[2+2] Photodimerization in the Solid State Aided by Molecular

Jun 8, 2010 - E-mail: [email protected]. Cite this:Organometallics 29, 13, 2842-2849. Synopsis. Half-sandwich iridium and rhodium complexes bearing o...
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Organometallics 2010, 29, 2842–2849 DOI: 10.1021/om901125r

[2þ2] Photodimerization in the Solid State Aided by Molecular Templates of Rectangular Macrocycles Bearing Oxamidato Ligands Wan-Zheng Zhang, Ying-Feng Han, Yue-Jian Lin, and Guo-Xin Jin* Shanghai Key Laboratory of Molecular Catalysis and Innovative Material, Department of Chemistry, Advanced Materials Laboratory, Fudan University, Shanghai, 200433, People’s Republic of China Received December 28, 2009

A series of binuclear half-sandwich iridium and rhodium complexes bearing oxamidato ligands [Cp*2M2(μ-N,N0 -bis(aryl)oxamidato)Cl2] (1a, M = Ir, aryl = Ph; 1b, M = Ir, aryl = C6H4-p-Me; 2a, M = Rh, aryl = Ph; 2b, M = Rh, aryl = C6H4-p-Me) functioned as potential ‘‘organometallic clip’’ linear templates were synthesized by the reactions of the lithium salts of oxamide with [Cp*MCl2]2 (M = Ir or Rh), respectively. Treatment of the binuclear complexes (1a-2b) with trans-1,2-bis(4pyridyl)ethylene (4,40 -bpe) in the presence of AgOTf (OTf = CF3SO3) gave the corresponding tetranuclear complexes of general formula [Cp*4M4(μ-N,N0 -bis(aryl)oxamidato)2(μ-4,40 -bpe)2](OTf)4 (3a, M = Ir, aryl = Ph; 3b, M = Ir, aryl = C6H4-p-Me; 4a, M = Rh, aryl = Ph; 4b, M = Rh, aryl = C6H4-p-Me) in high yields. Confirmed by the single-crystal X-ray analysis, the molecular structures of tetranuclear complexes 3a and 4a showed that two binuclear fragments as building blocks were connected by μ-4,40 -bpe to construct a rectangular cavity with the dimensions 5.58  13.59 A˚ (3a) and 5.55  13.65 A˚ (4a). The two μ-4,40 -bpe ligands in 3a and 4a are close to each other within 4.2 A˚ to allow the [2þ2] photoreaction in the solid state, which was proved by 1H NMR of the resulting products (5a-6b0 ) and single-crystal X-ray analysis.

Introduction To rationally design a molecule that adopts suitable geometric criteria that is not easy to achieve in the liquid phase for reaction in the solid state is currently attracting significant attention.1 That ecologically harmful organic solvents are not required in the solid state means not only the development of simple, more environmentally friendly synthetic method2 but also access in high efficiency and selectivity to a product that is less available or unavailable in solution.3 The foundation of the solid-state bimolecular reaction was based on the pioneering work of Schmidt and co-workers.4 Ever since Schmidt postulated conditions for the photochemical dimerization of CdC bonds in the solid state,5 extensive studies have been made on the templatecontrolled solid-state photochemical [2þ2] cycloadditions *Corresponding author. Tel: þ86-21-65643776. Fax: þ86-21-65641740. E-mail: [email protected]. (1) (a) MacGillivray, L. R.; Papaefstathiou, G. S.; Friscic, T.; Varshney, D. B.; Hamilton, T. D. Top. Curr. Chem. 2004, 248, 201–221. (b) Holman, K. T.; Pivovar, A. M.; Swift, J. A.; Ward, M. D. Acc. Chem. Res. 2001, 34, 107–118. (2) Metzger, J. O. Angew. Chem., Int. Ed. 1998, 37, 2975–2978. (3) (a) Georgiev, I. G.; MacGillivray, L. R. Chem. Soc. Rev. 2007, 36, 1239–1248. (b) Xiao, J.; Yang, M.; Lauher, J. W.; Fowler, F. W. Angew. Chem., Int. Ed. 2000, 39, 2132–2135. (4) (a) Cohen, M. D.; Schmidt, G. M. J. J. Chem. Soc. 1964, 1996– 2000. (b) Cohen, M. D.; Schmidt, G. M. J.; Sonntag, F. I. J. Chem. Soc. 1964, 2000–2013. (c) Schmidt, G. M. J. J. Chem. Soc. 1964, 2014–2021. (d) Bregman, J.; Osaki, K.; Schmidt, G. M. J.; Sonntag, F. I. J. Chem. Soc. 1964, 2021–2030. (e) Cohen, M. D.; Hirshberg, Y.; Schmidt, G. M. J. J. Chem. Soc. 1964, 2051–2059. (5) Schmidt, G. M. J. Pure Appl. Chem. 1971, 27, 647–678. pubs.acs.org/Organometallics

Published on Web 06/08/2010

by employing noncovalent bonds such as hydrogen bonds, coordination bonds, π 3 3 3 π stacking, and halogen-halogen forces, and it has still been a challenge to organize the two double bonds in the solid state.6,7 By using the strength and directionality of hydrogen bonds and coordination-driven self-assembly to functionalize molecules as linear templates, MacGillivray and co-workers have assembled olefinic compounds within discrete molecular assemblies in the solid state for the [2þ2] photoreaction.8 We9 and others10 have reported the synthesis of a series of tetranuclear complexes with half-sandwich Ir, Rh, and Ru fragments by the combination of binuclear complexes (6) (a) Ito, Y.; Borecka, B.; Trotter, J.; Scheffer, J. R. Tetrahedron Lett. 1995, 36, 6083–6086. (b) Amirsakis, D. G.; Garcia-Garibay, M. A.; Rowan, S. J.; Stoddart, J. F.; White, A. J. P.; Williams, D. J. Angew. Chem., Int. Ed. 2001, 40, 4256–4261. (c) Feldman, K. S.; Campbell, R. F. J. Org. Chem. 1995, 60, 1924–1925. (d) Coates, G. W.; Dunn, A. R.; Henling, L. M.; Ziller, J. W.; Lobkovsky, E. B.; Grubbs, R. H. J. Am. Chem. Soc. 1998, 120, 3641–3649. (e) Maekawa, Y.; Kato, S.; Hasegawa, M. J. Am. Chem. Soc. 1991, 113, 3867–3872. (f) Sharma, C. V. K.; Panneerselvam, K.; Shimoni, L.; Katz, H.; Carrell, H. L.; Desiraju, G. R. Chem. Mater. 1994, 6, 1282–1292. (g) Nie, W. L.; Erker, G.; Kehr, G.; Froehlich, R. Angew. Chem., Int. Ed. 2004, 43, 310–313. (7) (a) Nagarathinam, M.; Vittal, J. J. Angew. Chem., Int. Ed. 2005, 45, 4337–4341. (b) Toh, N. L.; Nagarathinam, M.; Vittal, J. J. Angew. Chem., Int. Ed. 2005, 44, 2237–2241. (c) Mir, M. H.; Koh, L. L.; Tan, G. K.; Vittal, J. J. Angew. Chem., Int. Ed. 2009, 49, 390–393. (d) Caronna, T.; Liantonio, R.; Logothetis, T. A.; Metrangolo, P.; Pilati, T.; Resnati, G. J. Am. Chem. Soc. 2004, 126, 4500–4501. (e) Iwama, N.; Kato, T.; Sugano, T. Organometallics 2004, 23, 5813–5817. (f) Lee, J. Y.; Hong, S. J.; Kim, C.; Kim, Y. Dalton. Trans. 2005, 3716–3718. (g) Lewis, F. D.; Quillen, S. L.; Hale, P. D.; Oxman, J. D. J. Am. Chem. Soc. 1988, 110, 1261–1267. (h) Vela, M. J.; Snider, B. B.; Foxman, B. M. Chem. Mater. 1998, 10, 3167–3171. (i) Ramamurthy, V.; Venkatesan, K. Chem. Rev. 1987, 87, 433–481. r 2010 American Chemical Society

