Tunable Hemilabile Ligands for Adaptive Transition Metal Complexes

Organometallics 2011, 30, 499–510 DOI: 10.1021/om100804k

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Tunable Hemilabile Ligands for Adaptive Transition Metal Complexes Ronald Lindner,† Bart van den Bosch,† Martin Lutz,‡ Joost N. H. Reek,† and Jarl Ivar van der Vlugt*,† †

van ’t Hoff Institute for Molecular Sciences, Supramolecular and Homogeneous Catalysis Group, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands, and ‡Department for Crystal and Structural Chemistry, University of Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands Received August 18, 2010

A new family of monoanionic hemilabile ligands L1H-L3H with a PNN donor set has been developed, based on Pd-catalyzed C-N bond formation and straightforward phosphorylation. For these structurally related compounds with a hybrid set of donor atoms, the coordination chemistry with both Rh and Ir has been studied. The anticipated hemilabile character of the dimethylamino group was assessed by NMR and IR competition experiments, using isopropyl isocyanide as exogenous substrate. Supporting DFT calculations were used to quantify the electronic differences between the various members of the ligand family. In effect, we have constructed a modular ligand class that exhibits tunable hemilability.

Introduction Hemilabile coordination is a frequently observed design principle in natural systems, wherein the coordination geometry around and hence the activity displayed by the active metal center are manipulated by controlled access to an open coordination site through a reversible but weak bonding interaction of specific donor groups.1-3 This in turn allows for selective transformation and efficient catalytic turnover. In contrast with the elegant use of these principles in Nature, man-made ligand analogues present in the first coordination sphere of transition metals are generally not poised to display flexible coordination during productive turnover. Typically, ligands employed in “traditional” homogeneous catalysis lead to stable, isolable, and well-defined metal complexes. However, it is precisely this lack of lability that can impede *Corresponding author. E-mail: [email protected]. (1) (a) Palenik, B.; Brahamsha, B.; Larimer, F. W.; Land, M.; Hauser, L.; Chain, P.; Lamerdin, J.; Regala, W.; Allen, E. E.; McCarren, J.; Paulsen, I.; Dufresne, A.; Partensky, F.; Webb, E. A.; Waterbury, J. Nature 2003, 424, 1037. (b) Wuerges, J.; Lee, J.-W.; Yim, Y.-I.; Yim, H.-S.; Kang, S.-O.; Djinovic Carugo, K. Proc. Natl. Acad. Sci. U. S. A. 2004, 101, 8569. (c) Barondeau, D. P.; Kassmann, C. J.; Bruns, C. K.; Tainer, J. A.; Getzoff, E. D. Biochemistry 2004, 43, 8038. (d) van der Vlugt, J. I.; Meyer, F. Met. Ions Life Sci. 2007, 2, 181. (2) (a) Springman, E. B.; Angleton, E. L.; Birkedal-Hansen, H.; van Wart, H. E. Proc. Natl. Acad. Sci. U. S. A. 1990, 87, 364. (b) van Wart, H. E.; Birkedal-Hansen, H. Proc. Natl. Acad. Sci. U. S. A. 1990, 87, 5578. (3) Traylor, T. G.; Chang, C. K.; Geibel, J.; Berzinis, A.; Mincey, T.; Cannon, J. J. Am. Chem. Soc. 1979, 101, 6716. (4) Jeffrey, J. C.; Rauchfuss, T. B. Inorg. Chem. 1979, 18, 2658. This paper described the first hemilabile coordination of (o-diphenylphosphino)anisole. For related bisphosphines with this motif, see: (a) van der Vlugt, J. I.; Bonet, J. M.; Mills, A. M.; Spek, A. L.; Vogt, D. Tetrahedron Lett. 2003, 44, 4389. (b) van der Vlugt, J. I.; Grutters, M. M. P.; Mills, A. M.; Kooijman, H.; Spek, A. L.; Vogt, D. Eur. J. Inorg. Chem. 2003, 4361. (c) van der Vlugt, J. I.; van Duren, R.; Batema, G. D.; den Heeten, R.; Meetsma, A.; Fraanje, J.; Goubitz, K.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Vogt, D. Organometallics 2005, 24, 5377. (d) Grutters, M. M. P.; van der Vlugt, J. I.; Pei, Y.; Mills, A. M.; Lutz, M.; Spek, A. L.; M€uller, C.; Moberg, C.; Vogt, D. Adv. Synth. Catal. 2009, 351, 2199. r 2011 American Chemical Society

Figure 1. Generalized description of adaptive coordination behavior of a hemilabile ligand (left) and schematic representation of a tridentate pincer-ligand scaffold with different types of donor groups.

certain activation and/or propagation pathways, and it might therefore prove beneficial to have systems (ligand-metal combinations) that can structurally adapt to changes at intermediate stages of a catalytic cycle and/or accommodate different geometric or electronic requirements (Figure 1). The term “hemilabile ligand” was first introduced in synthetic chemistry in 1979 by Jeffrey and Rauchfuss,4 and several reviews have highlighted the versatile coordination chemistry with various types of scaffolds.5,6 However, the targeted and tunable (catalytic) application of ligands that adapt to exogenous ligands or alternative external stimuli remains relatively unexplored.7,8 Recently, Milstein et al. (5) Braunstein, P.; Naud, F. Angew. Chem., Int. Ed. 2001, 40, 680. (6) (a) Bader, A.; Lindner, E. Coord. Chem. Rev. 1991, 108, 27. (b) Slone, C. S.; Weinberger, D. A.; Mirkin, C. A. Prog. Inorg. Chem. 1999, 48, 233. (7) For applications of hemilabile ligands in industrial catalysis, see: (a) Parshall, G. W.; Ittel, S. D. In Homogeneous Catalysis: The Applications and Chemistry of Catalysis by Soluble Transition Metal Complexes, 2nd ed.; Wiley-Interscience: New York, 1992; p 70. (b) Keim, W. Angew. Chem., Int. Ed. 1990, 29, 235. (8) (a) Deckers, P. J. W.; Hessen, B.; Teuben, J. Angew. Chem., Int. Ed. 2001, 40, 2516. (b) Moxham, G. L.; Randell-Sly, H. E.; Brayshaw, S. K.; Woodward, R. L.; Weller, A. S.; Willis, M. C. Angew. Chem., Int. Ed. 2006, 45, 7618. Published on Web 01/12/2011

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Scheme 1. Examples of Non-C2-symmetric, Potentially Hemilabile PNN Ligands

reported elegant applications of Ru complexes with hybrid, formally neutral PNN0 pincer system A,9 while our own group reported chemistry with Cu, Ni, and Pd.10 Besides the implied hemilabile character of this ligand system under catalytic conditions, which is suggested to be beneficial for the reaction rate and chemoselectivity, the lutidine core also enables a formal charge-switch from neutral to monoanionic by dearomatization of the heterocyclic framework. Chelating monoanionic “pincer” ligands11,12 are now well-known to display strong tridentate coordination to (late) transition metals. Within this class of rigid ligands, mixed donor sets ECE0 have been studied to some extent, although the symmetric ECE analogues prevail, which is mainly related to their synthetic simplicity. Particularly noteworthy are contributions by Zargarian, Dupont, Szab o/Klein Gebbink, and Jensen, describing both coordination chemistry and catalytic implications of various mixed donor geometries.13

(9) (a) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. J. Am. Chem. Soc. 2005, 127, 10840. (b) Gunanathan, C.; Ben-David, Y.; Milstein, D. Science 2007, 317, 790. (c) Kohl, S. W.; Weiner, L.; Schwartsburd, L.; Konstantinovski, L.; Shimon, L. J. W.; Ben-David, Y.; Iron, M. A.; Milstein, D. Science 2009, 324, 74. (d) For comprehensive highlights, see: (a) Friedrich, A.; Schneider, S. ChemCatChem 2009, 1, 72. (b) Hetterscheid, D. G. H.; van der Vlugt, J. I.; de Bruin, B.; Reek, J. N. H. Angew. Chem., Int. Ed. 2009, 48, 8178. (10) (a) van der Vlugt, J. I.; Reek, J. N. H. Angew. Chem., Int. Ed. 2009, 48, 8832. (b) van der Vlugt, J. I.; Pidko, E. A.; Vogt, D.; Lutz, M.; Spek, A. L.; Meetsma, A. Inorg. Chem. 2008, 47, 4442. (c) van der Vlugt, J. I.; Lutz, M.; Pidko, E. A.; Vogt, D.; Spek, A. L. Dalton Trans. 2009, 1016. (d) Lutz, M.; van der Vlugt, J. I.; Vogt, D.; Spek, A. L. Polyhedron 2009, 28, 2341. (e) van der Vlugt, J. I.; Pidko, E. A.; Vogt, D.; Lutz, M.; Spek, A. L. Inorg. Chem. 2009, 48, 7513. (f) van der Vlugt, J. I.; Siegler, M. A.; Janssen, M.; Vogt, D.; Spek, A. L. Organometallics 2009, 28, 7025. (g) See also: Feller, M.; Ben-Ari, E.; Iron, M. A.; Diskin-Posner, Y.; Leitus, G.; Shimon, L. J. W.; Konstantinovski, L.; Milstein, D. Inorg. Chem. 2010, 49, 1615. (11) (a) For recent accounts of pincer chemistry, see: (a) MoralesMorales, D.; Jensen, C. M. In The Chemistry of Pincer Compounds; Elsevier: Amsterdam, The Netherlands, 2007. (b) Liang, L.-C. Coord. Chem. Rev. 2006, 250, 1152. (c) Whited, M. T.; Grubbs, R. H. Acc. Chem. Res. 2009, 42, 1607. (12) (a) Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976, 1020. (b) Al-Salem, N. A.; Empsall, H. D.; Markham, R.; Shaw, B. L.; Weeks, B. J. Chem. Soc., Dalton Trans. 1979, 1972. (13) (a) Ankersmit, H. A.; Veldman, N.; Spek, A. L.; Vrieze, K.; van Koten, G. Inorg. Chim. Acta 1996, 252, 339. (b) Rietveld, M. H. P.; Hagen, H.; van de Water, L.; Grove, D. M.; Kooijman, H.; Veldman, N.; Spek, A. L.; van Koten, G. Organometallics 1997, 16, 168. (c) Ebeling, G.; Meneghetti, M. R.; Rominger, F.; Dupont, J. Organometallics 2002, 21, 3221. (d) Eberhard, M. R.; Matsukawa, S.; Yamamoto, Y.; Jensen, C. M. J. Organomet. Chem. 2003, 687, 185. (e) Gagliardo, M.; Selander, N.; Mehendale, N. C.; van Koten, G.; Klein Gebbink, R. J. M.; Szabo, K. J. Chem.;Eur. J. 2008, 14, 4800. (f) Moreno, I.; SanMartin, R.; Ines, B.; Herrero, M. T.; Domínguez, E. Curr. Org. Chem. 2009, 13, 878. (g) Spasyuk, D. M.; Zargarian, D.; van der Est, A. Organometallics 2009, 28, 6531. (h) Bonnet, S.; Li, J.; Siegler, M. A.; von Chrzanowski, L. S.; Spek, A. L.; van Koten, G.; Klein Gebbink, R. J. M. Chem.;Eur. J. 2009, 15, 3340. (i) Kozlov, V. A.; Aleksanyan, D. V.; Nelyubina, Y. V.; Lyssenko, K. A.; Vasil'ev, A. A.; Petrovskii, P. V.; Odinets, I. L. Organometallics 2010, 29, 2054. (j) Spasyuk, D. M.; Zargarian, D. Inorg. Chem. 2010, 49, 6203. (k) Niu, J.-L.; Chen, Q.T.; Hao, X.-Q.; Zhao, Q.-X.; Gong, J.-F.; Song, M.-P. Organometallics 2010, 29, 2148. (l) Zhang, B.-S.; Wang, W.; Shao, D.-D.; Hao, X.-Q.; Gong, J.-F.; Song, M.-P. Organometallics 2010, 29, 2579. (m) Li, J.; Siegler, M.; Lutz, M.; Spek, A. L.; Klein Gebbink, R. J. M.; van Koten, G. Adv. Synth. Catal. 2010, 352, 2474.

