Os and

Organometallics 2011, 30, 815–825 DOI: 10.1021/om101000y

815

Ethylene Substitution in a Bis-Ethylene Complex of Rh/Os and Unusual Brønsted-Lowry Basicity of an N-Heterocyclic Carbene Kyle D. Wells, Michael J. Ferguson,† Robert McDonald,† and Martin Cowie* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2. † X-ray Crystallography Laboratory Received October 20, 2010

Reactions of the bis-ethylene complex [RhOs(η2-C2H4)2( μ-CO)2(dppm)2][CF3SO3], with CO, MeCN, PMe3, IMe4, and [PPN][Cl] (IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene; PPNþ = bis(triphenylphosphine)iminium) yield the monoethylene products [RhOsL(C2H4)( μ-CO)2(dppm)2][CF3SO3] (L=CO (2), MeCN (3), PMe3 (4), IMe4 (5)) and [RhOsCl(C2H4)( μ-CO)2(dppm)2] (6), respectively, by substitution of the Rh-bound ethylene ligand. In the case of L=CO, carbon monoxide was added at -78 °C, since at higher temperatures substitution of both ethylene ligands occurred to give the known product [RhOs(CO)4(dppm)2][CF3SO3]. For L=IMe4, the stoichiometry had to be carefully controlled, since in the presence of more than 1 equiv of IMe4 deprotonation of a dppm methylene group by IMe4 occurred to yield [RhOs(IMe4)(C2H4)( μ-CO)2(Ph2PCHPPh2)(dppm)]. The IMe4 product (5) could also be obtained in THF solution by substitution of the acetonitrile ligand in compound 3. However, when this reaction was carried out in acetonitrile, deprotonation of the acetonitrile ligand by IMe4 occurred to give the acetonitrilide intermediate [RhOs(NCCH2)(C2H4)( μ-CO)2(dppm)2], which rapidly coupled with another 1 equiv of 3 to yield the enamino-nitrile-bridged product [(RhOs(C2H4)( μ-CO)2(dppm)2)2( μ-NHC(CH3)CHCN)][CF3SO3].

Introduction Much of the interest in bimetallic complexes stems from the possibility that adjacent metals can interact in a cooperative manner in the activation of substrate molecules leading to enhanced reactivity over that displayed by monometallic analogues.1 Metal-metal cooperativity effects can be manifested in a number of ways. Possibly the most obvious involves bridging of the substrate at some stage of the activation

process, leading to the simultaneous involvement of both metals.2-4 More subtle activation modes can involve the transmission of electronic effects, by either bridging ligands or metal-metal bonds, from one metal to the adjacent metal.5 Alternatively, electron transfer between metals can generate a reactive mixed-valence intermediate which can bring about substrate activation.6 In addition, pairs of low-valent metals have a greater potential for multiple activations, through oxidative additions, than single metals, since both metals can undergo their normal increases in oxidation state.7 Heterobimetallic complexes, employing pairs of different metals, add an additional dimension to metal-metal cooperativity effects by virtue of their inherently different properties that result from their different electronic and steric requirements, and such systems have demonstrated unique

*Corresponding Author: E-mail: [email protected] Fax: 01 780 492 8231; Tel: 01 780 492 5581. (1) (a) van den Beuken, E. K.; Feringa, B. L. Tetrahedron 1998, 54, 12985. (b) Catalysis by Di- and Polynuclear Metal Cluster Complexes; Adams, R. D., Cotton, F. A., Eds.; Wiley-VCH: New York, 1998. (c) Braunstein, P.; Ros
e, J. In Metal Clusters in Chemistry; Braunstein, P., Oro, L. A., Raithby, P. R., Eds.; Wiley-VCH: Weinheim, Germany, 1999; Vol. II. (d) Li, H.; Marks, T. J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15295. (e) Rowlands, G. J. Tetrahedron 2001, 57, 1865. (2) See for example: (a) Torkelson, J. R.; Antwi-Nsiah, F. H.; McDonald, R.; Cowie, M.; Pruis, J. G.; Jalkanen, K. J.; DeKock, R. L. J. Am. Chem. Soc. 1999, 121, 3666. (b) Mottalib, Md. A.; Begum, N.; Abedin, S. M. T.; Akter, T.; Kabir, S. E.; Miah, Md. A.; Rokhsana, D.; Rosenberg, E.; Hossain, G. M. G.; Hardcastle, K. I. Organometallics 2005, 24, 4747. (c) Cabeza, J. A.; P
erez-Carre~no, E. Organometallics 2008, 27, 4697. (d) Tunik, S. P.; Balova, I. A.; Borovitov, M. E.; Nordlander, E.; Haukka, M.; Pakkanen, T. A. Dalton Trans. 2002, 827. (3) (a) Anderson, D. J.; McDonald, R.; Cowie, M. Angew. Chem., Int. Ed. 2007, 46, 3741. (b) Slaney, M. E.; Anderson, D. J.; McDonald, R.; Ferguson, M. J.; Cowie, M. J. Am. Chem. Soc. 2010, 132, 16544. (c) Trepanier, S. J.; Dennett, J. N. L.; Sterenberg, B. T.; McDonald, R.; Cowie, M. J. Am. Chem. Soc. 2004, 126, 8046. (d) Cowie, M. Can. J. Chem. 2005, 83, 1043. (e) George, D. S. A.; Hilts, R. W.; McDonald, R.; Cowie, M. Organometallics 1999, 18, 5330. (4) Yuan, Y.; Jim
enez, M. V.; Sola, E.; Lahoz, F. J.; Oro, L. A. J. Am. Chem. Soc. 2002, 124, 752.

(5) (a) Esteruelas, M. A.; Garcia, M. P.; L
opez, A. M.; Oro, L. A. Organometallics 1991, 10, 127. (b) Sola, E.; Bakhmutov, V. I.; Torres, F.; Elduque, A.; L
opez, J. A.; Lahoz, F. J.; Werner, H.; Oro, L. A. Organometallics 1998, 17, 683. (c) Bosnich, B. Inorg. Chem. 1999, 38, 2554. (d) Sola, E.; Torres, F.; Jim
enez, M. V.; L
opez, J. A.; Ruiz, S. E.; Lahoz, F. J.; Elduque, A.; Oro, L. A. J. Am. Chem. Soc. 2001, 123, 11925. (e) Fackler, J. P. Inorg. Chem. 2002, 41, 6959. (f) Powers, D. C.; Benitez, D.; Tkatchouk, E.; Goddard, W. A.; Ritter, T. J. Am. Chem. Soc. 2010, 132, 14092. (6) (a) Gray, T. G.; Veige, A. S.; Nocera, D. G. J. Am. Chem. Soc. 2004, 126, 9760. (b) Takahashi, Y.; Fujita, K.; Yamaguchi, R. Eur. J. Inorg. Chem. 2008, 4360. (c) Fujita, K.; Takahashi, Y.; Nakaguma, H.; Humada, T.; Yamaguchi, R. J. Organomet. Chem. 2008, 693, 3375. (7) (a) Graham, W. A. G. J. Organomet. Chem. 1986, 300, 81. (b) Ogino, H.; Tobita, H. Adv. Organomet. Chem. 1998, 42, 223. (c) Corey, J. Y.; Braddock-Wilking, J. Chem. Rev. 1999, 99, 175. (d) Wang, W. D.; Eisenberg, R. J. Am. Chem. Soc. 1990, 112, 1833. (e) McDonald, R.; Cowie, M. Organometallics 1990, 9, 2468. (f) RisticPetrovic, D.; Torkelson, J. R.; Hilts, R. W.; McDonald, R.; Cowie, M. Organometallics 2000, 19, 4432.

