Au₁₅Ag₃（SPhMe₂）₁₄Nanocluster:Crystal Structure And Insights Into The Ligand-induced Variation

Ligand-protected nanoclusters（NCs）with precise atomic structure have attracted increasing interest in the past few decades,owing to their highly promising properties such as the electrochemical,catalytic and optical properties.Compared to the single-component systems,alloy NCs often exhibit unique physicochemical properties because of the synergistic effect between different metal atoms.These properties create more opportunities for applications in catalysis,biosensing,luminescence,surface patterning and molecular electronics.In particular,the gold-silver alloys are of interest because the atomic size（or the lattice constants）of Au and Ag are nearly identical,and the similar valence electron characters might lead to significant electronic structure perturbations in nanocluster range.For example,a 200-fold quantum yield boost in the fluorescence has been achieved when 13 silver atoms are doped into the[Au₂₅（PPh₃）io（SR_）5]²⁺ nanorod(i.e.[Agi₃Au₁₂（PPh₃）io（SR）₅]²⁺).Elsewhere,doping single gold in Ag₂₉ significantly enhanced the thermal stability.Nonetheless,the structure determination of the AuAg alloy NCs remains quite challenging.So far,the synthetic methods of alloy NCs can be mainly classified into the following two classes:1)in situ synthesis by reducing the bi-metallic Au-Ag complexes;2)doping the heteroatom（s）complexes to the single-component templated-NCs.In this context,the doping strategy might allow the mechanistic insights into the synergistic effect from the atomic-level,which is of major importance for synthesizing more functionalized NCs.According to our recent study on doping Au25（SR）i8 nanocluster,the framework of Au25 was retained and the AuxAg25-x with same overall structure was finally generated.By contrast,in Jin and co-workers’ recent study on doping Au23（SR）i6 with Ag（I）SR,the Au23 nanocluster is transformed to a different structure,i.e.,Au25-xAgx.These phenomena pose an interesting question:if other NCs with single-component are doped by heteroatom complexes,whether or not the framework（structure）will be perturbed?The answer to this question will not only help us better understand the heteroatom effect within a confined environment（in the nanocluster range）,but also expand the application of doping strategy to synthesize bi-metallic NCs with atomic precision.On the other hand,aside from the heteroatom effects,surface ligands also have a significant implications on the size,structure and physico-chemical properties of NCs.The ligand effects in NCs have been extensively studied through both experimental and theoretical studies.For instance,compared the Au25（SR）18（where SR=PhCH2CH2S）,the luminescence,electrochemical properties and the catalytic performance of the Au25（SePh）18 show noticeable differences.Understanding the profound effects on the structures and properties of the NCs by altering the coordination preference,bulkiness and electronic nature of ligands can help us have more fundamental insight into the interrelation on property between ligands and NCs.In addition,the relationship could shed light on synthetizing new NCs with controllable structure and property.We report here the discovery of framework-retained doping of Au₁₈（SR）14 with Ag（Ⅰ）SR,which generated the Au₁₅Ag3（SR）14 nanocluster（SR= 2,4-dimethyl benzenethiol,i.e.,SPhMe₂）.The structure of Au₁₅Ag3（SPhMe₂）14 nanocluster was successfully determined by X-ray crystallography,and further characterized by matrix-assisted laser desorption/ionization mass spectrometry（MALDI-MS）,inductively coupled plasma（ICP）and X-ray photoelectric spectroscopy（XPS）measurements.Compared to the previously reported Au₁₅Ag3（SC6H11）14 nanocluster with the same framework and metallic arrangement as those of Au₁₅Ag3（SPhMe₂）14,the significant changes in UV-vis spectra of Au₁₅Ag3（SPhMe₂）14 could be explained by different composition in energy levels.Density functional theory（DFT）calculations illustrate the differences in the UV-vis spectra were caused by the distinct electronic components of the frontier orbitals.In addition,analyzing the MALDI-MS spectra of the crude products and the recrystallized results,we found that the Au₁₃Ag5 and Au14Ag4 in crude products decomposes and the Au₁₅Ag3（SPhMe₂）14 nanocluster finally exists as the sole product.