Palladium cluster complex [Pd 13 (μ 4 -C 7 H 7 ) 6 ] 2+ (C 7 H 7 = tropylium) with an fcc-close-packed cuboctahedral Pd 13 core and isomers: theoretical insight into ligand-control of the Pd 13 core structure.

One of the challenging targets in today's chemistry is size-, shape- and metal-atom packing-controlled synthesis of nano-scale transition metal cluster complexes because key factors governing these features have been elusive. Here, we present a DFT study on a recently synthesized palladium cluster complex [Pd 13 (μ 4 -C 7 H 7 ) 6 ] 2+ (named Cubo-μ4; C 7 H 7 = tropylium) with an fcc-close-packed cuboctahedral Pd 13 core and possible isomers. The stability decreases in the order Cubo-μ4 > [Pd 13 (μ 3 -C 7 H 7 ) 3 (μ 4 -C 7 H 7 ) 3 ] 2+ with an hcp-close-packed anticuboctahedral Pd 13 core (Anti-μ3,4) > [Pd 13 (μ 3 -C 7 H 7 ) 6 ] 2+ with a non-close packed icosahedral Pd 13 core (Ih-μ3) > [Pd 13 (μ 4 -C 7 H 7 ) 6 ] 2+ with an anticuboctahedral Pd 13 core (Anti-μ4) > [Pd 13 (μ 3 -C 7 H 7 ) 6 ] 2+ with a cuboctahedral Pd 13 core (Cubo-μ3). This ordering disagrees with the stability of the Pd 13 core. The key factor governing the stability and metal-atom packing manner of these Pd 13 cluster complexes is not the stability of the Pd 13 core but the interaction energy between the Pd 13 core and the [(C 7 H 7 ) 6 ] 2+ ligand shell. The interaction energy is mainly determined by the charge-transfer from the Pd 13 core to the [(C 7 H 7 ) 6 ] 2+ ligand shell and the coordination mode of the C 7 H 7 ligand (μ 3 - vs. μ 4 -coordination bond). In the μ 4 -coordination, all seven C atoms of the C 7 H 7 ligand interact with four Pd atoms of the Pd 4 plane using two CC double bonds and one π-allyl moiety. On the other hand, in the μ 3 -coordination, one or two C atoms of C 7 H 7 cannot form bonding interaction with the Pd atom of the Pd 3 plane. Thus, the use of appropriate capping ligands is one of the key points in the synthesis of nano-scale metal cluster complexes.