DFT Studies on Reactions of Transition Metal Complexes with O2

Organometallics 2009, 28, 4443–4451 DOI: 10.1021/om9002957

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DFT Studies on Reactions of Transition Metal Complexes with O2 Haizhu Yu,†,‡ Yao Fu,*,† Qingxiang Guo,† and Zhenyang Lin*,‡ †

Department of Chemistry, University of Science and Technology of China, Hefei, People’s Republic of China, and ‡Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China Received April 21, 2009

DFT calculations have been carried out to study the mechanisms for reactions of O2 with a series of metal complexes, including d6 CpRuL2, d6 ML5, and d8 ML4 complexes. The calculation results indicate that the reaction is initiated by an end-on coordination of O2 to the metal center, which gives an (η1-O2)[M] intermediate. The uncoordinated oxygen atom of the η1-O2 ligand then approaches the metal center to give a new η1-O2 intermediate in which the η1-O2 ligand is oriented approximately the same as the one defined in the product. An intersystem conversion from the triplet to singlet energy surface (MECP) then occurs to enable the metal peroxide product to be formed. Introduction Transition metal-catalyzed oxidation of organic molecules, such as alcohols, carbonyl compounds, and epoxides, by the environmental friendly oxidant O2 has recently attracted much interest.1 In most of these oxidation processes, direct reaction of O2 with metal complexes to give M(η1-O2), M(η2-O2), and Mx(μ-O)y intermediates was often proposed to be an important step.2,3 In the past few decades, many transition metal complexes were also reported to easily react with O2 to form metal peroxide complexes.3

Despite the extensive experimental studies on reactions of O2 with transition metal complexes, the detailed aspects regarding the intermediates, transition states, and state crossing points together with their energetics were not theoretically well studied due to the lack of practical methods in calculating state crossing points. Recently, Harvey and co-workers developed a computer code4 that allows state crossing points to be calculated, and therefore reactions of O2 can be studied in detail. With the newly developed computer code, theoretical studies on reactions of O2 with palladium(0) and copper(I) complexes to give the O2-coordinated products were reported.1e,5 Insertion of O2 into metal-hydride bonds was also studied.6 Nevertheless, mechanisms for reactions of O2 with other transition metal complexes have not yet been well investigated. In this work, we set out to systematically study the mechanisms of the reactions of O2 with complexes of various transition metals including Ru, Os, and Ir with the aid of DFT calculations. We hope that the insight provided will be helpful for understanding reactions involving O2 and lead to more attractive transition metal catalytic processes in oxidation reactions using the environmental friendly oxidant O2.

