Hans_Bauer96

Xenon tetrafluoride and WOF₄ react in CFCl₃ solvent to form the first transition-metal coordination complex of Xe^IV, F₃XeF_b- - -WOF₄. The complex was structurally characterized by Raman spectroscopy and single-crystal X-ray diffraction. Quantum-chemical analyses, such as MEPS, show the W- - -F_b bond is primarily an electrostatic, σ-hole bond with a smaller degree of covalent character.

Abstract

The coordination complex, F₃XeF_b- - -WOF₄, was synthesized in CFCl₃ solvent by reaction of the weak fluoride-ion donor and strong oxidative fluorinating agent, XeF₄, with the moderate-strength fluoride-ion acceptor, WOF₄. The compound is the only transition-metal coordination complex of Xe^IV and was characterized at low temperatures by single-crystal X-ray diffraction and Raman spectroscopy. Xenon tetrafluoride and WOF₄ coordinate trans to the W=O bond through a W- - -F_b bond. The XeF₃ moiety of F₃XeF_b- - -WOF₄ acquires a degree of [XeF₃]⁺ character upon coordination that is reflected by its Xe−F stretching frequencies which are intermediate with respect to those of XeF₄ and [XeF₃]⁺. Calculations show W- - -F_b is predominantly an electrostatic, σ-hole bond with a significant orbital contribution that accounts for the bent Xe−F_b- - -W angle. The calculations show F₃XeF_b- - -MOF₄ (M=Cr, Mo) are less stable than their W analogue, consistent with failed attempts to synthesize F₃XeF_b- - -MoOF₄.

Nature Chemistry, Published online: 27 September 2022; doi:10.1038/s41557-022-01053-5

By the action of a strong Lewis acid, isomerization of the tricyanomethane to the ketenimine, HN=C=C(CN)₂, is triggered, which in turn directly attacks an aromatic species in an electrophilic aromatic substitution.

Abstract

Electrophilic aromatic substitution (EAS) can provide a straightforward approach to the efficient synthesis of functionalized complex aromatic molecules. In general, Lewis acids serve as a beneficial stimulus for the formation of a Wheland complex, the intermediate in the classical S_EAr mechanism of EAS, which is responsible for H/E (E=electrophile) substitution under formal H⁺ elimination. Herein, we report an unusual variant of EAS, in which a complex molecule such as the tricyanomethane, HC(CN)₃, is activated with a strong Lewis acid (B(C₆F₅)₃) to the point where it can finally be used in an EAS. However, the Lewis acid here causes the isomerization of the tricyanomethane to the ketenimine, HN=C=C(CN)₂, which in turn directly attacks the aromatic species in the EAS, with simultaneous proton migration of the aromatic proton to the imino group, so that no elimination occurs that is otherwise observed in the S_EAr mechanism. By this method, it is possible to build up amino-malononitrile-substituted aromatic compounds in one step.