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Organometallics, Vol. 30, No. 4, 2011

reactivity, in both heterogeneous8 and homogeneous9 systems. In this paper, we continue our investigation of the Rh/Os metal combination,10 in which the greater lability of Rh and its tendency for coordinative unsaturation are combined with the stability that results from the greater bond strength of the third-row metal. One of our interests in this and related systems is in the protonation of complexes containing unsaturated substrates10b,c,11 in order to determine how ancillary ligands can influence the kinetic and thermodynamic products, given the different protonation sites available (at either metal, at the metal-metal bond, or at the substrate). As part of this study we set out to determine how the electronic effects of ancillary ligands on one metal could be transmitted to the adjacent metal to which an ethylene ligand is bound, thereby influencing the tautomerism between ethylene/hydride and ethyl complexes upon protonation.12 In this paper we describe the synthesis of such a series of monoethylene complexes as well as some unexpected transformations occurring when an NHC group is the ancillary ligand. A subsequent paper will address the reactivity of this series of ethylene complexes.

Experimental Section General Comments. All solvents were dried using the appropriate desiccants, distilled before use, and stored under nitrogen. Reactions were performed under an argon atmosphere using standard Schlenk techniques. The 1,3,4,5-tetramethylimidazol2-ylidene ligand (IMe4) was prepared using a published procedure13 and then recrystallized from toluene to afford colorless crystals that were stored in a freezer at -30 °C. In cases in which isolation of the highly reactive free carbene ligand was not necessary, a solution of the carbene in THF (0.213 M) was stored over potassium metal at 0 °C and used without further purification. The compound [RhOs(η2-C2H4)2( μ-CO)2(dppm)2][CF3SO3] (1) was prepared according to the published procedure.3c Unless otherwise noted, reagents were purchased through Aldrich and used without further purification; PMe3 (1 M solution in THF) was transferred into Teflon-capped Schlenk flasks and stored at 2 °C under argon, and potassium bis(trimethylsilyl)amide (KN(Si(CH3)3)2) and bis(triphenylphosphine)iminium chloride (PPNCl) were purchased as solids. (8) (a) Sinfelt, J. H. Bimetallic Catalysts; Wiley: New York, 1983. (b) Satterfield, C. N. Heterogeneous Catalysis in Industrial Practice; McGraw-Hill: New York, 1991. (c) Topsøe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating Technology; Anderson, J. R., Broart, M., Eds.; SpringerVerlag: New York, 1996; Catalysis, Science and Technology, Vol. II. (d) Lee, C. E.; Bergens, S. H. J. Phys. Chem. B 1998, 102, 193. (e) Xiao, F. S.; Fukuoka, A.; Ichikawa, M. J. Catal. 1992, 138, 206. (f) Shen, C.; GarciaZayas, E. A.; Ser, A. J. Am. Chem. Soc. 2000, 122, 4029. (9) (a) Stephan, D. W. Coord. Chem. Rev. 1989, 95, 41. (b) Wheatlye, N.; Kalck, P. Chem. Rev. 1999, 99, 3379. (c) Chetcuti, M. J. In Comprehensive Organometallic Chemistry II; Able, E. W., Stone, F. G. A., Williamson, G., Eds.; Pergamon: Oxford, U.K., 1995; Vol 10, p 23. (d) Gade, L. H. Angew. Chem., Int. Ed. 2000, 39, 2658. (e) Ritleng, V.; Chetcuti, M. J. Chem. Rev. 2007, 107, 797. (10) See for example: (a) Sterenberg, B. T.; Hilts, R. W.; Moro, G.; McDonald, R.; Cowie, M. J. Am. Chem. Soc. 1995, 117, 245. (b) Trepanier, S. J.; McDonald, R.; Cowie, M. Organometallics 2003, 22, 2638. (c) Chokshi, A.; Rowsell, B. D.; Trepanier, S. J.; Ferguson, M. J.; Cowie, M. Organometallics 2004, 23, 4759. (d) Wigginton, J. R.; Trepanier, S. T.; McDonald, R.; Ferguson, M. J.; Cowie, M. Organometallics 2005, 24, 6194. (e) Wigginton, J. R.; Chokshi, A.; Graham., T. W.; McDonald, R.; Ferguson, M. J.; Cowie, M. Organometallics 2005, 24, 6398. (11) (a) Rowsell, B. D.; McDonald, R.; Cowie, M. Organometallics 2004, 23, 3873. (b) Samant, R. G.; Trepanier, S. J.; Wigginton, J. R.; Xu, L.; Bierenstiel, M.; McDonald, R.; Ferguson, M. J.; Cowie, M. Organometallics 2009, 28, 3407. (12) Wells, K. D. Ph.D. Thesis, University of Alberta, 2010. (13) Kuhn, N.; Kratz, T. Synthesis 1993, 561.

Wells et al. Carbon monoxide was purchased from Praxair, and ethylene was obtained from Union Carbide. The NMR spectra were recorded using a Varian iNova-400 spectrometer operating at 399.8 MHz for 1H, 161.8 MHz for 31 P, and 100.6 MHz for 13C nuclei. Infrared spectra were obtained on either Nicolet Magna 760 or NicPlan FTIR spectrometers, for solutions or solids, respectively. Elemental analyses were performed by the microanalytical service within the department using a CE 1108 CHNS-O analyzer. Electrospray ionization mass spectra (EIMS) were acquired on a Micromass ZabSpec spectrometer by the staff in the departmental mass spectrometry laboratory. In most cases the distribution of isotope peaks closely matched the calculated distribution for the appropriate parent ion. For complex 6 ion cyclotron resonance mass spectrometry was used to obtain the mass spectrum using an Apex-Qe instrument. Spectroscopic data for all compounds are summarized in Table 1. Preparation of Compounds. (a). [RhOs(CO)(η2-C2H4)(μ-CO)2(dppm)2][CF3SO3] (2). At -78 °C, CO was gently purged for 10 min through a solution containing compound 1 (100 mg, 0.076 mmol) dissolved in 5 mL of CH2Cl2. The solution was stirred for 1 h at -78 °C, after which time a vacuum was applied to the flask for 10 min to remove excess CO. The solution was then warmed to ambient temperature, and 15 mL of pentane was added to precipitate the product (2), which was further dried in vacuo (93 mg, 93%). Anal. Calcd for C56H48F3O6OsP4RhS: C, 50.84; H, 3.66; S, 2.42. Found: C, 50.75; H, 3.79; S, 2.20. HRMS: m/z calcd for C55H48O3P4RhOs (Mþ - CF3SO3) 1175.1218, found 1175.1208. When the addition of CO was performed at ambient temperature, substitution of both Rh- and Os-bound ethylene groups occurred, giving the known compound [RhOs(CO)4(dppm)2][CF3SO3].14 (b). [RhOs(MeCN)(η2-C2H4)( μ-CO)2(dppm)2][CF3SO3] (3). To a yellow solution containing compound 1 (0.250 g, 0.19 mmol) in 5 mL of CH2Cl2 was added 2.5 equiv of MeCN (25 μL, 0.48 mmol) in a single portion. The solution immediately darkened, accompanied by mild effervescence. The addition of 10 mL of pentane immediately caused precipitation of a flocculent yellow solid, which was separated by decanting the mother liquor and then rinsed using 2  5 mL of Et2O before being allowed to dry in vacuo (98%). Anal. Calcd for C57H51NF3O5P4RhOsS: C, 51.24; H, 3.85; N, 1.05. Found: C, 51.43; H, 3.95; N, 0.95. (c). [RhOs(PMe3)(η2-C2H4)( μ-CO)2(dppm)2][CF3SO3] (4). To a stirred solution containing either 1 (150 mg, 0.113 mmol) or 3 (150 mg, 0.112 mmol) in 5 mL of CH2Cl2 was added excess PMe3 (150 μL, 0.15 mmol, 1.3 equiv, 1 M in toluene). No color change was observed for either reaction; however, after stirring for 5 min 31P{1H} NMR spectra confirmed quantitative conversion to 4 along with unreacted PMe3. Slow addition of 15 mL of pentane caused precipitation of a flocculent yellow solid, which was separated by filtration, washed with 2  5 mL portions of pentane, and then dried in vacuo to give a near-quantitative yield (96% prepared from 1) of 4. Anal. Calcd for C58H57F3O5P5RhOsS: C, 50.81; H, 4.19. Found: C, 51.06; H, 4.40. (d). [RhOs(IMe4)(η2-C2H4)( μ-CO)2(dppm)2][CF3SO3] (5). To a slurry containing either partially dissolved 1 (150 mg, 0.113 mmol) or 3 (150 mg, 0.112 mmol) in 5 mL of THF was added 0.58 mL of an IMe4 solution (0.213 M in THF, 1.1 equiv) dropwise. The resulting solution was stirred overnight, yielding a flocculent yellow precipitate. A 5 mL portion of pentane was then added to the mixture to fully precipitate 5. Once the mother liquor was decanted, the remaining yellow solid was dissolved in 5 mL of CH2Cl2 and precipitated using 15 mL of Et2O, yielding a bright yellow solid which was dried in vacuo (80% prepared from 1). HRMS: m/z calcd for C61H60N2O2P4RhOs (Mþ - CF3SO3) 1271.2270, found 1271.2250. Anal. Calcd for C62H60N2F3O5P4RhOsS: C, 52.47; H, 4.26; N, 1.97. Found: C, 52.50; H, 4.51; N, 2.17. (14) Hilts, R. W.; Franchuk, R. A.; Cowie, M. Organometallics 1991, 10, 304.

