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

• Main Authors: Sudheer S. Kurup, Richard J. Staples, Richard L. Lord, Stanislav Groysman
• Format: Article
• Language: English
• Published: MDPI AG 2020-01-01
• Series: Molecules
• Subjects: alkoxides; azides; carbodiimides
• Online Access: https://www.mdpi.com/1420-3049/25/2/273

Synthesis of new chromium(II) complexes with chelating bis(alkoxide) ligand [OO]^Ph (H₂[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 Cr₂([OO]^Ph)₂ (in CH₂Cl₂/THF). The electronic structure and bonding in Cr[OO]^Ph(THF)₂ were probed by density functional theory calculations. Isolated Cr₂([OO]^Ph)₂ undergoes facile reaction with 4-MeC₆H₄N₃, 4-MeOC₆H₄N₃, or 3,5-Me₂C₆H₃N₃ to yield diamagnetic Cr(VI) bis(imido) complexes; a structure of Cr[OO]^Ph(N(4-MeC₆H₄))₂ was confirmed by X-ray crystallography. The reaction of Cr₂([OO]^Ph)₂ with bulkier azides N₃R (MesN₃, AdN₃) forms paramagnetic products, formulated as Cr[OO]^Ph(NR). The attempted formation of a Cr−alkylidene complex (using N₂CPh₂) instead forms chromium(VI) bis(diphenylmethylenehydrazido) complex Cr[OO]^Ph(NNCPh₂)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 Cr₂([OO]^Ph)₂ or H₂[OO]^Ph with NO⁺ leads to the formation of [1,2-b]-dihydroindenofluorene, likely obtained via carbocation-mediated cyclization of the ligand.