Published on Web 03/01/2002
Oxalate-Bridged Complexes of Dimolybdenum and Ditungsten Supported by Pivalate Ligands: (tBuCO2)3M2(µ-O2CCO2)M2(O2CtBu)3. Correlation of the Solid-State, Molecular, and Electronic Structures with Raman, Resonance Raman, and Electronic Spectral Data Bruce E. Bursten,*,† Malcolm H. Chisholm,*,† Robin J. H. Clark,*,‡ Steven Firth,‡ Christopher M. Hadad,*,† Ann M. MacIntosh,§ Paul J. Wilson,† Patrick M. Woodward,*,† and Jeffrey M. Zaleski*,§ Contribution from the Department of Chemistry, The Ohio State UniVersity, 100 West 18th AVenue, Columbus, Ohio 43210, Christopher Ingold Laboratories, UniVersity College London, 20 Gordon Street, London WC1H 0AJ, U.K., and Department of Chemistry, Indiana UniVersity, 800 East Kirkwood AVenue, Bloomington, Indiana 47405 Received July 23, 2001. Revised Manuscript Received December 14, 2001
Abstract: The compounds (tBuCO2)3M2(µ-O2CCO2)M2(O2CtBu)3 (M4OXA), where M ) Mo or W, are shown by analysis of powder X-ray diffraction data to have extended lattice structures wherein oxygen atoms from the oxalate and pivalate ligands of one M4OXA molecule are linked to metal atoms of neighboring molecules. Raman, resonance Raman, electronic absorption (2-325 K in 2-MeTHF), and emission spectra are reported, together with corresponding spectra of the µ-O213C13CO2 isotopomers. To aid in the assignment, the Raman spectra of K2C2O4‚H2O and K213C2O4‚H2O have also been recorded. The visible region of the electronic spectra is dominated by intense, fully allowed MLCT transitions, M2 δ to oxalate π*, which show pronounced thermochromism and extensive vibronic progressions associated with the oxalate ligand at low temperatures. With excitation into these charge-transfer bands, strong resonance enhancement is seen for Raman bands assigned to the oxalate ν1(ag) and, to a lesser extent, ν2(ag) modes. Electronic structure calculations for the model compounds (HCO2)3M2(µ-O2CCO2)M2(O2CH)3, employing density functional theory (gradient corrected and time-dependent) with the Gaussian 98 and ADF 2000 packages, predict the planar oxalate D2h configuration to be favored, which maximizes M2 δ to oxalate π* back-bonding, and indicate low barriers ( K, and also to back-bonding into the oxalate LUMO, which weakens the C-O bonds and is more extensive for tungsten than for molybdenum. As a further aid in interpreting the Raman and resonance Raman spectra, vibrational frequency calculations were performed on the optimized D2h model (HCO2)3M2(µ-O2CCO2)M2(O2CH)3 complexes using Gaussian 98.20 Despite the apparent complexity of the systems, for M ) Mo, there are three totally symmetric modes, ν1, ν2, and ν3, that are each almost totally localized on the oxalate ligand. The predicted normal modes are depicted as atomic displacements in Figure 9, and the predicted wavenumbers for the ν1, ν2, and ν3 modes of the 12C and 13C oxalate-bridged isotopomers are given in Table 8, along with the experimentally obtained values. The calculated gas-phase values, and their isotopic shifts, are clearly in good agreement with those obtained for the solid state, given the fact that the latter would also be influenced by the intermolecular M-O interactions described earlier. The atomic displacements qualitatively explain why ν1 shows greater resonance enhancement than ν2 or ν3. The HOMO 1b2g to lowest 1b3u electronic transition takes an electron from an orbital that is predominantly M4 δ bonding, but weakly C-C antibonding and C-O bonding, to an oxalate-based π orbital that is both C-C π bonding and C-O π antibonding (see Figure 4). The expected shortening of the C-C bond and lengthening
