Revisiting the Synthesis and Elucidating the Structure of Potassium

Article pubs.acs.org/Organometallics

Revisiting the Synthesis and Elucidating the Structure of Potassium Cyclopentadienyldicarbonylruthenate, K[CpRu(CO)2] Kai F. Kalz,† Nicole Kindermann,† Sheng-Qi Xiang,‡ Andreas Kronz,§ Adam Lange,‡ and Franc Meyer*,† †

Institute of Inorganic Chemistry, Georg-August-University Göttingen, Tammannstrasse 4, D-37077 Göttingen, Germany Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, D-37077 Göttingen, Germany § Geowissenschaftliches Zentrum, Georg-August-University Göttingen, Goldschmidtstrasse 1, D-37077 Göttingen, Germany ‡

S Supporting Information *

ABSTRACT: Known procedures for the synthesis of K[CpRu(CO)2] (KRp) via reductive cleavage of the ruthenium dimer Rp2 were found to be inconsistent and have thus been revisited, and a revised protocol using K[HB(sec-Bu)3] (K-Selectride) as the reducing agent is now reported that gives yellow KRp in crystalline form in around 40% yield. The structure of KRp· THF has been determined by X-ray diffraction, representing the first crystallographic characterization of an Rp− salt. Inevitably the reductive cleavage of Rp2 also gives a poorly soluble black solid as an additional product, which has now been analyzed by a variety of methods, including 13C MAS NMR spectroscopy using 13CO-labeled material. The black solid has been identified as a polymeric Cp/Ru/CO compound with both bridging and terminal CO ligands in a 3:1 ratio. The present report may stimulate the use of the [CpRu(CO)2]− (Rp−) anion, which has been barely exploited as yet in comparison to its popular congener [CpFe(CO)2]− (Fp−). ince the first preparation of the [CpFe(CO)2]− (Fp−) anion in 1955 by Fischer,1,2 this organometallic reagent has been used in a vast number of chemical transformations and its unique properties and reactions have been reviewed several times.3,4 For the heavier element analogue [CpRu(CO)2]− (Rp−), however, literature reports are much less numerous than for the iron congener. Its first preparation and use in the syntheses of ruthenium alkyl complexes was described by Wilkinson and co-workers.5 The Ru metalate was in fact more difficult to prepare,6 and in most cases solutions of the anion were directly used in subsequent reactions without prior isolation of the Rp− salt. In only very few cases were Rp− salts actually isolated,7,8 though clear analytical proof for the purity of the obtained product was missing. Furthermore, an X-ray crystallographic structure of an Rp− salt has not yet been reported. In the framework of an ongoing research project we intended to make use of the beneficial properties of the ruthenium anion’s potassium salt (KRp), these being for instance its lower nucleophilicity and redox activity as well as its higher basicity in comparison to the iron analogue.9−11 To our surprise, however, we encountered difficulties in isolating KRp as a pure material when following the published synthetic protocols. After careful investigation of the literature, we found inconsistencies in the reported observations, which prompted us to look into the preparation of this compound more thoroughly. In this note we wish to communicate our experiences with the preparation of KRp, its structural characterization, and the associated

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© 2014 American Chemical Society

discovery of a polymeric Cp/Ru/CO material that arises during the synthesis of this metalate. Commonly utilized methods for the preparation of the [CpRu(CO)2]− anion mostly parallel the procedures established for the iron analogue and comprise the reduction of the ruthenium dimer [CpRu(CO)2]2 (Rp2) by various reagents. In this context, for example, sodium amalgam,6,12,13 NaK alloy,7 Na metal,14 and alkyl borohydrides8,15−17 have been used. Appreciating the straightforward synthesis of the Fp− anion by borohydride cleavage of the Fp2 dimer18,19 and realizing the disadvantages of the sodium amalgam method,15,16 we decided to use K[HB(sec-Bu)3] (K-Selectride) for the preparation of KRp. Upon addition of K[HB(sec-Bu)3] to a solution of Rp2 in THF and stirring at room temperature, however, we observed the gradual darkening of the orange solution and the formation of a finely dispersed black precipitate. Since KRp was reported to be soluble in THF, we soon realized that we were dealing with a byproduct and anticipated that its formation might be the cause of the usually moderate yields encountered in the reported syntheses where solutions of KRp have been prepared for further use (20−40%, sometimes even less).6,15−17 The formation of a black precipitate was also observed when a potassium mirror was used to reduce the ruthenium dimer Rp2, thus ruling out the possibility of an involvement of the borohydride. When the filtrate of the reaction mixture was Received: January 30, 2014 Published: March 7, 2014 1475

