Lanthanide(III) 4,6-Dimethylpyrimidine-2-thionate Complexes as

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Organometallics 2011, 30, 208–214 DOI: 10.1021/om100671s

Lanthanide(III) 4,6-Dimethylpyrimidine-2-thionate Complexes as Efficient Catalysts for Isocyanate Cyclodimerization Hong-Xi Li,†,‡ Mei-Ling Cheng,†,§ He-Ming Wang,† Xiao-Juan Yang,† Zhi-Gang Ren,† and Jian-Ping Lang*,†,‡ †

College of Chemistry, Chemical Engineering and Materials Science, Suzhou University, Suzhou 215123, People’s Republic of China, ‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 210032, People’s Republic of China, and § School of Chemistry and Chemical Engineering, Changzhou University, Changzhou 213164, People’s Republic of China Received July 10, 2010

Protonolysis reactions of [(Me3Si)2N]3Ln( μ-Cl)Li(THF)3 (Ln = Pr, Nd, Sm, Eu) with 3 equiv of 4,6-dimethylpyrimidine-2-thiol (dmpymtH) gave rise to the four Ln(III) pyrimidine-2-thionate complexes Li[Ln(dmpymt)4] (Ln = Pr (1), Nd (2), Sm (3), Eu (4)). Compounds 1-4 were characterized by elemental analysis, IR and 1H NMR spectroscopy, and single-crystal X-ray diffraction. X-ray diffraction analysis shows that the structures of 1-4 are similar and each eightcoordinate Ln(III) ion is chelated by four dmpymt ligands. Complexes 1-4 display excellent catalytic performance in the cyclodimerization of isocyanates to produce substituted ureas via elimination of CO, which represents the first example of lanthanide thiolates exhibiting a high catalytic activity and a high selectivity in the cyclodimerization of isocyanates. The effects of the solvents, temperatures, catalyst loadings, and rare-earth metals on the catalytic activities of the complexes were examined.

Introduction In the past decades, the synthesis of lanthanide thiolate complexes has attracted much attention due to their interesting structures,1 luminescence properties,2 and potential applications in catalytic processes3 and the preparation of organic or organolanthanide complexes.4 So far, a number *To whom correspondence should be addressed. Fax and tel: int. code þ86 512 65880089. E-mail: [email protected]. (1) (a) Roger, M.; Arliguie, T.; Thuery, P.; Fourmigue, M.; Ephritikhine, M. Inorg. Chem. 2005, 44, 584. (b) Jin, G. X. Coord. Chem. Rev. 2004, 248, 587. (2) (a) Kornienko, A.; Banerjee, S.; Kumar, G. A.; Riman, R. E.; Emge, T. J.; Brennan, J. G. J. Am. Chem. Soc. 2005, 127, 14008. (b) Kumar, G. A.; Riman, R. E.; Torres, L. A. D.; Garcia, O. B.; Banerjee, S.; Kornienko, A.; Brennan, J. G. Chem. Mater. 2005, 17, 5130. (c) Banerjee, S.; Huebner, L.; Romanelli, M. D.; Kumar, G. A.; Riman, R. E.; Emge, T. J.; Brennan, J. G. J. Am. Chem. Soc. 2005, 127, 15900. (3) (a) Nakayama, Y.; Shibahara, T.; Fukumoto, H.; Nakamura, A.; Mashima, K. Macromolecules 1996, 29, 8014. (b) Hou, Z. M.; Zhang, Y. G.; Tezuka, H.; Xie, P.; Tardif, O.; Koizumi, T.; Yamazaki, H.; Wakatsuki, Y. J. Am. Chem. Soc. 2000, 122, 10533. (c) Li, H. R.; Yao, Y. M.; Yao, C. S.; Sheng, H. T.; Shen, Q. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 1312. (d) Kim, J. Y.; Livinghouse, T. Org. Lett. 2005, 7, 1737. (4) (a) Taniguchi, Y.; Maruo, M.; Takaki, K.; Fujiwara, Y. Tetrahedron Lett. 1994, 35, 7789. (b) Dowsland, J.; McKerlie, F.; Procter, D. J. Tetrahedron Lett. 2000, 41, 4923. (c) Shen, Q.; Li, H. R.; Yao, C. S.; Yao, Y. M.; Zhang, L. L.; Yu, K. B. Organometallics 2001, 20, 3070. (d) Zhang, J.; Ma, L. P.; Cai, R. F.; Weng, L. H.; Zhou, X. G. Organometallics 2005, 24, 738. (5) (a) Nief, F. Coord. Chem. Rev. 1998, 13, 178. (b) Aspinall, H. C.; Cunningham, S. A. Inorg. Chem. 1998, 37, 5396. (c) Tatsumi, K.; Amemiya, T.; Kawaguchi, H.; Tani, K. J. Chem. Soc., Chem. Commun. 1993, 773. (d) Stults, S. D.; Andersen, R. A.; Zalkin, A. Organometallics 1990, 9, 1623. (e) Wu, Z. Z.; Ma, W. W.; Huang, Z. E.; Cai, R. F. Polyhedron 1996, 15, 3427. (f ) Cendrowski-Guillaume, S. M.; Gland, G. L.; Nierlich, M.; Ephritikhine, M. Organometallics 2000, 19, 5654. pubs.acs.org/Organometallics

