Theoretical Study of Methane Activation by Re, Os, Ir, and Pt - The

Ole Swang, Knut Faegri Jr., and Odd Gropen. J. Phys. Chem. , 1994, 98 (11), pp 3006–3009. DOI: 10.1021/j100062a043. Publication Date: March 1994...
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J. Phys. Chem. 1994,98, 30063009

Theoretical Study of Methane Activation by Re, Os, Ir, and Pt Ole Swag;*+ Knut Faegri, Jr.,+ and Odd Groped Contributionfrom the Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway, and Department of Mathematical and Physical Sciences, University of Tromsr, N-9000 Tromse, Norway Received: October 1 1, 1993; In Final Form: January 3, 19940

The activation of methane C-H bonds by the third-row transition metal atoms Re, Os,Ir, and Pt has been studied using relativistic effective core potentials (RECPs) and electron correlation. Activation and reaction energies are calculated and compared to earlier results on dissociation of H2 on the same atoms and of CH4 and H2 on first- and second-row transition metal atoms.

1. Introduction

The complex electronic structure of transition metal (TM) containing systems leads to a particularly rich chemistry, and TM compounds have found use in various commerciallyimportant areas. One of these is catalysis, as annually some 10l2 dollars worth of products are made through catalytic processes, above all heterogeneous catalysis.' The commercialimportanceof such systems has led to an intenseactivity in applied research, so intense that mechanistic aspectsoften have been deemphasized in search of the quickest possible improvement of existing processes. Nevertheless, mechanistic studies are presently receiving increased attention, not only from academic workers but also in industrial R&D. In the last decades, considerable experimental and theoretical progress has been made.' The theoretical study of reactionsinvolving TM atoms presents several major difficulties: The TM compounds require extensive treatment of electron correlation. Also, the second-row and especially the third-row TM elements call for inclusion of relativistic effects in the numerical treatment. In this article, we present calculated activation and reaction energies for the oxidative addition reaction

M + CH, + HMCH,

(1)

where M is Re, Os,Ir, and Pt. In a recent theoretical paper,2 Blomberg et al. have studied the same reaction for second-row transition metal atoms. This, and our previous paper on reactions between third-row transition metal atoms and hydrogen,3 will serve as a basis for the discussion below. The simple, essentially gas-phase systems described here can be viewed as models both for heterogeneous and homogeneous catalysis, as earlier results*.4 indicate that a single atom in a carefully chosen electronic state may be a useful model for a metal surface or a complex. Also, the systems under study have been studied experimentallyper se,s+6so that a direct comparison can be attempted. In the following, we will try to relate our computational results to experimentaldata, as well as to calculations for related systems. 2. Computational Details

Geometriesfor energy minima and transition states have been calculated at the Hartree-Fock level of theory, while the energies have been calculated with the modified coupled pair functional (MCPF) method of Chong and Langhoff.7 The latter may be regarded as a size-extensive development of the configuration interaction method including single and double excitations. The metal atoms are described by relativistic effective core potentials t University of Oslo. t University of Tromse.

*Abstract published in Advance ACS Absrracrs, February

15,

1994.

0022-365419412098-3006$04.50/0

TABLE 1: Valence Basis Sets' atom metal C primitive exponents 8,6,8 995 0,2,1 S3) 0,OJ polarization exponents contracted functions 4,4,5,(1 1 3,2,1 a Functions are listed in the order 8, p. d, and f.

