**1. Introduction**

The first fullerene-transition metal complex, (η<sup>2</sup> -C60)Pt(Ph3 )2 , was prepared and structurally characterized by Fagan et al. in 1991 [1]. It was the starting point for a new class of study for fullerene chemistry. Since then, various fullerene-transition metal complexes have been synthesized and these have potential applications in solar cells, spintronics, catalysis and

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© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

drug delivery [2]. Balch et al. then studied the reactions of C60 with electron-rich fragments, IrCl(CO)(PPh<sup>3</sup> )2 and produced the fullerene-iridium complex (η<sup>2</sup> -C60)IrCl(CO)(PPh<sup>3</sup> )2 [3]. The formation of fullerene-iridium complex is a reversible process and the reversible binding of IrCl(CO)(PPh<sup>3</sup> ) 2 to fullerenes can be used as a structural probe because the adducts can build ordered single crystals that are suitable for X-ray diffraction [4, 5]. Fullerene-iridium complexes that contain an enantiomeric phosphine ligand are used as solar photoelements [6]. One of the significant characteristics of fullerenes is that they are capable of encaging atoms, ions and small molecules to form endohedral complexes. Endohedral metallofullerenes (EMFs) are those that encapsulate metal atoms within a hollow carbon cage. Proft et al. theoretically studied the interactions between encapsulated monoatomic ions (Li<sup>+</sup> to Rb<sup>+</sup> and F− to I− ) and C60 and its Si and Ge analogues [7] and found that, for these families, the interactions between Li<sup>+</sup> (Na+ ) and F<sup>−</sup> (Cl<sup>−</sup> ) ions and C60 are strongest and exothermic, which confirms the possibility of the existence of these species. Recently, Li<sup>+</sup> @C60 was successfully synthesized and isolated by Watanabe et al. [8].

Understanding the strength and nature of metal-ligand bonding is crucial for the design of new fullerene-transition metal complexes because the structure and stability of various intermediates are important to the formation of organometallics [9]. In an earlier work by the authors [10], {η<sup>2</sup> -(X@C60)}ML2 complexes (M = Pt, Pd; X = 0, Li+ , L = PPh3 ) were studied and it was found that there is a relationship between thermodynamic stability and π backbonding; that is, the greater the π back-bonding, the greater is thermodynamic stability. This shows that thermodynamic stability can be modified by tuning the π back-bonding. As far as the authors are aware, π back-bonding could be affected by several factors, including the encapsulated ions, the metal fragments and the cage sizes. This study determines the importance of π back-bonding to the thermodynamic stability of {η<sup>2</sup> -(X@Cn)}ML2 complexes by using M = Pt, Pd; X = F− , 0, Li+ and n = 60, 70, 76, 84, 90 and 96 to ascertain the role of these factors in π back-bonding. Since the system is very large, methyl-substituted N-heterocyclic carbenes (NHC) are used as a ligand (L), instead of PPh<sup>3</sup> . NHC is one of the frequently used and most powerful tools in organic chemistry [11]. In this work, the following reactions are studied:

$$\text{ML}\_2 + \text{X} \oplus \text{C}\_n \rightarrow \left\{ \eta^2 \cdot \left( \text{X} \oplus \text{C}\_n \right) \right\} \text{ML}\_2 \tag{1}$$

The geometry optimizations are performed without any symmetry restrictions by using the M06 [17]/LANL2DZ [18, 19] level of theory. The vibrational frequency calculations at 298.15 K and 1 atm use the same level of theory. The stationary points are confirmed by the absence of imaginary frequencies. The natural charges are obtained using NBO 5.9, as implemented in

How Important is Metal-Carbon Back-Bonding for the Stability of Fullerene-Transition Metal Complexes?...

**Scheme 1.** The sites of attack for addition to the fullerenes Ih-C60, D5h-C70, D2

The Hückel π bond orders (B) were calculated using the freeware program, HuLiS [16].

