Understanding the Coordination Modes of [Cu(acac)₂(imidazole)_<i>n</i>=1,2] Adducts by EPR, ENDOR, HYSCORE, and DFT Analysis

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)₂Im_<i>n</i>=1] monoadduct is formed in frozen solution with the spin Hamiltonian parameters <i>g</i>₁ = 2.063, <i>g</i>₂ = 2.063, <i>g</i>₃ = 2.307, <i>A</i>₁ = 26, <i>A</i>₂ = 15, and <i>A</i>₃ = 472 MHz with Im coordinating along the axial direction. At higher Im concentrations (Cu:Im 1:50), a 6-coordinate [Cu­(acac)₂Im_<i>n</i>=2] bis-adduct is formed with the spin Hamiltonian parameters <i>g</i>₁ = 2.059, <i>g</i>₂ = 2.059, <i>g</i>₃ = 2.288, <i>A</i>₁ = 30, <i>A</i>₂ = 30, and <i>A</i>₃ = 498 MHz with a poorly resolved ¹⁴N superhyperfine pattern. The isotropic EPR spectra revealed a distribution of species ([Cu­(acac)₂], [Cu­(acac)₂Im_<i>n</i>=1], and [Cu­(acac)₂Im_<i>n</i>=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 ^N<i>A</i> tensor of [34.8, 43.5, 34.0] MHz, with e²<i>qQ</i>/<i>h</i> = 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 e²<i>qQ</i>/<i>h</i> = 1.4 MHz and η = 0.9. ¹H ENDOR also revealed three sets of ^H<i>A</i> 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)₂Im_<i>n</i>=2], including <i>cis</i>-mixed plane, <i>trans</i>-axial, and <i>trans</i>-equatorial, were calculated. The best agreement between theory and experiment indicated the preferred coordination is <i>trans</i>-equatorial [Cu­(acac)₂Im_<i>n</i>=2]. A number of other Im derivatives were also investigated. 4(5)-methyl-imidazole forms a [Cu­(acac)₂(Im-<b>3</b>)_<i>n</i>=2] <i>trans</i>-equatorial adduct, whereas the bulkier 2-methyl-imidazole (Im-<b>2</b>) and benzimidazole (Im-<b>4</b>) form the [Cu­(acac)₂(Im-<b>2,4</b>)_<i>n</i>=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