Do Trinuclear Triplesalen Complexes Exhibit Cooperative Effects? Synthesis, Characterization, and Enantioselective Catalytic Sulfoxidation by Chiral Trinuclear Fe-III Triplesalen Complexes

Mukherjee C, Stammler A, Bögge H, Glaser T (2010)
CHEMISTRY-A EUROPEAN JOURNAL 16(33): 10137-10149.

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Abstract
The chiral triplesalen ligand H(6)chand provides three chiral salen ligand compartments in a meta-phenylene arrangement by a phloroglucinol backbone. The two diastereomeric versions H(6)chand(RR) and H(6)chand(rac) have been used to synthesize the enantiomerically pure chiral complex [(FeCl)(3)-(chand(RR))] (3(RR)) and the racemic complex [(FeCl)(3)(chand(rac))] (3(rac)). The molecular structure of the free ligand H(6)chand(rac) exhibits at the terminal donor sides the O-protonated phenolimine tautomer and at the central donor sides the N-protonated keto-enamine tautomer. The trinuclear complexes are comprised of five-coordinate square-pyramidal Fe-III ions with a chloride at the axial positions. The crystal structure of 3(rac) exhibits collinear chiral channels of similar to 11 angstrom in diameter making up 33.6% of the volume of the crystals, whereas the crystal structure of 3(RR) exhibits voids of 560 angstrom(3). Mossbauer spectroscopy demonstrates the presence of Fe-III high-spin ions. UV/Vis spectroscopy is in accordance with a large delocalized system in the central backbone evidenced by strong low-energy shifts of the imine pi-pi* transitions relative to that of the terminal units. Magnetic measurements reveal weak intramolecular exchange interactions but strong magnetic anisotropies of the FeIII ions. Complexes 3(rac) and 3(RR) are good catalysts for the sulfoxidation of sulfides providing very good yields and high selectivities with 3(RR) being enantioselective. A comparison of 3(RR) and [FeCl-(salen')] provides higher yields and selectivities but lower enantiomeric excess values (ee values) for 3(RR) relative to [FeCl(salen')]. The low ee values of 3(RR) appeared to be connected to a strong ligand folding in 3(RR), opening access to the catalytically active high-valent Fe-O species. The higher selectivity is assigned to a cooperative stabilization of the catalytically active high-valent Fe-O species through the phloroglucinol backbone in the trinuclear complexes.
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Mukherjee C, Stammler A, Bögge H, Glaser T. Do Trinuclear Triplesalen Complexes Exhibit Cooperative Effects? Synthesis, Characterization, and Enantioselective Catalytic Sulfoxidation by Chiral Trinuclear Fe-III Triplesalen Complexes. CHEMISTRY-A EUROPEAN JOURNAL. 2010;16(33):10137-10149.
Mukherjee, C., Stammler, A., Bögge, H., & Glaser, T. (2010). Do Trinuclear Triplesalen Complexes Exhibit Cooperative Effects? Synthesis, Characterization, and Enantioselective Catalytic Sulfoxidation by Chiral Trinuclear Fe-III Triplesalen Complexes. CHEMISTRY-A EUROPEAN JOURNAL, 16(33), 10137-10149.
Mukherjee, C., Stammler, A., Bögge, H., and Glaser, T. (2010). Do Trinuclear Triplesalen Complexes Exhibit Cooperative Effects? Synthesis, Characterization, and Enantioselective Catalytic Sulfoxidation by Chiral Trinuclear Fe-III Triplesalen Complexes. CHEMISTRY-A EUROPEAN JOURNAL 16, 10137-10149.
Mukherjee, C., et al., 2010. Do Trinuclear Triplesalen Complexes Exhibit Cooperative Effects? Synthesis, Characterization, and Enantioselective Catalytic Sulfoxidation by Chiral Trinuclear Fe-III Triplesalen Complexes. CHEMISTRY-A EUROPEAN JOURNAL, 16(33), p 10137-10149.
C. Mukherjee, et al., “Do Trinuclear Triplesalen Complexes Exhibit Cooperative Effects? Synthesis, Characterization, and Enantioselective Catalytic Sulfoxidation by Chiral Trinuclear Fe-III Triplesalen Complexes”, CHEMISTRY-A EUROPEAN JOURNAL, vol. 16, 2010, pp. 10137-10149.
Mukherjee, C., Stammler, A., Bögge, H., Glaser, T.: Do Trinuclear Triplesalen Complexes Exhibit Cooperative Effects? Synthesis, Characterization, and Enantioselective Catalytic Sulfoxidation by Chiral Trinuclear Fe-III Triplesalen Complexes. CHEMISTRY-A EUROPEAN JOURNAL. 16, 10137-10149 (2010).
Mukherjee, Chandan, Stammler, Anja, Bögge, Hartmut, and Glaser, Thorsten. “Do Trinuclear Triplesalen Complexes Exhibit Cooperative Effects? Synthesis, Characterization, and Enantioselective Catalytic Sulfoxidation by Chiral Trinuclear Fe-III Triplesalen Complexes”. CHEMISTRY-A EUROPEAN JOURNAL 16.33 (2010): 10137-10149.
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