Pyrazolylborate ligands

These versatile ligands have also shown applications in catalysis and more recently the formation of a variety of related ligands has led to varied donor sets and cavity properties from pyrazolylborate ligands. As this important area has been recently and extensively reviewed only a few representative examples will be discussed. Other examples appear elsewhere in relation to zinc coordination of other donors in the remaining coordination site or sites. A large volume of work has been produced recently in this area, particularly by the research groups of Parkin and V ahrenkamp.181,220-228... 

Wagner and co-workers have reported the synthesis of ferrocene-bridged tris(l-pyrazolylborate) oligomeric systems (80) (Fig. 49).62 Such polymers are of interest, as they offer a multiplicity of options for tailoring the access space to a metal held by the tris(l-pyrazolylborate) ligand, as well as a means to electronically influence the environment of a transition metal.113,114... 

An extensive series of compounds of formula Mo(pz)(NO)(X)(Y) have been synthesized and characterized, where pz = a substituted tridentate pyrazolylborate ligand and X, Y = various combinations of halides, SR, OR and NHR.201,202 Here again the values of v(NO) may be monitored... 

Figure 6.4. Pyrazolylborate ligands used for silver-based intermolecular C-H amination.

While the transition metal chemistry of the pyrazolylborate ligands, [H cB(pz)4 c] (v = 0 - 2, pz = pyrazolyl) has been extensively studied, it is only recently that the first gallium compounds have been reported. Reaction of GaCb with the bis(pyrazolyl)borato ligands (equation 15) yields a five-coordinate complex (Figure 19). ... 

The most stable type of compounds with Pb N bonds are those with pyrazolylborate ligands. This type of complex... 

Figure 6.27 The crystal structure of Sm(Tp )2(OH) where is tris(dimethylpyrazolyl)borate, a sterically cumbersome ligand [100]. (Redrawn from N. Marques, A. Sella and J. Takats, Chemistry of the lanthanides using pyrazolylborate ligands, Chemical Reviews, 102, 2137-2160, 2002.)...

A titanium complex with a pyrazolylborate ligand was studied by Campbell and Malanga [19]. Similarities between the cyclopentadienyl ligand and the hydridotris(pyrazolyl)borate ligand have been noted for transition metal complexes. Catalyst efficiencies are much lower than those of the analogous penta-methylcyclopentadienyl complexes. 

Synthesis and structural chemistry of non-cyclopentadienyl organolanthanide complexes have been reviewed by Edelmann and Schumann et /.13,13a 24 Marques et al. have published a review on the chemistry of the lanthanide using pyrazolylborate ligands.36 Theoretical investigations on lanthanide alkyls have been published by Eisenstein et al. under the title Lanthanide complexes electronic structure and H—H, C-H, and Si-H bond activation from a DFT perspective. ... 

Recently. Alvarado et al. reported a related set of active catalysts for quinoline hydrogenation, formed in situ by addition of pyrazolylborate ligands Tp, hydrotris(pyrazolyl)borate and Tp. hydrotris(3,5-dimethylpyrazolyl)borate -which are isolectronic with cyclopentadienyl ligands- to [M(COD)C1]2 (M = Rh, Ir, COD = 1,5-cyclooctadiene), [Ir(COE)Cll2 (COE = cyclooctenc), and RuCl2(NCMe)4. Again, rhodium turned out to be the most efficient metal, and the activity was related with the ease of formation of Tp or Tp complexes. No mechanistic details were provided [101]. 

Several papers appeared in literature which report on the mechanism of action of LADH, showing examples for the formation of alkokides140-142 and hydride transfer.143 Zinc enzyme modeling with zinc complexes containing polar pyrazolylborate ligands have been reported by Vahrenkamp and coworkers.144... 

Although the pyrazolylborate ligands have been around for 40 years, there is still more to explore including ligands that can be tridentate to polydentate, as the third generation scorpionates, recently reported by Reger,1,2 which can find application in the advanced materials field. 

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