NHC-Gold-Alkyne Complexes: Influence of the Carbene Backbone on the Ion Pair Structure

We have studied the ion pair structure of [NHC-Au(eta(2)-3-hezyne)] [BF4] (NHC = nitrogen-heterocyclic carbene) by solution NOE NMR spectroscopy and relativistic DFT calculation. The neutral complexes [NHC-AuCl] have been synthesized through an improved, silver-free one-pot synthesis, by reaction (in air and using solvents and substrates without any previous purification) of the appropriate [NHC(H)]Cl, gold precursor, and KHCO3. Ion pairs were generated in situ in NMR tubes. In our previous work, two main ion pair orientations were observed for unsaturated NHC ligands: one with the anion close to the carbene backbone (A, most populated) and another with the anion close to the alkyne (B). Here we focus on the effect of the carbene backbone on the ion pair structure, comparing the unsaturated NHC (1BF(4)) with two different variants: a saturated NHC (2BF(4)) and a polycyclic ligand with an extended aromatic system (3BF(4)). For 2BF(4), the A:B ratio remains almost the same as for 1BF(4), while the ion pair structure of 3BF(4) becomes mainly nonspecific, with a slight preference for the orientation B. Both cases can be explained analyzing the DFT Coulomb potential map, which shows an attractive region on the backbone of 2BF(4) and a flat weak potential around the whole 3BF(4).