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

Abstract

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 ZnCl 2, ZnEt 2, and Zn(OAc) 2 with Tl[Tp Ms] or Tl[Tp Ms∗] cleanly afforded the corresponding compounds Tp MsZnCl ( 1), Tp MsZnEt ( 2), Tp Ms∗ZnEt ( 3), and Tp MsZnOAc ( 5). Compound 3 slowly disproportionates in benzene solution to afford the bis(ligand) complex (κ 2-Tp Ms∗) 2Zn ( 4). Acetate complex 5 as well as Tp MsZnOCOPh ( 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, 1H and 13C 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.