1769 (s)

[RhOs(CH2CN)(η2-C2H4)(μ-CO)2(dppm)] (8) [[{RhOs(η2-C2H4)(μ-CO)2(dppm)2}2(μ-NCCHC(Me)NH)][CF3SO3] (9) 3:1 (cis:trans);d cis at 28.9 (dm, 1 JPRh = 116), 27.9 (dm, 1 JPRh = 125), 9.0 (m), 8.1 (m, dppm); trans at 29.5 (dm, 1JPRh = 116), 27.1 (dm, 1 JPRh = 123), 9.0 (m), 8.6 (m, dppm)d

PA, 31.3 (dddd); PD, 9.1 (ddd, dppm-H); PB, 23.6 (dddd); PC, 4.4 (ddd, dppm)e (J: PAPB, 352. PAPC, 21; PAPD, 142; PBPC, 79; PBPD, 20; PCPD, 271; PARh, 118; PBRh, 124) 26.0 (dm, 1JPRh = 127); 8.3 (m)e

3.04 (m, 4H, 2JHP(Os) = 2JHP(Rh) = 4.5, dppm); 1.29 (br t, 2H, 3JHP = 1.5, CH2CN); 0.80 (t, 4H, 3JHP = 7.4, C2H4)e cis at 3.21, 3.17 (m, 4H, dppm); 2.88 (s, 1H, CH), 2.12 (bs, 1H, NH), -0.41 (s, 3H, Me); 0.24 (t, 8H, 3 JHP = 7.1, C2H4);d trans at 3.14, 3.08 (m, 4H, dppm); 2.90 (s, 1H, CH); 1.89 (bs, 1H, NH), -0.38 (s, 3H, Me); 0.19 (t, 8H, 3JHP = 7.1, C2H4)d

3.32 (m, 2H, 2JHP(Os) = 2JHP(Rh) = 9.7, dppm); 2.48 (bs, 1H, dppm-H); 2.58 (s, 6H, NMe), 1.67 (s, 6H, IMe4); 0.39, 0.30 (AB m, 4H, 2JHH = 4.7, C2H4)e

215.2 (dm, 2C, 1JCRh = 19, μ-CO); 29.8 (m, 2C, dppm); 167.4 (s, 1C, CdN), 24.2 (s, 1C, CH2dN, CH2CN); 12.3 (s, 2C, C2H4)e 209.2 (m, 4C þ 4C, μ-CO), 172.3 (br s, 1C þ 1C, C(Me), 141.1 (br s, 1C þ 1C, CtN), 52.7 (br s, 1C þ 1C, C(H)), 23.6 (br s, 1C þ 1C, CH3, μ-N(H)C(Me)dC(H)CtN); 31.0, 29.6 (m, 4C þ 4C, dppm); 13.9, 12.5 (m, 4C þ 4C, C2H4)d

195.4 (dt, 2C, 1JCRh = 2JCP(Os) = 8, μ-CO); 64.5 (d, 2C, 1JCRh = 12, Rh-C2H4); 23.4 (s, 2C, Os-C2H4); 34.5 (m, 2C, dppm)d 194.8 (dt, 2C, 1JCRh = 3 Hz, 2JCP(Os) = 11, 2JCP(Rh) = 1.5, μ-CO); 181.4 (dt, 1C, JCRh = 76, 2JCP(Rh) = 16); 28.5 (s, 2C, Os-C2H4); 34.5 (m, 2C, dppm)d 204.9 (dtt, 2C, 1JCRh = 16 Hz, 2JCP(Os) = 7, 2JCP(Rh) = 2.5, μ-CO); 117.1 (bs, 1C, CN), 2.2 (s, 1C, Me, MeCN); 16.1 (t, 2C, 2JCP = 2.4, C2H4); 29.6 (tt, 2C, 1 JCP(Os) = 1JCP(Rh) = 12, dppm)d 202.5 (ddtt, 2C, 1JCRh = 11, 2JCP(Os) = 11, 2JCP(Me3) = 11, 2JCP(Rh) < 2, μ-CO); 19.5 (d, 3C, 1JCP = 29, PMe3); 17.7 (t, 2C, 2JCP < 2, C2H4); 41.4 (m, 2C, dppm)d 208.9 (dt, 2C, 1JCRh = 2JCP(Os) = 8, μ-CO); 185.3 (dt, 1C, 1JCRh = 55, 2JCP = 19, Ccar), 35.3 (s, 2C, NMe), 13.5 (s, 2C, CMe, IMe4); 9.4 (t, 2C, 2 JCP < 2, C2H4); 31.0 (m, 2C, dppm)d 213.6 (dtt, 2C, 1JCRh = 19, 2JCP(Os) = 7, 1JCP(Rh) = 3, μ-CO); 12.8 (t, 2C, 2JCP = 3, C2H4); 31.4 (tt, 2C, 1 JCP(Os) = 1JHP(Rh) = 11, dppm)d 215.4 (dm, 2C, 1JCRh = 23, μ-CO); 205.3 (ddd, 1C, 1 JCRh = 60, 2JCP = 17, Ccar), 34.9 (s, 2C, NMe), 13.2 (s, 2C, CMe, IMe4); 9.3 (s, 2C, C2H4)e

δ(13C{1H})h

a

IR abbreviations: s = strong, m = medium, br = broad. b FTIR microscope. In units of cm-1. c NMR abbreviations: s = singlet, d = doublet, t = triplet, m = multiplet, br = broad. NMR data at 298 K. All δ values are given in ppm and J values in Hz. d In CD2Cl2. e In C6D6. f 31P chemical shifts referenced to external 85% H3PO4. g Chemical shifts for the phenyl hydrogens and carbons not given. h 1H and 13C chemical shifts referenced to TMS.

