Chemistry
Dual Behavior of Bromine Atoms in Supramolecular Chemistry: The Crystal Structure and Magnetic Properties of Two Copper(II) Coordination Polymers
Abstract
The crystal structures of two copper(II) chloride coordination polymers, (Cu2(2bp)2Cl4)n, henceforth "polydimer", and (Cu3(2bp)2Cl6)n, henceforth "polytrimer", where 2bp = 2-bromopyridine, and the temperature dependence of their magnetic susceptibilities are reported. Also, the temperature dependence of the magnetic susceptibility of the monomeric complex, Cu(2bp)2Cl2, is reported. The magnetic data were measured in the range 2-310 K. The organic bromine atom of the 2bp ligand shows electrophilic and nucleophilic character: an electrophilic nature via C-Br···Cl interactions and a nucleophilic nature via Cu···(π)Br interactions. The Cu···(π)Br interactions, previously underestimated in the literature, turn the 2bp ligand to a bidentate rather than monodentate ligand via the N-Cu coordination bond and the Cu···(π)Br noncovalent interactions. Thus, the 2bp ligand behaves as a molecular bender; the Cu···(π)Br interactions result in the bending of the (CuCl2)n infinite linear extended chains at specific locations. The magnetic data of the monomer were fit to the two-dimensional S = 1/2 Heisenberg model (Curie constant = 0.43(1) emu-K/mol-Oe and an exchange constant J/k = 0.41(4) K). The magnetic data of the polydimer were best fit to an alternating antiferromagnetic chain rather than a dimer model (Curie constant = 0.415(3), J1/k = -11.4(1) K, J(2)/k = -2.2(3) K). The magnetic data of the polytrimer may be approximated by a model including both nearest neighbor (J1) and next nearest neighbor (J2) interactions within the polytrimer as well as a Curie-Weiss correction for intertrimer exchange within a single chain [J1 = -53(1) K, J2 = -6.0(5) K, θ = -1.5(5) K]. Viewing the reported structures as bent coordination polymers is supported by observations that (a) the Cu···Cl coordination distance is quite short (avg. = 2.672 Å) and (b) the magnetic data of the polydimer are best fit to the alternating antiferromagnetic chain model.
