Structural characterization of two solvates of a luminescent copper(II) bis(pyridine)-substituted benzimidazole complex

The acetonitrile solvate and the ethanol hemisolvate of bis(acetato-κO){5,6-dimethyl-2-(pyridin-2-yl)-1-[(pyridin-2-yl)methyl]-1H-benzimidazole-κ2 N 2,N 3}copper(II) have been structurally characterized. Both compounds exhibit π–π interactions.


Chemical context
Copper(II) complexes containing benzimidazole ligands exhibit anticancer properties involving reactive oxygen species and DNA interactions (Prosser et al., 2017;Lewis et al., 2016;Mal et al., 2014). Similar complexes show antibacterial activity (Chen et al., 2012). The biological activity suggests that Cu II -benzimidazole complexes have potential as chemotherapeutic and other pharmaceutical uses.
In addition to biological applications, Cu II complexes containing benzimidazole have been explored as catalysts. For example, one complex behaves as a ring-opening polymerization catalyst (Zaca et al., 2016). Others have been used as building blocks for the construction of metal-organic frameworks and coordination polymers (Li et al., 2011;Machura et al., 2010).

Spectroscopy
The absorption and emission spectra of Cu(Me 2 BzImpy 2 )-(OAc) 2 are shown in Fig. 1. In the UV region, the absorption spectrum is similar to that of the free ligand, Me 2 BzImpy 2 ,  but red-shifted ( max = 340 nm, 3.65 eV; " = 17,500 M À1 cm À1 ), as was observed for the zinc(II)  and platinum(II) (DeStefano & Geiger, 2017) complexes of Me 2 BzImpy 2 . In the previously reported complexes, the bands were assigned as ligandcentered * in nature based on the results of molecular orbital calculations. In addition to the features in the UV region of the spectrum, a ligand-field band is observed in the visible region ( max = 695 nm, 1.78 eV; " = 77 M À1 cm À1 ). The emission spectrum obtained using an excitation wavelength of 320 nm exhibits a band ( max = 383 nm, 3.24 eV) similar to those of the Zn II and Pt II complexes where there is evidence of involvement of the diimine in the emissive state (DeStefano & Geiger, 2017;Hissler et al., 2000).

Figure 2
View of 1 showing the atom-labeling scheme. Anisotropic displacement parameters of non-H atoms are drawn at the 30% probability level.

Figure 3
View of 2 showing the atom-labeling scheme. Anisotropic displacement parameters of non-H atoms are drawn at the 30% probability level. Hydrogen atoms are not shown. Only the major contributor to the disordered ethanol molecule is shown.

Structural commentary
The two copper complexes explored in this study differ in the co-crystallized solvent: 1 contains one acetonitrile molecule per copper complex, whereas 2 is an ethanol solvate with two symmetry-independent molecules of the copper complex per molecule of ethanol. The two independent molecules will be referred to as 2a and 2b. Representations of the asymmetric units of 1 and 2 along with the respective atom-labeling schemes are found in Figs. 2 and 3. The ethanol molecule in 2 is threefold disordered (see Refinement section for details).
In both 1 and 2, the coordination geometries of the copper ions are best described as distorted square planar with monodentate coordination of two acetate ligands in addition to the Me 2 BzImpy 2 ligand (see Figs. 2 and 3, and Tables 1 and  2). In 1, the uncoordinated oxygen atoms are 2.651 (3) and 2.676 (4) Å from the Cu II atom. In 2a, the corresponding distances are 2.471 (2) and 2.698 (3) Å ; in 2b, the distances are 2.546 (3) and 2.554 (3) Å . The oxygen atoms of the N 2 O 2 coordination sphere have a twist angle from the nitrogen atoms of 6.7 (2) for 1. These values are 17.2 (2) and 7.9 (2) for 2a and 2b, respectively. In 1, 2a and 2b, the two acetate ligands adopt anti conformations.
In 1 and 2, the coordinated pyridine and benzimidazole ring systems are approximately coplanar. The torsion angles are reported in Tables 1 and 2. In 1 the angle between the mean planes of the benzimidazole ring system and the coordinated pyridine is 0.89 (19) and in 2a and 2b the corresponding angles are 3.5 (2) and 4.91 (16) , respectively.

