Tetrasilacyclobutadiene and Cyclobutadiene Tricarbonylruthenium

Jan 30, 2009 - Synopsis. The ruthenium tricarbonyl complexes [η4-(tBu2MeSi)4Si4]Ru(CO)3 2 and [η4-(Me3Si)4C4]Ru(CO)3 4 were synthesized, and the ...
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Organometallics 2009, 28, 1248–1251

Tetrasilacyclobutadiene and Cyclobutadiene Tricarbonylruthenium Complexes: [η4-(tBu2MeSi)4Si4]Ru(CO)3 and [η4-(Me3Si)4C4]Ru(CO)3 Kazunori Takanashi, Vladimir Ya. Lee, and Akira Sekiguchi* Department of Chemistry, Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan ReceiVed NoVember 25, 2008

The novel ruthenium tricarbonyl complexes (η4-tetrasilacyclobutadiene)tricarbonylruthenium [η4( Bu2MeSi)4Si4]Ru(CO)3 2 and (η4-cyclobutadiene)tricarbonylruthenium [η4-(Me3Si)4C4]Ru(CO)3 4 were synthesized by the reaction of either the dipotassium salt of a tetrasilacyclobutadiene dianion derivative [(tBu2MeSi)4Si4]2-•2K+ 1 or the dilithium salt of a cyclobutadiene dianion derivative [(Me3Si)4C4]2-•2Li+ 3 with [Ru(CO)3Cl2]2. The spectral and structural characteristics of both 2 and 4 were compared with each other to reveal the general tendencies of the influence of the nature of skeletal atoms and transition metal on the geometry of the four-membered ring ligand and its electronic properties. t

Introduction Cyclobutadiene transition metal complexes, in which the antiaromatic 4π-electron cyclobutadiene might be stabilized by coordination to a transition metal fragment, were originally theoretically predicted by Longuet-Higgins and Orgel in 19561 and experimentally realized for the first time in the form of the tricarbonyliron complex (η4-Ph4C4)Fe(CO)3 by Hu¨bel and Braye in 1959.2 The parent unsubstituted (η4-H4C4)Fe(CO)3 was prepared by Pettit and co-workers in 1965,3 who later extensively developed the chemistry of such 18-electron aromatic complex. Since then, the chemistry of iron cyclobutadiene complexes has been very comprehensively studied with many compounds of this type being synthesized and structurally characterized.4 In marked contrast, the chemistry of the cyclobutadiene complexes of ruthenium, which is a heavier neighbor of iron in group 8, has been studied far less extensively. Although the synthesis of the first ruthenium cyclobutadiene complex (η4-H4C4)Ru(CO)3 by Pettit and co-workers was reported in 1967,5 followed by preparation of its tetraphenyl derivative (η4-Ph4C4)Ru(CO)3,6 information about the structure and reactivity of the ruthenium cyclobutadiene complexes remained unavailable until the middle of the 1980s.7 It should be noted that the most common method for preparation of such complexes is the cyclodimerization of alkynes catalyzed by ruthenium complexes.4a,b However, the structural characterization of the tricarbonyl ruthenium complexes featuring a cyclobutadiene ligand has not yet been accomplished. The same is true for the cyclobutadiene ruthenium complexes incorporating heavy group 14 elements in the four-membered ring, which are * To whom correspondence should be addressed. Phone: +81-29-8534314. Fax: +81-29-853-4314. E-mail: [email protected]. (1) Longuet-Higgins, H. C.; Orgel, L. E. J. Chem. Soc. 1956, 1969. (2) (a) Hu¨bel, W.; Braye, E. H.; Clauss, A.; Weiss, E.; Kru¨erke, U.; Brown, D. A.; King, G. S. D.; Hoogzand, C. J. Inorg. Nucl. Chem. 1959, 9, 204. (b) Hu¨bel, W.; Braye, E. H. J. Inorg. Nucl. Chem. 1959, 10, 250. (3) Emerson, G. F.; Watts, L.; Pettit, R. J. Am. Chem. Soc. 1965, 87, 131. (4) (a) Efraty, A. Chem. ReV. 1977, 77, 691. (b) Baker, P. K.; Silgram, H. Trends Organomet. Chem. 1999, 22, 21. (c) Seyferth, D. Organometallics 2003, 22, 2. (5) Amiet, R. G.; Reeves, P. C.; Pettit, R. J. Chem. Soc., Chem. Commun. 1967, 1208. (6) Sears, C. T., Jr.; Stone, F. G. A. J. Organomet. Chem. 1968, 11, 644.

