eta(4)-coordination of dienes and heterodienes to the tripodcobalt(I) template [CH3C(CH2PPh2)(3)Co](+): Synthesis, structure, and dynamics

CH3C(CH2PPh2)(3)CoCl (1) is easily accessible from CH3C(CH2PPh2)(3)CoCl2 by reduction with activated zinc powder. Upon dehalogenation with TlPF6, 1 re acts with dienes to give [tripodCo(I)-(eta(4)-diene)](+) (2) The heterodien es acrolein and methyl vinyl ketone produce the analogous eta(4)-heterodien e compounds 3. When crotonaldehyde is used as the potential eta(4)-diene li gand, decarbonylation is observed leading to [tripodCo(I)-(CO)(2)](+) (4). Reaction of [tripodCo(ag)](BF4)(2) with allyl mercaptan produces [tripodCo( I)-(eta(4)-thioacrolein)](+) (3a) through dehydrogenation of the Ligand pre cursor. 1,2-Diketones such as benzil and phenanthrenequinone do not coordin ate in a eta(4) fashion but rather generate eta(2)-coordinate enediolato li gands by an electron-transfer process, resulting in compounds of the type { tripodCo(III)-[eta(2)-RC(O)=C(O)R]}(+) (5). All the compounds have been cha racterized by standard analytical and spectroscopic techniques, including X -ray analysis in some cases. Compounds 4 and 5 show trigonal-bipyramidal co ordination in the solid state, whereas the coordination polyhedra in compou nds 2 and 3 are better described as square-pyramidal. While a minimum of tw o phosphorus resonances might be expected for each of these coordination ge ometries, only one time-averaged signal is normally observed. Only with the eta(4)-coordinated heterodienes acrolein, methyl vinyl ketone, and thioacr olein present in 3 is there a resolution of the signals of the three chemic ally distinct phosphorus nuclei at low temperatures. By P-31-NMR Line-shape analysis, the activation barriers for the rotational reorientation of the heterodienes are found to be around Delta H-not equal = 47 kJmol(-1) for al l three compounds 3.