Organometallics 2001, 20, 401-407
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Solvent Study of the Kinetics of Molybdenum Radical Self-Termination John C. Linehan,*,† Clement R. Yonker,† R. Shane Addleman,† S. Thomas Autrey,† J. Timothy Bays,‡ Thomas E. Bitterwolf,§ and John L. Daschbach† Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, Chemistry Department, United States Military Academy, West Point, New York, and The Department of Chemistry, University of Idaho, Moscow, Idaho 83844 Received August 18, 2000
The kinetics of (n-butylCp)Mo(CO)3 (n-butylCp is n-butyl-η5-cyclopentadienyl) radical selftermination to form a nonequilibrium mixture of trans- and gauche-[(n-butylCp)Mo(CO)3]2 and the kinetics of the gauche-to-trans isomerization have been determined in the liquid solvents n-heptane, tetrahydrofuran, xenon (350 bar), and CO2 (350 bar) at 283 K by stepscan FTIR spectroscopy. The overall rate constant for the disappearance, 2kR, of the (nbutylCp)Mo(CO)3 radical increases with decreasing solvent viscosity as expected, except in CO2, which is anomalously slower. The slower overall termination rate in liquid CO2 is consistent with the formation of a transient molybdenum radical-CO2 complex. The observed overall rate constants for (n-butylCp)Mo(CO)3 self-termination, 2kR, are (7.9 ( 0.5) × 109 M-1 s-1 in xenon; (3.2 ( 0.5) × 109 M-1 s-1 in heptane; (2.2 ( 0.8) × 109 M-1 s-1 in THF; and (1.7 ( 0.5) × 109 M-1 s-1 in CO2. The first determinations of the radical self-terminationto-gauche rate constants, kG, are presented. The values of kG are much slower than the corresponding recombination to trans, kT, reflecting a steric contribution to the rate. The rate of isomerization (rotation about the molydenum-molybdenum bond) from gauche to trans is unaffected by the solvent and is 3 times faster than the reported isomerization rate for the nonsubstituted [CpMo(CO)3]2 molecule. Introduction The photolytic chemistry of [CpM(CO)3]2 (Cp is η5cyclopentadienyl, M is Cr, Mo, W) has been well studied.1-5 Photolysis of these metal-metal dimers can lead to carbonyl loss with UV photons to produce a pentacarbonyl intermediate which can then add another ligand, recombine with the displaced carbonyl, or lose a second carbonyl to produce a stable tetracarbonyl species. In contrast visible irradiation leads almost exclusively to metal-metal bond scission to produce the CpM(CO)3 radical.6,7 The metal radical can react to reform the dimer, abstract an atom or group from a substrate, or disproportionate. Rates of CpM(CO)3 recombination, rates of atom abstraction by CpM(CO)3, and the possible solvent effects on cage recombination rates/cage radical escape have all been reported.8-10 †
Pacific Northwest National Laboratory. United States Military Academy. University of Idaho. (1) Atruc, D. Electron Transfer and Radical Processes in TransitionMetal Chemistry; VCH Publishers: New York, 1995. (2) Meyer, T. J.; Caspar, J. V. Chem. Rev. 1985, 85, 187-218. (3) Tyler, D. R. Prog. Inorg. Chem. 1988, 36, 125-194. (4) Tyler, D. R. Acc. Chem. Res. 1991, 24, 325-331. (5) Trogler, W. C. Organometallic Radical Processes; Elsevier: Amsterdam, 1990; Vol. 22. (6) Peters, J.; George, M. W.; Turner, J. J. Organometallics 1995, 14, 1503-1506. (7) Knorr, J. R.; Brown, T. L. J. Am. Chem. Soc. 1993, 115, 40874092. (8) Male, J. L.; Lindfors, B. E.; Covert, K. J.; Tyler, D. R. Macromolecules 1997, 30, 6404-6406. ‡ §
Early research has shown that there are two isomers of [CpM(CO)3]2 in solution, a gauche rotamer in which the cyclopentadienyl rings are adjacent and a trans rotamer in which the cyclopentadienyl rings are far removed from each other, as shown in Figure 1. The equilibrium constant between these two rotamers varies proportionally with the dielectric constant of the solvent. The proportion of the gauche isomer increases with increasing solvent dielectric constant.11,12 Recent work has shown that the recombination of CpMo(CO)3 radicals leads to formation of both the gauche and trans rotamers, but the rate constant for the formation of the gauche rotamer from the radical has not been reported.6,7 Continuing our interest in the study of organometallic chemistry in compressible fluids13,14 and our interests in organic/organometallic radical chemistry,15-17 we (9) Male, J. L.; Lindfors, B. E.; Covert, K. J.; Tyler, D. R. J. Am. Chem. Soc. 1998, 120, 13176. (10) Male, J. L.; Yoon, M.; Glenn, A. G.; Weakley, T. J. R.; Tyler, D. R. Macromolecules 1999, 32, 3898-3906. (11) Adams, R. D.; Cotton, F. A. Inorg. Chim. Acta 1973, 7, 153156. (12) Adams, R. D.; Collins, D. M.; Cotton, F. A. Inorg. Chem. 1974, 13, 1086-1090. (13) Linehan, J. C.; Yonker, C. R.; Bays, J. T.; Autrey, S. T. J. Am. Chem. Soc. 1998, 120, 5826-5827. (14) Linehan, J. C.; Wallen, S. L.; Yonker, C. R.; Bitterwolf, T. E.; Bays, J. T. J. Am. Chem. Soc. 1997, 119, 10170-10177. (15) Franz, J. A.; Linehan, J. C.; Birnbaum, J. C.; Hicks, K. W.; Alnajjar, M. S. J. Am. Chem. Soc. 1999, 121, 9824-9830. (16) Autrey, S. T.; Devadoss, C.; Suerwien, B.; Franz, J. A.; Schuster, G. B. J. Phys. Chem. 1995, 99, 869-871.
