Synthetic Diversity in the Formation of Triazoles from Nitriles and

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Organometallics 2009, 28, 2897–2903

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Synthetic Diversity in the Formation of Triazoles from Nitriles and Diazo Compounds Using Metallocenes of Electropositive Metals William J. Evans,*,† Elizabeth Montalvo,† Timothy M. Champagne,† Joseph W. Ziller,† Antonio G. DiPasquale,‡ and Arnold L. Rheingold‡ Department of Chemistry, UniVersity of California, IrVine, California 92697-2025, and Department of Chemistry and Biochemistry, UniVersity of California, San Diego, 9500 Gilman DriVe, MC 0358, La Jolla, California 92093-0358 ReceiVed December 22, 2008

Reactions of [(C5Me5)2Ln][(µ-Ph)2BPh2] complexes with Li[Me3SiCN2] form dimeric isocyanotrimethylsilyl amide complexes {(C5Me5)2Ln[µ-N(SiMe3)NC]}2, not only for Ln ) Sm (1-Sm) and La (1-La) but also for the intermediate and small size metal ions Ln ) Nd, Y, Yb, and Lu. Complexes 1-Y and 1-Yb were characterized by X-ray crystallography and are structurally similar to 1-Sm and 1-La. A more convenient synthesis of 1-Sm and 1-La from the corresponding (C5Me5)2LnCl2K(THF)2 “ate” salts with Li[Me3SiCN2] is also reported. Analogues of reactions of 1-Sm and 1-La with Me3CCN that form the 1,2,3-triazolato complexes (C5Me5)2Ln(NCCMe3)[NNC(SiMe3)C(CMe3)N] (2-Sm, 2-La) were examined with C6H5CH2CN and Me3SiCN to investigate the diversity of the triazoles accessible by this route. Complex 1-La reacts with C6H5CH2CN to make a 1,2,3-triazole complex, but in contrast to 2-La, the product is a base-free dimer in which each triazole is ligated by two metallocenes, {(C5Me5)2La[µη1:η2-NNC(SiMe3)C(CH2C6H5)N]}2, 3. The reaction of 1-La with Me3SiCN involves Si-C bond cleavage, and a nitrile-solvated cyanide trimer, [(C5Me5)2La(µ-CN)(NCSiMe3)]3, 4, was isolated. The reaction of 1-La with the isocyanide reagent Me3SiCH2NC also generated a cyanide trimer, this time via N-C bond cleavage as an isocyanide solvate, [(C5Me5)2La(µ-CN)(Me3SiCH2NC)]3, 5. Unsubstituted 1,2,3-triazoles {(C5Me5)2Ln[µ-η1:η2-NNC(SiMe3)C(H)N]}2 (6-Sm, 6-La) can be isolated directly from 1-Sm and 1-La in reactions that involve N-N bond cleavage. Introduction Recently, the reactions of the cationic metallocene complexes of the tetraphenylborate anion [(C5Me5)2Ln][(µ-Ph)2BPh2]1-5 (Ln ) Sm, La) with the salt obtained from iBuLi and Me3SiCHN26 were examined in efforts to expand the chemistry of diazoalkanes with the lanthanides.6,7 These reactions formed isomorphous bimetallic isocyanotrimethylsilyl amide complexes, {(C5Me5)2Ln[µ-N(SiMe3)NC]}2 (1-Sm, 1-La), eq 1, on the basis of structural and reactivity data.6 Eq 1 requires a 1,3-silyl migration from carbon to nitrogen if the lithium salt has a

carbon-bound silyl group. Theoretical and experimental studies of these lithium salts suggest that several isomers are similar * Corresponding author. E-mail: [email protected]. † University of California, Irvine. ‡ University of California, San Diego. (1) Evans, W. J.; Seibel, C. A.; Ziller, J. W. J. Am. Chem. Soc. 1998, 120, 6745–6752. (2) Evans, W. J.; Perotti, J. M.; Ziller, J. W. J. Am. Chem. Soc. 2005, 127, 3894–3909. (3) Evans, W. J.; Perotti, J. M.; Kozimor, S. A.; Champagne, T. M.; Davis, B. L.; Nyce, G. W.; Fujimoto, C. H.; Clark, R. D.; Johnston, M. A.; Ziller, J. W. Organometallics 2005, 24, 3916–3931.

in energy and accessibility.8-10 An N-bound silyl product, [(MeC5H4)TiCl(µ-NSiMe3)]2, has been previously observed in the reaction of Li[Me3SiCN2] with (MeC5H4)TiCl3.11 When efforts were made to obtain more structural data on these complexes by making monometallic base adducts, it was found that 1-Sm and 1-La react with Me3CCN to make products containing 1,2,3-triazole rings, (C5Me5)2Ln(NCCMe3)[NNC(SiMe3)C(CMe3)N] (2-Sm, 2-La), eq 2.6

Since the 1,2,3-triazole molecular skeleton is found in heterocycles that have pharmaceutical, industrial, and agrochemical applications,12 these unusual transformations were examined further. This reaction sequence has been examined with different metals since it is often possible in organolanthanide chemistry to modify reaction pathways by varying the size of the metal. In addition, nitriles other than Me3CCN were examined to explore the range of triazoles that could be formed.6 Herein we report reactions with the smallest diamagnetic metals in the series, Y and Lu, for comparison with the large diamagnetic La, as well as a metal intermediate in size between La and Sm, namely, Nd. In addition, we have studied Yb, a metal with a divalent precursor option like Sm. Reactions with

10.1021/om8012103 CCC: $40.75  2009 American Chemical Society Publication on Web 04/06/2009

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C6H5CH2CN and Me3SiCN as well as Me3SiCH2NC are reported in addition to triazole syntheses that occur in the absence of added Lewis base.

