Properties of a mixed-valence (FeII)2(Fe III)2 square cell for utilization in the quantum cellular automata paradigm for molecular electronics

Abstract

The di-mixed-valence complex [{(η5-C5H 5)Fe(η5-C5H4)} 4-(η4-C4)Co(η5-C 5H5)]2+, 12+, has been evaluated as a molecular four-dot cell for the quantum cellular automata paradigm for electronic devices. The cations 11+ and 12+ are prepared in good yield by selective chemical oxidation of 10 and are isolated as pure crystalline materials. The solid-state structures of 10 and 11+ and the midrange- and near-IR spectra of 10, 1 1+, 12+, and 13+ have been determined. Further, the variable-temperature EPR spectra of 11+ and 12+, magnetic susceptibility of 11+ and 12+, Mössbauer spectra of 10, 11+, and 12+, NMR spectra of 10, and paramagnetic NMR spectra of 11+ and 1 2+have been measured. The X-ray structure determination reveals four ferrocene "dots" arranged in a square by C-C bonds to the corners of a cyclobutadiene linker. The four ferrocene units project from alternating sides of the cyclobutadiene ring and are twisted to minimize steric interactions both with the Co(η5-C5H5) fragment and with each other. In the solid state 12+ is a valence-trapped Robin and Day class II compound on the 10-12 s infrared time scale, the fastest technique used herein, and unambiguous evidence for two FeIIand two FeIII sites is observed in both the infrared and Mössbauer spectra. Both EPR and magnetic susceptibility measurements show no measurable spin-spin interaction in the solid state. In solution, the NMR spectra show that free rotation around the C-C bonds connecting the ferrocene units to the cyclobutadiene ring becomes increasingly hindered with decreasing temperature, leading to spectra at the lowest temperature that are consistent with the solid-state structure. Localization of the charges in the cations, which is observed in the paramagnetic NMR spectra as a function of temperature, correlates with the fluxional behavior. Hence, the alignment between the π systems of the central linker and the ferrocene moieties most likely controls the rate of electron exchange between the dots. © 2005 American Chemical Society.