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  • 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 [102]. 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 [103], 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 [104]. The stacking interactions
    between neighboring complex molecules expand binuclear units into 1D supramolecular chains running along [100] 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 [105]. 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 AWD 131-138 comprising 1D supramolecular chains running along [100] 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 [106], displayed in Fig. 2C and D.