The splitting of dinitrogen (1 atm, THF, 25°C) by Mo(N[R]Ar) 3 (R ) C(CD 3 ) 2 CH 3 , Ar ) 3,5-C 6 H 3 Me 2 ) giving 2 equiv of nitride NtMo(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) x Na][(N 2 )Mo(N[R]Ar) 3 ], where x is from 0 to 3. Via electrochemical experiments and synthetic studies, [(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 sodium 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 (µ-N 2 ){Mo(N[R]Ar) 3 } 2 , which in turn splits to 2 equiv of nitride NtMo(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-N 2 ){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) 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 dinitrogen 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 ClSiMe 3 to give (Me 3 SiNN)Mo(N[R]-Ar) 3 . This complex underwent reaction with methyl triflate to give the dimethyl hydrazido cationic species, [(Me 2 NN)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[ t Bu]N) 3 Ti(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 6 H 3 Me 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) 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 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 ) 2 CH 3 ) is attributed to the enhanced steric protection at Mo afforded by the 1-adamantyl substituents.

Dinitrogen gas rarely is employed as the nitrogen source in the synthesis of molecular nitridometal compounds, 1 but recent advances involving well-defined N 2 -splitting reactions are improving the outlook for the direct utilization of molecular nitrogen. Dimolybdenum systems 2-4 have been found to effect N 2 splitting in the absence of added reagents, while certain diniobium and divanadium systems 5-7 also split dinitrogen when used in conjunction with alkali metals. While dinuclear dinitrogen complexes represent an emerging theme for N 2 -splitting systems, 2-6,8-12 thus far the systems in question have been homobimetallic in nature. The present work shows that molybdenum and niobium can be used together to effect the splitting of molecular nitrogen in a cooperative fashion. Nitridoniobium compounds stemming from this intriguing heterodinuclear N 2 -scission process include a niobazene cyclic trimer, the structure of which provides insight into the bonding in M 3 N 3 rings (M ) transition metal). [13][14][15] Recent work with anionic molybdenum dinitrogen complexes has shown them to be good nucleophiles, in that they are smoothly alkylated or silylated at the (terminal) nitrogen atom. [16][17][18] For example the anion 3 and MeNNMo(N[R]Ar) 3 upon reaction with chlorotrimethylsilane and methyl tosylate, respectively. Furthermore, it has been possible to assemble a variety of heterodinuclear dinitrogen complexes by reaction of dinitrogen molybdenum complex anions with various metal halide (M ) Zr, V, Fe, and U) electrophiles. 16,17,19,20 Accordingly, we find that the reaction of the purple chloroniobium( Figure 2). Complex 2 is characterized by an intense ν NN at 1583 cm -1 , a value to be compared with that (1548 cm -1 ) for the isotopomer prepared from 15 N 2 . While EPR spectroscopy (25°C, toluene) revealed a classic 10-line pattern resulting from coupling to the I ) 9/2 niobium nucleus 93 Nb (100%), no coupling to the spin 5/2 molybdenum nuclei 95 Mo and 97 Mo (15.92% and 9.55%, respectively) was resolved. 21 The magnitude of the 93 Nb coupling was found by simulation to be 99.3 G, a value typical for niobium(IV) systems of the type Nb(X)(N[ i Pr]Ar) 3 (X -) I, Cl). 22 Inclusion in the simulation of a 32.0 G coupling to 95/97 Mo (this value being typical for S ) 1/2 molybdenum systems of the type MoCl 2 (N[ i Pr]Ar) 3 ) 23 did not lead to resolution of the molybdenum hyperfine. Thus one cannot conclude that Solari, E.; Giannini, L.; Floriani, C.; ChiesiVilla, A.; Rizzoli, C. J. Am. Chem. Soc. 1998, 120, 437. (6) Ferguson, R.; Solari, E.; Floriani, C.; Osella, D.; Ravera, M.; Re, N.; Chiesi-Villa, A.; Rizzoli, C. Musaev, D. G.; Morokuma, K.; Fryzuk, M. D.; Love, J. B.; Seidel, W. W.; Albainati, A.; Koetzle, T. F.; Klooster, W. T.; Mason, S. A.; Eckert, J. J. Am. Chem. Soc. 1999, 121, 523. (13) Plenio, H.; Roesky, H. W.; Noltemeyer, M.; Sheldrick, G. M. IV) complex NbCl(N[ i Pr]Ar) 3 21 with [Na(THF) x ][(N 2 )Mo(N[R]Ar) 3 ] 18 or [Mg(THF) 2 ][(N 2 )Mo-(N[R]Ar) 3 ] 2 21 provides in high yield (74-78%) the ne...

Reaction of Mo(N[R]Ar)(3) (R = (t)Bu or C(CD(3))(2)CH(3)) with N(2)O gives rise exclusively to a 1:1 mixture of nitride NMo(N[R]Ar)(3) and nitrosyl ONMo(N[R]Ar)(3), rather than the known oxo complex OMo(N[R]Ar)(3) and dinitrogen. Solution calorimetry measurements were used to determine the heat of reaction of Mo(N[R]Ar)(3) with N(2)O and, independently, the heat of reaction of Mo(N[R]Ar)(3) with NO. Derived from the latter measurements is an estimate (155.3 +/- 3.3 kcal.mol(-1)) of the molybdenum-nitrogen bond dissociation enthalpy for the terminal nitrido complex, NMo(N[R]Ar)(3). Comparison of the new calorimetry data with those obtained previously for oxo transfer to Mo(N[R]Ar)(3) shows that the nitrous oxide N-N bond cleavage reaction is under kinetic control. Stopped-flow kinetic measurements revealed the reaction to be first order in both Mo(N[R]Ar)(3) and N(2)O, consistent with a mechanism featuring post-rate-determining dinuclear N-N bond scission, but also consistent with cleavage of the N-N bond at a single metal center in a mechanism requiring the intermediacy of nitric oxide. The new 2-adamantyl-substituted molybdenum complex Mo(N[2-Ad]Ar)(3) was synthesized and found also to split N(2)O, resulting in a 1:1 mixture of nitrosyl and nitride products; the reaction exhibited first-order kinetics and was found to be ca. 6 times slower than that for the tert-butyl-substituted derivative. Discussed in conjunction with studies of the 2-adamantyl derivative Mo(N[2-Ad]Ar)(3) is the role of ligand-imposed steric constraints on small-molecule, e.g. N(2) and N(2)O, activation reactivity. Bradley's chromium complex Cr(N(i)Pr(2))(3) was found to be competitive with Mo(N[R]Ar)(3) for NO binding, while on its own exhibiting no reaction with N(2)O. Competition experiments permitted determination of ratios of second-order rate constants for NO binding by the two molybdenum complexes and the chromium complex. Analysis of the product mixtures resulting from carrying out the N(2)O cleavage reactions with Cr(N(i)Pr(2))(3) present as an in situ NO scavenger rules out as dominant any mechanism involving the intermediacy of NO. Simplest and consistent with all the available data is a post-rate-determining bimetallic N-N scission process. Kinetic funneling of the reaction as indicated is taken to be governed by the properties of nitrous oxide as a ligand, coupled with the azophilic nature of three-coordinate molybdenum(III) complexes.