Lately, especially pincer systems with a non-carbon pivot atom have drawn significant attention, and various classes of monoanionic tridentate scaffolds are now available that have revitalized traditional pincer chemistry with new approaches, methodologies, and modes of reactivity.14-19 Notably, very few of these examples bearing a noncarbon pivot deal with non-C2-symmetric frameworks (Scheme 1).20,21 Peters and co-workers recently described Pt complexes with a potentially tridentate asymmetric, all-nitrogen donor ligand B, showing that square-planar coordination of the Pt center prevailed, thereby leaving a dimethylamine side-group uncoordinated and rotated out of the Pt(NN) (14) (a) Fryzuk, M. D.; MacNeil, P. A.; Rettig, S. J.; Secco, A. S.; Trotter, J. Organometallics 1982, 1, 918. (b) Winter, A. M.; Eichele, K.; Mack, H.-G.; Potuznik, S.; Mayer, H. A.; Kaska, W. C. J. Organomet. Chem. 2003, 682, 149. (c) Liang, L.-C.; Ling, J.-M.; Hung, C.-H. Organometallics 2003, 22, 3007. (d) Fan, L.; Foxman, B. M.; Ozerov, O. V. Organometallics 2004, 23, 326. (15) (a) Mankad, N. P.; Rivard, E.; Harkins, S. B.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 16032. (b) Whited, M. T.; Rivard, E.; Peters, J. C. Chem. Commun. 2006, 1613. (16) (a) Peters, J. C.; Harkins, S. B.; Brown, S. D.; Day, M. W. Inorg. Chem. 2001, 40, 5083. (b) Betley, T. A.; Qian, B. A.; Peters, J. C. Inorg. Chem. 2008, 47, 11570. (c) Csok, Z.; Vechorkin, O.; Harkins, S. B.; Scopelliti, R.; Hu, X. J. Am. Chem. Soc. 2008, 130, 8156. (17) (a) Sessler, J. L.; Gebauer, A.; Kral, V.; Lynch, V. Inorg. Chem. 1996, 35, 6636. (b) Br€oring, M.; Brandt, C. D. Chem. Commun., 2001, 499. (c) McManus, H. A.; Guiry, P. J. J. Org. Chem. 2002, 67, 8566. (d) Mazet, C.; Gade, L. H. Chem. Eur. J., 2003, 9, 1759. (e) Konrad, F.; Lloret Fillol, J.; Wadepohl, H.; Gade, L. H. Inorg. Chem. 2009, 48, 8523. (e) Konrad, F.; Lloret Fillol, J.; Rettenmeier, C.; Wadepohl, H.; Gade, L. H. Eur. J. Inorg. Chem. 2009, 4950. (18) MacInnis, M. C.; MacLean, D. F.; Lundgren, R. J.; McDonald, R.; Turculet, L. Organometallics 2007, 26, 6522. Korshin, E. E.; Leitus, G.; Shimon, L. J. W.; Konstantinovski, L.; Milstein, D. Inorg. Chem. 2008, 47, 7177. (19) (a) Segawa, Y.; Yamashita, M.; Nozaki, K. J. Am. Chem. Soc. 2009, 131, 9201. See also: van der Vlugt, J. I. Angew. Chem., Int. Ed. 2010, 49, 252. (b) Spokoyny, A. M.; Reuter, M. G.; Stern, C. L.; Ratner, M. A.; Seideman, T.; Mirkin, C. A. J. Am. Chem. Soc. 2009, 131, 9482. (c) van der Vlugt, J. I. Angew. Chem. Int. Ed. 2010, 49, 252. (20) Examples of ENE0 pivot pincer ligands with mixed donor sets: (a) Debono, N.; Iglesias, M.; Sanchez, F. Adv. Synth. Catal. 2007, 349, 2470. (b) Br€oring, M.; Kleeberg, C. Chem. Commun. 2008, 2777. (c) Br€oring, M.; Kleeberg, C.; K€ohler, S. Inorg. Chem. 2008, 47, 6404. (d) Gu, S.; Chen, W. Organometallics 2009, 28, 909. (e) del Pozo, C.; Debono, N.; Corma, A.; Iglesias, M.; Sanchez, F. ChemSusChem 2009, 2, 650. (f) Baratta, W.; Benedetti, F.; Del Zotto, A.; Fanfoni, L.; Felluga, F.; Magnolia, S.; Putignano, E.; Rigo, P. Organometallics 2010, 29, 3563. (21) A variety of other hybrid, potentially tridentate PNN ligand scaffolds have been reported in the past decade, but so far their chemistry has remained relatively unexplored: (a) Horibe, H.; Fukuda, Y.; Kondo, K.; Okuno, H.; Murakami, Y.; Aoyama, T. Tetrahedron 2004, 6, 10701. (b) Dalili, S.; Caiazzo, A.; Yudin, A. K. J. Organomet. Chem. 2004, 689, 3604. (c) Zhang, C.; Sun, W.-H.; Wang, Z.-X. Eur. J. Inorg. Chem. 2006, 4895. (d) Boubekeur, L.; Ulmer, S.; Ricard, L.; Mezailles, N.; Le Floch, P. Organometallics 2006, 25, 315. (e) Kawamura, K.; Fukuzawa, H.; Hayashi, M. Org. Lett. 2008, 10, 3509. (f) Han, F.-B.; Zhang, Y.-L.; Sun, X.-L.; Li, B.G.; Guo, Y.-H.; Tang, Y. Organometallics 2008, 27, 1924. (g) He, L.-P.; Liu, J.-Y.; Pan, L.; Wu, J.-Q.; Xu, B.-C.; Li, Y.-S. J. Polym. Sci. A: Polym. Chem. 2009, 47, 713. (h) Lagaditis, P. O.; Mikhailine, A. A.; Lough, A. J.; Morris, R. H. Inorg. Chem. 2010, 49, 1094. (i) Picot, A.; Dyer, H.; Buchard, A.; Auffrant, A.; Vendier, L.; Le Floch, P.; Sabo-Etienne, S. Inorg. Chem. 2010, 49, 1310. (22) Coordination of the dimethylamino group in ligand B was not observed to date. B should therefore as yet not be regarded as a potentially hemilabile ligand.

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Scheme 2. Synthetic Methodologies for the Preparation of Compounds L1H and L2Ha

Figure 2. Concept of novel PNN-based tunable hemilabile ligands.

plane.16a,22 Furthermore, mixed (or hybrid) donor sets have 6 C). hardly been investigated to date with EXE0 ligands (X ¼ Gusev prepared ligand C and described its (Noyori-type) cooperative behavior in Ir-catalyzed transfer hydrogenation.23 Surprisingly, to the best of our knowledge, no (pincer) ligand classes exist in which the hemilabile character is selectively changed and tuned to alter the coordination behavior selectively from tri- to bidentate, a tool that is considered very relevant from a conceptual point of view. Such control over the ligand denticity is crucial to fully understand the impact of this property on catalytic reactions. Furthermore, novel modes of adaptive complexes could also lead to new or tunable reactivity. We therefore sought a modular approach to a family of hemilabile ligands, amenable to both steric and electronic modification and tunable variation in the side-group functionalities without impeding the intrinsic coordinating properties of the core atom.24 With these conceptual prerequisites in mind, we set out to design a family of PNN ligands with a variable backbone (Figure 2). We herein report on the synthesis of these ligands with tunable hemilabile character and general trends in their coordination chemistry to Rh and Ir. Notably, we also determined the degree of hemilability by means of competition experiments between various closely related members of this class, combined with various DFT calculations, to establish the first selection rules and hemilability scale for these novel systems.

Results and Discussion Ligand Synthesis. To introduce an inherent linkage asymmetry in our target ligand framework, we devised a three-step synthetic route, starting from commercially available precursors. To prepare the diarylamine backbones, we opted for Buchwald-Hartwig-type Pd-catalyzed amination chemistry. Coupling the appropriate aryl iodide reagent with the desired ortho-substituted aniline in the presence of Pd2(dba)3/dppf as catalyst yielded the corresponding disubstituted amines in good yields. However, introduction of the phosphorus fragment proved to be less straightforward then initially assumed. Phosphorylation of ortho-bromodiarylamine using 2 equiv of n-BuLi and 1 equiv of ClPPh 2 and subsequent aqueous workup unexpectedly led to mixtures of monoand diphosphorus compounds. Hence, it appears that the character of the secondary amine is slightly different from the diarylamine precursors used to prepare the known PNP (23) Choualeb, A.; Lough, A. J.; Gusev, D. G. Organometallics 2007, 26, 5224. (24) Alternative approaches to introduce “masked” vacant coordination sites: (a) Allgeier, A. M.; Mirkin, C. A. Angew. Chem., Int. Ed. 1998, 37, 894. (b) Gianneschi, N. C.; Masar, M. S., III; Mirkin, C. A. Acc. Chem. Res. 2005, 38, 825.

a

[Pd] is formed in situ by reaction of equimolar Pd2(dba)3 and dppf.

Scheme 3. Synthetic Methodologies for the Preparation of Compound L3H

ligands, for which these selectivity issues have not been observed to this extent.15 However, targeted double lithiation and subsequent phosphorylation provided the phosphinephosphinamine species in good yield.25 This was directly converted by selective hydrolysis of the P-N bond, using concentrated aqueous HCl in MeOH, to obtain the desired PNN0 ligand L1H (Scheme 2). Notably, the archetypical, symmetric pincer ligands, when coordinated to a given metal center, give rise to tridentate coordination with two five-membered chelate rings (a 5,50 system). To probe the effect of different geometries on the potentially tunable hemilabile character of the overall ligand structure, we also prepared a backbone that leads to a 5,60 complex upon coordination. Such asymmetry in coordination geometry, by means of differing chelate ring size, is rarely explored to probe or tune the labilization of a particular donor functionality.26 Therefore, we devised a synthetic approach to vary the amine side arm, which led to ligand L2H. We sought extension of our dedicated, novel hybrid ligand family by preparing the semialiphatic analogue L3H of the diarylamide backbones (Scheme 3).27 Liang previously reported the preparation of this species after (25) Some bidentate phosphine-phosphinamines have been reported in the literature; see: (a) Aucott, S. M.; Slawin, A. M. Z.; Woollins, J. D. J. Organomet. Chem. 1999, 582, 83. (b) Xavier, K. O.; Smolensky, E.; Kapon, M.; Aucott, S. M.; Woollins, J. D.; Eisen, M. S. Eur. J. Inorg. Chem. 2004, 4795, and references therein. (c) For related bidentate aminophosphine ligands, see: Zijp, E. J.; van der Vlugt, J. I.; Tooke, D. M.; Spek, A. L.; Vogt., D. Dalton Trans. 2005, 512. (26) Tridentate diphosphinoazines, featuring a PNP donor set, ak, J.; Vojtı´ sek, coordinate in a 5,60 -chelate fashion: (a) Posta, M.; Cerm P.; Sykora, J.; Cı´ sarova, I. Inorg. Chim. Acta 2009, 362, 208, and references therein. Also a PNN ligand based on lutidine has been reported: (b) Poverenov, E.; Leitus, G.; Shimon, L. J .W.; Milstein, D. Organometallics 2005, 24, 5937. (c) Schaub, T.; Radius, U.; Diskin-Posner, Y.; Leitus, G.; Shimon, L. J. W.; Milstein, D. Organometallics 2008, 27, 1892. For related 5,60 -PCN ligands see: (d) Poverenov, E.; Gandelman, M.; Shimon, L. J .W.; Rozenberg, H.; Ben-David, Y.; Milstein, D. Chem.;Eur. J. 2004, 10, 4673. (e) Poverenov, E.; Gandelman, M.; Shimon, L. J .W.; Rozenberg, H.; Ben-David, Y.; Milstein, D. Organometallics 2005, 24, 1082. (27) In principle, the same general synthetic protocol could also be used to install other functionalities besides dialkylamines or allow for different substitution patterns.

