Formation of a [ONN (allyl) O]− Anion via NO Insertion and Coupling

Jun 15, 2010 - Nan Xu , Jonathan H. Christian , Naresh S. Dalal , Erwin G. Abucayon , Colin Lingafelt , Douglas R. Powell , George B. Richter-Addo...
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Organometallics 2010, 29, 5209–5214 DOI: 10.1021/om100364k

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Formation of a [ONN(allyl)O]- Anion via NO Insertion and Coupling Using Yttrium and Lanthanide Allyl Metallocenes† Ian J. Casely, YoungSung Suh, Joseph W. Ziller, and William J. Evans* Department of Chemistry, University of California Irvine, Irvine, California 92697-2025 Received April 28, 2010

Nitric oxide (NO) reacts with (C5Me5)2Ln(η3-CH2CHCH2)(THF) to form the first crystallographically characterized group 3 and organolanthanide NO insertion products, namely, {(C5Me5)2Ln[μONN(CH2CHdCH2)O]}2 (Ln = Y, La, Sm). The [ONN(allyl)O]- anions adopt an unusual trans geometry and presumably arise from insertion of NO into the Ln-C(allyl) bond followed by coupling of the (allyl-NO) radical anion with a second molecule of NO. Heating a solution of the yttrium product at 110 °C for 20 h affords (C5Me5)2Y[ONN(CH2CHdCH2)O-κ2O,O0 ], resulting from cleavage of the dimer and formation of the monomer as the thermodynamic product. The NdN, N-O, and Ln-O bond distances suggest that a zwitterionic (-)O-NdN(þ)(R)-O(-) resonance structure is a main contributor to the bonding of these N-allyl-N-nitrosohydroxylaminato ligands.

Introduction NO has been conspicuously absent from the list of substrates investigated in the numerous studies of small-molecule activation by organolanthanide complexes.1-5 As an oddelectron species, NO is an interesting reagent since it can generate radical centers in situ, a process inherent in the extensive use of Sm2þ.6-10 Although the coordination and subsequent reaction chemistry of NO with transition metals is of environmental11 and biological12-16 importance, and has

been extensively studied,17-21 little is reported on lanthanidebased NO chemistry. Nitric oxide insertion into metal-carbon bonds is known for a wide range of metals. Bergman et al. reported the migratory insertion of NO into the Co-C bond of CpCo(Me)NO in the presence of triphenylphosphine to afford the nitrosomethane complex CpCo[N(O)Me](PPh3),22,23 a process that was later examined theoretically.24 Similar reactivity was reported with Fe25 and Ru26 alkyl nitrosyl complexes, and NOþ in the form of [NO][PF6] was found by Legzdins et al. to react with Cr-Me bonds in CpCr(NO)2Me to form {CpCr(NO)2[N(CH2)OH]}[PF6].27 Bottomley and co-workers reported that the reaction of Cp2Ti(CO)2 with NO gives the isocyanate Cp2Ti(NCO), along with other titanium-containing products.28 Several examples of the photochemical formation of metal-bound isocyanates from carbonyl nitrosyl complexes of Ru,29 Mo,30 W,31 and Pt32 are also known.

† Dedicated to Professor Dietmar Seyferth for his extensive efforts to advance organometallic chemistry and for his insight, judgment, and leadership in establishing Organometallics as an outstanding journal. Part of the Dietmar Seyferth Festschrift. *To whom correspondence should be addressed. Fax: 949-824-2210. E-mail: [email protected]. (1) Evans, W. J.; Wayda, A. L.; Hunter, W. E.; Atwood, J. L. J. Chem. Soc., Chem. Commun. 1981, 706. (2) Evans, W. J.; Grate, J. W.; Doedens, R. J. J. Am. Chem. Soc. 1985, 107, 1671. (3) Evans, W. J.; Seibel, C. A.; Ziller, J. W.; Doedens, R. J. Organometallics 1998, 17, 2103. (4) Evans, W. J.; Lee, D. S. Can. J. Chem. 2005, 83, 375. (5) Watson, P. L. J. Am. Chem. Soc. 1983, 105, 6491. (6) Girard, P.; Namy, J. L.; Kagan, H. B. J. Am. Chem. Soc. 1980, 102, 2693. (7) Molander, G. A.; Harris, C. R. Chem. Rev. 1996, 96, 307. (8) Molander, G. A.; Harris, C. R. J. Am. Chem. Soc. 1996, 118, 4059. (9) Evans, W. J.; Drummond, D. K. J. Am. Chem. Soc. 1988, 110, 2772. (10) Evans, W. J.; Ulibarri, T. A.; Ziller, J. W. J. Am. Chem. Soc. 1988, 110, 6877. (11) Pandey, K. K. Coord. Chem. Rev. 1983, 51, 69. (12) McCleverty, J. A. Chem. Rev. 2004, 104, 403. (13) Ford, P. C.; Fernandez, B. O.; Lim, M. D. Chem. Rev. 2005, 105, 2439. (14) Rosen, G. M.; Tsai, P.; Pou, S. Chem. Rev. 2002, 102, 1191. (15) Wasser, I. M.; de Vries, S.; Moenne-Loccoz, P.; Schroeder, I.; Karlin, K. D. Chem. Rev. 2002, 102, 1201. (16) Moller, J. K. S.; Skibsted, L. H. Chem. Rev. 2002, 102, 1167. (17) Ford, P. C.; Lorkovic, I. M. Chem. Rev. 2002, 102, 993. (18) Hayton, T. W.; Legzdins, P.; Sharp, W. B. Chem. Rev. 2002, 102, 935.