The interatomic interactions are determined using energy decomposition analysis (EDA). Two types of EDA are used in this work. The first is the basic EDA that was developed individually by Yang et al. [21] and by Ziegler and Rauk [22]. For this basic EDA, the bonding energy (∆E) is partitioned into two terms, ∆E = ∆E(DEF) + ∆E(INT). In this work, basic EDA is used for the

carbon cages (B) and metal ions (C) as shown in **Scheme 2**. The deformation energy (∆E(DEF))

X@Cn complexes, which are categorized into transition metal complexes (A),


http://dx.doi.org/10.5772/intechopen.70068

113

the Gaussian 09 program [20].

optimized ML2

#### **2. Computational details**

The following fullerenes that comply with the isolated pentagon rule are used to develop a correlation: Ih-C60, D5h-C70, D2 -C76, D2d(23)-C84, D5h(1)-C90 and D3d(3)-C96. These are experimentally isolated and identified [12–14]. The symmetry and numbering scheme for fullerene isomers are in accordance with an approved classification [15]. Hückel molecular orbital calculations show that the 6:6 ring junctions at the poles of the molecules usually have highest π bond orders (B) and are expected to be the most reactive, so these are the sites of attack (see **Scheme 1**) [12, 16].

How Important is Metal-Carbon Back-Bonding for the Stability of Fullerene-Transition Metal Complexes?... http://dx.doi.org/10.5772/intechopen.70068 113

drug delivery [2]. Balch et al. then studied the reactions of C60 with electron-rich fragments,

formation of fullerene-iridium complex is a reversible process and the reversible binding

build ordered single crystals that are suitable for X-ray diffraction [4, 5]. Fullerene-iridium complexes that contain an enantiomeric phosphine ligand are used as solar photoelements [6]. One of the significant characteristics of fullerenes is that they are capable of encaging atoms, ions and small molecules to form endohedral complexes. Endohedral metallofullerenes (EMFs) are those that encapsulate metal atoms within a hollow carbon cage. Proft et al.

) and C60 and its Si and Ge analogues [7] and found that, for these families, the interac-

complexes (M = Pt, Pd; X = 0, Li+

Understanding the strength and nature of metal-ligand bonding is crucial for the design of new fullerene-transition metal complexes because the structure and stability of various intermediates are important to the formation of organometallics [9]. In an earlier work by

and it was found that there is a relationship between thermodynamic stability and π backbonding; that is, the greater the π back-bonding, the greater is thermodynamic stability. This shows that thermodynamic stability can be modified by tuning the π back-bonding. As far as the authors are aware, π back-bonding could be affected by several factors, including the encapsulated ions, the metal fragments and the cage sizes. This study determines

role of these factors in π back-bonding. Since the system is very large, methyl-substituted

frequently used and most powerful tools in organic chemistry [11]. In this work, the follow-

ML2 + X@Cn → {η<sup>2</sup> ‐(X@Cn)} ML2 (1)

The following fullerenes that comply with the isolated pentagon rule are used to develop

mentally isolated and identified [12–14]. The symmetry and numbering scheme for fullerene isomers are in accordance with an approved classification [15]. Hückel molecular orbital calculations show that the 6:6 ring junctions at the poles of the molecules usually have highest π bond orders (B) and are expected to be the most reactive, so these are the sites of attack (see

the importance of π back-bonding to the thermodynamic stability of {η<sup>2</sup>

N-heterocyclic carbenes (NHC) are used as a ligand (L), instead of PPh<sup>3</sup>

, 0, Li+

to fullerenes can be used as a structural probe because the adducts can

) ions and C60 are strongest and exothermic, which confirms


@C60 was successfully synthesized

, L = PPh3

and n = 60, 70, 76, 84, 90 and 96 to ascertain the


) 2

to Rb<sup>+</sup>

) were studied

com-


. NHC is one of the

and

[3]. The

and produced the fullerene-iridium complex (η<sup>2</sup>

theoretically studied the interactions between encapsulated monoatomic ions (Li<sup>+</sup>

IrCl(CO)(PPh<sup>3</sup>

F− to I−

of IrCl(CO)(PPh<sup>3</sup>

tions between Li<sup>+</sup>

the authors [10], {η<sup>2</sup>

)2

) 2

112 Fullerenes and Relative Materials - Properties and Applications

(Na+

and isolated by Watanabe et al. [8].

plexes by using M = Pt, Pd; X = F−

ing reactions are studied:

**2. Computational details**

a correlation: Ih-C60, D5h-C70, D2

**Scheme 1**) [12, 16].

) and F<sup>−</sup>


(Cl<sup>−</sup>

the possibility of the existence of these species. Recently, Li<sup>+</sup>

**Scheme 1.** The sites of attack for addition to the fullerenes Ih-C60, D5h-C70, D2 -C76, D2d(23)-C84, D5h(1)-C90 and D5h(1)-C96. The Hückel π bond orders (B) were calculated using the freeware program, HuLiS [16].

