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-(CF 3 ) 2 Pz]M} 3 with M ) Cu, Ag, and Au (i.e., Cu 3 , Ag 3 , and Au 3 , respectively). They form isomorphous crystals and exist as trimers featuring nine-membered M 3 N 6 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-(CF 3 ) 2 Pz]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 Ag 3 (3.204 Å) versus Au 3 (3.885 Å) or Cu 3 (3.813 Å) despite the significantly larger covalent radius of Ag I . Remarkable luminescence properties are found for the three M 3 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) 2 Pz]M} 3 trimers with R ) H or Me are bases with the relative basicity order Ag , Cu < Au while fluorination (R ) CF 3 ) renders even the Au trimer acidic. These predictions were substantiated experimentally by the isolation of the first acid−base adduct, {[Au 3 ] 2 :toluene} ∞ , in which a trinuclear Au I complex acts as an acid.

The correlation-consistent composite approach ͑ccCA͒, an ab initio composite technique for computing atomic and molecular energies, recently has been shown to successfully reproduce experimental data for a number of systems. The ccCA is applied to the G3/99 test set, which includes 223 enthalpies of formation, 88 adiabatic ionization potentials, 58 adiabatic electron affinities, and 8 adiabatic proton affinities. Improvements on the original ccCA formalism include replacing the small basis set quadratic configuration interaction computation with a coupled cluster computation, employing a correction for scalar relativistic effects, utilizing the tight-d forms of the second-row correlation-consistent basis sets, and revisiting the basis set chosen for geometry optimization. With two types of complete basis set extrapolation of MP2 energies, ccCA results in an almost zero mean deviation for the G3/99 set ͑with a best value of −0.10 kcal mol −1 ͒, and a 0.96 kcal mol −1 mean absolute deviation, which is equivalent to the accuracy of the G3X model chemistry. There are no optimized or empirical parameters included in the computation of ccCA energies. Except for a few systems to be discussed, ccCA performs as well as or better than Gn methods for most systems containing first-row atoms, while for systems containing second-row atoms, ccCA is an improvement over Gn model chemistries.

The interactions present in cyclic trinuclear coinage metal pyrazolates were studied computationally. Cuprophilic interaction was found to bind the singlet ground state of the dimer of trimers {[Cu(Pz)] 3 } 2 , overcoming electrostatic repulsion. The large variation in intertrimer separations found in the literature for coinage metal pyrazolates is consistent with the relatively weak metallophilic interaction. The emissive triplet excited-state geometry of {[M(Pz)] 3 } 2 is predicted by density functional calculations to show major geometric distortion caused by Jahn-Teller instability and excimeric M-M bonding. Large calculated Stokes' shifts, which are also observed for experimental models, are consistent with significant excited-state distortions for these materials. The major finding derived from the present study is that the intertrimer M‚‚‚M contraction in the emissive T 1 state is much more than the intratrimer contraction in all {[M(Pz)] 3 } 2 models, giving rise to a lower T 1 f S 0 phosphorescence energy in these models than in analogous monomer-of-trimer models. The observations made here point to a great potential for rationally tuning the emission properties of trinuclear coinage metal complexes through choice of the metal and ligands.

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