Metal Sandwich Molecules: Planar Metal Atom Arrays Between Aromatic Hydrocarbons

Materials Transactions, Vol. 48, No. 4 (2007) pp. 693 to 699 Special Issue on ACCMS Working Group Meeting on Clusters and Nanomaterials #2007 Society of Nano Science and Technology Metal Sandwich Molecules: Planar Metal Atom Arrays between Aromatic Hydrocarbons Michael R Philpott1;2 and Yoshiyuki Kawazoe1 1Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan 21631 Castro Street, San Francisco, CA 94114, USA Sandwich molecules MnS2 consisting of a layer of transition metal atoms M (palladium) between planar polyacene aromatic hydrocarbon molecules (S) are studied using ab initio density functional theory. The polyacenes range from tetracene C18H12 (four rings) to circumcoronene C54H18 (nineteen rings). Geometry optimization shows that in simple arrangements, where one metal is assigned to each ring, there is a preference for metal atoms to coordinate to carbon atoms on the circumference of the sandwich. This can result in metal-metal distances greater than in bulk metal and so the establishment of planar metal clusters with metal-metal bonds in small systems is frustrated. This effect is studied by changing the number of metal atoms and relaxing symmetry constraints. A neutral molecule Pd5(C18H12)2 with C2v symmetry and a lopsided arrangement of metal atoms is shown to be consistent with experimental work on dications by Murahashi et al. [Science 313 (2006) 1104–1107]. In tetracene sandwiches with n ¼ 5 and 9 the palladium atoms adopt 2- and 3-coordination to the edge carbon atoms. In addition to edge bonding, motifs for interior metal atoms are identified from a series of sandwiches with increasing size containing: coronene (C24H12), ovalene (C32H14), circumanthracene (C40H16) and circumcoronene (C54H18). [doi:10.2320/matertrans.48.693] (Received December 14, 2006; Accepted February 6, 2007; Published March 25, 2007) Keywords: density functional theory, transition metal, palladium, sandwich molecule, aromatic hydrocarbon, tetracene, coronene, ovalene, circumanthracene, circumcoronene, linear polyacene, catacondensed polyacene 1. Introduction nated by aromatic and inorganic ligands.6,10) For simplicity we consider only systems which are: This paper reviews the status of an ab initio and density neutral, have identical linear or catacondensed hydrocarbon functional theory (DFT) study of the properties of molecules moieties, and contain a palladium middle layer. We generally Mn(S)2 in a sandwich structure, that is with an array of metal impose symmetry D2h in order to keep the calculations atoms M between planar aromatic hydrocarbon molecules S. tractable. In the case of tetracene we consider n ¼ 4 (linear Geometry is the focus of this report. We consider only array of Pd atoms), n ¼ 5 (irregular pentagonal array, sandwiches with identical outer molecules of the polyacene corresponding to the experimental geometry of Murahashi class. The basic geometry is shown as in Fig. 1A. There is a et al.9)) and n ¼ 9 (to explore saturation of edge sites). The layer of metal atoms (central layer) sandwiched between Pd5[C18H12]2 system provides a good test of the efficacy of planar aromatic molecules. The top left Fig. 1B, shows the the computational scheme used and we note the methodology manifold of electronic states schematically subdivided into used in this paper provides a good account of the geometrical - and -levels of the carbon framework and a d-manifold of structure of the experimental system. Space does not permit a metal atom levels. The aromatics considered explicitly are full account of all geometric and electronic structure from Fig. 1: tetracene C18H12 (Fig. 1C) the smallest with calculations including bonding. In this paper we concentrate four rings, coronene C24H12 (Fig. 1D), ovalene C32H14 on reporting geometry, pointing out emergent motifs in the (Fig. 1E), circumanthracene C40H14 (Fig. 1F) and circum- chemical bonding as the hydrocarbon size or number of metal coronene C54H24 (Fig. 1G) the largest with nineteen rings. atoms is increased. Identifying bonding motifs is important Synthesis and properties of the molecules C, D, E and F are because the mismatch between palladium neighbour distance described in the treatise by Clar.1) 275 pm (fcc, bulk) and the aromatic ring centers (240– Broadly speaking sandwich systems of the type contem- 245 pm) implies a significant level of frustration. Supportive plated here may be considered as extensions of the smaller work not explicitly described here includes spin polarized known mono-metallocenes like ferrocene,2–6) dimetallocenes calculations to verify that the ground states are not magnetic, like dizincocene7,8) and the recently synthesised compounds and total energy of formation from calculations of the energy 9) 2þ of palladium with n ¼ 3,R=C7H7 (cyclo-heptatienyl of separated hydrocarbons and palladium arrays and clusters. cation; with chlorides attached to the Pd atoms) and n ¼ 5, There are many configurations for a given