Development of Rhenium Catalysts for Amine Borane

Aug 18, 2009 - For instance in the case of d6square-pyramidal hydride complexes ...... (c) Getty , K.; Delgado-Jaime , M. U.; Kennepohl , P. J. Am. Ch...
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Organometallics 2009, 28, 5493–5504 DOI: 10.1021/om900458e

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Development of Rhenium Catalysts for Amine Borane Dehydrocoupling and Transfer Hydrogenation of Olefins Yanfeng Jiang, Olivier Blacque, Thomas Fox, Christian M. Frech, and Heinz Berke* Anorganisch-Chemisches Institut, Universit€ at Z€ urich, Winterthurerstrasse 190, CH-8037, Z€ urich, Switzerland Received May 29, 2009

Five-coordinated rhenium(I) hydride complexes of the type [Re(Br)(H)(NO)(PR3)2] (R=Cy 2a, iPr 2b) were prepared from [Re(Br)2(NO)(PR3)2(η2-H2)] (R=Cy 1a, iPr 1b) via deprotonation of the η2-H2 ligands with various bases. Filling the vacant site of 2a or 2b by various less bulky two-electron donors produced the 18-electron complexes [Re(Br)(H)(NO)(PR3)2(L)] (L = O2 3, CH2dCH2 4, acetylene 5, H2 6, CO 7, CH3CN 8). The influence of the trans-coordinated ligand on the Re-H bond was examined. The 1H NMR chemical shift of the hydride depends on L in the order O2 > acetylene > CH2dCH2 > H2 > CO > CH3CN. The reactions of 2a or 2b with the IMes or SIMes ligands afforded the five-coordinated complex [Re(Br)(H)(NO)(PR3)(NHC)] (NHC=IMes 9 (IMes=1,3bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), SIMes 10 (SIMes = 1,3-bis(2,4,6-trimethylphenyl)4,5-dihydroimidazol-2-ylidene)) via replacement of one phosphine. The reaction of 2a or 2b with n-BuLi leads to the formation of the n-butene-coordinated dihydride complexes [Re(H)2(NO)(PR3)2(η2-n-CH2dCHC2H5)] (R = Cy 12a, iPr 12b). Species 1a and 1b reacted also with NaNMe2 3 BH3, affording the tetrahydride complexes [Re(H)4(NO)(PR3)2] (R=Cy 14a, iPr 14b) via the intermediacy of 2a and 2b. The molecular structures of complexes 8b, 10a, and 10b were established by single-crystal X-ray diffraction studies. The five-coordinated rhenium(I) hydride complexes 2a, 2b, 9a, and 9b catalyzed the dehydrocoupling of Me2NH 3 BH3 and the transfer hydrogenation of olefins using Me2NH 3 BH3 as a hydrogen donor, which showed high activities. Mechanistic studies were carried out indicating that these rhenium(I) hydride catalyses allowed formation of dihydrogen hydride complexes. A plausible catalytic cycle for both dehydrocoupling and transfer hydrogenation was proposed, which implies the ability of rhenium(I) complexes to activate B-H and N-H bonds by the facile redox interplay of Re(I) and Re(III) species.

Introduction “Hydrogen” as an energy carrier would crucially involve storage of hydrogen in suitable chemical compounds. Furthermore “fuelling” and “refuelling” requires chemical reactions that are dehydrogenations/hydrogenations or dehydrocoupling/hydrocoupling processes.1 Amine borane compounds are considered valuable candidates for storage materials due to their high H2 storage capacity and low thermicity in their “fuelling” and “refuelling” reactions.2

Recently mainly dehydrocoupling reactions of amine boranes were studied for applications in dehydrogenative processes utilizing transition metal catalyses with various Rh, Ru, Ir, Pd, Ni, Re, and Ti compounds (Scheme 1).3-10 For instance, the catalytic dehydrocoupling of amine boranes, such as Me2NH 3 BH3, can be achieved in a homogeneous

*Corresponding author. E-mail: [email protected]. (1) (a) Coontz, R.; Hanson, B. Science 2004, 305, 957. (b) Grochala, W.; Edwards, P. P. Chem. Rev. 2004, 104, 1283. (c) Welch, G. C.; Juan, R. R. S.; Masuda, J. D.; Stephan, D. W. Science 2006, 314, 1124. (d) Gutowska, A.; Li, L.; Shin, Y.; Wang, C. M.; Li, X. S.; Linehan, J. C.; Smith, R. S.; Kay, B. D.; Schmid, B.; Shaw, W.; Gutowski, M.; Autrey, T. Angew. Chem., Int. Ed. 2005, 44, 3578. (2) (a) Hamilton, C. W.; Baker, R. T.; Staubitz, A.; Manners, I. Chem. Soc. Rev. 2009, 38, 279. (b) Pons, V.; Baker, R. T. Angew. Chem., Int. Ed. 2008, 47, 9600. (c) Stephens, F. H.; Pons, V.; Baker, R. T. Dalton. Trans. 2007, 2613. (d) Marder, T. B. Angew. Chem., Int. Ed. 2007, 46, 8116. (e) Bluhm, M. E.; Bradley, M. G.; Butterick, R.; Kusari, U.; Sneddon, L. G. J. Am. Chem. Soc. 2006, 128, 7748. (f) Ramachandran, P. V.; Gagare, P. D. Inorg. Chem. 2007, 46, 7810. (g) Nguyen, V. S.; Matus, M. H.; Nguyen, M. T.; Dixon, D. A. J. Phys. Chem. C 2007, 111, 9603. (h) Nguyen, V. S.; Matus, M. H.; Grant, D. J.; Nguyen, M. T.; Dixon, D. A. J. Phys. Chem. A 2007, 111, 8844.

