Interplay between Gold(I)-Ligand Bond Components and Hydrogen Bonding: A Combined Experimental/Computational Study

The influence of
weak interactions on the donation/back-donation
bond components in the complex [(NHC)­Au­(SeU)]+ (NHC = N-heterocyclic carbene; SeU = selenourea) has
been studied by coupling experimental and theoretical techniques.
In particular, NMR 1H and pulsed-field gradient spin-echo
titrations allowed us to characterize the hydrogen bond (HB) between
the −NH2 moieties of SeU and the anions
PF6– and ClO4–, whereas 77Se NMR spectroscopy allowed us to characterize
the Au–Se bond. Theoretically, the Au–Se and Au–C
orbital interactions have been decomposed using the natural orbital
for the chemical valence framework and the bond components quantified
through the charge displacement analysis. This methodology provides
the quantification of the Dewar–Chatt–Duncanson (DCD)
components for the Au–C and Au–Se bonds in the absence
and presence of the second-sphere HB. The results presented here show
that the anion has a dual mode action: it modifies the conformation
of the cation by ion pairing (and this already influences the DCD
components) and it induces new polarization effects that depend on
the relative anion/cation relative orientation. The perchlorate polarizes SeU, enhancing the Se → Au σ donation and the
Au → C back-donation and depressing the C → Au σ
donation. On the contrary, the hexafluorophosphate depresses both
the Se → Au and C → Au σ donations.