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 [L(X)Rh(eta4-arene)] (arene = benzene, toluene, m-xylene, mesitylene), which for X = Me disproportionate to fluxional dinuclear complexes [[L(Me)Rh]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 eta4(1,2,3,4):eta4(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 eta3(1,2,3):eta3(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.
Budzelaar
PH
Department of Inorganic Chemistry, University of Nijmegen, The Netherlands. budz@sci.kun.nl
Moonen
NN
de Gelder R
Smits
JM
Gal
AW