Role of ancillary ligands in -nitrosothiol and NO generation from nitrite-thiol interactions at mononuclear zinc(ii) sites.

Generation of -nitrosothiol (RSNO) and nitric oxide (NO) mediated by zinc(ii) coordination motifs is of prime importance for understanding the role of zinc(ii)-based cofactors in redox-signalling pathways. This study uniquely employs a set of mononuclear [LZn] cores (where L = MePzPz/MePzPy/MePzQu) for introducing subtle alterations of the primary coordination sphere and investigates the role of ligand tuning in the transformation of NO in the presence of thiols. Single crystal X-ray diffraction (SCXRD) analyses on [LZn-X](X) (where X = perchlorate/triflate) illustrate consistent changes in the bond distances, thereby showing variations of the metal-ligand interactions depending on the nature of the heterocyclic donor arms (pyrazole/pyridine/quinoline). Moreover, such tuning of the ligands affects the Lewis-acidity of the [LZn] cores as evaluated by P NMR and SCXRD studies on the 1 : 1 acid-base adducts [LZn(OPEt)]. Crystallographic and N NMR spectroscopic analyses on the nitrite complexes [LZn(κ-nitrite)](ClO) reveal that the chemical environments of the nitrite anions in these complexes are nearly identical, despite the dissimilarity in the Lewis-acidity of the [LZn] cores. Interestingly, RSNO and NO generation from the reactions of [LZn(κ-nitrite)](ClO) with 4--butylbenzylthiol ( BuBnSH) exhibits that the [(MePzQu)Zn] core is the most efficient in promoting nitrite-thiol interactions due to the ease of available hemilabile coordination sites at the Lewis acidic [Zn]. Detailed UV-vis studies in tandem with computational investigation, for the first time, provide an unambiguous demonstration of the nitrous acid (HNO) intermediate generated through an intramolecular proton-transfer from thiol to nitrite at zinc(ii).