Dalton Transactions Dynamic Article Links Cite this: DOI: 10.1039/c1dt10838g www.rsc.org/dalton PAPER Calix[4]arenes with one and two N -linked imidazolium units as precursors of N -heterocyclic carbene complexes. Coordination chemistry and use in Suzuki–Miyaura cross-coupling† Eric Brenner,* a Dominique Matt,* a Micka¨ el Henrion, a Matthieu Teci a and Lo¨ ıc Toupet b Received 4th May 2011, Accepted 19th July 2011 DOI: 10.1039/c1dt10838g The calix[4]arene-imidazolium salts 5-(3-butyl-1-imidazolylium)-25,26,27,28-tetrabenzyloxy- calix[4]arene bromide (cone)(2), and 5,11-bis(3-alkyl-1-imidazolylium)-25,26,27,28- tetrabenzyloxycalix[4]arene diiodide (cone) (R = methyl, 3a;R = n-butyl, 3b) have been synthesised. Reaction of 2 in dioxane with PdCl 2 in the presence of CsCO 3 and KBr (80 ◦ C, 24 h) gives the carbene complex trans-[PdBr 2 (calix-monocarbene) 2 ](14), containing two N-heterocyclic carbene ligands derived from 2 (yield: 63%). Repeating the reaction in pyridine instead of dioxane gives the mixed pyridine-carbene complex trans-[PdBr 2 (calix-carbene)(pyridine)] (15) in 75% yield. Treatment of the bis-imidazolium salt 3a with [Pd(OAc) 2 ] affords a chelate complex, trans-[PdI 2 {calix-bis(carbene)}] (16), in which a metallo-(bis-carbene) fragment caps the upper rim of the calixarene basket. Complex 16, as well as its analogue 17, obtained from 3b, display apparent C s -symmetry in solution. This is not the case in the solid state, a single X-ray diffraction study carried out for 16 revealing a pinced cone structure for the calixarene skeleton, which reduces the symmetry to C 1 . The chelate complex 17 shows poor activity in Suzuki–Miyaura cross-coupling of phenyl boronic acid and p-tolyl halides, an observation that suggests the presence of a strained metallocyclic unit preventing easy stereochemical rearrangement to an active species. Unlike 17, complexes 14 and 15 show good activities in cross-coupling. A comparative study using the carbene precursor 1-butyl-3-(2,6-diisopropylphenyl)- imidazolium bromide (18), which is devoid of the receptor fragment, strongly suggests that the carbene ligands of 14 and 15 operate typically as bulky NHC-ligands. Introduction From the point of view of a coordination chemist, the use of the calix[4]arene skeleton has essentially two advantages. First, it constitutes an attractive platform for the construction of sophisticated coordination spheres by assembling a set of convergent podand arms on one of the two calixarene rims. 1–6 Ligands of this type often display unusual properties, most depending upon the particular features of the calixarene core, notably its flexibility, 7–9 shape, 10 and bulkiness 11,12 as well as its multifunctional character allowing, in particular, multiple metal binding. 13 Prominent examples of ligands of this type that have found applications in transition metal chemistry include those which are hemispherical, 14,15 hemilabile, 13 or chiral, 16 as well as a Laboratoire de Chimie Inorganique Mol´ eculaire et Catalyse, Institut de Chimie, UMR 7177 CNRS Universit´ e de Strasbourg, 1 rue Blaise Pascal, F-67008, Strasbourg Cedex, France. E-mail: brenner@unistra.fr, dmatt@chimie.u-strasbg.fr b Groupe Mati` ere Condens´ ee et Mat´ eriaux UMR 6626, Universit´ e de Rennes 1, Campus de Beaulieu, 35042, Rennes cedex, France † CCDC reference numbers 748587 and 821364. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c1dt10838g highly strained bidentates. 17,18 The second valuable asset of the calix[4]arene backbone is its capacity to function as a molecular receptor, a property that is usually found only with calixarenes in the so-called cone conformation. By tethering functional groups suitable for transition metal binding on such receptors, ligands result that constitute potential candidates for supramolecular catalysis. To date, the only known ligands of this class are calixarene-monophosphines of type 1, which, once associated with palladium salts, become very efficient Suzuki cross-coupling catalysts. 19 This property seemingly depends on the binding of metal–arene units within the cavity of the conical calixarene. Wondering whether calixarenes equipped with podand arms other than a phosphino group would also be suitable for supramolecular catalysis, we decided to synthesise and investigate the properties of the calixarene-imidazolium salts 2 and 3a and 3b, which qualify as precursors for N-heterocylcic carbene (NHC) ligands. 20–25 In these derivatives, the NHC units are connected through one of the nitrogen atoms to the calixarene upper rim, without any linker, a feature which we expected would facilitate metal–cavity interactions. The only reported calixarene complexes having a NHC unit directly attached to the calixarene skeleton are Dinar` es complexes 4a and 4b, which are active catalysts for the This journal is © The Royal Society of Chemistry 2011 Dalton Trans. Downloaded by Universite Louis Pasteur on 30 August 2011 Published on 30 August 2011 on http://pubs.rsc.org | doi:10.1039/C1DT10838G View Online