Reactions of Low-Valent Titanocene(II) Fragments with trans-4,4

Organometallics 2009, 28, 2799–2807

2799

Reactions of Low-Valent Titanocene(II) Fragments with trans-4,4′-Azobispyridine (RNdNR, R ) C5H4N): Formation of Tetranuclear Molecular Squares by trans-cis Isomerization Oliver Theilmann, Wolfgang Saak, Detlev Haase, and Ru¨diger Beckhaus* Institute of Pure and Applied Chemistry, Carl Von Ossietzky UniVersity Oldenburg, 26111 Oldenburg, Germany ReceiVed NoVember 25, 2008

The reactions of the low-valent titanocene sources [Cp*2Ti(η2-C2(TMS)2)] (2) and [tBuCp2Ti(η2-C2(TMS)2)] (3) with trans-4,4′-azobispyridine (8) leads to novel supramolecular squares [(Cp*2Ti)4(µ2N,N′;η2-N,N′-C10H8N4)2] (10) and [(tBuCp2Ti)4(µ2-N,N′;η2-N,N′-C10H8N4)2] (11). These complexes consist of four bent-titanocene corner units and two azo ligands 8. Within this self-assembly process the azo ligands experience a conformational rearrangement from trans to cis. The titanocene moieties are embedded with two different N-donor environments, provided by the pyridyl rings and the azo functionality of 8. Single-crystal X-ray analyses of the tetranuclear compounds 10 and 11 revealed the structural features of these molecular polygons. Exclusively cis-configurated azopyridine units are detected in 10 and 11. Comparison of bond lengths and angles in coordinated and free ligands shows a reduced state of the bridging ligands in the low-valent titanium complexes 10 and 11. In such a way, the Nazo-Nazo distances are elongated from 1.251(2) Å in 8 to av 1.405(3) Å (10) and 1.434(3) Å (11), respectively. The syntheses and attributes of these novel compounds are discussed. From the reaction of [Cp2Ti(η2-C2(TMS)2)] (1) with 8, 1,2-bis(4-pyridyl)hydrazine (13) was isolated and fully characterized as a subsequent product. Introduction The synthesis of highly ordered supramolecular architectures is of considerable chemical and structural interest. These molecular architectures are typically formed via the selfassembly of simple building blocks. In performing this strategy, it has successfully led to the formation of a multitude of twoand three-dimensional structures.1-3 Concerning the electronic and steric properties, aromatic N-heterocycles have been applied as building blocks in supramolecular compounds4-11 or as bridging ligands in binuclear derivatives.12-14 Besides the capability of connecting metal centers, aromatic N-heterocycles provide the property of π-back-donating. This effect provokes an electron transfer and delocalization.15-18 * Corresponding author: Phone +49 441 798 3656. Fax: +49 441 798 3851. E-mail: [email protected]. (1) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. ReV. 2000, 100, 853– 907. (2) Swiegers, G. F.; Malefetse, T. J. Chem. ReV. 2000, 100, 3483–3537. (3) Holliday, B. J.; Mirkin, C. A. Angew. Chem., Int. Ed. 2001, 40, 2022– 2043; Angew. Chem. 2001, 113, 2076-2097. (4) Kitagawa, S.; Kitaura, R.; Noro, S.-I. Angew. Chem., Int. Ed. 2004, 43, 2334–2375; Angew. Chem. 2004, 116, 2388-2430. (5) Coles, M. P.; Swenson, D. C.; Jordan, R. F.; Young, V. G., Jr. Organometallics 1997, 16, 5183–5194. (6) Fujita, M. Chem. ReV. 1998, 27, 417–425. (7) Baxter, P. N. W.; Lehn, J.-M.; Baum, G.; Fenske, D. Chem.-Eur. J. 1999, 5, 102–112. (8) Olenyuk, B.; Fechtenkotter, A.; Stang, P. J. J. Chem. Soc., Dalton Trans. 1998, 1707–1728. (9) Batten, S. R.; Robson, R. Angew. Chem., Int. Ed. 1998, 37, 1461– 1494; Angew. Chem. 1998, 110, 1558-1595. (10) Stang, P. J.; Olenyuk, B. Acc. Chem. Res. 1997, 30, 502–518. (11) Robson, R. J. Chem. Soc., Dalton Trans. 2000, 3735–3744. (12) Berg, D. J.; Boncella, J. M.; Andersen, R. A. Organometallics 2002, 21, 4622–4631. (13) Kaim, W. Coord. Chem. ReV. 2002, 230, 127–139. (14) Kaim, W. Angew. Chem., Int. Ed. 1983, 22, 171–190; Angew. Chem. 1983, 95, 201-221. (15) Launay, J.-P. Chem. Soc. ReV. 2001, 30, 386–397.

Comparing the well-established supramolecular chemistry of late transition metals with early transition metals, only a few attempts have been made to adopt the reducing attributes and well-defined coordination behavior of early transition metals.17-20 The aim of forming molecular squares and rectangles requires nearly 90° angles at the vertices. This is typically available in octahedral or square-planar late transition metal compounds.1 In contrast, only a few examples of molecular squares and rectangles are known in early transition metal chemistry because of the tendency of early transition metal complexes to adopt tetrahedral coordination geometries.17,18,21,22 A great variety of self-assembly processes are possible if the aromatic N-heterocyclic ligands carry structural moieties that allow conformational rearrangements. For instance, these reorganizations are known in azo compounds.23-25 In addition, reconfigurations of Nheterocyclic ligands carrying these specific azo moieties are seldom performed in self-assembly processes with early transi(16) Serroni, S.; Campagna, S.; Puntoriero, F.; Di Pietro, C.; McClenaghan, N. D.; Loiseau, F. Chem. Soc. ReV. 2001, 30, 367–375. (17) Kraft, S.; Beckhaus, R.; Haase, D.; Saak, W. Angew. Chem., Int. Ed. 2004, 43, 1583–1587; Angew. Chem. 2004, 116, 1609-1614. (18) Kraft, S.; Hanuschek, E.; Beckhaus, R.; Haase, D.; Saak, W. Chem.Eur. J. 2005, 11, 969–978. (19) Schafer, L. L.; Nitschke, J. R.; Mao, S. S. H.; Liu, F.-Q.; Harder, G.; Haufe, M.; Tilley, T. D. Chem.-Eur. J. 2002, 8, 74–83. (20) Schafer, L. L.; Tilley, T. D. J. Am. Chem. Soc. 2001, 123, 2683– 2684. (21) Schinnerling, P.; Thewalt, U. J. Organomet. Chem. 1992, 431, 41– 45. (22) Stang, P. J.; Whiteford, J. A. Res. Chem. Intermed. 1996, 22, 659– 665. (23) Mayer, G.; Heckel, A. Angew. Chem., Int. Ed. 2006, 45, 4900–4921; Angew. Chem. 2006, 118, 5020-5042. (24) Brown, E. V.; Granneman, G. R. J. Am. Chem. Soc. 1975, 97, 621– 627. (25) Ashton, P. R.; Brown, C. L.; Cao, J.; Lee, J.-Y.; Newton, S. P.; Raymo, F. M.; Stoddart, J. F.; White, A. J. P.; Williams, D. J. Eur. J. Org. Chem. 2001, 957–965.

