A New Manganese Dinuclear Complex with Phenolate Ligands and a Single Unsupported Oxo Bridge. Storage of Two Positive Charges within Less than 500 mV. Relevance to Photosynthesis.

The compound Mn(III)(2)OL(2)(2).2.23CHCl(3).0.65CH(2)Cl(2) where L(-) is the monoanionic N,N-bis(2-pyridylmethyl)-N'-salicyliden-1,2-diaminoethane ligand, has been synthesized. The complex dication Mn(III)(2)OL(2) contains a linear Mn(III)-O-Mn(III) unit with a Mn-Mn distance of 3.516 Å. The pentadentate ligand L(-) wraps around the Mn(III) ion. Electrochemically, it is possible to prepare the one electron oxidized trication Mn(2)OL(2) which crystallizes as Mn(2)OL(2)(2.37)(PF(6))(0.63).1.5CH(3)CN. The complex trication Mn(2)OL(2) contains a Mn(III)-O-Mn(IV) unit with a Mn-Mn distance of 3.524 Å and a Mn-O-Mn angle of 178.7(2) degrees. The contraction of the coordination sphere around the Mn(IV) is clearly observed. The Mn(2)OL(2) dication possesses a S = 0 electronic ground state with J = -216 cm(-)(1) (H = -JS(1)().S(2)()), whereas the Mn(2)OL(2) trication shows a S = (1)/(2) ground state with J = -353 cm(-)(1). The UV-visible spectrum of Mn(2)OL(2) exhibits an intense absorption band (epsilon = 3040 M(-)(1) cm(-)(1)) centered at 570 nm assigned to a phenolate --> Mn(IV) charge-transfer transition. The potentials of the redox couples determined by cyclic voltammetry are E degrees (Mn(2)OL(2)/Mn(2)OL(2)) = 0.54 V/saturated calomel electrode (SCE) and E degrees (Mn(2)OL(2)/Mn(2)OL(2)) = 0.99 V/SCE. Upon oxidation at 1.3 V/SCE, the band at 570 nm shifts to 710 nm (epsilon = 2500 M(-)(1) cm(-)(1)) and a well-defined band appears at 400 nm which suggests the formation of a phenoxyl radical. The Mn(2)OL(2)( )()complex exhibits a 18-line X-band electron paramagnetic resonance (EPR) spectrum which has been simulated with rhombic tensors |A(1)(x)()| = 160 x 10(-)(4) cm(-)(1); |A(1)(y)()| = 130 x 10(-)(4) cm(-)(1); |A(1)(z)()| = 91 x 10(-)(4) cm(-)(1); |A(2)(x)()| = 62 x 10(-)(4) cm(-)(1); |A(2)(y)()| = 59 x 10(-)(4) cm(-)(1); |A(2)(z)()| = 62 x 10(-)(4) cm(-)(1) and g(x)() = 2.006; g(y)() = 1.997; g(z)() = 1.982. This EPR spectrum( )()shows that the 16-line paradigm related to a large antiferromagnetic exchange coupling and a low anisotropy can be overcome by a large rhombic anisotropy. Molecular orbital calculations relate this rhombicity to the nature of the orbital describing the extra electron on Mn(III). This orbital has a majority but not pure d(z)()2 contribution (with the z axis perpendicular to the Mn-Mn axis). Low-temperature resonance Raman spectroscopy on an acetonitrile solution of Mn(2)OL(2) prepared at -35 degrees C indicated the formation of a phenoxyl radical. This suggests that the ligand was oxidized rather than the Mn(III)Mn(IV) pair to Mn(IV)Mn(IV), which illustrates the difficulty to store a second positive charge in a short range of potential in a manganese mono-&mgr;-oxo pair. The relevance of these results to the study of the photosynthetic oxygen evolving complex is discussed.