Published on Web 12/15/2010

of these complexes with alkane- or arenethiolates5,6 have been synthesized. Among them, the tendency for thiolates to serve as bridging ligands and for the lanthanide ions to maximize coordination numbers generally leads to the formation of oligomeric or polymeric lanthanide thiolate complexes unless the lanthanide ion is coordinated by strong donor molecules or very bulky thiolate ligands. Nitrogendonor-containing thiolates such as pyridine-2-thione have attracted much attention, because these ligands could chelate Ln(II)/Ln(III) metal atoms via both sulfur and nitrogen atoms to stabilize them.7,8 Another kind of N,S-donor ligand, pyrimidine-2-thione and its derivatives, has been (6) (a) Li, H. X.; Zhu, Y. J.; Cheng, M. L.; Ren, Z. G.; Lang, J. P.; Shen, Q. Coord. Chem. Rev. 2006, 250, 2059. (b) Banerjee, S.; Emge, T. J.; Brennan, J. G. Inorg. Chem. 2004, 43, 6307. (c) Roger, M.; Arliguie, T.; Thuery, P.; Fourmigue, M.; Ephritikhine, M. Inorg. Chem. 2005, 44, 584. (d) Mashima, K.; Nakayama, Y.; Shibahara, T.; Fukumoto, H.; Nakamura, A. Inorg. Chem. 1996, 35, 93. (e) Melman, J. H.; Rohde, C.; Emge, T. J.; Brennan, J. G. Inorg. Chem. 2002, 41, 28. (f ) Cetinkaya, B.; Hitchcock, P. B.; Lappert, M. F.; Smith, R. G. J. Chem. Soc., Chem. Commun. 1992, 932. (g) Mashima, K.; Nakayama, Y.; Fukumoto, H.; Kanehisa, N.; Kai, Y.; Nakamura, A. J. Chem. Soc., Chem. Commun. 1994, 2523. (h) Niemeyer, D. R. Eur. J. Inorg. Chem. 2001, 1969. (7) (a) Hillier, A. C.; Liu, S. Y.; Sella, A.; Elsegood, M. R. J. Inorg. ^ Chem. 2000, 39, 2635. (b) Lopes, I.; Hillier, A. C.; Liu, S. Y.; Domingos, A.; Ascenso, J.; Galv~ao, A.; Sella, A.; Marques, N. Inorg. Chem. 2001, 40, 1116. (c) Mashima, K.; Shibahara, T.; Nakayama, Y.; Nakamura, A. J. Organomet. Chem. 1998, 559, 197. (d) Zhou, X. G.; Zhang, L. X.; Zhang, C. M.; Zhang, J.; Zhu, M.; Cai, R. F.; Huang, Z. E.; Huang, Z. X.; Wu, Q. J. J. Organomet. Chem. 2002, 655, 120. (8) (a) Berardini, M.; Brennan, J. Inorg. Chem. 1995, 34, 6179. (b) Berardini, M.; Lee, J.; Freedman, D.; Lee, J.; Emge, T. J.; Brennan, J. G. Inorg. Chem. 1997, 36, 5772. (c) Niu, D. Z.; Ma, H. J.; Mu, L. L.; Song, B. L.; Chen, J. T. Chinese J. Struct. Chem. 2002, 21, 520. (d) Xiong, R. G.; Zuo, J. L.; You, X. Z.; Huang, X. Y. Polyhedron 1996, 15, 3321. r 2010 American Chemical Society

Article

applied extensively to synthesizing main-group and transitionmetal complexes, in which pyrimidine-2-thionate works as a monodentate ligand by coordination through the S atom, as an N,S chelating ligand, or as a bridging ligand through the S atom and one or two N atoms.9-12 However, only one paper has reported the synthesis of lanthanide pyrimidine-2-thionate complexes.13 Weak coordination of the soft base sulfur at a hard acidic Ln center may cause the resulting lanthanide thiolate complexes to possess high catalytic activity for the intramolecular alkene hydroamination reaction,3d olefin polymerization,3a,b and homo- and copolymerization of 2,2-dimethyltrimethylene carbonate (DTC) and ε-caprolactone.3c,14 In 2002, phenyl isocyanate (PhNCO) was reported to be inserted stoichiometrically into the Ln-S bond of [(C5H4CH3)2Nd( μ-SPh)(THF)]2 to give the corresponding insertion product [(C5H4CH3)2Nd( μ,η2-O(SPh)NPh]2.4 Isocyanates could undergo cyclodimerization, cyclotrimerization, and polymerization reactions, catalyzed by the lanthanocene complexes (MeC5H4)2LnNiPr2(THF) (Ln = Y, Er, Yb)15 and (C5H5)2LnCl/n-BuLi16 or lanthanide amides {(CH2SiMe2)(9) (a) Zhao, Y. J.; Hong, M. C.; Liang, Y. C.; Cao, R.; Li, W. J.; Weng, J. B.; Lu, S. F. Chem. Commun. 2001, 1020. (b) Han, L.; Hong, M. C.; Wang, R. H.; Wu, B. L.; Xu, Y.; Lou, B. Y.; Lin, Z. Z. Chem. Commun. 2004, 2578. (c) Arag on, P. J.; Carrillo-Hermosilla, F.; Villase~nor, E.; Otero, A.; Anti~ nolo, A.; Rodríguez, A. M. Eur. J. Inorg. Chem. 2006, 965. (d) Eichh€ ofer, A.; Buth, G. Eur. J. Inorg. Chem. 2005, 4160. (e) Rodríguez, A.; Sousa-Pedrares, A.; García-Vazquez, J. A.; Romero, J.; Sousa, A.; Russo, U. Eur. J. Inorg. Chem. 2007, 1444. (10) (a) Poelhsitz, G. V.; Batista, A. A.; Ellena, J.; Castellano, E. E.; Lang, E. S. Inorg. Chem. Commun. 2005, 8, 805. (b) Falcomer, V. A. S.; Lemos, S. S.; Batista, A. A.; Ellena, J.; Castellano, E. E. Inorg. Chim. Acta 2006, 359, 1064. (c) Yamanari, K.; Fukuda, I.; Yamamoto, S.; Kushi, Y.; Fuyuhiro, A.; Kubota, N.; Fukuo, T.; Arakawa, R. Dalton Trans. 2000, 2131. (d) Rojas, S.; Fierro, J. L. G.; Fandos, R.; Rodríguez, A.; Terreros, P. Dalton Trans. 2001, 2316. (e) Fandos, R.; Lanfranchi, M.; Otero, A.; Pellinghelli, M. A.; Ruiz, M.; Terreros, P. Organometallics 1996, 15, 4725. (f ) Cini, R.; Tamasi, G.; Defazio, S.; Corsini, M.; Berrettini, F.; Cavaglioni, A. Polyhedron 2006, 25, 834. (11) (a) Tzeng, B. C.; Fu, W. F.; Che, C. M.; Chao, H. Y.; Cheung, K. K.; Peng, S. M. J. Chem. Soc., Dalton Trans. 1999, 1017. (b) Yap, G. P. A.; Jensen, C. M. Inorg. Chem. 1992, 31, 4823. (c) Huang, C. H.; Gou, S. H.; Zhu, H. B.; Huang, W. Inorg. Chem. 2007, 46, 5537. (d) Lee, Y. A.; Eisenberg, R. J. Am. Chem. Soc. 2003, 125, 7778. (e) Lang, E. S.; Oliveira, G. M.; de; Casagrande, G. A.; Vazquez-Lopez, E. M. Inorg. Chem. Commun. 2003, 6, 1297. (f ) Fernandez-Galan, R.; Manzano, B. R.; Otero, A.; Poujaud, N.; Kubicki, M. J. Organomet. Chem. 1999, 579, 321. (g) Hadjikakou, S. K.; Antoniadis, C. D.; Hadjiliadis, N.; Kubicki, M.; Binolis, J.; Karkabounas, S.; Charalabopoulos, K. Inorg. Chim. Acta 2005, 358, 2861. (h) Au, Y.-K.; Cheung, K.-K.; Wong, W.-T. Inorg. Chim. Acta 1995, 228, 267. (12) (a) Ma, C. L.; Zhang, J. H.; Tian, G. R.; Zhang, R. F. J. Organomet. Chem. 2005, 690, 519. (b) Ng, V. W. L.; Leong, W. K.; Koh, L. L.; Tan, G. K.; Goh, L. Y. J. Organomet. Chem. 2004, 689, 3210. (c) Hong, F. E.; Huang, Y. L.; Chen, P. P.; Chang, Y. C. J. Organomet. Chem. 2002, 655, 49. (d) Dilshad, R.; Hanif, K. M.; Hursthouse, M. B.; Kabira, S. E.; Malik, K. M. A.; Rosenberg, E. J. Organomet. Chem. 1999, 585, 100. (e) Shi, J. C.; Chen, L. J.; Huang, X. Y.; Wu, D. X.; Kang, B. S. J. Organomet. Chem. 1997, 535, 17. (f ) Battistuzzi, G.; Corradi, A. B.; Dallari, D.; Saladini, M.; Battistuzzi, R. Polyhedron 1999, 18, 57. (g) Divanidis, S.; Cox, P. J.; Karagiannidis, P.; Aslanidis, P. Polyhedron 2005, 24, 351. (h) Castro, A.; Romero, J.; García-Vazquez, J. A.; Sousa, A.; Zubieta, J.; Chang, Y. Polyhedron 1996, 15, 2741. (13) Ma, W.; Huang, Z. E.; Zhou, X. G.; Cai, R. F. Synth. React. Inorg. Met.-Org. Chem. 1998, 28, 1469. (14) (a) Li, H. X.; Ren, Z. G.; Zhang, Y.; Zhang, W. H.; Lang, J. P.; Shen, Q. J. Am. Chem. Soc. 2005, 127, 1122. (b) Li, H. X.; Cheng, M. L.; Ren, Z. G.; Zhang, W. H.; Lang, J. P.; Shen, Q. Inorg. Chem. 2006, 45, 1885. (c) Li, H. X.; Zhang, Y.; Ren, Z. G.; Cheng, M. L.; Wang, J.; Lang, J. P. Chin. J. Chem. 2005, 23, 1499. (d) Cheng, M. L.; Li, H. X.; Zhang, W. H.; Ren, Z. G.; Zhang, Y.; Lang, J. P. Eur. J. Inorg. Chem. 2007, 1889. (15) Mao, L.; Shen, Q.; Xue, M.; Sun, J. Organometallics 1997, 16, 3711. (16) Zhou, X.; Zhang, L.; Zhu, M.; Cai, R.; Weng, L. Organometallics 2001, 20, 5700.

