Published: June 03, 2011 r2011 American Chemical Society 9738 dx.doi.org/10.1021/ja204522e | J. Am. Chem. Soc. 2011, 133, 9738–9740 COMMUNICATION pubs.acs.org/JACS Iridium Complexes of the Doubly Cyclometalated NHC Ligand IMes 00 Janeth Navarro, Olga Torres, Marta Martín, and Eduardo Sola* Instituto de Síntesis Química y Catalisis Homogenea (ISQCH), CSIC-Universidad de Zaragoza, E-50009 Zaragoza, Spain b S Supporting Information ABSTRACT: Two consecutive CH bond activations at the coordination sphere of Ir transform the commonly employed NHC ligand IMes into the new k 3 -C fac-coordi- nating ligand IMes 00 . The preparation and structure of Ir(III) complexes featuring this ligand together with se- lected reactions toward small molecules that illustrate their reactivity keys are described. N -Heterocyclic carbene (NHC) ligands have found wide- spread application in transition-metal catalysis and organo- metallic chemistry, 1 as they offer an extraordinarily wide range of stereoelectronic possibilities. 2 These may be further increased by combining NHCs with themselves or other donor moieties to form bi- or multidentate ligands. 3 For certain metals and oxida- tion states, bidentate NHC ligands can be generated via CH bond activations at the N-bound wingtips. 4,5 Just as for cyclo- metalated phosphines, 6 such bond activations are often reversible, 7 so metalation can be regarded as an additional reactivity resource provided by the ligand. 8 This work shows that one of the most commonly employed NHC ligands, 1,3- bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), is suscep- tible to double cyclometalation at the coordination sphere of iridium to form the k 3 -C fac-coordinating ligand IMes 00 . In addition to this stabilizing coordination mode, the new ligand offers strong trans-labilizing capabilities and can be demetalated in a stepwise fashion or functionalized using a variety of reagents. We recently reported that treatment of the iridium tris- (acetonitrile) dihydride [IrH 2 (NCMe) 3 (IMes)]PF 6 with hydro- gen acceptors such as ethylene or propylene produces the Ir(III) alkyl derivatives [Ir(IMes 0 )(R)(NCMe) 2 ]PF 6 (R = ethyl or n- propyl, respectively), most likely after a sequence of hydrogena- tion, CH oxidative addition, and alkene insertion reactions. 9 We have now observed that the slightly more reactive precursor [IrH 2 (η 6 -C 6 H 6 )(IMes)]PF 6 can drive activation of the NHC ligand beyond the first metalation step, affording Ir(IMes 00 ) complexes. The optimal experimental conditions for this process were found to be warming of propylene-saturated acetone solutions of the precursor arene complex at 328 K for several hours within a closed flask. Even though we have not been able to characterize the intricate mixture of complexes in equilibrium that results from such a treatment, the Ir(IMes 00 ) fragment can be “extracted” from these reacting solutions in good yield in the form of various isolable complexes, as illustrated in Scheme 1. This scheme features cationic, zwitterionic, and neutral Ir- (IMes 00 ) complexes coordinating exclusively through carbon atoms (1, 3, and 4, respectively), together with the cationic tris(acetonitrile) complex 2. The reactivity and synthetic versatility of these complexes decrease as their numbering increases. Thus, the cyclopentadienyl complex 4 can be synthe- sized from any of 13, while 3 can be obtained from the cationic compounds but not vice versa. The dehydrogenation pathway observed for our Ir(IMes) precursor contrasts with that previously reported for related Ir(IPr) and Ir(ICy) compounds, 5 where the initial CH bond activation was followed by a hydrogen β-elimination to yield an alkene, an elementary step not possible for IMes. Comparison of the structures in Figure 1 with those of related Ir(IMes) and Ir(IMes 0 ) derivatives show that the double cyclo- metalation does not imply significant distortions of the IMes skeleton. 9 The IrN bond distances in the cation of 2 (all >2 Å) confirm that the three acetonitrile ligands trans to IMes 00 are weakly bonded 9 and also indicate that the structural trans effect of the alkyl arms exceeds that of the carbene. Accordingly, the acetonitrile ligands of 2 coordinated trans to the methylene carbons were found to exchange readily with acetonitrile-d 3 in CDCl 3 solution at room temperature, while the ligand trans to the carbene carbon remained unaffected. The activation para- meters for the facile acetonitrile dissociations were estimated by 1 H NMR spin saturation transfer and line-width measurements as ΔH q = 20 ( 1 kcal mol 1 and ΔS q =7 ( 3 cal K 1 mol 1 (see the Supporting Information). In spite of these kinetic properties and as previously observed in parent Ir(III) octahedral complexes, 9,10 substitution reactions of 2 with monodentate ligands other than acetonitrile were found to yield the less encumbered substitution products, namely, those with the incoming ligand in the position trans to the carbene carbon. A representative example of such thermodynamic substitution products, the triisopropylphosphine complex [Ir(IMes 00 )- (NCMe) 2 (PiPr 3 )]PF 6 (5), is shown in Scheme 2. Despite their ability to generate coordination vacancies, none of the complexes 13 were observed to react with conventional sources of hydrogen atoms to reverse the cyclometalation process. Neither dihydrogen at a pressure of 60 bar nor other potential oxidative addition reagents such as triethylsilane, pinacolborane, or phenylacetylene indicated any reaction with the Ir(IMes 00 ) fragment under harsh reaction conditions. 11 On the contrary, the phosphine derivative 5 was found to react with all of these reagents (Scheme 2). First of all, this suggests that these reactions might involve intermediates or transition states with iridium in the Ir(V) oxidation state, which would be attainable by the Ir(IMes 00 ) moiety only with the help of an additional very basic ligand such as the phosphine. The reaction of 5 with either dihydrogen (1 bar) or excess triethylsilane was found to yield the known dihydride complex Received: May 17, 2011