Experimental and Theoretical Investigation into the Formation and

 Copyright 2009

Volume 28, Number 11, June 08, 2009

American Chemical Society

Articles Experimental and Theoretical Investigation into the Formation and Reactivity of M(Cp)(CO)2(CO2) (M ) Mn or Re) in Liquid and Supercritical CO2 and the Effect of Different CO2 Coordination Modes on Reaction Rates with CO, H2, and N2 Jixin Yang,† Boka Robert N’Guessan,‡,§ Alain Dedieu,*,‡ David C. Grills,†,⊥ Xue-Zhong Sun,† and Michael W. George*,† School of Chemistry, UniVersity of Nottingham, UniVersity Park, Nottingham, NG7 2RD, U.K., and Laboratoire de Chimie Quantique, Institut de Chimie, LC3-UMR 7177 CNRS/ULP, UniVersite´ Louis Pasteur, 67000 Strasbourg, France ReceiVed October 24, 2008 UV photolysis of M(Cp)(CO)3 (M ) Mn or Re) in liquid or supercritical CO2 (scCO2) solution led to the formation of the CO2 complexes M(Cp)(CO)2(CO2), which were detected by nanosecond time-resolved infrared spectroscopy (TRIR). The coordination of CO2 to the metal centers was confirmed by carrying out experiments in supercritical Kr (scKr) and scKr doped with CO2. Differences between the positions of the ν(C-O) IR bands of the CO ligands in Mn(Cp)(CO)2(CO2) and Re(Cp)(CO)2(CO2) suggest that the CO2 ligand has different coordination modes to the metal centers in these complexes. The rate constants and activation enthalpies for the reactions of M(Cp)(CO)2(CO2) with CO, H2, and N2 in scCO2 have been measured and compared with those previously reported for the analogous Xe and heptane complexes in scXe and heptane, respectively. Striking differences and similarities between these kinetic data and also the IR spectra of the CO2, Xe, and heptane complexes provided evidence that the CO2 coordination mode is η1-O end-on bound in Mn(Cp)(CO)2(CO2), and η2-C,O side-on bound in Re(Cp)(CO)2(CO2). These different coordination modes lead to dramatic differences in reactivity with CO, H2, and N2, with the Re complexes being significantly less reactive. To provide more evidence for the nature of the CO2 binding modes, a series of DFT calculations were performed at the B3LYP/SDD-6-311G** level. The calculations supported the experimentally proposed CO2 coordination modes. For Mn(Cp)(CO)2(CO2), a two-electron stabilizing interaction leads to the η1-O coordination mode, with a major component of the bonding being an electrostatic attraction, with little charge transfer between CO2 and Mn. For Re(Cp)(CO)2(CO2), the η2-C,O structure was more stable than η1-O by 1.7 kcal mol-1. A significant charge transfer from Re to CO2 occurs, resulting in partial oxidation of Re. Frequency calculations corroborate these conclusions and also reveal that the IR band observed at ca. 1860 cm-1 in the TRIR spectrum of Re(Cp)(CO)3 obtained after irradiation in scCO2 is due to the asymmetric CO2 stretch of bound CO2. Reaction enthalpies and activation barriers were also calculated for the reactions of the CO2 complexes with CO, H2, and N2.

Introduction There has recently been considerable research effort in the use of transition metal-based catalysts to convert CO2 into useful chemicals and fuels. Such catalytic processes offer great potential for using CO2 as a cheap, abundant C1 feedstock for * Corresponding authors. E-mail: [email protected]; [email protected]. † University of Nottingham. ‡ Universite´ Louis Pasteur. § Present address: Laboratoire de Chimie Organique Structurale, UFR SSMT, Universite´ d’Abidjan-Cocody, Abidjan, Coˆte d’Ivoire. ⊥ Present address: Chemistry Department, Brookhaven National Laboratory, PO Box 5000, Upton, NY 11973.

fine chemical production and for the recycling of CO2 from industrial emissions to help alleviate global warming.1 CO2 complexes of transition metal species are thus of major current interest. CO2 is also a remarkable solvent in itself, and there is considerable interest in performing chemical reactions in supercritical CO2 (scCO2), particularly since scCO2 is seen as a promising alternative to conventional, toxic organic solvents in green chemical applications.2 Apart from its environmental benefits, a major attraction of scCO2 as a solvent is the fact that supercritical fluids are highly miscible with gases. This (1) (a) Allen, S. D.; Byrne, C. M.; Coates, G. W. Feedstocks for the Future: Renewables for the Production of Chemicals and Materials; ACS Symp. Ser. 921; American Chemical Society: Washington, DC, 2006; pp 116-129. (b) Arakawa, H.; et al. Chem. ReV. 2001, 101, 953.

10.1021/om8010195 CCC: $40.75  2009 American Chemical Society Publication on Web 05/11/2009

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should be contrasted to the solubility of gases in liquid solvents, where the concentration of dissolved gas is relatively low and also decreases as temperature increases. For example, the concentration of hydrogen in a supercritical mixture of hydrogen (85 atm) and carbon dioxide (120 atm) at 50 °C is 3.2 M, whereas the concentration of hydrogen in tetrahydrofuran under the same pressure is only 0.4 M.3 This property of supercritical fluids therefore offers the potential for dramatic improvements to chemical processes in which gaseous reagents have traditionally been used in the solution phase and has been widely investigated for homogeneous, heterogeneous, and biocatalytic processes.4 The coordination of the CO2 solvent to a metal center is an important factor when considering catalytic reactions in scCO2. If a molecule of CO2 reacts with a transition metal center, there are three observed CO2 binding modes (carbon-bound η1-C, side-on bidentate η2-C,O, and end-on oxygen-bound η1-O).5 Several examples of η1-C and η2-C,O CO2 complexes have been isolated, and their modes of CO2 coordination were confirmed by X-ray crystallography.6 By contrast, the η1-O coordination mode has mainly been predicted theoretically and deduced from spectra obtained in low-temperature matrixes.7 Only recently has one example been characterized crystallographically by Meyer and co-workers.8 The electron-rich, six-coordinate trisaryloxide uranium(III) complex ((AdArO)3tacn)UIII (where (AdArOH)3tacn ) 1,4,7-tris(3-adamantyl-5-tert-butyl-2-hydroxybenzyl)1,4,7-triazacyclononane) reacted rapidly with CO2 to yield ((AdArO)3tacn)UIV(CO2), a complex in which the CO2 ligand is linearly coordinated to the metal through its oxygen atom. Early transition metal η1-O complexes have also been reported. For example, Downs and co-workers characterized9 M(CO)5(CO2) (M ) Cr and W) in low-temperature matrixes and suggested that the mode of coordination was η1-O, with subsequent calculations having supported this assignment.10 We reported the characterization of M(CO)5(CO2) (M ) Cr, Mo, and W) in scCO2 using fast time-resolved infrared spectroscopy (TRIR), a combination of UV flash photolysis with fast infrared detection, thus providing the first tentative observation of the reactivity of organometallic η1-O CO2 complexes, in solution (2) (a) Hyde, J.; Leitner, W.; Poliakoff, M. In High Pressure Chemistry; Eldik,R.; Kla¨rner, F.-G., Eds.; Wiley-VCH: Weinheim, 2002; p 369. (b) Matsui, K.; Kawanami, H.; Ikushima, Y.; Hayashi, H. Chem. Commun. 2003, 2502. (c) Oakes, R. S.; Clifford, A. A.; Rayner, C. M. J. Chem. Soc., Perkin Trans. 1 2001, 917. (3) Jessop, P. G.; Hsiao, Y.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1996, 118, 344. (4) (a) Baiker, A. Chem. ReV. 1999, 99, 453. (b) Jessop, P. G.; Ikariya, T.; Noyori, R. Chem. ReV. 1999, 99, 475. (c) Mesiano, A. J.; Beckman, E. J.; Russell, A. J. Chem. ReV. 1999, 99, 623. (d) Leitner, W. Acc. Chem. Res. 2002, 35, 746. (5) (a) Pandey, K. K. Coord. Chem. ReV. 1995, 140, 37. (b) Gibson, D. H. Chem. ReV. 1996, 96, 2063. (c) Leitner, W. Coord. Chem. ReV. 1996, 153, 257. (6) (a) Aresta, M.; Nobile, C. F.; Alnano, V. G.; Fornie, E.; Manassero, M. J. Chem. Soc., Chem. Commun. 1975, 636. (b) Gambarotta, S.; Arena, F.; Floriani, C.; Zanazzi, P. F. J. Am. Chem. Soc. 1982, 104, 5082. (c) Calabrese, J. C.; Herskovitz, T.; Kinney, J. B. J. Am. Chem. Soc. 1983, 105, 5914. (d) Do¨hring, A.; Jolly, P. W.; Kru¨ger, C.; Roma˜o, M. J. Z. Naturforsch. 1985, 40B, 484. (e) Bristow, G. S.; Hitchcock, P. B.; Lappert, M. F. J. Chem. Soc., Chem. Commun. 1981, 1145. (f) Gambarotta, S.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. J. Am. Chem. Soc. 1985, 107, 2985. (g) Alvarez, R.; Carmona, E.; Marı´n, J. M.; Poveda, M. L.; Gutie´rrezPuebla, E.; Monge, A. J. Am. Chem. Soc. 1986, 108, 2286. (h) Tanaka, H.; Nagao, H.; Peng, S.-M.; Tanaka, K. Organometallics 1992, 11, 1450. (i) Komiya, S.; Akita, M.; Kasuga, N.; Hirano, M.; Fukuoka, A. J. Chem. Soc., Chem. Commun. 1994, 1115. (j) Wang, T. F.; Hwu, C. C.; Tsai, C. W.; Lin, K. J. Organometallics 1997, 16, 3089. (k) Wang, T. F.; Hwu, C. C.; Wen, Y. S. J. Organomet. Chem. 2004, 689, 411. (7) Mascetti, J.; Tranquille, M. J. Phys. Chem. 1988, 92, 2177. (8) Castro-Rodriguez, I.; Nakai, H.; Zakharov, L. N.; Rheingold, A. L.; Meyer, K. Science 2004, 305, 1757. (9) Almond, M. J.; Downs, A. J.; Perutz, R. N. Inorg. Chem. 1985, 24, 275–281. (10) Pidun, U.; Frenking, G. Organometallics. 1995, 14, 5325.

