[acs symposium series] inorganic fluorine chemistry volume 555 (toward the 21st century) || new and...

Chapter 15

New and Revisited Transition Metal Chemistry of Fluoro-olefins

and Fluorodienes

Russell P. Hughes1, Owen J. Curnow1, Peter R. Rose1, Xiaoming Zheng1, Erin N. Mairs1, and Arnold L. Rheingold2

1Chemistry Department, Burke Laboratory, Dartmouth College, Hanover, NH 03755-3564

2Chemistry Department, University of Delaware, Newark, DE 19716

Organometallic chemistry of three classes of fluorinated ligands is reported. New Ru(II) complexes, [Ru(C5Me5)(acac)(C2F4)] (acac = acetylacetonate, and substituted analogues), containing η2-tetrafluoro-ethylene are described. Dynamic 19F NMR studies show unprecedentedly low barriers to propeller rotation of the C2F4 ligands. The first complex, [Ru(C5Me5)Cl(C4F6)], containing an η4-hexafluoro-butadiene ligand is reported; it exists as two conformational isomers, one of which has been characterized by X-ray crystallography. Hexafluorobutadiene reacts with [RhCl(PPh3)3] to afford initially a five-coordinate hexafluororhodacyclopentene complex, which is extraordinarily sensitive to water and undergoes facile hydrolysis to generate an α-ketone complex. In contrast, hexafluorobutadiene reacts with [RhCl(PMe3)3] to afford a six-coordinate hexafluororhodacyclopentene complex, mer-[RhCl(PMe3)3(C4F6)], which reacts with water to give a β-ketone complex. Finally, the first successful synthesis of a complex containing an η5-pentafluorocyclopentadienyl ligand is described. The ruthenocene, [Ru(C5Me5)(C5F5)], is prepared by flash vacuum pyrolysis of the corresponding η5-pentafluorophenoxide complex, with extrusion of CO.

Organometallic compounds with perfluorinated ligands have been known since the earliest days of the renaissance of die field in the 1950s, and advances in the field have been reviewed by a number of authors (1-6). This paper will deal with recent work in our group in three principal areas which illustrate the kinds of bonding trends, structural preferences, and reaction chemistry encountered in fluorocarbon-transition metal chemistry: a new class of ruthenium-tetrafluoroethylene complexes which exhibit unusually low barriers to propeller rotation of the olefin; new metallacyclic complexes, derived from hexafluorobutadiene, which exhibit facile hydrolytic activation of CF2 groups; and the first successful synthesis of a transition metal complex containing an -̂pentafluorocyclopentadienyl ligand.

0097-6156/94/0555-0252$08.00/0 © 1994 American Chemical Society

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In Inorganic Fluorine Chemistry; Thrasher, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

15. H U G H E S E T A L . Chemistry of Fluoro-olefins and Fluorodienes 253

Tetrafluoroethylene Complexes with Metallacyclopropane Structures, but with Low Activation Barriers to Olefin Rotation.

The d8-rhodium(I) complex 1 is often used as a paradigm for illustrating the differences between the bonding of the hydrocarbon olefin, ethylene, and its fluorinated analogue, tetrafluoroethylene, to transition metal centers. The molecular structure of 1 shows a more metallacyclopropane interaction for the C2F4 ligand than for the C2H4 (7), and while the activation barrier for propeller rotation of the C2H4 ligand can be measured using NMR techniques to be 13.6 ± 0.6 kcal mol - 1 [57 kJ mol" 1] (8), the rate of rotation of C2F4 is so slow on the NMR timescale so as not to result in any broadening of the 1 9 F resonances in 1 even at 100°C. This latter observation, taken with many other analogous observations of apparently static behavior in other d 8

and d 1 0 complexes of C2F4 (7) has led to the assumption that propeller rotation of C2F4 should invariably have a high activation barrier. It is sometimes assumed that the observation of a metallacyclopropane structure should necessarily mean a high barrier to propeller rotation, but this is not the case, as the metallacyclopropane structure is simply one extreme of a continuum of bonding possible within the Dewar-Chatt-Duncanson model of the transition-metal olefin bond (9). Theoretical studies of conformational preferences and rotational barriers in metal-olefin complexes suggested clearly that the lowest barriers to propeller rotation of olefins should occur in d 6

complexes (9), and led to our attempts to make tetrafluoroethylene complexes of d 6

metal centers. Prior to embarking on a study of d6-complexes we revisited one d 8 compound