10.1021/om801123k CCC: $40.75  2009 American Chemical Society Publication on Web 03/24/2009

2800 Organometallics, Vol. 28, No. 9, 2009 Chart 1. Coordination Modes of 4,4′-Azopyridine Ligands

tion metals, in contrast to the late transition metals.3,26-32 These mentioned conformational rearrangements of azo-bridge-carrying compounds can be easily induced by light.26,29,33-41 Since 4,4′-azopyridine (8) is well known for its effective bridging coordination mode, we coupled this structural capability with lowvalent titanocene fragments. Generally, azopyridines exhibit two general coordination sites involving the nitrogen atoms. The pyridyl moieties represent the coordination mode A (Chart 1), and the nitrogen atoms of the azo bridge the coordination mode B (Chart 1). The coordination mode A has been used to construct a variety of coordination geometries depending on the metal center.42,43 In such a way, multinuclear complexes (such as dimers, squares, and rectangles),32,41,44-47 molecular frameworks (for instance brick stone, herringbone, or wave-like motifs),26,29,48-55 and coordination polymers54,56-58 become available. In contrast, the coor(26) Withersby, M. A.; Blake, A. J.; Champness, N. R.; Cooke, P. A.; Hubberstey, P.; Schroder, M. New J. Chem. 1999, 23, 573–575. (27) Baldwin, D. A.; Lever, A. B. P.; Parish, R. V. Inorg. Chem. 1969, 8, 107–115. (28) Beadle, P. J.; Goldstein, M.; Goodgame, D. M. L.; Grzeskowiak, R. Inorg. Chem. 1969, 8, 1490–1493. (29) Withersby, M. A.; Blake, A. J.; Champness, N. R.; Cooke, P. A.; Hubberstey, P.; Realf, A. L.; Teat, S. J.; Schroder, M. J. Chem. Soc., Dalton Trans. 2000, 3261–3268. (30) Noro, S.; Kitagawa, S.; Nakamura, T.; Wada, T. Inorg. Chem. 2005, 44, 3960–3971. (31) Kaim, W.; Reinhardt, R.; Greulich, S.; Fiedler, J. Organometallics 2003, 22, 2240–2244. (32) Schalley, C. A.; Muller, T.; Linnartz, P.; Witt, M.; Schafer, M.; Lutzen, A. Chem.-Eur. J. 2002, 8, 3538–3551. (33) Yutaka, T.; Mori, I.; Kurihara, M.; Mizutani, J.; Kubo, K.; Furusho, S.; Matsumura, K.; Tamai, N.; Nishihara, H. Inorg. Chem. 2001, 40, 4986– 4995. (34) Yutaka, T.; Mori, I.; Kurihara, M.; Mizutani, J.; Tamai, N.; Kawai, T.; Irie, M.; Nishihara, H. Inorg. Chem. 2002, 41, 7143–7150. (35) Yutaka, T.; Mori, I.; Kurihara, M.; Tamai, N.; Nishihara, H. Inorg. Chem. 2003, 42, 6306–6313. (36) Kume, S.; Kurihara, M.; Nishihara, H. Chem. Commun. 2001, 1656– 1657. (37) Kume, S.; Kurihara, M.; Nishihara, H. Inorg. Chem. 2003, 42, 2194– 2196. (38) Kume, S.; Murata, M.; Ozeki, T.; Nishihara, H. J. Am. Chem. Soc. 2005, 127, 490–491. (39) Tang, H.-S.; Zhu, N.; Yam, V. W.-W. Organometallics 2007, 26, 22– 25. (40) Sakamoto, R.; Murata, M.; Kume, S.; Sampei, H.; Sugimoto, M.; Nishihara, H. Chem. Commun. 2005, 1215–1217. (41) Sun, S.-S.; Anspach, J. A.; Lees, A. J. Inorg. Chem. 2002, 41, 1862– 1869. (42) Kitagawa, S.; Kitaura, R.; Noro, S. I. Angew. Chem., Int. Ed. 2004, 43, 2334–2375; Angew. Chem. 2004, 116, 2388-2430. (43) Roesky, H. W.; Andruh, M. Coord. Chem. ReV. 2003, 236, 91–119. (44) Wang, J.-Q.; Ren, C.-X.; Jin, G.-X. Organometallics 2006, 25, 74– 81. (45) Yam, V. W.-W.; Lau, V. C.-Y.; Cheung, K.-K. Chem. Commun. 1995, 259–261. (46) Launay, J. P.; Tourrel-Pagis, M.; Lipskier, J. F.; Marvaud, V.; Joachim, C. Inorg. Chem. 1991, 30, 1033–1038. (47) Molnar, G.; Cobo, S.; Real, J. A.; Carcenac, F.; Daran, E.; Vieu, C.; Bousseksou, A. AdVanced Materials; Wiley-VCH: Weinheim, 2007; pp 2163-2167. (48) Kennedy, A. R.; Brown, K. G.; Graham, D.; Kirkhouse, J. B.; Kittner, M.; Major, C.; McHugh, C. J.; Murdoch, P.; Smith, W. E. New J. Chem. 2005, 29, 826–832. (49) Halder, G. J.; Kepert, C. J. Aust. J. Chem. 2005, 58, 311–314. (50) Halder, G. J.; Kepert, C. J.; Moubaraki, B.; Murray, K. S.; Cashion, J. D. Science 2002, 298, 1762–1765. (51) Tang, J.-K.; Zhu, L.-N.; Li, X.-Z.; Dong, W.; Liao, D.-Z.; Jiang, Z.-H.; Yan, S.-P.; Cheng, P. Z. Anorg. All. Chem. 2003, 629, 2000–2003.