*Corresponding authors. E-mail: [email protected]; [email protected]. (1) For example: (a) Punniyamurthy, T.; Velusamy, S.; Iqbal, J. Chem. Rev. 2005, 105, 2329. (b) De La Lande, A.; G
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erard, H.; Moliner, V.; Reinaud, O. Chem. Eur. J. 2008, 14, 6465. (f) Stahl, S. S. Angew. Chem., Int. Ed. 2004, 43, 3400. (g) El-Sayed, M. A.; Salam, A. H. A.; Abo-El-Dahab, H. A.; Refaat, H. M.; El-Dissouky, A. J. Coord. Chem. 2009, 62, 1015. (h) Collman, J. P. Acc. Chem. Res. 1968, 1, 136. (i) Sawyer, D. T.; Sobkowiak, A.; Matsushita, T. Acc. Chem. Res. 1996, 29, 409. (j) Gallo, E.; Solari, E.; Re, N.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Angew. Chem., Int. Ed. Engl. 1996, 35, 1981. (2) For example: (a) Jones, R. D.; Summerville, D. A.; Basolo, F. Chem. Rev. 1979, 79, 139. (b) Korendovych, I. V.; Kryatov, S. V.; RybakAkimova, E. V. Acc. Chem. Res. 2007, 40, 510. (c) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L. Chem. Rev. 2004, 104, 939. (d) Pecoraro, V. L.; Baldwin, M. J.; Gelasco, A. Chem. Rev. 1994, 94, 807. (e) Busch, D. H.; Alcock, N. W. Chem. Rev. 1994, 94, 585. (f) Lanci, M. P.; Roth, J. P. J. Am. Chem. Soc. 2006, 128, 16006. (g) El-Sayed, M. A.; Kassem, T. S.; AboEldahab, H. A.; El-Kholy, A. E. Inorg. Chim. Acta 2005, 358, 22. (h) Bol, J. E.; Driessen, W. L.; Ho, R. Y. N.; Maase, B.; Que, L.; Reedijk, J. Angew. Chem., Int. Ed. Engl. 1997, 36, 998. (i) Schr€oder, D.; Fiedler, A.; Herrmann, W. A.; Schwarz, H. Angew. Chem., Int. Ed. Engl. 1995, 34, 2517. (j) Chuf
an, E. E.; Puiu, S. C.; Karlin, K. D. Acc. Chem. Res. 2007, 40, 563. (k) Lewis, E. A.; Tolman, W. B. Chem. Rev. 2004, 104, 1047. (l) Suzuki, M. Acc. Chem. Res. 2007, 40, 609. (3) For example: (a) Shen, J. Y.; Stevens, E. D.; Nolan, S. P. Organometallics 1998, 17, 3875. (b) Ibers, J. A.; Laplaca, S. J. Science 1964, 145, 920. (c) Williams, D. B.; Kaminsky, W.; Mayer, J. M.; Goldberg, K. I. Chem. Commun. 2008, 4195. (d) Esteruelas, M. A.; Sola, E.; Oro, L. A.; Meyer, U.; Werner, H. Angew. Chem., Int. Ed. Engl. 1988, 27, 1563. (e) Kirchner, K.; Mauthner, K.; Mereiter, K.; Schmid, R. J. Chem. Soc., Chem. Commun. 1993, 892. (f) Hikichi, S.; Akita, M.; Moro-oka, Y. Coord. Chem. Rev. 2000, 198, 61. (g) Lindner, E.; Haustein, M.; Fawzi, R.; Steimann, M.; Wegner, P. Organometallics 1994, 13, 5021. (h) Shen, J. Y.; Stevens, E. D.; Nolan, S. P. Organometallics 1998, 17, 3875.

(4) (a) Harvey, J. N.; Aschi, M.; Schwarz, H.; Koch, W. Theor. Chem. Acc. 1998, 99, 95. (b) Harvey, J. N.; Aschi, M. Phys. Chem. Chem. Phys. 1999, 1, 5555. (5) (a) Popp, B. V.; Wendlandt, J. E.; Landis, C. R.; Stahl, S. S. Angew. Chem., Int. Ed. 2007, 46, 601. (b) Landis, C. R.; Morales, C. M.; Stahl, S. S. J. Am. Chem. Soc. 2004, 126, 16302. (c) Mukherjee, A.; Smirnov, V. V.; Lanci, M. P.; Brown, D. E.; Shepard, E. M.; Dooley, D. M.; Roth, J. P. J. Am. Chem. Soc. 2008, 130, 9459. (d) Lanci, M. P.; Smirnov, V. V.; Cramer, C. J.; Gauchenova, E. V.; Sundermeyer, J.; Roth, J. P. J. Am. Chem. Soc. 2007, 129, 14697. (e) Aboelella, N. W.; Kryatov, S. V.; Gherman, B. F.; Brennessel, W. W.; Young, V. G.; Sarangi, R.; Rybak-Akimova, E. V.; Hodgson, K. O.; Hedman, B.; Solomon, E. I.; Cramer, C. J.; Tolman, W. B. J. Am. Chem. Soc. 2004, 126, 16896. (6) (a) Chowdhury, S.; Rivalta, I.; Russo, N.; Sicilia, E. J. Chem. Theor. Comp. 2008, 4, 1283. (b) (c) Chowdhury, S.; Rivalta, I.; Russo, N.; Sicilia, E. Chem. Phys. Lett. 2008, 456, 41. (d) Gligorich, K. M.; Sigman, M. S. Angew. Chem., Int. Ed. 2006, 45, 6612. (e) Nielsen, R. J.; Goddard, W. A. J. Am. Chem. Soc. 2006, 128, 9651. (f) (g) Keith, J. M.; Goddard, W. A.; Oxgaard, J. J. Am. Chem. Soc. 2007, 129, 10361. (h) Chowdhury, S.; Rivalta, I.; Russo, N.; Sicilia, E. Chem. Phys. Lett. 2007, 443, 183. (i) Popp, B. V.; Stahl, S. S. J. Am. Chem. Soc. 2007, 129, 4410. (j) Chowdhury, S.; Rivalta, I.; Russo, N.; Sicilia, E. Chem. Phys. Lett. 2007, 443, 183. (k) Keith, J. M.; Muller, R. P.; Kemp, R. A.; Goldberg, K. I.; Goddard, W. A.; Oxgaard, J. Inorg. Chem. 2006, 45, 9631.