Supramolecular features
There are several types of hydrogen-bonding interactions present in 1, as seen in Table 3. The acetonitrile solvate participates as acceptor in C-HÁ Á ÁN hydrogen bonds in which an aromatic hydrogen atom (H9) is donor. Additionally, C-HÁ Á ÁO hydrogen bonds involving the uncoordinated acetate oxygen atom O2 as acceptor and the aromatic carbon atoms C10 and C16 as donors result in chains that run parallel to [1 10] (Fig. 4). In 2, the most significant hydrogen-bonding interactions (Table 4) involve the disordered ethanol solvate molecule, which participates as donor in O-HÁ Á ÁO hydrogen bonding with the uncoordinated acetate oxygen atoms O2 and O6 as acceptors.
stacking is prevalent in Cu II 1,10-phenanthroline complexes (Melnic et al., 2014) and it has been suggested as a necessary structural feature for the DNA-cleavage activity exhibited by these and similar complexes (McCann et al., 2013). stacking has also been implicated in the fluorescence quenching of amyloid-peptide, which could be of relevance to possible therapeutic applications of Cu II chelators in the treatment of Alzheimer's disease (Melnic et al., 2014).  Zaca et al., 2016). Excluding those complexes exhibiting Jahn-Teller distorted geometries, the average Cu-N(pyridine) and Cu-N(imidazole) bond distances found are 2.04 (2) and 2.00 (4) Å , respectively.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 6. Early in the refinement of 2, the ethanol hemisolvate molecule was found to be disordered. The disorder was modeled using three contributors. Successful refinement required the use of O-H, C-C and C-O distance restraints of 0.84, 1.53 and 1.43 Å , respectively, and restraints on the U ij components of the anisotropically refined atoms in the disordered ethanol molecule. The disorder model refined to occupancies of 0.411 (3):0.362 (3):0.227 (3). All H atoms were located in difference-Fourier maps for 1 and 2, except those associated with the disordered ethanol molecule. H atoms bonded to C atoms were refined using a riding model, with C-H = 0.95 Å and U iso (H) = 1.2U eq (C) for the aromatic positions; C-H = 0.99 Å and U iso (H) = 1.2U eq (C) for the methylene groups; and C-H = 0.98 Å and U iso (H) = 1.5U eq (C) for the methyl groups. The hydroxy H atoms in the disordered ethanol contributors were refined using a rotatinggroup model with C-O-H tetrahedral, distance restraints to acceptor atoms (O6 and symmetry-generated O2) and with U iso (H) = 1.5U eq (O).

Funding information
This work was supported by a Congressionally directed grant from the US Department of Education (grant No. P116Z100020) for the X-ray diffractometer and a grant from the Geneseo Foundation. . E73, 1616-1621 research communications Table 5 Significantinteractions (Å ) in 1 and 2.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Crystal data, data collection and structure refinement details are summarized in Table 1. Early in the refinement of (2), the ethanol hemisolvate molecule was found to be disordered. The disorder was modeled using three contributors. Successful refinement required the use of O-H, C-C and C-O distance restraints of 0.84 Å, 1.53 Å and 1.43 Å, respectively, and restraints on the U ij components of the anisotropically refined atoms in the disordered ethanol. The disorder model refined to occupancies of 0.411 (3) : 0.362 (3) : 0.227 (3). All H atoms were located in difference Fourier maps, except those associated with the disordered ethanol molecule. H atoms bonded to C atoms were refined using a riding model, with C-H = 0.95 Å and U iso (H) = 1.2U eq (C) for the aromatic positions; C-H = 0.99 Å and U iso (H) = 1.2U eq (C) for the methylene groups; and C-H = 0.98 Å and U iso (H) = 1.5U eq (C) for the methyl groups. The hydroxy H atoms in the disordered ethanol contributors were refined using a rotating group model with C-O-H tetrahedral, distance restraints to acceptor atoms (O6 and symmetry-generated O2) and with U iso (H) = 1.5U eq (O).