Activation of Propane C-H and C-C Bonds by Gas-Phase Pt Atom: A Theoretical Study

The reaction mechanism of the gas-phase Pt atom with C3H8 has been systematically investigated on the singlet and triplet potential energy surfaces at CCSD(T)//BPW91/6-311++G(d, p), Lanl2dz level. Pt atom prefers the attack of primary over secondary C-H bonds in propane. For the Pt + C3H8 reaction, the major and minor reaction channels lead to PtC3H6 + H2 and PtCH2 + C2H6, respectively, whereas the possibility to form products PtC2H4 + CH4 is so small that it can be neglected. The minimal energy reaction pathway for the formation of PtC3H6 + H2, involving one spin inversion, prefers to start at the triplet state and afterward proceed along the singlet state. The optimal C-C bond cleavages are assigned to C-H bond activation as the first step, followed by cleavage of a C-C bond. The C-H insertion intermediates are kinetically favored over the C-C insertion intermediates. From C-C to C-H oxidative insertion, the lowering of activation barrier is mainly caused by the more stabilizing transition state interaction ΔE≠int, which is the actual interaction energy between the deformed reactants in the transition state.


Introduction
In recent years, the dehydrogenation of lower alkanes has gained great importance in natural and petroleum gas utilization [1,2]. Propane is a cheap and easily available raw material as it is produced through a number of petrochemical processes, while the propylene market demand is rapidly increasing. Accordingly, the dehydrogenation of propane is an interesting alternative route to propylene production. In the catalytic cracking of alkanes and catalytic dehydrogenation of alkanes, platinum based catalysts have received much attention. Elucidation of the role of isolated platinum units in heterogeneous catalysis can be aided by gas-phase study, which can provide insight into the intrinsic properties and reactivities of discrete and well-characterized catalytic species [3].
In particular, the activation of C-H and C-C bonds of propane by transition metals (neutral, cationic, or clusters) in the gas phase has been an active area of research that provides fundamental information on catalytic reaction mechanisms, kinetics, and thermodynamics [4,5]. Of the first-row transition metal series, for the activation of propane, the early members (Sc + [6], Ti + [7], and V + [8]) exhibit efficiency for the dehydrogenation of propane. Co + cation favors H 2 over CH 4 [9,10], whereas Fe + and Ni + cations favor CH 4 over H 2 [9,11]. Cr + cation does not show any efficiency for the activation of propane [12]. Of the second-row transition metal series, for Nb + [13], Mo + [12,14,15], and Rh + cations [16], the dehydrogenation of propane is efficient and the dominant process at low energies, whereas products resulting from both C-H and C-C cleavage processes are observable at high energies. Rh atoms are also effective for the H 2 elimination from ethane and larger alkanes under kinetics technique experiment [17]. For the reaction of Ag + with propane, the dehydrogenation and formation of AgH + + R products are not observed, whereas the C-C bond cleavage is the predominant process [18]. Of the actinide ions, for the activation of propane, Th + , Pa + and U + cations are efficient for the dehydrogenation of propane. For the other cations (Np + , Pu + , Am + , and Cm + ), no reactions are observed experimentally [19]. For Th 2+ and U 2+ , both C-H and C-C cleavage products are effectively observed [20]. Finally, for [(MgO) n ] + clusters, the higher reactivity with propane is not specific to [(MgO) 2 ] + , but has been also observed for (MgO) + [21,22].
In the present study, a complete mechanism of neutral Pt atom with propane along with both the C-H and C-C bond activation processes is investigated, which is necessary to enable us to determine the crucial steps and to either block or enhance particular steps to steer the reaction in the desired direction. The goals of the present investigation are as follows: (a) to provide reliable structures and chemically accurate energetics of the reactants, intermediates, transition states (TSs), and products; (b) to elucidate the rate-determining step and the selectivity-controlling step; and (c) to gain a better understanding of the preference of reaction pathway. Particularly, to shed some light on the intrinsic reactivity of platinum atom toward the propane activation, the trends in reactivity and competition among the C-H and C-C bond cleavage mechanisms are analyzed using the activation strain model of chemical reactivity [30,31]. The potential energy profiles on the singlet and triplet states are investigated, because spin crossing is often involved in the transition metal-containing reactions [7,15,20].

