Note pubs.acs.org/Organometallics
Ru4(CO)8(μ-OOCAd)4(PPh3)2: Phosphine Derivative of an ElectronDeficient Linear Tetraruthenium Cluster Suat Ping Oh,† Bo Yang Chor,‡ Wai Yip Fan,*,† Yongxin Li,‡ and Weng Kee Leong*,‡ †
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 Division of Chemistry and Biological Chemistry, Nanyang Technological University, 21 Nanyang Link, Singapore 637371
‡
S Supporting Information *
ABSTRACT: The reaction of Ru3(CO)12 with 1-adamantanecarboxylic acid afforded a purple solid with the formulation Ru2(CO)4(μ-OOCAd)2 (Ad = 1-adamantyl). Reaction of this solid with PPh3 gave Ru4(CO)8(μ-OOCAd)4(PPh3)2, a 64electron linear tetraruthenium cluster.
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OOCR)2]n. Phosphine derivatives have also been obtained, and they have a similar structure.5 Much of the recent work on this class of compounds has been carried out by the group of Petrukhina,3,6 who have developed solvent-free synthetic routes to them. The propensity of these polymeric complexes to form the coordinatively unsaturated, and electron-deficient, diruthenium [Ru2(CO)4(μ-OOCR)2] units has also been exploited in catalytic investigations.7 In all these compounds, the basic building block is the diruthenium [Ru2(CO)4(μ-OOCR)2] unit, and those involving more than one such unit showed that these are linked via Ru···O interactions. As far as we are aware, the only known exception is the “dimer of dimer” complex [Ru2(CO)5(μ-OOC6H3-3,5-(CF3)2)2]2, in which the two [Ru2(CO)4(μ-OOCR)2] units are linked via a weak Ru−Ru bond.6 Recently, we have found cause to examine the reaction of clusters 1 with 1-adamantanecarboxylic acid (2). We had expected the chemistry to mirror that found for the other carboxylic acids, but instead the compound we obtained from the reaction of 1a was quite different. We wish to report our findings here.
INTRODUCTION Reactions of the group 8 trimetallic carbonyl clusters M 3(CO)12 (1, M = Ru (a), Os (b)) with carboxylic acids have been known for a long time. Interestingly, the reactivities for the ruthenium and osmium analogues are very different. While the reaction of 1b with carboxylic acids, RCOOH, gave the trinuclear derivatives Os3(CO)10(μ-H)(μ-OOCR),1 the analogous reaction with 1a afforded polymeric [Ru2(CO)4(μ-OOCR)2]n.2 Single-crystal X-ray structural analysis of these for R = CF3, for example, showed that they comprised dinuclear [Ru2(CO)4(μOOCCF3)2] units held together by strong interactions between an O atom of the carboxylato bridge and an Ru atom of the neighboring unit.3 These polymers dissolved reversibly in coordinating solvents (L), such as acetonitrile and tetrahydrofuran, to form dinuclear complexes Ru 2 (CO) 4 (μOOCR)2L2 (Scheme 1). The single-crystal X-ray structural Scheme 1
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RESULTS AND DISCUSSION The reaction of 1a with 2 in refluxing acetonitrile resulted in a light yellow solution, which, upon removal of the solvent, gave a purple solid (3), which analyzed as Ru2(CO)4(μ-OOCAd)2, but the color was in sharp contrast to the yellow color reported for the polymeric analogues such as [Ru2(CO)4(μ-OOC-
analysis of Ru2(CO)4(μ-OOCF3)2(NCMe)2, for example, showed that it comprised a diruthenium unit bridged by two mutually cis carboxylate groups and with the acetonitriles occupying axial positions. 4 These can easily lose the coordinated molecules L to re-form polymeric [Ru2(CO)4(μ© 2011 American Chemical Society
