Poly borates metal complexes

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. 

Like many late transition metals, nickel has featured, albeit briefly, in the pursuit of poly(pyrazolyl)borate-metal-carbollide complexes. Thus, [c/oso-3-(r 2-Ph2Bp)-3,l,2-NiC2B9H11] (90 ) has been obtained as the tet-ramethylammonium salt,39 via the reaction of [Me4N][Ph2Bp] with the neutral bisphosphine nickel dicarbollide [doso-3,3-(PhEt2P)2-3,l,2-... 

It is an axiom of modern organometallic chemistry that the pursuit of late transition metal complexes is ultimately driven by the need to formulate ever more efficient catalysts and reagents for chemical synthesis. In this respect, the field of poly(pyrazolyl)borate chemistry is no different from any other, albeit that in the case of the group 10 triad the breadth of study is perhaps more limited than for other metals and/or ligands. This section provides an overview of prominent results in respect of both catalysis and the C—H activation processes that underpin them. 

POLY(l-PYRAZOLYL)BORATES, THEIR TRANSITION -METAL COMPLEXES,... 

Poly(l -pyraxolyl)borates, Their Transition-metal Complexes 103... 

Many new transition metal complexes of these ligands continue to be prepared. The most interesting aspects of this chemistry are not so much the evolution of specific new ML, ML2, and ML3 compositions, but the purposes that the hgands serve or are suggested to serve. For example, poly(pyrazolyl)borate anions have been used to stabilize a 17-electron radical fragment, a seven-coordinate W complex, half-sandwich HB(pz )MCl compounds, and to mimic a variety of natural bioinorganic coordination conditions. ... 

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. ... 

In 1988 Lalor reported the synthesis and spectroscopic characterization of some new poly(benzo-triazolyl)borate salts (Scheme 8).180 The regiospecifity of the synthesis of I I B( Btz)4 differs from that of pyrazole/BH4 reaction in that B—N bond formation takes place in a manner that maximizes steric crowding at boron (i.e., at the triazole N(l) atoms). In 1989 Shiu describes some new metal carbonyls of HB(Btz)3.181 The synthesis and spectroscopic characterization of some first-row transition metal complexes was reported by Cecchi,182 whereas Hill described some new Rh183 and Ru184 carbonyl and isonitrile complexes. The formation of isonitrile derivatives was proposed to proceed through an associative mechanism involving an intermediate complex... 

The B shifts of a series of metal complexes of poly(l-pyrazolyl)-borates, M[H2- B(pz)2+ ]2> where pz = 1-pyrazolyl group, have been discussed.A rather different type of complex of iron with the 1,3,4-triaza-2,5-diborine system has been described by Noth and Regnet. Two B resonance signals were observed at —29 7 and —26-8 p.p.m. (from Et20,BF3) and the complex was considered to have a metallocene structure. 

The first alkylidyne-metal complexes were prepared by Fischer et al. more than 30 years ago. Since this pioneering event, the chemistry of the transition metal-carbon triple bond present in such complexes has developed into a major field of research and though the poly(pyrazolyl)borate ligands were discovered 7 years prior to the synthesis of the first alkylidyne complexes, their importance and significance in this field has only more recently been truly appreciated. 

Up to March 2006 the molecular structures of about 40 alkylidyne complexes ligated by poly(pyrazolyl)borates had been reported, and structural data for the crystallographically characterised alkylidyne-metal complexes are compiled in Table II. Of these, Tp- and Tp -ligated complexes represent more than 80%, with one third of the total known structures being bi- and polymetallic complexes. Structural features of alkylidyne complexes in general have been discussed elsewhere and, accordingly, an attempt to restrict this discussion to features which are unique or characteristic of the scorpionate alkylidyne complexes has been made. 

Alkylidyne Complexes Ligated by Poly(pyrazolyl)borates Table V Infrared Absorptions for Alkylidyne-Metal Complexes Tp W( = CR)(CO)2 45... 

Reactivity modes of the poly(pyrazolyl)borate alkylidyne complexes follow a number of recognised routes for transition metal complexes containing metal-carbon triple bonds, including ligand substitution or redox reactions at the transition metal centre, insertion of a molecule into the metal-carbon triple bond, and electrophilic or nucleophilic attack at the alkylidyne carbon, C. Cationic alkylidyne complexes generally react with nucleophiles at the alkylidyne carbon, whereas neutral alkylidyne complexes can react at either the metal centre or the alkylidyne carbon. Substantive work has been devoted to neutral and cationic alkylidyne complexes bearing heteroatom substituents. Differences between the chemistry of the various Tp complexes have previously been rationalised largely on the basis of steric effects. 

During the 40 years of poly(pyrazolyl)borate chemistry, complexes with group 9 metals have provided a particularly fertile area of research. Over 400 publications describe aspects ranging from the synthesis of homoleptic and heteroleptic coordination compounds to the application of complex organometallic molecules in catalysis, and the development of advanced specialist materials. 

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

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... 

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