Effect of Ancillary Ligands (A) on Oxidative Addition of CH4 to

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Effect of Ancillary Ligands (A) on Oxidative Addition of CH to Re(III) Complexes: A = B, Al, CH, SiH, N, P Using MP2, CCSD(T) and MCSCF Methods Riffat Parveen, and Thomas R. Cundari J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b04732 • Publication Date (Web): 26 Jun 2017 Downloaded from http://pubs.acs.org on June 29, 2017

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The Journal of Physical Chemistry

Effect of Ancillary Ligands (A) on Oxidative Addition of CH4 to Re(III) Complexes: A = B, Al, CH, SiH, N, P Using MP2, CCSD(T) and MCSCF Methods

Riffat Parveen† and Thomas R. Cundari†*

†

Department of Chemistry and Center of Advanced Scientific Computing and Modeling,

University of North Texas, 115 Union Circle, #305070, Denton, TX 76203-5017, United States

1

ACS Paragon Plus Environment


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ABSTRACT. A computational study of oxidative addition (OA) of methane to Re(OC2H4)3A (A = ancillary ligand, which thus may interact with the metal) was carried out. The choice of ancillary ligands has been made based on their electronic properties: A = B or Al (Lewis acid), CH or SiH (electron precise), N (σ-donor) and P (σ-donor/π-acid). The main objective of this study was to understand how variation in A affects the structural and electronic properties of the reactant d4-Re(III) complex, which can ultimately tune the kinetics and thermodynamics of OA. Results obtained from MP2 calculations revealed that for OA of CH4 to Re(OC2H4)3A, the order of ΔG≠ for a choice of ancillary ligand is B > Al > SiH > CH > N > P. Single point calculations for ΔG≠ obtained with CCSD(T) showed excellent agreement with those computed with MP2 methods. MCSCF calculations indicated that oxidative addition transition states are well described by a single electronic configuration, giving further confidence in the MP2 approach used for geometry optimization and ΔG≠ determination, and that the transition states are more electronically similar to the d4 Re(III) reactant than the d2-Re(V) product.

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Introduction One of the most challenging tasks in the field of chemistry is the development of efficient catalytic routes to alkane functionalization. Light alkanes are extremely nonreactive toward nucleophiles and electrophiles1 because of the high bond dissociation energies2,3 (typically 90 100 kcal/mol) of their Csp3-H bonds and their very low acidity (pKa ~ 50 - 60) and basicity. As alkanes are very poor bases (nucleophiles), they only weakly coordinate to metals. In fact, in some cases alkane coordination is the rate determining step in overall C-H activation process.4,5 Another substantial challenge encountered in alkane functionalization is selectivity as in general C-H bond dissociation energies follow the order: H3C-H > R(H)2C-H > R2(H)C-H > R3C-H. Selectivity of C-H bond cleavage can, however, be controlled by transition metal mediated C-H activation, including examples of selective activation of a stronger C-H bond in preference to a weaker bond.6 The C-H activation reaction is a fundamental step in the functionalization of inert alkanes and the incorporation of this reaction into catalytic processes is an important goal.6,7 The fundamental economic significance of these processes has led to many theoretical8-12 and experimental13,14 studies that have been carried out in search of highly active catalysts. There are number of transition metals complexes (Rh,4,14-18 Ir,19-21 Pt,22-24 Os,25,26 Re,27,28) that can perform C-H activation. A C-H bond in alkane can be functionalized by transition metal complexes via various mechanisms including sigma-bond metathesis, oxidative addition, electrophilic substitution, 1,2-CH addition and variants of these mechanisms.8,29 Oxidative addition (OA) reactions are common for low valent, electron rich complexes, generally of the later transition metals, for example Re, Fe, Ru, Rh, Os, Ir and Pt. In these reactions, the reactive species is coordinatively and electronically unsaturated and therefore almost always unstable.8 Oxidative 3



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addition is a metal-directed process. Initially, the metal acts as an acid to extract electron density from the σCH of the substrate and then as a base to populate the σ* of the C-H bond. Thus, both ends of the C-H bond are metal-ligated upon oxidative addition.30 Quantitative analysis of directional charge transfer stabilization energies between the metal-ligand fragment and the coordinated C-H bond of methane in a transition state for C-H bond cleavage reveals a continuum of electrophilic, amphiphilic and nucleophilic interactions. This classification of C-H activation reaction is based upon the net direction of charge transfer energy stabilization.31 Bergman et al. in 198527 reported the direct observation of intermolecular oxidative addition of C-H bonds in alkane to Re-complexes leading to the formation of metal-hydrido(alkyl) complexes; these type of reactions were previously observed only with iridium and rhodium complexes.4,19-20,32-34 But in contrast to Ir and Rh complexes, Cp and Cp* rhenium complexes are capable of both intermolecular C-H activation (complexes 3 - 4, 6 - 7 in Scheme 1) and intramolecular attack on C-H bonds in PMe3 ligands through cyclometalation (complex 5, Scheme 1). Moreover, the efficiency of oxidative addition depends greatly upon the nature of ligands attached to the metal center. As shown in Scheme 1, they reported that photolysis of the cyclopentadienyl-tris(trimethylphosphine)rhenium in benzene or cyclopropane resulted in the corresponding products of C-H oxidative addition (complexes 6 and 7). Similarly, irradiation of the same starting complex in n-hexane at -30 oC produces a C-H activation product (complex 3). The complexes are highly selective toward different type of alkanes, which allows one to activate useful hydrocarbons such as methane in the presence of large quantity of higher hydrocarbons.26

