Catalytic Activity of Molybdenum(II) Complexes in Homogeneous and Heterogeneous Conditions

The new complexes [MoBr­(η3-C3H5)­(CO)2(L)2] (C1) and [MX2(CO)3(L)2] (M = Mo­(II), X = I (C2); M = Mo­(II), X = Br (C3); M = W­(II), X = I (C4); M = W­(II), X = Br (C5)) were synthesized by reaction of 2-amino-1,3,4-thiadiazole (L) with [MoBr­(η3-C3H5)­(CO)2(NCCH3)2] (1), [MoI2(CO)3(CH3CN)2]­(M = Mo (2); M = W (4)), or [MoBr2(CO)3(CH3CN)2]­(M = Mo (3); M = W (5)) in 2:1 ratio. The five complexes were immobilized in MCM-41, yielding the materials MCM-Cn (n = 1–5), and C1 was also immobilized in silica gel (Silica-C1) and in a polyhedral oligomeric silsesquioxane (Cube-C1). Complexes and materials were fully characterized by spectroscopic techniques and elemental analysis. DFT calculations showed that several C1 isomers should coexist. The as synthesized and supported complexes were tested as catalysts on the oxidation of geraniol, cis-hex-3-en-1-ol, trans-hex-3-en-1-ol, (S)-limonene, and 1-octene. The conversions and TOF significantly depend on the complex and the nature of the substrate. The general conclusions are (i) complex C1 has the highest activity; (ii) tungsten complexes C4 and C5 are more active than the molybdenum analogues; (iii) the immobilization of the catalysts improves the performance; and (iv) silica gel and the polyhedral oligomeric silsesquioxane supports modify the selectivity, leading to products different from the one obtained with MCM for specific substrates.