Heterolytic Activation of COH Bond inMethane with (HNACHCHANH)M(CH3)(M � Pd�, Pt�, Rh�, Ir�, Rh, Ir):Comparative Density Functional Studyof Activation Mechanisms

HANNE HEIBERG,1 ODD GROPEN,1 OLE SWANG2

1Department of Chemistry, University of Tromsø, N-9037 Tromsø, Norway2SINTEF Applied Chemistry, Department of Hydrocarbon Process Chemistry, P.O.B. 124 Blindern,N-0314 Oslo, Norway

Received 10 May 2002; accepted 30 October 2002

DOI 10.1002/qua.10525

ABSTRACT: Activation of methane by oxidative addition and �-bond metathesis hasbeen investigated for (N-N)M(CH3) (M � Pd�, Pt�, Rh�, Ir�, Rh, Ir; N-N �(HNACHOCHANH) using different density functional approaches. The pathway ofoxidative addition is in general favored, the exceptions being Pd� and Rh�. Oxidativeaddition is clearly more favorable for the third-row metal complexes than those of thesecond row. The third-row metal complexes also tend to have a lower activation barrierfor �-bond metathesis than those of the second row. In each case, the oxidative additionis preceded by formation of a sigma complex. The bonding energies of these complexesare significantly stronger for the cationic systems. © 2003 Wiley Periodicals, Inc. Int JQuantum Chem 92: 391–399, 2003

Key words: COH bond activation; sigma complex; Pt; Pd; Ir; Rh; density functionaltheory

Introduction

M ethane is the major component of naturalgas, and thus represents a large part of the

world’s available hydrocarbon resources. Its com-

mercial utilization is, however, challenged by acombination of two factors: Methane is not liquifi-able at ambient temperatures, making it expensiveto transport, and its reactivity is lower than that ofhigher hydrocarbons, making its conversion intoliquid products expensive. The present conversionroute, based on synthesis gas, involves large mon-etary investments and requires a large portion ofCorrespondence to: O. Swang; e-mail: ole.swang@sintef.no

International Journal of Quantum Chemistry, Vol 92, 391–399 (2003)© 2003 Wiley Periodicals, Inc.

the gas for heating purposes, despite the fact thatthe overall reaction to, e.g., methanol, is exother-mal. Hence, a direct conversion scheme involvingmoderate temperatures may have great commercialpotential.

Oxidative coupling of methane over heteroge-neous catalysts based on transition metals andtheir oxides [1] has been investigated as a possi-ble improvement over present technology. Unfor-tunately, the oxidation products are at least asreactive as methane over these catalysts, a factthat imposes severe difficulties on process engi-neering. Another approach involves metal com-plex catalysis in a liquid phase under mild con-ditions. This approach was pioneered by Shilov,who reported three decades ago that catalyticconversion of methane/alkanes to mixtures of thecorresponding chlorides and alcohols could beachieved using an aqueous solution of Pt(II) andPt(IV) salts [2]. In the 1980s it was discovered thatalkane COH bonds could undergo stoichiometricadditions at highly reactive, coordinatively, andelectronically unsaturated organotransition metalspecies such as Cp*Rh(PMe3), Cp*Ir(PMe3), andCp*Rh(CO) (Cp* � �5-C5Me5) [3]. This type ofreactions has also been the subject of a number oftheoretical studies [4 –7]. The conversion of theorganometallically COH activated alkane to or-ganic products ROX has been difficult to achieve,even stoichiometrically.

ROHO¡“M”

M}

{

R

H

O¡“X”

ROX. (1)

The high air sensitivity of the neutral, low-oxi-dation-state organometallic complexes constitutesone major problem for the conversion of alkanes tovaluable products under oxidizing conditions.