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Scheme 1. Photodimerization of [Cp*4M4(μ-4,40 -bpe)2(μ-C2O4)2](OTf)4

bridged by oxalato or chloranilate and pyridyl-based subunits. In these complexes, the distances between two pyridylbased ligands are close to each other because of the presence of π 3 3 3 π interactions and fall within the ranges for [2þ2] photoreaction.5 By taking into account the dimensions of the tetranuclear rectangle macrocycles and using the μ-oxalato binuclear iridium and rhodium complexes as ‘‘organometallic clip’’ linear templates, [2þ2] photodimerization has been investigated (Scheme 1), which was based on ‘‘organometallic macrocycles’’ through single-crystal to single-crystal transformation.9f UV irradiation of [Cp*2Ir4(μ-η4-C2O4)2(μ-4,40 bpe)2](OTf)4 and [Cp*2Rh4(μ-η4-C2O4)2(μ-4,40 -bpe)2](OTf)4 produced the expected rctt-tetrakis(4-pyridyl)cyclobutane (rctt-tpcb) in high yields after a period of 25 or 50 h (Scheme 1). Therrien’s group reported the formation of [Ru4(η6-pcymene)4(μ-oxalato)2(μ-rctt-tpcb)](OTf)4 by [2þ2] cycloaddition of the olefinic double bonds in the supramolecular rectangle [Ru4(η6-p-cymene)4(μ-oxalato)2(μ-4,40 -bpe)2](OTf)4.10d Compared with μ-oxalato binuclear iridium and rhodium complexes, we anticipate that μ-oxamidato binuclear iridium and rhodium complexes could also play a role as potential ‘‘organometallic clip’’ linear templates to assemble 4,40 -bpe in tetranuclear rectangle macrocycles with ordered geometry, which would be suitable for exploring [2þ2] photodimerization in the solid state. Herein we report the stepwise formation of bi- and tetranuclear half-sandwich iridium and rhodium complexes bearing oxamidato bridges, based on (8) (a) Gao, X.; Friscic, T.; MacGillivray, L. R. Angew. Chem., Int. Ed. 2002, 41, 2070–2073. (b) Friscic, T.; MacGillivray, L. R. Chem. Commun. 2003, 1306–1307. (c) Dushyant, B.; Papaefstathiou, G. S.; MacGillivray, L. R. Chem. Commun. 2002, 1964–1965. (d) Papaefstathiou, G. S.; Zhong, Z. M.; Geng, L.; MacGillivray, L. R. J. Am. Chem. Soc. 2004, 126, 9158–9159. (e) MacGillivray, L. R.; Reid, J. L.; Ripmeester, J. A. J. Am. Chem. Soc. 2000, 122, 7817–7818. (f) Hamilton, T. D.; Papaefstathiou, G. S.; MacGillivray, L. R. J. Am. Chem. Soc. 2002, 124, 11606–11607. (9) (a) Wang, J.-Q.; Ren, C.-X.; Jin, G.-X. Organometallics 2006, 25, 74–81. (b) Wang, J.-Q.; Zhang, Z.; Weng, L.-H.; Jin, G.-X. Chin. Sci. Bull. 2004, 49, 735–738. (c) Han, Y.-F.; Lin, Y.-J.; Jia, W.-G.; Weng, L.-H.; Jin, G.-X. Organometallics 2007, 26, 5848–5853. (d) Han, Y.-F.; Lin, Y.-J.; Jia, W.-G.; Jin, G.-X. Organometallics 2008, 27, 4088–4097. (e) Han, Y.-F.; Jia, W.-G.; Lin, Y.-J.; Jin, G.-X. Organometallics 2008, 27, 5002–5008. (f) Han, Y.-F.; Lin, Y.-J.; Jia, W.-G.; Wang, G.-L.; Jin, G.-X. Chem. Commun. 2008, 1807–1809. (g) Han, Y.-F.; Lin, Y.-J.; Jia, W.-G.; Jin, G.-X. Angew. Chem., Int. Ed. 2009, 48, 6234–6238. (h) Jia, W.-G.; Han, Y.-F.; Lin, Y.-J.; Weng, L.-H.; Jin, G.-X. Organometallics 2009, 28, 3459–3464. (i) Zhang, W.-Z.; Han, Y.-F.; Lin, Y.-J.; Jin, G.-X. Dalton. Trans. 2009, 8426–8431. (10) (a) Yamamoto, Y.; Suzuki, H.; Tajima, N.; Tatsumi, K. Chem.Eur. J. 2002, 8, 372–379. (b) Suzuki, H.; Tajima, N.; Tatsumi, K.; Yamamoto, Y. Chem. Commun. 2000, 1801–1802. (c) Yan, H.; S€uss-Fink, G.; Neels, A.; Evans, H. S. J. Chem. Soc., Dalton Trans. 1997, 4345–4350. (d) Barry, N. P. E.; Therrien, B. Inorg. Chem. Commun. 2009, 12, 465–468.

Scheme 2. Synthesis of Binuclear and Tetranuclear Complexes

4,40 -bpe subunits. A new series of bi- and tetranuclear iridium and rhodium complexes were synthesized and characterized, and the solid-state photochemical [2þ2] cycloaddition activity of [Cp*4M4(μ-N,N0 -bis(aryl)oxamidato)2(μ-4,40 -bpe)2](OTf)4 (M=Ir, 3a, aryl =Ph; 3b, aryl =C6H4p-Me; M = Rh; 4a, aryl = Ph; 4b, aryl = C6H4-p-Me) was explored. The structures of binuclear complex [Cp*2Rh2(μN,N0 -bis(4-methylphenyl)oxamidato)Cl2] (2b) and tetranuclear complexes 3a, 4a, and irradiated product [Cp*4Ir4(μ-N, N0 -diphenyloxamidato)2(μ-rctt-tetrakis(4-pyridyl)cyclobutane)](OTf)4 were confirmed by X-ray analysis.

