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](Ph) (H(2)[OO](Ph) = [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](Ph)(THF)(2) (in toluene/THF, THF = tetrahydrofuran) and dinuclear Cr(2)([OO](Ph))(2) (in CH(2)Cl(2)/THF). The electronic structure and bonding in Cr[OO](Ph)(THF)(2) were probed by density functional theory calculations. Isolated Cr(2)([OO](Ph))(2) undergoes facile reaction with 4-MeC(6)H(4)N(3), 4-MeOC(6)H(4)N(3), or 3,5-Me(2)C(6)H(3)N(3) to yield diamagnetic Cr(VI) bis(imido) complexes; a structure of Cr[OO](Ph)(N(4-MeC(6)H(4)))(2) was confirmed by X-ray crystallography. The reaction of Cr(2)([OO](Ph))(2) with bulkier azides N(3)R (MesN(3), AdN(3)) forms paramagnetic products, formulated as Cr[OO](Ph)(NR). The attempted formation of a Cr–alkylidene complex (using N(2)CPh(2)) instead forms chromium(VI) bis(diphenylmethylenehydrazido) complex Cr[OO](Ph)(NNCPh(2))(2). 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(2)([OO](Ph))(2) or H(2)[OO](Ph) with NO(+) leads to the formation of [1,2-b]-dihydroindenofluorene, likely obtained via carbocation-mediated cyclization of the ligand.