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Visualizing diastereomeric interactions of chiral amine-chiral copper salen adducts by EPR spectroscopy and DFT

Murphy, Damien Martin, Caretti, Ignacio, Carter, Emma, Fallis, Ian Andrew, Göbel, Marcus, Landon, James Hugh, Doorslaer, Sabine Van and Willock, David James 2011. Visualizing diastereomeric interactions of chiral amine-chiral copper salen adducts by EPR spectroscopy and DFT. Inorganic Chemistry 50 (15) , pp. 6944-6955. 10.1021/ic200113u

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Single enantiomers of R/S-methylbenzylamine (MBA) were found to selectively form adducts with two chiral Cu–salen 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 Cu–MBA 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 mol–1 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.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Chemistry
Cardiff Catalysis Institute (CCI)
Subjects: Q Science > QD Chemistry
Publisher: American Chemical Society
ISSN: 0020-1669
Funders: EPSRC
Last Modified: 03 Feb 2022 10:43

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