Result and Discussion As shown in Scheme 2, when oxamide compounds were treated successively with 2 equiv of n-BuLi in THF at -78 °C and 1 equiv of [Cp*IrCl2]2 or [Cp*RhCl2]2 at 50 °C, the binuclear complexes [Cp*2M2(μ-N,N0 -bis(aryl)oxamidato)Cl2] (1a, M = Ir, aryl = Ph; 1b, M = Ir, aryl = C6H4-p-Me; 2a, M =Rh, aryl = Ph; 2b, M = Rh, aryl =C6H4-p-Me) were formed in about 70% yields. The 1H NMR spectra showed a sharp singlet at δ = 1.34 for 1a and 1.33 for 1b due to the Cp* protons, and the signals of N-H were absent. For 2a and 2b,

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Figure 1. Complex cation of 2b with thermal ellipsoids drawn at the 30% level. All hydrogen atoms are omitted for clarity. Selected distances (A˚) and angles (deg): Rh(1)-N(1) 2.097(3), Rh(1)-O(1) 2.127(2), Rh(1)-Cl(1) 2.4012(13), N(1)-C(1A) 1.301(4), N(1)-C(2) 1.429(4), O(1)-C(1) 1.275(4), C(1)-C(1A) 1.500(6), N(1)-Rh(1)-O(1) 77.63(9), N(1)-Rh(1)-Cl(1) 87.48(9), O(1)-Rh(1)-Cl(1) 87.37(8), C(1A)-N(1)-Rh(1) 115.6(2), C(1)O(1)-Rh(1) 113.41(19), O(1)-C(1)-N(1A) 127.0(3), O(1)-C(1)C(1A) 118.4(3), N(1A)-C(1)-C(1A) 114.6(3).

the 1H NMR showed similar spectra compared with 1a and 1b. These spectroscopic data indicate a dimeric structure in which the Ir or Rh centers are connected by an oxamidato ligand. The structure of 2b was confirmed by X-ray crystal analysis at 293 K. The red crystal was obtained by slowly evaporating a CHCl3 solution of 2b. A perspective drawing of 2b with the atomic numbering scheme is given in Figure 1. The crystal structure of 2b consists of binuclear units, connected by an oxamidato ligand, and each rhodium atom is surrounded by one oxygen atom and one nitrogen atom from the oxamidato ligand and one chlorine atom. All of the rhodium centers have six-coordinate geometry, assuming that the Cp* ligand occupies three coordination sites. The two rhodium atoms are separated by 5.5917(17) A˚, similar to the Rh 3 3 3 Rh distance (5.548 A˚) of [Cp*2Rh2(μ-η4-C2O4)Cl2].8c The Rh-N and Rh-O bond distances are 2.097(3) and 2.127(2) A˚, respectively. The bond distance of Rh-Cl is 2.4012(13) A˚, and the two chlorine atoms are oriented in a trans manner. Tetranuclear complexes bearing 4,40 -bpe ligands, which are formulated as [Cp*4Ir4(μ-N,N0 -bis(aryl)oxamidato)2(μ4,40 -bpe)2](OTf)4 (3a, aryl = Ph; 3b, aryl = C6H4-p-Me), were prepared by direct reaction of 1a or 1b with 4,40 -bpe in the presence of AgOTf (Scheme 2) in high yields. The 1H NMR spectrum of 3a showed resonances at approximately δ=7.89 (d) and 8.14 (d) ppm, which are assigned to dipyridyl protons. Similarly, in the 1H NMR spectrum, the CHdCH protons appear at 7.63 ppm for 3b as a singlet, while the resonances for the pyridyl unit appear at approximately δ = 7.86 (d) and 8.12 (d). A sharp singlet at δ = 1.25 is observed due to the protons of Cp* of 3a and 3b. 4a and 4b were obtained from 2a or 2b, AgOTf, and 4,40 bpe by a similar procedure to that described for 3a and 3b in about 70% yields. The 1H NMR spectrum of 4a showed singlets at δ = 1.29, 7.55, 7.80, and 8.04 ppm due to Cp*, olefinic, and the pyridyl protons, respectively. A similar 1H NMR spectrum of 4b was also observed.

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The structures of 3a and 4a have also been confirmed by X-ray analyses at 293 K. The yellow single crystal 3a was obtained by slow diffusion of Et2O into CH2Cl2 solutions of 3a, while red single crystal 4a was obtained by slow diffusion of hexane into CH2Cl2 solutions of 4a. Perspective drawings of 3a and 4a with the atomic numbering scheme are given in Figures 2a and 3, respectively. The crystal units of 3a and 4a are composed of [Cp*2M4(μ-N,N0 -diphenyloxamidato)2(μ-4,40 -bpe)2]4þ (3a, M = Ir; 4a, M = Rh) cations and OTfcounteranions in the solid. Assuming that the Cp* ligand occupies three coordination sites, each Ir or Rh center adopts a three-legged piano-stool conformation, which has six-coordinate geometry and is coordinated by one nitrogen atom and one oxygen atom from the oxamidato ligand and one nitrogen atom from 4,40 -bpe. The complex cations have a rectangular cavity bridged by oxamidato ligands and 4,40 bpe molecules with the dimensions 5.58  13.59 A˚ for 3a and 5.55  13.65 A˚ for 4a, respectively. The complex cations of [Cp*2Ir4(μ-η4-C2O4)2(μ-4,40 -bpe)2]4þ are known to consist of a cavity with the dimension 5.56 13.25 A˚,9f which is similar to that of 3a and 4a. As anticipated, the structure of 3a showed that the two 4,40 -bpe ligands are close to each other. Unlike the double bonds of the two 4,40 -bpe ligands in [Cp*2Ir4(μ-η4-C2O4)2(μ4,40 -bpe)2](OTf)4, which lie parallel to each other with a distance of 3.23 A˚,9f the double bonds are criss-crossed with a torsion angle of 75.3° and the center-to-center distance 3.79 A˚, as shown in Figure 2b. The C25 3 3 3 C26 and C25(A) 3 3 3 C26(A) distances are 3.87 A˚. The contacts between CdC bonds of adjacent assemblies are 10.33 and 8.65 A˚, as shown in Figure 2c,d. Consequently, [2þ2] photodimerization is unlikely to occur between adjacent assemblies according to the distance criterion of Schmidt for [2þ2] photoreactions.5 As a result, the double bonds of the assembly are the sole olefins of the solids that conform to the criterion of photodimerization in a solid. As shown in Scheme 3, a powdered crystalline sample of 3a was subjected to UV irradiation using an Hg lamp for a period of approximately 30 h to investigate its photoreaction activity. The cycloaddition was accompanied by a color change from yellow to pale yellow. The dimerization of 4,40 bpe gave two stereoisomers of cyclobutane rings 5a and 5a0 in 100% photochemical conversion, of which 64.5% is due to rctt-tpcb (5a) (where rctt-tpcb = rctt-tetrakis(4-pyridyl)cyclobutane) and 35.5% is due to the rtct-tpcb isomer (5a0 ) (where rtct-tpcb = rtct-tetrakis(4-pyridyl)cyclobutane) (Table 1), as evident by 1H NMR spectroscopy in CDCl3. The 1H NMR spectrum showed complete disappearance of the singlet for the pyridyl protons at δ = 7.89 and 8.14 ppm, the appearance of signals at δ=5.70 and 4.98 ppm due to rctt-tpcb protons, and signals at δ = 5.62 and 5.24 ppm due to rtct-tpcb protons, as shown in Figure 4. X-ray crystallographic analysis of [Cp*4Ir4(μ-N,N0 -diphenyloxamidato)2(μ-rctt-tpcb](OTf)4 (5a) at 293 K confirmed the formation of new bonds, as shown in Figure 5. The pyridyl rings are bent toward each other. Similarly to 3a, each Ir atom is coordinated by one nitrogen atom from rctt-tpcb and one oxygen atom and one nitrogen atom of bridging oxamidato ligands with Ir 3 3 3 Ir separations of 5.6 and 13.45 A˚, as defined by the iridium centers. For the complex [Cp*2Ir4(μ-η4-C2O4)2(μ-4,40 -tpcb)2](OTf)4,9f each Ir atom is coordinated by one nitrogen atom from rctt-tpcb and two oxygen atoms of bridging oxalato ligands with Ir 3 3 3 Ir separations of 5.56 and 13.12 A˚, as defined by the

