Redox-catalyzed binding of dinitrogen by molybdenum N-tert-hydrocarbylanilide complexes: Implications for dinitrogen functionalization and reductive cleavage

The splitting of dinitrogen (1 atm, THF, 25 degrees C) by Mo(N[R]Ar)(3) (R = C(CD3)(2)CH3, Ar = 3,5-C6H3Me2) giving 2 equiv of nitride N=Mo(N[R]Ar)(3) is found to be accelerated in the presence of sodium amalgam. Careful cont rol of the Mo(N[R]Ar)(3) concentration led to the isolation and characteriz ation of the anionic dinitrogen complex, [(THF)(x)Na] [(N-2)Mo(N[R]Ar)(3)], where x is from 0 to 3. Via electrochemical experiments and synthetic stud ies, [(THF)(x)Na] [(N-2)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 sodi um amalgam. Accordingly, in the presence of an electron acceptor, [(THF)(x) Na] [(N-2)Mo(N[R]Ar)(3)] reacts with Mo(N[R]Ar)(3) to give the neutral N-2- bridged complex (mu-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 (mu-N-2){Mo(N[R]Ar)(3)}(2), a dinuclear dinitrogen complex tha t does not lose N-2 readily. Electrochemical or chemical outer-sphere oxida tion of [(THF)(x)Na] [(N-2)Mo(N[R]Ar)(3)] leads to rapid N-2 evolution with regeneration of Mo(N[R]Ar)(3), presumably via the neutral mononuclear dini trogen complex (N-2)Mo(N[R]Ar)(3). In situ generated [(THF)(x)Na][(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, [(Me2NN)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 (N2)Mo(NIR)Ar)3 gave rise to efficient syntheses of heterodinuclear dinitrogen complexes i ncluding (Ph[Bu-t]N)(3)Ti(mu-N-2)Mo(N[R]-Ar)(3), which also was synthesized in its N-15(2)-labeled form. Synthesis and characterization data for the n ew N-adamantyl-substituted three-coordinate molybdenum(III) complex Mo(N[Ad ]Ar)(3) (Ad = 1-adamantyl, Ar = 3,5-C6H3Me2) are presented. The complex is found to react with dinitrogen (1 atm, THF, 25 degrees C) in the presence o f sodium amalgam to give the dinitrogen anion complex [(THF)(x)Na] [(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) b een observed to split dinitrogen or to give rise to a dinuclear mu-N-2 comp lex; this striking contrast with the reactivity of Mo(N[R]Ar)(3) (R = C(CD3 )(2)CH3) is attributed to the enhanced steric protection at Mo afforded by the 1-adamantyl substituents.