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Organometallics 2010, 29, 5358–5365 DOI: 10.1021/om1003754
Thermal Rearrangement of Osmabenzenes to Osmium Cyclopentadienyl Complexes† Paul M. Johns, Warren R. Roper, Scott D. Woodgate, and L. James Wright* Department of Chemistry, The University of Auckland, Private Bag 92019, Auckland, New Zealand Received April 30, 2010
The purple osmabenzene complex Os(C5H4{SMe-1})(CF3SO3)(CO)(PPh3)2 (1) is formed in high yield through reaction between Os(C5H4{S-1})(CO)(PPh3)2 and methyl triflate. The neutral blue osmabenzenes Os(C5H4{SMe-1})(cis-X)(CO)(PPh3)2 (X = I (2a), Cl (2b), SCN (2c), CF3CO2 (2d)) are readily obtained through treatment of 1 with the appropriate anion X-. In these complexes the geometry about osmium is approximately octahedral, with the two PPh3 ligands being mutually trans and X being cis to the SMesubstituted carbon of the metallabenzene ring. When solutions of 2a in benzene are heated under reflux, the I and CO ligands interchange positions and the brown isomeric osmabenzene Os(C5H4{SMe-1})(transI)(CO)(PPh3)2 (3a) is formed. However, if a solution of either 2a or 3a is heated under reflux in toluene, the metal-bound carbon atoms of the osmabenzene fragment couple and a mixture of the two cyclopentadienyl complexes [Os(η5-C5H4SMe)(CO)(PPh3)2]I (4a) and Os(η5-C5H4SMe)I(CO)(PPh3) (5a) is formed. Heating solutions of 2b-d is not a viable route to the corresponding brown isomers of these compounds, because cyclopentadienyl products are formed directly. In the case of 2b a mixture of the cyclopentadienyl complexes [Os(η5-C5H4SMe)(CO)(PPh3)2]Cl (4b) and Os(η5-C5H4SMe)Cl(CO)(PPh3) (5b) is formed, while in the case of 2c [Os(η5-C5H4SMe)(CO)(PPh3)2]SCN (4c) and Os(η5-C5H4SMe)(SCN)(CO)(PPh3) (5c) are formed. In contrast, [Os(η5-C5H4SMe)(CO)(PPh3)2](CF3CO2) (4d) is the only cyclopentadienyl complex formed on heating solutions of 2d. The brown osmabenzene isomers Os(C5H4{SMe-1})(trans-X)(CO)(PPh3)2 (X = Cl (3b), SCN (3c), CF3CO2 (3d)) are accessible through treatment of 3a with AgCF3SO3, followed by addition of the appropriate anion X-. Heating the brown isomers 3a,b under the same conditions gives the same mixture of cyclopentadienyl complexes that are formed when 2a,b, respectively, are heated. However, 3c,d are resistant to thermal rearrangement and remain unchanged when heated under reflux in toluene. The crystal structures of 2c, 3c, 4d, and 5a have been obtained.
Introduction Metallabenzenes are now a well established class of metallaaromatic compounds. Although a considerable number of papers have appeared that address the reaction chemistry of these species, this is still a relatively underdeveloped area.1-3 One of the fundamental reactions of metallabenzenes is the process whereby the two metal-bound carbon atoms couple to form a cyclopentadienyl ligand. This has been identified as a key decomposition route for metallabenzenes, and calculations suggest that most metallabenzenes are thermodynamically unstable with respect to this rearrangement.4,5 A deeper understanding of this process is therefore essential for the † Part of the Dietmar Seyferth Festschrift. Dedicated to Dietmar Seyferth in recognition of his outstanding contributions to the advancement of organometallic chemistry, particularly as Editor of Organometallics. *To whom correspondence should be addressed. E-mail: lj.wright@ auckland.ac.nz. (1) Bleeke, J. R. Chem. Rev. 2001, 101, 1205–1227. (2) Landorf, C. W.; Haley, M. M. Angew. Chem., Int. Ed. 2006, 45, 3914–3936. (3) Wright, L. J. Dalton Trans. 2006, 1821–1827. (4) Iron, M. A.; Lucassen, A. C. B.; Cohen, H.; van der Boom, M. E.; Martin, J. M. L. J. Am. Chem. Soc. 2004, 126, 11699–11710. (5) Iron, M. A.; Martin, J. M. L.; van der Boom, M. E. J. Am. Chem. Soc. 2003, 125, 13020–13021.