of the C-O bonds on electronic excitation is mirrored by the atomic displacements of ν1. Related calculations for the tungsten model complex give qualitatively similar results for the identities of the ν1, ν2, and ν3 modes. Electronic Spectra. Given that the Raman data support the previously expressed view that the lowest energy bands in the visible spectra of the M4OXA complexes are MLCT transitions,1 and recognizing that the features seen for the W4OXA compound in the room-temperature solution electronic spectrum in THF appear to be a vibronic progression, we have made a more thorough examination by recording absorption spectra as a function of temperature in 2-MeTHF. The spectra of the complexes have also been obtained as Nujol mulls. Comparisons of the absorption spectra obtained at 300 and 2 K in 2-MeTHF, and the mull spectra at 80 K, are shown for Mo4OXA and W4OXA in Figures 10 and 11, respectively. Two important observations are immediately apparent from the spectra: (i) The mull spectra are red-shifted with respect to the solution spectra, and furthermore, the spectra recorded at 2 K are red-shifted with respect to those at 300 K. (ii) Vibronic features are apparent at low temperatures for both the mull and glass spectra in each case, and most obviously for W4OXA. A third observation, although not evident in Figures 10 and 11, is that the spectra recorded as mulls do not show thermochromic shifts. In the spectra of the 2-MeTHF glasses at 2 K, there appear to be two vibronic progressions, with spacings of ca. 1540 cm-1 for W4OXA and ca. 1490 cm-1 for Mo4OXA. The two progressions are separated by ca. 860 cm-1 in the former case and by ca. 670 cm-1 in the latter. At 2 K the progressions are near-baseline-resolved for W4OXA, whereas the Mo4OXA spectra reveal similar behavior but with poorer resolution. The influence of temperature on the electronic absorption spectra of the complexes has been studied by recording spectra at 2, 10, 25, 50, 77, 100, 125, 150, 175, 200, 225, 250, 275, 300, and 325 K in 2-MeTHF solution. Selected spectra are shown for W4OXA in Figure 12. Upon lowering the temperature to 2 K, a smooth bathochromic shift of each component of each progression is observed as the peaks begin to sharpen, with the lowest energy transition shifting by ca. 50 nm, and the intensity of the transition trebling. The observed vibrational bands, which relate to the photoexcited molecules, M4OXA*, would be expected to have significant oxalate character based on our earlier description of J. AM. CHEM. SOC.
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Figure 10. Electronic absorption spectra of Mo4OXA as a 2-methyltetrahydrofuran solution at 300 K, a Nujol mull at 80 K, and a 2-methyltetrahydrofuran glass at 2 K.
Figure 11. Electronic absorption spectra of W4OXA as a 2-methyltetrahydrofuran solution at 300 K, a Nujol mull at 80 K, and a 2-methyltetrahydrofuran glass at 2 K.
the lowest vacant b3u oxalate-based orbital and the reported resonance Raman data. To probe this matter further, spectra were also obtained for the µ-O213C13CO2 isotopomers. A comparison of the spectra for the 12C (natural abundance) and 13C (99% abundance) isotopomers of W OXA is shown in 4 Figure S6 in the Supporting Information. The wavenumbers of the transition origins are unaffected by isotopic substitution, but the progression intervals are shifted by ca. 50 cm-1 on 13C labeling. A summary of the vibronic features in the excitedstate complexes, M4OXA*, is given in Table 9. The thermochromic shifts of the electronic transitions in solution are understandable in terms of a preference toward 3060 J. AM. CHEM. SOC.
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adoption of the more planar structure at lower temperatures. Note that even in the 2-MeTHF glass (below ∼140 K), there is significant thermochromism. With decreasing temperature, the molar absorptivity increases dramatically because the absorptivity of this MLCT band is proportional to the square of the overlap of the b3u M4 δ and oxalate LUMO orbitals, which is at a maximum for θ ) 0°. The calculated oscillator strengths for the M4OXA compounds (Table 2) are in the order W > Mo and are large for the D2h formate model compound. The emission spectra of M4OXA have also been examined in 2-MeTHF solution at room temperature. The spectra for W4OXA are shown in Figure 13 along with the absorption
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Figure 12. Electronic absorption spectra of W4OXA at 2, 50, 100, 150, 200, 250, and 300 K in 2-methyltetrahydrofuran solution.
