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
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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