10.1021/om000724j CCC: $20.00 © 2001 American Chemical Society Publication on Web 01/05/2001
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Figure 1. Schematic representations of the gauche and trans rotomers of [(n-butylCp)Mo(CO)3]2.
have examined the radical recombination rates of (nbutylCp)Mo(CO)3 in four dissimilar solvents using stepscan FTIR spectroscopy. We have chosen the monobutylcyclopentadienyl derivative (n-butylCp)Mo(CO)3 to initiate investigations into the differences of the radical self-termination rate between substituted and nonsubstituted cyclopentadienyl ligands. The phosphine-forcarbonyl substitution in the manganese pentacarbonyl radical, Mn(CO)4L (L is phosphine), significantly slows the radical self-termination rate. For this radical the rate of self-termination was inversely proportional to the cone-angle of the phosphine.18 The CpCr(CO)3 and Cp*Cr(CO)3 (Cp* is pentamethyl-η5-cyclopentadienyl) radical systems have been compared, with the Cp*Cr(CO)3 self-termination rate being 40 times slower than CpCr(CO)3.19 But in the chromium system the rate constants for radical recombination are much slower than the diffusion- or near diffusion-controlled rates reported for both the molybdenum and tungsten radicals. The addition of a butyl side chain to the cyclopentadienyl ring also has the benefit of increasing the solubility of the organometallic compound. The alkyl side chain increases the solubility of the organometallic radical complex in nonpolar solvents such as aliphatics and CO2. The solubility of the hydride, R5CpMo(CO)3H (a model for the R5CpMo(CO)3 radical), significantly increased in dodecane when R is methyl as opposed to R being hydrogen.15 In this study we present our preliminary results into the investigations of organometallic radical self-termination rates as a function of viscosity over a large viscosity range through the use of compressible gases and noncompressible liquids. We also report the first rates for radical-to-gauche isomerization. Future experimental efforts will examine the organometallic radical self-termination rates based on radical size as a function of temperature, pressure, and viscosity for both liquids and compressible gas solvents. Results Figure 2 shows the FTIR spectra of [(n-butylCpMo(CO)3]2 at 283 K in the four solvents used in this study. The IR spectra in heptane, CO2, and xenon are all very similar with only a slight band broadening apparent in the CO2 spectrum. The IR spectrum of the molybdenum dimer in THF shows a significant amount of gauche rotamer, band maximum at 2018 cm-1. The gauche-[(n(17) Camaioni, D. M.; Autrey, S. T.; Salinas, T. B.; Franz, J. A. J. Am. Chem. Soc. 1996, 118, 2013-2022. (18) Walker, H. W.; Herrick, R. S.; Olsen, R. J.; Brown, T. L. Inorg. Chem. 1984, 23, 3748-3752. (19) Yao, Q.; Bakac, A.; Espenson, J. H. Organometallics 1993, 12, 2010-2012.
Figure 2. FTIR spectra of [(n-butylCp)Mo(CO)3]2 solutions in xenon, carbon dioxide, tetrahydrofuran, and n-heptane collected at 8 cm-1 resolution.
butylCpMo(CO)3]2 rotamer shows an equilibrium concentration of 20% by 1H NMR in THF.20 This is consistent with the previously reported gauche-to-trans ratio for [CpMo(CO)3]2 in acetone of approximately 1:1, with the ratio in THF being slightly smaller.11,12 In heptane the amount of gauche-[(n-butylCpMo(CO)3]2 was undetectable by 1H NMR (200 µs) the contributions of the radical to the intensities in the IR spectrum in the 2000-2020 cm-1 region are small relative to those of the gauche dimer. If, as is observed in this study, there are no other species formed and the system returns to equilibrium before the next laser pulse, then the mass balance is denoted in eq 2.22
-[trans]bleached ) [radical]/2 + [gauche]
(2)
When the [radical] , [gauche], which occurs at long observation times, then [gauche] ) -[trans]bleached. This allows the determination of the molar absorptivities for the gauche transient. The radical molar absorptivity is determined from a fit of the mass balance, eq 2, over the entire time range once the molar absorptivity of
Figure 4. First 10 µs of the concentration profile vs time for radical (n-butylCp)Mo(CO)3, trans-[(n-butylCp)Mo(CO)3]2, and gauche-[(n-butylCp)Mo(CO)3]2 species at 283 K in heptane. The solid lines represent the LevenbergMarquardt fit of the data. The inset shows the linear plot of ln[gauche-[(n-butylCp)Mo(CO)3]2] against time after most of the radical has decayed. Notice the difference in the time scales between the two plots.