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Scheme 4. Preparation of Metal Complexes X1-X5

reaction of intermediate A with KPPh2 in refluxing dioxane for seven days.28 With our improved microwave-assisted methodology, this reaction only took six hours to reach full conversion to obtain L3H. Overall yields for the synthesis of compounds L1H-L3H, which feature subtle but significant backbone modifications and represent the first members of a new class of hemilabile ligands, were in the range 30-70%. Rh(I) and Ir(I) Alkene Complexes of PNN Pincer Ligands. In order to get a first approximation of the complexation characteristics of these new ligands, we investigated their coordination with RhI and IrI. However, using common precursors of group 8 metals, e.g., chloro-bridged RhI and IrI dimer species, we failed to obtain PNN pincer-based complexes. More successful was the use of hydroxy- or methoxybridged RhI and IrI dimers, wherein the internal base is sufficiently reactive to ensure formation of the desired [M(PNN0 )(COE)] complexes in high yields. For example, reaction of 1 equiv of the readily accessible dimer [{Rh(μ-OH)(COE)2}2]29 or [{Ir(μ-OMe)(COD)}2]30 with 2 equiv of the appropriate ligand in diethyl ether at -80 °C led to smooth formation of the desired well-defined MI(cycloalkene) species (28) Lee, W.-Y.; Liang, L.-C. Dalton Trans. 2005, 1952. (29) (a) Werner, H.; Bosch, M.; Schneider, M. E.; Hahn, C.; Kukla, F.; Manger, M.; Windm€ uller, B.; Webernd€ orfer, B.; Laubender, M. J. Chem. Soc., Dalton Trans. 1998, 3549. (b) Vicente, J.; Gil-Rubio, J.; Bautista, D.; Sironi, A.; Masciocchi, N. Inorg. Chem. 2004, 43, 5665. (30) Wanninger-Weiss, C.; Wagenknecht, H.-A. Eur. J. Org. Chem. 2008, 64. (31) The 31P NMR chemical shifts are in the range δ 56.5 (X1) to 63.0 ppm (X2) for Rh and δ 15.2 (X4) to 28.6 ppm (X5) for Ir. For the Rh complexes, characteristic coupling constants JRhP in the range of ∼200 Hz were observed. Recent work from our laboratories involving other Rh-phosphine complexes: (a) Chikkali, S. H.; Bellini, R.; BerthonGelloz, G.; van der Vlugt, J. I.; de Bruin, B.; Reek, J. N. H. Chem. Commun. 2010, 49, 1244. (b) Wassenaar, J.; de Bruin, B.; Siegler, M. A.; Spek, A. L.; Reek, J. N. H.; van der Vlugt, J. I. Chem. Commun. 2010, 46, 1232. (c) Wassenaar, J.; Siegler, M. A.; Spek, A. L.; de Bruin, B.; Reek, J. N. H.; van der Vlugt, J. I. Inorg. Chem. 2010, 49, 6495.

Figure 3. ORTEP plot (50% probability thermal ellipsoids) of complex X4, [Ir(κ3-P,N,N0 -L3)(COE)]. Selected bond lengths (A˚) and angles (deg): Ir-P 2.2010(4); Ir-N1 2.002(1); Ir-N2 2.212(1); Ir-C1 2.109(2); Ir-C2 2.156(2); P-Ir-N1 82.25(4); N1-Ir-N2 79.13(6), P-Ir-N2 161.34(4); P-Ir-C1 105.58(5); P-Ir-C2 98.66(5); N1-Ir-C1 154.05(7); N1-Ir-C2 166.15(6); N1-Ir-{C1-C2}mid 172.83(6); N2-Ir-{C1-C2}mid 95.85(5). Torsion angles (deg): Ir-N1-C9-C10 -18.5(2); Ir-N1-C11C12 15.1(2).

in moderate to good yields (Scheme 4). Strikingly, all compounds X1-X5 were found to be very air- and moisturesensitive, the Ir derivatives even more so than the Rh analogues. The 31P chemical shifts and Rh-P coupling constants are in the expected ranges.31 The coordination mode of the hemilabile side arm of the pincer ligand was determined by the chemical (in)equivalency of the olefinic protons. When the meridinal κ3-coordination mode of the ligand prevails, the complexes exhibit mirror symmetry in the coordination plane of the central metal atom, resulting in one signal for the olefinic protons of the COE fragment in X1-X4. For species X5, the olefinic protons were detected as two broad signals at room temperature. To determine the barrier for rotation around the Namido-Calkyl bond in complex X5, we performed variable-temperature NMR experiments. According to the coalescence temperature (approximately 10 °C, with a ν0 value of 60.5 Hz), the corresponding energy barrier for rotation is calculated to be around 58 kJ/mol.32,33 X-ray Crystallographic Analysis. The molecular structure for species X4, depicted in Figure 3, could be determined via X-ray crystallography. Relevant bond lengths for the complex [Ir(κ3-P,N,N0 -L3)(COE)] include Ir-N1 (2.002(1) A˚) and Ir-N2 (2.112(1) A˚). The small difference between the Ir-amide and Ir-amine bond is in accord with previous reports by Peters and Hu on related complexes.16 The (32) The signal of the methylene protons NCH2CH2NEt2 close to the amido group was used to determine the coalescence temperature. (33) Johnson, E. S. In Advances in Magnetic Resonance, Vol. 1; Waugh, J. S., Ed.; Academic Press: New York, 1956; pp 64-68. Preliminary NMR investigations with ligand L1H or L2H in combination with [{M(m-OMe)(COD)}2] (M = Rh or Ir) also indicate the presence of four inequivalent olefinic protons in the 1H NMR spectra, indicating κ2-P,Ncoordination with a dangling dimethylamino arm.

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Scheme 5. Hemilabile Behavior of the Diaryl PNN Backbone in Complex [Rh(L1)(COE)] with CNiPr

Scheme 6. Formation of Square-Planar MI Species X00 Is Favored over Five-Coordinated Analogues Y, As Determined by DFT Calculations

Figure 4. Sequential NMR spectra (in toluene-d8 at -45 °C) for the stoichiometric reaction of compound X1 (top) with 1.2 equiv (middle) and 2 equiv of CNiPr (bottom).

geometry around the IrI center is distorted square planar, as suggested by the rather acute angles P-Ir-N1 (82.25(4)°) and P-Ir-N2 (161.34(4)°) and the N1-Ir-C1 angle of 154.05(6)°. Surprisingly, there are only very few examples of Ir complexes featuring a PN-N donor set.22,34 The C1dC2 bond of the cyclooctene ligand in X4 is lengthened slightly due to back-bonding induced by the strongly π-donating transamide,35-37 compared to other Ir(COE) complexes.38-40 Substitution Reactions of [RhI(L)(cycloalkene)] Complexes. We investigated the reactivity of complex X1 using the strongly coordinating isopropyl isocyanide CNiPr. This approach allows for the facile addition of near-stoichiometric amounts of exogenous ligand. The addition of one equivalent of isopropyl isocyanide (CNiPr) resulted in clean substitution of the cycloalkene ligand and formation of mono(isocyanide) complex [Rh(L1)(CNiPr)] (X6). We observed similar behavior for [Rh(L2)(COE)] and [Rh(L3)(COE)] (X2 and X3, respectively), leading to the clean formation of [Rh(L2)(CNiPr)] (X8) and [Rh(L3)(CNiPr)] (X10). The bonding situation in the (mono)isocyanide complexes can be examined by an analysis of various geometric parameters. Most commonly the CtN bond length is used as a measure for the back-donation in isocyanide complexes. Alternatively, the CtN-C bond angle in the isocyanide ligand should decrease with increased M-C back-donation. We therefore performed DFT calculations for all (mono)isocyanide complexes relevant to the current study. An inspection of these values leads to the following order for the CtN bond lengths and CtN-C angles: [Rh(L1)(CNiPr)] 1.205 A˚/152.5°; [Rh(L2)(CNiPr)] 1.208 A˚/150.1°; [Rh(L3)(CNiPr)] 1.211 A˚/146.7°. The trend in lengthening of the CtN bond as well as the significant change of the CtN-C angle in the same order indicates an increasingly π-accepting (34) Casalnuovo, A. L.; Calabrese, J. C.; Milstein, D. Inorg. Chem. 1987, 26, 971. (35) For an X-ray structure of the related [Ir(PNP)(COE)] complex, see: Calimano, E.; Tilley, T. D. J. Am. Chem. Soc. 2009, 131, 11161. (36) Ben-Ari, E.; Leitus, G.; Shimon, L. J. W.; Milstein, D. J. Am. Chem. Soc. 2006, 128, 15390. (37) Friedrich, A.; Ghosh, R.; Kolb, R.; Herdtweck, E.; Schneider, S. Organometallics 2009, 28, 708. (38) Iimura, M.; Evans, D. R.; Flood, T. C. Organometallics 2003, 22, 5370. (39) Budzelaar, P. H. M.; Moonen, N. N. P.; de Gelder, R.; Smits, J. M. M.; Gal, A. W. Eur. J. Inorg. Chem. 2000, 753. (40) Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. Chem. Commun. 2004, 764.

Figure 5. Structure of different conformers of [Rh(L1)(CNiPr)] (XC6) and calculated harmonic CtN vibration frequencies. H atoms of the pincer ligands are omitted for clarity.

isocyanide ligand due to the increase in the π-donating character of the amide in the trans position.41 Similar observations were made for the corresponding Ir complexes. Addition of a second equivalent of CNiPr to complex X6 resulted in splitting of the hemilabile Ir-N(Me2) bond of the pincer ligand with formation of [Rh(L1)(CNiPr)2] (X7) (Scheme 5). To exclude the possibility of five-coordinated RhI complexes with overall formula [Rh(κ3-P,N,N0 -L)(CNiPr)2] (see Scheme 6), we performed comparative DFT calculations on the conversion of complexes X0 (with ligands L1-L3) to the corresponding four- and five-coordinated bis(isocyanide) analogues X00 and Y. These investigations revealed that for each case no stable isomer Y exists (i.e., no minimum could be located on either respective potential energy surface). Also the 31P NMR spectroscopic data for complex Z [Rh(L4)(CNiPr)2], based (41) It could also be related to the different substitution pattern on the amino side-group, although this appears less intuitive due to the mutual cis-arrangement rather than the direct trans-influence exerted by the amide donor.

