Pyrazolyl, ligand structure

A series of unsymmetric bis(pyrazolylate) ligands have been synthesized and complexed with zinc via metathesis of a thallium derivative, (17) and (18). This allowed the synthesis and complexation of NNS and NNO pyrazolylborate derivatives.1 9,160 Parkin and co-workers synthesized zinc complexes of a series of unsymmetric pyrazole-containing ligands.161-163 The asymmetric bis(pyrazolyl)hydroborato zinc complex has been structurally characterized,164 and will also be discussed in the context of its interaction with the B H hydrogen in Section 6.8.9.2. 

B H bonds can also participate in similar interactions, as observed in the crystal structure of bis bis(3-rer -butyl-5-isopropylpyrazol)hydroborato -cobalt(n) complex, where two nitrogen atoms of each pyrazolyl ligand bind the metal in a square planar arrangement and the two B H units approach axially (Co- -H distance 1.95 A). The involvement of the B-H bond is also reflected in... 

Analogously, pyrazolyl-aluminate and -indate ligands have been prepared <75JCS(D)749) and their chelating properties evaluated with cobalt, nickel, copper and zinc. Gallyl derivatives of pyrazoles and indazoles have been extensively studied by Storr and Trotter e.g. 75CJC2944) who determined several X-ray structures of these compounds. These derivatives exist in the solid state as dimers, such as (212) and (288). A NMR study in acetone solution showed the existence of a slow equilibrium between the dimer (212) and two identical tautomers (289) and (290) (Section 4.04.1.5.1) (81JOM(215)157). 

In an attempt to change the electronics of the chromium atom, we are replacing the carbon based cyclopentadienyl ring with ligands containing harder donor atoms. For example, we have employed the tris(pyrazolyl)borate moiety, an isoclectronic replacement for Cp featuring tridentate N-coordination.[9] Figure 2 shows the molecular structure of Tp SU Cr-Ph, a representative Cr° alkyl. It will be noted, that this complex is mononuclear, due to the steric protection of the extremely bulky tris(pyrazolyl)borate. 

II. Syntheses, Structures, and Steric Properties of Poly(pyrazolyl)hydroborato Ligands... 

Another illustration of the structural changes that may result as a consequence of alkyl substitution at the 3-position of the pyrazolyl group is provided by the structures of the dimeric copper(I) complexes [Tp]Cu 2 (36), [TpMe2]Cu 2 (36), [TpPh2]Cu 2 (37), and [TpBut]Cu 2 (37), which differ in the manner in which the tris(pyrazo-lyDhydroborato ligand bridges the two copper centers (Fig. 16). 

The structure of [pzTp]2GaMe, determined by x-ray diffraction, demonstrates that the [pzTp] ligands adopt both bidentate and tridentate coordination. However, in solution, the complexes [Tp]2GaMe and [pzTp]2GaMe are fluxional on the NMR time scale at room temperature, exhibiting a single set of resonances for the pyrazolyl groups. 

Finally, the tris(pyrazolyl)hydroborato ligand system has also provided an interesting example in which a crystallographic site is disordered between a vacancy and a chain of three atoms. Thus, the x-ray structure of the cobalt complex [TpAnt]CoNCS (Ant = 9-anthryl) revealed the presence of the cocrystallized thallium derivative... 

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