Experimental Section The manipulations described below were conducted under argon or nitrogen with rigorous exclusion of air and water using Schlenk, vacuum line, and glovebox techniques. Solvents were sparged with UHP argon and dried over columns containing Q-5 and molecular sieves. NMR solvents (Cambridge Isotope Laboratories) were dried over sodium-potassium alloy, degassed, and vacuum-transferred before use. [(C5Me5)2Ln][(µ-Ph)2BPh2]1 and Li[Me3SiCN2]6 were prepared according to literature methods. (C5Me5)2LnCl2K(THF)2 (Ln ) Sm, La) complexes were prepared using methods similar to those used for the yttrium analogue.13 Me3SiCHN2 (2.0 M in hexanes, Sigma-Aldrich) was dried over activated 4 Å molecular sieves and degassed by three freeze-pump-thaw cycles before use. C6H5CH2CN and Me3SiCN (Sigma-Aldrich) were dried over calcium hydride, distilled over activated 4 Å molecular sieves, and degassed by three freeze-pump-thaw cycles before use. Me3SiCH2NC (Sigma-Aldrich) was dried over activated 4 Å molecular sieves and degassed before use. 1H NMR and 13C NMR spectra were recorded on a Bruker DRX500 spectrometer at 25 °C. Infrared spectra were recorded as KBr pellets on a Varian 1000 FTIR spectrophotometer at 25 °C. Elemental analyses were performed by Analytische Laboratorien (Lindlar, Germany) on a Perkin-Elmer Series II CHN Analyzer 2400. Lanthanide metal analyses were carried out by complexometric titration.14 {(C5Me5)2Sm[µ-N(SiMe3)NC]}2, 1-Sm, from (C5Me5)2SmCl2K(THF)2. Li[Me3SiCN2] (19 mg, 0.16 mmol) was added to a stirred yellow solution of (C5Me5)2SmCl2K(THF)2 (106 mg, 0.16 mmol) in THF (10 mL). No color change was observed, and after 3 h insolubles were removed from the mixture via centrifugation and filtration, leaving a clear yellow solution. Removal of solvent under reduced pressure yielded a yellow tacky solid, which was redissolved in the minimum amount of toluene. Insolubles were removed by centrifugation and filtration, and the clear yellow solution filtrate was evaporated to dryness, yielding 1-Sm as a yellow solid (67 mg, 80%). Yellow X-ray quality crystals of the previously characterized 1-Sm6 were grown by slow diffusion of hexane in THF at 25 °C. 1H NMR (THF-d8): δ 1.27 (s, 60H, C5Me5), -0.27 (s, 18H, SiMe3). {(C5Me5)2La[µ-N(SiMe3)NC]}2, 1-La, from (C5Me5)2LaCl2K(THF)2. As described for 1-Sm, 1-La was obtained as a white solid (70 mg, 81%) from Li[Me3SiCN2] (20 mg, 0.17 mmol) and (C5Me5)2LaCl2K(THF)2 (112 mg, 0.17 mmol) in THF (10 mL). (4) Evans, W. J.; Lee, D. S.; Lie, C.; Ziller, J. W. Angew. Chem., Int. Ed. 2004, 43, 5517–5519. (5) Evans, W. J.; Davis, B. L.; Ziller, J. W. Inorg. Chem. 2001, 40, 6341–6348. (6) Evans, W. J.; Montalvo, E.; Champagne, T. M.; Ziller, J. W.; DiPasquale, A. G.; Rheingold, A. L. J. Am. Chem. Soc. 2008, 130, 16–17. (7) Siebald, H.; Dartiguenave, M.; Dartiguenave, Y. J. Organomet. Chem. 1992, 438, 83–87. (8) Boche, G.; Lohrenz, J. C. W.; Schubert, F. Tetrahedron 1994, 50, 5889–5902. (9) Feeder, N.; Hendy, M. A.; Raithby, P. R.; Snaith, R.; Wheatley, A. E. H. Eur. J. Org. Chem. 1998, 861–864. (10) Armstrong, D. R.; Davies, R. P.; Haigh, R.; Hendy, M. A.; Raithby, P. R.; Snaith, R.; Wheatley, A. E. H. Eur. J. Inorg. Chem. 2003, 3363– 3375. (11) Bai, G.; Roesky, H. W.; Hao, H.; Noltemeyer, M.; Schmidt, H.-G. Inorg. Chem. 2001, 40, 2424–2426. (12) Fan, W. Q.; Katritzky, A. R. In ComprehensiVe Heterocyclic Chemistry II; Katritzky, A. R., Rees, C. W., Scriven, E. F. V., Eds.; Pergamon: Oxford, 1996; Vol. 4, pp 1-126. (13) Evans, W. J.; Peterson, T. T.; Rausch, M. D.; Hunter, W. E.; Zhang, H.; Atwood, J. L. Organometallics 1985, 4, 554–559. (14) Evans, W. J.; Engerer, S. C.; Coleson, K. M. J. Am. Chem. Soc. 1981, 103, 6672–6677.

