Synthesis and characterization of xanthene-bridged Schiff-base dimanganese(iii) complexes: bimetallic catalysts for asymmetric oxidation of sulfides

Abstract

Dimanganese(III) complexes of salen-type ligands anchored by 9,9-dimethylxanthene-4,5-diyl spacers were synthesized. Two types of structures, cyclic and acyclic forms, are presented. The 2 + 2 Schiff-base condensation of 5,5'-(9,9-dimethylxanthene-4,5-diyl)bis(salicylaldehyde) and 1,2-diaminobenzene gave a macrocyclic ligand, from which a cyclic dimanganese(III) complex was synthesized. A similar dimanganese(III) complex with stereogenic centers was synthesized by the metal-assisted condensation reaction using (1R,2R)-1,2-diaminocyclohexane. Acyclic dimanganese(III) complexes anchored by one xanthene spacer were obtained by metal-assisted stepwise condensation reactions using the xanthene-bridged bis(salicylaldehyde), (1R,2R)-1,2-diaminocyclohexane monohydrochloride, and salicylaldehydes. X-Ray crystal structure analysis confirmed that both cyclic and acyclic complexes have two cofacially oriented salen-type manganese(III) units in a complex cation, showing an Mn...Mn distance of ca. 5.1 A. Asymmetric oxidation of sulfides was performed by using the chiral dimeric complexes as catalysts, which showed enantiomeric excesses ranging from 5 to 19%. The addition of 4-(dimethylamino)pyridine to the reaction system improved the enantioselectivity up to 39% ee. This effect was not observed for the corresponding mononuclear catalyst.

Class III Delocalization and Exciton Coupling in a Bimetallic Bis‐ligand Radical Complex

“…[45][46][47] The low energies required to alter the metalmetal distance in the co-facial porphyrin systems likely enhances substrate activation. This contrasts with the report of Hirotsu et al, [41] in which the two salen units are aligned trans. The connecting twist angles for 2 (the dihedral angle between the 1,2-phenylene bridge and the salen units) are 558 and 37.58.…”

“…The coordination sphere bond lengths in the two Ni salen units of 2 match the Ni monomer data (Table 2), indicating that the 1,2-phenylene bridge does not perturb the coordination sphere in 2 relative to the monomer species 1. [41] The dihedral angle between the para-phenyl substituent and the phenolate ring is 418 in 1. [45][46][47] The low energies required to alter the metalmetal distance in the co-facial porphyrin systems likely enhances substrate activation.…”

“…These values can be compared to twist angles of 50.68 and 50.48 for a recently reported Mn salen dimer species, in which the salen units are linked by a 9,9-diA C H T U N G T R E N N U N G meth-A C H T U N G T R E N N U N G ylxanthene-4,5-diyl spacer. [41] The dihedral angle between the para-phenyl substituent and the phenolate ring is 418 in 1. Interestingly, the two salen units in the solid state structure of 2 are oriented in a cis arrangement ( Figure 2), with the bulky tert-butyl substituents aligned on the same side of the molecule.…”

“…The structure of 2 bears resemblance to the co-facial porphyrins studied by Collman et al, [37] Nocera et al, [38] and calixpyrroles studied by Love and co-workers. [40,41] By a combination of experiments and calculations we show that 2 can be oxidized to a bis-ligand radical species in solution. [40,41] By a combination of experiments and calculations we show that 2 can be oxidized to a bis-ligand radical species in solution.…”

“…[39] Although bimetallic salen derivatives with flexible linkers, and bis-strapped (two symmetrically placed linkers) are relatively common, a single rigid pillar linking salen units is rare. [40,41] By a combination of experiments and calculations we show that 2 can be oxidized to a bis-ligand radical species in solution. The spectroscopic signature of the bis-ligand radical complex [2CC] 2 + , and comparison to an analogous oxidized monomer [1C] + , provides significant insight into the geometric and electronic structure of the these systems.…”