Received: September 28, 2011 Published: November 29, 2011 6774
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Me)2]n.2a The complex 3 was soluble in a number of solvents to give orange-yellow solutions, but reverted back to a purple solid upon removal of the solvent. The IR spectra of these solutions showed a similar pattern, although the absorption maxima showed some solvent-dependent shifts. In particular, a solution in acetonitrile required a more thorough removal of the solvent before the initially obtained yellow residue reverted to purple. These observations suggested that 3 was electron-deficient and dissolved through the reversible formation of dinuclear adducts Ru2(CO)4(μ-OOCAd)2(S)2, where S = solvent; a more coordinating solvent such as CH3CN dissociated less readily. In agreement with the above suggestion, 3 reacted reversibly with gaseous CO to afford a faint yellow complex, Ru2(CO)6(μ-OOCAd)2 (5); its identity was proposed from the similarity of its IR spectrum in the CO region with those of known analogues and, like those, continued bubbling of CO led to decomposition to an almost colorless solution.8 Its reaction with PPh3 afforded yellow Ru2(CO)4(μ-OOCAd)2(PPh3)2 (4a); a similar reaction with the water-soluble triphenylphosphine-3,3′,3″-trisulfonic acid trisodium salt (TPPTs) yielded the water-soluble analogue Ru2(CO)4(μ-OOCAd)2(TPPTs)2 (4b). A single-crystal X-ray crystallographic analysis on 4a showed that, similar to the known analogues, the Ru2(CO)4 moiety has a sawhorse geometry and the PPh3 ligands occupy the axial positions;9 the molecular structure of 4a, together with selected bond parameters, are given in Figure 1.
Figure 2. Molecular structure of 6. Selected bond lengths (Å) and angles (deg): Ru(1)−Ru(2) = 2.6851(5), Ru(2)−Ru(2A) = 2.8754(7), Ru(1)−P(1) = 2.3945(9), Ru(1)−O(31) = 2.138(3), Ru(1)−O(41) = 2.110(2), Ru(2)−O(32) = 2.076(3), Ru(2)−O(42) = 2.089(3); Ru(2)−Ru(1)−O(31) = 81.82(7), Ru(2)−Ru(1)−O(41) = 82.41(7), Ru(2)−Ru(1)−P(1) = 171.44(3), O(31)−Ru(1)−O(41) = 83.6(1), O(32)−Ru(2)−O(42) = 83.2(1), Ru(1)−Ru(2)−Ru(2A) = 162.51(2).
The structure of 6 is the same as that of [Ru2(CO)5(μOOC 6 H 3 -3,5-(CF 3 ) 2 ) 2 ] 2 ; it comprises two dinuclear [Ru2(CO)4(μ-OOCAd)2] units held together by an Ru−Ru bond instead of the more usual staggered Ru···O interactions.8 The total valence electron count based on the molecular formula for 6 is 64; for a linear transition-metal complex of the type M−M−M−M, the 18-electron rule would predict 4 × 18 − 3 × 2 = 66 electrons. As such, complex 6 is electron-deficient. The Ru−Ru bond lengths of 2.8754(7) and 2.6851(5) Å for Ru(2)−Ru(2A) and Ru(1)−Ru(2), respectively, suggest a single metal−metal bond for Ru(2)−Ru(2A) and a bond order of 1.5 for Ru(1)−Ru(2). This same variation in the Ru− Ru bond lengths is also observed in [Ru2(CO)5(μ-OOC6H33,5-(CF3)2]2; the corresponding values being 2.9065(9) and 2.6859(8) Å, respectively.6 We have also carried out a DFT calculation on a simplified model of 6, in which the adamantyl was replaced by a methyl group and PPh3 with PMe3. This shows that the HOMO (highest occupied molecular orbital) is located mostly along