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Scheme 1. Formation of Li[Ln(dmpymt)4] from Reactions of [(Me3Si)2N]3Ln( μ-Cl)Li(THF)3 (Ln = Nd, Sm, Eu) with dmpymtH

[(2,6- i Pr2 C 6H3 )N]2 }LnN(SiMe 3 )2(THF) (Ln = Yb, Y, Dy, Sm, Nd) and {(Me2Si)[(2,6-R12-4-R2-C6H2)N]2}LnN(SiMe3)2(THF) (R1 = iPr, R2 = H, Ln = Yb, Y, Eu, Sm, Nd; R1 = R2 = H, Ln = Yb, Sm).17 However, a systematic study of the catalytic transformation of the isocyanates to isocyanurates with lanthanide thiolates as catalysts has been far less explored. Recently, we have been interested in studying the synthesis and catalytic properties of lanthanide thiolate complexes.14 In a continuing effort to make new lanthanide thiolate complexes, we chose the (bis(trimethylsilyl)amino)lanthanide(III) chloride complexes ([(Me3Si)2N]3Ln( μ-Cl)Li(THF)3 (Ln = Pr, Nd, Sm, Eu)) as starting materials to react with a pyrimidine-2-thione derivative, 4,6-dimethylpyrimidine-2-thiol (dmpymtH). The four anionic mononuclear Ln(III) complexes Li[Ln(dmpymt)4] (1, Ln = Pr; 2, Ln = Nd; 3, Ln = Sm; 4, Ln = Eu) were isolated; they displayed high catalytic activity and selectivity in the cyclodimerization of isocyanates. Herein we report their synthesis, characterization, and catalytic activity.

Results and Discussion Synthesis. As shown in Scheme 1, reactions of [(Me3Si)2N]3Pr( μ-Cl)Li(THF)3 with dmpymtH in a 1:3 molar ratio afforded a large amount of slightly yellow precipitate. After filtration, the precipitate was further dissolved in MeCN and filtered again. The filtrate was allowed to stand at -18 °C for 2 days, forming colorless crystals of Li[Pr(dmpymt)4] (1) in 67% yield. The analogous reaction of the precursor and dmpymtH in 1:1 or 1:2 molar ratio did not give rise to the corresponding 1:1 or 1:2 product. Compound 1 was the only product isolated in a crystalline form regardless of the precursor/dmpymtH molar ratios. However, the change of the precursor to dmpymtH molar ratio from 1:3 to 1:4 only slightly increased the yield of 1. The reason for it may be due to the formation of other unidentified materials containing Pr/dmpymt/Cl complexes. Similar reactions of [(Me3Si)2N]3Ln( μ-Cl)Li(THF)3 (Ln = Nd, Sm, Eu) with 3 equiv of dmpymtH in THF resulted in the formation of Li[Nd(dmpymt)4] (2) in 73% yield, Li[Sm(dmpymt)4] (3) in 63% yield, and Li[Eu(dmpymt)4] (4) in 71% yield, respectively (Scheme 1). Compounds 1-4 were air and moisture sensitive and were soluble in MeCN, slightly soluble in THF, and insoluble in toluene and n-hexane. Elemental analyses of 1-4 were consistent with the formulas. In the IR spectra of 1-4, the strong absorption at 1200-1300 cm-1 could be assigned to (17) (a) Wu, Y. J.; Wang, S. W.; Zhu, X. C.; Yang, G. S.; Wei, Y.; Zhang, L. J.; Song, H. B. Inorg. Chem. 2008, 47, 5503. (b) Zhu, X. C.; Fan, J. X.; Wu, Y. J.; Wang, S. W.; Zhang, L. J.; Yang, G. S.; Wei, Y.; Yin, C. W.; Zhu, H.; Wu, S. H.; Zhang, H. T. Organometallics 2009, 28, 3882.

