ABSTRACT
The syntheses and donor-acceptor properties of some novel, halo-bridged dicopper(II) complexes of alpha,alpha'-bis(5,7-dimethyl-1,4,8,11-tetraazacyclotetradecane-6-yl)-o-xylene are reported. These complexes were characterized by their magnetic and electrochemical behavior, X-ray structure analysis, FAB mass spectroscopy, and electronic spectra. The bromo-bridged complex crystallized in the tetragonal system, space group P4(3)2(1)2, with a = 12.6584(5) A, c = 28.6483 (14) A, Z = 4, R = 0.071, and Rw = 0.147. The chloro-bridged complex crystallized in the monoclinic system, space group C2/c, with a = 32.749(2) A, b = 18.8915(9) A, c = 26.022(2) A, beta =114.831 degrees, Z = 12, R = 0.080, and Rw = 0.132. Both molecules have C2 symmetry. The two copper(II) ions are axially bridged by a bromine or a chlorine, and the two macrocycles are bridged by an o-xylene group. Each complex displays a cofacial ring arrangement. The Cu-X distance (where X = Cl, Br) is shorter than the sum of van der Waals radii of Cu and X. The phenyl ring is approximately orthogonal to the Cu-X-Cu axis. The nonhalo-bridged complex has a significant affinity for halides (Kf approximately 10(4) M(-1)). The chloride-bridged complex had barely resolved differential pulse polarographic waves (DeltaE1/2 approximately 28 mV), while the bromide-bridged complex exhibited two CV waves in the 1.0-1.5 V range (DeltaE1/2 = 0.24 V). All the Cu(II)/Cu(I) couples were irreversible with a cathodic peak at about - 0.9 V. The magnetic susceptibility results below 20 K follow Curie-Weiss behavior, indicating that the magnetic interaction between the two Cu centers is weakly antiferromagnetic with J < or = -1 cm(-1) for all three complexes. A bridging-ligand-mediated superexchange model is used to treat the magnetic and electron-transfer coupling in the Cu(II)(X-)Cu(II) complexes. A single set of perturbation theory parameters is consistent with the magnetic and electrochemical observations on the chloride-bridged complex and the magnetic properties of the bromide-bridged complex. The electrochemical behavior of the latter suggests a relatively low-energy, high-spin configuration for the Cu(III)(Br-)Cu(II) complex. The analysis attributes the weak Cu(II)/Cu(II) coupling to the orthogonality of the donor and acceptor orbitals to the bridging axis. It is inferred that bridging halide-mediated metal-metal dsigma/psigma coupling significantly alters the chemical properties of the bimetallic complexes only when the donor and acceptor orbitals are coaxial with the bridging ligand. In such a limit, the coupling takes the form of a three-center bonding contribution.
