Understanding the Coordination Modes of [Cu(acac)2(imidazole)n=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_n=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)₂Im_n=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)₂Im_n=1], and [Cu(acac)₂Im_n=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)₂Im_n=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)₂Im_n=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.

Spectroscopic and reactivity differences in metal complexes derived from sulfur containing Triphos homologs

Spectroscopic, computational, and reactivity studies shed light on the different coordination behavior of sulfur containing Triphos derived complexes.