Further Exploring the “Sting of the Scorpion ... - ACS Publications

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Organometallics 2009, 28, 5222–5232 DOI: 10.1021/om900499v

Further Exploring the “Sting of the Scorpion”: Hydride Migration and Subsequent Rearrangement of Norbornadiene to Nortricyclyl on Rhodium(I) Nikolaos Tsoureas, Thomas Bevis, Craig P. Butts, Alex Hamilton, and Gareth R. Owen* The School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K. Received June 12, 2009

A new boron-based flexible scorpionate ligand based upon 7-azaindole, Li[Ph(H)B(azaindolyl)2] (Li[PhBai]), has been prepared. This ligand, together with the previously reported ligand K[HB(azaindolyl)3] (K[Tai]), have been used to prepare a range of monovalent group 9 transition-metal complexes. The complexes [M(COD){κ3N,N,H-Ph(H)B(azaindolyl)2}] (where M = rhodium, iridium and COD = 1,5-cyclooctadiene) and [Rh(NBD){κ3N,N,H-HB(R)(azaindolyl)2}] (where NBD = 2,5-norbornadiene and R = Ph, azaindolyl) have been prepared. Structural characterization of [M(COD){κ3NNH-Ph(H)B(azaindolyl)2}] (where M = rhodium, iridium) and [Rh(NBD){κ3N, N,H-HB(azaindolyl)3}] reveal strong interactions of the B-H functional group with the metal centers, particularly in the case of [Ir(COD){κ3N,N,H-Ph(H)B(azaindolyl)2}]. The complex [Rh(NBD){κ3N,N,H-HB(azaindolyl)3}] undergoes a further reaction, resulting from hydride migration from boron to the norbornadiene group. Subsequent rearrangement results in the formation of the rhodium-nortricyclyl complex [Rh(nortricyclyl){κ4N,N,B,N-B(azaindolyl)3}], providing the first nitrogen-based metallaboratrane complex to contain the tetradentate (κ4N,N,B,N) coordination mode.

Introduction For some time now there has been great interest in finding new methodologies for the generation of transition-metal hydrides.1 This is due to their broad reactivity, their application in homogeneous catalysis, and their potential within hydrogen storage technologies.2 Within this area there has been great interest in bonding interactions between a wide range of boron-hydrogen-containing functionalities and transition-metal centers,3 and recent discoveries have provided a number of remarkable bonding structures.4 We envisioned the role of the borohydride functional group as *To whom correspondence should be addressed. Tel: þ44 (0)117 928 7652. E-mail: [email protected]. (1) (a) Pearson, R. G. Chem. Rev. 1985, 85, 41. (b) McGrady, G. S.; Guilera, G. Chem. Soc. Rev. 2003, 32, 383. (c) del Rosal, I.; Maron, L.; Poteau, R.; Jolibios, F. Dalton Trans. 2008, 3959. (2) Kubas, G. J. Chem. Rev. 2007, 107, 4152. (3) (a) Hartwig, J. F.; De Gala, S. R. J. Am. Chem. Soc. 1994, 116, 3661. (b) Hartwig, J. F.; Muhoro, C. N.; He, X.; Eisenstein, O.; Bosque, R.; Maseras, F. J. Am. Chem. Soc. 1996, 118, 10936. (c) Muhoro, C. N.; Hartwig, J. F. Angew. Chem. 1997, 109, 1536; Angew. Chem., Int. Ed. Engl., 1997, 36, 1510. (d) Muhoro, C. N.; He, X.; Hartwig, J. F. J. Am. Chem. Soc. 1999, 121, 5033. (e) Schlecht, S.; Hartwig, J. F. J. Am. Chem. Soc. 2000, 122, 9435. (f) Montiel-Palma, V.; Lumbierres, M.; Donnadieu, B.; Sabo-Etienne, S.; Chaudret, B. J. Am. Chem. Soc. 2002, 124, 5624. (g) Lachaize, S.; Essalah, K.; Montiel-Palma, V.; Vendier, L.; Chaudret, B.; Barthelat, J.-C.; Sabo-Etienne, S. Organometallics 2005, 24, 2935. (h) Crestani, M. G.; Mu~ noz-Hernandez, M.; Arívalo, A.; Acosta-Ramírez, A.; García, J. J. J. Am. Chem. Soc. 2005, 127, 18066. (i) Douglas, T. M.; Chaplin, A. B.; Weller, A. S. J. Am. Chem. Soc. 2008, 130, 14432. (j) Alcaraz, G.; Clot, E.; Helmstedt, U.; Vendier, L.; Sabo-Etienne, S. J. Am. Chem. Soc. 2007, 129, 8704. (4) Braunschweig, H.; Dewhurst, R. D. Angew. Chem., Int. Ed. 2009, 48, 1893. pubs.acs.org/Organometallics