(19) McCleverty, J. A. Chem. Rev. 1979, 79, 53. (20) Richter-Addo, G. B.; Legzdins, P. Metal Nitrosyls; Oxford University Press: New York, 1992. (21) Kahn, M. M.; Martell, A. E., Eds. Homogeneous Catalysis by Transition Metal Complexes; Academic Press: New York, 1974. (22) Weiner, W. P.; White, M. A.; Bergman, R. G. J. Am. Chem. Soc. 1981, 103, 3612. (23) Weiner, W. P.; Bergman, R. G. J. Am. Chem. Soc. 1983, 105, 3922. (24) Niu, S.; Hall, M. B. J. Am. Chem. Soc. 1997, 119, 3077. (25) Seidler, M. D.; Bergman, R. G. Organometallics 1983, 2, 1897. (26) Seidler, M. D.; Bergman, R. G. J. Am. Chem. Soc. 1984, 106, 6110. (27) Legzdins, P.; Wassink, B. J. Am. Chem. Soc. 1986, 108, 317. (28) Bottomley, F.; Lin, I. J. B. J. Chem. Soc., Dalton Trans. 1981, 271. (29) Stevens, R. E.; Fjare, D. E.; Gladfelter, W. L. J. Organomet. Chem. 1988, 347, 373. (30) McPhail, A. T.; Knox, G. R.; Robertson, C. G.; Sim, G. A. Inorg. Phys. Theor. 1971, 205. (31) Hitam, R. B.; Rest, A. J.; Herberhold, M.; Kremnitz, W. J. Chem. Soc., Chem. Commun. 1984, 471. (32) Bhaduri, S. A.; Bratt, I.; Johnson, B. F. G.; Khair, A.; Segal, J. A.; Walters, R.; Zuccaro, C. J. Chem. Soc., Dalton Trans. 1981, 234.

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Double NO insertion into an M-C bond with NO coupling and N-N bond formation produces a complex bearing an N-R-N-nitrosohydroxylaminato (R = alkyl or aryl) ligand, [ONN(R)O]-.33,34 This functional group is known to be biologically significant34,35 and has been widely utilized in the analytical reagent cupferron, [NH4][ONN(Ph)O].36-38 Double insertion reactivity has been reported for a number of early transition metal complexes such as Cp2Zr(CH2Ph)2, CpW(NO)(CH2SiMe3)2, WMe6, and Me3TaCl2, which, following treatment with NO, afford Cp2Zr(CH2Ph)[ONN(CH2Ph)O-κ 2 O,O0 ],39 CpW(NO)(CH2 SiMe 3 )[ONN(CH 2SiMe 3)O-κ2O,O0 ],40 WMe4[ONN(Me)O-κ2O,O0 ]2,41 and MeTaCl2[ONN(Me)O-κ2O,O0 ]2,42 respectively. Examples of the group 13 complexes Me2M[ONN(Me)O-κ2O,O0 ] (M = Al, Ga) are also known, being derived from NO and the corresponding MMe3 reagent.43,44 Examples of late transition metal complexes displaying this double NO insertion reactivity are [Me2NN]Ni(Et)(2,4-lutidine) ([Me2NN] = {CH[C(Me)NC6H3Me2-2,6]2}) and (η6-C7H8)Co(Ar*-3,5-iPr2) [Ar*-3,5i Pr2 = C6H-2,6-(C6H2-2,4,6-iPr3)2-3,5-iPr2], which react with NO to provide [Me2NN]Ni[ONN(Et)O-κ2O,O0 ]45 and [ONN(Ar*-3,5-iPr2)O-κ2O,O0 ]Co(NO)2,46 respectively. In contrast to the transition metals, nitrosyl complexes of yttrium and the f element metals are unknown, and there is a dearth of chemistry described for the reactions of complexes of these metals with NO. We have previously reported the formation of the oxo-bridged organosamarium complex [(C5Me5)2Sm]2(μ-O) by treatment of (C5Me5)2Sm(THF)2 with NO,47 and Burns et al. have synthesized [(ArO)3U]2(μ-O) (OAr = OC6H3-2,6-tBu) by treatment of U(OAr)3 with NO.48 A reaction with a different outcome was reported by Sella and co-workers, in which the divalent tris-pyrazolylborate complexes Ln(TpMe2,4-Et)2 (Ln = Sm, Eu, Yb; TpMe2,4-Et =HB[N2C3Me2-4-Et]3) react with NO to furnish the corresponding trivalent nitrite complexes Ln(TpMe2,4-Et)2(NO2- κ2O,O0 ).49 The only other example we are aware of for the reaction of an f element complex with nitric oxide is our recent isolation of the first radical dianion of NO, (NO)2-, from the two-electron reduction of NO facilitated by the (33) Hrabie, J. A.; Keefer, L. K. Chem. Rev. 2002, 102, 1135. (34) Keefer, L. K.; Flippen-Anderson, J. L.; George, C.; Shanklin, A. P.; Dunams, T. M.; Christodoulou, D.; Saavedra, J. E.; Sagan, E. S.; Bohle, D. S. Nitric Oxide 2001, 5, 377. (35) Ziche, M.; Donnini, S.; Morbidelli, L.; Monzani, E.; Roncone, R.; Gabbini, R.; Casella, L. Chem. Med. Chem. 2008, 3, 1039. (36) Popov, A. I.; Wendlandt, W. W. Anal. Chem. 1954, 26, 883. (37) Wendlandt, W. W. Anal. Chem. 1955, 27, 1277. (38) Elving, P. J.; Olson, E. C. Anal. Chem. 1955, 27, 1817. (39) Fochi, G.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. J. Chem. Soc., Dalton Trans. 1986, 445. (40) Legzdins, P.; Rettig, S. J.; Sanchez, L. Organometallics 1988, 7, 2394. (41) Fletcher, S. R.; Skapski, A. C. J. Organomet. Chem. 1973, 59, 299. (42) Wilkins, J. D.; Drew, M. G. B. J. Organomet. Chem. 1974, 69, 111. (43) Amirkhalili, S.; Conway, A. J.; Smith, J. D. J. Organomet. Chem. 1978, 149, 407. (44) Amirkhalili, S.; Hitchcock, P. B.; Smith, J. D.; Stamper, J. G. J. Chem. Soc., Dalton Trans. 1980, 2493. (45) Puiu, S. C.; Warren, T. H. Organometallics 2003, 22, 3974. (46) Lei, H.; Ellis, B. D.; Ni, C.; Grandjean, F.; Long, G. J.; Power, P. P. Inorg. Chem. 2008, 47, 10205. (47) Evans, W. J.; Grate, J. W.; Bloom, I.; Hunter, W. E.; Atwood, J. L. J. Am. Chem. Soc. 1985, 107, 405. (48) Avens, L. R.; Barnhart, D. M.; Burns, C. J.; McKee, S. D.; Smith, W. H. Inorg. Chem. 1994, 33, 4245. (49) Maunder, G. H.; Russo, M. R.; Sella, A. Polyhedron 2004, 23, 2709.