H 5 091 3,1

(RECPs) calibrated against one-component, "no-pair approximation'' relativistic calculations on the atoms.* The RECP procedure is described in detail elsewhere? and the specific RECR used here are previously published.) The atomic orbitals 5s,5p, and 4d for the metals are kept frozen in all calculations, whereas the inner shells are described by a local potential. Spin4rbit interaction is not accounted for in this model. Possible consequences of this are discussed below. Frozen shells and valence shells are described by four s-type, four ptype, and five d-type contracted Gaussian type orbitals (CGTOs). For the energy calculations,one f-type CGTO was added. The effect of omitting f-functionson the geometries has been shown to be minor.) The f-type polarization functions consist of three primitive exponents found by optimizing the SDCI energyof the atoms in their ground states. In the molecular calculations, these three functionswere contracted to one. The two ptype polarization exponents were found by minimizing the Hartree-Fock energy of the lowestlying atomic state with 6p occupation; see ref 3 for details. The d-type polarization exponent was set equal to 0.1 for all metal atoms. The metal basis sets are available upon request (Email: [email protected] or [email protected]). For carbon, the (9,5) basis reported by van Duijneveldt" was contracted to (3,2), and a d exponent of 0.63 was added. In order to minimize basis set superposition errors, the 1s orbital of carbon was kept frozen in its atomic shape at all times. The basis for hydrogen is the 5s set reported by Huzinaga,'z contracted to triple-{ and augmentedwitha ptypeexponent of magnitude0.8. Thevalence basis sets are summarized in Table 1. Multireference test calculationsshowed that all systems dissociated correctly within the Hartree-Fock formalism. C, symmetry was imposed during all calculations. A HMCH3 molecule with Cs symmetry restrictions yields 8 geometrical degrees of freedom, which were all optimized (see Figure 1 for an illustration). Cartesian coordinates were chosen as the variables to optimize. The geometrical energy minima were found through the use of the downhill simplex method of Nelder and Mead.13J4 This code was interfaced with the MOLECULESWEDEN program system,lS which was used for all calculations reported here. To ensure correct convergence, the simplex was reinitiated from each minimum and the algorithm was allowed to converge again. Invariably, in our calculations it then converged to the same geometry, but any saddle points or pathological convergence should be unveiled by this procedure. The geometries Q 1994 American Chemical Society

Methane Activation by Re, Os, Ir, and Pt

The Journal of Physical Chemistry, Vol. 98, No, 11, 1994 3007

n

TABLE 3: Energies for Transition States and Miaim of HMCHj Systems' metal state E-, Eui. Re 4Af +55 +23 Re I (Cd +53 +14 ~~

os os

+35 +27

'Al(C3u) 2A'

Ir

~

+3 -7 -22 -34

Pt 'A' a Energies are in kilocalories/mole, relative to the metal atom and a methane molecule at infinite separation, both in their ground states.

TABLE 4: Geometry Parameters for Trausition States and Energy Minima of HMCH, Systems' metal state geometry RMX RM-H a /3 Re 'A' TS 2.21 1.63 41 131 99 Re 'A' Min 2.06 1.65 126 115 97 2.31 1.70 47 99 132 6A1(Ck) TS Re 2.23 1.78 180 112 112 6Al(C3u) Min Re Os 'A' TS 1.60 2.22 37 133 96 0s 'A' Min 1.98 1.58 149 111 101 TS 2.17 1.57 51 99 130 OS 'Ai(C30) C$

Figure 1. Geometry parameters for the HMCH, systems.

TABLE 2 metal Re

os Ir Pt

Atomic States' ground state 6s 'D 'F

low-spin state 4P 3P 2P

'S a Calculated energy differences in kilocalories/mole. 3D

AE 51 32 17 7

arecomputationally stable in all reported digits. For the transition states, the distance between the carbon atom and the dissociating hydrogen atom was chosen as the reaction coordinate. This distance was stepped downward from its value at the energy minimum, and the other 7 degrees of freedom were allowed to relax for each step. The energy maximum on the reaction path was found by iterative parabolic fitting in the reaction coordinate.

3. Results and Discussion The HMCH3 systems described here have two conformers, staggered and eclipsed. Both conformers were optimized, and in all cases, the staggered conformer was found to be the most stable. However, the energy difference was in no case larger than 1.5 kcal/mol, and in the following,only the staggered conformer will be considered. For the high-spin C3, structures of Re and Os, there is of course only one conformer. The electronic state.of the naked metal atom must be chosen with care if it is to be a good model for a metal atom in a complex or in a surface. Upon reaction, two covalent bonds are formed between the metal and the hydrogen and methyl fragments. Because all the metal atoms studied have high-spin ground states, the bound complex must have lower spin than the ground state of the atom when only s and d orbitals are involved in the bonding. The most stable reaction products with Re and Os have high spin; they have C3, symmetry and sp hybrid bonding orbitals on the metal. The reaction energy for the gas-phase reaction in eq 1 is calculated as the difference between the energy of the product and the energy of the reactants. This procedure does not give good approximation to the reaction energies for less unsaturated complexes, but calculations on different models of a rhodium complex2 suggest that the reaction energy for the atom is a good approximation to the reaction energy for a complex if the metal atom is restricted to be in a low-spin state. Hence, we will need two different reference states, hereafter called separated system limits (SSLs), for each metal atom: the ground state and the most stable state with a lower multiplicity than the ground state (all the atoms studied have high-spin ground states). The term symbol and calculated excitation energies of the low-spin atomic terms are given in Table 2. For a detailed discussion of the calculated atomic spectra see ref 3. Below, the reaction with each metal will be described on the background of energetics and electronicstructure. Then, comparisonswill be made with earlier theoretical results, experimental results on active complexes, and