(3) Douglas, T. M.; Chaplin, A. B.; Weller, A. S. J. Am. Chem. Soc. 2008, 130, 14432. (4) (a) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. Chem. Commun. 2001, 962. (b) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. J. Am. Chem. Soc. 2003, 125, 9424. (c) Jaska, C. A.; Manners, I. J. Am. Chem. Soc. 2004, 126, 2698. (5) (a) Clark, T. J.; Russell, C. A.; Manners, I. J. Am. Chem. Soc. 2006, 128, 9582. (b) Clark, T. J.; Lee, K.; Manners, I. Chem.;Eur. J. 2006, 12, 8634. (6) Jiang, Y.; Berke, H. Chem. Commun. 2007, 3571. (7) Pun, D.; Lobkovsky, E.; Chirik, P. J. Chem. Commun. 2007, 3297. (8) (a) Keaton, R. J.; Blacquiere, J. M.; Baker, R. T. J. Am. Chem. Soc. 2007, 129, 1844. (b) Yang, X.; Hall, M. B. J. Am. Chem. Soc. 2008, 130, 1798. (9) (a) Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2006, 128, 12048. (b) Hebden, T. J.; Denney, M. C.; Pons, V.; Piccoli, P. M. B.; Koetzle, T. F.; Schultz, A. J.; Kaminsky, W.; Goldberg, K. I.; Heinekey, D. M. J. Am. Chem. Soc. 2008, 130, 10812. (c) Paul, A.; Musgrave, C. B. Angew. Chem., Int. Ed. 2007, 46, 8153. (10) Staubitz, A.; Soto, A. P.; Manners, I. Angew. Chem., Int. Ed. 2008, 47, 6212.

r 2009 American Chemical Society

Published on Web 08/18/2009

pubs.acs.org/Organometallics

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Organometallics, Vol. 28, No. 18, 2009 Scheme 1

fashion by a η2-amine borane σ-complex of Rh3 or heterogeneously by Rh(0) colloids4 and the Cp2TiCl2/n-BuLi system.5 The latter two catalysts are also active in tandem dehydrocoupling-hydrogenation reactions of amine boranes and olefins, which in total are transfer hydrogenations. Similar reactions in solution were observed with rhenium(I) dibromides6 and titanocene dinitrogen catalysts.7 More practical and active catalysts for hydrogen release from NH3 3 BH3 are an NHC nickel8 (NHC = N-heterocyclic carbene) and pincer-ligated iridium dihydride complexes,9 by which the formation of the quite volatile dehydrocoupling product borazine is efficiently avoided. More recently, high molecular weight polyaminoboranes were produced in the dehydrocoupling of primary amine boranes catalyzed by the pincer iridium dihydride.10 Ruthenium complexes activated by KOtBu were found to be exceptionally active catalysts for hydrogen release from NH3 3 BH3 and Me2NH 3 BH3.11 In all these reactions transition metal hydrides are thought to play a pivotal role with their M-H bond strengths in a range that they enable a wide variety of key transformations, such as hydride transfers and insertions.12 These are not only generally synthetically useful but also crucial to the homogeneous dehydrocoupling reactions of amine boranes.3,9-11 Most of the known transition metal hydrides obey the 18electron rule.12 Open-shell 16-electron complexes are less abundant and usually bear early or late transition metal centers. 16-Electron complexes with middle transition metal elements need for the stabilization as catalytic intermediates the help of the residual coordination sphere to create vacant sites. Utilization of π-donor effects or the trans influence is prevalent. For instance in the case of d6 square-pyramidal hydride complexes [M(X)(H)(CO)(PR3)2] (M=Ru, Os, X= halide) the coordinative vacancy is supported among other effects by the electronic interplay of the π-donor X and the strong σ-donor H.13 Metal hydride complexes possessing a vacant site, like those with ruthenium and osmium, also display catalytic chemistry.14 Therefore we attempted exploration of a related isoelectronic nitrosyl hydride chemistry of rhenium. (11) Blaquiere, N.; Diallo-Garcia, S.; Gorelsky, S. I.; Black, D. A.; Fagnou, K. J. Am. Chem. Soc. 2008, 130, 14034. (12) (a) Peruzzini, M.; Poli, R. In Recent Advances in Hydride Chemistry; Elsevier: Amsterdam, Holland, 2001. (b) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals; Wiley-Interscience, 2001. (13) Esteruelas, M. A.; Oro, L. A. In Advances in Organometallic Chemistry, Vol. 47; Academic Press Inc: San Diego, 2001. (14) (a) Esteruelas, M. A.; Sola, E.; Oro, L. A.; Meyer, U.; Werner, H. Angew. Chem., Int. Ed. Engl. 1988, 27, 1563. (b) Esteruelas, M. A.; Sola, E.; Oro, L. A.; Werner, H.; Meyer, U. J. Mol. Catal. 1988, 45, 1. (c) Lay, P. A.; Harman, W. D. Adv. Inorg. Chem. 1991, 37, 219. (d) Albeniz, M. J.; Buil, M. L.; Esteruelas, M. A.; Lopez, A. M.; Oro, L. A.; Zeier, B. Organometallics 1994, 13, 3746. (e) Bakhmutov, V. I.; Bertran, J.; Esteruelas, M. A.; Lledos, A.; Maseras, F.; Modrego, J.; Oro, L. A.; Sola, E. Chem.;Eur. J. 1996, 2, 815. (f) Esteruelas, M. A.; Gomez, A. V.; Lahoz, F. J.; Lopez, A. M.; Onate, E.; Oro, L. A. Organometallics 1996, 15, 3423. (g) Esteruelas, M. A.; Lahoz, F. J.; Onate, E.; Oro, L. A.; Sola, E. J. Am. Chem. Soc. 1996, 118, 89. (h) Buil, M. L.; Elipe, S.; Esteruelas, M. A.; Onate, E.; Peinado, E.; Ruiz, N. Organometallics 1997, 16, 5748. (i) Yi, D. C. S.; Lee, W. Organometallics 1999, 18, 5152. (j) Esteruelas, M. A.; Oro, L. A. Chem. Rev. 1998, 98, 577.

