Proton motion inside [(DMF)2H]2[W6Cl14]: structural, Raman and luminescence studies. Issue 43 (3rd November 2020) - Kolesov, Boris A.
Abstract : Proton coupled [(DMF)2 H] + dimers in the crystal structure of [(DMF)2 H]2 [W6 Cl14 ] strongly differentiate upon cooling, resulting in different proton motion behavior. Abstract : Protonation of DMF by (H7 O3 )2 [W6 Cl14 ] results in the appearance of strongly proton coupled [(DMF)2 H] + dimers. Such units are captured as the cationic part of [(DMF)2 H]2 [W6 Cl14 ] (1 ). The proton behavior in such dimers was studied for the first time with single crystal X-ray diffraction (XRD) and 1 H MAS NMR, Raman and photoluminescence (PL) spectroscopic techniques. The experimental data reveal the presence of two types of [(DMF)2 H] + dimers in 1 (cisoidal and transoidal, with respect to the mutual orientations of their C–O groups) which differ in terms of the degree to which they interact with the cluster anions as the temperature decreases. At room temperature all the O⋯O distances in the [(DMF)2 H] + dimers are very short (2.375 Å) and almost equal. 1 H 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 belonging to a single-well potential of proton motion. The temperature decrease leads to the differentiation of [(DMF)2 H] + dimers due to the elongation of the O⋯O distance in one pair while keeping a practically constant O⋯O distance in the second pair. The analysis of the cation–anion interactions reveals a strong difference between these two types of dimers which results from the 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 the PL of such dimeric units in the solid state. The temperature screening of the PL behavior demonstrates two types of luminescent centers at low temperatures which coalesce at 298 K. The proton motion in the hydrogen bond was studied using Raman spectroscopy, which was beneficial to monitor the complex behavior over a very broad temperature range from 5 to 298 K. According to the Raman data, we are dealing with a symmetric double-well potential for the hydrogen bond at room temperature, which becomes a broad single well potential below 110 K for the [(DMF)2 H] + cation with a longer O⋯O distance (the cisoidal isomer) and below 60 K for the [(DMF)2 H] + cation with a shorter O⋯O distance (the transoidal isomer).