Organometallics 2011, 30, 191–194 DOI: 10.1021/om100884y
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Synthesis and Structure of [Cp*Ru(CO)2(μ-H){RuFe3(CO)9}]: An Unusual Mixed-Metal Tetrahedral Cluster with an Exopolyhedral Metal Fragment K. Geetharani, Shubhankar Kumar Bose, and Sundargopal Ghosh* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India Received September 13, 2010 Summary: Mild pyrolysis of arachno-(Cp*RuCO)2B2H6, 1 (Cp* = η5-C5Me5), with Fe2(CO)9 leads to the isolation of a mixed-metal tetrahedral cluster, [Cp*Ru(CO)2(μ-H){RuFe3(CO)9}], 2, in which [Cp*RuH(CO)2] is attached to the tetrahedral cluster in an exopolyhedral manner. Cluster 2 can be viewed as 52-cve (cluster valence electron) tetrahedral cluster, in which the Ru fragment occupies the low connectivity cluster vertexes and participates in an exopolyhedral hydrogenbridged ruthenium-ruthenium double bond. Cluster 2 has been characterized by mass spectrometry, IR, and 1H, 11B, and 13C NMR spectroscopy, and the geometric structure was unequivocally established by crystallographic analysis.
Introduction Mixed-metal clusters1-4 have attracted considerable interest because of their unique reactivity and catalytic properties and their role as molecular precursors to metal particles and novel heterogeneous catalysts.5 They have also been shown to be good precursors for the preparation of a variety of supported bimetallic nanoparticles.6 Even though numerous heterometallic cluster complexes have been prepared,3 their number and combinations of metals are not satisfactory, particularly from the viewpoint of the above-mentioned applications. As a result, there have been major efforts to
prepare bimetallic cluster complexes containing group 8 transition metals.7-11 On the other hand, polyhedral boroncontaining compounds are shown to be intimately connected with organometallic and other p-block transition-element compounds.12,13 However, until recently, limitations of synthetic methods have precluded systematic study of metallaboranes, and as a result, their reactivity has remained largely unexplored relative to that of organometallic compounds.14 Compared to organometallic chemistry, less is known about the role of metals in borane chemistry;13 however that which is known, for example, Suzuki coupling,15 functionalization of hydrocarbyl groups16,17 and boranes,18 and oxidative coupling of carboranes,19 is important. Transition metal carbonyl compounds, for example, Fe2(CO)9 or Co2(CO)8, have received considerable attention in metallaborane chemistry in connection with their potential as versatile reagents in metal cluster building reactions.2,20-24 For example, the reaction of Fe2(CO)9 with [(Cp*Ru)2B6H12] results in the formation of [Fe2(CO)6(Cp*RuCO)(Cp*Ru)B6H10].22 In a similar fashion, reaction of Co2(CO)8 with [(Cp*ReH2)2B4H4] yielded [(Cp*Re)2(μ-η6:η6-B4H4Co2(CO)5)].25
*Corresponding author. Fax: (þ91) 44 2257 4202. E-mail:
[email protected]. (1) Waterman, S. M.; Lucas, N. T.; Humphrey, M. G. In Advances in Organometallic Chemistry; Hill, A., West, R., Eds.; Academic Press: New York, 2000; Vol. 46, p 47. (2) Shriver, D. F.; Kaesz, H. D.; Adams, R. D. The Chemistry of Metal Cluster Complexes; VCH: New York, 1990. (3) Braunstein, P.; Rose, J. In Metal Clusters in Chemistry; Braunstein, P., Raithby, P. R., Oro, L. A., Eds.; Wiley-VCH: Weinheim, Germany, 1999; Vol. 2, pp 616-677. (4) Collman, J. P.; Boulatov, R. Angew. Chem, Int. Ed. 2002, 41, 3948–3961. (5) Braunstein, P.; Rose, J. In Catalysis by Di- and Polynuclear Metal Cluster Complexes: Heterometallic Clusters for Heterogeneous Catalysis; Adams, R. D., Cotton, F. A., Eds.; Wiley-VCH: New York, 1998; p 443. (6) (a) Johnson, B. F. G. Coord. Chem. Rev. 1999, 192, 1269–1285. (b) Nashner, M. S.; Frenkel, A. I.; Somerville, D.; Hills, C. W.; Shapley, J. R.; Nuzzo, R. G. J. Am. Chem. Soc. 1998, 120, 8093–8101. (7) Braunstein, P.; Graiff, C.; Massera, C.; Predieri, G.; Rose, J.; Tiripicchio, A. Inorg. Chem. 2002, 41, 1372–1382. (8) Eadie, D. T.; Holden, H. D.; Johnson, B. F. G.; Lewis, J. J. Chem. Soc., Dalton Trans. 1984, 301–304. (9) Gladfelter, W. L.; Geoffroy, G. L. Inorg. Chem. 1980, 19, 2579– 2585. (10) Geoffroy, G. L.; Gladfelter, W. L. J. Am. Chem. Soc. 1977, 99, 7565–7573. (11) Nakajima, T.; Shimizu, I.; Kobayashi, K.; Wakatsuki, Y. Organometallics 1998, 17, 262–269.
