Zinc(II) complexes based on sterically hindered hydrotris(pyrazolyl)borate ligands: Synthesis, reactivity and solid-state structures

The coordination chemistry and reactivity of zinc(II) complexes supported by monoanionic hydrotris( pyrazolyl) borate ligands substituted by 3,3,3-mesityl groups (Tp(Ms)) and 3,3,5-mesityl groups (Tp(Ms*)) have been investigated. Salt metathesis of ZnCl2, ZnEt2, and Zn(OAc)(2) with Tl[Tp(Ms)] or Tl[Tp(Ms*)] cleanly afforded the corresponding compounds Tp(Ms)ZnCl (1), Tp(Ms)ZnEt (2), Tp(Ms*)ZnEt (3), and Tp(Ms)ZnOAc (5). Compound 3 slowly disproportionates in benzene solution to afford the bis(ligand) complex (kappa(2)-Tp(Ms*))(2)Zn (4). Acetate complex 5 as well as Tp(Ms)ZnOCOPh (6) and [Tp(Ms*)ZnOAc](2) (7) were alternatively prepared by acidolysis of the parent ethyl complexes (2, 3) with the corresponding carboxylic acid. No reaction was observed between 2 and 3 and alcohols (ROH; R = Et, iPr, Bn), while salt metathesis reactions of ZnEt(OR) with Tl[Tp(Ms)] led to 2 instead of the desired zinc-alkoxide complex. Compounds 1-7 were characterized by elemental analysis, H-1 and C-13 NMR spectroscopy, as well as by X-ray diffraction studies for 1, 2, 4, 5 and 7. The former compounds adopt a monomeric structure in the solid state while [Tp(Ms*)ZnOAc](2) (7) exists as an anti-syn bridged acetate dimer. Complex 4 is four-coordinated, featuring a rare bidentate coordination mode of the Tp(Ms*) ligands. The results are rationalized in terms of the variable steric constraint around the zinc atom provided by the Tp(Ms) and Tp(Ms*) ligands. (C) 2009 Elsevier B. V. All rights reserved.