Bidentate pyrazolyl-based ligands

Bidentate pyrazolyl-based ligands have shown to be versatile building blocks for the synthesis of a plethora of coordination assemblies. Initial work by McCleverty and Ward in the late 1990s showed that the synthesis of cages [Co4(L )6CX](X)7 (where L and = bidentate pyrazolyl-pyridine ligands ... 

More recently, Ward has explored the effect that the length of bidentate pyrazolyl-based ligands has on the formation of the metallo-cages [34], When... 

Fig. 5 A selection of bidentate pyrazolyl-based ligands employed for the synthesis of metaUo assemblies...

The 12-dentate compartmental ligand hexakis [3-(2-pyridyl)-pyrazol]-lyl diboran(IV)ate (BisTpPy) has two hexadentate tris (pyrazolyl)borate-based cavities, linked back-to-back by a B-B bond, which have been reported.238 [Ln(BisTpPy)(NC>3)2]2 (Ln = La or Gd) is dinuclear with each metal center being 10-coordinate from a tripodal hexadentate ligand cavity and two bidentate nitrates. 

Catalyst initiation and overall catalytic activity are not always opposing effects. For example, complexes that contain chelating ligands, such as bidentate phosphines [90], Schiff bases [91], and tris(pyrazolyl)borates [92], generally show both low rates of initiation and low overall olefin metathesis activity (Fig. 4.34). Both effects appear to result from slow rates of ligand dissociation from the starting complexes, which leads to low concentrations of the catalytically active 14-electron species in solution. 

McCleverty, Ward and co-workers have characterized a [Cu2L2(OAc)2] " helicate based on square-pyramidal Cu. " One bidentate pyridyl/pyrazolyl chelate from each ligand and one monodentate terminal acetate ligand complete the copper coordination sphere. 

Structurally related species that exhibit C-H activation include the bis(pyrazolyl) borate complex in Scheme 21, for which (as in the above dmdpb system) protonation (or methide abstraction) generates an intermediate that reacts readily with benzene [74] the bis(azaindolyl)methane complex in Scheme 22, which activates both aromatic and benzylic C-H bonds [75,76] (some stable Pt(TV) complexes based on the same architectiRe have also been isolated and shown to undergo reductive elimination of MeX [77]) and complexes based on anionic bidentate ligands such as 2-(2 -pyridyl)indolide [78]. Intramolecular C-H activation was observed for one example of a series of A -heterocyclic carbene complexes of Pt(II) distortions induced by steric crowding appear to influence reactivity strongly [79]. 

The Perez group has also synthesized a range of d transition metal-derived anion receptors based on rhenium and ruthenium, for example, complexes 137 and 138. Compound 137 contains a bidentate pyrazolylamidino ligand which provides one NH hydrogen-bond donor and an additional pyrazolyl NH donor. The complex binds chloride particularly strongly because of its rigid nature, with... 

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