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Figure 2. (a) Complex cation of 3a with thermal ellipsoids drawn at the 30% level. All hydrogen atoms, anions, and solvent molecules are omitted for clarity. (b) The double bonds are criss-crossed with a torsion angle of 75.3°. (c) Stacking of the molecules in crystals of 3a viewed along the c-axis. (d) Stacking of the molecules in crystals of 3a viewed along the a-axis. d1 (3.79 A˚) represents the center-to-center distance between double bonds of the two 4,40 -bpe ligands in the organometallic macrocycles, while d2 (=10.33 A˚) and d3 (=8.65 A˚) represent the distance of CdC bonds between adjacent assemblies. Ir, N, O, and C atoms are represented by green, blue, red, and gray, respectively. Selected distances (A˚) and angles (deg): Ir(1)-O(1) 2.116(5), Ir(1)-N(1) 2.119(6), Ir(1)-N(3) 2.127(7), Ir(2)-N(2) 2.100(6), Ir(2)-O(2) 2.122(5), Ir(2)-N(4) 2.144(9), O(1)-Ir(1)-N(1) 77.4(2), O(1)-Ir(1)-N(3) 83.4(2), N(1)-Ir(1)-N(3) 87.0(2), C(1)-N(1)-Ir(1) 115.5(5), C(3)-N(1)-Ir(1) 127.2(5), C(2)-O(1)-Ir(1) 115.5(5), N(1)-C(1)-C(2) 115.8(7), O(1)-C(2)-C(1) 115.3(7).

iridium centers, which are similar to that of the corresponding distances for [Cp*4Ir4(μ-N,N0 -diphenyloxamidato)2(μ-rctt-tpcb](OTf)4. As a result, the carbon-carbon bond lengths of rctt-tpcb are 1.53, 1.54, 1.56, and 1.61 A˚, while the carbon-carbon bond lengths of rctt-tpcb of

[Cp*2 Ir4 (μ-η4 -C 2O 4)2 (μ-4,40 -tpcb)2](OTf)4 are 1.45 and 1.87 A˚. UV irradiation of a powdered crystalline sample of 3b similarly also gave the products as a mixture of rctt-tpcb (5b) and rtct-tpcb (5b0 ) in 100% photochemical conversion

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CDCl3 (Figure 6). The cycloaddition was also accompanied by a color change from yellow to pale yellow. The formation of products was characterized by a complete disappearance of the olefinic singlet at δ = 7.63 ppm; in addition, the appearance of singlets at δ = 5.66 and 4.99 ppm due to rctttpcb protons and δ = 5.55 and 5.20 ppm due to rtct-tpcb protons also evidenced the transformation. Building on the orientation of the criss-crossed double bonds of 3a, either the cycloaddition of C25, C26 and C25(A), C26(A) or C25, C26(A) and C25(A), C26 will lead to rtct-tpcb. The formation of rctt-tpcb requires both the reactive olefins to be aligned in a parallel fashion. Therefore, a conversion of the criss-crossed olefins to a parallel alignment must have occurred prior to dimer formation. This could be interpreted by the double bonds of the 4,40 -bpe ligands undergoing pedal-like motion prior to photodimerization. Such dynamic behavior of double bonds (CdC, NdN, CdN) has been studied by Ogawa and co-workers.11 Some recent examples of solid-state [2þ2] photoreactions that gave regiospecific cyclobutane isomers can also be rationalized on the assumption of pedal motion.12 UV irradiation of 3b, which gave the products as a mixture, could also be explained by the assumption of pedal motion. The incomplete pedal-like motion of 4,40 -bpe observed here is reflected in the residual formation of rtct-tpcb during photodimerization.

(rctt-tpcb, 75.2%; rtct-tpcb, 24.8%) after a period of 30 h (Table 1), as evident by 1H NMR spectroscopy in

Figure 3. Complex cation of 4a with thermal ellipsoids drawn at the 30% level. All hydrogen atoms, anions, and solvent molecules are omitted for clarity. Selected distances (A˚) and angles (deg): Rh(1)-N(1) 2.112(4), Rh(1)-O(1) 2.124(3), Rh(1)-N(3) 2.140(4), Rh(2)-N(2) 2.112(4), Rh(2)-O(2) 2.111(3), Rh(2)N(4) 2.147(4), N(1)-Rh(1)-O(1) 78.06(14), N(1)-Rh(1)-N(3) 87.98(16), O(1)-Rh(1)-N(3) 85.82(16), C(1)-N(1)-Rh(1) 114.3(3), C(3)-N(1)-Rh(1) 127.2(3), C(2)-O(1)-Rh(1) 113.3(3), O(1)-C(2)-C(1) 118.6(4), N(1)-C(1)-C(2) 115.1(4).

