Chemistry of bulky tetrakis(pyrazolyl)borate ligands [B(pzR)4]- (R = p-CH3OC6H4 or C6H11)

The ligands [B(pz(R))(4)](-) [R = p-CH3OC6H4 (An) or C6H11 (Cy)] and their corresponding complexes [Mo(pz(R)Tp(R))(CO)(2)(NO)] (R = An 1 or Cy 2) and [MoCl2(pz(R)Tp(R))(NO)] (R = An 3 or Cy 4) have been prepared. The molecular structures determined by NMR and X-ray crystallography reveal in all cases the formation of only one isomer having a three-co-ordinated tetrakis(pyrazolyl)borate ligand with the R substituents at the 3 position of the pyrazolyl rings, ie. the symmetric form (3R,3R,3R,3R). The incorporation of the fourth pyrazolyl arm coupled to the use of different bulky substituents on the pyrazolyl rings has been analysed over the cavities produced around the molybdenum centres, and the results compared to those for related tris(pyrazolyl)borate derivatives. A greater protection of the molybdenum atom is provided by the p-CH3OC6H4 substituents, which also give rise to more rigid frameworks.