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

The di-mixed-valence complex [{(eta(5)-C5H5)Fe(eta(5)-C5H4)}4(eta(4)-C4)Co(eta(5)-C5H5)]2+, 1(2+), has been evaluated as a molecular four-dot cell for the quantum cellular automata paradigm for electronic devices. The cations 1(1+) and 1(2+) are prepared in good yield by selective chemical oxidation of 1(0) and are isolated as pure crystalline materials. The solid-state structures of 1(0) and 1(1+) and the midrange- and near-IR spectra of 1(0), 1(1+), 1(2+), and 1(3+) have been determined. Further, the variable-temperature EPR spectra of 1(1+) and 1(2+), magnetic susceptibility of 1(1+) and 1(2+), Mössbauer spectra of 1(0), 1(1+), and 1(2+), NMR spectra of 1(0), and paramagnetic NMR spectra of 1(1+) 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(eta(5)-C5H5) fragment and with each other. In the solid state 1(2+) 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 Fe(II) and two Fe(III) 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 pi systems of the central linker and the ferrocene moieties most likely controls the rate of electron exchange between the dots.