Communication pubs.acs.org/Organometallics
Aminoborane σ Complexes: Significance of Hydride Co-ligands in Dynamic Processes and Dehydrogenative Borylene Formation David A. Addy, Joshua I. Bates, Michael J. Kelly, Ian M. Riddlestone, and Simon Aldridge* Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, U.K. S Supporting Information *
ABSTRACT: Systems of the type [(p-cym)Ru(PR 3 )(H)(H2BNiPr2)]+ (R = Cy, Ph) can be synthesized from (p-cym)Ru(PR3)Cl2 and H2BNiPr2/Na[BArf4] and are best formulated as (hydrido)ruthenium κ1-aminoborane complexes. VT-NMR measurements have been used to probe the σ-bond metathesis process leading to Ru−H/H−B exchange, yielding an activation barrier of ΔG⧧ = 7.5 kcal mol−1 at 161 K. Moreover, in contrast to the case for related non-hydride-containing systems, reactivity toward alkenes constitutes a viable route to a metal borylene complex via sacrificial hydrogenation.
T
less common, and alternative structural descriptions as metal hydroborate or boryl dihydride species have been investigated (Chart 1).7b,12,13 In the current study, the reactions of (p-
he chemistry of metal borylene complexes, LnM(BX)m, has developed rapidly over the past 10 years, to the extent that systems previously viewed as chemical novelties are now well understood from the perspective of geometric and electronic structure.1 Potentially useful reactivity patterns have also begun to be mapped outfor stoichiometric systems at least (e.g., in the controlled functionalization of organic/ organometallic substrates).1 Catalytic applications, on the other hand, remain all but unknown,2 primarily due to the incompatibility of existing synthetic routes (e.g., salt metathesis, photolytic transfer, and halide abstraction) with catalytic turnover.1 Direct borylene synthesis by the dehydrogenation of a dihydroborane,3 in contrast, offers an exciting new approach which, by analogy with related silylene systems, might be more amenable to incorporation into a catalytic cycle.4 Moreover, the viability of direct borane to borylene conversion has been demonstrated by the dehydrogenation of Ru(PCy3)2(H)Cl(κ2H2BMes) to Ru(PCy3)2(H)Cl(BMes) (Mes = C6H2Me32,4,6),3 and the intermediates and discrete mechanistic steps implicit in such a transformation have recently been elucidated.5 That said, the scope of this chemistry beyond one groundbreaking example has yet to be realized, and with this in mind, we hypothesized that the hydrogenation of a sacrificial alkene might constitute an alternative and more broadly applicable methodology. Central to the success of such an approach is controlling the competition between alkene hydrogenation and hydroboration.6 While the choice of alkene is potentially important, the formation of the target borylene systems appears in the current study to be crucially dependent on using a borane complex precursor featuring an ancillary hydride ligand. In recent work we have been interested in the chemistry of aminoborane complexes [LnM(H2BNR2)],7−10 systems which have been implicated in processes such as the dehydrocoupling/polymerization of amineboranes 11 but for which relatively little fundamental chemistry has been mapped out. σ-Borane complexes featuring additional hydride coligands are © 2013 American Chemical Society
Chart 1. Alternative Descriptions of a (Hydrido) σ-Borane Complex: Metal Hydroborate or Boryl Dihydride
cym)Ru(PR3)Cl2 (1a, R = Cy; 1b, R = Ph; p-cym = pMeC6H4iPr)14 with excess H2BNiPr2/Na[BArf4] led to the formation of the cationic complexes [(p-cym)Ru(PR3)(H)(H2BNiPr2)][BArf4] (3a,b; Arf = C6H3(CF3)2-3,5) as yelloworange crystalline materials in 50−60% isolated yield (Scheme 1). For the PCy3 system, stepwise addition of the borane and halide abstraction agent reveals that (p-cym)Ru(PCy3)H(Cl) (2a)14 is accessible (together with H(Cl)BNiPr2)15a via initial hydride/halogen exchange. Subsequent addition of Na[BArf4] confirms that 2a is a viable intermediate in the formation of 3a. 3a,b have both been characterized by standard spectroscopic and analytical techniques. Of interest are 11B chemical shifts Scheme 1. Syntheses of [(p-cym)Ru(PR3)(H)(κ1H2BNiPr2)][BArf4] (3a, R = Cy; 3b, R = Ph)a
a
Reagents and conditions: (a) H2BNiPr2 (2.0 equiv), C6H5F;14 (b) Na[BArf4] (1.1 equiv), C6H5F, room temperature, 5 min, 50−60%.