not known to date. Meanwhile, recently, we have developed a novel effective method for the synthesis of heavy cyclobutadiene transition metal complexes based upon utilization of the tetrasilacyclobutadiene dianion as a convenient source for the tetrasilacyclobutadiene ligand. Accordingly, tetrasilacyclobutadiene carbonyl complexes of iron and cobalt, [η4-(tBu2MeSi)4Si4]Fe(CO)38 and [(η4-(tBu2MeSi)4Si4)Co(CO)2]-,9 as well as sandwich complexes of the type [η4-(tBu2MeSi)4Si3E]CoCp10 (E ) Si, Ge), were prepared as the first examples of transition metal complexes featuring an all heavy group 14 elements-cyclic polyene ligand.11 In the present paper, we wish to report the synthesis, structural characterization, and comparative study of novel ruthenium tricarbonyl complexes, featuring either η4tetrasilacyclobutadiene or η4-cyclobutadiene ligands.

Results and Discussion The target [tetrakis(di-tert-butylmethylsilyl)tetrasilacyclobutadiene]tricarbonylruthenium complex [η4-(tBu2MeSi)4Si4]Ru(CO)3 2 was readily available by the reaction of the dipotassium salt of a tetrasilacyclobutadiene dianion derivative [(tBu2Me(7) (a) Connelly, N. G.; Kelly, R. L.; Whiteley, M. W. J. Chem. Soc., Dalton Trans. 1981, 34. (b) Crocker, M.; Green, M.; Orpen, A. G.; Thomas, D. M. J. Chem. Soc., Chem. Commun. 1984, 1141. (c) Crocker, M.; Green, M.; Orpen, A. G.; Neumann, H.-P.; Schaverien, C. J. J. Chem. Soc., Chem. Commun. 1984, 1351. (d) Crocker, M.; Froom, S. F. T.; Green, M.; Nagle, K. R.; Orpen, A. G.; Thomas, D. M. J. Chem. Soc., Dalton Trans. 1987, 2803. (e) Crocker, M.; Green, M.; Nagle, K. R.; Williams, D. J. J. Chem. Soc., Dalton Trans. 1990, 2571. (f) Campion, B. K.; Heyn, R. H.; Tilley, T. D. Organometallics 1990, 9, 1106. (g) Bailey, P. J.; Blake, A. J.; Dyson, P. J.; Ingham, S. L.; Johnson, B. F. G. J. Chem. Soc., Chem. Commun. 1994, 2233. (h) Dyson, P. J.; Ingham, S. L.; Johnson, B. F. G.; McGrady, J. E.; Mingos, D. M. P.; Blake, A. J. J. Chem. Soc., Dalton Trans. 1995, 2749. (i) Wesendrup, R.; Schwarz, H. Organometallics 1997, 16, 461. (j) Yamazaki, S.; Taira, Z. J. Organomet. Chem. 1999, 578, 61. (k) Le Paih, J.; Derien, S.; Bruneau, C.; Demerseman, B.; Toupet, L.; Dixneuf, P. H. Angew. Chem., Int. Ed. 2001, 40, 2912. (l) Ruba, E.; Mereiter, K.; Schmid, R.; Sapunov, V. N.; Kirchner, K.; Schottenberger, H.; Calhorda, M. J.; Veiros, L. F. Chem.sEur. J. 2002, 8, 3948. (m) Yamamoto, Y.; Arakawa, T.; Itoh, K. Organometallics 2004, 23, 3610. (8) Takanashi, K.; Lee, V. Ya.; Ichinohe, M.; Sekiguchi, A. Angew. Chem., Int. Ed. 2006, 45, 3269. (9) Takanashi, K.; Lee, V. Ya.; Matsuno, T.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2005, 127, 5768. (10) Takanashi, K.; Lee, V. Ya.; Ichinohe, M.; Sekiguchi, A. Eur. J. Inorg. Chem. 2007, 5471.

