A Cationic AgI(PNPtBu) - American Chemical Society

Organometallics 2009, 28, 7025–7032 DOI: 10.1021/om900862w

7025

A Cationic AgI(PNPtBu) Species Acting as PNP Transfer Agent: Facile Synthesis of Pd(PNPtBu)(alkyl) Complexes and Their Reactivity Compared to PCPtBu Analogues Jarl Ivar van der Vlugt,*,† Maxime A. Siegler,‡,^ Michele Janssen,§ Dieter Vogt,§ and Anthony L. Spek‡ †

Supramolecular & Homogeneous Catalysis Group, van’t Hoff Institute for Molecular Sciences, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands, ‡Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands, and §Homogeneous Catalysis Group, Schuit Institute of Catalysis, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands. ^ Current address: Department of Chemistry, John Hopkins University, Baltimore, Maryland Received October 5, 2009

The straightforward synthesis of cationic complex 1, [Ag(PNPtBu)]BF4 (PNPtBu =1,2-bis[(di-tertbutylphosphino)methyl]pyridine), and its facile transmetalating properties toward gold and palladium are described. The corresponding Au complex [Au(PNPtBu)]2(BF4)2 (2) exists as a dimer in the solid state, as deduced by X-ray crystallography. The transmetalating properties were expanded to include the formation of Pd-alkyl species. Reaction of 1 with 0.5 equiv of Pd(allyl)(μ-Cl) dimer led to clean formation of the [Pd(PNPtBu)(η1-allyl)]BF4 complex 3. The analogous cationic methyl (4), cyanophenyl (5), and chloro species (6) could also be prepared in good yields from appropriate Pd precursors via this transmetalation methodology. The molecular structures for complexes 3-5 were established by X-ray crystallography. Reaction of complexes 4 and 6 with NaN(SiMe3)2 led to deprotonation and dearomatization of the PNP backbone with formation of the neutral Pd species 7 and 8, which can be regarded as the diphosphino-monoamido N-ligated analogues of well-studied Pd(PCPtBu) complexes. These neutral Pd(PN-PtBu) complexes 7 and 8 displayed reasonable activity in the Suzuki-Miyaura coupling of phenylboronic acid pinacol ester and bromoarenes.

Introduction Palladium alkyl and aryl species have been shown to be intermediates in the ethylene homo- and CO/ethylene copolymerization, Wacker-type oxidation reactions, allylic alkylations, different types of cross-coupling reactions, the hydrovinylation of vinyl arenes, and the Pd-catalyzed hydroformylation of alkenes to alcohols.1 Standard synthetic routes to such Pd complexes include direct addition of suitable Pd-alkyl precursors with the desired ligand system, introduction of the alkyl fragment by substitution of a halide ligand using alkyllithium reagents, or insertion of alkenes into Pd-H bonds. Among the plethora of palladium catalysts employed for a good number of modern organic transformations, pincertype palladium complexes constitute an appealing niche due to their suitable balance between stability and reactivity.2 In the context of cross-coupling reactions, the latter complexes allow for the use of minimal amounts of such homogeneous catalysts.3 To date, the majority of research has focused on aryl-based pincer systems, although Kirchner et al. have shown the catalytic activity of 2,6-bis(diaminophosphino)pyridine-derived Pd complexes.4 *Corresponding author. E-mail: [email protected]. r 2009 American Chemical Society

Complexes composed of the neutral PNPtBu ligand 2,6bis[(di-tert-butylphosphino)methyl]pyridine5—or closely related analogues, such as the neutral PNN derivative6— preferentially undergo deprotonation of a backbone methylene spacer with strong bases rather than nucleophilic substitution on the metal center. This in turn leads to a formal charge-switch of the PNPtBu ligand, from neutral to monoanionic.7 Recently, this feature has been exploited for various types of cooperative catalysis, uncovering unprecedented reactivity.8-10 The noninnocence of the ligand scaffold effectively prohibits the facile introduction of functional groups such as alkyl or allyl fragments onto the metal center by halide abstraction, e.g., with Grignard or lithium reagents. We have previously studied the noninnocent character and reactivity of the PNP backbone in CuI complexes in some detail.11 We also explored selective sequential dearomatization-reprotonation as well as dearomatizationmetal functionalization protocols to arrive at Ni-alkyl species (Scheme 1).12 The direct combination of a halide-containing metal precursor with the tridentate ligand PNPtBu leads to the formation of ionic species with halide as the counterion. In case such “coordinating” counterions are undesirable, an additional ion-exchange step is necessary. Transmetalation using a well-defined cationic Ag precursor, and utilizing the Published on Web 12/03/2009

pubs.acs.org/Organometallics

7026

Organometallics, Vol. 28, No. 24, 2009

van der Vlugt et al.

Scheme 1. Selective Dearomatization-Functionalization Sequences of PNPtBu with NiII and CuI

