Preparation, Structure, Bonding, and Preliminary Reactivity of a Six

Communication pubs.acs.org/Organometallics

Preparation, Structure, Bonding, and Preliminary Reactivity of a SixCoordinate d4 Osmium−Boryl Complex Miguel A. Esteruelas,*,† Israel Fernández,*,‡ Ana M. López,† Malka Mora,† and Enrique Oñate† †

Departamento de Quı ́mica Inorgánica-Instituto de Sı ́ntesis Quı ́mica y Catálisis Homogénea (ISQCH), Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain ‡ Departamento de Quı ́mica Orgánica I, Facultad de Ciencias Quı ́micas, Universidad Complutense de Madrid, 28040 Madrid, Spain S Supporting Information *

ABSTRACT: The d4 ML6 bis-boryl complex OsH(Bcat)2Cl(PiPr3)2 (Bcat = catecholborate) has been prepared by the reaction of OsH2Cl2(PiPr3)2 with HBcat, via the monoboryl intermediate OsH2(Bcat)Cl(PiPr3)2, and its structure determined by X-ray diffraction analysis. The Os−B bonds and the stability and disposition of the boryl ligands have been analyzed by DFT calculations. The bis-boryl complex loses HBcat to give Os(Bcat)Cl(CO)2(PiPr3)2, under a CO atmosphere.

fragment OsHCl(PiPr3)2.7 There are, however, significant differences in the reactions of HBpin and HBcat with 1. Thus, we have now observed that the treatment of dichloromethane solutions of the latter complex with two portions of 2.2 equiv of HBcat for 4 h at room temperature leads to the bisboryl derivative OsH(Bcat)2Cl(PiPr3)2 (2). Its formation takes place via the monoboryl intermediate OsH2(Bcat)Cl(PiPr3)2 (A in Scheme 1),8 which can be detected by NMR spectroscopy9

d4 ML6 complexes are a class of little-known unsaturated transition-metal derivatives, which undergo distortion from an octahedral geometry to destabilize one orbital from the t2g set and simultaneously to stabilize some occupied orbitals. Thus, they prefer to be diamagnetic.1 The typical distortions for these species are represented by the structures of complexes OsH2X2(PR3)2 and OsH3X(PR3)2 (I and II in Chart 1). In Chart 1

Scheme 1

the beginning, structure I was viewed as a distorted square antiprism of ideal D4h symmetry with two vacant coordination sites.2 Later it was described as a bicapped tetrahedron and as a trigonal prism.3 Structure II has essentially C2v symmetry. The distortion partially cancels the electron deficiency at the metal center, which receives electron density from the hydrides via strong σ bonds and additionally from one lone pair of X via a π bond.4 Boryl complexes, M−BR2, are the subclass of compounds with metal−boron bonds exhibiting the strongest potential for the functionalization of hydrocarbons.5 In particular, the use of pinacolborane (HBpin) and catecholborane (HBcat) has given access to a large number of useful species for organic chemists.6 Here, we report the first d4 ML6 boryl complex. In addition, we show its structure and analyze the M−B bond, the stability and disposition of the boryl ligand, and some of its reactivity. We have previously reported that complex OsH2Cl2(PiPr3)2 (1) reacts with HBpin to afford the σ-borinium derivative OsH2Cl{η2-H-B(OCMe2CMe2OBpin)}(PiPr3)2, as a result of the dimerization of two HBpin molecules and the hydride transfer from one of them to the osmium atom of the metal © 2012 American Chemical Society

after the addition of the first portion of borane. We have recently reported that the complexes OsHCl(CO)(PR3)2 (PR3 = PiPr3, PCy3) react with HBpin to afford the boryl− dihydrogen derivatives Os(Bpin)Cl(η2-H2)(CO)(PR3)2, which lose H2 to give Os(Bpin)Cl(CO)(PR3)2.10 Early examples of rhodium and iridium hydride complexes that react with HBcat or HBpin in a similar manner are known.5a,11 Complex 2 was isolated as an orange solid in 75% yield and characterized by Xray diffraction analysis.12 The structure belongs to type II, with Received: May 14, 2012 Published: June 18, 2012 4646