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dination mode B is less common.59 To the best of our knowledge, structurally characterized complexes that display coordination mode B are unknown for 4,4′-azopyridines even though the coordination chemistry of azobenzene with early transition metals has been explored.60-69 Recently we described the synthesis and characterization of various tri- and tetranuclear complexes carrying low-valent titanium centers and aromatic N-heterocyclic ligands.17,18,70,71 Single-crystal X-ray structure analyses revealed the existence of square and rectangular structures of the metal complexes. Herein we report on the preparation and structural characterization of novel self-assembled polynuclear titanium complexes that have been obtained from reactions of [Cp*2Ti(η2C2(TMS)2)] (2) and [tBuCp2Ti(η2-C2(TMS)2)] (3) with 4,4′azobispyridine (8). In these reactions the ligand 8 has experienced a trans to cis rearrangement. In addition, we report on the reduction of the azo ligand to its hydrazine derivative.

Results and Discussion In the course of our studies we reported on the reactions of excellent titanocene sources carrying bis(trimethylsilyl)acetylene (C2(TMS)2) such as [Cp2Ti(η2-C2(TMS)2)] (1), [Cp*2Ti(η2-C2(TMS)2)] (2), and [tBuCp2Ti(η2-C2(TMS)2)] (3)72,73 with various aromatic N-heterocyclic ligands. By using different N-heterocyclic ligands neutral molecular squares (6, 7) can be synthesized (Scheme 1).17 Additionally, molecular rectangular titanium complexes become available.18 The titanocene source 1 in reaction with pyrazine (4) leads to the first structurally characterized neutral tetranuclear molecular square with low-valent titanium(II) as corner units, [(Cp2Ti(µ2C4H4N2))4] (6). The related tetranuclear complex [(Cp2Ti(µ2C10H8N2))4] (7) was similarly obtained from the reaction of 1 with 4,4′-bipyridine (5) (Scheme 1). When 4,4′-bipyridine (5) is modified (52) Zhu, L.-N.; Yan, O.-Y.; Liu, Z.-Q.; Liao, D.-Z.; Jiang, Z.-H.; Yan, S.-P.; Cheng, P. Z. Anorg. Allg. Chem. 2005, 631, 1693–1697. (53) Li, B.; Wang, X.; Zhang, Y.; Gao, S.; Zhang, Y. Inorg. Chim. Acta 2005, 358, 3519–3524. (54) Niu, Y.; Song, Y.; Chen, T.; Xue, Z.; Xin, X. CrystEngComm 2001, 36, 1–3. (55) Kondo, M.; Shimamura, M.; Noro, S.-i.; Minakoshi, S.; Asami, A.; Seki, K.; Kitagawa, S. Chem. Mater. 2000, 12, 1288–1299. (56) Li, B.; Zhu, L.; Wang, S.; Lang, J.; Zhang, Y. J. Coord. Chem. 2003, 56, 933–941. (57) Zhu, L.-N.; Yi, L.; Dong, W.; Wang, W.-Z.; Liu, Z.-Q.; Wang, Q.-M.; Liao, D.-Z.; Jiang, Z.-H.; Yan, S.-P. J. Coord. Chem. 2006, 59, 457–465. (58) Clegg, J. K.; Lindoy, L. F.; McMurtrie, J. C.; Schilter, D. J. Chem. Soc., Dalton Trans. 2005, 857–864. (59) Wong, W. Y.; Cheung, S. H.; Lee, S. M.; Leung, S. Y. J. Organomet. Chem. 2000, 596, 36–45. (60) Dermer, O. C.; Fernelius, W. C. Z. Anorg. Allg. Chem. 1934, 221, 83–96. (61) Avar, G.; Ruesseler, W.; Kisch, H. Z. Naturforsch., B: Chem. Sci. 1987, 42, 1441–1446. (62) Hill, J. E.; Fanwick, P. E.; Rothwell, I. P. Organometallics 1990, 9, 2211–2213. (63) Eisch, J. J.; Gitua, J. N. Organometallics 2003, 22, 24–26. (64) Tripepi, G.; Young, V. G., Jr.; Ellis, J. E. J. Organomet. Chem. 2000, 593-594, 354–360. (65) Durfee, L. D.; Hill, J. E.; Fanwick, P. E.; Rothwell, I. P. Organometallics 1990, 9, 75–80. (66) Dias, A. R.; Dias, P. B.; Diogo, H. P.; Galvao, A. M.; Minas da Piedade, M. E.; Simoes, J. A. M. Organometallics 1987, 6, 1427–1432. (67) Fochi, G.; Floriani, C.; Bart, J. C.; Giunchi, G. J. Chem. Soc., Dalton Trans. 1983, 1515–1521. (68) Gambarotta, S.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. Chem. Commun. 1982, 1015–1017. (69) Bart, J. C. J.; Bassi, I. W.; Cerruti, G. F.; Calcaterra, M. Gazz. Chim. Ital. 1980, 110, 423–436. (70) Piglosiewicz, I. M.; Beckhaus, R.; Saak, W.; Haase, D. J. Am. Chem. Soc. 2005, 127, 14190–14191.

Low-Valent Titanocene(II) Fragments

Organometallics, Vol. 28, No. 9, 2009 2801 Scheme 1

with an azo moiety connecting both pyridine units in position 4, the resulting molecule becomes trans-4,4′-azobispyridine (8) (Scheme 2). The 4,4′-azobispyridine ligand (8) is efficiently prepared by oxidative coupling of 4-aminopyridine (9).24,46 The crude product shows a trans/cis ratio of 4,4′-azobispyridine (8) of 37:1 determined by integration of 1H NMR signals. After column chromatography on silica gel, pure red-colored trans-4,4′-azobispyridine (8) is obtained in 77% yield. Scheme 2

The reaction of [Cp*2Ti(η2-C2(TMS)2)] (2) with trans-4,4′azobispyridine (8) in benzene is accompanied after 12 h by a color change from orange to black. Within this period of time black crystals of 10, suitable for X-ray analysis, were obtained directly from the reaction solution in yields of 41% (dec 170 °C). The related complex 11 was obtained from reacting 3 with 8 in THF. The color of the reaction solution also became black during the course of the reaction. Black crystals suitable for X-ray analysis are grown within several days at 60 °C from a THF/n-hexane mixture. Complex 11 can be obtained in yields of 51% (dec 150 °C). Compounds 10 and 11 were successfully characterized by 1H NMR measurements, IR spectroscopy, mass spectrometry, and elemental analysis. All compounds are intensively colored and highly sensitive to air and moisture. During the formation of 10 and 11 the azopyridine 8 undergoes a similar trans to cis rearrangement. The resulting tetranuclear complexes 10 and 11 carry exclusively cis-4,4′-azobispyridine (8) (Scheme 3). Scheme 3