Published on Web 08/12/2009

a “hydride source” following the discovery of the “metallaboratrane” complexes by Hill in 1999 (Figure 1).5 (5) Hill, A. F.; Owen, G. R.; White, A. J. P.; Williams, D. J. Angew. Chem., Int. Ed. 1999, 38, 2759. (6) (a) Kuzu, I.; Krummenacher, I.; Armbruster, F.; Breher, F. Dalton Trans. 2008, 5836. (b) Fontaine, F.-G.; Boudreau, J.; Thibault, M.-H. Eur. J. Chem. 2008, 5439. (c) Hill, A. F. Organometallics 2006, 25, 4743. (d) Parkin, G. Organometallics 2006, 25, 4744. (e) Braunschweig, H.; Kollann, C.; Rais, D. Angew. Chem., Int. Ed. 2006, 45, 5254. (7) (a) Westcott, S. A.; Marder, T. B.; Baker, R. T.; Harlow, R. L.; Calabrese, J. C.; Lam, K. C.; Lin, Z. Polyhedron 2004, 23, 2665. (b) Curtis, D.; Lesley, M. J. G.; Norman, N. C.; Orpen, A. G.; Starbuck, J. J. Chem. Soc., Dalton Trans. 1999, 1687. (c) Braunschweig, H.; Radacki, K.; Rais, D.; Whittell, G. R. Angew. Chem., Int. Ed. 2005, 44, 1192. (d) Fehlner, T. P. Angew. Chem., Int. Ed. 2005, 44, 2056. (8) (a) Bontemps, S.; Gornitzka, H.; Bouhadir, G.; Miqueu, K.; Bourissou, D. Angew. Chem., Int. Ed. 2006, 45, 1611. (b) Bontemps, S.; Bouhadir, G.; Miqueu, K.; Bourissou, D. J. Am. Chem. Soc. 2006, 128, 12056. (c) Sircoglou, M.; Bontemps, S.; Mercy, M.; Saffon, N.; Takahashi, M.; Bouhadir, G.; Maron, L.; Bourissou, D. Angew. Chem., Int. Ed. 2007, 46, 8583. (d) Bontemps, S.; Bouhadir, G.; Gu, W.; Mercy, M.; Chen, C.-H.; Foxman, B. M.; Maron, L.; Ozerov, O. V.; Bourissou, D. Angew. Chem., Int. Ed. 2008, 47, 1481. (e) Sircoglou, M.; Bontemps, S.; Bouhadir, G.; Saffon, N.; Miqueu, K.; Gu, W.; Mercy, M.; Chen, C.-H.; Foxman, B. M.; Maron, L.; Ozerov, O. V.; Bourissou, D. J. Am. Chem. Soc. 2008, 130, 16729. (9) (a) Foreman, M. R. St.-J.; Hill, A. F.; Owen, G. R.; White, A. J. P.; Williams, D. J. Organometallics 2003, 22, 4446. (b) Foreman, M. R. St.-J.; Hill, A. F.; White, A. J. P.; Williams, D. J. Organometallics 2004, 23, 913. (c) Crossley, I. R.; Hill, A. F. Organometallics 2004, 23, 5656. (d) Crossley, I. R.; Foreman, M. R. St-J.; Hill, A. F.; White, A. J. P.; Williams, D. J. Chem. Commun. 2005, 221. (e) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2005, 24, 1062. (f) Crossley, I. R.; Hill, A. F.; Humphrey, E. R.; Willis, A. C. Organometallics 2005, 24, 4083. (g) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2006, 25, 289. (h) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2007, 26, 3891. (i) Crossley, I. R.; Hill, A. F. Dalton Trans. 2008, 201. (j) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2008, 27, 312. (k) Crossley, I. R.; Foreman, M. R. St.-J.; Hill, A. F.; Owen, G. R.; White, A. J. P.; Williams, D. J.; Willis, A. C. Organometallics 2008, 27, 381. r 2009 American Chemical Society

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Figure 1. Metallaboratrane complex [M{κ4S,S,B,S-B(mt)3L0 L] (1; N-S = mt).

Figure 3. Complexes [Rh(COD){κ3N,N,H-HB(azaindolyl)3] (2) and [Ir(COD){κ3N,N,H-HB(azaindolyl)3] (3).

Figure 2. Reversible migration of “hydride” between boron and metal centers (N N represents the 7-azaindole rings).

Scheme 1. Hydride Migration Induced by Addition of Carbon Monoxide

-

Metallaboratrane complexes, which feature a transitionmetal-to-boron dative interaction, have since attracted much attention.6,7 The originally postulated σ-acceptor coordination within these compounds has been called into question and is currently a matter of debate.6c,d This debate has been fueled by the unexpected geometries of some of the resulting compounds.8c The majority of the reported examples have been based on Tm [HB(mt)3] or similar derivatives.9,10 Moreover, recent examples by Bourissou8 based on phosphane amphiphilic ligands have augmented the field by providing further insight into the metal-boron bonding interaction. Additionally we have very recently reported the first examples to be supported by a flexible ligand containing nitrogen donors.11 Our focus lies in metal-borane complexes, since the boron atom could potentially be utilized as a “hydride store” and could have a number of catalytic applications (Figure 2). Both Hill9i and ourselves12 have provided evidence for reversible hydride migration between boron and transitionmetal centers. As part of our investigations we have recently reported the coordination chemistry of the relatively unexplored11,13-16 flexible tridentate nitrogen scorpionate ligand hydrotris(7-azaindolyl)borate (Tai).14 We further reported that this nitrogen based ligand, a more flexible analogue of Trofimenko’s original scorpionate ligand (Tp),17 could be utilized to provide facile hydride migration leading to the generation of a new family of nitrogen based metallaboratrane complexes.11 Our studies with this ligand led to the synthesis of [M(COD){κ3N,N,H-HB(azaindolyl)3}] (M = Rh (2), Ir (10) (a) Landry, V. K.; Melnick, J. G.; Bucella, D.; Pang, K.; Ulichny, J. C.; Parkin, G. Inorg. Chem. 2006, 45, 2588. (b) Pang, K.; Quan, S. M.; Parkin, G. Chem. Commun. 2006, 5015. (c) Mihalcik, D. J.; White, J. L.; Tanski, J. M.; Zakharov, L. N.; Yap, G. P. A.; Incarvito, C. D.; Rheingold, A. L.; Rabinovich, D. Dalton Trans. 2004, 1626. (d) Blagg, R. J.; Charmant, J. P. H.; Connelly, N. G.; Haddow, M. F.; Orpen, A. G. Chem. Commun. 2006, 2350. (e) Figueroa, J. S.; Melnick, J. G.; Parkin, G. Inorg. Chem. 2006, 45, 7056. (11) Tsoureas, N.; Haddow, M. F; Hamilton, A; Owen, G. R. Chem. Commun. 2009, 2538. (12) Rudolf, G. C.; Hamilton, A.; Orpen, A. G.; Owen, G. R. Chem. Commun. 2009, 553. (13) Tsoureas, N.; Owen, G. R.; Hamilton, A.; Orpen, A. G. Dalton Trans. 2008, 6039. (14) Song, D.; Jia, W. L.; Wu, G.; Wang, S. Dalton Trans. 2005, 433. (15) Saito, T.; Kuwata, S.; Ikariya, T. Chem. Lett. 2006, 35, 1224. (16) Wagler, J.; Hill, A. F. Organometallics 2008, 27, 2350. (17) (a) Trofimenko, S. In Scorpionates: The Coordination of Poly(pyrazolyl)borate Ligands,; Imperial College Press: London, 1999. (b) Trofimenko, S. Chem. Rev. 1993, 93, 943. (c) Trofimenko, S. Polyhedron 2004, 23, 197.