The geometry optimizations are performed without any symmetry restrictions by using the M06 [17]/LANL2DZ [18, 19] level of theory. The vibrational frequency calculations at 298.15 K and 1 atm use the same level of theory. The stationary points are confirmed by the absence of imaginary frequencies. The natural charges are obtained using NBO 5.9, as implemented in the Gaussian 09 program [20].

The interatomic interactions are determined using energy decomposition analysis (EDA). Two types of EDA are used in this work. The first is the basic EDA that was developed individually by Yang et al. [21] and by Ziegler and Rauk [22]. For this basic EDA, the bonding energy (∆E) is partitioned into two terms, ∆E = ∆E(DEF) + ∆E(INT). In this work, basic EDA is used for the optimized ML2 X@Cn complexes, which are categorized into transition metal complexes (A), carbon cages (B) and metal ions (C) as shown in **Scheme 2**. The deformation energy (∆E(DEF))

**Scheme 2.** Basic EDA for {η<sup>2</sup> -(Li+ @Cn)}PtL2 .

is the sum of the deformation energy of A (∆E(DEF)A, which is defined as the energy of A in the product relative to the optimized isolated structure (A0 ) and B (∆E(DEF)B)). The interaction energy term, ∆E(INT)A(BC), is the interaction energy between A and (BC) for their respective optimized product structures.

Advanced EDA unites the natural orbitals for chemical valence (NOCV), so it is possible to separate the total orbital interactions into pairwise contributions [23]. The advanced EDA (i.e., EDA-NOCV) further divides the interaction energy (∆E(INT)) into three main components: ∆E(INT) = ∆Eelstat + ∆EPauli + ∆Eorb. It is used for a quantitative study of π back-bonding to fullerene ligands that uses the M06/TZ2P level of theory with the ADF 2016 program package [24]. The relativistic effect is considered by applying a scalar zero-order regular approximation (ZORA) [25]. The interaction energy and its decomposition terms are obtained from a singlepoint calculation using the M06/TZ2P basis set from the Gaussian 09 optimized geometry.

C1 , C2

Li+

C1 (C2

and Li+

the atomic charges on the C<sup>1</sup>

**Figure 1.** Optimized geometries for {η<sup>2</sup>

the atomic charges on C<sup>1</sup>

ion, the atomic charges on the C<sup>1</sup>

atoms are also less for X = 0 and F<sup>−</sup>

sulated ions (F<sup>−</sup>

are 2.29 Å and 2.29 Å. As the encapsulated ion is changed to F−

transition metals because of electrostatic interaction. The metal-coordinated carbon atoms of C60 are negatively charged because there is π back-donation from the metal center. For the Pt-C60 complex without encapsulated ions, the natural population analysis (NPA) shows that

(M = Pt, Pd; X = Li+

How Important is Metal-Carbon Back-Bonding for the Stability of Fullerene-Transition Metal Complexes?...

, 0, F<sup>−</sup> ).

atoms. In terms of Pd-C60 complexes, it is worthy of note that the geometrical distances are generally similar to the corresponding distances for Pt-C60 complexes, but the charge distributions are different. Specifically, the encaged Li atom has a charge (+0.86) but the charges on

) atoms are reduced to −0.27 (−0.27). The negative charges on metal-coordinated carbon

bonds remain almost unchanged (2.13 and 2.13 Å), but the distance between C1

(C2

tively charged C atoms. However, as the encapsulated ion is changed to F−

) increases (3.18 and 3.18 Å). The Li<sup>+</sup>


(C2

seen for n = 70, 76, 84, 90 and 96 and are presented elsewhere.

charge on the Li atom is +0.86. Therefore, the encaged Li+

(C2

the F atom is negative (−0.93), so the encaged F−

, the metal-carbon

, NPA shows that

and encap-

, C2

ion is located at a site that is close to the

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115

ion is attracted toward these nega-

ion is repelled by the negatively charged C

) atoms are −0.27 (−0.27). When the cage is encapsulated by a

) atoms are decreased to −0.23 (−0.23) and the atomic charge on

. Similar geometric changes and charge distributions are

) atoms are increased to −0.32 (−0.32) and the atomic