Yang et al.

at or above room temperature.11 We found that the rate constant for the reaction of W(CO)5(CO2) with CO was identical to that observed for the reaction of W(CO)5(Xe) (generated following photolysis of W(CO)6 in supercritical Xe (scXe)) with CO. The ability to dissolve high concentrations of H2 in supercritical fluids was taken advantage of for the characterization of thermally labile nonclassical dihydrogen complexes in scXe.12 The difference between the coordination of dihydrogen to Mn and Re complexes is particularly relevant to this study. Mn(Cp)(CO)3 and Re(Cp)(CO)3 (Cp ) η5-C5H5) were irradiated in supercritical Xe in the presence of H2.12 Mn(Cp)(CO)3 generated the dihydrogen complex MnI(Cp)(CO)2(η2-H2), which exhibited ν(C-O) bands that were shifted down in frequency relative to Mn(Cp)(CO)3. In contrast, Re(Cp)(CO)3 generated the dihydride ReIII(Cp)(CO)2H2, with ν(C-O) bands lying between those of Re(Cp)(CO)3. In addition to examining the role of the solvent in catalytic reactions in supercritical fluids, part of the motivation for the current study arose from follow-up experiments in supercritical fluids reported by Lee.13 In those experiments, attempts were made to scale-up the synthesis of Re(Cp)(N2)3 in scCO2. Re(Cp)(N2)3 had previously been characterized using FTIR spectroscopy following photolysis of Re(Cp)(CO)3 in scXe in the presence of a high concentration of N2.14 However, Re(Cp)(N2)3 could not be generated using a similar approach in scCO2, and this was explained in terms of CO2 facilitating oxidation at the metal center.14 In this paper, we report the generation of M(Cp)(CO)2(CO2) (M ) Mn or Re) in scCO2 and demonstrate that in these species Mn and Re have different CO2 binding modes, resulting in a dramatic effect on the reactivity of these complexes in scCO2. DFT calculations have been performed to provide further understanding of the nature of the binding of CO2 to the metal center.

Experimental Section Materials. Mn(Cp)(CO)3 (Aldrich), Re(Cp)(CO)3 (Strem), Xe, Kr (BOC, research grade), H2, N2, and CO (Air Products) were used as received. Time-Resolved Infrared Measurements. Two different types of TRIR instrumentation were used to monitor the transient IR absorptions, both employing a pulsed Nd:YAG laser (Spectra Physics Quanta-Ray GCR-12; 355 or 266 nm) to initiate the photochemical reactions. The time-resolved spectra were obtained using a step-scan FTIR interferometer, details of which have been reported elsewhere.15 Briefly, the apparatus comprises a commercially available step-scan FTIR spectrometer (Nicolet Magna 860) equipped with a 100 MHz 12-bit digitizer and a 50 MHz MCT detector interfaced to a Nd:YAG laser with data collection synchronized by a pulse generator (Stanford DG535). IR kinetic experiments were performed using a continuous wave IR diode laser (Mu¨tek MDS 1100). Details of the Nottingham diode laser-based TRIR apparatus have been described elsewhere.16 In these experiments, the change in IR transmission at one IR frequency was measured following UV excitation, and if desired, a spectrum was built up on a “point-by-point” basis by repeating this measurement at different infrared frequencies. Supercritical noble gas solutions (11) Sun, X. Z.; George, M. W.; Kazarian, S. G.; Nikiforov, S. M.; Poliakoff, M. J. Am. Chem. Soc. 1996, 118, 10525. (12) Howdle, S. M.; Poliakoff, M. J. Chem. Soc., Chem. Commun. 1989, 1099. (13) Lee, P. D. PhD Thesis, University of Nottingham, 1996. (14) Howdle, S. M.; Grebenik, P.; Perutz, R. N.; Poliakoff, M. J. Chem. Soc., Chem. Commun. 1989, 1517. (15) Sun, X. Z.; Nikiforov, S. M.; Yang, J.; Colley, C. S.; George, M. W. Appl. Spectrosc. 2002, 56, 31. (16) George, M. W.; Poliakoff, M.; Turner, J. J. Analyst 1994, 119, 551.