Jiang et al.

Concerning the synthetic access to transition metal hydrides, there is, besides the classic ways of protonation of basic metal complexes or halide replacement with hydride donors,15 the prominent route of deprotonation of η2-H2 complexes. Most dihydrogen ligands are acidic and can be deprotonated with bases, leaving a hydride on the metal and a proton on the base, resulting in heterolytic cleavage of the H2 molecule. In fact, the discovery of dihydrogen complexes by Kubas in 1984 initiated this research including catalytic hydrogen chemistry occurring with formal heterolysis of the H2 ligand.16 In recent years our group focused on the exploration of a related hydride chemistry with middle transition metal centers,17 in particular with respect to the development of polar homogeneous catalyses involving H-H, H-Si, and H-B activation.6,18,19 For example in the realm of rhenium, the dihydrogen rhenium(I) complexes [Re(Br)2(NO)(PR3)2(η2H2)] (R=Cy 1a, iPr 1b) were found to catalyze dehydrocoupling of Me2NH 3 BH3 and transfer hydrogenations of olefins with Me2NH 3 BH3 as a hydrogen donor.6 Recently, fivecoordinated rhenium(I) hydride complexes [Re(Br)(H)(NO)(PR3)2] (R = Cy 2a, iPr 2b) were isolated from dehydrogenative silylations of alkenes using 1a and 1b as catalysts.19 Such mononitrosyl Re(I) hydrides, which resemble the mentioned monocarbonyl Ru(II) or Os(II) hydrides, were expected to bear great potential in particular with regard to homogeneous catalysis. In extension of our earlier work, we report here the organometallic chemistry of 2a and 2b as a new platform for catalysis including the preparations of these compounds from dihydrogen complexes, their reactions toward less bulky two-electron donors to give 18electron complexes, their transformations into more bulky NHC derivatives, and their reactions with n-BuLi or NaNMe2 3 BH3 to generate dihydride and tetrahydride species. Furthermore the catalytic activities of appropriate rhenium hydrides in the dehydrocoupling of Me2NH 3 BH3 and NH3 3 BH3, as well as transfer hydrogenations of olefins using Me2NH 3 BH3, are probed.

Results and Discussion Preparation of [Re(Br)(H)(NO)(PR3)2]. Previously we reported that the five-coordinate 16-electron complex 2a or 2b could be prepared from the dihydrogen complex 1a or 1b by (15) James, B. R.; Ashby, M. T. In Inorganic Reactions and Methods; Norman, A. D., Ed.; VCH Publishers: New York, 1991. (16) (a) Kubas, G. J. Chem. Rev. 2007, 107, 4152. (b) Kubas, G. J. Adv. Inorg. Chem. 2004, 56, 127. (c) Kubas, G. J. Metal Dihydrogen and σ-Bond Complexes; Kluwer: New York, 2001. (d) Kubas, G. J. Acc. Chem. Rev. 1988, 21, 120. (e) Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.; Wasserman, H. J. J. Am. Chem. Soc. 1984, 106, 451. (17) (a) Choualeb, A.; Blacque, O.; Schmalle, H. W.; Fox, T.; Hiltebrand, T.; Berke, H. Eur. J. Inorg. Chem. 2007, 5246. (b) Frech, C. M.; Blacque, O.; Schmalle, H. W.; Berke, H.; Adlhart, C.; Chen, P. Chem.; Eur. J. 2006, 12, 3325. (c) Frech, C. M.; Blacque, O.; Schmalle, H. W.; Berke, H. Chem.;Eur. J. 2006, 12, 5199. (d) Kurz, P.; Rattat, D.; Angst, D.; Schmalle, H.; Spingler, B.; Alberto, R.; Berke, H.; Beck, W. Dalton Trans. 2005, 804. (e) Jaunky, P.; Schmalle, H. W.; Blacque, O.; Fox, T.; Berke, H. J. Organomet. Chem. 2005, 690, 1429. (f) Liu, X. Y.; Venkatesan, K.; Schmalle, H. W.; Berke, H. Organometallics 2004, 23, 3153. (g) Messmer, A.; Jacobsen, H.; Berke, H. Chem.;Eur. J. 1999, 5, 3341. (h) Frech, C. M.; Blacque, O.; Schmalle, H. W.; Berke, H. Dalton Trans. 2006, 4590. (18) (a) Choualeb, A.; Maccaroni, E.; Blacque, O.; Schmalle, H. W.; Berke, H. Organometallics 2008, 27, 3474. (b) Llamazares, A.; Schmalle, H. W.; Berke, H. Organometallics 2001, 20, 5277. (c) Gusev, D.; Llamazares, A.; Artus, G.; Jacobsen, H.; Berke, H. Organometallics 1999, 18, 75. (d) Jiang, Y.; Berke, H. Chem. Commun. 2007, 3571. (19) Jiang, Y.; Blacque, O.; Fox, T.; Frech, C. M.; Berke, H. Chem.; Eur. J. 2009, 15, 2121.

