Pyrazolyl ligands

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. 

Based on the analysis of the reactions in Scheme 3 and on previous studies (46, 47), a mechanism for the reaction was proposed in which the /x-peroxo complex, 16, may simultaneously abstract two hydrogen atoms from iso-propyl groups on the pyrazolyl ligands. Alternatively, because of the weak 0—0 bond, 16 may homolytically dissociate to form two Tp"Co(0-) oxo-radical moieties, and these species would then abstract hydrogen from the iso-propyl groups. In either case, the resulting carbon-centered radical can either react with solvent, as was observed for the Tp complex (46), or with another carbon-centered radical so as to regenerate the Tp"Co(OH) complex and produce a derivative of the Tp" complex with an iso-propenyl substituent, 18. Ultimately, either route would produce the (/x-OH)2 complex, 17. 

Starting from alkynes, cyclopropenes were obtained under mild reaction conditions at room temperature (30-51 %). Aziridines were isolated in 40-90 % yield with catalyst 15 using [(p-tolylsulfonyl)imino]phenyliodinane. Phi = NTs. The first chiral tris(pyrazolyl) ligand was prepared as early as 1992 by Tolman and co-workers starting from a camphor-pyrazole derivative and... 

Pyrazolylpyrimidines (pzpm) also provide a method for detecting the Pd-N bond rupture (see equation 16). This has been extended to tri-substituted triazines, wherein Pd-N bond rupture can be detected, but which also allows the metal to pass around the entire ligand via a series of (1) metallotropic shifts involving retention of the triazene N-Pd bond and (2) pyrazolyl ligand assisted metal hurdling from one triazene N to another (see equation 17). The subject of metals shuttling... 

The trifunctional pyrazolyl ligands (61) afford a Ru11 complex, in which the coordinated metal is simultaneously involved in rf -bonding to the benzene ring and tripodal pyrazole coordination to a suitably placed ligand.88... 

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... 

Table 1.2. Selected parameters (A, deg) defining the flexibility of all RTp ligands in a data set of RB(pz)3 complexes with any group at positions 3,4 and 5 of the pyrazolyl ligands, including hydrogen (RTpx) or only hydrogen (RTp) (adapted from Ref. 38).

New pyrazolyl ligands containing an aminomethyl group, in position 4, have been coordinated to rhodium(I) and show a good solubility in water, presumably because the ammonium group does not interact with the metal center. The resulting complexes have been characterized and, in our opinion, they are good candidates for biphasic catalysis [82], such as l,2,4-triazol-2-ium-5-ylidene complexes of Rh(I), Ir(I), Ni(0), Ni(II), Pd(II), and Hg(II) [83]. Due to their ionic character, these carbene complexes are extremely soluble in water. 

On the other hand when 3,5-bis(trifluoromethyl)- was used instead of 3,5-dimethylpyrazole, compound 2 (Fig. 1), a derivative of the hydrogen-rich SiHiCh, could be isolated and characterized by X-ray analysis. The silicon atom is additionally stabilized by two tetrahydrofuran molecules to achieve an overall octahedral coordination, which is most probably due to the nature of the electron-withdrawing pyrazolyl ligands. 

Thus, in the dinuclear d -d iridium (III) complex, bridged by two pyrazolyl ligands, the intermetallic trans-influence of the terminal hydride ligand on the bridging hydride is crucial for decoordination of acetonitrile and activation of diphenylacetylene. The transmission of the trans-effect along the binuclear backbone results in the selectivity of the hydrogenation into cis-stilbene. 

Complexes of these pyrazolyl groups are typically prepared by reaction of the metal halide with the anionic pyrazolyl ligand. For example, bis-olefin complexes of group 9 metals, such as rhodium and iridivtm, are prepared from complexes of the general type [M(olefin)2Cl]2, in which M = Ir and Rh, and olefin = ethylene, COE, COD, norbomadiene, etc., upon the addition of Tp salts (Equation 4.46). Although a majority of Tp complexes are obtained from the reaction of a metal halide with a salt of the anionic Tp ligand, some Tp complexes have been synthesized by alternate routes. For example, Tp VO(acac)... 

FIGURE 25.12 Achiral tris(pyrazolyl)borate (Tp) metal catalysts used for LA polymerization (third pyrazolyl ligand represented as N-N for clarity). 

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