(3); COD = 1,5-cyclooctadiene) and their application as catalysts for the transfer hydrogenation of ketones.14 In an effort to explore the concept further, we began to investigate the effect of changing one of the substituents at boron and, in addition, of varying the coligands at the metal center. During these investigations, it was found that hydride migration is dependent on both of these factors. We therefore wish to report the synthesis and characterization of group 9 transition-metal complexes containing 7-azaindole-based ligands together with the results of our investigations exploring the propensity of hydride migration between boron and transition-metal centers.

Results and Discussion We have recently reported the synthesis and catalytic investigations of rhodium and iridium cyclooctadiene complexes, containing Tai.14 The isostructural complexes 2 and 3 were found to bind to the metal centers with a κ3N,N,H coordination mode (Figure 3). Both complexes showed fluxional behavior at room temperature due to the rapid exchange between the free and coordinated azaindole rings. Within these examples, there was no evidence to indicate that hydride migration between the boron and the metal centers occurred even at elevated temperatures (up to 80 °C). It was shown, however, that hydride migration can be induced by disturbing the coordination sphere at the metal center.11 Addition of carbon monoxide to a solution of 3 results in the loss of one of the double bonds of the cyclooctadiene moiety from the coordination sphere, followed by subsequent hydride migration from boron to the metal center and then insertion of the olefin into the resulting iridium-hydride bond. This led to the isolation of the square-based-pyramidal complex Ir(CO)(C8H13){κ3N,N,BB(azaindolyl)3} (5) via the dicarbonyl complex 4 (Scheme 1). In contrast to the case for the phosphine- and sulfur-based systems, we had not observed the tetradentate coordination mode (κ4N,N,B,N) for the azaindole-based ligands and had attributed this to the rigid structure provided by the fused bicyclic ring system of this heterocycle. With this in mind, it

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Figure 4. Monosubstituted borate ligand Na[(7-azain-7)BH3] (6). Scheme 2. Synthesis of Lithium Hydrophenylbis(azaindole)borate (7, Li[PhBai])

was of interest to prepare the corresponding bis-substituted compound [H2B(azaindolyl)2]- (Bai). Hill and Wagler have recently reported the synthesis of a monosubstituted borate anion (6) together with some preliminary complexes.16 In this case, the 7-azaindole unit was found to bind to the boron atom via the pyridine nitrogen following tautomerization of the azaindole group (Figure 4). Our attempts to synthesize and isolate the bis-substituted ligand Bai, however, proved unsuccessful, leading to mixtures of the two possible bis-substituted products (bound via both pyridine and pyrrole nitrogen atoms) and small quantities of Tai. Although the target compound Na[Bai] was found to be the major component (approximately >80% as determined by 11B{1H} NMR), it was not possible to separate it from the other components in the mixture.16 We therefore targeted an analogous compound which also contained two azaindole units.18 The lithium salt Li[Ph(H)B(azaindolyl)2] (7, Li[PhBai] could be readily synthesized by reaction of Li[PhBH3] 3 2THF (see the Experimental Section) with an excess (2.4 equiv) of 7-azaindole (Scheme 2). These two components were heated together in toluene at 120 °C, and the progress of the reaction was monitored by 11B{1H} NMR spectroscopy. The reaction was complete within a period of 48 h, and a single peak at -6.9 ppm (hhw 132 Hz) was observed in the boron NMR spectrum. A protoncoupled boron (11B) NMR experiment was carried out in order to confirm the reaction of two azaindole units at the boron center. The experiment was not conclusive, since the signal was not fully resolved. Nevertheless, the signal had broadened with respect to the boron-decoupled experiment (hhw 198 Hz), suggesting coupling to a single adjacent proton. Filtration of the reaction mixture while hot and cooling at -30 °C overnight yielded the desired borate ligand as a crystalline solid in moderate yield. The spectroscopic and analytical data were consistent with the formation of Li[PhBai]. Confirmation of bis-substitution was obtained from the 1H{11B} NMR spectrum, which revealed a peak at 5.68 ppm (δ C7D8) integrating for one proton and was assigned as the hydrogen of the B-H group. This was not observed in the standard 1H spectrum, as a result of quadrupolar broadening from the boron atom. The infrared spectrum of 7 showed a band at 2196 cm-1, which corresponded (18) Garcia, R; Paulo, A.; Domingos, A.; Santos, I. J. Organomet. Chem. 2001, 632, 41.

Figure 5. Molecular structure of [Li{Ph(H)B(azaindolyl)2}]2. Hydrogen atoms, with the exception of H100, have been omitted for clarity (thermal ellipsoids are drawn at the 50% probability level). Selected bond lengths (A˚) and angles (deg): B(1)-H(100) = 1.20(3), Li(1)-H(100) = 1.90(3), Li(1i)-H(100) = 2.08(3), N(1)-B(1)=1.541(4), N(2)-Li(1)=1.984(6); N(1)-B(1)-H(100) = 103.6(16), Li(1i)-B(1)-H(100) = 52.0(16), N(2)-Li(1)-H(100) = 85.0(10), B(1i)-Li(1)-H(100) = 92.2(10), H(100i)-Li(1)-H(100) = 65.1(17), N(2)-Li(1)-H(100i) = 138.8(10), B(1)-H(100)-Li(1) = 140(2), B(1)-H(100)-Li(1i) = 100.9(19), N(1)-B(1)-Li(1) = 88.7(3), Li(1)-H(100)-Li(1i) = 111.03(5).

