Theoretical Study of Addition Reactions of L4M (M= Rh, Ir) and L2M (M

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Article

A Theoretical Study of Addition Reactions of LM(M = Rh, Ir) and LM(M = Pd, Pt) to Li+@C 4

2

60

Ming-Chung Yang, Akhilesh Kumar Sharma, W M Chamil Sameera, Keiji Morokuma, and Ming-Der Su J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b01086 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 16, 2017

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The Journal of Physical Chemistry

A Theoretical Study of Addition Reactions of L4M(M = Rh, Ir) and L2M(M = Pd, Pt) to Li+@C60

Ming-Chung Yang1,2, Akhilesh K. Sharma2, WMC Sameera2,3, Keiji Morokuma2 and Ming-Der Su*1,4,a) 1

2

Department of Applied Chemistry, National Chiayi University, Chiayi 60004, Taiwan

Fukui Institute for Fundamental Chemistry, Kyoto University, Kyoto 606-8103,

Japan Department of Chemistry, Faculty of Science, Hokkaido University, North-10 West-8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan. 4 Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, 3

Kaohsiung 80708, Taiwan

Abstract: The addition reaction of M(Cl)(CO)(PPh3)2 (M = Rh, Ir) and M(PPh3)2 (M = Pd, Pt) fragments with X@C60 (X = 0, Li+) were characterized by density functional theory (DFT) and the artificial force-induced reaction (AFIR) method. The calculated free energy profiles suggested that the η2[6:6]-addition is the most favorable reaction, which is consistent with the experimental observations. In the presence of Li+ ion, the reaction is highly exothermic, leading to η2[6:6] product of L4IrLi+@C60. In contrast, an endothermic reaction was observed in the absence of a Li+ ion. The encapsulated Li+ ion can enhance the thermodynamic stability of the η2[6:6] product. The energy decomposition analysis showed that the interaction between metal fragment and X@C60 fragment is the key for the thermodynamic stability. Among the group IA and IIA metal cations, Be2+ encapsulation is the best candidate for the development of new fullerene-transition metal complexes, which will be useful for future potential 1

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applications such as solar cells, catalysts, and electronic devices.

a)

E-mail: [email protected]

2


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1. Introduction Since the first fullerene transition-metal complex, namely (η2-C60)Pt(Ph3)2 was prepared and structurally characterized by Fagan and co-workers,1 the fullerenes has combined with different electron donor transition metal fragments through donor-acceptor interactions. The resultant compounds can be used for potential applications, such as solar cells, spintronics, catalysis, and drug delivery.2 One of the remarkable characteristics of fullerenes is that they are capable of encaging atoms, ions,

and

small

molecules,

forming

endohedral

complexes.

Endohedral

metallofullerenes (EMFs) are those encapsulating metal atoms within the hollow carbon cage. They are also good candidates for molecular devices such as a single molecular memory3,4 Although the yield of EMFs is generally low, Li+@C60 can be macroscopically synthesized and isolated.5 The other marked nature of fullerenes is that they have several different coordination modes. For example, eight synthetic routes were proposed for the addition reactions of metal fragments with C60.6 Depending on the binding sites, the resulting complexes can be denoted as η1-coordination, η2[6:6]-coordination ([6:6] indicates the midpoint of two six-membered rings junction), η2[6:5]-coordination ([6:5] indicates the midpoint of a six- and a five-membered ring junction), η3[6:5,6:5]-coordination([6:5,6:5] indicates the fusion of two six- and five-membered rings), η3[6:6,6:5]-coordination, η4[6:6,6:6]-coordination, η5-coordination, and η6-coordination. Balch and co-workers studied the reactions of C60 with electron-rich fragments, IrCl(CO)(PPh3)2,

giving

rise

to

the

fullerene-iridium

complex

(η2-C60)IrCl(CO)(PPh3)2.7 Formation of fullerene-iridium complex is reversible, and the reversible binding of IrCl(CO)(PPh3)2 to fullerenes can be used as a structural probe because of the adducts can build ordered single crystals that are suitable for X-ray diffraction.8-9 3



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Recently, Tobita et al. have reported the structures of iridium and platinum complexes

of

lithium

cation

encapsulated

fullerene

([{η2-(Li+@C60)}IrCl(CO)(PPh3)2](PF6-) and [{η2-(Li+@C60)}Pt(dppf)])(PF6-), where dppf = 1,1′-bis(diphenylphosphino)ferrocene).5 These encapsulated complexes are identified as η2[6:6]-coordination, and the most importantly, no other types of coordination was found. It was also reported that the reaction of IrCl(CO)(PPh3)2 with Li+@C60 is irreversible.5 In other words, the iridium fullerene complex becomes more stable when Li+ is encapsulated into an empty C60. In other studies, the encapsulated metal cation was reported to affect the reactivity of the C60 in the Diels-Alder reaction.10-12 These experimental and computational findings raised our interest to study the effect of encapsulated cation in metal fullerene complexes. To the best of our knowledge, no theoretical studies were reported to show the effect of a Li+ ion on the C60-Ir and C60-Pt complexes, and the mechanism of C60-metal complex formation. In this study, we have used AFIR method to investigate reaction paths for the addition reaction of L4M (M = Ir, L4 = Cl(CO)(PPh3)2) and L′2M (M = Pt, L′2 = (PPh3)2) with X@C60 (X = 0 and Li+). AFIR method is useful to determine reaction paths, and to locate approximate transition state (TSs) and local minima (LMs) on the reaction paths. Approximate stationary points can be refined by using the standard computational methods. After studying the reaction mechanism for L4Ir and L′2Pt with X@C60 (X = 0 and Li+), we have extended our strategy to the elements in the second row transition metals; M = Rh for L4M and M = Pd for L′2M. In this study, the following reactions were investigated;