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the molecular structure of KRp·THF, which crystallizes in the space group Pbca. The Rp− anions cross-link polymeric chains of alternating THF and potassium ions. As each potassium ion features a 4-fold O coordination by the oxygen atoms of two CO ligands as well as by two THF molecules, a threedimensional network with close K···O interactions is generated. Furthermore, two weak interactions between the K+ and C atoms of the carbonyl groups (3.150(2) and 3.202(2) Å) and two contacts K···Ru (3.607(1) and 3.981(1) Å) are observed, resulting in a formal coordination number of 8 (or {4 + 4}) for the potassium cation. Similar interactions of the alkali-metal ion are common for carbonyl metalates21 and were observed before for KFp as well.22 Black Solid. A typical Rp2 reduction as described above gave not only the desired product KRp but alsoirrespective of the chosen reducing agenta black solid. For further characterization this was separated by filtration, washed with THF and MeCN, and finally dried in vacuo. The chemical characterization of this second product, however, proved to be tedious, as it was not soluble in any of the commonly used organic solvents. Only when very polar solvents such as DMF and DMSO were used, was a (partial) dissolution of the substance achieved, giving black to dark purple solutions. Efforts to obtain crystals suitable for X-ray crystallography were largely unsuccessful. Only crystallization attempts from a solution of the black material in degassed water yielded crystals; however, this turned out to be the dimer Rp2. At least this finding demonstrates that the “CpRu(CO)2” unit is in some form preserved in the black solid and that decomposition of the compound under aqueous conditions partially recovers its chemical precursor Rp2. Figure 2a depicts the IR spectrum of the black solid dissolved in DMSO, DMF, and MeOH in comparison with the spectrum

layered with toluene, yellow crystals of KRp were obtained. This shows that in fact two different products result from the reductive cleavage of Rp2, namely yellow KRp and a black solid. In the following we report our efforts aiming at elucidating what happens during the reaction and at characterizing both obtained products.

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RESULTS AND DISCUSSION KRp (Yellow Crystals). A typical reaction gave reproducible yields between 37 and 41% of yellow crystalline KRp after filtration of the reaction mixture and subsequent crystallization of the product by addition of toluene. Elemental analysis data (CHN) match the expected values, and IR data were found in accordance with literature data.7,13,17 As was observed for Fp− salts, also for KRp the CO stretching frequencies are solvent dependent, indicative of competing tight, contact, and solventseparated ion pairs.4,20 To the best of our knowledge, NMR data of KRp have not been reported previously. Spectroscopic properties of KRp are summarized in Table 1. When the Table 1. Spectroscopic Data of K[CpRu(CO)2] (KRp) method

value

−1

IR (ν̃(CO), cm ) MeCN THF DMSO 1 H NMR (δ, ppm) CD3CN DMSO-d6 13 C NMR (δ, ppm) CD3CN DMSO-d6 solid

1888 (vs), 1803 (vs) 1895 (vs), 1812 (vs) 1882 (vs), 1796 (vs) 4.83 (Cp) 4.68 (Cp) 216 (CO), 80 (Cp) 215 (CO), 80 (Cp) 218 (CO), 87 (Cp)

compound is recrystallized from THF/toluene, single crystals appropriate for X-ray diffraction were obtained. Figure 1 shows

Figure 2. (a) IR spectrum of KRp in DMSO (blue) and IR spectra of the black solid in DMSO (black), DMF (red), and MeOH (green). (b) ATR IR spectra of the black solid (black) and 13CO-enriched black solid (red).

of KRp dissolved in DMSO (values for the measured reflectance are not comparable, since a variable scaling parameter was applied). It is evident that solutions of the black solid in those polar solvents show the same two CO stretching vibrations as KRp (1883 and 1795 cm−1); however, these two bands protrude more or less from broad bands centered at those positions. This observation perhaps hints at an association phenomenon which strongly depends on the polarity of the solvent. When protic MeOH is used as the solvent, the black solid dissolves completely and two significantly shifted IR bands occur in the recorded spectrum (2026 and 1963 cm−1), indicating that a reaction took place. Similar bands have been observed for the protonated metalate HRp;5,10,23,24 however, in the present case the bands are highly

Figure 1. (a) Molecular structure of KRp·THF. (b) Packing diagram showing the cross-linking of the polymeric chains. (c) {4 + 4} coordination environment of the potassium ions. Thermal displacement ellipsoids are given at 50% probability; hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru1− C1 = 1.826(2), C1−O1 = 1.175(3), O1−K1 = 2.754(2), Ru1−C2 = 1.818(2), C2−O2 = 1.179(3), O2−K1 = 2.737(2), K1−O3 = 2.861(2), O3−K1 = 2.848(2); K1−O1−C1 = 164.5(2), O1−C1− Ru1 = 177.3(2), C1−Ru1−C2 = 87.4(1), Ru1−C2−O2 = 179.8(2), C2−O2−K1 = 142.5(2), O2−K1−O3 = 93.97(5), K1−O3−K1 = 107.6(1). 1476