of C2F4, the acetylacetonato (acac) complex 2. The 56 MHz 1 9 F NMR spectrum was reported to exhibit an apparent single environment for all 4 F atoms, (10) possibly indicative of facile propeller rotation, yet its molecular structure is consistent with a metallacyclopropane RÎ1-C2F4 interaction like that proposed in 1 (11). We found that the 1 9 F NMR spectrum of this compound at 282 MHz (CDCI3; 20°C) can be simulated as an AA'BB'X pattern [VA = -115.1, VR = -116.1 ppm rel. CFCI3; J A A ' = J B B ' = -2.4, J A B = JA'B' = 104.3, J A B ' = J B A ' = -56.6, J R H A = 9.6, JRJIB = 3.3 Hz], confirming that rotation is slow on the NMR time scale. A small chemical shift difference is probably the reason for the single F environment reported for the dipivalomethane analogue of this acac complex (72).

Our synthetic approach to d 6 complexes of C2F4 started with the well known tetrameric complex [RuCp*Cl]4 (Cp* = CsMes) (13), which was shown to react with C2F4 to afford the crystallographically characterized dimer complex 3 (14). This compound proved too insoluble for the required variable temperature NMR studies, and so mononuclear β-diketonate analogues 4a-c were prepared by reaction of 3 with the appropriate thallium salt (75). The molecular structure of 4a was determined crystallographically, and an ORTEP is shown in Figure 1. The key bond and dihedral angles associated with the RU-C2F4 subunit, also shown in Figure 1, were found to be almost identical with those found in 1, clearly indicating that a metallacyclopropane description of the bonding was quite appropriate for 4a (75). The 1 9 F NMR spectrum of 4a showed two broad resonances which sharpened into an AA'XX' pattern at low temperatures, but which coalesced into a single peak at about 100°C, with AGi = 55 ± 2 kJ mol - 1 . This behavior is consistent with propeller rotation, or with a dissociation/recombination process for 1 9 F site-exchange. However, clear support for the former mechanism is provided by the variable temperature 1 9 F NMR spectrum of 4b, which shows the expected four resonances of an AGMX pattern at low temperature, and coalescence to two resonances at 100°C with AG* = 53 ± 3 kJ mol"1. Dissociation/recombination would scramble all four 1 9 F environments, whereas propeller rotation only accomplishes a pairwise interchange of mutually trans-fluorines (Fi~>F3,F3^Fi; F4~>F2). That the latter process occurs without any pseudorotation at the metal is confirmed by the observation that 4c exhibits fluorine

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254 INORGANIC FLUORINE CHEMISTRY: TOWARD THE 21ST CENTURY

RuCp*(C2F4)(acac) RhCp(C2ll4)(C2F4) d(C-F) 1.349(7) 1.351(3) d(C-M) 2.047(6) 2.024(2) cl(C-C) 1.395(9) 1.405(7) α 73.9° 74.3° β 53.0° 52.8° δ 114.5° 114.3° γ 131.0° 131.4°

Figure 1. ORTEP of 4a and comparison of its structural features with those of 1 (7). Definition of angles α, β, δ, and γ, is provided in reference (36).

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15. HUGHES ET AL. Chemistry of Fluoro-olefins and Fluorodienes 255

site exchange analogous to that observed for 4a, without any corresponding site exchange of diastereotopic methyl groups within the isopropyl groups in the1!! NMR spectrum over the same temperature range (16). The only remaining explanation for the observed site exchange of fluorines is that of propeller rotation of the tetrafluoroethylene ligand. Trie activation barriers for C 2 F 4 rotation are at the low end of those usually found for ethylene rotation in d 8 and d 1 0 complexes, and are unprecedented^ low for tetrafluoroethylene. It remains to be seen whether other d 6 complexes containing C 2 F 4 can be prepared, and whether the facility of propeller rotation in these compounds proves to be a general phenomenon. It is noteworthy that the lack of facile propeller rotation of C 2 F 4 in d 8

and d 1 0 complexes may contribute to the lack of reactivity of C 2 F 4 in these compounds towards insertion into metal-carbon bonds. Whether such insertion proves to be more facile in d 6 systems remains to be explored.

New and Revisited Transition Metal Chemistry of Hexafluorobutadiene.