(1) Carbonylation: (a) Jennerjahn, R.; Piras, I.; Jackstell, R.; Franke, R.; Wiese, K. -D.; Beller, M. Chem.—Eur. J. 2009, 15, 6383–3688. (b) Beller, M., Catalytic Carbonylation Reactions; Springer: Berlin, 2006. (c) Kollar, L. Modern Carbonylation Methods; Wiley-VCH: Weinheim, 2008. Wacker: (d) Keith, J. A.; Henry, P. M. Angew. Chem., Int. Ed. 2009, 48, 9038-9049. Allylic alkylation: (e) Wassenaar, J.; van Zutphen, S.; Mora, G.; Le Floch, P.; Siegler, M. A.; Spek, A. L.; Reek, J. N. H. Organometallics 2009, 28, 2724–2734. (f) Tsuji, J. Pure Appl. Chem. 1999, 71, 1539–1547. (g) Trost, B. M; Crawley, M. L. Chem. Rev. 2003, 103, 2921–2944. (h) Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Reek, J. N. H. Acc. Chem. Res. 2001, 34, 895–904. (i) Jensen, T.; Fristrup, P. Chem.—Eur. J. 2009, 15, 9632-9636. Polymerization: (j) Flapper, J.; Reek, J. N. H. Angew. Chem., Int. Ed. 2007, 46, 8590–8592. (k) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc. 1996, 118, 267–268. (l) No€el, G.; R€oder, J. C.; Dechert, S.; Pritzkow, H.; Bolk, L.; Mecking, S.; Meyer, F. Adv. Synth. Catal. 2006, 348, 887–897. (m) Kochi, T.; Nakamura, A.; Ida, H.; Nozaki, K. J. Am. Chem. Soc. 2007, 129, 7770–7771. (n) Drent, E; Budzelaar, P. H. M. Chem. Rev. 1996, 96, 663–681. (o) Durand, J.; Milani, B. Coord. Chem. Rev. 2006, 250, 542-560. Cross-coupling reactions: (p) Negishi, E. Handbook of Organopalladium Chemistry for Organic Synthesis; Wiley-VCH: Weinheim, 2002. (q) Fors, B. P.; Davis, N. R.; Buchwald, S. L. J. Am. Chem. Soc. 2009, 131, 5766–5768. (r) Shen, Q.; Hartwig, J. F. J. Am. Chem. Soc. 2006, 128, 10028-10029. Hydroformylation: (s) Konya, D.; Almeida Lenero, K.; Drent, E. Organometallics 2006, 25, 3166–3174. (t) Drent, E. Budzelaar, P. H. M. J. Organomet. Chem. 2000, 593-594, 211-225. Formylation: (u) Sergeev, A. G.; Spannenberg, A.; Beller, M. J. Am. Chem. Soc. 2008, 130, 15549–15563. Hydrovinylation: (v) Eggeling, E. B.; Hovestad, N. J.; Jastrzebski, J. T. B. H.; Vogt, D.; van Koten, G. J. Org. Chem. 2000, 65, 8857–8865. (w) Hovestad, N. J.; Eggeling, E. B.; Heidbuchel, H. J.; Jastrzebski, J. T. B. H.; Kragl, U.; Keim, W.; Vogt, D.; Van Koten, G. Angew. Chem., Int. Ed. 1999, 38, 1655–1658. (x) Englert, U.; Haerter, R.; Vasen, D.; Salzer, A.; Eggeling, E. B.; Vogt, D. Organometallics 1999, 18, 4390–4398. (y) Bayersd€orfer, R.; Ganter, B.; Englert, U.; Keim, W.; Vogt, D. J. Organomet. Chem. 1998, 552, 187–194. (2) (a) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750–3781. (b) Singleton, J. T. Tetrahedron 2003, 59, 1837–1857. (c) Peris, E.; Crabtree, R. H. Coord. Chem. Rev. 2004, 248, 2239–2246. (3) (a) DeVasher, R. B.; Moore, L. R.; Shaughnessy, K. H. J. Org. Chem. 2004, 69, 7919–7927. (b) Braunstein, P. J. Organomet. Chem. 2004, 689, 3953–3967. (c) Hahn, F. E.; Jahnke, M. C.; Pape, T. Organometallics 2006, 25, 5927–5936. (4) Benito-Garagorri, D.; Kirchner, K. Acc. Chem. Res. 2008, 41, 201-213, and references therein. (5) (a) Kawatsura, M.; Hartwig, J. F. Organometallics 2001, 20, 1960– 1964. (b) Hermann, D.; Gandelman, M.; Rozenberg, H.; Shimon, L. J. W.; Milstein, D. Organometallics 2002, 21, 812–818. (6) Gunanathan, C.; Ben-David, Y.; Milstein, D. Science 2007, 317, 790–792. (7) (a) Sacco, A.; Vasapollo, G.; Nobile, C. F.; Piergiovanni, A.; Pellinghelli, M. A.; Lanfranchi, M. J. Organomet. Chem. 1988, 356, 397– 409. (b) Vasapollo, G.; Nobile, C. F.; Sacco, A. J. Organomet. Chem. 1988, 296, 435–441. (c) Kloek, S. M.; Heinekey, D. M.; Goldberg, K. I. Angew. Chem., Int. Ed. 2007, 46, 4736–4738. (8) (a) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. J. Am. Chem. Soc. 2005, 127, 10840–10841. (b) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. Angew. Chem., Int. Ed. 2006, 45, 1113–1115. (c) Gunanathan, C.; Milstein, D. Angew. Chem., Int. Ed. 2008, 47, 8661–8664. (d) Gunanathan, C.; Shimon, L. J. W.; Milstein, D. J. Am. Chem. Soc. 2009, 131, 3146–3147. (9) (a) 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. (b) Highlight: Hetterscheid, D. H. G.; van der Vlugt, J. I.; de Bruin, B.; Reek, J. N. H. Angew. Chem., Int. Ed. 2009, 48, 8178–8181.

formation of solid AgCl as a driving force for selective coordination, might provide a selective route toward the preparation of functionalized Pd complexes of PNPtBu, including species with reactive alkyl functionalities. Moreover, it could offer a versatile platform for the modular synthesis of a range of related metal species from a common starting point with little experimental manipulations. Transmetalation is a well-established methodology to prepare transition metal complexes selectively and in a straightforward fashion. Recent transmetalation reactivity of Ag-containing complexes mainly focused on carbenebased systems, using Ag2O as the silver source. This methodology, first described by Lin et al.,13 has been mainly applied to form Pd-, Pt-, Rh-, Ru-, Ir-, and Cu-NHC complexes. Using a structurally related monoanionic PNP scaffold, Ozerov and Mindiola demonstrated the transmetalation of its Ag dimer (as well as the Tl monomer) into monomeric Ni, Pd, and Pt species.14 We now report on the formation of crystalline [Ag(PNPtBu)]BF4, complex 1, and its use as starting material for the preparation of a dinuclear AuIAuI complex 2 as well as three cationic Pd complexes bearing allyl (3), methyl (4), or 4-cyanophenyl (5) functionalities, which have all been fully characterized, including X-ray crystallography. Furthermore, the corresponding neutral derivatives have been generated by deprotonation of the ligand backbone using NaN(SiMe3)2. These species, carrying the formally monoanionic dearomatized PN-PtBu ligand, can be regarded as direct analogues of the “classic” Pd(PCPtBu) complexes, and as such, we have investigated the relative reactivity of these two closely related families in the Suzuki-Miyaura coupling of aryl bromides with boronic acids and the pinacol ester derivatives thereof.