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H (C5H5)(PMe3)2Re

(n-C6H13)

3a

H

Cp'L1L2Re n-hexane

CH3

4a,c

ne

exa

h /c-

H

CH 4 -PMe3

Re(PMe3)3 1

hv

[Cp'ReL1L2]

Cp'L1Re

CH2

PMe2

2

5a,b,c

c-C3H6

H a series: Cp' = C5H5: L1 = L2 = PMe3 b series: Cp' = C5Me5; L1 = CO, L2 = PMe3 c series: Cp' = C5H5; L1 = L2 = PMe3

Cp'L1L2Re

H 6a,b,c

Cp'L1L2Re

7a,b,c

Scheme 1: Intermolecular and intramolecular oxidative addition of C-H bonds to Re-complexes observed by Bergman et al.27 Adapted with permission from reference 27. Timothy et al.35 described the irradiation of CpRe(PMe3)3 in cyclohexane at 5 - 10 oC under 25 atm of CH4 to produce CpRe(PMe3)3(H)(CH3) as the major product. The proposed intermediate in this reaction was a 16-electron Re fragment CpRe(PMe3)2. Later in 1986, Jones and Maguire28 also reported the activation of methane by a rhenium complex. As an analogue to the known complexes of Ir and Rh, (C5Me5)Re(PMe3)H2 undergoes oxidative addition of alkane after losing H2 upon irradiation. However, contrary to the behavior of Rh and Ir complexes, the CpRe(PPh3)2H2 loses PPh3 upon irradiation,36,37 perhaps due to the trans disposition of the hydride ligands. However, the intermediate species that is responsible for alkane oxidative addition in this case must contain at least one H ligand, making it different from the one that Bergman et al.27 reported. The presence of two hydride ligands suggests that an even electron intermediate is involved, suggesting that a Re(III)/Re(V) couple is responsible for the alkane activation reported by Jones and Maguire. For C-H activation by Re(PPh3)2H7, Felkin38-40 also proposed that a Re(III)/Re(V) couple is responsible for the process.

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In 2002, a computational investigation of C-H activation with Re-complexes that possess scorpionate (i.e., Tp (tris(pyrazolyl)borate) and Cp (cycopentadienyl) ligands was reported by Bergman, Gunnoe, Cundari and their coworkers.41 The model tris(azo)borate ligand

(Tab:

[HB(N=NH)3]-) was used as an alternative to the parent Tp ligand. It is observed that OA of CH4 to CpRe(CO)2 was exothermic, while the same reactions for TpRe(CO)2 and (Tab)Re(CO)2 are endothermic. The reactivity differences between Cp and Tp were attributed to steric effects. Following earlier work42 on the effect of ancillary ligands on oxidative addition of methane to TaIII, in this study we systematically examine the oxidative addition of methane to ReIII organometallic complexes with the goal of understanding the effect of the ancillary ligand and metal ion on the reactivity of these complexes through computational studies. Results from the previous study showed that having an electron-deficient moiety as the ancillary ligand (A) such as B or Al, raises the activation energy barrier for oxidative addition of CH4 to the TaIII reactant complex Ta(OC2H4)3A and an electron-donating group (like N or P) lowers the free energy barrier. It was also concluded that the oxidative addition energy barrier is mostly effected by a change in the HOMO energy of the reactant complex, as the reactant complexes with lowest energy HOMO have the highest activation barrier and vice versa.

Computational Methods The Gaussian 09 software package43 was used for geometry optimization and frequency calculations. Reaction and activation free energies were calculated at the unrestricted secondorder Møller-plesset44,45 (UMP2) level of theory in combination with the CEP-31G46-48 pseudopotential/valence basis set for Re while for all other elements the 6-31+G(d)49,50 basis sets were used. Calculations for geometry optimization were performed in the gas phase; MP2 single

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point calculations were conducted in acetone (ɛ = 20.7) using a continuum solvent model (SMD)51 with the larger basis set on main group elements 6-311++G(d,p). Coupled cluster calculations with single and double excitations and perturbative triples, CCSD(T),52-57 single point calculations were also carried out in the gas phase with same basis set used for UMP2 optimizations. Test CCSD calculations with both 6-31+G(d) and 6-311++G(d,p) basis sets suggested a small impact from this change. All species were modeled as neutrals and free energies calculations were performed at 298.15 K and 1 atm and are reported in kcal/mol. According to the energy Hessian all the stationary points and transition states are defined by having zero and one imaginary frequency, respectively. For reactants, transition states and products all possible spin states were analyzed. Multiconfiguration self-consistent field (MCSCF)58 calculations have been performed for MP2-calculated oxidative addition TSs for each Re(OC2H4)3A complex using the GAMESS59,60 software package. In order to understand the impact of different methods used for computational study on the free energies of reactions, calculations for reactions of Ta(OC3H4)3A using UMP2 were performed and compared with the reported results42 obtained for oxidative addition (OA) of CH4 by Ta(OC2H4)3A using DFT methods. Therefore, one can also investigate the effect of metal on the OA of CH4 to M(OC2H4)3A complexes by comparing the results obtained for (M = Re and Ta) with UMP2 methods.

