Nanogymnastics: Visualization of Intercluster Reactions by High-Resolution Trapped Ion Mobility Mass Spectrometry

Abstract

Although single-crystal X-ray diffraction is a proven technique to determine the structure of monolayer-protected coinage metal clusters in solid state, it is not readily applicable to the characterization of such cluster structures in solution. The complexity of the characterization problem increases further when intercluster reactions are studied, in which two reactive cluster ions interact to form final products using a sequence of structural changes involving exchange of metal atoms and ligands.Here, we present the first time-resolved structural study of such processes which occur when solutions of [TOA] + [Au 25 (PET) 18 ] − and [PPh 4 ] 4 4+ [Ag 44 (FTP) 30 ] 4− react upon mixing (PET: phenylethanethiolate; FTP: 4-fluorothiophenolate; and TOA: tetraoctylammonium ion). This is achieved using high-resolution trapped ion mobility mass spectrometry (TIMS). Specifically, we have used electrospray transfer to the TIMS apparatus followed by ion mobility measurements to probe the time-dependent structure of mass-selected Au x Ag 44−x (FTP) 30 4− (x = 0−12) exchange products, with limited FTP for PET exchanges, formed in the reaction medium. Over the roughly 40 min reaction time before equilibration, with a product distribution centered around Au 12 Ag 32 (FTP) 30 4− , we observe intermediate species, Au x Ag 44−x (FTP) 30 4−, whose collision cross sections (CCSs) at a given x increase first relative to that of the Ag 44 (FTP) 30 4− parent and decrease subsequently. We attribute this to an energy-driven migration of the incorporated Au atoms from the ligated "staples" at the cluster surface to its icosahedral core. Upon collisional heating of Au x Ag 44−x (FTP) 30 4− , analogous back-migration of the heavier Au atoms from the core to the staples was observed in tandem mass spectrometry. To support our experimental observations, several isomeric structures (with all ligands) were calculated using density functional theory, and their CCS values were modeled using trajectory method calculations.

Atomically Precise Alloy Nanoclusters

“…[56,57] This illustrates that [Ag 44 [57] The groups of Xie, Pradeep, and Kappes, and Bigioni independently reported the synthesis of [Ag 44Àx Au x (SR) 30 ] 4À alloy NCs (n = 0-12;S R = p-MBA or SPhF). [89,102,195] In the same paper,X ie et al [89] also reportedt he synthesis of [Ag 44Àx Cu x (p-MBA) 30 ] 4À alloy NCs (x = 0-6). .…”

“…Because of these advantages, metal exchange has often been used to synthesize alloy NCs in recent years. The heteroatoms introduced by this method are not always located outside the alloy NCs; the heteroatoms move to the most stable position in the NCs after their introduction by metal exchange [102] . Recently, deep understanding of the mechanism of heteroatom transfer in alloy NCs, which involves a shuttle process, [103] has been obtained [39, 104] …”

“…Therefore, it can be considered that the alloy NC, in which 12 Ag inside the core werer eplaced with Au, was stably formed. Pradeep et al [102] reported that in the reaction of [Au 25 (PET) 18 ] À with [Ag 44 (SPhF) 30 ] 4À ,A uf irst replaced Ag in the staples of [Ag 44 (SPhF) 30 ] 4À and then movedi nto the core via metal exchange within the NC (Figure 37 B). Based on these results, it is presumed that [Ag 32 Au 12 (SPhF) 30 ] 4À with the geometricals tructure shown in Figure 37 Ai sg enerated regardless of the synthesis method.…”

“…Zheng and colleagues reported the synthesis and geometrical structure of [Ag 32 Au 12 (SR) 30 ] 4− (SR=SPhF, SPhF 2 , or SPhCF 3 ) in the same paper as [Ag 44 (SR) 30 ] 4− [57] . The groups of Xie, Pradeep, and Kappes, and Bigioni independently reported the synthesis of [Ag 44− x Au x (SR) 30 ] 4− alloy NCs ( n= 0–12; SR= p ‐MBA or SPhF) [89, 102, 195] . In the same paper, Xie et al [89] .…”

“… (A) Geometrical structure of [Ag 32 Au 12 (SPhF 2 ) 30 ] 4− [57] . (B) Schematic of Au atom migration from the outer staple to the inner core of the NC [102] . Reproduced with permission from references [57] and [102].…”