Optically Active Transition Metal Complexes. 129.1 Novel Cycloheptatrienyl−Molybdenum and Cyclopentadienyl−Ruthenium Complexes with Chiral Pyridinecarbaldiminato Chelate Ligands: Syntheses, Molecular Structures, Properties, and Stereochemistry at the Metal Atom

Brunner, Henri ; Klankermayer, Jürgen

; Zabel, Manfred

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The reaction of [{eta(7)-C7H7)Mo(eta(6)-C6H5CH3)BF4 with the optically active Schiff base ligands LL1-LL3 (LL1 = pyridine-2-carbald-(S)-1-phenylethylimine, LL2 = 6-methylpyridine-2-carbald-(S)-1-phenylethylimine, LL3 = pyridine-2-carbald-(S)-1-cyclohexylethylimine) in acetonitrile afforded the diastereomers (R-mo,S-C)- and (S-Mo,S-C)-[(eta(7) -C7H7)Mo(LL1-3)(NCMe)]BF4 (1ab, 3ab, and 5ab). Reaction with carbon monoxide resulted in a mixture of the diastereoniers (R-Mo,Sc)- and (S-Mo,S-C)-[(eta(7)-C7H7)Mo(LL1-3)(CO)]BF4 (2ab, 4aA and 6ab). The ligand LL4 = (S)-2-(4,5-dihydro-4-isopropyloxazol-2-yl)pyridine gave the pure diastereomer 9b, In both series a and b the diastereomers only differed in the configuration of the metal atom. Complex 1a,b reacted with (CNBu)-Bu-t and PPh3 to give (R-Mo,S-C)- and (SMo,SC)-{(eta(7)- C7H7)Mo(LL1)((CNBu)-Bu-t)]BF4 (7a,b) and (R-Mo,Sc)- and (S-Mo,S-C)-[(eta(7)-C7H7)Mo(LL1)(PPh3)]BF4 (8a,b). Substitution experiments of 8a,b with PPhMe2 revealed a significantly different reactivity of the two diastereomers. The neutral complexes (R-Mo,S-C)- and (S-Mo,S-C)-[(eta(7)-C7H7)-Mo(LL1)II (10a,b) were obtained by refluxing [(eta(7)-C7H7)Mo(CO)(2)I] with LL1 in toluene. The isoelectronic ruthenium diastereomers (R-Ru,Sc)- and (S-Ru,S-C)-[(eta(5)-C5R5)] Ru(LL1,4)(L')]PF6 (R = H, Me, L = CO, PPh3) (11ab-14a,b) were prepared analogously to their (eta(7)-C7H7)MO counterparts by starting from [(eta(5)-C5R5)Ru(NCMe)(3)]PF6. The diastereomers were separated by fractional crystallization. Xray structure analyses established the conspicuously similar molecular structures of the isoelectronic molybdenum and ruthenium complexes and the absolute configurations of 2a, 7b, 8b, 9b, 10a, 12a, and 13b, In the diastereomers (Rm,Sc)-and (S-Mo,S-C)-[(eta(7)-C7H7)Mo(LL1)(CO)BF4 (2a,b), the molybdenum configuration was configurationally labile at room temperature. The epimerization 2a reversible arrow 2b was a clean first-order reaction in acetone-d(6) solution (tau(1/2) = 335 min at 21.8 degreesC; DeltaH(dagger) = 93 +/- 14 kJ mol(-1) and DeltaS(dagger) = -20 +/- 20 J K-1 mol(-1)). Additional experiments with the sterically more hindered 6-methylpyridine complexes (R-Mo,S-C)- and (S-Mo,S-C)-[(eta(7)- C7H7)Mo(LL2)(CO)BF4 (4a,b) indicated that the mechanism of the epimerization involved a chelate ring opening at the imine side of LL1 and LL2. For the carbonyl complexes 2ab the diastereomer ratio at equilibrium was 2a:2b = 76:24, whereas for the corresponding triphenylphosphane complexes (R-Mo,S-C)-and (S-Mo,S-C) [(eta(7) -C7H7)Mo(LL1)(PPh3)BF4 (8a,b) it was 8a:8b = 4:96. These diastereomer ratios reflected the thermodynamic chiral induction from the stable ligand configuration to the labile metal configuration. In contrast to the molybdenum complexes 2ab and 8a,b, which were configurationally labile at room temperature in acetone solution, the related ruthenium complexes (RRuSC)- and (S-Ru,S-C)-[(eta(5)-C-5,Me-5)Ru(LL1,4)(CO)]PF6 (12a,b and 14a,b) turned out to he configurationally stable.

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