Organometallics 2010, 29, 3387–3391 DOI: 10.1021/om100416w
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Dihydrogen Complexes of the Chromium Group: Synthesis and Characterization of (Arene)M(CO)2(H2) Complexes Jonathan D. Egbert and D. Michael Heinekey* Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700 Received May 3, 2010
Irradiation of (arene)M(CO)3 complexes (M = Cr, Mo, W; arene = various benzene derivatives) in the presence of H2 results in CO extrusion and subsequent reaction with H2. For chromium and molybdenum, the resulting complexes have moderate thermal stability and are identified as sigma complexes of H2 by NMR spectroscopy. Analogous tungsten reactions require prolonged irradiation in the presence of H2 to afford moderate yields of dihydride complexes. This study allows for comparison of related complexes for Cr, Mo, and W.
Introduction Since the seminal work of Kubas1 and co-workers, the coordination chemistry of dihydrogen has rapidly developed into an active field of study. The vast majority of isolable dihydrogen complexes reported to date are cationic. Relatively few neutral dihydrogen complexes have been reported. In the initial work by Kubas, complexes of the form (PR3)2M(CO)3(H2) were prepared for M = Cr, Mo, and W. The tungsten complex is the most amenable to study. The molybdenum analogue is slightly less stable, and the chromium analogue readily releases hydrogen in solution. We recently reported extensions of this chemistry to the simple dihydrogen complexes Cr(CO)5(H2) and W(CO)5(H2), which were prepared by photoextrusion of CO from the corresponding hexacarbonyls in the presence of hydrogen.2,3 These complexes had been previously reported in matrix isolation studies and in liquid xenon solutions, but turned out to be readily prepared in conventional solvents. While the tungsten and especially the chromium species were quite thermally stable, efforts to prepare the molybdenum analogue were not successful. It has long been known that UV irradiation of (arene)M(CO)3 (M = Cr, Mo) complexes liberates a carbonyl ligand. George and co-workers have studied the resulting *To whom correspondence should be addressed. E-mail: heinekey@ chem.washington.edu. (1) Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.; Wasserman, H. J. J. Am. Chem. Soc. 1984, 106, 451–452. (2) Matthews, S. L.; Pons, V.; Heinekey, D. M. J. Am. Chem. Soc. 2005, 127, 850–851. (3) Matthews, S. L.; Heinekey, D. M. J. Am. Chem. Soc. 2006, 128, 2615–2620. (4) (a) Creaven, B. S.; George, M. W.; Ginzburg, A. G; Hughes, C.; Kelly, J. M.; Long, C.; McGrath, I, M.; Pryce, M. T. Organometallics 1993, 12, 3127–3131. (b) Goff, S.; Nolan, T. F.; George, M. W.; Poliakoff, M. Organometallics 1998, 17, 2730–2737. (c) Breheny, C. J.; Kelly, J. M.; Long, C.; O'Keeffe, S.; Pryce, M. T.; Russel, G.; Walsh, M. M. Organometallics 1998, 17, 3690–3695. (d) Alamiry, A. H. M.; Boyle, N. M.; Brookes, C. M.; George, M. W.; Long, C.; Portius, P.; Pryce, M. T.; Ronayne, K. L.; Sun, X.; Towrie, M.; Vuong, K. Q. Organometallics 2009, 28, 1461–1468. r 2010 American Chemical Society
transient species for M = Cr and Mo under various matrix conditions in the presence of alkane, H2, and N2 ligands.4 Related studies of H2 and N2 binding to photogenerated chromium dicarbonyl species in a porous metal organic framework have been reported by Long and Kaye.5 In order to study a homologous series of chromium group complexes, we have investigated the photoextrusion of CO from arene tricarbonyl complexes of the form (arene)M(CO)3 (M = Cr, Mo, W). We find that under similar conditions the efficiency of photochemically driven reactions between H2 and (arene)M(CO)3 complexes varies greatly, with the tungsten analogues requiring the longest irradiation times. In the presence of H2, both dihydrogen and dihydride complexes are observed as products, depending upon the nature of the metal. The products have highly variable thermal stability. We have employed a range of arene ligands, with the prospect of exploring arene ring substituent effects on the H2-activating ability of the metal centers.
