Poly l-pyrazolyl borates

During the time of the Olin reports, the first examples of oligomeric boron-bridged (l-pyrazolyl)borate systems appeared from the laboratory of Trofimenko at DuPont Chemicals 24 He reported the synthesis of poly(l-pyrazolyl)borates (6) (Fig. 5) from the reactions of alkali metal borohydrides with the pyrazole ligand. The (l-pyrazolyl)borate ligand was obtained from two pyrazole units when bridged by a BR2 unit on one side and by a metal or onium ion on the other. Even though reports... 

Figure 5 The first reported poly(l-pyrazolyl)borate (6) containing a cobalt bridge. (Adapted from ref. 24.)...

Poly(l-pyrazolyl)borates are dealt with in Section 54.1.2.6. 

POLY(l-PYRAZOLYL)BORATES, THEIR TRANSITION -METAL COMPLEXES,... 

The filtrates may be saved for subsequent recovery of pyrazole and poly-(l-pyrazolyl)borates. 

B. POTASSIUM HYDROTRIS(1 -PYRAZOLYL)BORATE— ALKALI METAL POLY(l-PYRAZOLYL)BORATES... 

Poly l -pyrazolyl)borates, Their Transition-metal Complexes 105... 

Despite the multitude of well characterized boron compounds, the knowledge of azole derivatives of boron is still rather limited. For example, only three C-borylated pyrazoles are known. Although N-borylated pyrazole derivatives are considerably more abundant, until most recently these were restricted to compounds containing only four-coordinate boron. Of these, the poly(l-pyrazolyl)borate anions have been a bonanza for the coordination chemist, since the steric and electronic features of these ions render them as extremely useful (polydentate and chelating) ligands. However, recent studies of the chemistry of boron derivatives of pyrazoles have provided for some noteworthy developments and a survey of such compounds and their chemistry appears to be a timely subject. 

Historically, C-borylated pyrazoles were first described in 1962. A few years later, N-borylated species containing four-coordinate boron were prepared, i.e., the poly(l-pyrazolyl)borate anions and the (neutral) dimeric pyrazol-l-ylboranes = pyrazaboles. The first monomeric pyrazol-l-ylborane containing trigonal boron was reported in 1980. In addition, several cationic species in which two or more boron atoms or a boron and a carbon atom are bridged by pyrazolyl groups are known, but their chemistry has not yet been explored at all. 

This latter formulation may give access to metal derivatives in which such a di(l-pyrazolyl)borate unit rves as a chiral ligand, i.e., by employing a C-substituted pyrazole as donor molecule. There exists only one report in the literature on forming poly(l-pyrazolyl)borate ions containing two different pyrazole groups, which were obtained in quite laborious manner... 

The largest class of pyrazole derivatives of boron are the poly(l-pyrazolyl)borates. These are species of the general formula M[R B(pz% where M is a cation of effective charge m (as it may also contain other ligands), R is a substituent other than pz (usually H, alkyl or aryl), pz is a 1-pyrazolyl moiety which may contain various substituents at the carbon sites, and n can be 0, 1 or 2. 

It is understandable, therefore, that since its introduction in 1966 this ligand system has been explored to a considerable extent, and by 1984 poly(l-pyrazolyl)borate derivatives of most transition metals (except for Sc, Hf and Os) up to Np have been described and a few brief reviews of this subject matter have appeared Since... 

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 ]... 

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). 

Table 6. Poly(l-pyrazolyl)borates of Structure 21 with n = 2...

Numerous physical studies of poly(l-pyrazolyl)borate complexes have been carried out, some as part of structure determinations, others to study different phenomena inherent in the complexed transition metal. 

A large number of poly(l-pyrazolyl) borate complexes had their structure determined by X-ray crystallography these are listed in Table 8. It should be remembered, however, that in some instances the structure in solution will have a different denticity of the poly(l-pyrazolyl)borate ligand than that found in the crystal. 

Table 8. Listing of Poly(l-pyrazolyl)borate Complexes of Known Crystal Structures...

VI.C.6 Poly(l-pyrazolyl)borates Containing Other Donor Groups... 

While early work emphasized high symmetry of this ligand, the current trends may take advantage of asymmetric substitution to adapt the basic poly(l-pyrazolyl)borate framework to various enzyme-like tasks. This could be done by placing appropriate functionalities in such a manner that they will interact, as needed, with a transition metal (or metals). Achievement of these refinements of the ligand stem is likely to result from advances in boron chemistry, boron being here the pivotal element,... 

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