Results and Discussion 1. Reactant Complexes In addition to methane the reactant complex for the present study is Re(OC2H4)3A, where A may act as an ancillary ligand (and thus interact with the metal), A = B, Al, CH, SiH, N, P. The

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choice of ancillary ligands have been made on the basis of their electronic properties: A = B or Al (Lewis acid), CH or SiH (electron precise), N (σ-donor) and P (σ-donor/π-acid). The main objective of the present study was to understand how variation in A affects the structural and electronic properties of the reactant Re complex, which can ultimately tune the kinetics and thermodynamics of oxidative addition. The ReIII reactant complexes have a d4 electronic configuration and the three oxygen are attached to Re in a nearly trigonal planar fashion; A is located almost on the opposite side of the polycyclic ring across from Re (Scheme 2). The calculated sum of the three O-Re-O angle for the reactant complex with B, Al, SiH and N is ~ 358o, suggesting that the arrangement of three oxygen around Re is trigonal planar but for CH and P the sum of angles are 347.6o and 352.7o, indicating a slightly greater distortion towards a pyramidal coordination geometry. O

?

Re

O O

A

Scheme 2 For reactant complexes, all plausible spin states (singlet, triplet and quintet) were calculated. No significant difference have been observed in geometries of Re reactants with different ancillary ligands, except that the optimized structures showed variation in Re-A distances ranging from 2.05 Å (N) to 3.27 Å (P), Table 1.

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Table 1. Comparison of Re-A distances (Å) in reactants for different spin states with the sum of Re-A covalent radii (Å)61 and van der Waals radii (Å).62 (values in red represent the ground spin state) A

Al B SiH CH P N

Re-A distance reactant singlet 2.57 2.21 2.95 3.15 3.44 2.05

in Sum of ReA covalent radii triplet quintet 2.58 2.84 2.77 2.22 2.70 2.41 2.95 3.13 2.70 3.12 3.04 2.36 3.41 3.27 2.65 2.29 2.32 2.34

Sum of Re-A van der Waals radii 3.8963 3.9763 4.15 3.75 3.85 3.60

For Re(OC2H4)3Al the triplet was calculated to be the ground state by 11.5 and 14.3 kcal/mol relative to the singlet and quintet, respectively. The Al-Re distance for the triplet is 2.58 Å, which is 0.01 Å greater than that of the singlet; however, the quintet has a far greater Al-Re distance (2.84 Å) and in contrast to other states it shows Al-O interaction (the Al-O distance in the quintet is 1.97 Å). As shown in Table 1, the Re-Al distance in the triplet ground state is very close to the sum of Re and Al covalent radii (Δr = 0.19 Å). As for A = Al, for Re(OC2H4)3B, the triplet was found to be the ground state by 11.3 and 30.2 kcal/mol as compared to singlet and quintet states, respectively. The Re-B distance (2.22 Å) in the ground state optimized structure when compared with the estimated sum of Re and B covalent and van der Waals radii showed that the Re-B interaction is closer to the covalent limit, Table 1; perhaps more importantly, after Re-N, the Re-B distance is the shortest among all Re-A distances in the ground state reactant complexes. Like A = B and Al, Re(OC2H4)3SiH was predicted to have a triplet ground state (12.8 and 3.6 kcal/mol more stable as compared to singlet and quintet states, respectively). This complex also showed an increasing trend in Re-Si distance from lower to higher multiplicity. The Re-Si

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distance in the triplet reactant is ~ 2.95 Å, which is much closer to the sum of Re and Si covalent radii (2.70 Å) as compared to their van der Waals radii sum (4.15 Å). Re(OC2H4)3N is the only reactant for which the singlet state was computed to be most stable: triplet and quintet are higher by 8.90 and 0.11 kcal/mol, respectively. The Re-N distances increase with increase in spin state and for the ground state the calculated Re-N distance of 2.05 Å is even shorter than the sum of Re and N covalent radii, 2.34 Å. For Re(OC2H4)3P and Re(OC2H4)3CH, quintets were found to be the most stable multiplicities as compared to singlets (by 18.1 and 18.3 kcal/mol for CH and P, respectively) and triplets (by 6.1 kcal/mol for both CH and P). Interestingly, for both reactants the Re-A (A = P and CH) distance decreases from 3.44 and 3.15 Å (for singlet) to 3.27 and 3.04 Å (for quintet) in the P and CH complexes respectively. The comparison of Re-P/CH distances in the quintet ground state with the corresponding sums of covalent and van der Waals radii suggested intermediate character to the Re-P/CH bonds. The Re-P distance in the reactant complex is highest among all Re-A complexes, then followed by the Re-CH distance. The optimized structures, therefore, suggest appreciable bonding between Re and Al, B, N, SiH, indicating the possibility of electronic interaction between the metal and ancillary ligands and presumably impact of the latter upon the reactivity at the rhenium center.