to the B-H group, the low stretching frequency of this band suggesting a strong interaction of the B-H group with the lithium center. The 7Li{1H} (s, 4.8 ppm (δ C7D8)/s, 5.1 ppm (δ CDCl3)) and 13C{1H} NMR spectra were also consistent with the formation of Li[PhBai], although in the latter the phenyl aromatic carbon ipso to the boron center could not be located even when longer acquisition times or relaxation delays were employed. The formation of 7 was further confirmed by a singlecrystal diffraction study (Figure 5). The molecular structure in the solid state reveals a dimeric structure involving two lithium centers and two PhBai moieties. One of the azaindole units from each ligand is connected to a lithium atom, and the B-H unit from each ligand bridges the two lithium centers (Li-H distances 1.90(3) and 2.08(3) A˚). The short lithium-hydrogen distances confirm the moderately strong interactions of the B-H group with the metal centers. Additionally, there is a short distance between the ipso carbon of the boron-bound phenyl group and a lithium center (2.546(7) A˚). The calculated molecular structure also confirmed that the azaindole units were connected to boron center via the “pyrrole” nitrogen atoms. A similar dimeric motif has also been observed in the solid-state structure of K[Tai].14 When the reaction was repeated on a larger scale, full consumption of Li[PhBH3] 3 2THF was observed after a period of 48 h. On this occasion after the same workup (see the Experimental Section), the 11B{1H} NMR (δ CDCl3) spectrum consisted of two new signals at -8.8 and -3.7 ppm. A 11B NMR experiment indicated that both these peaks corresponded to species containing one B-H bond with 1JBH coupling constants of 63.5 Hz (-8.8 ppm) and 89.4 Hz (-3.7 ppm). The difference in the B-H coupling constants for these species suggests an interaction of the B-H group with

Article Scheme 3. Synthesis of [Rh(COD){Ph(H)B(azaindolyl)2}] (8) and [Ir(COD){Ph(H)B(azaindolyl)2}] (9)

the metal center in the former species and no interaction in the latter. The 7Li{1H} NMR (δ CDCl3) spectrum also consisted of two peaks at 5.8 and 4.1 ppm. The 1H NMR (δ CDCl3) spectrum revealed a similar pattern, with some differences in the chemical shifts in addition to the presence of two THF molecules per molecule of ligand. Furthermore, the signal corresponding to the BH proton had shifted upfield to 4.91 ppm, suggesting a different chemical environment for this group. We tentatively assigned these differences observed in the 11B{1H} and 7Li{1H} NMR spectra to the THF solvent perturbing the B-H-Li interaction by coordination to the lithium center. N€ oth has previously shown that both 11B and 7Li NMR chemical shifts are highly dependent on the number of coordinated etherate solvents within lithium hydroborate compounds.19 In order to confirm this, a CDCl3 solution of crystalline material synthesized previously was prepared and the 1H NMR, 11B{1H} NMR (-7.0 ppm), and 7Li{1H} NMR (5.1 ppm) spectra were recorded. Upon addition of a few drops of THF, spectra similar to those described above were observed, confirming our assumption. We were unable to further verify this by a crystallographic study, despite repeated attempts to obtain single crystals. Once the identity of the new ligand (PhBai) was confirmed, we investigated its coordination chemistry with group 9 transition-metal complexes containing diene coligands. The complexes [Rh(COD){Ph(H)B(azaindolyl)2}] (8) and [Ir(COD){Ph(H)B(azaindolyl)2}] (9) were isolated, as orange microcrystalline powders, employing a synthetic methodology analogous to that for the preparation of 2 and 3 (Scheme 3).14 The compounds were fully characterized by spectroscopic and analytical methods (Table 1). The 11B{1H} NMR spectra revealed single peaks at -4.6 ppm (hhw 123 Hz) and -0.3 ppm (hhw 86 Hz) for 8 and 9, respectively (downfield from the ligand (7), -6.9 ppm). A 11B NMR experiment resulted in further broadening of the signals observed for 8 and 9 (hhw = 180 and 156 Hz, respectively), suggesting coupling to an adjacent proton. The BH resonances were located as broad signals at 4.21 ppm (δ C6D6) for 8 and 3.51 ppm (δ C6D6) for 9 in the 1H{11B} NMR spectra. The upfield shift of this signal in comparison to that for the free ligand (1H{11B}, δ(C7D8) 5.68 ppm) is suggestive of a strong interaction of the BH groups with the metal centers. This was also confirmed by the IR spectra of 8 and 9 in the solid state (2117 and 2004 cm-1, respectively) and in solution (2088 and 1989 cm-1), both indicating a κ3N,N,H coordination which is retained in solution (Table 1). The 1H and 13C{1H} NMR data for both 8 and 9 were consistent with the formation of [Rh(COD){κ3N,N,H-Ph(19) (a) Giese, H.-H.; N€ oth, H.; Schwenk, H.; Thomas, S. Eur. J. Inorg. Chem. 1998, 941. (b) Knizek, J.; N€oth, H. J. Organomet. Chem. 200, 614-615, 168.

