Aurophilicity as a structure-directing principle in hyperchlorinated Au_nCl_n+5 ^- (2 ≤ n ≤ 7) clusters

This study addresses the gap concerning the structural landscape of experimentally observed high-chloride Au_nCl_n+5 ^- cluster anions, whose stable geometries, aurophilic interaction patterns, thermodynamic stability, and growth pathways remain largely unexplored compared to well-characterized low- and medium-chloride analogues. To resolve this, we systematically investigate Au_nCl_n+5 ^- (n = 2-7) using laser desorption/ionization mass spectrometry (LDI-MS) and density functional theory at the TPSSh/aug-cc-pVTZ/ECP60MDF level. The results show that elevated laser intensity preferentially stabilizes these chlorine-rich clusters. Except for the smallest cluster (Au₂Cl₇ ^-), the global minima adopt nearly planar zigzag geometries, where terminal gold atoms are in a tetracoordinate Au(III) state and internal gold atoms are in an Au(I) state. Bond-length analysis reveals that only non-terminal Au-Au distances are significantly shorter than the van der Waals sum, indicating aurophilic attraction, while terminal distances show no such interaction. Aurophilicity emerges at Au₄Cl₉ ^- and rapidly stabilizes for larger clusters. Natural bond orbital (NBO) and electron localization function (ELF) analyses attribute this interaction to 6s/5d orbital delocalization between adjacent Au(I) centers, with no contribution from Au(III)-Au(I) pairs. Comparison with lower-chloride series (Au_nCl_n+1 ^- and Au_nCl_n+3 ^-) indicates that the core thermodynamic trend remains unchanged, although higher laser fluence favors chlorine-rich species via thermodynamically favored addition of two Cl atoms. This study highlights the structure-directing role of aurophilicity in hyperchlorinated gold clusters and provides a comprehensive understanding of their size-dependent structural evolution.