Early transition metal poly(methimazolyl)borate complexes

Poly(methimazolyl)borates ‘HxB(mt)4-x’ (x = 1, 2; mt = methimazolyl) are a class of chelating ligands that present two or three ‘soft’ sulfur donors to a metal centre and as such are especially suited to the later ‘soft’ transition metals in low oxidation states. The question arises, therefore, as to whether these ligands might also bind effectively to early ‘hard’ transition metals in high oxidation states. We have begun to explore this possibility and whilst our initial efforts indicated that there were considerable synthetic hurdles associated with attendant redox, hydrolysis and ligand degradation processes, these have now been circumvented. The first examples of complexes of these ligands bound to titanium, zirconium, tantalum and niobium have now been successfully prepared with metals in high formal oxidations states III – V. Selected examples include (a) the first homoleptic titanium complex [Ti{HB(mt)3}2]+; (b) An alkyne complex of tantalum(III) in which the alkyne (red) binds in a chiral pocket [TaCl22-PhC≡CPh){HB(mt)3}] and (c) A cyclooctatetraene complex of zirconium [ZrCl(η-C8H8){H2B(mt)2}] that is devoid of B–H–Zr interactions despite the metal being coordinatively unsaturated.