Results and Discussion Synthesis of a Cationic Ag(PNP) Complex and Its Use as a Transmetalating Agent to Generate a Dinuclear Cationic Gold(I) Complex. Addition of an equimolar amount of (10) For a recent overview of the noninnocent character of lutidine based ligands, see: van der Vlugt, J. I.; Reek, J. N. H. Angew. Chem., Int. Ed. 2009, 48, 8832–8846. (11) (a) van der Vlugt, J. I.; Pidko, E. A.; Vogt, D.; Lutz, M.; Spek, A. L.; Meetsma, A. Inorg. Chem. 2008, 47, 4442–4444. (b) van der Vlugt, J. I.; Pidko, E. A.; Vogt, D.; Lutz, M.; Spek, A. L. Inorg. Chem. 2009, 48, 7513–7515. (12) (a) van der Vlugt, J. I.; Lutz, M.; Pidko, E. A.; Vogt, D.; Spek, A. L. Dalton Trans. 2009, 1016–1023. (b) Lutz, M.; van der Vlugt, J. I.; Vogt, D.; Spek, A. L. Polyhedron 2009, 28, 2341–2346. (13) Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972–975. (14) DeMott, J. C.; Basuli, F.; Kilgore, U. J.; Foxman, B. M.; Huffman, J. C.; Ozerov, O. V.; Mindiola, D. J. Inorg. Chem. 2007, 46, 6271–6276.

Article

Figure 1. ORTEP plot (50% probability displacement ellipsoids) of complex 1, [Ag(PNPtBu)]BF4. Hydrogen atoms are omitted for clarity. Selected bond lengths (A˚) and angles (deg): Ag-P1 2.3986(5); Ag-P2 2.3991(5); Ag 3 3 3 N 2.4642(17); P1-Ag-P2 157.87(2); P1-Ag 3 3 3 N 79.51(4); P2-Ag 3 3 3 N 78.89(4); Ag-P1-C6 101.31(6); Ag-P2-C15 102.47(6). Scheme 2. Formation of the Cationic AgI Complex 1

AgBF4 to a suspension of PNPtBu in MeCN led to gradual dissolution of the ligand and formation of a colorless solution. Evaporation of the solvent yielded white, analytically pure complex 1 (Scheme 2). The 31P NMR spectrum showed one signal for the two equivalent phosphorus atoms at δ 62.9 ppm with J(31P-107Ag) and J(31P-109Ag) coupling constants of 502 and 580 Hz, respectively; the ratio of these constants is very close to the theoretical ratio of 1:1.15 expected because of the relative gyromagnetic ratios. An X-ray crystallographic analysis of single crystals, grown from THF-pentane, revealed a nonlinear coordination of the AgI ion between the two phosphine groups of the PNPtBu ligand, with Ag-P distances of ∼2.39 A˚, which is in the range of what is observed for AgI complexes with electron-rich phosphine ligands (Figure 1).15 The IR spectrum indicated some interaction of the pyridine nitrogen atom, with bands at 1585 and 1569 cm-1, but the Ag-N distance of 2.4642(17) A˚ is just outside the typical range of 2.2-2.4 A˚, so the interaction is most likely best described as (very) weak. The P1-Ag-P2 angle of 157.87(2) is smaller than normally displayed by two-coordinate AgI complexes with a linear arrangement of the three atoms, due to the semirigid chelate ring of the PNPtBu scaffold. The observed geometry is strikingly different from the only report, by Venanzi and co-workers, concerning the structurally similar PCP ligand, in which case halide-bridged dimeric complexes were found.16 Also with the monoanionic PNP ligand used by Ozerov and Mindiola a clear preference for the formation of a ligand-bridged dinuclear species was observed.14 (15) Meijboom, R.; Bowen, R. J.; Berners-Price, S. J. Coord. Chem. Rev. 2009, 253, 325–342. (16) Camso, F.; Camalli, M.; Rimml, H.; Venanzi, L. M. Inorg. Chem. 1995, 34, 673–679.


7027

To be able to correlate this monomeric two-coordinate cationic structure of 1 with other d10 metals besides the CuI complexes described by us before,11 which displayed a T-shaped coordination, its stoichiometric reaction with AuCl(SMe2) in CH2Cl2 was studied. This reaction could also serve as a first test for the transmetalating properties of complex 1. Upon addition of a CH2Cl2 solution of 1 into a solution of the gold precursor in CH2Cl2, immediate formation of AgCl was observed, and workup yielded a white solid with one broadened signal in the 31P NMR spectrum at δ 85.5 ppm, indicating full conversion to the AuI complex. Strikingly, little literature precedent for AuI complexes with (pseudo)-pincer ligands exists.17 As the 1H NMR spectrum also displayed broadened signals for, for example, the tBu fragments, we postulate the formation of a dimeric species with general formula ([Au2(PNPtBu)2](BF4)2 in solution, which would better accommodate a strictly linear two-coordinate geometry for the AuI ions. To shed more light on this possibility, single crystals, grown by slow diffusion of hexane into a CH2Cl2 solution, were subjected to an X-ray crystallographic analysis; the molecular structure is depicted in Figure 2. As anticipated, a nonlinear coordination of the AuI ion between the two phosphine groups of the PNPtBu ligand exists, with Au-P distances of ∼2.31-2.32 A˚, which are in the range observed for Au-phosphine complexes.18 The pyridine N atoms are pointing away from the virtual metallo-based rectangle. The P1-Au-P2 angle of 173.91(3) is in line with the expected preference for a two-coordinate linear geometry for gold(I) species. Synthesis and Characterization of Pd-Alkyl Complexes with PNPtBu via Transmetalation. Osborn et al. have previously described the synthesis of crotyl-based Pd complexes with the less hindered PNPPh ligand (compared to PNPtBu) via Ag-based halide abstraction from the Pd precursor and thereafter addition of the ligand.19 Vitagliano and co-workers have studied the hydrovinylation activity of η2-coordinated, cationic olefin complexes of Pd(PNPPh), which after electrophilic addition resulted in spectroscopically characterized Pd-alkyl species (Scheme 3).20 The group of Michael recently reported on the intramolecular hydroamination activity of cationic Pd(PNPPh) with the implied intermediacy of Pd-alkyl components, which could also be isolated in stoichiometric reactions.21 (17) (a) Sircoglou, M.; Mercy, M.; Saffon, N.; Coppel, Y.; Bouhadir, G.; Maron, L.; Bourissou, D. Angew. Chem., Int. Ed. 2009, 48, 3454–3457. (b) Escalle, A.; Mora, G.; Gagosz, F.; Mezailles, N.; F. Le Goff, X.; Jean, Y.; Le Floch, P. Inorg. Chem. 2009, 48, 8415–8422. (c) Stoccoro, S.; Alesso, G.; Agostina Cinellu, M.; Minghetti, G.; Zucca, A.; Manassero, M.; Manassero, C. Dalton Trans. 2009, 3467–3477. (d) Tunyogi, T.; Deak, A.; Tarkanyi, G.; Kiraly, P.; Palinkas, G. Inorg. Chem. 2008, 47, 2049–2055. (e) de la Riva, H.; Nieuwhuyzen, M.; Mendicute Fierro, C.; R. Raithby, P.; Male, L.; Lagunas, M. C. Inorg. Chem. 2006, 45, 1418–1420. (f) Robitzer, M.; Bouamaïed, I.; Sirlin, C.; Chase, P. A.; van Koten, G.; Pfeffer, M. Organometallics 2005, 24, 1756–1761. (g) Yam, V. W. -W.; Wong, K. M.-C.; Hung, L.-L.; Zhu, N. Angew. Chem., Int. Ed. 2005, 44, 3107–3110. (h) Contel, M.; Stol, M.; Casado, M. A.; van Klink, G. P. M.; Ellis, D. D.; Spek, A. L.; van Koten, G. Organometallics 2002, 21, 4556–4559. (i) Shieh, S.-J.; Hong, X.; Peng, S.-M.; Che, C.-M. J. Chem. Soc., Chem. Commun. 1994, 3068–3069. (j) Gimeno, M. C.; Laguna, A.; Sarroca, C.; Jones, P. G. Inorg. Chem. 1993, 32, 5926–5932. (18) Stol, M.; Snelders, D. J. M.; Kooijman, H.; Spek, A. L.; van Klink, G. P. M.; van Koten, G. Dalton Trans. 2007, 2589–2593. (19) Barloy, L.; Ramdeehul, S.; Osborn, J. A.; Carlotti, C.; Taulelle, F.; De Cian, A.; Fischer, J. Eur. J. Inorg. Chem. 2000, 2523–2532. (20) (a) Hahn, C.; Morvillo, P.; Vitagliano, A. Eur. J. Inorg. Chem. 2001, 419–429. (b) Cucciolito, M. E.; D'Amora, A.; Vitagliano, A. Organometallics 2005, 24, 3359–3361. (21) (a) Michael, F. E.; Cochran, B. M. J. Am. Chem. Soc. 2006, 128, 4246–4247. (b) Michael, F. E.; Sibbald, P. A.; Cochran, B. M. Org. Lett. 2008, 10, 793–796. (c) Cochran, B. M.; Michael, F. E. J. Am. Chem. Soc. 2008, 130, 2786–2792.