ReactiVity of M(Cp)(CO)2(CO2) (M ) Mn or Re) were prepared in a high-pressure cell described previously for conventional spectroscopic15 monitoring. The cell was used with CaF2 windows and a pressure transducer (RDP Electronics). Theoretical Calculations. The calculations were carried out at the DFT-B3LYP level17 with the Gaussian 98 program.18 The geometries were first optimized by the gradient technique, using the standard LANL2DZ pseudopotential and basis sets for all atoms.19 The nature of the optimized structures, either transition states or intermediates, was assessed through a frequency calculation. The effect of a larger basis set (hereafter referred to as SDD6-311G**) on the optimized geometries and on the energies was tested by repeating the calculations with this SDD-6-311G** basis set. In this basis set, the innermost core electrons of the metal atoms are described by the quasi-relativistic energy adjusted spin-orbit averaged effective core potential from the Stuttgart group and the remaining outer core and valence electrons by the associated triple-ζ basis set.20 The polarized triple-ζ 6-311G** basis set is used for C, O, N, and H.21 The frequencies were computed within the harmonic oscillator approximation and left unscaled. The dissociation energies were corrected from the basis set superposition error (BSSE) through the use of the counterpoise method. The enthalpies were obtained by taking into account zero-point energies and thermal motion at standard conditions (temperature of 298.15 K, pressure of 1 atm). The dissociation enthalpies were also corrected from the BSSE, using the value obtained for the dissociation energies. We will, in the following, concentrate on the results obtained with the larger basis set, which seem reliable; see the discussion below and ref 34. In some instances these results turned out to differ from the LANL2DZ results. We will mention these cases only.

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Figure 1. (a) FTIR spectrum of Mn(Cp)(CO)3 in liqCO2 (1630 psi, 25 °C) in the presence of CO (15 psi) and (b) TRIR spectrum obtained 100 ns after photolysis (355 nm) of this solution. (c) TRIR spectrum obtained 100 ns after photolysis of Mn(Cp)(CO)3 dissolved in scKr (4160 psi, 25 °C) in the presence of CO (30 psi) and CO2 (400 psi).

Results and Discussion Figure 1a shows the FTIR spectrum of Mn(Cp)(CO)3 in liqCO2 (1630 psi) in the presence of CO (15 psi). The TRIR spectrum obtained 100 ns after 355 nm excitation of this solution is shown in Figure 1b. It is clear that the parent ν(CO) bands are bleached, and two new transient bands are formed at 1958 and 1885 cm-1. These new absorptions are shifted down in wavenumber relative to those of Mn(Cp)(CO)3 and are tentatively assigned to the transient carbon dioxide complex Mn(Cp)(CO)2(CO2) by comparison with previous matrix isolation and TRIR measurements.22 Two kinetic decay traces obtained from a similar experiment in scCO2, corresponding to Mn(Cp)(CO)3 and Mn(Cp)(CO)2(CO2), are shown in Figure 2. Mn(Cp)(CO)3 is re-formed at a rate (kobs ) (1.1 ( 0.1) × 105 s-1) that is identical to the rate of decay of Mn(Cp)(CO)2(CO2) (kobs ) (1.1 ( 0.1) × 105 s-1). However, these experiments do not definitively prove the coordination of CO2 to the Mn(Cp)(CO)2 moiety. In an effort to confirm the assignment of Mn(Cp)(CO)2(CO2), the experiment was repeated in scKr doped (17) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (c) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (18) Frisch, M. J.; et al. Gaussian 98; Gaussian Inc.:Pittsburgh, PA, 1998. (19) (a) Hay, P. J. J. Chem. Phys. 1985, 82, 299. (b) Dunning, T. H.; Hay, P. J. In Modern Theoretical Chemistry; Plenum: New York, 1976; pp 1-28. (c) Huzinaga, S. J. Chem. Phys. 1965, 42, 1293. (20) Andrae, D.; Hau¨ssermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123. (21) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. (22) (a) Rest, A. J.; Sodeau, J. R.; Taylor, D. J. J. Chem. Soc., Dalton Trans. 1978, 651. (b) Creaven, B. S.; Dixon, A. J.; Kelly, J. M.; Long, C.; Poliakoff, M. Organometallics 1987, 6, 2600. (c) Grills, D. C.; Sun, X. Z.; Childs, G. I.; George, M. W. J. Phys. Chem. A 2000, 104, 4300. (d) Childs, G. I.; Colley, C. S.; Dyer, J.; Grills, D. C.; Sun, X. Z.; Yang, J. X.; George, M. W. J. Chem. Soc., Dalton Trans. 2000, 1901.

Figure 2. TRIR kinetic traces obtained following photolysis (355 nm) of Mn(Cp)(CO)3 in scCO2 at 35 °C in the presence of CO (15 psi) at (a) 1948 cm-1 and (b) 1959 cm-1. (c) Plot of kobs versus CO concentration for the reaction of Mn(Cp)(CO)2(CO2) with CO in scCO2 at 35 °C.

with 10% CO2 (p/p). We have previously demonstrated that Kr is an extremely weakly coordinating ligand, which allows the binding of more strongly coordinating ligands, e.g., Xe, to be probed.11,23 Therefore, if CO2 does coordinate to the metal center, ν(C-O) bands should be observed in the doped scKr experiment at similar positions to those observed in the pure scCO2 experiment. However, if CO2 does not coordinate to the metal center, the TRIR spectrum obtained in the doped experiment should be identical to that in pure scKr. The ν(C-O) (23) (a) Sun, X. Z.; Grills, D. C.; Nikiforov, S. M.; Poliakoff, M.; George, M. W. J. Am. Chem. Soc. 1997, 119, 7521. (b) Grills, D. C.; George, M. W. AdV. Inorg. Chem. 2001, 52, 113. (c) Ball, G. E.; Darwish, T. A.; Geftakis, S.; George, M. W.; Lawes, D. J.; Portius, P.; Rourke, J. P. Proc. Natl. Acad. Sci. 2005, 102, 1853.


Yang et al.

Table 1. Infrared Frequencies of M(Cp)(CO)2L (M ) Mn or Re; L ) CO, H2, N2, heptane, CO2, Kr, Xe, or THF (Mn only)) Complexes (gas-phase DFT calculated IR frequencies are shown in parentheses) complex Mn(Cp)(CO)3

solvent/matrix a

Ar matrix CH4 matrixa liqCO2b scCO2b scKrc scXec scKr + 10% CO2b n-heptaned THFe DFT calculations Mn(Cp)(CO)2(Kr) scKrc Mn(Cp)(CO)2(Xe) scXec DFT calculations Mn(Cp)(CO)2(heptane) n-heptaned Mn(Cp)(CO)2 DFT calculations liqCO2b Mn(Cp)(CO)2(CO2) scCO2b scKr + 10% CO2b DFT calculations Mn(Cp)(CO)2(N2) scXef heptane DFT calculations liqCO2 DFT calculations Mn(Cp)(CO)2(H2) scXef heptaneb liqCO2b Mn(Cp)(CO)2(THF) THFe a

temperature (K) ν(C-O)/cm-1 12 12 298 308 298 298 298 298 298 298 298 298 298 308 298 298 298 298 298 298 298 298

2033, 1951 2029, 1943 2030, 1945 2031, 1944 2035, 1956 2033, 1952 2034, 1955 2028, 1947 2017, 1929 (2101, 2038) 1973, 1908 1970, 1903 (2056, 2002) 1961, 1894 (2066, 2010) 1958, 1884 1958, 1885 1968, 1900 (2054, 2000) 1981, 1925 1979, 1928 (2073, 2032) 1979, 1922 (2069, 2023) 1986, 1923 1986, 1927 1986, 1921 1925, 1850

complex Re(Cp)(CO)3

Re(Cp)(CO)2(Kr) Re(Cp)(CO)2(Xe) Re(Cp)(CO)2(heptane) Re(Cp)(CO)2 Re(Cp)(CO)2(CO2) Re(Cp)(CO)2(N2) cis-Re(Cp)(CO)2(H)2 trans-Re(Cp)(CO)2(H)2 trans-Re(Cp)(CO)2(H)2 cis-Re(Cp*)(CO)2(H)2 trans-Re(Cp*)(CO)2(H)2

solvent/matrix g

Nujol matrix N2 matrixh liqCO2b scCO2b scKri scXei scKr + 3% CO2b n-heptaneb DFT calculations scKri scXei DFT calculations n-heptanei DFT calculations liqCO2b scCO2b scKr + 3% CO2b DFT calculations scXef heptane DFT calculations liqCO2h DFT calculations liqCO2h scXef liqXe liqXe

temperature (K) ν(C-O)/cm-1 77 20 298 308 298 298 298 298 298 298 298 298 308 298 298 298 298 298 298 203 203

