Aerobic Oxidation of an Osmium(III) N-Hydroxyguanidine Complex To Give Nitric Oxide

Abstract

The aerobic oxidation of the N-hydroxyguanidinum moiety of N-hydroxyarginine to NO is a key step in the biosynthesis of NO by the enzyme nitric oxide synthase (NOS). So far, there is no chemical system that can efficiently carry out similar aerobic oxidation to give NO. We report here the synthesis and X-ray crystal structure of an osmium(III) N-hydroxyguanidine complex, mer-[Os(III){NH═C(NH2)(NHOH)}(L)(CN)3](-) (OsGOH, HL = 2-(2-hydroxyphenyl)benzoxazole), which to the best of our knowledge is the first example of a transition metal N-hydroxyguanidine complex. More significantly, this complex readily undergoes aerobic oxidation at ambient conditions to generate NO. The oxidation is pH-dependent; at pH 6.8, fac-[Os(NO)(L)(CN)3](-) is formed in which the NO produced is bound to the osmium center. On the other hand, at pH 12, aerobic oxidation of OsGOH results in the formation of the ureato complex [Os(III)(NHCONH2)(L)(CN)3](2-) and free NO. Mechanisms for this aerobic oxidation at different pH values are proposed.

Tunable Luminescent Properties of Tricyanoosmium Nitrido Complexes Bearing a Chelating O^N Ligand

“…The synthetic route for osmium­(VI) nitrido complexes bearing various substituents on the bidentate L ligands is shown in Figure , which is similar to that of our recently reported complexes 1 and 8 . − These complexes were obtained from oxidation of their guanidine precursors by iodosylbenzene in CH 3 CN under Ar, and they were isolated as light-yellow microcrystalline PPh 4 + salts in moderate yields. − All nitrido complexes were characterized by IR, UV/vis, 1 H NMR, and electrospray ionization mass spectrometry (ESI-MS). Five of them were also structurally characterized by X-ray crystallography.…”

Dual Pathways in the Oxidation of an Osmium(III) Guanidine Complex. Formation of Osmium(VI) Nitrido and Osmium Nitrosyl Complex

“…Although γ -OsGOH readily undergoes oxidation by air or H 2 O 2 , a 1 mM solution of OsG in CH 3 CN is stable in the presence of 1 atm O 2 or 0.1 M H 2 O 2 for more than one week at room temperature. However, OsG is readily oxidized by m -chloroperbenzoic acid ( m -CPBA) within minutes at ambient conditions, as monitored by electrospray ionization mass spectrometry (ESI/MS).…”

“…When only 0.2–0.4 equiv of m -CPBA was added to OsG at −20 °C, an intermediate species at m / z 553 could be observed, which is tentatively assigned to the nitrosoamidine species [Os III {N­(O)­CNH­(NH 2 )}­( L ) (CN) 3 ] − that is formed from the addition of an O atom to the guanidine ligand together with the loss of two protons (see Discussion below) (Figure S2). In the positive region, there is also a peak at m / z 43 (+ve mode), which can be assigned to [NH 2 CN + H] + (Figure S3); this species should arise from the oxidative cleavage of the guanidine ligand (NHC­(NH 2 ) 2 ) that give NO and NH 2 CN.…”

“…We recently reported the synthesis of novel osmium­(III) guanidine, mer -[Os­{N­(H)­C­(NH 2 ) 2 }­( L ) (CN) 3 ] − ( OsG , HL = 2-(2-hydroxyphenyl)­benzoxazole) and N -hydroxyguanidine ( mer -[Os­{NHC­(NH 2 ) (NHOH)}­( L ) (CN) 3 ] − , γ -OsGOH ) complexes via the nucleophilic addition of NH 3 and NH 2 OH, respectively, at an osmium­(III) hydrogen cyanamide precursor ([Os­(NHCN)­( L ) (CN) 3 ] 2– , OsNHCN ) (Figure ). , Inspired by the second step of NO biosynthesis by NOS, we also studied the aerobic oxidation of γ -OsGOH to give NO . Interestingly we found that at pH 6.8 the NO produced is bound to the osmium center, while at pH 12 free NO is produced (Figure ).…”

“…Depending on the oxidant, OsG can be either oxidized to the osmium nitrosyl complex, mer -[Os­(NO)­( L ) (CN) 3 ] − ( mer - OsNO ), or to the osmium­(VI) nitrido complex mer -[Os VI (N)­( L ) (CN) 3 ] − ( mer - OsN ) (Figure ). This work is not a simple extension of our recent work on the oxidation of γ -OsGOH , since the oxidation of OsG goes through different pathways and does not involve the intermediate formation of γ -OsGOH .…”