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

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 phenol-imine 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 approximately 11 A in diameter making up 33.6 % of the volume of the crystals, whereas the crystal structure of 3(RR) exhibits voids of 560 A(3). Mössbauer 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 Fe(III) 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.

From Triplesalen to Triplesalophen: Ferromagnetic Interactions through Spin‐Polarization in a Trinuclear NiII Triplesalophen Complex

“…[2,3] These triplesalen ligands are intended to combine phloroglucinol A, as a ferromagnetic coupler by the spin-polarization mechanism, [4] and the salen-like coordination environment B, which enforces a strong magnetic anisotropy in selected metal ions. [5] Indeed, the characterization of trinuclear Cu II and V IV =O complexes demonstrate the targeted ferromagnetic interactions, [6][7][8] while in Mn III[9] and Fe III [10] complexes, antiferromagnetic interactions prevail. Moreover, the trinuclear complexes of the tert-butyl derivative H 6 -talen tBu 2 exhibit a strong ligand folding, which leads to an overall bowl-shaped molecular structure (Scheme 1).…”

Electronic and Molecular Structures of Heteroradialenes: A Combined Synthetic, Computational, Spectroscopic, and Structural Study Identifying IR Spectroscopy as a Simple but Powerful Experimental Probe

“…This calculated model nicely reproduces the experimental structure by the central backbone of H 6 chand rac , although H 6 chand rac exhibits stronger deviations from C 3 symmetry. Nevertheless, the mean O···N distances of 2.50Å compares well to that of H 3 14 Me [53]. The strongest difference is in the C(sp 2 )-N bond (mean value 1.31Å in H 6 chand rac vs. 1.34Å in H 3 14 Me ), which may be related to a cyclohexyl vs. a methyl substituent at the nitrogen atom.…”

Aromatic Versus Heteroradialene Character in Extended Thiophloroglucinol Ligands and their Trinuclear Nickel(II) Complexes

“…Detailed magnetic studies demonstrated that our trinuclear Cu II 3 [27,36] and V IV 3 [37] triplesalen complexes indeed exhibit ferromagnetic but only weak interactions. However, Fe III [38,39] and Mn III [28-33, 40, 41] ions appeared to be almost uncoupled and even slightly antiferromagnetically coupled through the triplesalen ligand, respectively. Thus, the anticipated spin-polarization through the central phloroglucinol is not efficient.…”