2ReRe(CO)2Cp* and Fragmentation and Rearrangement Reactions

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Organometallics 1996, 15, 2640-2649

Ligand Additions to Cp*(CO)2RedRe(CO)2Cp* and Fragmentation and Rearrangement Reactions of Cp*(CO)2Re(µ-CO)Re(CO)(L)Cp* Charles P. Casey,* Ronald S. Carinõ, Hiroyuki Sakaba, and Randy K. Hayashi Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706 Received December 11, 1995X

Reaction of Cp*(CO)2RedRe(CO)2Cp* (1) with CO produced the stable adduct Cp*(CO)2Re(µ-CO)Re(CO)2Cp* (2). Reaction of 1 with CH3CN gave the stable adduct Cp*(CO)2Re(µCO)Re(CO)(CH3CN)Cp* (6). Reaction of 1 with PMe3 or CH2dCH2 at low temperature produced the adducts Cp*(CO)2Re(µ-CO)Re(CO)(PMe3)Cp* (5) and Cp*(CO)2Re(µ-CO)Re(CO)(CH2dCH2)Cp* (7), which fragment at -20 °C to Cp*Re(CO)2(THF) and either Cp*Re(CO)2(PMe3) or Cp*Re(CO)2(CH2dCH2). Reaction of 1 with HCtCH gave the dimetallacyclopentenone Cp*(CO)2Re(µ-η1,η3-CHdCHCO)Re(CO)Cp* (10) without the observation of an intermediate. Reaction of 1 with CH3CtCCH3 at -60 °C initially produced the 1:1 adduct Cp*(CO)2Re(µ-CO)Re(CO)(η2-CH3CtCCH3)Cp* (8). At -40 °C, the 2-butyne complex 8 slowly converted to a mixture of dimetallacyclopentenone Cp*(CO)2Re[µ-η1,η3-(CH3)CdC(CH3)CO]Re(CO)Cp* (9) and two fragmentation products: Cp*Re(CO)3 and Cp*Re(CO)(CH3CtCCH3) (14). At room temperature, 9 converted to additional Cp*Re(CO)3 and Cp*Re(CO)(CH3CtCCH3) (14). A fluxional process that interchanges the environments of the Cp* groups and the methyl groups of Cp*(CO)2Re[µ-η1,η3-(CH3)CdC(CH3)CO]Re(CO)Cp* (9) is suggested to proceed via the dimetallacyclobutene intermediate Cp*(CO)2Re(µ-η1,η1CH3CdCCH3)Re(CO)2Cp* (A). Introduction Cp*(CO)2RedRe(CO)2Cp* (1) is a rare example of a dimer of a d6 16 electron metal fragment.1 The reactions of 1 with 2-electron donors are being investigated as a convenient route to new dirhenium derivatives. In a preliminary communication, we reported the rapid reactions of 1 with CO to produce Cp*(CO)2Re(µ-CO)Re(CO)2Cp* (2) and with H2 to produce Cp*(CO)2Re(µ-H)2Re(CO)2Cp*. We have also reported that the reaction of 1 with the monosubstituted alkyne HCtCC(Me)dCH2 produced the dimetallacyclopentenone Cp*(CO)2Re{µ-η1,η3-CHdC[C(CH3)dCH2]CO}Re(CO)Cp* (3)2 and that reaction of 1 with dimethyl acetylenedicarboxylate (DMAD) produced the dimetallacyclobutene Cp*(CO)2Re(µ-η1,η1-CH3O2CCdCCO2CH3)Re(CO)2Cp* (4).3

Here we report that the addition of 2-electron donor ligands to 1 produces a series of tetracarbonyl complexes Cp*(CO)2Re(µ-CO)Re(CO)(L)Cp* [L ) CO (2), PMe3 (5), CH3CN (6), CH2dCH2 (7), CH3CtCCH3 (8)] which have interesting stereochemistries and display fascinating Abstract published in Advance ACS Abstracts, April 15, 1996. (1) Casey, C. P.; Sakaba, H.; Hazin, P. N.; Powell, D. R. J. Am. Chem. Soc. 1991, 113, 8165. (2) Casey, C. P.; Ha, Y.; Powell, D. R. J. Am. Chem. Soc. 1994, 116, 3424. (3) Casey, C. P.; Carinõ, R. S.; Hayashi, R. K.; Schladetzky, K. D. J. Am. Chem. Soc. 1996, 118, 1617. X

S0276-7333(95)00950-2 CCC: $12.00

fluxional behavior. The observation of a 2-butyne complex is particularly important since such alkyne complexes are likely intermediates in the formation of both dimetallacyclopentenones and dimetallacyclobutenes. The conversion of the 2-butyne adduct 8 to the dimetallacyclopentenone Cp*(CO)2Re[µ-η1,η3-(CH3)CdC(CH3)CO]Re(CO)Cp* (9) and the fluxional behavior of 9 that requires a symmetric dimetallacyclobutene or dimetallabicyclobutane intermediate is also presented. Results CO Addition. A green THF solution of Cp*(CO)2RedRe(CO)2Cp* (1) reacted with CO to give a yellow solution of the known Cp*(CO)2Re(µ-CO)Re(CO)2Cp* (2),4 which was isolated in 95% yield after chromatography. The reaction of 1 with CO is extremely fast. When a THF solution of 1 at -78 °C was rapidly added by cannula to a saturated solution of CO (e6 mM)5 in THF at -78 °C, an instantaneous (t1/2 e 0.1 s) color change from green to yellow was observed.6 A related (4) Hoyano, J. K.; Graham, W. A. G. J. Chem. Soc., Chem. Commun. 1982, 27. (5) (a) Field, L. R.; Wilhelm, E.; Battino, R. J. Chem. Thermodyn. 1974, 6, 237. Mole fraction solubility data of CO in toluene at 283313 K was extrapolated to 195 K assuming a linear relationship of ln [CO] and 1/T. (b) Nudelman, N. S.; Doctorovich, F. J. Chem. Soc., Perkin Trans. 2 1994, 1233. Solubility of CO in THF at room temperature was calculated to be 0.011 M at 25 °C under 1 atm CO. (c) Krauss, W.; Gestrich, W. Chem.-Tech. (Heidelberg) 1977, 6, 513. The temperature dependence of the solubility of CO in THF was assumed to be similar to that of 1,4-dioxane. [CO] was estimated to be 6 ( 3 mM in THF at 195 K under 1 atm CO. (6) Assuming a second-order rate law for addition of CO to 1 allowed estimation of a rate constant of k2 g 103 M-1 s-1 and an activation barrier of ∆G‡ e 9 kcal mol-1.

© 1996 American Chemical Society

Ligand Additions to Cp*(CO)2RedRe(CO)2Cp*

Organometallics, Vol. 15, No. 11, 1996 2641

Figure 1. Carbonyl region (2100-1600 cm-1) of the infrared spectra of Cp*(CO)2Re(µ-CO)Re(CO)(L)Cp* adducts: 2 (L ) CO), 5 (L ) PMe3), 7 (L ) CH2dCH2), 6 (L ) NCCH3, KBr), 6 (L ) NCCH3, in THF), 8 (L ) CH3CtCCH3).

addition of CO across the MtM triple bond of Mo2(Ot-Bu)6 to produce (t-BuO)2Mo(µ-O-t-Bu)2(µ-CO)Mo(O-tBu)2 has been reported.7

In the X-ray structure of Cp(CO)2Re(µ-CO)Re(CO)2Cp,8 the Cp analog of 2, each Re center has a four-legged piano stool geometry with the adjacent Re, the bridging carbonyl, and the two terminal carbonyls forming the legs. The terminal carbonyls trans to the bridging carbonyl are designated axial and the others equatorial (carbonyls 3/4 and 2/5, respectively, in the Newman projection on the left).

