Nickel(II), Copper(II) and Palladium(II) Complexes with Bis-Semicarbazide Hexaazamacrocycles: Redox-Noninnocent Behavior and Catalytic Activity in Oxidation and C–C Coupling Reactions

Abstract

Nickel­(II), copper­(II), and palladium­(II) complexes ML H , where M = Ni (1), Cu (2), Pd (3), and ML OMe , where M = Ni (4), Cu (5), Pd (6), have been prepared by reactions of NiCl2·6H2O, Cu­(OAc)2·H2O, and PdCl2(MeCN)2 with 14-membered bis-semicarbazide hexaazamacrocycles H 2 L H and H 2 L OMe in dimethylformamide (DMF). The compounds were characterized by elemental analysis, ESI mass spectrometry, IR, UV–vis, and 1D (1H, 13C) and 2D (1H-1H COSY, 1H-1H TOCSY, 1H-1H NOESY, 1H-13C HSQC, 1H-13C HMBC) NMR spectra (1, 3, 4, and 6), and X-ray diffraction (2, 4–6). The complexes with MIIN4 coordination environment have S = 0, 1/2, 0 ground states for Ni, Cu, and Pd, respectively. The electrochemical behavior of 1–6 was investigated in detail. The electronic structures of 1e-oxidized species were studied by EPR, UV–vis–NIR spectroelectrochemistry, and DFT calculations, indicating the redox-noninnocent behavior of the ligands. Compounds 1–6 were tested in the oxidation of styrene and C–C coupling (Henry and Knoevenagel condensations). Compounds 2 and 5 selectively catalyze the microwave-assisted oxidation of neat styrene to benzaldehyde (up to 88% yield), whereas the 1 and 4 catalytic systems afforded up to 99% β-nitroethanol yield with an appreciable diastereoselectivity toward the formation of the anti isomer.

Unprecedented synthesis of a 14-membered hexaazamacrocycle

“…Recently, we developed some approaches to 14-membered cyclic bis-semicarbazones [32][33][34][35] and bis-thiosemicarbazone [36], namely 7,14-dimethyl-1,2,4,8,9,11-hexaazacyclotetradeca-7,14-diene-3,10-diones and -3,10-dithiones. The prepared com- pounds were able to chelate various metal cations through the N1, N4, N8, and N11 atoms [37,38]. In continuation of our research on 1,2,4,8,9,11-hexaazamacrocycles, which are underexplored representatives of the PAMs family, we were particularly interested in the synthesis of more unsaturated and therefore more conformationally rigid compounds.…”

A general and stereoselective approach to 14-membered cyclic bis-semicarbazones involving BF3-catalyzed amidoalkylation of 2-(trimethylsilyloxy)propene

“…The prepared macrocycles were found to be able to chelate various metal cations through the N1, N4, N8, and N11 atoms. 15…”

Fe(III) Complexes in Cyclohexane Oxidation: Comparison of Catalytic Activities under Different Energy Stimuli

“…In continuation of our work on the catalytic oxidation of alkanes and related species with metal-hydrazone catalysts [24][25][26][27][34][35][36][37][38][39][40][41][42][43][44], herein, we report the synthesis and characterization of the new mononuclear Fe(III) complex [Fe(HL)(NO 3 )(H 2 O) 2 ]NO 3 (1) obtained…”