EVans et al. Colorless X-ray quality crystals of previously characterized 1-La6 were grown by slow diffusion of hexane in THF at 25 °C. 1H NMR (THF-d8): δ 1.95 (s, 60H, C5Me5), 0.11 (s, 18H, SiMe3). {(C5Me5)2Y[µ-N(SiMe3)NC]}2, 1-Y. Li[Me3SiCN2] (24 mg, 0.19 mmol) was added to a stirred solution of [(C5Me5)2Y][(µ-Ph)2BPh2] (121 mg, 0.18 mmol) in benzene (20 mL). After 30 min the stirred mixture changed color from pale to bright yellow. After 3 h, an insoluble material was removed from the mixture via centrifugation and filtration, leaving a clear yellow solution. Removal of solvent under reduced pressure yielded 1-Y as a yellow solid (70 mg, 83%). Colorless X-ray quality crystals of 1-Y were grown from a concentrated benzene solution at 25 °C. Anal. Calcd for C24H39N2SiY: C, 60.99; H, 8.33; N, 5.92; Y, 18.81. Found: C, 60.78; H, 8.16; N, 5.80; Y, 19.05. 1H NMR (C6D6): δ 2.13 (s, 60H, C5Me5), 0.37 (s, 18H, SiMe3). 1H NMR (THF-d8): δ 1.93 (s, 60H, C5Me5), 0.11 (s, 18H, SiMe3). 13C NMR (C6D6): δ 119.2 (C5Me5), 12.6 (C5Me5), 1.6 (SiMe3). IR: 2955m, 2906m, 2859m, 2723w, 2058m, 1983s, 1438w, 1377w, 1313w, 1061w, 1021w, 837s, 763w, 742w, 677w, 627w, 535w cm-1. {(C5Me5)2Yb[µ-N(SiMe3)NC]}2, 1-Yb. A solution of Me3SiCHN2 in hexanes (32 µL, 0.064 mmol) was added dropwise via syringe to a stirred solution of (C5Me5)2Yb (28 mg, 0.063 mmol) in 1 mL of C6D6. The dark red solution immediately changed to bright violet, and the solution was transferred to an NMR tube. Pale purple crystals of 1-Yb suitable for X-ray diffraction were grown via slow evaporation of a C6D6 solution at 25 °C in an NMR tube (19 mg, 54% crystalline yield). Anal. Calcd for C24H39N2SiYb: Yb, 31.1. Found: Yb, 30.8. IR: 2958m, 2912m, 2858m, 2729w, 2087s, 1439w, 1378w, 1251w, 1065w, 1032w, 841s, 747w, 707w, 636w cm-1. 1H NMR (C6D6): δ 8.90 (br s, 60H, C5Me5, ∆ν1/2 ) 169 Hz), -0.70 (br s, 18H, SiMe3, ∆ν1/2 ) 44 Hz). {(C5Me5)2Nd[µ-N(SiMe3)NC]}2, 1-Nd. As described for 1-Y, 1-Nd was obtained as a bright green solid (74 mg, 82%) from Li[Me3SiCN2] (23 mg, 0.19 mmol) and [(C5Me5)2Nd][(µ-Ph)2BPh2] (125 mg, 0.17 mmol) in benzene (15 mL). Anal. Calcd for C24H39N2SiNd: C, 54.60; H, 7.46; N, 5.30; Nd, 27.32. Found: C, 54.75; H, 7.34; N, 5.43; Nd, 27.70. 1H NMR (C6D6): δ 8.79 (s, 60H, C5Me5), -21.55 (s, 18H, SiMe3). IR: 2957m, 2905m, 2858m, 2724w, 2060m, 1973s, 1437w, 1377w, 1323w, 1247m, 1061w, 1021w, 836s, 760w, 737w, 687w, 624w, 534w cm-1. {(C5Me5)2Lu[µ-N(SiMe3)NC]}2, 1-Lu. As described for 1-Y, 1-Lu was obtained as a bright yellow solid (33 mg, 85%) from Li[Me3SiCN2] (10 mg, 0.08 mmol) and [(C5Me5)2Lu][(µ-Ph)2BPh2] (53 mg, 0.07 mmol) in benzene (10 mL). Anal. Calcd for C24H39N2SiLu: C, 51.59; H, 7.05; N, 5.01; Lu, 31.27. Found: C, 51.50; H, 6.98; N, 4.90; Lu, 31.70. 1H NMR (C6D6): δ 2.15 (s, 60H, C5Me5), 0.38 (s, 18H, SiMe3). 13C NMR (C6D6): δ 118.6 (C5Me5), 12.8 (C5Me5), 1.8 (SiMe3). IR: 2957m, 2908m, 2859m, 2724w, 2061m, 1987s, 1440w, 1377w, 1305w, 1247m, 1058w, 1020w, 988w, 837s, 764w, 744w, 650w, 627w cm-1. {(C5Me5)2La[µ-η1:η2-NNC(SiMe3)C(CH2C6H5)N]}2, 3. C6H5CH2CN (13 µL, 0.11 mmol) was added via syringe to a stirred yellow solution of 1-La (59 mg, 0.06 mmol) in benzene (10 mL). The color of the reaction mixture immediately faded to a pale yellow color. After 3 h, the reaction mixture was filtered and solvent was removed under vacuum, leaving 3 as a pale yellow solid (60 mg, 83%). Colorless crystals of 3 suitable for X-ray diffraction were grown from a solvent mixture of benzene and hexane at 25 °C. Anal. Calcd for C32H46N3SiLa: C, 60.08; H, 7.25; N, 6.57. Found: C, 60.23; H, 7.39; N, 6.41. 1H NMR (C6D6): δ 4.54 (s, 4H, CH2C6H5), 1.99 (s, 60H, C5Me5), 0.31 (s, 18H, SiMe3). The phenyl protons were located in the 7.23-7.00 ppm region, but integrals are not reported since the resonances overlapped with the C6D6 resonance. 13C NMR (C6D6): δ 0.4 (SiMe3), 12.3 (C5Me5), 35.7 (CH2C6H5), 120.9 (C5Me5). The phenyl carbons were located in the 129.0-128.0 ppm region, but are not reported since they overlapped with the C6D6 resonances. IR: 3058w, 3031w, 2952m,

Synthetic DiVersity in the Formation of Triazoles 2903s, 2857s, 2724w, 1624w, 1604w, 1496m, 1449m, 1383w, 1328w, 1250w, 1187w, 1157w, 1121w, 1050w, 936w, 840s, 792w, 755w, 695w, 676w, 637w, 614w cm-1. [(C5Me5)2La(µ-CN)(NCSiMe3)]3, 4. Excess Me3SiCN was added to a stirred pale yellow solution of 1-La (60 mg, 0.06 mmol) in benzene (10 mL). After 3 h, solvent was removed under reduced pressure, yielding a pale yellow solid (60 mg, 97%). Colorless crystals of 4 suitable for X-ray diffraction were grown from a concentrated benzene solution at 25 °C. 1H NMR (C6D6): δ 2.34 (s, 90H, C5Me5), 0.18 (s, 27H, NCSiMe3). 13C NMR (C6D6): δ 0.17 (NCSiMe3), 12.1 (C5Me5), 120.0 (C5Me5). IR: 2974s, 2950s, 2856s, 2725w, 2178m, 2104m, 1444m, 1377w, 1257m, 1018w, 847m, 756w cm-1. The 1H NMR spectrum of 4 is shown in the Supporting Information. [(C5Me5)2La(µ-CN)(Me3SiCH2NC)]3, 5. Excess Me3SiCH2NC was added to a stirred pale yellow solution of 1-La (61 mg, 0.06 mmol) in benzene (10 mL). After 3 h, solvent was removed under reduced pressure, yielding a pale yellow solid. This yellow solid was washed with hexanes to yield the product. Pale yellow crystals of 5 suitable for X-ray diffraction were grown from a concentrated benzene solution at 25 °C (17 mg, 26%). 1H NMR (C6D6): δ 2.47 (s, 6H, Me3SiCH2NC), 2.40 (s, 90H, C5Me5), -0.02 (s, 27H, Me3SiCH2NC). 13C NMR (C6D6): δ -3.1 (Me3SiCH2NC), 12.8 (C5Me5), 119.0 (C5Me5). IR: 2968s, 2953s, 2852s, 2725w, 2184m, 2109m, 1551w, 1438m, 1379w, 1254m, 1021w, 956w, 858m, 761w cm-1. The 1H NMR spectrum of 5 is shown in the Supporting Information. X-ray Data Collection, Structure Determination, and Refinement. Crystallographic information on 1-Y, 1-Yb, 3, 4, 5, and 6-La is summarized in tables in the Supporting Information.