1778 (s)

1788 (s)

25.8 (dm, 1JPRh = 128); 6.3 (m, dppm)d

1797 (s); 1903 (m)

[RhOs(IMe4)(η2-C2H4)(μ-CO)2(dppm-H)(dppm)] (7)

3.40 (m, 4H, 2JHP(Os) = 2JHP(Rh) = 4.3, dppm); 2.66 (s, 6H, NMe), 1.65 (s, 6H, CMe, IMe4); 0.45 (t, 4H, 3JHP = 7.1, 1JHC = 152.1, C2H4)d

26.5 (ddm, 1JPRh = 130, 2JPP = 40); 2.3 (m, dppm); -23.3 (dtt, 1JPRh = 157, 2 JPP = 40, 2JPC = 11, PMe3)d

1823 (s); 1901 (m)

3.13 (m, 4H, 2JHP(Os) = 2JHP(Rh) = 4.7, dppm); 0.10 (t, 4H, 1JHP = 7.6, Os-C2H4)d

3.73 (m, 4H, 2JHP(Os) = 4.3, 2JHP(Rh) = 4.3, dppm); 0.39 (d, 9H, 3JHP = 8.8, PMe3); 0.64 (t, 4H, 3 JHP = 7.4, 1JHC = 154.8, C2H4)d

30.0 (dm, 1JPRh = 115); 7.2 (m, dppm)d

1803 (s); 1897 (m)

29.6 (dm, 1JPRh = 119); 9.9 (m)d

3.37 (m, 4H, 2JHP(Os) = 2JHP(Rh) = 4.2, dppm); 1.20 (br s, 3H, MeCN); 0.45 (t, 4H, 3JHP = 7.2, 1JHC = 153.2, Os-C2H4)d

26.1 (dm, 1JPRh = 118); 1.0 (m, dppm)d

1876 (s); 2000 (s)

1757 (s); 1810 (m)

3.76 (m, 4H, 2JHP(Os) = 2JHP(Rh) = 4.4, dppm); 2.89 (m, 4H, 1JHC = 159.6, Rh-C2H4); 0.91 (t, 4H, 1JHC = 157.1, 3JHP = 6.4, Os-C2H4)d 3.99 (m, 4H, 2JHP(Os) = 2JHP(Rh) = 4.4, dppm); 1.18 (t, 4H, 1JHC = 158.9, 3JHP = 6.4, Os-C2H4)d

δ(1H)g,h

33.7 (dm, 1JPRh = 118); 2.8 (m, dppm)d

δ(31P{1H})f

1858 (s)

IR (cm-1)a,b

[RhOsCl(η2-C2H4)(μ-CO)2(dppm)2] (6)

[RhOs(η2-C2H4)2(μ-CO)2(dppm)2][CF3SO3] (1) [RhOs(CO)(η2-C2H4)(μ-CO)2(dppm)2][CF3SO3] (2) [RhOs(MeCN)(η2-C2H4)(μ-CO)2(dppm)2][CF3SO3] (3) [RhOs(PMe3)(η2-C2H4)(μ-CO)2(dppm)2][CF3SO3] (4) [RhOs(IMe4)(η2-C2H4)(μ-CO)2(dppm)2][CF3SO3] (5)

compd

NMRc

Table 1. Spectroscopic Data for the Compounds

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(e). [RhOsCl(η2-C2H4)( μ-CO)2(dppm)2] (6). To a stirred solution containing either 1 (50 mg, 0.038 mmol) or 3 (50 mg, 0.037 mmol) in 1 mL of CH2Cl2 was added [PPN][Cl] (23.9 mg, 0.042 mmol, 1.1 equiv) in a single portion. Compound 6 precipitated from CH2Cl2 in near-quantitative yield, and the solid was rinsed with a further 0.2 mL portion of cold CH2Cl2 before drying in vacuo (92% prepared from 1). Anal. Calcd for C54H48ClO2OsP4Rh: C, 54.90; H, 4.10. Found: C, 54.99; H, 4.12. (f). [RhOs(IMe4)(η2-C2H4)( μ-CO)2(Ph2PCHPPh2)(dppm)] (7). To a slurry containing 50 mg of 1 (0.038 mmol) in 0.7 mL of C6D6 was added 0.39 mL of IMe4 (2.133  10-1 M in THF, 2.2 equiv) via syringe. Monitoring the reaction by 31P{1H} NMR spectroscopy confirmed that it had proceeded to completion. The resulting red solution was reduced in vacuo to give an oil, which was redissolved in 0.2 mL of C6D6, and to this solution was added 0.3 mL of CH3CN, causing yellow crystals to form on the walls of the NMR tube. These crystals were separated from the supernatant and allowed to dry in vacuo (39 mg, 80%). Anal. Calcd for C61H59N2O5P4RhOs 3 0.5CH3CN 3 0.5C6D6: C, 58.62; H, 5.03; N, 2.63. Found: C, 58.72; H, 5.14; N, 2.76. The presence of the solvents of crystallization was verified by 1 H NMR spectroscopy in CD2Cl2, in which protonation of the compound also occurred to generate compound 5, containing a chloride counterion and having a single deuterium in the dppm methylene groups. (g). [RhOs(CH2CN)(η2-C2H4)( μ-CO)2(dppm)2] (8). To a rapidly stirred solution of excess KN(Si(CH 3 )3)2 (32 mg, 0.16 mmol, 4 equiv) in 1 mL of CH3CN was added, via cannula, 50 mg (0.04 mmol) of compound 3 in 1 mL of CH3CN, resulting in the immediate formation of in an orange precipitate. The remaining solvent was removed via cannula, and the crude product was dried in vacuo. The NMR spectra of the resulting mixture of compounds 8 and 9, provided as Supporting Information, show only the two products in an approximate 2:1 ratio. Owing to our failure to separate 8 and 9, characterization of 8 is based on the spectral results and on its subsequent transformation to compound 9. (h). [(RhOs(η2-C2H4)( μ-CO)2(dppm)2)2( μ-NHC(CH3)dCHCN)][CF3SO3] (9). To a solution of 3 (200 mg, 0.15 mmol) in 3 mL of acetonitrile was added 0.77 mL of an IMe4 solution (2.13310-1 M in THF, 1.1 equiv), resulting in a color change to deep orange followed by the rapid formation of a yellow precipitate. The mixture was stirred for an additional 5 min and left standing for 1 h before the light yellow mother liquor was decanted to waste, yielding 148 mg (78%) of the solid product. In an NMR tube the microcrystalline material was dissolved in a minimum volume of CH2Cl2 (approximately 1 mL) and the resulting solution was layered with 5 mL of Et2O, yielding X-ray-quality red crystals after 1-2 days. All electrospray MS methods failed to detect the parent ion of 9; however, Fourier transform ion cyclotron resonance MS was successful in determining the structural formula. HRMS: m/z calcd for C112H101N2O4P8Rh2Os2 (Mþ - CF3SO3) 2375.2996, found 2375.2985. Anal. Calcd for C113H101F3N2O7Os2P8Rh2S 3 3.25CH2Cl2: C, 49.90; H, 3.87; N, 1.00. Found: C, 49.95; H, 3.89; N, 0.99. X-ray Structure Determinations. (a). General Considerations. Crystals were grown via slow diffusion using the following solvent systems: nitromethane/n-pentane (4) and dichloromethane/diethyl ether (9). Both data sets were collected using a Bruker APEX II CCD detector/D8 diffractometer15 with Mo KR radiation, with the crystals cooled to -100 °C. The data were corrected for absorption through Gaussian integration from indexing of the (15) Programs for diffractometer operation, unit cell indexing, data collection, data reduction and absorption correction were those supplied by Bruker. (16) Beurskens, P. T.; Beurskens, G.; de Gelder, R.; Garcia-Granda, S.; Israel, R.; Gould, R. O.; Smits, J. M. M. The DIRDIF-99 program system; Crystallography Laboratory, University of Nijmegen, Nijmegen, The Netherlands, 1999.

Wells et al. Scheme 1

crystal faces15 (4) or through use of a multiscan model (SADABS)15 (9). Structures were solved using Patterson search/structure expansion (DIRDIF-2008)16 (4) or direct methods (SHELXS-97)17 (9). Refinements were completed using the program SHELXL-97.17 Hydrogen atoms were assigned positions based on the idealized sp2 or sp3 geometries of their attached carbon or nitrogen and were given thermal parameters 20% greater than those of the parent atoms. See the Supporting Information for a listing of crystallographic experimental data. (b). Notes. (i) For compound 4, distances involving corresponding pairs of atoms within the disordered solvent nitromethane molecule were constrained to be equal (within 0.03 A˚) during refinement: d(O(1S)-N(1S)) = d(O(3S)-N(2S)); d(O(2S)-N(1S)) = d(O(4S)-N(2S)); d(N(1S)-C(1S)) = d(N(2S)C(2S)); d(O(1S) 3 3 3 O(2S)) = d(O(3S) 3 3 3 O(4S)); d(O(1S) 3 3 3 C(1S))= d(O(3S) 3 3 3 C(2S)); d(O(1S) 3 3 3 C(1S)) = d(O(3S) 3 3 3 C(2S)). (ii) For compound 9, the geometry of the minor orientation of the disordered triflate ion was restrained to have the same geometry (by use of the SHELXL-9717 SAME instruction) as that of the major orientation. The solvent dichloromethane molecules in the 0.25 occupied sites were restrained to have C-Cl and Cl 3 3 3 Cl distances of 1.770(2) and 2.870(4) A˚, respectively.