4


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IrCl(CO)(PPh3)2

+

C60

→

[{η2-C60}IrCl(CO)(PPh3)2]

(1a)

IrCl(CO)(PPh3)2

+ Li+@C60

→

[{η2-(Li+@C60)}IrCl(CO)(PPh3)2]

(1b)

+

→

[{η2-C60}RhCl(CO)(PPh3)2]

(2a)

2 + → [{η -(Li @C60)}RhCl(CO)(PPh3)2]

(2b)

RhCl(CO)(PPh3)2

C60

RhCl(CO)(PPh3)2 + Li+@C60 C60

→

[{η2-C60}Pt(PPh3)2]

(3a)

Pt(PPh3)2 + Li+@C60

→

[{η2-(Li+@C60)}Pt(PPh3)2]

(3b)

Pd(PPh3)2 +

C60

→

[{η2-C60}Pt(PPh3)2]

(4a)

+ Li+@C60

→

[{η2-(Li+@C60)}Pt(PPh3)2]

(4b)

Pt(PPh3)2

Pd(PPh3)2

+

The present paper is organized as follows. In section 3.1, we briefly presented the AFIR method to study approximate reaction paths, and then compare the calculated Gibbs free energy profiles with experimental results. In section 3.2, we expanded encapsulated ions (X) to group IA (Li+, Na+, K+, Rb+, Cs+) and IIA (Be2+, Mg2+, Ca2+, Sr2+, Ba2+) metal cations, and discussed the energy decomposition analysis (EDA) to understand the origin of the thermodynamic stability. In section 3.3, we discussed geometrical comparisons with the X-ray structures, and π back-bonding strength in section 3.4.

2. Computational details The AFIR method,13-15 as implemented in the GRRM strategy16 was used to determine the approximate TSs and LMs on the reaction pathways.17-18 For this purpose, ONIOM (M0619:PM620) method in the Gaussian0921 program was used. Partitioning of the molecular system is shown in Scheme 1. For the high-level, M06 functional was used, because this functional is suitable for kinetic and thermodynamic studies of organometallic systems.22 The SDD basis set and associate effective core 5



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potential (ECP) was applied for metal atoms,23-24 and 6-31G(d,p) basis sets were employed for the other atoms (BS1).25-28 AFIR paths were determined by adding the artificial force between the metal atom and the reactive carbon atoms as shown in Scheme 1. The artificial force parameter (γ) of 100 kJ mol-1 was used. Approximate LMs and TSs were refined with M06/BS1 method. The vibrational frequency calculations at 298.15 K and 1 atm were carried out at the same level of theory. The LMs were confirmed by zero imaginary frequencies, and TSs were characterized by one imaginary frequency. The EDA29 was performed for the optimized products, which is divided into transition metal complex (A), C60 cage (B) and metal ion (C) as shown in Scheme 2. The deformation energy (DEF) is the sum of the deformation energy of A (DEFA, defined as the energy of A in product relative to the optimized isolated structure (A0) and B (DEFB). The interaction energy term, INTA(BC), is the interaction energy between A and (BC) at their respective optimized product structures.

Scheme 1. ONIOM Partitioning (High- and Low-Layers) used for AFIR calculations between L4Ir and Li+@C60 and AFIR Artificial Force was applied between metal atom of L4Ir and the reactive carbon atoms of Li+@C60 fragment.

6


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Scheme 2. EDA for L4IrLi+@C60 3. Results and discussion 3.1 AFIR calculations We performed 8 AFIR minimizations as shown in Scheme 1. In the first search, artificial force was added between metal atom and C1, giving rise to the η1-coordination mode. In the second search, artificial force was added between metal atom and C1-C2, leading to the η2[6:6]-coordination mode. In a similar vein, η2[6:5], η3[6:5,6:5], η3[6:6,6:5], η4[6:6,6:6], η5 and η6-coordination modes were obtained from the remaining six AFIR minimizations. After refining the approximate TSs and LMs on the AFIR paths, the free energies profiles were developed. More details about the AFIR calculations can be found in supporting information.

7



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Figure 1. Free energy profile (kcal/mol) for the reaction between L4Ir and X@C60. The enthalpies are given in parentheses (italics). The black profile indicate the reaction in the absence of Li+ ion (Eqn. 1a) and red profile indicates the reaction in the presence of Li+ ion (Eqn. 1b). The abbreviations OP, 66, and 65 stand for η1-addition, η2[6:6]-addition, and η2[6:5]-addition, respectively. Free energy profiles for the reaction routes 1a and 1b are summarized in Figure 1. The optimized structures are shown in Figure 2. Key structural parameters of the stationary points are summarized in Table 1. Despite several attempts, we were unable to locate the η3-η6 types of products, where the structure optimizations ultimately converged to η1 or η2 products. The reference point of energy profile is the sum of the energy of two reactants (L4Ir and X@C60) at infinite separation. First, we discuss the reaction in the absence of Li+ ion. The reaction starts with the formation of a pre-reaction-complex (PRC), which is 2.8 kcal/mol higher in free energy than the separated reactants. Subsequent TS leads to η2[6:6] product in a single step with the overall free energy barrier of 7.1 kcal/mol. The TS involves interaction of metal and 8


