Redox-Catalyzed Binding of Dinitrogen by Molybdenum N-tert-Hydrocarbylanilide Complexes: Implications for Dinitrogen Functionalization and Reductive Cleavage

Abstract

The splitting of dinitrogen (1 atm, THF, 25 °C) by Mo(N[R]Ar)_3 (R = C(CD_3)_2CH_3, Ar = 3,5-C_6H_3Me_2) giving 2 equiv of nitride N⋮Mo(N[R]Ar)3 is found to be accelerated in the presence of sodium amalgam. Careful control of the Mo(N[R]Ar)_3 concentration led to the isolation and characterization of the anionic dinitrogen complex, [(THF)xNa][(N_2)Mo(N[R]Ar)_3], where x is from 0 to 3. Via electrochemical experiments and synthetic studies, [(THF)xNa][(N2)Mo(N[R]Ar)_3] is found to be a key intermediate in the acceleration of N_2 splitting by Mo(N[R]Ar)_3 in the presence of sodium amalgam. Accordingly, in the presence of an electron acceptor, [(THF)xNa][(N_2)Mo(N[R]Ar)_3] reacts with Mo(N[R]Ar)_3 to give the neutral N2-bridged complex (μ-N_2){Mo(N[R]Ar)_3}_2, which in turn splits to 2 equiv of nitride N⋮Mo(N[R]Ar)3. It is seen that the function of sodium amalgam in this system is as a redox catalyst, accelerating the conversion of Mo(N[R]Ar)_3 to (μ-N2){Mo(N[R]Ar)3}2, a dinuclear dinitrogen complex that does not lose N_2 readily. Electrochemical or chemical outer-sphere oxidation of [(THF)xNa][(N2)Mo(N[R]Ar)_3] leads to rapid N_2 evolution with regeneration of Mo(N[R]Ar)_3, presumably via the neutral mononuclear dinitrogen complex (N2)Mo(N[R]Ar)_3. In situ generated [(THF)xNa][(N_2)Mo(N[R]Ar)_3] was efficiently trapped by ClSiMe3 to give (Me3SiNN)Mo(N[R]Ar)_3. This complex underwent reaction with methyl triflate to give the dimethyl hydrazido cationic species, [(Me_2NN)Mo(N[R]Ar)_3][OTf]. The synthesis of the monomethyl complex (MeNN)Mo(N[R]Ar)_3 also was achieved. Experiments designed to trap the neutral mononuclear dinitrogen complex (N_2)Mo(N[R]Ar)_3 gave rise to efficient syntheses of heterodinuclear dinitrogen complexes including (Ph[tBu]N)3Ti(μ-N_2)Mo(N[R]Ar)_3, which also was synthesized in its ^(15)N_2-labeled form. Synthesis and characterization data for the new N-adamantyl-substituted three-coordinate molybdenum(III) complex Mo(N[Ad]Ar)_3 (Ad = 1-adamantyl, Ar = 3,5-C_6H_3Me_2) are presented. The complex is found to react with dinitrogen (1 atm, THF, 25 °C) in the presence of sodium amalgam to give the dinitrogen anion complex [(THF)xNa][(N_2)Mo(N[Ad]Ar)_3]; the synthesis does not require careful regulation of the Mo(N[Ad]Ar)_3 concentration. Indeed, under no conditions has Mo(N[Ad]Ar)_3 been observed to split dinitrogen or to give rise to a dinuclear μ-N_2 complex; this striking contrast with the reactivity of Mo(N[R]Ar)_3 (R = C(CD_3)_2CH_3) is attributed to the enhanced steric protection at Mo afforded by the 1-adamantyl substituents.