Interactions of an asymmetric amine with a non-C2 symmetric Cu-salen complex: an EPR/ENDOR and HYSCORE investigation

Phys Chem Chem Phys. 2011 Dec 7;13(45):20427-34. doi: 10.1039/c1cp22522g. Epub 2011 Oct 13.

Abstract

Single enantiomers of R-/S-methylbenzylamine (MBA) were found to selectively form adducts with the chiral non-C(2) symmetric Cu-salen complex N-(3,5-di-tert-butylsalicylidene)-N'-(salicylidene)-cyclohexane-1,2-diamine copper(II), hereafter labelled [Cu(3)]. The g/A spin Hamiltonian parameters of this Cu(II) complex showed a decrease in symmetry from axial to rhombic upon formation of the [Cu(3)] + MBA adducts. The selectivity in enantiomeric discrimination was found to be only 59 ± 5% in favour of the heterochiral R,R'-[Cu(3)] + S-MBA and S,S'-[Cu(3)] + R-MBA adducts. This was directly evidenced by W-band EPR spectroscopy. The observed low selectivity for enantiomer discrimination is primarily attributed to the loss of the bulky tert-butyl groups from the 3,5 positions of [Cu(3)] compared to the parent N,N'-bis(3,5-di-tert-butylsalicylidene)-cyclohexane-1,2-diamine copper(II) ligand (labelled [Cu(1)]). The structure of the [Cu(3)] complex in the presence and absence of coordinating amine was further investigated by analysis of the ligand hyperfine interactions, as revealed through Q-band CW-ENDOR, X-band Davies ENDOR and HYSCORE. (1)H couplings from the -NH(2) group of the amine, observed by ENDOR and HYSCORE, provided direct evidence of amine coordination.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Coordination Complexes / chemistry*
  • Copper / chemistry*
  • Cyclohexylamines / chemistry
  • Electron Spin Resonance Spectroscopy* / methods
  • Ethylenediamines / chemistry*
  • Isomerism
  • Phenethylamines / chemistry*

Substances

  • Coordination Complexes
  • Cyclohexylamines
  • Ethylenediamines
  • Phenethylamines
  • Copper
  • disalicylaldehyde ethylenediamine
  • 1,2-cyclohexanediamine
  • 1-phenethylamine