gauche transient has been determined. The molar absorbtivities determined for the three species at 8 cm-1 resolution are presented in the Supporting Information table. The concentration profiles over time for these species in heptane at 283 K determined by the above method are shown in Figure 4. Similar profiles were obtained for the other three solvents in this study. The mass balance determined from this analysis is very good, with less than a 1% deviation from zero over the entire time range in each of the solvents except xenon, in which a 2% deviation was found. A more complicated three-part deconvolution was used to determine the molar absorptivity for the radical band at 1905 cm-1 in THF since a solvent band interferes with the quantification of the radical band at 2000 cm-1. This complication leads to a greater uncertainty in the THF results when compared to the other solvents. The molar absorptivities of the other species in THF were obtained in a manner similar to that for the other solvents as described above. The rate constants for the reactions were determined by nonlinear least-squares fit of the concentrations of each species using approximations to the analytical solutions of the differential equations, eqs 3-5 (shown below), via the Levenberg-Marquardt technique. The rate constants determined in heptane, along with those obtained in xenon, THF, and CO2, are shown in Table 1. (22) There is a very small IR band at 1667 cm-1 observed in these experiments. This corresponds to the pentacarbonyl dimer intermediate at a concentration of less than 0.5% of the trans bleached. The absorbances due to this species are negligible in the 1963 cm-1 region used for trans quantification. The 1667 cm-1 band grows in intensity proportional with reaction temperature.
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Table 1. Rate Constants for the [(n-butylCp)Mo(CO)3]2 Photolyzed Systems at 283 Ka solvent rate constant
heptane
xenon
CO2
THF
2kR (M-1 s-1) 2kRb (M-1 s-1) kT (M-1 s-1) kG (M-1 s-1) kGT (s-1) kGTc (s-1) kT/kG kR/(kG + kT)d
(3.2 ( 0.5) × 109 3.2 × 109 (1.5 ( 0.5) × 109 (0.10 ( 0.07) × 109 (1.1 ( 0.1) × 103 1.1 × 103 15 1.005
(7.9 ( 0.5) × 109 7.7 × 109 (3.5 ( 0.5) × 109 (0.32 ( 0.07) × 109 (1.0 ( 0.1) × 103 1.1 × 103 11 1.006
(1.7 ( 0.5) × 109 1.6 × 109 (0.70 ( 0.1) × 109 (0.13 ( 0.07) × 109 (0.91 ( 0.05) × 103 0.88 × 103 5.4 1.019
(2.2 ( 0.8) × 109 2.3 × 109 (0.84 ( 0.3) × 109 (0.23 ( 0.2) × 109 (1.0 ( 0.05) × 103 0.96 × 103 3.7 1.016
a Uncertainties are estimates of 2σ. A major source of uncertainty comes from the uncertainties in the molar absorptivities. b Linear fit of 1/[R] vs time. c Linear fit of ln[G] vs time. d kR/(kG + kT) should equal 1 and is another check of the correctness of the fit.
d[R] ) -2kR[R]2 dt
(3)
d[G] ) kG[R]2 - kGT[G] dt
(4)
d[T] ) kT[R]2 + kGT[G] dt
(5)
where R ) (n-butylCp)Mo(CO)3, G ) gauche-[(n-butylCp)Mo(CO)3]2, and T ) trans-[(n-butylCp)Mo(CO)3]2. The overall rate constant for radical recombination, 2kR, could also be determined by a linear graphical fit of 1/[R] vs time, as shown in Figure 5. The first-order rate constant, kGT, for the isomerization of gauche-totrans could also be obtained from a linear fit of ln[G] vs time after the radical concentration approaches zero (see inset in Figure 4). These graphical solutions are also presented in Table 1 for comparison with the values derived from nonlinear least-squares fit to the differential equations. The 2kR and the kGT values obtained by the two methods yield almost identical results. The use of the differential analysis to calculate the rate constants not only yielded the overall rate constant for radical recombination, 2kR, but also yielded the individual rates of formation of the gauche dimer, kG, and the trans dimer, kT, from the radical. From Table 1 we can see that the rate of recombination of the radical to form the trans isomer is much faster than gauche formation. The last row in Table 1 shows another fit parameter, kR/(kG + kT). Since the values for the rate constants were determined independently, the good agreement,