10.1021/om801128h CCC: $40.75  2009 American Chemical Society Publication on Web 01/30/2009

[η4-(tBu2MeSi)4Si4]Ru(CO)3 and [η4-(Me3Si)4C4]Ru(CO)3

Organometallics, Vol. 28, No. 4, 2009 1249

Scheme 1. Synthesis of [η4-(tBu2MeSi)4Si4]Ru(CO)3 2

Si)4Si4]2-•2K+, 12-•2K+,12 with 0.5 equiv of [Ru(CO)3Cl2]2 in THF (Scheme 1). Complex 2 was isolated from the reaction mixture by recrystallization from hexane as highly air- and moisture-sensitive, but thermally rather stable, pale-yellow crystals in 67% yield. The tetrahaptocoordination of the Si4 ligand to the Ru atom was clearly manifested by the NMR spectral characteristics of the Si4 ring. Thus, both 1H and 13C NMR spectra of 2 displayed only one set of signals for the tBu2MeSi substituents. The resonances of the skeletal Si atoms were observed at -33.4 ppm, clearly beyond the low-field range diagnostic of doubly bonded Si atoms13 and even further shifted upfield compared with those of the isostructural iron complex [η4-(tBu2MeSi)4Si4]Fe(CO)3 (-15.8 ppm).8 The CO carbon atoms in 2 are also shielded compared with those in [η4-(tBu2MeSi)4Si4]Fe(CO)3: 208.6 versus 221.1 ppm. This trend can be rationalized in terms of the higher π-basicity of Fe versus Ru atoms because of the difference in the electronegativities of the metals (Fe: 1.8, Ru: 2.2), resulting in different predominant formulations of the transition metal-carbonyl ligand bond, as expressed in the following resonance extremes: FedCdO versus Ru--CtO+. Such a tendency would also be responsible for the characteristic decrease in the values of the CO stretching vibration frequencies on going from Ru complex 2 to Fe complex [η4-(tBu2MeSi)4Si4]Fe(CO)3: 1946 and 1998 cm-1 versus 1922 and 1973 cm-1, reflecting the strengthening of the transition metal-carbon bond because of an increase in the degree of π-backdonation from the transition metal to the carbonyl ligands (Fe > Ru). The crystal structure of 2 was determined by X-ray crystallography, which revealed the presence of two crystallographically independent molecules in the unit cell of 2. Because both molecules are structurally very similar, below we will discuss the structural features of only one of them (Figure 1 and Table 1). (11) Several heavy analogues of the cyclic polyenes have been recently utilized as ligands in transition metal complexes. For heavy cyclopentadienyl ligands, see: (a) Freeman, W. P.; Tilley, T. D.; Rheingold, A. L.; Ostrander, R. L. Angew. Chem., Int. Ed. Engl. 1993, 32, 1744. (b) Freeman, W. P.; Tilley, T. D.; Rheingold, A. L. J. Am. Chem. Soc. 1994, 116, 8428. (c) Dysard, J. M.; Tilley, T. D. J. Am. Chem. Soc. 1998, 120, 8245. (d) Dysard, J. M.; Tilley, T. D. J. Am. Chem. Soc. 2000, 122, 3097. (e) Dysard, J. M.; Tilley, T. D. Organometallics 2000, 19, 2671. (f) Dysard, J. M.; Tilley, T. D. Organometallics 2000, 19, 4720. (g) Freeman, W. P.; Dysard, J. M.; Tilley, T. D.; Rheingold, A. L. Organometallics 2002, 21, 1734. For heavy arene ligands, see: (h) Nakata, N.; Takeda, N.; Tokitoh, N. Angew. Chem., Int. Ed. 2003, 42, 115. (i) Shinohara, A.; Takeda, N.; Sasamori, T.; Matsumoto, T.; Tokitoh, N. Organometallics 2005, 24, 6141. (j) Mizuhata, Y.; Sasamori, T.; Takeda, N.; Tokitoh, N. J. Am. Chem. Soc., 2006, 128, 1050. (k) Lee, V. Ya.; Kato, R.; Sekiguchi, A.; Krapp, A.; Frenking, G. J. Am. Chem. Soc. 2007, 129, 10340. (l) Tokitoh, N.; Nakata, N.; Shinohara, A.; Takeda, N.; Sasamori, S. Chem.sEur. J. 2007, 13, 1858. For heavy cyclobutadiene ligand, see: (m) Kon, Y.; Sakamoto, K.; Kabuto, C.; Kira, M. Organometallics 2005, 24, 1407. Recent review: (n) Lee, V. Ya.; Sekiguchi, A. Angew. Chem., Int. Ed. 2007, 46, 6596. (12) Lee, V. Ya.; Takanashi, K.; Matsuno, T.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2004, 126, 4758. (13) The 29Si NMR chemical shifts of the sp2-Si atoms incorporated in the four-membered ring lie in the range of 141.4-182.7 ppm: (a) Kira, M.; Iwamoto, T.; Kabuto, C. J. Am. Chem. Soc. 1996, 118, 10303. (b) Wiberg, N.; Auer, H.; No¨th, H.; Knizek, J.; Polborn, K. Angew. Chem., Int. Ed. 1998, 37, 2869. (c) Sekiguchi, A.; Matsuno, T.; Ichinohe, M. J. Am. Chem. Soc. 2001, 123, 12436. (d) See ref 8. (e) See ref 12.

