Kinetic element effect for atropisomerization of an organometallic complex of the misdirected ligand 1,1′-biisoquinoline

The quinoline moieties of the metal-bound eta(2)-1,1'-biisoquinoline ligand of (eta(6)-benzene)(delta/lambda-1,1'-biisoquinoline)halometal(II) hexafluorophosphate (metal = ruthenium, osmium; halo = chloro, iodo; 1(M = Ru, Os; X = Cl, I)) are stereotopic. The rates of atropisomerization of the delta/lambda-1,1'-biisoquinoline ligand, measured by spin-labeling NMR methods, indicate the energy barrier is higher for 1(Ru) than 1(Os); e.g., Delta H-double dagger[1(M = Ru, X = Cl)] = 77.3(2) and Delta H-double dagger[1(M = Os, X = Cl)]= 71.2(2) kJ mol(-l). Since the crystal structures of 1(M = Ru, X Cl) and 1(M = Os, X = Cl) reveal comparable metric parameters, steric factors associated with atropisomerization of the 1,1'-biisoquinoline ligand, essentially the deformation of the 1,1'-binaphthylene skeleton that is necessary to pass H-8 and H-8' past one another, are presumably equivalent for the Ru and Os derivatives. Assuming that normal bond energies are greater for the third-row transition metal than for second-row transition metals, we conclude the difference in reactivity can be attributed to electronic factors-the sigma-donor orbitals and pi-acceptor orbitals of the 1,1-biisoquinoline ligand are misdirected in the ground state but redirected in the syn transition state of atropisomerization. Thus, an inverse relationship between the kinetic and thermodynamic stabilities of 1 is observed for the misdirected reversible arrow [directed](double dagger) reversible arrow misdirected (MDM) isomerization of 1 (the more thermodynamically stable bond is more reactive). Atropisomerization of I represents only the second example of such an inverse free-energy relationship for a thermodynamically controlled reaction, and it contrasts with the regular relationship that has been found for the atropisomerization of related directed reversible arrow [misdirected](double dagger) reversible arrow directed (DMD) systems.