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P NMR Competition Experiment Involving Complexes X1 and X3

on bidentate ligand L4,42,43 which showed a doublet at δ 47.2 ppm (1JRhP = 138.7 Hz), provide further strong indications that the assignment for X7 is valid. Thus, the reaction of mono(isocyanide) complex [Rh(L1)(CNiPr)] (X6) with an additional equivalent of isocyanide leads to rupture of the hemilabile Rh-N(Me2) bond, as observed by NMR spectroscopy and supported by DFT calculations. The corresponding 31P NMR spectra after reaction of the analogous Rh complex X1 with 1.2 and 2 equiv of CNiPr are depicted in Figure 4. The chemical shifts and the coupling constants for the three relevant species are clearly distinguishable, which enables in situ monitoring of substitution reactions (vide infra). In all experiments with more than one equivalent of added CNiPr, low-temperature NMR spectra were recorded as broad signals were observed at ambient temperature. This indicates a rapid equilibrium between the mono- and bis(isocyanide) complex (X6/X7) and a small amount of uncoordinated CNiPr at room temperature, which severely hampered the isolation of bis(isocyanide) complexes. The complexes proved very air-sensitive, both in solution and as solid materials. As a result, attempts to fully characterize these compounds or to obtain solid-state information via (42) Eggenstein, M.; Thomas, A.; Theuerkauf, J.; Franci o, G.; Leitner, W. Adv. Synth. Catal. 2009, 351, 725. (43) Notably, complex Z is the first example of a [Rh(PN)(L0 )2] complex with a monoanionic phosphine-amide PN coordination sphere. For related work on amido-phosphinoalkenyl ligands, see: (a) Sasamori, T.; Matsumoto, T.; Tokitoh, N. Polyhedron 2010, 29, 425. Rare dinuclear examples: (b) Schenk, T. G.; Downes, J. M.; Milne, C. R. C.; Mackenzie, P. B.; Boucher, H.; Whelan, J.; Bosnich, B. Inorg. Chem. 1985, 24, 2334. (c) Dubs, C.; Yamamoto, T.; Inagaki, A.; Akita, M. Organometallics 2006, 25, 1359.

X-ray crystallography failed. We observed similar adaptive behavior in NMR experiments with either monoisocyanide complex X8 or X10, which resulted in the corresponding bis(isocyanide) complex [Rh(L2)(CNiPr)2] (X9) or [Rh(L3)(CNiPr)2] (X11). Thus, our PNN ligands act as a bidentate PN- ligand rather than a tridentate PN-N ligand in the presence of strong coligands such as isocyanides. To further substantiate the formation of a bis(isocyanide) species, we monitored the substitution reaction of [Rh(L1)(COE)] (X1) with CNiPr by in situ IR spectroscopy in C6D6. DFT calculations on the relevant mono- and bis(isocyanide) species suggested an effect of decreasing bond order of the linear C-tNþR fragment into a bent CdNR isomer, due to π-back-bonding, on the IR signature of the isocyanide ligand. For the mono(isocyanide) complexes [Rh(L1)(CNiPr)] the DFT calculations (Figure 5) indicate that the precise conformation of the resulting nonlinear CNiPr ligand has a significant influence on the vibration frequency, leading to multiple bands in the νCN (trans-N) region of the IR spectrum of [Rh(L1)(CNiPr)] (X6), which was experimentally verified. For the bis(isocyanide) complex [Rh(L1)(CNiPr)2] the DFT calculations suggest a significantly higher bond order of the isocyanide ligand trans to the P atom upon introduction of the second equivalent of isocyanide. For complex X7, we observed a strong band at νCN 2161 cm-1. Similar observations were made for complexes with L2 and L3. Comparison with data obtained for complex Z, [Rh(L4)(CNiPr)2], and literature values44 strongly suggest the formation of four-coordinate (44) Northcutt, T. O.; Lachicotte, R. J.; Jones, W. D. Organometallics 1998, 17, 5148.

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Figure 6. Sequential 31P NMR spectra for a mixture of complexes X1 and X3 with 1 equiv (top), 2 equiv (middle). and 3 equiv of CNiPr, relative to total [Rh].

bis(isocyanide) species and PN rather than PNN coordination in all cases. NMR Competition Experiments. We further examined the hemilabile behavior in a direct competition experiment using Rh complexes [Rh(L1)(COE)] (X1) and [Rh(L3)(COE)] (X3) (Scheme 7 and Figure 6). In situ monitoring of a 1:1 reaction mixture of both complexes with 1 equiv of CNiPr (relative to total molar concentration of Rh complexes) by NMR spectroscopy at room temperature revealed that the first equivalent of CNiPr substitutes either cycloalkene ligand without specific preference or selectivity and that no side-arm substitution occurs. Thus, a mixture of unreacted X1 and X3 together with the intermediate species [Rh(L1)(CNiPr)] (X6) and [Rh(L3)(CNiPr)] (X10) was present in solution, as well as free 1,5-cyclooctene. Subsequent addition of 2 equiv of CNiPr led to the selective formation of bis(isocyanide) species X7, concomitant with freshly generated X6 (from X1) and an increased concentration of X10. In order to probe the existence and degree of tunable hemilability among the different members of this novel ligand class, we also performed an NMR competition experiment with the [Rh(L)(COE)] complexes of L2H vs L3H (X2 and X3). In this case, only minor preference for either scaffold to undergo rupture of the Rh-NMe2 bond was observed, as both complexes [Rh(L2)(CNiPr)2] (X9) and [Rh(L3)(CNiPr)2] (X11) could be identified with 31P NMR spectroscopy. Note that the interconversion between monoand bis(isocyanide) complex is fast on the NMR time scale, as shown by coalescence effects at temperatures above -45 °C. Furthermore, it was proven that the relative concentration of these complexes is constant for more than 24 h, which indicates that the reaction reaches equilibrium and the observed complexes are the thermodynamic products. Determination of Tunable Hemilabile Character. As formation of the bidentate bis(isocyanide) complexes [Rh(L)(CNiPr)2], via the corresponding monoisocyanide complexes [Rh(L)(CNiPr)] featuring tridentate coordination of L, was proven to be very fast, any kinetic hindrance for coordination of the second equivalent of CNiPr is considered to be small and was therefore neglected in this investigation. On the other hand, the thermodynamic balance between both coordination modes was found to be only slightly exergonic and seems to be a decisive factor for the stability of the bidentate coordination mode and the hemilabile coordination behavior. The various equilibrium constants were extracted from the NMR spectroscopic competition experi-

Figure 7. Relative scale of hemilability within the novel family of PNN ligands, expressed as ΔΔG values, as determined from the NMR competition experiments.

ments, and these were used to calculate the free energy differences, ΔΔG. This reflects the driving force for “substitution” of the amine side-arm of the pincer ligand for the more strongly coordinating isocyanide ligand in squareplanar [MI(L)(CNiPr)] complexes, which relates to the hemilability of the pincer side-arm or more specifically the thermodynamic stability of the chelating Rh-N bond. To substantiate these experimental findings, we calculated the formation of these rhodium and iridium bis(isocyanide) complexes, starting from the corresponding [MI(L)(COE)] (M = Rh, Ir; L = L1, L2, L3) complexes, at the DFT level of theory (BP86/TZVP). The concept of bond strength has been shown to be quite successful for a rationalization of reaction energies. The particular reaction ([MI(L)(CNiPr)] þ CNiPr f [MI(L)(CNiPr)2]) can be broken down into two different processes: (1) cleavage of the M-N bond of the pincer sidearm and (2) M-C bond formation of the new isocyanide ligand. The first contribution is expected to differ significantly between the observed ligands and contains the differences of the electronic properties as well as ring-strain effects, whereas the second contribution is expected to be quite similar for all expected pincer ligands (similar phosphorus donor in the trans position) and is effected mainly by the (difficult to rationalize) steric interaction between the “new” isocyanide ligand and the other ligands. The advantage of this formal partition is that the first contribution is independent of the nature of the new ligand, and therefore this dissociation energy is a much more general description of the hemilability of these pincer ligands than the overall reaction ([MI(L)(CNiPr)] þ CNiPr f [MI(L)(CNiPr)2]). Note also that such a formalistic breakdown makes no statement about the mechanism of the overall reaction, which could be either associative or dissociative.45 The energies of the corresponding calculated structures are displayed in Figure 8. Obviously, the complete reaction is strongly exergonic for both the Rh and the Ir complexes (ΔRG = -30.4 - -42.1 kcal/mol). Furthermore, it can be

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Figure 8. Calculated energies of (formal) intermediates for the reaction of the COE complexes [M(L)(COE)] (M = Rh, Ir; L = L1-L3) with isopropyl isocyanide.

seen that the mono- and bis(isocyanide) complexes exhibit almost similar stabilities for rhodium, whereas a substantial stabilization of the bis(isocyanide) species can be observed for iridium. The order of the spectroscopically determined relative stability of the bis(isocyanide) complexes is accurately reproduced. The differences in stability of the bis(isocyanide) complexes between the various ligands are mainly attributed to differences in the dissociation of the peripheral M-N bond. Thus, the most labile tridentate coordination mode was found for complexes exhibiting the bisaryl pincer ligand L1, whereas the saturated ligand L3 and the 5,60 -membered ligand L2 lead to stronger M-N bonds. The trend in the lability of the N side-arm might be rationalized by the difference in the electronic environment of the N donor (alkyl vs aryl) as well as the resonance stabilization between the noncoordinated NR2 donor of the side-arm with the aryl ring, which is important for complexes of L1. The chelate ring size and conjugative effects of the central amide donor are less important.

Conclusions We have described the facile and straightforward synthesis of a series of three novel hybrid, tridentate PNN ligand (45) The mechanism is likely to be associative, given the d8-ML4 configuration. However, these investigations are beyond the scope of this paper, and the end results in terms of absolute ΔG values are identical. We have no indication for thermal lability of the M-Nalkyl bond.

scaffolds via a series of optimized three-step synthetic methodologies, starting from readily available materials to yield L1H-L3H in good yields. The Rh- and Ir-cyclooctenyl complexes of these ligands have been characterized, and a relationship between the substitution pattern of the central amido fragment and the degree of hemilability is deduced. The presence of both hard and soft donor groups in the ligand scaffold, implying different coordination strength to either Rh or Ir metal centers, infers hemilabile behavior of the tridentate ligand under certain conditions, as demonstrated by NMR-based competition experiments. We have further supported this by DFT calculations for both Rh- and Ir-based complexes. This has led to the formulation of a scale for the bond strength of the chelating N-M bond, characterizing the hemilability for the first proponents of this new ligand class. These novel hybrid frameworks may be applied as adaptive ligands, as their coordination complexes contain a “masked” vacant site that can be accessible to incoming substrate under specific reaction conditions, thereby enabling productive reactivity. We foresee various interesting applications for these adaptive, hybrid ligand systems in catalytic reactions, especially taking advantage of the different, tunable binding strength of the P and N side-arm donor groups.46 Current efforts are directed to the controlled activation of N-H bonds47 and subsequent reactivity, utilizing the hemilabile nature of our ligand scaffolds. (46) We have noted very interesting redox chemistry with these Rh(PNN) species. These results will be disclosed in a separate account. (47) van der Vlugt, J. I. Chem. Soc. Rev. 2010, 39, 2302.

