Theoretical Study of the 1:1 and 2:1 (Homo- and Heterochiral) Complexes of XOOX′ (X, X′ = H, CH₃) with Lithium Cation

Abstract

A theoretical study of the 1:1 and 2:1 complexes of XOOX' (X, X' = H, CH(3)) with the lithium cation has been carried out by means of ab initio computational methods up to the MP2/aug-cc-pVTZ level. The optical rotatory power and NMR parameters (absolute chemical shielding and indirect coupling constants) have been calculated. In addition, the racemization barriers within the complexes formed have been evaluated. Special attention has been paid concerning the differences between the 2:1 homo- and heterochiral complexes.

Can Transition Metals and Group II Mono- and Dications Discriminate between Homo- and Heterochiral XYYX’ Dimers (X,X’=H,Me; Y=O,S,Se)?

“…These ratios are consistent with the availability of 4s orbitals of transition metals to accept charge from the ligands leading to dative bonds. In agreement with a previous study, 28 no important cooperative effects were found for these systems; that is, the interaction energies calculated for the 2:1 complexes are approximately twice those computed for the 1:1 complexes. The only exception are Zn 2+ /Se complexes for which B3LYP/MP2 predict a difference of 1.5 between the interaction energies of the ML and ML2 complexes.…”

“…In agreement with the previous results for complexes with axial chirality in the gas phase, the heterochiral isomers were in the majority of cases lower in energy than their homochiral counterparts. 28,29 The small but consistent differences in energy may indeed serve as grounds for chiral distinction.…”

“…These two possible transition state structures are in agreement with those found in previous studies for similar model systems. 28,29,45 Since the monoligand Cu(I)(HSSH) complex yielded both cis and trans transition structures, we have run IRC calculations on both structures to confirm that both relax to the two expected isomers in the forward and backward directions (see Figure 9). The preferred transition pathway displays a dependency upon the nature of the bonding -for the ionically-bonded alkali and alkaline earth metals Li + and Ca…”

“…28,30 To account for the correlation of core electrons of the metal 31 and since calcium is absent from the standard aug-cc-pVTZ basis set, calcium was instead described using the CVTZ basis of J. Martin et al 32 Depending on the nature of the ligands and the axial chirality of the complexes, the ligands were optimized under the following symmetry constraints: ligands in heterochiral hydrogen and dimethylated complexes were arranged around the central metal according to S 4 symmetry, whereas no symmetry constraints were considered for unimethylated complexes, recall Figure 2. In turn, D 2 and C 2 environments around the metal were considered for hydrogenated or dimethylated and unimethylated homochiral complexes, respectively.…”

“…The study had determined that although heterochiral complexes were consistently more stable than their homochiral counterparts, the stabilization differences were less than chemical accuracy. 28 Sánchez-Sanz et al carried out a similar study on homochiral and heterochiral dimers of chalcogen-chalcogen bridges ((HTeYH) 2 where Y=O, S, Se, Te) using DFT methods. Though metal coordination was not considered, their investigation concluded that stability differences between the two stereoisomers amounted to less than 0.7 kcal/mol, with the sole exception of (HTeSeH) 2 exhibiting a relative energy difference of 2 kcal/mol in favor of the heterochiral configuration.…”