The Synthesis, Redox Properties, and Ligand Binding of Heterobinuclear Transition-Metal Macrocyclic Ligand Complexes. Measurement of an Apparent Delocalization Energy in a Mixed-Valent CuICuII Complex

Article abstract of DOI:10.1021/ja00404a017

A series of binuclear complexes M_A^IIM_B^IIL²⁺ have been synthesized and characterized.The binuleating macrocyclic ligand L²⁺ is a symmetric Schiff base derived by condensing 2 equiv. of 2,6-diformyl-4-methylphenol with 2 equiv of 1,3-diaminopropane, resulting in two identical N2O2 coordination sites.In all cases, M_A(II) = Cu(II) while M_B was varied across the series M_B(II) = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II).The electrochemical properties of these species were examined by cyclic voltammetry, differential pulse polarography, sampled DC polarography, and coulometry.In each case reversible to quasi-reversible Cu(II)C(I) electrochemistry was observed.The Cu(II)Cu(I) reduction potential was, within experimental error, invariant with respect to the remote metal M_B; E_f(Cu(II)Cu(I)) = -1.068 V vs. ferrocene/ferrocinium(1+).The one exception is the homonuclear complex M_A(II) = M_B(II) = Cu(II); the homobinuclear complex was more readily reduced, E^f = -0.925 V vs.Fc/Fc⁺, than the heteronuclear species.After a correction due to magnetic stabilization the difference between the heteronuclear and homonuclear reduction potantials, 143 mV = 3.3 kcal/mol, has been ascribed to a special stability associated with the mixed-valent Cu^IICu^IL⁺ species, where some electronic delocalization has been previously demonstrated.In addition, the electrochemical properties of homonuclear complexes (M_A = M_B) are reported.The ligand-binding properties of the species Cu^IM_B^IIL⁺, M_B(II) = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II), have been examined.Cu(I) shows an affinity for the axial bases carbon monoxide, ethylene, tris(o-methoxyphenyl)phosphine, and 4-ethylpyridine.In contrast to the Cu(II)/Cu(I) reduction potentials, the binding of axial bases to Cu(I) does seem to depend on the nature of the remote metal, M_b.