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Communication

trans phosphines (P−Os−P = 174.35(6) and 176.59(6)°), where the metal center and the boryl, hydride, and chloride ligands are placed in a plane perpendicular to the P−Os−P axis. As expected, the Cl−Os−B (121.3(2)−125.54(19)°) and H− Os−B angles (54(2)−58(2)°) deviate markedly from 90°. Os− B distances between 2.019(7) and 2.043(8) Å support the σboryl formulation.10,13 In agreement with the presence of the hydride ligand, the 1H NMR spectrum in benzene-d6, at room temperature, shows at −12.3 ppm a triplet with a H−P coupling constant of 6.0 Hz. A singlet at 36.8 ppm and a broad signal at 39 ppm in the 31P{1H} and 11B NMR spectra, respectively, are also characteristic of 2. The bonding situation in 2 was analyzed by means of DFT calculations. The nature of the metal−boron interactions was investigated with AIM (atoms in molecules)14 and NBO (natural bond orbital)15 methods. The geometry of 2 was optimized at the BP86/def2-SVP and M06/def2-SVP levels, a better agreement between the computed and the experimental structures being found with the latter method (see the Supporting Information). The Laplacian distribution in the Os−B bonding regions (Figure 1) exhibits an area of charge concentration at the boron

Figure 2. Molecular orbitals of complex 2 (isosurface value of 0.035 au).

Scheme 2.

a

a

Free energy values are given in parentheses. All data have been computed at the M06/def2-SVP level.

negativity of Os compared to B (2.20 versus 2.04, in the Pauling scale). This indicates that the most favorable way to break the Os−B bond is homolytic rupture, which would give a covalent bond as found in related species.18 However, the AIM method clearly shows that the intrinsic interactions can be described as donor−acceptor bonds (these interactions are indeed stronger than the homolytic ones).19 In addition, the computed NBO charges at osmium and boron centers (−1.14e and +1.09e, respectively) also suggest a significant electrostatic interaction between the metal fragment and the boryl ligands. A noticeable feature of the structure of 2 is the perpendicular disposition of the boryl ligands with regard to the P−Os−P axis. Our calculations reveal that this fact is in part due to the presence of stabilizing CH−π interactions between the closest methyl groups of the phosphines and the vacant pz(π*) atomic orbital of the boron atoms.20 Figure 3 visualizes the overlap between the involved orbitals. The SOP method is able to find such stabilizing delocalizations (σ-C−H → p z -B, one interaction per phosphine and boryl ligand) whose associated

Figure 1. Contour line diagram ∇2ρ(r) for 2 in the Os−B−O plane.

atom (∇2ρ(r) < 0, solid lines), which has the shape of a droplet-like appendix directed toward the metal center, while the osmium end carries an area of charge depletion (∇2ρ(r) > 0, dashed lines). This is typical for closed-shell donor−acceptor bonds, as previously found for transition-metal complexes with group 13 ER ligands.16 In contrast to the Os−B region, the B− O and C−O bonds exhibit the typical shape for an electronsharing bond. The main bonding interactions are the result of the B(sp2)→ Os(dσ orbital) and the Os(dπ orbital)→B(vacant pz orbital) electronic donations, as seen from the molecular orbitals depicted in Figure 2. The NBO second-order perturbation (SOP) energy associated with the Os(dπ orbital)→B(vacant pz orbital) delocalization is significant (ΔE(2) = −17.9 kcal mol−1). In agreement with this, the computed Os−B NBO-Wiberg bond index of 0.86 is comparable to that calculated for terminal aminoborylene complexes, which feature a distinctive metal− boron π component.17 The bond dissociation energies (BDE) for the two consecutive losses of Bcat ligands as radicals (a), anions (b), and cations (c) have been computationally explored (Scheme 2). Homolytic cleavage, which affords radicals, gives the lowest BDE, whereas the computed BDE’s leading to charged species are considerably higher, in particular for the reductive dissociation of [Bcat]+ cations due to the higher electro-

Figure 3. NBO orbitals involved in the σ-C−H→pz-B(π) interaction in complex 2. 4647

