Co-crystallization of atomically precise metal nanoparticles driven by magic atomic and electronic shells

This paper reports co-crystallization of two atomically precise, different-size ligand-stabilized nanoclusters, a spherical (AuAg)267(SR)80 and a smaller trigonal-prismatic (AuAg)45(SR)27(PPh3)6 in 1:1 ratio, characterized fully by X-ray crystallographic analysis (SR = 2,4-SPhMe2). The larger cluster has a four concentric-shell icosahedral structure of Ag@M12@M42@M92@Ag120(SR)80 (M = Au or Ag) with the inner-core M147 icosahedron observed here for metal nanoparticles. The cluster has an open electron shell of 187 delocalized electrons, fully metallic, plasmonic behavior, and a zero HOMO-LUMO energy gap. The smaller cluster has an 18-electron shell closing, a notable HOMO-LUMO energy gap and a molecule-like optical spectrum. This is the first direct demonstration of the simultaneous presence of competing effects (closing of atom vs. electron shells) in nanocluster synthesis and growth, working together to form a co-crystal of different-sized clusters. This observation suggests a strategy that may be helpful in the design of other nanocluster systems via co-crystallization.

The authors report (1) the synthesis of atomically pure Ag-Au metal-nanoclusters, a two-layered M45 and a four-layer M267 one, regularly protected basically by −thiolate ligands and also by some ←phosphine ligands ; (2) the preparation of 1:1 co-crystals ; (3) the full structure resolution of the co-crystal by single crystal X-ray diffraction, resolving the structural details, exhibiting the well described regularities ; (4) the UV-Vis absorption spectra, confirming the smaller molecular and the larger nano metallic character of the two units ; (5) the further characterization by differential pulse voltammetry ; (6) and finally the quantum-chemical PBE-DFT calculational reproduction and electronic analysis of the two units.
The report and discussion is clear and concise. It is a great paper.
I have only three questions: (1) A small point: Concerning Fig. 4a, the red curve A is for the small unit in solution, the curve B for the co-crystal. What is curve C? According to the legend (which may be written more clearly) I would guess that C is for the large unit in solution -however in the Figure the wording suggests that C is the difference of co-crystal minus small-unit-solution spectra. Please clarify.
(2) Another minor point: How well do the experimentally derived and the theoretically optimized geometries agree with each other? Please add a short sentence concerning typical deviations and trends.
(3) Did the authors account for the seriously relativistic behaviour of the metallic (gold) valence electrons? I could not find any word about that. If a nonrelativistic model was applied, the calculations are somewhat meaningless, and the agreement with reality must be due to large error cancellation. Then we MUST think what to do.
The authors present a fascinating discovery of a co-crystal of two different sizes of noble metal nanoparticles. It is the first observation to my knowledge of a co-crystal of two differently-sized clusters. It is hard to purposefully create co-crystals yet they have the potential for novel properties, so anything that can be learned about this type of system is very valuable. The individual nanoparticles themselves also have unique and highly interesting structures. I recommend publication in Nature Communications after the following minor points are considered.
I have only minor comments about the manuscript: p.4 rephrase "or by simulated based on energy consideration " p.4: this doesn't make sense to me: "x=93.8 ± 0.1 for AuxAg1-x " with this formula, I would expect x between 0 and 1. it doesn't seem to fit with a 2.33:1 ratio. should subscript 1 be 312 instead? p.5: is the wrong figure listed in the following statement "the 9 atoms in M9 tricapped trigonal prism core of the (AuAg)45 cluster, are simply composed with occupancies exceeding 95% (see Supplementary Fig. 7)." ? the figure is for 267, not 45. p.6: how were the nanoparticles separated in order to get the individual components of the absorption in Fig. 4? p.7: spelling "1J-symmteric" Caption of Figure 1: rephrase "The individual structures of the plasmonic (AuAg)267 nanoparticle cocrystallized and of the molecule-like (AuAg)45 cluster. " it is called "tri-prism" in caption, but is normally referred to as trigonal prism Figure 2 caption: forth should be fourth; meaning of "severally capped" is unclear The authors could mention something about the single crystal [(AuAg)45] results in the SI -a picture would be nice to show its differences from the co-crystal figures.
We appreciate the helpful comments from the reviewer. According to the reviewer's suggestions, we have carefully revised our manuscript as follows: 1. Comment: This paper reports a joint experimental and theoretical study of atomically precise synthesis of ligand protected metal nanoclusters. The important result of this paper is that the authors have discovered co-crystallization of two nano-clusters composed of (AuAg)45 and (AuAg)267 core. The former is stabilized by electronic shell closure while the later is stabilized by atomic shell closure. One of the most important discoveries in cluster science has been the presence of magic numbers and their potential as budding blocks of a new class of materials -cluster assembled materials.
Ways have been found to make this possible. This papers shows that in a single experiment one can observe both the electronic shell closure and atomic shell closure effect. The paper is well written, the results are important, and I recommend its publication in Nature Communications, subject to the following implementation. That cluster assembled materials could be synthesized with magic clusters as building blocks and that both electronic and atomic shell closings are important was first brought into focus by Khanna and Jena (Phys. Rev. Lett. 69, 1664 (1992); Phys. Rev. B 51, 13705 (1995)). These papers should be cited.
Response: Thank the reviewer for bringing the references to our attention. We have cited these two references in our revised manuscript as Ref. 11 and 12.
Thank the reviewer for the publication recommendation and also helpful comments.
1. Comment: A small point: Concerning Fig. 4a, the red curve A is for the small unit in solution, the curve B for the co-crystal. What is curve C? According to the legend (which may be written more clearly) I would guess that C is for the large unit in solutionhowever in the Figure the wording suggests that C is the difference of co-crystal minus small-unit-solution spectra. Please clarify.
Response: Thank for the reviewer' carefully attentions. We have added curve C=B-A=(AuAg)267 in Fig. 4a. And yes, the curve C is for the large (AuAg)267 unit in solution. Since the pure (AuAg)267 unit have not been obtained yet, we just indirectly calculated its molar absorptivity via the difference of co-crystal minus small-unit-solution spectra.

Response:
We thank the referee for asking about inclusion of the relativistic effects. We do it routinely for all heavy metals. We have added a clarification of this into the methods description