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recently discovered (N2)3- complex {[(Me3Si)2N]2(THF)Y}2(μ3-η2:η2:η2-N2)K,50 affording the paramagnetic complex {[(Me3Si)2N]2(THF)Y}2(μ-η2:η2-NO). Subsequent treatment of this complex with a second equivalent of NO affords the diamagnetic (ONdNO)2- product {[(Me3Si)2N]2Y}4(μ3-ONdNO)2(THF)2.51 The number of f element N-R-N-nitrosohydroxylaminato complexes in the CCDC52 database is limited to a uranyl cupferrate example, [NH4]{UO2[ONN(Ph)O-κ2O,O0 ]3}, formed via salt metathesis,53 although lanthanide-cupferrate complexes are known for analytical determinations.36,37 To address the lack of information on the chemistry of NO with yttrium and the lanthanide metals, we have examined the reactivity of NO with group 3 and lanthanide carbon bonds in the allyl metallocenes (C5Me5)2Ln(η3-CH2CHCH2)(THF), Ln = Y, La, Sm. These complexes take advantage of the protective and crystophilic nature of the bis(pentamethylcyclopentadienyl) ligand set and the fact that the allyl complexes have proven to be synthetically expedient lanthanide metallocenes bearing a reactive Ln-C bond.54

Experimental Section All manipulations and syntheses described below were conducted with the rigorous exclusion of air and water using standard Schlenk, vacuum line, and glovebox techniques under an argon atmosphere. Solvents were sparged with UHP argon and dried by passage through columns containing Q-5 and molecular sieves prior to use. Deuterated NMR solvents were purchased from Cambridge Isotope Laboratories, dried over NaK alloy, degassed by three freeze-pump-thaw cycles, and vacuum transferred before use. The starting materials (C5Me5)2Ln(η3-CH2CHCH2)(THF), Ln = Y, La, Sm, were prepared according to literature procedures.3,55 NO gas was purchased from Aldrich and purified by passage through two U-shaped glass tubes cooled to -78 °C immediately before use. All other reagents were purchased from Aldrich and used as received. 1H NMR spectra were recorded on Bruker DR400, GN500, or CRYO500 MHz spectrometers (13C spectra on 500 MHz spectrometer operating at 125 MHz), at 298 K unless otherwise stated, and referenced internally to residual protio-solvent resonances. Infrared spectra were recorded as KBr pellets on a Varian FTS 1000 FT-IR spectrometer. Elemental analyses were recorded on a Perkin-Elmer 2400 CHNS elemental analyzer. General Procedures. A toluene solution of (C5Me5)2Ln(η3CH2CHCH2)(THF) and a Teflon-coated stirbar were placed in a 100 mL Schlenk tube fitted with a greaseless stopcock and attached to a high-vacuum line. The reaction mixture was frozen in liquid nitrogen and placed under high vacuum (10-5 Torr) for 30 min. The desired quantity of NO gas was measured into a second, similar Schlenk tube of known volume using a calibrated manometer and was subsequently condensed over the frozen reaction mixture. The reaction vessel was sealed and removed from the high-vacuum line, and the frozen mixture was rapidly warmed to room temperature. The resulting solution (50) Evans, W. J.; Fang, M.; Zucchi, G.; Furche, F.; Ziller, J. W.; Hoekstra, R. M.; Zink, J. I. J. Am. Chem. Soc. 2009, 131, 11195. (51) Evans, W. J.; Fang, M.; Bates, J. E.; Furche, F.; Ziller, J. W.; Kiesz, M. D.; Zink, J. I. Nat. Chem. 2010, DOI: 10.1038/NCHEM.701. (52) Allen, F. H. Acta Crystallogr., Sect. B: Struct. Sci. 2002, B58, 380. (53) Horton, W. S. J. Am. Chem. Soc. 1956, 78, 897. (54) Evans, W. J.; Seibel, C. A.; Ziller, J. W. J. Am. Chem. Soc. 1998, 120, 6745. (55) Evans, W. J.; Kozimor, S. A.; Brady, J. C.; Davis, B. L.; Nyce, G. W.; Seibel, C. A.; Ziller, J. W.; Doedens, R. J. Organometallics 2005, 24, 2269.

