Synthesis of Chromium(II) Complexes with Chelating Bis(alkoxide) Ligand and Their Reactions with Organoazides and Diazoalkanes.

Synthesis of new chromium(II) complexes with chelating bis(alkoxide) ligand [OO] (H[OO] = [1,1':4',1''-terphenyl]-2,2''-diylbis(diphenylmethanol)) and their subsequent reactivity in the context of catalytic production of carbodiimides from azides and isocyanides are described. Two different Cr(II) complexes are obtained, as a function of the crystallization solvent: mononuclear Cr[OO](THF) (in toluene/THF, THF = tetrahydrofuran) and dinuclear Cr([OO]) (in CHCl/THF). The electronic structure and bonding in Cr[OO](THF) were probed by density functional theory calculations. Isolated Cr([OO]) undergoes facile reaction with 4-MeCHN, 4-MeOCHN, or 3,5-MeCHN to yield diamagnetic Cr(VI) bis(imido) complexes; a structure of Cr[OO](N(4-MeCH)) was confirmed by X-ray crystallography. The reaction of Cr([OO]) with bulkier azides NR (MesN, AdN) forms paramagnetic products, formulated as Cr[OO](NR). The attempted formation of a Cr-alkylidene complex (using NCPh) instead forms chromium(VI) bis(diphenylmethylenehydrazido) complex Cr[OO](NNCPh). Catalytic formation of carbodiimides was investigated for the azide/isocyanide mixtures containing various aryl azides and isocyanides. The formation of carbodiimides was found to depend on the nature of organoazide: whereas bulky mesitylazide led to the formation of carbodiimides with all isocyanides, no carbodiimide formation was observed for 3,5-dimethylphenylazide or 4-methylphenylazide. Treatment of Cr([OO]) or H[OO] with NO leads to the formation of [1,2-b]-dihydroindenofluorene, likely obtained via carbocation-mediated cyclization of the ligand.