Synthesis of Tungsten and Molybdenum Carbon Dioxide Complexes

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Synthesis of Tungsten and Molybdenum Carbon Dioxide Complexes Robert G. Carden, Jr.,† James J. Ohane,† Robert D. Pike,‡ and Peter M. Graham*,† †

Department of Chemistry, Saint Joseph’s University, 5600 City Avenue, Philadelphia, Pennsylvania 19131, United States Department of Chemistry, The College of William & Mary, P.O. Box 8795, Williamsburg, Virginia 23187-8795, United States

‡

S Supporting Information *

ABSTRACT: Tungsten and molybdenum η2-CO2 complexes of the form TpM(NO)(L)(η2-CO2), where M = W, Mo, Tp = tris(pyrazolyl)borate, and L = PMe3, 1-methylimidazole, have been synthesized by displacement of an η2-aromatic ligand under elevated CO2 pressure. These complexes have been characterized by single-crystal X-ray diffraction. The η2-CO2 complexes are air stable and resist decomposition in solution. In addition, the molybdenum η2-CO2 complex undergoes chemical reduction of the η2-CO2 ligand to form TpMo(NO)(1-methylimidazole)(CO).

D

of dry CO2 (50−100 psi initial pressure) for 16 h, the resulting metal fragments coordinate CO2 and form TpMo(NO)(1methylimidazole)(η2-CO2) (3; Scheme 1) and TpW(NO)-

espite the inherent economic and environmental problems associated with a carbon-based economy, the world remains dependent on fossil fuels as the primary energy and chemical-feedstock source.1 Consequently, scientists are seeking new CO2-utilizing processes as alternatives to existing energy-intensive and fossil-feedstock-reliant technologies.2,3 Because of the inherent stability of CO2, many of these emerging methods require a transition metal that can activate CO2 toward reactions with other substrates.4−6 Since CO2 coordination can be a prerequisite for activation, the preparation and study of isolable CO2 complexes is of great interest.4,7 Side-on (η2) coordination is often observed for monomeric complexes of CO2.8,9 Since the landmark report of the CO2 complex Ni(η2-CO2)(PCy3)2 by Aresta in 1975,10,11 others have prepared monomeric η2-CO2 complexes of other transition metals, including titanium,12 niobium,13−15 tantalum,16 molybdenum,17−21 tungsten,22,23 rhenium,24 iron,25−27 ruthenium,28 rhodium,29,30 iridium,31 and palladium.26 η2-CO2 complexes are most commonly prepared using lowvalent, electron-rich metals with filled d orbitals capable of back-bonding into the high-energy π* orbital of CO2. Thus, our pursuit of isolable η2-CO2 complexes led us to the metal fragments {TpMo(NO)(1-methylimidazole)}32 and {TpW(NO)(PMe3)},33 which are adept π- bases capable of coordinating a range of conjugated π systems. Here we report the ability of these metal fragments to coordinate CO2 and form thermally stable η2-CO2 complexes. η 2-CO2 complexes are most frequently prepared by substitution of a labile ligand such as nitrogen,19 an alkene,24 or a diene.26,34 Thus, TpMo(NO)(1-methylimidazole)(η2furan) (1) and TpW(NO)(PMe3)(η2-trifluorotoluene) (2) seemed prudent precursors to CO2 complexes, since heating results in the dissociation of the aromatic ligand with half-lives for 1 and 2 of approximately 2−4 h at 80 °C in solution.32,33 Indeed, when they are heated under slightly elevated pressures © 2013 American Chemical Society

Scheme 1. Synthesis of the Molybdenum η2-CO2 Complexa

a Reagents and conditions: (i) 1,2-dimethoxyethane (DME), CO2 (100 psi, initial pressure), 70 °C, 16 h.

(PMe3)(η2-CO2) (4; Scheme 2), as mixtures of coordination diastereomers. For molybdenum, a small amount of the carbonyl complex TpMo(NO)(1-methylimidazole)(CO) (5) is also formed. Complexes 3 and 4 were isolated after silica chromatography in 55% and 70% yields, respectively. The solid-state (ATR) infrared spectra show strong stretching frequencies at 1619 cm−1 (3) and 1592 cm−1 (4), corresponding to the nitrosyl. In addition, both spectra display a strong absorption consistent with the antisymmetric C−O stretching mode for η2-CO2 at 1755 cm−1 (3) and 1710 cm−1 (4).8 The 13C NMR spectra of 3 and 4 feature resonances near 200 ppm attributable to the CO2 carbons. Received: March 12, 2013 Published: April 29, 2013 2505

