Structural properties of metal - benzene , M n ð benzene Þ m , M 1⁄4 Ni , V complexes : an ab initio study

Interactions of Ni and V with benzene (Bz)-molecules are investigated using ab initio methods and tight-binding molecular dynamics (TBMD) simulations. The differences in the behavior for Ni and V is found to be consistent with their similar contrasting bonding behavior found in interactions with graphite, C60 and carbon nanotubes. 2001 Elsevier Science B.V. All rights reserved. The interactions of transition metal atoms (TMA) with various low dimension forms of carbon has been the subject of recent experimental and theoretical investigations [1–17]. All these investigations affirm the contrasting bonding behaviors of the 3d-TMA in their interaction with graphite, C60, single wall carbon nanotubes (SWCN), and benzene (Bz)-molecules beyond any reasonable doubt. In particular, detailed theoretical studies of the interaction of graphite with TMA have shown that the early 3d-elements (Sc, Ti, V) are energetically more stable at hole sites, while the late 3d-elements (Fe, Co, Ni) are found energetically more stable on atop sites [8,12]. Similarly, the experimental results dealing with the structural properties of small MnðC60Þm clusters, where M denotes a TMA of the 3d-series ðmþ n < 10Þ, have revealed that early 3d-elements form an M1[g-ðC60Þ2] bonding configuration, while the late 3d-elements form either an M1[gðC60Þ2] or an M1[g-ðC60Þ2] configuration [1,3,4]. Here gk (k an integer) denotes that there are k ligand atoms (carbon ring atoms) bonded to the metal (M) atoms. Based on these findings, it was proposed that the early 3d-elements form ‘dumbbell’ structures in which the M-atom is sandwiched between six-membered rings of C60. It was also proposed that the late 3d-elements form bent or ring-type structures in which the M-atom forms two or three bonds with each C60 molecule. These experimental conclusions were theoretically confirmed by recent tight-binding molecular dynamics 28 December 2001 Chemical Physics Letters 350 (2001) 393–398 www.elsevier.com/locate/cplett * Corresponding author. Fax: +30-859-323-1029. E-mail address: [email protected] (M. Menon). 0009-2614/01/$ see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0009-2614 (01 )01224-6 (TBMD) simulations of MnðC60Þm, M 1⁄4 Ni, V ðmþ n6 5Þ clusters [9,10]. The experimental results obtained for the interaction of TMA with Bz-molecules have further reaffirmed the contrasting bonding configuration of the 3d-TMAs in their interaction with carbon [5]. Specifically, these results led to the conclusion that the early 3d-elements should form structures in which the M-atom is sandwiched symmetrically between two Bz-molecules in a double g-bonding configuration. On the other hand, it was concluded that the late 3d-elements should form structures of the ‘rice-ball’ type with gand/or g-bonding configurations. In the latter case, the TMA is encapsulated in an oyster-like opening or within a small volume surrounded by Bz-molecules. In addition to the contrasting bonding behavior discussed above, the TMAs were found to undergo a significant change in their magnetic moment when they interact with graphite, C60, SWCN and Bz-molecules. A striking characteristic feature of this interaction was the quenching of the magnetic moment of Ni on graphite, C60, SWCN and Bzmolecules [8,9,13] on the one hand, and the enhancing of the magnetic moment of the early 3datoms when supported on Bz-molecules on the other hand [13]. Our recent investigations on the interaction of TMAs with other carbon materials of low dimension have revealed many interesting features [8–11]. This along with the lack of detailed theoretical studies of the organometallic systems in the literature has provided us the necessary motivation to study the MnðBzÞm systems in a systematic way in the present work. In this Letter, we report our results of a theoretical investigations of TMA–Bz complexes of the form MnðBzÞm, M 1⁄4 Ni, V with mþ n6 5, obtained using ab initio computational methods. We also use TBMD relaxations of the systems to complement our ab initio work. The ab initio calculations were performed using the GAUSSIAN 98 program [18]. Structural optimizations were carried out using both ab initio and TBMD methods. While TBMD simulations were used to perform fully symmetry-unconstrained optimizations in all cases, only the smaller clusters were so optimized using ab initio methods due to the computational complexity. Fig. 1a,b show our ab initio results for the symmetry-constrained ðD6hÞ and symmetry-unconstrained optimization of NiðBzÞ2 clusters, respectively. Our calculations show the symmetryunconstrained relaxed structure to be energetically more stable (by 1.16 eV). It is then apparent from these figures that Ni prefers the g-type bonding with each Bz-molecule following the same general trend found in the bonding configuration of the TMAs with graphite, C60 and SWCN. Our results, however, are at odds with the ‘rice-ball’ structure used to interpret the experimental findings [5]. Instead, we find that the two Bz-molecules repel each other and form a Z-like (or step-like) bonding with the Ni atom instead of the ‘oyster-like’ opening of the ‘rice-ball’ structure proposed [5]. It is worth noting that even though we start from a rice-ball like structure, the symmetry-unconstrained relaxation of NiðBzÞ2 results in the structure shown in Fig. 1b, indicating no local energy minimum for the rice-ball structure. As seen in Fig. 1b, the planes of the Bz-molecules are not parallel to each other. Furthermore, the plane containing the C–Ni–C bonds of Ni with one Bzmolecule is rotated by approximately 68 with respect to the bonding plane containing the C–Ni–C bonds of the Ni atom with the other Bz-molecule. We have also performed TBMD optimization of the structure shown in Fig. 1a. This relaxation also resulted in the structure shown in Fig. 1b, showing very good agreement with the ab initio results. We next repeat our ab initio calculations for the Ni2Bz2 cluster. Fig. 2a,b, respectively, show the results for the symmetry-constrained ðD2hÞ and symmetry-unconstrained optimization of this Fig. 1. Final structures obtained by performing: (a) symmetryconstrained ðD6hÞ, and (b) symmetry-unconstrained ðC1Þ relaxations for the NiðBzÞ2 complex using ab initio methods. 394 G.E. Froudakis et al. / Chemical Physics Letters 350 (2001) 393–398