Phenyl tris(3-tert-butylpyrazolyl)borato complexes of lithium and thallium, [PhTpBut]M (M = Li, Tl): a novel structure for a monomeric tris(pyrazolyl)boratothallium complex and a study of its stereochemical nonrigidity by 1H and 205Tl NMR spectroscopy

Abstract

The syntheses and structures of the phenyl substituted tris(3-tert-butylpyrazolyl)borato complexes, [PhTp^But]M (M = Li, Tl, H), are reported. In contrast to other monomeric [Tp^RR′]Tl derivatives, which exhibit symmetric tridentate coordination of the tris(pyrazolyl)borate ligand, [PhTp^But]Tl exhibits an unprecedented structure. Specifically, one of the tert-butylpyrazolyl groups is rotated by ca. 90° and the Tl interacts with the nitrogen attached directly to the boron via a p-orbital component of the aromatic π-system of the pyrazolyl nucleus. [PhTp^But]Tl is stereochemically nonrigid on the NMR spectroscopic timescale in solution at room temperature, but cooling to ca. –80 °C slows down the dynamic processes sufficiently to allow “axial” and “equatorial” isomers to be identified, with the descriptors denoting the position of the pyrazolyl group relative to the boat configuration of the six-membered [BN₄Tl] ring.