Pt(II) complexes containing bidentate nitrogen li-gands have proved active toward COH bond activa-tion [8–11]. The Bercaw and Labinger group demon-strated activation of methane COH bonds byobserving exchange of labeled 13CH3 for CH3 be-tween (tmeda)PtII(NC5F5)(CH3)� (tmeda � (CH3)2NCH2CH2N(CH3)2) and CH4 in pentafluoropyridine,NC5F5 [10]. A computational density functional theory(DFT) study [12] indicates that the activation occurs byan oxidative addition mechanism. Wick and Goldberg[13] demonstrated activation of methane employing(�2-Tp�)PtII(CH3) (Tp� � hydridotris(3,5-dimethylpyr-azolyl)borate) and managed to trap the normally elusivefive-coordinated oxidative addition product by intra-molecular pyrazole coordination, ultimately producingPt(IV) complexes (�3-Tp�)PtII(H)(CH3)2.

Activation of methane under mild conditions hasbeen reported for (Nf-Nf)Pt(CH3)(H2O)� (Nf-Nf �ArNAC(CH3)OC(CH3)ANAr, Ar � 3,5-(CF3)2C6H3) when dissolved in trifluorethanol [8a, 9].

�2�

The reaction mechanism has been theoretically stud-ied in our group using the model ligand HNACHOCHANH and the present results of platinumcalculated with the BP functional were presented inRef. [9] (note that the BP functional is called BP86 inthat reference). The computational results favor acti-vation by oxidative addition, although �-bond met-athesis cannot be ruled out entirely. Besides being aconventional �-(lone pair) donor, the nitrogen ligandalso contributes with extra electronic flexibility due topossible interaction between the �-system and themetal center. In the present study, we studied oxida-tive addition and �-bond metathesis reactions formethane on complexes in which Pt� is replaced byPd�, Rh�, Ir�, Rh, and Ir in (N-N)Pt(CH3) (N-NA

HNACHOCHANH). The Pd�, Pt�, Rh, and Ir spe-cies are isoelectronic and offer the possibility of com-parison between second- and third-row metals, aswell as between cationic and neutral complexes. TheRh� and Ir� reactions involve the metals in the un-common oxidation states II and IV, and were in-cluded in an attempt to describe the difference be-tween group 9 and 10 systems while keeping thecharge constant.

Computational Details

The geometries were optimized with DFT usingthe ADF program [14]. Slater exchange and the

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392 VOL. 92, NO. 4

VWN [15] parameterization of the LDA correlationenergy were used with the gradient corrections ofBecke [16] for exchange and of Perdew [17] forcorrelation. The frozen core approximation wasused for all atoms except hydrogen. Orbitals up to3d and 4f were frozen in their atomic shapes forsecond- and third-row transition metals, respec-tively, and up to 1s for the first-row atoms. Thebasis sets had the overall quality of TZV for thetransition metals and DZVP for the other atoms.*For open-shell systems, the calculations were car-ried out unrestrictedly, with a separate set of orbit-als for each spin. Relativistic effects were describedby the quasirelativistic (first-order Pauli Hamilto-nian) approach [18]. These calculations will be de-noted BP/ADF.

The stationary points were characterized bycalculating the vibrational spectra, showing ex-actly one imaginary frequency for transitionstates and no imaginary frequency for energyminima. For some structures we did not succeedin eliminating a single, small imaginary fre-quency (�87i cm�1) due to methyl rotation. Aslight rotation of a methyl group requires littleenergy, and we find it reasonable to assume thatthe energies of these structures are close to that ofthe true stationary structures. Frequencies werecomputed in ADF by numerical differentiation ofenergy gradients in slightly displaced geometries.

Geometries were displaced in two directions ofeach degree of freedom, allowing for greater nu-merical precision. The accuracy of the numericalintegration was set to 10�6.5 for vibrational cal-culations and 10�5.0 (or higher as necessary) toconverge the geometries.

Energies for the optimized geometries were alsocalculated using the Gaussian 98 program [19] andthe B3LYP [20] functional (denoted B3LYP/G98).Relativistic effects were treated by including theorbitals up to either 3s, 3p, and 3d (for Rh and Pd) or4s, 4p, 4d, and 4f (for Ir and Pt) in the relativisticeffective core potentials (RECPs) from the Stutt-gart/Dresden group [21a].† The valence basis setsof the transition metals were augmented with twocontracted f-functions [22], [3f]3 (2f ), to an overallcontracted (6s, 5p, 3d, 2f ) basis set. The correlationconsistent cc-pVTZ [21b] basis sets were used for allother atoms.