Figure 13. Emission spectra of W4OXA at room temperature in 2-methyltetrahydrofuran solution, measured using 705, 635, and 585 nm excitation, respectively, along with the electronic spectrum of the complex recorded at room temperature. Table 9. Summary of the Wavenumbers of the Progression Forming Modes Observed in the Electronic Spectra of Mo4OXA, W4OXA, and W413C-OXAa
first origin second origin progression interval a
Mo4OXA ν/cm-1
W4OXA ν/cm-1
W413C-OXA ν/cm-1
18 080 18 080 + 670 1490
13 420 13 420 + 860 1540
13 420 13 420 + 860 1490
As frozen glasses in 2-methyltetrahydrofuran at 2 K.
spectrum, and the spectra for Mo4OXA are given in Figure S7 in the Supporting Information. There is clearly vibronic structure with an interval of ca. 930 cm-1 in the emission spectrum of
W4OXA, though this is not as well resolved as in the absorption spectrum. Estimates of the lifetimes of the emissive states are currently under investigation as well as other spectroscopic investigations of these compounds. For compounds with M-M quadruple bonds, the assignment of the δ f δ* electronic transition has a long and interesting history.2 At low temperatures, this absorption band often reveals a vibronic progression in ν(M-M) relating to the excited-state molecule. In several instances, other totally symmetric M-L modes are also observed. In the emission spectra, vibronic features have also been observed, and their assignment has not been without considerable discussion, as it is evident that the J. AM. CHEM. SOC.
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excited-state geometry may or may not be closely related to that of the ground-state molecule, and extensive coupling of M-M and M-L vibrational modes can occur.2 In our study, it is clear that we are not dealing with M4 δ-toδ* transitions but, instead, with MLCT transitions to the oxalate. The origins of the low-energy absorption and emission bands do not coincide, and the spectra are not mirror images of one another. The M4OXA and M4OXA* compounds have different geometries, and, because of the Duschinsky effect, the coupling of the totally symmetric vibrational modes in the excited state is likely to be quite different from that in the ground state. We propose the assignment for the progression based on 1540 (M ) W) and 1490 cm-1 (M ) Mo) in the excited-state M4OXA* species to be the ν1 mode localized on the oxalate ligand. Similarly, the origin of the second progression is shifted by 860 (M ) W) or 670 cm-1 (M ) Mo) from the origin of the first progression, which is likely to be ν2. In the photoexcited state, we expect to find a more planar oxalate unit as a result of populating a C-C π bonding molecular orbital, which is also C-O π antibonding. Populating this orbital leads to ν1 for the M4OXA* species having a higher wavenumber than in the ground-state species M4OXA. The isotopic shift (13C/12C) of ca. 50 cm-1 seen in ν1 for W4OXA* is comparable to that of 43 cm-1 observed for ground-state W4OXA. The isotopic shift of ν2 for M4OXA* is insignificant in comparison to that of ν1, which parallels the trend in the enhancement of the Raman spectra of ground-state M4OXA. Concluding Remarks
The use of XRPD has allowed us to establish the nature of the molecular packing in the solid-state structures of the M4OXA compounds (M ) Mo or W). The techniques we described are likely to gain in utility and popularity as programs become more sophisticated and chemists become more and more interested in molecular design and architectural control of assemblies in the solid state. While the technique is not likely to rival single-crystal X-ray crystallography, particularly with respect to the determination of precise bond distances and angles, it can provide valuable structural information that might otherwise be unavailable in the absence of single crystals. The Raman, resonance Raman, and variable-temperature electronic spectra of these oxalate-linked “dimers of dimers” reveal the richness of orbital mixing in these extended π systems, both in the ground and excited states. Although much can now be rationalized based on the electronic structure calculations, there clearly remains more to be understood. In particular, the vibrational features that are seen in the electronic absorption spectra at low temperatures pose a challenge. Geometry and electronic structure optimization of species in electronically excited states is currently unfeasible for other than very small model systems, and hence meaningful predictions of the vibrational spectra of such excited state species are not currently possible. The barrier to rotation about the central oxalate C-C bond in the M4OXA compounds is clearly very small, as indicated by the DFT calculations and by the thermochromism displayed in solution. In the ground state, the coupling between the M2‚‚‚ M2 centers is at a maximum for planar oxalate, θ ) 0, and a minimum for a twisted (D2d) geometry, θ ) 90°. This correlates with the δ orbital splitting which is, furthermore, sensitive to 3062 J. AM. CHEM. SOC.