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Li et al. Scheme 2. Catalytic Cyclodimerization of Isocyanates

Table 2. Influence of the Conditions on the Cyclodimerization of Phenyl Isocyanate

Figure 1. Perspective view of the anion of 1 with a labeling scheme and 30% thermal ellipsoids. All hydrogen atoms are omitted for charity. Symmetry codes: (A) -x, -y, z; (B) -y, x, -z; (C) y, -x, -z. Table 1. Selected Bond Lengths (A˚) and Angles (deg) for 1, 3, and 4a Ln = Pr (1)

Ln = Sm (3)

Ln = Eu (4)

Ln(1)-N(2) Ln(1)-S(1)

2.654(4) 2.9071(11)

2.605(5) 2.8611(13)

2.598(5) 2.8494(13)

N(2A)-Ln(1)-N(2) N(2)-Ln(1)-N(2B) S(1)-Ln(1)-S(1A) S(1)-Ln(1)-S(1B) N(2A)-Ln(1)-S(1) N(2)-Ln(1)-S(1) N(2B)-Ln(1)-S(1) N(2)-Ln(1)-S(1B)

165.15(15) 90.957(19) 82.49(5) 124.43(3) 138.67(8) 56.18(8) 84.53(8) 84.31(8)

164.64(18) 91.02(3) 81.36(5) 125.11(3) 138.36(10) 57.00(10) 84.09(10) 84.28(10)

164.64(19) 91.02(3) 81.27(5) 125.16(3) 138.32(10) 57.05(10) 84.07(10) 84.29(10)

a Symmetry codes: (A) -x, -y, z, (B) -y, x, -z, and (C) y, -x, -z for 1; (A) -x þ 1, -y, z, (B) -y þ 1/2, x - 1/2, -z, and (C) y þ 1/2, -x þ 1/2, -z þ 3/2 for 3; (A) -x þ 1, -y, z, (B) -y þ 1/2, x - 1/2, -z þ 3/2, and (C) y þ 1/2, -x þ 1/2, -z þ 3/2 for 4.

the ν(C-S) vibration of dmpymt ligands. The 1H NMR spectra of 1-3 in DMSO-d6 showed two singlets at δ 1.653 and 3.511 ppm (1), δ 1.741 and 3.584 ppm (2), δ 2.218 and 2.282 ppm (3), which were assignable to the six protons of the two methyl groups of the dmpymt ligand. The singlet at δ 8.510 (1), 7.834 (2), and 6.483 ppm (3) was ascribed to the one proton of the heterocycle of the dmpymt ligand. The identities of 1, 3, and 4 were finally confirmed by X-ray crystallography. Crystal Structures of Li[Ln(dmpymt)4] (Ln = Pr (1), Sm (3), Eu (4)). In the tetragonal space group I4, the asymmetric units for 1, 3, 4 contain one-fourth of a [Ln(dmpymt)4]anion, one-fourth of a Liþ, and half of a MeCN solvent molecule. As the structures of the [Ln(dmpymt)4]- anions in 1, 3, and 4 are essentially identical, only the perspective view of the anion of 1 is shown in Figure 1. The pertinent bond distances and angles for 1, 3, and 4 are compared in Table 1. In the structures of the anions of 1, 3, and 4, the Pr, Sm, or Eu atom is coordinated by four S and four N atoms from four η2-dmpymt ligands to form a distorted-dodecahedral coordination geometry. The four dmpymt anions chelate to each Ln ion in a bidentate chelating S,N mode. The average Ln-S (2.9071(11) (1), 2.8611(13) (3), 2.8494(13) A˚ (4)) and Ln-N (2.654(4) (1), 2.605(5) (3), 2.598(5) A˚ (4)) bond distances decrease along with the ionic radius in the order Pr > Sm > Eu. In 1, the average Pr-S bond distance of 2.9071(11) A˚ is

entry

cat. (mol %)a

temp (°C)

time (h)

solvent

yield (%)b

1 2 3 4 5 6 7 8 9

1 1 1 1 1 1 1 2 3

20 20 20 20 20 40 60 20 20

12 12 12 12 12 12 12 12 12

n-hexane CH2Cl2 toluene THF MeCN MeCN MeCN MeCN MeCN

NR NR NR 14 66 90 99 85 98

a The catalyst is complex 1, and the catalyst loading is given in mole percent (mol %). b Isolated yield.