The pattern of IR absorbances of 2 is similar to that of its Cp analog.8 In the IR spectrum of 2, four terminal

and one bridging CO stretching bands are observed (Figure 1). A medium-intensity band at 1967 cm-1 (0.15) and a high-intensity band at 1922 cm-1 (1.00) are assigned to νsym and νasym of the equatorial CO’s, a highintensity band at 1893 cm-1 (0.75) and a mediumintensity band at 1869 cm-1 (0.19) are assigned to νsym and νasym of the axial CO’s, and a medium-intensity band at 1707 cm-1 is assigned to the bridging carbonyl.9 The observation of only a single carbonyl resonance at δ 214.7 in the 13C NMR spectrum of 2 at -80 °C in THF-d8 requires a rapid fluxional process to interchange the environments of the carbonyl groups. This fluxional process converts 2 into its enantiomer by bridging an axial CO and unbridging the µ-CO ligand. In this process, an axial CO becomes equatorial and an equatorial CO becomes axial. We favor a concerted mechanism for this exchange process. However, we cannot exclude stepwise mechanisms involving unbridged or doubly bridged intermediates as suggested by Caulton for CO scrambling of the Cp analog of 2.10 The exchange (7) Chisholm, M. H.; Kelly, R. L.; Cotton, F. A.; Extine, M. W. J. Am. Chem. Soc. 1978, 100, 2256. (8) Foust, A. S.; Hoyano, J. K.; Graham, W. A. G. J. Organomet. Chem. 1971, 32, C65. (9) Since the intensities of the IR bands are crucial in assigning stereochemistry in this paper, the relative absorbances of bands are given in parentheses following the peak position reported in cm-1.

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Casey et al. Scheme 1

of carbonyl environments of Cp(CO)Rh(µ-CO)Rh[P(OPh)3]Cp has been suggested to occur by concerted interchange of a terminal and a bridging CO.11,12 PMe3 Addition. The reaction of PMe3 with 1 in THF-d8 at -78 °C gave a yellow solution of Cp*(CO)2Re(µ-CO)Re(CO)(PMe3)Cp* (5), which was characterized spectroscopically at -78 °C. The 1H NMR spectrum of 5 exhibited two Cp* resonances at δ 1.89 and 1.94 and a PMe3 doublet (J ) 9 Hz) at δ 1.52. In the 13C NMR spectrum, two carbonyl resonances of equal intensity were seen: a doublet (JCP ) 11 Hz) at δ 232.4 and a singlet at δ 212.4. The high-frequency resonance requires a pair of CO ligands fluctuating between bridging (δ ∼250)13 and terminal (δ ∼210) positions and closely associated with the coupled PMe3 ligand; the lower frequency resonance requires a pair of terminal CO ligands not interacting with the PMe3 ligand. On the basis of this 13C NMR spectrum, the major species in solution must be a rapidly fluxional equatorial PMe3 isomer. It is clear from the drawings shown in Scheme 1 that, in a rapidly fluxional equatorial isomer, carbonyls 3 and 5 remain bonded to the Re bearing the PMe3 ligand and, consistent with the 13C NMR result, only their averaged signal would show coupling to 31P. Although an axial PMe3 isomer may also be in rapid equilibrium, it cannot be a major species. Fluxionality in the axial isomer exchanges carbonyls 1 with 2 and 3 with 5 which would result in coupling of both 13C resonances with 31P, contrary to the experimental results. The infrared spectrum of 5 gives a signature for an equatorial substituted complex (Figure 1). The lowenergy band at 1835 cm-1 (0.28) is assigned to the axial CO on the Re bearing the PMe3 ligand; its lower intensity is attributed to intensity being “stolen” from this ν“asym” stretch by interaction with the ν“asym” stretch of the other axial CO stretch at 1861 cm-1 (1.00).9 An intense band at 1918 cm-1 (0.60) is assigned to the equatorial CO, and a band at 1662 cm-1 is assigned to the bridging CO. The key feature of the pattern of terminal CO stretches for an equatorial isomer is that the lowest energy band is the least intense. (10) Lewis, L. N.; Caulton, K. G. Inorg. Chem. 1981, 20, 1139. (11) Evans, J.; Johnson, B. F. G.; Lewis, J.; Matheson, T. W. Chem. Commun. 1975, 576. (12) Band, E.; Muetterties, E. L. Chem. Rev. 1978, 78, 639. (13) Typically δ 220-280: Harris, D. C.; Rosenberg, E.; Roberts, J. D. J. Chem. Soc., Dalton Trans. 1974, 2398. Gansow, O. A.; Burke, A. R.; Vernon, W. D. J. Am. Chem. Soc. 1972, 94, 2550. Zhuang, J.-M.; Batchelor, R. J.; Einstein, F. W. B.; Jones, R. H.; Hader, R.; Sutton, D. Organometallics 1990, 9, 2723. Evans, J.; Johnson, B. F. G.; Lewis, J.; Norton, J. R. J. Chem. Soc., Chem. Commun. 1973, 79.

Addition of a two-electron donor ligand to one metal of a bimetallic complex is rare. Ru2(µ-O2CR)4 forms weakly coordinated bis adducts Ru2(µ-O2CR)4L2 with THF and CH3CN.14 Addition of ethylene to photochemically generated (CO)4OsdOs(CO)4 gave a 1:1 adduct (CO)4Os(µ-CO)Os(CO)3(H2CdCH2) both in inert matrices15 and at low temperature in solution.16 Upon warming of the sample to -20 °C, the 1:1 adduct 5 slowly (t1/2 ≈ 12 min) underwent cleavage of the Re-Re bond and formation of a 1:1 mixture of Cp*Re(CO)2(PMe3)17 and Cp*Re(CO)2(THF). Upon warming to 10 °C in the presence of excess PMe3 (95 mM), Cp*Re(CO)2(THF) was converted to additional Cp*Re(CO)2(PMe3) (t1/2 ≈ 3 h).

CH3CN Addition. A green solution of Cp*(CO)2RedRe(CO)2Cp* (1) in THF turned orange upon addition of CH3CN. Addition of pentane followed by cooling to -40 °C led to the isolation of Cp*(CO)2Re(µ-CO)Re(CO)(NCCH3)Cp* (6) as a yellow powder in 73% yield. The conversion of 1 to 6 was extremely rapid at -78 °C. A green THF solution of 1 (6.5 mM) turned yelloworange in about 30 s upon addition of acetonitrile (1 M).18 Unlike the PMe3 adduct 5, acetonitrile adduct 6 was stable to fragmentation at room temperature. The X-ray crystal structure of 6 (Table 1, Figure 2) showed axial coordination of CH3CN in the solid state. The 1H NMR spectrum of 6 consisted of two Cp* resonances (δ (14) Lindsay, A. J.; Wilkinson, G.; Motevalli, M.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1985, 2321 and 2723. (15) Haynes, A.; Poliakoff, M.; Turner, J. J.; Bender, B. R.; Norton, J. R. J. Organomet. Chem. 1990, 383, 497. (16) Grevels, F.-W.; Klotzbu¨cher, W. E.; Seils, F.; Schaffner, K.; Takats, J. J. Am. Chem. Soc. 1990, 112, 1995. (17) Angelici, R. J.; Facchin, G.; Singh, M. M. Synth. React. Inorg. Met.-Org. Chem. 1990, 20, 275. (18) Estimating t1/2 ≈ 5 s and assuming a second order rate law for addition of CH3CN to 1 allowed estimation of a rate constant of k2 ≈ 0.14 M-1 s-1 and an activation barrier of ∆G‡ ≈ 12 kcal mol-1.