Computational Details
All calculations were carried out with the Gaussian 03 program [32]. Full geometry optimizations were run to locate all of the stationary points and TSs on the singlet and triplet potential energy surfaces (PESs) for the reactions of Pt atom with C 3 H 8 , using the BPW91 [33,34] method with 6-311++G(d, p) basis set for the carbon and hydrogen atoms [35,36], and the Lanl2dz basis set and the corresponding effective core potential (ECP) for platinum [37], namely BPW91/6-311++G(d, p), Lanl2dz. Meantime, the stability of the density function theory (DFT) wavefunction was tested [38,39]. If an instability was found, the wavefunction was reoptimized with appropriate reduction in constraints, and the stability tests and reoptimizations were repeated until a stable wavefunction was found [38,39]. Harmonic frequency calculations were run to characterize stationary points and to take corrections of zero-point energy (ZPE) into account. The intrinsic reaction coordinate (IRC) method was performed to track minimum energy paths from transition structure to the corresponding minima [40,41]. The dominant occupancies of natural bond orbitals for some species have been analyzed with the help of the natural bond orbital (NBO) analysis [42,43]. To further determine electron correction energies, the single-point calculation of various species based on the optimized BPW91/6-311++G(d, p), Lanl2dz geometries were then refined using CCSD(T) [44] method with the same basis sets, namely CCSD(T)//BPW91/6-311++G(d, p), Lanl2dz. Unless otherwise mentioned, all energies are relative to the ground-state reactants [Pt( 3 D) + C 3 H 8 ] at the CCSD(T)//BPW91/6-311++G(d, p), Lanl2dz level, including ZPE correction obtained at the BPW91/6-311++G(d, p), Lanl2dz level.
In this work, we will mainly discuss the following reactions of Pt atom with C 3 H 8 : The above three product channels were divided into five sections: (i) C-H bond activation: dehydrogenation, (ii) C-C bond activation: deethanization, (iii) C-C bond activation: demethanation, (iv) comparison of C-H with C-C bond activation, and (v) activation strain analysis of the direct C-H and C-C bond cleavage.

C-H Bond Activation: Dehydrogenation
For the dehydrogenation of C 3 H 8 by Pt atom, the reaction pathway and the optimized geometric structures of various species are depicted in Scheme 1.
The triplet state Pt atom, 3 D(d 9 s 1 ), is the ground state. The singlet state Pt atom, 1 S(d 10 ), lies 59.4 kJ·mol −1 above the ground triplet state 3 D(d 9 s 1 ), in good agreement with the estimated value of 56.6 kJ·mol −1 [26]. The superscript prefixes " 1 " and " 3 " will be used to indicate the singlet and triplet states, respectively. As depicted in Scheme 1, there are two primary (tans-C-H (1) and cis-C-H (1) in the trans and cis position with respect to the CH 3 group, respectively) and one secondary (C-H (2) ) C-H bonds in propane. Then, with regard to the initial interaction between Pt atom and C 3 H 8 , three molecular complexes are considered: (i) Pt atom attacking the H-end of primary tans-C-H (1) (1-PtC 3 H 8 ), (ii) Pt atom attacking the H-end of secondary C-H (2) approaching to Pt atom (2-PtC 3 H 8 ), (iii) Pt atom attacking the H-end of primary cis-C-H (1) (3-PtC 3 H 8 ).
For the formation of PtC 3 H 6 (Pt-propene) + H 2 , there are five reaction pathways beginning at the three kinds of molecular complexes (two from 1-PtC 3 H 8 , two from 2-PtC 3 H 8 , and one from 3-PtC 3 H 8 ), respectively, as shown in Scheme 1. Alternatively, for the formation of Pt(CH 2 ) 3 (Pt-cyclopropane) + H 2 , there is an unique reaction pathway starting from 3-PtC 3 H 8 .   2 , with the EHHP of 69.5 kJ·mol −1 at 3 TS5. From 3-PtC 3 H 8 , the MERP should progress via MECP between 1 cis-3-HPtC 3 H 7 and 3 cis-3-HPtC 3 H 7 , with the EHHP of 162.1 kJ·mol −1 at TS11. Since the EHHP at 3 TS1 from 1-PtC 3 H 8 is the lowest among the three reaction channels, this reaction channel for the formation of PtC 3 H 6 + H 2 is the most feasible kinetically. Furthermore, these results reveal a high preference of Pt atom for the attack of primary C-H bonds in propane, which is analogous to that of MgO + cation for the attack of alkanes [21]. This feature represents a notable distinction of the transition-metal atom from various transition-metal oxide cations, which show a clear preference for the attack of secondary C-H bonds [21].
