An EPR and ENDOR investigation of a series of Cu(II) transition metal complexes

Copper complexes have an enormous potential as cytotoxic compounds. Understanding the subtle interactions of these complexes and the variable modes of coordination with biologically relevant bases is mandatory from a biological perspective. However, in this thesis the more specific context is to investigate in detail the electronic and structural aspects of various copper complexes including [Cu(acac)2], [Cu(acac)(N-N)]+ and the multidentate ligand based complex [Cu(aNbN-aN)], and their interaction with target nitrogen bases. Therefore, an in-depth study on the configurational aspects of adducts formed between [Cu(acac)2] and imidazole (Im) was performed using ENDOR, HYSCORE and DFT calculations, providing a detailed analysis of the decoded trans-equatorial [Cu(acac)2(Im)n = 2] conformation. Based on N-imine coordinated complexes, a series of square planar Casiopeina Cu2+ complexes, consisting of one acetylacetonate and one diamine ligand, have been investigated by EPR and ENDOR spectroscopy. These diimine ligands were selected in light of the fact that the size of the aromatic diimine ligand may influence the therapeutic activity. Subtle but not neglectable electronic and structural changes of these [Cu(acac)(N-N)]+ complexes were detected by using X-,Q- and W-band CW EPR and 1H and 14N ENDOR measurements, revealing distortion and strain effects in the ligand system. Multidentate ligand based complexes [Cu(aN-bN-aN)] formed with e.g. di(2-picolyl)amine, tris(2-pyridylmethyl)amine and tris(2-aminoethyl)amine have been investigated by EPR and ENDOR spectroscopy. The focus of this investigation is to utilise ENDOR spectroscopy to examine copper complexes bearing inequivalent coordination nitrogen nuclei. Interesting electronical and structural features have been observed, illustrating the nature of the multidentate mixed nitrogen ligand

Probing the structure of copper(II)-casiopeina type coordination complexes [Cu(O-O)(N-N)]+ by EPR and ENDOR spectroscopy

Although copper based complexes have been widely used in homogeneous catalysis, more recently they are attracting considerable attention as pharmaceutical therapeutic agents. Of paramount importance in their efficacy of use is their structure and electronic properties, which can be thoroughly probed using advanced EPR techniques. In this study, a series of [Cu(acac)(N-N)]+ Casiopeina type complexes were investigated, bearing a series of diimine N-N ligands (including bipy, phen, Py-bipy and dppz). All complexes displayed rhombic g and CuA tensors, although the extent of rhombicity was dependent on the N-N ligand. Greater Cu(II)-N2 in-plane distortion, away from the square planar arrangement, was detected by CW W-band EPR for the smaller bipy and phen ligands compared to the larger Py-bipy and dppz ligands. Changes in ligand spin density distributions (over the 1H and 14N nuclei) were revealed by CW Q-band ENDOR. The largest components of the 1H imine and 14N hyperfine coupling decreased as the ligand size increased, following the trend bipy > phen > Py-bipy > dppz. These results indicate how even small structural and electronic (spin density) perturbations within the Casiopeina family of Cu(II) complexes can be probed by advanced EPR methods

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)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

The role of copper speciation in the low temperature oxidative upgrading of short chain alkanes over Cu/ZSM-5 catalysts

Partial oxidative upgrading of C1–C3 alkanes over Cu/ZSM‐5 catalysts prepared by chemical vapour impregnation (CVI) has been studied. The undoped ZSM‐5 support is itself able to catalyse selective oxidations, for example, methane to methanol, using mild reaction conditions and the green oxidant H2O2. Addition of Cu suppresses secondary oxidation reactions, affording methanol selectivities of up to 97 %. Characterisation studies attribute this ability to population of specific Cu sites below the level of total exchange (Cu/Al<0.5). These species also show activity for radical‐based methane oxidation, with productivities exceeding those of the parent zeolite supports. When tested for ethane and propane oxidation reactions, comparable trends are observe

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