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SOP energies range between −1.70 and −2.50 kcal mol−1 (CH···B distances ranging from 2.518 to 2.644 Å). In addition, the lone pairs of the oxygen atoms directly attached to the boron centers produce an additional stabilization of the boryl ligands, as they can delocalize into the atomic pz boron orbital (associated SOP energy ΔE(2) = −44.0 and −44.5 kcal mol−1). As a consequence of these π donations, the atomic pz-B orbital presents a significant electronic occupation (0.47e). In order to further support the strong influence of the noncovalent CH−π interactions on the disposition of the boryl ligands, we have also optimized the structure of the complex OsH(Bcat)2Cl(PMe3)2 (2′), where the isopropyl substituents of the phosphines have been replaced by methyl groups. In this situation, these CH−π interactions cannot exist and, consequently, the M06 optimized structure shows that the boryl ligands are no longer coplanar but are perpendicular to the equatorial plane (see the Supporting Information).21 Preliminary studies on the reactivity of 2 show that this compound has a marked tendency to lose HBcat. Thus, under a CO atmosphere, it affords the cis-dicarbonyl derivative Os(Bcat)Cl(CO)2(PiPr3)2 (3), which can be also prepared via the monocarbonyl Os(Bcat)Cl(CO)(PiPr3)2 (4), according to Scheme 3. In contrast to the case for 1, the five-coordinate

hydrides. The Os−B bonds are closed-shell donor−acceptor bonds with a remarkable Os→B π donation. The boryl ligands are further stabilized by π donation from the lone pairs of the oxygen atoms of the substituents and, strikingly, from the σ-CH bonds of a phosphine methyl group to the vacant pz-B orbital, featuring a genuine CH−π interaction that is in part responsible for the observed perpendicular arrangement of the boryl ligands. Preliminary studies suggest that the reactivity of the new compound will be governed by a marked tendency to eliminate HBcat, affording monoboryl derivatives.

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and CIF files giving general experimental details and synthesis, characterization, and crystallographic data for 2−4 and details of the computational studies. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.A.E.) [email protected] (I.F.). Notes

The authors declare no competing financial interest.

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Scheme 3

ACKNOWLEDGMENTS Financial support from the Spanish MICINN (Projects CTQ2011-23459, CTQ2010-20714-C02-01, and Consolider Ingenio 2010 (CSD2007-00006) and Ramón y Cajal contract to I.F.), the DGA (E35), and the European Social Fund (FSE) is acknowledged. M.M. thanks the Spanish MEC for her FPU grant. We thank Profs. G. Frenking and M. Bickelhaupt for fruitful discussions.

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REFERENCES

(1) (a) Templeton, J. L.; Ward, B. C. J. Am. Chem. Soc. 1980, 102, 6568. (b) Kubácek, P.; Hoffman, R. J. Am. Chem. Soc. 1981, 103, 4320. (c) Baya, M.; Esteruelas, M. A.; Oñate, E. Organometallics 2011, 30, 4404. (2) (a) Aracama, M.; Esteruelas, M. A.; Lahoz, F. J.; López, J. A.; Meyer, U.; Oro, L. A.; Werner, H. Inorg. Chem. 1991, 30, 288. (b) Atencio, R.; Esteruelas, M. A.; Lahoz, F. J.; Oro, L. A.; Ruiz, N. Inorg. Chem. 1995, 34, 1004. (3) Gusev, D. G.; Kuhlman, R.; Rambo, J. R.; Berke, H.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1995, 117, 281. (b) Maseras, F.; Eisenstein, O. New J. Chem. 1998, 5. (4) (a) Gusev, D. G.; Kuhlman, R.; Sini, G.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1994, 116, 2685. (b) Kuhlman, R.; Clot, E.; Leforestier, C.; Streib, W. E.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1997, 119, 10153. (c) Esteruelas, M. A.; García-Raboso, J.; Oliván, M Organometallics 2011, 30, 3844. (5) (a) Irvine, G. J.; 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. (b) Braunschweig, H.; Colling, M. Coord. Chem. Rev. 2001, 223, 1. (c) Aldridge, S.; Coombs, D. L. Coord. Chem. Rev. 2004, 248, 535. (d) Braunschweig, H.; Kollann, C.; Rais, D. Angew. Chem., Int. Ed. 2006, 45, 5254. (e) Kays, D. L.; Aldridge, S. Struct. Bonding (Berlin) 2008, 130, 29. (f) Braunschweig, H.; Dewhurst, R. D.; Schneider, A. Chem. Rev. 2010, 110, 3924. (6) (a) Miyaura, N. Bull. Chem. Soc. Jpn. 2008, 81, 1535. (b) Dang, L.; Lin, Z.; Marder, T. B. Chem. Commun. 2009, 3987. (c) Crudden, C. M.; Glasspoole, B. W.; Lata, C. J. Chem. Commun. 2009, 6704. (d) Mkhalid, I. A. I.; Barnard, J. H.; Marder, T .B.; Murphy, J. M.; Hartwig, J. F. Chem. Rev. 2010, 110, 890. (e) Braunschweig, H.; Guethlein, F. Angew. Chem., Int. Ed. 2011, 50, 12613.