Results and Compound Characterization (a). Ethylene Substitution Products. The bis-ethylene compound [RhOs(η2-C2H4)2( μ-CO)2(dppm)2][CF3SO3] (1), having one ethylene bound to each metal opposite the Rh-Os bond, was chosen as the precursor to the targeted series of monoethylene complexes on the assumption that the Rhbound ethylene ligand would be more labile than that bound to Os. This has proven to be the case, with facile substitution of this ethylene group occurring, yielding the mono-ethylene products [RhOs(L)(η 2-C 2H 4)( μ-CO)2 (dppm)2][CF3SO 3 ] (L=CO (2), MeCN (3), PMe 3 (4), IMe 4 (5)) and [RhOsCl(η2-C2H4)( μ-CO)2(dppm)2] (6), upon addition of the respective ligands, as outlined in Scheme 1. The Os-bound ethylene ligand of 1 is substantially less labile and remains inert to substitution even in the presence of a several-fold excess of MeCN, PMe3, IMe4, or [PPN][Cl]. Even for the strongly coordinating CO ligand, selective substitution of only the Rh-bound ethylene can be effected if the reaction is carried out at -78 °C, while reaction with CO at ambient temperature results in displacement of both ethylene groups, generating the previously reported species14 [RhOs(CO)4(dppm)2]þ. Addition of 1 equiv of PMe3 or IMe4 to 1 yields the monosubstitution products, but attempts to replace both ethylene groups in 1 by these strongly coordinating ligands, by adding them in excess under ambient conditions, does not give the bis-PMe3 or bis-IMe4 targets. Presumably, substitution of the second ethylene group by either PMe3 or IMe4 is inhibited by their steric bulk. Although double substitution would result in the added ligands being on different (17) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.

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Figure 1. Perspective view of the [RhOs(PMe3)(η2-C2H4)(μCO)2(dppm)2]þ cation of 4, showing the atom-labeling scheme. Numbering of the phenyl rings starts at the ipso carbon and works sequentially around the rings. Non-hydrogen atoms are represented by Gaussian ellipsoids at the 20% probability level. Hydrogen atoms are shown with arbitrarily small thermal parameters, except for phenyl hydrogens, which are not shown. Relevant parameters (distances in A˚ and angles in deg): Os-Rh= 2.8530(3), Os-C(1) = 1.967(3), Os-C(2) = 1.914(3), Os-C(3)= 2.167(3), Os-C(4)=2.175(3), C(3)-C(4) = 1.438(4), Rh-P(5)= 2.3155(8), Rh-C(1)=2.238(3), Rh-C(2) = 2.647(3); Rh-Os-C(1)=51.42(8), Rh-Os-C(2)=63.96(9), Os-C(1)-O(1)=158.9(2), Os-C(2)-O(2) = 169.8(3), Rh-C(1)-O(1) = 115.2(2), Os-C(3)-C(4) =70.9(2), Os-C(4)-C(3) =70.4(2), C(3)-Os-C(4) = 38.7(1).

metals at opposite ends of the complex, where direct mutual contact is minimized, we anticipate that significant repulsions would nevertheless result with the dppm phenyl groups, which align themselves in such a way as to avoid ligands in the equatorial plane (see Figure 1, for example). However, the addition of excess IMe4 does lead to subsequent, unanticipated reactivity, as will be described later. Complexes 1-6 have similar 31P{1H}, 13C{1H}, and 1H NMR spectral parameters (see Table 1) which support the structures shown in Scheme 1. All 31P{1H} NMR spectra, except for that of 4 (vide infra), which has the additional phosphine group, display patterns that are characteristic of AA0 BB0 X spin systems in which the dppm ligands are mutually trans at both metals. The Rh-bound ends of the dppm ligands appear at lower field (δ 25.8-33.7), displaying typical couplings to 103Rh of between 115 and 130 Hz, while the Os-bound ends appear at higher field (δ 1.0-9.9). In the 1H NMR spectra all species display a single multiplet (showing coupling to both pairs of Rh- and Os-bound 31P nuclei) in the range δ 3.1-4.0 for the dppm methylene protons, indicative of symmetrical environments on either side of the RhOsP4 plane (at least on the NMR time scale), and triplets in the range δ 0.1-1.2 for the Os-bound ethylene groups. In all compounds, the Os-bound ethylene protons display well-resolved three-bond coupling (6.4-7.6 Hz) to the adjacent 31P nuclei, whereas those on the Rh-bound ethylene of 1 appear as a broad signal having no resolvable phosphorus coupling. The 1H NMR resonance for the Rhbound ethylene in 1 is also significantly downfield (at δ 2.89)

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from the Os-bound ethylene groups (range δ 0.1-1.2) in all compounds. In samples prepared using 13C2H4, one-bond C-H coupling of 152-160 Hz is observed for all ethylene groups (Rh and Os) in the 1H NMR spectra, close to the value for free ethylene (154 Hz), while in the 13C{1H} NMR spectra the Osbound ethylene groups appear as a single resonance in the range δ 9.4-28.5, significantly upfield from the Rh-bound ethylene ligand of 1 at δ 64.5 and that of free ethylene (δ 122.8). The highfield chemical shifts for the Os-bound ethylene groups in both the 1H and 13C{1H} NMR spectra and the resolvable coupling to phosphorus in the former are consistent with these groups being more tightly bound owing to better π back-donation, which is also consistent with their lower lability. In all compounds, only a single resonance is observed for the pair of semibridging carbonyl groups in the range δ 194.8-213.6, having coupling to 103Rh of between 3 and 19 Hz. The corresponding IR spectra show a strong band between 1757 and 1858 cm-1 and a medium-intensity band in the range 1810-1903 cm-1, both of which are consistent with some degree of bridging but inconsistent with a terminally bound carbonyl in these cationic or neutral low-valent systems. On the basis of the structures of related species reported herein, we propose that both carbonyls are semibridging, although they possibly have different degrees of interaction with rhodium, and exchange rapidly on the NMR time scale. The spectral parameters for the Rh-bound C2H4, CO, MeCN, PMe3, and IMe4 ligands of compounds 1-5, respectively, appear as expected. For 2 the Rh-bound carbonyl group appears as a doublet of triplets at δ 181.4 in the 13C{1H} NMR spectrum, displaying an expected coupling to 103Rh of 76 Hz and to the adjacent 31P nuclei of 16 Hz. For 3 the acetonitrile group appears as a broad singlet at δ 1.20 in the 1H NMR spectrum, while in the 13C{1H} NMR spectrum the nitrile and the methyl carbons appear at δ 117.1 and 2.2, respectively; no 103 Rh coupling is observed for either. For compound 4, the PMe3 group is clearly identified as bound to Rh, appearing as a doublet of triplets at δ -23.3 in the 31P{1H} NMR spectrum and displaying 157 Hz coupling to 103Rh and 40 Hz coupling to the Rh-bound 31P nuclei of the dppm groups. Consequently, the 31P{1H} resonance for the Rh-bound phosphorus atoms of dppm also display coupling to the PMe3 group. In a sample that is enriched in 13CO, the PMe3 group displays additional 11 Hz coupling to both carbonyls. In the 1H NMR spectrum, the IMe4 group of 5 displays one resonance for each pair of N- and C-bound methyl groups at δ 2.66 and 1.65, respectively, and displays a doublet of triplets at δ 185.3 in the 13C{1H} NMR spectrum for the carbene carbon, with typical 55 Hz coupling to 103 Rh and 19 Hz coupling to the Rh-bound 31P nuclei, again confirming the Rh-bound formulation. The X-ray structure determination of 4 confirms the structural assignments proposed above, as shown for the complex cation in Figure 1, in which the PMe3 ligand is bound to Rh while the ethylene ligand is bound to Os, each opposite the metal-metal bond. The main ambiguity regarding the spectral parameters of compounds 1-6 (concerning the binding modes of the carbonyl ligands) is clarified in this structure, which shows that one carbonyl (C(1)O(1)) is semibridging and is primarily bound to Os, as demonstrated by the unsymmetrical Os-C(1)-O(1) and Rh-C(1)-O(1) angles (158.9(2) and 115.2(2)°) and Os-C(1) and Rh-C(1) distances (1.967(3) and 2.238(3) A˚), respectively. The second carbonyl is nominally terminally bound to Os, having an Os-C(2) bond length of 1.914(3) A˚ and an Os-C(2)-O(2) angle of 169.8(3)°, although the slight bending of this group suggests a weak interaction with Rh

