A COMPOUND WITH A NEW TYPE OF BUTTERFLY METAL ATOM CLUSTER - [N(C4H9)4]2[MO4OBR12]

The reaction of molybdenum(III) bromide with a mixture of acetic acid and acetic anhydride in the presence of [N(C4H9)4][BF4] affords the ti tle compound. It was isolated as a crystalline solvate containing 0.5( CH3)2CO and the structure determined. The anion has a butterfly array of molybdenum atoms with an internal dihedral angle of 118.6-degrees. The Mo-Mo distances around the periphery have an average value of 2.67 1[3] angstrom while that between hinge atoms is 2.594(1) angstrom. The re is a mu4-O atom making distances of 2.061[9] angstrom to the wingti p Mo atoms and 2.104[3] angstrom to the hinge Mo atoms, with Mo-O-Mo a ngles of 153.9(3) and 76.1(2)-degrees, respectively. The structure can be regarded as a fragment of a Mo6(mu3-Br)8Br6 octahedral cluster fro m which two adjacent Mo-Br units have been deleted, followed by insert ion of the mu4-O atom. Alternatively, it may be regarded as two triang le Mo3(mu3-O)(mu3-Br)(mu2-Br)3Br6 clusters fused on a common Mo-Mo-O f ace. The crystallographic parameters are as follows: space group P1BAR , a = 12.755(2) angstrom, b = 21.192(3) angstrom, c = 11.692(2) angstr om, alpha = 103.53(1)-degrees, beta = 107.99(1)-degrees, gamma = 82.00 (1)-degrees, V = 2914.6(7) angstrom3, Z = 2. By using 6714 observed re flections, 472 parameters were refined to R = 0.054, R(w) = 0.087. Mol ecular orbital calculations (Fenske-Hall and SCF-Xalpha-SW) have been carried out for [Mo4OX12]2-, [Mo4Cl12]3-, and Mo4Cl12 clusters, and th e results have been analyzed on the basis of the clusters in molecules concept. It is found that a set of five strong Mo-Mo bonds exists wit h additional structural support by interaction of the oxygen 2p orbita ls with all of the molybdenum atoms.