complex OsHCl(CO)(PiPr3)2 (5) reacts with HBcat in the same manner as with HBpin.10 Thus, the treatment of its toluene solutions with 1.0 equiv of HBcat, for 5 min, at room temperature leads to 4, which adds CO to afford 3. Complexes 3 and 4 have both been characterized by X-ray diffraction analysis. The structure of 3 can be rationalized as a distorted octahedron with the boryl ligand (Os−B = 2.145(15) Å) lying in the perpendicular plane to the P−Os−P axis, whereas the coordination polyhedron around the osmium atom of 4 can be described as a square pyramid with the boron in the apex (Os− B = 1.991(11) Å). In conclusion, the first example of a new class of boryl complexes, the unsaturated d4 ML6 derivative OsH(Bcat)2Cl(PiPr3)2, has been prepared by the reaction of OsH2Cl2(PiPr3)2 with 3 equiv of HBcat. Its structure has essentially C2v symmetry and resembles that of the trihydride derivatives OsH3X(PR3)2 with the boryl ligands in the positions of two 4648

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(d) Fernández, I.; Lugan, N.; Lavigne, G. Organometallics 2012, 31, 1155. (21) The adopted perpendicular disposition of the boryl ligands with respect to the P−Os−P axis is also a consequence of the steric hindrance of the bulky PiPr3 ligands, which destabilizes the corresponding parallel disposition (which is adopted by OsH(Bcat)2Cl(PMe3)2 (2′); computed dihedral angle O−B−Os−B of −1.6° in complex 2 vs 94.6° in 2′). See also: Lam, W. H.; Shimada, S.; Batsanov, A. S.; Lin, Z.; Marder, T. B.; Cowan, J. A.; Howard, J. A. K.; Mason, S. A.; McIntyre, G. J. Organometallics 2003, 22, 4557.