†Basis sets and ECPs Stuttgart RSC 1997 ECP in [21a] andcc-pVTZ in [21b] (code version Gaussian 94) were obtained fromthe Extensible Computational Chemistry Environment Basis SetDatabase, as developed and distributed by the Molecular ScienceComputing Facility, Environmental and Molecular Sciences Lab-oratory, which is part of the Pacific Northwest Laboratory (Rich-land, WA) and funded by the U.S. Department of Energy. ThePacific Northwest Laboratory is a multiprogram laboratory op-erated by Battelle Memorial Institute for the U.S. Department ofEnergy under Contract DE-AC06-76RLO 1830. Contact DavidFeller or Karen Schuchardt for further information. http://www.emsl.pnl.gov:2080/forms/basisform.html (May 2002).

SCHEME 1.

*Basis sets from the library of ADF2.3.0: quasirelativisticapproach—IV/M.3d, M � {Rh, Pd}, IV/M.4f, M � {Ir, Pt}, III/N,III/C, and III/H.

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INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY 393

Results and Discussion

The two mechanisms in Scheme 1, i.e., oxidativeaddition (OA) and �-bond metathesis (SB), are com-monly considered when COH bond activation atthe transition metal is discussed. We will use thenumbering system (A–E) for the different types ofmolecular structures as indicated in Scheme 1 andFigure 1. The numbering system for the atoms isillustrated in Figure 2.

There is a general consent that the activation ofalkylic COH bonds on organometallic complexesmost likely involves an intermediate �-complex,where CH4 coordinates weakly to the metal center[10, 23], see (B) in Scheme 1 and Figure 1. Thisprovides a straightforward explanation for the oc-currence of multiple H/D exchange into CD4 whenCD4 are used instead of C�H4 in Scheme 1 [9].Alkane �-complexes have rarely been directly ob-served [24], but a large body of evidence stronglyimplies their existence [25–27].

The focus in the present study will be put onhow different metal atoms in organometallic dii-mine complexes (N-N)M(CH3) (HNACHCHANH) affect the activation of methane. First, somefindings about the geometries of the stationarypoints will be described, followed by a discussion

of the energy profiles with respect to different met-als and electronic structures of the complexes.

GEOMETRIES

Geometry data is given in Table I (see Figs. 1 and2 for numbering system for molecules and atoms,respectively). We have previously shown [9] thatthe present computational approach gives excellentgeometries for the similar complex (p-MeOC6H4-NACHCHAN-C6H4-p-OMe)Pt(CH3)2. Differencesbetween X-ray crystallographic data of the complexand the computationally optimized geometry weretypically less than 0.01 Å for bond distances andless than 2° for bond angles (maximum deviationswere 0.02 Å and 4°).

The metal complexes in the present study ingeneral have a square planar coordination. How-ever, the reactants (A) and �-complexes (B) of Ir�

and Rh� deviate from this because their methylgroups are pointing out of the N-N-M plane, givingbent geometries. The radical structures with singlyoccupied HOMO are bent, while the closed-shellstructures are planar. We are not aware of anyreports of the geometry for similar Ir(II)- and Rh(II)-complexes in the literature. Indeed, oxidation state(II) is uncommon for these metals [28].

When the COH bond undergoes OA to the metalatom [i.e., C(1)OH(1) in Fig. 2], the COH bond isstretched and cleaved, while the MOC and MOHdistances decrease toward equilibrium bondlengths. The transition states for OA, (OA#, C), ap-parently depend heavily on the MOH distance,which is similar for all the optimized OA# struc-tures and close (3–6 pm) to the equilibrium distanceof the final five-coordinated OA complex (D). Onthe contrary, the COH distances vary widely (1.44–2.08 Å). Further, the structures with short COH

FIGURE 2. �-complex with numbering system.

FIGURE 1. Calculated structures of relevance to themethane COH activation reaction.

