N-Heterocyclic Carbene Complexes DOI: 10.1002/anie.201003177 Reduction of a Chelating Bis(NHC) Palladium(II) Complex to [{m- bis(NHC)} 2 Pd 2 H] + : A Terminal Hydride in a Binuclear Palladium(I) Species Formed under Catalytically Relevant Conditions** Peter D. W. Boyd, Alison J. Edwards, Michael G. Gardiner,* Curtis C. Ho, Marie-HØlne LemØe- Cailleau, David S. McGuinness, Anung Riapanitra, Jonathan W. Steed, Damien N. Stringer, and Brian F. Yates Catalytic processes featuring N-heterocyclic carbene (NHC) ligands have been extensively studied following the isolation of free imidazol-2-ylidenes. [1] An important reaction class is the base-assisted palladium-mediated C–C/N coupling, such as the Sonogashira, Buchwald–Hartwig, and Mizoroki–Heck reactions. [2] Interestingly, mechanistic debate spans from the in situ generation of active NHC complexes by reaction of simple palladium(0) or palladium(II) species with imidazo- lium-based ionic liquid solvents [3] to the extreme of NHC complexes being precatalysts to NHC-free colloidal palla- dium materials effecting heterogeneous catalysis. [4] Pd II NHC precatalyst usage prevails, and is generally thought to give an active palladium(0) species; however, the first palladium(IV) NHC complex was recently reported via a catalytically relevant oxidative addition to a palladium(II) precursor. [5] Pd 0 NHC complexes have been prepared from palladium(0) sources and used as catalysts. [6] However, we are unaware of any isolated, well-characterized, low-valent Pd NHC com- plexes (Pd 0 or Pd I ) prepared by reduction of palladium(II) precursors under conditions that are relevant to the catalysis of C–C/N coupling reactions. [7] Herein we present high-yield access to a Pd 2 H + species, 2, stabilized by a bulky bridging bis(NHC) ligand by base- assisted reduction of a chelating bis(NHC)Pd II complex. DFT calculations have established the bonding in 2 and its fluxional behavior is described. To the best of our knowledge, 2 is the first reported well-characterized 1) Pd I NHC complex, 2) Pd NHC complex prepared via base-assisted reduction, and 3) structurally authenticated Pd 2 H + complex and Pd ÀPd bonded complex of any oxidation state with a terminal hydride ligand. A colorless solution of 1 over anhydrous Na 2 CO 3 in dry MeOH was heated at 50 8C for two hours to give a red solution, from which red crystals of 2 were obtained in 84 % yield after filtration and concentration (Scheme 1). [8] Com- plex 2 has high stability as a solid (more than one year) and in MeOH and THF (several months). The complex is tolerant to moisture, but reacts quickly with atmospheric oxygen in solution and the solid state. Initial characterization of 2 was based on an X-ray crystal structure determination, which establishes the Pd/NHC con- nectivity and its monocationic status. 1 H NMR spectral data subsequently provided initial evidence of a hydride ligand, which was confirmed by mass spectrometry. The terminal hydride ligand has been established by a neutron crystal structure analysis (Figure 1). The Pd À H bond length in 2 determined from the neutron structure analysis (1.58(3) ) is typical, and is comparable to the m-HÀPd I distance in [(dipp) 2 Pd 2 (m-H)(m-CO)] + (1.531(11) and 1.540(10) ; X-ray; dipp = 1,3-bis(diisopropylphosphi- no)propane). [9] A terminal hydride (1.53(5) ) is known for Scheme 1. Synthesis of the cationic low-valent binuclear hydride 2. [*] Prof. P. D. W. Boyd, [+] Dr. M. G. Gardiner, C. C. Ho, Dr. D. S. McGuinness, A. Riapanitra, [++] Dr. D. N. Stringer, Prof. B. F. Yates School of Chemistry, University of Tasmania Private Bag 75, Hobart TAS 7001 (Australia) Fax: (+ 61) 3-6229-2858 E-mail: michael.gardiner@utas.edu.au Dr. A. J. Edwards Bragg Institute Australian Nuclear Science and Technology Organisation PMB 1, Menai NSW 2234 (Australia) Dr. M.-H. LemØe-Cailleau Institut Laue-Langevin, B.P.156 6 rue Jules Horowitz, 38042 Grenoble Cedex 9 (France) Prof. J. W. Steed Department of Chemistry, South Road, University of Durham Durham DH1 3LE (UK) [ + ] Current address: Department of Chemistry The University of Auckland Auckland 1142 (New Zealand) [ ++ ] Current address: Department of Chemistry Universitas Jenderal Soedirman Jl. Dr. Soeparno 61, Purwokerto 53123 (Indonesia) [**] X-ray data were obtained on MX1 at the Australian Synchrotron, Victoria, Australia. [29] We thank the Australian Government for an Australian Partnership Scholarship (APS) under the Australia Indonesia Partnership for Reconstruction and Development (AIPRD), The University of Auckland for research and study leave support, and Dr. Ross O. Piltz for assistance with neutron data reduction. NHC = N-heterocyclic carbene ligand. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201003177. A ngewandte Chemi e 6315 Angew. Chem. Int. Ed. 2010, 49, 6315 –6318  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim