Kinetic element effect for atropisomerization of an organometallic complexof 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) hexaflu orophosphate (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, i ndicate the energy barrier is higher for 1(Ru) than 1(Os); e.g., Delta H-do uble 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 fact ors associated with atropisomerization of the 1,1'-biisoquinoline ligand, e ssentially the deformation of the 1,1'-binaphthylene skeleton that is neces sary to pass H-8 and H-8' past one another, are presumably equivalent for t he Ru and Os derivatives. Assuming that normal bond energies are greater fo r the third-row transition metal than for second-row transition metals, we conclude the difference in reactivity can be attributed to electronic facto rs-the sigma-donor orbitals and pi-acceptor orbitals of the 1,1-biisoquinol ine ligand are misdirected in the ground state but redirected in the syn tr ansition state of atropisomerization. Thus, an inverse relationship between the kinetic and thermodynamic stabilities of 1 is observed for the misdire cted reversible arrow [directed](double dagger) reversible arrow misdirecte d (MDM) isomerization of 1 (the more thermodynamically stable bond is more reactive). Atropisomerization of I represents only the second example of su ch an inverse free-energy relationship for a thermodynamically controlled r eaction, and it contrasts with the regular relationship that has been found for the atropisomerization of related directed reversible arrow [misdirect ed](double dagger) reversible arrow directed (DMD) systems.