Article was stirred at room temperature for 12 h while sealed, after which time the crude product was obtained by removal of the volatiles under reduced pressure in the glovebox. {(C5Me5)2Y[μ-ONN(CH2CHdCH2)O]}2, 1. Warming a frozen yellow solution of (C5Me5)2Y(η3-CH2CHCH2)(THF) (255 mg, 0.54 mmol) in toluene (15 mL) with condensed NO (2 equiv) to room temperature caused a transient darkening of the color. Removal of solvent from the final yellow solution yielded a yellow solid (244 mg, 98%). Slow cooling of a hot toluene solution of this solid to room temperature afforded yellow 1 (171 mg, 69%) as X-ray quality single crystals. The 1H and 13C NMR spectra show the product consists of major (A) and minor (B) components in an approximate 56:44 ratio. 1H NMR (C6D6) δ: 6.02-5.92(AþB) (m, 2H, 3Jtrans = 17.0 Hz, 3 Jcis = 10.1 Hz, 3JH-H = 6.9 Hz, CH2CHdCH2), 5.19-5.11(AþB) (m, 4H, 3Jtrans = 17.1 Hz, 3Jcis = 10.1 Hz, CH2CHd CH2), 4.58(B) and 4.56(A) (d, 4H total, 3JH-H = 6.9 Hz, CH2CHdCH2), 2.02, 2.01, and 2.00 (3s, 60H, C5Me5). 13C NMR (C6D6) δ: 127.7(B) and 127.6(A) (CH2CHdCH2), 121.9(B) and 121.7(A) (CH2CHdCH2), 116.8, 116.7, and 116.4 (C5Me5), 57.6(B) and 56.9(A) (CH2CHdCH2), 11.3 and 11.2 (C5Me5). IR (cm-1): 2905s, 2857s, 2721w, 1495w, 1436m, 1400m, 1319m, 1297m, 1226m, 1212m, 1144s, 1122s, 1025m, 981m, 919m, 838m, 728w, 636w, 590w, 533w, 446w. Anal. Calcd for C46H70N4O4Y2: C 59.98, H 7.68, N 6.08. Found: C 60.22, H 7.49, N 5.99. {(C5Me5)2La[μ-ONN(CH2CHdCH2)O]}2, 2. Following a procedure analogous to that described above, (C5Me5)2La(η3-CH2CHCH2)(THF) (217 mg, 0.42 mmol) gave similar color changes and a yellow solid (211 mg, 99%). Slow cooling of a hot toluene solution of this solid to room temperature afforded yellow 2 (113 mg, 53%) as X-ray quality single crystals. The 1H and 13C NMR spectra show the product consists of major (A) and minor (B) components in an approximate 57:43 ratio. 1H NMR (C6D6) δ: 5.96-5.89(AþB) (m, 2H, 3Jtrans = 16.9 Hz, 3 Jcis = 10.1 Hz, 3JH-H = 6.8 Hz, CH2CHdCH2), 5.18-5.09(AþB) (m, 4H, 3Jtrans = 17.1 Hz, 3Jcis = 10.1 Hz, CH2CHd CH2), 4.50(B) and 4.48(A) (d, 4H total, 3JH-H = 6.8 Hz, CH2CHdCH2), 2.06, 2.05, and 2.03 (3s, 60H, C5Me5). 13C NMR (C6D6) δ: 127.4(B) and 127.2(A) (CH2CHdCH2), 121.9(B) and 121.8(A) (CH2CHdCH2), 118.8 and 118.6 (C5Me5), 58.1(B) and 56.9(A) (CH2CHdCH2), 10.9, 10.8, and 10.7 (C5Me5). IR (cm-1): 2906s, 2856s, 2724w, 1495w, 1436s, 1380m, 1320m, 1296m, 1276w, 1225 m, 1143s, 1129s, 1022m, 978m, 918m, 836m, 768w, 728w, 640w, 588w, 533w, 501w, 439w. Anal. Calcd for C46H70N4O4La2: C 54.11, H 6.92, N 5.49. Found: C 54.71, H 6.80, N 5.36. {(C5Me5)2Sm[μ-ONN(CH2CHdCH2)O]}2, 3. Warming a frozen orange-red solution of (C5Me5)2Sm(η3-CH2CHCH2)(THF) (224 mg, 0.42 mmol) in an analogous procedure caused a pale orange solution to form. Removal of solvent yielded an orange solid in quantitative yield, and slow cooling of a hot toluene solution of this solid to room temperature afforded orange 3 (171 mg, 78%) as X-ray quality single crystals. The 1H and 13C NMR spectra show the product consists of major (A) and minor (B) components in an approximate 60:40 ratio. 1H NMR (C6D6) δ: 5.28-5.20(B) and 5.11-5.02(A) (m, 2H total, 3 Jtrans = 17.0 Hz, 3Jcis = 10.1 Hz, 3JH-H = 6.9 Hz, CH2CHd CH2), 4.88(B) and 4.80(A) (d, 2H total, 3Jcis = 10.1 Hz, CH2CHdCH2), 4.60(B) and 4.52(A) (d, 2H total, 3Jtrans = 17.1 Hz, CH2CHdCH2), 2.84(B) and 2.77(A) (d, 4H total, 3JH-H = 6.9 Hz, CH2CHdCH2), 1.10, 0.96, and 0.76 (3s, 60H, C5Me5). 13C NMR (C6D6) δ: 126.9(B) and 126.7(A) (CH2CHdCH2), 121.3(B) and 121.1(A) (CH2CHdCH2), 112.2, 111.9, and 111.3 (C5Me5), 53.8(B) and 52.1(A) (CH2CHdCH2), 16.1, 15.9, and 15.7 (C5Me5). IR (cm-1): 2905s, 2857s, 2722w, 1496w, 1436m, 1398w, 1380w, 1319m, 1296w, 1225m, 1213m, 1143s, 1123s, 1023m, 978m, 918m, 837m, 728w, 637w, 589w, 532w, 501w, 443w. Anal. Calcd for C46H70N4O4Sm2: C 52.92, H 6.77, N 5.37. Found: C 53.16, H 6.46, N 5.27.

