B Preparation of Poly l-pyrazolyl borates

The parent ligand system, [H B(pz 4 has been synthsized by heating K[BHJ in molten pyrazole. Through careful temperature control it is possible to stop the reaction at the di- or tri-substitution stage 

It should be noted that the reaction of K[BHJ with pyrazole cannot be stopped at the K[H3B(pz)] stage from an incomplete reaction only K[BHJ and K[H2B(pz)2] could be isolated. It has been possible, though, to synthesize the ion [H3B pz-3,5-(CHj) ] by reaction of the borane adduct of 3,5-dimethylpyrazole with sodium hydride, and from it by a carefully controlled reaction with pyrazole in N,N-dimethyl-acetamide to prepare the first example of an asymmetric poly(l-pyrazolyl)borate ligand, i.e., Na[H2B(pz) pz-3,5-(CH3)2 ] 

When K[BHJ is heated in an even more sterically hindered pyrazole, i.e., 3,5-bis(t-butyl)pyrazole, up to its boiling point, no reaction occurs at all. 

In some instances complexes were obtained derived from poly(l-pyrazolyl)borate ligands which never existed by themselves. This was the case with halogenation of the 4-position in HB[pz-3,5-(CH3)2]3Mo(CO)2NO to produce derivatives of the type HB[pz-3,5-(CH3)2-4-X]3Mo(NO)X2 and in HB[pz-3,5-(CH3y3Re(CO)3 which, on bromination, yielded the species HB[pz-3,5-(CH3)2-4-Br]3Re(CO)3 Another example is the formation of 20 by the reaction depicted in Eq. (7). 

Direct preparation of the ligand [HB(pz)2(pz-4-CN)] would have been impractical, due to the difficulties in separating the complex mixture of products which would have resulted, as the pyrazolyl groups are prone to a rambling under the experimental conditions 

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