Received: January 18, 2013 Published: March 11, 2013 1583
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BNC2 unit itself. Thus, the B−N distance (1.392(8) Å) is strongly indicative of BN double-bond character, being (i) similar to those found in Cl2BNPh2 (1.380(6) Å)17 and κ2 complexes containing the H2BNiPr2 ligand (e.g., 1.379(8) Å for [Ir(IMes)2(H)2(κ2-H2BNiPr2)]+)9b and (ii) significantly shorter than either the sum of the respective covalent radii (1.58 Å)18 or than distances in complexes of the H3B·NHiPr2 ligand (ca. 1.60 Å).9d Moreover, the geometry at N(32) is planar (Σ(angles) = 359.8°). Given the limitations in locating hydrogen atoms by X-ray diffraction, additional information was sought from DFT studies. While the potential energy hypersurface for 3b is clearly a shallow one, a conformation lying 0.76 kcal mol−1 above the global minimum was found to model well the alignment of the planar BNC2 heavy-atom framework19 and additionally features B−μ-H (1.62, 1.38 Å) and Ru−H distances (1.65, 1.72 Å) in good agreement with the crystallographic study (Supporting Information). The geometric data therefore imply that a hydroborate structure of type I (Figure 1) involving significant covalent bonding interactions with three hydrogen atoms is unlikely and that an alternative motif based around a κ1-aminoborane fragment (i.e., II) is plausible.20 With this in mind, we synthesized for structural comparison the isoelectronic [CpRuL2]+ system 4b, which features the same H2BNiPr2 fragment but no hydride coligand.7h While a number of structural features (Figure 2), namely the short BN distance
(36 and 35 ppm, respectively), which are similar to those of both “free” H2BNiPr2 (35.2 ppm)15b,c and [CpRu(PPh3)2(κ1H2BNiPr2)][BArf4] (4b; 38 ppm), the latter of which features an unambiguously κ1-bound aminoborane fragment (vide infra). By comparison, the κ2-aminoborane ligand in [Cp*Ru(PCy3)(κ2-H2BNiPr2)][BArf4] (5a; see the Supporting Information), for example, gives rise to a much lower field signal (δB 54 ppm) consistent with a tighter M···B contact.7g That said, 11B chemical shifts similar to those for 3a,b have also been reported for four-coordinate ruthenium hydroborate systems,16 and the additional facts (i) that only one Ru−H−B resonance could be discerned in the room-temperature 1H NMR spectrum of either 3a or 3b and (ii) that selective 11B{1H} experiments yielded broad resonances giving no definitive coupling constant data prompted us to examine in detail the mode of attachment of the boron-containing ligand by crystallographic and computational approaches. Crystallographically, the solid-state structure of 3a features a disordered borane fragment; that of 3b, however, suffers from no such problems, and hydrogen atoms could also be located in the difference Fourier map (Figure 1). Superficially, the solid-
Figure 1. Molecular structures and limiting structural descriptions for 3a,b. Here and elsewhere, the anion, most H atoms, and minor disorder component(s) are omitted and phosphine substituents are shown in wireframe format for clarity. H atoms in 3b were located in the difference Fourier map and refined isotropically with no restraints. Key bond lengths (Å) and angles (deg) for 3b: Ru(1)−P(2) = 2.324(1), Ru(1)−B(31) = 2.264(6), Ru(1)−H(11) = 1.63(6), Ru(1)−H(17) = 1.69(6), B(31)−N(32) = 1.392(8), B(31)···H(11) = 1.62(6), B(31)−H(17) = 1.33(6), B(31)−H(311) = 1.08(6); Ru(1)−B(31)−N(32) = 128.3(4), B(31)−N(32)−C(33) = 119.7(5), B(31)−N(32)−C(36) = 124.7(5), C(33)−N(32)−C(36) = 115.4(5).
Figure 2. Molecular structure of [CpRu(PPh3)2(κ1-H2BNiPr2)][BArf4], 4b. Key bond lengths (Å) and angles (deg): Ru(1)−B(50) = 2.353(4), Ru(1)−P(2) = 2.338(1), Ru(1)−P(4) = 2.334(1), Ru(1)−H(28) = 1.70(5), B(50)−N(51) = 1.386(5), B(50)−H(28) = 1.23(4), B(50)−H(501) = 1.08(5); Ru(1)−B(50)−N(51) = 131.4(3), B(50)−N(51)−C(52) = 120.2(3), B(50)−N(51)−C(55) = 123.9(3), C(52)−N(51)−C(55) = 115.6(3).