7028


van der Vlugt et al. Scheme 4. Formation of Pd-Alkyl Species 3-5 via Transmetalation of Cationic Silver Complex 1

Figure 2. ORTEP plot (50% probability displacement ellipsoids) of the cationic part of 2, [Au2(PNPtBu)2](BF4)2, including a side view of the bimetallic core of the molecule. Hydrogen atoms are omitted for clarity. Selected bond lengths (A˚) and angles (deg): Au1-P1 2.3139(8); Au10 -P2 2.3186(8); Au1 3 3 3 Au10 7.0203(3); P1 3 3 3 P20 4.6259(11); P1-Au1-P20 173.91(3); Au1-P1-C6 113.65(9). Scheme 3. Application of Pd(PNPR) Complexes in the Stochiometric (top) and Catalytic (bottom) Activation of CdC Double Bonds

To date, no full report on the synthesis, structural characterization, and reactivity of Pd-alkyl species with ligands such as PNPtBu has appeared. This might be related, at least in part, to the noninnocent character of the PNP backbone, making the direct alkylation via, for example, lithium-alkyl

species impossible as a synthetic route. It was reasoned that compound 1 could be a suitable starting material for the efficient preparation of functionalized PdII(PNPtBu) species (Scheme 4). Addition of 2 equiv of complex 1 to a CH2Cl2 solution of [Pd(allyl)(μ-Cl)] dimer led to immediate precipitation of AgCl. The resultant off-white solid displayed a singlet in the 31P NMR spectrum, indicative of equivalent trans-coordinated phosphine groups, at δ 55.8 ppm. The presence of the allyl ligand in complex 3 was evidenced by signals at δ 6.43 (-CHd), 5.14 (dCHtrans), 4.85 (dCHcis), and 3.09 ppm (CH2) in the 1H NMR spectrum, and these signals also imply the rearrangement of the allyl fragment to η1 end-on bonding. Similar to the formation of complex 3, the transmetalation of the silver complex with Pd(cod)Cl(CH3) cleanly generated the palladium-methyl species [Pd(PNPtBu)(CH3)]BF4, 4 (Scheme 4). Note that this complex is not accessible via reaction of a cationic Pd(PNPtBu)Cl derivative with MeLi, as the noninnocent PNPtBu backbone will instead be deprotonated by this strong base. The presence of the methyl group was established by the characteristic triplet signal at δ 0.89 ppm (with a JP-H of 56 Hz) in the 1 H NMR spectrum, and the singlet in the 31P NMR spectrum had virtually the same chemical shift as for complex 3, at δ 55.5 ppm. Reaction of 1 with half an equivalent of the dimer [Pd(P{otolyl}3)(4-CN-phenyl)(μ-Br)]222 yielded a yellow solid with characteristic NMR (δ 56.2 ppm) and IR signatures (νCN 2219 cm-1) for complex 5, [Pd(PNPtBu)(4-CN-phenyl)]BF4. Single crystals for all three complexes could be obtained by diffusion of pentane into a concentrated THF solution, and the molecular structures are depicted in Figure 3. The palladium center in all complexes is in a distorted square-planar geometry, with P1-Pd-P2 angles of around 165 and Pd-N bond lengths in the range ∼2.11-2.15 A˚, reflecting the subtle differences due to the coligated alkyl/ aryl species. The orientation of the phenyl ring in complex 5 is almost perpendicular to the pyridine ring, to minimize steric hindrance from the tert-butyl groups on the phosphorus atoms as well as to maximize the orbital overlap with the Pd center. Finally, we also prepared the corresponding chloro species 6—either by transmetalation of 1 with PdCl2(PhCN)2 or by (22) Wolfe, J. P.; Singer, R. A.; Yang, B. H.; Buchwald, S. L. J. Am. Chem. Soc. 1999, 121, 9550–9561.

