Low-valent, multiply bonded, trigonal-planar Sb complex: Rational syntheses, dual acidic/Basic properties, and unexpected semiconducting characteristics

A 4-center, 6π-conjugated, multiply bonded trigonal-planar complex, [Sb{Cr(CO)5}3]- (1), was synthesized via the hydride abstraction of [HSb{Cr(CO)5}3]2- (1-H) with HBF4·H2O, with the release of high yields of H2. The oxidation state of the Sb atom in [Et4N][1] was well-defined as 0, which was evidenced by X-ray photoelectron spectroscopy and X-ray absorption near-edge structure. The distinct color-structure relationship of this low-valent Sb complex 1 toward a wide range of organic solvents was demonstrated, as interpreted by time-dependent density functional theory calculations, allowing the trigonal-planar 1 and the tetrahedral solvent adducts to be probed, revealing the dual acid/base properties of the Sb center. In addition, 1 showed pronounced electrophilicity toward anionic and neutral nucleophiles, even with solvent molecules, to produce tetrahedral complexes [(Nu)Sb{Cr(CO)5}3]n- [1-Nu; n = 2, Nu = H, F, Cl, Br, I, OH; n = 1, Nu = PEt3, PPh3, N,N-dimethylformamide (DMF), acetonitrile (MeCN)]. On the contrary, the Fe/Cr hydride complex [HSb{Fe(CO)4}{Cr(CO)5}2]2- (2-H) was obtained by treating 1 with [HFe(CO)4]-. Upon hydride abstraction of 2-H with HBF4·H2O or [CPh3][BF4], a multiply bonded Fe/Cr trigonal-planar complex, [Sb{Fe(CO)4}{Cr(CO)5}2]- (2), was produced in which the oxidation coupling Sb2-containing complexes [Sb2Cr4Fe2(CO)28]2- (3-Cr) and [HSb2Cr3Fe2(CO)23]- (3-H) were yielded as final products. Complex 3-Cr exhibited dual Lewis acid/base properties via hydridation and protonation reactions, to form 2-H or 3-H, respectively. Surprisingly, [Et4N][1] possessed a low energy gap of 1.13 eV with an electrical conductivity in the range of (1.10-2.77) × 10-6 S·cm-1, showing that [Et4N][1] was a low-energy-gap semiconductor. The crystal packing, crystal indexing, and density of states results of [Et4N][1] further confirmed the efficient through-space conduction pathway via the intermolecular Sb···O(carbonyl) and O(carbonyl)···O(carbonyl) interactions of the 1D anionic zigzag chain of 1.