{1,1’-(Dimethylsilylene)bis[methanechalcogenolato]}diiron Complexes [2Fe2E(Si)] (E ¼ S, Se, Te) – [FeFe] Hydrogenase Models by Ulf-Peter Apfel a ) b ), Helmar Gçrls a ), Greg A. N. Felton c ), Dennis H. Evans* d ), Richard S. Glass* c ), Dennis L. Lichtenberger* c ), and Wolfgang Weigand* a ) a ) Institut für Anorganische und Analytische Chemie, Friedrich-Schiller-Universität Jena, Humboldtstrasse 8, D-07743 Jena (e-mail: wolfgang.weigand@uni-jena.de) b )Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, 02139, USA c )Department of Chemistry and Biochemistry, The University of Arizona, Tucson, AZ, 85721, USA d )Department of Chemistry, Purdue University, West Lafayette, IN, 47907, USA Dedicated to Professor Dieter Seebach on the occasion of his 75th birthday (Bis-selenolato) and (bis-tellurolato)diiron complexes [2Fe2E(Si)] were prepared and compared with the known (bis-thiolato)diiron complex A to assess their ability to produce hydrogen from protons. Treatment of [Fe 3 (CO) 12 ] with 4,4-dimethyl-1,2,4-diselenasilolane (1) in boiling toluene afforded hexacarbonyl{m-{[1,1’-(dimethylsilylene)bis[methaneselenolato-kSe : kSe]](2  )}}diiron( Fe Fe) (2). The analog bis-tellurolato complex hexacarbonyl{m-{[1,1’-(dimethylsilylene)bis[methanetellurolato- kTe : kTe]](2  )}}diiron( Fe Fe)( 3) was obtained by treatment of [Fe 3 (CO) 12 ] with dimethylbis(telluro- cyanatomethyl)dimethylsilane, which was prepared in situ. All compounds were characterized by NMR, IR spectroscopy, mass spectrometry, elemental analysis and single-crystal X-ray analysis. The electro- catalytic properties of the [2Fe2X(Si)] (X ¼ S, Se, Te) model complexes A, 1, and 2 towards hydrogen formation were evaluated. Introduction. – Since the structure of the active site of the [FeFe] hydrogenase has been reported [1], finding a suitable model with comparable properties, i.e. , high turnover rates with low overpotential for the reversible reduction of protons to molecular hydrogen, is still a challenge. Recent progress in this topic was achieved by Darensbourg and co-workers concerning the investigations of the rotated geometry on [2Fe2S] model complexes [2] and by Barton and Rauchfuss discerning the role of phosphine ligands in directing protons to a bridging or terminal site [3]. Beside the direct protonation of the Fe centers, experiments and calculations pointed out a possible addition of a proton to the bridgehead S-atoms [4]. An experimental proof of these calculations was displayed by silicon containing [2Fe2S] models A – D ( Fig. 1) [5]. Recent investigations of our group suggested that in contrast to the propane-1,3- dithiolato (pdt) iron complexes [6], an increase of electron density on the S-atoms by interaction of the s(Si C) and 3p(S) orbitals can be assumed. Similar interactions were found for comparable tin complexes, described by Glass and co-workers [7]. During electrochemical investigations, protonation of the thiolato S-atoms was observed, and a subsequent proton shift to the iron core suggested. More recently, investigations on hexacarbonyl(2-oxaspiro[3.4]octane-6,7-dichalcogenolato)diiron complexes revealed diminished activity towards hydrogen production and negatively shifted potentials for Helvetica Chimica Acta – Vol. 95 (2012) 2168  2012 Verlag Helvetica Chimica Acta AG, Zürich