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

Abstract

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 1 H and pulsed-field gradient spin-echo titrations allowed us to characterize the hydrogen bond (HB) between the −NH 2 moieties of SeU and the anions PF 6 – and ClO 4 – , whereas 77 Se 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.

Disentanglement of orthogonal hydrogen and halogen bonds via natural orbital for chemical valence: A charge displacement analysis

“…As a result, the charge transfer involved in the H … Br HB is slightly underestimated by CT 1 . Δ ρ ′ 2 describes a polarization of the selenourea, with a charge flux from the nitrogen atoms to the selenium, as already observed for other XB selenourea‐adducts and complexes . Finally, Δ ρ ′ 3 (−1.7 kcal/mol) describes the H … O HB, depletion regions on the oxygen with a shape resembling a p orbital and accumulation regions between the two atoms and on the nitrogen.…”

“…Δρ 0 2 describes a polarization of the selenourea, with a charge flux from the nitrogen atoms to the selenium, as already observed for other XB selenourea-adducts [52] and complexes. [34,43] Finally, Δρ 0 3 (−1.7 kcal/mol) describes the H … O HB, depletion regions on the oxygen with a shape resembling a p orbital and accumulation regions between the two atoms and on the nitrogen. This contribution is particularly weak, but it is expected that a HB of this kind is mainly electrostatic in nature and contains only a small orbital contribution, [53] which is the only analyzed here.…”

“…For single‐determinant wavefunctions, Δ ρ ′ can be brought into diagonal contributions using the “valence operator” , producing pairs of orbitals with the same eigenvalue (in absolute value) that describes the donor and acceptor molecular orbitals involved in the charge rearrangement . When applied to organometallic complexes, also the NOCV demonstrated to correctly decompose donation and back donation for asymmetric systems . The method can be also applied in combination with the Extended Transition State—Natural Orbital for the Chemical Valence (ETS‐NOCV), so that an energy stabilization can be associated with the different bonding components.…”

“…[33] When applied to organometallic complexes, also the NOCV demonstrated to correctly decompose donation and back donation for asymmetric systems. [34][35][36][37] The method can be also applied in combination with the Extended Transition State-Natural Orbital for the Chemical Valence (ETS-NOCV), [38] so that an energy stabilization can be associated with the different bonding components. We point out that both the NOCV-CD and ETS-NOCV are based on the assumption that the molecular system can be separated into chemically meaningful reference fragments and clearly this assumption is not always valid or unambiguous.…”

An ETS‐NOCV‐based computational strategies for the characterization of concerted transition states involving CO2

“…As the problem concerns the decomposition and quantitative characterization of two different interactions, a possible solution can be given by the Energy Decomposition Analysis (EDA) 16 , 17 and the Extended Transition State‐Natural Orbital for Chemical Valence (ETS‐NOCV) 18 , 19 , 20 analysis, which recently demonstrated to be a useful tool for the bond analysis of complex adducts, 21 , 22 as those held together by both halogen and hydrogen bonds, 23 characterization of donation and back‐donation in metal–ligand interactions 18 , 24 , 25 , 26 and for giving insights into the recently‐coined spodium bond. 27 While the EDA decomposes the interaction between two fragments (Δ E int ) into different contributions (mainly related to electrostatics, Pauli repulsion and orbital mixing), the ETS‐NOCV focuses only on the orbital term, by decomposing it into chemically meaningful contributions.…”