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development of rational syntheses of kinetically stable metallabenzenes. Although undetected metallabenzenes have often been proposed as intermediates in the formation of cyclopentadienyl complexes,6-16 examples that involve the well-defined transformation of isolated metallabenzenes into the corresponding (6) Ferede, R.; Allison, N. T. Organometallics 1983, 2, 463–465. (7) Ferede, R.; Hinton, J. F.; Korfmacher, W. A.; Freeman, J. P.; Allison, N. J. Organometallics 1985, 4, 614–616. (8) Mike, C. A.; Ferede, R.; Allison, N. T. Organometallics 1988, 7, 1457–1459. (9) Schrock, R. R.; Pedersen, S. F.; Churchill, M. R.; Ziller, J. W. Organometallics 1984, 3, 1574–1583. (10) Hung, W. Y.; Zhu, J.; Wen, T. B.; Yu, K. P.; Sung, H. H. Y.; Williams, I. D.; Lin, Z.; Jia, G. J. Am. Chem. Soc. 2006, 128, 13742– 13752. (11) He, G.; Zhu, J.; Hung, W. Y.; Wen, T. B.; Sung, H. H.-Y.; Williams, I. D.; Lin, Z.; Jia, G. Angew. Chem., Int. Ed. 2007, 46, 9065– 9068. (12) Clark, G. R.; Lu, G.-L.; Roper, W. R.; Wright, L. J. Organometallics 2007, 26, 2167–2177. (13) Clark, G. R.; O’Neale, T. R.; Roper, W. R.; Tonei, D. M.; Wright, L. J. Organometallics 2009, 28, 567–572. (14) Jacob, V.; Landorf, C. W.; Zakharov, L. N.; Weakley, T. J. R.; Haley, M. M. Organometallics 2009, 28, 5183–5190. (15) Jacob, V.; Weakley, T. J. R.; Haley, M. M. Organometallics 2002, 21, 5394–5400. (16) Jacob, V.; Weakley, T. J. R.; Haley, M. M. Angew. Chem., Int. Ed. 2002, 41, 3470–3473. r 2010 American Chemical Society
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cyclopentadienyl complexes are surprisingly rare. The best example is provided by the iridabenzene Ir(C5H3{Ph-1}{t-Bu2})(CO)(PPh3)2, which in solution at 50 °C is smoothly converted into the corresponding cyclopentadienyl complex Ir(η5-C5H3{Ph-1}{t-Bu-2})(CO)(PPh3) over 15 h.17 The closely related trimethylsilyl-substituted iridabenzenes (in mixtures with the corresponding precursor iridabenzvalenes) are also slowly converted in solution to the corresponding iridium cyclopentadienyl complexes, and these reactions have been monitored by NMR spectroscopy.18,19 In addition to these examples a reactive ruthenabenzene intermediate has been detected by NMR spectroscopy in solution at low temperature and shown to rearrange to the corresponding cyclopentadienyl complex on warming to 0 °C.20 In a related reaction it has been reported that on heating solid samples of the ruthenabenzene [Ru(C5H3{PPh3-2}{PPh3-4})Cl2(PPh3)2]Cl the free cyclopentadienyl derivative [C5H3{PPh3-1}{PPh3-3}]Cl is formed.21 Herein we report (i) the synthesis of the set of blue osmabenzenes Os(C5H4{SMe-1})(cis-X)(CO)(PPh3)2 (X = I (2a), Cl (2b), SCN (2c), CF3CO2 (2d); PPh3 ligands mutually trans, X and the SMe-substituted carbon mutually cis) through treatment of purple Os(C5H4{SMe-1})(cis-CF3SO3)(CO)(PPh3)2 (1) with the appropriate anion X-, (ii) the conversion of 2a into the corresponding brown isomeric osmabenzene Os(C5H4{SMe-1})(trans-I)(CO)(PPh3)2 (3a; where CO and I have interchanged positions and the I and the SMe-substituted carbon are mutually trans) through heating 2a under reflux in benzene and the formation of the remaining brown osmabenzene isomers Os(C5H4{SMe-1})(trans-X)(CO)(PPh3)2 (X=Cl (3b), SCN (3c), CF3CO2 (3d)) through sequential treatment of 3a with AgCF3SO3 and X-, (iii) heating toluene solutions of 2a-c under reflux gives mixtures of the two corresponding cyclopentadienyl complexes [Os(η5-C5H4SMe)(CO)(PPh3)2]X (4a-c) and Os(η5-C5H4SMe)X(CO)(PPh3) (5a-c), while 2d gives [Os(η5-C5H4SMe)(CO)(PPh3)2]CF3CO2 (4d) exclusively, (iv) heating toluene solutions of the brown isomers 3a,b under reflux also gives mixtures of the two corresponding cyclopentadienyl complexes (4a,b and 5a,b), although 3c,d are unreactive under these conditions, and (v) the crystal structures of 2c, 3c, 4d, and 5a.
