SYNTHESIS AND DYNAMIC NMR-STUDIES OF ETA(3)-TRIPHENYLCLOPROPENYL AND ETA(3)-TRIMETHYLCYCLOPROPENYL COMPLEXES OF RUTHENIUM, [RU(ETA(5)-C5R5)(ETA(3)-C3R'3)X2] (R = H, ME R' = ME, PH X = CL, BR, I) - EXTENDED HUCKEL MOLECULAR-ORBITAL STUDY OF BARRIERS TO ROTATION OF ETA(3)-CYCLOPROPENYL LIGANDS IN ISOELECTRONIC RUTHENIUM AND MOLYBDENUM COMPLEXES

Oxidative addition reactions of triphenylcyclopropenyl halides to Ru(I I) precursors [Ru(eta5-C5H5)(eta4-COD)Cl] and [Ru(eta5-C5Me5)Cl]4 affo rd [Ru(eta5-C5R5)(eta3-C3Ph3)Cl2] (3a, R = H; 3e, R = Me). Derivative complexes [Ru(eta5-C5R5)(eta3-C3Ph3)X2] (3b R = H, X = Br; 3c R = H, X = I; 3f R = Me, X = Br; 3g R = Me, X = I) were prepared by halide met athesis reactions on 3a and 3e. Solution NMR evidence for the mixed ha lide complex [Ru(eta5-C5H5)(eta3-C3Ph3)BrCl] (3d) has also been obtain ed. Rotation of the triphenylcyclopropenyl ring about the Ru-C3 axis i n 3 is not observable on the NMR time scale. Similar reactions of trim ethylcyclopropenyl tetrafluoroborate with the same Ru(II) precursors, in the presence of lithium chloride, lead to [Ru(eta5-C5R5)(eta3-C3Me3 )Cl2] (4a, R = H; 4d, R = Me), which were converted to the dibromo and diiodo analogues [Ru(eta5-C5R5)(eta3-C3Me3)X2] (4b R = H, X = Br; 4c R = H, X = 1; 4e R = Me, X = Br; 4f R = Me, X = I) by halide metathesi s. The H-1 NMR spectra of complexes 4 exhibit variable temperature beh avior, consistent with rotation of the trimethylcyclopropenyl ligand o n the NMR time scale. Line shape analysis of the variable temperature spectra leads to values for the free energy of activation (DELTAG(doub le dagger) for this rotation of ca. 60 kJ mol-1, which are insensitive to the nature of the halide or the cyclopentadienyl ligand. These rel atively high values of DELTAG(double dagger) stand in contrast to the maximum value of DELTAG(double dagger) for triphenylcyclopropenyl rota tion estimated previously for an isoelectronic complex [Mo(eta5-Cr5H5) (eta3-C3Ph3)(CO)2](2). A detailed theoretical analysis at the extended Huckel level has been carried out for the bonding interactions betwee n both [Ru(eta5-C5H5)Br2] and [Mo(eta5-C5H5)(CO)2] fragments and the[C 3H3] ligand. This analysis predicts a significantly higher barrier to cyclopropenyl rotation in the Ru system and provides a rational pictur e of the electronic structural features which give rise to the observe d differences in values of DELTAG(double dagger) for cyclopropenyl rot ation in the two systems.