Novel calix[4]arene complexes of tetravalent molybdenum

The reaction of p-tert.-Butylcalix[4]aren Cax(OH)(4) with [Mo(NMe2)(4)] in equimolar amounts at 80 degreesC in toluene affords after extraction into acetonitrile a mixture of [Cax-O4Mo(NHMe2)(NCMe)] (1(NCMe)) and [(CaXO4MO)(2)(NCMe)(2)] (2(NCMe)2). If the same reaction is carried out in acetonitrile or in mixtures of toluene and acetonitrile instead of toluene, the formation of 2(NCMe)(2) is suppressed and only 1(NCMe) has been isolated. Both compounds have been characterized by X ray crystal structure determinations. 1(NCMe): space group: C2/c, lattice constants: a = 37,987(8) Angstrom, b = 13,012(3) Angstrom, c = 20,271(4) Angstrom, beta = 103,39degrees; 2(NCMe)(2): space group: P2(1)/n, lattice constants: a = 11,937(2) Angstrom, b = 21,078(4) Angstrom, c = 19,620(4) Angstrom, beta = 107,31(3)degrees. The molybdenum atom in 1(NCMe) is coordinated with four oxygen atoms of the calix[4]arene ligand, a nitrogen atom of the amine ligand, and the nitrogen atom of the endohedrally coordinated acetonitrile molecule in a slightly distorted octahedron. Two of these monomer units are linked via hydrogen bridges. In 2(NCMe)2 two complex fragments [CaXO4Mo(NCMe)] are linked via phenolate units of the calix[4]arene. The Mo-Mo' distance of 261.2(1) pm is in accordance with a Mo-Mo double bond. EH-, DFT-, und MP2- calculations have been performed on model complexes [Cax-O4Mo(NH3)(NCH)] (1'(NCH)) and [(p-H-CaxO(4)MO)(2)(NCH)(2)] (2'(NCH)(2)) for a closer inspection of the binding in these compounds. The results of the calculations suggest that addition of the electron rich, basic oxygen atom is the structure determining feature of 2'(NCH)(2) and not a metal metal bond.