the Ru−Ru vector within the dinuclear [Ru2(CO)4(μ-OOCAd)2] unit, i.e., between Ru(1)−Ru(2) (Figure 3). On the other hand, the LUMO (lowest unoccupied molecular orbital) is located in between the dinuclear [Ru2(CO)4(μ-OOCAd)2] units, i.e., between Ru(2)−Ru(2A). This would seem to indicate the central Ru−Ru bond as the site of nucleophilic attack. A rigid PES scan in which the Ru(2)−Ru(2A) bond was lengthened, however, showed that the energy corresponding to breakage of this bond was ∼20 kJ mol−1; optimization of a structure corresponding to the dinuclear [Ru2(CO)4(μ-OOCMe)2] unit also suggested that the bond dissociation enthalpy for this would be ∼27 kJ mol−1. Indeed, the calculated free energy for this process is −23 kJ mol−1, suggesting that fragmentation of 6 at the central Ru−Ru bond could also have occurred under ambient conditions. An NBO calculation within the natural localized molecular orbitals (NLMO) formalism indicated a bond order of 0.47 and 0.54 for the outer Ru−Ru bonds and 0.28 for the central Ru− Ru bond.10 The values for a double and single Ru−Ru bond have earlier been reported to be ∼0.8 and ∼0.2, respectively.11
Figure 1. Molecular structure of 4a. Selected bond lengths (Å) and angles (deg): Ru(1)−Ru(2) = 2.71827(16), Ru(1)−P(1) = 2.4295(4), Ru(2)−P(2) = 2.4502(4), Ru(1)−O(3) = 2.11(1), Ru(1)−O(4) = 2.143(1), Ru(2)−O(7) = 2.137(1), Ru(2)−O(8) = 2.122(1); Ru(2)− Ru(1)−O(3) = 83.10(3), Ru(2)−Ru(1)−O(4) = 81.76(3), Ru(1)− Ru(2)−O(7) = 82.07(3), Ru(1)−Ru(2)−O(8) = 83.57(3), Ru(2)− Ru(1)−P(1) = 171.49(1), Ru(1)−Ru(2)−P(2) = 168.02(1), O(3)− Ru(1)−O(4) = 89.22(3), O(7)−Ru(2)−O(8) = 84.46(5).
More interestingly, when 3 was reacted with PPh3 in a 1:0.5 (Ru:P) ratio, the linear tetranuclear cluster Ru 4(CO)8(μOOCAd)4(PPh3)2 (6) was isolated as a purple solid. Complex 6 has been characterized by a single-crystal X-ray crystallographic study; the molecular structure and selected bond parameters are given in Figure 2. Complex 6 dissolved reversibly in DCM or CH3CN, and it could react further with PPh3 to afford 4a. The 31P{1H} NMR spectrum of 6 in CD3CN showed a major resonance at δ 8.5 ppm and a minor resonance at 13.7 ppm; the latter may be due to a species with the formulation Ru2(CO)4(OOCAd)2(PPh3)(NCCD3). 6775
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LanL2DZ (Los Alamos effective core potential double-ζ) basis set. Bond order analysis was carried out with the NBO program implemented within Gaussian 03W.15 Reaction of 1a and 2. Clusters 1a (112.6 mg, 0.176 mmol) and 2 (191.6 mg, 1.06 mmol) were refluxed in CH3CN (15 mL) for 2 h in a one-necked round-bottom flask. After removal of the solvent, the residue was taken up in DCM, and the resulting orange solution filtered through a thin pad of silica gel. The filtrate was evaporated to dryness to afford [Ru2(CO)4(μ-OOCAd)2] (3) as a purple solid. Yield: 156 mg (88%). IR (DCM): νCO 2041 (vs), 1990 (s), 1960 (vs), 1924 (w) cm−1. IR (KBr): νCO 2032 (vs), 1990 (s), 1955 (vs) cm−1. 1 H NMR (C6D6): δ 1.66 (s, CH2CH, 12H), 1.96 (s, CH, 6H), 2.13 (s, CH2CCOO, 12H). 