Palladium cluster complex [Pd(μ-CH)] (CH = tropylium) with an fcc-close-packed cuboctahedral Pd core and isomers: theoretical insight into ligand-control of the Pd 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(μ-CH)] (named Cubo-μ4; CH = tropylium) with an fcc-close-packed cuboctahedral Pd core and possible isomers. The stability decreases in the order Cubo-μ4 > [Pd(μ-CH)(μ-CH)] with an hcp-close-packed anticuboctahedral Pd core (Anti-μ3,4) > [Pd(μ-CH)] with a non-close packed icosahedral Pd core (Ih-μ3) > [Pd(μ-CH)] with an anticuboctahedral Pd core (Anti-μ4) > [Pd(μ-CH)] with a cuboctahedral Pd core (Cubo-μ3). This ordering disagrees with the stability of the Pd core. The key factor governing the stability and metal-atom packing manner of these Pd cluster complexes is not the stability of the Pd core but the interaction energy between the Pd core and the [(CH)] ligand shell. The interaction energy is mainly determined by the charge-transfer from the Pd core to the [(CH)] ligand shell and the coordination mode of the CH ligand (μ- μ-coordination bond). In the μ-coordination, all seven C atoms of the CH ligand interact with four Pd atoms of the Pd plane using two CC double bonds and one π-allyl moiety. On the other hand, in the μ-coordination, one or two C atoms of CH cannot form bonding interaction with the Pd atom of the Pd plane. Thus, the use of appropriate capping ligands is one of the key points in the synthesis of nano-scale metal cluster complexes.