Pyrazole organoboranes. VII. Addition complexes of pyrazolylboranes.
Slow addn. of a toluene soln. of 1 equiv of pyrazoles to a soln. of (9H-9-BBN)2 gave adducts I (R = H, R1 = H, Me, Ph; R = R1 = Me; R = Ph, R1 = Me, Ph). Reversing the mode of addn. of the reagents and choosing a 1:2 ratio of (9H-9-BBN)2 to pyrazole afforded adducts II (R = H, R1 = H, Me, Ph; R = R1 = Me; R = Ph, R1 = Me). Spectroscopic data (NMR, IR) suggest and the solid state structure of I (R = R1 = Me) confirms the presence of a central B-N heterocycle in complexes I. In solns. of II, NMR data at room temp. indicate a rapid exchange of the 4 N atoms of the two pyrazole rings at the B atom of the 9-BBN moiety. An x-ray crystal structure of II (R = H, R1 = Me) indicates intermol. N-H...N bonds lead to dimeric structures. A polymeric structure for II (R = R1 = H) is suggested. Pyrazoles with large substituents, e.g. 3,5-diphenylpyrazole, give addn. complexes of type I only in soln. in equil. with a monomeric species, and none of the type II.
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