br The results of elemental analysis
The results of elemental analysis confirmed that aphaOMe ligand was formed and that there is one chloride, one acetate and one cad-mium ion per ligand molecule. The existence of acetate anion in 1 was confirmed by IR spectroscopy. Two very strong bands, characteristic for bidentate acetate coordination, were observed at 1541 (νas) and 1478 (νs) cm−1 . The molar conductivity of 1 in water is 20.0 Ω−1 cm2 mol−1, which is significantly less than threshold value for 1:1 electrolytes , confirming that 1 is molecular coordination compound. The general formula of 1 [Cd2Cl2(AcO)2(aphaOMe)2], is unequivocally derived from a single crystal X-ray diffraction experi-ment (vide infra).
3.2. Description of crystal structure
ORTEP drawings of the asymmetric unit and molecular structure of binuclear complex 1 are depicted in Fig. 1. The complex lies at the centre of inversion at 1 ½ ½. Each cadmium ion is coordinated with pyridine and imine nitrogen atoms from in situ obtained aphaOMe li-gand, two oxygen atoms from acetate ion and two bridging chloride ions [Cl(1) and its symmetry equivalent at 2 − x, 1 − y, 1 − z]. Due to deviation of ligands' and acetate ion bite angles from an ideal value of 90°, the geometry around each cadmium ion is distorted octahedral. The cis bond angles are in the wide range from 54.8 to 116.9°, while the trans ones vary from 142.0 to 160.1° (Table 2). All coordinative bond lengths are in the usual range (Table 2). As previously noted for related Cu(II) and Cd(II) complexes [74,76], the ester oxygen AWD 131-138 from aphaOMe ligand is not involved in coordination.
3D crystal packing is based on hydrogen bonds and π-π stacking in-teractions. Each NH group of the ligand is involved in hydrogen bonding
Scheme 1. (A) Structures of aphaOEt and aphaOMe ligands. (B) Synthesis of 1.
idine fragments are involved in π-π interactions with centroid-centroid distance of 3.675(4) Å. Stacked rings are almost perfectly align with face-
to-face orientation, as indicated by the corresponding displacement angle of 13.5° and respective shift of 0.861 Å . The stacking interactions
between neighboring complex molecules expand binuclear units into 1D supramolecular chains running along  direction (Fig. 2B). Within these chains, two out of four aromatic protons are involved in weak
To better understand crystal packing determinants, a qualitative ranking of intermolecular interactions is necessary, as qualitative rea-soning on intermolecular cohesion, based only on geometrical para-meters, can sometimes lead to erroneous conclusions . Therefore, a calculation of pairwise intermolecular interaction energies by whole-of-molecule approach, which avoids the focus on specific atom–atom interactions, was performed. The resulting interaction energies are summarized in Table S1 (Supplementary material). A general conclu-sion is that both electrostatic and dispersion terms play equally im-portant role in total interaction energies. The largest stabilizing energy (−158 kJ mol−1 per pair) is associated with two pairs comprising 1D supramolecular chains running along  direction, while the inter-action energy between four molecular pairs involved in formation of 2D
Fig. 1. Asymmetric unit (A) and perspective view and labelling of the molecular structure of 1 (B). Thermal ellipsoids are at the 40% probability level. Equivalent atoms are generated by the transformation i = 2 − x, 1 − y, 1 − z.
S. Bjelogrlić et al.
Table 2 r> Selected bond lengths (Å) and angles (°) for 1, with esd's in parentheses.
layers parallel to (1 −1) is lower (−100 kJ mol−1 per pair). Conse-quently, interactions between these molecular pairs stand out as structure determining, since other molecular pairs of the first co-ordination sphere have significantly lower stabilizing energies (< 9 kJ mol−1, see Table S1, Supplementary material). A topology of the intermolecular interaction energies for the crystal structure of 1 is summarized by an energy framework , displayed in Fig. 2C and D.