Compared to the abundant chemistry of its hydrocarbon analogue, 1,3-butadiene, that of hexafluorobutadiene is remarkably sparse. Two well characterized complexes containing this ligand have been reported prior to our work. In contrast to butadiene itself, which binds to the tricarbonyliron fragment in the well-known T|4-diene mode, hexafluorobutadiene reacts with iron carbonyl to afford the fluorinated metallacyclopentene complex 5 (77), which has been characterized unambiguously by X-ray crystallography (18). The driving force for this mode of reactivity may well be the formation of stronger metal-carbon σ-bonds to fluorinated carbon atoms, and the gem-difluoro effect which arises from the exothermicity associated with the pyrarmdalization of a planar C F 2 group (19). In addition to 5 the only other complex containing hexafluorobutadiene appears to be the r|2-diene complex of platinum(O), 6, which has been characterized spectroscopically (20). Examples of reaction chemistry of hexafluorobutadiene are also few: the insertion of hexafluorobutadiene into a Co-H bond to give allylic complexes of cobalt has been reported (27), and some early reports of defluorination reactions involving rhodium are revisited below.

We were intrigued that no examples of T|4-hexafluorobutadiene complexes had been reported, and buoyed by our success at binding C 2 F 4 to d 6 centers, we carried out the reaction of hexafluorobutadiene with the tetrameric reagent [RuCp*Cl]4. The reaction proceeds smoothly to give two isomeric products 7 and 8, which differ only in the exo/endo conformation of the diene ligand. The isomer 7 appears to be the more thermodynamically stable, and a single crystal X-ray diffraction study has confirmed the conformation of the rj4-diene (22). An ORTEP is shown in Figure 2, together with a schematic diagram showing relevant distances and angles. There is extensive pyramidalization at the terminal C F 2 groups, and the C-C bond distances in the coordinated diene are consistent with the valence bond description shown in the line drawing of 7. This type of coordination is analogous to that found previously in the tricarbonyliron compound containing rj4-bound perfluoro-1,3-cyclohexadiene (23). The chemistry of complexes 7 and 8 is under ongoing investigation in our group.

We were puzzled by an early report that reaction of hexafluorobutadiene with Wilkinson's compound, [RhCl(PPh3)3], in hot benzene solution, afforded a tetrafluorometallacyclopentadiene complex, with apparent loss of one PPI13 ligand and two fluorines! While the proposed structures 9 or 10 would require four different fluorine and two different phosphorus environments, the 1 9 F NMR spectrum of this compound showed only three 1 9 F resonances in a 2:1:1 ratio, leading to the assumption that two fluorines in the ring were accidentally isochronous. Unfortunately no 3 Φ NMR data were reported. In addition, the IR spectrum of this compound showed two bands at 1739 and 1672 cnr1, which were attributed to the two fluorinated double bonds in the ring.

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Me

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15. HUGHES ET AL. Chemistry of Fluoro-olefins and Fïuorodienes 257

Figure 2. ORTEP of 7, with a schematic representation of some key bond distances (in Â). There is a crystallographic plane of symmetry relating the two halves of the molecule.

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The apparent cleavage of two C-F bonds, and loss of two fluorines to an unspecified fate intrigued us, and we re-investigated this reaction. Repetition under the literature conditions afforded 60-70% yields of yellow crystalline material with almost identical 1 9 F NMR and IR data to those reported in the literature. Subjection of single crystals of this compound for X-ray crystallographic analysis revealed the true structure of the compound as 11, in which the hexafluorobutadiene had been converted into a tetrafluorometallacyclopentenone ligand. The interpretation of the 1:1:2 pattern in the 1 9 F NMR spectrum is now clear, and the IR bands can be reassigned as 1672 (i)c=o) and 1734 (x>c=c) cm"1. An ORTEP of the structure of 11 is shown in Figure 3, together with a schematic drawing containing key bond distances (24).

Clearly one CF2 group in hexafluorobutadiene has undergone conversion to a ketone. We regarded water as the most likely reagent for this transformation, with formation of the C=0 and 2 equivalents of HF providing the thermodynamic compensation for cleavage of 2 C-F bonds, but careful drying of solvents and reagents still resulted in formation of good yields (60%) of 11. Only when the internal glass surfaces of the reaction vessel were silylated was formation of 11 suppressed; in its place was formed the metallacyclopentene complex 12, containing a metallacyclic ring analogous to that found in the iron complex 5. Characterization of 12 was accomplished spectroscopically using 1 9 F and 3 1 P NMR. When solutions of 12 were transferred to unsilylated glass vessels, or when water was added to dry solutions of 12, conversion to 11 was rapid.