Table 1. Selected Bond Lengths (Å) and Bond Angles (deg) for Cp*(CO)2Re(µ-CO)Re(CO)(CH3CN)Cp* (6) Re(1)-Re(2) Re(1)-C(3) Re(2)-C(3) Re(1)-C(5) Re(2)-Re(1)-C(3) Re(1)-Re(2)-C(3) Re(1)-C(3)-Re(2) Re(2)-Re(1)-N(1)

2.951(2) 2.06(1) 2.09(1) 1.88(1) 45.2(2) 44.3(2) 90.5(3) 84.0(2)

Re(2)-C(4) Re(2)-C(6) Re(1)-N(1) N(1)-C(1) Re(1)-Re(2)-C(4) Re(2)-Re(1)-C(5) Re(1)-Re(2)-C(6)

1.88(1) 1.90(1) 2.09(1) 1.11(1) 72.7(3) 102.7(3) 103.6(3)

Figure 2. ORTEP drawing and atomic numbering scheme for Cp*(CO)2Re(µ-CO)Re(CO)(CH3CN)Cp* (6). Ellipsoids are drawn at the 35% probability level, and hydrogens are omitted for clarity.

1.98 and 1.80) and a broad resonance for coordinated CH3CN (δ 1.33).

In the 13C NMR spectrum of 6 at -50 °C, only a single CO resonance was observed at δ 213.7 in THF-d8 and at δ 213.1 in toluene-d8. There are two possible explanations for the observation of only one 13CO resonance: (1) accidental degeneracy of the carbonyl resonances and (2) interchange of the environments of all four carbonyls by an additional fluxional process. The fluxional process previously discussed that involves bridging of an axial CO accompanied by opening of the carbonyl bridge leads to two sets of two CO ligands. In the equatorial substituted isomer, the first pair of carbonyls are axial and equatorial on the unsubstituted rhenium while the second pair are bridging and axial on the substituted rhenium. The situation is reversed for the axial substituted isomer. If a single isomer dominates at equilibrium, then only one set of carbonyls will be involved in bridging and will have a substantial shift to higher frequency. The single carbonyl resonance for the CH3CN adduct 6 could be due to similar concentrations of the axial and equatorial isomers so that all carbonyls spend about 25% of the time in a bridging position and the two sets of carbonyls are accidentally degenerate (Scheme 2).

Alternatively, the two sets of carbonyls might be interchanged by an additional fluxional process. One possibility involves opening of the carbonyl bridge, rotating about the Re-Re bond and reclosing a bridge to give a higher energy conformer with cis Cp* ligands and eclipsing (CO, CO) and (CO, CH3CN) ligands, reopening the carbonyl bridge to the opposite rhenium, rotating, and closing the bridge again. This type of process might be less unfavorable for the slim CH3CN ligand. Similar bridge-opening mechanisms which involve cis-trans isomerizations of Cp ligands have been proposed for Cp2Fe2(CO)4, Cp2Mn2(CO)2(NO)2, and Cp2Cr2(NO)4.19 A second possibility involves bridging both axial CO’s of the equatorial isomer to form a triply bridged intermediate and then unbridging the same two CO’s in the opposite direction. A triply bridged intermediate was initially considered to explain the equilibration of terminal and bridge carbonyls in Cp(CO)Rh(µ-CO)Rh(CO)Cp.12 Later, this explanation was rejected since Cp(CO)Rh(µ-CO)Rh[P(OPh3)3]Cp, which cannot form a triply bridged intermediate, also underwent rapid CO interchange.11 Fluxional processes that involve simultaneous bridging of two terminal ligands or unbridging of two bridging ligands have been suggested for Cp2Pt2(CO)2,20 Cp(CO)3MoMo(CO)2(CNCH3)Cp,21 Cp(CO)Mn(µ-CO)(µ-NO)Mn(NO)Cp,22 and Cp(CO)Fe(µ-CO)(µGeMe2)Fe(CO)Cp23 (Scheme 3). The solid-state IR spectrum of the axially substituted CH3CN adduct 6 in KBr has a pattern of CO stretches quite different from that of the equatorially substituted PMe3 adduct 5 (Figure 1). In particular, the lowest energy terminal CO band is very intense. The lowenergy band at 1828 cm-1 (1.00) is assigned to the equatorial CO on the Re bearing the CH3CN ligand; its high intensity is attributed to intensity being “stolen” by this ν“asym” stretch from the ν“asym” stretch of the other equatorial CO stretch at 1901 cm-1 (0.51).9 An intense band at 1857 cm-1 (0.92) is assigned to the axial CO, and a band at 1655 cm-1 is assigned to the bridging CO. The key feature of the pattern of terminal CO stretches for an axial isomer is that the highest energy band is the least intense. The IR spectrum of 6 in THF solution is similar to that in the solid state in that the highest energy band at 1905 cm-1 is the least intense (Figure 1). This provides evidence that the major isomer in solution is the same axial isomer as seen by X-ray crystallography. Since the intensity pattern and positions of bands are somewhat different in the solid state and solution [1905 (0.27), 1863 (1.00), and 1838 (0.75) cm-1], we cannot exclude a moderate amount of an equatorial isomer in equilibrium with the predominant axial isomer.9 Evidence for CH3CN dissociation from 6 was provided by exchange and reactivity studies. Addition of ∼4 equiv of CD3CN to 6 in C6D6 at room temperature led (19) Kirchner, R. M.; Marks, T. J.; Kristoff, J. S.; Ibers, J. A. J. Am. Chem. Soc. 1973, 95, 6602. (20) Boag, N. M.; Goodfellow, R. J.; Green, M.; Hessner, B.; Howard, J. A. K.; Stone, F. G. A. J. Chem. Soc., Dalton Trans. 1983, 2585. (21) Adams, R. D.; Brice, M.; Cotton, F. A. J. Am. Chem. Soc. 1972, 94, 6193. (22) (a) Adams, R. D.; Cotton, F. A. J. Am. Chem. Soc. 1973, 95, 6589. (b) Cotton, F. A. Bull. Soc. Chim. Fr. 1973, 2587. (c) Jackman, L. M.; Cotton, F. A. Dynamic Nuclear Magnetic Resonance; Academic Press: New York, 1975. (23) Adams, R. D.; Brice, M. D.; Cotton, F. A. Inorg. Chem. 1974, 13, 1080.

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Casey et al. Scheme 2

Scheme 3

to complete exchange of free and bound CH3CN in less than 15 min as determined by 1H NMR spectroscopy. Line shape analysis of the variable-temperature 1H NMR spectra of a 1:8 mixture of 6:CH3CN in C6D6 indicated a barrier of about ∆G‡ ≈ 15 kcal mol-1 for exchange of free and bound CH3CN. In the 1H NMR spectrum of an 1:8 ratio of 6:CH3CN in C6D6, the Cp* resonances of 6 coalesced at 59 °C (k ) 200 s-1, ∆G‡ ≈ 16 kcal mol-1). The coalescence of the Cp* resonances is consistent with reversible dissociation of CH3CN to regenerate Cp*(CO)2RedRe(CO)2Cp* (1). The highest mass ion observed in the mass spectrum of 6 was (6 CH3CN)+. This may be due to lability of CH3CN in either 6 or its radical cation. In an attempt to trap 1 in equilibrium with 6, dimethyl acetylenedicarboxylate (DMAD) was added to a solution of 6. A rapid irreversible reaction (t1/2 ) 10 min) occurred to produce the dimetallacyclobutene complex Cp*(CO)2Re(µ-η1,η1-CH3O2CCdCCO2CH3)Re(CO)2Cp* (4)3 in support of the intermediacy of 1. CH2dCH2 Addition. The reaction of ethylene with a green solution of 1 in THF-d8 at -78 °C gave a yellow solution of Cp*(CO)2Re(µ-CO)Re(CO)(η2-C2H4)Cp* (7), which was characterized spectroscopically at low temperature. The 1H NMR spectrum of 7 at -40 °C exhibited two Cp* resonances at δ 1.93 and 1.73 and a single resonance for coordinated ethylene at δ 1.42. In the 13C NMR spectrum at -50 °C, two carbonyl resonances of equal intensity were seen at δ 229.6 and 206.1. The high-frequency resonance requires a pair of CO ligands fluctuating between bridging (δ ∼250)13 and terminal (δ ∼210) positions, while the lower frequency resonance requires a pair of terminal CO ligands

