Why intermolecular nitric oxide (NO) transfer? Exploring the factors and mechanistic aspects of NO transfer reaction

Abstract

Small molecule activation & 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)} sup(8), [(12-TMC)Co sup(III)(NO-)] sup(2+) (1-CoNO, S = 0), and {Cr(NO)} sup(5), ([(BPMEN)Cr(NO)(Cl)]+) (4-CrNO, S =1/2), complexes. 1-CoNO transfer its bound NO moiety to a high-spin [(BPMEN)Cr sup(II)(Cl sub(2))] (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 sup(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 observation supports the formation of mu-NO bridged intermediate species ({Cr-NO-Co} sup(4+)). Mechanistic investigations using sup(15)N-labeled- sup(15)NO and tracking the sup(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 sup(II)-complex, [(12-TMC)Cr sup(II)(Cl)] sup(+) (5-Cr, S = 2), and 1-CoNO, showing the generation of the low-spin [(12-TMC)Cr(NO)(Cl)] sup(+) (6-CrNO, S = 1/2); however, again there was no opposite reaction, i.e., from Cr-center to Co-center. The above results advocates clearly that the NO transfer from Co-center generates thermally stable and low-spin & inert {Cr(NO)} sup(5) complexes (4-CrNO & 6-CrNO) from high-spin & 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.