Tris-pyrazolyl borate ligand

Properties of nickel poly(pyrazol-l-yl)borate complexes such as solubility, coordination geometry, etc., can be controlled by appropriate substituent groups on the pyrazol rings, in particular in the 3- and 5-positions. Typical complexes are those of octahedral C symmetry (192)°02-604 and tetrahedral species (193). In the former case, two different tris(pyrazolyl)borate ligands may be involved to give heteroleptic compounds.602,603 Substituents in the 5-position mainly provide protection of the BH group. Only few representative examples are discussed here. 

During the course of modeling copper dioxygen chemistry, Kitajima et al. reported the synthesis of a yu-peroxo dinuclear complex with a 3,5-dimethyl-substituted tris(pyrazolyl)borate ligand, which showed remarkable physicochemical similarities to oxy-Hc and oxy-Tyr. Using a 3,5-di-isopropyl-substituted terminal ligand, they provided the first structural proof of the existence of peroxo dicopper(II) core (108) (copper geometry distorted square pyrami-... 

Using a fluorinated tris(pyrazolyl)borate ligand, Gorun and co-workers126 synthesized a complex (117) (t = 0.06). This group very successfully isolated and structurally characterized a dinuclear oxygenated complex, revealing that the crystal contains about 80% peroxo... 

A large body of work with particular reference to the mimicry of mononuclear zinc enzymes has utilized tris(pyrazolyl)borate ligands. This ligand class offers a facial coordination mode of three pyrazole A-donors and can be functionalized in the three and five positions on the rings to increase steric bulk and vary the electronic properties of the ligand. The synthesis and complex formation with these ligands has been extensively reviewed.2,21 219... 

Pyridyl functionalized tris(pyrazolyl)borate ligands show some interesting properties including the formation of polynuclear zinc complexes.23,1 Some of these contain extensive H bonding and have potential as models for multinuclear zinc enzymes such as phospholipase C or PI nuclease.235 A bis-ligand complex of the hydrotris(5-methyl-3-(3-pyridyl)pyrazolyl)borate ligand (23) shows octahedral coordination of all six pyrazole nitrogen donors despite the steric bulk. 

The zinc acetate complex of tris(3-/-butyl-5-methylpyrazol-l-yl)borate was prepared as a structural model for carbonic anhydrase and comparison with the enzyme active site structures confirmed that the complexes are excellent structural models.239 A mononuclear zinc hydroxide complex can also be formed with the tris(pyrazolyl) borate ligand system as a structural model for carbonic anhydrase.240... 

A structurally characterized example of a dinuclear zinc complex with a bridging phosphate monoester was provided by Kitajima and co-workers using the tris(pyrazolyl)borate ligand system. The P—O bond in a tris- or bis-phosphate ester is cleaved by a hydroxo zinc complex resulting in the monoester compound.443... 

A Cd2 complex with one of the tris(pyrazolyl)borate ligands mentioned in Section 6.9.4.2.3(ii) has been identified in solution by its large coupling constant 1/(113Cd,111Cd) of 20.626 kHz also a Cd11 hydride complex has been detected by NMR.410... 

Using a similar strategy, Dias and coworkers were able to prepare classical AgI-CO and AgJ-C2H4 complexes supported by the mesityl-substituted tris(pyrazolyl)borate ligand L9 (Fig. 11) (102). The isolation of such complexes is a challenging task because silver(I) is a poor u-acceptor, a poor 7t-donor, and in... 

Recently, a somewhat different synthetic approach has been reported. Halcrow et al. (215) synthesized a series of five-coordinate copper(II) complexes comprising a tridentate tris(pyrazolyl)borate ligand and a bidentate phenol derivative. Neutral complexes [Cun(TpPh)(bidentate phenolate)] were synthesized and structurally characterized [Tpph] = hydrido-tris(3-phenylpyrazol-l-yl)borate. The species [Cun(TpPh)(2-hydroxy-5-methyl-3-methylsulfanylbenzaldehydato)] can electro-chemically be converted to the (phenoxyl)copper(II) monocation, which has been characterized in solution by UV-vis spectroscopy. It displays two intense absorption maxima at 907 nm (e = 1.2 x 103 M 1 cm-1), and 1037 (1.1 x 103 M l cm-1), resembling in this respect the radical cofactor in GO (Fig. 7). 