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(H)B(azaindolyl)2}] and [Ir(COD){κ3N,N,H-Ph(H)B(azaindolyl)2}], respectively. In contrast to the analogous complexes 2 and 3, the 1H NMR spectra of 8 and 9 showed no fluxional behavior at room temperature due to the PhBai ligand scaffold not having a free N-donor coordinating moiety available to exchange with coordinated azaindolyl rings. This observation also suggests that the coordination of the B-H unit to the metal center is not fluxional. The 1H NMR spectra of 8 and 9 consisted of eight signals between 6.25 and 8.55 ppm, corresponding to the 5 azaindole ring proton environments (integrating for 10 protons) and 3 environments of the phenyl ring (integrating for 5 protons), indicating a Cs symmetry. The 13C{1H} NMR spectra of 8 and 9 showed 10 signals in the aromatic region, 7 corresponding to the azaindole ring carbons and 3 corresponding to the phenyl ring (the resonance of the aromatic carbon ipso to the boron center could not be located, even when longer acquisition times or relaxation delays were employed). The geometries of complexes 8 and 9 were further confirmed by X-ray diffraction studies on single crystals obtained by cooling their pentane solutions at -30 °C (Figures 6 and 7, respectively). Complexes 8 and 9 are isostructural (and their crystals are isomorphous). Selected bond lengths and angles are highlighted in Table 2. Both complexes adopt a square-pyramidal geometry with one COD ligand, two azaindolyl moieties coordinating through the nitrogen atom of the pyridine heterocycle, and a B-H interaction at the apical site on the metal. The M-H bond distances in 8 and 9 are 2.00(4) and 1.78(4) A˚, respectively. Even though these are similar (within esds) to the corresponding M-H distances found in the analogous complexes 2 and 3, there appears to be a greater difference between the distance found in the case of complex 8 as compared to that found in 9. The elongation of the M-H bond distances in 8 and 9 are consistent with the type 1 (for 9) and type 2 (for 8) categorizations of 3c-2e M 3 3 3 H-B bonding interactions proposed byP Spicer and Reglinski,20 using their criteria of d(M 3 3 3 H) vs rcov (rcov = sum of the covalent radii of the metal center and hydrogen).20b Accordingly, the Ir 3 3 3 H bond distance inP9 is 1.78(4) A˚ and is therefore 0.06 A˚ greater than the rcov value, indicating a significant covalent interaction with the metal center (type 1). This is further reflected by a parallel elongation of the B-H distance in 9. In the case of P 8, the Rh 3 3 3 H distance is 2.00(4) A˚, 0.27 A˚ greater than the rcov value, consistent ˚ with P the type 2 category (where 0.25 A < d(M 3 3 3 .H) - rcov < 0.75 A˚). The B-H stretching frequencies in their infrared spectra also reflect the structural data (cf. 2117 cm-1 (8) and 2004 cm-1 (9)), suggesting a marked reduction in the bond order of the B-H bond in 9 compared to 8. This is further supported by the chemical shift of the BH resonances described above (4.21 ppm (8) and 3.51 ppm (9)). There also appears to be a significant difference between the longest metal-nitrogen bond distances for complex 9 and the previously reported complex 3 (cf. 2.138(3) A˚ in 9 vs 2.163(3) A˚ in 3), which is not observed in rhodium examples 2 and 8. Our studies with the new ligand architecture revealed no evidence to suggest that hydride migration was occurring in these complexes. We have recently shown that hydride (20) (a) Spicer, M. D.; Reglinski, J. Eur. J. Inorg. Chem. 2009, 1553. Covalent radii were taken from: (b) Cordero, B.; Gomez, V.; Platero-Prats, A. E.; Reves, M.; Echeverría, J.; Cremades, E.; Barragan, F.; Alvarez, S. Dalton Trans. 2008, 2832.

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Table 1. 11B{1H} NMR Data for Scorpionate Complexes of 7-Azaindole-Based Ligands 11

B{1H} NMR

complex

δ (ppm)a

[Rh(COD)(κ3N,N,H-Tai)] (2) [Rh(NBD)(κ3N,N,H-Tai)] (10) [Ir(COD)(κ3N,N,H-Tai)] (3) [Rh(COD)(κ3N,N,H-PhBai)] (8) [Rh(NBD)(κ3N,N,H-PhBai)] (12) [Ir(COD)(κ3N,N,H-PhBai)] (9)

-2.2 -1.6b, -1.5c 2.1 -4.6 -3.5 -0.3

a

JB-H (Hz)

IR (B-H) power film (cm-1)

ref

78 78 59 unresolvedd unresolvedd unresolvedd

2106 2015 2102 2117 2022 2004

14 this work 14 this work this work this work

1

In C6D6 unless otherwise stated. b In C7D8. c In CD2Cl2. d The B-H coupling constant was unresolved due to the broadness of the signal.

Table 2. Comparison of Structural Data for Complexes 2, 3, 8, and 9 bonding distance (A˚)/ angle (deg) b

M-N M-Nc M-H B-H N-M-N B-H-M N-M-Hb N-M-Ha N-B-N

2a

3a

8

9

2.159(2) 2.134(2) 1.99(2) 1.17(3) 89.24(8) 138.9(18) 83.4(7) 80.7(4) 110.7(2)

2.163(3) 2.125(3) 1.87(4) 1.25(5) 89.06(11) 139.9(3) 85.9(13) 80.5(14) 111.7(3)

2.159(3) 2.139(3) 2.00(4) 1.15(4) 89.22(11) 143(5) 85.4(11) 82.4(11) 109.8(3)

2.138(3) 2.119(3) 1.78(4) 1.281(19) 89.18(12) 136(3) 85.2(19) 82.7(19) 109.9(3)

a Reference 14. b Longest M-N distance within structure. c Shortest M-N distance within structure.

Table 3. 11B{1H} NMR Data for Scorpionate and Metallaboratrane Complexes of 7-Azaindole-Based Ligands complex

Figure 6. Molecular structure of 8. Hydrogen atoms, with the exception of H100, have been omitted for clarity (thermal ellipsoids are drawn at the 50% probability level).

[Rh(C7H9){κ4N,N,B,N-B(azaindolyl)3}] (11) Ir(CO)2(C8H13){κ3N,N,B-B(azaindolyl)3}]b Ir(CO)(C8H13){κ3N,N,B-B(azaindolyl)3}]b Ir(CO)(CNC8H9)(C8H13){κ3N,N,B-B(azaindolyl)3}]b Ir(CO)(CNC4H9)(C8H13){κ3N,N,B-B(azaindolyl)}]b a

Figure 7. Molecular structure of 9. Hydrogen atoms, with the exception of H100, have been omitted for clarity (thermal ellipsoids are drawn at the 50% probability level).

migration was prompted when complex 3 was exposed to a carbon monoxide atmosphere. We wondered whether hydride migration could be facilitated by the use of a strained olefin such as 2,5-norbornadiene in place of 1,5-cyclooctadiene. In an attempt to test this, the rhodium complex

11

B{1H} (ppm)a

5.0 5.8 -9.3 4.3 3.7

In C7D8 solvent. b Reference 11.