13C{1H} NMR (C6D6): δ 29.2 (CH2CH), 37.4 (CH 2 CH), 40.7 (CH 2 CCOO), 42.8 (CH 2 CCOO), 191.2 (CH2CCOO), 201.6 (RuCO). ESI-MS− m/z: 674.01 [Ru2(CO)4(μOOCAd)2]−, 704.83 [Ru2(CO)4(μ-OOCAd)2 + MeO]−, 1379.61 [[Ru2(CO)4(μ-OOAd)2]2 + MeO]−. Anal. Calcd for C26H30O8Ru2: C 46.42, H 4.50. Found: C 46.76, H 4.58. Reaction of 3 with PPh3. Complex 3 (16.3 mg, 0.0242 mmol) and PPh3 (30.4 mg, 0.116 mmol) were stirred in DCM (3 mL) for 45 min. The mixture was purified by TLC with hexane/DCM (4:1, v/v) as eluant. Complex 4a was obtained as a yellow band following a colorless band of unreacted PPh3. Yield: 9.7 mg (33%). IR (DCM): νCO 2021 (vs), 1977 (m), 1947 (vs), 1916 (w) cm−1. 1H NMR (C6D6): δ 1.58 (s, CH2CH, 12H), 1.72 (s, CH2CCOO, 12H), 1.83 (s, CH, 6H), 7.08 (m, Ph, 18H), 7.83 (m, Ph, 12H). 31P{1H} NMR (C6D6): δ 14.3 (s). ESI-MS+ (m/z): 1198 [M]+. Anal. Calcd for C62H60O8P2Ru2: C 62.20, H 5.05. Found: C 61.83, H 5.33. Synthesis of 4b. Complex 3 (7.6 mg, 0.0113 mmol) in DCM (2 mL) was added to TPPTs (5.1 mg, 0.000897 mmol) in deionized water (1 mL), and the resulting biphasic mixture stirred vigorously for 2 h. The aqueous layer was separated, washed with DCM (3 × 5 mL), and then dried in vacuo to afford 4b as a yellow solid. Yield: 11.6 mg (57%). IR (KBr): νCO 2021 (vs), 1980 (m), 1949 (vs) cm−1. 1H NMR (MeOD): δ 0.8−2.2 (m, adamantyl-H, 30H), 7.1−8.3 (m, Ph, 24H). 31 1 P{ H} NMR (MeOD): δ 13.2 (s). ESI-MS+ m/z: 1629.09 [M − C10H15COO]+, 1602.75 [M − C10H15COO − CO]+. Hi-res MS+: calcd for C62H54Na6O26P298Ru99RuS6 1803.8280; found 1803.8289. Anal. Calcd for C62H54Na6O26P2Ru2S6.5H2O: C 39.21, H 3.40. Found: C 39.07, H 3.48. Reaction of 3 with CO. A solution of 3 in DCM (2 mL) was placed in a Carius tube, degassed by three cycles of freeze−pump− thaw, and then filled with CO gas (1 bar). There was an immediate color change to a pale yellow. The IR spectrum indicated the formation of Ru2(CO)6(μ-OOCAd)2 (5). IR (DCM): νCO 2103 (m), 2078 (vs), 2032 (vs), 1999 (vs) cm−1 (cf. 2110 (s), 2080 (vs), 2038 (vs), 2000 (vs) cm−1 for Ru2(CO)6(μ-OOCPh)2 and 2110 (vs), 2080 (vs), 2038 (vs), 2005 (vs) cm−1 for Ru2(CO)6(μ-OOCC6H4-pOMe)2.8 Removal of solvent in vacuo afforded 3 again. Standing the solution for a few minutes led to a darker yellow solution (νCO 2099 (s), 2078 (m), 2033 (vs), 2002 (vs), 1961 (w), 1935 (m) cm −1). Synthesis of 6. A sample of 3 (18 mg, 0.0268 mmol) and PPh3 (3.5 mg, 0.0134 mmol) [ratio of Ru:PPh3 = 2:1] was stirred in DCM (3 mL) for 1 h. After removal of the solvent, the residue was separated by TLC with hexane/DCM (1:1, v/v) as eluant. Three bands were obtained; in order of elution, they were a yellow band of 4a (24%), a purple band of 6 (5.5 mg, 22%), and a pale yellow band of unreacted 3 (52%). IR (DCM): νCO 2036 (vs), 1985 (m), 1963 (s), 1916 (w) cm−1. IR (KBr): νCO 2027 (vs), 1987 (s), 1966 (vs), 1929 (m) cm −1. 1 H NMR (C6D6): δ 1.63 (s, CH2CH, 12H), 1.91 (s, CH and CH2CCOO, 18H), 7.06 (m, Ph, 9H), 7.71 (m, Ph, 6H). 31P{1H} NMR (C6D6): δ 8.8 (s). 13C{1H} NMR (C6D6): δ 29.2 (CH2CH), 37.4 (CH2CH), 40.2 (CH2CCOO), 43.0 (CH2CCOO), 193.8 (CH2CCOO), 206.5 (RuCO). ESI-MS+ (m/z): 1871 [M]+, 1690 [M − C10H15COO]+. Anal. Calcd for C88H90O8P2Ru4: C 56.52, H 4.85. Found: C 56.54, H 5.12. X-ray Crystallographic Studies. Crystals of diffraction quality were grown by slow evaporation of DCM solutions. X-ray data was collected at 103 K on a Bruker Kappa diffractometer equipped with a CCD detector, employing Mo Kα radiation (λ = 0.71073 Å), with the
Figure 3. HOMO (top) and LUMO (bottom) calculated for Ru4(CO)8(OOCMe)4(PMe3)2.
