Ab Initio MO Study of Silver Ion Complexation in [2.2.2]Cyclophane π-Prismands

Abstract

Ab initio Hartree-Fock and DFT MO calculations have been used to study the conformations of [2.2.2]cyclophane π-prismands and the formation of π-complexes with silver ion and [2.2.2]cyclophanes. The lowest energy cyclophane conformations have been calculated up to the HF/6-31+G* level of theory. The silver π-complexes of the lowest energy conformations have been calculated with HF/3-21G* and B3LYP/3-21G* levels of theory. The nature of bonding in silver ion π-complexes has been studied with natural bond orbital analysis (NBO). Energies of the calculated cyclophanes and complexes, together with formation energies of those complexes, have also been discussed. The results obtained have been compared to X-ray crystal structures whenever such structures were available. Calculated and experimental X-ray structures agreed reasonably well given provision to the rather small 3-21G* basis set and the omission of triflate anion in the calculation model. The NBO analysis showed that when Ag + is bonded to the cyclophane cavity, the bonds are formed by σ-donation and d-π*-back-donation between the silver ion and the hydrocarbon skeleton, resulting in hexahapto (3×η 2 ) overall π-bonding. This is in agreement with the well-known "bondingback-bonding" scheme in transition metal carbonyl complexes. In this case the σ-donation from hydrocarbon to silver ion is the main contribution to the metal-cyclophane bonding. One dihapto (η 2 ) bonding to one aromatic ring in the present π-prismands relates by strength to a single strong hydrogen bond, which is up to 100 kJ/mol; thus the strength of this 3×η 2 bonding in these [2.2.2]cyclophane π-prismands can be compared to three simultaneous strong hydrogen bonds.

Small Hydrocarbon Cyclophanes: Synthesis, X-ray Analysis and Molecular Modelling

“…Some selected bond and torsion angles of the most stable conformations of the B3LYP-optimised cyclophanes are shown with the experimental values obtained from X-ray analysis in Table 2. These calculated bond parameters agree well with previously reported values in similar cyclophanes, [16,17] and are also in very good agreement with experimental Xray values. Based on the very good agreement between the calculated structure and the X-ray structure for 21, the calculated structure presented here for [2.2.0] m,m,m-cyclophane (20) (Figure 9) can be accepted as a good model of its structure, [a] Numbering according to Figure 8.…”

“…The structure of the [2.2.2]p,p,p-cyclophane (1) ionophore seems to be particularly favourable for Ag ϩ -π interactions, resulting in ISEs with the best Ag ϩ selectivity among the cyclophanes studied. [14,15] The nature of the bonding in π-prismands and the conformations of [ [16,17] Following the syntheses and testing of [2.2.1]cyclophanes, we were extremely interested to study even smaller and more strained hydrocarbon cyclophanes, i.e. [2.2.0]m,m,mcyclophane (20) [18] and [2.2.0]p,m,m-cyclophane (21) [19] to see if even more rigid and distorted cyclophanes than [2.2.1]cyclophanes can act as π-prismands.…”

Complexation Behavior of a Highly Preorganized 7,7‐Diphenylnorbornane‐Derived Macrocycle: Towards the Design of Molecular Clocks

“…We performed computations in order to gain information about the structure of 4 ⋅Ag + and confirm the results obtained by NMR spectroscopy. It has been shown that HF/3‐21G* and B3LYP/3‐21G* calculations are able to reproduce the geometries of cyclophane⋅Ag + complexes reasonably well, with omission of the triflate anion in the computation model 39. The calculated structures are in agreement with those obtained by X‐ray structural analysis.…”

A Stable Salt of the Tris(ethene)silver Cation: Structure and Characterization of [Ag(η2‐C2H4)3]+[Al{OC(CF3)3}4]−

“…In contrast, Ag + and Pd have an accessible 4d 10 shell. The large energetic differences Δ E of the planar versus the prismatic form, as well as the short MC separation, suggest that d‐orbital bonding contributions are most important for the Pd complex but are, to a lesser extent, also important for the Ag complex 37. A full analysis of the nature of this bonding is in progress 35…”