Collision-Induced Reductive Elimination of 1,3-Diynes from [MAu24(C≡CR)18]2– (M = Pd, Pt) Yielding Clusters of Superatoms

Abstract

We studied collision-induced dissociation (CID) processes of alkynyl-protected alloy clusters [MAu24(C≡CR)18]2– (M = Pd, Pt; R = 3,5-(CF3)2C6H3) having an icosahedral M@Au12(8e) superatomic core using mass spectrometry and density functional theory (DFT) calculations. [MAu24(C≡CR)18]2– underwent CID into [MAu24(C≡CR)18–2n]2– (n = 1–6) by sequential desorption of 1,3-diynes (RC≡C–C≡CR). Notably, the number of the valence electrons of the fragments increased by 2 upon every reductive elimination of a single 1,3-diyne. DFT calculations on the [MAu24(C≡CR)16]2– fragment revealed that the increased two electrons are not accommodated into a 1D superatomic orbital, but are localized at the Au2(C≡CR)1 site newly formed on the M@Au12(8e) core from the original Au2(C≡CR)3 unit by the desorption of 1,3-diyne. This desorption step continued to n = 6, leading to the formation of M@Au12[Au2(C≡CR)1]6. Thus, the [MAu24(C≡CR)18–2n]2– fragments can be viewed as clusters of the M@Au12(8e) superatomic core and n units of Au(2e) in terms of the electronic structure. The average number of 1,3-diynes eliminated from the precursor with M = Pd was larger than that from M = Pt, suggesting that Pd doping may facilitate the reductive elimination step of the homocoupling of terminal alkynes by Au cluster catalysts. These results illustrate that the CID is not only a useful probe for elementary steps for catalysis but also a tool for synthesizing novel superatomic compounds.