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Computational Details

(∼22.4 kcal/mol).13 All the DFT calculations were performed with the Gaussian 03 package.14

Molecular geometries of complexes were optimized without constraints via DFT calculations using the UBecke3LYP (UB3LYP) functional.7 Frequency calculations at the same level of theory were also performed to identify the stationary points as minima (zero imaginary frequencies) or transition states (one imaginary frequency). Transition states were located using the Berny algorithm. Intrinsic reaction coordinates (IRC)8 were calculated for the transition states to confirm that such structures indeed connect two relevant minima. The 6-31G* Pople basis set9 was used for C, O, N, and H atoms, while the effective core potentials (ECPs) of Hay and Wadt with a double-ζ valence basis set (Lanl2DZ)10 were used to describe Ru, Ir, Os, P, and Cl atoms. The polarization functions were added for Ru (ζ(f)=1.235), Ir (ζ(f)=0.938), Os (ζ(f)=0.886), P (ζ(d)=0.340), and Cl (ζ(d)=0.514).11 Reactions of transition metal complexes with O2 often involve state crossing between triplet and singlet states. Reactants normally have a triplet ground state, while products are usually in a singlet ground state. We employed a code developed by Harvey and co-workers to optimize the geometry of minimum energy crossing points (MECPs) between potential energy surfaces of different spin states.4 We have carefully examined the ÆS2æ values and the spin densities for all the species calculated to make sure that the electronic configurations have been correctly characterized from the converged SCF wave functions. For species with a triplet state, the ÆS2æ values are all close to 2; that is, no obvious spin contamination was observed. The local minimum structures and transition state structures that connect the reactants and the state crossing points have triplet ground states and were easily optimized. For these triplet-optimized structures, their corresponding open-shell singlet state energies were evaluated by performing single-point energy calculations. The wave functions obtained for these structures of open-shell singlet state are found to be contaminated with triplet state wave functions, and the ÆS2æ values calculated are close to 1. In order to exclude the triplet state contribution, spin-projection corrections were applied to re-evaluate the spin-corrected singlet state total energy Esinglet according to the following equation:12

It is well known that DFT-based methods are still not able to give a univocal description of open-shell systems.15 Although hybrid functionals often closely approximate the experimental behavior, they tend to increase the relative stability of high-spin states as the proportion of the Hartree-Fock exchange is increased.15 To test the functional dependence, we employed the pure DFT methods, UBP86 and UBLYP, to calculate all of the species involved in the reaction of [CpRu(PMe3)2]+ with O2 (Figure 1a). On the basis of these calculations (see Supporting Information for more details), we obtained the following interesting findings. If we take [CpRu(PMe3)2]+ + O2(triplet) as the energy reference point, both of the pure DFT methods increase the relative stability of all other species involved in the reactions, with the triplet species by 6-7 kcal/mol and the open-shell singlet species (except the O2 singlet) by 10-18 kcal/mol. It appears that the pure DFT methods overcorrect the deficiencies of the UB3LYP hybrid method. For example, the coordination energy of the singlet O2 to the [CpRu(PMe3)2]+ metal fragment was calculated to be -14.9 kcal/ mol with UB3LYP. However, the coordination energy was calculated to be ca. -24.0 kcal/mol with the pure DFT methods. These additional calculations suggest that the results obtained from the hybrid UB3LYP method appear more reasonable.

