On the Electronic Structures of the 1,3‐Diboracyclobutane‐1,3‐diyls and Their Valence Isomers with a B₂E₂ Skeleton (E=N, P, As)

Abstract

The concept of through-space versus through-bond interactions on the stabilization of biradical structures with a singlet or triplet ground state is evaluated for the 1,3-diboracyclobutane-1,3-diyls and related congeners. Singlet biradicals are favored when the intermediate units E feature singlet character (PH(2) (+), AsH(2) (+)), while E fragments with triplet character (NH(2) (+)) induce small energy separations between the lowest singlet and triplet states. These considerations are supported by quantum chemical calculations with energy optimization at 1) MCSCF level plus MR-MP2 correction, 2) MR-MP2 level, and 3) two different types of density functional levels for the planar (D(2h)) geometries. The singlet-triplet energy separations in the planar compounds increase with increasing singlet stability of the corresponding E fragments. In addition to this newly developed principal features for singlet stabilization, which primarily occurs in bonded structures with higher main-group elements, the corresponding valence isomers with bicyclobutane, cyclobutene and cis-butadiene structures are investigated.

Electronic Structure of N2P2 Four‐Membered Rings

“…Oppositely, the optimized geometries of compounds 1 and 2 in their lowest triplet excited states bear anti-pyramidalization of the two N atoms, leading to a loss of planarity of the fourmembered ring. The DE S-T values for compounds 1 and 2 are similar to other reported B 2 E 2 (E = N, P, As) four-ring compounds, [24] but smaller than in N 2 S 2 . Hence, the population of the antibonding p* LUMO orbital leads to a loss of aromaticity and a concomitant increase in the biradicaloid character.…”

“…The adiabatic DE S-T values are similar at both levels of theory, but they are relatively large for yielding pure biradicaloid species. The DE S-T values for compounds 1 and 2 are similar to other reported B 2 E 2 (E = N, P, As) four-ring compounds, [24] but smaller than in N 2 S 2 .…”

Let’s Play with Valence Isomers: The Influence of Different Main Group Elements on Their Relative Stability

“…Indeed, calculations predict the BNBN diradical to be some 350 kJ mol À1 higher in energy than its butadiene isomer. 42 The synthesis of 11 P , 14 P , and 19 P suggests that the unique electronic properties of heavier main group elements will allow for the preparation of a variety of new structural moieties that are difficult or impossible to access in the corresponding carbon and nitrogen series. …”

“…Our own calculations, 40,42 carried out on the parent compound XIX P (H atoms at phosphorus and boron centers) showed that the singlet state is lower in energy than the triplet state by 17.2 kcal/mol, suggesting an interaction between the two radical sites. The nature of the coupling of the two radical sites (positive p-orbital overlap) was apparent from the highest occupied molecular orbital (HOMO) (Fig.…”

Substituent effect on the energy barrier for σ-bond formation from π-single-bonded species, singlet 2,2-dialkoxycyclopentane-1,3-diyls

“…The 2,2-electron-withdrawing-group-substituted singlet 1,3-diradicals are categorized as Type-1 diradicals [1,27], which possess a π-single-bonding character (–π–, closed-shell character) between the two radical sites. The role of the alkoxy group (OR) on the lifetime ( k = 1/τ) was investigated by combined studies of experiments and quantum chemical calculations [26,28].…”