number of metal PAC = C18H12 (tetracene cation; toluene coordinated to atoms trapped between polyacenes in registry. An exhaustive the apical Pd). Both synthetic compounds are composed of search through all possible configurations has not been organometallic dications containing palladium. The tetracene performed. Instead we have used intuitive guess work compound is particularly interesting because the crystal modified by ongoing calculations to guide the survey plus structure shows that the five Pd atoms are not evenly the knowledge of the experimental geometry from the 9) distributed, being located as an irregular pentagon in one half ‘‘lopsided’’ experimental sandwich Pd5(C18H12)2. of the sandwich and bonded through edge sites. The This paper is organized as follows. Next, there is a geometry of this compound is a departure from a majority description of the computational method, in essential of organo-metallics, which commonly consist of transition minimal detail. This is followed by a section on several metal atoms connected by bridging ligands and/or coordi- sandwiches of tetracene. Then comes a section surveying the 694 M. R. Philpott and Y. Kawazoe B A σ* Π* d Π σ C G D E F Fig. 1 Schematic diagram of the structure of metal sandwich molecules. The top right A, schematic showing a layer of metal atoms between planar aromatic molecules. Top left B, schematic of the electronic levels manifold of the sandwich subdivided into - and - levels of the hydrocarbon and a 4d manifold of metal atom levels. Bottom C, D, ..., G show the carbon skeletons of the aromatic hydrocarbon molecules. calculated geometry and bonding in sandwiches with the cases. Harmonic analysis of the wave function inside spheres catacondensed polyacenes: coronene (C24H12, seven rings), with centers on individual atoms also assisted the analysis of ovalene (C32H14, ten rings), circumanthracene (C40H16,13 charge density of the chemical bonds. The starting geo- rings), and circumcoronene (C54H18, 19 rings). The last metries were aromatic hydrocarbons with standard bond section presents a brief summary and discussion of the lengths, and metal atoms located at the ring centers in many bonding motifs that have been found in this study. but not all cases. 2. Computational Method 3. Coordination in Sandwich Molecules All calculations were performed using the Vienna ab initio Figure 2 (a, top left) shows schematically the possible simulation package VASP.11–16) The plane wave based binding sites A, B, C, D of the metal atom to the hydrocarbon calculations used PAW pseudo-potentials17) and the spin layer using the carbon skeleton of the linear polyacene polarized generalized gradient approximation for the ex- pentacene. The coordination of carbon atoms to a metal atom change-correlation energy functional parameterized by Per- in a sandwich is shown in more detail at the bottom as dew et al.11,12) The valence shell of palladium was treated as a Fig. 2(b). The site A (2-coordination) shows bonding to two ten electron system (4d85s2). Valence electrons were as- adjacent C atoms in each layer. Site B (3-coordination) on signed as follows: Pd (10), C (4), and H (1). All calculations the circumference of a molecule has two flavours, convex (B were performed using periodic boundaries with a cubic cell, inside) or concave (B outside). At the edge of the hydro- edge lengths in the range 0.75 nm to 2.50 nm, so the carbon the metal atom on a concave site would lie outside the molecules were at least 0.9 to 1.0 nm apart. All Brillouin footprint of the hydrocarbon and might be important in zone integrations were done at the gamma point. The catalysis reactions. The site C (4-coordination) involves four geometry optimizations were carried out using the conjugate carbon atoms and site D (6-coordination) involves coordi- gradient method usually until the forces acting on each atom nation of the metal atom to a ring of six carbons. The were approximately 5 meV/A˚ . We routinely calculated quantization axis z is parallel to the metal plane (molecular L geometry, total energy for given total spin, isometric surfaces axis) and x-axis is perpendicular to the sandwich plane. In of total charge, spin density and in some cases ELF. this axis system direct Pd-Pd bonding involves palladium Decomposition of total charge density into partial charges dz2-, s- and pz-functions. The orbitals involved in Pd-C based on Kohn-Sham levels was also performed in selected bonding are schematically shown in Fig. 2 (c, top right). Note Metal Sandwich Molecules 695 x (c) px C (a) top layer B dx2-y2 dxz dxy A C D B Pd y2 z (b) A η2 B η3 C η4 D η6 M M M M Fig. 2 Bonding sites of palladium inside the sandwich: (a) hydrocarbon bonds comprising the A-, B-, C-, D-sites; (b) position of the metal atom and the Pd-C bonds; (c) schematic of the relative orientation of metal d-orbitals and the carbon p-orbitals. Note the axis system with x-axis perpendicular and z-parallel to the hydrocarbon plane.

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