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Experimental Section General Procedures. All reactions were carried out under an atmosphere of argon using standard Schlenk techniques. With exception of the compounds given below, all reagents were purchased from commercial suppliers and used without further purification. The following compounds were synthesized according to published procedures: 2-(dimethylamino)methylaniline,48 [{Rh(μ-OH)(COE)2}2],29 [{Ir(μ-OMe)(COD)}2],30 [{Ir(μ-OH)(COE)2}2],49 and 2-iodo-N,N-dimethylaniline.50 THF, pentane, hexane, and diethyl ether were distilled from sodium benzophenone ketyl. CH2Cl2, isopropyl alcohol, and methanol were distilled from CaH2, and toluene was distilled from sodium under nitrogen. NMR spectra (1H, 31P, and 13C) were measured on a Varian INOVA 500 MHz or a Varian Mercury 300 MHz. IR spectra (C6D6 solution) were measured on a FT-IR Vertex-70 (Bruker). Fast atom bombardment (FAB) mass spectrometry was carried out on a JEOL JMS SX/SX 102 A four-sector mass spectrometer, coupled to a JEOL MS-MP9021D/UPD system program. Samples were loaded in a matrix solution (3-nitrobenzyl alcohol) onto a stainless steel probe and bombarded with xenon atoms with an energy of 3 keV. During the high-resolution FAB-MS measurements a resolving power of 10 000 (10% valley definition) was used. Preparation of 2-(Bromophenyl)-20 -(dimethylaminophenyl)amine. To a mixture of 2-iodo-N,N-dimethylaniline (2.47 g, 10.0 mmol), 2-bromoaniline (1.71 g, 10.0 mmol), and NaOtBu (1.92 g, 20.0 mmol) in toluene (20 mL) were added DPPF (111 mg, 0.20 mmol) and Pd2(dba)3 (91.6 mg, 0.10 mmol) at room temperature. After heating at 80 °C for 12 h the reaction mixture was evaporated to dryness, and the resulting crude product extracted in CH2Cl2 (50 mL) and washed with H2O (2 20 mL). The organic phase was further purified by filtration over silica (3 A˚) and evaporated to dryness, which gave the product as a dark oil. Yield: 2.6 g (90%). 1H NMR (300 MHz, CDCl3): δ 7.56 (m, 1H, CH), 7.42 (m, 1H, CH), 7.31 (m, 1H, ArH), 7.20 (m, 1H, ArH), 7.12 (m, 1H, ArH), 7.07 (m, 1H, ArH), 6.76 (m, 1H, ArH), 2.71 ppm (s, 6H, NCH3). 13C NMR (121 MHz, CDCl3): δ 141.3 (s, Ar), 136.7 (s, Ar), 133.4 (s, Ar), 128.4 (s, Ar), 124.0 (s, Ar), 121.7 (s, Ar), 121.2 (s, Ar), 120.0 (s, Ar), 116.9 (s, Ar), 116.4 (s, Ar), 113.5 (s, Ar), 44.3 ppm (s, NCH3). Preparation of 2-Bromo-N-(2-((dimethylamino)methyl)phenyl)aniline. To a mixture of 2-bromoiodobenzene (2.83 g, 10.0 mmol), 2-((dimethylamino)methyl)aniline (1.50 g, 10.0 mmol), and NaOtBu (1.92 g, 20.0 mmol) in toluene (20 mL) were added DPPF (111 mg, 0.20 mmol) and Pd2(dba)3 (91.6 mg, 0.10 mmol) at room temperature. After heating at 80 °C for 12 h the reaction mixture was evaporated to dryness, and the resulting crude product extracted in CH2Cl2 (50 mL) and washed with H2O (2 20 mL). The organic phase was further purified by filtration over silica (3 A˚) and evaporated to dryness, which gave the product as a dark oil. Yield: 2.69 g (88%). 1H NMR (300 MHz, CDCl3): δ 9.15 (br, 1H, NH), 7.54 (m, 1H, ArH), 7.38 (m, 2H, ArH), 7.19 (m, 3H, ArH), 6.87 (m, 1H, ArH), 6.71 (m, 1H, ArH), 3.46 (s, 4H, CH2), 2.28 ppm (s, 6H, NCH3). N,N-Diethyl-N0 -(2-fluorophenyl)-1,2-ethane. To a mixture of 1-bromo-2-fluorobenzene (1.75 g, 10.0 mmol), N,N-(diethylamino)ethylamine (1.16 g, 10.0 mmol), and NaOtBu (1.92 g, 20.0 mmol) in toluene (30 mL) were added BINAP (63.0 mg, 0.10 mmol) and Pd2(dba)3 (46 mg, 0.050 mmol) at room temperature. After heating at 80 °C for 18 h the reaction mixture was filtered and washed with toluene (2 10 mL), and the filtrate dried in vacuo. To the resulting brown oil, 15 mL of water was added. This mixture was washed with CH2Cl2 (3 15 mL). (48) Barybin, M. V.; Diaconescu, P. L.; Cummins, C. C. Inorg. Chem. 2001, 40, 2892. (49) Ortmann, D. A.; Werner, H. Z. Anorg. Allg. Chem. 2002, 628, 1373. (50) Yue, D.; Larock, R. C. Org. Lett. 2004, 6, 1037.

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The combined organic layers were dried over MgSO4, and the solvent was removed in vacuo, yielding the desired product as a brown oil. Yield: 1.7 g (81%). 1H NMR (300 MHz, CDCl3): δ 6.95-7.04 (br m, 2H, ArH), 6.60-6.74 (br m, 2H, ArH), 4.63 (br s, 1H, NH), 3.16 (dd, 2H, NCH2CH2NEt2), 2.74 (t, 2H, CH2CH2NEt2), 2.59 (q, 4H, NCH2CH3), 1.06 ppm (t, 6H, NCH2CH3). 13C NMR (76 MHz, CDCl3): δ 152.1 (d, JFC = 28.6 Hz, Ar), 137.1 (d, JFC = 11.7 Hz, Ar), 124.4 (d, JFC = 3.5 Hz, Ar), 116.2 (d, JFC = 6.9 Hz, Ar), 114.2 (d, JFC = 18.4 Hz, Ar), 114.2 (d, JFC = 18.4 Hz, Ar), 112.2 (d, JFC = 3.6 Hz, Ar), 51.5 (s, N(CH2)2N), 46.7 (s, NCH2CH3), 41.0 (s, N(CH2)2N), 11.8 ppm (s, NCH2CH3); 19F NMR (282 MHz, CDCl3): δ 136.41 ppm (s). N-(2-Bromophenyl)aniline. A modification of the procedure from Rossi et al.51 was used for the synthesis of N-(2-bromophenyl)aniline. To a mixture of aniline (745 mg, 8.0 mmol), 2-bromoiodobenzene (2.26 g, 8.0 mmol), and NaOtBu (1.53 g, 16.0 mmol) in toluene (15 mL) were added DPPF (110.8 mg, 0.20 mmol) and Pd2(dba)3 (91.6 mg, 0.10 mmol) at room temperature. After heating at 80 °C for 12 h the reaction mixture was evaporated to dryness, and the resulting crude product was extracted in CH2Cl2 (50 mL) and washed with H2O (2 20 mL). The organic phase was further purified by filtration over silica (3 A˚) and evaporated to dryness, which gave the product as a dark oil. Yield: 1.83 g (92%). 1H NMR (300 MHz, CDCl3): δ 7.51 (m, 1H, ArH), 7.32 (m, 2H, ArH), 7.23 (m, 1H, ArH), 7.15 (m, 3H, ArH), 7.03 (m, 1H, ArH), 6.92 (m, 1H, ArH), 6.08 ppm (br, 1H, NH). Preparation of (2-Diphenylphosphinophenyl)-20 -(dimethylaminophenyl)amine (L1H). A 2.5 M solution of BuLi in hexane (4.00 mL, 10.0 mmol) was added to a solution of (2-bromophenyl-20 -dimethylaminophenyl)amine (1.46 g, 5.00 mmol) in Et2O (20 mL) at -80 °C. The mixture was allowed to warm to ambient temperature and stirred for 12 h. After cooling of this solution to -80 °C ClPPh2 (1.84 mL, 10.0 mmol) was added, and the resulting reaction mixture warmed to room temperature again. After 12 h the suspension was evaporated to dryness, redissolved in MeOH (50 mL), and acidified with concentrated HCl (2 mL). After stirring for 1 h, the solution was neutralized with an excess of Na2CO3 (4.24 g, 40.0 mmol) and evaporated to dryness. The crude product was obtained by extraction with CH2Cl2 and further purified by chromatography over silica (3 A˚) with chloroform. Yield: 807 mg (41%). 1H NMR (300 MHz, CDCl3): δ 7.34 (m, 10H, ArH), 7.20-7.42 (m, 4H, ArH), 6.75-7.00 (m, 4H, ArH), 2.31 ppm (s, 6H, NCH3). 13C NMR (121 MHz, CDCl3): δ 145.8 (d, JPH = 18.5 Hz, Ar), 143.1 (s, Ar), 138.1 (s, Ar), 136.0 (d, JPH = 10.4 Hz, Ar), 134.4 (d, JPH = 20.8 Hz, Ar), 134.2 (d, JPH = 2.3 Hz, Ar), 129.9 (s, Ar), 129.2 (s, Ar), 128.9 d, JPH = 8.1 Hz, Ar), 128.5 d (JPH = 6.9 Hz, Ar), 126.9 (d, JPH = 9.2 Hz, Ar), 124.1 (s, Ar), 121.4 (s, Ar), 120.1 (d, JPH = 32.3 Hz, Ar), 118.8 (s, Ar), 115.0 ppm (s, Ar). 31P NMR (121 MHz, CDCl3): δ -16.7 ppm (s). HR-MS (FAB): m/z calcd for C26H26N2P 397.1834 [M þ H]þ; found 397.1839. Preparation of 2-((Dimethylamino)methyl)-N-(2-(diphenylphosphino)phenyl)aniline (L2H). A 2.5 M solution of BuLi in hexane (4.00 mL, 10.0 mmol) was added to a solution of 2-bromoN-(2-((dimethylamino)methyl)phenyl)aniline (1.53 g, 5.00 mmol) in Et2O (20 mL) at -80 °C. The mixture was allowed to warm to ambient temperature and stirred over 12 h. After cooling of this solution to -80 °C ClPPh2 (1.84 mL, 10.0 mmol) was added, and the resulting reaction mixture stirred for 12 h at room temperature again. Thereafter the suspension was evaporated to dryness, redissolved in MeOH (50 mL), and acidified with concentrated HCl (2 mL). After stirring for 1 h the solution was neutralized with an excess of Na2CO3 (4.24 g, 40.0 mmol) and evaporated to dryness. The crude product was obtained by extraction with CH2Cl2 and further purified by chromatography over silica (3 A˚) with chloroform. Yield: 1.08 g (53%). 1H NMR (300 MHz, CDCl3): δ 8.92 (s, 1H, NH), 7.18-7.45 (m, 14H, ArH), 7.14 (51) Buden, M. E.; Vaillard, V. A.; Martin, S. E.; Rossi, R. A. J. Org. Chem. 2009, 74, 4490.

