Abstract
The salen complex CuL and the corresponding tetrahydrosalen complex Cu[H
4]L were investigated by ESR spectroscopy and molecular orbital calculations [H
2L =
N,
N′-bis(3-
tert-butyl-5-methylsalicylidene)-2,3-diamino-2,3-dimethylbutane; H
2[H
4]L =
N,
N′-bis(2-hydroxy-3-
tert-butyl-5-methylbenzyl)-2,3-diamino-2,3-dimethylbutane]. X-ray structure determination of CuL confirmed a slightly distorted planar geometry of the CuN
2O
2 coordination core. The ESR spectra of CuL at both 78 K and room temperature revealed that very well resolved lines cannot be attributed to the interaction with copper nuclear spin and
14N donor nuclei alone. Computer simulation showed that in addition to copper hyperfine (
g
iso = 2.094, |
A
Cu(iso)| = 270 MHz, room temperature) and nitrogen superhyperfine structure [|
A
N(iso)| = 46 MHz] a distant interaction with two equivalent protons is also present [|
A
H(iso)| = 23 MHz]. These protons are attached to the carbon atoms adjacent to
14N nuclei. In contrast the number of lines in the spectrum of the hydrogenated analogue Cu[H
4]L is greatly reduced. At room temperature only a quintet with considerable smaller nitrogen shf constant [|
A
N(iso)|] = 25 MHz is observed. Thus, both factors planarity and conjugation, are essential for the observation of distant hydrogen shf splitting in CuL. The ESR findings are in good agreement with calculated spin densities by QR-INDO/1 method.