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olefinic bond of C60. The distance between metal (Ir) and carbon atoms (C1 and C2) becomes shorter by ~ 0.80Å in the TS (see Figure 2). The product, Prod.(66, X = 0), is only 1.3 kcal/mol higher in energy than the separated reactants. The stationary point for η2[6:5] product or η1 product could not be located at M06 level.30 For the reaction route (1a), the activation free energy barrier for forward reaction is 7.1 kcal/mol, while the reverse process has barrier of 5.8 kcal/mol. As the activation barrier for both forward and reverse direction is small the reaction should be reversible at room temperature. The equilibrium in (1a) may be shifted towards reactants, as the Prod.(66, X = 0) is higher in free energy (1.3 kcal/mol). These results are in agreement with the experimental observations.5,7

Figure 2. Optimized geometries of the LMs and TSs for the reactions 1a and 1b.

For the reaction between L4Ir and Li+@C60 (Eqn. 1b), initial adduct (PRC) formation is exothermic, and the resultant complex is 9.2 kcal/mol stable than the separated reactants (L4Ir and Li+@C60) (Figure 1). In PRC, the metal is relatively 9



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closer to C60 ((3.22Å, 3.05Å) vs. (3.45Å, 3.40Å) in absence of Li+ ion, Table 1 and Figure 2). Formation of the Prod.(65, X = Li+) occurs in a single step. The free energy barrier for this process (TS(65, X = Li+)) is 7.0 kcal/mol, and the Prod.(65, X = Li+) is -3.5 kcal/mol lower in energy than the separated reactants. The Prod.(66, X = Li+) is however formed in a two step process.31 The first step leads to the formation of bond between metal and C2 atom of C60 (η1-coordination mode) through TS(OP, X = Li+). The free energy barrier for this step is very small (2.7 kca/mol). The resultant η1 product is energetically similar to the PRC(X = Li+). In the next step, the η1 product changes to η2[6:6] product with a barrier of 2.4 kcal/mol (TS1(X = Li+)). In TS1(X = Li+), the Ir-atom is coordinated to C2 and the Li+ is mainly interacting with C1. The Prod.(66, X = Li+) is -19.9 kcal/mol below the separated reactants. Our calculations indicated that the formation of C60-Ir complexes (η1, η2[6:6], η2[6:5]) are exergonic when Li+ is encapsulated in C60. For reaction route (1b), three possible products are located, specifically Int.(OP, X = Li+), Prod.(65, X = Li+) and Prod.(66, X = Li+). All three products are kinetically accessible at room temperature, but the most likely one is Prod.(66, X = Li+), because it is more exergonic (∆G = -19.9 kcal/mol) than Prod.(65, X = Li+) (∆G = -3.4 kcal/mol) and Int.(OP, X = Li+) (∆G = -9.4 kcal/mol). Further, the formation of Prod.(66, X = Li+) would not be reversible at the room temperature. These results are consistent with experimental observation, where only η2[6:6] product was isolated.5

10


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Table 1. Selected Geometrical parameters (bond distances in Å) and Relative Gibbs Free Energies (kcal/mol) at the M06/BS1 Level of Theory for the addition of L4Ir on X@C60. The abbreviations OP, 66, and 65 stand for η1-addition, η2[6:6]-addition, and η2[6:5]-addition, respectively.

M = Ir, X = Li+ Geometrical parameters

System ML4 + X@C60 +

PRC(X = Li )

∆G(b)

M-C1

M-C2

Li-C1

Li-C2

―

―

2.26(a)

2.26(a)

0.0

(a)

(a)

-9.2

3.22

3.05

2.36

+

―

2.65

―

2.41

-6.5

+

2.44

2.48

2.72

2.54

-2.2

TS1(X = Li )

2.89

2.36

2.69

3.02

-7.0

TS(OP, X = Li ) TS(65, X = Li ) +

+

Int.(OP, X = Li )

2.29

―

2.50

―

2.49

-9.4

+

2.24

2.24

2.68

2.50

-3.4

+

2.19

2.19

2.37

2.38

-19.9

Prod.(65, X = Li ) Prod.(66, X = Li )

M = Ir, X = 0 System

(a)

Geometrical parameters

∆G(b)

M-C1

M-C2

Li-C1

Li-C2

ML4 + X@C60

―

―

―

―

0.0

PRC(X = 0)

3.45

3.40

―

―

2.8

TS(66, X = 0)

2.65

2.59

―

―

7.1

Prod.(66, X = 0)

2.20

2.21

―

―

The distance between Li atom and nearest two carbons of C60.