2029, 1934 2032, 1943 2031, 1938 2031, 1939 2038, 1952 2035, 1946 2038, 1951 2031, 1940 (2095, 2016) 1966, 1903 1957, 1894 (2025, 1970) 1952, 1886 (2034, 1979) 2024, 1953 2028, 1951 2028, 1961 (2082, 2027) 1978, 1926 1975, 1923 (2051, 2002) 2010, 1934 (2079, 2015) 2025, 1952 2024, 1955 1922, 1918 1995, 1921

Ref 22a. b This work. c Ref 22c. d Ref 22b. e Ref 37. f Ref 12. g Ref 36. h Ref 14. i Ref 23a; Cp*) (η5-C5Me5).

absorptions of Mn(Cp)(CO)3 in scKr are not significantly perturbed by the presence of CO2 (see Table 1). Figure 1c shows the TRIR spectrum obtained 100 ns after 355 nm excitation of Mn(Cp)(CO)3 dissolved in scKr (4160 psi, 25 °C) in the presence of CO (30 psi) and CO2 (400 psi). Again, the parent bands are bleached, and two new bands are observed at 1968 and 1900 cm-1. These bands are 5-8 cm-1 lower in frequency than those assigned to Mn(Cp)(CO)2Kr in pure scKr (see Table 1). Furthermore, by comparison with the bands assigned to Mn(Cp)(CO)2(CO2) in pure scCO2 (and taking into account solvent shifts from scCO2 to scKr), these new bands can be assigned to the CO2 complex Mn(Cp)(CO)2(CO2) in scKr (see Table 1). Thus, on the time scale of the TRIR experiments, in scKr doped with CO2, there is preferential coordination of CO2 to the Mn(Cp)(CO)2 moiety. The coordination of CO2 is further supported by a comparison of the kinetic behavior of the ν(C-O) absorptions obtained in this experiment with those in pure scKr. Figure 3 compares analogous TRIR decay traces corresponding to the transient dicarbonyl complexes, obtained in pure and doped scKr. It can be seen that the presence of only a modest quantity of CO2 increases the lifetime of Mn(Cp)(CO)2 by approximately 3 times (kobs ) 3.0 × 106 s-1 in doped scKr compared with 8.3 × 106 s-1 in pure scKr). The decay of Mn(Cp)(CO)2(CO2) in scCO2 depends linearly upon CO concentration, and this affords the second-order rate constant for the reaction of Mn(Cp)(CO)2(CO2) with CO in scCO2 at 35 °C, kMnCO ) (3.5 ( 0.4) × 106 M-1 s-1, Figure 2c. The ν(C-O) bands of Mn(Cp)(CO)2(CO2) in scCO2 are shifted down in frequency by an average of ca. 66 cm-1 relative to those of Mn(Cp)(CO)3. Mn(Cp)(CO)2Xe also exhibits ν(C-O) bands in scXe solution that are shifted down in wavenumber relative to Mn(Cp)(CO)3 (see Table 1). However, the ν(C-O) bands of Mn(Cp)(CO)2(CO2) are at significantly lower wavenumbers compared to those of Mn(Cp)(CO)2Xe,22c,22d despite the similar ν(C-O) positions of Mn(Cp)(CO)3 in scCO2 and scXe (Table 1). Furthermore, in THF solution the ν(C-O) bands

Figure 3. TRIR kinetic traces recorded (a) at 1908 cm-1 after photolysis of Mn(Cp)(CO)3 in scKr (3280 psi, 25 °C) in the presence of CO (30 psi) and (b) at 1910 cm-1 following irradiation of Mn(Cp)(CO)3 in scKr (4160 psi, 25 °C) in the presence of CO (30 psi) and CO2 (400 psi). The traces have been normalized.

of Mn(Cp)(CO)2(THF) are significantly shifted down in wavenumber (by an average of ca. 86 cm-1) relative to those of Mn(Cp)(CO)3. THF is a simple σ-coordinating ligand, binding via a lone pair of its oxygen atom, and this analogy indicates that CO2 is likely bound to Mn via η1-O coordination, since the other possible CO2 binding modes are not expected to result in such a downward shift in the positions of the ν(C-O) bands. We therefore tentatively assign this complex to be a η1-O coordinated CO2 complex. The rates of reaction of Mn(Cp)(CO)2(CO2) and Mn(Cp)(CO)2Xe with CO at 35 °C in scCO2 and scXe, respectively, are almost identical within experimental error (see Table 2). This is analogous to our previous observation that M(CO)5(CO2) and M(CO)5Xe (M ) Cr, Mo, or W) have similar reactivity, particularly since the

ReactiVity of M(Cp)(CO)2(CO2) (M ) Mn or Re)


Table 2. Second-Order Rate Constants and Activation Enthalpies for the Reactions of M(Cp)(CO)2L (M ) Mn or Re; L ) heptane, Kr, Xe, or CO2) with CO, H2, and N2 in n-Heptane, scKr, scXe, or scCO2 solutiona complex Mn(Cp)(CO)2(CO2) Mn(Cp)(CO)2(CO2) Mn(Cp)(CO)2(CO2) Mn(Cp)(CO)2(Kr) Mn(Cp)(CO)2(Xe) Mn(Cp)(CO)2(heptane) Mn(Cp)(CO)2(heptane) Mn(Cp)(CO)2(heptane) Re(Cp)(CO)2(CO2) Re(Cp)(CO)2(CO2) Re(Cp)(CO)2(CO2) Re(Cp)(CO)2(Kr) Re(Cp)(CO)2(Xe) Re(Cp)(CO)2(heptane) Re(Cp)(CO)2(heptane) Re(Cp)(CO)2(heptane) a

solvent scCO2 scCO2 scCO2 scKr scXe heptane heptane heptane scCO2 scCO2 scCO2 scKr scXe heptane heptane heptane

temperature (K) 308 308 308 298 298 298 298 298 308 308 308 298 298 298 298 298

reagent

rate constant (M-1 s-1)

∆H‡ (kcal mol-1)

CO H2 N2 CO CO CO H2 N2 CO H2 N2 CO CO CO H2 N2

3.5 × 10 2.3 × 106 1.2 × 106 7.2 × 107 1.6 × 106 8.1 × 105 8.8 × 105 7.4 × 105 1.6 × 102 3.0 × 102 3.0 × 102 8.1 × 106 4.8 × 103 2.1 × 103 4.0 × 103 3.0 × 103

8.7 (5.1) 8.2 (6.6) 5.9 (5.2) n.d.b 7.1 8.6 9.4 7.9 16.4 (6.1) 16.1 (9.4) 15.9 (7.7) n.d. 11.3 11.0 9.0 9.6

6

Gas-phase DFT calculated values are shown in parentheses. b n.d. ) not determined.

