Search

The dimanganese(ll,ll) complexes 1a [Mn[sub 2](L)(OAc)[sub 2](CH[sub 3]OH)](CIO[sub 4]) and lb [Mn2(L)(OBz)[sub 2](H[sub 2]O)](CIO[sub 4]), where HL is the unsymmetrical phenol ligand 2-(bis-(2-pyridylmethyl)aminomethyl)-6-((2-pyridylmethyl)(benzyl)aminomet hyl)4-methylphenol, react with hydrogen peroxide in acetonitrile solution. The disproportionation reaction was monitored by electrospray ionization mass spectrometry (ESI-MS) and EPR and UV-visible spectroscopies. Extensive EPR studies have shown that a species (2) exhibiting a 16-line spectrum at g ∼ 2 persists during catalysis. ESI-MS experiments conducted similarly during catalysis associate 2a with a peak at 729 (791 for 2b) corresponding to the formula [Mn[sup III]Mn[sup IV]V(L)(O)[sub 2](OAc)][sup +] ([Mn[sup III]Mn[sup IV](L)(O)[sub 2](OBz)][sup +] for 2b). At the end of the reaction, it is partly replaced by a species (3) possessing a broad unfeatured signal at g ∼ 2. ESI-MS associates 3a with a peak at 713 (775 for 3b) corresponding to the formula [Mn[sup II]Mn[sup III](L)(O)(OAc)][sup +] ([Mn[sup II]Mn[sup III](L)(O)(OBz)][sup +] for 3b). In the presence of H[sub 2, sup 18]O, these two peaks move to 733 and to 715 indicating the presence of two and one oxo ligands, respectively. When H[sub 2, sup 18]O[sub 2] is used, 2a and 3a are labeled showing that the oxo ligands come from H[sub 2]O[sub 2]. Interestingly, when an equimolar mixture of H[sub 2]O[sub 2] and H[sub 2, sup 18]O[sub 2] is used, only unlabeled and doubly labeled 2a/b are formed, showing that its two oxo ligands come from the same H[sub 2]O[sub 2] molecule. All these experiments lead to attribute the formula [Mn[sup III]Mn[sup IV](L)-(O)[sub 2](OAc)][sup +] to 2a and to 3a the formula [Mn[sup II]Mn[sup III](L)(O)(OAc)][sup +]. Freeze-quench/EPR experiments revealed that 2a appears at 500 ms and that another species with a 6-line spectrum is formed transiently at ca. 100 ms. 2a was prepared by reaction of la with reft-butyl hydroperoxide as shown by EPR and UV-visible spectroscopies and ESI-MS experiments, its structure was studied by X-ray absorption experiments which revealed the presence of two or three O atoms at 1.87 Å and three or two N/O atoms at 2.14 Å. In addition one N atom was found at a longer distance (2.3 Å) and one Mn at 2.63 Å. 2a can be one-electron oxidized at E[sub ½] = 0.91 V[sub NHE] (ΔE[sub ½] = 0.08 V) leading to its Mn[sup IV]Mn[sup IV] analogue. The formation of 2a from la was monitored by UV-visible and X-ray absorption spectroscopies. Both concur to show that an intermediate Mn[sup II]Mn[sup III] species, resembling 4a [Mn[sub 2](L)(OAc)[sub 2](H[sub 2]O)](ClO[sub 4])[sub 2], the one-electron-oxidized form of 1a, is formed initially and transforms into 2a. The structures of the active intermediates 2 and 3 are discussed in light of their: spectroscopic properties, and potential mechanisms are considered and discussed in the context of the biological reaction. [ABSTRACT FROM AUTHOR]

Reactions of the unsymmetrical phenol ligand 2-(bis(2-pyridyimethyl)aminomethyl)-6-((2-pyridylmethyl)(benzyl)aminometh yl)-4-methylphenol with Mn(OAc)[sub 2]·4H[sub 2]O or Mn(H[sub 2]O)[sub 6](ClO[sub 4])[sub 2] in the presence of NaOBz affords the dimanganese(II) complexes 1(CH[sub 3]OH), [Mn[sub 2](L)(OAc)[sub 2](CH[sub 3]OH)](ClO[sub 4]), and 2(H[sub 2]O), [Mn[sub 2](L)(OBz)[sub 2](H[sub 2]O)](ClO[sub 4]), respectively. On the other hand, reaction of the ligand with hydrated manganese(III) acetate furnishes the mixedvalent derivative 3(H[sub 2]O), [Mn[sub 2](L)(OAc)[sub 2](H[sub 2]O)](ClO[sub 4])[sub 2]. The three complexes have been characterized by X-ray crystallography. 1(CH[sub 3]OH) crystallizes in the monoclinic system, space group P2[sub 1]/c, with a = 10.9215(6) Å, b = 20.2318(12) Å, c = 19.1354(12) Å, α = 90°, β = 97.5310(10)°, γ = 90°, V = 4191.7 A³, and Z = 4. 2(H[sub 2]O) crystallizes in the monoclinic system, space group P2[sub 1]/n, with a = 10.9215(6) Å, b = 20.2318(12) Å, c = 19.1354(12) Å, α = 90°, β = 97.5310(10)°, γ = 90°, V = 4191.7 ų, and Z = 4.3(H[sub 2]O) crystallizes in the monoclinic system, space group P2[sub 1]/c, with a = 11.144(6) Å, b = 18.737(10) Å, c = 23.949(13) Å, α = 90°, β = 95.910(10)°, γ = 90°, V-4974(5) ų, and Z = 4. Magnetic measurements revealed that the three compounds exhibit very similar magnetic exchange interactions -J = 4.3(3) cm[sup -1]. They were used to establish tentative magnetostructural correlations which show that for the dimanganese(II) complexes -J decreases when the Mn-O[sub phenoxo] distance increases as expected from orbital overlap considerations. For the dimanganese(II,III) complexes, crystallographic results show that the Mn[sup II]-O[sub phenoxo] and Mn[sup III]-O[sub phenoxo] bond lengths are inversely correlated. An... [ABSTRACT FROM AUTHOR]