Os

Ir Pt

'Al(C3,) ZA' 'A'

Min Min Min

2.06 2.03 2.00

1.65 1.56 1.52

180 107 93

111 112 110

111 105 106

a Distances are in angstroms and angles in degrees. See Figure 1 for information on the geometry parameters.

experimental results on the methyl hydrides themselves. The activation and reaction energies relative to the atomic ground states are listed in Table 3. The geometries are listed in Table 4. The mechanism for addition is the same for the low-spin potential energy surfaces (PES) of all the systems: Toward the transition state, electron density moves from a (CH)a bonding orbital to the LUMO on the metal, resulting in a negative charge of about 0.2 electrons on the metal. The reaction then proceeds to the minimum under back-donation of charge from the metal atom to an antibonding orbital on methane and formation of the covalent metal-carbon and metal-hydrogen bonds. In the minima, the metals have a positive charge of about 0.1 electrons. 3.1. Rhenium. The rhenium atom in its 6S ground state is uncommonly inert compared to other third-row transition metals, due to its half-filled 5d shell. However the most stable quartet term, 4P, inserts spontaneously into a H2 molecule according to previous calculations.3 The reaction between Re('P) and CH, proceeds via a transition energy barrier of 4 kcal/mol. This minimum is 23 kcal/mol less stable than the 6s ground-state SSL,but it is bound with 28 kcal/mol relatively to the 4P lowspin SSL. The significant p character (0.32 electrons) of the bonding orbitals manifests itself in the H-Re-C angle of 126O. A pure sd bonding scheme should give a bonding angle of 90°. The most stable bound state of HReCH3 is found on the PES corresponding to the 6S ground state SSL,14 kcal/mol above it. The bonding orbitals can best be described as sp hybrids, which is consistent with the linear arrangement of the "ligands". The Mulliken population of the bonding orbitals indicates0.5 electrons of p character. The transition state on this PES is located 53 kcal/mol above the ground state and is hence 2 kcal/mol more stable than the transition state on the low-spin PES. 3.2. Os".On the low-spin PES, the transition and reaction energies are +3 and -29 kcal/mol, respectively, which is very close to the values for rhenium. The bonding angle is even more obtuse at 149O. Relative to the 5D ground-state SSL, the transition and reaction energies are +35 and +3 kcal/mol, which is significantly lower than for rhenium. Also for Os,the molecular ground state is an sp-bonded, C3"high-spin structure, bonded by 7 kcal/mol relative to the5D high-spin SSL. The transition state on this PES is located 27 kcal/mol above the ground state and is hence 7 kcal/mol more stable than the transition state on the low-spin PES.

3008 The Journal of Physical Chemistry, Vol. 98, No. 11, 199'4 TABLE 5: Reaction Ewr- for M + HZand M + CH4 Reactiws (in kcal/mol) Taken as the Energy Difference between the Ground State of the Reactants and the Ground State of tbe Product' Hz Fe [21]+1 Co [21]+3 Ni [22]-8 TC[23]-18 Rh [18]-17 Pd [24]-6 RU[23]-16 Re [3]-1 Os [3]+1 Ir [3]-41 Pt [3]-50 CH4 Co [18]+10 Ni [18]+4 Fe [la]+8 Rh [2]-5 Pd [2]+9 Ru [2]+9 Re +14 os -1 Ir -22 Pt -34 a Hence, PES crossings are allowed here. References for results not from this study are enclosed in brackets. See text for details. TABLE 6 Reaction Energies for M + H2 and M + (3% Reactions (in kcal/mol) with Low-Spin ResMctioP H2 FC[21]-1 c o [21]-3 Ni [22]-4 TC[23]-37 RU[23]-35 Rh [18]-31 Pd [24]-6 Re [3]-32 Os [3]-26 Ir [3]-56 Pt [3]-57 CHq

