Sodium hydrotris(methimazolyl)borate, a novel soft, tridentate ligand: preparation, structure and comparisons with sodium hydrotris(pyrazolyl)borate

The hydrotris(methimazolyl)borate anion (Tm), a soft analogue of the hydrotris(pyrazolyl)borate anion (Tp), has been synthesized. This novel ligand system has been designed to maintain the tripodal geometry around the boron while allowing the replacement of the three nitrogen donor atoms by three sulfur (thione) donor atoms, thus providing a complementary soft, tridentate, face capping ligand system. The two ions, Tm and Tp were compared by X-ray analysis and ab initio calculations in an attempt to explore the effects of exchanging the hard donor atoms for soft donor atoms in this type of ligand. The compound NaTm is essentially salt like with discrete anions and hydrated sodium cations. The structure of NaTp crystallised under identical conditions is observed to be an infinite ribbon containing monodentate, bridging and pendant pyrazolyl units. The co-ordination sphere of the sodium cation in NaTp is completed by two water molecules. Ab initio calculations at the Hartree-Fock level using a 6-31G* basis set on these anions and their sodium complexes suggested that while both ions are in general similar in nature, there are subtle differences which will influence their chemistry. Ab initio calculations were also used to provide a rational analysis of the formation of the two sodium salts obtained and on the analogous copper complexes further to clarify the hard and soft nature of the two ligand systems.