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Organometallics, Vol. 28, No. 18, 2009 Scheme 2

the reaction with a hydride donor, such as Et3SiH or Ph3SiH.19 We supposed that 2a or 2b could also be generated via the heterolytic cleavage of the η2-H2 ligand of 1a or 1b by hydrodebromination with various bases, such as HNiPr2, Na[N(SiMe3)2], or Me2NH 3 BH3 (Scheme 2). For example, treatment of 1a with 2 equiv of HNiPr2 in benzene solution afforded at room temperature 2a in 88% yield within 10 min; 1 equiv of HNiPr2 did not furnish satisfying conversions. In comparison, the reaction of 1b with HNiPr2 was much more sluggish and took 15 h at 80 °C to reach 80% conversion, implying that the dihydrogen ligand of 1b is presumably less acidic than that of 1a. Treatment of 1a with 2 equiv of the stronger base Na[N(SiMe3)2] afforded 2a within 5 min at room temperature and in 92% NMR yield. In contrast, no reaction occurred between 1b and Na[N(SiMe3)2] at room temperature. At the higher temperature of 80 °C the reaction between 1b and Na[N(SiMe3)2] led within 2 h to the formation of free PiPr3 in 100% conversion probably with substitution by the amide anion. The relatively high dissociation tendency of the PiPr3 in this complex was further demonstrated by the reaction with BH3 3 THF, producing the phosphine abstraction product H3B 3 PiPr3 in 99% yield. In contrast, the reaction of 1a with BH3 3 THF yielded 2a in 50% yield. However, this pathway was observed concomitant with the abstraction of a phosphine ligand to form H3B 3 PCy3. The reaction of BH3 3 THF with the ethylene complex [Re(Br)2(NO)(η2-C2H4)(PCy3)2] did not yield the hydride complex 1a, but solely a phosphine abstraction product, H3B 3 PCy3. This indicates that the reaction of 1a with BH3 3 THF induces heterolytic cleavage of the η2-H2 ligand. It should be noted that such a phosphine borane adduct H3B 3 PR3 is also formed in the dehydrocoupling reaction of Me2NH 3 BH3 catalyzed by the complex 1a or 1b.6 We re-examined the stoichiometric reaction of 1a or 1b with Me2NH 3 BH3 (10 equiv) and found that besides the main H3B 3 PR3 products a small amount (less than 33%) of the hydride complex 2a or 2b appeared within 30 min at 75 °C. We assume that either the B-H bond or the freed Me2NH, generated by H3B 3 PR3 formation, acts as a base here. Reaction of [Re(Br)(H)(NO)(PR3)2] with Less Bulky TwoElectron Donors. The unsaturated hydrides 2a and 2b are stable under N2 atmosphere. Exposure of hexane solutions of 2a or 2b to air under ambient condition (23 °C, 1 bar) led to the immediate formation of the 18-electron complexes [Re(Br)(H)(NO)(PR3)2(η2-O2)] (R = Cy 3a, iPr 3b), which were isolated as gray powders in 92% (3a) and 88% (3b) yield. These saturated complexes not only withstand in the solid state treatment with high vacuum but survive also

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Scheme 3

relatively high temperatures in solution (80 °C). We suspected that the relatively high stability of both derivatives possessing an oxidant (O2) and a reductant (H-) in the coordination sphere at the same time is of kinetic origin and caused by the trans position of both ligands at “maximum” separation and apparently hindered rearrangement of the coordination sphere.17h 3a and 3b were fully characterized by elemental analysis and various spectroscopic methods. In the IR spectra the O-O stretching vibration was observed at 869 cm-1 (3a) or 873 cm-1 (3b), which is comparable in its position to that of the structurally analogous η2-O2 complex of osmium.14a In the 1H NMR spectra the Re-H signals were observed as triplets at δ 7.11 ppm (2J(HP)=40.0 Hz) for 3a and 6.99 ppm (2J(HP)=39.0 Hz) for 3b, which are shifted strongly to low field in comparison with the related signals of 2a (-17.6 ppm) and 2b (-16.7 ppm). This indicates that the coordinated O2 acts as a π acceptor, most probably oriented parallel to the P-Re-P axis in compliance with the structure of the previously reported η2-styrene adducts.19 The unusual low-field shift of the hydride can be explained via strong d-electron back-donation from rhenium to the π* orbital of η2-O2, which decreases the electron density at the trans hydride site. The influence of trans-coordinated two-electron ligands on the Re-H bond was further explored by 1H NMR observation of changes in the hydride chemical shift. In solution complexes 2a and 2b reacted readily with acetylene, CO, and CH3CN to yield the corresponding 18-electron complexes [Re(Br)(H)(NO)(PR3)2(η2-C2H2)] (R = Cy 5a, iPr 5b), [Re(Br)(H)(NO)(PR3)2(CO)] (R = Cy 7a, iPr 7b), and [Re(Br)(H)(NO)(PR3)2(CH3CN)] (R = Cy 8a, iPr 8b), which were isolated as pure solids in excellent yields (Scheme 3, Table 1). All the complexes appear to be stable in the solid state, and decomposition did not take place even after prolonged treatment under vacuum. The structure of complex 8b was established by a single-crystal X-ray diffraction study, as shown in Figure 1.20 8b crystallizes with one THF solvate molecule in the asymmetric unit. The coordination sphere of 8b is pseudo-octahedral, with the hydride trans to CH3CN (N-Re-H angle of 180.00(1)o). The bromide and nitrosyl group are trans to each other with a Br-Re-N angle of 178.48(13)o. The phosphine ligands in 8b are bent toward the hydride ligand (P-Re-P angle of 164.14(3)o) (20) X-ray crystal-structure data for 8b: yellow crystals, C26H52BrN2OP2Re, M=736.75, crystal size 0.23  0.23  0.17 mm3, trigonal, space group P3221; a=11.22500(10) A˚, b=11.22500(10) A˚, c= 21.6967(2) A˚, R=90°, β=90°, γ=120°, V=2367.54(4) A˚3, Z=3, Fcalcd= 1.550 Mg 3 m-3, F(000) = 1110, μ = 5.236 mm-1, 25 459 reflections (2θmax = 61°), 4814 unique (Rint = 0.0442), 175 parameters, R1 (I > 2σ(I))=0.0175, wR2 (all data)=0.0428, GooF=1.070, largest difference peak and hole 0.701 and -0.821 e 3 A˚-3.

