Heteroleptic copper(i) complexes bearing functionalized 1_H-pyrazole-bipyridine ligands: synthesis, photophysical properties, crystal structures, and applications in halogen sensing

A series of new heteroleptic copper(i) complexes [Cu(NN)(P<^>P)][BF4] (1-6), where N<^>N is a pyrazole-substituted 2,2'-bipyridine (5-(1H-pyrazol-4-yl)-2,2'-bipyridine (5-pzbpy, L-1), 4,4 & PRIME;-di(1H-pyrazol-4-yl)-2,2'-bipyridine (4,4 & PRIME;-pz(2)bpy, L-2), 5,5 & PRIME;-di(1H-pyrazol-4-yl)-2,2'-bipyridine (5,5'-pz(2)bpy, L-3), and 3,8-di(1H-pyrazol-4-yl)-1,10-phenanthroline (3,8-pz(2)phen, L-4)) and PP is either oxybis(2,1-phenylene)bis(diphenylphosphane) (POP) or 1,3-bis(diphenylphosphanyl)propane (dppp), were successfully prepared and investigated. The crystal structures of 1, 2, 3 and 6 were determined by single X-ray diffraction. The novel copper(i) complexes 1, 2 and 3 show turn-on emission sensing for halogen ions. The fluorescence enhancement of the copper(i) complexes increases in the order of F- < I- < Br- < Cl-, and the differences in fluorescence intensity could be easily distinguished with the naked eye under UV light illumination. The H-1 NMR titration indicated that the interaction involves hydrogen-bond formation between the halide (Cl-, Br- and I-) and the acidic H of the pyrazole group, with N-H deprotonation occurring at increasing F- concentrations. The result was further confirmed by carrying out theoretical calculations.