Trinuclear (aminonitrone)ZnII complexes as key intermediates in zinc(ii)-mediated generation of 1,2,4-oxadiazoles from amidoximes and nitriles

Abstract

Aliphatic and aromatic amidoximes RC(NH₂)NOH (R = Et, ^tBu, Ph, o-ClC₆H₄) react with Zn(OAc)₂·2H₂O in Me₂CO giving [Zn(OAc)₂{RC(NH₂)NOH}₂] complexes bearing N-bound amidoximes, which are involved in a moderate strength (7.3–11.9 kcal mol⁻¹ by the DFT calculations) intramolecular resonance-assisted hydrogen bonding between the oxime HO group and the oxo group of the acetate ligand. The complexes [Zn(OAc)₂{RC(NH₂)NOH}₂] react with excess Zn(OTf)₂ in acetone accomplishing trinuclear species [Zn₃(μ₂-OAc)₂{μ₂-RC(NH₂)N(H)O}₄(H₂O)₆](OTf)₄ featuring both O-ligated amidoximes—stabilized in the aminonitrone tautomeric form—and bridging acetate ligands. The aminonitrone trinuclear species were also prepared directly via the reaction of the amidoximes with Zn(OTf)₂ in EtOAc; ethyl acetate in this reaction plays the role of the acetate donor and OAc⁻ is generated in situ via Zn^II-mediated hydrolysis of EtOAc. Although [Zn(OAc)₂{RC(NH₂)NOH}₂] are inactive toward dimethylcyanamide, the [Zn₃(μ₂-OAc)₂{μ₂-RC(NH₂)N(H)O}₄(H₂O)₆](OTf)₄ complexes readily react with Me₂NCN giving, as a result of Zn^II-mediated amidoxime–cyanamide coupling, the O-carbamidine amidoxime complexes [Zn(OTf)₂{RC(NH₂)NOC(NMe₂)NH}₂]. All synthesized compounds were characterized by HRESI-MS, FTIR, ¹H-, CP-MAS TOSS ¹³C{¹H}-, and ¹³C{¹H} NMR, and additionally by single-crystal X-ray diffraction for eight species. Different types of non-covalent interactions in the obtained solid-state structures were studied by DFT calculations (M06-2X/6-311+G(d,p) level of theory) and topological analysis of the electron density distribution within the formalism of Bader's theory (QTAIM method).