Carbon-Bound Diazeniumdiolates from the Reaction of Nitric Oxide with Amidines

Photolytic C-Diazeniumdiolate Disassembly in the β-Diketiminate Complexes [MeLM(O2N2CPh3)] (M = Fe, Co, Cu)

The reaction of [K(18-crown-6)][O2N2CPh3] with [MeLCo(μ-Br)2Li(OEt2)] (MeL = {(2,6-iPr2C6H3)NC(Me)}2CH) generates the trityl diazeniumdiolate complex, [MeLCo(O2N2CPh3)] (1), in moderate yield. Similar metathesis reactions result in the formation of the Fe and Cu analogues, [MeLM(O2N2CPh3)] (Fe, 2; Cu, 3), which can also be isolated in moderate yields. Complexes 1-3 were characterized by ultraviolet-visible (UV-vis) spectroscopy, and their solid-state structures were determined by X-ray crystallography. These complexes were further characterized via 1H NMR spectroscopy (in the case of 1 and 2) or EPR spectroscopy (in the case of 3). Irradiation of complexes 1 and 2 with 371 nm light generates the known dinitrosyl complexes, [MeLM(NO)2] (M = Co, 4; Fe, 5), along with Ph3CH and 9-phenylfluorene. We propose that 4 and 5 are formed via the putative hyponitrite intermediates, [MeLM(κ2-O,O-ONNO)], which are formed by photoinduced homolysis of the C-N bond of the [O2N2CPh3] ligand. In contrast, irradiation of complex 3 with 371 nm light, in the presence of 1 equiv of PPh3, led to the formation of the Cu(I) complexes, [MeLCu(PPh3)], [(ArNCMeC(NO)CMeNAr)Cu(PPh3)] (6), and [(ArNCMeC(NO)CMeNAr)Cu]2 (7), of which the latter two are products of γ-nitrosation of the β-diketiminiate ligand. Also formed in this transformation are Ph3CN(H)OCPh3, Ph3PO, and N2O, along with trace amounts of NO.

NO and N2O Release from the Trityl Diazeniumdiolate Complexes [M(O2N2CPh3)3]- (M = Fe, Co)

Reaction of MBr₂ with 3 equiv of [K(18-crown-6)][O₂N₂CPh₃] generates the trityl diazeniumdiolate complexes [K(18-crown-6)][M(O₂N₂CPh₃)₃] (M = Co, 2; Fe, 3) in good yields. Irradiation of 2 and 3 using 371 nm light led to NO formation in 10 and 1% yields (calculated assuming a maximum of 6 equiv of NO produced per complex), respectively. N₂O was also formed during the photolysis of 2, in 63% yield, whereas photolysis of 3 led to the formation of N₂O, as well as Ph₃CN(H)OCPh₃, in 37 and 5% yields, respectively. These products are indicative of diazeniumdiolate fragmentation via both C-N and N-N bond cleavage pathways. In contrast, oxidation of complexes 2 and 3 with 1.2 equiv of [Ag(MeCN)₄][PF₆] led to N₂O formation but no NO formation, suggesting that diazeniumdiolate fragmentation occurs exclusively via C-N bond cleavage under these conditions. While the photolytic yields of NO are modest, they represent a 10- to 100-fold increase compared to the previously reported Zn congener, suggesting that the presence of a redox-active metal center favors NO formation upon trityl diazeniumdiolate fragmentation.

Nitrogen-bound diazeniumdiolated amidines

In contrast to amidines bearing ionizable alpha-CH bonds, which react with nitric oxide (NO) to add diazeniumdiolate groups at their alpha-carbons, benzamidine forms an N-bound diazeniumdiolate that can be further derivatized at the other amidine nitrogen and/or the terminal oxygen to form caged NO compounds as potential NO prodrugs.

Nitric oxide reacts with methoxide

Despite over a century of reports to the contrary, sodium methoxide has been found to react with nitric oxide (NO). The reaction, whose final organic product is sodium formate, is postulated to occur via an intermediate O-bound diazeniumdiolate [CH3O-N(O)=NO-] that decomposes to formaldehyde and nitrous oxide. Sodium formate forms from the aldehyde via a Cannizzaro reaction. Carboxylate salts have similarly been obtained by exposing sodium benzylate and sodium neopentoxide to NO in dioxane solution. Accordingly, sodium trimethylsilanoate should be considered as a substitute for sodium methoxide as the base used to accomplish the replacement of active hydrogens by the diazeniumdiolate functional group via the Traube reaction.

Theoretical investigation of the mechanisms of reactions of H2CN and H2SiN with NO

The mechanisms of the reaction of H2XN (X = C, Si) with NO were studied at the level CCSD(T)/aug-cc-PVTZ//B3LYP/6-31++G(d,p). The results indicate that there are two most favorable reaction pathways in the reaction H2CN + NO that have similar energy barriers; these two pathways lead to the formation of HCN + HNO (P1) and H2CO + N2 (P3), with the calculated barriers 11.1 and 10.2 kcal/mol, respectively, with respect to the reactants (H2CN + NO). In the reaction H2SiN + NO the difference of the barriers in these two analogous pathways becomes large, and the preferable pathway shifts to the production of H2SiO + N2 (P3s), which has no barrier with respect to the reactants (H2SiN + NO). A direct reduction of NO into a stable and nontoxic nitrogen molecule with no energy input becomes possible. As a consequence, H2SiN might be an effective reagent to convert the reactive and toxic NO into a benign gas N2 in several NO-producing combustion systems. We offer a possible explanation of the differences between H2CN and H2SiN toward NO as well as the calculated potential energies for these reactions.