Calixarene and Trimethylphosphine Complexes of Molybdenum

The synthesis of molybdenum, tungsten and nickel mono- and dinuclear complexes using low-valent trimethylphosphine metal precursors and calixarene ligands, namely p-tert-butylcalix[4]arene and its sulfur variants p-tert-butylthiacalix[4]arene, p-tert-butylsulfonylcalix[4]arene and p-tert-butyl-mercaptothiacali[4]arene, is explored in an effort to develop small-molecule models for species relevant to hydrodesulfurization (HDS) catalysis. p-tert-Butylcalix[4]arene reacts with Mo(PMe3) 6 and W(PMe3)4(eta2-CH2 PMe2)H to yield complexes of composition {[ CalixBut (OH)2(O)2]M(PMe3) 3H2}. The new compounds exhibit unprecedented binding modes for the calixarene ligand, making use of one of its methylene groups as a binding functionality. X-ray diffraction studies reveal that in the molybdenum complex the methylene coordinates to the metal via an agostic interaction, whereas the tungsten derivative undergoes oxidative addition of the C--H bond yielding the metallated trihydride species [ CalixBut -H(OH)2(O)2]W(PMe3)3H 3. However, NMR spectroscopic studies suggest that, in solution, the agostic and metallated isomers are in equilibrium for the tungsten system. Evidence for facile dissociation of the agostic interaction and metal migration around the phenolic rim is provided by dynamic NMR spectroscopic studies and labeling experiments. The importance of this facile dissociation as a means to generating possible reactive intermediates is illustrated by the formation of the kappa4-coordinated metal alkylidene [ CalixBut (O)2]W=C(Ph)R [R = C(Ph)C(Ph)CH2Ph] from the reaction of {[ CalixBut (OH)2(O)2]W(PMe3)3H 2} with diphenylacetylene. The bisphenol 2,2'-methylenebis(4,6-dimethylphenol) affords non-macrocyclic molybdenum and tungsten counterparts to the aforementioned calixarene complexes, namely [kappa2, eta2-CH2(Ar Me2O)2]M(PMe3)3H 2. Comparison between the compounds derived from the two ligands suggests that the methylene group of the calixarene is more susceptible to participating in agostic interactions and undergoing subsequent oxidative addition than is the methylene of the bisphenoxide ligand. The sulfur-containing calixarene variants p-tert-butylthiacalix[4] arene, p-tert-butylsulfonylcalix[4]arene and p-tert-butylmercaptothiacalix[4]arene are employed for the synthesis of metal complexes with sulfur-rich coordination, in an attempt to mimic the sulfur-rich environment on the surface of HDS catalysts. (Abstract shortened by UMI.)