Tris borate complex, transition metal

NMR (CeDg, 125.7 MHz, 25°C) 139.8, 132.5, 128.8-129.1 (overlapping resonances), 124.6, 17.0 (br). B NMR (CgDg, 128.3 MHz, 25°C) -10.96 ppm. The compound is a versatile precursor to a wide range of transition metal complexes supported by the tris(phosphino)borate ligand. It is air- and water-stable for extended periods, and, unlike the lithium and ammonium salts of [PhB(CH2PPh2)3] , it is both soluble and stable in chloroform and dichloromethane for days, making these useful solvents available for subsequent trans-metallation chemistry. 

Transitions of this type are associated with polyatomic ligands which have electronic spectra in their own right. As an example, the tris(3,5-dimethylpyrazolyl)borate (tp 2-) proligand (Figure 4.2) exhibits a tc 7C transition in the ultraviolet region at about 230 nm, which is also observed in its transition metal complexes. Usually, such absorptions do not reveal much about the metal centre and so they are of less interest to transition metal chemists than the remaining types of absorption which directly involve the metal centre. 

Dialkyl metallocenes and other dialkyl Group 4 transition metal complexes are useful as precatalysts in combination with co-catalysts such as tris(perfluoro-aryl)boranes or tetrakis(periluoroaryl)borate salts [18]. Recently, an expedient procedure for the production of dimethyl metallocenes and Cp-amido dimethyl metal complexes in high yields and purity has been reported. The direct synthesis of Group 4 dimethylmetallocenes [19] consists of the one-pot reaction between the r-ligand, a 2-fold excess of MeLi, and MtCU. This simple method produces the dimethylated complexes in higher overall yield, and saves on reaction time and solvents, compared to the classic two-step route, which consists in the synthesis of the metallocene dichloride followed by its methylation with 2 equiv. MeLi. 

The ligand hydrotris[3-(2/-thienyl)pyrazol-l-yl)]borate (= TpTn) is reported to have the second-lowest steric hindrance among the known poly(pyrazolyl)borates, also if a priori tris(pyrazolyl)borates with the 3-(2/-thienyl) substituent were expected to fit between those with 3-phenyl and those with 3-methyl substituents in terms of steric hindrance. TpTn forms octahedral [M(TpTn)2] complexes with first-row transition-metal ions, but fails to yield stable [MX(TpTn)j species, except with Zn(II).12... 

The vast majority of tris(pyrazolyl)borate ruthenium complexes prepared until now contains the parent Tp ligand. In contrast to other transition metals, ruthenium compounds which bear bulky Tp ligands remain still rare and are, thus far, limited to Tp, Tp Tp and Tp as shown in Chart 1. 

Alkyne hydroamination has been extensively reviewed [3, 4, 10] and important contributions using late transition metals have been realized to give the Markovnikov-type products most typically. Interestingly, in 2007, Fukumoto reported a tris(pyrazolyl borate)rhodium(l) complex for the anti-Markovnikov hydroamination of terminal aUcynes with both primary and secondary amine substrates, although yields with primary amines are always reduced compared to those with secondary amines (Scheme 15.26). Desirable functional group tolerance is also noteworthy for this regioselective hydroamination catalyst [187]. 

Other recent PRDDO studies on transition metal complexes include an examination of inversion barriers in metal-phosp-hido complexes. such a.s Cp2ClTiP4)2, and substituent effects in trifluoromethyl[tris(pyrazoly)J borate complexes. ... 

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