Binuclear copper(II) complexes of bis(pentadentate) ligands derived from alkyl-bridged bis(1,4,7-triazacyclonane) macrocycles

Three alkyl-bridged bis(pentadentate) ligands, 1,2-bis[N,N-bis(2-pyridylmethyl)-1,4,7-triazacyclononyl]ethane (tmpdtne), 1,3-bis[N,N'-bis(2-pyridylmethyl)-1,4,7-triazacyclononyl]propane (tmpdtnp), and 1,4-bis[N,N'-bis(2- pyridylmethyl)-1,4,7-triazacyclononyl]butane (tmpdtnb), have been synthesized by reaction of the corresponding bis(tridentate) macrocycles with 2-picolyl chloride at pH similar to 10. Copper(II) complexes of the three ligands have been prepared and were found to have the composition [Cu(2)L](ClO4)(4) . xH(2)O where x = 2 for L = tmpdtne and x = 0 for L = tmpdtnp and tmpdtnb. Single-crystal X-ray diffraction studies established the molecular structure of the monomeric complex, [Cu(dmptacn)](ClO4)(2) (1), where dmptacn = 1,4-bis(2-pyridylmethyl)-1,4,7-triazacyclononane, and the corresponding binuclear complex [Cu-2(tmpdtne)](ClO4)(4) . 2H(2)O (2). The mononuclear complex 1 crystallizes in the orthorhombic space group Pbcn with a = 19.467(4) Angstrom, b = 16.798(6) Angstrom, c = 14.204(4) Angstrom, V = 4644(1) Angstrom(3), and Z = 8. The structure was refined to final R = 0.066 and R(w) = 0.056 for 1806 observed reflections. The binuclear complex 2 crystallizes in the monoclinc space group P2(1)/c with a = 13.885(1) Angstrom, b = 13.218(2) Angstrom, c = 13.531(3) Angstrom, beta = 106.04(1)degrees, V = 2386(1) Angstrom(3), and Z = 2 (dimers). Refinement gave final R = 0.055 and R(w) = 0.063 for 3509 observed reflections. The binuclear unit is centrosymmetric about the Cu(II) centers. The UV-vis spectra of the binuclear complexes [Cu-2(tmpdtne)](ClO4)(4) . 2H(2)O (2), [Cu-2(tmpdtnp)](ClO4)(4) (3), and [Cu-2(tmpdtnb)](ClO4)(4) (4) show a band in the range 598-602-nm which, for five-coordinate Cu(II) complexes, is indicative of a square pyramidal geometry. The slightly higher lambda(max) value of 612 nm for the mononuclear complex 1 suggests more distortion toward trigonal bipyramidal geometry in solution. These data are in agreement with the solid state structures which show that 1 and 2 are 32% and 11% distorted from SP, respectively. Variable-temperature magnetic susceptibility measurements of 2-4 show no evidence of exchange coupling between the Cu(II) centers. ESR measurements in frozen DMF and nitromethane solutions indicate the presence of weak dipole-dipole interactions in 2 and 3, which have ethane and propane linkages between the pentadentate compartments. Complex 4, however, shows little evidence of coupling, with its ESR spectrum closely resembling that of 1.