Abstract
The interaction of imidazole with a [Cu(acac)2] complex was studied using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), hyperfine sublevel correlation spectroscopy (HYSCORE), and density functional theory (DFT). At low Im ratios (Cu:Im 1:10), a 5-coordinate [Cu(acac)2Imn=1] monoadduct is formed in frozen solution with the spin Hamiltonian parameters g1 = 2.063, g2 = 2.063, g3 = 2.307, A1 = 26, A2 = 15, and A3 = 472 MHz with Im coordinating along the axial direction. At higher Im concentrations (Cu:Im 1:50), a 6-coordinate [Cu(acac)2Imn=2] bis-adduct is formed with the spin Hamiltonian parameters g1 = 2.059, g2 = 2.059, g3 = 2.288, A1 = 30, A2 = 30, and A3 = 498 MHz with a poorly resolved 14N superhyperfine pattern. The isotropic EPR spectra revealed a distribution of species ([Cu(acac)2], [Cu(acac)2Imn=1], and [Cu(acac)2Imn=2]) at Cu:Im ratios of 1:0, 1:10, and 1:50. The superhyperfine pattern originates from two strongly coordinating N3 imino nitrogens of the Im ring. Angular selective 14N ENDOR analysis revealed the NA tensor of [34.8, 43.5, 34.0] MHz, with e2qQ/h = 2.2 MHz and η = 0.2 for N3. The hyperfine and quadrupole values for the remote N1 amine nitrogens (from HYSCORE) were found to be [1.5, 1.4, 2.5] MHz with e2qQ/h = 1.4 MHz and η = 0.9. 1H ENDOR also revealed three sets of HA tensors corresponding to the nearly equivalent H2/H4 protons in addition to the H5 and H1 protons of the Im ring. The spin Hamiltonian parameters for the geometry optimized structures of [Cu(acac)2Imn=2], including cis-mixed plane, trans-axial, and trans-equatorial, were calculated. The best agreement between theory and experiment indicated the preferred coordination is trans-equatorial [Cu(acac)2Imn=2]. A number of other Im derivatives were also investigated. 4(5)-methyl-imidazole forms a [Cu(acac)2(Im-3)n=2] trans-equatorial adduct, whereas the bulkier 2-methyl-imidazole (Im-2) and benzimidazole (Im-4) form the [Cu(acac)2(Im-2,4)n=1] monoadduct only. Our data therefore show that subtle changes in the substrate structure lead to controllable changes in coordination behavior, which could in turn lead to rational design of complexes for use in catalysis, imaging, and medicine.
Highlights
The interaction of metal ions with bioligands, including proteins, nucleic acids, and their components, forms a central part of medicinal inorganic chemistry.[1,2] These interactions are important from a biological perspective because metals ions play an essential role in many biological processes.[3]
We demonstrate how the coordination mode and structure of the resulting adducts can be investigated using a combination of advanced electron paramagnetic resonance (EPR) techniques and density functional theory (DFT)
The 1H electron nuclear double resonance (ENDOR) spectra of this ligand in the unbound [Cu(acac)2] complex is deceptively complex, bearing couplings that arise from the methine protons, the fully averaged methyl group protons, and a subset of methyl group protons undergoing hindered rotation on the EPR time scale such that a very anisotropic hyperfine tensor is produced.[26]
Summary
The interaction of metal ions with bioligands, including proteins, nucleic acids, and their components, forms a central part of medicinal inorganic chemistry.[1,2] These interactions are important from a biological perspective because metals ions play an essential role in many biological processes.[3]. The experimental and simulated EPR spectra of the [Cu(acac)2Imn=1,2] mono- or bis-adducts (obtained at Cu:Im ratios of 1:5 and 1:50) are shown in Figures 1b and c, respectively.
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