(1.386(5) Å for 4b) and planar BNC2 skeleton, are common to the two systems, the presence of a hydride coligand in 3b does lead to some geometric perturbation of the aminoborane fragment. Most notably, the geometry at the boron center in 3b (as defined by N(32) and the two aminoborane hydrogens, H(17) and H(311)) is less planar than in 4b (Σ(angles) = 343.5°; cf. 352.3°). From a structural perspective, 3a,b therefore offer a platform on which to study the degenerate exchange between a single metal-bound hydride and a σ-coordinated BH bond (i.e., M− H/H−B σ-bond metathesis).4,21 In the case of 3a, the dynamics of exchange can be probed by VT-NMR in CDCl2F solution. Thus, at room temperature, a single high-field resonance integrating to 2H is measured at δH −12.53 ppm (for HA and HB), and an AB multiplet is observed for the p-cymene aromatic hydrogens. In effect, at this temperature, the cation possesses a mirror plane on the NMR time scale. At very low temperatures (T = 161 K), however, two distinct hydride resonances are observed (at δH −12.73 and −12.48 ppm), and four arene CH signals can be resolved, consistent with a static
state structure resembles those of isoelectronic charge-neutral systems such as Cp*Ru(PiPr3)(κ2-H2BXMes) (X = Cl, H; Mes =2,4,6- Me3C6H2), which feature a [BH2XMes]− ligand bound to the ruthenium center via two equivalent bridging hydrogen atoms.16b Closer inspection, however, reveals that this is unlikely to be a valid structural description in the case of 3b. Thus, in comparison to the Cp*Ru complexes, there is greater asymmetry in the binding of the boron ligand, manifested by (i) the B−H(Ru) distances (1.33(6) and 1.62(6) Å, cf. 1.25(2) and 1.27(2) Å for Cp*Ru(PiPr3)(κ2-H3BMes)),16b with the longer B···H distance being slightly shorter than a similar contact measured for {C6H3(OPtBu2)2}Ir(H)2(κ1-BH3) (1.74(5) Å), which is described as being “outside the limits of a bonding interaction”,12 and (ii) the BNC2 heavy-atom framework, which is canted to one side of the metal center, such that Ru(1) lies 0.40 Å out of the least-squares plane defining B(31), N(32), C(33), and C(36). More telling still is the geometry of the 1584
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Scheme 3. Reaction of [Cp*Ru(PR3)(κ2-H2BNiPr2)][BArf4] (5a) with TBE: Formation of [Cp*Ru(H){PCy2(C6H8)}][BArf4] and t BuCH2CH2B(H)NiPr2a
structure featuring nonequivalent Ru−H bonds and which is chiral at ruthenium. Modeling of the coalescence behavior allows the activation barrier ΔG⧧ = 7.5 kcal mol−1 to be estimated at 161 K. While fluxional processes have previously been identified in (hydrido)metal σ-borane complexes,7a,b,12,13c 3a offers, for the first time, unambiguous determination of the barrier for simple M−H/H−B pairwise exchange. Given the low energy but nonproductive (i.e., degenerate) σbond metathesis processes occurring for 3a,b, we hypothesized that useful (facile) reactivity might stem from insertion of unsaturated substrates into the Ru−H bond. In particular, with borane dehydrogenation in mind, we examined the reactivity toward the well-known dihydrogen acceptor 3,3-dimethylbutene (TBE). Comparative studies utilizing the related (but nonhydride-containing) complexes 4b and 5a were also undertaken. In practice, the reaction of 3a with TBE leads to the formation of two new boron-containing species. The major product (65% isolated yield) gives rise to a broad 11B NMR signal at δB 77 ppm and can subsequently be shown to be the borylene complex [(p-cym)Ru(PCy3)(H)(BNiPr2)][BArf4] (6a), formed by net dehydrogenation of 3a (Scheme 2). The
Reagents and conditions: (a) TBE (10.0 equiv), C6H5F, 60 °C, 8 days, 45%.
a
elimination,28 it should be noted that the hydrido (κ2-borane) complex Ru(PCy3)2(H)Cl(κ2-H2BMes) undergoes spontaneous H−H bond formation without the need for a sacrificial alkene.3 On the other hand, 3a does not undergo dehydrogenation in the absence of TBE under any conditions we have examined. While these experimental observations hint at the mechanistic relevance of both the alkene and ancillary hydride ligand, in-depth investigations by quantum chemical methods of potential steps leading to the dehydrogenation of 3a (and related systems) are clearly necessary and will be reported in due course.