(12) The Borane, Carborane, Carbocation Continuum; Casanova, J., Ed.; Wiley: New York, 1998. (13) Inorganometallic Chemistry; Fehlner, T. P., Ed.; Plenum: New York, 1992. (14) (a) Gilbert, K. B.; Boocock, S. K.; Shore, S. G. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford, 1982; Vol. 6, pp 879-945. (b) Kennedy, J. D. Prog. Inorg. Chem. 1984, 32, 519–670. (c) Kennedy, J. D. Prog. Inorg. Chem. 1986, 34, 211–434. (d) Barton, L.; Srivastava, D. K. In Comprehensive Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Eds.; Pergamon: Oxford, 1995; Vol. 1, pp 275-372. (15) Suzuki, A. Organoboranes in Organic Syntheses; Hokkaido Univ.: Sapporo, 2004. (16) Chen, H.; Schlecht, S.; Semple, T. C.; Hartwig, J. F. Science 2000, 287, 1995–1997. (17) Duan, Z.; Hampden-Smith, M. J.; Sylwester, A. P. Chem. Mater. 1992, 4, 1146–1148. (18) Sneddon, L. G.; Pender, M. J.; Forsthoefel, K. M.; Kusari, U.; Wei, X. J. Eur. Ceram. Soc. 2005, 25, 91–97. (19) Grimes, R. N. In Current Topics in the Chemistry of Boron; Kabalka, G. W., Ed.; Royal Society of Chemistry: Cambridge, UK, 1994; p 269. (20) Housecroft, C. E. In Boranes and Metalloboranes; Ellis Horwood: Chichester, UK, 1990. (21) Ghosh, S.; Beatty, A. M.; Fehlner, T. P. Angew. Chem., Int. Ed. 2003, 42, 4678–4680. (22) Ghosh, S.; Fehlner, T. P.; Noll, B. C. Chem. Commun. 2005, 3080–3082. (23) Lei, X.; Shang, M.; Fehlner, T. P. Organometallics 1998, 17, 1558–1563. (24) Lei, X.; Shang, M.; Fehlner, T. P. Chem. Commun. 1999, 933– 934. (25) Ghosh, S.; Shang, M.; Fehlner, T. P. J. Am. Chem. Soc. 1999, 121, 7451–7452.
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Scheme 1. Synthesis of Mixed-Metal Tetrahedral Cluster 2 and Triply Bridged Borylene Complexes
As part of our interest in synthesizing metallaboranes containing a range of early and late transition metals, we have recently described the synthesis and structure of diruthenatetraborane arachno-[(Cp*RuCO)2B2H6], 1, by the reaction of [1,2-(Cp*RuH)2B3H7] and monometalcarbonyl fragment [Mo(CO)3(CH3CN)3] in good yield.26 Consequently, upon availability of 1, the chemistry is elaborated by the use of a cluster expansion reaction with Fe2(CO)9, which yielded 2, in parallel with the formation of some triply bridged borylene complexes, reported elsewhere.26 In this vein, we now report the structure and bonding of the unusual mixed-metal tetrahedral cluster 2.
