Weak interactions in chain polymers [M(μ-X)₂ L ₂]_∞ (M = Zn, Cd; X = Cl, Br; L = substituted pyridine) – an electron density study

Abstract

The experimental electron-density distributions in crystals of five chain polymers [M(mu-X)2(py)2] (M = Zn, Cd; X = Cl, Br; py = 3,5-substituted pyridine) have been obtained from high-resolution X-ray diffraction data sets (sin theta/lambda > 1.1 A(-1)) at 100 K. Topological analyses following Bader's 'Atoms in Molecules' approach not only confirmed the existence of (3, -1) critical points for the chemically reasonable and presumably strong covalent and coordinative bonds, but also for four different secondary interactions which are expected to play a role in stabilizing the polymeric structures which are unusual for Zn as the metal center. These weaker contacts comprise intra- and inter-strand C-H...X-M hydrogen bonds on the one hand and C-X...X-C interhalogen contacts on the other hand. According to the experimental electron-density studies, the non-classical hydrogen bonds are associated with higher electron density in the (3, -1) critical points than the halogen bonds and hence are the dominant interactions both with respect to intra- and inter-chain contacts.

Tunable Crystal-to-Crystal Phase Transition in a Cadmium Halide Chain Polymer

“…Several compounds in this class undergo single-crystal to single-crystal transformations: We have encountered solid state reactivity [2], thermal degradation [3], and phase transitions [4]. In the context of a charge density study [5], we envisaged the well-crystallizing chain polymer [{Cd(µ-Br) 2 py 2 } 1 ∞ ] as a suitable candidate for high resolution X-ray diffraction experiments. We had previously determined its room temperature structure [6]; upon cooling, the compound undergoes a phase transition.…”

Halide-bridged polymers of divalent metals with donor ligands – structures and properties

“…Obvious possibilities for such a stabilization include attractive halogen· · ·halogen interactions along the polymer strand as well as non-classical hydrogen bonds [69]. A detailed study of these secondary interactions based on high resolution X-ray diffraction experiments followed by the topological analysis of the experimental electron density has very recently been completed [77]. In addition to the above-mentioned chain polymers, a small number of structurally related 2D networks involving octahedrally coordinated Zn(II) and crosslinking ditopic ligands is known; these compounds will be discussed in Section 3.2.…”

Synthesis, Structures, Thermal and Luminescence Properties of Zn and Cd Halide Coordination Polymers with 2‐Cyanopyrazine

“…[11,[19][20][21][22][23][24][25][26] For compounds based on metal halides, different substructures are reported, in which the metal cations are linked by the halide anions into dimers, chains or layers. [27][28][29][30][31][32][33] For, e.g. compounds based on cadmium halides and mono coordinating ligands, compounds are frequently observed, in which the Cd cations are linked by pairs of halide anions, sharing common edges into chains, such as for compounds with pyridine as coligand.…”