Results and Discussion Treatment of the osambenzene Os(C5H4{S-1})(CO)(PPh3)222 with methyl triflate is a high-yielding and direct route to the purple osmabenzene Os(C5H4{SMe-1})(cis-CF3SO3)(CO)(PPh3)2 (1) (Scheme 1). Characterization data for 1 and all the other new compounds are given in the Experimental Section. The ring-numbering system used for discussion of the NMR spectra is indicated in Scheme 1. The 1H and 13C NMR spectra for 1 are entirely consistent with a metallabenzene formulation.1,2 The metallabenzene metal-bound carbon atoms C1 and C5 have characteristically downfield-shifted signals at 224.63 (17) Wu, H.-P.; Weakley, T. J. R.; Haley, M. M. Chem. Eur. J. 2005, 11, 1191–1200. (18) Wu, H.-P.; Ess, D. H.; Lanza, S.; Weakley, T. J. R.; Houk, K. N.; Baldridge, K. K.; Haley, M. M. Organometallics 2007, 26, 3957–3968. (19) Wu, H.-P.; Lanza, S.; Weakley, T. J. R.; Haley, M. M. Organometallics 2002, 21, 2824–2826. (20) Yang, J.; Jones, W. M.; Dixon, J. K.; Allison, N. T. J. Am. Chem. Soc. 1995, 117, 9776–9777. (21) Zhang, H.; Feng, L.; Gong, L.; Wu, L.; He, G.; Wen, T.; Yang, F.; Xia, H. Organometallics 2007, 26, 2705–2713. (22) Elliott, G. P.; Roper, W. R.; Waters, J. M. J. Chem. Soc., Chem. Commun. 1982, 811–13.
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Scheme 1. Syntheses of the Blue Osmabenzenes 2a-d and the Corresponding Isomeric Brown Osmabenzenes 3a-d
and 243.36 ppm, while the remaining ring carbon atoms appear in the normal aromatic region (C2 122.58 ppm, C4 126.87 ppm, and C3 146.09 ppm). The proton on C5 (H5) also has a characteristically low-field signal (at 13.93 ppm) and the remaining ring protons (H2 6.50 ppm, H3 7.04 ppm, and H4 6.94 ppm) have regular aromatic shifts. The single resonance observed for the two PPh3 ligands in the 31P NMR spectrum and the coupling to phosphorus observed for the phenyl carbon atoms in the 13C NMR spectrum indicates the two PPh3 ligands are mutually trans. We have represented the structure of 1 with the CF3SO3anion coordinated to osmium in the position cis to the SMesubstituted metallabenzene carbon (C1) because of the similarity between the IR and NMR spectral properties of 1 and the closely related osmabenzenes 2a-c. The structure of 2b has been reported previously,23 and the X-ray crystal structure determination of 2c is discussed below. However, the possibility that the CF3SO3- anion is present as an uncoordinated counteranion, with either a water molecule or the sulfur atom of the methanethiolate group coordinated to osmium, cannot be ruled out. Osmabenzene 1 is a convenient starting material for the synthesis of the set of closely related blue osmabenzenes Os(C5H4{SMe-1})(cis-X)(CO)(PPh3)2 (X = I (2a), Cl (2b), SCN (2c), CF3CO2 (2d)) through simple anion metathesis reactions (Scheme 1). In each of these cases the ligand X is cis to the SMe-substituted carbon of the metallabenzene ring. We have previously reported an alternative synthesis of 2a that involves treatment of Os(C5H4{S-1})(CO)(PPh3)2 with methyl iodide.22 Furthermore, 2b can also be prepared by treating 2a first with AgCF3SO3 and then LiCl, although this is a less efficient route than that depicted in Scheme 1. As might be expected, the 1H and 13C NMR spectra of each of the blue compounds 2a-d are all very similar and are consistent with osmabenzene formulations. As an example, (23) Johns, P. M.; Roper, W. R.; Woodgate, S. D.; Wright, L. J. Acta Crystallogr., Sect. E 2009, E65, m1319.