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the metal. The energy of the electronic transition M2 δ to the low-lying vacant oxalate-based orbital also varies greatly with respect to the torsional angle θ. The M4OXA molecules have several potential modes of binding substrates, as is evident from the solid-state structure. We would therefore expect that their spectroelectrochemical properties will be very solvent dependent and, furthermore, substrate binding perhaps through hydrogen bonding which favors a twisted or a planar oxalate-group will have a most dramatic effect. Such changes could form the basis for signaling and molecular recognition, and investigations of these notions are currently underway. Experimental Section Due to the air-sensitive nature of the complexes, all were synthesized and manipulated under dinitrogen or argon atmospheres using either standard Schlenk-line or drybox techniques. The preparation of the M4OXA compounds followed previously described procedures.1 Toluene and hexane were predried over activated molecular sieves and refluxed over molten sodium and potassium, respectively, under an atmosphere of dinitrogen, collected by distillation, and then degassed. 13C-labeled oxalic acid (Cambridge Isotope Laboratories Inc., >99% 13C) was dried and degassed in vacuo at 50 °C prior to use. Samples of the complexes were stored in sealed ampules or capillaries in a dinitrogen-filled drybox. K2C2O4‚H2O (BDH, Analar >99.99%) was used as received. K213C2O4‚ H2O was prepared from the reaction between 13C-labeled oxalic acid and potassium bicarbonate. Solid-State CPMAS NMR Spectroscopy. Solid-state cross polarization, magic angle spinning40,41 (CPMAS) NMR spectra were obtained on a Bruker DMX400 NMR spectrometer operating at a 1H frequency of 400.14 MHz and a 13C frequency of 100.64 MHz utilizing a 4 mm high-power CPMAS probe, on samples of the ground compound sealed in a glass spinner insert. Spectra were acquired with ramped cross polarization42,43 and high-power two-pulse phase-modulated (TPPM) decoupling.44 CP contact times were 5 ms, the decoupling field strengths were approximately 80 kHz, the recycle delay was 2 s, and the sample spinning frequency was 10 kHz. A total of 1K of complex data with a dwell time of 33.2 µs (spectral width of 30 kHz or 300 ppm) were zero filled to 8K of complex data points, exponentially apodized with a 5 Hz line broadening, and Fourier transformed to produce the NMR spectra, which were then referenced to the carbonyl resonance of solid glycine at 176.03 ppm with respect to TMS. 13C NMR data for Mo4OXA (solid, 100.64 MHz, 298 K): δ (ppm) 190.8 (cis and trans O2CC(CH3)3), 156.9 (µ-O2CCO2), 40.9 (cis O2CC(CH3)3), 40.1 (trans O2CC(CH3)3), 29.8 29.4 28.7 (cis and trans O2CC(CH3)3). Powder X-ray Diffraction and Structure Solution. Powder samples of M4OXA were ground using a pestle and mortar and then packed and sealed in 0.7 mm diameter glass capillaries (W. Mu¨ller). X-ray powder diffraction patterns were collected either at the Brookhaven National Laboratory or in-house. At Brookhaven, the data were collected at the X7A beamline, rotating the capillaries at 1-2 Hz. Monochromatic radiation with a wavelength of 0.8000 Å was obtained using a channelcut double-crystal Si(111) monochromator. The data were collected by step scanning over the angular range 4° < 2θ < 30° using a linear position-sensitive detector (PSD). In-house, data were collected using a Bruker D8 diffractometer equipped with copper radiation and an incident beam Ge(111) monochromator. Again the capillaries were (40) Pines, A.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1973, 59, 569-590. (41) Stejskal, E. O.; Schaefer, J.; Waugh, J. S. J. Magn. Reson. 1977, 28, 105112. (42) Hediger, S.; Meier, B. H.; Kurur, N. D.; Bodenhausen, G.; Ernst, R. R. Chem. Phys. Lett. 1994, 223, 283-288. (43) Metz, G.; Wu, X.; Smith, S. O. J. Magn. Reson., Ser. A 1994, 110, 219227. (44) Bennett, A. E.; Rienstra, C. M.; Auger, M.; Lakshmi, K. V.; Griffin, R. G. J. Chem. Phys. 1995, 103, 6951-6958.