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(m, 1H, ArH), 7.03 (m, 1H, ArH), 6.83 (m, 1H, CH), 6.75 (m, 1H, ArH), 3.31 (s, 2H, CH2), 1.90 ppm (s, 2H, NCH3). 13C NMR (126 MHz, C6D6): δ 145.6 (d, JPC = 18.2 Hz, Ar), 142.9 (s, Ar), 137.8 (s, Ar), 135.8 (d, JPC = 9.8 Hz, Ar), 134.1 (d, JPC = 19.9 Hz, Ar), 133.9 (d, JPC = 2.6 Hz, Ar), 129.6 (s, Ar), 128.9 (s, Ar), 128.6 (s, Ar), 128.6 (s, Ar), 126.7 (d, JPC = 10.1 Hz, Ar), 123.8 (s, Ar), 121.2 (s, Ar), 119.8 (d, JPC = 32.0 Hz, Ar), 117.5 (s, Ar), 114.8 (s, Ar), 118.8 (s, Ar), 118.1 (s, Ar), 95.9 (d, JPC = 12.8 Hz, Ar), 65.9 (s, CH2), 52.5 (s, CH2), 52.0 (s, CH2), 43.3 (s, NCH3), 33.7 (s), 30.3 (s, CH2), 15.5 ppm (s, CH2). 31P NMR (121 MHz, CDCl3): δ -16.3 ppm (s). N-(2-(Diphenylphosphino)phenyl)-N0 ,N0 -diethylethane-1,2-diamine (L3H). A microwave vessel was charged with N,N-diethyl-N0 -(2fluorophenyl)-1,2-ethane (410 mg, 2.0 mmol) and KPPh2 (0.5 M in THF, 4.0 mL, 2.0 mmol). After irradiation for 6 h (150 W, 3 bar, 110 °C), the solvent was removed in vacuo. CH2Cl2 (20 mL) was added, and this mixture was washed with water (20 mL). The two layers were separated, and the aqueous layer was extracted with CH2Cl2 (2 10 mL). The combined organic layers were dried over MgSO4, and the solvent was removed in vacuo. The product was purified over a silica column (3 A˚) using acetone as eluent, yielding L3H as a wax-like white-yellow solid. Yield: 489 mg (65%). 1H NMR (300 MHz, CDCl3): δ 7.40 (d, 1H, ArH), 7.16 (dd, 1H, ArH), 6.60 (d, 1H, ArH), 6.52 (dd, 1H, ArH), 5.47 (br s, 1H, NH), 3.19 (m, 2H, NHCH2CH2), 2.55 (m, 4H, NCH2CH3), 1.80 (m, 2H, CH2CH2NEt2), 1.05 ppm (t, 6H, NCH2CH3). 13C NMR (126 MHz, CDCl3): δ 149.9 (d, JPC = 17.3 Hz, Ar), 135.1 (d, JPC = 6.9 Hz, Ar), 134.3 (d, JPC = 3.5 Hz, Ar), 133.5 (d, JPC = 18.5 Hz, Ar), 130.6 (s, Ar) 128.7 (s, Ar), 128.4 (d, JPC = 6.9 Hz, Ar), 119.5 (d, JPC = 6.9 Hz, Ar), 117.7 (s, Ar), 110.1 (s, Ar), 49.3 (s, N(CH2)2NEt2), 46.4 (s, NCH2CH3), 40.9 (s, N(CH2)2NEt2), 8.7 ppm (s, NCH2CH3). 31P NMR (121 MHz, CDCl3): δ -20.75 ppm (s). HR-MS (FAB): m/z calcd for C27H49N2P 377.2147 [M þ H]þ; found 377.2151. N-(2-(Diphenylphosphino)phenyl)aniline (L4H). Modified procedure from literature:42 A 2.5 M solution of BuLi in hexane (4.00 mL, 10.0 mmol) was added to a solution of N-(2-bromophenyl)aniline (1.24 g, 5.00 mmol) in Et2O (20 mL) at -80 °C. The mixture was allowed to warm to ambient temperature and stirred over 12 h. After cooling of this solution to -80 °C ClPPh2 (1.84 mL, 10.0 mmol) was added, and the resulting reaction mixture stirred for 12 h at room temperature again. Thereafter the suspension was evaporated to dryness, redissolved in MeOH (50 mL), and acidified with concentrated HCl (2 mL). After stirring for 1 h the solution was neutralized with an excess of Na2CO3 (4.24 g, 40.0 mmol) and evaporated to dryness. The crude product was obtained by extraction with CH2Cl2 and further purified by chromatography over silica (3 A˚) with pentane/Et2O (2:1). Yield: 1.18 g (67%). 1H NMR (121 MHz, CDCl3): δ 7.16-7.45 (m, 10H, ArH), 6.80-6.98 (m, 1H, ArH), 6.22 ppm (br, 1H, NH). 31P NMR (121 MHz, CDCl3): δ -18.9 ppm (s). [Rh(L1)(COE)] (X1). A solution of L1H (218 mg, 0.55 mmol) in Et2O (15 mL) was added to [{Rh(μ-OH)(COE)2}2] (187 mg, 0.28 mmol) at -80 °C. The reaction mixture was slowly heated to room temperature overnight, which resulted in the precipitation of the product as an orange-brown solid, which was filtered, washed with hexane (3 1 mL), and dried in vacuo. Yield: 304 mg (91%). 1H NMR (300 MHz, C6D6): δ 8.12 (m, 4H, ArH), 7.62 (dd, 1H, ArH), 7.54 (d, 1H, ArH), 7.20 (dd, 2H, ArH), 7.03 (s, 8H, ArH), 6.58 (dd, 1H, ArH), 6.73 (d, 1H, ArH), 6.48 (dd, 2H, ArH), 3.15 (d, 2H, dCH), 2.50 (s, 6H, NCH3), 2.20 (dt, 2H, COE CH2), 1.30-1.62 (m, 8H, COE CH2), 0.87-1.03 ppm (m, 2H, COE CH2). 13C NMR (126 MHz, C6D6): δ 159.6 (d, JPC = 20.3 Hz, Ar), 150.8 (s, Ar), 148.3 (d, JPC = 2.7 Hz, Ar), 135.5 (d, JPC = 42.0 Hz, Ar), 134.6 (s, Ar), 134.5 (s, Ar), 132.6 (d, JPC = 50.1 Hz, Ar), 132.3 (s, Ar), 131.1 (d, JPC = 1.4 Hz, Ar), 129.7 (d, JPC = 2.3 Hz, Ar), 127.3 (s, Ar), 119.1 (s, Ar), 117.0 (d, JPC = 7.2 Hz, Ar), 115.9 (s, Ar), 114.1 (s, Ar), 113.5 (d, JPC = 11.5 Hz, Ar), 70.1 (d, JPC = 13.0 Hz, Ar), 47.4 (s, NCH3), 31.8 (s, CH2),

Lindner et al. 31.4 (s, CH2), 26.8 ppm (s, CH2). 31P NMR (121 MHz, C6D6): δ 56.5 ppm (d, 1JRhP = 207.8 Hz). [Rh(L2)(COE)] (X2). A solution of L2H (42.0 mg, 0.200 mmol) in Et2O (10 mL) was added to [{Rh(μ-OH)(COE)2}2] (68.1 mg, 0.100 mmol) at -80 °C. The reaction mixture was slowly heated to room temperature overnight and concentrated to 3 mL. Layering with hexane (5 mL) resulted in the precipitation of the product as a yellow solid, which was filtered, washed with hexane (3 1 mL), and dried in vacuo, 27 mg (22%). 1H NMR (500 MHz, toluene-d8): δ 8.34 (m, 2H, ArH), 7.62 (m, 2H, ArH), 6.95-7.13 (m, 10H, ArH), 6.92 (m, 1H, ArH), 6.88 (m, 1H, ArH), 6.72 (m, 1H, ArH), 6.45 (m, 1H, ArH), 3.80 (d, 1H, ArCH2N), 3.07 (m, 2H, COE CH2), 2.32 (m, 1H, ArCH2N), 2.03-2.32 (m, 2H COE CH2), 2.15 (s, 3H, NCH3), 1.76 (s, 3H, NCH3), 1.66 (m, 1H, COE CH2), 1.45 (m, 3H, COE CH2) 1.28 (m, 3H, COE CH2), 1.15 (m, 1H, COE CH2), 0.99 (m, 1H, COE CH2), 0.40 ppm (m, 1H, COE CH2). 13C NMR (126 MHz, C6D6): δ 164.7 (d, 3JRhC = 164.7 Hz, Ar), 155.0 (s, Ar), 137.2 (d, 3JRhC = 40.4 Hz, Ar), 135.5 (s, Ar), 135.4 (s, Ar), 134.8 (s, Ar), 134.7 (s, Ar), 134.3 (d, JRhC = 41.6 Hz, Ar), 132.4 (d, JRhC = 9.2 Hz, Ar), 131.7 (s, Ar), 131.2 (s, Ar), 130.6 (s, Ar), 130.0 (s, Ar), 129.8 (s, Ar), 129.7 (s, Ar), 120.6 (s, Ar), 119.7 (d, JRhC = 11.5 Hz, Ar), 117.5 (s, Ar), 117.2 (d, JRhC = 6.9 Hz, Ar), 66.9 (d, JRhC = 12.7 Hz, COE CH), 66.0 (s, ArCH2N), 66.9 (d, JRhC = 12.7 Hz, COE CH), 50.2 (s, NCH3), 45.6 (s, NCH3), 32.7 (s, COE CH2), 32.2 (s, COE CH2), 31.5 (s, COE CH2), 30.6 (s, COE CH2), 26.9 (s, COE CH2), 26.8 ppm (s, COE CH2). 31P NMR (122 MHz, C6D6): δ 63.0 ppm (d, 1JRhP = 206.2 Hz). [Rh(L3)(COE)] (X3). A solution of L3H (105 mg, 0.279 mmol) in Et2O (15 mL) was added to [{Rh(μ-OH)(COE)2}2] (100 mg, 0.147 mmol) at -80 °C. The reaction mixture was slowly heated to room temperature overnight, which resulted in the precipitation of the product as an orange-brown solid, which was filtered, washed with hexane (3 1 mL), and dried in vacuo. Yield: 95 mg (58%). 1H NMR (500 MHz, C6D6): δ 8.17 (m, 4H, ArH), 7.22 (m, 1H, ArH), 7.11 (d, 1H, ArH), 7.05 (m, 6H, ArH), 6.42 (s, 1H, ArH), 6.36 (m, 1H, ArH), 3.04 (m, 2H, NCH2CH2N), 2.88 (m, 2H, NCH2CH2N), 2.42 (m, 2H, COE CH þ CH2CH3), 2.12 (m, 2H, COE CH2), 1.61 (m, 2H, COE CH2), 1.42 (m, 4H, COE CH2), 1.19 (m, 2H, COE CH2), 0.98 (m, 2H, COE CH2), 0.91 ppm (t, 6H, CH2CH3). 13C NMR (126 MHz, C6D6): δ 137.0 (d, JPC = 41.6 Hz, Ar), 134.7 (s, JPC = 11.5 Hz, Ar), 132.2 (d, JPC = 38.1 Hz, Ar), 129.6 (s, Ar), 113.2 (s, Ar), 107.7 (br, Ar) 66.9 (d, JRhC = 7.2 Hz, COE CH), 56.6 (s, NCH2CH2N), 48.4 (s, NCH2CH3), 48.1 (s, COE CH2), 32.3 (s, COE CH2), 32.0 (s, COE CH2), 27.0 (s, COE CH2), 10.0 ppm (s, NCH2CH3). 31P NMR (122 MHz, C6D6): δ 57.6 ppm (JRhP= 210.8 Hz). [Ir(L3)COE] (X4). A solution of L3H (75.2 mg, 0.200 mmol) in Et2O (10 mL) was added to [{Ir(μ-OH)(COE)2}2] (85.9 mg, 0.100 mmol) at -80 °C. The reaction mixture was slowly heated to room temperature overnight and concentrated to 3 mL. Layering with hexane (5 mL) resulted in the precipitation of the product as a yellow solid, which was filtered, washed with hexane (3 1 mL) at -70 °C, and dried in vacuo. Yield: 53 mg (39%). Single crystals were grown from Et2O/hexane. 1H NMR (500 MHz, C6D6): δ 8.18 (m, 4H, ArH), 6.98-7.25 (m, 8H, ArH), 6.44 (m, 2H, ArH), 3.05 (m, 2H, N(CH2)2N), 2.55 (m, 2H, COE), 2.44 (m, 2H, N(CH2)2N), 2.40 (m, 2H, COE), 2.21 (m, 2H, COE), 2.07 (m, 2H, COE), 1.71 (m, 2H, COE), 1.48 (m, 4H, NCH2CH3), 1.26 (m, 2H, COE), 0.91 (m, 2H, COE), 0.85 ppm (m, 6H, NCH2CH3). 13C NMR (126 MHz, C6D6): δ 165.7 (d, JPC = 18.0 Hz, Ar), 135.4 (d, JPC = 50.3 Hz, Ar), 134.5 (d, JPC = 10.6 Hz, Ar), 132.2 (s, Ar), 131.9 (d, JPC = 1.9 Hz, Ar), 130.8 (d, JPC = 58.7 Hz, Ar), 129.6 (d, JPC = 1.9 Hz, Ar), 127.6 (d, JPC = 10.0 Hz, Ar), 115.0 (d, JPC = 8.7 Hz, Ar), 108.2 (d, JPC = 10.8 Hz, Ar), 58.3 (d, JPC = 2.6 Hz, N(CH2)2N), 49.4 (s, N(CH2)2N), 48.8 (d, JPC = 2.0 Hz, COE CH2), 46.9 (d, JPC = 1.0 Hz, NCH2CH3), 32.7 (s, COE CH2), 32.0 (d, JRhC = 3.2 Hz, COE CH), 27.3 (s, COE CH2), 10.1 ppm (s, NCH2CH3). 31P NMR (202 MHz, C6D6): δ 15.2 ppm (s).

