Inorg. Chem. 2000, 39, 1573-1577
1573
Electronic Effects on the Rates of Coupled Two-Electron/Halide Self-Exchange Reactions of Substituted Ruthenocenes Timothy M. Shea and T. David Westmoreland* Department of Chemistry, Wesleyan University, Middletown, Connecticut 06459 ReceiVed June 15, 1999
The complexes (MemCp)(MenCp)Ru (m,n ) 1,1; 0,4; 0,5; 1,5) have been synthesized along with the corresponding halometalloceniums [(MemCp)(MenCp)RuX]+ (X ) Cl, Br, I). The two-electron/halide transfer self-exchange kinetics have been determined and compared to those of the Cp2Ru/[Cp2RuX]+ parent system. Methylation decreases the rate of exchange monotonically, and plots of ln(kex) vs number of methyl groups are linear for constant X. It is concluded that steric effects do not contribute significantly to the observed kinetics. Thermodynamic studies of halide substitution equilibria support the conclusion that the electronic effects of methylation are essentially additive.
Introduction The characterization of multielectron/group transfer reactions, often termed “atom-transfer reactions”,1 is of considerable recent interest.2 The self-exchange reactions of ruthenocene and osmocene derivatives given in eq 1 have provided a synthetically accessible series of complexes for the study of fundamental aspects of these reactions.3-6
[Cp2MX]+ + Cp2M f Cp2M + [Cp2MX]+
(1)
(M ) Ru, Os; X ) Cl, Br, I) Previous studies3-6 have focused on the effects of varying the transferred atom, the counterions, and the solvent on the observed self-exchange kinetics and have resulted in mechanistic proposals that invoke a halogen-bridged transition state. Thermodynamic and structural studies6 have also led to the conclusion that electronic factors are responsible for the large halide dependence of the rates. Another approach to modifying the electronic properties of the complexes is to add substituents to the cyclopentadienyl rings. For methyl substituents, it has been shown,3 for example, that the self-exchange rates for the [Cp*2RuX]+/Cp*2Ru couples (Cp* ) pentamethylcyclopentadienyl) were too slow for detection by dynamic NMR techniques. This is in contrast to the observed increase in the rate of one-electron outer-sphere (1) Taube, H. In Mechanistic Aspects of Inorganic Reactions; Rorabacher, D. B., Endicott, J. F., Eds.; ACS Symposium Series 198; American Chemical Society: Washington, DC, 1982; pp 151-179. (2) (a) Holm, R. H. Chem. ReV. 1987, 87, 1401-1449 and references therein. (b) Woo, L. K. Chem. ReV. 1993, 93, 1125-1136 and references therein. (3) Smith, T. P.; Iverson, D. J.; Droege, M. W.; Kwan, K. S.; Taube, H. Inorg. Chem. 1987, 26, 2882-2884. (4) (a) Kirchner, K.; Dodgen, H. W.; Wherland, S.; Hunt, J. P. Inorg. Chem. 1989, 28, 604-605. (b) Kirchner, K.; Dodgen, H. W.; Wherland, S.; Hunt, J. P. Inorg. Chem. 1990, 29, 2381-2385. (c) Kirchner, K.; Han, L. F.; Dodgen, H. W.; Wherland, S.; Hunt, J. P. Inorg. Chem. 1990, 29, 4556-4559. (d) Anderson, K. A.; Kirchner, K.; Dodgen, H. W.; Hunt, J. P.; Wherland, S. Inorg. Chem. 1992, 31, 2605-2608. (5) Watanabe, M.; Sano, H. Chem. Lett. 1991, 555-558. (6) Shea, T. M.; Deraniyagala, S. P.; Studebaker, D. B.; Westmoreland, T. D. Inorg. Chem. 1996, 35, 7699-7703.