Scheme 3. Photodimerization of 3a-4b

Table 1. Formation of rctt-tpcb and rtct-tpcb on Photolysis of Complexes 3a-4b starting material

metal

cavity (A˚)

da (A˚)

time (h)

conversion (%)

yield of rctt-tpcb (%)

yield of rtct-tpcb (%)

3a 3b 4a 4b

Ir Ir Rh Rh

5.58  13.59

3.79

5.55  13.65

4.05

30 30 260 260

100 100 41.7 35.9

64.5 75.2 29.2 25.6

35.5 24.8 12.5 10.3

a

d represents the center-to-center distance of the reactive carbon-carbon double bond.

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Figure 4. 1H NMR spectra of 3a (400 MHz, CDCl3) before UV irradiation (a) and after 30 h of UV irradiation (b). The red circle represents the signal of rctt-tpcb (5a), while the blue circle represents the signal of rtct-tpcb (5a0 ), and the green circle represents the signal of CH2Cl2.

Figure 5. Complex cation of 5a with thermal ellipsoids drawn at the 30% level. All hydrogen atoms, anions, and solvent molecules are omitted for clarity. Selected distances (A˚) and angles (deg): Ir(1)-N(5) 2.117(9), Ir(1)-N(1) 2.122(9), Ir(1)-O(1) 2.130(8), Ir(2)-O(2), 2.090(9), Ir(2)-N(2) 2.120(9), Ir(2)-N(7) 2.146(12), C(29)-C(35) 1.522(15), C(29)-C(30) 1.527(16), C(29)-C(32) 1.610(18), C(30)-C(40) 1.499(17), C(30)-C(31) 1.543(18), C(31)-C(32) 1.563(16), N(5)-Ir(1)-N(1) 87.2(4), N(5)-Ir(1)-O(1) 84.3(4), N(1)-Ir(1)-O(1) 77.1(4), O(2)-Ir(2)-N(2) 78.3(4), O(2)-Ir(2)-N(7) 83.0(4), N(2)-Ir(2)-N(7) 86.3(4), C(2)-N(1)-Ir(1) 115.7(9), C(1)-N(2)-Ir(2) 113.3(8), C(1)-O(1)-Ir(1) 114.7(8), C(2)-O(2)-Ir(2) 113.8(8), N(2)C(1)-O(1) 126.8(11), N(2)-C(1)-C(2) 115.9(12), O(1)-C(1)C(2) 117.3(12), O(2)-C(2)-N(1) 128.2(12), O(2)-C(2)-C(1) 117.7(12), N(1)-C(2)-C(1) 114.1(13).

The double bonds of the two 4,40 -bpe ligands in 4a are also criss-crossed with a torsion angle of 84.8° and the centerto-center distance 4.05 A˚. It is of interest that when the data (11) (a) Harada, J.; Ogawa, K. J. Am. Chem. Soc. 2001, 123, 10884– 10888. (b) Harada, J.; Ogawa, K.; Tomoda, S. J. Am. Chem. Soc. 1995, 117, 4476–4478. (c) Ogawa, K.; Sano, T.; Yoshimura, S.; Takeuchi, Y.; Toriumi, K. J. Am. Chem. Soc. 1992, 114, 1041–1051. (d) Harada, J.; Ogawa, K.; Tamoda, S. Acta Crystallogr. Sect. B 1997, 53, 662–672. (e) Harada, J.; Harakawa, M.; Ogawa, K. Acta Crystallogr. Sect. B 2004, 60, 589–597. (12) (a) Furukawa, D.; Kobatake, S.; Matsumoto, A. Chem. Commun. 2008, 55–57. (b) Ohba, S.; Hosomi, H.; Ito, Y. J. Am. Chem. Soc. 2001, 123, 6349–6352. (c) Ito, Y.; Hosomi, H.; Ohba, S. Tetrahedron 2000, 56, 6833–6844. (d) Natarajan, A.; Mague, J. T.; Venkatesan, K.; Ramamurthy, V. Org. Lett. 2005, 7, 1895–1898. (e) Peedikakkal, A. M. P.; Vittal, J. J. Chem.-Eur. J. 2008, 14, 5329–5334.

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Figure 6. 1H NMR spectra of 3b (400 MHz, CDCl3) before UV irradiation (a) and after 30 h of UV irradiation (b). The red circle represents the signal of rctt-tpcb (5b), while the blue circle represents the signal of rtct-tpcb (5b0 ), and the green circle represents the signal of CH2Cl2.

were re-collected at 100 K, we found that the double bonds of the two 4,40 -bpe ligands in 4a adopt two conformations in the structure: parallel and antiparallel. This type of phenomenon that the occupancy factor of the misoriented conformer decreases as the temperature decreases could well compare with the conclusions drawn by Ogawa and co-workers.11 The distances of C20 3 3 3 C26 and C20(A) 3 3 3 26(A) are 4.14 A˚, which is longer than that of 3a. UV irradiation of powdered crystalline samples of 4a for a period of approximately 260 h also gave the products as a mixture of rctt-tpcb (6a) and rtcttpcb (6a0 ) in 29.2% and 12.5% yields, respectively, which was evidenced by the 1H NMR (Table 1). Similarly, the mixture of products rctt-tpcb (6b) and rtct-tpcb (6b0 ) in 25.6% and 10.3% yields was also obtained by UV irradiation of powdered crystalline samples of 4b for a period of approximately 260 h.

Conclusion We have presented the synthesis and characterization of a series of bi- and tetranuclear half-sandwich iridium and rhodium complexes. All the binuclear complexes have been shown to function as linear templates and to organize the double bonds in criss-cross conformation in the solid state. Prior to photodimerization, the conformation of these CdC bonds of 4,40 -bpe ligands was changed from antiparallel to parallel through pedal-like motion. Hence the 4,40 -bpe ligands gave quantitative conversion to the cyclobutane derivative with exclusively rctt stereochemistry on UV irradiation of the powdered crystalline sample. The incomplete pedal-like motion of 4,40 -bpe resulted in the residual formation of rtct-tpcb during photodimerization. This is again an example that proved that the olefinic bonds do not need to be parallel (within 4.2 A˚) to one another in the solid state for photoreaction. It is noticed that different half-sandwich complex templates result in different geometries of the organized CHdCH in the solid state compared with our previous templates of μ-oxalato binuclear iridium and rhodium complexes. As a result, new products were obtained. The interesting reactions provided in this report also cause us to explore this kind of reaction by modifying the metal

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templates and olefinic compounds. Work is still in progress following this pursuit.