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Table 1. Yields for the Preparation of [Re(Br)(H)(NO)(PR3)2(L)] Complexes and 1H NMR Chemical Shiftsa of the Hydride Ligands complex [Re(Br)(H)(NO)(PCy3)2(η2-O2)] (3a) [Re(Br)(H)(NO)(PiPr3)2(η2-O2)] (3b) [Re(Br)(H)(NO)(PCy3)2(η2-C2H4)] (4a) [Re(Br)(H)(NO)(PiPr3)2(η2-C2H4)] (4b) [Re(Br)(H)(NO)(PCy3)2(η2-C2H2)] (5a) [Re(Br)(H)(NO)(PiPr3)2)2(η2-C2H2)] (5b) [Re(Br)(H)(NO)(PCy3)2(η2-H2)] (6a) [Re(Br)(H)(NO)(PiPr3)2)2(η2-H2)] (6b) [Re(Br)(H)(NO)(PCy3)2(CO)] (7a) [Re(Br)(H)(NO)(PiPr3)2)2(CO)] (7b) [Re(Br)(H)(NO)(PCy3)2(CH3CN)] (8a) [Re(Br)(H)(NO)(PiPr3)2)2(CH3CN)] (8b)

yield (%)b

1 Hδ (Re-H, ppm)

83 86 66 55 85 91 99c 99c 87 90 90 67

7.11 6.99 4.11 3.75 5.68 5.11 3.66 3.29 2.82 2.50 -1.53 -1.82

a In benzene-d6 solution. b Isolated yield. c Determined in situ by the integral in the 31P NMR spectrum.

Scheme 4

mainly due to the optimization of d-π*CN orbital overlap between rhenium and the acetonitrile. The CH3CN moiety in 8b is linear, with a Re-N-C angle of 180.00°. The Re-NCCH3 distance is 2.174(3) A˚ and close to the average value of 2.098 A˚.21 In the presence of H2, the dihydrogen rhenium(I) hydride complexes [Re(Br)(H)(NO)(PR3)2(η2-H2)] (R = Cy 6a, iPr 6b) could be generated in situ from 2a and 2b. However, upon removal of the H2 atmosphere, 6a and 6b were immediately transformed back into the starting materials 2a and 2b, which also prevented isolation of 6a and 6b. The 1H NMR

spectrum of compound 6a or 6b, recorded under H2 atmosphere, showed a low-field resonance for the dihydrogen ligand (3.46 ppm for 6a, T1=49 ms, and 3.38 ppm for 6b, T1= 87 ms). Both signals appear broad apparently due to fast rotation of the H2 moiety and unresolved coupling with the phosphorus nuclei. The long T1 time of 6b indicates an elongated H2 ligand approaching dihydride character.22,16 Reactions of the [Re(Br)(H)(NO)(PR3)2] Complexes with Bulky NHCs. At room temperature 2a or 2b did not undergo ligand addition by treatment with 1.2 equiv of the bulkyl Nheterocyclic carbene (NHCs) IMes (IMes = 1,3-bis(2,4,6trimethylphenyl)imidazol-2-ylidene); rather a ligand exchange reaction was initiated, affording the five-coordinate rhenium(I) IMes hydrides [Re(Br)(H)(NO)(PR3)(IMes)] (R=Cy 9a, iPr 9b) in good yield (71% for 9a and 86% for 9b). Complexes 9a and 9b were fully characterized. The IR spectra of 9a and 9b showed a strong ν(NO) absorption at 1631 cm-1 (9a) and 1626 cm-1 (9b). In comparison with the ν(NO) absorptions of 2a (1649 cm-1) or 2b (1640 cm-1), the lower wavenumbers of 9a or 9b speak for a stronger electrondonating ability of the IMes ligand in comparison with the PCy3 and PiPr3 derivatives. The 31P{1H} NMR spectra showed a singlet at δ 30.3 (9a) and 41.5 ppm (9b). In the 1 H NMR spectra the methyl signals of the IMes ligand were observed at δ 2.43, 2.36, 2.12 ppm (9a) and 2.43, 2.35, 2.12 ppm (9b), respectively, and the signals for the imidazole ring protons were found at δ 6.24 (9a) or 6.25 ppm (9b). The HRe resonance of 9a was observed at δ -18.18 ppm (doublet with 2J(HP) =12.0 Hz), confirming the presence of only one phosphine ligand. The relatively small JHP coupling constant was consistent with the hydride cis to the PCy3 ligand. The HRe signal of 9b, however, appeared as a broad resonance at δ -17.94 ppm. Interestingly, complex 9a or 9b could be prepared directly from 1a or 1b by treatment with 2 equiv of IMes (Scheme 4). The reaction proceeded via initial formation of 2a and 2b with heterolytic cleavage of the η2-H2 ligand of 1a or 1b induced by IMes as a strong base. HBr was removed with formation of the IMes 3 HBr salt, which could be isolated and proven by 1H NMR spectroscopy. Similarly, 2a or 2b was reacted in a 1:1.2 molar ratio with SIMes (SIMes = 1,3-bis(2,4,6-trimethylphenyl)-4,5dihydroimidazol-2-ylidene) in benzene at room temperature, which gave purple solutions. The complexes

(21) Jacobsen, H.; Schmalle, H. W.; Messmer, A.; Berke, H. Inorg. Chim. Acta 2000, 306, 153.

