Reactivity of Two-Electron-Reduced Boron Formazanate Compounds with Electrophiles: Facile N–H/N–C Bond Homolysis Due to the Formation of Stable Ligand Radicals

Abstract

The reactivity of a boron complex with a redox-active formazanate ligand, LBPh2 [L = PhNNC(p-tol)NNPh], was studied. Two-electron reduction of this main-group complex generates the stable, nucleophilic dianion [LBPh2]2–, which reacts with the electrophiles BnBr and H2O to form products that derive from ligand benzylation and protonation, respectively. The resulting complexes are anionic boron analogues of leucoverdazyls. N–C and N–H bond homolysis of these compounds was studied by exchange NMR spectroscopy and kinetic experiments. The weak N–C and N–H bonds in these systems derive from the stability of the resulting borataverdazyl radical, in which the unpaired electron is delocalized over the four N atoms in the ligand backbone. We thus demonstrate the ability of this system to take up two electrons and an electrophile (E+ = Bn+, H+) in a process that takes place on the organic ligand. In addition, we show that the [2e–/E+] stored on the ligand can be converted to E• radicals, reactivity that has implications in energy storage applications such as hydrogen evolution.

Structure and bonding in reduced boron and aluminium complexes with formazanate ligands

“…Also, the 1 H NMR spectrum shows two inequivalent resonances for the para-protons of the N-Ph rings at δ 6.09 and 6.34 ppm due to a descent in symmetry from the C 2v -symmetric precursor 2 2− . The NMR data of the reaction product are similar to those of the boron analogue, 32 and suggest that it contains a benzyl group that is attached to a N-atom of the ligand backbone. Consequently, we formulate the product as an anionic, ligand-benzylated complex…”

“…Both these values are somewhat smaller than those found for the boron analogue [ Bn 1][Na] (ΔH ‡ = 121 ± 5 kJ mol −1 ; ΔS ‡ = 77 ± 14 J mol −1 K −1 ). 32 A likely explanation for these differences is the ground-state destabilization of the N-C(Bn) bond which is indicated by the somewhat larger N-C(Bn) distance found by both experiment and theory (vide supra).…”

“…30,31 The 2-electron reduced formazanate boron compound (1 2− ) subsequently reacted with electrophiles (E + ) such as benzyl bromide (BnBr) and water (H 2 O) to form ligand-benzylated and -protonated products ( Bn 1 − and H 1 − in Scheme 1). 32 In these compounds, the formazanate ligands are modified by the 'storage' of [2e − /E + ], which could be converted to Bn • and H • radicals by the homolytic cleavage of the N-C (Bn) and N-H bonds, respectively (Scheme 1). 32 These reactions occur readily, because the boron-containing radical that is generated (1 •− ) is relatively stable due to the presence of a low-energy SOMO that is delocalized over all four N-atoms in the ligand backbone.…”

“…32 In these compounds, the formazanate ligands are modified by the 'storage' of [2e − /E + ], which could be converted to Bn • and H • radicals by the homolytic cleavage of the N-C (Bn) and N-H bonds, respectively (Scheme 1). 32 These reactions occur readily, because the boron-containing radical that is generated (1 •− ) is relatively stable due to the presence of a low-energy SOMO that is delocalized over all four N-atoms in the ligand backbone. 30,32 It is anticipated that the basicity (nucleophilicity), radical stability and N-C/N-H bond strength of compounds bearing functionalized formazanate ligands could be tuned via either ligand substituent effects or incorporation of a different central element (main group or transition metal) in the formazanate chelate ring.…”

Formazanate coordination compounds: synthesis, reactivity, and applications

“…Reaction of dianion 76 2À with benzyl bromide (BnBr) or H 2 O resulted in the production of novel anionic analogues of leucoverdazyls (e.g., 77) that demonstrated the ability of the formazanate ligand framework to store two electrons and an electrophile (Scheme 28b). 91 The N-Bn or N-H bonds formed were highly reactive, and favored homolytic cleavage pathways due to the inherent stability of the relevant radical anions (e.g., 76 À ). This was experimentally demonstrated, for example, by reacting 77 with the (2,2,6,6-tetramethylpiperidin-1yl)oxyl radical (TEMPO) in THF, resulting in the regeneration of 76 À and the formation of adduct 78.…”