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(consistent with the low stretching frequency in the IR spectrum). However, the very long Rh-C(2) separation of 2.647(3) A˚ indicates that any such interaction (at least in the solid state) must be weak. The metal-metal separation of 2.8530(3) A˚ is consistent with the presence of a single bond, showing significant contraction in comparison to the intraligand P-P separations of 3.026(1) and 3.059(1) A˚. Furthermore, the Rh-P(5) distance of 2.3155(8) A˚ is normal for a Rh-P single bond, as demonstrated in other PMe3 complexes.10d The ethylene ligand is symmetrically bound to Os, having normal Os-C distances of 2.167(3) and 2.175(3) A˚ and a C(3)-C(4) separation of 1.438(4) A˚,18 showing substantial elongation in comparison to the C-C separation in free ethylene (1.33 A˚).19 (b). IMe4 as a Brønsted Base. (i). dppm Deprotonation. As noted above, the reaction of the precursor bis-ethylene complex (1) with excess CO at ambient temperature resulted in substitution of both ethylene groups, while with the MeCN, PMe3, and Cl- ligands substitution of the Os-bound ethylene did not occur. Although attempts to substitute the Os-bound ethylene group in 5 by IMe4 were also unsuccessful, a different reactivity pathway was observed. Rather than displacing the Os-bound ethylene ligand, the additional IMe4 group serves as a Brønsted base, abstracting a proton from one dppm methylene group, generating [RhOs(IMe4)(η2-C2H4)( μ-CO)2(dppm-H)(dppm)] (7), (dppm-H refers to the anionic bis(diphenylphosphino)methanide bridging group), as shown in Scheme 2. The methylene group of coordinated dppm is known to be acidic, and dppm-H species similar to 7, in which the resulting dppm-H group remains in the bridging position trans to the unaltered dppm group, have been reported by our group20a,b and others.20c-m Furthermore, the strong basicity of NHC ligands is also well recognized.21 We are aware of one previous report of an NHC ligand functioning as a Brønsted base in organometallic complexes.22 The 31P{1H} NMR spectrum of 7 displays four separate resonances, characteristic of an ABCDX spin system with the Rh-bound and Os-bound ends of dppm appearing at δ 23.6 and 4.4, respectively, and of dppm-H at δ 31.3 and 9.1, respectively. The large two-bond P-P couplings across Rh (352 Hz) and Os (271 Hz) are consistent with a mutually trans arrangement of dppm and dppm-H groups at both metals, while the intraligand two-bond 31P-31P coupling within the dppm-H group (142 Hz) is significantly larger than that Scheme 2

within the dppm ligand (79 Hz), consistent with the delocalized bonding shown for the former in Scheme 2 and consistent with the coupling observed in analogous dppm-H-bridged complexes.20a,b In the 1H NMR spectrum, the dppm-H methanide proton is located at δ 2.48, upfield from the dppm methylene protons at δ 3.32, again consistent with previous determinations.20a,b The Os-bound ethylene protons appear as AA0 BB0 multiplets, as anticipated from the “top/bottom” asymmetry present in the compound, and these resonances at δ 0.39 and 0.30 are further complicated by the coupling to the Os-bound phosphorus nuclei. In the 13C{1H,31P} NMR spectrum, only a single resonance at δ 215.4 is observed for the carbonyl ligands with coupling to 103Rh of 23 Hz, suggesting a bridging coordination mode for both, while only a single broad band is observed at 1788 cm-1 in the IR spectrum. These spectral data suggest similar semibridging geometries for both carbonyls, as diagrammed in Scheme 2. The carbene carbon of the Rh-bound IMe4 group, observed at δ 205.3 in the 13C{1H} NMR spectrum, displays 60 Hz coupling to 103Rh, while the signal for the Os-bound ethylene ligand, located at δ 9.3, shows no distinguishable coupling to the adjacent Os-bound phosphines. (ii). Acetonitrile Deprotonation. As noted earlier, compound 5 can be prepared in THF either by the replacement of the Rh-bound ethylene group in 1 or of the acetonitrile ligand in 3. However, if the reaction of 3 with IMe4 is carried out in acetonitrile instead of THF, very different reactivity is observed, initiated by deprotonation of the coordinated acetonitrile by IMe4, as outlined in Scheme 3. The final product in this reaction is a mix of two isomers of [(RhOs(C2H2)( μ-CO)2(dppm)2)2( μ-NHC(Me)dCHCN)]þ (9) in which two “[RhOs(η2-C2H4)( μ-CO)2(dppm)2]” units are linked by the “HNC(Me)dC(H)CN” ligand in either a cis or a trans arrangement about the olefinic bond, as depicted in the simplified drawings of these isomers in Scheme 3. As will be discussed later, this bridging ligand results from coupling of the acetonitrilide ligand on one complex with an acetonitrile ligand on another. The conformers 9-cis and 9-trans exist in variable proportions, depending on solvent polarity. For example, in either CH2Cl2 or THF the cis:trans ratio is approximately 3:1, while in acetonitrile the ratio is 28:1. Identification of the cis and trans isomers was established by NOE, in which a correlation between the protons on the mutually cis methyl and olefinic proton of 9-cis was observed in the 2D-NOESY spectrum, while none was observed for the trans isomer. This transformation of 3 to 9 can also be more effectively carried out using a more conventional base such as KN(Si(CH3)3)2 to deprotonate the acetonitrile ligand. The intermediate deprotonated species (8) is observed as a minor component (5-10%) alongside the final product (9) in the 31P{1H} NMR spectrum; however, it disappears within

