Solution- and solid-state NMR study of intermediate eta(3)-allyl-cobalt tricarbonyl type complexes in 3-methyl-1,2-butadiene polymerization

H-1- and C-13-NMR of eta(3)-allyl-cobalt tricarbonyl complexes {Co(CO)(3)}( 2){(eta(3)-Me2CCCH2)(2)CO} (1), {Co(CO)(3)}2{(eta(3)-Me2CCCH2)(3)CO} (2) an d {Co(CO)(3)}(2){(eta(3)-Me2CCCH2)(4)CO} (3) were studied. The X-ray struct ures obtained for 1 and 2 suggested that the 1,2-polymer chain would grow i n a helical manner. Due to the coordination of the Co(CO)(3) units to the c hain-closing prochiral dimethylallylic planes and/or to the hindered rotati on of the dimethylallylic units conjugated to some extent with the ketonic carbonyl several diastereomers and even more rotamers can be envisaged. In solution, however, only two symmetric forms in different ratio could be det ected in all cases studied so far. We have undertaken a detailed NMR study on these compounds both in solution- and solid-state, in order to assign th e resonances unambiguously and understand clearly the nature of the isomers present in solutions. The results obtained suggest the presence of meso an d rac diastereomers due to the coordination of the Co(CO), units to the pro chiral allyl planes. These dimethylallylic groups rotate relatively free ar ound the ketonic carbonyl. (C) 1999 Elsevier Science S.A. All rights reserv ed.