Results and Discussion Reaction of 1 with Fe2(CO)9. When an excess of Fe2(CO)9 was thermolyzed in the presence of 1 at 75 °C for a prolonged time (24 h), 2 was produced along with the formation of borylene complexes [(μ3-BH)(Cp*RuCO)2(μ-CO){Fe(CO)3}], [{(μ3-BH)(Cp*Ru)(μ-CO)}2Fe2(CO)5], and [{(μ3-BH)(Cp*Ru)Fe(CO)3}2(μ-CO)] (Scheme 1). This reaction also produced other products, which were observed during the chromatographic workup, but due to instability and insufficient amounts, isolation and characterizations were not possible. Although compound 2 is produced in a mixture, these compounds can be separated by preparative thin-layer chromatography (TLC), which allowed spectroscopic and structural characterization of pure materials. The FAB mass spectrum gave molecular ion peaks corresponding to C21H16O11Ru2Fe3, while the IR spectrum displayed intense bands at 2015 and 1968 cm-1, characteristic of terminal carbonyl groups. Besides the Cp* protons the 1H NMR spectrum reveals one sharp singlet at δ = -23.03 ppm, similar to those observed in [H2FeRu3(CO)13]9 and [H2Ru5(CO)13Cp*BH2],27 in which the hydride is bridging the Ru-Ru edge. Thus, this high-field resonance has been assigned to one Ru-H-Ru proton. In order to confirm the spectroscopic assignments and determine the full molecular and crystal structure of 2, an X-ray analysis was undertaken. The crystal structure of 2 corresponds to discrete molecules of [Cp*Ru(CO)2(μ-H){RuFe3(CO)9}] separated by normal van der Waals distances. (26) Geetharani, K.; Bose, S. K.; Varghese, B.; Ghosh, S. Chem.; Eur. J. 2010, 16, 11357–11366. (27) Galsworthy, J. R.; Housecroft, C. E.; Rheingold, A. L. Organometallics 1993, 12, 4167–4171.
Figure 1. Molecular structure and labeling diagram for [Cp*Ru(CO)2(μ-H){RuFe3(CO)9}], 2. Thermal ellipsoids are shown at the 30% probability level. Relevant bond lengths (A˚) and angles (deg): Ru1-Ru2 2.5049(6), Ru2-Fe1 2.3026(10), Ru2-Fe2 2.3017(9), Ru2-Fe3 2.2839(9), Fe1-Fe2 2.6876(12), Fe1-Fe3 2.6251(12), Fe2-Fe3 2.6285(12); Ru1-Ru2-Fe1 138.38(3), Ru1-Ru2-Fe3 144.14(3), Ru2-Fe2-Fe3 54.71(3), Fe1-Fe2Fe3 59.17(3).
The molecular structure, shown in Figure 1, may be regarded as being composed of one Cp*Ru(CO)2 fragment, one Ru atom, and three Fe(CO)3 fragments, which are linked by metal-metal bonds. The cluster 2 shows a tetrahedral arrangement of three iron atoms and one ruthenium atom (Ru2), in which an apical Ru group is symmetrically coordinated to a basal Fe3(CO)9 fragment containing three Fe(CO)3 groups located at the corners of an equilateral triangle.