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394 VOL. 92, NO. 4

distances tend to have low activation barriers andvice versa, e.g., (N-N)Ir(CH3)(CH4) and (N-N)Pd(CH3)(CH4)� (C) have COH distances of 1.44and 2.08 Å with activation energies relative to the�-complex of 22 and 101 kJ/mol (B3LYP/G98), re-spectively. This is probably because the activationenergy depends on how much the COH bond mustbe stretched and weakened before a proper MOHbond is formed. Despite several attempts, no oxi-dative addition pathway for Rh(II3 IV) was found.

During oxidative addition the MON(1) distanceincreases due to the formation of MOC(1) in trans-position of MON(1) (see Fig. 2 and Table I). This is

due to the trans-effect, which is known to be inparticular strong for platinum [29]. The results forour platinum complexes indeed show the largestMON(1) bond increase (0.23 Å), closely followedby Pd� (0.22 Å), while Rh-, Ir-, and Ir�-complexesshow corresponding values of 0.16–0.19 Å.

RELATIVE ENERGIES

In the following, we discuss energies of the op-timized geometries. The energies are calculated us-ing the ADF program with the BP functional (BP/ADF) and the Gaussian 98 program with the B3LYP

TABLE I ______________________________________________________________________________________________Calculated bond distances (Å) and angles (°).a

Complex M M-H(1) H(1)-C(1) M-C(1) M-N(1) H(1)-M-C(1) C(2)-M-N(1)-N(2)

(A) Singlet (N-N)M(CH3)Rh — — — 1.91 — 180Ir — — — 1.89 — 180Pd� — — — 1.98 — 180Pt� a — — — 1.93 — 181Rh� — — — 1.93 — 124Ir� — — — 1.92 — 143

(B) (N-N)M(CH3)(CH4)Rh 1.98 1.13 2.34 1.94 29 180Ir 1.95 1.14 2.33 1.93 29 180Pd� 2.02 1.12 2.37 1.99 28 180Pt� a 1.84 1.16 2.34 1.96 29 180Rh� 1.92 1.13 2.41 1.96 27 141Ir� 1.91 1.15 2.37 1.95 29 155

(C) (N-N)M(CH3)(CH4) OA#

Rh 1.56 1.65 2.10 2.06 51 177Ir 1.60 1.44 2.22 2.02 40 �165Pd� 1.53 2.08 2.04 2.20 70 �175Pt� a 1.54 1.73 2.11 2.13 54 179Ir� 1.58 1.61 2.15 2.08 48 143

(D) (N-N)M(CH3)2(H) OARh 1.53 2.41 2.04 2.11 83 �177Ir 1.54 2.59 2.06 2.09 91 �176Pd� 1.52 2.18 2.04 2.21 74 �174Pt� a 1.51 2.42 2.04 2.19 84 �177Ir� 1.53 2.62 2.03 2.14 94 �175

(E) (N-N)M(CH3)(CH4) SB#

Rh 1.60 1.57 2.15 2.03 47 180Ir 1.57 1.99 2.13 2.04 63 180Pd� 1.66 1.47 2.14 2.10 43 180Pt� a 1.59 1.56 2.14 2.08 47 180Rh� 1.64 1.51 2.15 2.10 44 180Ir� 1.56 1.84 2.11 2.08 58 �178

See Figs. 1 and 2 for numbering systems. The following numbering system has been used (see Fig. 2): H(1), hydride or bridging Hin �-CH4; C(1), carbon of �-CH4 ligand or of methyl group derived from this ligand; C(2), carbon of methyl ligand; N(1), diimine cisto original methyl; N(2), diimine N trans to original methyl.a Ref. [9].

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INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY 395

functional (B3LYP/G98). The reaction energies andactivation barriers include zero-point energy (ZPE)corrections, and are presented in Table II. The ZPEsare based on frequencies calculated with the ADFprogram. We will mainly focus on the B3LYP ener-gies, which are those plotted in Figures 3 and 4.Note that the overall net reaction is a hydrogentransfer from the incoming methane molecule to themethyl group, transforming the latter into a meth-ane molecule. The intermediates studied here, viz.the OA product (D) and the SB transition state (E),have planes of symmetry. The reaction energy pro-file of the total reaction is therefore symmetrical.

