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Despite the current interest in structure and reactivity of sub-valent main group compounds, neutral boron analogues of N-heterocyclic carbenes have been elusive due to their high reactivity. Here we provide evidence that 2-electron reduction of a (formazanate)BF2 precursor leads to NaF elimination and formation of an N-heterocyclic boron carbenoid, and describe the formation of a series of unusual BN heterocycles that result from trapping of this fragment. Subsequent chemical oxidation by XeF2 demonstrates that the trapped (formazanate)B fragment retains carbenoid character and regenerates the boron difluoride starting material in good yield. These results indicate that the formazanate ligand framework provides a unique entry into sub-valent boron chemistry. [ABSTRACT FROM AUTHOR]

A range of tetrahedral bis(formazanate)zinc complexes with different steric and electronic properties of the formazanate ligands were synthesized. The solid-state structures for several of these were determined by X-ray crystallography, which showed that complexes with symmetrical, unhindered ligands prefer coordination to the zinc center via the terminal N atoms of the NNCNN ligand backbone. Steric or electronic modifications can override this preference and give rise to solid-state structures in which the formazanate ligand forms a 5-membered chelate by binding to the metal center via an internal N atom. In solution, these compounds show dynamic equilibria that involve both 5- and 6-membered chelates. All compounds are intensely colored, and the effect of the ligand substitution pattern on the UV-vis absorption spectra was evaluated. In addition, their cyclic voltammetry is reported, which shows that all compounds may be electrochemically reduced to radical anionic (L2Zn-) and dianionic (L2Zn2-) forms. While unhindered NAr substituents lie in the plane of the ligand backbone (Ar = Ph), the introduction of sterically demanding substituents (Ar = Mes) favors a perpendicular orientation in which the NMes group is no longer in conjugation with the backbone, resulting in hypsochromic shifts in the absorption spectra. The redox potentials in the series of L2Zn compounds may be altered in a straightforward manner over a relatively wide range (~700 mV) via the introduction of electron-donating or -withdrawing substituents on the formazanate framework. [ABSTRACT FROM AUTHOR]