Steric control over arene coordination to beta-diiminate rhodium(I) fragments

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 [LxR h(eta(4)-arene)] (arene = benzene, toluene: m-xylene, mesitylene), which fo r X = Me disproportion ate to fluxional dinuclear complexes [{LMeRh}(2)(ant i-mu-arene)]. For both mononuclear and dinuclear complexes, steric interact ions do not stop the fluxionality but govern the preferred orientation of t he methyl-substituted arenes, thus allowing indirect determination of the s tatic NMR parameters. For the mu-arene complexes, two distinct types of flu xionality are proposed on the basis of calculations: ring rotation and meta l 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, ho wever, 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 r eversal of stabilities.