Pyrazole‐Organoboranes, VII. Addition Complexes of Pyrazolylboranes

The monomeric 9‐pyrazolyl‐9‐borabicyclo[3.3.1]nonanes react in situ with either excess of 9H‐9‐BBN or of some pyrazoles to form the 2:1 or 1:2 adducts 1 – 5 or 7 – 11 , respectively. Spectroscopic data (NMR, IR) suggest and the solid state structure of 4 confirms the presence of a central heterocycle in complexes of the type 1 – 5 . In solutions of 7 – 11 NMR data at room temperature indicate a rapid exchange of the four nitrogen atoms of the two pyrazole rings at the boron atom of the 9‐BBN moiety. In the solid state (X‐ray structure of 8 ) intermolecular N–H ⃛N bonds lead to dimeric structures. In the almost insoluble adduct 7 a polymeric structure is suggested. Pyrazoles with large substituents, e. g. 3,5‐diphenylpyrazole ( P 2 Pz ) give addition complexes of the type 1 – 5 only in solution in equilibrium with the corresponding monomeric species ( 11 B NMR) and none of the type 7 – 11 .