Results Synthesis of the Isocyanotrimethylsilyl Amides, 1-Ln. The original synthesis of the {(C5Me5)2Ln[µ-N(SiMe3)NC]}2 complexes, 1-Ln, accomplished with Ln ) Sm and La, eq 1, was readily extended to Ln ) Nd, Y, and Lu. Yields were consistently at least 80%. The 1-Yb member of this series was made via a divalent route from (C5Me5)2Yb, eq 3. Attempts to synthesize 1-Sm via this route from (C5Me5)2Sm had previously been described,6 but this route was found to be less successful for Sm than for Yb. This may be due to the higher reactivity of Sm2+ vs Yb2+, which leads to more side reactions.

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Figure 1. Thermal ellipsoid plot of {(C5Me5)2Y[µ-N(SiMe3)NC]}2, 1-Y, drawn at the 50% probability level. Hydrogen atoms have been excluded for clarity. Table 1. Bond Distances (Å) and Angles (deg) in {(C5Me5)2La[µ-N(SiMe3)NC]}2, 1-La, {(C5Me5)2Sm[µ-N(SiMe3)NC]}2, 1-Sm, {(C5Me5)2Y[µ-N(SiMe3)NC]}2, 1-Y, and {(C5Me5)2Yb[µ-N(SiMe3)NC]}2, 1-Yb bond distances/angles

1-La

1-Sm

1-Y

1-Yb

Ln(1)-Cnt(C5Me5) Ln(2)-Cnt(C5Me5) Ln(1)-C(C5Me5) av Ln(2)-C(C5Me5) av Ln(1)-N(1) Ln(2)-N(3) Ln(1)-C(42) Ln(2)-C(41) Si(1)-N(1) Si(2)-N(3) N(1)-N(2) N(2)-C(41) N(3)-N(4) N(4)-C(42) Cnt1-Ln(1)-Cnt2 Cnt-Ln(1)-N(1) Cnt-Ln(1)-C(42) Cnt3-Ln(2)-Cnt4 Cnt-Ln(2)-N(3) Cnt-Ln(2)-C(41) Ln(1)-N(1)-N(2) N(1)-N(2)-C(41) N(1)-Ln(1)-C(42) Ln(1)-C(42)-N(4) Ln(2)-N(3)-N(4) N(3)-N(4)-C(42) N(3)-Ln(2)-C(41) Ln(2)-C(41)-N(2)

2.550/2.550 2.542/2.551 2.82(2) 2.82(1) 2.5672(14) 2.5450(14) 2.6909(18) 2.6704(18) 1.7585(15) 1.7545(15) 1.3347(19) 1.162(2) 1.3354(19) 1.161(2) 132.3 107.8/114.9 100.4/102.0 135.4 106.6/110.1 98.1/103.2 107.73(9) 177.45(16) 86.93(5) 169.95(14) 100.93(9) 176.66(16) 94.51(5) 157.15(14)

2.454/2.453 2.452/2.449 2.74(2) 2.73(1) 2.4979(17) 2.4749(17) 2.524(2) 2.536(2) 1.7576(18) 1.7595(17) 1.329(2) 1.170(3) 1.327(2) 1.168(3) 133.1 115.1/108.1 101.1/101.7 136.0 107.4/111.4 99.5/103.1 113.27(12) 177.8(2) 82.34(6) 166.31(17) 110.72(12) 177.4(2) 85.81(6) 157.90(17)

2.387/2.388 2.387/2.385 2.67(2) 2.67(1) 2.4421(17) 2.4147(16) 2.4927(19) 2.496(2) 1.7660(17) 1.7626(17) 1.338(2) 1.162(2) 1.338(2) 1.156(2) 133.3 114.1/108.2 102.6/101.8 135.9 107.6/110.8 100.0/104.1 113.51(11) 177.12(19) 82.04(6) 166.40(16) 111.55(11) 177.22(19) 85.04(6) 159.34(16)

2.350/2.352 2.350/2.351 2.64(2) 2.64(2) 2.414(3) 2.388(3) 2.379(4) 2.406(4) 1.760(3) 1.766(3) 1.329(4) 1.182(5) 1.324(4) 1.174(5) 132.8 108.3/113.9 102.5/103.1 135.2 107.9/111.1 100.9/104.2 113.6(2) 178.3(3) 81.97(11) 165.5(3) 113.0(2) 178.0(3) 83.83(11) 158.6(3)

in approximately 80% yield according to eq 4. This is now the preferred route to 1-Sm and 1-La. Although the synthesis of the 1-Ln series from the tetraphenylborate salts was quite convenient, a shorter synthesis was subsequentlyexaminedstartingfromthe“ate”salts,(C5Me5)2LnCl2K(THF)2,13,15 that are precursors to the tetraphenylborate starting materials of eq 1. Such “ate” salts are often problematic in organolanthanide synthesis because the elimination of 2 equiv of alkali metal chloride from the products can be difficult.16,17 However, this was not the case in the formation of 1-Ln, and the isocyanotrimethylsilyl amides 1-Sm and 1-La can be made (15) Evans, W. J.; Keyer, R. A.; Ziller, J. W. Organometallics 1993, 12, 2618–2633. (16) Schumann, H.; Albrecht, I.; Loebel, J.; Hahn, E.; Hossain, M. B.; Van der Helm, D. Organometallics 1986, 5, 1296–1304. (17) Evans, W. J.; Boyle, T. J.; Ziller, J. W. Inorg. Chem. 1992, 31, 1120–1122.