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(C5Me5)2Y[ONN(CH2CHdCH2)O-K2O,O0 ], 4. In the glovebox, a 100 mL Schlenk tube was charged with a yellow toluene (10 mL) solution of 1 (114 mg), sealed, and heated at 110 °C for 20 h. The solvent was removed from the resulting yellow solution under reduced pressure to afford 4 in quantitative yield (114 mg) as a yellow oil that solidified upon standing. X-ray quality single crystals were grown by overnight storage of a concentrated diethyl ether solution at -30 °C. 1H NMR (C6D6) δ: 5.83-5.78 (m, 1H, 3Jtrans = 17.0 Hz, 3Jcis = 10.2 Hz, 3JH-H = 6.8 Hz, CH2CHdCH2), 4.97-4.93 (m, 2H, cis and trans coupling constants not observable, CH2CHdCH2), 4.07 (d, 3JH-H = 6.8 Hz, CH2CHdCH2), 1.94 (s, 30H, C5Me5). 13C NMR (C6D6) δ: 129.1 (CH2CHdCH2), 121.1 (CH2CHdCH2), 117.4 (C5Me5), 59.8 (CH2CHdCH2), 10.2 (C5Me5). IR (cm-1): 2969 m, 2908s, 2859s, 2728w, 1645w, 1491w, 1427m, 1400s, 1269m, 1229s, 1193m, 1163w, 1118w, 1024w, 989w, 943m, 916w, 769m, 590w, 496m. Anal. Calcd for C23H35N2O2Y: C 59.98, H 7.68, N 6.08. Found: C 59.02, H 7.26, N 5.85.

Results and Discussion Synthesis. Treatment of toluene solutions of the allyl complexes (C5Me5)2Ln(η3-CH2CHCH2)(THF), Ln = Y, La, Sm, with nitric oxide in a 1:2 ratio affords the double NO insertion products {(C 5Me 5 )2Ln[μ-ONN(CH 2CHd CH2)O]}2 (Ln = Y, 1; La, 2; and Sm, 3, respectively), as shown in eq 1. Reactions with a 1:1 stoichiometry form the same product in a 1:1 mixture with starting material.

All three of these insertion products are insoluble in hexane, contrary to the high solubility of the allyl starting materials, but they can be crystallized from hot toluene or benzene solutions as yellow (1 and 2) and orange (3) crystalline solids. The isomorphous complexes were identified by X-ray crystallography to be dimers in the solid state, as shown with the yttrium analogue as a representative example in Figure 1. X-ray crystallographic data and selected metrical parameters are listed in Tables S1 (see SI) and 1, respectively. However, all three crystal structures exhibited disorder about the central NdN bond, precluding detailed discussion of the [ONN(CH2CHdCH2)O]- ligands. Spectroscopy. The infrared spectra of all three complexes, prepared as KBr pellets, display absorptions indicative of the [ONN(R)O]- ligand in the ranges 1220-1270, 1135-1190, and 915-975 cm-1.40,43,56-58 Since the ligands adopt both η1-NO and η2-NO binding modes (vide infra), pairs of absorptions are observed in the NO stretching region. The yttrium complex, 1, shows two pairs of absorptions assigned to the νNdO mode at 1226 and 1212 cm-1 and 1144 and 1122 cm-1. Presumably the η1-NO component is responsible for the higher frequency stretches and η2-NO for the lower frequency absorptions. The IR spectrum of the lanthanum complex, 2, is not as well resolved, with absorptions at 1225 cm-1, bearing a (56) Deak, A.; Haiduc, I.; Parkanyi, L.; Venter, M.; Kalman, A. Eur. J. Inorg. Chem. 1999, 1593. (57) Middleton, A. R.; Wilkinson, G. J. Chem. Soc., Dalton Trans. 1981, 1898. (58) Parkanyi, L.; Kalman, A.; Deak, A.; Venter, M.; Haiduc, I. Inorg. Chem. Commun. 1999, 2, 265.

