Electron/atom transfer in halo-bridged homobimetallic complexes. Structureand donor-acceptor properties of face-to-face dicopper complexes with teraazamacrocyclic ligands

The syntheses and donor-acceptor properties of some novel, halo-bridged dic opper(II) complexes of alpha,alpha ' -bis (5,7-dimethyl-1,4,8,11-tetraazacy clotetradecane-6-yl)-o-xylene are reported. These complexes were characteri zed by their magnetic and electrochemical behavior, X-ray structure analysi s, FAB mass spectroscopy, and electronic spectra. The bromo-bridged complex crystallized in the tetragonal system, space group P4(3)2(1)2, with a 12.6 584(5) Angstrom, c = 28.6483 (14) Angstrom, Z = 4, R = 0.071, and R-w = 0.1 47. 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 = 26.0 22(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 brid ged 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 d istance (where X = Cl, Br) is shorter than the sum of van der Waals radii o f 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 a pproximate 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 r ange (DeltaE(1/2) = 0.24 V). All the Cu-II/Cu-I couples were irreversible w ith a cathodic peak at about - 0.9 V. The magnetic susceptibility results b elow 20 K follow Curie-Weiss behavior, indicating that the magnetic interac tion between the two Cu centers is weakly antiferromagnetic with J less tha n 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 co upling in the Cu-II(X-)Cu-II complexes. A single set of perturbation theory parameters is consistent with the magnetic and electrochemical observation s on the chloride-bridged complex and the magnetic properties of the bronzi de-bridged complex. The electrochemical behavior of the latter suggests a r elatively low-energy, high-spin configuration for the Cu-III(Br-)Cu-II comp lex. The analysis attributes the weak Cu-II/Cu-II coupling to the orthogona lity of the donor and acceptor orbitals to the bridging axis. It is inferre d that bridging halide-mediated metal-metal d sigma /p sigma coupling signi ficantly alters the chemical properties of the bimetallic complexes only wh en 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 contri bution.