Accurate Ring Strain Energy Calculations on Saturated Three-Membered Heterocycles with One Group 13–16 Element

Abstract

Accurate ring strain energy (RSE) data for parent (CH2)2X rings are reported, where X are group 13–16 elements (El) in their lowest oxidation state, from the second to the sixth row, with their covalence completed by bonds to H. They are obtained from appropriate homodesmotic and hyper-homodesmotic reactions at different levels up to the CCSD­(T) level, thus providing a benchmark of high-quality reference RSE values, as well as acceptably accurate faster lower-level options. Derivatives of indium, thallium, and lead cannot be properly described by a three-member ring connectivity, because they display a unique donor–acceptor structure from an ethylene π­(CC) orbital to an empty p orbital of a metallylene subunit. The RSE of groups 13 and 14 heterocycles increases on descending in the group (except for Ga and Ge), while it decreases for groups 15 and 16. The latter is presumably due to a strain-releasing mechanism favored by the increase of p-character at the sp3-type atomic orbital used by El in the endocyclic El–C bonds, %p­(El)El‑C, originated by the tendency of the El lone pairs in groups 15–16 to increase their s-character. This strain-releasing mechanism does not exist in heavier tetrels, which keep almost unchanged the p-character in the endocyclic bonds at El, whereas for triels the p-character is still lower owing to their sp2-like hybridization. Remarkable linear correlations were found between the RSE and either the above-mentioned %p­(El)El‑C, the distal C–C bond distance or the relaxed force constants for the endocyclic bond angles.

Analysis of Non‐innocence of Phosphaquinodimethane Ligands when Charge and Aromaticity Come into Play

“…As expected, the easiest oxidizable molecules are those with P ‐amino substituents ( II ). The chemical hardness (η) of a molecule is connected to its chemical reactivity and has been used, [25] for instance, to discuss the ring strain in saturated three‐membered rings [26] or the reactivity in conjugated ring systems. [27] Large HOMO‐LUMO gap (chemical hardness) points to unfavourable extraction of electrons from a low‐lying HOMO and unfavourable addition of electrons to a high‐lying LUMO, therefore corresponding to low reactivity.…”

Between Oxirane and Phosphirane: The Spring‐loaded Oxaphosphirane Ring

“…As previously studied for a variety of saturated three‐membered heterocycles with only one heteroelement, [29] as well as for parent oxaphosphirane and some O ‐ and P ‐borane adducts, [24] the predictable relationship of RSE with the intuitively related relaxed force constants ( k 0 ) for endocyclic bond distances and bond angles were inspected. Unexpectedly, only the relaxed force constants for the P−C bond (Figure 3) and for the related P−O−C bond angle (Figure S1) displayed acceptable linear correlation (even better than the one in Figure 2) for the same aforementioned set of derivatives also correlating with G(r)/ρ(r) at RCPs.…”

“…RSE values [27] or directly related QTAIM‐derived parameters [28] are easily accessible nowadays through theoretical calculations. Recently, a benchmark study for high‐quality RSE values in saturated three‐membered rings containing one group 13–16 element was reported [29] . The RSE for parent σ 3 λ 3 ‐oxaphosphirane was estimated [13,24] to be 22.4 kcal mol −1 , halfway between those of oxirane (26.4 kcal mol −1 ) [13,28] and phosphirane (19.4 kcal mol −1 ) [13,28] .…”

“…Recently, a benchmark study for high-quality RSE values in saturated threemembered rings containing one group 13-16 element was reported. [29] The RSE for parent σ 3 λ 3 -oxaphosphirane was estimated [13,24] to be 22.4 kcal mol À 1 , halfway between those of oxirane (26.4 kcal mol À 1 ) [13,28] and phosphirane (19.4 kcal mol À 1 ). [13,28] k-P-complexation with pentacarbonylmetal(0) units leading to complexes VI turned out to have little effect on RSE, [30] whereas a significant RSE enhancement was found by complexation with BH 3 .…”

Synthesis of azadiphosphiridine complexes. Theoretical studies on ring formation, the P-to-P′ metal shift and the resulting nitrogen geometry

“…20 kcal mol −1 ). 3,4 In the series of P-heterocycles bearing two phosphorus atoms, mainly diphos-phiranes ( Ia , Fig. 1) have been intensively investigated.…”