A GENERAL-ROUTE TO PROCHIRAL AND CHIRAL DICATIONIC MOLYBDENUM-COBALT ACETYLENIC COMPLEXES - SYNTHESIS, STRUCTURES, AND REACTIVITY

Several [Co-Co]-alkyne adducts u-eta(2),eta(2)-(HOCRR2)-R-1-C=C-(CRROH )-R-1-O-2)] were prepared (R-1 = R-2 = H (1); R-1 = H, R-2 = CH3 (2); R-1 = R-2 = CH3 (3)) and their X-ray molecular structures determined, showing that the mu-alkyne unit adopts a specific geometry depending o n the nature of the alkyne unit. The prochiral [Mo-Co]-alkyne adduct [ CpMoCo(CO)(5)(mu-eta(2),eta(2)-HOCH2-C=C-CH2OH)] (4), identified by X- ray analysis, was obtained from [Co-2-(CO)(6))(mu-eta(2),eta(2)-HOCH2- C=C-CH2OH)] (1) via replacement of a -Co(CO)3 vertex by the isolobal m oiety -CpMo(CO)(2). Complex 4 reacts with aqueous HBF4 to produce the dicationic tetranuclear species O)(5)(mu-eta(2),eta(3)-CH2-C=C-CH2-)}( 2)O][BF4](2) (5), which reacts further with nucleophiles to give a new dicationic tetranuclear species mu-eta(2),eta(2)-Nu(1)-CH2-C=C-CH2-)} (2)O][BF4](2) (Nu(1) pyridine (6), 3-picoline (7), triphenylphosphine (8)), where the nucleophile has been introduced to the carbenium cente r (-CH2+). Subsequent treatment of these dications in acidic medium pr oduces the carbenium ions O)(5)(mu-eta(2),eta(3)-Nu(1)-CH2-C=C-CH2)][B F4](2) (Nu(1) pyridine (9), 3-picoline (10), triphenylphosphine (11)), which further react with nucleophile Nu(2) to produce dications mu-et a(2),eta(2)-Nu(1)-CH2-C=C-CH2-Nu(2))][BF4](2) {Nu(1)= Nu(2) = pyridine (13); Nu(1) = pyridine, Nu(2) = triphenylphosphine (14); Nu(1) = Nu(2 ) = triphenylphosphine (15); Nu(1) = Nu(2) = 3-picoline (16); Nu(1) = pyridine = Nu(2) = 3-picoline(17)). This synthetic approach is unprece dented because it allows the placement of the same or different types of nucleophiles in a stepwise fashion at the two terminal carbon atoms of the acetylenic complex, yielding, for instance, the chiral dicatio nic [Mo Co] cluster a(2),eta(2)-C6H5N-CH2-C=C-CH2-NC6H4(CH3))][BF4](2) (17). The X-ray molecular structures of 13, 16, and 17 belonging to t his family were determined, including the first chiral [Mo-Co] complex (17) possessing two different functional groups. Further, the structu re shows that the coordinated functionalized alkyne can adopt a specif ic geometry, whereby the two positively functionalized groups are situ ated on the same side of the cluster unit, available to bind molecules of opposite charge.