Synthesis and Reactivity of Stable Open-Shell Gallylene

Treatment of an excess of gallium metal with iodine and 1,2-bis[(2,6-dibenzhydryl-4-methylphenyl)imino]acenaphthene (Ar^BIG-bian) in toluene at reflux affords the deep green radical [Ar^BIG-bianGaI₂] (<b>1</b>). Product <b>1</b> can also be synthesized by reacting Ar^BIG-bian with "GaI" in toluene. The chloride and bromide derivatives, [Ar^BIG-bianGaCl₂] (<b>2</b>) and [Ar^BIG-bianGaBr₂] (<b>3</b>), were prepared by the reactions of corresponding gallium(III) halides with Ar^BIG-bian in the presence of an excess of gallium metal in toluene. The reduction of radical <b>1</b> by potassium metal in toluene produces stable gallylene with a paramagnetic ligand [Ar^BIG-bianGa] (<b>4</b>). The reaction of <b>4</b> with <i>n</i>-propyl bromide (1:1) affords gallium(III) derivatives, <b>3</b> and [Ar^BIG-bianGa^<i>n</i>Pr] (<b>5</b>). In the course of the reaction of <b>4</b> with tetramethylthiuram disulfide (2:1), the gallium(I) center transfers two electrons: one to metallacycle and another one to the S-S bond of the substrate. The resulting product is the gallium(III) derivative [Ar^BIG-bianGaS₂CNMe₂] (<b>6</b>). Complexes <b>1-6</b> have been characterized by elemental analysis and IR spectroscopy, and their molecular structures have been determined by single-crystal X-ray analysis. The paramagnetic complexes <b>1-4</b> have been characterized by ESR spectroscopy and the diamagnetic compounds <b>5</b> and <b>6</b> by NMR spectroscopy. Based on DFT calculations, the electronic structures of molecules <b>2-5</b> and the thermodynamics of reactions accompanying the interaction of <b>4</b> with <i>n</i>-propyl bromide were studied. The computational results confirm the localization of the radical center in molecules <b>2</b>-<b>4</b> on the bian fragment and visualize the gallium lone pair directed along the NGaN bisector in <b>4</b>. Thermodynamically favorable reactions leading to the <b>3</b> and <b>5</b> products have been identified.