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 A r ]Zn(mim A r )}[ClO 4 ], which feature tetrahedrally coordinated zinc centers in a sulfur rich environment, may be prepared by the reaction of [Tm A r ]Li with Zn(ClO 4 ) 2 in the presence of 2-mercapto-1-arylimidazole (mim A r ; Ar = Ph, p-Tol). X-ray diffraction studies demonstrate that the mim A r 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 M e )} + . The orthogonal arrangement observed experimentally for both {[Tm P h ]Zn(mim P h )} + and {[Tm p - T o l ]Zn(mim p - T o l )} + is a consequence of the existence of a N-H...O hydrogen bond between the N-H group of the mim A r ligand and a molecule of methanol. Evidently, the hydrogen bonding interactions in {[Tm A r ]Zn(mim A r )...(HOMe)} + are sufficiently strong to counter the electronic destabilization resulting from rotation of the mim A r ligand from the Zn-S-C plane. The mim P h ligand of {[Tm P h ]Zn(mim P h )} + may be immediately displaced by I - to give [Tm A r ]ZnI; however, {[Tm P h ]Zn(mim P h )} + does not react rapidly with MeI, thereby demonstrating that the zinc coordinated mim P h ligand is less susceptible to electrophilic attack than is a zinc thiolate ligand.