The most important crystal structure feature of 2 is the tetrahaptocoordination of the Si4 ring to the Ru atom. Accordingly, the tetrasilacyclobutadiene ring is nearly planar with a negligible folding angle of 1.38°.14 The distances between the skeletal Si atoms and the Ru center in 2 range from 2.5977(9) to 2.6384(9) Å.15 All skeletal Si-Si bond lengths are intermediate between those of typical Si-Si single (av. 2.34 Å)16 and SidSi double (av. 2.20 Å)17 bonds; however, they are nonequivalent, with the Si3-Si4 bond (2.2676(12) Å) being a bit shorter than the other skeletal bonds (Si1-Si2 ) 2.2878(12), Si2-Si3 ) 2.2853(12), Si1-Si4 ) 2.2803(12) Å). This

Figure 1. ORTEP drawing of 2 (30% thermal ellipsoids). Hydrogen atoms are omitted for clarity. Selected bond lengths (Å): Si1-Si2 ) 2.2878(12), Si2-Si3 ) 2.2853(12), Si3-Si4 ) 2.2676(12), Si1-Si4 ) 2.2803(12), Si1-Ru1 ) 2.6002(9), Si2-Ru1 ) 2.6112(9), Si3-Ru1 ) 2.5977(9), Si4-Ru1 ) 2.6384(9), Si1-Si5 ) 2.3768(11), Si2-Si6 ) 2.3850(13), Si3-Si7 ) 2.3763(11), Si4-Si8 ) 2.4015(12). Selected bond angles (deg): Si4-Si1-Si2 ) 90.55(4), Si1-Si2-Si3 ) 88.94(4), Si2-Si3-Si4 ) 90.94(4), Si1-Si4-Si3 ) 89.56(4). Dihedral angle (deg): Si1-Si2-Si3/ Si1-Si3-Si4 ) 1.17(2). Table 1. Selected Bond Lengths and Bond Angles (Experimental and Calculated) for Tetrasilacyclobutadiene (2 and 2′) and Cyclobutadiene (4 and 4′) Tricarbonyl Ruthenium Complexesa

a

2: [η4-(tBu2MeSi)4Si4]Ru(CO)3; 2′: [η4-(Me3Si)4Si4]Ru(CO)3; 4: [η4(Me3Si)4C4]Ru(CO)3; 4′: [η4-(Me3Si)4C4]Ru(CO)3.

1250 Organometallics, Vol. 28, No. 4, 2009

Takanashi et al.

Figure 2. Schematic representation of the interaction between the R4E4 (E ) Si, C) p- and the transition metal (Fe or Ru) d-orbitals in [η4-(Me3Si)4Si4]Fe(CO)3, 2′ and 4′.

