Boron‐Boron One‐Electron Sigma Bonds versus B‐X‐B Bridged Structures

Abstract

The existence of one-electron B-B σ bonds, for two different sets of compounds, was investigated by analyzing their electron density with different tools, namely QTAIM, ELF, NCIPLOT, and NBO approaches. Our results indicate that although the generic label "one-electron sigma bond" is often used in the literature, the nature of these bonds varies considerably, or they even do not exist. The [B2 X6 ](-) radical anions give place to true covalent one-electron σ bonds, the stronger the more electronegative is the X substituent. When both boron atoms are substituents in a rigid aromatic moiety, such as naphthalene, to yield 1,8-disubstituted derivatives, two kinds of equilibrium structures are found, those also stabilized through a one-electron σ bond (X=OH, F, Cl, CN) and those stabilized by the formation of B-X-B bridges (X=H, OMe). These 1,8-BX2 naphthalene derivatives can be considered as analogues of 1,8-NX2 naphthalene proton sponges. While the latter are able to stabilize a proton between the two basic sites, the former are able to stabilize an electron between the two electron-deficient B atoms. Interestingly, when all the H atoms attached to B are substituted by phenyl groups no one-electron σ bonds B-B bonds are formed, due to the dispersion of the unpaired electron in the aromatic substituents.

Trapping One Electron between Three Beryllium Atoms: Very Strong One‐Electron Three‐Center Bonds

“…Strong one‐electron Be−Be bonds had been reported before in the literature, but for cationic (HBeBeH + ) rather than anionic species . Fullerenes also yield very stable anions, acting as electron traps, through the formation of one‐electron intermolecular C−C σ‐bonds and multicenter two‐electron bonds to give trimers analogous to H 3 + , whereas the high E ea of the radical anionic 1,8‐X,X′‐naphtalene boron‐containing derivatives is also related to the formation of B‐B one‐electron bonds . A very interesting review on odd‐electron bonds has been recently published by Clark …”

“…[15] Fullerenes also yield very stable anions, [16] acting as electron traps,t hrough the formation of one-electron intermolecular CÀC s-bonds and multicenter two-electron bondst og ive trimers analogous to H 3 + , [17,18] whereas the high E ea of the radical anionic 1,8-X,X'-naphtalene boron-containing derivatives is also related to the formation of B-B one-electron bonds. [19] Av ery interesting review on odd-electron bonds has been recently published by Clark. [20] In this context, we would like to highlight the role played by the scaffold that maintains the BeX groups close together.I n this field, like in supramolecular chemistry,n ot only the geom-The ability of as et of beryllium-substitutedc yclohexane derivatives to trap electrons wasd etermined by evaluating their electron affinities at the G4(MP2) level of theory.T he nature of bondinga nd the effect of the different substituents attached to berylliumw ere studied by different computationalm ethods (quantumt heory of atoms in molecules, electron localization function, natural bond orbital, and analysisoft he spin density), revealing the existence of ao ne-electron/Be 3 cyclic bondingi n trisubstituted species.…”

Hexahalodiborate Dianions: A New Family of Binary Boron Halides

“…Theo bserved BÀBb ond lengths are also in stark contrast with predicted values for the more weakly bound hypothetical [B 2 X 6 ]C À radical anions (> 2.11 ). [9] TheB ÀXb ond lengths in 1-4 (Scheme 1) are all significantly larger than those predicted for B 2 X 4 ( Table 1). TheB -X bond lengths in 1-4 are comparable to,although noticeably longer than, the BÀXbond length in the respective [BX 4 ] À anion.…”

One‐Electron Bonds in Frustrated Lewis Pair TPB Ligands: Boron Behaving as a Lewis Base

“…[1][2][3][4][5][6][7][8][9][10][11] Very often this peculiar bond is formed by two identical first-row atoms from Groups 13,14, and 15, [3][4][5][6][7][8][9][12][13][14][15] and dimers of boron compounds are particularly abundant among them. [3-7, 14,15] Very recently,i th as been shown that electronegative substituents directly attached to boron in [X 3 B·BX 3 ] À systems yield ELF (electron localization function) populations around one electron between the two boron atoms, [16] whereas conversely,aromatic substituents lead only to weak interactions.T hese results are in agreement with electron paramagnetic resonance (EPR) measurements, which evidence av ery different degree of coupling for these two types of substituents. [4,9,[14][15][16] Moreover,o ne-electron bonds may also appear in rigid frameworks.…”

“…[3-7, 14,15] Very recently,i th as been shown that electronegative substituents directly attached to boron in [X 3 B·BX 3 ] À systems yield ELF (electron localization function) populations around one electron between the two boron atoms, [16] whereas conversely,aromatic substituents lead only to weak interactions.T hese results are in agreement with electron paramagnetic resonance (EPR) measurements, which evidence av ery different degree of coupling for these two types of substituents. [4,9,[14][15][16] Moreover,o ne-electron bonds may also appear in rigid frameworks. [16] Indeed, structure constraints may be the key for the stability of some particular species.AB ÀCu one-electron bond in the neutral (TPB)Cu compound has been reported to be stable in the solid state (see Scheme 1).…”