1.3 (b)

The reaction

Gibbs free energy of product, relative to the corresponding reactants. Now we turn our attention into the reaction between L′2Pt and C60 (Eqn. 3a).32 The free energy profile of reaction is shown in Figure 3 (in black). Key structural parameters of the stationary points are summarized in Table 2 and Figure 4. During reaction L′2Pt and C60 approach each other and PRC(X = 0)′ is formed, which is 0.4 11



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kcal/mol above the entry point. The Prod.(66, X = 0)′ and Prod.(65, X = 0)′ are both formed in a two step process. First metal coordinates to one carbon atom (C2), leading to the η1 product with a barrier of 1.5 kcal/mol (TS(OP, X = 0)′). The η1 product (Int.(OP, X = 0) is slightly higher than the entry point (+0.7 kcal/mol). The coordination of metal with second carbon (C1) occurs in next step, which can lead the formation of either η2[6:6] or η2[6:5] products. The free energy barrier for both Prod.(66, X = 0)′ and Prod.(65, X = 0)′ are 0.5 (TS1(X = 0)′ and 0.6 kcal/mol (TS2(X = 0)′), respectively. The formation of both η2[6:6] and η2[6:5] product is exergonic. However, formation of Prod.(66, X = 0)′ is thermodynamically favorable. In contrast to the reaction of Ir-complex, formation of the Prod.(66, X = 0)′ for Pt-complex is more exothermic (-16.4 kcal/mol), and therefore not reversible.

12


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Figure 3. Free energy profiles (kcal/mol) for the reaction between L′2Pt and X@C60. The enthalpies are given in parentheses (italics). The black profile indicates the reaction in the absence of Li+ ion (Eqn. 3a), and red profile indicates the reaction in the presence of Li+ ion (Eqn. 3b). The abbreviations OP, 66, and 65 stand for η1-addition, η2[6:6]-addition, and η2[6:5]-addition, respectively.

13



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Figure 4. Optimized geometries of the LMs and TSs for Eqn. 3a and 3b.

The reaction between L′2Pt and Li+@C60 (Eqn. 3b) begins from the formation of PRC(X = Li+)′. Similar to the reaction in absence of Li+ ion, the formation of product involves two steps. The reactants come closer to form a transition state, TS(OP, X = Li+)′, leading to Int.(OP, X = Li+)′. The first metal-carbon bond formation may be low barrier or barrierless.33 The metal carbon bond become shorter with encapsulation of Li+ ion (Int.(OP, X = Li+)′) by 0.16 Å (Table 2 and Figure 4). The M-C bond is also shorter for Pt in comparison to Ir (Table 1-2). The second metal-carbon bond formation from (Int.(OP, X = Li+)′) can either lead to formation of Prod.(66, X = Li+)′ or Prod.(65, X = Li+)′. The activation barriers for these two paths are 2.3 (TS1(X = Li+)′) and 3.7 kcal/mol (TS2(X = Li+)′), respectively. For reaction route (3a), three minima viz., OP, 66 and 65 were located. All minima are kinetically accessible as activation barrier is very small. Hence, the product distribution is guided by thermodynamics of the reaction. Formation of 14


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Prod.(66, X = 0) is more likely as the ∆G(66, X = 0) = -16.4 is much lower than ∆G(65, X = 0) = -1.9 kcal/mol. Similarly, in presence of encapsulated Li+ ion in reaction route (3b), formation of both Prod.(65, X = Li+) and Prod.(66, X = Li+) are kinetically feasible, but the former one is less likely, because activation barrier for the reverse process is relatively smaller (9.1 kcal/mol), and it is thermodynamically less stable. Hence, formation of Prod.(66, X = Li+) is favorable, which is in agreement with experimental observation.5

15



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Table 2. Selected Geometrical parameters (bond distances in Å) and Relative Gibbs free Energies (kcal/mol) at the M06/BS1 Level of Theory for the addition of L′2Pt on X@C60. The abbreviations OP, 66, and 65 stand for η1-addition, η2[6:6]-addition, and η2[6:5]-addition, respectively.

M = Pt, X = Li+ Geometrical parameters

System

∆G(b)

M-C1

M-C2

Li-C1

Li-C2

―

―

2.26(a)

2.26(a)

0.0

PRC(X = Li )′

―

―

―

―

―

+

―

―

―

―

―

TS1(X = Li )′

2.89

2.16

2.60

3.01

-16.6

TS2(X = Li+)′

2.46

2.21

2.80

2.94

-15.2

ML2 + X@C60 +

TS(OP, X = Li )′ +

+

―

2.20

―

2.95

-18.9

+

2.15

2.14

2.56

2.72

-24.3

+

2.11

2.12

2.31

2.35

-38.7

Int.(OP, X = Li )′ Prod.(65, X = Li )′ Prod.(66, X = Li )′

M = Pt, X = 0 System

(a)

Geometrical parameters

∆G(b)

M-C1

M-C2

Li-C1

Li-C2

ML2 + X@C60

―

―

―

―

0.0

PRC(X = 0)′

3.40

3.16

―

―

0.4

TS(OP, X = 0)′

―

2.74

―

―

1.5

TS1(X = 0)′

2.83

2.41

―

―

1.2

TS2(X = 0)′

2.48

2.32

―

―

1.3

Int.(OP, X = 0)′

―

2.36

―

―

0.7

Prod.(65, X = 0)′

2.16

2.16

―

―

-1.9

Prod.(66, X = 0)′

2.12

2.13

―

―

-16.4

The distance between Li atom and nearest two carbons of C60

(b)

The reaction Gibbs

free energy of product, relative to the corresponding reactants.