Scheme 3

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30 min of addition of base under ambient conditions. In an attempt to isolate 8, we added 2.2 equiv of the bis(trimethylsilyl)amide to 3 in acetonitrile, anticipating that excess base would convert all of 3 to the desired deprotonated species, leaving none of the precursor available for coupling; however, the formation of 9 was unavoidable and was still obtained as the major product (ca. 10:1) under these conditions. Reversing the order of addition, adding 3 to a 4-fold excess of KN(SiMe3)2), still yielded some of the final product (9) but gave 8 as the major species (in a 2:1 ratio). Attempts to obtain 8 by the reverse addition of 3 to IMe4 only generated compound 7-the product resulting from initial acetonitrile substitution by IMe4 and subsequent dppm deprotonation, as described earlier. In the 31P{1H} NMR spectrum, compound 8 displays resonances at δ 26.0 (1JPRh = 127 Hz) and δ 8.3, assigned to Rh- and Os-bound ends of the diphosphines. The dppm methylene protons appear in the 1H NMR spectrum as a multiplet at δ 3.04, integrating as four protons, confirming that deprotonation of these groups, as is commonly observed under strongly basic conditions20 and as observed earlier in 7, has not occurred. The ethylene protons appear as a triplet of appropriate integration at δ 0.80, showing 7.4 Hz coupling to the Os-bound 31P nuclei. A broad triplet at δ 1.29, assigned to the ketenimino protons, shows a small 1.5 Hz coupling to the Rh-bound phosphines. The 13C{1H} NMR spectrum of 8 shows the expected multiplet for the bridging carbonyl groups at δ 215.2 (1JCRh = 19 Hz) and the ethylene group at δ 12.3. In addition, 13C{1H} resonances, corresponding to the anionic CH2dCdN- ligand, were observed as singlets at δ 24.2 and 167.4; the absence of coupling to Rh in the former signal confirms that this group has remained N-bound and has not rearranged to a cyanomethyl group (vide infra).20b The 31P{1H} spectrum of 9 is quite complicated, due to the presence of two sets of Rh- and Os-bound ends of the diphosphines for each of the cis and trans isomers. For the cis isomer the resonances (in CD2Cl2) for the Rh-bound 31 P nuclei appear at δ 28.9 (1JPRh=116 Hz) and 27.9 (1JPRh= 125 Hz), while those for the Os-bound ends of the diphosphine appear at 9.0 and 8.1. For the trans isomer the equivalent resonances appear at δ 29.5 (1JRhP = 116 Hz), 27.1 (1JRhP = 123 Hz), 9.0, and 8.6, respectively. In the 1 H NMR spectrum the signals for the cis-bridging NtCC(H)dC(CH3)N(H) ligand are observed as singlets in a 1:3:1 intensity ratio at δ 2.88, -0.41 and 2.12, respectively, (18) Bender, B. R.; Hembre, R. T.; Norton, J. R. Inorg. Chem. 1998, 37, 1720. (19) Kuchitsu, K. J. Chem. Phys. 1966, 44, 906. (20) (a) Torkelson, J. R.; Oke, O.; Muritu, J.; McDonald, R.; Cowie, M. Organometallics 2000, 19, 854. (b) Sterenberg, B. T.; Hilts, R. W.; Moro, G.; McDonald, R.; Cowie, M. J. Am. Chem. Soc. 1995, 117, 245. (c) Ge, Y. W.; Peng, F.; Sharp, P. R. J. Am. Chem. Soc. 1990, 112, 2632. (d) Sharp, P. R.; Ge, Y. W. J. Am. Chem. Soc. 1987, 109, 3796. (e) Ye, C.; Sharp, P. R. Inorg. Chem. 1995, 34, 55. (f) Brown, M. P.; Yavari, A.; Manojlovic-Muir, L.; Muir, K. W. J. Organomet. Chem. 1983, 256, C19. (g) Dominguez, R.; Lynch, T. J.; Wang, F. J. Organomet. Chem. 1988, 338, C7. (h) Camus, A.; Marsich, N.; Nardin, G.; Randaccio, L. J. Organomet. Chem. 1973, 60, C39. (i) Browning, J.; Bushnell, G. W.; Dixon, K. R. J. Organomet. Chem. 1980, 198, C11. (j) Briant, O. E.; Hall, K. P.; Mingos, D. M. P. J. Organomet. Chem. 1982, 229, C5. (k) Al-Jibori, S.; Shaw, B. L. Inorg. Chim. Acta 1983, 74, 235. (l) Us
on, R.; Laguna, A.; Laguna, M.; Manzano, B. R.; Jones, P. G.; Sheldrick, G. M. J. Chem. Soc., Dalton Trans. 1984, 839. (m) Hashimoto, H.; Nakamura, Y.; Okeya, S. Inorg. Chim. Acta 1986, 122, L9. (21) (a) Kim, Y.-J.; Streitwieser, A. J. Am. Chem. Soc. 2002, 124, 5757. (b) Magill, A. M.; Cavell, K. J.; Yates, B. F. J. Am. Chem. Soc. 2004, 126, 8717. (c) Alder, R. W.; Allen, P. R.; Williams, S. J. Chem. Commun. 1995, 1267. (22) Edwards, P. G.; Hahn, F. E.; Limon, M.; Newman, P. D.; Kariuki, B. M.; Stasch, A. Dalton Trans. 2009, 5115.

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while the corresponding signals for the trans isomer appear at 2.90, -0.38, and 1.89. For each isomer only one set of dppm methylene resonances and one set of olefin resonances appear, with the latter appearing as a triplet at δ 0.24 for the cis isomer and at δ 0.19 for the trans isomer; both display 7.1 Hz coupling to the Os-bound phosphines. In the 13C{1H} NMR spectrum all resonances have been assigned for the bridging ligand, the dppm methylene carbons, the carbonyls, and the ethylene ligands (see Table 1). For the carbonyl ligands only one broad resonance is observed for both isomers at δ 209.2, for which no coupling to Rh is resolved; however, the relative downfield shift suggests a semibridging arrangement, consistent with the carbonyl stretch in the IR spectrum at 1778 cm-1. Two ethylene resonances are observed at δ 13.9 and 12.5 in the 13C{1H} NMR for the major and minor isomers, respectively. The structure proposed, on the basis of the spectral data, has been confirmed for the trans isomer of 9 by an X-ray structure determination, and a representation of the complex cation is shown in Figure 2. Although 9-trans is the only isomer in the crystal studied, dissolution of these crystals in CD2Cl2 gives the equilibrium isomer ratio noted earlier, further suggesting interconversion of the two isomers in solution. Both “RhOs(CO)2(C2H4)(dppm)2” ends of this tetranuclear system are structurally almost identical and also are comparable to the structure reported for the PMe3 complex 4. Both carbonyls on each “RhOs” unit are semibridging, in which both are primarily bound to Os but one displays a slightly stronger interaction with Rh, as shown by the Rh-CO bond lengths and the unsymmetrical angles at the carbonyl carbons. The semibridging geometries in 9 are somewhat less pronounced than the strongly semibridging CO in 4 but more pronounced than the very weakly semibridging CO in this earlier species, with Os-C-O angles ranging from 156.9(3) to 168.2(3)°, Rh-C-O angles from 113.6(3) to 118.0(3)°, Os-CO distances from 1.935(4) to 1.978(3) A˚, and Rh-CO distances from 2.138(3) to 2.448(4) A˚. These slight differences in the bridging carbonyl geometries between the two complexes may be a result of packing effects and may not be of chemical significance. Within the nitrile end of the bridging ligand the N(1)-C(171) distance (1.156(4) A˚) is comparable to a typical distance for nitriles (1.14 A˚),23 while the formally olefinic bond (C(172)-C(173) = 1.386(5) A˚) is somewhat elongated and is surprisingly close to the adjacent C(171)-C(172) bond length (1.384(5) A˚), suggesting delocalization over this ligand framework as shown by the canonical extremes in Chart 1. Similarly, the N(2)-C(173) bond (1.319(4) A˚) is slightly shorter than would be expected for a C(sp2)-NR2 bond (1.36 A˚), again consistent with a delocalized bonding model.11 Furthermore, the torsion angles within this ligand, C(171)-C(172)C(173)-C(174)= -3.4(5)° and C(171)-C(172)-C(173)N(2) = 175.4(3)°, indicate a close-to-planar arrangement for this bridging group, as required for the delocalization proposed. All parameters for this bridging group are in close agreement with those in the closely related complex [(Re(CO)3(bpy))2( μ-NHC(CH3)CHCN)]þ, for which a delocalized model for the bridging ligand was also proposed.24 (23) (a) Allen, F. H.; Kennard, O.; Watson, D. G. J. Chem. Soc. Perkin Trans. 2 1987, S1. (b) Kaneti, J; Schleyer, P; Clark, T.; Kos, A; Spitznagel, G; Andrade, J.; Moffats, J. J. Am. Chem. Soc. 1986, 108, 1481. (24) Yam, V. W.-W.; Wong, K. M.-C.; Cheung, K.-K. Chem. Commun. 1998, 135.

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Organometallics, Vol. 30, No. 4, 2011

Wells et al.

