Towards structurally versatile mesoionic N-heterocyclic olefin ligands and their coordination to palladium, gold, and boron hydride

We have developed an efficient and versatile approach for the synthesis of a family of 1,2,3-triazole-based mesoionic N-heterocyclic olefin (mNHO) ligands and investigated their coordination to palladium, gold, and boron hydride experimentally and computationally. We reacted mNHOs obtained through deprotonation of the corresponding methylated and ethylated 1,3,4-triaryl-1,2,3-triazolium salts with [Pd(allyl)Cl]$_2$ to give the corresponding [Pd(η$^3$-allyl)Cl(mNHO)] coordination complexes. $^{13}$C NMR data revealed the strong σ-donor character of the mNHO ligands, consistent with the calculated bond orders and atom-condensed charges. Furthermore, we also synthesized [AuCl(mNHO)] and a BH$_3$–mNHO adduct by reacting the triazolium salts with AuCl(SMe$_2$) and BH$_3$·THF, respectively. The BH$_3$–mNHO adduct was tested in the reduction of select aldehydes and ketones to alcohols.