Scheme 2. Synthesis of Borylene Complex 6a by Dehydrogenation of 3a and Molecular Structure of 6aa
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Text, tables, figures, and CIF files giving synthetic, spectroscopic, and crystallographic data for compounds 3a,b, 4b, 5a, 6a, [Cp*Ru(H){PCy2(C6H8)}][BArf4] and tBuCH2CH2B(H)NiPr2 and details of the DFT calculations on 3b and 6a, and all CIFs. This material is available free of charge via the Internet at http://pubs.acs.org.
a
Reagents and conditions: (a) TBE (10.0 equiv), C6H5F, room temperature, 30 min, 65%. Key bond lengths (Å) and angles (deg): Ru(1)−P(2) = 2.336(1), Ru(1)−B(31) = 1.911(4), Ru(1)−H(8) = 1.54(6), B(31)−N(34) = 1.361(5), B(31)...H(8) = 1.71(6); Ru(1)− B(31)−N(34) = 173.0(4).
Corresponding Author
positions of both the metal-bound hydrogen atom and the linear Ru−B−N unit determined crystallographically for this species are well reproduced by DFT calculations (Supporting Information). The second (minor) boron-containing species, t BuCH2CH2B(H)NiPr2 (Supporting Information), arises from alkene hydroboration, presumably via a metal-mediated process, since the reaction of TBE with H2BNiPr2 does not proceed under otherwise analogous conditions in the absence of 3a.22,23 In contrast, the reaction of the simple (i.e. nonhydride-containing) κ1-H2BNiPr2 complex 4b with TBE results in simple borane displacement chemistry. Interestingly, t BuCH2CH2B(H)NiPr2 is the sole boron-containing product formed in the reaction of the κ2-H2BNiPr2 system 5a with TBE. The metal-containing product in this case (Scheme 3 and the Supporting Information) features a chelating phosphino-allyl ligand resulting from multiple C−H activation processes in the putative [Cp*Ru(PCy3)]+ cation.24 3a is therefore unique in the current study not only in featuring both hydride and σ-borane ligands but also in its ability to undergo dehydrogenative borylene formation. As such, it represents, to our knowledge, the first unambiguous report of the use of a sacrificial H2 acceptor in such chemistry.1,25 While there is mechanistic precedent for both (i) alkene insertion into a Ru−H bond26,27 and (ii) a subsequent Ru−C/B−H exchange step leading to alkane
*S.A.: email,
[email protected]; tel, +44 1865 285201; fax, +44 1865 272690; web, http://users.ox.ac.uk/ ∼quee1989. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the EPSRC PDRA (D.A.A.), studentships (M.J.K., I.M.R.), the National Mass Spectrometry Service Centre, and the NSERC PDF (J.I.B.).
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REFERENCES
(1) Recent reviews of borylene chemistry: (a) Braunschweig, H.; Kollann, C.; Rais, D. Angew. Chem., Int. Ed. 2006, 45, 5254. (b) Aldridge, S.; Kays, D. L. Main Group Chem. 2006, 5, 223. (c) Braunschweig, H.; Kollann, C.; Seeler, F. Struct. Bonding (Berlin) 2008, 130, 1. (d) Vidovic, D.; Pierce, G. A.; Aldridge, S. Chem. Commun. 2009, 1157. (e) Braunschweig, H.; Dewhurst, R. D.; Schneider, A. Chem. Rev. 2010, 110, 3924. (2) Apostolico, L.; Braunschweig, H.; Crawford, A. G.; Herbst, T.; Rais, D. Chem. Commun. 2008, 497. (3) Alcaraz, G.; Helmstedt, U.; Clot, E.; Vendier, L.; Sabo-Etienne, S. J. Am. Chem. Soc. 2008, 130, 12878. (4) See for example: Calimano, E.; Tilley, T. D. J. Am. Chem. Soc. 2009, 131, 11161.