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asymmetric and especially the band at 1963 cm−1 has broad tails. An attenuated total reflectance (ATR) IR spectrum of the black solid features no distinct CO stretching vibrations but rather broad bands centered at 1815 and 1500 cm−1 (Figure 2b). These broad bands shift to lower wavenumbers in the 13 CO-enriched compound (for details of the preparation see the Supporting Information), indicating that they indeed have significant contributions from CO stretches, albeit at unexpected frequencies and with unusual band shapes.25 The low energy of the stretching vibrations suggested the presence of bridging CO ligands, μ-CO and μ3-CO, and/or η2-bonded CO. Efforts to measure Raman spectra of the compound at different excitation wavelengths were unsuccessful, likely because of problems with the morphology of the solid sample (see below) and/or damage of the material caused by the laser irradiation. Due to the poor solubility of the black solid as well as ionization problems, only limited conclusions could be drawn from mass spectrometry. ESI measurements of a DMSO solution of the compound diluted with MeCN, performed under inert conditions, gave no meaningful signals. Also mass spectrometry measurements with EI, FD, and MALDI ionization turned out to be unsuitable for analyzing the black solid. On the other hand, it was possible to measure solid-state UV/vis spectra of the compound. Against the continuous absorption over a large wavelength range, two distinct absorptions at 236 and 284 nm are clearly discernible. These two absorptions are very similar to the absorptions found for KRp in the solid state (236 and 281 nm) and are also similar to those found for THF solution spectra of KRp (242 and 281 nm). This would support the assumption that the Rp− unit is an essential component of the black solid. To further analyze the material, we then focused our attention on NMR spectroscopy. Again, the solution spectra (DMSO-d6) provided no clear picture of the black solid’s structure, since in addition to the signal for the C5H5 ring of KRp (δ 4.68 ppm) only weak and partially broad signals (δ 5.35−4.73 ppm) were observed in the 1H NMR spectrum. The 13 C NMR spectrum exclusively showed signals characteristic of KRp at 215 (CO) and 80 ppm (Cp). In order to obtain more information about the structure of the black compound, solid-state 13C MAS NMR spectra at different spinning rates were recorded. The spectra clearly showed at least two different kinds of CO ligands, but a quantification was not possible, as the signals were relatively broad and difficult to integrate. For this reason, a 13CO-enriched sample was prepared (roughly 90% 13C in the CO part of the compound inferred from NMR measurements of the labeled starting material Rp2) and the measurements were repeated. Figure 3 depicts the 13C crosspolarization MAS (CP/MAS) NMR spectra at different spinning rates. The spectra clearly establish that Cp rings (δ 94 ppm) as well as two different CO molecules, which could be assigned to bridging or η2-bonded (δ 280 ppm) and terminal COs (δ 215 ppm), are present. For comparison, a 13C CP/ MAS spectrum of solid KRp features sharp resonances for the terminal CO ligands at 218 ppm and for the Cp ring at 87 ppm (see Figure S5, Supporting Information). The terminal CO molecules in the black solid material show a larger chemical shift anisotropy (CSA) than the bridging molecules, and hence more intensity is distributed into the spinning sidebands of the signal; this has been observed before for terminal CO signals.26−30 In fact, even second-order spinning sidebands could be assigned for the signal of the terminal CO molecules.

Figure 3. 13C CP/MAS spectra of the 13CO-enriched black solid at 9 kHz (red), 11 kHz (green) and 12 kHz (blue) spinning rate showing bridging (280 ppm) and terminal CO molecules (215 ppm). First- and second-order sidebands of the terminal CO signal are marked with an asterisk (*) and first-order sidebands of the Cp ring (94 ppm) with a circle (○). The two signals marked with a question mark could not be assigned but are presumably due to impurities.