Results Photolysis of (Arene)Cr(CO)3 Complexes in the Presence of Hydrogen. Photolysis of (arene)Cr(CO)3 (arene = 1,3,5-iPr3C6H3; 1,3-tBu2,5-BrC6H3; mesitylene; toluene; F-C6H5) in fluorobenzene-d5 or cyclopentane solution at 230 K in the presence of H2 leads to new signals in the 1H NMR spectra corresponding to the bound arene moiety and high-field resonances consistent with the binding of dihydrogen (Table 1). Equation 1 depicts the photochemical reaction of (1,3,5-iPr3C6H3)Cr(CO)3 with H2, which was later characterized as (1,3,5-iPr3C6H3)Cr(CO)2(H2) (1) (see Discussion). For all complexes, experiments using HD gas in place of H2 were also performed to determine the coupling constants JHD, as reported in Table 1. The coupling constants JHD were extracted by fitting the observed signals as Lorentzian lineshapes. This was necessary due to overlap of the residual HH resonance with the HD resonance. (5) Kaye, S. S.; Long, J. R. J. Am. Chem. Soc. 2008, 130, 806–807. Published on Web 07/02/2010
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Organometallics, Vol. 29, No. 15, 2010
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Table 1. Chemical Shift and JHD Values with Isotopic Perturbation for Chromium Dihydrogen Complexes and the Frequencies for the A1 Mode of the Parent Tricarbonyls Measured in Cyclohexane compound
δ H2 (ppm)
ΔHD (ppb)
JHD (Hz)
(Ar)Cr(CO)3 A1 (cm-1)
(1,3,5-iPr3C6H3)Cr(CO)2(H2) (1) (1,3-tBu25-BrC6H3)Cr(CO)2(H2) (1,3,5-Me3C6H3)Cr(CO)2(H2) (MeC6H5)Cr(CO)2(H2) (F-C6H5)Cr(CO)2(H2)
-12.5 -12.4 -12.3 -12.3 -12.4
-50 -48 -48 -48 -46
33.4 33.2 34.1 33.5 34.3
1966 1969 1970 1978 1990
Similar results were obtained in cyclopentane-h10 with 1H NMR spectra recorded using solvent suppression.6 To confirm that the high-field dihydrogen resonance corresponded to the new arene ligand signals, the experiment of eq 1 was repeated in methylcyclohexane-d14 with a sufficient delay between scans to allow for reliable integration. The resulting integrals of the signals due to the photoproduct 1 were in the expected ratio of 3:3:18:2. For 1, the relaxation time constant (T1) for the metal dihydrogen resonance was measured as a function of temperature. The minimum value of T1 was 14 ms (205 K, 500 MHz). In general, the complexes listed in Table 1 have limited thermal stability. Warming the samples to room temperature led to decomposition. The dihydrogen complex 1 formed from (1,3,5-iPr3C6H3)Cr(CO)3 showed the greatest stability. At room temperature under an atmosphere of H2, complex 1 persists for several days. The reaction of eq 1 was also monitored by in situ IR spectroscopy in cyclopentane solution at 200 K. Under these conditions, the carbonyl stretching bands due to (1,3,5iPr3C6H3)Cr(CO)3 are observed at 1964 and 1895 cm-1. Irradiation while purging with H2 showed a reduction in the intensity of these bands in concert with the appearance of two new bands at 1922 and 1870 cm-1. Photolysis of (Arene)Mo(CO)3 Complexes in the Presence of Hydrogen. Similar reactions were carried out using (1,3,5iPr3C6H3)Mo(CO)3 and (mesitylene)Mo(CO)3. The 1H NMR spectra resulting from photolysis in the presence of H2 gas were recorded at low temperature due to the thermal lability of the products. The 1H NMR spectrum of the photoproducts generated from (1,3,5-iPr3C6H3)Mo(CO)3 in the presence of H2 showed two high-field resonances at -6.55 and -7.35 ppm (273 K), which were ultimately assigned as the hydridic resonances of [(1,3,5-iPr3C6H3)2(CO)2Mo(μ-H)]2 (3) and (1,3, 5-iPr3C6H3)2Mo(CO)2(H2) (2), respectively (see Discussion). In methylcyclohexane-d14, the T1min for the resonance at -6.55 ppm was 678 ms (225 K, 500 MHz), while the T1min for the -7.35 ppm resonance was 24 ms (205 K, 500 MHz). Repeating the experiment with HD gas in place of H2 gave two additional resonances, with the higher field signal exhibiting a 1:1:1 triplet due to HD coupling with JHD = 32.8 Hz (Figure 1). Similar photolysis of (mesitylene)Mo(CO)3 in the presence of HD gas gave a high-field resonance with JHD = 32.9 Hz. In order to identify the initial photoproduct, irradiation of (1,3,5-iPr3C6H3)Mo(CO)3 with H2 in cyclopentane was (6) Huang, T. L.; Shaka, A. J. J. Magn. Reson. Ser. A 1995, 112, 275.
