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 ∼ 10. Copper(II) complexes of the three ligands have been prepared and were found to have the composition [Cu 2 L](ClO 4 ) 4 · x H 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)](ClO 4 ) 2 ( 1 ), where dmptacn = 1,4-bis(2-pyridylmethyl)-1,4,7-triazacyclononane, and the corresponding binuclear complex [Cu 2 (tmpdtne)](ClO 4 ) 4 ·2H 2 O ( 2 ). The mononuclear complex 1 crystallizes in the orthorhombic space group Pbcn with a = 19.467(4) Å, b = 16.798(6) Å, c = 14.204(4) Å, V = 4644(1) Å 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 P 2 1 /c with a = 13.885(1) Å, b = 13.218(2) Å, c = 13.531(3) Å, β = 106.04(1)°, V = 2386(1) Å 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 ethane bridge, and the pentadentate compartments are orientated away from one another such that the complex adopts an anti -configuration. Both complexes 1 and 2 exhibit distorted square pyramidal geometries about the Cu(II) centers. The UV−vis spectra of the binuclear complexes [Cu 2 (tmpdtne)](ClO 4 ) 4 ·2H 2 O ( 2 ), [Cu 2 (tmpdtnp)](ClO 4 ) 4 ( 3 ), and [Cu 2 (tmpdtnb)](ClO 4 ) 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 λ 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 .