A dimethylaluminum complex [HB(3,5-(CF3)2Pz)3]AlMe2 (Figure 22) of the highly fluorinated tris(pyrazolyl)borate ligand [HB(3,5-(CF3)2Pz3] was obtained from the reaction between the silver adduct [HB(3,5-(GF3)2Pz)3]Ag(THF)... 

A side-on p,-Tq2 Tq2-peroxo dicopper(II) complex. A very important development in copper-dioxygen chemistry occurred in 1989 with the report by Kitajima et al. [10,108] that another Cu202 species could be prepared and structurally characterized by using copper complexes with a substituted anionic tris(pyrazolyl)borate ligand. This intensely purple compound, Cu[HB(3,5-iPr2pz)3] 2(02) (5), was prepared either by reaction of Cu[HB(3,5-iPr2pz)3] (4) with 02 or by careful addition of aqueous hydrogen peroxide to the p-dihydroxo... 

Keywords Bioorganometallic chemistry Medicinal organometallic chemistry Metal carbonyls Metallocenes N-heterocyclic carbenes Peptides Peptide Bioconjugates Peptide synthesis Solid phase synthesis Tris(pyrazolyl)borate ligands... 

Some of the first catalytic model systems for the simulation of the function of methane monooxygenase comprise monomeric as well as dimeric iron-containing model complexes bearing hydro-tris(pyrazolyl)borate ligands [6]. These complexes, e.g. 3, catalyze the oxidation of aromatic and aliphatic carbon-hydrogen bonds in the presence of oxygen (1 atm), acetic acid and zinc powder at room temperature (Scheme 2). 

Substituted tris(pyrazolyl)borates were recently employed as silver(l) ligands due to their ability to yield heat and air stable metal complexes. Two research groups have prepared very similar sUver(l)-tris(pyrazolyl)borate compounds for catalyzing carbene insertion chemistry. Both systems require electron-withdrawing groups on the tris(pyrazolyl)borate ligands to stabilize the sUver(l) complexes and enhance the electrophilicity of the putative silver-carbenoids (129,130). Dias et al. (129) found that, with CF3 substitutes, a complex of [HB(3,5-(Cp3)2pz)3] Ag(thf) could activate ethyl diazoacetate (EDA) to react with different C X (X = Cl, Br, 1) bonds to afford either C—X bond insertion products or 1,1-hydrochlorination products. At the time of this writing, this unique reactivity was reported only with a silver catalyst (Fig. 27). 

R = Me, Et, -Bu, CH2SiMc3, Ph). They feature short Cr-Cr distances ( 2.26A), low effective magnetic moments, and attenuated reactivity, all consistent with strong metal-metal bonding. Steric saturation of chromium with a sterically encumbered tris(pyrazolyl)borate ligand (i. e. =... 

In contrast, the substituted tris(pyrazolyl)borate ligand allows for the formation of the six-coordinate complex cation (13) (equation 16). The 3,5-dimethylpyrazolate analog has... 

A recent report has shown that tris(pyrazolyl)borate (Tp) (see Tris(pyrazolyl)borates) complexes of Ni tend to form 5-coordinate, formally 18-electron complexes. The hapticity of the tris(pyrazolyl)borate ligand in these complexes appears to be more sensitive to the steric environment around the Ni center as opposed to the electronic factors (i.e. formal electron count of the complex). Thus, the -allyl derivative of tris (3,5-i-Pr2-pyrazolyl)borate NiR (3) displays a -coordination, whereas the -prenyl derivative (4) adopts a /c -coordination... 

For the nomenclature of these ligands (Scheme 2) we adopt a scheme derived from that of the tris(pyrazolyl)borate ligands. We call the bis(thioimidazolyl)hydroborate RBmx and the tris(thio-midazolyl)hydroborate RTmx with x naming the substituent at the 1-positions of the imidazole rings and R the substituent at boron atom. 

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