Article


7029

Scheme 5. Dearomatization of Complexes 4 and 6 to Give the Neutral Analogues 7 and 8 and the Electronic Correlation of Species 7 and 8 with the PCP Analogues 9 and 10

Figure 3. ORTEP plot (50% probability displacement ellipsoids) of the cationic portions of complexes [Pd(η1-C3H5)(PNPtBu)]BF4 (3), [Pd(CH3)(PNPtBu)]BF4 (4), and [Pd(4-CN-phenyl)(PNPtBu)]BF4 (5). Hydrogen atoms are omitted for clarity. Selected bond lengths (A˚) and angles (deg) for 3: Pd-P1 2.3468(5); Pd-P2 2.3230(5); Pd-N 2.1446(16); Pd-C24 2.095(2); C24-C25 1.463(4); C25-C26 1.312(5); P1-Pd-P2 164.23(2); P1-Pd-N 83.01(4); P1-Pd-C24 97.56(6); N-Pd-C24 176.86(9); Pd-P1-C6 99.10(6); Pd-C24-C25 115.4(2); C24-C25-C26 128.8(3); C6 3 3 3 N 3 3 3 C15 169.56(10); Pd-N 3 3 3 C3 163.83(9); for 4: Pd-P1 2.3004(5); Pd-P2 2.2971(5); Pd-N 2.1143(14); Pd-C24 2.0557(17); P1-Pd-P2 167.63(2); P1-Pd-N 84.27(4); P1-Pd-C24 95.59(6); N-Pd-C24 178.99(6); Pd-P1-C6 99.52(6); C6 3 3 3 N 3 3 3 C15 170.26(9); Pd-N 3 3 3 C3 171.07(8); for 5: Pd-P1 2.3219(6); Pd-P2 2.318(4); Pd-N1 2.117(2); Pd-C24 2.013(2); P1-Pd-P2 164.15(13); P1-Pd-N1 82.84(5); P1-Pd-C24 97.62(7); N1-Pd-C24 177.05(9); Pd-P1-C6 98.97(8); C6 3 3 3 N1 3 3 3 C15 174.00(19); Pd-N1 3 3 3 C3 174.52(11).

addition of this Pd precursor with PNPtBu and 1 equiv of NaBF4—as well as the dearomatized complexes 7 and 8. Upon addition of 1 equiv of NaN(SiMe3)2 to the monocationic Pd-methyl species 4 in diethyl ether, instantaneous reaction concomitant with a color change from off-white to brown was observed. The resulting neutral Pd-alkyl complex 7 carried the monoanionic ligand PN-PtBu, as indicated by the sharp, single band at ν 1605 cm-1 for the Pd-Namide bond stretch, which is strikingly different from the two observed bands at ν 1596 and 1568 cm-1 in complex 4, which corresponded to the Pd-NPy bond. The 31P NMR spectrum for complex 7 showed an AB quartet for two inequivalent phosphorus atoms at δ 53.4 and 50.2 ppm, with coupling constants 2JP-P of 356 Hz, whereas for complex 8 similar signals were found at δ 64.4 and 52.8 ppm (JP-P 398 Hz) in C6D6. The 1H NMR spectra for 7 and 8 (the latter given in parentheses) displayed well-resolved signals (1:1:1 ratio) at 6.55 (6.35), 6.40 (6.25), and 5.45 (5.26) ppm for the dearomatized ring protons and two sets of doublets (integral ratio 1:2) at 3.48 (3.30) and 2.68 (2.49) ppm for the methine and methylene protons, respectively. Two sets of doublets are observed for the tert-butyl substituents on the phosphorus atoms at 1.39 (1.49) and 1.02 (1.14) ppm, with a coupling constant 2JP-H of 11.6 (13.6) Hz. Comparative Reactivity of Pd(Alkyl)(PN-P) vs PdCl(PN-P) Species in the Suzuki-Miyaura Coupling. Dearomatization of the pyridine ring in 4 and 6 smoothly yields two pentane-soluble complexes that are in principle isoelectronic with well-studied PCP-palladium compounds 9 and 10 (Scheme 5). As a first test to estimate the reactivity of the neutral “lutidinyl”-based complexes, we examined their activity in the Suzuki-Miyaura coupling of bromoarenes and phenylboronic acid as well as its pinacol ester. Palladium complexes with either PNP and PCP pincer ligands have been reported as efficient (pre)catalysts for the Pd-catalyzed Suzuki-Miyaura coupling of arylboronic acids and aryl halides,23 but to the best of our knowledge only halide, acetate, or cationic complexes have been utilized to date; little is known about the possible activity of palladium alkyl species in this kind of catalysis. Furthermore, given the (23) Pincer-based Pd catalysts for the Suzuki-Miyaura coupling: (a) Bedford, R. B.; Draper, S. M.; Scully, P. N.; Welch, S. L. New J. Chem. 2000, 24, 745–747. (b) Lee, H. M.; Zeng, J. Y.; Hu, C.-H.; Lee, M.-T. Inorg. Chem. 2004, 43, 6822–6829. (c) Benito-Garagorri, D.; Bocokic, V.; Mereiter, K.; Kirchner, K. Organometallics 2006, 25, 3817–3823. (d) Kimura, T.; Uozumi, Y. Organometallics 2006, 25, 4883–4887. (e) Bolliger, J. L.; Blacque, O.; Frech, C. M. Angew. Chem., Int. Ed. 2007, 46, 6514–6517. (f) Hahn, F. E.; Jahnke, M. C.; Pape, T. Organometallics 2007, 26, 150–154. (g) Gong, J.-F.; Zhang, Y.-H.; Song, M.-P.; Xu, C. Organometallics 2007, 26, 6487–6492. (h) Sheloumov, A. M.; Tundo, P.; Dolgushin, F. M.; Koridze, A. A. Eur. J. Inorg. Chem. 2008, 572–576.

7030


van der Vlugt et al.

