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

Abstract

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.

Catalytic synthesis of azoarenes via metal-mediated nitrene coupling

“…Two bulky alkoxide donors in H 2 [OO] Ph were linked together with a terphenyl linker. 76,77 The resulting Fe(II) complex Fe[OO] Ph (THF) 2 (Fig. 16) replicated key structural features of the complexes with monodentate alkoxides Fe (OR) 2 (THF) 2 and Fe(OR′) 2 (THF) 2 , featuring overall seesaw geometry.…”

Nitrene-transfer from azides to isocyanides: Unveiling its versatility as a promising building block for the synthesis of bioactive heterocycles

“…They investigated its reactivity in catalyzing nitrene transfer to isocyanide 4b within a bis(alkoxide) ligand environment (as illustrated in Scheme 4 ). 35 Their research team explored the use of less bulky aryl azides, which, however, did not lead to the formation of carbodiimides due to the absence of Cr(IV) imido, a crucial intermediate. Their research deduced that carbodiimide synthesis via nitrene transfer is feasible only in the presence of bulky aryl azides or aryl isocyanides.…”

Transition metal-catalysed carbene- and nitrene transfer to carbon monoxide and isocyanides

“…80 In 2020 the same research group reported the use of dichromium complex 167 with chelating bis(alkoxide) ligands in the context of catalytic synthesis of carbodiimides (165) from azides (130) and isocyanides (5) (Scheme 43). 81 The authors observed that only sterically encumbered azides were able to participate in the nitrene transfer reaction. Based on their previous work (Scheme 42) the authors postulate that non-bulky azides form a saturated catalytically inactive Cr IVbis(imido) complex.…”