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Figure 1. Molecular structure of 2c with thermal ellipsoids at the 50% probability level. Hydrogen atoms have been omitted for clarity.
Figure 2. Molecular structure of 3c with thermal ellipsoids at the 50% probability level. Hydrogen atoms have been omitted for clarity.
the resonances of the ring protons of 2c are observed at 6.36 (H2), 7.03 (H3), 6.38 (H4), and 13.17 ppm (H5) and the ring carbon atoms appear at 244.70 (C1), 120.00 (C2), 146.72 (C3), 125.98 (C4), and 225.32 ppm (C5). The crystal structure of 2c has been determined, and there are two molecules in the asymmetric unit. The molecular structure of one of these is shown in Figure 1, and for simplicity the discussion of the bond lengths and angles below refers to only one of these molecules. The crystal data and refinement details for 2c and for the other crystal structures reported in this paper are available in the Supporting Information. The overall geometry is approximately octahedral with the two triphenylphosphine ligands arranged mutually trans. The cis disposition of the SCN- ligand and the SMe-substituted carbon atom is confirmed. The SCN- ligand is bonded through nitrogen in an approximately linear geometry with the angle Os-N1-C8 = 168(1)°. The two osmabenzene Os-C distances (Os-C1 = 2.112(8) A˚, Os-C5 = 2.025(8) A˚) are not equivalent but are consistent with there being some multiple character to each of these bonds. The longest of these two distances (Os-C1) involves the carbon trans to the CO ligand. The C-C bond distances within the planar osmabenzene ring are entirely appropriate for a metallabenzene (C1-C2 = 1.428(11) A˚, C2-C3 = 1.375(12) A˚, C3-C4 = 1.384(12) A˚, C4-C5 = 1.393(11) A˚). During investigations into the thermal stability of the metallabenzenes 2a-d it was found that, on heating a benzene solution of 2a under reflux for a period of 10 min, the color changes from blue to brown. The brown product isolated in good yield from solution is the isomeric osmabenzene Os(C5H4{SMe-1})(trans-I)(CO)(PPh3)2 (3a), which differs from 2a in that the iodide and carbonyl ligands have interchanged positions so that the iodide is now trans to the SMe-substituted carbon atom (Scheme 1). This different geometry gives rise to small changes in the resonances of the osmabenzene ring protons in the 1H NMR spectrum of 3a. These are observed at 5.67 (H2), 6.90 (H3), 7.28 (H4), and
12.40 ppm (H5). Subtle changes are also apparent in the 13C NMR spectrum of 3a (244.18 (C1), 120.48 (C2), 146.07 (C3), 128.22 (C4), and 219.34 ppm (C5)), with the largest change (6.7 ppm) occurring for C1. Although we have not been able to obtain crystals of 3a that are suitable for an X-ray crystal structure determination, we have obtained the crystal structure of the thiocyanate analogue 3c (see below), and this confirms the geometry depicted in Scheme1. In contrast to the thermal isomerization process that transforms blue 2a to brown 3a, heating benzene solutions of 2b-d does not lead cleanly to the corresponding brown isomers Os(C5H4{SMe-1})(trans-X)(CO)(PPh3)2 (X = Cl (3b), SCN (3c), CF3CO2 (3d)). Instead, conversion to cyclopentadienyl complexes of osmium very slowly ensues (see below). Nevertheless, 3b-d can be conveniently prepared by the treatment of 3a with AgCF3SO3, removal of the AgI precipitate, and then addition of the appropriate anion X(Scheme 1). As might be expected, the resonances of the metallabenzene ring atoms in the 1H and 13C NMR spectra of 3a-d are all very similar. The crystal structure of 3c has been obtained, and the molecular structure is shown in Figure 2. The structure confirms that the CO and SCN ligands have exchanged positions compared with the case for 2c and the thiocyanate ligand is now trans to the SMesubstituted carbon of the metallabenzene ring. As in 2c, the SCN ligand in 3c is bound through nitrogen and has an approximately linear geometry (Os-N-C8 = 176.2(3)°). The two osmabenzene Os-C distances are inequivalent (Os-C1 = 2.029(4) A˚, Os-C5 = 2.091(4) A˚), and again it is the bond trans to the CO ligand (in this case Os-C5) that is the longest. The bond distances within the planar osmabenzene ring (C1-C2 = 1.434(6) A˚, C2-C3 = 1.369(6) A˚, C3-C4 = 1.425(6) A˚, C4-C5 = 1.366(6) A˚) all fall within the range observed for other metallabenzenes but clearly show some alternation in length. Unfortunately the relatively high esd values obtained for the corresponding distances in 2c means that no significant differences
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Scheme 2. Rearrangement of the Blue Osmabenzenes 1 and 2a-d and the Isomeric Brown Osmabenzenes 3a,b to the Corresponding Cyclopentadienyl Osmium Complexes 4a-e and 5a-c
Figure 3. Molecular structure of 5a with thermal ellipsoids at the 50% probability level. Hydrogen atoms have been omitted for clarity.
