Proton motion inside [(DMF)2H]2[W6Cl14]: structural, Raman and luminescence studies
Protonation of DMF by (H7O3)2[W6Cl14] results in appearance of strongly proton coupled [(DMF)2H]+ dimers. Such units are captured as the cationic part of [(DMF)2H]2[W6Cl14] (1) The proton behavior in such dimers was studied for the first time with single crystal diffraction (XRD), 1H MAS NMR, Raman spectroscopy and photoluminescence (PL) techniques. The experimental data reveal the presence of two types of the [(DMF)2H]+ dimers in 1 (cisoidal and transoidal, respective to mutual orientations of their C-O groups) which differentiate by the degree in which they interact with the cluster anions as the temperature decreases. At room temperature all the O…O distances in the [(DMF)2H]+ dimers are very short (2.375 Å) and almost equal. 1H MAS NMR spectra show a resonance line at 18.7 ppm which is very close to that observed in sodium hydrogen maleate with a strong hydrogen bond with a single-well potential of proton motion. The temperature decrease leads to the differentiation of [(DMF)2H]+ dimers due to elongation of the O…O distance in one pair while keeping practically constant O…O distance in the second pair. Analysis of the cation-anion interactions reveals a strong difference between these two types of dimers which results from shifting of one DMF molecule toward a terminal Cl– ligand of the cluster anion. The DFT calculations were used to show the difference in O..H+..O stretches for two different dimers. Moreover, we have found PL of such dimeric units in the solid state. The temperature screening of the PL behavior demonstrates two types of the luminescent centers at low temperatures which coalesce at 298 K. The proton motion in the hydrogen bond was followed with Raman spectroscopy, taking an advantage to monitor complex behavior over a very wide temperature interval form 5 to 298 K. Accordin to Raman data, we are dealing with a symmetric double-well potential for hydrogen bond at room temperature, which becomes broad single well potential below 110 K for the [(DMF)2H]+ cation with longer O…O distance (the cisoidal isomer), and below 60 K for the [(DMF)2H]+ cation with shorter O…O distance (the transoidal isomer).