Activation of nitrogen for organic synthesis

Benzonitrile Extrusion from Molybdenum(IV) Ketimide Complexes Obtained via Radical C−E (E = O, S, Se) Bond Formation: Toward a New Nitrogen Atom Transfer Reaction

Beta-elimination is explored as a possible means of nitrogen-atom transfer into organic molecules. Molybdenum(IV) ketimide complexes of formula (Ar[t-Bu]N)3Mo(N=C(X)Ph), where Ar = 3,5-Me2C6H3 and X = SC6F5, SeC6F5, or O2CPh, are formally derived from addition of the carbene fragment [:C(X)Ph] to the terminal nitrido molybdenum(VI) complex (Ar[t-Bu]N)3Mo identical with N in which the nitrido nitrogen atom is installed by scission of molecular nitrogen. Herein the pivotal (Ar[t-Bu]N)3Mo(N=C(X)Ph) complexes are obtained through independent synthesis, and their propensity to undergo beta-X elimination, i.e., conversion to (Ar[t-Bu]N)3MoX + PhC identical with N, is investigated. Radical C-X bond formation reactions ensue when benzonitrile is complexed to the three-coordinate molybdenum(III) complex (Ar[t-Bu]N)3Mo and then treated with 0.5 equiv of X2, leading to facile assembly of the key (Ar[t-Bu]N)3Mo(N=C(X)Ph) molecules. Treated herein are synthetic, structural, thermochemical, and kinetic aspects of (i) the radical C-X bond formation and (ii) the ensuing beta-X elimination processes. Beta-X elimination is found to be especially facile for X = O2CPh, and the reaction represents an attractive component of an overall synthetic cycle for incorporation of dinitrogen-derived nitrogen atoms into organic nitrile (R-C identical with N) molecules.

Synthesis of tungsten complexes that contain hexaisopropylterphenyl-substituted triamidoamine ligands, and reactions relevant to the reduction of dinitrogen to ammonia

[HIPTN3N]WCl (WCl) can be synthesized readily by adding H3[HIPTN3N] to WCl4(DME) followed by LiN(SiMe3)2 ([HIPTN3N]3– = [(HIPTNCH2CH2)3N]3– where HIPT = 3,5-(2,4,6-i-Pr3C6H2)2C6H3 = HexaIsoPropylTerphenyl). Reduction of WCl with KC8 in benzene under N2 yields WN=NK. WN=NK is readily oxidized in THF by ZnCl2 to yield zinc metal and WN2. Reduction of WN2 to [WN2]– is reversible at –2.27 V vs. FeCp2+/0 in 0.1 mol/L [Bu4N][BAr′4]/PhF electrolyte (Ar′ = 3,5-(CF3)2C6H3), while oxidation of WN2 to [WN2]+ is also reversible at –0.66 V. Protonation of WN=NK by [Et3NH][OTf] in benzene yields WN=NH essentially quantitatively. Protonation of WN=NH at Nβ with [H(OEt)2][BAr′4] in ether affords [W=NNH2][BAr′4] quantitatively. Electrochemical reduction of [W=NNH2][BAr′4] in 0.1 mol/L [Bu4N][BAr′4]/PhF is irreversible at scan rates of up to 1 V/s. Addition of NaBAr′4 and NH3 to WCl in PhF yields [W(NH3)][BAr′4]. Electrochemical reduction of [W(NH3)][BAr′4] in 0.1 mol/L [Bu4N][BAr′4]/PhF is irreversible at –2.06 V vs. FeCp2+/0 at a scan rate of 0.5 V/s. Treatment of [W(NH3)][BAr′4] with triethylamine and [FeCp2][PF6] in C6D6, followed by LiN(SiMe3)2, yielded W≡N. Treatment of [W(NH3)][BAr′4] with LiBHEt3 (1 mol/L in THF) results in formation of WH, which is converted to WH3 upon exposure to an atmosphere of H2. Attempts to prepare WN=NH by treating WN2 with [2,6-LutH][BAr′4] and CoCp2 yielded only [W=NNH2][BAr′4]. [W=NNH2][BAr′4] is reduced to W=NNH2 by CoCp*2, but this species disproportionates to yield WN=NH, W≡N, and ammonia. Reduction of [W(NH3)][BAr′4] with CoCp*2 does not yield any observable W(NH3). Attempted catalytic reduction of dinitrogen using WN2 as the catalyst under conditions identical or similar to those employed for catalytic reduction of dinitrogen by MoN2 and related Mo complexes failed. Single crystal X-ray studies were carried out on W-N=NK, WN2, W-N=NH, [W=NNH2][BAr′4], and [W(NH3)][BAr′4].Key words: dinitrogen, reduction, tungsten, ammonia.