E singlet ¼

2E OSS -E triplet ÆS 2 æOSS 2 -ÆS 2 æOSS

ð1Þ

where EOSS and Etriplet are the total electronic energies of the open-shell singlet and triplet states, respectively, obtained from the UB3LYP calculations. ÆS2æOSS corresponds to the ÆS2æ value obtained from the UB3LYP calculations of the open-shell singlet state. With the spin-projection corrections, the singlet-triplet splitting calculated for O2 is 20.7 kcal/mol, close to the one experimentally measured (7) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989, 157, 200. (c) Lee, C.; Yang, W.; Parr, G. Phys. Rev. B 1988, 37, 785. (d) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F. J. Phys. Chem. 1994, 98, 11623. (8) (a) Fukui, K. J. Phys. Chem. 1970, 74, 4161. (b) Fukui, K. Acc. Chem. Res. 1981, 14, 363. (9) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. (10) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (11) Ehlers, A. W.; Bohme, M.; Dapprich, S.; Gobbi, A.; Hollwarth, A.; Jonas, V.; Kohler, K. F.; Stegmman, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993, 208, 111. (12) (a) Yamaguchi, K.; Jensen, F.; Dorigo, A.; Houk, K. N. Chem. Phys. Lett. 1988, 149, 537. (b) Yamanaka, S.; Kawakami, T.; Nagao, H.; Yamaguchi, K. Chem. Phys. Lett. 1994, 231, 25. (c) Lim, M. H.; Worthington, S. E.; Dulles, F. J.; Cramer, C. J. In Chemical Applications of Density Functional Theory; Laird, B. B., Ross, R. B., Ziegler, T., Eds.; American Chemical Society: Washington, DC, 1996; Vol. 629, p 402. (d) Isobe, H.; Takano, Y.; Kitagawa, Y.; Kawakami, T.; Yamanaka, S.; Yamaguchi, K.; Houk, K. N. Mol. Phys. 2002, 100, 717.

Results and Discussion Before we discuss the detailed energy profiles calculated for reactions of O2 with transition metal complexes, we here give a brief summary of the relevant experimental work reported in the past several decades. We are mainly interested in those reactions that give metal peroxide complexes (η2-O2)[M] as products since these reactions always proceed smoothly under mild conditions and the (η2-O2)[M] complexes are well characterized. In 1970s, Vaska and Ibers et al. found that IrX(CO)(PR3)2 (X=Cl, I) and [M(PR3)4]X (M=Ir, Rh; X=Cl) react with O2 and give the metal peroxides (η2-O2)IrX(CO)(PR3)2 and (η2-O2)[M(PR3)4]+ as products.3a,3b,16,17 Analogous reactions leading to the formation of the metal peroxides (η2O2)RuCl(NO)(PR3)2,18a (η2-O2)Ru(CO)(CNp-tolyl)(PR3)2,18b (η2-O2)M(CO)2(PR3)2 (M=Ru, Os),19 and (η2-O2)[Co(PR3)4]+ were also reported.20 Recently, reactions of O2 with (13) Okabe, H. Photochemistry of Small Molecules; John Wiley & Sons: New York, 1978. (14) Frisch, M. J. et al. Gaussian 03, revision B05; Gaussian, Inc.: Pittsburgh, PA, 2003. (15) (a) Reiher, M.; Salomon, O.; Hess, B. A. Theor. Chem. Acc. 2001, 107, 48. (b) Poli, R.; Harvey, J. N. Chem. Soc. Rev. 2003, 32, 1. (c) Harvey, J. N.; Aschi, M. Faraday Discuss. 2003, 124, 129. (d) Cohen, R.; Weitz, E.; Martin, J. M. L.; Ratner, M. A. Organometallics 2004, 23, 2315. (e) Estephane, J.; Groppo, E.; Vitillo, J. G.; Damin, A.; Lamberti, C.; Bordiga, S.; Zecchina, A. Phys. Chem. Chem. Phys. 2009, 11, 2218. (16) (a) McGinnety, J. A.; Doedens, R. J.; Ibers, J. A. Science 1967, 155, 709. (b) Vaska, L.; Chen, L. S.; Senoff, C. V. Science 1971, 174, 587. (c) Weininger, M. S.; Taylor, I. F.; Amma, E. L. J. Chem. Soc. D., Chem. Commun. 1971, 1172. (d) Weininger, M. S.; Griffith, E. A. H.; Sears, C. T.; Amma, E. L. Inorg. Chim. Acta 1982, 60, 67. (e) Lanci, M. P.; Brinkley, D. W.; Stone, K. L.; Smirnov, V. V.; Roth, J. P. Angew. Chem., Int. Ed. 2005, 44, 7273. (f) Vaska, L. Acc. Chem. Res. 1968, 1, 335. (g) Vaska, L. Science 1963, 140, 809. (17) Mcginnety, J. A.; Payne, N. C.; Ibers, J. A. J. Am. Chem. Soc. 1969, 91, 6301. (18) (a) Laing, K. R.; Roper, W. R. J. Chem. Soc., Chem. Commun. 1968, 1568. (b) Christian, D. F.; Roper, W. R. J. Chem. Soc., Chem. Commun. 1971, 1271. (19) (a) Hiraki, K.; Kira, S.; Kawano, H. Bull. Chem. Soc. Jpn. 1997, 70, 1583. (b) Ogasawara, M.; Macgregor, S. A.; Streib, W. E.; Folting, K.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1996, 118, 10189. (c) Cavit, B. E.; Grundy, K. R.; Roper, W. R. J. Chem. Soc., Chem. Commun. 1972, 60. (20) Terry, N. W.; Amma, E. L.; Vaska, L. J. Am. Chem. Soc. 1972, 94, 653.