phenomenon can be explained by taking into account the staggered conformation of the three carbonyl ligands toward the Si4 ring, in which only the Si3-Si4 bond (the shortest skeletal bond) is not superimposed with a CO group (see below in Table 1). The bulkiness of the tBu2MeSi substituents can cause such a staggered conformation because the optimized structure of the model complex [η4-(Me3Si)4Si4]Ru(CO)3 2′ with the less voluminous Me3Si substituents showed an eclipsed conformation (Table 1).18 The methyl groups of the bulky t Bu2MeSi substituents in 2 are uniformly directed toward the carbonyl groups to minimize the possible steric interactions between the bulky tBu groups of the tBu2MeSi substituents and the carbonyl ligands. The geometry of the Si4 ring in ruthenium complex 2 is slightly different from that of the previously reported iron complex [η4-(tBu2MeSi)4Si4]Fe(CO)3.8 Namely, the endocyclic Si-Si bonds in 2 (av. 2.2803(12) Å) are slightly longer than those in [η4-(tBu2MeSi)4Si4]Fe(CO)3 (av. 2.2714(7) Å), a tendency that was well reproduced computationally: 2.274 Å (av.) in 2′ versus 2.262 Å (av.) in [η4-(Me3Si)4Si4]Fe(CO)3. One of the obvious reasons for such a tendency would be the difference in the size of the transition metal bonding orbitals: bigger 4d(Ru) versus smaller 3d(Fe). This would also be responsible for the greater degree of pyramidalization of the skeletal Si atoms in [η4-(tBu2MeSi)4Si4]Fe(CO)3 compared with that in 2. Thus, the silyl substituents in the model iron complex [η4-(Me3Si)4Si4]Fe(CO)3 are more bent toward the transition metal (av. 0.15 Å away from the Si4 mean plane) than those in the model ruthenium complex 2′ (av. 0.09 Å away from the Si4 mean plane). Such an arrangement of substituents in both 2′ and [η4-(Me3Si)4Si4]Fe(CO)3 provides the optimum orientation of p-orbitals on Si atoms for their maximum overlap with either Fe or Ru d-orbitals (Figure 2).19 The carbon analogue of 2, [tetrakis(trimethylsilyl)cyclobutadiene]tricarbonylruthenium [η4-(Me3Si)4C4]Ru(CO)3 4, was synthesized in a similar way, that is, by the treatment of the (14) Planarity of the cyclobutadiene ligand is a general structural feature of cyclobutadiene transition metal complexes. See ref 4a. (15) The Si-Ru bond distance of the only reported ruthenium complex possessing a heavy cyclic polyene ligand, {(η5-Me5C5)Ru(H)[η5-Me4C4SiSi(SiMe3)3]}+•[Ph4B]-, is 2.441(3) Å. See ref 11b. (16) Kaftory, M.; Kapon, M.; Botoshansky, M. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, UK, 1998; Vol. 2, Part 1, Chapter 5. (17) (a) Okazaki, R.; West, R. AdV. Organomet. Chem. 1996, 39, 231. (b) Power, P. P. Chem. ReV. 1999, 99, 3463. (c) Weidenbruch, M. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, UK, 2001; Vol. 3, Chapter 5. (18) All theoretical calculations were performed with the GAUSSIAN 98 program package at the B3PW91/6-31G(d) level for H, C, O, Si, and Fe atoms and the B3PW91/3-21G(d) level for the Ru atom. (19) In complex 2, the two tBu2MeSi groups occupy alternating up and down positions relative to the Si4 mean plane to minimize the steric hindrances: those attached to the Si2 (0.05 Å) and Si4 (0.02 Å) atoms are bent toward the Ru center, whereas those attached to the Si1 (0.10 Å) and Si3 (0.51 Å) atoms are bent away from the Ru atom.

Scheme 2. Synthesis of [η4-(Me3Si)4C4]Ru(CO)3 4

dilithium salt of a cyclobutadiene dianion derivative [(Me3Si)4C4]2-•2Li+, 32-•2Li+,20 with 0.5 equiv of [Ru(CO)3Cl2]2 in THF (Scheme 2). The Ru complex 4 was isolated by recrystallization from hexane as thermally stable but moderately moisture-sensitive pale yellow crystals in 70% yield. The skeletal and carbonyl carbon atoms resonated at 89.5 and 202.3 ppm, respectively. The CO stretching vibrations in the IR spectrum of 4 were observed at higher frequencies (1980 and 2046 cm-1) than those of tetrasilacyclobutadiene ruthenium complex 2 (1946 and 1998 cm-1), indicating the significantly stronger electron-donating ability of tetrasilacyclobutadiene compared with that of cyclobutadiene because of the higherlying HOMOs of the former ligand. In other words, the tetrasilacyclobutadiene complex 2 has a more important contribution from the RudCdO resonance form, whereas the cyclobutadiene complex 4 has a preference for the resonance structure Ru--CtO+. This was also manifested in the slight low-field shift of the carbonyl group resonance in the 13C NMR spectrum of 2 (208.6 ppm) compared with that of 4 (202.3 ppm). The crystal structure of 4 is shown in Figure 3. One of the skeletal C-C bonds (C2-C3 ) 1.501(6) Å) is stretched compared with other C-C bonds (C1-C2 ) 1.480(6), C3-C4 ) 1.474(6), C1-C4 ) 1.469(6) Å) (Table 1). The distances from the skeletal C atoms to the Ru atom of 2.177(5)-2.207(4) Å are within the normal range of 2.1042.375 Å.7 A comparison of the orientation of the silyl substituents in the tetrasilacyclobutadiene Ru complex 2 and the cyclobutadiene Ru complex 4 revealed an interesting regularity; the tBu2MeSi substituents are more bent toward the Ru center in 2 (av. 0.09 Å), whereas the Me3Si substituents are more bent away from the Ru center in 4 (av. 0.37 Å).19 Such a tendency, supported by the theoretical calculations, can be explained in terms of the difference in the size of the Si4 and C4 rings, which dictates the optimum orientation of the silyl substituents to achieve the best matching of the interacting p(cyclobutadiene ligand)-4d(Ru) orbitals (Figure 2).