For the formation of Pt(CH 2 ) 3 + H 2 , from 3-PtC 3 H 8 , the MERP should advance via MECP between 1 3-HPtC 3 H 7 and 3 3-HPtC 3 H 7 , with the EHHP of 65.5 kJ·mol −1 at 3 TS8. Furthermore, for Pt atom (10 valence electrons), Pt( 1 S) singlet state, has an empty orbital and five doubly occupied nonbonding orbitals, whereas Pt( 3 D) triplet state has all of its s and d valence orbitals occupied, with four doubly occupied nonbonding orbitals, and two singly occupied nonbonding orbitals. That is to say, the bonding capacity of Pt( 1 S) singlet state to C 3 H 8 is stronger than that of Pt( 3 D) triplet state. Then, the binding of Pt to C 3 H 8 in the C-H insertion intermediates (1-HPtC 3 H 7 , cis-1-HPtC 3 H 7 , HPtCH(CH 3 ) 2 , 3-HPtC 3 H 7 , and cis-3-HPtC 3 H 7 ) inverts the energies of the singlet and triplet states from the Pt atom. Therefore, the ground state of the C-H insertion intermediates is the singlet state, as depicted in Scheme 1. The reaction goes forward from the excited state reactants Pt( 1 S) + C 3 H 8 to the C-H inserted intermediates, without energy barrier. This can be ascribed to the fact that the singlet state Pt( 1 S) has an empty orbital, which should greatly facilitate the interaction with propane and its bond activation, leading smoothly to the formation of two covalent bonds to H and C 3 H 7 . This phenomenon has also appeared in the analogous Pt + CH 4 system [26].

C-C Bond Activation: Deethanization
For the deethanization of C 3 H 8 by Pt atom, the reaction pathway and the optimized geometric structures of various species are depicted in Scheme 2. The change of Gibbs free energies (ΔG 298 ) for the reaction of Pt( 3 D) + C 3 H 8 → 1 PtCH 2 + C 2 H 6 is calculated to be −45.9 kJ·mol −1 . Thereupon, the deethanization of C 3 H 8 is thermodynamically favorable. Then, it is necessary to discuss kinetically the above reaction infra.
As shown in Scheme 2, for the deethanization of C 3 H 8 by Pt atom, there are seven reaction pathways, three from 1-PtC 3 H 8 , one from 2-PtC 3 H 8 , and three from 3-PtC 3 H 8 . These seven reaction pathways are separated into two kinds of reaction pathways, one through the initial C-C bond direct cleavage, and another through the σ-complex assisted C-C σ-bond metathesis. For simplicity, we will primarily discuss the three reaction pathways from 1-PtC 3 H 8 infra, which are analogous to those from 2-PtC 3 H 8 and 3-PtC 3 H 8 . The nuances in energy mainly stem from their configuration differences among the three kinds of reaction channels.
For the second and third reaction pathways from 1-PtC 3 H 8 , there are two reaction pathways to produce cis-CH 3 PtC 2 H 5 . That is, Pt atom directly inserts the C-C bond via three-member TS13, leading to the intermediate cis-CH 3 PtC 2 H 5 . Alternatively, Pt atom firstly inserts the C-H bond via the three-member TS1, resulting in the intermediate 1-HPtC 3 H 7 . Next, from 1-HPtC 3 H 7 , a σ-complex assisted C-C σ-bond metathesis occurs via a four-member TS14 with both 1,2-H shift and C-C bond cleavage, also yielding cis-CH 3 PtC 2 H 5 . Then, from CH 3 PtC 2 H 5 , the σ-complex assisted C-H σ-bond metathesis takes place via a four-member TS15 with 1,3-H shift, yielding the molecular complex C 2 H 6 PtCH 2 . As mentioned earlier, C 2 H 6 PtCH 2 releases C 2 H 6 molecule, staying PtCH 2 behind. For the two reaction pathways, each MERP should advance via the MECP between 1 cis-CH 3 PtC 2 H 5 and 3 cis-CH 3 PtC 2 H 5 , with the HER of 299.0 kJ·mol −1 at the 1 cis-CH 3 PtC 2 H 5 → 1 TS15 reaction step. The two MERPs involve the EHHP of 156.1 and 120.3 kJ·mol −1 at 3 TS13 and 1 TS15, respectively. Comparing these three reaction pathways, one can conclude that the reaction pathway via TS2 involving the first C-H cleavage and via TS12 involving C-C cleavage with synchronous 1,3-H migration is the gross MERP for the 1-PtC 3 H 8 →PtCH 2 + C 2 H 6 reaction, owing to its comparatively low HER (273.2 vs. 299.0 kJ·mol −1 ) and low EHHP (91.6 vs. 120.3 and 156.1 kJ·mol −1 ), with the rate-determining step of 1 cis-1-HPtC 3 H 7 → 1 TS12→ 1 C 2 H 6 PtCH 2 .