2. Oxidative Addition Re(OC2H4)3 activates the methane C-H bond through a three centered transition state (TS) Re--C--H as expected for oxidative addition reactions, which results in the formation of two coordination isomers for the penta-coordinated d2-ReV products Re{(OC2H4)3A(CH3)(H)}: one with the H ligand trans to A and the other one with the CH3 trans to A. Based on the position of

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H/CH3 (either axial or equatorial), two types of TSs are possible for each reaction type. Based on a previous study conducted for TaIII complexes,42 only the kinetically more feasible TSs, i.e., that is with H trans to A were considered in this present study. In order to accommodate the new H and CH3 ligands, the Re coordination geometry changed from distorted trigonal planar to distorted trigonal bipyramid in both the TSs and products with two oxygen atoms occupying equatorial coordination sites, while the third oxygen occupied an axial position. Depending upon the type of ancillary ligand present, the Re-A distance and A-Oaxial distance showed significant variation. All TSs and products involved in the studied OA reactions are most stable in the triplet state regardless of the ancillary ligand. Results for the oxidative addition of methane to ReIII are discussed below, starting from electron deficient ancillary ligands and proceeding to electron rich ancillary ligand (2p elements followed by 3p elements for each type of A).

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Re

Ta

B

53.0

50.8

Al

32.3

36.8

CH

27.5

27.8

H3C O O M

∆G

31.3 26.8

N

24.3

25.4

P

20.7

19.2

H

O

A SiH

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∆G Re

Ta

B

24.4

7.9

Al

8.3

-11.1

CH

-1.3

-29.9

SiH

7.4

-21.0

N

-11.3

-30.2

P

4.0

-31.4

0 O O M O

+ CH4

A

H O M CH3 O O A

Scheme 3. The calculated free energy (kcal/mol) profile for oxidative addition of CH4 to M(OC2H4)3A: where M = Re and Ta and A = B, Al, CH, SiH, N, P. Oxidative Addition Transition State for A = B A transition state that leads to the formation of Re{(OC2H4)3B(CH3)(H)} with H trans to B (1) was located with an imaginary frequency of 586i cm-1, which corresponds primarily to C-H bond breaking (formation for the microscopic reverse). The free energy barrier for this transition state is 53.0 kcal/mol relative to separate reactants. This value of activation energy (ΔG≠) is the highest among all the Re reactants investigated in this study.

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The structural parameters revealed that as compared to reactants, the Re-B distance greatly increased in the TS (2.22 to 2.86 Å). However, in contrast to products (which are discussed below), the transition state showed no apparent interaction between B with the axial oxygen, although the distance (2.65 Å) is slightly less than the B-O distance in the reactant (2.78 Å), Figure 1.

1

2

Figure 1. Optimized geometries of: oxidative addition transition state for Re{(OC2H4)3B(CH3)(H)} with H trans to B (1) and the product of the oxidative addition of CH4 to Re(OC2H4)3B (2); distances are given in Å. Oxidative Addition Product for A = B As shown in Scheme 3, the oxidative addition of methane to Re(OC2H4)3B is calculated to be endothermic by 24.4 kcal/mol for product with H trans to B (2). The free energy (ΔG) value is calculated relative to separate reactants. When compared with all other Re-A reactant complexes in this study, this OA is the most endergonic. The structural parameters of the product showed that upon conversion of the TS to products, the Re-B distances increased from 2.86 to 3.04 Å. However, interestingly the B and axial oxygen distance decreases drastically from 2.65 to 1.61 Å, which is indicative of strong B-O interaction. This is also consistent with the Lewis acid character of B, as in OA products the ReV is d2 thus not as electron rich as in the reactant state (d4). Therefore, the Lewis acidic B site 13



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prefers a stronger interaction with oxygen as indicated by a gradual decrease in B-O distance and gradual increase in Re--B distance along the reaction coordinates.

Oxidative Addition Transition State for A = Al For the TS (3) with H at the trans position relative to Al (Figure 2), the imaginary vibrational frequency corresponds to C-H bond breaking and is 1013i cm-1. This TS is 32.2 kcal/mol uphill in free energy relative to reactants at 298.15 K and 1 atm. When compared with the same type of TS obtained for B, this TS is (ΔΔG≠ (B-Al)) 20.8 kcal/mol more favorable in free energy, but still the second highest calculated activation energy barrier among all reactions studied here. Along the reaction coordinated from reactant to TS, the Al-Re distance increases from 2.58 to 3.05 Å. Meanwhile, Al bonds to the axial oxygen atom with a short distance of only 1.93 Å, Figure 2. This is contrary to the analogous transition state with A = B, for which B showed no interaction with the axial oxygen.

3 4 Figure 2. Optimized geometries of: oxidative addition transition state for Re{(OC2H4)3Al(CH3)(H)} with H trans to Al (3) and the product of the oxidative addition of CH4 to Re(OC2H4)3Al (4); distances are given in Å.

Oxidative Addition Product for A = Al

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The oxidative addition of methane to Re(OC2H4)3Al results in the formation of two coordination isomers having a distorted trigonal bipyramid geometries. The relative free energy of isomer with H trans to Al (4) is 8.3 kcal/mol. As the free energy value indicates, just like A = B the oxidative addition reaction for A = Al is an endergonic process (Scheme 3). However, the free energy for A = Al is significantly less endergonic than that of B, ΔΔG(B-Al) = 16.1 kcal/mol. Structural comparison showed that the Re-Al distance increases from TS (3.06 Å) to product (3.25 Å), Figure 2, while the Al-O distance slightly decreases (1.90 Å), which is analogous to the B-O distance variation as trivalent Al is also a Lewis acid.

