Organometallics 2000, 19, 5257-5259
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Acidic C-H Activation of Methyltrioxorhenium(VII): Isolation and Characterization of Two Compounds Having the Novel Group C5H5N-CH2-ReO2 Coordinated to Tin(IV) Cungen Zhang, Ilia A. Guzei, and James H. Espenson* Ames Laboratory and Department of Chemistry, Iowa State University of Science and Technology, Ames, Iowa 50011 Received August 8, 2000 Summary: Pyridine acts as a nucleophile toward the methylene group of the tautomer of CH3ReO3, CH2d Re(O)2OH. The structures of two complexes, [{C5H5NCH2-Re(NC5H5)3(O)2}2](SnMe2Cl2)(ReO4)2 and [{3CH 3 C 5 H 4 NCH2 -Re(NC5 H 4 -3-Cl)3 (O)2 } 2 ](SnMe2 Cl 2 )(ReO4)2‚2CHCl3, prepared by the reaction of pyridine with CH3ReO3 in the presence of SnMe2Cl2, were confirmed by NMR and X-ray determinations. Compounds that feature a high-valent metal, one or more oxo groups, and an alkyl group, a combination at one time regarded as exceptional, are now more widely known. One of the most remarkable early members of this group is methyltrioxorhenium(VII) (CH3ReO3 or MTO). The original synthesis1 gave this compound in minor yield, but better synthetic procedures for it have since been developed.2 The C-H bond of CH3ReO3 appears to be somewhat acidic. It is soluble in water, and in D2O its NMR spectrum has three additional peaks of low intensity, indicating slow and incomplete H/D exchange. Although direct proof of this mechanism is lacking, it might reasonably be attributed to tautomeric forms of MTO, CH3ReO3 and CH2dRe(O)2OH. The addition of pyridine to MTO in nitromethane led to an enhanced rate of this H/D exchange into the methyl group of MTO.3 Solvent exchange with ketones is catalyzed by bases as well as acids because of the keto-enol transformation.4 We have now isolated two compounds that may arise from the nucleophilic trapping of a tautomer of MTO. Pyridine and 3-methylpyridine add to the methylene group of the proposed CH2dRe(O)2OH intermediate under the added influence of an electropositive Sn(IV) center. The two compounds are [{C5H5N-CH2-Re(NC5H5)3(O)2}2](SnMe2Cl2)(ReO4)2 (1) and [{3-CH3C5H4N-CH2-Re(NC5H4-3-Cl)3(O)2}2](SnMe2Cl2)(ReO4)2‚ 2CHCl3 (2). These compounds were prepared from MTO (0.5 g, 2 mmol) and an excess of pyridine for 1 (0.79 g, 10 mmol) or of 3-methylpyridine for 3 (1.13 g, 10 mmol) in the presence of an amount of dimethyltin dichloride * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Beattie, I. R.; Jones, P. J. Inorg. Chem. 1979, 18, 2318. (2) Herrmann, W. A.; Kratzer, R. M.; Fischer, R. W. Angew. Chem., Int. Ed. Engl. 1997, 36, 2652-2654. (3) Wang, W.-D.; Espenson, J. H. J. Am. Chem. Soc. 1998, 120, 11335-11341. (4) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic Chemistry, 3rd ed.; Harper Collins: New York, 1987; pp 723731.
