STRUCTURAL, EPR, AND ELECTROCHEMICAL STUDIES OF BINUCLEAR COPPER(II) COMPLEXES OF BIS(PENTADENTATE) LIGANDS DERIVED FROM BIS(1,4,7-TRIAZACYCLONANE) MACROCYCLES

Structural, electrochemical, and EPR studies of binuclear copper(II) c omplexes of bis(pentadentate) ligands, obtained by attaching 2-pyridyl methyl arms to the four secondary nitrogens of bis(tacn) macrocycles l inked by ethyl (tmpdtne, [Cu-2(tmpdtne)](ClO4)(4). 2H(2)O 2), propyl ( tmpdtnp, [Cu-2(tmpdtnp)](ClO4)(4)), 3) butyl (tmpdtnb, [Cu-2(tmpdtnb)] (ClO4)(4) 4), m-xylyl (tmpdtnm-X, [Cu-2(tmpdtnm-X)](ClO4)(4). 2DMSO . 2H(2)O, 5), and 2-propanol (tmpdtnp-OH, [Cu-2(tmpdtnp-OH)](ClO4)(4) . 2H(2)O, 6) bridges, are reported, together with further analysis of th e mononuclear complex, [Cu(dmptacn)](ClO4)(2) 1, dmptacn = 1,4-bis(2-p yridylmethyl)-1,4,7-triazacyclononane. Single-crystal X-ray diffractio n studies established;the molecular structure of 3 and the tetrakis(DM F) solvate of 4. Complex 3 crystallizes in the monoclinic space group P2(1)/c (No. 14) with a = 13.867(3), b = 13.548(6), c = 28.055(4) Angs trom, beta = 102.63(1)degrees, V = 5143(2) Angstrom(3), and Z = 4. Ref inement gave R = 0.085 and R-w = 0.089 for 3696 observed reflections. 4 crystallizes in the triclinic space group (P) over bar (No. 2) with a = 11.775(4), b = 12.718(3), c = 13.201(3) Angstrom, alpha = 61.25(2) , beta = 75.99(3), gamma = 77.66(3)degrees, V = 1673(1) Angstrom(3), a nd Z = 1. Refinement gave R = 0.072 and R-w = 0.065 for 3876 observed reflections. In 4, the pentadentate compartments are oriented in an an ti configuration, while in 3, constraints introduced by the propane br idging group result in a syn configuration. Both complexes exhibit dis torted square pyramidal (SP) geometries about the Cu(II) centers with Cu-N(apical) approximate to 2.25 Angstrom and Cu-N(equatorial) approxi mate to 2.0 Angstrom. Molecular mechanics calculations have been carri ed out on these types of complexes for the first time in order to pred ict the solution structures of 5 and 6. The calculations revealed that the SP geometry is also,preferred by these complexes and that there i s little energy difference between the syn and anti configurations. Cy clic, square-wave, and steady-state voltammetric studies on 1-6 indica te that, on the time scale of the measurements, 1 undergoes a one-elec tron reduction to the Cu(I) state while 2-6 undergo an overall two-ele ctron 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 tw o partially resolved reduction processes for 6 suggest that the alcoho l group in the ligand backbone promotes a small level of interaction b etween copper centers.