Oxidative Addition Transition State for A = CH The free energy barrier for OA of methane to Re(OC2H4)3CH is 27.5 kcal/mol (versus separate reactants) for the TS (5) with H trans to CH (Figure 3) with the corresponding imaginary frequency of 709i cm-1, Scheme 3. The energy barrier is lower as compared to Group 13 ancillary ligands (B, Al): ΔΔG≠ (B-CH) = 25.5 and ΔΔG≠ (Al-CH) = 4.7 kcal/mol, indicating a significant decrease upon moving from Group 13 to 14 across the second period.

5

6

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Figure 3. Optimized geometries of: oxidative addition transition state for Re{(OC2H4)3CH(CH3)(H)} with H trans to CH (5) and the product of the oxidative addition of CH4 to Re(OC2H4)3CH (6); distances are given in Å. Oxidative Addition Product for A = CH As Scheme 3 depicts, the oxidative addition of methane to Re(OC2H4)3CH is exothermic with a free energy of -1.3 kcal/mol for the product with H trans to CH (6). When calculated ΔG value for CH4 oxidative addition for A = Al, B was compared with A = CH the results indicated that OA is more exothermic for the latter by 9.6 and 25.7 kcal/mole, respectively, for the products with H trans to A. Both thermodynamic and kinetic data therefore, indicated that OA is more favorable for A = CH as compared to A = Al and B. In moving from TS to product CH--Re distance further increases (3.39 to 3.63 Å), Figure 3. However, the CH--Oaxial distance decreases from the TS (2.98 Å) to product (2.90 Å).

Oxidative Addition Transition State for A = SiH The

free

energy

barrier

for

the

OA

of

CH4

to

Re(OC2H4)3SiH

to

form

Re{OC2H4)3SiH(CH3)(H)} with H trans to SiH (7) is 31.3 kcal/mol relative to reactants with the corresponding imaginary frequency of 749i cm-1, Scheme 3. There is a 1 kcal/mol decrease in activation energy barrier upon moving across the period from Al (Group 13) to SiH (Group 14), which is consistent with the trend but much less significant as compared to stabilization of the TS achieved while moving from B (Group 13) to CH (Group 14) ΔΔG≠ (B-CH) = 25.5 kcal/mol. However, comparison of ΔG≠ within Group 13 and Group 14 has the opposite trend as there is a notable decrease in the activation energy barrier from B to Al as ΔΔG≠ (B-Al) = 20.8 kcal/mol in contrary to Group 14 which showed slight increase in ΔG≠ as ΔΔG≠ (CH-SiH) = -3.8 kcal/mol. The optimized structure of the TS (7) showed that the Re--Si distance is 3.4 Å, which is

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significantly greater than corresponding distances in reactant (2.95 Å) indicating no apparent interaction between Re--SiH in TS, Figure 4.

7 8 Figure 4. Optimized geometries of: oxidative addition transition state for Re{(OC2H4)3 SiH(CH3)(H)} (7) and for the products of the oxidative addition of CH4 to Re(OC2H4)3SiH (8); distances are given in Å. Oxidative Addition Products for A = SiH Like A = Al and B, the reaction is overall endergonic for the formation of product with H trans to SiH (8) (ΔG = 7.4 kcal/mol); this value for ΔG values is comparable to A = Al but much less than A = B. Thermodynamic and kinetic data showed that overall OA with A = SiH (Scheme 3) is somewhat less feasible than CH. The Re--SiH distance increases as the reaction proceeds from the TS toward the product formation and as for the TS there is no apparent interaction between Si and the axial oxygen in the product.

Oxidative Addition Transition State for A = N For A = N, the transition state with H trans to N was located with a free energy barrier of 24.3 kcal/mol (9), Scheme 3. The imaginary frequency for this transition state was 1152i cm-1. So far among the all oxidative additions discussed, this reaction with A = N is kinetically the most favorable reaction as ΔG≠ (B-N) = 28.7 kcal/mole, ΔG≠ (Al-N) = 7.9 kcal/mol, ΔG≠ (CH-N) = 3.2 kcal/mol and ΔG≠ (SiH-N) = 7.0 kcal/mol. These values reveal that the trend of decreasing

17



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in activation barrier persists across the period in moving from CH to N, however, it is not as significant as was calculated from B to CH. Structural parameters of the transition states revealed that the N-Re distance increases along the reaction coordinate from reactant (2.05 Å) to TS (2.29 Å), Figure 5, which is less of an increase as compared to the previous reactions. Furthermore, this is the shortest Re--A distance among all TSs suggesting a strong covalent interaction between Re and N (Table 1).

9 10 Figure 5. Optimized geometries of: oxidative addition transition state for Re{(OC2H4)3N (CH3)(H)} (9) the products of the oxidative addition of CH4 to Re(OC2H4)3N (10); distances are given in Å. Oxidative Addition Product for A = N For the Re complexes with N as the ancillary ligand Re(OC2H4)3N, methane OA is calculated to be exothermic by 11.3 kcal/mol for the product with H trans to N (10). This is the only reaction that is highly exothermic for the formation of product (Scheme 3) and also thermodynamically the most favorable among all candidates for ancillary ligand investigated, with a thermodynamic preference of 35.7, 19.6, 18.7, 10 kcal/mol over B, Al, SiH, and CH respectively.

18


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The optimized structure of the product showed that Re-N and N--Oaxial distances slightly decrease but are still comparable to those observed in the TS. This is the only case that did not show large variation in the Re--A and A--Oaxial distance upon conversion from TS to product. A CCDC ConQuest64 search for experimental values of Re-N bond showed the mean value of 2.15 ± 0.08 Å (for 1646 samples). Comparison of experimental Re-N bond distance with that of Re complexes with N as ancillary ligands suggest the strong interaction between Re and N.