Table 1. Suzuki-Miyaura Cross-Coupling of Aryl Bromides with Phenylboronic Acid Derivatives Using Pd Complexes 4 and 7-10a catalyst 4 7 9 7 8 9 7 8 9 10

substrate

conversion (%)

phenylboronic acid bromobenzene phenylboronic acid bromobenzene phenylboronic acid bromobenzene phenylboronic acid pinacol ester bromobenzene phenylboronic acid pinacol ester bromobenzene phenylboronic acid pinacol ester bromobenzene phenylboronic acid pinacol ester 4-bromobenzonitrile phenylboronic acid pinacol ester 4-bromobenzonitrile phenylboronic acid pinacol ester 4-bromobenzonitrile phenylboronic acid 4-bromobenzonitrile

3 0 48 51 52

transmetalation using complex 1 should make this methodology accessible with other lutidine-derived ligands and first-row transition metals, and research along these lines is ongoing in our laboratories. There are striking similarities between the dearomatized PNP backbone and the archetypical PCP-pincer system in terms of electronics, and the Pd complexes perform equal in the Suzuki-Miyaura coupling of aryl bromides and boronic esters. There is also no difference between Me- and Cl-based Pd precatalysts for this reaction. A study of the reactivity of PdPNP-alkyl species toward electrophiles using cooperative catalysis is currently ongoing in our laboratories.

52 66 69 66 58

a Reaction conditions: THF, 1 mol % Pd, 1.5 mmol of PhBr, 3.0 mmol of PhB(OH)2, 60 C, 16 h. Conversion determined by GC analysis. Conversion is averaged over two runs.

structurally similar but electronically different complexes 4 and 7, it is worthwhile to investigate the influence of the nature of the ligand backbone on the catalytic activity of the resulting Pd complexes. Reactions were performed for 16 h in THF at 60 C with 1 mol % Pd catalyst and a relative ratio of bromoarene to boronic acid derivative of 1:2. Under these mild, nonoptimized reaction conditions (for these ligand systems), notable differences in catalyst activity were observed. The cationic derivative 4 proved very inactive (3% conversion) in the cross-coupling of phenylboronic acid and bromobenzene, while neutral species 7 is totally inactive and its PCP analogue 9 gave about 48% conversion (Table 1). This difference is possibly due to unwanted side-reactions of the boronic acid substrate with catalyst precursor 7. Therefore we switched to the reaction of phenylboronic acid pinacol ester with bromobenzene or 4-bromobenzonitrile. Using these substrates, almost identical results were obtained with both precatalyst species, i.e., 51% vs 52% for PhBr and 66% vs 69% for p-BrC6H4CN; it is clear that both precatalysts 7 and 8 perform equally well in these reactions. Moreover, the isoelectronic PCP derivative 9 performs equally well under these conditions, showing the high degree of similarity between the noninnocent lutidine-derived ligand and the “standard” PCP-pincer system. Obviously there is very little difference between methyl- and chloroderived (pre)catalyst species in terms of activity.

Conclusions The synthesis and full characterization of the new twocoordinate cationic Ag complex 1 with the sterically encumbered pincer ligand PNPtBu has been described. The nitrogen donor of the pyridine group was shown not to interact significantly with the Ag ion by IR spectroscopy and X-ray crystallographic analysis. Complex 1 showed efficient transmetalation activity. Thus, reaction of 1 with AuCl(SMe2) resulted in the clean formation of the dimeric species 2, with perfectly linear coordination, as indicated by the angle P-Au-P of ∼174. The ease of formation of Pdalkyl complexes with the noninnocent PNPtBu ligand via

Experimental Section General Procedures. Solvents were purified by established procedures. Starting materials were purchased from commercial sources and used as received. Microanalyses were performed by Kolbe Laboratories, M€ ulheim a/d Ruhr. IR spectra (ATR reflectance mode) were recorded with a Nicolet Avatar 360 FT-IR spectrometer, and NMR spectra were recorded with a Varian 200 spectrometer at room temperature (1H: 200 MHz, 13 C: 50 MHz, 31P: 80 MHz) and calibrated to the residual proton and carbon signals of the solvent (CDCl3: δH 7.26, δC 77.0) or external 85% aqueous H3PO4. Ligand PNPtBu,11,12 Pd(1,5cyclooctadiene)Cl(CH3),24 and Pd(tris{o-tolyl}phosphine)(4-cyanophenyl)Br dimer25 as well as complexes 926 and 1027 were prepared according to literature procedures. Silver Complex 1, [Ag(PNPtBu)]BF4. To a slurry of PNPtBu (395.5 mg, 1.00 mmol) in MeCN (30 mL) was added a solution of AgBF4 (194.7 mg, 1.00 mmol) in MeCN (20 mL) by means of a cannula at room temperature. The resultant suspension was stirred for 2 h to leave a colorless solution. After removal of solvent in vacuo, recrystallization of the white solid yielded colorless crystals by slow diffusion of pentane into an MeCN solution in good yield (442.7 mg, 0.75 mmol, 75%). 31P NMR (80.9 MHz, acetone-d6): δ 62.9 (dd, J(31P-107Ag) 502; J(31P-109Ag) 580 Hz). 1H NMR (200 MHz, acetone-d6): δ 7.72 (t, J1 = 7.4 Hz, 1H, p-ArH), 7.40 (d, J1 = 7.4 Hz, 2H, m-ArH), 3.55 (m, 4H, -CH2-), 1.35 (d, J1 = 4.0 Hz, 18H, tBu), 1.29 (d, J1 = 4.0 Hz, 18H, tBu). IR (ATR, cm-1): ν 1585, 1569. Anal. Calcd for C23H43AgBF4NP2: C, 46.80; H, 7.34; N, 2.37. Found: C, 47.21; H, 7.29; N, 2.39. Gold Complex 2, [Au(PNPtBu)2](BF4)2. A solution of 1 (150 mg, 254.1 μmol) in CH2Cl2 (10 mL) was added to a solution of AuCl(SMe2) (75.1 mg, 254.1 μmol) in CH2Cl2 (10 mL) by means of a cannula at room temperature. Precipitation of AgCl commenced almost instantaneously and was complete after 3 h of stirring, leaving a nearly colorless solution after filtration. The resulting cream-colored solid left after evaporation of all volatiles in vacuo was subsequently recrystallized by layering a CH2Cl2 solution with hexane to yield white single-crystalline material. Yield (234.8 mg, 0.68 mmol, 68%). 31P NMR (80.9 MHz, acetone-d6): δ 85.5 (br s). 1H NMR (200 MHz, acetone-d6): δ 7.82 (br t, 1H, p-ArH), 7.46 (br d, 2H, m-ArH), 3.69 (m, 4H, -CH2-), 1.42 (br d, 36H, tBu). Anal. Calcd for C23H43AuBF4NP2: C, 40.67; H, 6.38; N, 2.06. Found: C, 41.05; H, 6.25; N, 1.77. Palladium Complex 3, [Pd(PNPtBu)(η1-allyl)]BF4. A solution of 1 (150.0 mg, 254.1 μmol) in MeCN (10 mL) was added dropwise to a solution of Pd(η3-allyl)Cl dimer (46.5 mg, 127.0 μmol) in MeCN (10 mL) by means of a cannula at room temperature. (24) Ladipo, F. T.; Anderson, G. K. Organometallics 1994, 13, 303– 306. (25) Widenhoefer, R. A.; Zhong, H. A.; Buchwald, S. L. J. Am. Chem. Soc. 1997, 119, 6787–6795. (26) Johansson, R.; Jarenmark, M.; Wendt, O. F. Organometallics 2005, 24, 4500–4502. (27) Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976, 1020–1024.