From 2-PtC 3 H 8 for the formation of PtCH 2 + C 2 H 6 , only one reaction pathway is obtained, which includes the initial C-C bond cleavage and σ-complex assisted C-H σ-bond metathesis. This reaction pathway is homologous to that via the initial C-C cleavage from 1-PtC 3 H 8 with the HER of 299.0 kJ·mol −1 and the EHHP of 156.1 kJ·mol −1 at 3 TS13.
From 3-PtC 3 H 8 for the formation of PtCH 2 + C 2 H 6 , there are also three reaction pathways. These three reaction pathways are similar to those from 1-PtC 3 H 8 . The reaction pathway of σ-complex assisted C-C σ-bond metathesis via 3 TS9 and 1 TS19 is kinetically most preferable in the three reaction pathways, because of its lowest HER (274.9 vs. 298.7 kJ·mol −1 ) and lowest EHHP (91.6 vs. 120.3 and 156.1 kJ·mol −1 ).
In summary, for the formation of the C-C cleavage products PtCH 2 + C 2 H 6 , the optimal pathway proceeds through the σ-complex cis-1-HPtC 3 H 7 or cis-3-HPtC 3 H 7 from initial C-H bond cleavage, which assists the C-C σ-bond metathesis. This reactivity mode is also complementary for the classical reactivity picture through the direct C-C cleavage intermediate (M = Fe + [48] and Ta + [49]).

C-C Bond Activation: Demethanation
For the demethanation of C 3 H 8 by Pt atom, the reaction pathway and the optimized geometric structures of various species are depicted in Scheme 3. The change of Gibbs free energies (ΔG 298 ) for the reactions of Pt(3D) + C 3 H 8 →1PtC 2 H 4 + CH 4 are calculated to be −143.6 kJ·mol −1 , which is thermodynamically favorable. Afterwards, we will discuss the kinetics of the above reaction infra. As shown in Scheme 3, there are four kinds of reaction pathways, which are through cis-CH 3  Third, from HPtCH(CH 3 ) 2 , a σ-complex assisted C-C σ-bond metathesis occurs via a four-member TS21 with 1,3-H shift, also leading to the molecular complex CHCH 3 PtCH 4 . The MERP should go forward via the MECP between 1  Fourth, from 3-HPtC 3 H 7 , an oxidative insertion of C-C bond to the platinum center takes place via a four-member TS23, producing a methyl hydrid complex C 2 H 4 PtH(CH 3 3 ). Thereby, the optimal pathway proceeds through the σ-complex 3-HPtC 3 H 7 from initial C-H bond cleavage, which assists the C-C σ-bond metathesis. That is to say, the optimal C-C bond cleavages are assigned to C-H bond activation as the first step, followed by cleavage of a C-C bond. This reactivity mode is complementary for the classical reactivity picture through the direct C-C cleavage intermediate (M = Fe + [48], and Ta + [49]).

Comparison of C-H with C-C Bond Activation
As shown in Schemes 1-3, the C-H insertion intermediates ( 1 1-HPtC 3 H 7 , 1 cis-1-HPtC 3 H 7 , 1 HPtCH(CH 3 ) 2 , 1 3-HPtC 3 H 7 , 1 cis-3-HPtC 3 H 7 ) and the C-C insertion intermediates ( 1 CH 3 PtC 2 H 5 , 1 cis-CH 3 PtC 2 H 5 , 1 C 2 H 4 PtH(CH 3 ), and 1 CH 4 PtC 2 H 4 ) deposit in a deep well, respectively. It is indicated that these intermediates are thermodynamically preferred. For the formation of the C-H and the C-C insertion intermediates, the corresponding MERP should involve the HER of about 60~70 and 140~230 kJ·mol −1 , respectively. Thereby, the C-H insertion intermediates are kinetically favored, while the C-C insertion intermediates are kinetically hindered by energy barriers. These results are in qualitative agreement with the experimental results, in which the C-H insertion product is experimentally observed and the C-C insertion product is not formed in observable quantity in Pt + C 2 H 6 system [27].
