Theoretical study of neutral and charged Fe7-(C6H6)m, m = 1, 2 rice-ball clusters

Bonding of benzene molecules on the surface of neutral and charged Fe7 clusters, which have pentagonal bipyramids (PBP), was studied by means of all-electrons density functional calculations. Dispersion corrections were done with the BPW91-D2 method using the 6-311++G(2d,2p) basis sets. With two less coordinated equatorial sites (bonded to four iron atoms) and one axial site (bonded to five atoms), a triangular face of Fe7 emerges as the basic unit for the absorption of benzene moieties. Bonding of benzene (Bz) on such triangle yields the ground state (GS) for Fe7Bz, Fe7Bz(-), and Fe7Bz(+). Without dispersion, in the GS of Fe7Bz(-), the ligand is η(6) coordinated with a single equatorial iron site, and in the GSs of Fe7Bz2 and Fe7Bz2(-), each benzene moiety is η(6) bonded on opposite equatorial sites. However, BPW91-D2 yields GS structures for Fe7Bz2, Fe7Bz2(-), and Fe7Bz2(+), where the absorption is done on opposite triangles. Therefore, dispersion corrections are crucial for a proper study of Fe7Bz2. The multiplicities (M = 2S + 1, where S is the total spin) of these species, 17, 16, and 18, respectively, are smaller than those of Fe7(23), Fe7(-)(22) and Fe7(+)(24) showing important quenching of the magnetic moment of Fe7. Bond dissociation energies (BDE), in kcal/mol, for Fe7Bz (32.7), Fe7Bz(+) (47.3), and Fe7Bz(-) (27.2) show bigger (smaller) values for the cation (anion). A similar picture was found for the BDEs of Fe7Bz2. Ionization energies, 5.37 and 4.94 eV, for m = 1 and 2 are smaller than that of Fe7, 6.00 eV; which is due to delocalization of the electrons through the network of 3d-π bonds. Electron affinities of Fe7Bz1,2 are also smaller than that of Fe7, being mainly due to the increased repulsion.