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Table 1. Selected Bond Distances (A˚) and Angles (deg) for {(C5Me5)2Y[μ-ONN(CH2CHdCH2)O]}2, 1, {(C5Me5)2La[μ-ONN(CH2CHdCH2)O]}2, 2, {(C5Me5)2Sm[μ-ONN(CH2CHdCH2)O]}2, 3, and (C5Me5)2Y[ONN(CH2CHdCH2)O-κ2O,O0 ], 4

M-(centroid) M1-O1, -O2 O1-N1 O2-N2 N1-N2 N1-C21 C21-C22 C22-C23 (centroid)-M-(centroid) O1-M1-O2 O1-N1-N2 O2-N2-N1

1

2

3

4

2.396, 2.390 2.3210(16), 2.2695(15)

2.558, 2.553 2.494(3), 2.419(3)

2.460, 2.452 2.403(3), 2.340(3)

1.491(4) 1.293(4) 135.1 114.69(6)

1.473(8) 1.271(9) 134.9 118.73(12)

1.475(9) 1.282(9) 134.9 118.07(13)

2.348, 2.349 2.2677(11), 2.2744(12) 1.3204(19) 1.3014(19) 1.281(2) 1.471(2) 1.517(5) 1.365(6) 139.4 67.24(4) 123.39(14) 112.86(14)

Figure 1. ORTEP drawing of the disordered structure of 1 with ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity.

lower frequency shoulder, and at 1143 and 1129 cm-1, whereas the samarium complex, 3, shows absorptions at 1225 and 1213 cm-1 and 1143 and 1123 cm-1. The δONNO mode is observed at 918-919 cm-1 in all three complexes. The 1H and 13C NMR spectra of the dimers show a doubling of all expected resonances, consistent with the presence of two forms of the complex in each case (see Structural Features section below). For each complex, three singlets are observed in the (C5Me5)- region of the 1H NMR spectra with an approximate 1:2:1 ratio, while the 13C NMR has three (C5Me5)- and three (C5Me5)- signals. This apparently results from overlapping spectra due to the two forms of the complex, each with inequivalent (C5Me5)- groups. Integration of the separated allyl resonances revealed these complexes to be a mixture of major and minor components, with the paramagnetic nature of the samarium in 3 resulting in the

Figure 2. Chemdraw representation of the two ligand conformations used to model the data of 1-3 in Figure 1.

resonances of the allyl group being shifted so that both components can be completely identified. The 13C NMR spectra clearly indicate the presence of both components, as two resonances are attributed to each allyl carbon in each complex. The NMR spectra show an approximate 3:2 ratio of components in each case. The 1H NMR spectrum of the yttrium dimer, 1, gives a 56:44 ratio of components based on the two allyl-NCH2 doublets observed at 4.58 (minor) and 4.56 (major) ppm, with a coupling constant of 3JH-H = 6.9 Hz. The resonances from the CdCH2 group of both components are superimposed upon each other, appearing as a multiplet between 5.19 and 5.11 ppm, although it is possible to identify vicinal alkene cis and trans coupling constants, 3Jtrans = 17.1 Hz and 3Jcis = 10.1 Hz. These coupling constants can also be observed for the alkene CH multiplet between 6.02 and 5.92 ppm, which also provides 3JH-H = 6.9 Hz. The 1H NMR spectrum of the lanthanum dimer, 2, is essentially the same as the yttrium complex, displaying a 57:43 major to minor component ratio, with chemical shifts and coupling constants similar to within 0.1 ppm and 0.2 Hz of those observed for 1, respectively. The presence of two paramagnetic samarium atoms in 3 has the effect of shifting and separating the individual resonances for both the major and minor components, which were measured in a 59:41 ratio. Two allyl-NCH2 doublets are again observed, with a coupling constant of 3JH-H = 6.9 Hz, although they are shifted to lower frequency by ∼1.7 ppm to 2.84 (minor) and 2.77 (major) ppm. Two doublets for each component corresponding to cis and trans alkene coupling are clearly visible, at 4.88 (minor) and 4.80 (major) ppm (3Jcis = 10.1 Hz) and 4.60 (minor) and 4.52 (major) ppm (3Jtrans =17.1 Hz), respectively. Two multiplets for the alkene CH resonance are also observed at 5.28-5.20 (minor) and 5.11-5.02 (major) ppm, which display 3J coupling constants of 17.0, 10.1, and 6.9 Hz. Thermolysis. To investigate whether the two components observed by NMR spectroscopy in solution were interconvertible and if a monomer/dimer equilibrium was present, variable-temperature NMR experiments were performed on a toluene-d8 solution of the yttrium dimer, 1. Heating the NMR sample from 25 to 75 °C in 10 °C intervals elicited very little change in the appearance of the spectra, and the ratios of the resonances in the 1H and 13C NMR spectra did not change. Heating was continued in similar increments to 105 °C, over which range a new set of resonances began to appear. These new resonances were subsequently found to belong to the monomeric species (C5Me5)2Y[ONN(CH2CHdCH2)O-κ2O,O0 ], 4, which was isolated quantitatively

