Systematic structural control of multichromic platinum(ii)-diimine complexes ranging from ionic solid to coordination polymer

Abstract

Reactions of a Pt(II)-diimine-based metalloligand Na₂[Pt(CN)₂(4,4′-dcbpy)] (4,4′-H₂dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) with alkaline-earth metal salts Mg(NO₃)₂·6H₂O, CaCl₂, SrCl₂·6H₂O, and BaBr₂·2H₂O in aqueous solution gave luminescent complexes formulated as [Mg(H₂O)₅][Pt(CN)₂(4,4′-dcbpy)]·4H₂O (MgPt-4·9H₂O), {[Ca(H₂O)₃][Pt(CN)₂(4,4′-dcbpy)]·3H₂O}_∞ (CaPt-4·6H₂O), {[Sr(H₂O)₂][Pt(CN)₂(4,4′-dcbpy)]·H₂O}_∞ (SrPt-4·3H₂O), and {[Ba(H₂O)₂][Pt(CN)₂(4,4′-dcbpy)]·3H₂O}_∞ (BaPt-4·5H₂O), respectively. The crystal structures of all MPt-4 complexes were determined by X-ray crystallography. In these structures, the alkaline-earth metal ions are commonly coordinated to the carboxyl groups of the [Pt(CN)₂(4,4′-dcbpy)]²⁻ metalloligand. In the case of MgPt-4·9H₂O, the Mg(II) ion is bound by five water molecules and one oxygen atom of a carboxyl group to form a neutral complex molecule [Mg(H₂O)₅][Pt(CN)₂(4,4′-dcbpy)]. In contrast, the alkaline-earth metal ion and metalloligand form two-dimensional (CaPt-4·6H₂O) and three-dimensional (SrPt-4·3H₂O and BaPt-4·5H₂O) coordination networks, respectively. All fully hydrated complexes exhibited a strong phosphorescence from the triplet π–π* transition state. Luminescence spectroscopy revealed that MgPt-4·9H₂O exhibited interesting multichromic (i.e., thermo-, mechano-, and vapochromic) luminescence, whereas CaPt-4·6H₂O showed only thermochromic luminescence. The other two complexes did not exhibit any chromic behaviour. Combination analysis of powder X-ray diffraction, thermogravimetry, and IR spectroscopy suggests that the dimensionality of the coordination network contributes considerably to both the structural flexibility and luminescence properties; that is, the low-dimensional flexible coordination network formed in MPt-4 complexes with smaller alkaline-earth metal ions enables a structural rearrangement induced by thermal and mechanical stimuli and vapour adsorption, resulting in the observed multichromic behaviour.