Blue-Emitting Platinum(II) Complexes Bearing both Pyridylpyrazolate Chelate and Bridging Pyrazolate Ligands: Synthesis, Structures, and Photophysical Properties

A new Pt(II) dichloride complex [Pt(fppzH)Cl2] (1), in which fppzH = 3-(trifluoromethyl)-5-(2-pyridyl)pyrazole, was
prepared by the treatment of a pyridylpyrazole chelate fppzH with K2PtCl4 in aqueous HCl solution. Complex 1
could further react with its parent pyrazole (pzH), 3,5-dimethylpyrazole (dmpzH), or 3,5-di-tert-butylpyrazole (dbpzH)
to afford the monometallic [Pt(fppz)(pzH)Cl] (2), [Pt(fppz)(dmpzH)Cl] (3), [Pt(fppz)(dmpzH)2]Cl (4), or two structural
isomers with formula [Pt(fppz)(dbpzH)Cl] (5a,b). Single-crystal X-ray diffraction studies of 2, 4, and 5a,b revealed
a square planar Pt(II) framework, among which a strong interligand hydrogen bonding occurred between fppz and
pzH ligands in 2. This interligand H-bonding is replaced by dual N−H···Cl interaction in 4 and both intermolecular
N−H···O (with THF solvate) and N−H···Cl interaction in 5a,b, respectively; the latter are attributed to the bulky
tert-butyl substituents that force the dbpzH ligand to adopt the perpendicular arrangement. Furthermore, complex
2 underwent rapid deprotonation in basic media to afford two isomeric complexes with formula [Pt(fppz)(μ-pz)]2
(6a,b), which are related to each other according to the spatial orientation of the fppz chelates, i.e., trans- and
cis-isomerism. Similar reaction exerted on 3 afforded isomers 7a,b. Both 6a,b (7a,b) are essentially nonemissive
in room-temperature fluid state but afford strong blue phosphorescence in solid state prepared via either vacuum-deposited thin film or 77 K CH2Cl2 matrix. As also supported by the computational approaches, the nature of
emission has been assigned to be ligand-centered triplet ππ* mixed with certain metal-to-ligand charge-transfer
character.