Experiment Section All manipulations were performed using standard Schlenk techniques under an atmosphere of nitrogen. CH2Cl2 was dried over CaH2, and CH3OH was distilled over Mg/I2. THF, diethyl ether, and hexane were dried over Na and then distilled under nitrogen immediately prior to use. The 1H NMR spectra were measured on a VAVCE-DMX 400 spectrometer in CDCl3 or CD3OD. Elemental analysis was performed on an Elementar Vario EL III analyzer. IR (KBr) spectra were recorded on a Nicolet FT-IR spectrophotometer. The oxamide compounds13 and [Cp*IrCl2]214 and [Cp*RhCl2]214 were prepared according to the reported procedures, while other chemical reagents were purchased from commercial sources and used without further purification. Preparation of Binuclear Complexes. [Cp*2Ir2(μ-N,N0 -diphenyloxamidato)Cl2] (1a). Two equivalents of n-BuLi (0.30 mL, 0.48 mmol) was added to a solution of N,N0 -diphenyloxamide (48 mg, 0.2 mmol) in 15 mL of THF at -78 °C. The reaction mixture was allowed to reach room temperature gradually with stirring in about 2 h. Then the mixture was added to a suspension of [Cp*IrCl2]2 (159.2 mg, 0.2 mmol) in 5 mL of THF and stirred overnight at 50 °C. The solvent was evaporated in vacuo, and the residue was extracted with CH2Cl2. The extract was evaporated to dryness and washed with diethyl ether to give the products (yellow, 148 mg, 76.8%). IR (KBr): ν/cm-1 3441.7, 2984.2, 2916, 1604.3, 1579.8, 1488.1, 1448.6, 1382.5, 1352, 1031.1, 944.6, 755.1, 697.8. 1H NMR (400 MHz, CDCl3): δ (ppm) 7.60 (d, 4H; CH, oxamidato), 7.11-7.31 (m, 6H; CH, oxamidato), 1.34 (s, 30H; Cp*). Anal. Calcd for C34H40N2Cl2O2Ir2: C, 42.36; H, 4.18; N, 2.91. Found: C, 42.03; H, 4.20; N, 2.64. [Cp*2Ir2(μ-N,N0 -bis(4-methylphenyl)oxamidato)Cl2] (1b). This complex (yellow, 150 mg, 75.6%) was prepared in a way analogous to that for complex 1a using N,N0 -bis(4-methylphenyl)oxamide (53.6 mg, 0.2 mmol), n-BuLi (0.30 mL, 0.48 mmol), and [Cp*IrCl2]2 (159.2 mg, 0.2 mmol). IR (KBr): ν/cm-1 3458.2, 3024.4, 2989, 2914.6, 1586.3, 1507.4, 1449.7, 1382.7, 1352.4, 1190.6, 1032.4, 803.1, 729.7, 536.2. 1H NMR(400 MHz, CDCl3): δ (ppm) 7.48 (d, 4H; CH, oxamidato), 7.07 (d, 4H; CH, oxamidato), 2.34 (s, 6H; CH3, oxamidato), 1.33 (s, 30H; Cp*). Anal. Calcd for C36H44N2Cl2O2Ir2: C, 43.58; H, 4.47; N, 2.82. Found: C, 43.17; H, 4.58; N, 2.54. [Cp*2Rh2(μ-N,N0 -diphenyloxamidato)Cl2] (2a). Two equivalents of n-BuLi (0.30 mL, 0.48 mmol) was added to a solution of N,N0 -diphenyloxamide (48 mg, 0.2 mmol) in 15 mL of THF at -78 °C. The reaction mixture was allowed to reach room temperature gradually with stirring in about 2 h. Then the mixture was added to a suspension of [Cp*RhCl2]2 (123.6 mg, 0.2 mmol) in 5 mL of THF and stirred overnight at 50 °C. The solvent was evaporated in vacuo, and the residue was extracted with CH2Cl2. The extract was evaporated to dryness and washed with diethyl ether to give the products (orange, 115.6 mg, 73.6%). IR (KBr): ν/cm-1 3445.8, 2914.3, 1599.4, 1576.4, 1487.2, 1350, 1076.3, 1026.4, 756.1, 698.9. 1H NMR (400 MHz, CDCl3): δ (ppm) 7.69 (d, 4H; CH, oxamidato), 7.10-7.32 (m, 6H; CH, oxamidato), 1.34 (s, 30H; Cp*). Anal. Calcd for C34H40N2Cl2O2Rh2: C, 51.99; H, 5.13; N, 3.57. Found: C, 51.69; H, 5.49; N, 3.57. [Cp*2Rh2(μ-N,N0 -bis(4-methylphenyl)oxamidato)Cl2] (2b). This complex (orange, 129.2 mg, 79.3%) was prepared in a way analogous to that for complex 2a using N,N0 -bis(4-methylphenyl)oxamide (53.6 mg, 0.2 mmol), n-BuLi (0.30 mL, 0.48 mmol), (13) Stylianides, N.; Danopoulos, A. A.; Pugh, D.; Hancock, F.; Zanotti-Gerosa, A. Organometallics 2007, 26, 5627–5635. (14) White, C.; Yates, A.; Maitles, P. M. Inorg. Synth. 1992, 29, 228– 234.