We reasoned that the loss of the third triphenylphosphine might be sterically driven, and that the presence of a vacant coordination site in 12 might be important in facilitating its conversion to 11. Consequently we carried out the corresponding reaction of hexafluorobutadiene with the analogous rhodium complex [RhCl(PMe3)3]. In this reaction a clean conversion to the metallacyclic complex 13 was observed. This product was characterized spectroscopically and by a single crystal X-ray diffraction study (24). An ORTEP and key bond distances are shown in Figure 4. The complex contains a metallacyclopentene ring analogous to that found in 12, but with pseudo-octahedral coordination around rhodium and retention of all three PMe3 ligands meridionally bound in the coordination sphere. Complex 13 shows no tendency to react with water at room temperature but on heating hydrolysis did occur to give a mixture of two isomeric complexes 14 and 15. Isomer 15 could be crystallized from the mixture and its structure was confirmed by X-ray crystallography (24). An ORTEP and key bond distances are shown in Figure 5. The metallacyclic ring is isomeric with that observed in 11, with the ketone function β to the metal rather than a.

Thus we conclude that in a 5-coordinate system such as 12 hydrolysis of an a -CF2 in the metallacycle is extraordinarily facile; notably, subsequent hydrolysis of the second CF2 group is much slower, and certainly does not occur at 60°C. In the 6-coordinate complex 13 the additional phosphine on the metal shuts down facile hydrolysis of an 0C-CF2 in the hexafluorometallacyclopentene ring, but under more forcing conditions hydrolysis occurs at the β-position. Some mechanistic speculation as to the possible role of a vacant coordination site on rhodium is presented in Scheme 1. The transition metal chemistry of hexafluorobutadiene is clearly significantly different from that of its hydrocarbon analogue, and is under continuing investigation in our group.

The Pentafluorocyclopentadienyl Ligand.

While there are thousands of examples of transition metal complexes containing the cyclopentadienyl ligand and its substituted analogues, there are few examples of compounds containing pentachloro-, pentabromo-, and pentaiodo-cyclopentadienyl ligands (25). The absence of the pentafluorocyclopentadienyl ligand from this group is

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15. H U G H E S E T A L . Chemistry of Fluoro-olefins and Fïuorodienes 259

Figure 3. a) ORTEP of 11; b) ORTEP of 11 with phenyl groups omitted from die PPh3 ligands; c) a schematic representation of some key bond distances (in A).

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Figure 4. ORTEP of 13, with a schematic representation of some key bond distances (in Â).

Figure 5. ORTEP of 15, with a schematic representation of some key bond distances (in Â).

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noteworthy, especially in view of the considerable effort that has gone into attempts to synthesize this missing member of the family. One might expect that the perfluorocyclopentadienyl ligand might impart enhanced thermal stability, and significantly different physical and chemical properties to its transition metal complexes, as compared to its hydrocarbon analogue. In fact, only one example of a complex containing a monofluorocyclopentadienyl ligand, monofluoroferrocene, has been characterized with any degree of certainty (26,2 7); although (monofluorocyclopentadienyl)tricarbonylmanganese has also been reported, no 1 9 F NMR data were given (28).

The most sustained efforts to prepare pentafluorocyclopentadienyl-transition metal compounds came in the 1980s, following the elegant synthesis of the pentafluorocyclopentadienyl anion (29). Remarkably, attempts to bind this anion to transition metal centers via metathesis reactions with transition metal halides were unsuccessful (30). Other routes involving the 5H-pentafluorocyclopentadiene and its 5-bromo analogue also met with no success (37), leading to suggestions that η 5 -pentafluorocyclopentadienyl ligands bound to transition metals were perhaps unstable (30).

