Exchange of Cl⁺ between Lone-Pair Donors and π-Donors: A Computational Study

Abstract

The chemistry of mono-adducts ([Cl–X]+) between Cl+ and a Lewis base (X = NH3, H2O, HF, PH3, H2S or HCl) has been investigated using ab initio molecular orbital calculations at the G2 level. The reactions of such mono-adducts with additional Lewis bases (Y) are found to give [Y–Cl]+ plus X, generally without an intermediate barrier, via a bis-adduct [Y–Cl–X]+. The binding energies of the bis-adduct and the reaction energies are related to the donor properties of the Lewis bases. The reactions between the mono-adducts [Cl–X]+ and the π-donors ethylene and acetylene yield chloriranium and chlorirenium ions, respectively. These reactions proceed via complexes that resemble either the reactants or products depending on the sign of the reaction energy, the latter in turn being determined by the donor ability of the Lewis base. Results for the chlorine systems are compared with those for the corresponding phosphorus systems investigated previously.

Nucleophilicity—Periodic Trends and Connection to Basicity

“…[36,37] The reactant complex, the product complex, and the transition structures have all merged into one single structure located at the bottom of a symmetrical potential energy well. This is also the case for most di-adducts, [X···EH n ···X] + , formed between nucleophiles X and the larger third-row cations, EH n = PH 2 , SH, and Cl, [27][28][29][30] reflecting the ability of third-row atoms to form stable pentacoordinate molecules. Even neutral aluminum has this ability, in contrast to boron, [38] which is most probably the result of its enhanced tendency for electrostatic binding at the expense of covalency.…”

“…For neutral nucleophiles, it can be inferred from literature data that going down in the periodic table to EH n = PH 2 , SH, and Cl the barrier decreases. [27][28][29][30] Among the third-row groups, the barrier dependency on the nature of EH n is PH 2 < SH < Cl for second-row nucleophiles, but PH 2 < SH > Cl for third-row nucleophiles. These results show that both size and electron-accepting properties determine the barrier height.…”

“…[22] Ten years ago, Lee and co-workers published an article reporting MP2 barriers for identity reactions on allyl for the anionic nucleophiles X À (X = NH 2 , OH, F, PH 2 , SH, Cl). [23] Interestingly, there exist in the literature G2 barrier heights for identity S N 2 reactions on elements other than carbon, including reactions of anionic nucleophiles with substrates in which the central carbon (CH 3 ) has been changed to nitrogen (NH 2 or NR 2 ), [24,25] oxygen (OH), [26] and phosphorus (PH 2 ), [27] as well as reactions of neutral nucleophiles at phosphorus, [28] sulfur, [29] and chlorine, [30] some of them including periodically systematic variation of the nucleophile.…”

The Interplay between Steric and Electronic Effects in SN2 Reactions

“…Effect of the central atom: Although knowledge of pentacoordinated main group elements is old, Sølling and Radom were probably the first to apply quantum chemical methods to characterise identity S N 2 reactions without a central barrier (E # = ÀE 0 ; Figure 1). [43][44][45] The result is a single-well potential energy function in which the prototypical symmetric TS has become a potential energy minimum. This condition often occurs when the central atom is changed from carbon to main group elements of the third row, or for electropositive nucleophiles/electrofuges.…”

Steric and electronic effects in SN2 reactions

“…Even more support for the electrostatic model comes from the study of S N 2 reactions with noncarbon central atoms. Substituting carbon by a more electronegative element (N, O) increases the barrier height [43,44], while substituting by a more electropositive atom (Si, S, P) decreases the barrier, in many cases to the extent that the symmetrical central TS becomes the potential energy minimum [45][46][47][48][49].…”