Vapor-Controlled Linkage Isomerization of a Vapochromic Bis(thiocyanato)platinum(II) Complex: New External Stimuli To Control Isomerization Behavior

We synthesized a novel Pt­(II)–diimine complex
with a typical
ambidentate thiocyanato ligand, [Pt­(thiocyanato)2(H2dcbpy)] (1; H2dcbpy
=4,4′-dicarboxy-2,2′-bipyridine), and found that the
complex 1 exhibits unique linkage isomerizations with
drastic color and luminescence changes driven by exposure to volatile
organic chemical (VOC) vapors in the solid state. Reaction between
[PtCl2(H2dcbpy)] and KSCN in aqueous solution
at 0 °C enabled successful isolation of an isomer with the S-coordinated
thiocyanato ligand, [Pt­(SCN)2(H2dcbpy)] (1SS·H2O), as a nonluminescent orange solid. Interestingly, 1SS·H2O was isomerized
completely to one isomer with the N-coordinated isothiocyanato ligand,
[Pt­(NCS)2(H2dcbpy)] (1NN·3DMF) by exposure to DMF vapor, and this isomerization
was accompanied by significant color and luminescence changes from
nonluminescent orange to luminescent red. IR spectroscopy and thermogravimetric
analysis revealed that adsorption of the DMF vapor and transformation
of the hydrogen-bonded structure both played important roles in this
vapor-induced linkage isomerization. Another isomer containing both
S- and N-coordinated thiocyanato ligands, [Pt­(SCN)­(NCS)­(H2dcbpy)] (1SN), was obtained as a nonluminescent yellow solid simply by exposure
of 1SS·H2O to
acetone vapor at room temperature, and about 80% of 1SS·H2O was found to be converted
to 1SN. In the solution state, each isomer changed gradually
to an isomeric mixture, but pure 1SS was regenerated
by UV light irradiation (λirr. = 300 nm) of an MeOH
solution of the mixture. In the crystal structure of 1SN, the complex molecules were hydrogen-bonded to each other through
the carboxyl groups of the H2dcbpy ligand and the N site
of the thiocyanato ligand, whereas the 1NN molecules
in the 1NN·4DMF crystal were hydrogen-bonded to
the solvated DMF molecules. Competition of the hydrogen-bonding ability
among the carboxyl groups of the H2dcbpy ligand, N and
S atoms of the thiocyanato ligand, and the vapor molecule was found
to be one of the most important factors controlling linkage isomerization
behavior in the solid state. This unique linkage isomerization controlled
by vapor can provide an outstanding vapochromic system as well as
a new molecular switching function driven by vapor molecules.