Zhang et al. and [Cp*RhCl2]2 (123.6 mg, 0.2 mmol). IR (KBr): ν/cm-1 3448.1, 2916.9, 1569.7, 1507.8, 1347.5, 1188.4, 1026.1, 803.6, 727.8, 527.9, 504.1. 1H NMR (400 MHz, CDCl3): δ (ppm) 7.56 (d, 4H; CH, oxamidato), 7.08 (d, 4H; CH, oxamidato), 2.33 (s, 6H; CH3, oxamidato), 1.33 (s, 30H; Cp*). Anal. Calcd for C36H44N2Cl2O2Rh2: C, 53.15; H, 5.45; N, 3.44. Found: C, 53.05; H, 5.42; N, 3.31. Preparation of Tetranuclear Complexes. [Cp*4Ir4(μ-N,N0 -diphenyloxamidato)2(μ-4,40 -bpe)2](OTf)4 (3a). Two equivalents of AgOTf (36 mg, 0.14 mmol) was added to a suspension of binuclear complex 1a (48 mg, 0.05 mmol) in CH3OH (15 mL) at room temperature, and the mixture was stirred for 4 h, followed by filtration. One equivalent of 4,40 -bpe (9 mg, 0.05 mmol) was added to the filtrate, and the mixture was stirred for 15 h before being filtered. The solvent was removed and the residue was extracted with CH2Cl2. Yellow powders were obtained after evaporation of the solvent under reduced pressure, washed with diethyl ether, and dried in vacuo. Yield: 52 mg, 75.7%. IR (KBr): ν/cm-1 3494.1, 3061.2, 2921.3, 1605.1, 1578.4, 1488.6, 1451.2, 1345.8, 1260.1, 1156.8, 1030.2, 748.3, 638.6, 574.2, 517.8. 1H NMR (400 Hz, CDCl3): δ (ppm) 8.14 (d, 8H; CH, Py-H), 7.89 (d, 8H; CH, Py-H), 7.65 (m, 12H; CH, oxamidato (8H); HCdCH, 4,40 -bpe (4H)), 7.40 (m, 12H; CH, oxamidato), 1.25 (s, 60H; Cp*). Anal. Calcd for C96H100N8F12S4O16Ir4: C, 41.98; H, 3.67; N, 4.08. Found: C, 41.96 ; H, 4.06; N, 4.06. [Cp*4Ir4(μ-N,N0 -bis(4-methylphenyl)oxamidato)2(μ-4,40 -bpe)2](OTf)4 (3b). This complex (yellow, 62.4 mg, 89%) was obtained from 1b (50 mg, 0.05 mmol), AgOTf (36 mg, 0.14 mmol), and 4,40 -bpe (10 mg, 0.05 mmol) by a similar procedure to that described for 3a. IR (KBr): ν/cm-1 3506.2, 2922.6, 1613.3, 1585.1, 1507.3, 1343.9, 1260.1, 1158.2, 1031.4, 839, 639.5, 515.3. 1H NMR (400 Hz, CD3OD): δ (ppm) 8.12 (d, 8H; CH, Py-H), 7.86 (d, 8H; CH, Py-H), 7.63 (s, 4H; HCdCH, 4,40 -bpe), 7.46 (d, 8H; CH, oxamidato), 7.25 (d, 8H; CH, oxamidato), 2.49 (s, 12H; CH3, oxamidato), 1.25 (s, 60H; Cp*). Anal. Calcd for C100H108N8F12S4O16Ir4: C, 42.85; H, 3.88; N, 4.00. Found: C, 42.78; H, 4.06; N, 3.50. [Cp*4Rh4(μ-N,N0 -diphenyloxamide)oxamidato)2(μ-4,40 -bpe)2](OTf)4 (4a). This complex (orange, 40 mg, 67%) was obtained from 2a (40 mg, 0.05 mmol), AgOTf (36 mg, 0.14 mmol), and 4,40 -bpe (10 mg, 0.05 mmol) by a similar procedure to that described for 3a. IR (KBr): ν/cm-1: 3494.6, 3062.2, 2919.9, 1601.8, 1576.1, 1488.1, 1429.8, 1261.6, 1224.1, 1157.9, 1029.5, 750, 639.1, 567.2, 518.5. N. 1H NMR (400 Hz, CDCl3): δ (ppm) 8.04 (d, 8H; CH, Py-H), 7.80 (d, 8H; CH, Py-H), 7.66 (m, 8H; CH, oxamidato), 7.55 (s, 4H; HCdCH, 4,40 -bpe), 7.40 (m, 12H; CH, oxamidato), 1.29 (s, 60H; Cp*). Anal. Calcd for C96H100N8F12S4O16Rh4: C, 48.25; H, 4.22; N, 4.69. Found: C, 48.14; H, 4.42; N, 4.42. [Cp*4Rh4(μ-N,N0 -bis(4-methylphenyl)oxamide)2(μ-4,40 -bpe)2](OTf)4 (4b). This complex (orange, 46.5 mg, 76%) was obtained from 2b (41 mg, 0.05 mmol), AgOTf (36 mg, 0.14 mmol), and 4,40 -bpe (10 mg, 0.05 mmol) by a similar procedure to that described for 3a. IR (KBr): ν/cm-1 3495.2, 2921.6, 1610.5, 1577.3, 1505.6, 1429.6, 1341.4, 1260.6, 1223.8, 1157.3, 1029.4, 837.9, 639, 512.8. 1H NMR (400 Hz, CDCl3): δ (ppm) 8.02 (d, 8H; CH, Py-H), 7.77 (d, 8H; CH, Py-H), 7.53 (s, 4H; HCdCH, 4,40 -bpe), 7.43 (d, 8H; CH, oxamidato), 7.18 (d, 8H; CH, oxamidato), 2.47 (s, 12H; CH3, oxamidato), 1.28 (s, 60H; Cp*). Anal. Calcd for C100H108N8F12S4O16Rh4: C, 49.11; H, 4.45; N, 4.58. Found: C, 49.01; H, 4.65; N, 4.33. UV Irradiation of 3a. A powdered sample of 3a was placed between two glass slides and irradiated using a broad Hg lamp (300 W) for approximately 40 h. Yellow crystals of 5a suitable for X-ray diffraction study were obtained by slow diffusion of diethyl ether into a concentrated solution of the complexes in CH3OH. Conversion: 100% (rctt-tpcb: 64.5%; rtct-tpcb: 35.5%). 1H NMR (400 Hz, CDCl3): δ (ppm) rctt-tpcb: 8.86 (d, 2H; CH, Py-H), 8.66 (m, 2H; CH, Py-H), 8.52 (d, 2H; CH, Py-H), 8.08 (m, 2H; CH, Py-H), 7.32 - 7.68 (m, 6H; CH, Py-H), 6.94 (m, 2H; CH, Py-H), 5.70, (d, 2H; CH, cyclobutane), 4.98,

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Table 2. Crystallographic Data and Structure Refinement Parameters for 2b, 3a, 4a, and 5a

formula fw T, K cryst syst space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z Fcalcd (g/cm3) cryst size (mm3) μ(Mo KR) (mm-1) no. collected reflns no. unique reflns no. of params no. of obsd data GOF R1, wR2 [I > 2σ(I)]a R1, wR2 (all data)a largest diff peak and hole (e/A˚-3)

R1 = 2Fc2]/3 (also

3a

4a(1)

4a(2) 3 2CH2Cl2 3 2H2O

5a

C36H44Cl2N2O2Rh2 1052.19 293(2) triclinic P1 8.577(3) 11.861(4) 11.888(4) 98.828(4) 102.200(4) 107.769(4) 1094.3(7) 1 1.597 0.10  0.10  0.08 1.277 4970 4189 241 83.8% 0.995 0.0366, 0.0878 0.0453, 0.0914 0.580/-0.414

C96H100F12N8O16S4Ir4 2746.88 293(2) monoclinic C2/c 43.871(14) 15.719(4) 16.759(5) 90 97.747(7) 90 11452(6) 4 1.593 0.30  0.20  0.10 4.785 25 587 11 208 606 50.4% 0.812 0.0457, 0.0925 0.0904, 0.0988 1.144/-0.612

C96H100F12N8O16S4Rh4 2389.72 293(2) monoclinic C2/c 44.077(15) 15.603(5) 16.711(6) 90 98.426(5) 90 11368(7) 4 1.396 0.17  0.11  0.08 0.723 25 485 11 111 631 52.7% 0.835 0.0515, 0.1196 0.0940, 0.1288 0.663/-0.470

C96H100F12 N8O16Rh4S4 2595.60 100(2) monoclinic C2/c 43.839(4) 15.3993(15) 16.7973(16) 90 97.9470(10) 90 11230.7(19) 4 1.535 0.18  0.17  0.10 0.832 29 753 11 008 721 79.5% 1.087 0.0476, 0.1445 0.0605, 0.1548 1.072/ -1.116