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Figure 1. Energy profiles calculated for the reactions of (a) [CpRu(PMe3)2]+ and (b) CpRuCl (PMe3) with O2. The relative electronic energies are given in kcal/mol. Bond distances and angles are given in angstroms and degrees, respectively. Data in italics are spin densities.

five-coordinated metal complexes [ML4X]n+ (M = Ru, Rh, Os; X = H, Cl; and L = phospine or olefin) to give (η2-O2)[ML4X]n+ were also found.21 In these reactions, the peroxide complexes are formed by direct addition of O2 to metal fragments. Reactions in which dissociation of one weakly coordinated ligand occurs before addition of O2 to the metal center are also common. For example, reaction of O2 with (21) (a) Vigalok, A.; Shimon, L. J. W.; Milstein, D. Chem. Commun. 1996, 1673. (b) Bartucz, T. Y.; Golombek, A.; Lough, A. J.; Maltby, P. A.; Morris, R. H.; Ramachandran, R.; Schlaf, M. Inorg. Chem. 1998, 37, 1555. (c) Mezzetti, A.; Zangrando, E.; DelZotto, A.; Rigo, P. J. Chem. Soc., Chem. Commun. 1994, 1597.

RuH(H2)(X)(PR3)2 (X = 2-phenylpyridine) to give (η2-O2)RuH(X)(PR3)2 was supposed to undergo an H2 dissociation followed by O2 addition.22 For reactions between [Cp*RuL2]+Cl- and O2,3e,3h,23 it was also suggested that the (22) Matthes, J.; Gr€ undemann, S.; Toner, A.; Guari, Y.; Donnadieu, B.; Spandl, J.; Sabo-Etienne, S.; Clot, E.; Limbach, H. H.; Chaudret, B. Organometallics 2004, 23, 1424. (23) (a) Palacios, M. D.; Puerta, M. C.; Valerga, P.; Lledos, A.; Veilly, E. Inorg. Chem. 2007, 46, 6958. (b) Jia, G. C.; Ng, W. S.; Chu, H. S.; Wong, W. T.; Yu, N. T.; Williams, I. D. Organometallics 1999, 18, 3597. (c) De Ios Ríos, I.; Tenorio, M. J.; Padilla, J.; Puerta, M. C.; Valerga, P. J. Chem. Soc., Dalton Trans. 1996, 377.

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reactants first dissociate Cl- in the MeOH solvent to give [Cp*Ru(PR3)2]+. Addition of O2 to the ruthenium center then occurs to give (η2-O2)[Cp*Ru(PR3)2]+. The metal fragments that directly react with O2 can be broadly classified into three main categories: d6 Cp*ML2, d6 ML5, and d8 ML4 complexes. The detailed energy profiles for the following model reactions shown in eqs 1-7 were theoretically calculated with the aid of DFT calculations. On the basis of the calculation results, the detailed mechanism regarding the intermediates, transition states, and state crossing points together with their energetics and electronic states were obtained, from which a general understanding can be deduced for the reactions of O2 with transition metal complexes.

Reactions of d6 CpRuL2 with O2. We first studied the reactions of CpRuL2 with O2 shown in eqs 1 and 2. Figures 1 and 2 show the detailed energy profiles. Selected structural parameters and spin densities for selected atoms (given in italics) for the optimized structures are also shown in the figures. For [CpRu(PMe3)2]+, the reaction is initiated by an end-on coordination of O2 to the metal center, which is

Yu et al.