16


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Then, we studied the mechanism of coordination of C60 with Rh (equation 2a-2b) and Pd complexes (equation 4a-4b). Similar to the Ir and Pt, M-C60 complexes for Rh and Pd also become stable upon with encapsulation of Li+ ion in C60 (Table S1-S2 in SI). For example, Rh η2[6:6]-complex (∆G(66, X = Li+) = -12.2 kcal/mol) of reaction route 2b is lower than route 2a (∆G(66, X = 0) = +4.7 kcal/mol) (see Table S1 in SI). In comparison to Rh system, Ir species forms more stable complexes with C60 (Table 1-S1). The η2[6:6] adducts between Ir and Li+@C60 with ∆G(66, X = Li+) = -19.9 kcal/mol of route (1b) is of lower energy than Rh and Li+@C60 adduct (∆G(66, X = Li+) = -12.2 kcal/mol) of route (2b). Similarly, Pt complex (∆G(66, X = Li+) = -38.7) is found to be lower energy in comparison to Pd (∆G(66, X = Li+) = -34.3 kcal/mol). We also found that Pt/Pd forms more stable complexes in comparison to Ir/Rh. Hence, the order of stability of complexes is L4IrX@C60 > L4RhX@C60 and L′2PtX@C60 > L′2PdX@C60.

17



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3.2 The effect of the encapsulated ions As we discussed above, the reactions of L4Ir and L′2Pt with X@C60 (X = 0 or Li+) have low activation energy, and thermodynamic stability of the products can be increased by encapsulation of Li+ ion. To understand the effect of other encapsulated ions, we have studied the group IA and IIA metal cations for reaction between L4Ir and X@C60. It can be seen in Table 3 that ∆G is more negative in group IIA. The same tendency is observed for η2[6:6] as well as η2[6:5] product of L4IrX@C60. Hence, the metal ions with more positive charge would make L4IrX@C60 complex thermodynamically more stable. The ∆G become less negative when we go down the periodic table.34 Among the group IA metal ions Li+ or Na+ are the best candidates for making L4IrX@C60 complexes. The encapsulation of Be2+ is best candidate to form η2[6:6] and η2[6:5] products of L4IrX@C60. Among all the target metal cations, only mononuclear iridium and platinum complexes of Li+@C60 are synthesized and characterized to date.5 M06-2X calculations suggested that Na+ is the best catalyst among group IA metal cations for the Diels-Alder cycloaddition of cyclopentadiene to C60, and Ca2+ is more favorable than the group IA metal cation.12 However, our current findings suggest that Be2+ is the best candidate for making L4IrX@C60 complexes with η2[6:6] or η2[6:5] coordination. We hope that this work will guide the design of novel metal-fullerene complexes in the future.

18


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Table 3. Relative free energies of L4IrX@C60 formation(a) η2[6:6] product ∆G(kcal/mol)(b) X = Li+ X = Na

+

+

X=K

X = Rb

+

+

X = Cs

∆G(kcal/mol)(b)

-19.9

X = Be2+

-58.8

-20.1

2+

-54.4

2+

-53.0

2+

-51.8

2+

-49.7

X = Mg

-18.7

X = Ca

-18.5

X = Sr

-17.2

X = Ba 2

η [6:5] product ∆G(kcal/mol)(b) X = Li+ X = Na

+

+

X=K

X = Rb

+

+

X = Cs (a)

∆G(kcal/mol)(b)

-3.4

X = Be2+

-47.1

-3.4

2+

-39.6

2+

-37.3

2+

-36.8

2+

-32.9

X = Mg

-2.5

X = Ca

-2.0

X = Sr

-1.1

Energies at M06/BS1 levels of theory.

X = Ba (b)

The reaction Gibbs free energy of

product, relative to the corresponding reactants.

3.2.1 Energy decomposition analysis (EDA) In order to get a better understanding of the factors that govern the thermodynamic stability, we have performed EDA. According to the EDA, both metal fragment and empty C60 fragment are distorted during the metal-carbon bond formation (Table 4). The metal fragment is distorted more (DEFA = 33.0 kcal/mol) in comparison to C60 (DEFB = 11.7 kcal/mol). The similar results were obtained with encapsulation of metal ions into the C60 cage. It is found that encapsulation of metal ion induces more distortion in the fragment A and B of metal-Li+@C60 complex (∆DEF(X = Li+) = 6.4 kcal/mol). However, the interaction energy is more with encapsulation of metal ion (∆INTA(BC)(X = Li+) = -23.5 kcal/mol), implying the thermodynamic stability originates from the interaction energy. Similarly, we found that Be2+ induces the largest distortion in fragment A and B (∆DEF(X = Be2+) = 31.5 kcal/mol), but large interaction energy (∆INTA(BC)(X = Be2+) = -76.0 kcal/mol) 19



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leading to most stable complex (∆∆E(X = Be2+) = -70.4 kcal/mol). Similar EDA results were obtained with other encapsulated metal cations. Hence, the encapsulated metal ions induces stronger interaction between metal fragment and X@C60 fragment and make L4IrX@C60 more stable.35

Table 4. EDA of η2[6:6] product of L4IrX@C60 at M06/BS1(a-b) X 0 Li+ Na+ K+ Rb+ Cs+ Be2+ Mg2+ Ca2+ Sr2+ Ba2+ (a)

DEF

∆DEF

(DEFA,DEFB)

(c)

INTA(BC)

∆INTA(BC)(c)

∆E(d)

∆∆E(c,d)