(7) Esteruelas, M. A.; Fernández-Alvarez, F. J.; López, A. M.; Mora, M.; Oñate, E. J. J. Am. Chem. Soc. 2010, 132, 5600. (8) Complex 1 initially reacts with HBcat to give ClBcat and OsH3Cl(PiPr3)2. The reaction of the latter with a second molecule of HBcat affords A and H2. (9) Selected NMR data for A in benzene-d6, at room temperature: 1 H, δ −16.9 (t, JH−P = 10.2 Hz); 31P{1H}, δ 47.3 (s, t under protoncoupled conditions); 11B, δ 39 (br). (10) Esteruelas, M. A.; López, A. M.; Mora, M.; Oñate, E. Organometallics 2012, 31, 2965. (11) See for example: (a) Burgess, K.; van der Donk, W. A.; Wescott, S. A.; Marder, T. B.; Baker, R. T.; Calabrese, J. C. J. Am. Chem. Soc. 1992, 114, 9350. (b) Cleary, B. P.; Eisenberg, R. Organometallics 1995, 14, 4525. (c) Shimada, S.; Batsanov, A. S.; Howard, J. A. K.; Marder, T. B. Angew. Chem., Int. Ed. 2001, 40, 2168. (12) The structure has two chemically equivalent but crystallographically independent molecules in the asymmetric unit. Scheme 1 shows a drawing of one of them. (13) (a) Irvine, G. J.; Roper, W. R.; Wright, L. J. Organometallics 1997, 16, 2291. (b) Rickard, C. E. F.; Roper, W. R.; Williamson, A.; Wright, L. J. Organometallics 1998, 17, 4869. (c) Irvine, G. J.; Rickard, C. E. F.; Roper, W. R.; Williamson, A.; Wright, L. J. Angew. Chem., Int. Ed. 2000, 39, 948. (d) Rickard, C. E. F.; Roper, W. R.; Williamson, A.; Wright, L. J. Organometallics 2000, 19, 4344. (e) Rickard, C. E. F.; Roper, W. R.; Williamson, A.; Wright, L. J. Organometallics 2002, 21, 4862. (f) Rickard, C. E. F.; Roper, W. R.; Williamson, A.; Wright, L. J. Organometallics 2002, 21, 1714. (g) Clark, G. R.; Irvine, G. J.; Roper, W. R.; Wright, L. J. J. Organomet. Chem. 2003, 680, 81. (h) Rickard, C. E. F.; Roper, W. R.; Williamson, A.; Wright, L. J. J. Organomet. Chem. 2004, 689, 1609. (i) Buil, M. L.; Esteruelas, M. A.; Garcés, K.; Oñate, E. J. Am. Chem. Soc. 2011, 133, 2250. (14) Bader, R. F. W. Atoms in Molecules. A Quantum Theory; Oxford University Press: Oxford, U.K., 1990. (15) (a) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102, 7211. (b) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899. (16) (a) Weiss, J.; Stetzkamp, D.; Nuber, B.; Fischer, R. A.; Boehme, C.; Frenking, G. Angew. Chem., Int. Ed. Engl. 1997, 36, 70. (b) Fischer, R. A.; Schulte, M. M.; Weiss, J.; Zsolnai, L.; Jacobi, A.; Huttner, G.; Frenking, G.; Boehme, C.; Vyboishchikov, S. F. J. Am. Chem. Soc. 1998, 120, 1237. (c) Braunschweig, H.; Fernández, I.; Frenking, G.; Radacki, K.; Seeler, F. Angew. Chem., Int. Ed. 2007, 46, 5215. (d) Braunschweig, H.; Herbst, T.; Rais, D.; Ghosh, S.; Kupfer, T.; Radacki, K.; Crawford, A. G.; Ward, R. M.; Marder, T. B.; Fernández, I.; Frenking, G. J. Am. Chem. Soc. 2009, 131, 8989. (17) Blank, B.; Colling-Hendelkens, M.; Kollann, C.; Radacki, K.; Rais, D.; Uttinger, K.; Whittell, G. R.; Braunschweig, H. Chem. Eur. J. 2007, 13, 4770. (18) DFT calculations on Os(BX2)Cl(CO)(PH3)2 and Os(BX2) Cl(CO)2(PH3)2 complexes indicate that the Os−B bond in both types of compounds is provided by a covalent σ bond, which is polarized toward osmium by strong charge attraction. The stability of the boryl ligand comes mainly from B←X π donation, which is clearly stronger than the Os→B π donation. See: (a) Giju, K. T.; Bickelhaupt, F. M.; Frenking, G. Inorg. Chem. 2000, 39, 4776. (b) Frenking, G.; Fröhlich, N. Chem. Rev. 2000, 100, 717. (19) It is possible that the intrinsic interactions in a molecule are not between those fragments which are formed in the energetically most favorable dissociation pathway. For example, [Ir(CO)6]3+ is an experimentally stable trication where the bonding takes place between Ir3+ and 6 CO. However, the lowest dissociation pathway for dissociation of one ligand is [Ir(CO)5]2+ + CO+. See: Diefenbach, A.; Bickelhaupt, F. M.; Frenking, G. J. Am. Chem. Soc. 2000, 122, 6449. (20) Similar weak interactions have been suggested to influence the conformation adopted by different carbene complexes. See: (a) Fernández, I.; Cossío, F. P.; Arrieta, A.; Lecea, B.; Mancheño, M. J.; Sierra, M. A. Organometallics 2004, 23, 1065. (b) Andrada, D. M.; Michoff, M. E. Z.; Fernández, I.; Granados, A. M.; Sierra, M. A. Organometallics 2007, 26, 5854. (c) Valyaev, D. A.; Brousses, R.; Lugan, N.; Fernández, I.; Sierra, M. A. Chem. Eur. J. 2011, 17, 6602. 4649

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