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

Abstract

The compound [Mn(III)(2)OL(2)](ClO(4))(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)](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)](3+) which crystallizes as [Mn(2)OL(2)](ClO(4))(2.37)(PF(6))(0.63).1.5CH(3)CN. The complex trication [Mn(2)OL(2)](3+) 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)](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)](3+) trication shows a S = (1)/(2) ground state with J = -353 cm(-)(1). The UV-visible spectrum of [Mn(2)OL(2)](3+) 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)](3+)/[Mn(2)OL(2)](2+)) = 0.54 V/saturated calomel electrode (SCE) and E degrees ([Mn(2)OL(2)](4+)/[Mn(2)OL(2)](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 [Mn(2)OL(2)](3+)( )()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)](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-&mgr;-oxo pair. The relevance of these results to the study of the photosynthetic oxygen evolving complex is discussed.

Synthesis, Structure, Electronic, Redox, and Magnetic Properties of a New Mixed-Valent Mn-Oxo Cluster: [Mn2III,IVO2(N,Nbispicen)2]3+ (N,Nbispicen =N,N-bis(2-pyridylmethyl)-1,2-diaminoethane)

“…The ligand N,Nbis(2-pyridylmethyl)-1,2-diaminoethane (abbreviation: N,Nbispicen) was prepared by following the procedure in ref [28] . This ligand has also recently been described in ref [10] .…”

Redox Reactions of a Dinuclear Manganese Complex – the Influence of Water

“…[29] If 1 is oxidized electrochemically in the presence of 0.5  H 2 O at an applied potential of +0.64 V, the solution after electrolysis gives rise to a characteristic Mn 2 complexes. [30][31][32] The mono(µ-oxido)-bridged Mn III,IV 2 core species exhibits a ca. 1110 G wide signal, distinguishable from the signal of a bis(µ-oxido)-bridged Mn III,IV 2 core that is typically 140 G wider.…”

Synthesis, Structure and Characterisation of a New Trinuclear Di‐μ‐phenolato‐μ‐carboxylato MnIIIMnIIMnIII Complex with a Bulky Pentadentate Ligand: Chemical Access to Mononuclear MnIV–OH Entities

“…Lastly, the absence of a broad absorption band near 1630 cm -1 confirms the absence of the imine function in 1. [53] The product of the molar magnetic susceptibility, χ M , and the temperature, T, recorded on a powder sample of 1(BPh 4 [44] The magnitude of the exchange interaction is indeed controlled by the presence of two alkoxo or phenoxo bridges. Such doubly bridged Mn II Mn III core units present J values ranging from -3.4 to 13.2 cm -1 .…”

“…The UV/Visible spectrum of complex 1 in acetonitrile is reproduced in Figure 2 (dashed line-left scale [50,53,57,58] whereas the other two transitions may originate from either phenolatoǞMn III or acetatoǞMn III LMCT transitions. [59] The large ε values of the LMCT transitions prevent the observation of the Mn III d-d transitions, which are theoretically expected with lower ε values.…”

“…The intense absorption band in the UV region may therefore be attributed to a phenolatoǞMn IV or to a hydroxoǞMn IV LMCT transition. The shoulders observed in the visible region can be attributed to a poorly resolved phenolatoǞMn IV MLCT [50,53,57] or to Mn IV d-d transitions, the strong intensity of which originates from the tail of the absorption in the UV region.…”