[RhCl(NBD)]2 (NBD = bicyclo[2.2.1]hepta-2,5-diene) was reacted with 2 molar equiv of K[Tai] and Li[PhBai].21 Two equivalents of K[Tai] was added to 1 equiv of [Rh(NBD)Cl]2 in THF at -90 °C; the resulting mixture was warmed to room temperature, during which time the color of the solution changed from yellow to orange. A 11 B{1H} NMR spectrum of the reaction mixture revealed two signals at -1.6 and 5.0 ppm (THF, unlocked). The former peak was in the region expected for the formation of [Rh(NBD){κ3N,N,H-HB(azaindolyl)3}] (10) with a 1JBH coupling constant of 78 Hz, as obtained from the 11B NMR experiment (cf. [Rh(COD){κ3N,N,H-HB(azaindolyl)3}] (2): δ(C6D6) 11B{1H} -2.2 ppm, 1JBH=78 Hz) (see also Table 1). The signal at 5.0 ppm in the 11B{1H} NMR experiment appeared as a doublet (J = 14 Hz, partially resolved hhw 25 Hz), suggesting coupling of the boron atom to the rhodium center. Furthermore, the boron-coupled experiment (11B) of this mixture revealed no changes for the signal at 5.0 ppm, indicating that a direct B-H bond was absent in this species. This chemical shift is also similar to our (21) (a) Bucher, U. E.; Currao, A.; Nesper, R.; R€ uegger, H.; Venanzi, L. M.; Younger, E. Inorg. Chem. 1995, 34, 66. (b) Sanz, D.; Santa María, M. D.; Claramunt, R. M.; Cano, M.; Heras, J. V.; Campo, J. A.; Ruíz, F. A.; Pinilla, E.; Monge, A. J. Organomet. Chem. 1996, 526, 341. (c) Akita, M.; Ohta, K.; Takahashi, Y.; Hikichi, S.; Moro-oka, Y. Organometallics. 1997, 16, 4121. (d) Adams, C. J.; Anderson, K. M.; Charmant, J. P. H.; Connelly, N. G.; Field, B. A.; Hallett, A. J.; Horne, M. Dalton Trans. 2008, 2680.

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Scheme 4. Synthesis of 10 and Subsequent Conversion to Complex 11

previously reported octahedral iridaboratranes containing the κ3N,N,B coordination mode for Tai (Table 3). We were therefore intrigued to discover whether the signal at 5.0 ppm also corresponded to a product resulting from hydride migration. The reaction mixture was heated to 65 °C, and the 11B{1H} spectrum was recorded at regular intervals. After 1 h, the 11B{1H} NMR spectrum revealed 97% conversion to the product (11), corresponding to the peak at 5.0 ppm, while no further conversion occurred over longer periods of heating or when higher temperatures were employed (Scheme 4). Evaporation of all volatiles followed by extraction in Et2O, filtration through diatomaceous earth, and removal of diethyl ether gave a mixture of 10 (97%), in a good yield (82%). Unfortunately, it was not possible to isolate 11 in spectroscopically pure form due to the contamination with traces of complex 10. Further spectroscopic analysis was carried out on the isolated solid in order to confirm the identity of species 11. The 11B NMR and 11B{1H} NMR (δ C7D8) spectra of the isolated solid were identical with those of the reaction mixture discussed above. The broad nature of the signals in the boron spectra typically precludes the resolution of the rhodium-boron coupling in the vast majority of complexes containing a rhodium-boron bond. The coupling constant only becomes resolved when the boron center is within a highly symmetrical environment, where the line width of the peak becomes small.22f There has been only one rhodium-borane complex where the 11B{1H} NMR signal was sufficiently resolved to observe a 1JRhB coupling constant of 80 Hz.10d It is unclear why the rhodium-boron coupling constant is so small in the case of 11. The infrared spectrum of the isolated solid showed no observable bands in the region expected for B-H or Rh-H functional groups. Furthermore, the 1H NMR spectrum (δ C7D8) confirmed the absence of metal hydrides, since no signals could be located in the upfield region of the spectrum. The 1H NMR spectrum of 11 revealed 11 signals in the downfield region of the spectrum, 7 signals integrating for 1 proton each and 4 other signals integrating for 2 protons (resulting from overlap of 2 independent signals, respectively, and confirmed by 1H/13C{1H} correlation (22) (a) Braunschweig, H. Angew. Chem., Int. Ed. 1998, 37, 1786. (b) Irvine, G. I.; Lesley, M. J. G.; Marder, T. B.; Norman, N. C.; Rice, C. R.; Robins, E. G.; Roper, W. R.; Whittell, G. R.; Wright, L. J. Chem. Rev. 1998, 98, 2685. (c) Smith, M. R.III Prog. Inorg. Chem. 1999, 48, 505. (d) Braunschweig, H.; Colling, M. Coord. Chem. Rev. 2001, 223, 1. (e) Aldrich, S.; Coombs, D. L. Coord. Chem. Rev. 2004, 248, 535. (f) Khattar, R.; Puga, J.; Fehlner, T. P. J. Am. Chem. Soc. 1989, 111, 1877.

experiments), indicating 3 different environments for the azaindole rings. This was also verified by the presence of 21 signals in the aromatic region of the 13C{1H} NMR spectrum. The 1H NMR spectrum also showed the absence of (un)coordinated alkene protons (except for signals showing 3% of 10), while the alkyl region of the spectrum consisted of broad peaks at 2.17, 1.77, 1.58, 1.24, 1.10, 0.88, and -0.49 ppm (δ C7D8) integrating in total for 9 protons. The same region in the 13C{1H} NMR spectrum consisted of 7 signals, 2 of which were determined to be methylene groups and the other 5 as methyne by a DEPT-135 experiment. This evidence led us to the conclusion that the hydride moiety had migrated from the boron center onto the former norbornadiene group. However, the absence of olefinic protons indicated that a further transformation had occurred. The unusual rearrangement of the norbornadiene ligand to a nortricyclyl group was further confirmed by COSY, HMQC, and HMBC correlation experiments, which agree with the assignment presented in Figure 8, and a comparable rearrangement has previously been reported.23 Our assignment of the stereochemistry of the nortricyclyl moiety was further confirmed by 1D-NOESY spectroscopy, specifically NOE interactions between protons Ha and Hc, as well as Hi and the aromatic He proton of the azaindole heterocycle (Figure 12). Of particular note in the 13C{1H} NMR spectrum of 11 was the presence of a doublet at 37.3 ppm corresponding to a CH with a 1JRhC coupling constant of 28 Hz. This value is in agreement with the previously reported examples.23 In order to confirm our assignment, an X-ray crystallography study was carried out. Single colorless crystals were obtained from a saturated pentane solution. The calculated molecular structure of the resulting crystals is shown in Figure 9. Crystallographic parameters are given below. Complex 11 adopts a distorted-square-pyramidal geometry with all three azaindolyl moieties coordinating through the nitrogen atom of the pyridine heterocycles, a boron atom (at the apical site), and one nortricyclyl group (Figure 9). There is no ligand in the site trans to boron. Complex 11 represents the first example of a κ4N,N,B,N coordination mode for this flexible scorpionate ligand. In all previously reported examples, only two out of the possible three azaindole rings coordinated to the metal center in the metallaboratrane (23) (a) Mata, J. A.; Peris, E.; Incarvito, C.; Crabtree, R. H. Chem. Commun. 2003, 184. (b) Dotra, R.; Konstantinovski, L.; Shimon, L. J. W.; Ben-David, Y.; Milstein, D. Eur. J. Inorg. Chem. 2003, 70. (c) Mail, R. E.; Garralda, M. A.; Hernandez, R.; Ibarlucea, L.; Pinilla, E.; Torres, M. R.; Zarandona, M. Eur. J. Inorg. Chem. 2005, 1671. (d) Trauthwein, H.; Tillack, A.; Beller, M. Chem. Commun. 1999, 2029.

