Dual Behavior of Bromine Atoms in Supramolecular Chemistry: The Crystal Structure and Magnetic Properties of Two Copper(II) Coordination Polymers

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.