Our alternative approach to this fascinating ligand was stimulated by recent observations that the affinity of the [RuCp*]+ fragment for binding to arene rings allows the r|5-phenoxide complexes 16 to be prepared (32,33). We were quickly able to establish that the pentafluorophenoxide analogue 17 could be generated easily, and could be characterized by its 2:2:1 pattern in the 1 9 F NMR spectrum and the characteristic C=0 stretch at 1620 cm'1 in its IR spectrum (34). Attempts to extrude CO from this molecule by extended reflux or photolysis in various solvents failed to do so. However, when subjected to flash vacuum pyrolysis (FVP) at 770°C complex 17 underwent extrusion of CO, to give a 20% yield of white crystalline 18, in which the presence of the pentafluorocyclopentadienyl ring was confirmed by a combination of mass spectrometric, and J H , 1 9 F , and 1 3 C NMR spectroscopic data. (34). Complex 18 melts in air without decomposition at 235 °C. Its 1 9 F NMR spectrum appears as a sharp singlet at δ -213.2 ppm to high field of CFCI3. The 1 3C{ lU] NMR spectrum of the pentafluorocyclopentadienyl ring can be simulated successfully as an ABB'CC'X system, with X = 13C, and a small isotopic perturbation of the chemical shift of the unique fluorine (A) compared to the other four ring fluorines (35). Unfortunately, despite repeated attempts, we have been unable to obtain a crystal structure of 18, due to an end for end disorder problem. Attempts to circumvent this problem by preparing the ethyltetramethylcyclopentadienyl analogue of 18 were fruitless; while the compound was easily prepared by the same method, its crystals showed identical disorder problems. Attempts to prepare other derivatives are underway.

This successful synthesis of the pentafluorocyclopentadienyl ligand, albeit by an unorthodox route, demonstrates clearly that complexes of this ligand can be prepared, and that they are likely to be thermally stable, thus re-opening the synthetic challenge to develop suitable general syntheses of complexes of this ligand.

The work outlined here shows that there are still many interesting discoveries to be made in the organometallic chemistry of fluorinated ligands. The synthesis of new ligands, evaluation of their effects on the chemical and physical properties of organometallic compounds, and investigation of their chemical reactivity, continue to present significant intellectual challenges.

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15. HUGHES ET AL. Chemistry of Fïuoro-olefins and Fïuorodienes 263

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Acknowledgments.

We are grateful to the Air Force Office of Scientific Research (Grant AFOSR-91-0227) and the National Science Foundation for generous support of our research. Generous loans of precious metal salts from Johnson Matthey Aesar/Alfa are gratefully acknowledged.

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114, 3153. 16. Curnow, O. J.; Hughes, R. P.; Mairs, Ε. N. unpublished results. 17. Hunt, R. L.; Roundhill, D. M.; Wilkinson, G. J. Chem. Soc. (A), 1967, 982. 18. Hitchcock, P. B.; Mason, R. J. Chem. Soc. Chem. Commun., 1967, 242. 19. Smart, Β. E. In "The Chemistry of Functional Groups, Supplement D," (S.

Patai and Z. Rappoport, eds), 1983, Chapter 14, John Wiley & Sons, New York.

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21. Sasaoka, S. -I.; Joh, T.; Tahara, T.; Takahashi, S. Chem. Lett., 1989, 1163. 22. Hughes, R. P.; Rose, P. R.; Zheng, X.; Rheingold, A. L. unpublished results. 23. Churchill, M. R.; Mason, R. Proc. Roy. Soc. A, 1967, 301, 433. 24. Hughes, R. P.; Rose, P. R.; Rheingold, A. L. unpublished results. 25. For a recent compilation of such compounds, see: Winter, C. H.; Han, Y. -H.;

Heeg, M. J. Organometallics 1992, 11, 3169. 26. Hedberg, F. L.; Rosenberg, H. J. Organomet. Chem. 1971, 28, C14. 27. Popov, V. I.; Lib, M.; Haas, A. Ukr. Khim. Zh. (Russ. Ed.) 1990, 56, 1115. 28. Cais, M.; Narkis, N. J. Organomet. Chem. 1965, 3, 269. 29. Paprott, G; Seppelt, K. J. Am. Chem. Soc. 1984, 106, 4060. 30. Paprott, G.; Lehmann, S.; Seppelt, K. Chem. Ber. 1988, 121, 727. 31. Paprott, G.; Lenz, D.; Seppelt, K. Chem. Ber. 1984, 117, 1153. 32. Koelle, U.; Wang, M. H.; Raabe, G. Organometallics 1991, 10, 2573. 33. Loren, S. D.; Campion, B. K.; Heyn, R. H.; Tilley, T. D.; Bursten, Β. E.;

Luth, K. W. J. Am. Chem. Soc. 1989, 111, 4712. 34. Curnow, O. J.; Hughes, R. P. J. Am. Chem. Soc., 1992, 114, 5895. 35. Curnow, O. J.; Hughes, R. P. unpublished results. We are grateful to Professor

David Lemal for suggesting this spin system for a successful simulation. 36. Ittel, S.D.; Ibers, J.A. Adv. Organomet. Chem., 1976, 14, 33. RECEIVED February 24, 1994

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