Article

by heating a toluene solution of 1 at 110 °C for 20 h, eq 2. Monomeric 4 is not related to 1 by a monomer/dimer equilibrium, as the new set of resonances remained upon cooling the sample back to 25 °C. No evidence was observed for the interconversion of the two components. It is apparent that the dimer is stable up to 75 °C, above which temperature it is cleaved and converted to the monomer, itself a stable compound. The fact that the two components present in 1 cleanly convert to the single compound, 4, suggests that the NMR spectra arise from two conformers rather than two different complexes. Similarly, heating toluene-d8 solutions of 2 and 3 does not change the ratio of components. After four days, the resonances for the starting materials disappear, but monomeric lanthanum and samarium complexes analogous to 4 were not isolable.

An X-ray diffraction study was performed on single crystals of 4, and the structure is shown in Figure 4. X-ray crystallographic data and selected bond lengths and angles are displayed in Tables S1 (see SI) and 1, respectively. The 1H NMR spectrum of 4 closely resembles that of the parent dimer, 1. The allyl CH and CdCH2 resonances appear as simplified multiplets and are shifted to lower frequency by ∼0.2 ppm, to 5.80 and 4.96 ppm, respectively. In place of the two doublets attributed to NCH2 in each of the components in dimeric 1, one doublet is observed for 4 at 4.07 ppm (3JH-H = 6.8 Hz), which is a shift of 0.5 ppm from the starting material. This suggests there is only a single allyl ligand environment present in 4, which also displays a single (C5Me5)- resonance at 1.94 ppm. The 13C NMR spectrum is in agreement with this simplified spectrum, and only a single set of resonances corresponding to the allyl and (C5Me5)- groups is observed, in contrast to the two sets seen in the starting material, 1. In agreement with this, the infrared spectrum of the monomer, 4, displays single absorptions for the νNdO mode at 1229 and 1193 cm-1, consistent with the oxygen atoms of the ligand binding to the metal in a similar fashion, and an absorption at 943 cm-1 for the δONNO mode. These results suggest that the initial NO insertion into the Ln-C(allyl) bond and subsequent NO coupling are fast and that the isolated dimeric complexes are the kinetic products trapped in two conformations. In the yttrium case, these are stable up to 75 °C, but heating to 110 °C rapidly cleaves the dimers and produces the monomeric species as the thermodynamic product. Structural Features. The isomorphous complexes 1, 2, and 3 crystallize in the monoclinic space group P21/n. They exist as dimers in the solid state and reside on a crystallographic inversion center. All three structures display disorder over the central NdN unit, limiting the structural discussion of the [ONN(allyl)O]- ligand. An ORTEP59 drawing of the yttrium analogue, 1, is shown in Figure 1. The X-ray data were modeled with the individual components shown in Figure 2. The model has two different bonding modes of the central [ONN(R)O]- ligand core, which, although the allyl carbons (59) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.

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Figure 3. ORTEP drawing of 1 with ellipsoids at the 50% probability level. Hydrogen atoms are omitted, and only one of the disordered [ONN(allyl)O]- ligand conformations is shown for clarity.

occupy the same spatial positions in the solid state, renders them spectroscopically inequivalent and could account for the observation of two sets of signals in the NMR and IR spectra. The occupancies of the central NdN core in both isomers (N1/N2 versus N3/N4) are 73:27 for 1, 65:35 for 2, and 64:36 for 3, which broadly agree with the component ratios determined in solution. In one of the disordered forms, the [ONN(allyl)O]- ligand bridges the metals with κ1O bonding on one end and an η2-NO attachment on the other. In the second disordered form, the bonding is κ1O to each metal. The fact that similar disorder was observed regardless of the size of the metal and that interconversion did not occur in solution was unexpected. Both forms have a trans arrangement of the oxygen atoms around the NdN bond, which is unprecedented:33,34 to the best of our knowledge, the [ONN(R)O]- ligands previously characterized by X-ray crystallography have adopted exclusively the cis geometry. This cis arrangement occurs even in complexes where the [ONN(R)O]- ligand bridges two metal centers, such as in the extended structure present in K3{CH[N(NO)O]3}60 or the 20-membered macrocycle formed in [Me3Sn(O2N2Ph)]4.58 The metallocene components of 1-3 are not unusual; for example, complex 1 displays typical 2.396 and 2.390 A˚ Y(C5Me5 ring centroid) bond lengths and a 135.1° (C5Me5 ring centroid)-Y-(C5Me5 ring centroid) bond angle.61-63 The [ONN(allyl)O]- bridge is large enough that the (C5Me5 ring centroid)-Y-(C5Me5 ring centroid) planes on each metal are approximately parallel to each other, as shown in Figure 3, as opposed to the commonly observed perpendicular arrangement in bimetallic complexes with smaller bridging ligands.60 The cyclopentadienyl rings have 22.4° C2-(C5Me5 ring centroid)-(C5Me5 ring centroid)-C12 dihedral angles, in between eclipsed (0°) and staggered (36°). Complex 4 crystallizes in the orthorhombic space group Pbcn and contains the [ONN(allyl)O-κ2O,O0 ]- ligand binding in the more conventional mode with the oxygen atoms cis to the NdN bond, Figure 4. The metallocene structure suggests that this complex is less crowded than 1 since the (60) Arulsamy, N.; Bohle, D. S. J. Am. Chem. Soc. 2001, 123, 10860. (61) Evans, W. J.; Fujimoto, C. H.; Ziller, J. W. Organometallics 2001, 20, 4529. (62) Evans, W. J.; Foster, S. E. J. Organomet. Chem. 1992, 433, 79. (63) Evans, W. J.; Lorenz, S.; Ziller, J. W. Chem. Commun. 2007, 4662.

