η 4 ‐Coordination of Dienes and Heterodienes to the TripodCobalt(I) Template [CH 3 C(CH 2 PPh 2 ) 3 Co] + : Synthesis, Structure, and Dynamics

CH 3 C(CH 2 PPh 2 ) 3 CoCl ( 1 ) is easily accessible from CH 3 C(CH 2 PPh 2 ) 3 CoCl 2 by reduction with activated zinc powder. Upon dehalogenation with TlPF 6 , 1 reacts with dienes to give [ tripod Co I ‐(η 4 ‐diene)] + ( 2 ). The heterodienes acrolein and methyl vinyl ketone produce the analogous η 4 ‐heterodiene compounds 3 . When crotonaldehyde is used as the potential η 4 ‐diene ligand, decarbonylation is observed leading to [ tripod Co I ‐(CO) 2 ] + ( 4 ). Reaction of [ tripod Co aq ](BF 4 ) 2 with allyl mercaptan produces [ tripod Co I ‐(η 4 ‐thioacrolein)] + ( 3a ) through dehydrogenation of the ligand precursor. 1,2‐Diketones such as benzil and phenanthrenequinone do not coordinate in a η 4 fashion but rather generate η 2 ‐coordinate enediolato ligands by an electron‐transfer process, resulting in compounds of the type { tripod Co III ‐[η 2 ‐RC(O)=C(O)R]} + ( 5 ). All the compounds have been characterized by standard analytical and spectroscopic techniques, including X‐ray analysis in some cases. Compounds 4 and 5 show trigonal‐bipyramidal coordination in the solid state, whereas the coordination polyhedra in compounds 2 and 3 are better described as square‐pyramidal. While a minimum of two phosphorus resonances might be expected for each of these coordination geometries, only one time‐averaged signal is normally observed. Only with the η 4 ‐coordinated heterodienes acrolein, methyl vinyl ketone, and thioacrolein present in 3 is there a resolution of the signals of the three chemically distinct phosphorus nuclei at low temperatures. By 31 P‐NMR line‐shape analysis, the activation barriers for the rotational reorientation of the heterodienes are found to be around Δ H ≠ = 47 kJmol –1 for all three compounds 3 .