interchanging environments. On the basis of this 13C NMR spectrum, there must be a large excess of either the axial or equatorial isomer in solution. The configuration of the isomer cannot be assigned unambiguously from 13C NMR data as in the case of PMe3 adduct 5 where phosphorus-carbon coupling gave added structural information. A second isomer may also be in rapid equilibrium, but it cannot be a major species. Since the IR spectrum of 7 at -80 °C in THF (Figure 1) showed an intensity pattern very similar to that of the equatorially substituted PMe3 complex 5, it too must be an equatorial isomer. The low-energy band at 1842 cm-1 (0.19) is assigned to the axial CO on the Re bearing the CH2dCH2 ligand, this ν“asym” stretch has lower intensity than the ν“asym” stretch of the other axial CO stretch at 1881 cm-1 (1.00).9 An intense band at 1950 cm-1 (0.62) is assigned to the equatorial CO, and a band at 1699 cm-1 is assigned to the bridging CO. Ethylene adduct 7 was stable below 0 °C in THF, but upon warming to 25 °C, it fragmented into Cp*Re(CO)2(η2-CH2dCH2)24 and Cp*Re(CO)2(THF-d8) (t1/2 ) 9 min). In the presence of excess ethylene (23 mM), Cp*Re(CO)2(THF-d8) was slowly converted to additional Cp*Re(CO)2(η2-CH2dCH2) (t1/2 ≈ 20 h). Reaction of 1 with Acetylene. When acetylene was added to a green solution of 1 in THF at -78 °C, the solution turned orange immediately (t1/2 e 0.1 s).25 The dimetallacyclopentenone Cp*(CO)2Re(µ-η1,η3-CHdCHCO)Re(CO)Cp* (10) was isolated from the reaction (24) Zhuang, J.-M.; Sutton, D. Organometallics 1991, 10, 1516. (25) Estimating that t1/2 e 0.1 s and assuming a second order rate law for addition of acetylene (25 mM) allowed estimation of a rate constant k2 g 2.5 × 102 M-1 s-1 and ∆G‡ e 9 kcal mol-1.


mixture as an orange powder in 84% yield. When the reaction of 1 with acetylene was monitored by 1H NMR spectroscopy at -78 °C, rapid formation of dimetallacyclopentenone 10 was observed without the detection of an intermediate.

The structure of 10 was assigned on the basis of the similarity of its spectra to that of Cp*(CO)2Re{µ-η1,η3CHdC[C(CH3)dCH2]CO}Re(CO)Cp* (3),2 the product of the reaction of 1 with HCtCC(CH3)dCH2. The structure of 3 was established by X-ray crystallography.2 The 1H NMR spectrum of 10 in C D exhibited two Cp* 6 6 resonances at δ 1.81 and 1.60 and two coupled doublets (J ) 8.5 Hz) for the CHdCH′ unit at δ 8.16 and 3.57. The high-frequency resonance was assigned unambiguously to the proton R to rhenium by comparison with the R-proton resonance of 3 at δ 8.27. A large difference between the 13C NMR chemical shifts of the CHdCH′ unit (δ 126.2, 30.8) was also observed. The higher frequency chemical shift is assigned to the CH carbon bridging between the two Re centers.26 Four distinct resonances were seen for the carbonyl carbons (214.7, 210.4, 208.8, 208.1) in a range consistent with terminal carbonyls or a ketone carbonyl. Similar dimetallacyclopentenone complexes of iron and ruthenium27 and (26) Mirza, H. A.; Vittal, J. J.; Puddephatt, R. J. Organometallics, 1994, 13, 3063. (27) Dyke, A. F.; Knox, S. A. R.; Naish, P. J.; Taylor, G. E. J. Chem. Soc., Dalton Trans. 1982, 1297. Hogarth, G.; Kayser, F.; Knox, S. A. R.; Morton, D. A. V.; Orpen, A. G.; Turner, M. L. J. Chem. Soc., Chem. Commun. 1988, 360. Cotton, F. A.; Jamerson, J. D.; Stults, B. R. Inorg. Chim. Acta 1976, 17, 235. Wong, A.; Pawlick, R. V.; Thomas, C. G.; Leon, D. R.; Liu, L.-K. Organometallics 1991, 10, 530. Johnson, K. A.; Gladfelter, W. L. Organometallics 1992, 11, 2534.


heterodimetallacyclopentenones of iron-platinum and osmium-cobalt have been reported.28 The dimetallacyclopentenone 10 is a fluxional molecule. At 70 °C, the Cp* resonances of 10 (∆ν ) 95 Hz) coalesced and narrowed to 7.5 Hz at 100 °C. Line shape analysis of the Cp* resonances indicated a barrier of ∆G‡ ) 16.7 kcal mol-1. The more widely separated (∆ν ) 2280 Hz) CH resonances of 10 broadened to broad singlets (ω1/2 ) 25 Hz) at 50 °C and had still not coalesced at 110 °C. The rate of exchange of the CH environments of 10 was determined to be 2.44 s-1 at 25 °C by magnetization transfer experiments. A fluxional process which simultaneously interchanges the environments of the Cp* ligands and of the CHdCH′ unit is required. We suggest that equilibration of 10 with an unseen dimetallacyclobutene or dimetallabicyclobutane complex accounts for the fluxionality. Since the dimetallacyclobutene 4 formed in the reaction of 1 with DMAD equilibrates with the dimetallabicyclobutane Cp*(CO)2Re(µ-η2,η2-CH3O2CCtCCO2CH3)Re(CO)2Cp* (11) upon heating at 70 °C, both types of intermediates need to be considered for the fluxionality of 10.3 Examination of molecular models show that dimetallacyclobutenes and dimetallacyclopentenones are poised for interconversion. The trans Cp* ligands in the X-ray crystal structure of 4 bracket a diagonal “groove” in which the bridging alkene lies [the twist of the four member ring in the drawing below is exaggerated; the dihedral angle between Re-(midpoint of Re-Re bond)(midpoint of CdC-bond)-C in 4 is 8°].3 Migration of the alkenyl bridge to an equatorial carbonyl and coordination of the alkene to rhenium would produce a dimetallacyclopentenone directly. In this process, the alkene carbon labeled * moves to a bridging position between the Re centers and the twist of the alkene carbon carbon axis relative to the Re-Re axis increases.

Interconversion between dimetallabicyclobutanes and dimetallacyclopentenones is also plausible but would require greater geometric changes including shortening the Re-Re distance from 3.77 to 2.93 Å and widening the Cp*-Re-Re-Cp* dihedral angle from 38 to 173°, in addition to formation of a new carbon-carbon bond to a carbonyl ligand.2,3

(28) Pt/Fe: Fontaine, X. L. R.; Jacobsen, G. B.; Shaw, B. L.; Thornton-Pett, M. J. Chem. Soc., Dalton Trans. 1988, 741. Os/Co: Burn, M. J.; Kiel, G.-Y.; Seils, F.; Takats, J.; Washington, J. J. Am. Chem. Soc. 1989, 111, 6850. Washington, J. Ph.D. Thesis, University of Alberta, 1994.

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Fluxionality of dimetallacyclopentenones has been seen in diiron complexes. cis-Cp(CO)Fe[µ-η1,η3-(CH3O2C)CdC(CO2CH3)CO](µ-CO)FeCp (12) shows equilibration of both the Cp and ester methyl resonances in the 1H NMR spectrum upon heating to 67 °C. The dimetallacyclobutene structure was ruled out as an intermediate because Cp(CO)Fe[(µ-η1,η1-(CH3O2C)CdC(CO2CH3)](µCO)Fe(CO)Cp was isolated as a stable complex. The perpendicularly bridged alkyne complex 13 was also ruled out since the Cp resonances in the adducts of asymmetric alkynes did not coalesce at higher temperatures.28 Interconversion between the enantiomers of (CO)3Fe(µ-η1,η3-CHdCHCO)(µ-Ph2PCH2PPh2)Fe(CO)2 occurs upon heating to 90 °C.29 For both iron compounds, Knox proposed a simultaneous carbon-carbon bond-breaking and bond-making process.

