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 ZnII-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 ZnII-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).