close to the corresponding distance found in [Pr(bipy)(S2CNEt2)3]18 (2.905(5) A˚; bipy = 2,20 -bipyridine) but longer than that observed in [(DME)3Pr(SC6F5)2]2[Hg2(SC6F5)6] (2.8619(8) A˚).6b The mean Pr-N bond distance (2.654(4) A˚) is comparable to that observed in [Pr(bipy)(S2CNEt2)3] (2.659(5) A˚).18 In 3, the mean Sm-S bond distance of 2.8611(13) A˚ is close to those of [Sm(SC5H4N)2(HMPA)3]I (2.870(3) A˚; SC5H4N = 2-thiopyridyl; HMPA = hexamethylphosphoric amide)7b and [Sm(Tp Me,Me )2 SC 5 H 4 N] (2.862(4) A˚; TpMe,Me = tris(3,5-dimethylpyrazolyl)borate)7c and between those of [(py)4Sm(SC6F5)3] (2.836(3) A˚, py = pyridine)6e and [(DMSO)2Sm(C5H4NOS)3] (2.894(3) A˚; DMSO = dimethyl sulfoxide; C5H4NOS = 1-hydroxy2(1H )-pyridinethionato).8b The average Sm-N bond distance (2.605(4) A˚) is slightly longer than those in [Sm(TpMe,Me)2SC 5 H4 N] (2.523(9) A˚ ), 7c [Sm(TpMe,Me )2 SC 6 H4 -Me-4] (2.531(6) A˚), and [Sm(SC5H4N)2(HMPA)3]I (2.542(7) A˚).7b In 4, the mean Eu-S bond length (2.8494(13) A˚) is shorter than that of [(PEt4){Eu(SC5H4N)4}] (2.87(1) A˚), while the mean Eu-N bond length (2.598(5) A˚) is consistent with that of [(PEt4){Eu(SC5H4N)4}] (2.52(1) A˚).7d Catalytic Activities of 1-4. To explore the reactivity of 1-4 toward isocyanates, complex 1 was mixed with phenyl isocyanate in a 1:100 molar ratio and stirred at room temperature for 12 h. The reaction mixture was hydrolyzed by water and extracted with diethyl ether. After removal of all the volatile species of the extract in vacuo, the residue was re-extracted with hot toluene/THF and filtered off. The filtrate was allowed to stand at -18 °C for several days, forming colorless crystals of 1,3-diphenylurea (5) in 66% yield. The formation of 5 is assumed to proceed through elimination of CO from the phenyl isocyanate cyclodimerization product (Scheme 2).17b To our knowledge, it represents the first example of employing lanthanide thiolate complexes to catalyze the cyclodimerization of isocyanates. (18) Bower, J. F.; Cotton, S. A.; Fawcett, J.; Hughes, R. S.; Russell, D. R. Polyhedron 2003, 22, 347.

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Table 3. Data for the Cyclodimerization of Different Isocyanatesd

a Catalyst loading: 1 mol %. b Isolated yields by running the reaction in MeCN. c [(Me3Si)2N]3Ln( μ-Cl)Li(THF)3 (Ln = Pr (10 ), Nd (20 ), Sm (30 ), Eu (40 )). d Conditions: isocyanate to catalyst ratio 100:1; solvent MeCN; T = 60 °C; time 12 h.

Complex 1 was employed to optimize the reaction conditions of cyclodimerization of phenyl isocyanate. Five different solvents (CH2Cl2, toluene, THF, n-hexane, and MeCN) were used to evaluate their influence on the catalytic performance of 1. As shown in Table 2, a strong solvent dependence of the cyclodimerization was observed. 1 did not work in n-hexane, toluene, or CH2Cl2, but it showed low activity in THF. Almost complete conversion of the phenyl isocyanate into 1,3-diphenylurea (1 mol % catalyst) was achieved within 12 h at 60 °C in MeCN (Table 2, entry 7). The catalytic activity of 1 in MeCN was enhanced as the reaction temperature was increased. A 66% yield of 1,3-diphenylurea could be obtained in the presence of 1 mol % catalyst in MeCN in 12 h at room temperature (Table 2, entry 5), while the temperature was raised to 40 or 60 °C, and the yields can be raised to 90% and 99%, respectively (Table 2, entries 6

and 7). The catalyst to phenyl isocyanate molar ratio may also affect the catalytic activity. The product yield was gradually increased from 66% to 85% to 98% when the catalyst loading was changed from 1 mol % to 2 mol % and 3 mol % in the presence of MeCN at 20 °C (Table 2, entries 5, 8, and 9). Thus, the following optimized conditions were determined: isocyanate:cat. = 100:1 (molar ratio); solvent MeCN; T = 60 °C; time 12 h. With these optimized conditions, the catalytic performances of the other lanthanide thiolates were also investigated. Complexes 2-4 also exhibited good to excellent catalytic activity for the cyclodimerization of phenyl isocyanate (Table 3, entries 1-4). In all cases, the 1,3-diphenylurea was the only product, indicating the high selectivity of these catalysts. For comparison, the catalytic activity of the precursor complexes [(Me3Si)2N]3Ln( μ-Cl)Li(THF)3 (Ln = Pr

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Scheme 3. Proposed Catalytic Mechanism for the Cyclodimerization of Isocyanates

Figure 2. View of the molecular structure of 10.

(10 ), Nd (20 ), Sm (30 ), Eu (40 )) was studied (Table 3, entries 21-24). However, 10 -40 exhibited good to high catalytic activity for the cyclotrimerization of phenyl isocyanate, which was observed in the case of [(Me3Si)2N]3Yb( μ-Cl)Li(THF)3.17a Therefore, the catalytic activities of 1-4 are completely different from those of lanthanide amides, suggesting ligand binding at the lanthanide center makes an great impact on the selectivity of the catalysts. Complexes 1-4 showed a similar catalytic activity for the cyclodimerization of other alkyl or aryl isocyanates such as cyclohexyl isocyanate, benzyl isocyanate, 4-isopropylphenyl isocyanate, and allyl isocyanate (Table 3, entries 5-20). The corresponding products were identified as 1,3-dicyclohexylurea (6), 1,3-dibenzylurea (7), 1,3-bis(4-isopropylphenyl)urea (8), and 1,3-diallylurea (9). In comparison to that for the cyclodimerization of aryl isocyanates, 1-4 exhibited lower catalytic activity for the cyclodimerization of alkyl isocyanates. It is noted that other byproducts such as trimerization products or polymerization products were observed in the cases of alkyl isocyanates, indicating that 1-4 exhibited not only a high catalytic activity but also a high selectivity for the cyclodimerization of aryl isocyanates. The structures of products 5, 6, and 8 were determined for the clear identification of their identities. The catalytic reaction mechanism for the above reactions is proposed as follows (Scheme 3). First, insertion of an isocyanate into the Ln-S bond of the catalyst may lead to the formation of the six-membered intermediate II. Second, the intermediate II may combine with the another isocyanate to give the eight-membered intermediate III. Third, the intermediate III may eliminate the cyclodimerization product that subsequently loses 1 equiv of carbon monoxide (CO), producing the urea and thus furnishing the catalytic cycle. To confirm the proposed mechanism, we carried out the reaction of 2 with 4 equiv of PhNCO in MeCN. Intriguingly, it did not produce the expected insertion species “Nd[OC(dmpymy)NPh]4” but gave rise to 5. When phenyl isocyanate was added to the mixture containing [(Me3Si)2N]3Sm( μ-Cl)Li(THF)3 and 3 equiv of dmpymtH in THF, a standard workup produced colorless crystals of phenylthiocarbamic acid S-4,6-dimethylpyrimidin-2-yl ester (10) (Figure 2). These results showed that the intermediate II should be formed but is protonated by (Me3Si)2NH released in the reaction.