Reaction of 1 with 2-Butyne. The reaction of 1 with 2-butyne was somewhat slower than with acetylene. In an NMR tube reaction, 1 (2.6 µmol) reacted with 2-butyne (5.8 µmol) in toluene-d8 (0.30 mL) at -60 °C (t1/2 ) 16 min) to produce the 1:1 adduct Cp*(CO)2Re(µ-CO)Re(CO)(η2-CH3CtCCH3)Cp* (8). 1H NMR spectroscopy at -60 °C showed inequivalent Cp*’s (δ 1.73, 1.56) and a single broad resonance for the methyl groups (δ 2.41, ω1/2 ) 8 Hz). Infrared spectroscopy at -78 °C showed bands for three terminal carbonyls (1925, 1892, 1855 cm-1) and a bridging carbonyl (1662 cm-1) (Figure 1). The IR spectrum of 8 suggests axial coordination of 2-butyne since the highest energy CO stretch is the least intense. The lowest energy terminal CO band at 1855 cm-1 (0.64) is assigned to the equatorial CO on the Re bearing the 2-butyne ligand; this ν“asym” stretch has higher intensity than the ν“asym” stretch of the other equatorial CO stretch at 1924 cm-1 (0.50).9 An intense band at 1891 cm-1 (1.00) is assigned to the axial CO, and a band at 1662 (0.15) cm-1 is assigned to the bridging CO.

At -40 °C, the 2-butyne complex 8 slowly converted (t1/2 ) 15 min) to the dimetallacyclopentenone Cp*(CO)2Re[µ-η1,η3-(CH3)CdC(CH3)CO]Re(CO)Cp* (9) and two fragmentation products, Cp*Re(CO)3 and Cp*Re(CO)(CH3CtCCH3) (14), which contains a 4e donor alkyne ligand. The ratio of dimetallacyclopentenone 9 to (29) Hogarth, G.; Knox, S. A. R.; Lloyd, B. R.; Macpherson, K. A.; Morton, D. A. V.; Orpen, A. G. J. Chem. Soc., Chem. Commun. 1988, 360.

Casey et al.

fragmentation products was roughly 1.2:1 and did not change below room temperature. At 25 °C, slow fragmentation (t1/2 ) 4.5 h) of the dimetallacyclopentenone 9 occurred to produce additional Cp*Re(CO)3 and 14. Since efforts to separate Cp*Re(CO)3 and 14 by sublimation or chromatography led to decomposition of 14, spectral characterization of 1430 was performed on mixtures containing Cp*Re(CO)3. Both 1H NMR and 13C NMR spectroscopy of the mixture showed inequivalent methyl groups for 14 (1H NMR, δ 2.66, 2.13; 13C NMR, δ 17.0, 14.0), suggesting that the alkyne was bound perpendicularly to the plane of the Cp* ligand and that rotation was slow. Subtraction of infrared bands due to Cp*Re(CO)3 at 2008 (relative absorbance ) 0.6) and 1911 cm-1 (relative absorbance ) 1.0) revealed additional intensity at 1920 cm-1 (relative absorbance ) 0.3) assigned to a terminal carbonyl stretch of 14. Addition of CO to the solution containing 14 gave the known Cp*Re(CO)2(η2-CH3CtCH3).31 The structure of the dimetallacyclopentenone Cp*(CO)2Re[µ-η1,η3-(CH3)CdC(CH3)CO]Re(CO)Cp* (9) was established spectroscopically by examining solutions which also contained the two fragmentation products Cp*Re(CO)3 and Cp*Re(CO)(CH3CtCCH3) (14). In the 1H NMR spectrum of 9 at -40 °C, inequivalent Cp*’s were observed at δ 1.74 and 1.57 and inequivalent methyl groups were observed at δ 2.93, 1.64. Infrared spectroscopy [Cp*Re(CO)3 and 14 subtracted] showed a stretch for the ketone carbonyl at 1686 cm-1 (0.4) and an intensity pattern for bands at 1944 (0.7), 1896 (1.0), and 1844 (0.5), similar to that of 3 and 10.9 The dimetallacyclopentenone 9 is a fluxional molecule. Coalescence of the Cp* resonances occurred at 10 °C (∆G‡ ) 13.6 kcal mol-1). The fluxional barrier of 9 is lower than that of dimetallacyclopentenone 10 derived from acetylene. Exchange of Cp* environments via a dimetallacyclobutene intermediate Cp*(CO)2Re(µ-η1,η1-CH3CdCCH3)Re(CO)2Cp* (A) (or via a dimetallabicyclobutane intermediate) explains this fluxional process. Discussion Free Energy Diagram for Reaction of 2-Butyne with Cp*(CO)2RedRe(CO)2Cp* (1). On the basis of free energies of activation calculated from kinetic studies, we have constructed the energy surface relating complexes 1, 8, 9, and 14 shown in Figure 3. At -60 °C, the half-life for the formation of the initial alkyne complex 8 from the reaction of 1 with 2-butyne was 16 min, which corresponds to ∆G‡ ) 14 kcal mol-1. Since the conversion of 1 to 8 proceeds to completion, the free energy of 8 must lie at least 3 kcal mol-1 below 1. Decomposition of 8 occurred at -40 °C with t1/2 ) 6 min to give a 1.2:1 mixture of dimetallacyclopentenone 9 and fragmentation products Cp*Re(CO)(MeCtCMe) (14) and Cp*Re(CO)3; this corresponds to ∆G‡ ) 16 kcal mol-1. The partitioning between 9 and 14 plus Cp*Re(CO)3 establishes a 0.1 kcal mol-1 difference in activation energies. At 25 °C, fragmentation of dimetallacy(30) 1H NMR (C6D6): δ 2.66 (q, J ) 0.7 Hz, CH3), 2.13 (q, J ) 0.7 Hz, CH3), 1.98 (s, Cp*), 1.67 [s, Cp*Re(CO)3]. 13C NMR (C6D6): δ 231.4 (CO), 169.9 (MeCt), 168.5 (MeCt), 89.7 (C5Me5), 17.0 (CH3Ct), 14.0 (CH3Ct), 12.2 (Cp*CH3). (31) Alt, H. G.; Engelhardt, H. E. J Organomet. Chem. 1988, 342, 235.



Figure 3. Free energy diagram for reaction of 2-butyne with Cp*(CO)2RedRe(CO)2Cp* (1).