From direct excitation spectra (see the Supporting Information) a ratio of 3:1 for bridging and terminal COs, respectively, could be determined. However, deriving a reliable CO:Cp ratio was not possible from the direct excitation spectra, due to inaccuracies in the determination of the 13CO enrichment and due to extensive overlapping of spinning sidebands and signals. The nature of the sharp, though weak, signal at 173 ppm could not be resolved, but its varying intensity in different measurements hints at an impurity (possibly residual solvent) being the source of the signal. Interstitial carbide in ruthenium complexes usually shows 13C NMR resonances in the range 360−450 ppm;31 hence, its presence can be excluded. Standard CHN elemental analysis of the black solid revealed a surprisingly low carbon content (23.72%) in comparison to the expected and experimentally determined values of KRp (Anal. Calcd: C, 32.18; H, 1.93. Found: C, 32.17; H, 2.05.). The spectrophotometrically determined contents of Ru and the K content determined by means of ICP-OES were 42.26 and 13.18%, respectively. In order to gain a more firm basis for the analytical data, a wavelength dispersive electron microprobe analysis (EMPA) of the compound was conducted. Despite the difficult bulk morphology of the sample (Figure 4),

Figure 4. Secondary electron image (SEI) of the black solid (Au coated) showing the granular structure of the compound.

reproducible values for the Ru, K, and O contents could be obtained. By means of these measurements, the elemental analysis data obtained before were largely confirmed, and an average atomic ratio Ru:K:O of 4:3:8 was deduced. This demonstrates that, in the black solid as well as in KRp, each ruthenium atom is coordinated by two CO molecules on average. In the black solid, however, the CO molecules are not only found as terminal ligands but also three out of four CO 1477

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ligands adopt a bridging (μ-CO or μ3-CO) and/or η2 binding mode. Figure 4 depicts a secondary electron image of a certain region of the Au-coated black solid. The material features a granular structure which possibly hints at a clustering of the individual molecular fragments of the compound. Obviously, aggregates of different size (a few micrometers and below) exist. This phenomenon might offer an explanation for the unusual properties of the black solid: the ATR IR spectrum and the 13C MAS NMR spectrum both feature relatively broad signals for the CO molecules, a finding which could be explained if one assumes a distribution of signals that varies with the size of the particles.32 In addition, the IR spectra in solution indicate a dependence on the degree of association, which in turn depends on the polarity of the used solvent. The position and shape of the bands in the ATR IR spectrum furthermore suggests strong ion-pairing effects in the black solid. In conclusion, we have presented herein a detailed analysis of the products emerging from the reductive cleavage of Rp2. KRp has been prepared via a revised protocol and isolated in crystalline form, and its X-ray crystal structure has been determined. We expect this to stimulate the use of the Rp− anion, which has as yet been largely neglected in comparison to its popular congener Fp−. Additionally, a poorly soluble Cp/ Ru/CO material has been obtained which features unusual spectroscopic properties. Solid-state 13C MAS NMR spectra of this material hint at the simultaneous presence of terminal and bridging (and/or η2) CO ligands. Nevertheless, the “CpRu(CO)2” unit seems to be preserved in some form in this compound, suggesting a polymeric or clusterlike structure. In fact, it has been noticed earlier that the accumulation of negative charge in ruthenium carbonyls might lead to a rearrangement giving bridging carbonyl ligands: Inkrott and Shore suggested that this might represent an efficient way to average the charge distribution in highly charged cluster compounds.33 The present case seems to support this hypothesis; however, the exact molecular structure of the obtained black material remains unclear.

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C, 32.17; H, 2.05. Analytical data and characterization details for the black solid can be found in the Supporting Information. Crystal Structure Determination. X-ray data were collected on a STOE IPDS II diffractometer with an area detector (graphitemonochromated Mo Kα radiation, λ = 0.71073 Å) by use of ω scans at 133 K (see Table S1 in the Supporting Information for crystal data and refinement details). The structure was solved by direct methods and refined against F2 using all reflections with SHELX2013.34 All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed in calculated positions and assigned to an isotropic displacement parameter of 1.2/1.5 times Ueq(C). Faceindexed absorption corrections were performed numerically with the program X-RED.35 CCDC 983836 (KRp·THF) contains supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.

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ASSOCIATED CONTENT

* Supporting Information S

Text, tables, figures, and a CIF file giving modified literature procedures for Rp2 and 13CO-labeled compounds with complete experimental details, crystallographic data for KRp, and additional spectroscopic and analytical data with corresponding discussion. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*F.M.: e-mail, [email protected]; web, http://www.meyer.chemie.uni-goettingen.de/index.html. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the DFG (International Research Training Group GRK 1422 “Metal Sites in Biomolecules: Structures, Regulation and Mechanisms”; see www.biometals.eu) and the Fonds der Chemischen Industrie (PhD fellowships for K.F.K. and N.K.) is gratefully acknowledged.