exothermic by ca. 10 cal/mol in the reaction energy (Figure 1a). From the η1-O2-coordinated intermediate 1a-in1-T, the dangling oxygen atom O(2) of the η1-O2 ligand approaches the metal center to give another intermediate having a significantly shortened Ru---O(2) distance, with a very small barrier via the transition state 1a-ts-T. The triplet state structures are lower in electronic energy than the corresponding open-shell singlet state structures for all species connecting the reactants and the intermediate 1a-in2-T. At the structure of 1a-in2-T, the energy difference between the triplet state and its open-shell singlet state becomes smaller. The state crossing point (1a-MECP) then connects the second intermediate 1a-in2-T and the product [(η2-O2)CpRu(PMe3)2]+, which has a singlet ground state. The results of calculations show that the MECP lies 4.2 kcal/ mol higher in electronic energy than the intermediate 1a-in2-T. The overall reaction is exothermic (ΔE = -14.9 kcal/mol), and the state crossing point is actually the key transition state for the reaction. It is interesting to note that there exist two η1-O2-coordinated intermediates, 1a-in1-T and 1a-in2-T, on the potential energy surface of the triplet state in Figure 1a prior to the formation of the metal peroxide product [(η2-O2)CpRu(PMe3)2]+. This feature is commonly found in all the reactions studied in this work (vide infra) and in the reactions of Pd(NHC)2 (NHC = N-heterocyclic carbene) with O2 studied by Stahl et al.5a,5b The structural parameters calculated for species from the first η1-O2-coordinated intermediate 1a-in1-T to the state crossing point 1a-MECP along the potential energy surface in Figure 1a clearly show a continuous change in the Ru--O(2) distance and the Ru-O(1)-O(2) angle. The Ru---O(2) distance shortens while the Ru-O(1)-O(2) angle decreases. In 1a-in1-T, the Ru-O(1)-O(2) bond angle is 134.1, suggesting that the hybridized orbital used from the O2 ligand to form the coordination bond with the metal center in 1a-in1-T possesses sp/sp2 character. In the second η1-O2-coordinated intermediate 1a-in2-T, an approximately pure p orbital from the O2 ligand is expected to be responsible for the coordination bond, as the Ru-O(1)-O(2) angle is ca. 90. The Ru-O(1) bond in 1a-in1-T (2.119 A˚) is shorter than that in 1a-in2-T (2.141 A˚), supporting the orbital argument given here. The relative orientation of the O2 ligand with respect to other ligands in the second η1-O2-coordinated intermediate (1a-in2-T) is close to that in the state crossing point (1a-MECP). The structural changes along the potential energy surface serve for formation of the state crossing point from the triplet to singlet energy surface, enabling the metal peroxide product to be formed. In order to better understand the two triplet intermediates, we performed natural bond orbital analyses24 to obtain the Wiberg bond indices (a measure of bond strength)25 for selected triplet species in Figure 1a (Table 1). The bond indices indicate that the two intermediates have similar bonding between Ru and O2. Both intermediates have a stronger Ru-O(1) bond and a very weak Ru-O(2) bond. Compared with the peroxide product 1a-prod-S, both intermediates retain the O-O double-bond character. The O-O bond in 1a-in2-T is slightly more activated than that in 1ain1-T. From these results together with the structural differences discussed above, we believe that the electronic reason (24) Reed, A.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899. (25) Wiberg, K. B. Tetrahedron 1968, 24, 1083.

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Figure 2. Energy profiles calculated for the reactions of [CpRu(PMe2(CH2)2PMe2)2]+ with O2. The relative electronic energies are given in kcal/mol. Bond distances and angles are given in angstroms and degrees, respectively. Data in italics are spin densities. Table 1. Wiberg Bond Indices Calculated for Selected Triplet Species in Figure 1a

Ru-O(1) Ru-O(2) O(1)-O(2)