(0.1 g, 0.5 mmol) equivalent to MTO in 10 mL of chloroform. The solvent was slowly evaporated, and brown crystals were obtained and isolated in ∼80% yield for both 1 and 2. Compounds 1 and 2 are stable in dry air. They are soluble in acetone and chloroform but insoluble in benzene and toluene. In the presence of water and free pyridine in acetone, both complexes decompose; the peak at 6.38 ppm decreases and the pyridine peaks shift until they change entirely to [Re(O)2(NC5H4-3-R)4]+ (R ) H, CH3). The new compounds were characterized by elemental analysis5 and NMR spectroscopy.6 A sharp peak in the 1H NMR spectrum at 6.38 ppm was found for both 1 and 2, indicative of the methylene group. In the range 7.0-9.0 ppm are found four to five multiplets that can be resolved into three sets of chemically distinct pyridine groups: that bonded to CH2, that trans to the CH2Py group, and the pair trans to one another. The CH3-Sn resonance occurs at 1.24 ppm in both 1 and 2. Shoulder peaks at 1.39 and 1.07 ppm of ∼15% intensity are consistent with the abundances of 119Sn (I ) 1/2, 8.6%) and 117Sn (I ) 1/2, 7.5%). The structure of 1 was determined by X-ray crystallography.7 The ORTEP diagram for 1 is shown in Figure 1, and selected bond distances and angles are given in Table 1. The X-ray data for 2, on the other hand, were of low quality and were refined only to reveal atomic connectivity; an ORTEP diagram is given in the Supporting Information (Figure S-1). Bond angles and distances for 2, therefore, cannot be cited. Both 1 and 2 comprise a dipositive cation with three nearly octahedral centers, balanced by two perrhenate anions. Three pyridines and one methylene carbon atom are coordinated to rhenium in the equatorial plane, and two oxo (5) Anal. Found (calcd) for 1, C44H50Cl2N8O12Re4Sn: C, 29.15 (29.08); H, 2.77 (2.77); N, 6.03 (6.17). Found (calcd) for 2, C54H68Cl8N8O12Re4Sn: C, 30.73 (30.02), H, 3.21 (3.16), N, 5.20 (5.19). (6) 1: 1H NMR (CDCl3) δ 8.74 (d, 4H, o-H); 8.64 (d, 4H, o-H); 8.50 (m, 3H, p-H); 7.78 (d, 1H, p-H); 7.50 (m, 4H, meso-H); 7.32 (m, 4H, meso-H); 6.37 (s, 2H, CH2); 1.21 (s, 3H, CH3-Sn); 13C NMR (acetoned6) δ 154.49, 151.65, 150.93, 129.22, 128.33, 127.61, 127.29 (pyridine); 49.0 (CH2), 20.46 (CH3-Sn). 2: 1H NMR (CDCl3) δ 8.75, 8.73, 8.66, 8.64, 8.62 (o-H, Py), 7.79, 7.77, 7.61, 7.54, 7.50, 7.49, 7.48 (p-H, Py), 7.36, 7.34, 7.33, 7.32, 7.30, (meso-H, Py); 13C NMR (CDCl3) δ 150.99, 148.46, 142.63, 141.41, 141.25, 140.21, 137.97, 135.90, 126.43, 124.93, 55.09, 18.55, 18.42, 18.24. (7) X-ray crystallography of 1: temperature, 173(2) K; wavelength, 0.710 73 Å; crystal system, monoclinic; space group, Pc1/c; Z ) 2; absorption coefficient, 9.586 mm-1; θ range for data collection, 2.0226.37°; absorption correction, SADABS; refinement, full-matrix least squares on F2; final R indices (I > 2σ(Ι)), R1 ) 0.220, wR2 ) 0.0463.
10.1021/om000683r CCC: $19.00 © 2000 American Chemical Society Publication on Web 11/11/2000
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Supporting Information presents the details of the crystallographic solution. The mechanism might be understood in terms of nucleophilic attack of pyridine at the methylene group of the enol of MTO, made electrophilic by the inductive effect of Re(VII) as compared to nucleophilic enols of ketones. Alternatively, as a reviewer pointed out, the intermediate can be regarded as a hydroxycarbene. Acid-base interaction of an oxo oxygen of the tautomer with Sn(IV) gives rise to a further inductive effect that allows the reaction to occur, because it reduces the electron density at the CH2 group, promoting attack of the pyridine nucleophile. The reaction does not proceed in the absence of the tin reagent. These events can be depicted as
Figure 1. ORTEP diagram for [{C5H5N-CH2-Re(NC5H5)3(O)2}2](SnMe2Cl2)(ReO4)2 drawn with 40% probability ellipsoids. Table 1. Selected Bond Distances (pm) and Angles (deg) for 1 Re-O(1) Re-O(2) Re-C Sn-O(2)
173.9(3) 180.6(2) 215.5(4) 221.9(2)
∠O(1)-Re-O(2) ∠N(1)-Re-N(3)
Re-N(1) (cis to CH2) Re-N(3) (cis to CH2) Re-N(2) (trans to CH2)
175.81(12) 178.89(13)
∠C(16)-Re-N(2) ∠N(4)-C(16)-Re
215.3(3) 215.1(3) 224.2(3) 176.39(14) 113.2(3)
ligands occupy trans positions that are slightly bent: ∠O-Re-O ) 175.8(1)°. The Sn atom has a trans disposition of two CH3 and two Cl groups equatorially, with an oxo group from Re in each axial position. The two rhenium atoms and their ligands are related by symmetry. The terminal Re-O(1) distance of 173.9(3) pm in 1 is similar to those in [Re(O)2(4-MePy)4]+ (175 pm)8 and in [Re(O)2(CH2But)(Py)3] (3) (174.5(9) pm).9 On the other hand, it is shorter than the Re-O(2) bond in 1 (180.6(2) pm). The O(2) atom also coordinates to Sn(IV), which leads to the partial delocalization of π electrons from RedO to Sn and a reduction of the Re-O(2) bond order. The Re(1)-C(16) bond distance in 1 is 215.5(4) pm, slightly shorter than those in 3 (217 and 219 pm)8 but slightly longer than in [MeReO(mtp)2]2 (4) (212.0 pm)10 and in [MeReO(mtp)PPh3] (5) (212.5 and 213.1 pm).10 The d(Re-Py) values for mutually trans Py ligands in 1 are 215.5(3) and 215.3(3) pm, nearly the same as the comparable distances of 214.7 pm in 3 and 213 pm in [ReO2(4-MePy)4]+ (5). The Re-N distance to the pyridine trans to the methylene group in 1 is 224.2(3) pm, about 10 pm longer than the average of the Re-N distances of the two pyridines trans to one another, owing to the strong trans influence of the alkyl ligand. A similar effect was found for the pyridines in 3. The (8) Johnson, J. W.; Brody, J. F.; Ansell, G. B.; Zentz, S. Inorg. Chem. 1984, 23, 2415-2418. (9) Cai, S.; Hoffman, D. M.; Wierda, D. A. Organometallics 1996, 15, 1023-1032. (10) Jacob, J.; Guzei, I. A.; Espenson, J. H. Inorg. Chem. 1999, 38, 1040-1041.