Conclusion We have presented the novel ruthenium tricarbonyl complexes 2 and 4, featuring either tetrasilacyclobutadiene or cyclobuta(20) (a) Sekiguchi, A.; Matsuo, T.; Watanabe, H. J. Am. Chem. Soc. 2000, 122, 5652. (b) Matsuo, T.; Sekiguchi, A. Bull. Chem. Soc. Jpn. 2004, 77, 211. (c) Sekiguchi, A.; Matsuo, T. Synlett 2006, 2683.

[η4-(tBu2MeSi)4Si4]Ru(CO)3 and [η4-(Me3Si)4C4]Ru(CO)3

Figure 3. ORTEP drawing of 4 (30% thermal ellipsoids). Hydrogen atoms are omitted for clarity. Selected bond lengths (Å): C1-C2 ) 1.480(6), C2-C3 ) 1.501(6), C3-C4 ) 1.474(6), C1-C4 ) 1.469(6), C1-Ru1 ) 2.199(4), C2-Ru1 ) 2.177(5), C3-Ru1 ) 2.179(5), C4-Ru1 ) 2.207(4), C1-Si1 ) 1.891(5), C2-Si2 ) 1.875(5), C3-Si3 ) 1.888(5), C4-Si4 ) 1.887(5). Selected bond angles (deg): C4-C1-C2 ) 90.6(4), C1-C2-C3 ) 89.1(4), C2-C3-C4 ) 89.6(4), C1-C4-C3 ) 90.6(4). Dihedral angle (deg): C1-C2-C3/C1-C3-C4 ) 0.34(2).

diene ligands, which were synthesized by the reaction of either the dipotassium salt of a tetrasilacyclobutadiene dianion derivative 12-•2K+ or the dilithium salt of a cyclobutadiene dianion derivative 32-•2Li+ with 0.5 equiv of [Ru(CO)3Cl2]2 in THF. The spectral and structural characteristics of both 2 and 4 were compared with each other, and also with those of the previously reported iron complex [η4-(tBu2MeSi)4Si4]Fe(CO)3, to reveal the general trends of the influence of the nature of skeletal atoms and transition metal on the geometry of the four-membered ring ligand and its electronic properties.

Experimental Section General Procedures. All experiments were performed using high-vacuum line techniques or in an argon atmosphere using a MBRAUN MB 150B-G glovebox. All solvents were dried and degassed over a potassium mirror in vacuum prior to use. NMR spectra were recorded on Bruker AC-300FT NMR (1H NMR at 300.1 MHz; 13C NMR at 75.5 MHz; 29Si NMR at 59.6 MHz) and AV-400FT NMR (1H NMR at 400 MHz; 13C NMR at 100.6 MHz; 29 Si NMR at 79.5 MHz) spectrometers. IR spectra were measured on a JASCO FT/IR-410 spectrophotometer. UV-vis spectra were recorded on a Shimadzu UV-3150 UV-vis spectrophotometer. Synthesis of [η4-(tBu2MeSi)4Si4]Ru(CO)3 (2). The dipotassium salt of a tetrasilacyclobutadiene dianion 12-•2K+ (83 mg, 0.075 mmol) and [RuCl2(CO)3]2 (20 mg, 0.039 mmol) were placed in a reaction tube with a magnetic stirring bar. Dry oxygen-free THF (2 mL) was introduced into this tube by vacuum transfer, and the reaction mixture was stirred at room temperature for 1 h to form an orange solution. Solvent was evaporated in vacuum, and dry hexane was introduced. After inorganic salt was filtered off, the residue was recrystallized from hexane to give 2 (47 mg, 67%) as pale yellow crystals. Mp 135-137 °C. 1H NMR (C6D6, δ): 0.34