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Figure 8. 1H and 13C{1H} NMR assignments of the nortricyclyl group within complex 11.

Tsoureas et al.

Figure 10. Molecular structure of 10. Hydrogen atoms (with the exception of H100) and two disordered molecules of toluene have been omitted for clarity (thermal ellipsoids are drawn at the 50% probability level). Selected bond lengths (A˚) and angles (deg): Rh(1)-N(1)=2.160(3), B(1)-H(100)=1.15(5), Rh(1)-H(100)=1.98(5); N(1i)-Rh(1)-N(1)=93.93(14), N(1)-Rh(1)-H(100) = 82.6(10), N(2)-B(1)-N(2i) = 108.9(4).

Figure 11. Change in coordination mode for Tai induced by carbon monoxide. Figure 9. Molecular structure of 11. Hydrogen atoms have been omitted for clarity (thermal ellipsoids are drawn at the 50% probability level). Selected bond lengths (A˚) and angles (deg): Rh(1)-N(5) = 2.057(3), Rh(1)-N(1)=2.063(3), Rh(1)-B(1)= 2.064(4), Rh(1)-Cc = 2.135(11), Rh(1)-N(3) = 2.186(3); N(5)-Rh(1)-B(1) = 83.41(15), N(1)-Rh(1)-B(1) = 81.92(15), N(5)-Rh(1)-N(3)=91.64(11), N(1)-Rh(1)-N(3)=94.30(11), B(1)-Rh(1)-N(3) = 84.53(14), Cc-Rh(1)-N(3) = 178.0(3), N(2)-B(1)-N(6) = 121.0(3), N(2)-B(1)-N(4) = 108.3(3), N(6)-B(1)-N(4) = 111.0(3), N(2)-B(1)-Rh(1) = 105.7(3), N(6)-B(1)-Rh(1) = 104.8(3), N(4)-B(1)-Rh(1) = 104.9(2), N(5)-Rh(1)-N(3)=91.64(11), N(1)-Rh(1)-N(3)=94.30(11), B(1)-Rh(1)-N(3) = 84.53(14), N(5)-Rh(1)-Cc = 90.3(3), N(1)-Rh(1)-Cc = 83.9(3), B(1)-Rh(1)-Cc = 96.0(4), N(5)-Rh(1)-N(1)=163.56(12). The carbons of the nortricyclyl moiety are named after the protons attached to them, as shown in Figure 8.

compounds.11 We had attributed this fact to the rigid nature of the fused bicyclic aromatic rings. Complex 11 highlights that the tetradentate coordination mode is indeed possible, although there appears to be some strain associated with its coordination. The N(6)-B(1)-N(2) angle is 121.0(3)°, representing a considerable distortion with respect to the idealized angle expected for a tetrasubstituted boron atom (cf. those found in K[Tai]; 108.66(17), 108.92(17), and 109.32(17)°).14 This angle is also significantly larger than those found in the sulfur-based rhodaboratranes reported by Hill9d,g and [Rh{B(taz)3}(CO)(PPh3)]PF6 (12; taz = 4-ethyl-3-methyl1H-1,2,4-triazole-5(4H)-thione) reported by Connelly10d

Figure 12. Labeling scheme for compounds described herein.

(which range between 105.61(6) and 115.5(4)°). This strained angle is accompanied by a displacement of the boron atom from the axial site (Cc-Rh(1)-B(1) = 96.0(4)°). Such a correlation between these two angles can also be seen in a previous example reported by Hill in [RhCl{B(mt)3(PPh3)] (13; Cl trans to Rh-B) where an even larger distortion is observed (P-Rh1-B = 98.27(16)°).9d Significant distortion is also observed in the plane of complex 11; for example, N(1)-Rh-N(5) = 163.56(12)°. It is as yet unclear whether this displacement from idealized geometry (as derived from crystallographic data)9d,g,10d can be correlated to the chemical shift and broadening of the resulting 11B{1H} NMR signals. In complex 11, the Rh(1)-B(1) bond length (2.064(4) A˚) is significantly shorter than in the case of complexes 12 and 13 (2.155(5) A˚ for 12; 2.132(6) and 2.122(7) A˚ for two independent molecules of 13 cocrystallized). Indeed, there is only one reported example which contains a shorter metalboron distance, [Pd{κ4S,S,B,S-(mttBu)3}(PMe3)], where the palladium-boron distance is 2.050(8) A˚.10b The Rh(1)-B(1)