Note
The other ruthenium atom (Ru1) is involved in an exopolyhedral Cp*Ru(CO)2 moiety, which is bound to the apical ruthenium atom of the tetrahedral core. The bridging hydrogen along the Ru(1)-Ru(2) edge has not been positioned by X-ray diffraction studies; however its connectivity with regard to Ru(1) and Ru(2) (Scheme 1) has been assertively determined by low-temperature 1H NMR. One of the most striking features of the structure 2 is the occurrence of a strong metal-metal bond as evidenced by the short Ru-Ru distance of 2.5049(6) A˚. The distance is considerably longer than true RudRu distances of 2.262.29 A˚ in tetracarboxylate ruthenium(II) dimers and other related species.28 It is, however, similar to those in other RudRu species, for example, [Ru2(μ-CO)(μ-C2 Ph2)(η-C 5H5 )2]29 (2.505(1) A˚) and [Ru2H6(N2)(PPh3)4]30 (2.556(3) A˚). All the terminal carbonyl groups are almost linear, with MC-O angles ranging from 176° to 179°. The terminal CO ligands display the usual geometry, and the dihedral angle between the mean plane of the iron atoms relative to the Cp* ligand is 132.1°. The 3-fold axis of the tetrahedron is not collinear with the Ru-Ru vector, which may be due to the presence of the bridging hydrogen. Ignoring the Cp*Ru(CO)2 moiety, the molecule has near-perfect C3v symmetry. The molecule contains an essentially regular RuFe3 metalatom core, with an average Ru-Fe separation of 2.2960 A˚ and Fe-Fe separation of 2.647 A˚. The average Ru-Fe distance is significantly shorter than the analogous tetrahedral cluster, [(Cp*Ru)2(CpFe)2(CO)4] (2.6275 A˚).11 However, the Fe-Fe bond distances are normal for tetranuclear species and can be compared with [Et4N]2[{SeFe3(CO)9}2Hg] (2.619(1)-2.816(1) A˚),31 [Et4N][SeFe3(CO)9(μ-HgI)] (2.615(3)-2.887(3) A˚), 32 and [(Cp*Ru)2 (CpFe)2(CO)4] (2.536(1) A˚).11 The cluster 2 is, to the best of our knowledge, the first metal-cluster found to possess an exopolyhedral metal fragment via a metal-metal double bond. Considering a {Cp*Ru(CO)2} fragment as a single-orbital, one-electron fragment (“into the t2g set”),33 we can assume the {Cp*Ru(CO)2H} fragment as a two-electron donor to the tetrahedral core RuFe3. A similar interpretation has also been suggested by Housecraft for the boride cluster [H2Ru5(CO)13Cp*BH2], in which an exo-{Cp*Ru(CO)2} fragment is contributing one electron toward the Ru4B core.27 Hence, a 52-electron count for compound 2 is achieved if the {Cp*Ru(CO)2H} fragment contributes two electrons and the single Ru and all three Fe atoms of the RuFe3 core contribute eight electrons each.34 With an electron count of 52, cluster 2 appears to be eight fewer than the total valence electrons needed for an electron-precise tetrahedral cluster. One can rationalize the unsaturated nature of the cluster by geometric (28) Chakravarty, A. R.; Cotton, F. A.; Tocher, D. A. Inorg. Chem. 1985, 24, 172–177. (29) Colborn, R. E.; Dyke, A. F.; Gracey, B. P.; Knox, S. A. R.; Macpherson, K. A.; Mead, K. A.; Orpen, A. G. J. Chem. Soc., Dalton Trans. 1990, 761–771. (30) Chaudret, B.; Devillers, J.; Poilblanc, R. J. Chem. Soc., Chem. Commun. 1983, 641–643. (31) Shieh, M.; Tsai, Y.-C. Inorg. Chem. 1994, 33, 2303–2305. (32) Shieh, M.; Tsai, Y.-C.; Cherng, J.-J.; Shieh, M.-H.; Chen, H.-S.; Ueng, C.-H.; Peng, S.-M.; Lee, G.-H. Organometallics 1997, 16, 456– 460. (33) Hoffmann, R. Science 1981, 211, 995–1002. (34) Rosal, I. D.; Jolibois, F.; Maron, L.; Philippot, K.; Chaudret, B.; Poteau, R. Dalton Trans. 2009, 2142–2156. (35) (a) Saillant, R.; Barcelo, G.; Kaesz, H. J. Am. Chem. Soc. 1970, 92, 5739–5741. (b) Wilson, R. D.; Bau, R. J. Am. Chem. Soc. 1976, 98, 4687–4689.
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Figure 2. Cyclic voltammogram of 2 in CH3CN at a scan rate of 0.10 V/s.