For the formation of C-C bond cleavage intermediates 1 CH 3 PtC 2 H 5 and 1 cis-CH 3 PtC 2 H 5 , one can see that the reaction pathways of the direct C-C activation via 3 TS16 and 3 TS13 are inferior to those of the σ-complex assisted C-C σ-bond metathesis via 1 TS17 and 1 TS14 from 3-HPtC 3 H 7 and 1-HPtC 3 H 7 , respectively, because of their higher EHHP (156.1 vs. 65.5 and 61.2 kJ·mol −1 ). This is reminiscent of the important role of σ-complex assistance for the C-C σ-bond metathesis. In other words, the direct C-C bond activation is associated with a sizable barrier, which would prohibit this channel.
A glance to the reaction pathways shown in Schemes 1-3 reveals that two kinds of σ-complexes (1-HPtC 3 H 7 , HPtCH(CH 3 ) 2 , and 3-HPtC 3 H 7 ) and (cis-1-HPtC 3 H 7 and cis-3-HPtC 3 H 7 ) from initial C-H bond cleavage are crucial for the selective formation of the final C-H and C-C cleavage products.
To estimate quantitatively the reactivity and selectivity for the two kinds of products [PtC 3 H 6 + H 2 and PtCH 2 + C 2 H 6 ], the rate constants have been evaluated according to conventional transition state theory (TST) [50], including tunneling correction based on Winger's formulation [51]. The formation of rate constant (T) k including tunneling correction coefficient ( ) T κ in transition state theory is given by The rate constant ( ) k T ′ is simply given by where k B is the Bolzmann constant, h is the Planck constant, T is thermodynamic temperature, c 0 is standard concentration, and ΔG ≠ is Gibbs free energy. The tunneling correction coefficient κ(T) is written in the form of ( ) where k B is the Bolzmann constant, h is the Planck constant, T is thermodynamic temperature, and v ≠ is the imaginary frequency of the unbound normal mode at the saddle point. The branching ratio (α i ) of product i is calculated by where k i is the rate constant of product i. From the identical reactants Pt + C 3 H 8 , the formation of PtC 3 H 6 + H 2 and PtCH 2 + C 2 H 6 are competitive, while their selectivity-controlling steps are Pt( 3 D) + C 3 H 8 → 3 TS1→ 3 1-HPtC 3 H 7 and Pt( 3 D) + C 3 H 8 → 3 TS9→ 3 cis-3-HPtC 3 H 7 on their MERPs, respectively. Thereby, the rate constants were taken into account, where Pt( 3 D) + C 3 H 8 were taken as reactants, while 3 TS1 and 3 TS9 served as TSs, respectively. The rate constants for the formation of 3 1-HPtC 3 H 7 (k 1 ) and 3 cis-3-HPtC 3 H 7 (k 2 ) calculated over 300-1100 K temperature range can be fitted by the following expressions (in dm 3 ·mol −1 ·s −1 ): 8 The branching ratios for the formation of PtC 3 H 6 + H 2 and PtCH 2 + C 2 H 6 are calculated to be 97.7~83.6% and 2.3~16.4%, respectively, over 300-1100 K temperature range. In other words, the dehydrogenation channel is predominant, and the deethanization channel is minor, while the demethanation channel is ruled out.

Activation Strain Analysis of the direct C-H and C-C Bond Cleavage
To gain insight into how the Pt atom affects the activation barriers of the initial C-H and C-C bond cleavage, i.e., insight into how this effect depends on the nature of concomitant geometrical deformation and electronic structures of Pt and C 3 H 8 , the trends in reactivity and competition among the initial C-H and C-C bond mechanisms are analyzed using the activation strain model of chemical reactivity [30,31]. In this model, activation energies ΔE ≠ of the TS are divided into the activation strain ΔE ≠ strain and the stabilizing TS interaction ΔE ≠ int : ΔE ≠ = ΔE ≠ strain + ΔE ≠ int . The activation strain ΔE ≠ strain is the strain energy associated with deforming the reactants from their equilibrium geometry to the geometry they adopt in the TS. The TS interaction ΔE ≠ int is the actual interaction energy between the deformed reactants in the TS [30,31].