clopentenone 9 produced additional 14 and Cp*Re(CO)3 with t1/2 ) 4.5 h, which corresponds to ∆G‡ ) 24 kcal mol-1. Since examination of molecular models showed that simple fragmentation of dimetallacyclopentenone 9 to 14 and Cp*Re(CO)3 was impossible, we suggest that 9 first reverts to alkyne complex 8 from which it was formed and that 8 fragments to additional 14 and Cp*Re(CO)3. This interpretation places the dimetallacyclopentenone 9 7 kcal mol-1 lower in energy than the initially formed alkyne complex 8 [∆G‡(8f9) ) 17 kcal mol-1; ∆G‡(9f8f14) ) 24 kcal mol-1]. Dynamic NMR studies of the fluxionality of dimetallacyclopentenone 9 gave a ∆G‡ ) 13.6 kcal mol-1 for symmetrization of 9 via symmetric dimetallacyclobutene intermediate A (or a symmetric dimetallabicyclobutane). Since we are unable to observe the symmetric intermediate directly, we place its energy somewhere between 3 and 13.6 kcal mol-1 higher than 9. There are several possible pathways for the interconversions of alkyne complex 8 with dimetallacyclopentenone 9 and the proposed intermediate dimetallacyclobutene A which are consistent with our rate data. (1) Conversion of 8 to 9 might proceed only via A. (2) Conversion of 8 to 9 might occur directly and access to A might be only via 9. (3) Conversion of 8 to either 9 or A might occur equally readily. The direct formation of dimetallacyclobutene A from 2-butyne and Cp*(CO)2RedRe(CO)2Cp* (1) is inconsistent with our rate data and can be excluded. If the assumption that dimetallacyclobutene A is initially formed and selectively converted (∆∆G‡ > 3 kcal mol-1) to alkyne complex 8 is taken together with the observed ∆G‡ ) 17 kcal mol-1 for conversion of 8 to dimetallacyclopentenone 9 and the observed ∆G‡ ) 13 kcal mol-1 for fluxionality of 9 via intermediate A, then the dimetallacyclopentenone 9 would have to be placed >4 kcal mol-1 above the energy of the alkyne complex 8. This is clearly inconsistent with the observed conversion of 8 to 9. Relative Stability of Dimetallacyclobutenes and Dimetallacyclopentenones. Only alkynes with two ester substituents react with 1 to form dimetallacyclobutene complexes such as 4. All other alkynes react with 1 to form dimetallacyclopentenones such as 3, 9, and 10. Even alkynes with a single ester substituent such as HCtCCO2Me and MeCtCCO2Me form only dimetallacyclopentenones.32 Ester substituents act as

strong electron-withdrawing groups that make the DMAD unit very similar to a bridging CO. Like a µ-CO, a µ-DMAD unit can effectively remove electron density from the electron-rich rhenium centers. There are numerous examples of dimetallacyclobutenes with electron withdrawing CF3 or CO2R substituents.33 Fragmentation of Cp*(CO)2Re(µ-CO)Re(CO)(L)Cp*. We do not understand why some adducts fragment to monometallic complexes while others such as CO adduct (2) and acetonitrile adduct (6) do not. Adducts of PMe3 (5) and CH2dCH2 (7) fragment in THF to produce Cp*Re(CO)2(THF) and Cp*Re(CO)2L. The regiochemistry of this fragmentation can be understood in terms of the stability of the fragments generated. Fragmentation places the vacant coordination site at the least unstable site. This is consistent with the greater ease of CO loss from Cp*Re(CO)3 compared with loss of CO from Cp*Re(CO)2PMe3. If L is a ligand such as MeCtCMe that can shift from being a 2e donor to a 4e donor, then fragmentation to give Cp*Re(CO)3 and Cp*Re(CO)(L) is favored.

Experimental Section General Methods. 1H NMR spectra were obtained on a Bruker WP200, AC300, or AM500 spectrometer. 13C{1H} spectra were obtained on a Bruker AM500 spectrometer (126 (32) Casey, C. P.; Carinõ, R. S. Unpublished results. (33) Hoffman, D. M.; Hoffmann, R.; Fisel, C. R. J. Am. Chem. Soc. 1982, 104, 3858. Holton, J.; Lappert, M. F.; Pearce, R.; Yarrow, P. I. W. Chem. Rev. 1983, 83, 135.

2648


MHz). Infrared spectra were measured on a Mattson Polaris (FT) or a Mattson Genesis (FT) spectrometer; the relative absorbances of bands are given in parentheses following the peak position reported in cm-1. High-resolution mass spectra were obtained on a Kratos MS-80 mass spectrometer. Toluene-d8, THF-d8, THF, C6D6, ether, hexane, and pentane were distilled from purple solutions of sodium benzophenone ketyl immediately prior to use. CH2Cl2 and CH3CN were distilled from CaH2. CD2Cl2 was dried over P2O5 and distilled immediately prior to use. Air-sensitive materials were manipulated by standard Schlenk techniques or in an inertatmosphere glovebox. Carbon monoxide (Matheson) and ethylene (AGA Red Arrow) were purchased and used without further purification. Acetylene was purified by passing through a -78 °C trap to remove acetone. 2-Butyne and trimethylphosphine were purchased from Aldrich and freeze-pumpthaw degassed. Cp*Re(CO)2THF.1 A solution of Cp*Re(CO)334 (1.00 g, 2.46 mmol) in THF (150 mL) was purged with nitrogen in a photolysis cell at 0 °C. The solution was irradiated with a Hanovia medium-pressure mercury lamp for 30 min at 0 °C under a nitrogen purge. When the resulting red-orange solution was concentrated to 1 mL under vacuum in a reversible frit apparatus, a yellow solid precipitated. Additional yellow solid precipitated when the dark red-brown suspension was cooled to -78 °C for 5 min. Hexane (∼20 mL) was vacuum-transferred into the flask at -78 °C. The solid was collected on the frit, washed with cold hexane to remove red hexane-soluble impurities, and dried under vacuum to give 1 (460 mg, 42%) as a bright yellow solid which was stored at -30 °C under nitrogen. 1H NMR (200 MHz, THF-d8): δ 3.78 (m, OCH2), 1.95 (s, Cp*), 1.81 (m, OCH2CH2). 13C{1H} NMR (126 MHz, toluene-d8, -80 °C): δ 208.9 (CO), 92.9 (C5Me5), 86.4 (OCH2), 27.3 (OCH2CH2), 10.8 (Cp*CH3). IR (THF): 1893 (s), 1823 (s) cm-1. Cp*(CO)2RedRe(CO)2Cp* (1).1 Solid Cp*Re(CO)2THF (640 mg, 1.43 mmol) was allowed to stand in a plastic-capped vial at room temperature under a nitrogen atmosphere. The color of the solid changed to green over a period of 1 week. The resulting dark green powder was washed with ether (5 × 10 mL) to remove red ether-soluble impurities and dried under vacuum to give 1 (465 mg, 86%) as a light green powder. 1H NMR (C6D6): δ 1.98 (s, Cp*). 1H NMR (THF-d8): δ 2.14. 13C NMR (toluene-d8): δ 209.4 (CO), 103.1 (C5Me5), 11.1 (Cp*CH3). IR (toluene) 1869 (s), 1824 (s) cm-1. HRMS: calcd (found) for C23H30O3187Re2 (M-CO)+, m/z 728.131 (728.135). Anal. Calcd (found) for C24H30O4Re2: C, 38.19 (37.49); H, 4.01(4.16). Cp*(CO)2Re(µ-CO)Re(CO)2Cp* (2). When a stirred green solution of 1 (30 mg, 40 µmol) in THF (30 mL) was exposed to 1 atm of CO, the color changed to yellow instantaneously. Solvent was evaporated under vacuum, and the solid yellow residue was purified by flash chromatography (silica gel, 1:10 CH2Cl2 : hexane) to give Cp*(CO)2Re(µ-CO)Re(CO)2Cp* (2)4 (30 mg, 95%) as a yellow solid. 1H NMR (200 MHz, CD2Cl2): δ 1.98 (C5Me5). 13C{1H} NMR (126 MHz, CD2Cl2): δ 214.6 (CO), 100.1 (C5Me5), 10.3 (Cp*CH3). IR (THF): 1967 (0.17), 1923 (1.00), 1894 (0.76), 1869 (0.21), 1707 (0.21) cm-1. Cp*(CO)2Re(µ-CO)Re(CO)(PMe3)Cp* (5). PMe3 (9.1 µmol) was condensed into an NMR tube containing a solution of 1 (2 mg, 2.6 µmol) in THF-d8 (0.29 mL) at -196 °C. After flame sealing and thawing at -78 °C, the tube was shaken, giving a color change from green to yellow in < 20 s. The NMR tube was then inserted into a precooled (-78 °C) NMR probe to record 1H and 13C{1H} spectra. A low-temperature infrared spectrum was obtained on a sample prepared from 1 (20 mg, 26 µmol) and PMe3 (100 µmol) in THF (3 mL) at -78 °C over 20 min and injected into an IR cell precooled to -78 °C. 1H NMR (500 MHz, THF-d8, -80 °C): δ 1.94 (s, Cp*), 1.89 (s, Cp*), 1.52 (d, J ) 9.4 Hz, PMe3). 13C{1H} NMR (126 MHz, (34) Patton, A. T.; Strouse, C. E.; Knobler, C. B.; Gladysz, J. A. J. Am. Chem. Soc. 1983, 105, 5804.