In summary, the present work demonstrates that the protonolysis reaction of lanthanide amide complexes with dmpymtH readily produces the four lanthanide pyrimidine2-thionate complexes 1-4. Complexes 1-4 exhibit a high catalytic activity and a high selectivity for the cyclodimerization of aryl isocyanates, producing the corresponding substituted ureas, which represents a useful approach to the synthesis of ureas under mild reaction conditions. It is anticipated that other pyrimidine-2-thiols may be applied to afford other lanthanide thiolate complexes to yield previously unknown species with better catalytic activities. Studies on these respects are under way in our laboratory.

Experimental Section All manipulations were carried out under argon using standard Schlenk techniques. Solvents were dried by distillation from sodium/benzophenone (tetrahydrofuran, toluene, hexane) or P2O5 (MeCN or CH2Cl2) under argon prior to use. LiN(SiMe3)219 and [(Me3Si)2N]3Ln( μ-Cl)Li(THF)320 were prepared according to the literature methods. The dmpymtH ligand and phenyl isocyanate, cyclohexyl isocyanate, benzyl isocyanate, 4-isopropylphenyl isocyanate, and allyl isocyanate were purchased from Aldrich. 1H NMR spectra were recorded at ambient temperature on a Varian UNITYplus-300 spectrometer. 1 H NMR chemical shifts were referenced to the solvent signal in DMSO-d6 or CDCl3. Elemental analyses for C, H, and N were performed on a Carlo-Erbo CHNO-S microanalyzer. The IR spectra (KBr disk) were recorded on a Nicolet Magna-IR550 FT-IR spectrometer (4000-400 cm-1). The uncorrected melting points were measured on a Mel-Temp II apparatus. Synthesis of Li[Pr(dmpymt)4] (1). To a solution of dmpymtH (0.346 g, 2.471 mmol) in THF (30 mL) was added a solution of [(Me3Si)2N]3Pr( μ-Cl)Li(THF)3 (0.670 g, 0.818 mmol) in THF (10 mL). The reaction mixture was stirred overnight. The solution turned light yellow with a large amount of light yellow precipitate. After filtration, the resulting precipitate was dissolved in 30 mL of MeCN. Colorless crystals of 1 were isolated by cooling the solution to -18 °C for 2 days. After filtration, the crystals were dried under vacuum. Yield: 0.381 g (67% based on Pr), Anal. Calcd for C24H28N8PrS4: C, 41.32; H, 4.04; N, 16.06; S, 18.38. Found: C, 41.53; H, 4.34; N, 16.24; S, 18.45. IR (KBr disk): 3046 (m), 2960 (m), 2923 (m), 1631 (w), 1573 (s), 1535 (s), 1432 (m), 1384 (s), 1341 (s), 1254 (s), 1029 (w), 987 (w), 955 (w), 886 (m), 758 (w), 554 (w) cm-1. 1H NMR (300 MHz, DMSO-d6, ppm): δ 8.510 (s, 4H, -CH), 3.511 (s, 12H, -CH3), 1.653 (s, 12H, -CH3). 13C NMR (DMSO-d6, ppm): δ 170.6, 163.0, 130.6, 80.5, 67.6, 25.7. (19) Ghotra, J. S.; Hursthouse, M. B.; Welch, A. J. J. Chem. Soc., Chem. Commun. 1973, 669. (20) Zhou, S. L.; Wang, S. W.; Yang, G. S.; Liu, X. Y.; Sheng, E. H.; Zhang, K. H.; Cheng, L.; Huang, Z. X. Polyhedron 2003, 22, 1019.

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Table 4. Crystallographic Data and Refinement Results for 1, 3, 4, and 10 1

3

4

10

empirical formula C24H28N8S4PrLi C24H28N8S4LiSm C24H28EuN8S4Li C13H13N3OS 704.67 714.12 715.73 259.32 formula mass (g mol-1) cryst syst tetragonal tetragonal tetragonal triclinic I4 I4 P1 space group I4 0.60  0.50  0.45 0.38  0.34  0.20 0.30  0.30  0.20 0.20  0.29  0.30 cryst dimens (mm3) a (A˚) 10.8874(15) 10.8871(15) 10.8407(15) 10.070(2) b (A˚) 10.8874(15) 10.8871(15) 10.8407(15) 10.085(2) c (A˚) 13.790(3) 13.748(3) 13.720(3) 14.158(3) R (deg) 103.00(3) β (deg) 93.27(3) γ (deg) 113.14(3) 1634.6(5) 1614.6(4) 1612.4(4) 1271.2(4) V (A˚3) Z 2 2 2 4 -3 1.432 1.469 1.474 1.355 Dcalcd (g cm ) F(000) 708 714 716 544 1.772 2.103 2.230 0.246 μ (mm-1) 2θ range for data collcn (deg) 7.49-50.70 7.52-50.69 6.88-54.98 6.09-54.91 total no. of rflns 7909 7887 7967 10 508 1492 (Rint = 0.0339) 1492 (Rint = 0.0347) 5569 (Rint = 0.0403) no. of unique rflns 1500 (Rint = 0.0283) no. of obsd rflns 1496 (I > 2.00σ(I )) 1490 (I > 2.00σ(I )) 1490 (I > 2.00σ(I )) 3674 (I > 2.00σ(I )) no. of variables 99 97 97 329 transmission factor 0.358-0.456 0.501-0.678 0.554-0.663 0.9299-0.9525 0.0223 0.0231 0.0222 0.0709 Ra 0.0623 0.0673 0.0646 0.1602 Rw b 1.072 1.146 1.141 1.019 GOFc 0.609, -0.511 0.504, -0.322 0.641, -0.278 0.651, -0.345 residual peaks (e A˚-3) P P P P a b 2 P 2 1/2 c 2 1/2 R = ||Fo| - |Fc||/ |Fo|. Rw = { w(|Fo| - |Fc|) / w|Fo| }} . GOF = { w(|Fo| - |Fc|) /(M - N)} , where M is the number of reflections and N is the number of parameters.