Oxalate-Bridged Complexes of Dimolybdenum and Ditungsten rotated during data collection, and a scintillation counter was used as the detector while step scanning over the angular range 3° < 2θ < 22°. Molecular and Electronic Structure Calculations. Molecular and electronic structure determinations on the models (HCO2)3M2(µ-O2CCO2)M2(O2CH)3 were performed with density functional theory (DFT) using both the Gaussian 9820 and ADF 2000.0121-24 programs. With Gaussian 98, we have used the B3LYP45-47 functional along with the 6-31G* basis set for H, C and O,48 and the SDD energy-consistent pseudopotentials for Mo and W.49 With ADF 2000, we have used the exchange and correlation functionals of Perdew and Wang,50,51 along with a double-ζ Slater basis set with d-type polarization functions, for H, C, and O (1s frozen core for C and O), and triple-ζ basis sets were employed for both Mo (3d frozen core) and W (4d frozen core). ZORA scalar relativistic corrections were applied to both Mo and W systems under ADF.52-56 All geometries were fully optimized at the above levels using the default optimization criterion of the respective programs. Orbital analyses were completed with GaussView.57 Electronic absorption spectra were predicted using the TDDFT method implemented within the Gaussian 98 and ADF 2000.01 programs and using the default optimization criteria.30-34 Raman and Resonance Raman Spectra. Spectra were recorded on a Spex 1401 spectrometer following excitation with a range of different lines from Coherent Innova 70 Ar+ and 301 Kr+ lasers. (45) Becke, A. D. Phys. ReV. A: Gen. Phys. 1988, 38, 3098-3100. (46) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652. (47) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B: Condens. Matter 1988, 37, 785-789. (48) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab initio Molecular Orbital Theory; John Wiley & Sons: New York, 1986. (49) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123-141. (50) Perdew, J. P.; Wang, Y. Phys. ReV. B: Condens. Matter 1992, 45, 13244. (51) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B: Condens. Matter 1992, 46, 6671-6687. (52) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1993, 99, 4597-4610. (53) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1994, 101, 9783-9792. (54) van Lenthe, E.; Snijders, J. G.; Baerends, E. J. J. Chem. Phys. 1996, 105, 6505-6516. (55) van Lenthe, E.; Van Leeuwen, R.; Baerends, E. J.; Snijders, J. G. Int. J. Quantum Chem. 1996, 57, 281-293. (56) van Lenthe, E.; Ehlers, A.; Baerends, E. J. J. Chem. Phys. 1999, 110, 89438953. (57) GaussView 2.1; Gaussian Inc.: Pittsburgh, PA, 1998.
ARTICLES Wavenumber calibration was achieved by superimposing neon emission lines onto the Raman spectra, resulting in an accuracy of (1 cm-1. For Mo4OXA and Mo2(O2CtBu)4, disks of ca. 50% sample in either KCl or K2SO4 were prepared. The spectra were recorded with the sample disks held at liquid nitrogen temperature (ca. 80 K) in a purposebuilt cryostat. For W4OXA and W2(O2CtBu)4, the capillary tubes were mounted on the tip of the cryostat without further preparation. Variable-Temperature Electronic Absorption Spectra. Spectra were obtained in the temperature range 2-325 K using an Oxford Instruments OptistatBath helium cryostat placed within the sample compartment of a Perkin-Elmer Lambda 19 UV-vis-NIR absorption spectrometer. Samples were prepared in an inert atmosphere drybox and injected into a 150 µL copper cell consisting of two quartz windows separated by a permeable synthetic rubber spacer. Spectra of the compounds in the solution state were obtained in thoroughly degassed optical quality anhydrous 2-methyltetrahydrofuran (2-MeTHF, Aldrich), which forms a glass at ca. 130-40 K, while spectra of the compounds in the solid state were obtained as Nujol (Wilmad) mulls. All spectra were cumulatively averaged from two or three scans taken with a scan rate of 120 nm min-1 and using a 1-2 nm band-pass. Emission Spectra. Spectra were recorded in the solution state in thoroughly degassed optical quality anhydrous 2-methyltetrahydrofuran (2-MeTHF, Aldrich) using a SPEX FluoroMax-2 spectrometer equipped with a 150 W xenon source, a red-sensitive R928P photomultiplier tube, and DataMax-Std software. All spectra were cumulatively averaged from three scans taken with a scan rate of 60 nm min-1.
Acknowledgment. We thank the National Science Foundation at Indiana University and The Ohio State University, the Engineering and Physical Sciences Research Council, and the University of London Intercollegiate Research Service for financial support. The Ohio Supercomputer Center is gratefully acknowledged for computational resources with which the DFT calculations were performed. The Campus Chemical Instrumentation Center is also gratefully acknowledged for use of NMR facilities. Supporting Information Available: X-ray powder diffraction analysis and Raman data (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. JA011786J
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