Retention of the octahedral metal framework of Nb and Mo halide clusters in catalytic decomposition of phenyl acetate to phenol and ketene

When molecular chloride clusters of Nb, Mo, Ta, and W ([(M6Cl12)Cl2(H2O)4]·4H2O (M = Nb, Ta) and (H3O)2[(M6Cl8)Cl6]·6H2O (M = Mo, W)) possessing an octahedral metal framework were treated in a helium stream above 200 °C, catalytic activity for the selective decomposition of phenyl acetate to phenol and ketene arose. The Fries rearrangement did not occur. The activity was ascribed to the Brønsted acidity of the hydroxo ligand that was formed by elimination of hydrogen chloride from the chloro and aqua ligands, and the selectivity was attributable to the large counter cluster anion that did not stabilize the intermediate protonated phenyl acetate and acetyl cations. The maximum activities of the Nb and Mo clusters appeared by activation at 250 °C and 400 °C, respectively, at which temperatures the retention of the octahedral cluster framework was ascertained by XDR analyses, Raman spectrometry, EXAFS spectrometry, and elemental analyses.

Graphical abstractWhen molecular halide clusters [(Nb6Cl12)Cl2(H2O)4]·4H2O and (H3O)2[(Mo6Cl8)Cl6]·6H2O were treated in a helium stream at 250 °C and 400 °C, respectively, the maximum catalytic activities for decomposition of phenyl acetate to phenol and ketene appeared. The activity was ascribed to the Brønsted acidity of the hydroxo ligand formed, and the retention of the octahedral cluster frameworks was ascertained. Figure optionsDownload full-size imageDownload as PowerPoint slide