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EXPERIMENTAL SECTION

REFERENCES

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General Procedures. All reactions were carried out under dried argon using standard Schlenk techniques or in a glovebox (MBRAUN LabMaster) under a nitrogen atmosphere. Solvents were dried and degassed by standard procedures before use. 13C-enriched CO gas (99% 13C) was purchased from Euriso-Top. Synthesis of K[CpRu(CO)2] (KRp) and the Black Solid. To a solution of Rp2 (0.50 g, 1.1 mmol, 1.0 equiv) in THF (20 mL) was added potassium tri-sec-butylborohydride (K-Selectride, 1 M in THF, 6.7 mL, 6.7 mmol, 6.1 equiv), and the resulting mixture was heated to 40 °C for 6 h. After a short time the initially orange solution turned darker and precipitation of a finely dispersed black solid was observed. Stirring of the mixture was continued overnight, and the precipitate was isolated by filtration and subsequent washing with THF (2 × 5 mL) and MeCN (2 × 10 mL). Exhaustive drying of the solid in vacuo afforded a black material (0.10 g). Toluene (60 mL) was added to the filtrate, and after 1 day the resulting crystals were collected by decantation, washed with toluene, and dried in vacuo. By this method, KRp was obtained as yellow crystals (0.24 g, 0.90 mmol, 41%). Crystals suitable for X-ray diffraction were obtained by layering a THF solution of the compound with toluene. Caution is required when handling the compound outside a glovebox, as any exposure to air immediately destroys the crystals. IR, 1H NMR, and 13C NMR data for KRp are compiled in Table 1. Anal. Calcd for C7H5KO2Ru (the THF is lost upon drying under reduced pressure): C, 32.18; H, 1.93. Found: 1478

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(15) Joseph, M. F.; Page, J. A.; Baird, M. C. Inorg. Chim. Acta 1982, 64, L121−L122. (16) Joseph, M. F.; Page, J. A.; Baird, M. C. Organometallics 1984, 3, 1749−1754. (17) Pannell, K. H.; Rozell, J. M.; Tsai, W.-M. Organometallics 1987, 6, 2085−2088. (18) Gladysz, J. A.; Williams, G. M.; Tam, W.; Johnson, D. L.; Parker, D. W.; Selover, J. C. Inorg. Chem. 1979, 18, 553−558. (19) Ohishi, T.; Shiotani, Y.; Yamashita, M. J. Org. Chem. 1994, 59, 250. (20) Pannell, K. H.; Jackson, D. J. Am. Chem. Soc. 1976, 98, 4443− 4446. (21) Darensbourg, M. Y. Prog. Inorg. Chem. 1984, 33, 221−274. (22) Hey-Hawkins, E.; von Schnering, H. G. Z. Naturforsch., B 1991, 46b, 621−624. (23) Humphries, A. P.; Knox, S. A. R. J. Chem. Soc., Dalton Trans. 1975, 1710−1714. (24) Bitterwolf, T. E.; Linehan, J. C.; Shade, J. E. Organometallics 2001, 20, 775−781. (25) CO stretching frequencies as low as 1612 cm−1 have been reported before for ruthenium clusters: Blackmore, T.; Cotton, J. D.; Bruce, M. I.; Stone, F. G. A. J. Chem. Soc. A 1968, 2931−2936. Bands around 1445 cm−1 have been assigned to a Ru-bound μ4-η2-CO ligand: see ref 31. (26) Gleeson, J. W.; Vaughan, R. J. Chem. Phys. 1983, 78, 5384− 5392. (27) Aime, S.; Botta, M.; Gobetto, R.; Orlandi, A. Magn. Reson. Chem. 1990, 28, S52−58. (28) Hawkes, G. E.; Sales, K. D.; Aime, S.; Gobetto, R.; Lian, L.-Y. Inorg. Chem. 1991, 30, 1489−1493. (29) Reven, L.; Oldfield, E. Inorg. Chem. 1992, 31, 243−252. (30) Canuto, H. C.; Masic, A.; Rees, N. H.; Heyes, S. J.; Gobetto, R.; Aime, S. Organometallics 2006, 25, 2248−2252. (31) Bailey, P. J.; Duer, M. J.; Johnson, B. F. G.; Lewis, J.; Conole, G.; McPartlin, M.; Powell, H. R.; Anson, C. E. J. Organomet. Chem. 1990, 383, 441. (32) It cannot be fully excluded that the broad NMR signals originate from paramagnetic impurities, anisotropic motion, or incomplete scrambling of the carbonyl ligands. (33) Inkrott, K. E.; Shore, S. G. J. Am. Chem. Soc. 1978, 100, 3954− 3955. (34) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, A64, 112−122. (35) X-RED; STOE & CIE GmbH, Darmstadt, Germany, 2002.

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