Formation of a penta- or hexacoordinated Cu-(II) semicarbazone complex: Revisiting semicarbazone metal complexes

The structural characterization of a new pentacoordinated semicarbazone copper complex, [Cu(N,N',OHSCPy)(N,N'-HSCPy)][ClO4](2) (1) is reported. The structure of 1 was compared to the hexacoordinated complex, [Cu(N,N',O-HSCPy)2][ClO4](2)center dot H2O (2), which was already published, to shed some light on the formation of both forms. Energetic data of Cu-ligand bonds showed that the absence of a Cu-O bond in cation 1 does not significantly change the total stabilization energy in the pentacoordinate cation relative to the hexacoordinate cation (less than 4%). This is because the Cu-ligand bonds have their energies redistributed into the five existing bonds of 1 . Additionally, we extended the study revisiting analogous HSCPy complexes with other coordinated metals (M), with general formula [M(N,N',OHSCPy)2][X]2 center dot solvent, where M = Ni (3), Co (4), Zn (5), Fe (6), and Mn (7); X = ClO4, and solvent = H2O (3-6) or EtOH (7); and complex [Cu(N,N',O-HSCPy)2][ClO4][I3] (8) was included in this series. In general, all additional complexes showed similar behavior to that of the Cu-hexacoordinated complex (2). In this sense, was possible to observe that the energies of the bonds M-O and M-N have strong linear correlation in the three groups: M-O3'//M-O3, M-N1'//M-N1, and M-N2'//M-N2, for complexes 2-7 (r = 0.9766; N = 18). Also, a strong correlation between stabilization energy data of metal-ligand and their respective bond lengths (r = 0.9393; N = 47) was observed in all studied complexes (1 8). The supramolecular geometric similarity index ID was calculated comparing 2 with 1, 3-8 . It was observed a high geometric similarity between 2 and 3-6 (ID < 0.95), indicating an isostructural behavior. The comparison between 2 and complexes 1,8 indicated values in the intermediate region (ID = 0.75) and close to a border region with an intermediate-to-high region of similarity (ID = 0.83), respectively. The similarity comparison between 2 and 7 had a value of 0.77, in the intermediate region of similarity, showing that the change of a water molecule for an ethanol molecule considerably affected the crystal packing. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study reports the structural characterization of a new pentacoordinated semicarbazone copper complex and evaluates its formation in comparison to a hexacoordinated complex. It was found that the energies of Cu-ligand bonds in the pentacoordinated cation are redistributed into the existing bonds, leading to minimal changes in total stabilization energy. Additionally, a strong linear correlation was observed between the energies of M-O and M-N bonds in various complexes.