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

摘要

Nickel(II), copper(II), and palladium(II) complexes MLH, where M = Ni (1), Cu (2), Pd (3), and MLOMe, where M = Ni (4), Cu (5), Pd (6), have been prepared by reactions of NiCl2 center dot 6H(2)O, Cu(OAc)(2)center dot H2O, and PdCl2(MeCN)(2) with 14-membered bis-semicarbazide hexaazamacrocycles H2LH and H2LOMe in dimethylformamide (DMF). The compounds were characterized by elemental analysis, ESI mass spectrometry, IR, UV-vis, and 1D (H-1, C-13) and 2D (H-1-H-1 COSY, H-1-H-1 TOCSY, H-1-H-1 NOESY, H-1-C-13 HSQC, H-1-C-13 HMBC) NMR spectra (1, 3, 4, and 6), and X-ray diffraction (2, 4-6). The complexes with (MN4)-N-II 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% beta-nitroethanol yield with an appreciable diastereoselectivity toward the formation of the anti isomer.