Thermal Activation of Solid-State Molybdenum Halide Clusters with an Octahedral Cluster Framework and Their Application to Catalytic Synthesis of 3-Methylpyridine from Piperidine and Methanol

Abstract Solid-state molybdenum halide clusters with an octahedral metal framework MoX2 (or [Mo6X8]X2X4/2) (X = Cl, Br, I) are applied to catalysis. When these clusters are thermally activated in a hydrogen stream above 300 °C, they exhibit catalytic activity for the dehydrogenative C-methylation of piperidine with methanol, to yield 3-methylpyridine. At 400 °C, the selectivity is as high as 74%. This catalytic behavior is different from that of the molecular clusters [(M6Cl12)Cl2(H2O)4]·4H2O (M = Nb, Ta) and (H3O)2[(M6Cl8)Cl6]·6H2O (M = Mo, W), which exhibit Brønsted acidity after thermal activation; piperidine is N-methylated to yield N-methylpiperidine selectively. Elemental analysis and thermogravimetric analysis demonstrate that the solid-state clusters partially eliminate halogen ligands during the activation. Infrared analysis of adsorbed pyridine on the activated clusters shows the presence of a Lewis acid site. This coordinatively unsaturated site of the molybdenum is catalytically active for dehydrogenative C-methylation. The formation of an η3-1-azaallyl species on the molybdenum facilitates the methylation at the 3-position of piperidine, followed by dehydrogenation to yield 3-methylpyridine.