The four Ti atoms of 10 adopt a pseudotetrahedral geometry. In contrast to the molecular squares displayed by [Cp2Ti(µ2N2C4H4)]4 (6) and [Cp2Ti(µ2-N2C10H8)]4 (7),18 two different coordination environments are observed about the Ti centers of 10. Two metal atoms are aligned by the pyridyl rings of the 4,4′-azobispyridine ligands (8). The coordination of the azo moiety of 8 is accompanied by a lengthening of the N-N bond length from 1.251(2) Å to an average value of 1.41 Å in 10. Azobenzene, as a congener of 4,4′-azobispyridine (8), experiences an analogous bond lengthening upon coordination with the N-N bond ranging from 1.33967 to 1.416 Å.65 The distances of the Ti-Npyridyl bonds in complex 10 (av 2.25 Å) are within the expected range of Ti-N dative bonds (2.20-2.44 Å).74 The Ti-Nazo bridge distances of the azo units (N1, N2; N5, N6) in 10 are slightly shorter (av 2.11 Å). With respect to the structurally characterized azobenzene-containing titanium complexes, Ti-Nazo bridge bond lengths are shorter. Depending on further ligands at the titanium center, Ti-N distances of 1.949 and 1.963 Å are found in the case of titanium alkoxide fragments,65 whereas for Cp2Ti units 1.96867 and 1.935 Å68 are observed. Moreover, the angle enclosed by Ti and the azo moiety (av Nazo bridge-Ti-Nazo bridge ) 38.8°) matches with literature known azobenzene-containing examples (av N-Ti-N ) 39.8°).67,69 As shown in Figure 2 the titanium atoms are almost located in one plane, and therefore complex 10 forms an almost perfect square with bent titanocene moieties as corner units. The overall averaged edge length in molecular square 10 is 8.38 Å. 4,4′-Bisazopyridine-containing late transition metal complexes show unambiguously longer metal-metal distances from approximately 13 to 13.5 Å.26,30,44,48,52,55,76-82 The distances for the diametrically opposed titanium centers Ti1 and Ti3 are 13.035(8) and 10.514(7) Å respectively to Ti2 and Ti4. Moreover, a slight distortion of the molecular square can be observed. The nitrogen atoms of the azo unit are located in an alternating manner above and below a plane constructed by the four Ti centers (Figure 2). In complex 10 the azo-coordinated titanocene fragments are twisted from each other, forming an average angle R of 30° (Chart 2). The torsion angle β shows that the two TiN2(azo) Chart 2. Twist Angles (r, β) of Diametric Titanocene Fragments

The molecular structure of 10 is shown in Figure 1. The crystallographic asymmetric unit contains one tetranuclear complex (10) and in total 10 independent benzene solvent molecules.

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Figure 1. ORTEP plot of the solid state molecular structure of 10. Thermal ellipsoids are drawn at the 50% probability level; protons and solvent molecules are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Ti1-N1 2.1135(22), Ti1-N2 2.1156(22), N1-N2 1.4051(29), N1-C1 1.3322(34), N2-C6 1.3287(30), Ti2-N3 2.2510(20), Ti2-N4 2.2381(22), Ti3-N5 2.1116(23), Ti3-N6 2.1125(21), N5-N6 1.4049(29), N5-C13 1.3359(33), N6-C16 1.3310(30), Ti4-N7 2.2551(20), Ti4-N8 2.2433(22), Ti1-Ct1 2.1064(50), Ti1-Ct2 2.1144(40), Ti2-Ct3 2.1546(40), Ti2-Ct4 2.1614(40), Ti3-Ct5 2.1082(40) Ti3-Ct6 2.1112(40), Ti4-Ct7 2.1544(40), Ti4-Ct8 2.1694(40), Ti1-Ti2 8.3838(70), Ti2-Ti3 8.3701(70), Ti3-Ti4 8.3881(70), Ti1-Ti4 8.3643(70), Ti1-Ti3 13.0348(80), Ti2-Ti4 10.5139(70), Ct1-Ti1-Ct2 140.51(2), Ct3-Ti2-Ct4 137.75(2), Ct5-Ti3-Ct6 140.39(2), Ct7-Ti4-Ct8 137.33(2), N1-Ti1-N2 38.81(8), N3-Ti2-N4 84.24(8), N5-Ti3-N6 38.85(8), N7-Ti4-N8 83.91(8), Ct1 ) ring centroid of C21-C25, Ct2 ) ring centroid of C31-C25, Ct3 ) ring centroid of C41-C45, Ct4 ) ring centroid of C51-C55, Ct5 ) ring centroid of C61-C66, Ct6 ) ring centroid of C71-C75, Ct7 ) ring centroid of C81-C85, Ct8 ) ring centroid of C91-C95.

Figure 2. Side view from the ORTEP plot of the solid state molecular structure of 10. Thermal ellipsoids are drawn at 50% probability; Cp* ligands, hydrogen atoms, and solvent molecules are omitted for clarity.

planes are twisted by an average of 45.6°. The comparable angles R and β of the pyridyl-substituted titanocene fragments are found to be smaller (av R: 18.5°, β: 8.8°). The tBuCp-substituted titanocene complex 11 crystallizes in the space group P21/c. The crystallographic asymmetric unit

contains one tetranuclear complex and a half-molecule of n-hexane. The molecular structure of 11 is shown in Figure 3. The molecular square 11 is equally configured as 10. As already depicted for 10 the azo moieties in complex 11 experience a widening to 1.43 Å as well. Furthermore, the average Ti-Npyridyl bonds, at 2.26 Å, are in the expected range of dative Ti-N bonds.74,75 The average Ti-Nazo bridge distances of 2.18 Å are comparable with 10 (2.10 Å). The N-Ti-N angles of the azo moiety (av 39.8°) are in the expected range of titanium-coordinated azo units.67,69The intramolecular Ti-Ti distances in 11 range from 8.076(8) Å for Ti1-Ti2 to 8.339(8) for Ti1-Ti4. The distances for the diametrically opposed titanium centers Ti1 and Ti3 are 12.409(8) and 10.796(8) Å respectively to Ti2 and Ti4. The angles R and β (Chart 2) reveal that the tBuCp-carrying tetranuclear complex 11 suffers like the permethylated titanocene-containing derivative 10 from distortion. In such a way the angles R and β are found at higher values (Ti-Nazo unit R: 84.6° β: 87.3°; Ti-Npyridyl unit R: 25.3°, β: 19.0°).