Article [Ir(L3)(COD)] (X5). A solution of L3H (100 mg, 0.25 mmol) in 10 mL of Et2O was slowly added to a mixture of [{Ir(μOMe)(COD)}2] (88 mg, 0.13 mmol) in 15 mL of Et2O at -80 °C. This resulted in a color change from orange to bright red after stirring for 30 min. This mixture was allowed to reach ambient temperature overnight, after which the solvent was removed in vacuo, yielding the desired product as a bright red solid. Yield: 140 mg (83%). 1H NMR (500 MHz, toluene-d8, -45 °C): δ 7.57 (dd, 2H, ArH), 7.45 (dd, 2H, ArH), 7.18 (dd, 1H, ArH), 7.01 (m, 3H, ArH), 6.88-6.83 (br m, 5H, ArH), 6.35 (dd, 1H, ArH), 5.09 (br s, 1H, COD CH), 5.01 (br s, 1H, COD CH), 4.14 (br m, 1H, COD CH), 3.75 (br m, 1H, COD CH), 3.03 (br s, 2H, NCH2CH2NEt2), 2.93 (br s, 2H, NCH2CH2NEt2), 2.57-2.41 (br m, 5H, COD CH2), 2.16-2.09 (br m, 3H, COE CH2), 1.74-1.66 (br m, NCH2CH3), 0.99 ppm (t, 6H, NCH2CH3). 13 C NMR (126 MHz, C6D6): δ 133.7 (d, JPC = 18.0 Hz, Ar), 133.3 (d, JPC = 10.9 Hz, Ar), 130.2 (s, Ar), 119.5 (d, JPC = 55.4 Hz, Ar), 114.8 (d, JPC = 7.5 Hz, Ar), 112.0 (d, JPC = 13.2 Hz, Ar), 55.8 (s, COD CH2), 53.8 (s, COD CH2), 52.5 (br s, COD CH), 50.7 (s, COD CH2), 48.4 (s, NCH2CH3), 33.3 (br s, COD CH), 31.8 (s, COD CH2), 12.8 ppm (s, NCH2CH3). 31P NMR (122 MHz, C6D6): δ 28.6 ppm (s). [Rh(L4)(CNiPr)2] (Z). A solution of L4H (70.7 mg, 0.200 mmol) in Et2O (10 mL) was added to [{Rh(μ-OH)(COE)2}2] (68.1 mg, 0.100 mmol) at -80 °C. The reaction mixture was slowly heated to room temperature overnight. CNiPr (38 μL, 0.40 mmol) was added, and the solution concentrated to 1 mL. Layering with pentane (5 mL) resulted in the precipitation of the product as a orange solid, which was filtered, washed with pentane (3 1 mL), and dried in vacuo. Yield: 71.2 mg (60%). 1 H NMR (300 MHz, C6D6): δ 8.03 (m, 4H, ArH), 7.49 (m, 2H, ArH), 7.31 (m, 1H, ArH), 7.23 (m, 2H, ArH), 7.07-7.10 (m, 6H, ArH), 7.00 (m, 1H, ArH), 6.92 (m, 1H, ArH), 6.75 (m, 1H, ArH), 6.40 (m, 1H, ArH), 2.99 (br, 1H, CH(CH3)2), 2.91 (br, 1H, CH(CH3)2), 0.65 (s, 6H, CH(CH3)2), 0.59 ppm (s, 6H, CH(CH3)2). 13C NMR (75 MHz, C6D6): δ 169.0 (d, JPC = 2.4 Hz, Ar), 168.6 (d, JPC = 2.4 Hz, Ar), 160.4 (s, Ar), 136.7 (d, JPC = 1.8 Hz, Ar), 136.1 (d, JPC = 1.8 Hz, Ar), 136.1 (s, Ar), 134.0 (s, Ar), 133.9 (s, Ar), 132.5 (d, JPC = 1.8 Hz, Ar), 129.5 (d, JPC = 1.8 Hz, Ar) 129.4 (s, Ar), 129.2 (s, Ar), 128.2 (s, Ar), 121.8 (s, Ar), 115.0 (s, Ar), 114.4 (s, Ar), 113.8 (d, JPC = 14.0 Hz, Ar), 111.9 (d, JPC = 6.7 Hz, Ar), 47.5 (br, CH(CH3)2), 23.6 (br, CH(CH3)2), 22.5 ppm (br, CH(CH3)2). 31P NMR (121 MHz, C6D6): δ 47.2 ppm (d, 1JRhP = 138.7 Hz). IR (C6D6): νCN 2040 (w), 2080 (m), 2154 (m) cm-1. NMR Competition Experiments. Three equivalents of CNiPr (2.3 μL, 24 μmol or 1.2 μL, 12.8 μmol) were added subsequently in three steps (1 equiv, 2 equiv, 3 equiv) to a solution of X1 (14.5 mg, 23.8 μmol) and X3 (13.0 mg, 22.1 μmol) or X2 (8.0 mg, 12.8 μmol) and X3 (7.5 mg, 12.7 μmol) in toluene-d8 (0.7 mL). After each addition the solution was analyzed by NMR spectroscopy and compared to spectroscopic data of independently prepared samples. NMR spectroscopically determined relative concentrations of the different COE and CNiPr complexes were used to calculate the relevant equilibrium constants for isocyanide exchange between complexes. [Rh(L1)(CNiPr)] (X6). CNiPr (3.8 μL, 40 μmol) was added to a solution of X1 (24.3 mg, 40.0 μmol) in benzene (3 mL). After 5 min the reaction mixture was concentrated to 1 mL and precipitated by layering with pentane (3 mL) at -70 °C. The resulting yellow precipitate was filtered, washed with pentane (3 1 mL) at -70 °C, redissolved in C6D6, and analyzed by NMR spectroscopy. 1H NMR (300 MHz, C6D6): δ 7.85-8.02 (m, 6H, ArH), 7.30 (m, 1H, ArH), 7.01-7.22 (m, 7H, ArH), 6.95 (m, 1H, ArH), 6.87 (m, 1H, ArH), 6.54 (m, 2H, ArH), 3.06 (s, 6H, NCH3), 3.01 (sep, 3JHH = 6.6 Hz, CH(CH3)2), 0.72 ppm (d, 2 JHH = 6.0 Hz, CH(CH3)2). 13C NMR (126 MHz, C6D6): δ 161.6 (dd, 1JRhC/2JPC = 63/20 Hz, CNiPr), 161.1 (d, JPC = 26.6 Hz, Ar), 151.5 (s, Ar), 146.8 (s, Ar), 137.2 (d, JPC = 46.2 Hz, Ar), 133.6 (d, JPC = 12.7 Hz, Ar), 131.2 (s, Ar), 129.2 (s, Ar), 126.9