1a-in1-T

1a-ts-T

1a-in2-T

1a-prod-S

0.3153 0.1311 1.3762

0.3110 0.1248 1.3720

0.3512 0.1757 1.3062

0.5546 0.5549 1.1906

behind the presence of a barrier between the two intermediates is related to the change in the Ru-O(1) bonding character. In bonding with the metal center, O(1) utilizes a hybridized orbital formally in between sp and sp2 in the first intermediate but utilizes an approximately pure p orbital in the second intermediate. We also studied the reactions of CpRuCl(PMe3) and [CpRu(PMe2(CH2)2PMe2)]+ with O2 to see how a halide ligand or a bidentate ligand affects the energy profiles. The calculation results shown in Figures 1b and 2 are similar to that shown in Figure 1a. The first step is the exothermic coordination of O2 to the metal center to give an η1-O2 intermediate, from which the dangling oxygen atom of the η1-O2 ligand approaches the metal center and gives another intermediate having a significantly shortened Ru---O(2) distance. Transformation between the two η1-O2-coordinated intermediates in each case is easily achieved via a low-energy barrier. From the second η1-O2-coordinated intermediate, the state crossing between the triplet and singlet state energy surfaces then occurs, enabling the singlet product to be formed. All the reactions of d6 CpRuL2 fragments with O2 (Figures 1 and 2) are exothermic, and the state crossing points act as the key transition states for the reactions. Comparing the potential energy curves of Figures 1a and 1b, we note that CpRuCl(PMe3) has weaker O2 binding ability than [CpRu(PMe3)2]+ does based on the interaction energies. Reaction of CpRuCl(PMe3) with O2 is also less exothermic. The results can be related to the π-donor ability of the halide ligand. The 16e metal fragment CpRuCl(PMe3)

shows strong Ru-Cl π bonding, weakening its O2 binding ability. The strong π interaction is evidenced by the relatively shorter Ru-Cl bond distance in the reactant (Figure 1b). Subsequent binding of O2 weakens the π interactions and the Ru-Cl bond distance is lengthened (Figure 1b). The reaction of [CpRu(PMe2(CH2)2PMe2)]+ with O2 is slightly more exothermic than that of [CpRu(PMe3)2]+. This is because the smaller bite angle of the (PMe2(CH2)2PMe2) ligand reduces the steric effect and favors the formation of the metal peroxide product. From Figures 1 and 2, we see that spin densities are mainly located on the two oxygen atoms and the metal center for each of the species connecting the first η1-O2-coordinated intermediate and the state crossing point. This feature is commonly seen for the other reactions that will be discussed below. Reactions of d6 ML5 with O2. Figure 3 shows the energy profiles calculated for reactions of the d6 ML5 metal fragments with O2 shown in reactions 3 and 4. Clearly, the energy profiles are similar to those shown in Figures 1 and 2. The results can be easily understood because d6 ML5 and CpML2 are isolobal. Comparing the potential energy curves shown in Figures 3a and 3b, we found that formation of the osmium peroxide product 3b-prod-S is obviously more exothermic than formation of its ruthenium analogue. The osmium complex [OsH(PMe2(CH2)2PMe2)2]+ also has stronger O2 binding ability than [RuH(PMe2(CH2)2PMe2)2]+ does, consistent with the fact that bonds to third-row transition metals are generally significantly stronger than those to their lighter congeners. The phenylpyridine-ligated metal fragment 3c-react also shows stronger O2 binding ability than [RuH(PMe2(CH2)2PMe2)2]+ (3a-react) (Figures 3c and 3a). The reason can be attributed to the fact that the π-acceptor phosphine ligand on [RuH(PMe2(CH2)2PMe2)2]+ (3a-react) relatively weakens the reducing ability of the ruthenium center.

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Figure 3. Energy profiles calculated for the reactions of (a) [RuH(PMe2(CH2)2PMe2)2]+, (b) [OsH(PMe2(CH2)2PMe2)2]+, and (c) Ru(H)(PMe3)2(ppy) with O2. The relative electronic energies are given in kcal/mol. Bond distances and angles are given in angstroms and degrees, respectively. Data in italics are spin densities.

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Figure 4. Energy profiles calculated for the reactions of (a) [Co(PMe2(CH)2PMe2)2]+ and (b) IrCl(CO)(PMe3)2 with O2. The relative electronic energies are given in kcal/mol. Bond distances and angles are given in angstroms and degrees, respectively. Data in italics are spin densities.

Reactions of d8 ML4 with O2. In this section, we examine the reactions of d8 ML4 metal fragments with O2 shown in reactions 5-7. The detailed energy profiles are shown in Figures 4 and 5. Similar to what we have seen in the reactions of d6 metal fragments with O2 (Figures 1-3), reaction of [Co(Me2P(CH)2PMe2)2]+ with O2 (Figure 4a) is also initiated by an end-on coordination of O2 to the metal center. The η1-O2-coordinated intermediate 4a-in1-T adopts a distorted square-pyramidal geometry, with the two diphosphine ligands lying on the basal plane. Since [Co(Me2P(CH)2PMe2)2]+ is a square-planar complex, 4a-

in1-T is a weakly O2-coordinated intermediate. From 4a-in1T, ligand rearrangement occurs easily via the transition state 4a-ts-T to give the second intermediate 4a-in2-T, in which the two bidentate ligands and the metal center form a distorted seesaw geometry. The state crossing between the triplet and singlet state energy surfaces then occurs, which enables the octahedral-like peroxide product 4a-prod to be formed. The energy profiles for the reaction of IrCl(CO)(PMe3)2 with O2 (Figure 4b) shows that the reaction is initiated by formation of the van der Waals complex 4b-in1-T, in which the Ir-O(1) bond is as long as 3.626 A˚. From 4b-in1-T, ligand rearrangement then occurs via the transition state

