Carbenes DOI: 10.1002/ange.200906702 Non-Innocent Behavior of a Tridentate NHC Chelating Ligand Coordinated onto a Zirconium(IV) Center** Charles Romain, Karinne Miqueu, Jean-Marc Sotiropoulos, StØphane Bellemin-Laponnaz,* and Samuel Dagorne* The use of N-heterocyclic carbenes (NHCs) as ancillary ligands for coordination to transition metal complexes has undoubtedly constituted a major breakthrough in the areas of organometallic chemistry and homogeneous catalysis over the past ten years. [1, 2] When compared to their phosphine analogues, NHC-containing metal complexes usually exhibit an inert NHC À metal bond that affords these complexes an enhanced stability; this has opened the way for the develop- ment of various and numerous robust NHC-incorporating metal catalysts that often feature high catalytic activity. However, several reports have highlighted that the supporting carbene moiety in NHC–metal complexes is, in some instances, quite reactive and thus could be the source of the various observed deactivation processes. Until now, examples in this area, with fully characterized products, have been restricted to NHC-bearing late-transition-metal complexes. These pathways include: NHC-involving migratory inser- tion, [3] reductive elimination, [4] and heterocycle cleavage; [5] the formation of “abnormal” NHC–metal complexes; [6] and, very recently, C À C bond formation arising from the reaction of an NHC-containing Ni ÀH species with an alkene. [7] In view of the fast-growing development of NHC–transition metal complexes in synthesis and catalysis, well-identified reactions of NHC–transition metal complexes are of crucial importance to better understand and rationalize the catalytic perfor- mance of this class of compounds. Despite their potential interest as catalysts, high-oxida- tion-state and oxophilic transition metal complexes that contain NHC ligands have received little attention, as these complexes are thought to be less stable because of easier M À C carbene bond dissociation. However, the use of anionic NHC-containing chelating ligands for coordination to Group 4 and 5 metal centers has been shown to be beneficial to the stability of the derived complexes, although the suitability of these species as catalysts remains to be addressed. [8] We are interested in the synthesis of robust NHC-containing Group 4 metal complexes [9] and have devel- oped a novel family of tridentate bis(aryloxide) NHC chelating ligands (A, Scheme 1) in which the NHC moiety is positioned as a central donor, a feature that is likely to disfavor deactivation processes. [10, 11] Herein, however, we report that the coordination of tridentate ligand A to Zr IV opens a way to an unprecedented rearrangement involving NHCs; this rearrangement constitutes the first instance in which an NHC that is coordinated to a high-oxidation-state and oxophilic transition metal exhibits such a behavior. As an entry to Zr IV complexes that are supported by tridentate ligand A, the well-established salt metathesis route was first considered as it is known to be a suitable route to closely related Zr IV NHC complexes. [12] Therefore, the reaction of ligand A, generated in situ at À78 8C in THF by addition of three equivalents of nBuLi and [ZrCl 4 (thf) 2 ], afforded the Zr À NHC dichloro complex [{ tBu (OCO)}ZrCl 2 - (thf)] where { tBu (OCO)} 2À = [h 3 -O,C,O-{(3,5-di-tert-butyl- C 6 H 2 O) 2 N 2 C 3 H 4 }] 2À as an air-stable colorless solid in consis- tently modest yield (2-thf, Scheme 2). The NMR spectro- Scheme 1. Structure of NHC-containing anionic ligand A. Scheme 2. Salt-metathesis route to access the zirconium NHC com- plex 2-thf. [*] C. Romain, Dr. S. Bellemin-Laponnaz, Dr. S. Dagorne Laboratoire DECOMET, Institut de Chimie CNRS-UniversitØ de Strasbourg 1 rue Blaise Pascal, 67070 Strasbourg (France) E-mail: dagorne@unistra.fr bellemin@unistra.fr Dr. K. Miqueu, Dr. J.-M. Sotiropoulos Institut Pluridisciplinaire de Recherche sur l’Environnement et les MatØriaux (UMR CNRS 5254) UniversitØ de Pau et des Pays de l’Adour Technopôle HØlioparc, 2 avenue du PrØsident Angot 64053 Pau cedex 09 (France) [**] Financial support from the CNRS, the Ministre de l’Enseignement SupØrieur et de la Recherche (MESR). C.R. is grateful for a MESR PhD fellowship. M3PEC-MØsocentre (Aquitaine-Bordeaux 1) is acknowledged for their calculation facilities. NHC = N-heterocyclic carbene. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200906702. Zuschriften 2244  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2010, 122, 2244 –2247