Synthesis of Li[Nd(dmpymt)4] (2). A similar reaction of dmpymtH (0.318 g, 2.271 mmol) with [(Me3Si)2N]3Nd( μ-Cl)Li(THF)3 (0.633 g, 0.718 mmol) in THF followed by a workup similar to that used in the isolation of 1 afforded colorless crystals of 2. Yield: 0.371 g (73% based on Nd). Anal. Calcd for C24H28LiN8NdS4: C, 40.72; H, 3.99; N, 15.83; S, 18.12. Found: C, 40.48; H, 3.55; N, 16.11, S, 17.98. IR (KBr disk): 3055 (m), 2963 (m), 2920 (m), 1627 (w), 1576 (s), 1535 (s), 1428 (m) 1385 (m), 1386 (s), 1343 (s), 1254 (s), 1027 (w), 987 (w), 958 (w), 888 (m), 767 (w) cm-1. 1H NMR (300 MHz, DMSO-d6, ppm): δ 7.834 (s, 4H, -CH), 3.584 (s, 12H, -CH3), 1.741 (s, 12H, -CH3). 13 C NMR (DMSO-d6, ppm): δ 179.4, 167.7, 147.0, 134.8, 66.9, 25.1. Synthesis of Li[Sm(dmpymt)4] (3). A similar reaction of dmpymtH (0.292 g, 2.086 mmol) with [(Me3Si)2N]3Sm( μ-Cl)Li(THF)3 (0.613 g, 0.689 mmol) in THF followed by a workup similar to that used in the isolation of 1 afforded colorless crystals of 3. Yield: 0.310 g (63% based on Sm). Anal. Calcd for C24H28LiN8S4Sm: C, 40.37; H, 3.95; N, 15.69; S, 17.96. Found: C, 40.75; H, 3.89; N, 15.52; S, 17.67. IR (KBr disk): 3055 (m), 2963 (m), 2920 (m), 1627 (w), 1576 (s), 1535 (s), 1428 (m), 1386 (s), 1343 (s), 1254 (s), 1027 (w), 987 (w), 958 (w), 888 (m), 767 (w) cm-1. 1H NMR (300 MHz, DMSO-d6, ppm): δ 6.483 (s, 4H, -CH), 2.282-2.218 (d, 24H, -CH3). 13C NMR (DMSOd6, ppm): δ 190.0, 164.9, 112.9, 112.0, 23.8, 23.2. Synthesis of Li[Eu(dmpymt)4] (4). A similar reaction of dmpymtH (0.387 g, 2.764 mmol) with [(Me3Si)2N]3Eu( μ-Cl)Li(THF)3 (0.808 g, 0.908 mmol) in THF followed by a workup similar to that used in the isolation of 1 afforded colorless crystals of 4. Yield: 0.461 g (71% based on Eu). Anal. Calcd for C24H28LiN8EuS4: C, 40.28; H, 3.94; N, 15.66; S, 17.92. Found: C, 40.56; H, 3.67; N, 15.35; S, 17.75. IR (KBr disk): 3058 (m), 2967 (m), 2931 (m), 1634 (w), 1578 (s), 1539 (s), 1428 (m), 1385 (m), 1343 (s), 1256 (s), 1025 (w), 989 (w), 957 (w), 887 (m), 761 (w) cm-1. Satisfactory 1H NMR and 13C NMR spectra of 4 were not obtained. General Procedure for the Synthesis of Substituted Ureas Catalyzed by Li[Ln(dmpymt)4] (5 as an Example). A 30 mL Schlenk tube under dried argon was charged with 1 (0.014 g, 0.020 mmol), MeCN (10 mL), and phenyl isocyanate (0.2387 g,

2.0 mmol). The resulting mixture was stirred at room temperature for 12 h. After the reaction was complete, the reaction mixture was hydrolyzed by water (1 mL), extracted with diethyl ether (3  10 mL), dried over anhydrous MgSO4, and filtered. The solvent was removed under reduced pressure. The final products were further purified by recrystallization from THF and toluene. Compound 521 was isolated colorless crystals. Mp: 234.5-235.3 °C (lit.21 mp 234-235 °C). 1H NMR (CDCl3, ppm): δ 7.363-7.330 (m, 8H, aromatic CH), 7.157-7.1380 (m, 2H, aromatic CH), 6.509 (s, 2H, NH); 13C NMR (CDCl3, ppm) δ 148.7, 134.1, 131.5, 128.2, 127.3. HRMS (EI) m/z: calcd. for C13H12N2O 212.0950; Found: 212.0951. Compound 6:22 white solid. Mp: 231.6-232.4 °C (lit.22 mp 229-230 °C). 1H NMR (CDCl3, ppm): δ 4.076-4.045 (d, 2H, NH), 3.501-3.488 (m, 2H, CH), 2.012-1.922 (m, 4H, CH2), 1.719-1.612 (m, 8H, CH2), 1.410-1.295 (m, 4H, CH2), 1.160-1.036 (m, 4H, CH2). 13C NMR (CDCl3, ppm): δ 156.9, 49.4, 35.7, 25.8, 25.2. HRMS (EI): m/z calcd for C13H24N2O 224.1889, found 224.1889. Compound 7:22 white solid. Mp: 167.9-168.6 °C (lit.22 mp 169-170 °C). 1H NMR (CDCl3, ppm): δ 7.319-7.227 (m, 10H, aromatic CH), 4.870 (s, 2H, NH), 4.338 (s, 4H, CH2). 13C NMR (CDCl3, ppm): δ 158.8, 139.3, 128.9, 127.6, 127.5, 44.8. HRMS (EI): m/z calcd for C15H16N2O 240.1263, found 240.1263. Compound 8:23 white solid. Mp: 237.5-238.6 °C (lit.23 mp 238.4 °C). 1H NMR (CDCl3, ppm): δ 7.266-7.262 (d, 2H, aromatic CH), 7.245-7.239 (d, 2H, aromatic CH), 7.171-7.167 (d, 2H, aromatic CH), 7.145-7.139 (d, 2H, aromatic CH), 6.774 (s, 2H, NH), 2.938-2.822 (m, 2H, CH), 1.233-1.210 (d, 12H, CH3). 13C NMR (CDCl3, ppm): δ 154.1, 145.3, 135.7, 127.4, 121.9, 33.8, 24.2. HRMS (EI): m/z calcd for C19H24N2O 296.1889, found 296.1888. (21) Franz, R. A.; Applegath, F.; Morriss, F. V.; Baiocchi, F.; Bolze, C. J. Org. Chem. 1961, 26, 3311. (22) (a) Franz, R. A.; Morriss, F. V.; Baiocchi, F. J. Org. Chem. 1961, 26, 3306. (b) Emma, A.; Lacopo, D.; Rita, F.; Claudio, M. Synthesis 2007, 3497. (23) Takumi, M.; Masatoshi, M.; Toshiyuki, I.; Takatoshi, I.; Yoshio, I. Synthesis 2006, 2825.