Oxidative Addition Transition State for A = P The transition state (11) corresponding to the product Re{(OC2H4)3P (CH3)(H)} with H trans to P (Figure 6) is located with an imaginary frequency of 950i cm-1 which corresponds to C-H bond making/breaking. The free energy barrier for this TS was 20.7 kcal/mol, Scheme 3, which indicated the greater kinetic feasibility of this reaction over other methane oxidative additions studied: ΔG≠ (Al-P) = 11.5 kcal/mole, ΔG≠ (B-P) = 22.3 kcal/mol), ΔG≠ (CH-P) = 7.8 kcal/mol and ΔG≠ (SiH-P) = 10.6 kcal/mol and ΔG≠ (N-P) = 3.6 kcal/mol. The optimized TS geometry indicated that the Re--P distance increased as compared to reactant by 0.4 Å. In TS the P--Oaxial distance is 3.35 Å which is the largest A--Oaxial distance among all TS discussed. Oxidative Addition Product of A = P As shown in Scheme 3, the oxidative addition of methane to Re(OC2H4)3P is endergonic: ΔG = 4.0 kcal/mol for the product with H trans to P (12). The OA of Re complexes for A = P is thermodynamically more favorable by 4.3, 20.4 and 3.4 kcal/mol versus A = Al, B and SiH respectively. However, it is less favorable by 5.3 and 15.5 kcal/mol as compared to A = CH and N. The optimized structure of the product showed that the P--Oaxial distance increase from 3.35 (in TS) to 3.72 Å (in product). 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11

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12

Figure 6. Optimized geometries of: oxidative addition transition state for Re{(OC2H4)3P (CH3)(H)} (11) and the products of the oxidative addition of CH4 to Re(OC2H4)3P (12); distances are given in Å.

3. MCSCF Calculations for Re Complexes For the transition states involved in oxidative addition of methane to Re(OC2H4)3A complexes, MCSCF calculations have been performed. The end points, i.e., reactant and product complexes can be reasonably described by a wavefunction that corresponds to a single Lewis structure. However, transition states often must be described with a more complex wavefunction in which several different arrangements of the electrons must be taken into consideration. Choosing the active space is the most important step in MCSCF calculations.58 As the formally ReIII ion in the reactant complexes has a d4 configuration, the MCSCF wavefunction has been calculated in order to determine the nature of the Re ion in the TSs – whether it is d4 (more similar to ReIII reactants) or d2 (more like ReV products). MCSCF calculations have been performed at all MP2-optimized TSs. The active space for these calculations consists of 6 active electrons and ten active orbitals after experimenting with different active space sizes. The six active electrons allow correlation of all metal-based d electrons and orbitals and any lone pair (if present) on the ancillary ligand. The results of the MCSCF calculations (natural orbitals for

20


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oxidative addition TSs: Re{(OC2H4)3Al(H)(CH3)} are presented in Figure 7) showed the occupancies of the natural orbitals that were chosen as a part of active space to be close to expected integer values of 2, 1 or 0 e-. These calculations indicate that the methane oxidative addition transition states are well described by a single electron configuration, giving further confidence in the MP2 approach used for geometry and free energy determination. For each transition state, out of 10 active orbitals, three showed significant contribution from Re, and with the sum of electron occupancies close to d4. Based on these results it can be deduced that the rhenium in the transition states is formally ReIII, and thus the TSs are “early,” i.e., closer to reactants, in an electronic sense.

Figure 7. Strongly occupied natural orbitals (CAS (6, 10) active space) for oxidative addition TS: Re{(OC2H4)3Al(H)(CH3)} (Occupancy (from left to right) = 1.95, 0.99, 0.99 e-). 4. CCSD(T) Results for OA of CH4 to Re(OC2H4)3A As the MCSCF results support that the systems are well described by a single electronic configuration, more highly correlated wavefunction methods were thus evaluated. The MP2 method is based on a Hartree-Fock starting wavefunction. A more elaborate way is to go beyond perturbation theory. One of the most reliable approaches is coupled-cluster methods, which are

21



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among the most reliable and popular in quantum mechanics, especially CCSD(T), which is a higher-level correlation method used widely for transition metal complexes.65 Table 2 summarizes the results obtained from the CCSD(T) method using MP2-optimized stationary point geometries. These single point calculations were performed with smaller basis set (6-31+G(d)) and in the gas phase only. Tests with the larger 6-311++G(d,p) basis sets, and with and without solvent effects, using the somewhat less expensive CCSD technique, suggested that basis set effects were small and did not change much among the different A. The activation energy barriers (ΔE≠) of all six transition states corresponding to six different ancillary ligands for OA reactions obtained from the CCSD(T) method showed excellent agreement with those computed with MP2 methods. As shown in Table 2 the average absolute value difference in energy barriers between the methods, Δ(UMP2-CCSD(T)), is very small, 1.6 ± 1.0 kcal/mol. The mean signed error is essentially zero, as the higher correlation effects of CCSD(T) raised some barriers and reduced others, albeit by 2 kcal/mol or less. The kinetic feasibility for oxidative addition of methane to Re(OC2H4)3A is highest for A = P and lowest for A = B for CCSD(T) with the overall order of B > Al > SiH > N > CH > P, which is consistent with the order calculated using MP2 methods. Hence, the impact of higher levels of electron correlation is minimal, significantly less than the chemical factors induced by modification of A. Table 2. Comparison of activation energy barriers ΔE≠ (kcal/mol) obtained from UMP2 and CCSD(T) method (gas phase calculations) ΔE≠ ΔE≠ A MP2 CCSD(T) ΔΔE≠ Absolute ΔΔE≠ B 51.9 49.1 2.8 2.8 Al 34.4 32.8 1.6 1.6 CH 13.4 14.6 -1.2 1.2 SiH 32.2 32.4 -0.2 0.2 N 14.6 17.3 -2.7 2.7 P 9.1 10.0 -0.9 0.9 Ave. = -0.1 St. dev. = 2.0

