Why intermolecular nitric oxide (NO) transfer? Exploring the factors and mechanistic aspects of NO transfer reaction. Issue 6 (21st January 2022) - Das, Sandip
Abstract : Nitric oxide activation and parameters influencing intermolecular transfer of nitric oxide. Abstract : Small molecule activation and their transfer reactions in biological or catalytic reactions are greatly influenced by the metal-centers and the ligand frameworks. Here, we report the metal-directed nitric oxide (NO) transfer chemistry in low-spin mononuclear {Co(NO)} 8, [(12-TMC)Co III (NO − )] 2+ (1-CoNO, S = 0), and {Cr(NO)} 5, ([(BPMEN)Cr(NO)(Cl)] + ) (4-CrNO, S = 1/2) complexes. 1-CoNO transfers its bound NO moiety to a high-spin [(BPMEN)Cr II (Cl2 )] (2-Cr, S = 2) and generates 4-CrNO via an associative pathway; however, we did not observe the reverse reaction, i.e., NO transfer from 4-CrNO to low-spin [(12-TMC)Co II ] 2+ (3-Co, S = 1/2). Spectral titration for NO transfer reaction between 1-CoNO and 2-Cr confirmed 1 : 1 reaction stoichiometry. The NO transfer rate was found to be independent of 2-Cr, suggesting the presence of an intermediate species, which was further supported experimentally and theoretically. The experimental and theoretical observations support the formation of μ-NO bridged intermediate species ({Cr–NO–Co} 4+ ). Mechanistic investigations using 15 N-labeled- 15 NO and tracking the 15 N-atom established that the NO moiety in 4-CrNO is derived from 1-CoNO . Further, to investigate the factors deciding the NO transfer reactivity, we explored the NO transfer reaction between another high-spin Cr II -complex, [(12-TMC)Cr II (Cl)] + (5-Cr, S = 2), and 1-CoNO, showing the generation of the low-spin [(12-TMC)Cr(NO)(Cl)] + (6-CrNO, S = 1/2); however, again there was no opposite reaction, i.e., from Cr-center to Co-center. The above results advocate clearly that the NO transfer from Co-center generates thermally stable and low-spin and inert {Cr(NO)} 5 complexes (4-CrNO & 6-CrNO ) from high-spin and labile Cr-complexes (2-Cr & 5-Cr ), suggesting a metal-directed NO transfer (cobalt to chromium, not chromium to cobalt). These results explicitly highlight that the NO transfer is strongly influenced by the labile/inert behavior of the metal-centers and/or thermal stability rather than the ligand architecture.