Organometallics, Vol. 28, No. 9, 2009 2803

Figure 3. ORTEP plot of the solid state molecular structure of 11. Thermal ellipsoids are drawn at the 50% probability level; protons and solvent molecules are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Ti1-N1 2.1023(21), T1-N2 2.1038(23), N1-N2 1.4342(30), N1-C1 1.3546(35), N2-C6 1.3364(33), Ti2-N3 2.2478(22), Ti2-N4 2.2468(21), Ti3-N5 2.1008(22), Ti3-N6 2.1112(21), N5-N6 1.4342(34), N5-C13 1.3469(32), N6-C16 1.3443(32), Ti4-N7 2.2644(22), Ti4-N8 2.2661(21), Ti1-Ct1 2.0953(70), Ti1-Ct2 2.0744(60), Ti2-Ct3 2.1074(50), Ti2-Ct4 2.1168(50), Ti3-Ct5 2.0882(50) Ti3-Ct6 2.0810 (50), Ti4-Ct7 2.1032(50), Ti4-Ct8 2.1057(50), Ti1-Ti2 8.0755(80), Ti2-Ti3 8.2746(80), Ti3-Ti4 8.2070(80), Ti1-Ti4 8.3391(80), Ti1-Ti3 12.4091(80), Ti2-Ti4 10.7964(80) Ct1-Ti1-Ct2 135.79(3), Ct3-Ti2-Ct4 134.88(2), Ct5-Ti3-Ct6 135.89(2), Ct7-Ti4-Ct8 135.22(2), N1-Ti1-N2 39.88(9), N3-Ti2-N4 82.34(8), N5-Ti3-N6 39.81(8), N7-Ti4-N8 82.815(80), Ct1 ) ring centroid of C21-C25, Ct2 ) ring centroid of C30-34, Ct3 ) ring centroid of C39-C43, Ct4 ) ring centroid of C48-C52, Ct5 ) ring centroid of C57-C61, Ct6 ) ring centroid of C66-C70, Ct7 ) ring centroid of C75-C79, Ct8 ) ring centroid of C84-C88.

Generally, the strong reducing properties of low-valent titanium compounds lead to reduced N-heterocycles accompanied by an electron transfer. Thus, C-C coupling reactions,17,70 C-H and C-F bond cleavage reactions,71 and formation of complexes carrying stable heterocyclic radical anions by electron transfer occur.83 It seems reasonable to adopt this electron transfer concept to these reactions shown in Schemes 1 and 3

Figure 4. ORTEP plot of the solid state molecular structure of 11. Side view from complex 11. Thermal ellipsoids are drawn at 50% probalility; tBuCp ligands, hydrogen atoms, and solvent molecules are omitted for clarity.

as well. Some evidence for this assumption is given by molecular structures in the solid state. A wide range of compounds consisting of low-valent metals and aromatic heterocyclic ligands show alterations in bond lengths and angles because of their sensitivity to electron transfer. For example, the supposed electron transfer in a Fe(0) complex of 2,2′-bipyridine leads to a decreasing bond length of 0.083 Å between the two pyridyl rings. In this context an extensive π-back-bonding from the iron(0) center to the π*molecular orbital of 2,2′-bipyridine is discussed. In this respect, the ligand can be considered as dianionic.86,75 Moreover, the tetrazine derivatives containing low-valent copper(I) complexes synthesized by Kaim et al. are known in which the N-N bonds of the ligands experience an elongation due to electron receptions.87-85,88 In our studies we observed these bond length alterations in many previously synthesized low-valent titanocene-containing complexes. For instance, the pyrazine (4)- and 4,4′-bipyridine (5)-containing complexes 6 and 7 experience modifications of their bond lengths, as the ligands are considered as dianionic,


resulting from a two-step reduction process based on the titanocene fragments. Regarding free 4,4′-bipyridine (5) as a well-known example, the aromaticity is located in the pyridyl rings. Additionally, a twisted angle is observed. A two-electron-reduced 4,4′-bipyridine (5) leads to a complete modification of the bonding system. In this particular case, a double bond connecting the two rings is formed and the fold angle γ between the pyridyl units amounts to 0°. Concerning the formulated C-C double bond, the bond length is shortened. In total the complete bonding system can be recognized as quinonoid-like. The data received from the solid state confirms these assumptions (Scheme 4).18 Scheme 4

This concept of electron transfer can be applied to the new synthesized complexes 10 and 11 as well. The elongation of the N-N bond and shortening of the C-Nazo bond lengths are caused by electron transfer from the low-valent titanocene fragments to the ligand 8. In this manner the ligand receives in total two electrons. The assumption of a quinonoid-like bonding system for 8 may also explain the trans to cis rearrangement. Because of the two-electron reduction process a N-N single bond has to be formulated referring to the azo bridge. Therefore, a free rotation along this N-N σ-bond is possible (Scheme 5).

Theilmann et al.

In contrast, the C-N bonds are shortened from 1.443(2) Å in 8 to 1.33 Å (av 10) and 1.35 Å (av 11). In total the comparison of the N-N and C-N bond distances agree with an electron transfer from the titanocene units to the LUMO (π*) of ligand 8. Due to the reduced properties of the 4,4′-azopyridines (8) in 10 and 11, the ligands do not show any planarity, exhibiting a fold angle of 48.0° (av 10) and 70.5° (av 11). These values are in agreement with 1,2-bis(4-pyridyl)hydrazine (13) (vide infra), exhibiting a fold angle of 84.2°. The NNC angles δ are found from 113.1° for 8 up to 125.4° (av) in the case of 10. Compared to the molecular squares 6 and 7,18,90 the tetranuclear complexes 10 and 11 appear to be diamagnetic as well. By following the reaction progress for the formation of 10 with 1 H NMR measurements at 214 K in toluene-d8, the release of the acetylene ligand can be detected immediately. In this regard, four high-field-shifted signals at 8.57, 8.52, 6.09, and 5.73 ppm are assigned to the protons of the pyridyl rings. The downfield singlets at 1.86 and 1.47 ppm arise from the permethylated titanocene units, reflecting their different coordination environments (Figure 5). 1H NMR measurements of 11 led only to poorly resolved spectra even at low temperatures, whereas 13C NMR experiments of 10 and 11 failed, due to their poor solubility.

Scheme 5

Figure 5. 1H NMR spectrum of 10 recorded from the reaction of 2 with 8 in toluene-d8, measured at 214 K. # solvent signals, * C2(Si(CH3)3)2 (high intensity), traces of silicon grease (low).

The comparison of our data received from crystal structures 10 and 11 with values of the free ligand 4,4′-azobispyridine (8) and free 4,4′-bipyridine (5)18 with the two-electron-reduced bis(trimethylsilyl)dihydro-4,4′-bipyridine (12)89 shows a significant match with the supposed quinonoid-like bonding system. Table 2 lists relevant bond lengths, fold angles, and distortion angles for compounds 5, 8, 10, 11, and 12. In particular, the N-N bond length for the trans isomer of 8 is given. The asymmetric units of structures 5 and 12 contain two independent molecules. For 4,4′-bipyridine (5) the important values are shown for both, as the angle between the pyridyl rings differs significantly. In 12 there is no major difference detected, so the average is given. Comparing the values of free with coordinated 4,4′-bisazopyridine (8) confirms the assumption of a two-electron transfer to the ligands. As expected, N-N bonds are extended from 1.251(2) Å in 8 to 1.41 Å (av) for 10 and 1.43 Å (av) for 11.

