Pyrazolyl borate ligand

Recent advances in complexes of group VIB metals (Mo, W) with poly-(pyrazolyl)borate ligands 98MI2. 

Cobalt(III)-alkylperoxo complexes find use in the oxidation of hydrocarbons.1342,1343 Since they release ROO and RO radicals upon mild heating in solution, they are effective oxidants under mild conditions, and produce catalytic systems in the presence of excess ROOH. Aliphatic C—11 bond oxidation by ConOOR (R = Con, alkyl, H) complexes including a hydrotris(pyrazolyl) borate ligand have also been reported, with homolysis of the peroxo O—O bond believed to be important in oxygenation of the C—H bond.1344... 

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 highly fluorinated bis(pyrazolyl)borate ligand, dihydridobis(3,5-bis(trifluoromethyl)pyrazo-lyl)borate has been synthesized. The zinc triflate complex was prepared from the potassium salt of the ligand and exhibits distorted tetrahedral coordination which allows comparison of the amount of distortion, in relation to the electronic properties of substituents, with the methylated analog.165... 

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... 

During the time of the Olin reports, the first examples of oligomeric boron-bridged (l-pyrazolyl)borate systems appeared from the laboratory of Trofimenko at DuPont Chemicals 24 He reported the synthesis of poly(l-pyrazolyl)borates (6) (Fig. 5) from the reactions of alkali metal borohydrides with the pyrazole ligand. The (l-pyrazolyl)borate ligand was obtained from two pyrazole units when bridged by a BR2 unit on one side and by a metal or onium ion on the other. Even though reports... 

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... 

Although the poly(pyrazolyl)borate complexes of iron(II) have been well known for many years, [1] it is only recently that the complexes with the tris(l-pyrazolylmethane ligand, HC(pz)3, [45-48] have been studied in detail. It should be noted that poly(pyrazolyl)methane ligands, such as the tris(l-pyrazolylmethane ligand, are neutral, whereas the poly(pyrazolyl)bo-rate ligands, such as the tris(l-pyrazolyl)borate ligand, HB(pz)3", are monoanions. As a consequence, the metal(II) poly(pyrazolyl)methane complexes are dications and often have quite different properties from those of the analogous metal(II) poly(pyrazolyl)borate molecular complexes. But, in spite of these differences there are often very close structural similarities between the dicationic complexes and the neutral complexes. Therefore the study of the pyrazolylmethane complexes will parallel that of the borate complexes discussed above. 

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)... 

Rhenium(V) 4-tolylimido complexes with the hydrotris(pyrazolyl)borate ligand, HB(pz)3, are formed on heating the 0x0 derivatives [ReOX2 HB(pz)3 ] (X = Cl, I) with 4-toluidine, whereas reactions with amines at ambient temperatures only give 0x0 complexes of the composition [ReO(NHR)Cl HB(pz)3 ]. The [Re(NC6H4-4-Me)X2 HB(pz)3] complexes react with ZnEt2 with formation of imido-ethyl complexes of the composition [Re(NC6H4-4-Me)Cl(Et) HB(pz)3 ]. " ... 

Hydridotris(3,5-dimethyl-l-pyrazolyl)borate]molybdenum-(i72-acyl) complexes, such as 1, are deprotonated by butyllithium or potassium hydride to generate enolate species, such as 488.8> jjie overa]] structure of these chiral complexes is similar to that of the iron and rhenium complexes discussed earlier the hydridotris(3,5-dimethyl-l-pyrazolyl)borate ligand is iso valent to the cyclopentadienyl ligand, occupying three metal coordination sites. However, several important differences must be taken into account when a detailed examination of the stereochemical outcome of deprotonation-alkylation processes is undertaken. 

Alkylation of enolates, such as 4, produces products that are consistent with the preferred approach of the electrophile from either the least hindered face of an T -cnolate of conformation C or the least hindered face of a Z-enolate of conformation D88. Steric factors influencing approach of the electrophile appear to be similar in both of these models since the steric bulk of the hydridotris(3,5-dimethyl-l-pyrazolyl)borate ligand and the phosphite are both considerable any stereoelectronic and dipolar factors due to interaction of the enolate ligand with the carbon monoxide ligand would likely be similar for both geometries. The is-enolate geometry C appears to benefit from reduced steric interactions between the R substituent and the metal ligands. 

The related bis(pyrazolyl)borate ligand [L5]- forms 10-coordinate complexes of the type [Ln(L5)2(N03)]. The terbium complex in particular has promising luminescence characteristics, with a lifetime of 2.6 ms and quantum yield of 0.36 in methanol (49). However, the complexes are not stable in water. 

The tris(N-(3-tm-butyl)pyrazolyl)methane ligand, with an extremely bulky 3-substituent, favors the formation of tetrahedral complexes2 in much the same way as observed in the chemistry of the hydrotris(3-tert-butyl-pyrazolyl)borate ligand.1... 

In contrast to nickel, simple homoleptic Tpx or Bpx, and heteroleptic pyrazolylborate-halide complexes of palladium and platinum are largely unexplored. Indeed, while for 3d metals these are "standard" targets for each successive generation of poly(pyrazolyl)borate ligand, for the... 

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