Structures of the Isocyanotrimethylsilyl Amides, 1-Ln. Crystallographic data were obtained on 1-Y, Figure 1, and 1-Yb that show structures isomorphous to those of 1-Sm and 1-La. Selected bond lengths and angles for 1-Y and 1-Yb are compared with those of 1-La and 1-Sm in Table 1. Complete crystallographic studies on two samples of each of 1-Sm, 1-La, and 1-Y and one sample of 1-Yb indicated that the (Me3Si)N-bound structure shown in Figure 1 rather than an

2900 Organometallics, Vol. 28, No. 9, 2009

alternative (Me3Si)C-bound structure, (C5Me5)2Ln-C(SiMe3)N2, was present. This assignment was supported in all seven structures by the Si-N bond length data. The 1.766(3)-1.754(2) Å Si-N distances in all the structures of 1-Ln are longer that the related Si-N distances found in (C5Me5)2Sm[N(SiMe3)2]15 [1.697(4) and 1.700(4) Å] and Sm[N(SiMe3)2]318 [1.714(2) Å], but all of these distances are similar to the 1.711-1.748 Å averages given in tabulated data on Si-N bonds.19 These distances are not in the Si-C range for example, the six independent distances in (C5Me5)2M[CH(SiMe3)2] (M ) Ce,20 Nd,21 and Y22) are 1.812(21)-1.909(5) Å. However, the bond lengths in 1-Ln suggest that the resonance structure shown in eq 1 is not the only contributor to these complexes: a carbene resonance form in which N(1) and N(3) have neutral donor character and C(41) and C(42) are formally anionic may also be involved, i.e., Ln r N(SiMe3)dNdC. For example, the 1.329(4)-1.338(2) Å N(1)-N(2) and 1.324(4)1.338(2) Å N(3)-N(4) distances in 1-Ln are shorter than expected for a single bond (1.41-1.45 Å).19 However, the 1.162(2)-1.182(5) Å N(2)-C(41) and 1.156(2)-1.174(5) Å N(4)-C(42) lengths are comparable to the corresponding average distances in bridged cyanide lanthanide metallocenes, 1.17(2) and 1.161(4) Å in [(C5Me5)2Sm(CNC6H11)(µ-CN)]3,23 and [(C5Me5)2Sm(CNCMe3)(µ-CN)]3,23,24 respectively. The 2.498(2) and 2.475(2) Å Sm(1)-N(1) and Sm(2)-N(3) distances in 1-Sm are longer than the 2.301(3) Å Sm-N bond in (C5Me5)2Sm[N(SiMe3)2]15 (possibly due in part to the more crowded dimeric nature of 1), and the 2.524(2) and 2.536(2) Å Sm(1)-C(42) and Sm(2)-C(41) distances are within the broad 2.49(2)-2.64(2) Å range observed for Sm-(CN)/Sm-(NC) connections in [(C5Me5)2Sm(CNC6H11)(µ-CN)]323 and [(C5Me5)2Sm(CNCMe3)(µ-CN)]3.23,24 The bond distances in isomorphous 1-La are similar except that the distances involving the metal are longer due to the larger size of lanthanum. Similarly, analogous distances are shorter in isomorphous 1-Y and 1-Yb due to the smaller size of yttrium and ytterbium.25 The bond distances observed in 1-Ln could also arise if the crystals contained some amount of a Ln-C(SiMe3)dNdN f Ln isomer.26 In support of this possibility, an eighth crystallographically analyzed sample of 1-Ln involving Sm showed 1.823(2) and 1.837(2) Å Me3Si-element bonds and refined slightly better with the trimethylsilyl group bound to carbon. The overall refinement, however, was not better than that reported for 1-Sm in Table 1. Interestingly, the IR spectra of all the analogues of 1-Ln except 1-Yb displayed two absorptions in the multiple-bond region. The most prominent absorptions for 1-La, 1-Nd, 1-Sm, 1-Y, and 1-Lu are at 1970-1987 cm-1, respectively, but substantial bands at 2057-2063 cm-1 are also observed. However, neither variable-temperature NMR studies nor an 800 MHz 1H NMR spectrum showed evidence of two (18) Brady, E. D.; Clark, D. L.; Gordon, J. C.; Hay, P. J.; Keogh, D. W.; Poli, R.; Scott, B. L.; Watkin, J. G. Inorg. Chem. 2003, 42, 6682–6690. (19) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A. G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1–S19. (20) Heeres, H. J.; Renkema, J.; Booij, M.; Meetsma, A.; Teuben, J. H. Organometallics 1988, 7, 2495–2502. (21) Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P. N.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091–8103. (22) Den Haan, K. H.; De Boer, J. L.; Teuben, J. H.; Spek, A. L.; KojicProdic, B.; Hays, G. R.; Huis, R. Organometallics 1986, 5, 1726–1733. (23) Evans, W. J.; Drummond, D. K. Organometallics 1988, 7, 797– 802. (24) Evans, W. J.; Forrestal, K. J.; Ziller, J. W. J. Am. Chem. Soc. 1998, 120, 9273–9282. (25) Shannon, R. D. Acta Crystallogr. 1976, A32, 751–767. (26) Parkin, G. Acc. Chem. Res. 1992, 25, 455–460.

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Figure 2. Thermal ellipsoid plot of {(C5Me5)2La[µ-η1:η2NNC(SiMe3)C(CH2C6H5)N]}2, 3, drawn at the 50% probability level. Hydrogen atoms have been excluded for clarity.

(C5Me5)- resonances. The infrared spectrum of 1-Yb showed only a single strong absorption band at 2087 cm-1. Addition of Lewis Bases to the Isocyanotrimethylsilyl Amide Complexes. C6H5CH2CN. The reactions of 1-Sm and 1-La with Me3CCN, eq 2,6 differed from those of MeCN, which gave products with low solubility and crystals of insufficient quality for X-ray analysis. The effect of the nitrile reagent on the synthesis of substituted triazoles was examined by reacting other nitriles with 1-Ln. The reaction of 1-La with C6H5CH2CN is shown in eq 5. The reaction is similar to the Me3CCN reaction, eq 2, in that a 1,2,3-triazole forms, but in this case, the product, {(C5Me5)2La[µη1:η2-NNC(SiMe3)C(CH2C6H5)N]}2, 3, did not form a nitrile adduct. Instead, as shown in Figure 2, the complex crystallizes as an unsolvated dimer in which one of the triazole nitrogens takes the place of a coordinating nitrile in 2-La.

As in 2-Sm and 2-La, the X-ray data clearly show the heterocycle as a 1,2,3-triazole, a result that once again requires silyl migration. The 1.881(4) Å C(21)-Si(1) bond lengths in 3 are similar to the analogous 1.862(2) Å distance in 2-La and close to the 1.81-1.84 range expected for C-Si bonds.19 The 1,2,3-triazolato ligands in 3 have their closest attachment to the La3+ ions through N(1), 2.548(3) Å. The La(1)-N(2) bond length (2.652(3) Å) is longer, which is consistent with more donor bond character. Both of these distances are longer than (27) Evans, W. J.; Montalvo, E.; Foster, S. E.; Harada, K. A.; Ziller, J. W. Organometallics 2007, 26, 2904–2910. (28) Maynadie, J.; Berthet, J.-C.; Thuery, P.; Ephritikhine, M. Organometallics 2007, 26, 2623–2629.

