Copper(II) complexes with derivatives of salen and tetrahydrosalen: a spectroscopic, electrochemical and structural study

Salen type complexes, CuL, the corresponding tetrahydrosalen type complexes , Cu[H-4]L, and N,N'-dimethylated tetrahydrosalen type complexes, Cu[H2Me2] L, were investigated using cyclic voltammetry, and electronic and ESR spect roscopy. In addition, the analogous copper(II) complexes with a derivative of the tetradentate ligand 'salphen' [salphen=H(2)salphen=N,N'-disalicylide ne-1,2-diaminobenzene] were studied. Solutions of CuL, Cu[H-4]L and Cu[H2Me 2]L are air-stable at ambient temperature, except for the complex Cu(Bu-t, Me)[H-4]salphen [H-2(Bu-t, Me)[H-4]salphen=N,N'-bis(2-hydroxy-3 tert-butyl- 5-methylbenzyl)-1,2-diaminobenzene]. Cu(Bu-t, Me)[H-4]salphen interacts wit h dioxygen and the ligand is oxidatively dehydrogenated (-CH2-NH-->-C=N-) t o form Cu(Bu-t, Me)[H-2]salphen and finally, in the presence of base, Cu(Bu -t, Me)salphen. X-ray structure analysis of Cu(Bu-t, Me)[H2Me2]salen confir ms a slightly tetrahedrally distorted planar geometry of the CuN2O2 coordin ation core. The complexes were subjected to spectrophotometric titration wi th pyridine, to determine the equilibrium constants for adduct formation. I t was found that the metal center in the complexes studied is only of weak Lewis acidity. In dichlormethane, the oxidation Cu(II)/Cu(III) is quasireve rsible for the CuL type complexes, but irreversible for the Cu[H-4]L and Cu [H2Me2]L type. A poorly defined wave was observed for the irreversible redu ction Cu(II)/Cu(I) at potentials less than -1.0 V. The ESR spectra of CuL a t both 77 K and room temperature reveal that very well resolved lines can b e attributed to the interaction of an unpaired electron spin with the coppe r nuclear spin, N-14 donor nuclei and to a distant interaction with two equ ivalent protons [\A(Cu)(iso)\approximate to 253 MHz, \A(N)(iso)\approximate to 243 MHz, \A(N)(iso)\approximate to 20 MHz]. These protons are attached to the carbon atoms adjacent to the N-14 nuclei. In contrast to CuL, the nu mber of lines in the spectra of the complexes Cu[H-4]L and Cu[H2Me2]L is gr eatly reduced. At room temperature, only a quintet with a considerably smal ler nitrogen shf splitting constant [\A(N)(iso)\approximate to 27 MHz] is o bserved. Both factors, planarity and conjugation, are thus essential for th e observation of distant hydrogen shf splitting in CuL. Due to the C=N bond hydrogenation, the coordination polyhedra of the complexes Cu[H,]L and Cu[ H,Me,]L is more flexible and more sensitive to ligand modification than tha t of CuL. The electron-withdrawing effect of the phenyl ring of the phenyle nediamine bridge is reflected in a reduction of the copper hyperfine coupli ng constants in Cu('Bu, Me)[H,]salphen and Cu('Bu, Me)[H,Me,]salphen comple xes [\A(Cu)(iso)\approximate to 215 MHz]. (C) 1999 Elsevier Science Ltd. Al l rights reserved.