Excited-State Turn-On of Aurophilicity and Tunability of Relativistic Effects in a Series of Digold Triazolates Synthesized via iClick

iClick reactions
between Au­(I) acetylides PPh3Au–CCR,
where R = nitrophenyl (PhNO2), phenyl (Ph), thiophene (Th),
bithiophene (biTh), and dimethyl aniline (PhNMe2), and
Au­(I)–azide PPh3AuN3 provide digold complexes
of the general formula R-1,5-bis-triphenylphosphinegold­(I)
1,2,3-triazolate (Au2-R). Within the digold triazolate complexes the Au­(I) atoms are held
in close proximity but beyond the distance typically observed for
aurophilic bonding. Though no bond exists in the ground state, time-dependent
density functional theory interrogation of the complexes reveals excited
states with significant aurophilic bonding. The series of complexes
allows for tuning of the excited-state “turn-on” of
aurophilicity, where ligand to metal charge transfer (LMCT) induces
the aurophilic bonding. Complexes containing ligand-localized excited
states, however, do not exhibit aurophilicity in the excited state.
As a control experiment, a monogold complex was synthesized. The computed
excited state of the monogold species exhibited LMCT to the gold ion
as in the dinuclear cases, but without a partnering gold ion only
a distinct N–Au–P bending occurs, revealing a potential
mechanism for the excited-state turn-on of aurophilic bonding. Analysis
of the steady-state electronic spectra indicates that LMCT states
are achievable for compounds with sufficiently strong electron-donating
ligands, and in digold complexes this is associated with enhanced
fluorescence, suggestive of an aurophilic interaction.