Steric control over arene coordination to beta-diiminate rhodium(I) fragments
Budzelaar PHM, Moonen NNP, de Gelder R, Smits JMM, Gal AW
CHEMISTRY-A EUROPEAN JOURNAL 6 (15): 2740-2747 AUG 4 2000

Abstract:
The bulky ligands L-x(-) (L-x = (2,6-C6H3X2)NC(Me)CHC(Me)N(2,6-C6H3X2), X = Cl, Me) can be used to generate fluxional mononuclear arene complexes [LxRh(eta(4)-arene)] (arene = benzene, toluene: m-xylene, mesitylene), which for X = Me disproportion ate to fluxional dinuclear complexes [{LMeRh}(2)(anti-mu-arene)]. For both mononuclear and dinuclear complexes, steric interactions do not stop the fluxionality but govern the preferred orientation of the methyl-substituted arenes, thus allowing indirect determination of the static NMR parameters. For the mu-arene complexes, two distinct types of fluxionality are proposed on the basis of calculations: ring rotation and metal shift. In the solid state, the toluene complex has an eta(4)(1,2,3,4):eta(4)(3,4,5,6)-bridged structure: the NMR analysis indicates that the benzene and m-xylene complexes have similar structures. The mesitylene complex, however, has an unprecedented eta(3)(1,2,3):eta(3)(3,4,5)-bridged structure, which is proposed to correspond to the transition state for arene rotation in the other cases. Steric factors are thought to be responsible for this reversal of stabilities.