─

-62.1

─

-17.4

─

6.4

-85.6

-23.5

-37.3

-19.9

4.7

-85.3

-23.2

-36.8

-19.4

4.2

-84.1

-22.0

-36.6

-19.2

4.3

-83.9

-21.8

-36.4

-19.0

4.1

-84.2

-22.1

-36.4

-19.0

31.5

-138.1

-76.0

-87.8

-70.4

18.9

-129.7

-67.6

-77.6

-60.2

10.9

-121.3

-59.2

-69.7

-52.3

12.1

-122.2

-60.1

-69.3

-51.9

9.1

-119.2

-57.1

-67.4

-50.0

44.7 (33.0, 11.7) 51.1 (34.6, 16.5) 49.4 (34.5, 14.9) 48.9 (34.5, 14.4) 49.0 (34.5, 14.5) 48.8 (34.6, 14.2) 76.2 (39.3,36.9) 63.6 (38.5,25.1) 55.6 (37.0,18.6) 56.8 (38.1,18.7) 53.8 (36.7,17.1)

The deformation energy (DEF) is the sum of the deformation energy of A (DEFA,

defined as the energy of A in product relative to the optimized isolated structure (A0) and B (DEFB). The interaction energy term, INTA(BC), is the interaction energy between A and (BC) at their respective optimized product structures. Energies are in kcal/mol

(b)

A and B indicate the metal fragment and C60 cage, respectively

(c)

the 20


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difference is relative to corresponding quantity at X = 0

(d)

The reaction energy

without zero-point energy (ZPE) correction of product, relative to the corresponding reactants. 3.3 Geometrical Parameters of Metal-Fullerene Complex The selected geometrical parameters for η2[6:6] product of L4IrX@C60 and L′2PtX@C60 (X = 0, Li+) are listed in Table 1 and Table 2. In the case of X = Li+, the calculated bond lengths of Ir-C1 (2.19 Å) Ir-C2 (2.19 Å) and Ir-coordinated C atoms(C1-C2 , 1.52 Å, Table 5) are in agreement with the X-ray structure.4 In the case of X = 0, the calculated bond lengths of Pt-C1 (2.12 Å), Pt-C2 (2.13 Å) and Pt-coordinated C atoms (C1-C2, 1.49 Å, Table 5), also agree with the X-ray structure.1 In addition, the natural bond orbital (NBO) analysis shows that there are pronounced positive and negative natural charges on the Li ion (+0.60) and the metal-coordinated C atoms (-0.149 and -0.145), whereas the other C atoms of C60 are nearly neutral (from -0.03 to 0.01, see SI). These results suggested that there are strong electrostatic attractions between Li+ ion and the metal-coordinated C atoms.5 The position of the encapsulated metal-cation in C60 is different in group IA and group IIA. The optimized structure of C60 with the encapsulated ions are given in Figure S3. Our present theoretical results suggest that the group IA metal cations, Li+, Na+, K+ are located away from the center of C60 cage by 1.25Å, 0.66 Å, 0.48 Å, respectively, whereas Rb+ and Cs+ reside nearly at the center. The group IIA metal cations, Be2+, Mg2+, Ca2+, Sr2+, Ba2+ are located away from the center by 2.17 Å, 1.40 Å, 0.90 Å, 0.87 Å and 0.43 Å respectively. The position of encapsulated metal cation in C60 may be dependent on strength of its interaction with C-atoms of C60 and size of metal cation. Our calculations are in accordance with the previous first-principles study.12,36 The optimized geometries obtained from the M06 computations are in good agreement with available experimental results. 21



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3.4 Estimate π back-bonding strength It is known that empty C60 is able to form stable complexes with Pt(PPh3)2 and IrCl(CO)(PPh3)2 complexes, where the metal-carbon bonds are in principal made up by π back-donation from a filled d orbital of the metal to the π* orbitals of C60.4, 37-39 In order to obtain some insights about the nature of metal-carbon bonds, key structural parameters and spectral characteristics of η2[6:6] product of L4IrX@C60, L4RhX@C60, L′2PtX@C60 and L′2PdX@C60 (X = 0, Li+) were analyzed. In other word, bond length variations (∆r/r0), bond angle changes (∆θav), vibrational frequency (∆ν), chemical shift variations (∆δ) are employed to describe their π-complex characters.40,41 In principle, the larger values of these parameters corresponds to the stronger π back-bonding. This analysis can be used to reveal the effect of a Li+ ion on the π back-bonding ability and to compare the π back-bonding strengths of metal complexes.

3.4.1 Effect of a Li+ ion on the π back-bonding ability The variations of bond lengths (∆r10/r0 and ∆r20/r0) and bond angles (∆θav) are listed in the Table 5. In the presence of a Li+ ion, the r2 and θav2 values increase (i.e. ∆r20/r0 and ∆θav values are positive). The M06/BS1 computations show a strong band at 2083 cm-1 for CO stretching (νco). When a Li+ ion is encapsulated into C60, the νco is shifted to 2097 cm-1, which is in agreement with the experimental values (shift from 2011 cm-1 to 2025 cm-1).5 Therefore, encapsulation of Li+ ion may increase the back-bonding from M to C60. Calculated

13

C NMR parameters are summarized in Table 6. In the case of an

isolated C60 molecule, the highly symmetric structure (Ih) leads to only one signal at 140.9 ppm, which is in agreement with the experimental values (142.68~143.2 22