(22) (a) Heinekey, D. M.; Oldham, W. J. Chem. Rev. 1993, 93, 913. (b) McGrady, G. S.; Guilera, G. Chem. Soc. Rev. 2003, 32, 383.

Figure 1. ORTEP drawing of the molecular structure of complex 8b (thermal ellipsoids are shown at the 20% probability level; the benzene solvent and selected hydrogen atoms are omitted). Selected bond lengths (A˚) and angles (deg): Re1-Br1, 2.5297(7); Re1-N1, 1.780(5); Re1-H1, 1.65(4); Re1-P1, 2.4368(5); Re1-N2, 2.174(3); N1-O1, 1.230(5). O1-N1-Re1, 178.2(5); N1-Re1-N2, 91.17(13); N1-Re1-P1, 90.02(12); P1-Re1-P1i, 164.14(3); N2-Re1-H1, 180.0; N1-Re1-H1, 88.83(13); P1-Re1-H1, 82.1(9); N2-Re1-Br1, 178.48(13); C1-N2-Re1, 180.0. Symmetry operation: (i) -xþ2, -xþyþ1, -zþ2/3.

Article

[Re(Br)(H)(NO)(PR3)(SIMes)] (R = Cy 10a, iPr 10b) were separated as purple solids in 51% (10a) or 75% (10b) yield. 10a and 10b were fully characterized. The ν(NO) absorptions of 10a and 10b were observed at 1636 (10a) and 1629 cm-1 (10b), respectively. The 31P{1H} NMR spectra showed singlets at δ 32.5 (10a) and 43.2 ppm (10b). In the 1H NMR spectra the methyl signals of the SIMes ligand were observed at δ 2.58, 2.57, 2.12 ppm (10a) or 2.58, 2.43, 2.10 ppm (10b), respectively. The signals for the dihydroimidazole ring protons appeared as two multiplet resonances at δ 3.36, 3.24 ppm (10a) and 3.44, 3.25 ppm (10b). The broad HRe signal of 10a was observed at δ -16.15 ppm, and the HRe signal of 10b appeared as a doublet at δ -15.50 ppm with 2J(HP)=14.8 Hz, indicating the presence of only one phosphine ligand in the coordination sphere. The spectroscopically derived molecular structures of 10a and 10b were confirmed by single-crystal X-ray diffraction studies.23 Complex 10a adopts a square-pyramidal geometry around the rhenium centers, with the hydride ligand occupying apical positions, while 10b shows a trigonal-bipyrimid geometry, as shown in Figure 2 and Figure 3, respectively. In 10a the bromide is approximately trans to the NO group (Br1-Re1-N1average = 174.9(2)o), as well as the SIMes and the PCy3 ligands (C1-Re1-P1=171.73(6)o). From a structural comparison it became clear that the SIMes is sterically more demanding than the PCy3 or the PiPr3 ligand. The average Re-NO bond distance of 10a (1.711(3) A˚) is significantly shorter than that of 2a (1.746 A˚). This can be explained by the better σ-donation of SIMes than that of PCy3, resulting in increased π-back-donation into the NO π* orbital of 10a and a longer NO bond distance in 10a (1.318(4) A˚) than in 2a (1.258 A˚). The Re-P distance in 10a (2.4307(5) A˚) is slightly shorter than that in 2a (2.4419 A˚).24 In 10b the SIMes and PiPr3 groups are more bent toward the hydride ligand, with an average C-Re-P angle of 167.9(1)o, which also accounts for the small Br-Re-N angle (146.5(2)o) in 10b. It should be noted at this point that still bulkier NHC ligands, such as SIPr (SIPr = 1,3-bis(2,6-isopropylphenyl)4,5-dihydroimidazol-2-ylidene), proved to be unreactive in such ligand exchange reactions. Similar to the chemistry of the bis(phosphine) complexes 2a and 2b, the vacant site of the carbene complexes 9a and 9b can be occupied by less bulky two-electron donors. For example, when solutions of 2a or 2b were treated with hydrogen at room temperature, rhenium(I) IMes dihydrogen hydride complexes [Re(Br)(H)(NO)(PR3)(IMes)(η2-H2)] (R = Cy 11a, iPr 11b) were generated in situ in 100% yield, as indicated by NMR spectroscopy. Complexes 11a and 11b are however stable only under hydrogen atmosphere and will immediately (23) X-ray crystal-structure data for 10a: purple crystals, C39H60BrN3OPRe, M = 883.98, crystal size 0.38  0.31  0.11 mm3, monoclinic, space group P21/n; a=12.3151(1) A˚, b=17.3984(1) A˚, c= 18.9751(2) A˚, R=90°, β=104.296(1), γ=90°, V=3939.76(6) A˚3, Z=4, Fcalcd=1.490 Mg 3 m-3, F(000)=1792, μ=4.172 mm-1, 54 723 reflections (2θmax =61°), 14 324 unique (Rint =0.0460), 452 parameters, R1 (I > 2σ(I))=0.0265, wR2 (all data)=0.0585, GooF=0.992, largest difference peak and hole: 1.816 and -0.823 e 3 A˚-3. X-ray crystal-structure data for 10b: purple crystals, C30H48BrN3OPRe, M=763.79, crystal size 0.38  0.28  0.06 mm3, triclinic, space group P1: a = 10.9984(4) A˚, b = 16.0869(5) A˚, c = 18.9301(4) A˚, R = 103.099(2)°, β = 91.571(2)°, γ = 91.916(3)o, V=3258.22(17) A˚3, Z=4, Fcalcd =1.557 Mg 3 m-3, F(000)= 1528, μ=5.031 mm-1, 35 535 reflections (2θmax=61°), 13 300 unique (Rint = 0.0511), 710 parameters, R1 (I > 2σ(I)) = 0.0377, wR2 (all data) = 0.0797, GooF=0.952, largest difference peak and hole 1.666 and -1.568 e 3 A˚-3. (24) Lee, H. M.; Smith, D. C.; He, Z.; Stevens, E. D.; Yi, C. S.; Nolan, S. P. Organometallics 2001, 20, 794.

