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.

Synthetic details, solid-state structures, and photophysical properties of a group of trimeric copper(I) complexes containing pyrazolate ligands are described. The reaction of copper(I) oxide and the fluorinated pyrazoles [3-(CF(3))Pz]H, [3-(CF(3)),5-(Me)Pz]H, and [3-(CF(3)),5-(Ph)Pz]H leads to the corresponding trinuclear copper(I) pyrazolates, {[3-(CF(3))Pz]Cu}(3), {[3-(CF(3)),5-(Me)Pz]Cu}(3), and {[3-(CF(3)),5-(Ph)Pz]Cu}(3), respectively, in high yield. The {[3,5-(i-Pr)(2)Pz]Cu}(3) compound was obtained by a reaction between [Cu(CH(3)CN)(4)][BF(4)], [3,5-(i-Pr)(2)Pz]H, and NEt(3). These compounds as well as {[3,5-(Me)(2)Pz]Cu}(3) and {[3,5-(CF(3))(2)Pz]Cu}(3) adopt trimeric structures with nine-membered Cu(3)N(6) metallacycles. There are varying degrees and types of intertrimer Cu...Cu interactions. These contacts give rise to zigzag chains in the fluorinated complexes, {[3-(CF(3))Pz]Cu}(3), {[3-(CF(3)),5-(Me)Pz]Cu}(3), {[3-(CF(3)),5-(Ph)Pz]Cu}(3), and {[3,5-(CF(3))(2)Pz]Cu}(3), whereas the nonfluorinated complexes, {[3,5-(Me)(2)Pz]Cu}(3) and {[3,5-(i-Pr)(2)Pz]Cu}(3) form dimers of trimers. Out of all the compounds examined in this study, {[3-(CF(3)),5-(Ph)Pz]Cu}(3) has the longest (3.848 Angstroms) and {[3,5-(Me)(2)Pz]Cu}(3) has the shortest (2.946 Angstroms) next-neighbor intertrimer Cu...Cu distance. The Cu...Cu separations within the trimer units do not vary significantly (typically 3.20-3.26 Angstroms). All of these trinuclear copper(I) pyrazolates show bright luminescence upon exposure to UV radiation. The luminescence bands are hugely red-shifted from the corresponding lowest-energy excitations, rather broad, and unstructured even at low temperatures, suggesting metal-centered emissions owing to intertrimer Cu...Cu interactions that are strengthened in the phosphorescent state. The {[3-(CF(3)),5-(Ph)Pz]Cu}(3) compound exhibits an additional highly structured phosphorescence with a vibronic structure corresponding to the pyrazolyl (Pz) ring. The luminescence properties of solids and solutions of the trimeric compounds in this study show fascinating trends with dramatic sensitivities to temperature, solvent, concentration, and excitation wavelengths.

Dicyanoaurate(I) and dicyanoargentate(I) ions undergo significant oligomerization in aqueous and methanolic solutions. The absorption edges of K[Au(CN) 2 ] and K[Ag(CN) 2 ] solutions undergo progressive red shifts with an increase in concentration up to near the saturation limits, whereupon total red shifts of 13.4 × 10 3 and 11.9 × 10 3 cm -1 are obtained from the respective maxima of the corresponding lowest energy monomer bands. Two types of deviations from Beer's law are observed: a negative deviation for the monomers' MLCT bands and a positive deviation for the oligomers' bands. Increasing the concentration within a given concentration range leads to red shifts in the oligomers' absorption and/or excitation bands dominant in that range, while further increases in concentration lead to the appearance of new lower energy bands. Electronic structure calculations suggest that this behavior is attributed to metal-metal interactions between neighboring Au(CN) 2or Ag(CN) 2ions. Formation constants of 1.50 ( 0.05 and 17.9 ( 2.0 M -1 are obtained for [Ag(CN) 2 -] 2 and [Au(CN) 2 -] 2 dimers, respectively, at ambient temperature.

Solutions of K[Au(CN)(2)] and K[Ag(CN)(2)] in water and methanol exhibit strong photoluminescence. Aqueous solutions of K[Au(CN)(2)] at ambient temperature exhibit luminescence at concentration levels of > or =10(-2) M, while frozen methanol glasses (77 K) exhibit strong luminescence with concentrations as low as 10(-5) M. The corresponding concentration limits for K[Ag(CN)(2)] solutions are 10(-1) M at ambient temperature and 10(-4) M at 77 K. Systematic variations in concentration, solvent, temperature, and excitation wavelength tune the luminescence energy of both K[Au(CN)(2)] and K[Ag(CN)(2)] solutions by >15 x 10(3) cm(-1) in the UV-visible region. The luminescence bands have been individually assigned to *[Au(CN)(2)(-)](n) and *[Ag(CN)(2)(-)](n) excimers and exciplexes that differ in "n" and geometry. The luminescence of Au(I) compounds is related for the first time to Au-Au bonded excimers and exciplexes similar to those reported earlier for Ag(I) compounds. Fully optimized unrestricted open-shell MP2 calculations for the lowest-energy triplet excited state of staggered [Au(CN)(2)(-)](2) show the formation of a Au-Au sigma single bond (2.66 A) in the triplet excimer, compared to a weaker ground-state aurophilic bond (2.96 A). The corresponding frequency calculations revealed Au-Au Raman-active stretching frequencies at 89.8 and 165.7 cm(-1) associated with the ground state and lowest triplet excited state, respectively. The experimental evidence of the exciplex assignment includes the extremely large Stokes shifts and the structureless feature of the luminescence bands, which suggest very distorted excited states. Extended Hückel (EH) calculations for [M(CN)(2)(-)](n) and *[M(CN)(2)(-)](n) models (M = Au, Ag; n = 2, 3) indicate the formation of M-M bonds in the first excited electronic states. From the average EH values for staggered dimers and trimers, the excited-state Au-Au and Ag-Ag bond energies are predicted to be 104 and 112 kJ/mol, respectively. The corresponding bond energies in the ground state are 32 and 25 kJ/mol, respectively.