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ppm).42 With addition of L4Ir to C60 in η2[6:6] mode, the δC of Ir-coordinated C atoms is shifted to 79.0 ppm. However, when the L4Ir fragment is bound to [6,6] bond of Li+@C60, the δC of Ir-coordinated C atoms is shifted to 72.8 ppm (∆δC = -6.2 ppm, Table 6). The above results show that the Li+ ion can lead to increment of bond lengths and bond angle, higher wavenumber shift and extra upfield shift which all suggest that presence of encapsulated Li+ ion lead to more π back-bonding from Ir-metal to coordinated carbon atoms of C60. Increased back-bonding leads to stronger metal-carbon bond. The same are also observed for η2[6:6] product of L4RhX@C60, L′2PtX@C60 and L′2PdX@C60 (X = 0, Li+). Therefore, the π back-bonding ability of studied metal complexes can be enhanced by the Li+ ion. Table 5. The variations of bond lengths (∆r/r0) and bond angles (∆θav) of η2[6:6] products of studied complexes at M06/BS1 level(a-d)

r0

r1

r2

∆r10/r0

∆r20/r0

Θav1

Θav2

∆θav

L4RhX@C60

1.39

1.46

1.49

5.0%

7.2%

36.3

37.6

+1.3

L4IrX@C60

1.39

1.49

1.52

7.2%

9.4%

37.9

39.0

+1.1

L′2PdX@C60

1.39

1.46

1.50

5.0%

7.9%

36.2

37.6

+1.3

L′2PtX@C60

1.39

1.49

1.53

7.2%

10.1%

37.8

38.9

+1.1

(a)

r0 indicates the CC bond length(Å) between two six-membered rings of empty C60

before addition reaction

(b)

The subscript i=1 and 2 refer to the X = 0 and X = Li+ 23



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Page 24 of 33

after addition reaction, respectively. (c)∆r10=r1-r0 ; ∆r20=r2-r0.(d) θav=(θa+θb)/2 ; ∆θav =θav2 –θav1. Table 6. The calculated vibration CO frequency and 13C chemical shift(a) of η2[6:6] products of studied complexes at M06/BS1 level of theory(b) Chemical shift (δi)

CO frequency (νi) (cm-1)

L4RhX@C60

ν1

ν2

δ1

(ppm) δ2

2086 2083

2110 2097

90.6

79.4

-11.2

79.0

72.8

-6.2

L4IrX@C60

(c)

(2011)

∆δ

(c)

(2025)

L′2PdX@C60

―

―

92.6

82.5

-10.1

L′2PtX@C60

―

―

83.7

78.5

-5.2

(a)

Calculated by GIAO method and use tetramethylsilane (TMS) as reference

compound (b) The subscript i=1 and 2 refer to the X = 0 and X = Li+ after addition reaction, respectively.(c) The experimental data are from ref.(4).

24


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3.4.2 The π back-bonding strengths of different metal complexes In section 3.4.1, the ∆r/r0, ∆θav, quantities are utilized to elucidate the effect of a Li+ ion on the π back-bonding ability. In this section, these quantities are used to compare the π back-bonding strengths of different metal complexes. As summarized in Table 5-6, the values of ∆r10/r0, θav of η2[6:6] product of L4IrC60 are larger than those of L4RhC60. For example, ∆r10/r0 = 7.2%, θav = 37.9 for the former and ∆r10/r0 = 5.0%, θav = 36.3 for the later. Similarly in presence of encapsulated Li+ ion the values for η2[6:6] product of L4IrLi+@C60 are larger than those of L4RhLi+@C60. These theoretical findings suggest that the π back-bonding strength of L4IrX@C60 are greater than that of L4RhX@C60 where X = 0 or Li+. The similar results are found for L′2PtX@C60 and L′2PdX@C60, and the π back-bonding strength of L′2PtX@C60 are greater than that of L′2PdX@C60 for both X = 0 and X = Li+.

4. Conclusion Present computational study explored the addition reaction mechanisms of LnM fragments (n = 2 and 4) with X@C60 (X = 0 or Li+) using DFT. According to our calculations, the η2[6:6]-addition is most favorable path, and this is in agreement with the experimental observations. The reaction is more exergonic with encapsulation of Li+ ion into C60. Similar behavior was also observed when we use the metal cations is group IA&IIA. EDA shows that it is due to enhancement of interaction between metal fragment and C60 by the encapsulated metal ions. Our chemical bonding analyses results show that ∆r/r0, ∆θav and ∆δC of the metal complexes become larger in the presence of Li+ ion. As a consequence, it can be concluded that the Li+ ion can enhance the π back-bonding ability. Moreover, our calculations suggest that the iridium and platinum complexes have stronger π back-bonding strength than the rhodium and palladium complexes, respectively. 25



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Supporting Information Available The detailed AFIR and EDA calculation, the dominant parameters of the optimized geometries, the Cartesian coordinates of optimized structures and Natural Population Analysis have been collected in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements The authors thank the National Center for High-Performance Computing of Taiwan and the computer resources at the Academic Center for Computing and Media Studies (ACCMS) at Kyoto University and Research Center of Computer Science (RCCS) at the Institute for Molecular Science for generous amounts of computing time. M. C. Yang and M. D. Su are grateful to the Ministry of Science and Technology of Taiwan for the financial support. W. M. C. Sameera acknowledges to the Japan Society for the Promotion of Science (JSPS) for a foreign postdoctoral fellowship (no. P14334) and Hokkaido University. This work was in part supported by Grants-in-Aid for Scientific Research (KAKENHI 15H00938 and 15H02158) to K. Morokuma at Kyoto University. Special thanks are also due to reviewers 1 and 2 for very help suggestions and comments.

