Building Blocks for the Molecular Expression of Quantum Cellular Automata. Isolation and Characterization of a Covalently Bonded Square Array of Two Ferrocenium and Two Ferrocene Complexes Jieying Jiao, ² Gary J. Long,* ,‡ Fernande Grandjean, § Alicia M. Beatty, ² and Thomas P. Fehlner* ,² Department of Chemistry & Biochemistry, UniVersity of Notre Dame, Notre Dame, Indiana 46556-5670, Department of Chemistry, UniVersity of MissourisRolla, Rolla, Missouri 65409-0010, and Department of Physics, B5, UniVersity of Lie ` ge, B-4000 Sart-Tilman, Belgium Received March 10, 2003; E-mail: fehlner.1@nd.edu The utilization of molecules as components of electronic circuits has caught the imagination of many. 1 The temptation to look for molecular mimics of existing electronic components is strong; however, molecules are exceedingly poor charge conductors and resistive heating rules out high device densitiessthe primary justification of the approach. On the other hand, molecules are excellent charge containers and a novel paradigm, quantum cellular automata (QCA), which is based on field-coupled charge containers, has been proven theoretically as well as operationally at low temperature using 50 nm quantum dots. 2-4 Systems based on 2 nm dots are expected to operate at room temperature, hence, our interest in developing molecular expressions of the QCA paradigm. 5 The smallest building block of QCA wires consists of two dots containing a single mobile electron. At the molecular level this building block is a mixed-valence complex about which much is known. 6-8 A more versatile building block for constructing QCA circuits is a square of four electronically coupled dots containing two mobile electrons. Although molecular squares containing redox active metal centers have been described 9-14 and mixed-valence complexes up to nuclearity three have been thoroughly analyzed, 8,15 there is no example of an isolated four-metal, mixed-valence complex containing two mobile electrons in a square geometry. The independent existence and compatible electronic properties of such a species are of fundamental importance to the realization of the QCA paradigm. Here we report the full characterization of a symmetrical square containing two ferrocene and two ferrocenium moieties possessing measured properties that make it suitable for use as a component for charge-coupled QCA circuits. The basic requirements to be met by a molecular QCA cell are dots consisting of metal complexes possessing two stable redox states, a planar array of four such complexes with 4-fold symmetry, sufficient through-bond or through-space interaction so that the 2-electron, 2-hole mixed-valence state is stable with respect to comproportionation to lower and higher oxidation states, type II or type III mixed-valence behavior appropriate for switching, 16 and capability of isolation as a pure compound in useable yields from readily available starting materials. These design restrictions led to several failures and caused us to focus efforts on a square of four ferrocene complexes already described in the literature, i.e., {(η 5 -C 5 H 5 )Fe(η 5 -C 5 H 4 )} 4 (η 4 -C 4 )Co(η 5 -C 5 H 5 ), 1. 17,18 Although the reported electrochemistry was discouraging (a wave at 0.29 V with the “remaining 3 redox waves blend(ed) into a broad wave centered at ... 0.4 V”), 18 visible and near-IR band positions were published for the 1+,2+, and 3+ ions generated by bulk electrolysis. None of these ions were isolated. On the other hand, these spectroscopic data provided sufficient impetus to revisit the system. The synthesis of 1 was carried out as described previously, and analytically pure material was isolated. 17,18 Single crystals permitted a solid-state structure, which provides the dimensions of the square array of ferrocene moieties. The structure is similar to that of [1]- [PF 6 ] shown in Figure 1. Other spectroscopic properties are shown in Figures 3 and 4 and in the Supporting Information. Cyclic and square wave voltametry (Figure 2) reveals four waves sufficiently separated to suggest isolation of the 1+ and 2+ cations is feasible. Note that choice of solvent is critical to prevent precipitation of the cations on the electrode, thereby leading to complex behavior. 19 The analytically pure 1+ ion, [1][PF 6 ], was prepared by oxidation of 1 with [(η 5 -C 5 H 5 ) 2 Fe][PF 6 ]. It too was characterized in the solid state by X-ray diffraction (Figure 1). In contrast to 1,[1][PF 6 ] exhibits a near-IR band (Figure 3) which is solvent dependent (Type II), an axial EPR spectrum at 4 K similar to that of ferrocenium ² University of Notre Dame. ‡ University of MissourisRolla. § University of Lie `ge. Figure 1. Molecular structure of [1][PF6]. Fe-Fe edge distance 5.980 Å. The η 5 -C5H5 ring bound to the Co atom (green) is not shown for clarity. Figure 2. Cyclic and square wave voltametry of 1 at 100 mv/s on a Pt electrode in CH2Cl2/CH3CN mixed solvent, TBA[PF6] electrolyte, and Pt wire reference electrode (E1/2(FcH + /FcH) ) 0.344 V). The solid and open dots in the diagrams represent Fe(II) and Fe(III), respectively. Published on Web 06/03/2003 7522 9 J. AM. CHEM. SOC. 2003, 125, 7522-7523 10.1021/ja035077c CCC: $25.00 © 2003 American Chemical Society