Intramolecular Hydride Transfer Reactions in (Formazanate)Boron Dihydride Complexes

Abstract

The solution-phase thermolysis of (formazanate)boron dihydrides (LBH 2 ; 2) results in the formation of aminoborane compounds (4) via a series of boron-to-ligand intramolecular hydride transfer reactions. Monitoring the reactions by NMR spectroscopy allowed identification of several intermediates, and a reaction mechanism is proposed. In the case of a ligand with an N-Mes substituent it was possible to characterize an intermediate (7b-i) during this transformation that shows an unexpected cyclohexadiene moiety, which results from hydride transfer to the ortho-position of the mesityl substituent. Two consecutive boron-to-ligand hydride transfers eventually result in reductive N−N bond cleavage to give triazaboroles as the final product.

Aluminum Complexes of N2O23– Formazanate Ligands Supported by Phosphine Oxide Donors

“…[29][30] Boron difluoride (BF 2 ) complexes of formazanate ligands have been extensively studied by the Gilroy 31 and Otten 32 groups. These compounds have been used as fluorescent cell-imaging agents, 33 acted as precursors to unusual boron-based heterocycles, [34][35] and employed as electrochemiluminescence (ECL) emitters. 36 By contrast, the coordination chemistry of formazanate ligands and heavier group-13 elements (i.e., Al, Ga, In) has been investigated to a lesser extent.…”

Formazanate coordination compounds: synthesis, reactivity, and applications

“…Boron substituents other than F. The production of boron dihydride (BH 2 ) complexes of chelating N-donor ligands offers unique opportunities for further reactivity involving the boron-bound hydrides and a representative reaction involving BH 2 formazanates is shown in Scheme 27. 89 The formazan was converted to its corresponding BH 2 complex 70 by treatment with BH 3 ÁSMe 2 in toluene at room temperature. Heating pure samples of 70 in C 6 D 6 resulted in consecutive thermallyinduced hydride transfer reactions to yield compounds 71-i/ii and 72-i/ii.…”

“…Step 4 generates a formally low-valent boron center via transfer of the remaining borohydride to the internal nitrogen atom in 75, which allows DFT calculations (B3LYP/6-31G(d)) indicated that the isomerization step was energetically uphill (5.0 kcal mol À1 for 70 -73) and that all other steps are energetically downhill. 89 These studies provided significant insight into the use of formazanate ligands as redox-active ligands as each hydride transfer corresponds to the transfer of two electrons and a proton and served as a starting point for further reactivity studies. As noted earlier (Scheme 24), a significant hurdle in the development of the two-electron reactivity of complexes of redox-active formazanate ligands is the realization of methods for the production of their dianion form.…”

“…10 nm compared to the parent BF 2 complex 59e and were relatively insensitive to the nature of the alkyne substituents Scheme 27 (a) Representative synthesis of BH 2 formazanate complex 70 and subsequent thermally-driven hydride transfer reactions. 89 (b) Proposed mechanism of thermally-induced hydride transfer reactions of BH 2 formazanate complex 70. 89 employed.…”

“…89 (b) Proposed mechanism of thermally-induced hydride transfer reactions of BH 2 formazanate complex 70. 89 employed. This insensitivity arises as a result of the tetrahedral geometry at boron, which prevents p-orbital overlap between the formazanate and substituted alkynes.…”