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

Abstract

The guanidine moiety of arginine is involved in the active sites of a variety of enzymes, such as nitric oxide synthase (NOS) and NiFe hydrogenase. In this paper we aim to investigate the effects of a metal center on the oxidation of guanidine, which should provide an interesting comparison with the biological aerobic oxidation of arginine catalyzed by NOS. We studied the oxidation of an osmium(III) guanidine complex, mer-[Os(L){N(H)C(NH)}(CN)], (OsG, HL = 2-(2-hydroxyphenyl)benzoxazole) by m-chloroperbenzoic acid (m-CPBA), which is potentially an O atom transfer reagent, and by (NH)[Ce(NO)], which is a one-electron oxidant. With m-CPBA, mer-[Os(NO)(L)(CN)] (mer-OsNO) is the product, while with Ce, mer-[Os(N)(L)(CN)] (mer-OsN) is formed instead. The crystal structures of mer-OsNO and mer-OsN were determined by X-ray crystallography. The mechanisms for the oxidation of OsG by m-CPBA and Ce are proposed.

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

“…On the other hand, this absorption is red-shifted for complexes with electron-donating (Me and OMe) substituents (Figure 3a,b). This trend, together with the calculated frontier molecular orbitals (MOs; see below) and previously reported timedependent DFT calculations, 11 is consistent with the LML′CT [L ligand to OsN] assignment for the lowest-energy band. Because metal-centered d−d transitions would also occur in a similar region, a mixing of d−d transitions with the LML′CT transition is also suggested.…”

“…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.…”

“…Except for the methoxysubstituted complex 3, all nitrido complexes in the solid state display orange-red luminescence with submicrosecond lifetimes ranging from 0.074 to 1.90 μs (Figures 4a and S15a). With reference to our previous transient Raman and transient absorption studies, 11 this is ascribed to 3 LML′CT [L ligand to OsN] phosphorescence. In general, complexes with strongly electron-withdrawing substituents, particularly NO 2 (8−10), exhibit higher-energy and more intense emissions, as reflected by the much higher quantum efficiency (1.1−11.7% for 6−10 vs ≤0.1% for 1−5).…”

“…Indeed, upon irradiation with visible light, this complex readily activates the strong C−H bonds of alkanes and arenes, including benzene. 11 In contrast to the nitro-substituted osmium(VI) nitrido complex, the parent complex is only weakly emissive. To gain more insight into the effects of substituents on the photophysical properties of these osmium (VI) nitrido complexes, we have designed a series of osmium (VI) nitrido complexes with various substituents on the anionic chelating 2-(2-phenolate)benzoxazole (L) ligand.…”

Visible light-induced oxidative N-dealkylation of alkylamines by a luminescent osmium(vi) nitrido complex

“…We have recently designed an osmium( vi ) nitrido complex [Os VI (N)(L)(CN) 3 ] − ( NO2-OsN , HL = 2-(2-hydroxy-5-nitrophenyl)benzoxazole) that is strongly luminescent in the solid state and in fluid solutions. 4 It readily absorbs visible light to generate a long-lived and highly oxidizing excited state with a redox potential of ca. 1.4 V. The excited state of this complex also possesses [Os N˙] nitridyl characteristics that enable it to readily abstract H-atoms from inert organic substrates.…”