Figure 4. (a) FTIR spectrum of Re(Cp)(CO)3 in liqCO2 (3000 psi, 25 °C) in the presence of CO (15 psi) and (b) step-scan FTIR spectrum obtained 100 ns after photolysis (266 nm) of this solution.

assigned CO2 coordination mode in the former complexes was η1-O type bonding.11 We have also examined the photochemistry of Re(Cp)(CO)3 in liquid CO2 and scCO2, by recording the TRIR spectrum after 266 nm excitation in the presence of CO (15 psi), Figure 4b. It should be noted that the ν(C-O) bands of Mn(Cp)(CO)3 and Re(Cp)(CO)3 in scCO2 are at almost identical positions, but those of their respective photoproducts are very different (see Table 1). While the two bands of the Mn photoproduct are shifted down in wavenumber relative to those of Mn(Cp)(CO)3, the bands of the Re photoproduct (at 2028 and 1951 cm-1) appear between those of Re(Cp)(CO)3 and are tentatively assigned to Re(Cp)(CO)2(CO2). A third transient band was observed at ca. 1860 cm-1 in the Re(Cp)(CO)3 experiment. We tentatively assign this to a vibration of bound CO2, and this is supported by our DFT calculations (see later). The difference in the ν(C-O) spectra of the Mn and Re photoproducts is reminiscent of results obtained when Mn(Cp)(CO)3 and Re(Cp)(CO)3 were irradiated in supercritical Xe in the presence of H2, resulting in the formation of the Mn(I) dihydrogen complex and the Re(III) dihydride species, respectively (see Table 1).12 Similar differences in H2 coordination have been observed between V(Cp)(CO)3(η2-H2) and Ta(Cp)(CO)3H2.24 Thus, comparison of the band positions of the CO2 complexes in scCO2 indicates that the Mn and Re centers of the photoproducts must also have different oxidation states. Again, we have further (24) (a) Haward, M. T.; George, M. W.; Hamley, P.; Poliakoff, M. J. Chem. Soc., Chem. Commun. 1991, 1101. (b) George, M. W.; Haward, M. T.; Hamley, P.; Hughes, C.; Johnson, F. P. A.; Popov, V. K.; Poliakoff, M. J. Am. Chem. Soc. 1993, 115, 2286.

Figure 5. TRIR kinetic traces recorded after photolysis (266 nm) of Re(Cp)(CO)3 in scKr (3250 psi, 25 °C) in the presence of CO (30 psi) and CO2 (100 psi) at (a) 1969 cm-1 and (b) 1960 cm-1.

investigated the assignment of Re(Cp)(CO)2(CO2) by repeating the TRIR experiment in scKr doped with 3% CO2 (p/p). The Re(Cp)(CO)2 moiety has previously been found to produce long-lived complexes23 with noble gases, such that Re(iPrCp)(CO)(PF3)Xe has been characterized by NMR.23c In an experiment where Re(Cp)(CO)3 was irradiated in scKr doped with Xe, the initial formation of Re(Cp)(CO)2Kr could be observed on the nanosecond time scale, followed by its decay and the generation of the more stable complex Re(Cp)(CO)2Xe. The TRIR spectrum of Re(Cp)(CO)3 in scKr (3250 psi, 25 °C) in the presence of CO (30 psi) and CO2 (100 psi) obtained immediately after the laser flash showed only two new bands (at ca. 1966 and 1902 cm-1) that are almost identical to the ν(C-O) bands of Re(Cp)(CO)2Kr in pure scKr (see Table 1). However, within 2 µs these disappear and two new bands grow in at 2028 and 1961 cm-1. By comparison with the ν(C-O) band positions observed in the TRIR spectra of Re(Cp)(CO)3 in pure scKr and pure scCO2, these are assigned to the CO2 complex Re(Cp)(CO)2(CO2) in scKr (see Table 1). This can be seen more clearly in Figure 5, which shows two TRIR decay traces from this experiment recorded at wavenumbers corresponding to (a) Re(Cp)(CO)2Kr and (b) Re(Cp)(CO)2(CO2), respectively. This demonstrates that following the laser flash Re(Cp)(CO)2Kr is instantly generated (on the time scale of these TRIR experiments) and then rapidly decays at a rate (kobs )


Figure 6. Step-scan FTIR spectra obtained over the first 10 µs after photolysis (355 nm) of Mn(Cp)(CO)3 in liqCO2 (3000 psi, 25 °C) in the presence of CO (30 psi) and N2 (400 psi).

(6.5 ( 0.6) × 106 s-1) that is the same, within experimental error, as the rate of growth of Re(Cp)(CO)2(CO2) (kobs ) (7.5 ( 0.7) × 106 s-1). In the presence of CO in scCO2, the formation of Re(Cp)(CO)2(CO2) is completely reversible, and from a CO concentration dependence experiment, we obtained the rate constant for the reaction of Re(Cp)(CO)2(CO2) with CO in scCO2 at 35 °C, kReCO ) (2.0 ( 0.2) × 103 M-1 s-1. It is clear from the positions of the ν(CO) bands of the two CO2 complexes characterized above that different CO2 coordination modes must be involved in the Mn and Re compounds. In contrast to Mn(Cp)(CO)2(CO2), the Re(Cp)(CO)2(CO2) complex is significantly less reactive toward CO than its analogous Xe complex, Re(Cp)(CO)2Xe, in supercritical solution at 35 °C (see Table 2, 308 K scXe data). This again shows a difference between the Mn and Re complexes. Further evidence for the coordination modes of the rhenium complexes comes from a recently synthesized series of half-sandwich cyclopentadienyl Re-CO2 complexes.6j,k The structures of these complexes were crystallographically determined and shown to contain η2-C,O-bound CO2 ligands. We have further examined the effect of the mode of CO2 coordination on reaction rates by monitoring the photoreactivity of Mn(Cp)(CO)3 and Re(Cp)(CO)3 with H2 and N2 in scCO2. Photolysis of Mn(Cp)(CO)3 and Re(Cp)(CO)3 in scCO2 in the presence of N2 is known to generate the monosubstituted dinitrogen complexes M(Cp)(CO)2(N2) (M ) Mn or Re).12,13 The analogous photochemical reaction with dihydrogen will allow us to probe the difference between substitution and oxidative addition since, as discussed above, photolysis of Mn(Cp)(CO)3 in the presence of dihydrogen forms the nonclassical dihydrogen complex Mn(Cp)(CO)2(η2-H2), while the corresponding reaction with Re(Cp)(CO)3 forms the classical dihydride Re(Cp)(CO)2(H)2. Figure 6 shows the TRIR difference spectra obtained following irradiation of Mn(Cp)(CO)3 in liquid CO2 in the presence of N2 (400 psi) and CO (30 psi). The spectrum at 1 µs after the flash shows that Mn(Cp)(CO)2(CO2) is generated first. This then decays to form two new bands at 1979 and 1922 cm-1, which are assigned to Mn(Cp)(CO)2(N2). The rates of decay of Mn(Cp)(CO)2(CO2) and formation of Mn(Cp)(CO)2(N2) depend linearly on the concentration of N2 in scCO2, and this affords the rate constant for reaction of Mn(Cp)(CO)2(CO2) with N2 (kMnN2 ) (1.2 ( 0.1) × 106 M-1 s-1), Figure 7. We have also measured the rates of the analogous reactions of Mn(Cp)(CO)2(CO2) with H2 (kMnH2

Yang et al.

Figure 7. TRIR kinetic traces recorded after photolysis (355 nm) of Mn(Cp)(CO)3 in liquid CO2 (3000 psi, 25 °C) in the presence of CO (30 psi) and N2 (400 psi) at (a) 1884 cm-1 and (b) 1922 cm-1. (c) Plot of kobs versus N2 concentration for the reaction of Mn(Cp)(CO)2(CO2) with N2 to form Mn(Cp)(CO)2(N2) in scCO2 at 35 °C.

