Synthesis of Metal−Hydrazone Complexes and Vapochromic Behavior of Their Hydrogen-Bonded Proton-Transfer Assemblies

We synthesized and investigated a new series of metal−hydrazone complexes, including deprotonated [MX(mtbhp)] and protonated forms [MX(Hmtbhp)](ClO4) (M = Pd2+, Pt2+; X = Cl−, Br−; Hmtbhp = 2-(2-(2-(methylthio)benzylidene)hydrazinyl)pyridine) and hydrogen-bonded proton-transfer (HBPT) assemblies containing [PdBr(mtbhp)] and bromanilic acid (H2BA). The mtbhp hydrazone ligand acts as a tridentate SNN ligand and provides a high proton affinity. UV−vis spectroscopy revealed that these metal−hydrazone complexes follow a reversible protonation−deprotonation reaction ([MX(mtbhp)] + H+ ⇋ [MX(Hmtbhp)]+), resulting in a remarkable color change from red to yellow. Reactions between proton acceptor [PdBr(mtbhp)] (A) and proton donor H2BA (D) afforded four types of HBPT assemblies with different D/A ratios: for D/A = 1:1, {[PdBr(Hmtbhp)](HBA)·Acetone} and {[PdBr(Hmtbhp)](HBA)·2(1,4-dioxane)}; for D/A = 1:2, [PdBr(Hmtbhp)]2(BA); and for D/A = 3:2, {[PdBr(Hmtbhp)]2(HBA)2(H2BA)·2Acetonitrile}. The proton donor gave at least one proton to the acceptor to form the hydrogen bonded A···D pair of [PdBr(Hmtbhp)]+···HBA−. The strength of the hydrogen bond in the pair depends on the kind of molecule bound to the free monoanionic bromanilate OH group. Low-temperature IR spectra (T < 150 K) showed that the hydrogen bond distance between [PdBr(Hmtbhp)]+ and bromanilate was short enough (ca. 2.58 Å) to induce proton migration in the [PdBr(Hmtbhp)]2(BA) assembly in the solid state. The hydrogen bonds formed not only between [PdBr(Hmtbhp)]+ and HBA− but also between HBA− and neutral H2BA molecules in the {[PdBr(Hmtbhp)]2(HBA)2(H2BA)·2Acetonitrile} assembly. The H2BA-based flexible hydrogen bond network and strong acidic host structure result in an interesting vapor adsorption ability and vapochromic behavior in this assembly because the vapor-induced rearrangement of the hydrogen bond network, accompanied by changes in π−π stacking interactions, provides a recognition ability of proton donating and accepting properties of the vapor molecule.