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-(Npy,Npy) and trans-(Npz,Npz), complex 3
reveals a different configuration, cis-(Npz,Npz) and trans-(Npy,Npy) (Npy 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 S1(ππ*) → S0 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 nm.
The remarkable differences in photophysical properties were rationalized by a combination
of π-electron accepting CO ligand, relative pypz orientation, and heavy-atom-enhanced spin−orbit coupling effects.