) (2.3 ( 0.2) × 106 M-1 s-1), Re(Cp)(CO)2(CO2) with N2 (kReN2 ) (3.0 ( 0.3) × 102 M-1 s-1) and H2 (kReH2 ) (3.0 ( 0.3) × 102 M-1 s-1) in scCO2 (Table 2). We have also compared these rate constants with those for the reactions of the analogous heptane complexes M(Cp)(CO)2(heptane) (M ) Mn or Re) with N2 and H2. We find that the rate constants for reaction of Mn(Cp)(CO)2(heptane) with N2 and H2 are ca. 2-3 times slower than the corresponding reactions of Mn(Cp)(CO)2(CO2) in scCO2, while the reactions of Re(Cp)(CO)2(heptane) are ca. 10 times faster than the equivalent reactions of Re(Cp)(CO)2(CO2) in scCO2. This again provides additional support for different modes of CO2 coordination in the Mn and Re complexes. We have also determined the activation parameters for all the reactions described above and find that the ∆H‡ values (6.5-9.3 kcal mol-1) for the reactions of Mn(Cp)(CO)2(CO2) with CO, H2, and N2 in scCO2 are similar to or slightly lower than the values obtained for the reaction of Mn(Cp)(CO)2(heptane) with the same ligands in n-heptane (Table 2). However, we find that the same comparison does not hold true in the case of the rhenium complexes, where the ∆H‡ values (16.5-17.0 kcal mol-1) for the reactions of Re(Cp)(CO)2(CO2) with CO, H2, and N2 in scCO2 are significantly higher than the corresponding reactions of the alkane intermediate in n-heptane. This is consistent with different modes of CO2 coordination in the Mn and Re complexes. However, such a change might also indicate a change in mechanism. In order to provide more evidence for the nature of the binding of CO2 to the metal center and to explore how these different binding modes of CO2 to the M(Cp)(CO)2 moiety may affect the reactivity, we have performed a series of DFT calculations. Figure 8 shows the optimized geometries obtained for the M(Cp)(CO)2(CO2) (M ) Mn or Re) complexes at the B3LYP/SDD-6-311G** level of theory. We calculated all three modes of CO2 coordination for the Mn and Re complexes, and a comparison of the relative energies of those that reached a global minimum revealed that the Mn system adopts only a η1-O binding mode, in contrast to the Re complex, where the η2-C,O binding mode is preferred. This preference, which is in line with the TRIR results, may


Figure 8. Optimized geometries obtained at the DFT-B3LYP/SDD6-311G** level for (a) Mn(Cp)(CO)2(CO2) and (b) Re(Cp)(CO)2(CO2).

be rationalized on the basis of orbital interaction arguments,25 based on the analysis of the wave functions of both systems. As shown in the orbital interaction diagram of Figure 9a, the HOMO of the Re(Cp)(CO)2 fragment is of dπ character. 26 It interacts primarily via a two-electron stabilizing interaction with the empty π* orbital of CO2. This interaction accounts for the dihapto binding mode of CO2. Note, however, that the corresponding bonding combination of these two orbitals is somewhat pushed up by the lower πCO2 orbital. But one finds an additional two-electron stabilizing interaction25 between the Re(Cp)(CO)2 LUMO, which is of dσ character,26 and the doubly occupied nπ orbital of CO2. At variance with Re(Cp)(CO)2, in Mn(Cp)(CO)2 the dσ LUMO is very low in energy; see Figure 9b. In fact the triplet state, in which the dσ and dπ frontier orbitals are both singly occupied, is known to be the ground state.27 The interaction between the dσ orbital and the σ lone pair (actually the 3σu orbital) of CO2 takes over, leading to the η1-O coordination mode. The antibonding combination between dσ and 3σu is rather high in energy, higher than the antibonding combination between dπ and the out-of-plane nπ orbitals; see Figure 9b. It is therefore empty and the situation is equivalent to having a two-electron stabilizing interaction between dσ and 3σu. The fact that the Mn-O-C arrangement is not linear, but makes an angle of 167.4° is best explained by a hybridization at the oxygen atom, which is due to some additional mixing, in this dσ/3σu interaction, of the in-plane π orbital of CO2. At this point one should stress that the stabilizing interaction between dσ and 3σu is somewhat offset by four-electron destabilizing interactions between the doubly occupied d orbital of Mn(Cp)(CO)2 and the doubly occupied nπ and π orbitals of CO2; see Figure 9b. Hence a quite important component of the bonding is most likely from an electrostatic origin, via an attractive interaction between the positively charged Mn atom and the negatively charged O atom. That there is in fact not much charge transfer from CO2 to Mn is corroborated by the frequency analysis (see below). Quite interestingly, when the geometry optimization was carried out with either the LANL2DZ basis set for all atoms or a mixed basis set made of the SDD pseudopotential and basis set for the metal and of the LANL2DZ basis set for C, N, O, and H, we could obtain two different structures for the (25) Sakaki, S.; Dedieu, A. Inorg. Chem. 1987, 26, 3278. (26) Schilling, B. E. R.; Hoffmann, R.; Lichtenberger, D. L. J. Am. Chem. Soc. 1979, 101, 585. (27) (a) Yang, H.; Asplund, M. C.; Kotz, K. T.; Wilkens, M. J.; Frei, H.; Harris, C. B. J. Am. Chem. Soc. 1998, 120, 10154. (b) Full, J.; Gonza´lez, L.; Daniel, C. J. Phys. Chem. A 2001, 105, 184.


Mn(Cp)(CO)2(CO2) system, corresponding to both the η1-O and η2-C,O coordination mode. The η1 structure was found to be more stable by 3.4 kcal mol-1 with the LANL2DZ basis set and by 2.4 kcal mol-1 with the mixed SDD-LANL2DZ basis set. In contrast, the geometry optimization carried out with the SDD-6-311G** basis set led to the η1-O structure only (even with an η2-C,O starting geometry). This points to some possible artifacts when basis sets that are too small and unpolarized are used to describe CO2. In line with this statement one may note that the optimized bond length of the free CO2 molecule decreases from 1.193 Å with the LANL2DZ basis set to 1.160 Å with the 6-311G** basis set, a value very close to the experimental value of 1.598 Å. Note that the two CO bond length values in the complex are quite close to the free CO2 value, 1.162 and 1.157 Å for the neighboring and terminal C-O bonds, respectively. This again is consistent with the strong electrostatic character of the metal-CO2 bonding and the absence of any significant overall charge transfer between manganese and CO2. The present situation is clearly different from the one encountered in the carbon dioxide complex of the electron-rich, six-coordinate, U(III) compound,8 where the corresponding C-O bond lengths of 1.222 and 1.277 Å point to a one-electron reduction of CO2 upon coordination. For the Re complex we could obtain both η1-O and η2-C,O structures with the SDD-6-311G** basis set, with the η2 structure being more stable by 1.7 kcal mol-1 (the difference is greater with the smaller LANL2DZ basis set, 3.6 kcal mol-1). Thus, ca. 95% of Re(Cp)(CO)2(CO2) should exist in the η2C,O form. In the η2 structure, the coordinated C-O bond length is 1.240 Å, while the other carbon-oxygen bond is 1.185 Å. The Re-C and Re-O bond lengths are 2.070 and 2.212 Å, respectively. All these values are in agreement with the experimental values from the X-ray crystal structures of the η5: η1-C5H4CH2CH2N(CH3)2Re(CO)(η2-CO2) and η5:η1-C5H4CH2CH2N(CH3)(n-C4H8OH)Re(CO)(η2-CO2) complexes.6j,k They are consistent with the associated charge transfer from an occupied dπ orbital to the π* orbital of CO2, which can be viewed as corresponding to partial oxidation of the rhenium atom. The η1-O structure, on the other hand, with C-O bond lengths of 1.163 and 1.156 Å, is very similar to that of the Mn complex (the Re-C bond is, as expected, longer than the Mn-C bond, 2.330 Å instead of 2.193 Å). The MP2-optimized C-O bond lengths in W(CO)5(η1-CO2) were found to be slightly longer, 1.188 and 1.172 Å, respectively.10 At this stage it is interesting to compare the calculated and experimental IR ν(C-O) frequencies (see the values quoted in parentheses in Table 1). The computed values are, as expected from calculations carried out at the DFT-B3LYP level,28 within a few percent of the experimental values. There is also a fair agreement in the variation on going from one system to the other. In particular, the computed values for η1-O Mn(Cp)(CO)2(CO2) are, as in the experiment, shifted down in wavenumber relative to those of Mn(Cp)(CO)3, while those for η2C,O Re(Cp)(CO)2(CO2) lie between those of Re(Cp)(CO)3. This gives additional credence to our tentative experimental assignment. In fact, the ν(C-O) frequency values calculated for the Mn(Cp)(CO)2(CO2) complex are just slightly below those calculated for the coordinatively unsaturated Mn(Cp)(CO)2 moiety (2066 and 2010 cm-1) and are well separated from the bound CO2 frequencies, which are at 2462 and 1385 cm-1 for the asymmetric and symmetric stretches, respectively. In free (28) (a) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502. (b) Zhou, M.; Andrews, L.; Bauschlicher, C. W., Jr. Chem. ReV. 2001, 101, 1931. (c) Carbonnie`re, P.; Barone, V. Chem. Phys. Lett. 2004, 399, 226.


