Inorganic Chemistry, Vol.40, No.7, 1614-1625, 2001
Electron/atom transfer in halo-bridged homobimetallic complexes. Structure and donor-acceptor properties of face-to-face dicopper complexes with teraazamacrocyclic ligands
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) Angstrom, c = 28.6483 (14) Angstrom, Z = 4, R = 0.071, and R-w = 0.147. The chloro-bridged complex crystallized in the monoclinic system, space group C2/c, with a = 32.749(2) Angstrom, b = 18.8915(9) Angstrom, c = 26.022(2) Angstrom, beta = 114.831 degrees, Z = 12, R = 0.080, and R-w = 0.132. Both molecules have C-2 symmetry. The two copper(II) ions are axially bridged by a bromine or a chlorine, and the two macrocycles an 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 (K-f approximate to 10(4) M-1). The chloride-bridged complex had barely resolved differential pulse polarographic waves (DeltaE(1/2) approximate to 28 mV), while the bromide-bridged complex exhibited two CV waves in the 1.0-1.5 V range (DeltaE(1/2) = 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 less than or equal to -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 bronzide-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 d sigma /p sigma 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.