Synthetic DiVersity in the Formation of Triazoles

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Figure 3. Thermal ellipsoid plot of [(C5Me5)2La(µ-CN)(NCSiMe3)]3, 4, drawn at the 50% probability level. Hydrogen atoms have been excluded for clarity.

Figure 4. Thermal ellipsoid plot of [(C5Me5)2La(µ-CN)(Me3SiCH2NC)]3, 5, drawn at the 50% probability level. Hydrogen atoms have been excluded for clarity.

the corresponding 2.472(2) and 2.551(2) Å distances, respectively, in 2-La, while the 2.718(3) Å La(1)-N(3) distance is even longer. These distances suggest that the coordination environment in 3 is more crowded than that in 2-La. Me3SiCN. The reaction of Me3SiCN with 1-La yielded a product very different from complex 3 above. As shown in eq 6, a cyanide trimer, [(C5Me5)2La(µ-CN)(NCSiMe3)]3, 4, was isolated, Figure 3. Cyanide trimers of formula [(C5Me5)2Ln(µCN)L]3 are known for Ln ) Sm; L ) CNC6H11,23 CNCMe3,23,24 NCCMe327 and Ln ) Ce; L ) CNCMe3.28 Typically these species require the presence of a coordinating Lewis base, L, to be soluble and crystallizable. In this case, 4 crystallized with Me3SiCN as the coordinating base, Figure 3.

Since the La3+ ion in 1-La is not capable of doing reduction chemistry like the Sm2+ ion in (C5Me5)2Sm(THF)2, which forms cyanides from nitriles and isocyanides,23,27 complex 4 must have formed in another way. σ-Bond metathesis involving Me3SiCN and the La-N bond in 1-La would produce “(C5Me5)2La(CN)” and the isocyanide “(Me3Si)2NNC”. Oligomerization of “(C5Me5)2La(CN)” and coordination of excess Me3SiCN would form 4. (Me3Si)2NNC was not isolated from this reaction, but a reaction prepared in C6D6 and monitored by 1H NMR spectroscopy suggested that a related species might form since a resonance was observed at 0.76 ppm, which is close to that reported for (Me3Si)2NNC (0.70 ppm).29,30 Me3SiCH2NC. The reactivity of 1-La with isocyanides was examined since it has been previously reported that the C-metalated diazoalkane complex Rh(PEt3)3[C(SiMe3)N2]31 reacts with isocyanides, RNtC (R ) tBu, nBu), to produce the corresponding 1,2,3-triazolato complexes (PEt3)3-n(RNC)nRh[CC(SiMe3)N2N(R)] (R ) tBu, n ) 2; R ) nBu, n ) 1) by 1,3-dipolar cycloaddition, eq 7.32

Although the NMR spectrum of 4 (shown in the Supporting Information) indicates that a single metallocene product is present, attempts to definitively characterize 4 as a pure product were not successful. After washing the crude product with hexanes, it began to lose solubility and decomposed over time, possibly due to the removal of coordinated Me3SiCN. Similar problems were encountered in the isolation of the previously characterized [(C5Me5)2Sm(NCCMe3)(µ-CN)]327 and [(C5Me5)2Sm(CNR)(µ-CN)]3 (R ) CNC6H11,23 CNCMe323,24) obtained from the reductive cleavage of nitriles and isocyanides with (C5Me5)2Sm(THF)2, respectively. The related unsolvated complexes [(C5Me5)2MCN]n (M ) U,28 Ce,28 Sm27) have been reported to have poor solubility in coordinating solvents and have been characterized only by infrared spectroscopy and elemental analysis. The infrared spectrum of 4 contains two absorptions in the νCN region at 2178 and 2104 cm-1 corresponding to the coordinated Me3SiCN and the bridging cyanide ligands, respectively. The absorption at 2104 cm-1 is close to the corresponding absorption in [(C5Me5)2Sm(CNR)(µ-CN)]323 (R ) CNC6H11,23 2105 cm-1; CNCMe3,23 2110 cm-1) and the unsolvated [(C5Me5)2MCN]n (M ) Ce,28 2104 cm-1; Sm,27 2103 cm-1).

Complex 1-La reacted with Me3SiCH2NC, but a complicated product mixture resulted. The trimeric cyanide-bridged complex [(C5Me5)2La(µ-CN)(Me3SiCH2NC)]3, 5, eq 8, Figure 4, was isolated. Complex 5 is similar to 4, but it has a coordinated isocyanide rather than a nitrile.

The infrared spectrum of 5 contains two absorptions in the νCN region at 2184 and 2109 cm-1, corresponding to the coordinated Me3SiCH2NC and the bridging cyanide ligands,

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Table 2. Bond Distances (Å) and Angles (deg) in {(C5Me5)2La[µ-η1:η2-NNC(SiMe3)C(CH2C6H5)N]}2, 3 Ln(1)-Cnt(C5Me5) Ln(1)-C(C5Me5) av Ln(1)-N(1) Ln(1)-N(2) Ln(1)-N(3) N(1)-N(2) N(1)-N(3A) N(3A)-C(25A) C(25A)-C(21) N(2)-C(21) C(25A)-C(26A) C(21)-Si(1) Cnt1-Ln(1)-Cnt2 Cnt-Ln(1)-N(1) Cnt-Ln(1)-N(2) Cnt-Ln(1)-N(3) N(2)-Ln(1)-N(1) N(1)-Ln(1)-N(3) N(2)-N(1)-Ln(1) N(1)-N(2)-Ln(1) N(3A)-N(1)-Ln(1) N(2)-N(1)-N(3A) N(1A)-N(3)-C(25) N(3)-C(25)-C(21A) C(25A)-C(21)-N(2) C(21)-N(2)-N(1) N(3)-C(25)-C(26) C(26)-C(25)-C(21A) Si(1)-C(21)-C(25A) Si(1)-C(21)-N(2)

