Thiacalix[4]arene: New protection for metal nanoclusters

Abstract

Surface organic ligands are critical for the formation and properties of atomically precise metal nanoclusters. In contrast to the conventionally used protective ligands such as thiolates and phosphines, thiacalix[4]arene has been used in the synthesis of a silver nanocluster, [Ag35(H2L)2(L)(C≡CBu(t))16](SbF6)3, (H4L, p-tert-butylthiacalix[4]-arene). This is the first structurally determined calixarene-protected metal nanocluster. The chelating and macrocyclic effects make the thiacalix[4]arene a rigid shell that protects the silver core. Upon addition or removal of one silver atom, the Ag35 cluster can be transformed to Ag36 or Ag34 species, and the optical properties are changed accordingly. The successful use of thiacalixarene in the synthesis of well-defined silver nanoclusters suggests a bright future for metal nanoclusters protected by macrocyclic ligands.

Keywords: Calixarene; alkynyl ligand; coordination mode; crystal structure; ligand-protected metal nanocluster; silver nanocluster.

Scheme 1. Structure of thiacalix[4]arene H₄L.

Fig. 1. Representative crystal structure of [Ag₃₅(H₂L)₂(L)(C≡CBu^t)₁₆]³⁺ cluster.

(A) Overall structure of cationic cluster 1. (B) Space-filling view of the metal core structure. (C) The arrangement of thiacalixarene ligands and the metal core. Pink/green, silver; red, oxygen; yellow, sulfur; gray/blue, carbon; bright green, chloride. Dash line represents hydrogen bonding. The hydrogen atoms were omitted for clarity.

Fig. 2. The arrangements of metal core and ligands.

(A to D) Top view: Position of 10 peripheral Ag atoms (green) held by thiacalixarene ligands onto the Ag₁₃ core (pink) (A and C). Bottom view: Position of 12 peripheral Ag atoms (green, triangular prisms) capped by alkynyl ligands (B and D). (E and F) Top and side views of the position of surface ligands with respect to the Ag₃₅ core. Purple, sphere, Ag₃₅ core; blue, thiacalixarene ligand; gray, alkynyl ligand.

Fig. 3. The coordination motifs A and B of the thiacalix[4]arene with silver atoms.

The silver atoms in pink belong to the Ag₁₃ core.

Fig. 4. ESI-MS spectra.

(A) Experimental (blue) and simulated (red) spectra of [Ag₃₅(H₂L)₂(L)(C≡CBu^t)₁₆]³⁺ (1). (B) Formation of Ag₃₆ cluster by addition of 2 equiv of NEt₃ and 1 equiv of AgSbF₆ to Ag₃₅. (C) The deprotonated product of 1 by addition of 2 equiv of NEt₃. (D) Formation of Ag₃₄ cluster by addition of 2 equiv of HBF₄ to Ag_35.

Fig. 5. TEM image of 1.

Scheme 2. Transformation among Ag₃₄, Ag₃₅ and Ag₃₆ cluster species.

The alkynyl ligands have been omitted for clarity.

Fig. 6. The optical absorption spectra of Ag₃₅ and analogous Ag₃₄ and Ag₃₆ species in CH₂Cl₂.

Abs., absorbance.

Fig. 7. The experimental absorption in comparison with the calculated spectrum of Ag₃₅.

a.u., arbitrary unit.