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Scheme 2. Synthesis of the Tungsten η2-CO2 Complexa

a

Reagents and conditions: (i) THF, CO2 (50 psi, initial pressure), 60 °C, 16 h. 1

H NMR spectra of both 3 and 4 reveal two sets of resonances that correspond to two η2-CO2 coordination diastereomers (A and B) which derive from a rotation of the CO 2 ligand. 35 For the molybdenum complex 3, this diastereomeric ratio is initially 10:1 and persists upon heating an acetone-d6 solution in a J. Young NMR tube at 50 °C for 3 days. For the tungsten complex 4, the initial diastereomeric ratio is 6:4 (4A:4B). 31P NMR spectra provide 1J(P−183W) coupling constants of 288 Hz (4A) and 277 Hz (4B), consistent with other η2 complexes of {TpW(NO)(PMe3)}.33 The 13C NMR spectrum features a doublet (2JCP = 35.5 Hz) for the minor η2-CO2 carbon signal (208.5 ppm) and a singlet for the major η2-CO2 carbon (207.6 ppm). The doublet indicates that the minor diastereomer is very likely 4B, in which the coordinated CO2 carbon is pointing toward PMe3. For η2 complexes of {TpW(NO)(PMe3)}, carbon−phosphorus coupling is typically observed only for the tungsten-bound carbon proximate to phosphorus.33 When 4 is dissolved in acetone-d6 and heated for 3 days at 90 °C in a J. Young NMR tube, the slow conversion of 4B to 4A gives a final diastereomeric ratio of >20:1 (4A:4B). The 1H NMR experiments above indicate the remarkable thermal stability of 3 and 4 to loss of CO2 in solution. In fact, when 3 is heated in acetone-d6 beyond 3 days at 50 °C, slow loss (∼1% per day) of the signals for 3A and 3B is observed, concomitant with the appearance of resonances for free 1methylimidazole. No other signals are observable by 1H NMR spectroscopy, and the fate of the CO2 ligand is thus far unknown. For 4, no decomposition of the complex is observed in acetone-d6 at 90 °C over 3 days on the basis of integration of the PMe3 resonances vs an internal standard. This is remarkable, since both previously reported tungsten η2-CO2 complexes decompose at room temperature.22,23 Cyclic voltammograms of 3 and 4 in N,N-dimethylacetamide (DMA) show an irreversible oxidation (Ep,a) at 1.13 and 1.00 V vs NHE, respectively. Both η2-CO2 complexes are air stable for days at ambient temperature in solution and for weeks as solids. Recrystallization of 3 by layering of THF solutions with hexane allows the isolation of single crystals suitable for analysis by X-ray diffraction. However, positional disorder of the nitrosyl and η2-CO2 ligands complicates the analysis of the diffraction data for the bulk of the crystals obtained from a THF solution of 3. Fortunately, a small number of THF solvate crystals have also been isolated that provide satisfactory structural data for 3A (Figure 1). As with other η2-CO2 complexes, 3A features characteristic lengthening of the coordinated C−O bond and contraction of the O−C−O angle. Single crystals of 4 can also be obtained by THF−hexane layering. Figure 2 depicts the data for the minor diastereomer 4B, in which the coordinated carbon is oriented toward PMe3.

Figure 1. Solid-state molecular structure (50% thermal ellipsoids) of the molybdenum η2-CO2 complex 3A·THF. The THF solvate has been omitted for clarity. Selected bond lengths (Å) and bond angles (deg): Mo1−C1a = 2.145(5), Mo1−O1a = 2.144(2), C1a−O1a = 1.218(5), C1a−O2a = 1.186(5); Mo1−C1a−O2a = 144.8(3), Mo1− C1a−O1a = 73.5(3), O1a−C1a−O2a = 141.7(4).

Figure 2. Solid-state molecular structure (50% thermal ellipsoids) of the tungsten η2-CO2 complex 4B. Selected bond lengths (Å) and bond angles (deg): W1−C1a = 2.060(13), W1−O1a = 2.162(7), C1a−O1a = 1.323(14), C1a−O2a = 1.212(16); W1−C1a−O2a = 157.2(10), W1−C1a−O1a = 76.0(7), O1a−C1a−O2a = 126.7(10).

