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

Reactions of a Pt(II)-diimine-based metalloligand Na-2[Pt(CN)(2)(4,4'-dcbpy)] (4,4'-H(2)dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) with alkaline-earth metal salts Mg(NO3)(2)center dot 6H(2)O, CaCl2, SrCl2 center dot 6H(2)O, and BaBr2 center dot 2H(2)O in aqueous solution gave luminescent complexes formulated as [Mg(H2O)(5)][ Pt(CN)(2)(4,4'-dcbpy)]center dot 4H(2)O (MgPt-4 center dot 9H(2)O), {[Ca(H2O)(3)][Pt(CN)(2)(4,4'-dcbpy)]center dot 3H(2)O}(infinity) (CaPt-4 center dot 6H(2)O), {[Sr(H2O)(2)][Pt(CN)(2)(4,4'-dcbpy)]center dot H2O}(infinity) (SrPt-4 center dot 3H(2)O), and {[Ba(H2O)(2)][ Pt(CN)(2)(4,4'-dcbpy)]center dot 3H(2)O}(infinity) (BaPt-4 center dot 5H(2)O), 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 center dot 9H(2)O, 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 center dot 6H(2)O) and three-dimensional (SrPt-4 center dot 3H(2)O and BaPt-4 center dot 5H(2)O) coordination networks, respectively. All fully hydrated complexes exhibited a strong phosphorescence from the triplet pi-pi* transition state. Luminescence spectroscopy revealed that MgPt-4 center dot 9H(2)O exhibited interesting multichromic (i.e., thermo-, mechano-, and vapochromic) luminescence, whereas CaPt-4 center dot 6H(2)O 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.