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.