Fe [18]+8 Co [18]+10 Ni [18]0 TC[2]-15 RU[2]-11 Rh [2]-20 Pd [2]+9 Re -28 OS-29 Ir -39 Pt -41 * References for results not from this study are enclosed in brackets. See text for details. 3.3. Iridium. The ZP state of the iridium atom reacts with methane without any barrier and with an exothermic energy of 39 kcal/mol. The reaction is spontaneous even at the HartreeFock level. Relative to the 4Flow-spin SSL, the minimum energy is -22 kcal/mol. The sharper bonding angle (107O) is an indication of weaker p character in the bonding orbitals. 3.4. Platinum. Both the ground state and the low-spin IS state of the platinum atom insert spontaneously into a methane C-H bond, with the reservation that a PES crossing is necessary between the ground state and the energy minimum. On the lowspin PES, there is no barrier even at the HartreeFock level. The 1 sstate of Pt has a closed-shell5dlOconfiguration. As the reaction proceeds,theoccupation moves toward 5d96s1,and the final bonds have a bonding angle very close to 90°. The binding energy is 34 and 4 1kcal/mol with respect to the IS low-spin and 3D groundstate SSLs, respectively. 3.5. Comparison with Previous Results. Reaction energies for addition of Hz and CH4 to different transition metal atoms in their ground states are summarized in Table 5 , and the reaction energies for the low-spin PES are listed in Table 6. Vibrational zero-point energies are not corrected for in the tables. From experimental vibration frequencies for the HOsCHp m~lecule,~ we tentatively estimate inclusion of such corrections to stabilize the reaction products by some 2 kcal/mol and the transition states by some 3 kcal/mol. We note that H2 binds more strongly than CHI for all listed metals except osmium, whether low-spin restrictions are imposed or not. The sourceof theosmium anomaly is not clear to us. On the ground-state PES, both H2 and CH4 bind more strongly to the heavier elements of the groups 9 and 10, while the opposite trend is seen for the group 7 and 8 metals. A possible explanation for the former phenomenon is the relativistic contraction of the valence s orbital, facilitating sd hybridization as the s and d orbitals are closer in space. For the ground-state PESs of Re and Os, the binding orbitals are sp hybrids, which are less influenced by relativistic effects. For the low-spin PESs, we note that Re and Os give very similar reaction energies (28 and 29 kcal/mol, respectively), and so do Ir and Pt (39 and 41 kcal/mol, respectively). The first observationsof alkane C-H insertion in solution were made for iridium complexes,16 where the active intermediates were believed to be coordinatively unsaturated fragments of the