Casey et al. THF-d8, -80 °C): δ 232.4 (d, J ) 10.7 Hz, 2 CO), 212.4 (2 CO), 98.3 (C5Me5), 97.7 (C5Me5), 20.0 (d, J ) 35.4 Hz, PCH3), 11.2 (Cp*CH3) 10.5 (Cp*CH3). IR (THF, -78 °C): 1917 (0.60), 1861 (1.00), 1836 (0.28), 1662 (0.10) cm-1. NMR Kinetics Experiment for the Conversion of 1 and PMe3 to Cp*Re(CO)2PMe3. Addition of PMe3 (9.1 µmol) to a solution of 1 (2 mg, 2.6 µmol) in THF-d8 produced 5. The conversion of 5 (Cp*: δ 1.94, 1.89) to Cp*Re(CO)2PMe3 (Cp*: δ 2.08) and Cp*Re(CO)2THF (Cp*: δ 1.95) at -20 °C was followed by 1H NMR spectroscopy. A plot of ln [5] vs time was linear and gave k1 ) 9.57 × 10-4 s-1, t1/2 ) 12 min, and ∆G‡ ) 18.2 kcal mol-1. The subsequent conversion of Cp*Re(CO)2THF to 5 at 10 °C was followed by 1H NMR spectroscopy. A plot of ln [Cp*Re(CO)2THF] vs time was linear and gave kobs ) 6.30 × 10-5 s-1 and t1/2 ) 3 h. Cp*Re(CO)2PMe3. Addition of PMe3 (200 µmol) to a green solution of 1 (40 mg, 53 µmol) in THF (3 mL) at -78 °C produced a yellow solution which turned orange upon warming to room temperature. After 12 h, evaporation of solvent gave Cp*Re(CO)2PMe316 (30 mg, 62%) as a tan powder that was >95% pure by 1H NMR spectroscopy. 1H NMR (300 MHz, CDCl3): δ 2.07 (s, Cp*), 1.57 (d, J ) 9.0 Hz, PMe3). IR (pentane): 1924 (vs), 1860 (vs) cm-1. HRMS: calcd (found) for C15H24O2P187Re, m/z 454.107 (454.108). Cp*(CO)2Re(µ-CO)Re(CO)(NCCH3)Cp* (6). Addition of CH3CN (500 µL) to a green solution of 1 (45 mg, 60 µmol) in THF (2 mL) produced an orange solution. Pentane (5 mL) was added giving a yellow precipitate. The suspension was cooled to -40 °C overnight, filtered, and washed with pentane to give a 6 as a yellow powder (35 mg, 73%). Crystals suitable for X-ray crystallographic analysis were obtained by slow diffusion of hexane into a saturated THF solution of 6. 1H NMR (200 MHz, C6D6): δ 1.98 (s, Cp*), 1.80 (s, Cp*), 1.33 (br s, CH3CN). 13 C{1H} NMR (126 MHz, THF-d8, -50 °C): δ 213.7 (4 CO), 129.8 (CH3CN), 98.7 (C5Me5), 97.4 (C5Me5), 10.8 (CH3CN), 9.9 (Cp*CH3), 3.6 (Cp*CH3). IR (THF): 1905 (0.29), 1863 (0.61), 1838 (0.47), 1678 (0.17) cm-1. IR (KBr): 1902 (s), 1857 (vs), 1828 (vs), 1655 (m) cm-1. HRMS: calcd (found) for C24H30O4185Re2 (M+ - CH3CN), m/z 752.120 (752.122). Anal. Calcd for C26H33NO4Re2: C, 39.24 ; H, 4.18. Found: C, 39.85; H, 4.37. X-ray Crystallographic Determination and Refinement of Cp*(CO)2Re(µ-CO)Re(CO)(NCCH3)Cp* (6). Intensity data for 6 were obtained with graphite-monochromated Mo KR radiation on a Nicolet (Siemens) P3/F diffractometer at -160 °C. Crystallographic computations were carried out with Siemens SHELXTL.35 A ψ-scan absorption correction was applied. Initial positions for the Re atoms were obtained by the automatic Patterson interpretation. The other nonhydrogen atoms were obtained from successive Fourier difference maps coupled with isotropic least-squares refinement. All non-hydrogen atoms were refined anisotropically. Idealized positions were used for the hydrogen atoms. Table 2 presents crystal data, data collection parameters, and leastsquares refinement parameters. Tables of atomic coordinates for non-hydrogen atoms, complete lists of bond lengths and angles, anisotropic displacement parameters, and hydrogen coordinates and isotropic displacement parameters are available as Supporting Information. Cp*(CO)2Re(µ-CO)Re(CO)(η2-CH2dCH2)Cp* (7). The reaction of 1 (2 mg, 2.6 µmol) with H2CdCH2 (5.2 µmol) in THF-d8 (0.29 mL) and in toluene-d8 was followed by 1H NMR spectroscopy from -85 to 25 °C. The 13C NMR spectrum of 7 at -50 °C was obtained from a sample prepared by mixing 1 (11 mg, 15 µmol) with H2CdCH2 (1 atm) in THF-d8 (0.30 mL) followed by removal of excess H2CdCH2 under vacuum at -80 °C. A low-temperature infrared spectrum was obtained on a sample prepared from 1 (11 mg, 15 µmol) in THF (1 mL) under 600 mmHg of H2CdCH2 at -78 °C over 1 h and injected into an IR cell precooled to -78 °C. 1H NMR (THF-d8, -40 °C): δ (35) Siemens Analytical X-Ray Instruments.

Ligand Additions to Cp*(CO)2RedRe(CO)2Cp* Table 2. Crystal Data for Cp*(CO)2Re(µ-CO)Re(CO)(CH3CN)Cp* (6) empirical formula fw temp wavelength cryst system space group unit cell dimens

V, Z D (calcd) abs coeff F(000) cryst size (mm) θ range for data collcn reflcns collcd indepdt reflcns refinement method data/restraints/params goodness-of-fit on F final R indices [I >2σ(I)] R indices (all data) largest diff peak and hole

C26H33NO4Re2 795.93 113(2) K 0.710 73 Å triclinic P1 h a ) 8.695(4) Å b ) 10.192(6) Å c ) 14.931(5) Å R ) 99.45(4)° β ) 92.15(3)° γ ) 100.93(4)° 1278.4(10) Å3, 2 2.068 Mg/m3 9.492 mm-1 756 0.2 × 0.2 × 0.5 2.07-25.07° 4828 4515 (Rint ) 0.0469) full-matrix least-squares on F2 4514/0/309 1.052 R1 ) 0.0379, wR2 ) 0.0905 R1 ) 0.0511, wR2 ) 0.0992 2.180 and -2.579 e Å-3