509

(d, JPC = 48.5 Hz, Ar), 120.3 (s, Ar), 116.0 (d, JPC = 8.1 Hz, Ar), 115.2 (s, Ar), 115.0 (s, Ar), 115.0 (d, JPC = 14 Hz, Ar), 52.5 (s, NCH3), 47.1 (s, CH(CH3)2), 23.9 ppm (s, CH(CH3)2). 31P NMR (121 MHz, C6D6): δ 58.1 ppm (d, 1JRhP = 177.2 Hz). IR (C6D6): νCN 2038 (m), 2070 (m) cm-1. [Rh(L1)(CNiPr)2] (X7). CNiPr (3.8 μL, 40 μmol) was added to a solution of X1 (12.5 mg, 20.0 μmol) in toluene-d8 (0.7 mL). The resulting mixture was analyzed by NMR spectroscopy. 1H NMR (500 MHz, toluene-d8, -45 °C): δ 8.08 (m, 2H, ArH), 7.95 (m, 2H, ArH), 7.63 (m, 1H, ArH), 7.28 (m, 1H, ArH), 6.97-7.18 (m, 9H, ArH), 6.90 (m, 1H, ArH), 6.67 (m, 1H, ArH), 6.45 (m, 1H, ArH), 5.71 (m, 2H, COE CH), 2.91 (s, 6H, NCH3), 2.84 (m, 2H, CH(CH3)2), 2.09 (m, 4H, COE CH2), 1.44 (m, 8H, COE CH2), 0.56 ppm (m, 12H, CH(CH3)2). 31P NMR (121 MHz, toluene-d8, -45 °C): δ 48.2 ppm (d, 1JRhP = 137.4 Hz). IR (C6D6): νCN 2038 (w), 2081 (m), 2161 (s) cm-1. [Rh(L2)(NCiPr)] (X8). CNiPr (1.9 μL, 20 μmol) was added to a solution of X2 (12.5 mg, 20.0 μmol) in C6D6 (0.7 mL) and analyzed by NMR spectroscopy. 1H NMR (500 MHz, C6D6): δ 7.89-8.00 (m, 3H, ArH), 7.72 (m, 1H, ArH), 7.66 (m, 1H, ArH), 7.27 (m, 1H, ArH), 6.92-7.15 (m, 10H, ArH), 6.73 (m, 1H, ArH), 6.55 (m, 1H, ArH), 3.91 (d, 2JHH = 10.7 Hz, 1H, ArCH2N), 2.96 (sep, 3JHH = 6.3 Hz, 1H, CH(CH3)2), 2.78 (m, 3H, NCH3), 2.50 (m, 1H, ArCH2N), 2.14 (s, 3H, NCH3), 0.68 (d, 3 JHH = 6.8 Hz, 3H, CH(CH3)2), 0.66 ppm (d, 3JHH = 6.8 Hz, 3H, CH(CH3)2). 13C NMR (126 MHz, C6D6): δ 165.5 (d, JPC = 27.7 Hz, Ar), 154.5 (s, Ar), 137.2 (dd, 1JRhC/2JPC = 42.7/39.0 Hz, CNiPr), 134.0 (d, JPC = 11.5 Hz, Ar), 133.1 (d, JPC = 11.5 Hz, Ar), 133.0 (s, Ar), 131.9 (s, Ar), 131.3 (d, JPC = 2.3 Hz, Ar), 129.8 (s, Ar), 129.1 (d, JPC = 20.9 Hz, Ar), 127.3 (s, Ar), 124.6 (d, JPC = 48.5 Hz, Ar), 119.6 (s, Ar), 119.3 (d, JPC = 12.7 Hz, Ar), 116.5 (s, Ar), 116.4 (d, JPC = 5.8 Hz, Ar), 65.2 (d, JRhC = 1.4 Hz, PhCH2N), 53.8 (s, NCH3), 47.6 (s, NCH3), 47.4 (s, CH(CH3)2), 24.0 (s, CH(CH3)2), 24.0 ppm (s, CH(CH3)2). 31P NMR (202 MHz, C6D6): δ 62.7 ppm (d, 1JRhP = 175.2 Hz). IR (C6D6): νCN 2036 (m), 2071 (m). [Rh(L2)(NCiPr)2] (X9). CNiPr (3.8 μL, 40 μmol) was added to a solution of X2 (12.5 mg, 20.0 μmol) in toluene-d8 (0.7 mL). The resulting mixture was analyzed by NMR spectroscopy. 1H NMR (500 MHz, toluene-d8, -45 °C): δ 8.12 (m, 3H ArH), 7.96 (m, 2H ArH), 7.78 (m, 1H, ArH), 7.50 (m, 1H, ArH), 6.98-7.35 (m, 8H, ArH), 6.92 (m, 1H, ArH), 6.41 (m, 2H, ArH), 5.71 (m, 2H, CH, COE), 4.14 (d, 2JHH = 15.1 Hz, CH2), 3.75 (d, 2 JHH = 15.1 Hz, 1H, CH2), 2.83 (m, 2H, CH(CH3)2), 2.29 (s, 6H, NCH3), 2.09 (m, 4H, CH2 COE), 1.45 (m, 8H, CH2 COE), 0.56 ppm (m, 12H, CH(CH3)2). 31P NMR (121 MHz, toluened8, -45 °C): δ 48.5 ppm (d, 1JRhP = 137.4 Hz). [Rh(L3)(CNiPr)] (X10). CNiPr (3.8 μL, 40 μmol) was added to a solution of X3 (23.5 mg, 40.0 μmol) in C6D6 (0.7 mL) and analyzed by NMR spectroscopy. 1H NMR (500 MHz C6D6): δ 8.04 (m, 4H, ArH), 7.30 (m, 2H, ArH), 6.96-7.18 (m, 6H, ArH), 6.53 (m, 1H, ArH), 6.48 (m, 1H, ArH), 3.23 (m, 2H, NCH2), 3.10 (sep, 3JHH = 6 Hz, 1H, CH(CH3)2), 2.72 (m, 2H, NCH2), 2.61 (m, 2H, NCH2), 2.51 (m, 2H, NCH2), 1.29 (t, 3JHH = 6.3 Hz, 6H, NCH2CH3), 0.82 ppm (d, 3JHH = 6.3 Hz, 12H, CH(CH3)2). 13 C NMR (126 MHz, C6D6): δ 166.0 (d, JPC = 25.1 Hz, Ar), 165.9 (dd, 1JRhC/2JPC = 62/19 Hz, CNiPr), 138.3 (dd, JPC/ JRhC = 47.8/1.9 Hz, Ar), 134.0 (s, Ar), 133.7 (d, JPC = 1.3 Hz, Ar), 133.6 (d, JPC = 1.3 Hz, Ar), 132.4 (d, JPC = 2.0 Hz, Ar), 128.9 (d, JPC = 2.2 Hz, Ar), 120.5 (d, JPC = 48.9 Hz, Ar), 111.3 (d, JPC = 7.5 Hz, Ar), 109.1 (d, JPC = 14.1 Hz, Ar), 59.9 (s, N(CH2)2N), 51.9 (s, NCH2CH3), 47.8 (s, N(CH2)2N), 47.1 (s, CH2(CH3)2), 24.3 (s, CH2(CH3)2) 12.5 ppm (s, NCH2CH3). 31P NMR (121 MHz, C6D6): δ 58.6 ppm (d, 1JRhP = 183.2 Hz). IR (C6D6): νCN 2026 (m), 2066 (m). [Rh(L3)(CNiPr)2] (X11). CNiPr (3.8 μL, 40 μmol) was added to a solution of X3 (11.8 mg, 20.0 μmol) in toluene-d8 (0.7 mL). The resulting mixture was analyzed by NMR spectroscopy. 1H NMR (500 MHz, toluene-d8, -45 °C): δ 7.94 (m, 3H ArH), 7.38 (m, 1H ArH), 7.24 (m, 1H ArH), 6.95-7.17 (m, 8H ArH), 6.48

510


(m, 1H, ArH), 5.70 (m, 2H, COE CH), 4.27 (m, 2H, N(CH2)N), 2.95 (m, 2H, N(CH2)N), 2.90 (m, 1H, CH(CH3)2), 2.59 (m, 4H, NCH2CH3), 2.09 (m, 4H, COE CH2), 1.44 (m, 8H, COE CH2), 1.04 (m, 6H, NCH2CH3), 0.80 (br, 6H, CH(CH3)2), 0.62 ppm (m, 6H, CH(CH3)2). 31P NMR (121 MHz, toluene-d8, -45 °C): δ 58.3 ppm (d, 1JRhP = 183.2 Hz). DFT Geometry Optimizations. DFT calculations of compounds were carried out by the Gaussian03 program package52 using the BP86 functional.53 The 6-311G(d,p)54 basis set as implemented in Gaussian03 was employed for C, H, N, O, and P atoms, while the relativistic pseudopotentials of the Ahlrichs group and related basis functions of TZVPP quality55 were employed for Rh and Ir atoms. All systems were fully optimized without any symmetry restrictions. The resulting geometries were characterized as equilibrium structures and transition states, respectively, by the analysis of the force constants of normal vibrations. X-ray Crystal Structure Determination of X4. C32H42IrN2P, fw = 677.85, orange-yellow block, 0.45 0.25 0.10 mm, triclinic, P1 (no. 2), a = 10.50720(2) A˚, b = 10.6841(2) A˚, c = 13.3462(2) A˚, R = 102.812(2)°, β = 97.385(1)°, γ = 93.308(1)°, V = 1443.06(4) A˚3, Z = 2, Dx = 1.560 g/cm3, μ = 4.70 mm-1; 33 596 reflections were measured on a Nonius KappaCCD diffractometer with rotating anode (graphite monochromator, λ = 0.71073 A˚) up to a resolution of (sin θ/λ)max = 0.65 A˚-1 at a (52) Calculations were performed using Gaussian software at the BP86/6-311G(d,p) level: Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; M. A. Al-Laham, Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.04; Gaussian, Inc.: Wallingford, CT, 2004. (53) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Perdew, J. P. Phys. Rev. B 1986, 33, 8822. (54) (a) Dewar, M. J. S.; Reynolds, C. H. J. Comput. Chem. 1986, 7, 140. (b) Raghavachari, K.; Pople, J. A.; Replogle, E. S.; Head-Gordon, M. J. Phys. Chem. 1990, 94, 5579. (55) (a) Weigend, F.; H€aser, M.; Patzelt, H.; Ahlrichs, R. Chem. Phys. Lett. 1998, 294, 143. (b) Andrae, D.; H€aussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chem. Acc. 1990, 77, 123. (56) Duisenberg, A. J. M; Kroon-Batenburg, L. M. J.; Schreurs, A. M. M. J. Appl. Crystallogr. 2003, 36, 220.

Lindner et al. temperature of 150(2) K. Intensity integration was performed with EvalCCD.56 SADABS57 was used for absorption correction and scaling based on multiple measured reflections (0.49-0.75 correction range). A total of 6622 reflections were unique (Rint = 0.021), of which 6247 were observed [I > 2σ(I)]. The structure was solved with direct methods using the program SHELXS-97.58 The structure was also refined with SHELXL-97 against F2 of all reflections. Non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were located in difference Fourier maps. The C-H hydrogen atoms of the coordinated double bond were refined freely with isotropic displacement parameters; all other hydrogen atoms were refined with a riding model. A total of 335 parameters were refined with no restraints. R1/wR2 [I > 2σ(I)]: 0.0129/0.0290. R1/wR2 [all reflns]: 0.0152/0.0297. S = 1.028. Residual electron density between -0.54 and 0.69 e/A˚3. Geometry calculations and checking for higher symmetry was performed with the PLATON program.59 CCDC 779570 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Acknowledgment. This research is financially supported by The Netherlands’ Council for Scientific Research-Chemical Sciences (NWO-CW), through a VENI Innovative Research Grant (to J.I.v.d.V.), with additional funding from the University of Amsterdam and the EU-FP7 COST network PhoSciNet. We thank Dr. Bas de Bruin for helpful advice with DFT calculations and Dr. Guillaume Berthon-Gelloz for useful synthetic tips and tricks. This paper is dedicated to Prof. Thomas B. Rauchfuss (Univ. Illinois at Urbana-Champaign) on the occasion of his 60th birthday. Note Added after ASAP Publication. This paper was published on the Web on Jan 12, 2011, with errors in Scheme 2 and the second paragraph of the Experimental Section. The corrected version was reposted on Jan 20, 2011. Additional changes were made to Scheme 2 and reposted on Feb 7, 2011. Supporting Information Available: Crystallographic information file (CIF) of complex X4, [Ir(κ3-P,N,N0 -L3)(COE)], further experimental details, and DFT computational details. This material is available free of charge via the Internet at http:// pubs.acs.org. (57) Sheldrick, G. M. SADABS: Area-Detector Absorption Correction, v2.10; Universit€at G€ottingen: Germany, 1999. (58) Sheldrick, G. M. Acta Crystallogr. A 2008, 64, 112. (59) Spek, A. L. Acta Crystallogr. D 2009, 65, 148.

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