Yang et al.

Figure 9. (a) Schematic orbital interaction diagram between the orbitals of the ReCp(CO)2 fragment and the orbitals of a bent CO2 molecule. (b) Schematic orbital interaction diagram between the orbitals of the MnCp(CO)2 fragment and the orbitals of a linear CO2 molecule. Table 3. Computeda Bond Enthalpies of X ) CO, H2, and N2 to M(Cp)(CO2) (M ) Mn or Re; X ) CO, H2, or N2),b,c Reaction Enthalpies,d and Enthalpy Barrierse (in parentheses) for the CO2/X Exchange Reaction MCp(CO)2 + X f MCp(CO2)X M, L

∆H

Mn, CO2 Mn, CO Mn, H2 Mn, N2 Re, CO2 Re, CO Re, H2 Re, N2

-5.0 -43.8 -18.8 -21.4 -7.5 -61.5 -33.6 -30.7

MCp(CO)2 (CO2) + X f MCp(CO) 2X + CO2 ∆H

∆H ‡

-38.8 -13.8 -16.4

(+ 5.1) (+6.6) (+5.2)

-54.0 -26.1 -23.2

(+6.1) (+9.4) (+7.7)

a Values (in kcal mol-1) computed at the DFT-B3LYP/SDD-6-311G level. b The BSSE is taken into account, by using the corresponding energy value obtained through the counterpoise method. c In the Re complex H2 is present as a dihydride (see above). d BSSE-corrected values obtained from a thermodynamic cycle using the BSSE-corrected bond dissociation enthalpy values. The uncorrected values (obtained as the difference between the enthalpy of the products and of the reactants) are found to differ from the corrected values by less than 1 kcal mol-1 only, except for the rhenium dihydride case, where the difference goes up to 3.4 kcal mol-1. e Not corrected from BSSE.

CO2, the computed values are lower (2436 and 1375 cm-1, respectively). The increase of the CO2 vibrational frequencies on going from free CO2 to bound CO2 is therefore in line with the absence of a strong charge transfer, either from CO2 to Mn or from Mn to CO2: charge transfer from CO2 to Mn would involve the 3σu molecular orbital of CO2 as a donor orbital, which would be therefore less populated in the complex. Since this orbital is C-O bonding (see the schematic representation given in Figure 9b), this would result in a weakening of the two carbon-oxygen bonds and hence in a decrease of the CO2 stretching frequencies. Charge transfer from Mn to CO2 would involve the π* molecular orbital of CO2 as an accepting orbital. Since this orbital is C-O antibonding, this would again lead to a weakening of the carbon-oxygen bonds and to a decrease of the CO2 stretching frequencies upon coordination. For Re(Cp)(CO)2(CO2) the vibrational analysis points to a quite different picture. In this case, we found a mixing of the two ν(C-O) modes with the asymmetric stretching mode of the η2-bound CO2. As a result of this mixing, one ν(C-O) vibrational frequency increases while the other decreases relative to Re(Cp)(CO)3. Both the asymmetric (1998 cm-1) and symmetric (1183 cm-1) stretching frequencies of the bound CO2 decrease relative to free CO2, as expected from the metal to CO2 charge transfer, but there is also some mixing of one CO

mode of appropriate symmetry in the asymmetric CO2 stretching mode. On the other hand, no mixing was found for the η1-O structure, with ν(CO) vibrational frequencies amounting to 2022 and 1967 cm-1, just below those of the coordinatively unsaturated Re(Cp)(CO)2 system (2034 and 1979 cm-1), as in the manganese case. Thus, the calculations provide a rationale for the experimental observation. It is also interesting to note that in the TRIR spectrum of irradiated Re(Cp)(CO)3 in scCO2 (see Figure 4b), a transient band was observed at ca. 1860 cm-1. On the basis of the DFT calculations, this is therefore likely to be the asymmetric CO2 stretching band of the bound CO2. Considering now the energetic features of these CO2 complexes, we note that the computed dissociation enthalpies of CO2 from Re and Mn amount to 7.5 and 5.0 kcal mol-1, respectively; see Table 3. Hence CO2 is slightly more strongly bound to Re than to Mn. The rhenium value compares relatively well with an experimental gas-phase estimate of 8.2 ((1) kcal mol-1 for W(CO)5(η1-CO2),29 the estimated CCSD(T) value of Pidun and Frenking for the W complex being 10.2 kcal mol-1.10 Unfortunately, such an experimental estimate, which would allow a similar comparison for the Mn complex, is lacking. The (29) Zheng, Y.; Wang, W.; Lin, J.; She, Y.; Fu, K.-J. Chem. Phys. Lett. 1993, 202, 148.


