Strong Intra- and Intermolecular Aurophilic Interactions in a New Series of Brilliantly Luminescent Dinuclear Cationic and Neutral Au(I) Benzimidazolethiolate Complexes

The structural and photophysical properties of a new series of cationic and neutral Au(I) dinuclear compounds (1
and 2, respectively) bridged by bis(diphenylphosphino)methane (dppm) and substituted benzimidazolethiolate (X−BIT) ligands, where X = H (a), Me (b), OMe (c), and Cl (d), have been studied. Monocationic complexes, [Au2(μ-X−BIT)(μ-dppm)](CF3CO2), were prepared by the reaction of [Au2(μ-dppm)](CF3CO2)2 with 1 equiv of X−BIT in
excellent yields. The cations 1a−1d possess similar molecular structures, each with a linear coordination geometry
around the Au(I) nuclei, as well as relatively short intramolecular Au(I)···Au(I) separations ranging between 2.88907(6) Å for 1d and 2.90607(16) Å for 1a indicative of strong aurophilic interactions. The cations are violet luminescent
in CH2Cl2 solution with a λemmax of ca. 365 nm, assigned as ligand-based or metal-centered (MC) transitions. Three
of the cationic complexes, 1a, 1b, and 1d, exhibit unusual luminescence tribochromism in the solid-state, in which
the photoemission is shifted significantly to higher energy upon gentle grinding of microcrystalline samples with ΔE
= 1130 cm-1 for 1a, 670 cm-1 (1b), and 870 cm-1 (1d). The neutral dinuclear complexes, [Au2(μ-X−BIT)(μ-dppm)] (2a−2d) were formed in good yields by the treatment of a CH2Cl2 solution of cationic compounds (1) with
NEt3. 2a−2d aggregate to form dimers having substantial intra- and intermolecular aurophilic interactions with
unsupported Au(I)···Au(I) intermolecular distances in the range of 2.8793(4)−2.9822(8) Å, compared with
intramolecular bridge-supported separations of 2.8597(3)−2.9162(3) Å. 2a-2d exhibit brilliant luminescence in the
solid-state and in DMSO solution with red-shifted λemmax energies in the range of 485−545 nm that are dependent
on X−BIT and assigned as ligand-to-metal−metal charge transfer (LMMCT) states based in part on the extended
Au···Au···Au···Au interactions.