyz) 3}Al(CH 3) 2 are readily cleaved by H 2O to give {η 3-HB(3,5-Me 2pyz) 3}Al(OH) 2 and CH 4. However, the reaction of {η 3-HB(3,5-Me 2pyz) 3}Al(CH 3) 2 with D 2O gives CH 4 in addition to CH 3D 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 D 2O. The complex {η 2-H 2B(3- Bu tpyz) 2}Al(CH 3) 2 undergoes a unimolecular rearrangement of the bis(3-t-butylpyrazolyl)hydroborato ligand to give the more stable isomer, {η 2 -H 2B(3-Bu tpyz)(5-Bu tpyz)} Al(CH 3) 2. The kinetics of this rearrangement are first-order with the activation parameters Δ h ‡ = 34.5(8) kcal mol −1 and Δ s ‡ = 6(2) e.u. and the proposed mechanism consists of steps involving a 1,2-shift of the t-butylpyrazolyl group.