Organometallics, Vol. 28, No. 4, 2009 1251 (s, 12 H), 1.18 (s, 72 H). 13C NMR (C6D6, δ): -3.4, 21.8, 30.3, 208.6 (CO). 29Si NMR (C6D6, δ): -33.4 (skeletal Si), 23.8. UV-vis (THF): λmax/nm () 330 (7500), 550 (900). IR (KBr, cm-1): 1946, 1998 (νCO). Anal. Calcd for C39H84O3RuSi8: C, 50.54; H, 9.13. Found: C, 50.33; H, 9.04. Synthesis of [η4-(Me3Si)4C4]Ru(CO)3 (4). The dilithium salt of a cyclobutadiene dianion 32-•2Li+ (200 mg, 0.401 mmol) and [RuCl2(CO)3]2 (105 mg, 0.205 mmol) were placed in a reaction tube with a magnetic stirring bar. Dry oxygen-free THF (2 mL) was introduced into this tube by vacuum transfer, and the reaction mixture was stirred at room temperature for 1 h to form an orange solution. Solvent was evaporated in vacuum, and dry hexane was introduced. After inorganic salt was filtered off, the residue was recrystallized to give 4 (147 mg, 70%) as pale yellow crystals. Mp 107 °C (sublimed). 1H NMR (C6D6, δ): 0.20 (s, 36 H). 13C NMR (C6D6, δ): -2.7 (Me3Si), 89.5 (skeletal C), 202.3 (CO). 29Si NMR (C6D6, δ): -8.3. UV-vis (THF): λmax/nm () 257 (6600). IR (KBr, cm-1): 1980, 2046 (νCO). Anal. Calcd for C19H36O3RuSi4: C, 43.39; H, 9.60. Found: C, 43.50; H, 6.81. X-ray Crystal Analyses. Single crystals suitable for X-ray diffraction study were grown from dimethoxyethane for 2 and hexane for 4. Diffraction data were collected at 120 K on a MacScience DIP2030 Image Plate diffractometer with a rotating anode (50 kV, 90 mA) employing graphite-monochromatized MoKR radiation (λ ) 0.71070 Å). The structures were solved by the direct method using SIR-92 program21 and refined by the full-matrix least-squares method by SHELXL-97 program.22 Crystal data for 2 at 120 K: MF ) C39H84O3RuSi8, MW ) 926.85, triclinic, P1j, a ) 15.1370(7), b ) 17.5450(7), c ) 19.5900(5) Å, R ) 93.585(2), β ) 90.930(3), γ ) 92.055(2)o, V ) 5188.3(3) Å3, Z ) 4, Dcalcd ) 1.187 g · cm-3. The final R factor was 0.0470 for 16 958 reflections with Io > 2σ(Io) (Rw ) 0.1255 for all data, 22 861 reflections), GOF ) 1.019. Crystal data for 4 at 120 K: MF ) C19H36O3RuSi4, MW ) 525.91, monoclinic, P21/c, a ) 9.5530(3), b ) 16.1940(6), c ) 19.2560(5) Å, β ) 116.648(2)°, V ) 2662.50(15) Å3, Z ) 4, Dcalcd ) 1.312 g · cm-3. The final R factor was 0.0595 for 5509 reflections with Io > 2σ(Io) (Rw ) 0.1746 for all data, 6267 reflections), GOF ) 1.158.

Acknowledgment. This work was financially supported by the Grants-in-Aid for Scientific Research Program (Nos. 18064004, 19105001, 19020012, 19022004, 20038006) from the Ministry of Education, Science, Sports, and Culture of Japan. Supporting Information Available: Tables of crystallographic data including atomic positional and thermal parameters for 2 and 4. This material is available free of charge via the Internet at http://pubs.acs.org. OM801128H (21) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435. (22) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure Refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997.