26


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Methods. IX. An Extended Gaussian‧Type Basis for Molecular‧Orbital Studies of Organic Molecules. J. Chem. Phys. 1971, 54, 724-728. 26. Hehre, W. J.; Ditchfie, R.; Pople, J. A. Self—Consistent Molecular Orbital Methods. XII. Further Extensions of Gaussian—Type Basis Sets for Use in Molecular Orbital Studies of Organic Molecules. J. Chem. Phys. 1972, 56, 2257-2261. 27. Hariharan, P. C.; Pople, J. A. The Influence of Polarization Functions on Molecular Orbital Hydrogenation Energies. Theor Chim Acta 1973, 28, 213-222. 28. Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; Defrees, D. J.; Pople, J. A. Self‧Consistent Molecular Orbital Methods. XXIII. A Polarization‧Type Basis Set for Second‧Row Elements. J. Chem. Phys. 1982, 77, 3654-3665. 29. Sameera W. M. C.; Hatanaka, M.; Kitanosono T.; Kobayashi S.; Morokuma, K. The Mechanism of Iron(II)-Catalyzed Asymmetric Mukaiyama Aldol Reaction in Aqueous Media: Density Functional Theory and Artificial Force-Induced Reaction Study. J. Am. Chem. Soc. 2015, 137, 11085-11094. 30. The geometries of η2[6:5] and η1 product dissociate during the geometry optimization process, which may be due to their relative higher energies or they are not minima on PES. Hence, these products are not given in Figure 1. 31. Despite several attempts for TS involving simultaneous addition of Pt-metal complex to two carbons of C60, we were not able to locate it. Instead, the TS for the one-point addition product was obtained, implying a two-step process is more favorable. 32. The (η2-C2H4)PtL′2 complex is generally used for generation of (η2-C60)PtL′2 complex. The PtL′2 is generated by the decoordination of C2H4 from 30


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(η2-C2H4)PtL′2 complex. The decoordination of C2H4 involves a small change in free energy ((η2-C2H4)PtL′2 → C2H4 + PtL′2, ∆G = 5.3 kcal/mol). 33. Repeated attempts to locate the PRC(X = Li+)′ and TS(OP, X = Li+)′ failed and all attempts lead to Int.(OP, X = Li+)′. 34. The reason for such a phenomenon is still uncertain. Presumably, it would be due to the fact that as the atomic radius of alkali metal increases, the ability of alkali metal to reduce electron density from C60 decreases. Hence, the electron density on C60 increases (i.e. the electrophilicity of X@C60 decreases). As electron density on C60 increases, the ability of accepting electron from metal decreases (i.e. less back-bonding). 35. As suggested by referee, to give the detailed explanation on the interaction that, for example, stabilizes the product of the addition of L4Ir to Li+@C60 more than that to empty C60, we have performed the EDA-NOCV calculation proposed by Frenking G. and co-workers on η2[6:6] product of L4IrX@C60 (L4 = Cl(CO)(PPh3)2, X = 0, Li+). The result shows that the enhanced orbital interaction (∆Eorb) should be responsible for the stabilization after the encapsulation of metal cation. For details, please see the Table S70 in the supporting information. 36. de Oliveira, O. V.; da Silva Goncalves, A. Quantum Chemical Studies of Endofullerenes (M@C60) Where M = H2O, Li+, Na+, K+, Be2+, Mg2+, and Ca2+.

Comput. Chem. 2014, 2, 51. 37. Koga, N.; Morokuma, K. Ab initio MO Calculation of (η2-C60)Pt(Ph3)2. Electronic Structure and Interaction between C60 and Pt. Chem. Phys. Lett, 1993,

202, 330-334. 38. Kameno, Y.; Ikeda, A.; Nakao, Y.; Sato, H.; Sakaki, S. Theoretical Study of M(PH3)2 Complexes of C60, Corannulene (C20H10), and Sumanene (C21H12) (M = 31



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Pd or Pt). Unexpectedly Large Binding Energy of M(PH3)2(C60). J. Phys. Chem.

A, 2005, 109, 8055-8063. 39. Ikeda, A.; Kameno, Y.; Nakao, Y.; Sato, H.; Sakaki, S. Binding Energies and Bonding Nature of MX(CO)(PH3)2(C60) (M = Rh or Ir; X = H or Cl): Theoretical Study. J. Organomet. Chem., 2007, 692, 299-306. 40. Kira, M.; Sekiguchi, Y.; Iwamoto, T.; Kabuto, C. 14-Electron Disilene Palladium Complex Having Strong π-Complex Character. J. AM. CHEM. SOC. 2004, 126, 12778-12779. 41. Abe, T.; Iwamoto, T.; Kira, M. A Stable 1,2-Disilacyclohexene and Its 14-Electron Palladium(0) Complex. J. AM. CHEM. SOC. 2010, 132, 5008-5009. 42. Sun, G.; Kertesz, M. Theoretical 13C NMR Spectra of IPR Isomers of Fullerenes C60, C70, C72, C74, C76, and C78 Studied by Density Functional Theory. J. Phys.

Chem. A 2000, 104, 7398-7403.

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TOC A Theoretical Study of Addition Reactions of L4M(M = Rh, Ir) and L2M(M = Pd, Pt) to Li+@C60

Ming-Chung Yang, Akhilesh K. Sharma, WMC Sameera, Keiji Morokuma and Ming-Der Su*

33


Theoretical Study of Addition Reactions of L4M (M= Rh, Ir) and L2M (M

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