Studies on the Consequences of Metal-Metal Interactions

Author: Spiro, Clifford Lawrence

Year: 1981

Degree: Dissertation (Ph.D.)

Advisor: Gagné, Robert R.

Committee Member: Unknown, Unknown

Option: Chemistry

Abstract

A series of binuclear complexes M_A(II)M_B(II)L⁺² have been synthesized and characterized. The binucleating macrocyclic ligand, L⁼, is a symmetric Schiff base composed of two 2, 6-diformyl, 4-methylphenol units condensed with two 1, 3-diaminopropane groups, resulting in two identical N₂O₂ coordination sites. The stepwise synthesis is described in detail. In all cases, M_A(II) = Cu(II) while M_B was varied across the series MB(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/I) electrochemistry was observed. The Cu(II/I) reduction potential was, within experimental error, invariant with respect to the remote metal, M_B; E_f (Cu II/I)= -1.068 V vs. ferrocene/ferricinium⁺. The one exception is the homo binuclear complex, M_A(II)=M_B(II)=Cu(II); the homobinuclear complex was more readily reduced, E^f = -0.925 V vs. Fe/Fe⁺, than the heteronuclear species. The difference between the heteronuclear and homonuclear reduction potentials, 143 mv=3.3 kcal/mole, has been ascribed, after a correction due to magnetic stabilization, to a special stability associated with the mixed valent Cu(II)Cu(I)L⁺ 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(I)M_B(II)L⁺, 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 axial based carbon monoxide, ethylene, tris (o-methoxyphenyl) phosphine, and 4-ethylpyridine. In contrast to the Cu(II/I) reduction potentials, the binding of axial bases to Cu(I) does seem to depend on the nature of the remote metal, M_B.

The magnetic properties of these square pyramidal 5-coordinate heteronuclear species were explored via variable temperature susceptibility measurements. Antiferromagnetism was observed in all cases. A trend toward increased coupling in traversing the series Cu(II)Mn(II)L⁺² (J = -22.5 cm^-1), Cu(II)Fe(II)L⁺² (J = -70.5 cm^-1), Cu(II)Co(II)L⁺² (J = -81.0 cm^-1), Cu(II)Ni(II)L⁺² (J = -103.0 cm^-1), and Cu(II)Cu(II)L⁺² (J = -294 cm^-1) was observed. This same trend had been previously reported for isostructural square-pyramidal 5-coordinate homonuclear complexes. An effort to understand whether this trend was dominated by structural changes or electronic changes was undertaken. New homonuclear complexes of L⁼ were prepared in a pseudo-octahedral environment with pyridyl and imidazole bases occupying trans-axial positions. A crystal and molecular structure confirmed this geometry. Since little change in magnetic coupling was observed in comparing 6-coordinate to 5-coordinate species, the variation in exchange coupling constants in traversing the first transition series has been primarily ascribed to the changing number and symmetry of the d-electrons.

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