Palladium cluster complex [Pd13(μ4-C7H7)6]2+ (C7H7 = tropylium) with an fcc-close-packed cuboctahedral Pd13 core and isomers: theoretical insight into ligand-control of the Pd13 core structure

Abstract

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 [Pd₁₃(μ₄-C₇H₇)₆]²⁺ (named Cubo-μ4; C₇H₇ = tropylium) with an fcc-close-packed cuboctahedral Pd₁₃ core and possible isomers. The stability decreases in the order Cubo-μ4 > [Pd₁₃(μ₃-C₇H₇)₃(μ₄-C₇H₇)₃]²⁺ with an hcp-close-packed anticuboctahedral Pd₁₃ core (Anti-μ3,4) > [Pd₁₃(μ₃-C₇H₇)₆]²⁺ with a non-close packed icosahedral Pd₁₃ core (Ih-μ3) > [Pd₁₃(μ₄-C₇H₇)₆]²⁺ with an anticuboctahedral Pd₁₃ core (Anti-μ4) > [Pd₁₃(μ₃-C₇H₇)₆]²⁺ with a cuboctahedral Pd₁₃ core (Cubo-μ3). This ordering disagrees with the stability of the Pd₁₃ core. The key factor governing the stability and metal-atom packing manner of these Pd₁₃ cluster complexes is not the stability of the Pd₁₃ core but the interaction energy between the Pd₁₃ core and the [(C₇H₇)₆]²⁺ ligand shell. The interaction energy is mainly determined by the charge-transfer from the Pd₁₃ core to the [(C₇H₇)₆]²⁺ ligand shell and the coordination mode of the C₇H₇ ligand (μ₃- vs. μ₄-coordination bond). In the μ₄-coordination, all seven C atoms of the C₇H₇ ligand interact with four Pd atoms of the Pd₄ plane using two CC double bonds and one π-allyl moiety. On the other hand, in the μ₃-coordination, one or two C atoms of C₇H₇ cannot form bonding interaction with the Pd atom of the Pd₃ plane. Thus, the use of appropriate capping ligands is one of the key points in the synthesis of nano-scale metal cluster complexes.