For clarity, we only included one half of the totalprofiles in the figures.

The energies indicate that OA is in general themost favorable pathway. Exceptions are Rh� andPd�: No OA pathway was found for Rh�. For Pd�,the SB pathway was favorable by 32 kJ/mol(B3LYP/G98). The potential surface (BP/ADF) hasan extremely shallow (0.15 kJ/mol) minimum forOA (D) and a TS that is only marginally different(C). However, including ZPE reverses the energyorder of these two stationary points, giving no min-imum for the OA mechanism. This indicates that

TABLE II ______________________________________________________________________________________________Calculated stabilization energies of �-complex (B), activation (C, OA#), and reaction (D, OA) energies ofoxidative addition and �-bond metathesis (E, SB#) relative to the reactants (A) (N-N)M(CH3) � CH4.

M DFT approach (B) (C) OA# (D) OA (E) SB# �Esta

Rh BP/ADF �52 �17 �31 13 56B3LYP/G98 �38 24 4 45 26

Ir BP/ADF �65 �56 �88 �34 64B3LYP/G98 �34 �12 �37 14 45

Pd� BP/ADF �62 19 20b �7 107B3LYP/G98 �45 56 57b 25 87

Pt� BP/ADF �92c �59c �69c �48c 105c

B3LYP/G98 �67 �16 �19 �4 108Rh� BP/ADF �57 — — 11 —

B3LYP/G98 �46 — — 9 —Ir� BP/ADF �79 �56 �74 �20 —

B3LYP/G98 �55 �11 �39 26 —

Calculated singlet–triplet excitation energiesa (�Est) for (N-N)M(CH3). Units in kJ/mol. ZPE corrections are included.a �Est � Etriplet � Esinglet.b This structure is a minimum on the potential surface but becomes a transition state when ZPE is included.c Ref. [9].

FIGURE 3. The potential energy profile of the oxida-tive addition reaction mechanism: CH4 � (N-N)M(CH3)(A) 3 �-complex (B) 3 transition state OA# (C) 3 oxi-dative addition product OA (D). (See Fig. 1.)

FIGURE 4. Potential energy profile of the �-bondmetathesis reaction mechanism: CH4 � (N-N)M(CH3)(A) 3 �-complex (B) 3 transition state SB# (E). (SeeFig. 1.)

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396 VOL. 92, NO. 4

(N-N)Pd(CH3)2(H)� (D) eliminates CH4 withoutany activation barrier.

The activation barrier of SB and OA for the Pt�-complexes is only separated by 12 kJ/mol (B3LYP/G98) and the favorable mechanism cannot be con-clusively determined from this difference. The SBpathway is in general characterized by a four-centerTS structure (SB#, E) with an approximate C2v sym-metry.

It can be clearly seen that, from both kinetic andthermodynamic viewpoints, the OA reactions aremore favorable for the third-row metal complexesthan for those of the second row. The second-rowmetal complexes also tend to have higher activationbarriers for SB. It is well known that the s-orbitalsare stabilized relatively to the d-orbitals when goingfrom second- to third-row transition metals. Thiseffect is, for instance, seen in the atomic ground-state configurations where Ir (d7s2) and Pt (d9s1)have one electron less in their d-shell compared toRh (d8s1) and Pd (d10), respectively. The valences-orbital is important to form optimal overlap andstrong bonding with ligands, and the stabilizationof s-orbitals of the third row compared to secondrow supports the lower activation energies for thethird-row elements [30]. Siegbahn [7] studied theactivation of methane on the complexes CpM(CO)(M � Rh, Ir), and discussed how the relative size ofthe valence orbitals affects the activation barrier.The d-orbitals of Rh are more contracted than the s-and p-orbitals, while the valence s-, p-, and d-orbit-als of Ir have more similar size. The similar size isan advantage for forming good bonding hydridesand contributes to the lower activation energy for Ircompared to Rh.

