Metal Effect on the Supramolecular Structure, Photophysics, and Acid−Base Character of Trinuclear Pyrazolato Coinage Metal Complexes^†

Varying the coinage metal in cyclic trinuclear pyrazolate complexes is found to significantly affect the solid-state packing, photophysics, and acid−base properties. The three isoleptic compounds used in this study are {[3,5-(CF₃)₂Pz]M}₃ with M = Cu, Ag, and Au (i.e., Cu₃, Ag₃, and Au₃, respectively). They form isomorphous crystals and exist as trimers featuring nine-membered M₃N₆ rings with linear two-coordinate metal sites. On the basis of the M−N distances, the covalent radii of two-coordinate Cu^I, Ag^I, and Au^I were estimated as 1.11, 1.34, and 1.25 Å, respectively. The cyclic {[3,5-(CF₃)₂Pz]M}₃ complexes pack as infinite chains of trimers with a greater number of pairwise intertrimer M···M interactions upon proceeding to heavier coinage metals. However, the intertrimer distances are conspicuously short in Ag₃ (3.204 Å) versus Au₃ (3.885 Å) or Cu₃ (3.813 Å) despite the significantly larger covalent radius of Ag^I. Remarkable luminescence properties are found for the three M₃ complexes, as manifested by the appearance of multiple unstructured phosphorescence bands whose colors and lifetimes change qualitatively upon varying the coinage metal and temperature. The multiple emissions are assigned to different phosphorescent excimeric states that exhibit enhanced M···M bonding relative to the ground state. The startling luminescence thermochromic changes in crystals of each compound are related to relaxation between the different phosphorescent excimers. The trend in the lowest energy phosphorescence band follows the relative triplet energy of the three M^I atomic ions. DFT calculations indicate that {[3,5-(R)₂Pz]M}₃ trimers with R = H or Me are bases with the relative basicity order Ag ≪ Cu < Au while fluorination (R = CF₃) renders even the Au trimer acidic. These predictions were substantiated experimentally by the isolation of the first acid−base adduct, {[Au₃]₂:toluene}_∞, in which a trinuclear Au^I complex acts as an acid.