transfer for decamethylferrocenium/decamethylferrocene relative to the corresponding ferrocenium/ferrocene rate.7a The results for the ferrocene-based systems have been rationalized in terms of increased delocalization of charge with increasing methylation and/or decreased solvent contribution to the reorganizational energy.7 The origin of the dramatic decrease in rate for the ruthenocenium/ruthenocene system, however, has not been established. In particular, the relative importance of electronic effects on the magnitude of the electronic coupling in the transition state and the steric effects that might be expected to inhibit the formation of a halogen-bridged transition state has not been defined. We have synthesized a series of methyl-substituted ruthenocenes and their haloruthenocenium derivatives and have determined their two-electron/halide transfer self-exchange rate constants. The results lead to the conclusion that relative selfexchange rates are governed by electronic effects, not steric effects, for complexes with up to six methyl substituents. In addition, the relative energetics of the reaction coordinates have been defined from the activation parameters and the relative reactant state free energies. Experimental Section Materials. Ruthenocene and decamethylruthenocene were obtained from Strem Chemicals, Inc. and were sublimed prior to use. Cyclopentadiene dimer and methylcyclopentadiene dimer were obtained from Aldrich Chemical Co. and were cracked by distillation immediately prior to use. Tetramethylcyclopentadiene and RuCl3‚xH2O were purchased from Strem Chemicals, Inc. and were used as received. Other materials were obtained commercially and were used as received. Published procedures were used for syntheses of [Cp2RuX]PF6 (X ) Cl, Br, I),4,6 [Cp*RuCl2]x,8 (MeCp)2Ru,9 Cp*CpRu,8 (Me5Cp)(MeCp)(7) At 298 K, kex is 5.3 × 106 M-1 s-1 for [Cp2Fe]+/Cp2Fe exchange and 3.8 × 107 M-1 s-1 for [Cp*2Fe]+/Cp*2Fe exchange. (a) Yang, E. S.; Chan, M.-S.; Wahl, A. C. J. Phys. Chem. 1980, 84, 3094-3099. (b) Chan, M.-S.; Wahl, A. C. J. Phys. Chem. 1978, 82, 2542-2549. (8) Gassman, P. G.; Winter, C. H. J. Am. Chem. Soc. 1988, 110, 61306135. (9) Lemay, G.; Kaliaguine, S.; Adnot, A.; Nahar, S.; Cuzak, D.; Monnier, J. Can. J. Chem. 1986, 64, 1943-1946.
10.1021/ic990690b CCC: $19.00 © 2000 American Chemical Society Published on Web 03/15/2000
1574 Inorganic Chemistry, Vol. 39, No. 7, 2000 Ru10 (Cp* ) pentamethylcyclopentadienyl anion; MeCp ) methylcyclopentadienyl anion), (R4N)X2Y, and (R4N)X311,12 (R ) Me, Et, n-Bu; X, Y ) Cl, Br, I). CD3CN for kinetic measurements was obtained from Cambridge Isotopes in sealed glass ampules and contained Cl. Furthermore, for constant X, the ∆∆G°(X,I) values remain essentially constant within the error of the experiment and are thus independent of the number of methyl substituents. This result suggests a simple additive effect of each methyl group on the overall energetics. That is, addition of any number of methyl groups shifts the free energies of the X ) Cl, Br, and I metalloceniums by approximately the same amount. This lack of change in ∆∆G°(X,I) as the number of methyl groups is increased is consistent with the notion that methyl groups exert a relatively straightforward electronic effect on the energetics of the system. These results may be interpreted in terms of a model in which methylation of the cyclopentadienyl rings essentially changes only the effective electronegativity of the ruthenium and thus the Ru-X bond strength. As the degree of methylation increases, the electronegativity of the metal decreases, the difference in electronegativity, χX - χRu, increases, and the Ru-X bond strength increases. Since the kinetic results indicate that net Ru-X bond breaking is a feature of the transition state, this interpretation would predict that increasing methylation would decrease the exchange rate, which is consistent with the experimental observations. From the data in Tables 1 and 2, it is possible to construct reaction coordinate diagrams for each self-exchange, as has been previously demonstrated for the Cp2Ru/[Cp2RuX]+ and Cp2Os/[Cp2OsX]+ systems.6 Specific energetic parameters for the systems for which complete data are available can be calculated from the parameters in Tables 1 and 2 and are given in the Supporting Information. It is noteworthy that for a constant degree of methyl substitution, the relative energetics of the three halide transfer reactions are remarkably constant. In all cases, the transition states are closer in energy than the corresponding reactant states, providing further support for the conclusion that the transition state involves net bond breaking.
Substituted Ruthenocenes Concluding Remarks The thermodynamic and kinetic results given above indicate that in both the reactant states and the transition states, the effects of up to six methyl groups are electronic and quantitatively conform to a Hammett relationship. Steric effects play no significant role in the overall reaction. These results suggest that all the self-exchanges proceed by a similar mechanism, i.e., through a linear Ru-X-Ru bridged transition state.
Inorganic Chemistry, Vol. 39, No. 7, 2000 1577 Supporting Information Available: Tables of NMR and optical absorbance data for the new compounds, reaction coordinate parameters and diagram, integrated EXSY peak intensities, and a table and figures of the optical absorbance data for equilibrium mixtures corresponding to reaction 3b. This material is available free of charge via the Internet at http://pubs.acs.org.
IC990690B
