ARTICLE pubs.acs.org/Organometallics
New Mixed-Donor Bidentate Ligands Based on N-Heterocyclic Carbene and Thione Donors Miriam Slivarichova, Ruaa Ahmad, Yu-Ying Kuo, Joshua Nunn, Mairi F. Haddow, Hafiizah Othman, and Gareth R. Owen*,‡ The School of Chemistry, University of Bristol, Bristol, BS8 1TS, U.K.
bS Supporting Information ABSTRACT:
A new family of mixed-donor bidentate ligands containing both N-heterocyclic carbene (NHC) and thione functionalities is reported. The imidazolium salt precursors 1,10 -methylene(3-R-imidazol-2-ylidene)(3-R-2H-imidazole-2-thione) [CSRH] halide (where R = methyl, benzyl; halide = bromide, iodide) have been synthesized. Their coordination to copper(I) salts has been explored, providing the complexes [Cu(CSR)X]2 (where R = methyl, benzyl; halide = bromide, iodide). Structural characterization of two of the complexes confirmed the dimeric nature of these complexes in the solid state but revealed different coordination modes for each case (k1-C, k1-S, μ-S and k1-C, k1-S). Solutions of the copper complexes slowly react with oxygen under aerobic conditions to form uncoordinated 1,10 -methylene(3-R-2H-imidazol-2-one)(3-R-2H-imidazole-2-thione), SOR (where R = methyl, benzyl), where the NHC moiety has been converted into a urea functional group. The complexes were investigated as candidates in coppercatalyzed cross-coupling reactions.
’ INTRODUCTION N-Heterocyclic carbene (NHC) ligands have featured prominently in the chemical literature since the isolation of the first stable carbene by Arduengo in 1991.1 Since that time, these strong and robust ligands have been comprehensively explored.2 It is without doubt that they are outstanding supporting ligands for an extensive range of homogeneous catalytic applications.3,4 Some investigations have focused on carbene ligands containing additional functional groups based on donors such as phosphorus, nitrogen, and oxygen.5 In the last year or so, there has been a sharp rise of interest in sulfur-based functionalization (Chart 1).6,7 Many of these bidentate and tridentate ligands have provided complexes with excellent activities over a range of catalytic applications.6,7 Despite the large number of sulfurfunctionalized NHC ligands reported, there are to our knowledge no reported examples of NHC-based ligands featuring the thione functionality.8 Many investigations concerning N-heterocyclic carbene ligands have focused on methylene-bridged bidentate ligands (CCR) (Chart 2).9 A number of synthetic methods are available that allow access to these ligands from their corresponding bisimidazolium salt precursors.9,10 In a related area, Williams first reported the synthesis of bidentate bis-thione ligands (SSR) by reaction of their corresponding imidazolium salts with elemental sulfur in the presence of a base.11,12 Given the straightforward nature of these syntheses, it is somewhat surprising that the mixed ligand CSR, which contains one each of these functionalities, is still unknown.8 In all studies of the methylene-bridged r 2011 American Chemical Society
Chart 1. Selected Sulfur-Functionalized N-Heterocyclic Carbene Ligands
Chart 2. Bidentate Ligands Based on Thione and NHC Donor Groups
imidazolium salts to date, both imidazolium groups have been converted to thione. We were therefore intrigued to determine whether the synthesis of these mixed ligands was possible. Herein, we wish to report the straightforward synthesis and characterization of this new family of mixed bidentate ligand precursors together with some preliminary copper(I) complexes. Received: July 12, 2011 Published: August 11, 2011 4779
dx.doi.org/10.1021/om200629j | Organometallics 2011, 30, 4779–4787
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ARTICLE
Figure 1. ORTEP representations of [CSMeH]Br (top, left), [CSMeH]I (top, right), and [CSBnH]Br (bottom). The halide counterions and hydrogen atoms, with the exception of H(6), have been removed for clarity (thermal ellipsoids drawn at 50% probability level).
Scheme 1. Synthesis of Methylene-Bridged Mixed Bidentate NHC/Sulfur Ligand Precursors from Their Corresponding Bisimidazolium Salts
’ RESULTS AND DISCUSSION Synthesis of Ligand Precursors. The imidazolium salts 1,10 -
methylene(3-methylimidazol-2-ylidene)(3-methyl-2H-imidazole2-thione) bromide, [CSMeH]Br, and 1,10 -methylene(3-benzylimidazol-2-ylidene)(3-benzyl-2H-imidazole-2-thione) bromide, [CSBnH]Br, were readily synthesized by a directly analogous method to that described by Williams (Scheme 1) using only one equivalent of sulfur (1/8 S8). The corresponding iodide salts [CSMeH]I and [CSBnH]I were also prepared via the same methodology. The extent of these reactions could be followed by taking aliquots from the reaction mixture and recording their 1 H NMR spectra (in DMSO-d6). The solid products [CSRH]X [R = Me, Bn; X = Br, I] were obtained in high yields as white powders following standard workup and were fully characterized by spectroscopic and analytical methods. The 1H NMR spectra confirmed the loss of one of the imidazolium protons by revealing a new signal in the region between 9.38 and 9.70 ppm, which integrated for a single proton. The 13C{1H} NMR spectra showed a characteristic signal confirming the presence of the CdS group in these new compounds with signals between 163.0 and 163.9 ppm, respectively. The formation of [CSMeH]Br, [CSBnH]Br, and [CSMeH]I was further confirmed by X-ray single-crystal diffraction studies.
Crystals suitable for X-ray diffraction were grown by layering concentrated DCM solutions with hexane. ORTEP representations of the three imidazolium cations are presented in Figure 1. Selected bond lengths and angles are highlighted in Table 1. The structures revealed similar parameters to the corresponding bisimidazolium [CCR(H)2]2+ and bis-thione (SSR) compounds.10,11b The C(1)S(1) distances in the three structures range between 1.6753(14) and 1.6846(18) Å and are typical of a imidazolethione function.11b The imidazolium and thione functionalities are orientated in the same direction in [CSMeH]Br, while they are in opposite directions in [CSMeH]I. This is perhaps related to the packing of the compound in the crystal lattice rather than any electronic factors within the cation. Synthesis of Copper Complexes. The coordination chemistry of NHC ligands and their derivatives has been extensively investigated using a wide range of transition metal centers. In particular, the coordination to copper has been reported in a number of examples.1,4,1317 We therefore aimed to prepare a series of copper(I) complexes containing these mixed ligands. Our initial attempts using Cu2O as a precursor to synthesize our target complexes were unsuccessful.15 The reactions were followed by 1H NMR spectroscopy, revealing incomplete reactivity of the imidazolium salts even after prolonged periods of time and 4780
dx.doi.org/10.1021/om200629j |Organometallics 2011, 30, 4779–4787
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harsh conditions. In a typical experiment involving [CSMeH]Br and Cu2O, signals corresponding to the target compound were observed albeit with limited conversions (ca.