consideration, like H4Re4(CO)12,35 the resulting unsaturation being mainly located at the Ru-Fe bonds. In connection with a formal allocation of the bonding in 2, it appears reasonable to formulate the three Ru-Fe bonds in the RuFe3 tetrahedron as double bonds with a σ and a π component, which would give each iron atom the favored 18-electron configuration (six from the three CO, two from the two Fe-Fe bonds, two from the RudFe double bond, and the Fe atom contributes eight electrons). The redox properties of 2 has been studied by cyclic voltammetry in MeCN using a glassy carbon working electrode in the presence of [n-Bu4NPF6] as supporting electrolyte. The cyclic voltammogram, depicted in Figure 2, exhibits one reversible redox wave at Ep = -1.79 V (A) and two irreversible reduction waves at Ep = -1.38 V (B) and Ep = -0.93 V (C), followed by an irreversible oxidation at Ep=0.41 V (D). Of them, wave A corresponds most likely to the one-electron redox process Ru(III)Ru(II)/Ru(II)Ru(II) (ΔEp =0.077 V), which is closer to the ideal value of ΔEp = 0.059 V expected for a reversible process.36,37 However, the second reduction (B), Ru(II)Ru(II)/Ru(II)Ru(I), is not reversible, as shown by the lack of a return wave. The irreversible reduction (C) is in the considerably negative region, probably caused by the diruthenium-based reduction process Ru(II)Ru(I)/Ru(I)Ru(I). The uncoupled oxidation wave (D) may be assign to the oxidation of Ru(I)Ru(I)/ Ru(II)Ru(I).38,39 The irreversibility of this oxidation is similar to the behavior observed for [Cp*2FeRu(C8H8)],40 which decomposes upon electrochemical oxidation. The pathway for the formation of 2 from 1 is of interest, but we were unable to obtain any direct information. However, note that 1 contains a {B2H4} fragment, which is a six-electron (36) Zanello, P. Molecular Electrochemistry in Inorganic Chemistry: Theory, Practice and Applications; Royal Society of Chemistry: Cambridge, 2003. (37) Lense, S.; Hardcastle, K. I.; MacBeth, C. E. Dalton Trans. 2009, 7396–7401. (38) Sarkar, B.; Kaim, W.; Fiedler, J.; Duboc, C. J. Am. Chem. Soc. 2004, 126, 14706–14707. (39) Heck, J.; Lange, G.; Malessa, M.; Boese, R.; Bl€aser, D. Chem.; Eur. J. 1999, 5, 659–668. (40) Manriquez, J. M.; Ward, M. D.; Reiff, W. M.; Calabrese, J. C.; Jones, N. L.; Carroll, P. J.; Brunel, E. E.; Miller, J. S. J. Am. Chem. Soc. 1995, 117, 6182–6193.
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donor to the cluster framework. Thus, formation of 2 could be performed by the replacement of the {B2H4} ligand of 1 with a {Fe3(CO)9} fragment (B2H4 is equivalent to the Fe3(CO)9 unit), followed by the elimination of Cp*H.
Conclusion With the goal of obtaining new boron-containing compounds, the cluster growth reaction of 1 with Fe2(CO)9 yielded an unusual tetrahedral cluster without precedent in metalcarbonyl chemistry. Although many parallels exist between metal carbonyl and boron hydride or metallaborane clusters, examples of mixed-metal tetrahedral carbonyl clusters with an exopolyhedral metal fragment are rare. Compound 2 has 52 cve, where the Ru-Fe bond distances are shorter, possibly due to the unsaturation of the valence electrons. This is one of the few examples where formation of clusters originates from 1 by fragment addition and fragment exchange in one pot.