Figure 1. Partial 1H NMR spectrum (500 MHz, 230 K) of the photolysis products of (1,3,5-iPr3C6H3)Mo(CO)3 with HD gas in cyclopentane-h10 at 233 K.
Figure 2. ORTEP diagram for [(1,3,5-iPr3C6H3)Mo(CO)2(μH)]2 (3) with 50% thermal ellipsoids. Non-hydride H atoms have been omitted for clarity.
carried out at 200 K. Examination of the high-field region of the 1H NMR spectrum at 200 K showed only the resonance at -7.35 ppm. On a preparative scale, prolonged photolysis of (1,3,5-iPr3C6H3)Mo(CO)3 with H2 in pentane at 273 K resulted in a dark purple solution. Concentration and cooling at 268 K afforded a purple crystalline solid 3. When compound 3 was dissolved, a single resonance at -6.55 ppm was observed in the high-field region of the 1H NMR spectrum. The IR spectrum of 3 in cyclohexane solution exhibits bands at 1902 and 1849 cm-1. Crystals of complex 3 suitable for X-ray diffraction were obtained by slow cooling of a pentane solution. The ORTEP diagram of 3 is shown in Figure 2. Details of the data collection and refinement procedures are given in the Experimental Section. Photolysis of (Arene)W(CO)3 Complexes in the Presence of Hydrogen. Similar photochemical reactions were carried out with (arene)W(CO)3 complexes (arene = 1,3,5-iPr3C6H3 or mesitylene). Under the same conditions used for the Cr and Mo complexes, photolysis of the W complexes with H2 was very slow. The (arene)W(CO)3 complexes are much less soluble in alkanes compared to the Cr and Mo analogues.
