N-H•••O hydrogen bonding interactions in tetrahedral [ZnS4] complexes of relevance to zinc enzymes:: the synthesis, structures and reactivity of tris(2-mercapto-1-arylimidazolyl)hydroborato zinc(2-mercapto-1-arylimidazole) complexes,{[TMAr]Zn(mimAr)}[ClO4](Ar = Ph, p-Tol)

The tris(2-mercapto-1-arylimidazolyl)hydroborato complexes {[Tm-Ar]Zn(mim(Ar))}[ClO], which feature tetrahedrally coordinated zinc centers in a sulfur rich environment, may be prepared by the reaction of [Tm-Ar]Li with Zn(ClO4)(2) in the presence of 2-mercapto-1-arylimidazole (mim(Ar); Ar = Ph, p-Tol). X-ray diffraction studies demonstrate that the mim(Ar) ligands are almost orthogonal to the Zn-S-C plane, in contrast to the almost planar arrangement that is calculated for {[Tm-H]Zn(mim(H))}(+) and {[Tm-H]Zn(mim(Me))}(+). The orthogonal arrangement observed experimentally for both {[Tm-Ph]Zn(mim(Ph))}(+) and {[TMp-Tol]Zn(Mim(p-Tol))}(+) is a consequence of the existence of a N-H...O hydrogen bond between the N-H group of the mim(Ar) ligand and a molecule of methanol. Evidently, the hydrogen bonding interactions in {[Tm-Ar]Zn(mim(Ar))...(HOMe)}(+) are sufficiently strong to counter the electronic destabilization resulting from rotation of the mimAr ligand from the Zn-S-C plane. The mim(Ph) ligand of {[Tm-Ph]Zn(mim(Ph))}(+) may be immediately displaced by I- to give [Tm-Ar]ZnI; however, {[Tm-Ph]Zn(mim(Ph))}(+) does not react rapidly with MeI, thereby demonstrating that the zinc coordinated mim(Ph) ligand is less susceptible to electrophilic attack than is a zinc thiolate ligand. (C) 2003 Published by Elsevier Ltd.