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 [(Mn2OL2)-O-III](ClO4)(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 [(Mn2OL2)-O-III](2+) contains a linear Mn(III)-O-Mn(III) unit with a Mn-Mn distance of 3.516 Angstrom. The pentadentate ligand L- wraps around the Mn(III) ion. Electrochemically, it is possible to prepare the one electron oxidized trication [Mn2OL2](3+) which crystallizes as [Mn2OL2](ClO4)(2.37)(PF6)(0.63) . 1.5CH(3)CN. The complex trication [Mn2OL2](3+) contains a Mn(III)O--Mn(IV) unit with a Mn-Mn distance of 3.524 Angstrom 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 [Mn2OL2](2+) dication possesses a S = 0 electronic ground state with J = -216 cm(-1) (H = -JS(1). S-2), whereas the [Mn2OL2](3+) trication shows a S = 1/2 ground state with J = -353 cm(-1). The UV-visible spectrum of [Mn2OL2](3+) exhibits an intense absorption band (epsilon = 3040 M-1 cm(-1)) centered at 570 nm assigned tb a phenolate --> Mn(IV) charge-transfer transition. The potentials of the redox couples determined by cyclic voltammetry are E-o ([Mn2OL2](3+)/[Mn2OL2](2+)) = 0.54 V/saturated calomel electrode (SCE) and E-o (Mn2OL2](4+)/[Mn2OL2](3+)) = 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 [Mn2OL2](3+) complex exhibits a 18-line X-band electron paramagnetic resonance (EPR) spectrum which has been simulated with rhombic tensors \A(1x)\ = 160 x 10(-4) cm(-1); \A(1y)\ = 130 x 10(-4) cm(-1); \A(1z)\ = 91 x 10(-4) cm(-1); \A(2x)\ = 62 x 10(-4) cm(-1); \A(2y)\ = 59 x 10(-4) cm(-1); \A(2z)\ = 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 [Mn2OL2](4+) 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-mu-oxo pair. The relevance of these results to the study bf the photosynthetic oxygen evolving complex is discussed.