Structural, EPR, and Electrochemical Studies of Binuclear Copper(II) Complexes of Bis(pentadentate) Ligands Derived from Bis(1,4,7-triazacyclonane) Macrocycles
journal contributionposted on 01.07.1998, 00:00 by Suzanne J. Brudenell, Leone Spiccia, Alan M. Bond, Peter Comba, David C. R. Hockless
Structural, electrochemical, and EPR studies of binuclear copper(II) complexes of bis(pentadentate) ligands, obtained by attaching 2-pyridylmethyl arms to the four secondary nitrogens of bis(tacn) macrocycles linked by ethyl (tmpdtne, [Cu2(tmpdtne)](ClO4)4·2H2O 2), propyl (tmpdtnp, [Cu2(tmpdtnp)](ClO4)4 3), butyl (tmpdtnb, [Cu2(tmpdtnb)](ClO4)4 4), m-xylyl (tmpdtnm-X, [Cu2(tmpdtnm-X)](ClO4)4·2DMSO·2H2O, 5), and 2-propanol (tmpdtnp-OH, [Cu2(tmpdtnp-OH)](ClO4)4·2H2O, 6) bridges, are reported, together with further analysis of the mononuclear complex, [Cu(dmptacn)](ClO4)2 1, dmptacn = 1,4-bis(2-pyridylmethyl)-1,4,7-triazacyclononane. Single-crystal X-ray diffraction studies established the molecular structure of 3 and the tetrakis(DMF) solvate of 4. Complex 3 crystallizes in the monoclinic space group P21/c (No. 14) with a = 13.867(3), b = 13.548(6), c = 28.055(4) Å, β = 102.63(1)°, V = 5143(2) Å3, and Z = 4. Refinement gave R = 0.085 and Rw = 0.089 for 3696 observed reflections. 4 crystallizes in the triclinic space group P̄ (No. 2) with a = 11.775(4), b = 12.718(3), c = 13.201(3) Å, α = 61.25(2), β = 75.99(3), γ = 77.66(3)°, V = 1673(1) Å3, and Z = 1. Refinement gave R = 0.072 and Rw = 0.065 for 3876 observed reflections. In 4, the pentadentate compartments are oriented in an anti configuration, while in 3, constraints introduced by the propane bridging group result in a syn configuration. Both complexes exhibit distorted square pyramidal (SP) geometries about the Cu(II) centers with Cu−N(apical) ≈ 2.25 Å and Cu−N(equatorial) ≈ 2.0 Å. Molecular mechanics calculations have been carried out on these types of complexes for the first time in order to predict the solution structures of 5 and 6. The calculations revealed that the SP geometry is also preferred by these complexes and that there is little energy difference between the syn and anti configurations. Cyclic, square-wave, and steady-state voltammetric studies on 1−6 indicate that, on the time scale of the measurements, 1 undergoes a one-electron reduction to the Cu(I) state while 2−6 undergo an overall two-electron reduction to the binuclear Cu(I) complexes. For 2−4, a shift in reduction potential to more negative values with increasing Cu···Cu separation reflects the stabilization of the Cu(II) state while the two partially resolved reduction processes for 6 suggest that the alcohol group in the ligand backbone promotes a small level of interaction between copper centers.