Title
Visualizing diastereomeric interactions of chiral aminechiral copper salen adducts by EPR spectroscopy and DFT Visualizing diastereomeric interactions of chiral aminechiral copper salen adducts by EPR spectroscopy and DFT
Author
Faculty/Department
Faculty of Sciences. Physics
Publication type
article
Publication
Easton, Pa ,
Subject
Chemistry
Source (journal)
Inorganic chemistry / American Chemical Society. - Easton, Pa
Volume/pages
50(2011) :15 , p. 6944-6955
ISSN
0020-1669
ISI
000293036000016
Carrier
E
Target language
English (eng)
Full text (Publishers DOI)
Affiliation
University of Antwerp
Abstract
Single enantiomers of R/S-methylbenzylamine (MBA) were found to selectively form adducts with two chiral Cusalen complexes, [CuII(1)] (H21 = N,N′-bis(3,5-ditert-butylsalicylidene)-1,2-diaminocyclohexane) and [CuII(2)] (H22 = N,N′-bis-salicylidene-1,2-cyclohexanediamino). The axial g/A spin Hamiltonian parameters of the CuMBA adducts were typical of 5-coordinate species. Enantiomer discrimination in the MBA binding was directly evidenced by W-band CW EPR, revealing an 86 ± 5% preference for formation of the R,R-[Cu(1)] + S-MBA adducts compared to R,R-[Cu(1)] + R-MBA; this was reduced to a 57 ± 5% preference for R,R-[Cu(2)] + S-MBA following removal of the tert-butyl groups. The structure of these diastereomeric adducts was further probed by different hyperfine techniques (ENDOR and HYSCORE), although no structural differences were detected between these adducts using these techniques. The diastereomeric adducts were found to possess lower symmetry, as evidenced by rhombic g tensors and inequivalent Himine couplings. This was caused by the selective binding mode of MBA onto one side of the chiral CuII complex. DFT calculations were performed on the R,R-[Cu(1)] + S-MBA and R,R-[Cu(1)] + R-MBA adducts. A distinct difference in orientation and binding mode of the MBA was identified in both adducts, confirming the experimental results. The preferred heterochiral R,R-[Cu(1)] + S-MBA adduct was found to be 5 kJ mol1 lower in energy compared to the homochiral adduct. A delicate balance of steric repulsion between the α-proton (attached to the asymmetric carbon atom) of MBA and the methine proton (attached to the asymmetric carbon atom) of [Cu(1)] was crucial in the stereoselective binding.
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