Conformational constraints and σ–π-vinyl interchange in μ3-η3-5,6-dihydroquinoline triosmium complexes

The rates of the σ–π-interchange processes in the series of 5,6-dihydro-μ3-η3-quinolyl complexes Os3(CO)9(C9H6(5-R,6-R′)N)(μ-H) (R=R′=H, 1, R=nBu, R′=H, 2; R=allyl, R′=CH3, 3, R=C(CH3)2CN, R′=CH3, 4; R=CHS(CH2)2S, R′=CH3, 5; R=CH2CO2tBu, R′=CH3, 6; R=CH2CO2tBu, R′=Cl, 7) have been examined by 1H-NMR spectroscopy using the 187Os1H satellites as a dynamical probe. It was found that the unsubstituted and monosubstituted derivatives 1 and 2 are dynamic with respect to the σ-π-interchange process throughout the temperature range examined (+25 to −80°C). The disubstituted derivatives 3–6 on the other hand are rigid on the NMR time scale up to +100°C where coalescence of the satellites is observed. For compound 7, the barrier to σ–π-interchange is intermediate between 1 and 2 and 3–6, exhibiting coalescence at +70°C. Compound 7 also exhibits evidence for population of a second isomer as observed from VT 1H-NMR of the vinylic proton. The results are discussed in the context of the conformational constraints around the 5,6-carbon–carbon bond of the carbocyclic ring in these dihydroquinolyl complexes and are compared with previously reported σ–π-vinyl interchange processes.