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Reactivity of Nitrido Complexes of Ruthenium(VI), Osmium(VI), and Manganese(V) Bearing Schiff Base and Simple Anionic Ligands

Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
 

Abstract

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Nitrido complexes (M≡N) may be key intermediates in chemical and biological nitrogen fixation and serve as useful reagents for nitrogenation of organic compounds. Osmium(VI) nitrido complexes bearing 2,2′:6′,2″-terpyridine (terpy), 2,2′-bipyridine (bpy), or hydrotris(1-pyrazolyl)borate anion (Tp) ligands are highly electrophilic: they can react with a variety of nucleophiles to generate novel osmium(IV)/(V) complexes.
This Account describes our recent results studying the reactivity of nitridocomplexes of ruthenium(VI), osmium(VI), and manganese(V) that bear Schiff bases and other simple anionic ligands. We demonstrate that these nitrido complexes exhibit rich chemical reactivity. They react with various nucleophiles, activate C–H bonds, undergo N···N coupling, catalyze the oxidation of organic compounds, and show anticancer activities. Ruthenium(VI) nitrido complexes bearing Schiff base ligands, such as [RuVI(N)(salchda)(CH3OH)]+ (salchda = N,N′-bis(salicylidene)o-cyclohexyldiamine dianion), are highly electrophilic. This complex reacts readily at ambient conditions with a variety of nucleophiles at rates that are much faster than similar reactions using OsVI≡N. This complex also carries out unique reactions, including the direct aziridination of alkenes, C–H bond activation of alkanes and C–N bond cleavage of anilines. The addition of ligands such as pyridine can enhance the reactivity of [RuVI(N)(salchda)(CH3OH)]+. Therefore researchers can tune the reactivity of Ru≡N by adding a ligand L trans to nitride: L–Ru≡N. Moreover, the addition of various nucleophiles (Nu) to RuVI≡N initially generate the ruthenium(IV) imido species RuIV–N(Nu), a new class of hydrogen-atom transfer (HAT) reagents.
Nucleophiles also readily add to coordinated Schiff base ligands in OsVI≡N and RuVI≡N complexes. These additions are often stereospecific, suggesting that the nitrido ligand has a directing effect on the incoming nucleophile. M≡N is also a potential platform for the design of new oxidation catalysts. For example, [OsVI(N)Cl4]− catalyzes the oxidation of alkanes by a variety of oxidants, and the addition of Lewis acids greatly accelerates these reactions. [MnV(N)(CN)4]2– is another highly efficient oxidation catalyst, which facilitates the epoxidation of alkenes and the oxidation of alcohols to carbonyl compounds using H2O2. Finally, M≡N can potentially bind to and exert various effects on biomolecules. For example, a number of OsVI≡N complexes exhibit novel anticancer properties, which may be related to their ability to bind to DNA or other biomolecules.
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