Attempts to prepare 4,4′-azobispyridine complexes employing unsubstituted titanocene fragments, generated from [Cp2Ti(η2C2(TMS)2)] (1), ended in the formation of a dark black insoluble precipitate. We suppose that 8 is coordinated primarily to the titanocene fragment, e.g., forming the molecular square 12. Then the hydrazine derivative 13 is obtained from a benzene solution (Scheme 6) in 21% yield as a subsequent product. We propose hydrogen transfer reactions from the metallocene fragments. However, the nature of the formed titanium species remains unknown. Scheme 6


The 1,2-bis(4-pyridyl)hydrazine (13) crystallizes in the space group I41/a with eight molecules per unit cell. Each hydrazine derivative carries one benzene solvent molecule. The molecular structure is shown in Figure 6.


and squares without breaking any bonds. Studies related to the electronic and magnetic studies would be able to give insight into the observed properties. Furthermore, it may be possible to synthesize higher aggregated, for instance three-dimensional, titanium compounds on the basis of self-assembling processes.

Experimental Section

Figure 6. ORTEP plot of the solid state molecular structure of 13. Thermal ellipsoids are drawn at the 50% probability level; solvent molecules are omitted for clarity. Selected bond lengths [Å] and angles [deg]: N1-N1a 1.3934(14), C1-N1 1.3591(14), C1-C2 1.4077(15) C2-C3 1.3703(16), C3-N2 1.3460(16), C4-N2 1.3437(16), C4-C5 1.3716(16), C5-C1 1.3965(15), C1-N1-N1a 120.39, fold angle : 84.37(3); symmetry transformation for the generation of equivalent atoms a: -x+1, -y+1,+z.

Comparison of our data received from the crystal structure of 13 with values of 1,2-dipyridylhydrazine (14) shows an explicit compliance and confirms additionally the reduction of 8 to its hydrazine derivative 13. Compound 13 shows in contrast to 8 an N-N bond elongation of 0.142 Å. The N-N bond length of 1.393(1) Å in 13 is in accordance with 1.394(5) Å in 1,2diphenylhydrazine (14). Furthermore, the fold angle of 84.37(3)° in 13 is larger than that in 14 (75.59(7)°). Table 3 gives an overview of selected bond lengths and fold angles for 1,2-bis(4-pyridyl)hydrazine (13) and 1,2-dipyridylhydrazine (14).

Conclusions and Outlook The reactions and compounds discussed in this paper expand the array of early transition metal based self-assembly processes. Two new molecular squares with bent titanocene fragments carrying different cyclopentadienyl-type ligands are shown. These new complexes are efficiently synthesized by reacting titanocene precursors with 4,4′-azobispyridine (8). To the best of our knowledge, this is the first trans to cis rearrangement of azo ligands induced by the electronic properties of titanocene fragments. In contrast to cationic and water-soluble polygons of late transition metal complexes, the presented low-valent titanium compounds are neutral. Moreover, in azobispyridinecontaining late transition metal complexes, the azo ligands provide only the pyridyl rings for coordination and coordination on the azo bridge is seldom perfomed. In contrast to this, titanocene fragments coordinate on the azo bridge of azobispyridines. This initiates a trans to cis isomerization of the azo ligands and leads to titanocene-containing molecular squares. Due to this, the Ti-Ti distances are notably smaller than the metal-metal distances in late transition azobispyridine compounds. Generally, the reduced properties of the side-oncoordinated azo bridge in the presented complexes might open the door to subsequent functionalization of the N-N unit. Further studies may provide insight into titanium-based complexes concerning switch-like altering of the conformation. These investigations would be able to result in the synthesis of squares and rectangles capable of changing their conformation (e.g., induced by light) in forming small and large rectangles

Reagents and General Techniques. All operations were performed in a nitrogen and argon atmosphere with rigorous exclusion of moisture and oxygen using Schlenk or glovebox techniques. All chemicals used were reagent grade or higher and purified according to standard protocols. Non-chlorinated solvents were distilled over Na/K alloy and benzophenone under a nitrogen atmosphere. 1 H and 13C NMR spectra were recorded on a Bruker AVANCE 500 (1H, 500.1 MHz; 13C, 125.8 MHz) spectrometer. 1H and 13C chemical shifts (δ) are reported in ppm and referenced to residual solvent protons. Chemical ionization (CI) mass spectra were taken on a Finnigan-MAT 95 spectrometer. IR spectra were recorded on a Bruker Vector22 using KBr pellets. Melting points were determined using a “Mel-Temp” by Laboratory Devices, Cambridge. Elemental analyses were carried out with using an EA Euro 3000 from EuroVector, Italy. Synthesis of trans-4,4′-Azobispyridine (8). According to an adaptation of published procedures,24,46 4,4′-azobispyridine (8) was prepared by oxidative coupling of 4-aminopyridine (9) by an aqueous solution of sodium hypochlorite. A solution of 5 g (0.053 mol) of 4-aminopyridine (9) was cooled to 0 °C. After dropwise addition of 300 mL of a 15% NaOCl solution the color turned from colorless to orange. The mixture was stirred at 0 °C until an orange precipitate was formed. The precipitate was filtered out, and the aqueous phase was extracted three times with chloroform. The organic phases were gathered and dried with Na2SO4, and the solvent was removed. The crude product was purified by column chromatography on silica gel (eluent: ethyl acetate/ethanol, 12:1; Rf 0.46) to obtain the pure trans isomer. The trans isomer of 4,4′azobispyridine (8) was isolated as an orange powder in 3.7 g (0.02 mol, 77% yield). Mp: 105-106 °C (lit. mp: 107-108 °C24,46). Crystals suitable for X-ray diffraction can be obtained by crystallization in water. 1 H NMR (C6D6, 500.1 MHz, 300 K) [ppm]: δ 8.62 (d, 4H, 3JHH ) 5.8 Hz, ortho-H), 7.31 (d, 4H, 3JHH ) 5.8 Hz, ortho-H). 13C NMR (C6D6, 125.8 MHz, 300 K) [ppm]: δ 156.64 (C4), 152.03 (C2, C6,), 116.13 (C3, C5). IR (KBr pellet) [cm-1]: V˜ 3018, 2959, 2859, 2731, 2672, 1587, 1567, 1466, 1409, 1378, 1341, 1292, 1215, 1054, 992, 883, 835, 761, 669. MS (CI, isobutane) m/z (relative intensity): 369 (59) [2 × C10H8N4 + H+], 185 (100) [C10H8N4 + H+], 95 (23) [C5H6N2 + H+]. MS (CI high resolution, isobutane): calcd for C10H8N4 + H+ 185.0827, found 185.0828 (100). Synthesis of [Cp2Ti(η2-C2(TMS)2)] (1), [Cp*2Ti(η2-C2(TMS)2)] (2), and [tBuCp2Ti(η2-C2(TMS)2)] (3). Compounds 1, 2, and 3 were prepared according to literature procedures.72,73 Synthesis of [(Cp*2Ti)4(µ2-N,N′;η2-N,N′-C10H8N4)2] (10). A solution of 0.1 g (0.2 mmol) of [Cp*2Ti(η2-C2(TMS)2)] (2) in 2 mL of benzene was added to a solution of 0.019 g (0.1 mmol) of trans-4,4′-azobispyridine (8) in 3 mL of benzene. The mixture resulted in an orange solution, which turned black within 12 h. The product 10 was obtained as black crystals directly from the mother liquor in 0.0671 g (0.041 mmol, 41% yield). Mp: 170 °C (dec). Crystals suitable for X-ray diffraction were obtained from benzene. 1H NMR (toluene-d8, 500.1 MHz, 214 K) [ppm]: δ 8.57 (s, pyridyl-H), 8.52 (s, pyridyl-H), 6.09 (s, pyridyl-H), 5.73 (s, pyridyl-H), 1.86 (s, Cp*-H), 1.47 (s, Cp*-H). IR (KBr pellet) [cm-1]: V˜ 2901, 2851, 2720, 1617, 1597, 1518, 1497, 1448, 1403, 1372, 1316, 1262, 1204, 1090, 1023, 997, 983, 800, 598, 529. MS (CI, isobutane) m/z (relative intensity): 504 (12) [C30H40N4Ti+], 187 (100) [C10H10N4 + H+], 137 (43) [C10H16 + H+], 95 (24) [C5H6N2