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Figure 4. ORTEP drawing of 4 with ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity.

Y-(C5Me5 ring centroid) bond lengths are slightly shorter, 2.348 and 2.349 A˚, and the 139.4° (C5Me5 ring centroid)Y-(C5Me5 ring centroid) bond angle is slightly larger. Consistent with this, the rings in 4 are more staggered, with 35.5° C3-(C5Me5 ring centroid)-(C5Me5 ring centroid)C14 dihedral angles. The N-O distances of 1.3204(19) and 1.3014(19) A˚ in 4 are intermediate between typical N-O single, e.g., 1.453 A˚ in H2N-OH,64 and NdO double, e.g., 1.209 A˚ in HNdO, bonds.65 The 1.281(2) A˚ NdN bond length in 4 is also between typical N-N single, e.g., 1.47 A˚ in H2N-NH2, and NdN double, e.g., 1.25 A˚ in PhNdNPh, bond values,66 although it is closer to the double-bond value. These distances indicate that there is delocalization in the [ONN(allyl)O-κ2O,O0 ]- unit, but with a zwitterionic (-)O-Nd N(þ)(R)-O(-) resonance structure as the main contributor to the bonding, a feature common to this class of ligand.33,34 The [ONN(R)O]- core is planar, consistent with trigonalplanar sp2-hybridized nitrogen atoms, and contains asymmetric O1-N1-N2 and O2-N2-N1 angles of 123.39(14)° (64) Gurvich, L. V.; Veyts, I. V.; Alcock, C. B. Thermodynamic Properties of Individual Substances, 4th ed.; Hemisphere Pub. Co.: New York, 1989. (65) Ogilvie, J. F. J. Mol. Struct. 1976, 31, 407. (66) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A. G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1.

Casely et al.

and 112.86(14)°, respectively, which is again commonly observed.33,34 The Y1-O1 and Y1-O2 bond lengths are 2.268(1) and 2.274(1) A˚, respectively, longer than those found in eight-coordinate [(C5Me5)2Y(THF)]2(μ-OCH2CH2O) for the yttrium alkoxide bond, 2.042(4) A˚, but shorter than the dative Y-O(THF) bond, 2.398(5) A˚,67 whereas they are commensurate with examples bearing delocalized oxygen ligands such as the 2.288(3) A˚ distance observed in the yttrium caprolactamate complex [(C5Me5)2Y]2(C6H10NO).61 The ligand bite angle now enforces a much smaller O1-Y1-O2 bond angle of 67.24(4)°. Although there are no lanthanide [ONN(R)O-κ2O,O0 ]- complexes in the CCDC with which to make direct comparisons, the ligand metrical parameters in 4 are similar to other complexes bearing this ligand, such as in [ONN(Ar*3,5-iPr2)O-κ2O,O0 ]Co(NO)2 which displays bond distances of 1.334(10) and 1.299(11) A˚ (N-O) and 1.274(12) A˚ (NdN).46

Conclusion Nitric oxide readily undergoes insertion with the Ln-C(allyl) linkages of (C5Me5)2Ln(η3-CH2CHCH2)(THF) metallocenes (Ln = Y, La, Sm) to generate [ONN(CH2CHd CH2)O]- ligands, formally derived by coupling of NO to an initially formed (allyl-NO) radical anion. The ligands in the kinetic products of the reactions, {(C5Me5)2Ln[μ-ONN(CH2CHdCH2)O]}2, are disordered in the solid state and can be modeled by two coordination modes, both of which involve oxygen atoms trans to the NdN bond. Thermolysis of the yttrium complex, 1, forms the monomeric thermodynamic product (C5Me5)2Y[ONN(CH2CHdCH2)O-κ2O,O0 ], 4, which has cis oxygen atoms and adopts a κ2O metal binding mode.

Acknowledgment. We thank the National Science Foundation for support and also Dr. Michael K. Takase for assistance with X-ray crystallography. Supporting Information Available: CIF files giving crystallographic data for all complexes and Table S1 detailing X-ray data collection parameters. This material is available free of charge via the Internet at http://pubs.acs.org. (67) Deelman, B.-J.; Booij, M.; Meetsma, A.; Teuben, J. H.; Kooijman, H.; Spek, A. L. Organometallics 1995, 14, 2306.