Structure determination of bound nitrogen-based adducts with copper(ii) acetylacetonato; an EPR, ENDOR and DFT study

Abstract

The adducts of bis(acetylacetonato)–copper(II), [Cu(acac)2], formed with a range of nitrogen heterocycles including pyridine (2), methylpyridines (3,4,5), amino-methylpyridines (6,7) and diazines (8,9,10) were investigated in frozen solution using X-band EPR and 1H ENDOR spectroscopy. The small perturbations to the EPR spin Hamiltonian parameters (g and CuA) were consistent with the axial coordination of the nitrogen bases to Cu(II), and found to be dependent on both the basicity and steric influence of the coordinating substrate. The detailed structure of two adducts was then investigated by angular selective (1)H ENDOR and DFT. For the [Cu(acac)2](pyridine) adduct, axial coordination of the substrate was found to occur via the pyridine nitrogen as expected, producing a characteristic (1)H hyperfine coupling ((H)Ai = −2.6, −2.04, 4.7 MHz; β = 36°; aiso = 0.2 MHz) arising from the ortho-(1)H in the ring 2 or 6 position. These results were confirmed by DFT. However, in the [Cu(acac)2](2-amino-6-methyl-pyridine) adduct, the ENDOR data revealed a substantially different (1)H hyperfine coupling ((H)Ai = −4.52, −3.35, 6.47 MHz; β = 14°; aiso = −0.47 MHz) arising from the –NH2 amino protons. Analysis of this experimentally derived tensor in conjunction with the calculated DFT tensors, revealed that the 2-amino-6-methyl-pyridine substrate binds to Cu(II) via the exocyclic amino pyridine nitrogen, but with a tilt angle of 20° of the pyridine ring away from the geometry optimised structure. These results reveal how important structural information on the coordination geometry of Cu(II) adducts can be obtained by (1)H ENDOR, but only when the complete angular dependency profile of the ENDOR data is thoroughly considered.

FeIII, CuII and ZnII Complexes of the Rigid 9-Oxido-phenalenone Ligand—Spectroscopy, Electrochemistry, and Cytotoxic Properties

“…Structure solution and refinement in the monoclinic space group C2/c revealed the structure of a tetranuclear complex of the formula [Ni 4 (OCH 3 ) 4 (acac) 4 (CH 3 OH) 4 ] (Figure S12 and Tables S3-S4 in the Supplementary Materials). This heterocubane-type compound was already reported [81] Our values recorded for [Cu(acac)2] are very similar to those from previous reports [84][85][86][87][88][89]. The higher HFS is in line with a lower degree of spin delocalisation from Cu II over the ligand for opocompared with acac -.…”

“…In solution [Cu(acac)2] adds coordinating solvent molecules in the two axial positions to form hexacoordinate species [Cu(acac)2(L)2] what has been studied in detail using EPR spectroscopy [84][85][86] and we assume the same behaviour for [Cu(opo)2]. Very similar spectra were obtained for the opo and acac complexes (Figure 3 The bond distances and angles are very similar to those reported previously for [Fe(opo) 3 ]•DMSO [57] and also to those of [Fe(acac) 3 ] [74], proving the high similarity of the two ligands.…”

The Origin of Catalytic Benzylic C−H Oxidation over a Redox‐Active Metal–Organic Framework

“…The magnitudes of the 14 N hyperfine ( A iso =−0.7 MHz) and quadrupole interactions are similar to those reported for pyridines coordinated at the axial position of Cu II , for example, in bis‐diketonate complexes. [ 29 , 30 ] Thus, ENDOR and HYSCORE data provide clear evidence that the paramagnetic signal observed is due to a Cu II site that is incorporated within the MOF framework. Overall, the experimental data provide good evidence for the proposed reaction cycle and complement the computational study recently reported for a Cu/POM system.…”

Dinuclear Nickel(II) and Copper(II) Complexes of 8‐Quinoline‐1H‐pyrazole‐3‐carboxamide: Crystal Structure, Magnetic Properties, and DFT Calculations

“…For both the Cu(II) and the Ni(II) complex the TD‐DFT calculated data deviates markedly from the experimentally found absorptions. This might be due to additional axial coordination through solvent molecules (EtOH) [59,60] . The DFT calculations do not consider coordinative bonding of the solvent but model only the polar surrounding generated by solvent molecules.…”