Theilmann et al.

Table 1. Bond Lengths and Angles for 4,4′-Bipyridine (5), Two-Electron-Reduced 4,4′-bipyridine (12), free trans-4,4′-Azobispyridine (8), and Coordinated 4,4′-Azobispyridine (10 and 11)

Table 2. Selected Bond Lengths and Fold Angles E of 1,2-Bis(4-pyridyl)hydrazine (13) and 1,2-Diphenylhydrazine (14)

+ H+]. Anal. Calcd for C100H136N8Ti4 + C6H6: C 73.05, H 8.32, N 6.52. Found: C 73.61, H 9.13, N 6.53. Synthesis of [(tBuCp2Ti)4(µ2-N,N′;η2-N,N′-C10H8N4)2] (11). Solutions of 0.16 g (0.347 mmol) of [tBuCp2Ti(η2-C2(TMS)2] (3) in 2 mL of THF and 0.032 g (0.174 mmol) of trans-4,4′azobispyridine (8) in 3 mL of THF were combined and resulted in an orange solution. After heating to 60 °C for 12 h the mixture changed color to black. The product 11 was obtained as a blackcolored powder after removing the solvent, washing with n-hexane, and drying under vacuum. Crystals suitable for X-ray diffraction

were obtained from a solution of THF/n-hexane within several days at 60 °C. The product 11 was isolated in 0.137 g (0.089 mmol, 51% yield). Mp: 150 °C (dec). 1 H NMR (C6D6, 500.1 MHz, 300 K) [ppm]: δ 8.62 (pyridyl-H, not resolved), 7.31 (pyridyl-H, not resolved), 6.24 (pyridyl-H, not resolved), 6.03-6.19 (m, Cp-H, not resolved), 5.98 (pyridyl-H, not resolved), 3.41 (s, tBu), 1.26 (s, tBu). IR (KBr pellet) [cm-1]: V˜ 2962, 2904, 2854, 2527, 2360, 1601, 1488, 1436, 1375, 1316, 1261, 1204, 1093, 1016, 843, 799, 758, 696, 614, 590. MS (CI, isobutane) m/z (relative intensity): 185 (14) [C8H8N4 + H+], 122 (62) [C9H11



Table 3. Crystal Structure Data for Compounds 8, 10, 11, and 13 8

10

11

empirical formula fw color habit

C10H12N4O2 220.24 orange-red needles

C160H196N8Ti4 2422.85 black prisms

C95H127N8Ti4 1572.65 black prisms

cryst dimens, mm cryst syst space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z Dcacld, g cm-3 µ, mm-1 T, K θ range, deg no. of rflns collected no. of indep rflns no. of rflns with I > 2σ(I) abs corr max., min. transmn final R indices (I > 2σ(I))

1.00 × 0.26 × 0.06 monoclinic P21/c 9.9257(19) 4.7020(6) 11.9780(3) 90 96.830(2) 90 555.05(18) 2 1.318 0.096 153(2) 3.43-26.23 5594 1041 [R(int) ) 0.0509] 705 none 0.9943 and 0.9019 R1 ) 0.0297, wR2 ) 0.0675 R1 ) 0.0482, wR2 ) 0.0714

0.50 × 0.32 × 0.26 triclinic P1j 13.8596(7) 19.4391(6) 26.2189(8) 84.213(4) 85.555(5) 84.295(5) 6976.8(5) 2 1.153 0.274 153(2) 1.99-26.12 87 010 25 792 [R(int) ) 0.0679] 15754 numerical 0.9322 and 0.8752 R1 ) 0.0505, wR2 ) 0.1153 R1 ) 0.0912, wR2 ) 0.1278

0.55 × 0.09 × 0.08 monoclinic P21/c 14.9049(6) 35.132(2) 17.5763(6) 90 99.638(5) 90 9073.8(7) 4 1.151 0.387 153(2) 2.05-26.13 77 007 17 098 [R(int) ) 0.1054] 7259 numerical 0.9697 and 0.8155 R1 ) 0.0382, wR2 ) 0.0634 R1 ) 0.1058, wR2 ) 0.0724

R indices (all data)

