Uranium Siloxides DOI: 10.1002/ange.201206955 Siloxides as Supporting Ligands in Uranium(III)-Mediated Small- Molecule Activation** Victor Mougel, ClØment Camp, Jacques PØcaut, Christophe CopØret, Laurent Maron, Christos E. Kefalidis, and Marinella Mazzanti* Trivalent uranium compounds have been identified as excel- lent candidates for small-molecule activation, [1] and their potential use in catalytic cycles has been proposed. [2] Of particular current interest is their unprecedented reactivity with carbon dioxide, [3] which is both a major contributor to environmental concerns and an attractive potential low-cost source of carbon. [4] To date only two ligand systems for well- defined uranium-mediated carbon dioxide reduction have been reported. These ligand systems are based on highly encumbered polydentate aryl oxides [1b,d, 5] or classic organo- metallic half-sandwich COT R /Cp R derivatives (COT R and Cp R are substituted cyclooctatetraenyl and cyclopentadienyl ligands). [6] Electron-rich homoleptic uranium(III) complexes of simple ligands, such as aryl oxides [1c] and silyl amides, [7] have shown useful reactivity with small molecules; however, their reaction with CO 2 led to multiple products. As ancillary ligands, siloxides (in particular, tris(tert-butoxy)siloxides) are characterized by their small size relative to that of silyl amides and aryl oxides [8] and their ability to adopt mono- or bidentate forms. [8, 9] These properties often lead to increased reactivity and stability, which have been exploited in homogeneous catalysis. [10] They may thus provide an attractive alternative for the synthesis of highly reactive homoleptic U III complexes. However, siloxides have not been used previously as support- ing ligands in uranium-mediated reduction processes, [11] probably because the high reactivity of uranium(III) siloxide precursors makes their synthesis and handling challenging. Herein we report a homoleptic siloxide complex of trivalent uranium, [{U(OSi(OtBu) 3 ) 2 (m-OSi(OtBu) 3 )} 2 ](1), and its reactivity towards CS 2 , CO 2 , and toluene. Whereas the reaction of 1 with CS 2 led to the two-electron reduction product [{U(OSi(OtBu) 3 ) 3 } 2 {m-h 2 (C,S):h 2 (S,S)-CS 2 }] (2), CO 2 underwent reductive disproportionation in the presence of 1 to yield CO and a dimeric U IV /U IV carbonate complex. Moreover, the reaction of 1 with toluene afforded a diuranium inverted-sandwich complex. When [U{N(SiMe 3 ) 2 } 3 ] [12] was treated with tris-tert-but- oxysilanol (HOSi(OtBu) 3 , 3 equiv) in hexane at À40 8C, the uranium(III) complex [U(OSi(OtBu) 3 ) 2 (m-OSi(OtBu) 3 )] 2 (1) was obtained as a brown crystalline solid in 82 % yield. X-ray diffraction analysis of single crystals of 1 revealed a centro- symmetric dinuclear structure in which the two uranium centers are bridged by two siloxide ligands (Figure 1). Both uranium ions are coordinated in a distorted pentagonal- bipyramid geometry by the oxygen atoms of two terminal siloxide groups and two bridging bidentate siloxide ligands, and by a neutral tert-butoxy group of a bridging siloxide ligand. The U–U distance is 3.9862(2) . The measured UÀO bond lengths for the terminal siloxides (mean UÀO = 2.193(4) ) lie in the range of typical U À O distances in uranium(III) alkoxide complexes. [1c, 13] Longer U À O distances are observed, as expected, for the bridging siloxides (U1 À O1 2.396(3), U1 ÀO1A 2.549(3) ) and for the neutral tert-butoxy group (2.540(2) ). These distances are similar to those found for bridging siloxides in the rare reported examples of siloxide Figure 1. Ellipsoid plot (50 % probability) for [{U(OSi(OtBu) 3 ) 2 (m-OSi- (OtBu) 3 )} 2 ](1) crystallized from hexane. Hydrogen atoms and methyl groups were omitted for clarity. [*] V. Mougel, C. Camp, Dr. J. PØcaut, Dr. M. Mazzanti Laboratoire de Reconnaissance Ionique et Chimie de Coordination SCIB, UMR-E3 CEA-UJF, INAC, CEA-Grenoble 17 rue des Martyrs, 38054 Grenoble Cedex 09 (France) E-mail: marinella.mazzanti@cea.fr Homepage: http://inac.cea.fr/Pisp/51/marinella.mazzanti.html C. CopØret Laboratory of Inorganic Chemistry, ETH Zurich Wolfgang Pauli Strasse 10, 8093 Zurich (Switzerland) Prof. L. Maron, Dr. C. E. Kefalidis LPCNO, CNRS & INSA, UPS, UniversitØ de Toulouse 135 Avenue de Rangueil, 31077 Toulouse (France) [**] We thank Colette Lebrun and Pierre A. Bayle for their help with spectroscopic characterization. We acknowledge support from the Commissariat à l’Energie Atomique, Direction de l’Energie NuclØ- aire, RBPCH program and by the “Agence Nationale de la Recherche” (ANR-10-BLAN-0729). CAlmip and Cines are acknowl- edged for generous grants of computing time. L.M. is a member of the Institut Universitaire de France. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201206955. . Angewandte Zuschriften 12446  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2012, 124, 12446 –12450