Research paperSynthesis and complexes of imidazolinylidene-based CCC pincer ligands
Graphical abstract
Introduction
The use of N-heterocyclic carbenes (NHCs) as ancillary ligands is ubiquitous in modern organometallic chemistry and has enabled the development of new generations of homogeneous transition-metal-based catalysts [1]. The structural versatility of these privileged ligands has proven to be a major contributing factor in their widespread application and has enabled access not only a plethora of monodenate ligands, but also an extensive range of polydentate variants [2]. In particular, “pincer” architectures featuring terminal NHC donors are becoming a prominent design motif, combining the strong σ-donor characteristics of NHCs with the favorable thermal stability and reaction control possible with a mer-tridentate geometry [3], [4]. For instance, first row transition metal adducts of bulky CNC ligands (e.g. A) are highly active catalysts for the hydrogenation of sterically hindered alkenes, while Ir(CCC) complexes of the type B have been shown to be effective catalysts for the thermally promoted acceptor-less dehydrogenation of alkanes (Chart 1) [5], [6]. Increasingly NHC-based pincer complexes are also becoming recognised for their useful photophysical properties: ruthenium-based C, for example, is notable for microsecond 3MLCT excited-state lifetimes, three orders of magnitude higher than [Ru(terpyridine)2]2+ [7], [8].
As we noted in our recent commentary, the emergence of terminal-NHC-based pincer compounds has largely involved systems bearing imidazolylidene donors, while saturated imidazolinylidene variants have curiously received little attention [4]. Such an observation is rather surprising given the successful application of mono-dentate imidazolinylidene ligands in catalysis; epitomised by their wide-spread use in olefin metathesis reactions, where they significantly out perform their unsaturated counterparts [9]. Indeed, in late 2015 when we compiled our review the only known imidazolinylidene-based example to our knowledge was zirconium adduct D [10]. In the intervening time, Chirik and co-workers have prepared iron-based E and demonstrated its ability to selectivity catalyse trituration reactions of C(sp2)-H bonds with high efficiently, highlighting the potential versatility of such NHC-pincers [11]. As part of our work developing the organometallic chemistry of NHC-based pincer ligands, which has involved investigation of macrocyclic variants, backbone geometry, atropisomerism and transmetallation methodology [12], in this report we describe the preparation and preliminary coordination chemistry of imidazolinylidene-based CCC pincer ligands 1 (Scheme 1, Scheme 2).
Section snippets
Pro-ligand 1·2HCl synthesis
A variety of procedures have been reported for the preparation of imidazolinylidene ligand precursors [13]. Similar to that employed for the pro-ligand associated with D [10], we chose a synthetic route involving amide coupling reactions between 1,3-diaminobenzene and aminooxoacetic acids 2 (R = Mes, a; Dipp, b; iPr, c; tBu, d; Scheme 1). In this way, bis-amides 3 were obtained in satisfactorily yields (52, 48, 57, and 53%, respectively) and subsequently reduced, protonated and cyclised in a
Summary
With a view to expanding the structural diversity of NHC-based pincer compounds, a series of imidazolinium-based CCC pro-ligands featuring N-Mes, Dipp, iPr and tBu substituents (1·2HCl) have been prepared. The corresponding free carbenes are readily generated through deprotonation by strong bases and, in addition to being characterised in situ by 1H and 13C NMR spectroscopy, were trapped through reaction with CuCl. Iridium pincer compounds of the N-Mes (5a) and N-Dipp (5b) substituted ligands,
General considerations
All manipulations were performed under an atmosphere of argon, using Schlenk and glove box techniques unless otherwise stated. Glassware was oven dried at 150 °C overnight and flamed under vacuum prior to use. Anhydrous DMF, THF, MeCN, pentane (<0.005% H2O) were purchased from ACROS or Aldrich and freeze-pump-thaw degassed three times before being placed under argon. CD2Cl2 was dried over CaH2, vacuum distilled, and freeze-pump-thaw degassed three times before being placed under argon.
Acknowledgements
We gratefully acknowledge financial support from the University of Warwick, CONACYT (L.G.S) and the Royal Society (A.B.C.). Crystallographic and high-resolution mass-spectrometry data were collected using instruments purchased through support from Advantage West Midlands and the European Regional Development Fund.
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