Synthesis and characterization of metal complexes possessing the 5-(2-pyridyl) pyrazolate ligands: The observation of remarkable osmium-induced blue phosphorescence in solution at room temperature

A total of three distinctive main group and transition metal complexes containing the 2-pyridyl pyrazolate (pypz) ligand were prepared, namely, [B(C6F5)(2)(pypz)] (1), [Ru(CO)(2)-(pypz)(2)] (2), and [Os(CO)(2)(pypz)(2)] (3), where (pypz)H = 3-trifluoromethyl-5-(2-pyridyl)pyrazole. Single-crystal X-ray diffraction studies were carried out on complexes 2 and 3, revealing octahedral coordination geometry with two CO ligands located at cis dispositions. While the pypz ligand arrangement for complex 2 is in cis-(N-py,N-py) and trans- (N-pz,N-pz), complex 3 reveals a different configuration, cis-(N-pz,N-pz) and trans-(N-py,N-py) (N-py for pyridine-N and Npz for pyrazolate donor sites). Similar to that of the in-situ-prepared pypz anion, the boron complex [B(C6F5)(2)(pypz)] (1) exhibits a strong emission centered at 380 nm, which is unambiguously assigned to fluorescence derived from the S-1(pipi*)-->S-0 transition. In contrast to the nonluminescent behavior for Ru complex 2, the Os complex 3 displays unique, strong room-temperature phosphorescence, showing vibronic progressions at 430, 457, and 480 rim. The remarkable differences in photophysical properties were rationalized by a combination of pi-electron accepting CO ligand, relative pypz orientation, and heavy-atom-enhanced spin-orbit coupling effects.