The results of the activation strain analysis are listed in Table 1. The activation energy ΔE ≠ increases from the initial C-H cleavage TSs ( 3 TS1, 3 TS2, 3 TS5, 3 TS9, and 3 TS8) of 60~70 kJ·mol −1 to the initial C-C cleavage TSs ( 3 TS13 and 3 TS16) of ~160 kJ·mol −1 . The activation strain ΔE ≠ strain decreases from the intial C-H cleavage TSs ( 3 TS1, 3 TS2, 3 TS5, 3 TS9, and 3 TS8) of 290~300 kJ·mol −1 to the initial C-C cleavage TSs ( 3 TS13 and 3 TS16) of ~200 kJ·mol −1 . The activation strain appears to be related to the bond strength of the activated bond and with the percentage-wise extent of bond stretching in the TS. Typical strengths and lengths of the C-H and C-C bonds are the following: 414 (C-H) and 347 kJ·mol −1 (C-C), and ~1.1 (C-H) and ~1.5 Å (C-C) [52]. Moreover, we recall that the percentage-wise extent of bond stretching in the TS for oxidative insertion is 70~80% (C-H) and ~30% (C-C). Hence, both the bond strength and the percentage-wise bond elongation in the TS decreases from C-H to C-C. This correlates nearly with the activation strain ΔE ≠ strain , which decreases in the same order. It is indicated that the activation strain prefers the C-C oxidative insertion over the C-H oxidative insertion. That is to say, the activation strain ΔE ≠ strain makes in the reverse order as the activation energy ΔE ≠ , for the C-H and C-C oxidative insertion. Alternatively, the strength of the TS interaction increases from ~−50 kJ·mol −1 (C-C) to −220~−240 kJ·mol −1 (C-H). From C-C to C-H bond activation, the strengthening of the TS interaction varies among 170-190 kJ·mol −1 , whereas the activation strain changes only by ~90 kJ·mol −1 . Thereby, the lowering activation energy from C-C to C-H bond activation stems mainly from the strengthening of the TS interaction. Thus, the stabilizing interaction ΔE ≠ int prefers C-H oxidative insertion, whereas the activation strain ΔE ≠ strain favors C-C oxidative insertion. From C-C to C-H oxidative insertion, the lowering of activation barrier is mainly caused by the TS interaction ΔE ≠ int becoming more stabilizing.

Conclusions
The reaction mechanism of the gas-phase Pt atom with C 3 H 8 has been systematically investigated on the singlet and triplet potential energy surfaces. Considering the initial interaction of Pt atom with C 3 H 8 , Pt atom prefers the attack of primary over secondary C-H bonds in propane. The major and minor reaction channels lead to the dehydrogenation products PtC 3 H 6 + H 2 and the deethanization products PtCH 2 + C 2 H 6 , respectively, whereas the possibility to form the demethanation products PtC 2 H 4 + CH 4 is so small it can be neglected. Over the 300-1100 K temperature range, the branching ratios for the formation of PtC 3 H 6 + H 2 and PtCH 2 + C 2 H 6 are calculated to be 97.7%~83.6% and 2.3~16.4%, respectively. The MERP for the formation of the main products PtC 3 H 6 + H 2 , involving one spin inversion, prefers to start at the triplet state and afterward proceed along the singlet state. The optimal C-C bond cleavages are assigned to C-H bond activation as the first step, followed by cleavage of a C-C bond. This reactivity mode is complementary for the classical reactivity picture through the direct C-C cleavage intermediate.
Furthermore ] are thermodynamically preferred. However, the C-H insertion intermediates are kinetically favored over the C-C insertion intermediates. These results are in qualitative agreement with the experimental results, in which the C-H insertion product is experimentally observed and the C-C insertion product is not formed in observable quantity in Pt + C 2 H 6 system.
Unlike the role of the activation strain ΔE ≠ strain , the stabilizing interaction ΔE ≠ int favors the initial C-H oxidative insertion over the initial C-C oxidative insertion. From C-C to C-H oxidative insertion, the lowering of activation barrier is mainly caused by the TS interaction ΔE ≠ int becoming more stabilizing.