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(5) O’Neill, M.; Addy, D. A.; Kelly, M.; Riddlestone, I.; Phillips, N.; Aldridge, S. J. Am. Chem. Soc. 2011, 133, 11500. (6) For a review of metal-catalyzed hydroboration chemistry see: Burgess, K.; Ohlmeyer, M. J. Chem. Rev. 1991, 91, 1179. (7) For examples of ruthenium σ-borane complexes see: (a) MontielPalma, V.; Lumbierres, M.; Donnadieu, B.; Sabo-Etienne, S.; Chaudret, B. J. Am. Chem. Soc. 2002, 124, 5624. (b) Lachaize, S.; Essalah, K.; Montiel-Palma, V.; Vendier, L.; Chaudret, B.; Barthelat, J.-C.; SaboEtienne, S. Organometallics 2005, 24, 2935. (c) Alcaraz, G.; Clot, E.; Helmstedt, U.; Vendier, L.; Sabo-Etienne, S. J. Am. Chem. Soc. 2007, 129, 8704. (d) Hesp, K. D.; Rankin, M. A.; McDonald, R.; Stradiotto, M. Inorg. Chem. 2008, 47, 7471. (e) Gloaguen, Y.; Alcaraz, G.; Vendier, L.; Sabo-Etienne, S. J. Organomet. Chem. 2009, 694, 2839. (f) Alcaraz, G.; Vendier, L.; Clot, E.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2010, 49, 918. (g) Alcaraz, G.; Chaplin, A. B.; Stevens, C. J.; Clot, E.; Vendier, L.; Weller, A. S.; Sabo-Etienne, S. Organometallics 2010, 29, 5591. (h) Vidovic, D.; Addy, D. A.; Krämer, T.; McGrady, J.; Aldridge, S. J. Am. Chem. Soc. 2011, 133, 8494. (i) Bénac-Lestrille, G.; Helmstedt, U.; Vendier, L.; Alcaraz, G.; Clot, E.; Sabo-Etienne, S. Inorg. Chem. 2011, 50, 11039. (j) MacInnis, M. C.; McDonald, R.; Ferguson, M. J.; Tobisch, S.; Turculet, L. J. Am. Chem. Soc. 2011, 133, 13622. (k) Gloguen, Y.; Alcaraz, G.; Petit, A. S.; Clot, E.; Coppel, Y.; Vendier, L.; Sabo-Etienne, S. J. Am. Chem. Soc. 2011, 133, 17232. See also: (l) Bontemps, S.; Vendier, L.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2012, 51, 1671. (8) For recent reviews of σ-borane complexes, see: (a) Perutz, R. N.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2007, 46, 2578. (b) Lin, Z. Struct. Bonding (Berlin) 2008, 130, 123. (c) Alcaraz, G.; Sabo-Etienne, S. Coord. Chem. Rev. 2008, 252, 2395. (d) Pandey, K. K. Coord. Chem. Rev. 2009, 253, 37. (e) Alcaraz, G.; Grellier, M.; Sabo-Etienne, S. Acc. Chem. Res. 2009, 42, 1640. (f) Alcaraz, G.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2010, 49, 7170. (9) (a) Tang, C. Y.; Thompson, A. L.; Aldridge, S. Angew. Chem., Int. Ed. 2010, 49, 921. (b) Tang, C. Y.; Thompson, A. L.; Aldridge, S. J. Am. Chem. Soc. 2010, 132, 10578. (c) Tang, C. Y.; Phillips, N.; Bates, J. I.; Thompson, A. L.; Gutmann, M. J.; Aldridge, S. Chem. Commun. 2012, 48, 8096. (d) Tang, C. Y.; Phillips, N.; Kelly, M. J.; Aldridge, S. Chem. Commun. 2012, 48, 11199. (10) For other examples of aminoborane complexes see refs 7f,g and: Stevens, C. J.; Dallanegra, R.; Chaplin, A. B.; Weller, A. S.; Macgregor, S. A.; Ward, B.; McKay, D.; Alcaraz, G.; Sabo-Etienne, S. Chem. Eur. J. 2011, 17, 3011. (11) For a recent review, see: Staubitz, A.; Robertson, A. P. M.; Manners, I. Chem. Rev. 2010, 110, 4079. (12) 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. (13) (a) Hartwig, J. F.; DeGala, S. R. J. Am. Chem. Soc. 1994, 116, 3661. (b) Lantero, D. R.; Motry, D. H.; Ward, D. L.; Smith, M. R., III J. Am. Chem. Soc. 1994, 116, 10811. (c) Lantero, D. R.; Ward, D. L.; Smith, M. R., III J. Am. Chem. Soc. 1997, 119, 9699. (14) Scolari, E.; Gauthier, S.; Scopelliti, R.; Severin, K. Organometallics 