Swang et al. general formula Cp*IrL (L = CO, PR3),formed by photochemical dehydrogenation of more stable dihydrides of the form Cp*IrL(H)z. Later, such reactions have also been observed for rhenium and osmi~m.1~ To our knowledge, no such reaction has been reported for platinum, in apparent contradiction with our results. Klabundeet a1.5 havestudiedthereactions betweenall transition metal atoms and methane in an Ar matrix at temperatures in the 10-30 K range. The reactivity was monitored as the amount of metal atoms, measured by electronic spectroscopy, that was consumed by the reaction with methane. The results indicate that Re and Os should be more reactive than Ir and pt, in apparent contradiction to our calculated results. A possible explanation is that some metal atoms may be consumed by forming weakly bound complexes with methane or with each other. Such complexes could be stable at matrix conditions and will not necessarily show up in the spectra. This explanation is supported by the observation that copper is found to be consumed by the reaction and hence interpreted to be highly reactive, in contradiction to earlier results,5J8which indicate that copper atoms are unreactive toward methane. Another possible explanation is that the products might have varying stability at matrix conditions. This is consistent with our finding that HReCH, and HOsCH3 have higher barriers of dissociation than the corresponding Ir and Pt compounds. IR spectra were taken for Os and Ir. Evidence was found for the existence of an HOsCH3 species in the matrix, while no such evidence was found for Ir, suggesting that the lower dissociation barrier of HIrCH3 (22 kcal/mol, vs 34 kcal/ mol for HOsCH3) leads to its decomposition before a spectrum can be taken. Another possibility is that the two-coordinated iridium species reacts further with more methane molecules to yield larger and unidentified molecules in the matrix. Weisshaar et aL6 have investigated the kinetics of reactions between methane and iridium and platinum, respectively. Also here, the consumptionof metal atoms was measured by electronic spectroscopy. The reactions took place in an atmosphere of about 1 Torr of helium. Platinum was found to be reactive, while no significant reaction was observed for iridium. Again, this might be explained by differences in the dissociationbarriers of the two compounds: HIrCH3 has a dissociation barrier of 22 kcal/mol, while the analogous platinum compound has a dissociationbarrier of 34 kcal/mol. Hence, the iridium compound will more easily dissociate after being formed. Another possibility is that spinorbit coupling, not accounted for in the present calculations, stabilizes the Ir atom relative to the compound molecule to the extent that it gives rise to a reaction energy barrier. Low and Goddardlg have published an extensive study on hydrogen, methane, and ethane dissociation over platinum and palladium atoms, using the generalized valence-bond (GVB) method. For methane, they found a transition energy barrier of 13 kcal/mol and a reaction energy of -16 kcal/mol. This is a significantly less attractive potential compared to the present results; the discrepancy can be ascribed to the lack of description of dynamical correlation on behalf of the earlier work. Our qualitative conclusionsare, however, completely in line with those of Low and Goddard. Earlier studies also include the recent paper by Hada et aLZ0 who have investigated addition of CH4 to Pt, Pt+,and Pt- using the symmetry-adapted cluster (SAC) method. Both the )Dand the 1sstate of the Pt atom are reported to surmount large transition energy barriers of 59 and 102 kcal/mol, respectively, before insertion into a C-H bond of methane. Also, the overall reaction is reported to be endothermic on both singlet and triplet PESs, in clear discrepancy with our results. This might be explained by their failure to optimize the molecular geometries and the use of a RECPcontahiig no basis functionsdescribing the subvalence shells of the Pt atom. Also, the valence basis set used was rather small.

Methane Activation by Re, Os, Ir, and Pt Addressing the question of spin-orbit (SO)coupling, we note that the coupling between the ground state and lowest low-spinstate atomic LS terms is small for the Re and Os atoms, while the corresponding terms for Ir and Pt mix significantly. Hence, SO coupling would be expected to influence the shape of the PESs for the latter atoms, especially the parts of the PESs that correspond to early stages of the reaction. More specifically, SO coupling will probably stabilize the free atoms relative to the reaction products, resulting in higher transition energy barriers and smaller exothermic reaction energies. From the present results and the results for the analogous dihydrides,’ it seems that none of the critical points of the presently investigated PESs have other, energetically close, PESs to which SO coupling is symmetricallyallowed. The qualitative agreement between our platinum dihydride results from ref 3 and the Dirac-Fock results of Dyall’O is supporting this view, indicating that the most important contribution of SO coupling is the stabilization of the Ir and Pt atoms mentioned above. 4. Concluding Remarks

We have investigated the reactions between late third-row transition metal atoms and a methane molecule. The reactivity is found to be higher than for the analogous first- and second-row systems. The enhanced sd hybridizationis ascribed to relativistic contraction of the s-type valence orbitals, making the radial expectation values for the valence s and d orbitals more similar. Experimental results on HOsCH3, HIrCH3, and HPtCH3 may be explained in terms of the calculated dissociation energies.

Acknowledgment. Grant No. V6414 from the Norwegian VISTA foundation and a grant of computing time from the Norwegian SupercomputingCommittee (TRU) are both gratefully acknowledged. We would also like to thank Prof. J. C. Weisshaar for giving us access to preliminary results. References and Notes (1) Schustorovich, E., Ed.; Metal-Surface Reaction Energetics; VCH Publishers: New York, 1991.

The Journal of Physical Chemistry, Vol. 98, No. 11, 1994 3009 (2) Blomberg, M. R. A.;Siegbahn, P. E. M.;Svensson, M. J. Am. Chem.

Soc. 1992, 114, 6095.

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