accuracy of our calculations for the Mn complex may be nevertheless assessed by looking at the CO dissociation from MnCp(CO)3. For this process the experimental estimates are scattered, ranging between 43 kcal mol-1 (the most recent value30) and 55 kcal mol-1.31,32 Our computed value of 43.8 kcal mol-1 is at the lower limit of this range. Thus the results of both Re and Mn comparisons seem to indicate that our DFTB3LYP calculations may yield bond energy values that are too low by a few kcal mol-1. We have also addressed, from a computational point of view, the reaction of M(Cp)(CO)2(CO2) (M ) Mn, Re) with X ) CO, H2, and N2 to yield M(Cp)(CO)3, M(Cp)(CO)2(H2), and M(Cp)(CO)2(N2), respectively.33 Note that an associatiVe mechanism was assumed, with the corresponding transition states featuring the concomitant dissociation of CO2 and association of X. The energetics for these exchange reactions are given in Table 3.34 The optimized structures of the products are shown in the Supporting Information. Not unexpectedly, the exchange reactions are all exothermic. This is traced to the relatively weak bonding of CO2 to either Mn or Re. The exothermicity is greater for Re, compared to Mn, due to the much stronger bonding of incoming X ligands (CO, H2 and N2) compared to CO2 (see the first column of Table 3). The enthalpy barriers are higher by 1.2 to 2.4 kcal mol-1 for Re, i.e., a value that is close to the difference found for the computed M-CO2 bond enthalpies. This is in line with the reactant-like nature of the transition states, which in turn is a consequence of the reaction exothermicity (Hammond postulate).36 Yet the computed DFT values for the enthalpy barriers are somewhat smaller than the experimental activation enthalpies (compare Tables 2 and 3), especially for the rhenium system. This may be again an indication that the mechanism in the experimental conditions probably has a strong dissociatiVe character. Since in the calculations an associative mechanism was assumed, the dissociation of CO2 from the M(Cp)(CO)2 fragment in the transition states is somewhat offset by the bonding of either CO, H2, or N2. However, the discrepancy is not relieved through the consideration of a pure dissociative mechanism. The Mn-CO2 and Re-CO2 computed bond enthalpies, which would be then taken as a measure of the activation energy for the overall process, lead to values that are too low, especially for the activation energies in the rhenium series (although the calculations are in line with the experimental conclusion that the exchange reaction is more difficult for the rhenium systems than (30) Li, Y.; Szta´rray, Baer, T. J. Am. Chem. Soc. 2001, 123, 9388. (31) Burkey, T. J. J. Am. Chem. Soc. 1990, 112, 8329. (32) Klassen, J. K.; Selke, M.; Sorensen, A. A.; Yang, G. K. J. Am. Chem. Soc. 1990, 112, 1267. (33) For the M(Cp)(CO)2(N2) system, the η2-N2 structure was also found to be a true minimum on the potential energy surface, but higher in energy than the η1-N2 structure (by 18.6 and 22.7 kcal mol-1 for Mn and Re, respectively). (34) The values quoted in Table 3 can be used in thermodynamic cycles to compute the enthalpies of reaction for the CO/X exchange reactions of Mn(Cp)(CO)3 with X ) H2 and N2, for which experimental estimates are available for the isoelectronic Cr(η6-C6H6)(CO)3 complex.35 One finds values of 25.1 and 22.4 kcal mol-1, respectively, which compare well with the experimental estimates of 21.7 and 20.3 kcal mol-1, the difference being particularly well reproduced. (35) Walsh, E. F.; George, M. W.; Goff, S.; Nikiforov, S. M.; Popov, V. K.; Sun, X.-Z.; Poliakoff, M. J. Phys. Chem. 1996, 100, 19425. (36) Within each metal series, the order of the barrier roughly follows the order of the bond enthalpies and of the exothermicity of the exchange reaction, the only exception being the rhenium reactions with dihydrogen and dinitrogen. We ascribe this feature to the dihydride nature of Re(Cp)(CO)2(H)2: for this system the enthalpy values computed for the reactions Re(Cp)(CO)2 + X f Re(Cp)(CO)2X and Re(Cp)(CO)2(CO2) + X f Re(Cp)(CO)2X + CO2 result from the balance between the two strong Re-H bonds that are formed and the strong H-H bond that is cleaved.


for the manganese systems). We have already discussed the accuracy of the computed values for the M-CO2 bond enthalpies. The discrepancy between the calculations and the experiment reported here may also involve some specific solvation effects of scCO2 that were not considered in the calculations. Work is planned to examine this further.

Conclusions In this paper we have shown that UV photolysis of M(Cp)(CO)3 (M ) Mn or Re) in liquid or supercritical CO2 solution leads to the formation of the transient CO2 complexes M(Cp)(CO)2(CO2), which were detected by nanosecond TRIR spectroscopy. The ν(C-O) IR spectra of these CO2 complexes are strikingly different, with the ν(C-O) bands of Mn(Cp)(CO)2(CO2) being shifted down in frequency relative to Mn(Cp)(CO)3, while those of Re(Cp)(CO)2(CO2) lie between the bands of Re(Cp)(CO)3. This suggests that the CO2 ligand has a different mode of coordination in the Mn and Re complexes. Rate constants and activation enthalpies have also been determined for the reactions of the CO2 complexes with CO, H2, and N2 in scCO2. Comparing these with the equivalent data for the analogous Xe and heptane complexes provides evidence that the mode of CO2 binding in Mn(Cp)(CO)2(CO2) is η1-O endon bound, while that in Re(Cp)(CO)2(CO2) is η2-C,O side-on bound. The different CO2 coordination modes lead to dramatic differences in the reactivity of the Mn and Re CO2 complexes toward CO, H2, and N2 in scCO2, with the Re complex being less reactive by a factor of 1750 for reaction with CO, 4000 for reaction with N2, and ca. 7670 for reaction with H2. This contrasts strongly with the differences in reactivity of the Xe and heptane complexes of Mn and Re, M(Cp)(CO)2(L) (M ) Mn, Re; L ) Xe, heptane). In that case, the Xe and heptane complexes of Re are only 2 orders of magnitude less reactive toward CO, H2, and N2 than the corresponding Mn complexes. These results show that reactions involving gaseous reagents in supercritical solution may not always proceed as fast as expected when a high pressure of gas is added to the solution. A strong interaction of the supercritical solvent with the reactive intermediates, particularly when transition metal species are involved, can have a significant impact on reaction rates, and this should be considered when carrying our reactions in supercritical fluids. DFT calculations at the B3LYP/SDD-6-311G** level of theory were also performed to provide more evidence for the nature of the CO2 coordination modes. These calculations supported the binding modes proposed on the basis of the experimental studies. For Mn(Cp)(CO)2(CO2), a two-electron stabilizing interaction between the dσ orbital and σ lone pair of CO2 leads to the η1-O coordination mode, with a major component of the bonding being an electrostatic attraction, with little charge transfer between CO2 and Mn. For Re(Cp)(CO)2(CO2), the η2-C,O structure was more stable than the η1-O structure by 1.7 kcal mol-1. A significant charge transfer from an occupied Re dπ orbital to the π orbital of CO2 occurs, resulting in partial oxidation of Re. Frequency calculations match well with the experimental TRIR spectra and also reveal that the IR band observed at ca. 1860 cm-1 in the TRIR spectrum of Re(Cp)(CO)3 in scCO2 is due to the asymmetric CO2 stretch of bound CO2. Reaction enthalpies and activation barriers were also calculated for the reactions of the CO2 complexes with CO, H2, and N2. The computed energy barriers were somewhat smaller than the experimentally measured activation enthalpies. This discrepancy may be due to some specific solvation effects occurring in scCO2.


In this study, fast TRIR spectroscopy and DFT calculations were used together to provide strong evidence for different CO2 binding modes in the transient CO2 complexes Mn(Cp)(CO)2(CO2) and Re(Cp)(CO)2(CO2). The powerful combination of experiment and theory was invaluable for identifying the nature of the CO2 complexes. It is clear that this combination will become increasingly important for unraveling the mechanisms of photochemical reactions of transition metal complexes.

Acknowledgment. We are grateful to the University of Nottingham and the EPSRC for funding. We thank (37) Butler, I. S.; Coville, N. J.; Cozak, D. J. Organomet. Chem. 1977, 133, 59. (38) Bitterwold, T. E.; Lott, K. A.; Rest, A. J.; Mascetti, J. J. Organomet. Chem. 1991, 419, 113.

Yang et al.

Professor M. Poliakoff for many helpful discussions. The calculations have been carried out on the workstations of our laboratory and of the Centre Universitaire Re´gional de Ressources Informatiques (CURRI) of Strasbourg. We thank Dr. L. Padel and Mrs. S. Fersing for their technical assistance. B.N.G. gratefully acknowledges the EGIDE organization for the financing of his stays in Strasbourg. M.W.G. gratefully acknowledges receipt of a Royal Society Wolfson Merit Award. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.

OM8010195

Experimental and Theoretical Investigation into the Formation and

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