The electron path in the diagram shows the reduction of Re(VII) to Re(V), which we have made explicit by showing the usually omitted lone pair of d electrons on Re(V). Accompanying this reduction is the 2e oxidation of the nitrogen atom of pyridine. Hydroxide ions are released, as shown; they hydrolyze MTO to perrhenate ions, as has been described,11-13 thus completing the overall stoichiometry leading to 1 and 2:
MeReO3 + OH- f ReO4- + CH4 Only a few reports of isolated Py-CH2-M compounds have appeared: [CpFe(CO)2-CH2-Py]+ (7),14,15 [Rh(βdiketonate)Cl-CH2Py] (8),16 [Pt(Py)Cl2-CH2Py]+ (9),17 [CpRe(NO)(PPh3)-CH2Py]+ (10),18 and [CpRh(PMe3)I2CH2Py]+ (11).19 Their syntheses were based on pyridine attack on a methylene carbene, on methylene insertion into a metal-pyridine bond,17 on nucleophilic displacement of halide from M-CH2X,14,15 and on displacement of Me2S from M-CH2SMe2.18 The compound most closely related to 1 and 2 is the spectroscopically (11) Abu-Omar, M.; Hansen, P. J.; Espenson, J. H. J. Am. Chem. Soc. 1996, 118, 4966-4974. (12) Laurenczy, G.; Luka´cs, F.; Roulet, R.; Herrmann, W. A.; Fischer, R. W. Organometallics 1996, 15, 848-851. (13) Espenson, J. H.; Tan, H.; Mollah, S.; Houk, R. S.; Eager, M. D. Inorg. Chem. 1998, 37, 4621-4624. (14) Barefield, E. K.; McCarten, P.; Hillhouse, M. C. Organometallics 1985, 4, 1682-1684. (15) Bellinger, G. C. A.; Friedrich, H. B.; Moss, J. R. J. Organomet. Chem. 1989, 366, 175-186. (16) Fennis, P. J.; Budzelaar, P. H. M.; Frijns, J. H. G.; Orpen, A. G. J. Organomet. Chem. 1990, 393, 287-298. (17) Hanks, T. W.; Ekeland, R. A.; Emerson, K.; Larsen, R. D.; Jennings, P. W. Organometallics 1987, 6, 28-32. (18) McCormick, F. B.; Gleason, W. B.; Zhao, X.; Heah, P. C.; Gladysz, J. A. Organometallics 1986, 5, 1778-1785. (19) Werner, H.; Paul, W.; Feser, R.; Zolk, R.; Thometzek, P. Chem. Ber. 1985, 118, 261-274.
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characterized species [(HBpz)3ReO(OH)CHMePy]+, which was obtained from [(HBPz)3ReO(OTf)Et] and pyridine.20 Perhaps the method we have used can be developed more broadly for oxometal systems in particular, when even a small amount of the tautomeric form can coexist with the precursor. The use of SnMe2Cl2 is an important feature of the method, as it stabilizes the reactive tautomer relative to the other. Without the tin reagent, no crystals could be obtained even after all the solvent (20) DuMez, D. D.; Mayer, J. M. J. Am. Chem. Soc. 1996, 118, 12416-12423.
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had been evaporated; only dark red solids were obtained. Acknowledgment. This research was supported by a grant from the National Science Foundation. Some experiments were conducted with the use of the facilities of the Ames Laboratory. Supporting Information Available: Crystallographic data for 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org. OM000683R