2.557/2.592 2.84(2) 2.548(3) 2.652(3) 2.718(3) 1.333(4) 1.348(4) 1.354(5) 1.400(5) 1.379(5) 1.498(5) 1.881(4) 118.8 105.9/121.2 97.3/104.9 109.7/104.9 29.62(9) 77.62(9) 79.5(2) 70.85(19) 167.8(2) 110.5(3) 106.9(3) 108.9(3) 105.3(3) 108.4(3) 120.9(3) 130.2(3) 133.7(3) 121.0(3)

respectively. These absorptions are similar to the corresponding absorptions of coordinated isocyanides in [(C5Me5)2Sm(CNR)(µCN)]323 (R ) CNC6H11, 2180 cm-1; CNCMe3, 2180 cm-1) and [(C5Me5)2Ce(CNCMe3)(µ-CN)]328 (2177 cm-1) and ν(µ-CN) in 4, [(C5Me5)2Sm(CNR)(µ-CN)]323 (R ) CNC6H11, CNCMe3) and [(C5Me5)2MCN]n (M ) Ce,28 Sm27). After washing crude 5 with hexane, more than one product was still observed in the 1H NMR spectrum. The hexaneinsoluble material was recrystallized from benzene, and pale yellow crystals of 5 were obtained and identified by X-ray crystallography, Figure 4. As for 4, complex 5 could not be isolated as a pure product without significant decomposition. The 1H NMR spectrum of 5 (shown in the Supporting Information) shows the isolation of a single metallocene product. Initial reactivity studies of 1-La with Me3CNC and C6H5CH2NC have also shown that multiple products are formed by 1H NMR spectroscopy. Structures of 4 and 5. Complexes 4 and 5 crystallize as trimers, composed of (C5Me5)2LaL units (L ) NCSiMe3 and CNCH2SiMe3, respectively) connected by cyanide ligands. The formally nine-coordinate La3+ ions are ligated by two (C5Me5)ligands, one nitrogen and carbon atom of the bridging (CN)ligands, and either the nitrogen of Me3SiCN or the terminal carbon of CNCH2SiMe3. The X-ray data could not differentiate between C and N atoms in the (µ-CN)- ligands in either case. Selected bond lengths and angles are provided in Table 3. The three lanthanum atoms define an equilateral triangle with La-C-N-La edges. The La(1)-C(14)-N(1), La(1B)-N(1)C(14), and N(1A)-La(1)-C(14) angles in 4 are 171.0(3)°, 174.0(3)°, and 75.00(9)°, respectively, and the analogous angles (29) Vasisht, S. K.; Sood, M.; Sood, N.; Singh, G. J. Organomet. Chem. 1986, 301, 15–25. (30) Verma, P. K. Z. Anorg. Allg. Chem. 1991, 605, 131–136. (31) Menu, M. J.; Desrosiers, P.; Dartiguenave, M.; Dartiguenave, Y.; Bertrand, G. Organometallics 1987, 6, 1822–1824. (32) Deydier, E.; Menu, M. J.; Dartiguenave, M.; Dartiguenave, Y. J. Chem. Soc., Chem. Commun. 1991, 809–810.

Table 3. Bond Distances (Å) and Angles (deg) in [(C5Me5)2La(µ-CN)(NCSiMe3)]3, 4, and [(C5Me5)2La(µ-CN)(Me3SiCH2NC)]3, 5 bond distances/angles

4

5

Ln(1)-Cnt(C5Me5) Ln(1)-C(C5Me5) av Ln(1)-N(µ-CN) Ln(1)-C(µ-CN) Ln(1)-C(Me3SiCH2NC) Ln(1)-N(NCSiMe3) N(µ-CN)-C(µ-CN) Cnt1-Ln(1)-Cnt2 Cnt-Ln(1)-N(µ-CN) Cnt-Ln(1)-C(µ-CN) Cnt-Ln(1)-C(Me3SiCH2NC) Cnt-Ln(1)-N(NCSiMe3) Ln-C(µ-CN)-N(µ-CN) N(µ-CN)-Ln(1)-C(µ-CN) C(µ-CN)-Ln(1)-C(Me3SiCH2NC) N(µ-CN)-Ln(1)-N(NCSiMe3)

2.562 2.83(2) 2.684(3) 2.671(3) N/A 2.694(3) 1.164(4) 135.5 112.0 98.7 N/A 94.1 171.0(3) 75.00(9) N/A 70.52(9)

2.557 2.83(1) 2.661(5) 2.677(4) 2.792(6) N/A 1.147(6) 136.3 98.9 111.5 94.6 N/A 176.4(4) 74.83(14) 68.18(17) N/A

in 5 are similar. The 1.164(4) Å N(1)-C(14) distance in 4 and the 1.147(6) Å N(1)-C(11B) distance in 5 are consistent with a N-C triple bond in the bridging cyanide ligands19 and are comparable to the corresponding average distances of 1.17(2), 1.161(4), and 1.16(2) Å in [(C5Me5)2Sm(CNC6H11)(µ-CN)]3,23 [(C5Me5)2Sm(CNCMe3)(µ-CN)]3,24 and [(C5Me5)2Ce(CNCMe3)(µCN)]3,28 respectively. The 2.661(5)-2.684(3) Å La-X(CN or NC) distances are the same within experimental error. In 4, the 2.694(3) Å La(1)-N(2) distance is comparable to the corresponding 2.6449(17) Å La(1)-N(1) distance in 2-La within the 3σ error range. In 5, the 2.792(6) Å La(1)-C(12) distance is comparable to the corresponding average 2.58(2) and 2.643(7) Ådistancesin[(C5Me5)2Sm(CNC6H11)(µ-CN)]323 and[(C5Me5)2Sm(CNCMe3)(µ-CN)]3,24 respectively, taking into account the 0.084 Å difference in ionic radii between La3+ and Sm3+ ions in a nine-coordinate environment.25 Triazole Formation in the Absence of a Nitrile Reagent. In the course of these studies, crystals of two triazole complexes were isolated without the addition of a nitrile during the recrystallization of 1-Sm and 1-La by slow evaporation of benzene. X-ray crystallography identified these as {(C5Me5)2Ln[µη1:η2-NNC(SiMe3)C(H)N]}2 (6-Sm, 6-La), eq 9, Figure 5. In these complexes the CN unit that adds to the CN2 unit in 1-Ln to make the C2N3 triazole must come from a CN2 unit in 1-Ln.

Figure 5. Thermal ellipsoid plot of {(C5Me5)2Sm[µ-η1:η2NNC(SiMe3)C(H)N]}2, 6-Sm, drawn at the 50% probability level. Hydrogen atoms have been excluded for clarity.

