Synthesis, Structural, Spectroscopic, Electrochemical, Magnetic, and Catalytic Properties of the Trinuclear MnIII Triplesalen Complex [(talen t‐Bu 2){Mn(OAc)}3] Exhibiting Three Salen‐Subunits in a β‐cis‐Conformation

Reaction of the triplesalen ligand H(6)talen(t-Bu2) with three equivalents Mn(OAc)(2)center dot 4H(2)O in MeOH results in the formation of a brown solid which upon recrystallization from CH3CN provides the trinuclear complex [(talen(t-Bu2)){Mn(OAc)}(3)]center dot 7CH(3)CN as evidenced by single-crystal X-ray diffraction. The triple tetradentate ligand (talen(t-Bu2))(6-) coordinates to three Mn-III ions in the rare beta-cis-conformation of the salen-like ligand compartments with the central oxygen donor (O-c) being rotated out of the plane. This results in a longer Mn-O-c bond length of 2.00 angstrom compared to the mean Mn-O-t bond lengths of the terminal phenolates at 1.86 angstrom. The six-coordination is saturated by bidentate OA(c-) ligands. The electronic absorption spectrum measured in MeOH appears to be almost identical to all other complexes already studied possessing a {(talen(t-Bu2))Mn-3(III)}(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(t-Bu2)){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 co-ordination mode of the OA(c-) ligands. Electrochemical measurements reveal unresolved irreversible processes in the range 0.9-1.4 V vs. Fc(+)/Fc corresponding to oxidations of the Mn-III-phenolate units, while irreversible reductive waves in the range -0.7-(-1.2) V vs. Fc(+)/Fc correspond to Mn-III to Mn-II 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(t-Bu2))Mn-3(III)}(3+) subunits in the trans-conformation consistent with the longer Mn-O-c distances and the asymmetric coordination environment, respectively. The complex [(talen(t-Bu2)){Mn(OAc)}(3)] catalyzes the epoxidation of 1,2-dihydronaphthalene with iodosylbenzene with complete conversion at room temperature.