Organometallics, Vol. 28, No. 18, 2009

5497

Figure 2. ORTEP drawing of the molecular structure of [Re(Br)(H)(NO)(PCy3)(SIMes)] (10a). Thermal ellipsoids are shown at the 20% probability level; the trans NO/Br disorder and selected hydrogen atoms are omitted. Selected bond lengths (A˚) and angles (deg): Re1-H, 1.69(2); (Re1-N1)average, 1.711(3); (Re1-Br1)average, 2.4876(8); Re1-P1, 2.4307(5); (N1-O1)average, 1.318(4); (O1-N1-Re1)average, 175.3(6); (N1-Re1-Br1)average, 174.9(2); H-Re1-N1a, 95.4(8); P1-Re1-H, 89.2(7); H-Re1-Br1a, 87.0(7); C1-Re1-P(1), 171.73(6).

Figure 3. ORTEP drawing of the molecule structure of [Re(Br)(H)(NO)(PiPr3)(SIMes)] (10b) (thermal ellipsoids are drawn at the 30% probability level). Selected bond lengths (A˚): N1a-O1a, 1.221(6); N1a-Re1, 1.721(5); P1-Re1, 2.4186(14); Br1-Re1, 2.5236(5); Re1-H1, 1.40(3). Selected bond angles (deg): O1a-N1a-Re1, 173.7(6); C1-Re1-P1, 168.42(12); N1a-Re1-Br1, 146.5(2); N1a-Re1-H1, 89.0(17); Br1-Re1-H1, 124.2(17).

convert into the starting complexes 9a and 9b once they are exposed to N2. The 1H NMR spectra of 11a and 11b, recorded under H2 atmosphere, showed low-field-shifted doublets for the hydride ligands (δ 4.63 ppm, 2J(HP) = 26.2 Hz for 11a and 4.46 ppm, 2J(HP) = 26.0 Hz for 11b) and broad signals for the dihydrogen ligand (δ 4.08 ppm for 11a, T1 =44 ms, and 4.09 ppm for 11b, T1 =59 ms). The 31P{1H} NMR spectra showed singlets at δ 30.4 (11a) and 40.2 ppm (11b).

5498

Organometallics, Vol. 28, No. 18, 2009 Scheme 5

Reaction of [Re(Br)(H)(NO)(PR3)2] with n-BuLi. Treatment of 2a or 2b with 1.5 equiv of n-BuLi afforded at room temperature the n-butene-coordinated dihydride complexes [Re(H)2(NO)(PR3)2(η2-n-CH2dCHC2H5)] (R=Cy 12a, iPr 12b), which were obtained as pure colorless oils in 78% (12a) and 83% (12b) yield. The reaction presumably proceeded via replacement of the bromide ligand with the n-butyl group to establish a [Re(Bu)(H)(NO)(PR3)2] species, which is then converted by β-H shift into the dihydride 12a or 12b (Scheme 5). In the IR spectra the coordinated NO ligands gave rise to ν(NO) bands at 1610 cm-1 (12a) and 1615 cm-1 (12b). The 31P{1H} NMR spectra of 12a and 12b displayed signals with AB splitting patterns at 24.9, 21.6 ppm (2J(PP)= 82 Hz) for 12a and 35.0, 33.7 ppm (2J(PP)=83 Hz) for 12b, in agreement with the asymmetry induced by the alignment of the n-butene ligand along the P-Re-P axis and hindered rotation. In the 1H NMR spectra the two hydride ligands HA (trans to NO) and HB (trans to n-butene) were observed as doublets of triplets at -1.21 (Re-HA), and -5.24 ppm (Re-HB) for 12a and -1.63 (Re-HA) and -5.89 ppm (Re-HB) for 12b. Such doublets of triplet signals are indicative of the given substitution pattern by both the J(PH) couplings (38.5, 27.2 Hz for 12a and 38.0, 28.0 Hz for 12b) and J(HH) couplings (7.5, 5.7 Hz for 12a and 6.9, 6.3 Hz for 12b). η2-Hydroborate complexes [Re(H)(NO)(PR3)2(η2BH4)] (R=Cy 13a, iPr 13b) can also be prepared from the hydride 2 by treatment with NaBH4. For instance, the reaction of 2a with 5 equiv of NaBH4 in THF-d8 solution afforded complex 13a in 74% yield at 55 °C for 15 h. The yield was determined on the basis of the integration of the 31 P{1H} NMR spectrum. The 1H NMR spectroscopic data were in accord with those reported.18c The relatively long reaction time is most likely due to the poor solubility of NaBH4 in THF. Indeed, 12a or 12b could also be formed at room temperature by treatment of the dibromide complexes 1a or 1b with 3 equiv of n-BuLi. This reaction proceeded via the primary formation of the hydride species 2a or 2b, which could be traced by NMR spectroscopy of the reaction mixture. The intermediate n-butene-coordinated complexes [Re(Br)(H)(NO)(PR3)2(η2-n-CH2dCHC2H5)] were spectroscopically detected in the reaction mixture. Reaction of [Re(Br)(H)(NO)(PR3)2] with Na[NMe2 3 BH3]. In order to gain more insight into the amine borane related rhenium chemistry, we further examined the reaction between 2 and the sodium borane amide Na[NMe2 3 BH3], which was prepared by treatment of Me2NH 3 BH3 with 1.0 equiv of NaH in THF at room temperature. At room temperature the reaction of 2a with 2.0 equiv of