1.93 (s, Cp*), 1.74 (Cp*, overlapped with the residual proton signal of THF-d8), 1.44 (s, CH2). 1H NMR (toluene-d8, -40 °C): δ 1.75 (s, Cp*), 1.65 (s, CH2), 1.57 (s, Cp*). 13C{1H} NMR (THF-d8, -50 °C): δ 229.6 (2 CO), 206.1 (2 CO), 100.4 (C5Me5), 98.2 (C5Me5), 32.3 (CH2), 10.6 (Cp*CH3), 9.6 (Cp*CH3). IR (THF, -78 °C): 1950 (0.62), 1881 (1.00), 1842 (0.19), 1699 (0.31) cm-1. NMR Kinetics Experiment for the Conversion of 7 to Cp*Re(CO)2(CH2dCH2). Addition of CH2dCH2 (15 µmol) to a solution of 1 (3 mg, 4.0 µmol) in THF-d8 containing C6Me6 (1 µmol) as an internal standard produced 7. The conversion of 7 (Cp*: δ 1.93, 1.74) to Cp*Re(CO)2(CH2dCH2) (Cp*: δ 1.97) and Cp*Re(CO)2THF (Cp*: δ 1.95) at -40 °C was followed by 1H NMR spectroscopy. A plot of ln [7] vs time was linear and gave k1 ) 1.35 × 10-3 s-1, t1/2 ) 8.5 min, and ∆G‡ ) 21.3 kcal mol-1. The subsequent conversion of Cp*Re(CO)2THF to 7 at 25 °C was followed by 1H NMR spectroscopy. A plot of ln [Cp*Re(CO)2THF] vs time was linear and gave kobs ) 9.5 × 10-6 s-1 and t1/2 ) 20 h. Cp*Re(CO)2(η2-CH2dCH2). When excess CH2dCH2 (1 atm) was added to a green solution of 1 (26 mg, 34 µmol) in THF (3 mL) at room temperature, an immediate color change to orange occurred. After 12 h, solvent was evaporated, and the resulting yellow powder was purified by preparatory TLC (4:1 pentane:CH2Cl2) to give Cp*Re(CO)2(η2-CH2dCH2)25 (12 mg, 42%) as a white solid. 1H NMR (300 MHz, CDCl3): δ 1.94 (s, Cp*), 1.61 (s, CH2dCH2). IR (THF): 1952 (vs), 1878 (vs) cm-1. HRMS: calcd (found) for C14H19O2187Re, m/z 406.094 (406.094). Cp*(CO)2Re(µ-η1,η3-CHdCHCO)Re(CO)Cp* (10). Acetylene (140 µmol) was condensed into a frozen solution of 1 (53 mg, 70 µmol) in THF (15 mL) at -196 °C. The mixture was warmed to room temperature and concentrated under vacuum. Hexane was added, and the suspension was cooled to -80 °C and filtered to give 10 (45 mg, 58 µmol, 84%) as an orange solid, which was washed with hexane. 1H NMR (200 MHz, C6D6): δ 8.16 (d, J ) 8.5 Hz, CH), 3.57 (d, J ) 8.5 Hz, CH), 1.81 (s, Cp*), 1.60 (s, Cp*). 13C{1H} NMR (126 MHz, CD2Cl2): δ 214.7 (CO), 210.4 (CO), 208.8 (CO), 208.1 (CO), 126.2 (ReCHd), 99.4 (C5Me5), 98.5 (C5Me5), 30.8 (dCHC(O)), 10.9 (Cp*CH3), 9.6 (Cp*CH3). IR (toluene): 1941 (m), 1898 (s), 1848 (m), 1712 (m) cm-1. HRMS: calcd (found) for C26H32O4187Re2, m/z 782.142 (782.139). Anal. Calcd for C26H32O4Re2: C, 39.99; H, 4.13. Found: C, 39.83; H, 3.90. A magnetization transfer experiment at 25 °C was employed to measure the exchange of the CH environments of 10.36 A

Organometallics, Vol. 15, No. 11, 1996 2649 selective inversion pulse was applied to the resonance at δ 8.13 (relative intensity ) 1.00) resulting in an inverted signal (relative intensity ) -0.78). 1H NMR spectra were taken using a variable delay between the inversion pulse and the observation. The intensities of the CH peaks were recorded relative to the Cp* signal at δ 1.80. The integrals of the CH peaks in the difference spectra (unperturbed - pulsed) were calculated to give intensities for A (integrated intensities for δ 8.13) and X (δ 3.57). Plotting ln(A + X) vs delay time gives a slope -(1/T1A), where 1/T1A is the relaxation rate for the total magnetization introduced by the selective pulse. Plotting ln(A - X) vs delay time gives a slope -(1/T1A + kAX + kXA), where kAX and kXA represent the rates of interconversion between A and X. Values were calculated for T1A ) 4.24 s, and kAX ) 2.44 s-1 (assuming that kAX ) kXA), and ∆G‡ ) 16.9 kcal mol-1. NMR Kinetics Experiment for the Conversion of 1 and 2-Butyne to Cp*Re(CO)(η2-CH3C≡CCH3) (14) and Cp*Re(CO)3. Addition of 2-butyne (5.8 µmol) to a solution of 1 (2 mg, 2.6 µmol) in 0.3 mL toluene-d8 produced Cp*(CO)2Re(µCO)Re(CO)(η2-CH3CtCCH3)Cp* (8). The conversion of 1 (Cp*: δ 1.98) to 8 (Cp*: δ 1.73, 1.56) at -60 °C was followed by 1H NMR spectroscopy. A plot of ln [1] vs time was linear and gave kobs ) 7.2 × 10-4 s-1, k2 ) 3.7 × 10-2 M-1 s-1, and ∆G‡ ) 14 kcal mol-1. The conversion of 8 in toluene-d8 (Cp*: δ 1.73, 1.56) to Cp*(CO)2Re[µ-η1,η3-(CH3)CdC(CH3)CO]Re(CO)Cp* (9) (Cp*: δ 1.76, 1.61), Cp*Re(CO)3 (Cp*: δ 1.74), and Cp*Re(CO)(CH3CtCCH3) (14) (Cp*: δ 1.97) at -40 °C was followed by 1H NMR spectroscopy using C6Me6 (1 mg) as an internal standard. A plot of ln [8] vs time was linear and gave k1 ) 1.99 × 10-3 s-1 and ∆G‡ ) 17 kcal mol-1. The subsequent conversion of 9 to Cp*Re(CO)3 and 14 at 25 °C was followed by 1H NMR spectroscopy. A plot of ln [9] vs time was linear and gave k1 ) 2.2 × 10-5 s-1, t1/2 ) 8.8 h, and ∆G‡ ) 23.8 kcal mol-1. Cp*Re(CO)2(η2-CH3CtCCH3).31 2-butyne (50 µmol) was condensed into a flask containing a green solution of 1 (20 mg, 26.4 µmol) in 3 mL toluene at -78 °C. After 1 day at room temperature, IR spectroscopy of the red solution showed only bands for Cp*Re(CO)3 and Cp*Re(CO)(η2-MeCtCMe) [by subtraction of Cp*Re(CO)3]. The solution turned orange when placed under 1 atm of CO. Solvent was evaporated, and the resulting yellow-brown powder was purified by preparatory TLC (4:1 pentane:CH2Cl2) to yield Cp*Re(CO)3 (5.8 mg, 54%) and Cp*Re(CO)2(η2-MeCtCMe) (4.4 mg, 39%) as a dark yellow powder. 1H NMR (300 MHz, CDCl3): δ 2.36 (s, CH3), 1.96 (s, Cp*). IR (pentane): 1956 (vs), 1875 (vs) cm-1. HRMS: calcd (found) for C16H21O2187Re, m/z 432.110 (432.106).

Acknowledgment. Financial support from the Department of Energy, Office of Basic Energy Sciences, is gratefully acknowledged. Grants from the NSF (CHE9105497) and from the University of Wisconsin for the purchase of the X-ray instruments and computers are acknowledged. Supporting Information Available: Tables of atomic coordinates and U values for non-hydrogen atoms, complete bond lengths and angles, anisotropic displacement parameters, and hydrogen coordinates and isotropic displacement parameters for compound 6 (6 pages). This material is contained in many libraries on microfiche, immediately follows this article in the microfilm version of the journal, can be ordered from the ACS, and can be downloaded from the Internet; see any current masthead page for ordering information and Internet access instructions. OM9509507 (36) Dahlquist, F. W.; Longmuir, K. J.; Du Vernet, R. B. J. Magn. Reson. 1975, 17, 406.

2ReRe(CO)2Cp* and Fragmentation and Rearrangement Reactions

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