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

Abstract

The reaction of [(η 7 -C 7 H 7 )Mo(η 6 -C 6 H 5 CH 3 )]BF 4 with the optically active Schiff base ligands LL 1 -LL 3 (LL 1 ) pyridine-2-carbald-(S)-1-phenylethylimine, LL 2 ) 6-methylpyridine-2-carbald-(S)-1-phenylethylimine, LL 3 ) pyridine-2-carbald-(S)-1-cyclohexylethylimine) in acetonitrile afforded the diastereomers (R Mo ,S C )-and (S Mo ,S C )-[(η 7 -C 7 H 7 )Mo(LL 1-3 )(NCMe)]-BF 4 (1a,b, 3a,b, and 5a,b). Reaction with carbon monoxide resulted in a mixture of the diastereomers (R Mo ,S C )-and (S Mo ,S C )-[(η 7 -C 7 H 7 )Mo(LL 1-3 )(CO)]BF 4 (2a,b, 4a,b, and 6a,b). The ligand LL 4 ) (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 CN t Bu and PPh 3 to give (R Mo ,S C )-and (S Mo ,S C )-[(η 7 -C 7 H 7 )Mo(LL 1 )(CN t Bu)]BF 4 (7a,b) and (R Mo ,S C )-and (S Mo ,S C )-[(η 7 -C 7 H 7 )Mo(LL 1 )(PPh 3 )]BF 4 (8a,b). Substitution experiments of 8a,b with PPhMe 2 revealed a significantly different reactivity of the two diastereomers. The neutral complexes (R Mo ,S C )-and (S Mo ,S C )-[(η 7 -C 7 H 7 )-Mo(LL 1 )I] (10a,b) were obtained by refluxing [(η 7 -C 7 H 7 )Mo(CO) 2 I] with LL 1 in toluene. The isoelectronic ruthenium diastereomers (R Ru ,S C )-and (S Ru ,S C )-[(η 5 -C 5 R 5 )Ru(LL 1,4 )(L′)]PF 6 (R ) H, Me, L′ ) CO, PPh 3 ) (11a,b-14a,b) were prepared analogously to their (η 7 -C 7 H 7 )Mo counterparts by starting from [(η 5 -C 5 R 5 )Ru(NCMe) 3 ]PF 6 . The diastereomers were separated by fractional crystallization. X-ray 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 (R Mo ,S C )-and (S Mo ,S C )-[(η 7 -C 7 H 7 )Mo(LL 1 )(CO)]BF 4 (2a,b), the molybdenum configuration was configurationally labile at room temperature. The epimerization 2a / 2b was a clean first-order reaction in acetone-d 6 solution (τ 1/2 ) 335 min at 21.8°C; ∆H q ) 93 ( 14 kJ mol -1 and ∆S q ) -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 )-[(η 7 -C 7 H 7 )Mo(LL 2 )(CO)]BF 4 (4a,b) indicated that the mechanism of the epimerization involved a chelate ring opening at the imine side of LL 1 and LL 2 . For the carbonyl complexes 2a,b 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 )-[(η 7 -C 7 H 7 )Mo(LL 1 )(PPh 3 )]BF 4 (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 2a,b and 8a,b, which were configurationally labile at room temperature in acetone solution, the related ruthenium complexes (R Ru ,S C )-and (S Ru ,S C )-[(η 5 -C 5 Me 5 )Ru(LL 1,4 )(CO)]PF 6 (12a,b and 14...

Chloro(η6‐p‐cymene)(phosphoramidite)ruthenium(II), a 16‐Electron Fragment Stabilized by an η2‐Aryl–Metal Interaction, and Its Use in Asymmetric Cyclopropanation

“…The one belonging to the h 6 -complexed moiety appears as a dq (d 4.14, Table 3). Again, we note very substantial ion pairing between cation 5a and [PF 6 ]…”

“…-After Henri Brunner's seminal studies of [Mn(Cp)(CO)(PPh 3 )-(NO)] (Cp = cyclopenta-2,4-dien-1-yl) [1], pseudotetrahedral complexes containing chirotopic metal atoms have been extensively investigated over the last 35 years, with particular emphasis on stereochemical aspects. As configurational stability is required for these studies, octahedral complexes of d 6 ions -mainly Mn I [1], Re I [2], Ru II [3a,b] (for seminal papers concerning [RuCl(Cp)(PÀP*)] complexes, see also [3c,d], [4a], for application as stoichiometric reagents in organic chemistry, see [4b,c]), Rh III [5], and Ir III [5] -have been used for the synthesis of substitutionally inert 18-electron species with stereogenic metal atoms (for the extension of this concept to formally seven-coordinate d 4 metal ions, see [6]). With Ru I ions, two major classes of compounds have been studied: [RuCl(Cp)(PÀP*)] (PÀP* = chiral diphosphine) [3] and [RuX(h plexes based on the 16-…”

Enantioselective CpRu‐Catalyzed Carroll Rearrangement – Ligand and Metal Source Importance

“…[16] In the context of arenemetal coordination chemistry, this ligand family has been successfully used by Brunner [17] and Davies [18] to control the configuration of pseudo-tetrahedral Cp and η 6 -areneRu piano-stool complexes, respectively. Several pymox ligands were synthesized following the procedure of Bolm and co-workers by condensing commercially available enantiopure 1,2-amino alcohols onto 2-cyanopyridine with a catalytic amount of ZnCl 2 .…”

Enantiomerically Pure Bis(phosphanyl)carbaborane(12) Compounds

“…[20] Zabel et al also observed these interactions for molybdenum carbonyl complexes, but the CO···π arene distances are longer than those observed in this study. [21] The distance from O4 to the planes of the C12/C12Ј phenyl rings is significantly larger [355.8(2) pm]. …”