The neutral Ir-complex is isoelectronic with thecationic Pt-complex and has lower activation barri-ers and more exothermic reaction energies of OA.This corresponds to the oxidative addition Ir(I 3III) and Pt(II 3 IV)�, where more energy is re-quired to oxidize the cationic Pt(II)-complex thanthe neutral Ir(I)-complex. The same trend is ob-served for Rh(I3 III) and Pd(II3 IV)�. This agreeswith the observed trend that electron-rich systemswith low oxidation states give lower activation en-ergies for COH activation by OA [31, 6a, 6c].

For the SB mechanism the trend is opposite: Pt�

and Pd� results show smaller activation energiesthan the Ir and Rh results. The SB mechanism is ingeneral observed for electron-deficient, typically d0,metals that have no d-electrons available for OA. Anatural consequence would be that a complex withless accessible electron charge would in general also

give lower activation barrier for the SB mechanism.The cationic complexes of Pt� and Pd� probablyhave less flexible electronic structures than the iso-electronic Ir- and Rh-complexes, due to the positivetotal charge and oxidation state of II rather than I.

Su and Chu [6a, 6c] studied the potential energysurface of the oxidative reaction CpM(CO) � CH43 CpM(CO)(H)(CH3) (M � Ru�, Os�, Rh, Ir, Pd�,and Pt�) using the B3LYP functional with double-zeta valence basis sets (and ECPs for the metals)with the Gaussian 94 program. They conclude thata heavier transition-metal center, i.e., the third rowvs. second row, leads to lower activation barriersand reaction energies, in agreement with our find-ings. However, they also conclude that a heaviertransition-metal center leads to a smaller singlet–triplet splitting (�Est � Etriplet � Esinglet) (Pd� andPt� apparently being exceptions from this). Ourresults show the opposite trend, i.e., Ir and Pt� havea higher �Est than Rh and Pd�, respectively. Theyreport that the �Est correlate with the activationbarrier and reaction energy of oxidative addition ofmethane for the 16-electron systems CpM(CO)(M � Ru�, Os�, Rh, Ir, Pd�, and Pt�) [6a, 6c] and14-electron systems ML2 (M � Pd, Pt; L � CO, L2 �PH2CH2CH2PH2) [32]. The excitation of the metalcomplex from singlet to triplet state may be re-garded as a part of the activation energy of theoxidative addition. This is explained in terms ofbond preparation and promotion energy at themetal center to form optimal overlap with the fron-tier orbitals of methane. Our results do not supporttheir conclusion that �Est for isoelectronic second-and third-row metal complexes in general correlatewith the activation energy and reaction energy. Suand Chu also find a linear correlation between theactivation barrier and reaction energy [6a, 6c],which is not supported by our results. In conclu-sion, this indicates that the relations Su and Chufound between second- and third-row 16-electronCpM(CO) systems [6a, 6c] and 14-electron ML2 sys-tems [32] do not directly apply to our 14-electron(N-N)M(CH3) systems. Su and Chu’s report of cor-relation does obviously strongly depend on the li-gands and cannot be taken as a general rule.**

The stabilization of the �-complex (B) relative tothe respective reactants is larger for the cationiccomplexes than the neutral. The stabilization in-

**This is also partly shown in their own results, e.g., M(PH3)2

(M � Pd, Pt) [32]. The energies in Ref [32] were calculated at theMP4SDTQ level with geometries optimized at the MP2, bothwith LANL2DZ basis sets.

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INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY 397

creases in the order Ir � Ir� � Pt� (34, 55, 67kJ/mol) and Rh � Rh� Pd� (38, 46, 45 kJ/mol)(B3LYP/G98). This effect is probably due to elec-trostatic interaction between the positively chargedmetal center and the charge-induced polarization inthe frontier orbitals of methane [6a, 30].

The results of our two density functional ap-proaches agree on the choice of favorable mecha-nism for all the metal complexes. However, thereare some general differences: The stabilization en-ergies of the �-complex relative to the reactants are(11–31 kJ/mol) larger for the BP/ADF than theB3LYP/G98 results, while the activation barriers(also relative to the reactants) are in general 30–50kJ/mol lower for BP/ADF than B3LYP/G98 (ex-cept for the SB# barrier of Rh�, where the twoapproaches yield approximately the same result).

