PUB - Publikationen an der Universität Bielefeld

The reaction of [{(C5Me5)-CrCl2}(2)] with [2.2](1,4)cyclophane gave [(C5Me5)Cr{[2.2](1,4)cyclophane}] (1) and [(C5Me5)Cr{[2.2](1,4)cyclophane}-Cr(C5Me5)] (2), depending on the reaction conditions. X-ray structure analysis showed 2 to be a ministack which in turn is stacked in the lattice. The chromium atoms are 6.035 angstrom apart, and the distortion of the benzene rings to boat-shaped moieties is less pronounced than in parent [2.2](1,4)cyclophane. The NMR and EPR spectra were consistent with a S = 1/2 ground state for 1 and with two interacting S = 1/2 centers in 2. Spin density was found in the ligand pi systems, where its sign was negative when the pi system was adjacent to chromium, while on the non-bonded benzene moiety of 1 it was positive. Cyclic voltammograms showed reductions to 1(-) and 2(2-), as well as oxidations to 1(+), 2(+), and 2(2+) which were quasireversible, whereas oxidations to 1(+), 2(+) and 2(2+) were irreversible. Interaction between the metal ions was revealed by a 260 mV separation of the redox waves belonging to 2(+), and 2(2+). Both cations were isolated as [B-(C6H5)(4)](-) salts, which in solution decomposed to [2.2](1,4)cyclophane and [(C5Me5)Cr{(eta(6)-C6H5)B(C6H5)(3)}] (3). The H-1 and C-13 NMR spectra of 3 were in accordance with an S=1 ground state. Solid-state magnetic measurements of the dimetallic compounds showed antiferromagnetic interaction with J = -122 cm(-1) for 2, J = -31 cm(-1) for 2(+) (ground state S = 1/2), and J = -23.5 cm(-1) for 2(2+) (with H = -JS(1)S(2)). The decrease of J in the series 2, 2(+), and 2(2+) was traced to the number of impaired electrons and, for the mixed-valent cation 2(+), to additional double exchange.