between these two sets of metallabenzene C-C distances can be discerned. With the establishment of convenient routes to the two sets of isomeric osmabenzenes 2a-d and 3a-d, the possibility that these osmabenzenes might undergo rearrangement to cyclopentadienyl compounds under more vigorous thermal conditions was investigated. It was found that if 2a is heated under reflux in the higher boiling solvent toluene for 1 h the initial blue color first changes to brown and then fades as the colorless cationic cyclopentadienyl complex [Os(η5-C5H4SMe)(CO)(PPh3)2]I (4a) precipitates from solution. This material is obtained in about 45% yield after isolation and purification. In addition, the neutral cyclopentadienyl complex Os(η5-C5H4SMe)I(CO)(PPh3) (5a) can be recovered from this reaction in about 42% yield (Scheme 2). Two sets of equivalent protons for the cyclopentadienyl ligand of 4a are observed in the 1H NMR spectrum at 4.91 and 5.03 ppm, while the SMe group appears at 2.55 ppm. In the 13C NMR spectrum the carbon atoms of the cyclopentadienyl ligand appear at 81.53, 90.85, and 113.09 ppm (C-SMe). The X-ray crystal structure of the trifluoroacetate analogue of 4a has been determined (see below), and this confirms the geometry depicted in Scheme 2. The neutral cyclopentadienyl complex 5a is chiral, and multiplet signals are observed for the four cyclopentadienyl protons at 4.71 (2H, overlapping signals), 4.78 (1H), and 5.00 ppm (1H). The five inequivalent carbon atoms of the cyclopentadienyl ligand are observed in the 13C NMR spectrum at 74.66, 77.99, 81.07, 83.88, and 110.69 (C-SMe) ppm. The X-ray crystal structure of 5a has been determined, and the molecular structure is shown in Figure 3. The distances between osmium and the carbon atoms of the cyclopentadienyl ligand are Os-C1 = 2.258(8) A˚, Os-C2 = 2.305(10) A˚, Os-C3 = 2.307(8) A˚, Os-C4 = 2.215(8) A˚, and Os-C5 = 2.221(8) A˚. The longest distances are Os-C2 and Os-C3, which are positioned nearly opposite the CO ligand. The other structural parameters associated with 5a are normal.
The transformation of the isolated and fully characterized osmabenzene 2a into the cyclopentadienyl complexes 4a and 5a provides a very rare example of this important metallabenzene decomposition route. Therefore the behavior of the other closely related metallabenzenes 2b-d and 3a-d in boiling toluene was also explored in an attempt to learn more about the influence that the nature of the ancillary ligands and the coordination geometry has on this reaction. It was found that on heating under reflux toluene solutions of the blue osmabenzenes 2b-d for 1 h conversion into osmium cyclopentadienyl products also occurred. The products formed are presented in Table 1. Osmabenzenes 2b and 2c form the chloride and thiocyanate analogues of 4a and 5a, respectively. The cyclopentadienyl products 4b-c and 5b-c have been characterized by high resolution ESI-MS as well as 1H and 31P NMR spectroscopy. As expected, the NMR spectra of these species are very similar to the spectra of the corresponding compounds 4a and 5a, respectively. In contrast to these results, heating under reflux a toluene solution of 2d results in the rapid (