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Scheme 5. Reaction of Li[PhBai] with [RhCl(NBD)]2, Revealing That No Hydrogen Migration Occurs with This Ligand

distance found in complex 11 is also within the range found for structurally characterized rhodium-boryl complexes.22 Of the three rhodium-nitrogen distances in 11, the Rh(1)-N(3) distance is longer (2.186(3) A˚), as a result of the large trans influence of the rhodium-alkyl σ-bond. Finally, the rhodium-carbon distance (2.135(11) A˚) is similar to that in the η1-nortricyclyl rhodium complex reported by Mail (2.12(2) A˚).23c All other metric data for the nortricyclyl moiety are similar to those reported in the literature.23 The isolation of complex 10 was targeted in order to probe the position of the hydride. We have previously reported the structural characterization of [Ru{κ4H,B,S,S-HB(CH2Ph)(mt)2}(Cl)(PCy3)], where the hydride was located at an intermediate point between the ruthenium and boron centers.12 Single crystals of 10 were obtained from a saturated toluene solution containing a mixture of 10 and 11. Further noncrystalline precipitate obtained from the mixture was found to contain both complexes in an approximately 1:1 ratio (as determined by 1H and 11B NMR spectroscopy). Upon standing, at room temperature, it was found that complex 10 slowly converted into complex 11 over time (see the Experimental Section). An X-ray diffraction study was performed on the single crystals obtained. The calculated structure, shown in Figure 10, revealed the expected structure assigned to species 10, [Rh{κ3N,N,H-HB(azaindolyl)3(NBD)]. Complex 10 shows structural features similar to those of complexes 3 and 8. The complex adopts a square-pyramidal geometry with one NBD ligand, two azaindolyl moieties coordinating through the nitrogen atom of the pyridine heterocycle, and a B-H interaction at the apical site on the metal. The M-H bond distance of 1.98(5) A˚ is similar to the cyclooctadiene examples (cf. 1.99(2) A˚ (3), 2.00(4) A˚ (8)) and is also consistent with 2 M 3 3 3 H-B interaction (0.25 A˚ P a type 20 greater than the rcov). In contrast to the structural evidence, the B-H stretching frequency for 10 (2015 cm-1) is somewhat lower than these 3 and 8 (2106 cm-1 (3) and 2117 cm-1 (8)). It is, however, similar to the stretching frequency for the complex [Rh{κ3N,N,H-H(Ph)B(azaindolyl)3}(NBD)] (12), which is outlined below (2022 cm-1), indicating a reduction in the bond order of the B-H bond in the norbornadiene examples. We found no evidence of a rhodium-hydride species (or any other intermediate) during the reaction, suggesting that insertion of the olefin into a transient rhodium-hydride bond occurs very rapidly. A further possibility could involve direct attack of the olefin by the “hydride” species. Finally, a third possibility could involve dissociation of a second azaindolyl ring from complex 10 to form a complex such as [Rh{κ2N,H-HB(azaindolyl)3(NBD)]. This compound could subsequently undergo a B-H activation followed by olefin insertion and recoordination of the second azaindolyl ring. There is precedent for the κ2N,H coordination mode for the Tai ligand; Kuwata and Ikariya15 showed that one of the

azaindole rings could be substituted from [Ru{κ3N,N,HHB(azaindolyl)3}(Cp*)] in the presence of carbon monoxide without the loss of the B-H interaction with the metal center (Figure 11). It was of interest to us whether the new ligand, PhBai, would show a hydride migration reactivity similar to that found for Tai. A different coordination mode would certainly be obtained, since the κ4N,N,B,N coordination mode is not possible in this case. Upon reaction of 2 equiv of Li[PhBai] with 1 equiv of [Rh(NBD)Cl]2 in THF at -90 °C followed by equilibration to room temperature and standard reaction workup (see the Experimental Section), the 11B{1H} NMR spectrum of the reaction mixture revealed only one signal at -3.9 ppm (s, hhw = 138 Hz; 11B NMR δ (C6D6) -3.5 (s, Δν1/2 = 189 Hz)). The compound was isolated in high yield and was fully characterized by spectroscopic and analytical methods. A comparison with the analogous complexes (see Table 1) suggested the formation of the nonactivated complex [Rh(NBD){κ3N,N,H-Ph(H)B(azaindolyl)2}] (12). This was further confirmed by the analysis of the spectroscopic data, most notably the 1H NMR spectrum, which indicated the coordination of the norbornadiene ligand in a η4 mode. The spectrum also suggests a Cs symmetrical molecule, as it exhibits three signals for the norbornadiene ligand at 0.85, 3.15, and 3.25 ppm (δ C6D6; in a 2:4:2 integral ratio, respectively). The geometry around the metal center is also verified by the aromatic region of the spectrum, where signals attributed to the azaindolyl rings show that these experience one chemical environment. Unlike the case for complex 10, we were unable to obtain any evidence for the migration of the hydride between boron and the metal center in complex 12, in various solvents at elevated temperatures. This observation is certainly due to the replacement of one azaindolyl ring with a phenyl group. This suggests that the κ4N,N,B,N coordination mode in the final product is a requirement for hydride migration to occur.

Concluding Remarks In summary, we have reported the synthesis of the novel analogue of the K[Tai] scorpionate ligand Li[PhBai] (7), where an azaindolyl moiety has been replaced by a phenyl ring, and have explored its coordination chemistry with group 9 transition metals. The subtle differences of the two ligand scaffolds were demonstrated by the structural and spectroscopic comparison of their coordination compounds with Rh(I) and Ir(I) metal centers. The effect of changing the substituents around the boron center was highlighted by the difference of reactivity of the two ligand sets toward [Rh(NBD)Cl]2. When 7 was reacted with [Rh(NBD)Cl]2, a product analogous to the η4-cyclooctadiene complexes 2 and 8 was obtained. On the other hand, when the same reaction was performed using K[Tai], the novel rhodaboratrane

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complex 11 was obtained in good yields. This complex is the first example where a κ4N,N,B,N coordination mode is observed with nitrogen-based scorpionate ligands. The formation of the metallaboratrane complex is accompanied by rearrangement of the η4-norbornadiene ligand to form a rhodium-nortricyclyl σ bond. The fact that this transformation is not observed in the case of PhBai suggests that the κ4 coordination mode is necessary for this transformation to occur. We are currently exploring the mechanism for the formation of 11 as well as its reactivity toward σ-bases and πacids, and the results will be presented in a forthcoming publication.

Experimental Section General Remarks. All manipulations were performed in a Braun glovebox with an O2 and H2O atmosphere of below 5 ppm or by using standard Schlenk techniques. Solvents (toluene, THF, Et2O) were dried using a Grubbs alumina system and were kept in Young ampules under N2 over molecular sieves (4 A˚). Dry n-pentane (