+ H+], 94 (100) [C5H5N2+]. Anal. Calcd for C92H120N8Ti4 + C6H14: C 72.98, H 8.36, N 6.94. Found: C 71.85, H 8.36, N 6.79. Synthesis of 1,2-Bis(4-pyridyl)hydrazine (13). A solution of 0.1 g (0.28 mmol) of [Cp2Ti(η2-C2(TMS)2] (1) in 5 mL of benzene was combined with a solution of 0.026 g (0.14 mmol) of trans4,4′-azobispyridine (8) in 5 mL of benzene. After the combination the mixture was heated to 60 °C, and after 12 h a black flocculent precipitate occurred. After hot filtration of the precipitate the obtained solution showed a green, slightly pale yellow color. The 1,2-bis(4-pyridyl)hydrazine (13) is obtained in 0.034 g (0.183 mmol, 21% yield). Mp: 240 °C. Crystals suitable for X-ray diffraction can be obtained after several days from the mother liquor. 1 H NMR (C6D6, 500.1 MHz, 300 K) [ppm]: δ 9.21 (d, 2H, 4JHH ) 2.0 Hz, meta-H), 8.72 (d, 2H, 3JHH ) 4.0 Hz, ortho-H), 8.16 (d, 2H, 3JHH ) 8.2 Hz, ortho-H), 7.46 (dd, 2H, 3JHH ) 8.2 Hz, orthoH, 3JHH ) 4.7 Hz, ortho-H) 4.0 (s, 2H, N-H. 13C NMR (C6D6, 125.8 MHz, 300 K) [ppm]: δ 152.69 (C6), 148.28 (C2), 147.89 (C3), 126.15 (C4), 123.81 (C5). MS (CI, isobutane): m/z (relative intensity) 187 (25) [C10H10N4 + H+], 95 (100) [C5H7N2 + H+]. MS (CI high resolution, isobutane): calcd for C10H10N4 + H+ 187.0978, found 187.0948 (100). X-ray Diffraction. Single-crystal experiments were performed on a Stoe IPDS diffractometer with graphite-monochromated Mo KR radiation (λ ) 71.073 pm). The structures were solved by direct phase determination and refined by full-matrix least-squares techniques against F2 with the SHELXL-97 program system.92 Crystallographic details are given in Table 3. (71) Piglosiewicz, I. M.; Beckhaus, R.; Wittstock, G.; Saak, W.; Haase, D. Inorg. Chem. 2007, 46, 7610–7620. (72) Rosenthal, U.; Burlakov, V. V.; Arndt, P.; Baumann, W.; Spannenberg, A. Organometallics 2003, 22, 884–900. (73) Burlakov, V. V.; Polyakov, A. V.; Yanovsky, A. I.; Struchkov, Y. T.; Shur, V. B.; Vol’pin, M. E.; Rosenthal, U.; Goerls, H. J. Organomet. Chem. 1994, 476, 197–206. (74) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D. G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1–S83. (75) Herrmann, W. A.; Denk, M.; Albach, R. W.; Behm, J.; Herdtweck, E. Chem. Ber. 1991, 124, 683–689. (76) Wong, W. Y.; Cheung, S. H.; Lee, S. M.; Leung, S. Y. J. Organomet. Chem. 2000, 596, 36–45. (77) Hayami, S.; Inoue, K.; Osaki, S.; Maeda, Y. Chem. Lett. 1998, 987–988.

13 C16H16N4 264.33 colorless, slightly blue glimmer 0.20 × 0.20 × 0.15 tetragonal I41/a 12.8618(11) 12.8618(11) 17.9097(17) 90° 90° 90° 2962.7(5) 8 1.185 0.074 153(2) 3.17-26.20 14 083 1480 [R(int) ) 0.0682] 834 none 0.9891 and 0.9854 R1 ) 0.0260, wR2 ) 0.0423 R1 ) 0.0634, wR2 ) 0.0465

Acknowledgment. This investigation was financially supported by the Deutsche Forschungsgemeinschaft (DFG SPP 1118). Supporting Information Available: CIF files giving additional crystallographic data for structures 8, 10, 11, and 13 have been deposited with the Cambridge Data Center as supplementary publications nos. CCDC 710359 (8), CCDC 710358 (10), CCDC 710360 (11), and CCDC 710361 (13). Copies of the data can be received free of charges on application to CCDC, 12 Union Road, Cambridge CB21EZ, U.K. (fax: (+44)1223-336-033; e-mail: [email protected]), or via the Internet at http://pubs.acs.org. OM801123K

(78) He, C.; Zhang, B.-G.; Duan, C.-Y.; Li, J.-H.; Meng, Q.-J. Eur. J. Inorg. Chem. 2000, 2549–2554. (79) Wang, J.-Q.; Ren, C.-X.; Jin, G.-X. Eur. J. Inorg. Chem. 2006, 3274–3282. (80) Noro, S.-I.; Kitaura, R.; Kitagawa, S.; Akutagawa, T.; Nakamura, T. Inorg. Chem. 2006, 45, 8990–8997. (81) Noro, S.-I.; Kondo, M.; Ishii, T.; Kitagawa, S.; Matsuzaka, H. J. Chem. Soc., Dalton Trans. 1999, 1569–1574. (82) Kondo, A.; Noguchi, H.; Kajiro, H.; Carlucci, L.; Mercandelli, P.; Proserpio, D. M.; Tanaka, H.; Kaneko, K.; Kanoh, H. J. Phys. Chem. B 2006, 110, 25565–25567. (83) McPherson, A. M.; Fieselmann, B. F.; Lichtenberger, D. L.; McPherson, G. L.; Stucky, G. D. J. Am. Chem. Soc. 1979, 101, 3425– 3430. (84) Gyepes, R.; Witte, P. T.; Horacek, M.; Cisarova, I.; Mach, K. J. Organomet. Chem. 1998, 551, 207–213. (85) Witte, P. T.; Klein, R.; Kooijman, H.; Spek, A. L.; Polasek, M.; Varga, V.; Mach, K. J. Organomet. Chem. 1996, 519, 195–204. (86) Radonovich, L. J.; Eyring, M. W.; Groshens, T. J.; Klabunde, K. J. J. Am. Chem. Soc. 1982, 104, 2816–2819. (87) Schwach, M.; Hausen, H.-D.; Kaim, W. Inorg. Chem. 1999, 38, 2242–2243. (88) Gloeckle, M.; Huebler, K.; Kuemmerer, H.-J.; Denninger, G.; Kaim, W. Inorg. Chem. 2001, 40, 2263–2269. (89) Kaim, W. J. Am. Chem. Soc. 1983, 105, 707–713. (90) Kraft, S. Ph.D. Thesis, University of Oldenburg, 2004, pp 1181. (91) Pestana, D. C.; Power, P. P. Inorg. Chem. 1991, 30, 528–535. (92) Sheldrick, G. M. SHELXL-97. A Program for Refining Crystal Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.

Reactions of Low-Valent Titanocene(II) Fragments with trans-4,4

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