Article
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Dihydrogen Complexes of Chromium. Photoextrusion of CO from arene chromium tricarbonyl complexes has been studied by several research groups. In alkane solvents, infrared spectroscopy has been used to identify short-lived solvent complexes of the form (arene)Cr(CO)2(solvent).4 George and co-workers reported that photolysis of (mesitylene)Cr(CO)3 in polyethylene matrixes in the presence of H2 affords a species identified as the dihydrogen complex (mesitylene)Cr(CO)2(H2).4 Under these conditions, this dihydrogen complex is observable at room temperature. The carbonyl stretching region in the IR spectrum consists of two bands, at 1925 and 1875 cm-1. In the present study, we find that irradiation of (1,3,5-iPr3C6H3)Cr(CO)3 in pentane with H2 affords a moderately stable photoproduct with CO stretching bands at 1922 and 1870 cm-1. By comparison to the previously reported infrared data, this complex is formulated as (1,3,5iPr3C6H3)Cr(CO)2(H2) (1). Examination of 1 by 1H NMR spectroscopy revealed a high-field resonance at -12.6 ppm (integration 2 H relative to the arene ligand). The relaxation time constant (T1) of this
resonance was found to reach a minimum of 14 ms at 205 K (500 MHz), strongly suggesting that this complex contains an intact dihydrogen ligand. Using the analysis developed by Halpern,7 this relaxation time corresponds to an H-H distance of 0.78 A˚ assuming fast rotation of the bound hydrogen ligand or 0.99 A˚ if slow rotation is assumed. Further confirmation of this formulation for complex 1 was provided by the preparation of 1-d1 using photolysis in the presence of HD gas. The high-field signal of 1-d1 is a 1:1:1 triplet with JHD =33.4 Hz. Using the previously developed correlation between JHD and H-H distance, the H-H distance in complex 1 is calculated to be 0.86 A˚.8 This observation suggests that the H2 ligand in complex 1 is rotating rapidly, compared to the rate of molecular tumbling. This situation is relatively rare, in that most dihydrogen complexes reported to date are in the slow-rotation regime. A series of arene tricarbonyl complexes of chromium were investigated to probe the electronic effect of substitution in the arene ring. The IR spectra of the parent tricarbonyl complexes can be used to assess the electron-donating ability of the arene ligand. Greater electron density at the metal center correlates well with lower CO stretching frequencies (Table 1). In principle, the same factors will lead to more back-donation into the σ* orbital of the bound H2 ligand, which will increase the H-H distance and diminish the value of JHD. While there is a slight variation among the observed values of JHD, we find that there is no meaningful correlation with the IR stretching frequencies. We conclude that the H-H distance is less sensitive to these variations in electron density at the metal center than is the CO stretching frequency. Dihydrogen Complexes of Molybdenum. In our previous studies of M(CO)5(H2) complexes, synthesis of the complexes with M = Cr and W was relatively straightforward, but the molybdenum analogue could not be prepared.2 One of the objectives of the current study was to examine the properties of a complete series of group 6 complexes. We find that low-temperature irradiation of (arene)Mo(CO)3 initially yields dihydrogen complexes as described above for chromium complexes. For the (1,3,5-iPr3C6H3) complex, the 1 H NMR spectrum reveals an upfield resonance at -7.35 ppm attributed to the dihydrogen complex (1,3,5-iPr3C6H3)Mo(CO)2(H2) (2). This formulation of 2 is based on the preparation of 2-d1 using HD gas. In the 1H NMR spectrum of 2-d1, the resonance at -7.35 ppm exhibits an HD couplingof 32.7 Hz, suggesting that this signal corresponds to a dihydrogen complex with an H-H bond distance of 0.87 A˚.8 Consistent with this observation, the minimum relaxation time constant (T1(min)) for the hydride resonance of complex 2 was determined to be 24 ms (205 K, 500 MHz). Using the methodology of Halpern and co-workers,7 the H-H distance in 2 is calculated to be 0.88 A˚, assuming the fast-rotation regime. Thus the HD coupling and the relaxation time data are fully consistent. Upon warming to room temperature or prolonged lowtemperature photolysis, samples of 2 are converted to a new complex (3) with a high-field resonance at -6.55 ppm, which exhibits no coupling when deuterium is incorporated. The T1min value for this resonance is 678 ms. The lack of HD coupling and slow relaxation suggest that complex 3 does not contain a dihydrogen ligand. The integration of the
(7) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J. Am. Chem. Soc. 1991, 113, 4173–4184.
(8) Gelabert, R.; Moreno, M.; Lluch, J. M.; Lled os, A.; Pons, V.; Heinekey, D. M. J. Am. Chem. Soc. 2004, 126, 8813–8822.
Figure 3. ORTEP diagram (50% thermal ellipsoids) of (1,3,5iPrC6H3)W(CO)2H2 (4). Non-hydride H atoms have been omitted for clarity.
The solubility of (1,3,5-iPr3C6H3)W(CO)3 is 0.15 mg mL-1, and for Cr and Mo analogues solubilities are >1 mg mL-1. Irradiation at room temperature of solutions in NMR tubes for 3-5 h resulted in
