Synergistic Effects of Lewis Bases and Substituents on the Electronic Structure and Reactivity of Boryl Radicals

Abstract

Boryl radicals have the potential for the development of new molecular entities and for application in new radical reactions. However, the effects of the substituents and coordinating Lewis bases on the reactivity of boryl radicals are not fully understood. By using first-principles methods, we investigated the spin-density distribution and reactivity of a series of boryl radicals with various substituents and Lewis bases. The substituents, along with the Lewis bases, only affect the radical reactivity when an unpaired electron is in the boron pz orbital, that is, for three-coordinate radicals. We found evidence of synergistic effects between the substituents and the Lewis bases that can substantially broaden the tunability of the reactivity of the boryl radicals. Among Lewis bases, pyridine and imidazol-2-ylidene show a similar capacity for stabilization by delocalizing the spin density. Electron-donating substituents, such as nitrogen, more efficiently stabilize boryl radicals than oxygen and carbon atoms. The reactivity of a boryl radical is always boron based, irrespective of the spin density on boron.

Theoretical study of poly(azolyl)borane radical anions

“…• + H • ) of LB-complexed boranes, [18,19,[21][22][23]40] we next evaluate the feasibility of generating borane radical anions via a similar process (BR 3 -H → BR 3 -• + H • ) involving various hydrotris(azolyl)borates. We then contrast the bond dissociation energies (BDEs) of the B-H bond with that of the B-N bond in the borates to obtain the homolytic bond dissociation order, which can provide us a better understanding of the possible production of the proposed borane radical anions.…”

“…Experimentally observed boron-centered radicals, either as radical cations, [1][2][3] neutral radicals, [4][5][6][7][8][9] or radical anions, [10][11][12][13][14][15][16][17] all have the four-center-seven-electron (4c-7e) configuration (Figure 1). Although the three-center-five-electron (3c-5e) boron-centered radicals have been studied theoretically, [18,19] their experimental realization is very unlikely in the foreseeable future due to the lack of proper means to stabilize the unpaired electron. [9,20] For boron-based 4c-7e cationic and neutral radicals, the building block pool is large even only the variation of the Lewis base (LB) is considered.…”

“…[9,20] For boron-based 4c-7e cationic and neutral radicals, the building block pool is large even only the variation of the Lewis base (LB) is considered. [18,[21][22][23][24] Generations of LBs including amines, [25,26] phosphines, [27,28] nitrogen heterocycles, [6,29,30] carbenes, [31][32][33][34][35][36][37][38][39] intramolecular LBs, [9] etc., have been successfully incorporated into the boron-centered systems to afford stable boryl radicals. The effects of substitution groups (R) have also been studied in detail.…”

“…The effects of substitution groups (R) have also been studied in detail. For example, alkyl and aryl groups have been examined in experiment [36] and substitution groups containing heteroatoms like oxygen and nitrogen have been evaluated in theory [18,19] . Like LBs, π-electron withdrawing (accepting) groups, such as -CN, enhance radical stability.…”

“…Moreover, through proper combination of π-electron accepting LB and π-electron donating substituents, namely the "push-pull" effect, the reactivity of boryl radicals can be cooperatively modified in a range much wider than if only one type of the two functional moieties exists. [18] Additionally, a carefully designed 3c-5e boryl radical (BR 2…”