1.6 1.0 22


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5. MP2 Calculations for Ta(OC2H4)3A Complexes The main emphasis of the present study was to correlate methane activation ability with molecular and electronic structure, primarily through changing the ancillary ligand in a series of complexes. Results with UMP2 and CCSD(T) methods indicated that changing A imparts a substantial effect upon the thermodynamics and kinetics of oxidative addition and the reaction can be tuned by variation of the ancillary ligand. Of course, the other obvious way to change the molecular and electronic structure of a complex is to change the central metal ion. In order to understand how changes in metal would affect the oxidative addition of methane, MP2 calculations were performed for activation of CH4 by Ta(OC2H4)3A. Table 3 summarizes the kinetics and thermodynamic data for both Ta(OC2H4)3A and Re(OC2H4)3A. Results showed that for Ta(OC2H4)3A complexes, the order of energy barrier for choice of ancillary ligand is B > Al > CH > SiH > N > P thus making Ta(OC2H4)3P the best candidate for OA of methane. The results of ΔG≠ for Ta complexes are in close agreement with those obtained for Re complex (for respective ancillary ligand). As shown in Table 3, the average absolute value difference in computed activation barriers between the ReIII and TaIII complexes, ΔΔG≠ (Re-Ta), is very small, 2.4 ± 1.8 kcal/mol for the same ancillary ligand. The average absolute difference shown by two metals with 2p ancillary ligands: B, CH, N is (1.2 ± 1.0 kcal/mol) less than with 3p ancillary ligand (3.5 ± 1.8 kcal/mol) (from same group). Overall, it can be inferred from the present results that the change in metal ion from ReIII (d4) to TaIII (d2) (while keeping the ancillary ligand same) affects the kinetics of CH4 oxidative addition less than a change of ancillary ligand does for a given

metal.

Table 3. Comparison of ΔG≠ kcal/mol and ΔG kcal/mol for oxidative addition of methane to ReIII

and TaIII complexes. A

ΔG≠(Re) ΔG≠(Ta)

ΔΔG≠ |ΔΔG≠|

ΔG(Re)

ΔG(Ta) ΔΔG

23


|ΔΔG|


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

B Al CH SiH N P

53.0 32.3 27.5 31.3 24.3 20.7

50.8 36.8 27.8 26.8 25.4 19.2

Ave. St.dev

2.2 -4.5 -0.3 4.5 -1.1 1.5

2.2 4.5 0.3 4.5 1.1 1.5

24.4 8.3 -1.3 7.4 -11.3 4.0

7.9 -11.1 -29.9 -21.0 -30.2 -31.4

2.4 1.8

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16.5 19.4 28.6 28.4 18.9 35.4

16.5 19.4 28.6 28.4 18.9 35.4 24.5 7.4

SUMMARY AND CONCLUSION The activation of methane by ReIII(OC2H4)3A (where A may act as ancillary ligand) has been studied using MP2, CCSD(T) and MCSCF methods. A primary focus of this study was to understand how changes in electronic and structural properties of reactant complex can affect their tendency to undergo CH4 oxidative addition. Additionally, the sensitivity of the predicted free energies to different levels of theory was assessed. Ancillary ligands were selected based on their electronic properties, A = B or Al (Lewis acid), CH or SiH (electron precise), N (σ-donor) and P (σ-donor/π-acid). Several important points were concluded from this research, which are summarized below. (1) Oxidative addition of CH4 to Re(OC2H4)3A is kinetically most feasible with P acting as the ancillary ligand as indicated by the lowest energy barrier (ΔG≠ = 20.7 kcal/mol) for OA among all ancillary ligands modeled. The next most promising candidate on the basis of calculated ΔG≠ is A = N (ΔG≠ = 24.3 kcal/mol). However, for Lewis acids (A = B and Al), the energy barriers for the transition states were quite high, 53.0 and 32.3 kcal/mol, respectively. (2) Calculated ΔG values for the ReV product showed that for A = CH and P the values are close to thermoneutral (ΔG = -1.3 and 4.0 kcal/mol, respectively). For A = N (σ-donor) the oxidative addition is most exergonic among all ancillary ligands used. On the other hand, for A = B (Lewis acid) the OA is most endergonic with the ΔG value of 24.4 kcal/mol. Thus, ancillary