Jiang et al.

Na[NMe2 3 BH3] in THF afforded immediately a yellow solution accompanied by formation of a white precipitate. The 11B NMR spectra indicated the presence of mainly [Me2N-BH2]2 (δ 4.50 ppm, t, JBH=112 Hz) along with trace amounts of Me2NdBH2 (δ 36.82 ppm, t, JBH=113 Hz) and (Me2N)2BH (δ 28.07 ppm, d, JBH =134 Hz). The 1H NMR and 31P{1H} NMR spectra indicated the formation of a tetrahydride rhenium complex, [Re(H)4(NO)(PCy3)2] (14a), in 43% spectroscopic NMR yield. Compound 14a was known and was previously prepared via the reaction of the hydroborate complex 13a with 2-propanol or H2.18c Recorded at room temperature the 1H NMR spectrum showed in the hydride region two broad resonances at -1.71 and -7.10 ppm in a ratio of 3:1. The 31P{1H} NMR spectra of the resulting mixture displayed besides the typical signal at 44.6 ppm for 14a two other resonances at 51.6 and 28.6 ppm, which were assigned to two presumably isomeric rhenium dihydride species. Further support for this assignment came from the 1H NMR spectrum showing additional broad signals in the chemical shift range from -8.0 to 0 ppm. Interestingly, by adding 1 equiv of Me2NH 3 BH3 to this mixture, the two dihydride species could be completely transformed to 14a, which became then the unique organometallic species in the reaction solution (100% NMR yield). In the 11B NMR spectrum, the trimer [Me2N-BH2]3 (δ 1.23 ppm, t, JBH=96 Hz) was observed besides the dimer [Me2N-BH2]2. Formation of the linear dimer Me2NH 3 BH2NMe2 3 BH3 was not detected.25 This indicated that the two unknown dihydride species are not only active in the abstraction of H2 from the added Me2NH 3 BH3 but are in addition mediating the conversion of [Me2N-BH2]2 into [Me2NBH2]3. In fact, it turned out that the best route to prepare the complexes 14a and 14b is the reaction of the dibromide compounds 1a and 1b with Na[NMe2 3 BH3]. For example, the reaction of 1b with 2 equiv of Na[NMe2 3 BH3] afforded instantaneously [Re(H)4(NO)(PiPr3)2] (14b) in 99% spectroscopic yield. The 11B NMR spectrum indicated the formation of [Me2N-BH2]2 in 99% yield. The facile reaction can be explained by the generation of hydrogen in the first step of the reaction of 1a and 1b with 1 equiv of Na[NMe2 3 BH3], which is then consumed in a subsequent step forming 14a and 14b by the reaction with one of the proposed isomeric rhenium dihydride species (Scheme 6). Dehydrocoupling of Me2NH 3 BH3 and Transfer Hydrogenation of Olefins Using Me2NH 3 BH3 As a Hydrogen Donor. The catalytic activity of the various rhenium(I) hydrides was tested with the catalytic reactions of Me2NH 3 BH3 carried out under comparable conditions (Scheme 7, Table 2). For the dehydrocoupling reaction typically a solution of Me2NH 3 BH3 (12 mg, 0.02 mmol) in benzene (0.5 mL) was treated with catalytic amounts of 2a (1.7 mg, 1.0 mol %) and kept at 75 °C for 80 min. The 11B NMR spectrum indicated in this case 100% conversion to the cyclic aminoborane dimer [Me2N-BH2]2 (δ 4.50 ppm, t, JBH=112 Hz) with a TOF of 77 h-1. This is a 3-fold enhancement in comparison with the reactions catalyzed by the dibromide complex 1a.6 Rate enhancement was also observed for the dehydrocoupling of the Me2NH 3 BH3 compound using the hydride complex 2b as a catalyst. The 18-electron rhenium(I) phosphine hydride complexes [Re(Br)(H)(NO)(PR3)2(L)] (type 3 to 8) were also (25) (a) N€ oth, H; Thomas, S. Eur. J. Inorg. Chem. 1999, 1373. (b) Hahn, G. A.; Schaefer, R. J. Am. Chem. Soc. 1964, 86, 1503.

Article

Organometallics, Vol. 28, No. 18, 2009

5499

Scheme 6

Scheme 7

Table 2. Dehydrocoupling of Me2NH 3 BH3 Catalyzed by Re(I) Complexesa entry

[Re]

time (h)

yield (%)b

TOF (/h)

1 2 3 4 5 6 7 8 9 10 11 12

1a 1b 2a 2b 9a 9b 10a 10b 12a 12b 13a 13b

1.0 4.0 1.3 1.5 1.0 1.0 3.0 2.0 1.0 1.0 1.0 1.0

11 96 100 83 100 92 99 88 47 28