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

Abstract

[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.