Dimers of N‐Heterocyclic Carbene Copper, Silver, and Gold Halides: Probing Metallophilic Interactions through Electron Density Based Concepts

Abstract

Homobimetallic metallophilic interactions between copper, silver, and gold-based [(NHC)MX]-type complexes (NHC=N-heterocyclic carbene, i.e, 1,3,4-trimethyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene; X=F, Cl, Br, I) were investigated by means of ab initio interaction energies, Ziegler-Rauk-type energy-decomposition analysis, the natural orbital for chemical valence (NOCV) framework, and the noncovalent interaction (NCI) index. It was found that the dimers of these complexes predominantly adopt a head-to-tail arrangement with typical M⋅⋅⋅M distance of 3.04-3.64 Å, in good agreement with the experimental X-ray structure determined for [{(NHC)AuCl}2 ], which has an Au⋅⋅⋅Au distance of 3.33 Å. The interaction energies between silver- and gold-based monomers are calculated to be about -25 kcal mol(-1) , whereas that for the Cu congener is significantly lower (-19.7 kcal mol(-1) ). With the inclusion of thermal and solvent contributions, both of which are destabilizing, by about 15 and 8 kcal mol(-1) , respectively, an equilibrium process is predicted for the formation of dimer complexes. Energy-decomposition analysis revealed a dominant electrostatic contribution to the interaction energy, besides significantly stabilizing dispersion and orbital interactions. This electrostatic contribution is rationalized by NHC(δ(+) )⋅⋅⋅halogen(δ(-) ) interactions between monomers, as demonstrated by electrostatic potentials and derived charges. The dominant NOCV orbital indicates weakening of the π backdonation in the monomers on dimer formation, whereas the second most dominant NOCV represents an electron-density deformation according to the formation of a very weak M⋅⋅⋅M bond. One of the characteristic signals found in the reduced density gradient versus electron density diagram corresponds to the noncovalent interactions between the metal centers of the monomers in the NCI plots, which is the manifestation of metallophilic interaction.

X‐ray Crystal Structure of a Metalled Double‐Helix Generated by Infinite and Consecutive C*‐AgI‐C* (C*:N1‐Hexylcytosine) Base Pairs through Argentophilic and Hydrogen Bond Interactions

“…Moreover, an energetically equivalent “retro‐donation” 4p x →5d xy from Ag(2) to Ag(1) is also observed. Therefore the total stabilization energy due to donor–acceptor orbital interactions is E (2) =−24.2 kcal mol −1 , which is in good agreement with other calculations involving Ag dimers …”

“…In the polymeric structure the turn (when the bases are coincident through the projection of the a ‐axis) is made by 11 base pairs comprising two asymmetric units and eleven Ag ions (see Figure ), similar to the turn of B‐DNA (10.5 base pairs). Moreover, the on‐top representation depicted in Figure b shows the typical pentagonal symmetry of the B‐DNA …”

Conceptual Quantum Chemical Analysis of Bonding and Noncovalent Interactions in the Formation of Frustrated Lewis Pairs

“…Since the ωB97x‐D functional is currently not available in the ADF code, we carried out the energy decomposition analysis at the PBE‐D3/TZ2P level, which we found to be a very reliable method in our earlier studies on various systems including weakly interacting ones 56–58. The trustworthiness of the latter method for frustrated and classical Lewis pairs is demonstrated by the very good correlation with a correlation coefficient R 2 of 0.96 and a slope of 0.99 (see the Supporting Information, Figure S1) between the PBE‐D3/TZ2P complexation energies Δ E PBE (Table 3) and the complexation energies Δ E computed at the ωB97x‐D/6‐311++G(d,p) level of theory.…”

“…57 Rather, one should extract structure‐specific net effects in one way or another, for example, by using constrained structures56 or the relative contributions of the stabilizing terms 57. 58, 61 We applied the latter method, whereby the relative contributions of stabilizing terms are expressed in percentages (Table 2). These values indicate that the bonding of all five Lewis adducts is of a similar nature, that is, electrostatics provides about 43–48 % of the stabilization, which is similar to the contribution of orbital interactions (46–50 %), whereas dispersion is of low importance (ca.…”

“…It is based on the relationship between the electron density and the reduced density gradient, and it is capable of detecting and distinguishing weak interactions such as hydrogen bonds, van der Waals interactions, and repulsive steric clashes 57. 58, 61–63 Thus, we investigated what insights the NCI can provide for FLP systems that are known to exhibit various types of noncovalent interactions. The plot of s versus ρ for Me 3 P – B(C 6 F 5 ) 3 (Figure 5) displays the characteristic feature of sharp spikes in the low‐gradient/low‐density region, a footprint of noncovalent interactions.…”