Synthesis, Structural, Spectroscopic, Electrochemical, Magnetic, and Catalytic Properties of the Trinuclear MnIII Triplesalen Complex [(talen){Mn(OAc)}3] Exhibiting Three Salen‐Subunits in a β‐cis‐Conformation

Abstract

AbstractReaction of the triplesalen ligand H6talen with three equivalents Mn(OAc)2·4H2O in MeOH results in the formation of a brown solid which upon recrystallization from CH3CN provides the trinuclear complex [(talen){Mn(OAc)}3]·7CH3CN as evidenced by single‐crystal X‐ray diffraction. The triple tetradentate ligand (talen)6– coordinates to three MnIII ions in the rare β‐cis‐conformation of the salen‐like ligand compartments with the central oxygen donor (Oc) being rotated out of the plane. This results in a longer Mn–Oc bond length of 2.00 Å compared to the mean Mn–Ot bond lengths of the terminal phenolates at 1.86 Å. The six‐coordination is saturated by bidentate OAc– ligands. The electronic absorption spectrum measured in MeOH appears to be almost identical to all other complexes already studied possessing a {(talen)MnIII3}3+ subunit (in the trans‐conformation). The spectra measured in CH2Cl2 and CH3CN exhibit significant variations of the absorption features in the CT region above 20000 cm–1 and a low‐energy shift of the d‐d transitions from a shoulder around 18000 cm–1 in CH3OH to maxima around 13000 cm–1 in CH2Cl2 and CH3CN. This indicates a physical dissolution of [(talen){Mn(OAc)}3] in CH2Cl2 and CH3CN solutions without major structural rearrangements, while in MeOH solution a structural rearrangement to the preferred trans‐conformation of the salen‐like coordination compartments occurs loosing the bidentate coordination mode of the OAc– ligands. Electrochemical measurements reveal unresolved irreversible processes in the range 0.9–1.4 V vs. Fc+/Fc corresponding to oxidations of the MnIII‐phenolate units, while irreversible reductive waves in the range –0.7–(–1.2) V vs. Fc+/Fc correspond to MnIII to MnII reductions. The analysis of the magnetic data reveals a weaker antiferromagnetic interaction of J = –0.067 cm–1 and a stronger zero‐field splitting of D = –5.57 cm–1 in comparison to the complexes with {(talen)MnIII3}3+ subunits in the trans‐conformation consistent with the longer Mn–Oc distances and the asymmetric coordination environment, respectively. The complex [(talen){Mn(OAc)}3] catalyzes the epoxidation of 1, 2‐dihydronaphthalene with iodosylbenzene with complete conversion at room temperature.