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Insights into the Electronic Structure of CuII Bound to an Imidazole Analogue of Westiellamide

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journal contribution
posted on 01.12.2014 by Peter Comba, Nina Dovalil, Graeme R. Hanson, Jeffrey R. Harmer, Christopher J. Noble, Mark J. Riley, Bjoern Seibold
Three synthetic analogues of westiallamide, H3Lwa, have previously been synthesized (H3L1–3) that have a common backbone (derived from l-valine) with H3Lwa but differ in their heterocyclic rings (imidazole, oxazole, thiazole, and oxazoline). Herein we explore in detail through high-resolution pulsed electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) spectroscopy in conjunction with density functional theory (DFT) the geometric and electronic structures of the mono- and dinuclear CuII complexes of these cyclic pseudo hexapeptides. Orientation-selective hyperfine sublevel correlation, electron nuclear double resonance, and three-pulse electron spin echo envelope modulation spectroscopy of [CuII(H2L1)­(MeOH)2]+ reveal delocalization of the unpaired electron spin onto the ligating and distal nitrogens of the coordinated heterocyclic rings and that they are magnetically inequivalent. DFT calculations confirm this and show similar spin densities on the distal heteroatoms in the heterocyclic rings coordinated to the CuII ion in the other cyclic pseudo hexapeptide [CuII(H2L2,3,wa)­(MeOH)2]+ complexes. The magnetic inequivalencies in [CuII(H2L1)­(MeOH)2]+ arise from different orientations of the heterocyclic rings coordinated to the CuII ion, and the delocalization of the unpaired electron onto the distal heteroatoms within these N-methylimidazole rings depends upon their location with respect to the CuII dx2y2 orbital. A systematic study of DFT functionals and basis sets was undertaken to examine the ability to reproduce the experimentally determined spin Hamiltonian parameters. Inclusion of spin–orbit coupling (SOC) using MAG-ReSpect or ORCA with a BHLYP/IGLO-II Wachters setup with SOC corrections and ∼38% Hartree–Fock exchange gave the best predictions of the g and A(63Cu) matrices. DFT calculations of the 14N hyperfine and quadrupole parameters for the distal nitrogens of the coordinated heterocyclic rings in [CuII(H2L1)­(MeOH)2]+ with the B1LYP functional and the SVP basis set were in excellent agreement with the experimental data, though other choices of functional and basis set also provided reasonable values. MCD, EPR, mass spectrometry, and DFT showed that preparation of the dinuclear CuII complex in a 1:1 MeOH/glycerol mixture (necessary for MCD) resulted in the exchange of the bridging methoxide ligand for glycerol with a corresponding decrease in the magnitude of the exchange coupling.