Ligand-Centered Oxidations and Electron Delocalization in a Tetranuclear Complex of a Tetradonor-Substituted Olefin

The ability of the easily reducible tetracyanoethylene (TCNE) ligand to bridge up to four metal centers and to adopt any oxidation state between 0 and -2 has generated a unique coordination chemistry with potential applications in molecular magnetism, electrochromism, and molecular computing. The coordination chemistry of electroactive, tetradonorsubstituted olefins is much less explored but may hold similar potential. The tetrairon complex [{Cp*(dppe)Fe}4{μ4-(CtCC6H4-4)4(CdC)}] of the (4-ethynylphenyl)ethene ligand [(CtCC6H4-4)4CdC] is a recent example. This compound undergoes four consecutive metal-centered oxidations, during which the bridging ligand acquires only a small fraction δ of the positive charge(s): i.e., [{Cp*(dppe)Fe}4-n{Cp*(dppe)Fe}n{μ4(CtCC6H4-4)4(CdC)}]. Here we report on the tetraruthenium complex [{(PPr3)2(CO)ClRu}4{μ4-(CHdCHC6H4)}4(CdC)}] (1), which features the tetrakis(4-styryl)ethene (TSTE) ligand (Chart 1). Complex 1 was prepared as outlined in Scheme 1. Treatment of the alkyne (HCtCC6H4)4(CdC) with 4.08 equiv of [HRu(CO)Cl(PPr3)2] gave 1 as a deep orange-red solid in 85% yield. H NMR spectroscopy shows the presence of four equivalent vinyl groups with signals at δ 8.42 (HR) and 5.90 (H ) that display well-resolved H-H and P-H couplings of 13.4 and 0.9 Hz (JP-H) and 2.1 Hz (JP-H), respectively, and two signals of an AB system at δ 6.77 and 6.71 ppm for the p-phenylene groups. In C NMR spectra the resonance signals of the ruthenium-bonded vinyl groups appear at 150.8 (CR) and 134.8 ppm (C ), again with well-resolved coupling to two equivalent phosphorus nuclei each. The central ethylene carbon atoms are observed at 139.8 ppm, while the phosphorus nuclei give rise to a sharp singlet at δ 38.3 ppm in P{H} NMR spectroscopy (see Figure S1, Supporting Information). The low ν̃(CO) value of 1909 cm and the low oxidation potentials of 1 attest to its electron-rich character. Cyclic voltammetry of 1 shows two reversible couples at -0.020 and +0.415 V vs the ferrocene/ferrocenium standard (see Figure * To whom correspondence should be addressed. E-mail: rainer.winter@ chemie.uni-regensburg.de. † Universität Regensburg. ‡ Universität Stuttgart. § J. Heyrovský Institute of Physical Chemistry v.v.i. (1) Kaim, W.; Moscherosch, W. Coord. Chem. ReV. 1994, 129, 157. (2) Miller, J. S. Angew. Chem., Int. Ed. 2006, 45, 2508. (3) Miller, J. S.; Epstein, A. J. Coord. Chem. ReV. 2000, 207, 651. (4) Wang, G.; Zhu, H.; Fan, J.; Slebodnik, C.; Yee, G. T. Inorg. Chem. 2006, 45, 1406. (5) Tanaka, Y.; Ozawa, T.; Inagaki, A.; Akita, M. Dalton Trans. 2007, 928.  Copyright 2008