Conformational constraints and sigma-pi-vinyl interchange in mu(3)-eta(3)-5,6-dihydroquinoline triosmium complexes

The rates of the sigma-pi-interchange processes in the series of 5,6-dihydr o-mu(3)-eta(3)-quinolyl complexes Os-3(CO)(9)(C9H6(5-R,6-R')N)(mu-H) (R = R ' = H, 1, R = Bu-n, R' = H, 2; R = allyl, R = CH3, 3, R = C(CH3)(2)CN, R' = CH3, 4; R = -CHS(CH2)(2)S-, R' = CH3, 5; R = (CH2CO2Bu)-Bu-t, R' = CH3, 6; R = (CH2CO2Bu)-Bu-t, R' = Cl, 7) have been examined by H-1-NMR spectroscop y using the Os-187-H-1 satellites as a dynamical probe. It was found that t he unsubstituted and monosubstituted derivatives 1 and 2 are dynamic with r espect to the sigma-pi-interchange process throughout the temperature range examined (+25 to -80 degrees C). The disubstituted derivatives 3-6 on the other hand are rigid on the NMR time scare up to +100 degrees C where coale scence of the satellites is observed. For compound 7, the barrier to sigma- pi-interchange is intermediate between 1 and 2 and 3-6, exhibiting coalesce nce at +70 degrees C. Compound 7 also exhibits evidence for population of a second isomer as observed from VT H-1-NMR of the vinylic proton. The resul ts are discussed in the context of the conformational constraints around th e 5,6-carbon-carbon bond of the carbocyclic ring in these dihydroquinolyl c omplexes and are compared with previously reported sigma-pi-vinyl interchan ge processes. (C) 2000 Elsevier Science S.A. All rights reserved.