Hoi-Ki Kwong

Cerium(IV)-Driven Water Oxidation Catalyzed by a Manganese(V)–Nitrido Complex†

The study of manganese complexes as water-oxidation catalysts (WOCs) is of great interest because they can serve as models for the oxygen-evolving complex of photosystem II. In most of the reported Mn-based WOCs, manganese exists in the oxidation states III or IV, and the catalysts generally give low turnovers, especially with one-electron oxidants such as Ce(IV) . Now, a different class of Mn-based catalysts, namely manganese(V)-nitrido complexes, were explored. The complex [Mn(V) (N)(CN)4 ](2-) turned out to be an active homogeneous WOC using (NH4 )2 [Ce(NO3 )6 ] as the terminal oxidant, with a turnover number of higher than 180 and a maximum turnover frequency of 6 min(-1) . The study suggests that active WOCs may be constructed based on the Mn(V) (N) platform.

C-N bond cleavage of anilines by a (salen)ruthenium(VI) nitrido complex.

We report experimental and computational studies of the facile oxidative C-N bond cleavage of anilines by a (salen)ruthenium(VI) nitrido complex. We provide evidence that the initial step involves nucleophilic attack of aniline at the nitrido ligand of the ruthenium complex, which is followed by proton and electron transfer to afford a (salen)ruthenium(II) diazonium intermediate. This intermediate then undergoes unimolecular decomposition to generate benzene and N2.

Photocatalytic oxidation of alkenes and alcohols in water by a manganese(V) nitrido complex

Mn(v) nitrido complex [Mn(N)(CN)4](2-) is an efficient catalyst for visible-light induced oxidation of alkenes and alcohols in water using [Ru(bpy)3](2+) as a photosensitizer and [Co(NH3)5Cl](2+) as a sacrificial oxidant. Alkenes are oxidized to epoxides and alcohols to carbonyl compounds.

Highly Selective and Efficient Ring Hydroxylation of Alkylbenzenes with Hydrogen Peroxide and an Osmium(VI) Nitrido Catalyst

The OsVI nitrido complex, OsVI (N)(quin)2 (OTs) (1, quin=2-quinaldinate, OTs=tosylate), is a highly selective and efficient catalyst for the ring hydroxylation of alkylbenzenes with H2 O2 at room temperature. Oxidation of various alkylbenzenes occurs with ring/chain oxidation ratios ranging from 96.7/3.3 to 99.9/0.1, and total product yields from 93 % to 98 %. Moreover, turnover numbers up to 6360, 5670, and 3880 can be achieved for the oxidation of p-xylene, ethylbenzene, and mesitylene, respectively. Density functional theory calculations suggest that the active intermediate is an OsVIII nitrido oxo species.

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

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.