The values obtained here are thus consistent with the bond orders assigned.
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CONCLUDING REMARKS
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EXPERIMENTAL SECTION
We have found that the reaction of Ru3CO)12 with 1adamantanecarboxylic acid afforded the purple complex 3. Although its structure has eluded us, attempts at growing a diffraction-quality crystals have been unsuccessful to date, we believe it to be electron-deficient and has the formulation Ru2(CO)4(μ-OOCAd)2. It is able to react with nucleophiles to form adducts, including the electron-deficient tetranuclear species 6. These are unlike the compounds obtained from other carboxylic acids, which are dimers or polymers of [Ru2(CO)4(OOCR)2] units linked via Ru···O interactions. We believe, however, that complexes such as 3 and 6 are not unique to 1-adamantanecarboxylic acid and that they are more readily available than may have been suggested hitherto. Work on showing this is in progress.
General Procedures. All reactions and manipulations, except for TLC separations, were performed under argon by using standard Schlenk techniques. All reagents were from commercial sources and used as supplied without further purification. Reaction mixtures were separated by preparative thin-layer chromatography (TLC) with 20 cm × 20 cm plates precoated with silica gel K60F254, purchased from Merck. NMR spectra were recorded on a Bruker AC300 or AMX500 spectrometer; 1H and 13C{1H} chemical shifts were referenced to residual solvent peaks in the respective deuterated solvents, and 31 1 P{ H} chemical shifts were referenced to 85% H3PO4 (external standard). ESI was recorded on a Thermo Deca Max (LCMS) mass spectrometer with an ion-trap mass detector, while high-resolution mass spectra were recorded in ESI mode on a Waters UPLC-Q-TOF mass spectrometer. Elemental analyses were performed by the microanalytical laboratories at the Nanyang Technological University or the National University of Singapore. Computational studies were carried out with the Gaussian 03W suite of programs,12 utilizing Becke’s three-parameter hybrid function,13 and Lee−Yang−Parr’s gradient-corrected correlation function (B3LYP),14 together with the 6776
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SMART suite of programs.16 The data were processed and corrected for Lorentz and polarization effects with SAINT and for absorption effects with SADABS.18 Structural solution and refinement were carried out with the SHELXTL suite of programs.19 There was disorder of one of the two adamantane cages in 6. This was modeled with two alternative positions, and the corresponding distances between the two were restrained to be the same. A dichloromethane solvate was also found in 4a. Crystal and refinement data are summarized in Table S1.
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W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03; Gaussian Inc.: Wallingford, CT, 2004. (13) Becke, A. D. J. Chem. Phys. 1993, 98, 568. (14) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1998, 37, 785. (15) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO 3.0. (16) SMART version 5.628; Bruker AXS Inc.: Madison, WI, USA, 2001. (17) SAINT+ version 6.22a; Bruker AXS Inc.: Madison, WI, USA, 2001. (18) Sheldrick, G. M. SADABS, 1996. (19) SHELXTL version 5.1; Bruker AXS Inc.: Madison, WI, USA, 1997.
ASSOCIATED CONTENT
S Supporting Information *
IR spectroscopic data for 3 in various solvents. This material is available free of charge via the Internet at http://pubs.acs.org.
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ACKNOWLEDGMENTS This work was supported by Nanyang Technological University and the Ministry of Education (Research Grant No. T208B1111), and one of us (S.P.O.) thanks the National University of Singapore for a Research Scholarship.
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REFERENCES
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