Protonation triggers N2 loss from a tungsten azide to form its nitride

Protonation of the PNP ligand lowers the barrier for multiple metal-nitrogen bond formation resulting in the reduction of azide to form nitride and the liberation of N2. [Display omitted] •One step oxidation and deprotonation of zerovalent W(CO)3(HPNP) using N-halosuccinimides affords several structurally characterized W(II) complexes.•First report where protonation of ligand triggers W(II)-azide conversion to W(IV) nitride.•DFT investigation of azide to nitride conversion in presence and absence of protons. The syntheses of [W(N3)(CO)2(PNP)], [W(N)(CO)(HPNP)]+ and a new route to the known [W(N)(CO)(PNP)], (PNP = (tBu2PCH2CH2)2N–) are described. The acid/conjugate base partners [W(N)(CO)(HPNP)]+ and [W(N)(CO)(PNP)] represent unusual d2 carbonyls, yet show low νCO frequencies of 1932 and 1886 cm−1. DFT study of the thermal conversion of [W(N3)(CO)2(PNP)] to [W(N)(CO)(PNP)] + CO + N2 reveals that the TS involves a bent N3 unit as N2 leaves. This feature, which is quite general in previously detected TS (“zig-zag”) for azide elimination of N2, is traced both with DFT and simple bonding reasoning to arise from the need to alter stepwise the WN/N2 interactions. The zig-zag TS structure is shown to involve initial NαNβ-π bond scission with retention of that σ bond, which then cleaves in what is, remarkably, a less energetically demanding event. The same zig-zag character is found in the transition state for N2 loss from [W(N3)(CO)n(HPNP)]+ (n = 1,2), yielding [W(N)(CO)(HPNP)]+, which can be deprotonated to form [W(N)(CO)(PNP)]. Protonation at pincer amide nitrogen lowers the barrier for loss of N2 by eliminating competitive amide π donation. The influence of the pincer nitrogen is also seen in deprotonation of [W(CO)3(HPNP)], which occurs with facile loss of one CO, giving the fully characterized [K][W(CO)2(PNP)].