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ligands have a substantial impact on the kinetics and thermodynamics of methane oxidative addition. (3) Oxidative addition of CH4 to Re(OC2H4)3A complexes becomes kinetically feasible with successive changes in ancillary ligand, upon proceeding from electron-deficient Al and B to electron-donor N and P across 2nd and 3rd period. The ΔΔG≠ upon moving from A = B to A = N is 28.7 kcal/mol, while ΔΔG≠ (P-Al) = 11.6 kcal/mol. (4) Generally, decreases in activation energy have been observed while moving down a periodic table group for a given choice of ancillary ligands ΔΔG≠ (B-Al) = 20.7 and ΔΔG≠ (N-P) = 3.6 kcal/mol. However, for Group 14 the trend changes and activation energy barrier increase by 3.8 kcal/mol upon moving from C to SiH. (5) In addition to d4-ReIII complexes, to assess the effect of metal ion upon oxidative addition of CH4 to M(OC2H4)3A complexes, MP2 calculations are also performed on related d2-TaIII complexes. Results showed that the order of activation energy barrier for the ancillary ligands used is: B > Al > SiH > CH > N > P, which is consistent with that obtained for ReIII complexes. Also, as Table 3 shows, the activation free energy barriers for both metals ions with same ancillary ligand are not significantly different, being within ±5 kcal/mol or less. (6) For Ta complexes, the computed thermodynamic data show that free energy values for most of the ancillary ligands used (except for A = B) are highly exergonic for the formation of TaV product, ranging from ΔG = -11.1 kcal/mol for A = Al to -30.2 kcal/mol for A = N. Oxidative addition is endergonic for A = B with ΔG = 7.9 kcal/mol). These values are in contrast with the corresponding free energy values of OA for Re complexes, which are mostly endergonic in nature, presumably reflecting stronger metal-hydride and metal-alkyl bonds for Ta versus Re. As with ReIII complexes, oxidative addition of CH4 to TaIII(OC2H4)3A complexes becomes

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kinetically more feasible with a change in ancillary ligand while proceeding from electron deficient (A = Al and B) to electron donor (A = N and P) across 2nd and 3rd period. Oxidative addition of CH4 to Ta(OC2H4)3A becomes kinetically favorable in moving down a group. (7) ΔG≠ values were plotted against ΔG values (Figure 8) for OA of methane to Re(OC2H4)3A (a) and Ta(OC2H4)3A (b) complexes. Except for P, the plots showed almost similar trend for ancillary ligands, as expected via the Hammond Postulate. (8) MCSCF calculations have been performed at all MP2-optimized TSs for Re complexes. The active space for these calculations consists of 6 active electrons and ten active orbitals. Calculations revealed that three of the active orbitals have significant contribution from Re with the corresponding sum of electron occupancy being close to d4. Based on these results it can be deduced that the rhenium in the transition states is formally ReIII, and thus the TSs are “early,” i.e., closer to reactants, in an electronic sense. (9) CCSD(T) calculations, using MP2-optimized stationary points, were performed for ReIII complexes to investigate how higher-level correlation methods affect the OA barriers. The activation energy barriers (ΔE≠) of all six transition states corresponding to six different ancillary ligands for OA reactions obtained from the CCSD(T) method showed excellent agreement with those computed with MP2 methods CCSD(T). Therefore, the impact of higher levels of electron correlation is of similar importance in comparison to the chemical factors induced by modification of the metal M (Re vs. Ta), and much less than the impact of the ancillary ligand (A) upon the kinetics and thermodynamics of methane activation.

(a)

(b)

26


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Re (ΔG≠ vs ΔG ) R² = 0.7245

ΔG≠ (kcal/mol)

60 50

Al

40 N

B

30 CH 20

P

SIH

10 0 -20

-10

0

10

20

30

ΔG (kcal/mol)

Ta(ΔG≠vs ΔG) ΔG≠ (kcal/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 R² = 0.9318 Al CH

SiH

40 30 20

N

P

B

50

10 0

-40

-30

-20

-10

0

10

ΔG (kcal/mol)

Figure 8. Plot of ΔG≠ versus ΔG for OA of CH4 to (a) Re(OC2H4)3A and (b) Ta(OC2H4)3A complexes. ASSOCIATED CONTENT Supporting Information. Cartesian coordinates of all calculated species. This material is available free of charge via the internet. AUTHOR INFORMATIOM Corresponding Author *E-mail for T.R.C.: [email protected]. The authors declare no competing financial interests. ACKNOWLEGDMENTS This research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences (Chemical Sciences, Geosciences and Biosciences Division) via grant number DE-FG0203ER15387. The authors acknowledge the National Science Foundation for their support of the UNT Chemistry CASCaM high performance computing facility through grant CHE-1531468.

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Computational Study of Methane Activation by

TpRe(CO)2 and CpRe(CO)2 with a Stereoelectronic Comparison of Cyclopentadienyl and Scorpionate Ligands. Organometallics 2003, 22, 2331-2337.

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Page 36 of 36

TOC Graphic

Re

Ta

B

53.0

50.8

Al

32.3

36.8

CH

27.5

27.8

H3C O O M A

SiH

31.3

N

24.3

25.4

P

20.7

19.2

0 O

M

O O

26.8

+ CH4

H

O

∆G

B

Re

Ta

24.4

7.9

Al

8.3

-11.1

CH

-1.3

-29.9

A

∆G H O O

O A

SiH

7.4

-21.0

N

-11.3

-30.2

P

4.0

-31.4

36


M

CH3

Effect of Ancillary Ligands (A) on Oxidative Addition of CH4 to

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