2009, 28, 4519. (15) Key aminoborane characterizations are as follows. H(Cl)BNiPr2: (a) Maringgele, W.; Noltemeyer, M.; Teichgraber, J.; Meller, A. Main Group Met. Chem. 2000, 23, 735. H2BNiPr2: (b) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. J. Am. Chem. Soc. 2003, 125, 9424. (c) Pasumansky, L.; Haddenham, D.; Clary, J. W.; Fisher, G. B.; Goralski, C. T.; Singaram, B. J. Org. Chem. 2008, 73, 1898. (16) E.g.: (a) Kawano, Y.; Shimoi, M. Chem. Lett. 1998, 935 (Cp*Ru(PMe3)(κ2-H3BCl), δB 37.7 ppm). (b) Hesp, K. D.; Kannemann, F. O.; Rankin, M. A.; McDonald, R.; Ferguson, M. J.; Stradiotto, M. Inorg. Chem. 2011, 50, 2431 (Cp*Ru(PiPr3)(κ2H3BMes), δB 41.8 ppm). (17) Zettler, F.; Hess, H. Chem. Ber. 1975, 108, 2269. (18) Cordero, B.; Gómez, V.; Platero-Prats, A. E.; Revés, M.; Echeverrı ́a, J.; Cremandes, E.; Barrigán, F.; Alvarez, S. Dalton Trans. 2008, 2832.
(19) Selected DFT optimized parameters relating to the heavy-atom skeleton, followed by their crystallographically determined counterparts: d(B-N) = 1.416/1.392(8) Å, d(Ru···B) = 2.305/2.264(6) Å, ∠(Ru···B-N) = 127.4/128.3(4)°, distance from the ruthenium center to the least-squares BNC2 plane 0.44/0.40 Å. (20) In common with previously reported systems,13 a third potential description for 3a,b is as a Ru(IV) boryl dihydride, [(p-cym)Ru(PR3) (H)2{B(H)NiPr2}]+. 11B NMR and Ru−B metrical data for authentic ruthenium aminoboryl systems (e.g., δB 54, 50 ppm; d(Ru−B) = 2.132(2), 2.141(2) Å for CpRu(CO)2{B(Cl)NR2}, where R = iPr, Cy, respectively) would appear to rule out such a description for 3a,b: (a) Pierce, G. A.; Vidovic, D.; Kays, D. L.; Coombs, N. D.; Thompson, A. L.; Jemmis, E. D.; De, S.; Aldridge, S. Organometallics 2009, 28, 2947. (b) Vidovic, D.; Pierce, G. A.; Coombs, N. D.; Kays, D. L.; Thompson, A. L.; Stasch, A.; Aldridge, S. Main Group Chem. 2010, 9, 57. (21) The possibility of exchange via a B−H oxidative addition/ reductive elimination sequence appears unlikely, on the basis of the faster rate of exchange observed for PPh3 (no decoalescence observed at any temperature examined) over PCy3. PCy3 would be expected to better stabilize a Ru(IV) intermediate. (22) No evidence for the hydroboration of 3,3-dimethylbutene by H2BNiPr2 could be obtained from in situ NMR monitoring even after 48 h at 60 °C in fluorobenzene solution. (23) For an early example of the hydroboration of an unsaturated C− C bond by a metal σ-borane complex, see: Hartwig, J. F.; Muhoro, C. N. Organometallics 2000, 19, 30. (24) See also: Arliguie, T.; Chaudret, B.; Jalon, F. A.; Otero, A.; Lopez, J. A.; Lahoz, F. J. Organometallics 1991, 10, 1888. (25) For an example of the use of TBE in (hydrido)iridium borane chemistry giving access to a species best described as an α-agostic hydridoboryl complex (δB 38 ppm), see ref 9b. (26) Hartwig, J. F. In Organotransition Metal Chemistry; University Science Books: Sausalito, CA, 2010. (27) For a related example of TBE insertion, see: Sewell, L. J.; Chaplin, A. B.; Weller, A. S. Dalton Trans. 2011, 40, 7499. (28) (a) Irvine, G. J.; Roper, W. R.; Wright, L. J. Organometallics 1997, 16, 2291. For Ru−C/B−H exchange in the reverse sense see: (b) Hartwig, J. F.; Bhandari, S.; Rablen, P. R. J. Am. Chem. Soc. 1994, 116, 1839−1844.
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