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Figure 5. Energy profiles calculated for the reactions of Ru(CO)2(PMe3)2 with O2. The relative electronic energies are given in kcal/ mol. Bond distances and angles are given in angstroms and degrees, respectively. Data in italics are spin densities.

4b-ts-T, resulting in an end-on coordination of O2 to the metal center. From the η1-O2 (end-on)-coordinated intermediate 4b-in2-T, the dangling oxygen atom O(1) then approaches the metal center to give the peroxide product 4b-prod via the state crossing point 4b-MECP. Comparing the potential energy curves shown in Figures 4b and 4a, we found that IrCl(CO)(PMe3)2 has an obviously weaker O2 binding ability than [Co(PMe2(CH)2PMe2)2]+ does. Formation of the metal peroxide (η2-O2)IrCl(CO)(PMe3)2 is also much less exothermic. The Cl and CO ligands are in an optimal structural arrangement in the reactant state in which a push-pull relationship exists, as Cl- is a π donor, while CO is a π acceptor. Therefore, the structural rearrangement in IrCl(CO)(PMe3)2 apparently costs much more energy than that in [Co(PMe2(CH)2PMe2)2]+. The rigidity in the structure of the iridium complex also explains the existence of the van der Waals complex 4b-in1-T on the potential surface for the reaction of IrCl(CO)(PMe3)2 with O2 but not on that for the reaction of [Co(Me2P(CH)2PMe2)2]+ with O2. The high energy barrier for ligand rearrangement involved in Figure 4b (4b-in1-T f 4b-in2-T) relative to that in Figure 4a (4a-in1-T f 4a-in2-T) provides further support for the rigidity argument given above. The relative exothermicity of the reactions shown in eqs 5 and 6 are also in good agreement with the experimental observations that the O2 binding to the IrCl(CO)P2 (P = PPh3, PPh2Et) is reversible, 3b,16 while the oxygen uptaken by [Co(PPh2(CH)2PPh2)2]+ is irreversible.20 We also calculated the reaction of the d8 complex Ru(CO)2(PMe3)2 with O2 (reaction 7). It is interesting to note that the four-coordinated d8 complexes [Co(PMe2(CH)2PMe2)2]+ and IrCl(CO)(PMe3)2 adopt the well-known square-planar structures, while Ru(CO)2(PMe3)2 was found to adopt a seesaw structure.19b The LUMO of Ru(CO)2(PMe3)2 has the maximum amplitude pointing away from the two legs of the seesaw structure. Therefore, the first intermediate formed through the coordination of O2 via an end-on mode adopts an

approximately trigonal-bipyramidal structure (5-in1-T in Figure 5). The intermediate is more stable by ca. 10 kcal/ mol than the reactants. From the η1-O2-coordinated intermediate 5-in1-T, the dangling oxygen atom of the η1-O2 ligand approaches the metal center and gives the second η1-O2 intermediate, from which the final peroxide product is formed via a state crossing point. The reaction path is similar to those shown in Figures 1-3.

Conclusions The mechanisms for formation of metal peroxide complexes from the reactions of transition metal complexes with O2 have been investigated with the aid of DFT calculations. A general mechanism can be drawn from this study. The reaction is initiated by an end-on coordination of O2 to the metal center, which gives an (η1-O2)[M] intermediate. The uncoordinated oxygen atom of the η1-O2 ligand then approaches the metal center to give a new η1-O2 intermediate in which the η1-O2 ligand is oriented approximately as the one defined in the product. At this new η1-O2 intermediate, the energy gap between the triplet state and its open-shell singlet state becomes smaller. An intersystem conversion from the triplet to singlet energy surface (MECP) then occurs easily, enabling the metal peroxide product to be formed. The state crossing point is actually a transition state connecting the second η1-O2-coordinated intermediate with a triplet ground state and the metal peroxide product with a singlet ground state. The energy barriers calculated for the reactions studied in this paper are all very small (