Experimental Section General Procedures and Instrumentation. All the operations were conducted under an Ar/N2 atmosphere by using standard Schlenk techniques or an inert-atmosphere glovebox. Solvents were distilled prior to use under argon. [(Cp*RuCl2)n], Mo(CO)6, Fe2(CO)9, and LiBH4 in THF (Aldrich) were used as received. [(Cp*RuCO)2B2H6] was prepared as described in the literature.26 Thin-layer chromatography was carried out on 250 mm diameter aluminum-supported silica gel TLC plates (Merck TLC plates). NMR spectra were recorded on a 400 and 500 MHz Bruker FT-NMR spectrometer. Residual solvent protons were used as reference (δ, ppm, benzene, 7.16). Infrared spectra were recorded on a Nicolet 6700 FT spectrometer. Microanalyses for C, H, and N were performed on Perkin Elmer Instruments series II model 2400. Mass spectrometry was performed on a JEOL JMS-AX505HA mass spectrometer with perfluoro kerosene as standard and a Jeol SX 102/Da-600 mass spectrometer using argon/xenon (6 kV, 10 mA˚) as the FAB gas. Cyclic voltammograms were recorded in 0.2 M n-Bu4NPF6 and a 1.0 mM solution of 2 (CH3CN, N2 degassed) on a CHI-920C electrochemical workstation (CH Instruments) with a glassy carbon working electrode (diameter = 2 mm), a Pt-wire counter electrode, and an Ag/AgCl (1 M KCl) reference electrode with 0.1 M KCl as the supporting electrolyte. Synthetic Procedure for 2. A solution of 1 (0.15 g, 0.27 mmol) in hexane (15 mL) was stirred at 75 °C in the presence of 6 equiv of Fe2(CO)9 (0.59 g, 1.62 mmol) for 24 h. The solvent was removed in vacuo; the residue was extracted into hexane and passed through Celite. The filtrate was concentrated and kept at -40 °C to remove Fe3(CO)12. The mother liquor was concentrated, and
Geetharani et al. the residue was chromatographed on silica gel TLC plates. Elution with a hexane/CH2Cl2 (8:2 v/v) mixture yielded yellow 2 (0.02 g, 9%), reddish-brown [(μ3-BH)(Cp*RuCO)2(μ-CO){Fe(CO)3}] (0.02 g, 14%), [{(μ3-BH)(Cp*Ru)(μ-CO)}2Fe2(CO)5] (0.04 g, 19%), and [{(μ3-BH)(Cp*Ru)Fe(CO)3}2(μ-CO)] (0.06 g, 31%). 1H NMR ([D6]benzene, 22 °C, 400 MHz): δ 2.04 (s, 15H, C5Me5), -23.03 (s, 1 Ru-H-Ru). 13C NMR ([D6]benzene, 22 °C, 100 MHz): δ 211.9 (CO), 93.6 (C5(CH3)5), 9.4 (C5(CH3)5). IR (hexane) ν/cm-1: 2015s and 1968s (CO). MS (FAB) Pþ(max): m/ z (%) 814 (isotopic pattern for 2 Ru, 3 Fe, and 11 CO atoms). Anal. Calcd for C21H16Fe3Ru2O11: C, 30.99; H, 1.98. Found: C, 31.60; H, 2.15. X-ray Structure Determination. Suitable X-ray quality crystals of 2 were grown by slow diffusion of a hexane/CH2Cl2 (9.5:0.5 v/v) solution, and single-crystal X-ray diffraction studies were undertaken. The crystal data for 2 were collected and integrated using a Bruker Axs kappa apex2 CCD diffractometer, with graphite-monochromated Mo KR (λ = 0.71073 A˚) radiation at 173 K. The structures were solved by heavy atom methods using SHELXS-97 or SIR9241 and refined using SHELXL-97 (Sheldrick, G. M., University of G€ ottingen).42,43 Crystal data for 2: C22H15Fe3O11Ru2, Mr = 813.02 g/mol, monoclinic, space group P21/c, a=8.4863(2) A˚, b=27.3559(8) A˚, c=11.5806(3) A˚, β=100.6710(10)°, V=2641.95(12) A˚3, Z=4, Fcalcd = 2.044 g/cm3, final R indices [I > 2σ(I)] R1 = 0.0455, wR2 = 0.1462, reflections collected 19 546, independent reflections 6454, Rint =0.0266, goodness-of-fit on F2 1.045.
Acknowledgment. Generous support of the Department of Science and Technology, DST (Project No. SR/ S1/IC-19/2006), New Delhi, is gratefully acknowledged. K.G. and S.K.B. thank the Council of Scientific and Industrial Research (CSIR) and University Grants Commission (UGC), India, respectively for a Junior and Senior Research Fellowship. We thank V. Ramkumar for X-ray crystallography analysis. We thank Prof. M. V. Sangaranarayanan for helpful discussions on the electrochemical study. We would also like to thank Mass Lab, SAIF, CDRI, Lucknow 226001, India, for FAB mass analysis. Supporting Information Available: CIF file for 2. This material is available free of charge via the Internet at http://pubs. acs.org. (41) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A. J. Appl. Crystallogr. 1993, 26, 343–350. (42) Sheldrick, G. M. SHELXS-97; University of G€ottingen: Germany, 1997. (43) Sheldrick, G. M. SHELXS-97; University of G€ottingen: Germany, 1997.