Synthetic DiVersity in the Formation of Triazoles

Analogous reactivity with the alkali metal salts of diazomethane derivatives has been observed.8,33

As in 2-Sm, 2-La, and 3, the X-ray data clearly show that 6-Sm is a 1,2,3-triazole, a result that once again requires silyl migration if silyl groups in 1-La are bound to N and not C. The Sm-N bond lengths in 6-Sm exhibit a pattern similar to that in 3 except all of the lengths are shorter due to the smaller size of Sm. The 1,2,3-triazolato ligands in 6-Sm have their closest attachment to the Sm3+ ions through N(2) and N(5), 2.470(2) and 2.472(2) Å, respectively. The Sm(1)-N(6) and Sm(2)-N(3) bond lengths, 2.580(2) and 2.588(2) Å, respectively, are longer, which is consistent with more donor bond character. The 2.615(2) Å Sm(1)-N(1) and 2.592(2) Å Sm(2)-N(4) distances are even longer. The C(41)-Si(1) and C(43)-Si(1) bond lengths in 6-Sm are the same, 1.870(2) Å, and close to the 1.81-1.84 Å range expected for C-Si bonds.19

Discussion The results of this study show that the synthesis of isocyanotrimethylsilyl amide complexes from the reaction of the lithium salt of trimethylsilyldiazomethane with lanthanide metallocenes, eq 1, is quite general. This reaction forms {(C5Me5)2Ln{µN(SiMe3)NC]}2 complexes, 1-Ln, not only for larger lanthanides, such as Sm and La, but also for metals of intermediate (Nd) and smaller sizes (Y, Yb, Lu). Regardless of size of the lanthanide, analogous structures are obtained. Since the lanthanide contraction often causes a change in coordination number somewhere between La and Lu for a given ligand set, the formation of an isomorphous set of complexes across the lanthanide series is actually somewhat unusual but not unprecedented.34,35 These results also showed that the trimethylsilyl migration initially observed for La and Sm is general for the other metals. Two new syntheses of complexes 1-Ln are reported here that have significance from the lanthanide perspective. The fact that 1-Yb can be prepared cleanly from (C5Me5)2Yb, eq 3, in contrast to the more complicated samarium reaction, demonstrates the value of examining the less reactive Yb2+ reagents when Sm2+ precursors are too strongly reducing for a given substrate and form multiple products. The fact that 1-La and 1-Sm can be made from (C5Me5)2LnCl2K(THF)2, eq 4, is a reminder that these “ate” salts can be useful precursors with some reagents. Since the “ate” salts are readily made from LnCl3, this has significant synthetic advantages. The formation of new triazolate complexes, e.g., complex 3, eq 5, and complex 6, eq 9, demonstrates that the reaction that (33) Boche, G.; Harms, K.; Marsch, M.; Schubert, F. Chem. Ber. 1994, 127, 2193–2195. (34) Scholer, R. P.; Merbach, A. E. Inorg. Chim. Acta 1975, 15, 15– 20. (35) Donoghue, J. T.; Fernandez, E.; McMillan, J. A.; Peters, D. A. J. Inorg. Nucl. Chem. 1969, 31, 1431–1433.

Organometallics, Vol. 28, No. 9, 2009 2903 Table 4. Bond Distances (Å) and Angles (deg) in {(C5Me5)2Sm[µ-η1:η2-NNC(SiMe3)C(H)N]}2, 6-Sm Ln(1)-Cnt(C5Me5) Ln(1)-C(C5Me5) av Ln(1)-N(1) Ln(1)-N(2) Ln(1)-N(6) Ln(2)-N(3) Ln(2)-N(4) Ln(2)-N(5) N(1)-N(2) N(2)-N(3) N(1)-C(41) N(3)-C(42) C(41)-C(42) C(41)-Si(1)/C(43)-Si(2) Cnt1-Ln(1)-Cnt2 Cnt-Ln(1)-N(1) Cnt-Ln(1)-N(2) Cnt-Ln(1)-N(6) N(2)-Ln(1)-N(1) N(2)-Ln(1)-N(6) N(1)-N(2)-N(3) N(2)-N(1)-C(41) N(2)-N(3)-C(42) N(3)-C(42)-C(41) N(1)-C(41)-C(42)

2.447/2.471 2.74(3) 2.615(2) 2.470(2) 2.580(2) 2.588(2) 2.592(2) 2.472(2) 1.334(2) 1.340(2) 1.365(3) 1.352(3) 1.394(3) 1.870(2) 130.3 101.5/110.9 106.7/120.4 105.1/103.4 30.26(5) 72.24(6) 111.27(17) 108.10(17) 105.78(17) 109.66(19) 105.18(18)

formed 2 can be accomplished with other lanthanide metallocenes of this type. However, it is clear from the reactions with MeCN and Me3SiCN that the specific choice of nitrile is crucial. The formation of the bridged cyanide products, [(C5Me5)2La(µCN)L]3, 4 and 5, via eqs 6 and 8, demonstrates that a readily accessible alternative reaction pathway is available in these reactions. Since, in the absence of a coordinating base the trimeric cyanides form insoluble precipitates that can drive any equilibria toward the cyanide product, this can be a significant problem. In the reaction of the isocyanotrimethylsilyl amides with the isonitrile, Me3SiCH2NC, this cyanide-forming reaction was predominant.

Conclusion Lanthanide metallocenes provide a new, general platform for the synthesis of 1,2,3-triazole ring structures using lithium salts of diazoalkanes and nitriles. The substitution pattern in the triazole product depends on the specific substrate and reaction conditions involved. The formation of the isocyanotrimethylsilyl amido lanthanide precursors is general across the lanthanide series and offers the possibility of metal-based size optimization for the generation of subsequent reaction products. The propensity of lanthanide metallocenes to form cyanide-bridged trimers of low solubility is a complicating side reaction that must be avoided for successful triazole syntheses.

Acknowledgment. We thank the Chemical Sciences, Geosciences, and Biosciences Division of the Office of Basic Energy Sciences of the Department of Energy for support. This research was facilitated in part by a National Physical Science Consortium Fellowship and by stipend support from Los Alamos National Laboratory (to E.M.). Supporting Information Available: X-ray diffraction data, atomic coordinates, thermal parameters, and complete bond distances and angles. This material is available free of charge via the Internet at http://pubs.acs.org. OM8012103