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Compound 9:22 white solid. Mp: 91.4-92.8 °C (lit.22 mp 96.897.5 °C). 1H NMR (CDCl3, ppm): δ 5.926-5.815 (m, 2H), 5.328-5.089 (m, 4H), 4.611 (s, 2H, NH), 3.825-3.799 (m, 4H). 13 C NMR (CDCl3, ppm): δ 153.2, 137.3, 115.1, 44.9, 34.4. HRMS (EI): m/z calcd for C7H12N2O 140.0960, found 140.0965. Synthesis of S-4,6-Dimethylpyrimidin-2-yl Ester 10. To a solution of dmpymtH (0.2208 g, 1.577 mmol) in MeCN (10 mL) was added a solution of [(Me3Si)2N]3Pr( μ-Cl)Li(THF)3 (0.4621 g, 0.525 mmol) in THF (20 mL). The reaction mixture was stirred overnight. The solution turned light yellow with a large amount of light yellow precipitate. At ambient temperature, a solution of PhNCO (0.1916 g, 1.610 mmol) in MeCN (10 mL) was added to the mixture and the suspension gradually. The resulting mixture was stirred for 12 h, and then the solution was concentrated to dryness in vacuo. The resulting solid was extracted with toluene (20 mL) and then filtered. The filtrate was kept at -18 °C for 2 days, and colorless crystals of 10 were formed. Yield: 1.80 g (59%). Anal. Calcd for C13H13N3OS: C, 60.21; H, 5.05; N, 16.20; S, 12.37. Found: C, 60.07; H, 4.97; N, 16.33; S, 12.65. IR (KBr disk): 3454 (s), 1710 (s), 1652 (m), 1594 (w), 1491 (m), 1414 (s), 1220 (w), 1073 (w), 1028 (w), 754 (m), 689 (m), 590 (m), 506 (w) cm-1. 1H NMR (400 MHz, ppm, C6D6): 7.791 (s, 2H, -Ph), 7.312 (s, 2H, -Ph), 7.042 (s, 1H, -Ph), 6.921 (s, 1H, -CH), 6.03 (s, 1H, -NH), 2.364 (s, 6H, -CH3). X-ray Crystallography. All measurements were made on a Rigaku Mercury CCD X-ray diffractometer (3 kV, sealed tube) by using graphite-monochromated Mo KR radiation (λ = 0.710 73 A˚). Single crystals of 1, 3, 4, and 10 were obtained directly from the above preparations. Single crystals of 1, 3, and 4 were mounted on a glass capillary and cooled in a liquid nitrogen stream at 193 K, while that of 10 was placed at the top of a glass fiber at 293 K. Diffraction data were collected in a ω mode with a detector to crystal distance of 35 mm. The collected data were reduced by using the program CrystalClear (Rigaku and MSC, Version 1.3, 2001), and an absorption correction (multiscan) was applied. The reflection data were also corrected for Lorentz and polarization effects. The crystal structures of 1, 3, 4, and 10 were solved by direct methods24 and refined by full-matrix least squares on F2.25 For (24) Sheldrick, G. M. SHELXS-97: Program for the Solution of Crystal Structure; University of G€ottingen, G€ottingen, Germany, 1997. (25) Beurskens, P. T.; Admiral, G.; Beurskens, G.; Bosman, W. P.; Garcia-Granda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla, C. PATTY: The DIRDIF Program System; Technical Report of the Crystallography Laboratory, University of Nijmegen, Nijmegen, The Netherlands, 1992.

Li et al. 1, 3, and 4, the solvent-accessible void occupies a volume of 192.3, 179.0, and 181.5 A˚3, respectively, and may be filled with highly disordered MeCN solvents. Since disorder models did not give satisfactory results, the solvent contribution to the scattering factors have been taken into account with PLATON/SQUEEZE.26 A total of 20 (1), 18 (3), and 10 (4) electrons were found in each unit cell, corresponding to 0.45 MeCN (1), 0.40 MeCN (3), and 0.22 MeCN (4) molecules per cell. While relevant, the crystal data reported in this paper do not include the contribution of the disordered solvent molecules. All nonhydrogen atoms were refined anisotropically. All non-hydrogen atoms were placed in geometrically idealized positions (C-H = 0.98 A˚ for methyl groups, C-H = 0.99 A˚ for methylene groups, or C-H = 0.95 A˚ for phenyl groups) and constrained to ride on their parent atoms with Uiso(H) = 1.2[Ueq(C)], or Uiso(H) = 1.5[Ueq(C)] for methyl groups. All the calculations were performed on a Dell workstation using the CrystalStructure crystallographic software package (Rigaku and MSC, Version 3.60, 2004). A summary of the important crystallographic information for 1, 3, 4, and 10 is given in Table 4.

Acknowledgment. This work was supported by the National Nature Science Foundation of China (Nos. 20871088, 20871038, and 90922018), the Nature Science Key Basic Research of Jiangsu Province for Higher Education (09KJA150002), the NSF of the Education Committee of Jiangsu Province (No. 07KJD150182), the State Key Laboratory of Organometallic Chemistry of Shanghai Institute of Organic Chemistry (08-25), the Qin-Lan Project of Jiangsu Province, and the SooChow Scholar Program and Program for Innovative Research Team of Suzhou University. We highly appreciate the careful comments and suggestions of the reviewers and the editor. Supporting Information Available: CIF files giving crystallographic data for 1, 3, 4, and 10 and figures and tables giving crystallographic data and a view of the molecular structure for 8 and the 1H NMR and 13C NMR spectra of 1-3 and dmpymtH. This material is available free of charge via the Internet at http:// pubs.acs.org. (26) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7.