In a report of a similar complex CpM(CO)(CH4)(M � Rh, Ir) [7], B3LYP results (DZP basis) arecompared with PCI-80 results (DZP basis). In thatwork, B3LYP underestimates the stabilization ener-gies (by 29 kJ/mol) and overestimates the activa-tion and reaction energies (by 41–48 kJ/mol) forCpRh(CO), while B3LYP overestimates the stabili-zation energy (by 14 kJ/mol) for CpIr(CO) relativeto the more accurate PCI-80 results (DZP basis).There are also reports of OA of methane for thecomplexes CpM(PH3)(CH3)(CH4)� (M � Rh, Ir) [4,6b] and M(PH3)2 (M � Pd, Pt) [33], where B3LYPresults (LANL2DZ basis [4], TZV basis [33] with theGaussian program) are compared with results fromQCISD/MP2 (LANL2DZ basis) [6b] and CCSD(T)(TZV basis) [33] calculations. In these studiesB3LYP underestimates the stabilization energies(up to 25 kJ/mol) and overestimates the activationand reaction energies (3–30 kJ/mol) compared tothe other methods. However, the suitability of theLANL2DZ valence basis sets for QCISD calcula-tions could be a matter of discussion. AlthoughB3LYP in general gives more accurate energies thanBP [34], it is therefore difficult to judge the relativeaccuracy in our B3LYP/G98 and BP/ADF applica-tions, as there are also other features than the func-tional that differ between the two approaches: InADF, we use frozen core orbitals in all-electroncalculations while RECPs are employed in Gauss-ian 98. The RECPs are designed to include thescalar-relativistic effects and no further relativistictreatment is performed. In ADF the scalar-relativ-istic effects are explicitly taken into account by thequasirelativistic approach. The use of frozen corevs. RECPs is another difference. Further, ADF ap-plies Slater functions, while Gaussian functions are

used in Gaussian 98 as in most other quantumchemical codes.

Conclusions

Activation of methane by OA and �-bond met-athesis (SB) has been investigated for (N-N)M(CH3)(M � Rh, Ir, Pd�, Pt�, Rh�, and Ir�) (A). Thereaction pathway of oxidative addition is in generalfavored with the exceptions of Pd� and Rh�. Nointermediate was found for Pd� in OA, which alsomeans that CH4 would eliminate from (N-N)PdIV(CH3)2(H)� (D) without any barrier. No OApathway was found for Rh�.

Su and Chu report [6a, 6c, 32] correlation be-tween �Est and the reaction energy, �Est and acti-vation energy, and a linear correlation between thereaction and activation energy for the OA reactionof CpM(CO) (M � Ru�, Os�, Rh, Ir, Pd�, Pt�). Ourresults do not support this and the correlationseems to depend strongly on the ligands.

The third-row metal complexes tend to havelower activation barrier for SB than the secondrow. The activation barrier of OA is also morefavorable for the third-row metal complexes thanthose of the second row. This may partly be ex-plained by the importance of s-orbitals in thehybridization and formation for optimal overlapwith methane, which is facilitated for third-rowmetals because the s-orbitals are stabilized rela-tive to d-orbitals when going from second- tothird-row metals.

The activation energy of OA is larger for Pt�

than the isoelectronic Ir and the same trend is foundfor Pd� relative to Rh. This agrees with the trendthat the COH activation by the OA is favored byrelatively low-oxidation-state, electron-rich late-transition-metal centers [6a, 6c, 32]. The alternativeSB mechanism is observed for electron-deficient,typically d0 metals that have no d-electrons avail-able for OA. This may explain that the Pt� and Pd�

activation energies are smaller than the correspond-ing Ir and Rh results because we expect Pt(II)� andPd(II)� to have less electronic flexibility than Ir(I)and Rh(I).

The stabilization of the �-complex (B) relative tothe respective reactants (A) is larger for the cationicspecies than for the neutral, probably due to elec-trostatic interaction and increases in the order Ir �Ir� � Pt� and Rh � Rh� Pd�.

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ACKNOWLEDGMENTS

Financial support and computer time [GrantNN0126K (http://www.notur.org)] from the Nor-wegian Research Council are gratefully acknowl-edged.

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