Tris-pyrazolylborate ligands

The family of poly(pyrazol-l-yl)borates has been widely used as supporting ligands in nickel coordination chemistry.556,557 Complex (191) is an example, where unusual cysteine coordination is achieved at a tris(pyrazolylborate)nickel(II) template.601 (191) undergoes rapid reaction with molecular oxygen to presumably form a sulfinate. 

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... 

Both ligand classes are modelled on long established ligands, the pincer ligands with amino, phosphane or ether sidechains on a pyridine or benzene base for the mer geometry [64-66] and the scorpionate ligands (tris-pyrazolylborates) for the fac geometry [67-69]. 

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.)...

Polyhydrides are complexes such as FeH4(PR3)3, with a H M ratio exceeding 3. Hydrogen is not as electronegative as carbon, and so the metal in a polyhydride is not as oxidized as in a polyalkyl. Polyhydrides therefore retain more of the properties of low-valent complexes than do polyalkyls. For example, many of them are 18e, and relatively soft ligands (in the vast majority of cases a phosphine or a cyclopentadienyl) are required to stabilize them. Rare examples of N-donor-stabilized polyhydrides are [TpReH ] and [BpReH7] (Tp = tris-pyrazolylborate (5J7) Bp = bis-pyrazolylmethane). ... 

In contrast, the p-diketiminato-stabilized calcium amide [ (ArNCMe)2CH Ca N(SiMe3)2 (THF)] and a tris(pyrazolylborate)-stabilized analogue were reported by Chisholm and coworkers as precatalysts for the ring-opening polymerization of rac-lactide to form heterotactic polylactide [25, 26], while substituted fluorenyl ligands have been reported by Harder and coworkers for the stereocontrolled polymerization of styrene to yield highly syndiotactic polystryene [23]. 

Recent developments include synthesis of optically active trispyrazolylborates118 and both bis and tris ligands with CF3 groups,119 e.g., [HB(3,5(CF3)2)pz3] . It may be noted that main group elements such as tin can form pyrazolylborates.120... 

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