Article


7031

Table 2. Crystallographic Data for Complexes 1-5 1

2

3

4

5

formula

[C23H43AgNP2] [BF4]

fw/Mr cryst size (mm) cryst syst space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A3) μ (mm-1) Z dcalc (g cm-3) θ range reflns collected unique reflns (Rint)b obsd reflns (I > 2σ(I)) params/restraints GOF R1 (I > 2σ(I))b wR2 (all data)b residual electron density range (e A˚-3)

590.20 0.19 0.16 0.08 triclinic P1 (No. 2) 8.8998(2) 11.1577(2) 14.3300(2) 75.657(1) 81.716(2) 81.148(2) 1353.84(4) 0.902 2 1.448 1.9-27.5 35 522 6238, 0.027 5483 301, 0 1.07 0.0265 0.0612 -0.61, 1.15

[C46H68Au2N2P4] 3 2[BF4] 1358.60 0.13 0.09 0.07 monoclinic P21/c (No. 14) 13.7230(4) 14.4624(6) 14.6730(4) 90 112.254(2) 90 2695.20(16) 5.616 2 1.674 1.6-26.5 39 614 5573, 0.036 4669 301, 0 1.06 0.0210 0.0402 -0.55, 0.87

[C26H48NP2Pd][BF4] 3 C4H8O 701.91 0.37 0.30 0.08 triclinic P1 (No. 2) 8.8066(4) 11.0128(6) 18.1525(12) 102.560(2) 98.055(3) 99.184(2) 1668.21(16) 0.698 2 1.397 1.9-27.5 43 580 7683, 0.028 6731 438, 180 1.07 0.0269 0.0674 -0.47, 0.62

[C24H46NP2Pd] [BF4] 603.77 0.26 0.19 0.15 triclinic P1 (No. 2) 7.7663(3) 12.3413(3) 15.6865(4) 71.286(2) 75.810(2) 89.144(2) 1377.23(8) 0.830 2 1.456 1.8-27.5 33 036 6316, 0.025 5629 311, 0 1.06 0.0227 0.0528 -0.30, 0.47

[C30H47N2P2Pd][BF4] þ solvent 690.85a 0.27 0.22 0.07 monoclinic C2/c 33.9886(11) 11.0088(3) 19.8282(5) 90 96.546 90 7370.8(4) 0.630a 8 1.245a 2.1-27.0 51 242 8045, 0.041 6096 470, 326 1.07 0.0360 0.0826 -0.33, 0.71

a

Without disordered solvent contribution. b Rint = P Fc||)/ |Fo|.

P

P P P P [|Fo2 - Fo2 (mean)|]/ [Fo2]; wR(F2) = [ [w(Fo2 - Fc2)2]/ [w(Fo2)2]]1/2; R(F) = (||Fo| - |

The resultant light yellow solution turned turbid within 5 min and was subsequently stirred for 2 h. Filtration (to remove AgCl) and evaporation of solvent in vacuo left an off-white solid. Recrystallization by slow diffusion of pentane into a concentrated THF solution yielded light yellow single crystals in good yield (96.0 mg, 152.5 μmol, 60%). 31P NMR (80.9 MHz, acetone-d6): δ 55.8 (s). 1H NMR (200 MHz, acetone-d6): δ 8.01 (t, J1 = 7.4 Hz, 1H, p-ArH), 7.70 (d, J1 = 7.4 Hz, 2H, m-ArH), 6.43 (p, J1 = 8. Hz, 1H, -CHd), 5.14 (d, J1 = 17.2 Hz, 1H, dCHtrans), 4.85 (d, J1 = 10.0 Hz, 1H, dCHcis), 4.07 (t, d, J1 = 7.4 Hz, 1H, -CH2P), 3.09 (dd, J1 = 13.6 Hz, J2 = 5.4 Hz, 2H, PdCH2), 1.48 (d, J1 = 7.2 Hz, 18H, tBu), 1.44 (d, J1 = 7.2 Hz, 18H, tBu). Anal. Calcd for C26H48BF4NP2Pd: C, 49.58; H, 7.68; N, 2.22. Found: C, 49.17; H, 7.39; N, 2.05. Palladium Complex 4, [Pd(PNPtBu)(CH3)]BF4. Using a similar procedure to that for complex 3, but with Pd(1,5cyclooctadiene)Cl(CH3) as precursor and CH2Cl2 as solvent, compound 4 was obtained as a white solid in good yield. Recrystallization from THF-pentane yielded colorless crystals. 31 P NMR (80.9 MHz, acetone-d6): δ 55.5 (s). 1H NMR (200 MHz, acetone-d6): δ 8.00 (t, J1 = 7.4 Hz, 1H, p-ArH), 7.68 (d, J1 = 7.4 Hz, 2H, m-ArH), 4.07 (t, J = 3.6 Hz, 4H, -CH2-), 1.43 (m, 36H, tBu), 0.89 (t, J = 5.4 Hz, 3H, CH3). IR (ATR, solid): ν 1596, 1568 cm-1. Anal. Calcd for C24H46BF4NP2Pd: C, 47.74; H, 7.68; N, 2.32. Found: C, 48.03; H, 7.61; N, 2.28. Palladium Complex 5, [Pd(PNPtBu)(4-PhCN)]BF4. Using a similar procedure to that for complex 3, but with Pd(tris{o-tolyl}phosphine)(4-cyanophenyl)Br dimer as precursor and CH2Cl2 as solvent, compound 5 was obtained as a yellow solid. Recrystallization from THF-pentane yielded light yellow crystals. 31P NMR (80.9 MHz, acetone-d6): δ 56.3 (s). 1H NMR (200 MHz, acetone-d6): δ 7.97 (t, J1 = 7.4 Hz, 1H, p-ArHPy), 7.87 (d, J1 = 8.2 Hz, 2H, ArHPhCN), 7.58 (d, J1 = 7.4 Hz, 2H, m-ArHPy), 7.36 (d, J1 = 8.2 Hz, 2H, ArHPhCN), 3.92 (m, 4H, CH2), 1.23 (m, 36H, tBu). IR (ATR, cm-1): ν 2219. Anal. Calcd for C30H47BF4N2P2Pd: C, 52.15; H, 6.86; N, 4.05. Found: C, 52.04; H, 6.59; N, 3.26. Palladium Complex 6, [Pd(PNPtBu)(Cl)]BF4. A solution of 1 (150.0 mg, 254.1 μmol) in MeCN (10 mL) was added dropwise to