In addition to NO and η2-CO2 positional disorder, the 4A/4B coordination diastereomers cocrystallize, further complicating the analysis of this diffraction data. Consequently, neither the NO nor η2-CO2 ligand can be modeled anisotropically. Nevertheless, the resulting metrical parameters do not deviate from those for the handful of monomeric η2-CO2 complexes of early transition metals that have been crystallographically characterized.13,14,24 Indeed, complete structural data have been reported for only one other tungsten23 and two other molybdenum η2-CO2 complexes.17,19 The characterization data are consistent in indicating that the tungsten fragment {TpW(NO)(PMe3)} exhibits greater π basicity in comparison to {TpMo(NO)(1-methylimidazole)}. This superior back-bonding ability results in a longer C−O bond length for the coordinated C−O bond of CO2, a more acute O−C−O bond angle, lower energy nitrosyl and carbonyl stretches, and a less positive oxidation potential for the tungsten complex. A known reaction pathway for molybdenum η 2 -CO 2 complexes is reduction to a CO complex via oxygen atom transfer to another substrate.18,36 IR spectra of the crude reaction mixture of 3 reveal a small peak at 1870 cm−1. The minor product that gives rise to this CO stretching frequency 2506

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also features a nitrosyl stretch at 1588 cm−1 and can be isolated in up to 18% yield by column chromatography during the workup of 3. 1H NMR spectra and cyclic voltammetry confirm that this minor product is TpMo(NO)(1-methylimidazole)(CO) (5), which has been reported previously.32 Formation of 5 is suppressed when the reaction is run at more dilute concentrations of 1 (Scheme 1). As noted above, the molybdenum η2-CO2 complex 3 is quite thermally stable, and signals indicating the presence of the carbonyl complex 5 have never been detected by IR or NMR spectroscopy upon heating of 3 in CDCl3, THF, or toluene under N2 or CO2. Thus, the presence of the furan complex 1 or its decomposition products is required to convert the η2-CO2 complex 3 to the CO complex 5. This implies that the metal fragment {TpMo(NO)(1-methylimidazole)}, which is produced upon heating, may be capable of abstracting an oxygen from the ligated CO2. Indeed, when an equimolar mixture of 1 and rigorously purified 3 are dissolved in THF and refluxed for 18 h under a nitrogen atmosphere, IR spectra reveal the presence of the CO complex 5 in small quantities. However, the bulk of the metal fragment decomposes. When furan is used as the solvent in an effort to impede metal fragment decomposition, larger amounts of 5 are produced. However, 57% of the η2-CO2 complex remains unreacted, and conversion of 3 to 5 is only 10% (isolated yields). Unfortunately, the putative molybdenum complex containing the abstracted oxygen has not been isolated thus far. The complex [TpMo(NO)(μ-O)]2 is one possible outcome, since the Tp* (3,5-dimethylpyrazolylborate) analogue [Tp*Mo(NO)(μ-O)]2 is known,37 although not in this context. It is likely that inefficient O atom abstraction by the metal fragment enables the isolation of the molybdenum η2-CO2 complex 3. The reduction of the η2-CO2 ligand in 3 can also be accomplished using sodium, giving 5 in 10% yield. Such reducing conditions could be promoting a disproportionation. In fact, other molybdenum η2-CO2 complexes20 and metallocarboxylate anions, such as Li2W(CO)5(η1-CO2),38,39 are known to undergo reductive disproportionation. However, the expected carbonate complex has not been detected. Thus, reduction may simply be generating quantities of the oxygenabstracting {TpMo(NO)(1-methylimidazole)} metal fragment. In summary, we have synthesized new examples of η2-CO2 complexes of molybdenum and tungsten supported by the πbasic metal fragments {TpMo(NO)(1-methylimidazole)} and {TpW(NO)(PMe3)}. These new η2-CO2 complexes join a small collection of monomeric CO2 complexes that can be isolated at ambient temperature, persist in solution, and resist spontaneous loss of CO2 at ambient pressure. The tungsten complex 4 is the first known thermally stable η2-CO2 complex of tungsten. The molybdenum η2-CO2 complex 3 is susceptible to chemical reduction to give the metal carbonyl complex 5, albeit in low yield. Further studies of these new η2-CO2 complexes are ongoing in our laboratory and will be reported in due course.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for P.M.G.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to thank Dr. Frederick J. Hollander for his vital contributions in resolving the disorder issues in the crystallographic work. This work was supported by Saint Joseph’s University. We also acknowledge the NSF and the College of William and Mary for the purchase of the X-ray equipment.

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, a table, and CIF files giving full synthetic details for the preparation of compounds 3−5, selected spectra of these compounds, and crystallographic information for 3 and 4. This material is available free of charge via the Internet at http://pubs.acs.org. 2507

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