Abstract
Three-center systems QCuQn contg. electron transfer-active and donor-substituted o-iminoquinone ligands were synthesized and studied as models for electronic, charge and magnetic coupling within materials involving coordinative bonding. The neutral precursors (n = 0) involve spin-bearing semiquinone radical ions, Q.bul.-, bridged by paramagnetic copper(II). Crystal structures and EPR measurements reveal a delicate sensitivity of structures and of spin-spin coupling in these three-center configurations, depending on weak secondary interactions involving substituents at the coordinatively ambivalent (hemilabile) ligands Q$\times$. Stepwise electron loss or uptake by QCuQn is accompanied by considerable redox potential splitting and by spectroelectrochem. monitored change of UV-visible-NIR absorption. Whereas the monoanions exhibit long-wavelength ligand-to-ligand intervalence charge transfer bands around 2000 nm for p(Q2-) $\rightarrow$ p*(Q.bul.-) transitions, the monocations are formed in a reorganization step, achieving coordinative satn. at the metal. Weak secondary interactions thus are sufficient to cause significant structural changes as well as qual. differences in spin-spin interaction and in excited state structures. on SciFinder(R)
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