a solution of Pd(NCPh)2Cl2 (97.4 mg, 254.1 μmol) in MeCN (10 mL) by means of a cannula at room temperature. The resultant light yellow solution turned turbid within 5 min and was subsequently stirred for 2 h. Filtration (to remove AgCl) and evaporation of solvent in vacuo left an off-white solid. Recrystallization by slow diffusion of pentane into a concentrated THF solution yielded light yellow single crystals in good yield (106.3 mg, 170.2 μmol, 67%). 31P NMR (80.9 MHz, acetone-d6): δ 63.7 (s). 1H NMR (200 MHz, acetone-d6): δ 8.12 (t, J1 =7.4 Hz, 1H, p-ArH), 7.79 (d, J1 =7.4 Hz, 2H, m-ArH), 4.21 (m, 4 Hz, -CH2P), 1.54 (pseudo-t, 36H, tBu). Anal. Calcd for C23H43BClF4NP2Pd: C, 44.25; H, 6.94; N, 2.24. Found: C, 44.68; H, 6.91; N, 2.12. Palladium Complex 7, Pd(PN-PtBu)(CH3). Stoichiometric amounts of 4 and NaN(SiMe3)2 were dissolved in 3 mL of THF. After 10 min, the solvent was evaporated and the brown solid product was extracted into 4 mL of pentane to give after evaporation a red-brown solid, which was directly dissolved in C6D6. 31P NMR (161.8 MHz, acetone-d6): δ 53.4 (d, 2JP-P = 356 Hz, 1P), 50.2 (d, 2JP-P = 356 Hz, 1P). 1H NMR (400 MHz, acetone-d6): δ 6.55 (t, 1P, 4-PyH), 6.40 (d, 1P, 4-PyH), 5.45 (d, 1P, 4-PyH), 3.48 (d, 2H, -CH2P), 2.68 (d, 1H, dCHP), 1.39 (d, 16H, tBu), 1.02 (d, 16H, tBu) Palladium Complex 8, Pd(PN-PtBu)(Cl). A similar procedure to that for 7 was used, but starting with 6. 31P NMR (162.0 MHz, C6D6): δ 64.45 (d, 2JP-P = 398 Hz, 1P), 52.8 (d, 2JP-P = 398 Hz, 1P). 1H NMR (400 MHz, C6D6): δ 6.35 (t, J = 6.4 Hz, 1P, 4-PyH), 6.25 (d, J = 8.0 Hz, 1P, 5-PyH), 5.26 (d, J = 6.4 Hz, 1P, 2-PyH), 3.30 (d, J = 8.0 Hz, 1H, dCHP), 2.49 (d, J = 10.0 Hz, 2H, -CH2P), 1.49 (d, 2JP-H = 13.6 Hz, 16H, tBu), 1.14 (d, 2 JP-H = 11.6 Hz,16H, tBu). Catalytic Activity in the Suzuki-Miyaura Coupling:22 A 15 mL Schlenk flask was charged with the palladium complex (10 μmol, 1 mol %), phenyl boronic acid (183 mg, 1.5 mmol), and Na2CO3 (318 mg, 3.0 mmol). The flask was evacuated and backfilled with argon (3 times). Two milliliters of THF was added followed by bromobenzene (157 mg, 1.0 mmol). The mixture was heated to 65 C for 16 h. After this reaction period, the solution was cooled to rt and filtered over a plug of alumina

7032


to remove solids before analyzing an aliquot by GC chromatography. X-ray Crystallography. X-ray data for 1-5 were collected using a Nonius KappaCCD diffractometer (rotating anode) with graphite-monochromated Mo KR radiation (λ = 0.71073 A˚) at 150 K (except for 5, which was measured at 200 K, above a phase transition) under control of the COLLECT software. Images were processed with EVALCCD. Scaling and correction for absorption was done with SADABS.28 The structure was solved with DIRDIF08 and refined with SHELXL97.29 Hydrogen atoms were introduced at calculated positions and refined riding on their carrier atoms. Disorder models were implemented (for 3, allyl and BF4 moieties, and for 5, one of the pincer moieties). The minor disorder component of 3 features an artificially short formal H28C 3 3 3 H30B contact. The associated (28) Sheldrick, G. M. SADABS: Area-Detector Absorption Correction, Version 2008/1; Universit€at G€ottingen: Germany. (29) Sheldrick, G. M. Acta Crystallogr. A 2008, 64, 112. (30) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7.

van der Vlugt et al. minor disorder of the THF solvent molecule was not resolved. Structure 5 contains four voids, each sized 284 A˚3, filled with disordered THF molecules. Their contribution to the model calculations was taken into account with the PLATON/ SQUEEZE technique. The structures were validated with PLATON/Check.30 Further numerical details are given in Table 2.

Acknowledgment. This work has been supported by the Dutch Research Council-Chemical Sciences (NWOCW) through a VENI Innovative Research Grant (to J.I.v.d.V.) and the National Research School Combination on Catalysis-NRSCC. We thank Prof. Joost Reek (UvA) for continuous support. Supporting Information Available: Crystallographic details for compounds 1-5 in CIF format (CCDC codes 748854748858). This material is available free of charge via the Internet at http://pubs.acs.org.

A Cationic AgI(PNPtBu) - American Chemical Society

Recommend Documents