Poly borate ligands

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

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 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... 

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. 

Examination of the backbones of the neopentane-based, carbon-anchored TIME and bulky TIME tripodal ligands reveals possible reasons for the lack of 1 1 complex formation with metal centers. A hypothetical complex of TIME would contain three eight-membered rings (Fig. 9, I) with much lower stability than the more commonly observed five, six, or seven-membered rings formed, for instance, by poly(p5irazolyl)borate or poly(phosphine) ligands. In order... 

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... 

A series of complexes (Chart 5) have been reported that comprise both a poly(pyrazolyl)borate ligand and an intramolecularly chelating, monoanionic coligand. These are typically obtained via halide displacement and dimer cleavage by the respective sodium or potassium poly(pyrazo-lyl)borate salt, a means by which 462—466,45 467—469,143 and 470 479144 145 were each prepared in good yield, while the related poly (pyrazolyl)methane complexes 480+— 483+ were isolated as their perchlorate salts in the presence of NaClO 145... 

It should be noted that the reaction of K[BHJ with pyrazole cannot be stopped at the K[H3B(pz)] stage from an incomplete reaction only K[BHJ and K[H2B(pz)2] could be isolated. It has been possible, though, to synthesize the ion [H3B pz-3,5-(CHj) ] by reaction of the borane adduct of 3,5-dimethylpyrazole with sodium hydride, and from it by a carefully controlled reaction with pyrazole in N,N-dimethyl-acetamide to prepare the first example of an asymmetric poly(l-pyrazolyl)borate ligand, i.e., Na[H2B(pz) pz-3,5-(CH3)2 ]... 

In some instances complexes were obtained derived from poly(l-pyrazolyl)borate ligands which never existed by themselves. This was the case with halogenation of the 4-position in HB[pz-3,5-(CH3)2]3Mo(CO)2NO to produce derivatives of the type HB[pz-3,5-(CH3)2-4-X]3Mo(NO)X2 and in HB[pz-3,5-(CH3y3Re(CO)3 which, on bromination, yielded the species HB[pz-3,5-(CH3)2-4-Br]3Re(CO)3 Another example is the formation of 20 by the reaction depicted in Eq. (7). 

A large number of poly(l-pyrazolyl) borate complexes had their structure determined by X-ray crystallography these are listed in Table 8. It should be remembered, however, that in some instances the structure in solution will have a different denticity of the poly(l-pyrazolyl)borate ligand than that found in the crystal. 

Beeby, A., Burton-Rye, B.P., Faulkner, S., et al. (2002) Synthesis and near-lR luminescence properties of neodymium(III) and ytterbium(III) complexes with poly(pyrazolyl)borate ligands. Journal of the Chemical Society, Dalton Transactions, 1923. 

Attempts to introduce a larger poly(pyrazolyl)borate ligand have demonstrated the steric limits of this system. Reaction of UCI4 with one equivalent of the thallium salt of [HB(3-Mspz)3] (Ms = mesityl) generates only the product containing an isomerized ligand, UCl3[[HB(3-Mspz)2(5-Mspz)]. ... 

A tris(imidazolyl)borate (138) complex of thallium(I) has been synthesized.The solid-state structure of hydrotris(imidazolyl)boratothallium(I) consists of one-dimensional, twisted, ladderlike strands, and three-coordinate thallium centers. Due to the position of the nitrogen donors, the tris(imidazolyl)borate ligand is not capable of forming metal chelates as are observed in tris(pyrazolyl)borates. Poly(benzotriazolyl)borate ligands have some features of both tris(pyrazo-lyl)borate and tris(imidazolyl)borate systems. Thallium(I) complexes of bis-, tris-, and tetrakis (benzotriazolyl)borates are reported. These adducts have been synthesized by treating the corresponding potassium derivative with an equimolar quantity of thallium(I) formate. ... 

Poly(pyrazolyl)borate ligands are prepared, through a more general reaction, by heating tetra-hydroborate ion in molten pyrazole (Scheme 1) ... 

Trofimenko, S. Scorpionates The Coordination Chemistry of Poly (pyrazolyl)borate Ligands 1999, Imperial College Press London. 

The poly(pyrazolyl)borate ligands were developed by Trofimenko10 and have been used by him and others as cyclopentadienyl-like ligands which are capable of forming complexes that are often more stable than the analogous T s-cyclo-pentadienyl complexes. We have used this property to prepare a series of isolable, relatively stable copper(I) carbonyl complexes. The detailed syntheses of three complexes of this type, Cu(CO)[(pz)3BH], Cu(CO)[(Me2pz)3BH] (Me2pz = 3,5-dimethyl-l-pyrazolyl), and Cu(CO)[(pz)4B], are described below. 

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