Figure 2. View of the [(RhOs(C2H4)(CO)2(dppm)2)2(μ-NCCHC(Me)NH)]þ complex cation of 9-trans. All dppm phenyl groups, except the ipso carbon atoms, have been removed for clarity. Thermal parameters are as described in Figure 1. Selected parameters (distances in A˚ and angles in deg): Os(1)-Rh(1) = 2.7625(3), Os(1)-C(1) = 1.978(3), Os(1)-C(2) = 1.935(4), Os(1)-C(5) = 2.167(3), Os(1)-C(6) = 2.169(3), C(5)-C(6) = 1.448(6), Rh(1)-C(1) = 2.138(3), Rh(1)-C(2) = 2.448(4), Rh(1)-N(1) = 2.030(3), N(1)-C(171) = 1.156(4), C(171)-C(172) = 1.384(5), C(172)-C(173) = 1.386(5), C(173)-N(2) = 1.319(4), Os(2)-Rh(2) = 2.7651(3), Os(2)-C(3) = 1.965(3), Os(2)-C(4) = 1.936(4), Os(2)-C(7) = 2.165(4), Os(2)-C(8) = 2.162(3), C(7)-C(8) = 1.454(5), Rh(2)-N(2) = 2.084(3), Rh(2)-C(3) = 2.182(4), Rh(2)-C(4)=2.370(4); Os(1)-C(1)-O(1)=156.9(3), Os(1)-C(2)-O(2) = 168.2(3), Rh(1)-C(1)-O(1) = 118.0(3), Rh(1)-C(2)-O(2) = 113.6(3), Rh(1)-N(1)-C(171)=159.2(3), N(1)-C(171)-C(172) = 177.7(4), C(171)-C(172)-C(173) = 122.1(3), C(172)-C(173)-N(2) = 122.2(3), C(173)-N(2)-Rh(2) = 130.8(3), Os(2)-C(3)-O(3) = 160.1(3), Os(2)-C(4)-O(4) = 164.5(3), Rh(2)-C(3)O(3) = 115.7(3), Rh(2)-C(4)-O(4) = 115.8(3). Chart 1

The close-to-orthogonal arrangement of the two “RhOsP4” planes in 9 (dihedral angle 78.98(2)°) presumably minimizes repulsions between the phenyl rings on both Rh ends of the diphosphines. The ethylene ligands on each end of the complex are symmetrically bound and display the expected lengthening of the olefinic bond (1.448(6), 1.454(5) A˚) upon coordination, again in close agreement with the parameters for 4.

Discussion (a). Substitution Products. The lability of the Rh-bound ethylene ligand in [RhOs(η2-C2H4)2( μ-CO)2(dppm)2]þ makes this species a convenient precursor for generating the series of monoethylene complexes [RhOs(L)(η2-C2H4)( μ-CO)2(dppm)2]þ (L = CO (2), MeCN (3), PMe3 (4), IMe4 (5)) and [RhOsCl(η2-C2H4)( μ-CO)2(dppm)2] (6). In all cases (even with the weakly coordinating acetonitrile ligand), substitution of the Rh-bound ethylene by the added ligand

Chart 2

takes place readily. Only in the case of carbon monoxide does replacement of both ethylene ligands occur at ambient temperature. Nevertheless, the target monosubstitution product is readily obtained by carrying out the reaction at -78 °C. Surprisingly perhaps, none of the other added ligands result in the substitution of both ethylene groups; for the strongly coordinating PMe3 and IMe4 groups, this is presumably due to their steric bulk, which inhibits binding of a second ligand, as described earlier. The carbonyl-substitution product [RhOs(CO)(η2-C2H4)( μ-CO)2(dppm)2]þ (2) allows for an interesting comparison to its previously reported isomer (A; shown in Chart 2), in which the olefin is bound to Rh rather than Os.3c Synthesis of this isomer also made use of the greater lability of Rh-bound ligands, through reaction of the tetracarbonyl precursor [RhOs(CO)4(dppm)2]þ with Me3NO in the presence of ethylene. We were unable to establish which isomer was thermodynamically favored, since we had previously shown that ethylene transfer from metal to metal apparently does not occur readily in these systems,3c and as a result interconversion of these isomers was never observed.

Article

Organometallics, Vol. 30, No. 4, 2011 Chart 3

On the basis of the greater coupling observed (6 Hz) between the semibridging 13CO groups and 103Rh in isomer A these bridging interactions appear to be stronger than in 2, for which the coupling between these carbonyls and 103Rh was only 3 Hz, in agreement with the slight downfield shift observed in the 13C NMR spectra of δ 198.0 for A vs δ 194.8 for 2. This suggests that stronger donor and/or weaker π-acceptor groups on Rh should promote greater π backdonation from this metal to the semibridging carbonyls, leading to a stronger bridging interaction. However, contradictory evidence is observed in the IR spectra, where the stretch corresponding to the bridging carbonyls of A (1903 cm-1) is at a higher frequency than that of 2 (1876 cm-1), implying a weaker bridging interaction in A and demonstrating a lack of consistency in correlating carbonyl stretching frequencies to 13C NMR shifts and 103 Rh-13C coupling constants, in attempts to determine the extent of back-donation from Rh. However, it must be recalled that ν(CO) for these semibridging carbonyls depends not only upon the degree of interaction with the adjacent metal (Rh) but also upon the amount of π back-donation from Os. In the case of isomer A the additional Os-bound CO will leave less electron density available for backdonation to the semibridging groups, leading in this case to a higher ν(CO) for these groups. Of these two competing effects, presumably in this case back-donation from Os dominates, consistent with the small coupling to Rh observed in the 13C{1H} NMR spectrum. The other accompanying CO stretch, observed at 2000 and 1985 cm-1 for 2 and A, respectively, is assigned to the terminally bound carbonyl group, and the difference in these is consistent with more π back-donation in the case of the Os-bound CO. In comparing the series of ethylene complexes [RhOsL(C2H4)( μ-CO)2(dppm)2]þ (L = C2H4 (1), CO (2), MeCN (3), PMe3 (4). IMe4 (5)) and [RhOsCl(C2H4)( μ-CO)2(dppm)2] (6), we had hoped that clear trends in spectral parameters might emerge that could subsequently help rationalize their reactivities (to be addressed in a subsequent paper) and allow structural comparisons to be made from spectral data. In particular, carbonyl ligands are notoriously effective as indicators for the electron richness of the metal centers. However, when a pair of carbonyls bridge a pair of metals, as is observed in this series of compounds, a number of complications arise. As we have seen in other studies involving mixed-metal systems,10d,12,20b the carbonyls usually do not bridge the pairs of metals symmetrically. For the compounds being considered in this paper (1-6), there is a range of possible geometries that lie between what can be considered as the two extremes shown in Chart 3. In one extreme (B) both carbonyl ligands are bound in identical semibridging environments in which each is primarily bound to Os and has a weaker interaction with Rh. At the other extreme, one carbonyl is in a more standard bridging arrangement (symmetrically or close to symmetrically bridged), while

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the other is terminally bound to Os, having no interaction with Rh (C). Extreme C is expected to be fluxional in solution, with the bridging CO moving to a terminal position while the originally terminal carbonyl moves to a bridging site, since this exchange involves very little movement of both carbonyls. As a result, this fluxionality is often rapid on the NMR time scale even at -78 °C and can remain undetected by this technique. Although this complication can in principle be deciphered by IR spectroscopy (having a faster time scale), peaks corresponding to carbonyls having different degrees of semibridging interactions can be coincidentally overlapping, giving rise to only one broad band. Although IR spectroscopy can readily differentiate terminal from bridging CO modes, in our experience, it is not very diagnostic for differentiating the subtleties of the different bridging geometries, which all appear at comparable frequencies, commonly in the range 1700-1850 cm-1. Furthermore, as noted above, it is difficult to deconvolute the contributions to back-donation from the adjacent metal (Rh in these compounds) and from the primary metal (Os). Nevertheless, a comparison of the 13C{1H} NMR parameters (although they are only averaged owing to fluxionality) in the series 1-6 does yield some helpful information. In progressing from those complexes for which L is a π acid (L=C2H4 (1), CO (2)) through neutral donors (MeCN (3), PMe3 (4), IMe4 (5)) to the anionic Cl- (6), there is a steady progression of 13CO chemical shifts to lower field. This arises as the Rh center becomes more electron rich, functioning as a better donor into the π* orbitals of the Osbound carbonyls and giving these carbonyls more bridging character. This trend in the order of increasing electron density at the metals for the series [RhOs(L)(η2-C2H4)( μ-CO)2(dppm)2]þ is (13C chemical shifts in parentheses in ppm): L=CO (194.8)