QUADRUPLY BONDED DIMOLYBDENUM COMPOUNDS WITH POLYDENTATE PHOSPHINES .1. MO2X4(TETRAPHOS-1) (TETRAPHOS-1 = PH2PCH2CH2P(PH)CH2CH2P(PH)CH2CH2PPH2, X = CL, BR), THE 1ST EXAMPLES OF ENANTIOMERIC 1,2,5,8/1,2,6,7 AND 1,2,5,7/1,2,6,8 TYPES

Five meso and racemic diastereomers, rac-Mo2Cl4(tetraphos-1).CH2Cl2 (1), meso-Mo2Cl4(tetraphos-1) (2), rac-Mo2Br4(tetraphos-1).0.5 CH2Cl2 (3), meso-Mo2Br4(tetraphos-1).CH2Cl2 (4), and meso-Mo2Br4(tetraphos-1).1.5 THF (5) (tetraphos-1 = Ph2PCH2CH2P(Ph)CH2CH2P(Ph)CH2CH2PPh2) have been synthesized and structurally characterized. Among them are two new types of geometrical isomers, namely, 1,2,5,7/1,2,6,8 and 1,2,5,8/1,2,6,7 enantiomeric pairs, of M2X4L4 complexes. Four significant differences emerge as we compare the structures of complexes with the meso and racemic ligands: (i) While the two central phosphorus donor atoms coordinate to different Mo centers in the racemic ligand, they coordinate to the same Mo atom in the complex of the meso ligand, thus giving the bischelating/single-bridging coordination mode in the former and the chelating/double-bridging mode in the latter. (ii) The average Mo-Mo bond distances, 2.154 angstrom and 2.190 angstrom for racemic and meso diastereomers, respectively, are different by 0.037 A, but they both fall within the range of the distances established for Mo-Mo quadruple bonds. (iii) The average of the four smallest torsional angles is different: 18.8 and 30.0-degrees for the racemic and meso diastereomers, respectively. (iv) While the racemic diastereomers comprise the 1,2,5,8/1,2,6,7 enantiomers, the meso ones are designated as the 1,2,5,7/1,2,6,8 enantiomers. The crystal structures of 1-5 are fully described. Crystallographic data for these complexes are as follows: 1, C2/c with a = 19.527(6) angstrom, b = 18.210(2) angstrom, c = 13.241(2) angstrom, beta = 104.85(2)degrees, V = 4551(2) angstrom3, and Z = 4; 2, C2/c with a = 26.174(5) angstrom, b = 20.297(5) angstrom, c = 16.047(7) angstrom, beta = 100.03(3)degrees, V = 8395(5) angstrom3, and Z = 8; 3, C2/c with a = 19.563(2) angstrom, b = 18.317(4) angstrom, c = 13.354(3) angstrom, beta = 104.22(1)degrees, V = 4638(2) angstrom3, and Z = 4; 4, P2(1)/c with a = 16.444(4) angstrom, b = 21.335(3) angstrom, c = 25.882(3) angstrom, beta = 91.20(2)degrees, V = 9078(3) angstrom3, and Z = 8; 5, P2(1)/c with a = 19.920(5) angstrom, b = 17.818(5) angstrom, c = 15.999(4) angstrom, beta = 111.27(2)degrees, V = 5292(5) angstrom3, and Z = 4. In addition to the structural data for these complexes, P-31{H-1} NMR and UV-vis spectroscopy have also been used to characterize the complexes.