C96H100F12N8O16S4Ir4 2746.88 293(2) triclinic P1 15.272(6) 16.769(7) 22.522(10) 101.900(6) 95.976(6) 94.781(6) 5581(4) 2 1.635 0.12  0.10  0.08 4.909 25 399 21 291 1165 43.1% 0.833 0.0654, 0.1551 0.1287, 0.1699 1.149/-0.605

Fo| - |Fc (based on reflections with Fo2 > 2σF2).2 wR2 = [ with Fo2 > 2σF2). )

P

)

a

2b 3 2CHCl3

P

(d, 2H; CH, cyclobutane); rtct-tpcb: 8.72 (d, 2H; CH, Py-H), 8.58 (d, 2H; CH, Py-H), 8.33 (m, 2H; CH, Py-H), 7.98 (m, 2H; CH, Py-H), 7.32-7.68 (m, 6H; CH, Py-H), 7.06 (m, 2H; CH, PyH), 5.62, (m, 2H; CH, cyclobutane), 5.24, (m, 2H; CH, cyclobutane); 7.32-7.68 (m, 40H; CH, oxamidato), 1.31, (s, 60H; Cp*), 1.26, (s, 60H; Cp*). Anal. Calcd for C96H100N8F12S4O16Ir4: C, 41.98; H, 3.67; N, 4.08. Found: C, 41.97 ; H, 3.58; N, 4.16. UV Irradiation of 3b. A powdered sample of 3b was placed between two glass slides and was irradiated using a broad Hg lamp (300 W) for approximately 40 h. Conversion: 100% (rctttpcb: 75.2%; rtct-tpcb: 24.8%). 1H NMR (400 Hz, CDCl3): δ (ppm) rctt-tpcb: 8.83 (d, 2H; CH, Py-H), 8.62 (m, 2H; CH, PyH), 8.50 (d, 2H; CH, Py-H), 8.04 (m, 2H; CH, Py-H), 7.58 (d, 2H; CH, Py-H), 6.95 (d, 2H; CH, Py-H), 7.14-7.48 (m, 4H; CH, Py-H), 5.66, (d, 2H; CH, cyclobutane), 4.99, (d, 2H; CH, cyclobutane); rtct-tpcb: 8.70 (d, 4H; CH, Py-H), 8.55 (d, 2H; CH, Py-H), 8.28 (m, 2H; CH, Py-H), 7.94 (d, 2H; CH, Py-H), 7.63 (m, 2H; CH, Py-H), 7.05 (m, 2H; CH, Py-H), 7.14-7.48 (m, 4H; CH, Py-H), 5.55, (m, 2H; CH, cyclobutane), 5.20, (m, 2H; CH, cyclobutane); 7.14-7.48 (m, 32H; CH, oxamidato), 2.46 (d, 24H; CH3), 1.30 (s, 60H; Cp*), 1.27 (s, 60H; Cp*). Anal. Calcd for C100H108N8F12S4O16Ir4: C, 42.85; H, 3.88; N, 4.00. Found: C, 42.67; H, 3.84; N, 3.85. UV Irradiation of 4a. A powdered sample of 4a was placed between two glass slides and was irradiated using a broad Hg lamp (300 W) for approximately 260 h. Conversion: 41.7% (rctt-tpcb: 29.2%; rtct-tpcb: 12.5%). 1H NMR (400 Hz, CDCl3): δ (ppm) rctt-tpcb: 5.60 (m, 2H; cyclobutane), 4.92 (m, 2H; cyclobutane); rtct-tpcb: 5.51 (m, 2H; cyclobutane), 5.07 (m, 2H; cyclobutane). UV Irradiation of 4b. A powdered sample of 4b was placed between two glass slides and was irradiated using a broad Hg lamp (300 W) for approximately 260 h. Conversion: 35.9% (rctt-tpcb: 25.6%; rtct-tpcb: 10.3%). 1H NMR (400 Hz, CDCl3): δ (ppm) rctt-tpcb: 5.56 (m, 2H; cyclobutane), 4.92 (m, 2H; cyclobutane); rtct-tpcb: 5.50 (m, 2H; cyclobutane), 5.06 (m, 2H; cyclobutane). X-ray Structure Determination. Data were collected on a CCD-Bruker SMART APEX system. All the determinations (15) Sheldrick, G. M. SHELXL-97; Universit€at G€ottingen: Germany, 1997. (16) van der Sluis, P.; Spek, A. L. Acta Crystallogr. 1990, A46, 194.

P [w(Fo2 - Fc2)2]/ [w(Fo2)2]]1/2; w = 1/[σ2(Fo2) þ (0.095P)2]; P = [max(Fo2, 0) þ

of unit cell and intensity data were performed with graphitemonochromated Mo KR radiation (λ=0.71073 A˚). All the data were collected at room temperature or 100 K using the ω-scan technique. These structures were solved by direct methods, using Fourier techniques, and refined on F2 by a full-matrix leastsquares method. All the calculations were carried out with the SHELXTL-97 program.15 In all complexes, hydrogen atoms were placed in geometrically calculated positions with fixed isotropic thermal parameters. In complex 2b, all non-hydrogen atoms were refined anisotropically. There were disordered solvent molecules in the crystal structures of 3a, 4a, and 5a, which cannot be refined properly. New data sets corresponding to omission of them were generated with the SQUEEZE algorithm,16 and then the structures were refined to convergence. In complexes 3a and 4a, all non-hydrogen atoms were refined anisotropically except one anion, while in 5a all nonhydrogen atoms were refined anisotropically except for one anion. Bond lengths restraints were applied to one of the refined OTf- anions in 3a and 4a to avoid unreasonable distortion of its molecular geometry. Bond distances C26-C26(A) (3a), C20C20(A) (4a), and C(26)-C(26A) (4a) were also restrained by DFIX instructions. Two of four pentamethylcyclopentadienyl ligands in the cation of 5a were disordered because of rotation at room temperature. They were refined to two idealized positions (56:44), and their carbon atoms were refined with ISOR instructions. For 5a, bond lengths were applied to model the disorder of two OTf- anions. A summary of the crystallographic data and structure refinement parameters is listed in Table 2.

Acknowledgment. This work was supported by the National Science Foundation of China (20531020, 20721063, 20771028), Shanghai Science and Technology Committee (08DZ2270500, 08DJ1400103), Shanghai Leading Academic Discipline Project (B108), and the National Basic Research Program of China (2009CB825300). Supporting Information Available: 1H NMR data for photodimerization reaction of complexes 3a-4b and the crystallographic data for 2b, 3a, 4a, and 5a are available free of charge via the Internet at http://pubs.acs.org.