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.