Examines the properties of the dinuclear manganese complex. Outcome on the reaction of manganese compounds with ethylamines compounds; X-ray crystallography of the binuclear manganese compounds; Electrochemical behavior of the manganese complex.

The heterodinuclear complexes [FeIII,MnII,(L-Bn)(μ-OAc)2](ClO4)2 (1) and [FeIIMnII(L-Bn)(μ<-OAc)2](ClO4) (2) with the unsymmetrical dinucleating ligand HL-Bn {[2-bis[(2-pyridylmethyl)aminomethyl]]-6-[benzyl-2-(pyridylmethyl)-aminomethyl]-4-methylphenol} were synthesized and character-ized as biologically relevant models of the new Fe/Mn class of nonheme enzymes. Crystallographic studies have been completed on compound 1 and reveal an FeIIIMnIIμ-phenoxobis(μ-carbox- ylato) core. A single location of the FeIII ion in 1 and of the FeII ion in 2 was demonstrated by Mossbauer and ¹H NMR spectroscopies, respectively. An investigation of the temperature dependence of the magnetic susceptibility of 1 revealed a moderate antiferromagnetic interaction (J = 20 cm-1) between the high-spin FeIII and MnII ions in 1, which was confirmed by Mossbauer and electron paramagnetic resonance (EPR) studies. The electrochemical properties of complex 1 are described. A quasireversible electron transfer at --40 mV versus Ag/AgCl corresponding to the FeIIIMnII/FeIIMnII couple appears in the cyclic voltammogram. Thorough investigations of the Mossbauer and EPR signatures of complex 2 were performed. The analysis allowed evidencing of a weak antiferromagnetic interaction (J = 5.72 cm-1) within the FeuMn" pair consistent with that deduced from magnetic susceptibility measurements (j = 6.8 cm-1). Owing to the similar value of the Fe11 zero-field splitting (DFe = 3.55 cm-1), the usual treatment within the strong exchange limit was precluded and a full analysis of the electronic structure of the ground state of complex 2 was developed. This situation is reminiscent of that found in many diiron and iron--manganese enzyme active sites. [ABSTRACT FROM AUTHOR]

Reaction of the unsymmetrical phenol ligand 2-((bis(2-pyridylmethyl)amino)methyl)-6-(((2-pyridylmethyl)benzylamino)methyl)-4-methylphenol (HL-Bn) or its 2,6-dichlorobenzyl analogue (HL-BnCI2) with Fe(H2O)6(ClO4)2 in the presence of disodium m-phenylenedipropionate (Na2(mpdp)) followed by exposure to atmosphere affords the diiron(II,III) complexes [Fe2(L-Bn)(mpdp)(H2O)](ClO4)2 and [Fe2(L-BnCI2)(mpdp)(CH3OH)](CIO4)2, respectively. The latter complex has been characterized by X-ray crystallography. It crystallizes in the monoclinic system, space group P21/n, with a -13.3095(14) Å, b = 20.1073(19) Å, c = 19.4997(19) Å, α = 90°, β = 94.471(2)°, γ = 90°, V= 5202.6(9) ų, and Z = 4. The structure of the compound is very similar to that of [Fe2(L-Bn)(mpdp)(H2O)](BPh4)2 determined earlier, except for the replacement of a water by a methanol on the ferrous site. Magnetic measurements of [Fe2(LBn)(mpdp)(H2O)](BPh4)2 reveal that the two high-spin Fe ions are moderately antiferromagnetically coupled (J = -3.2(2) cm-1). Upon dissolution in acetonitrile the terminal ligand on the ferrous site is replaced by a solvent molecule. The acetonitrile-water exchange has been investigated by various spectroscopic techniques (UV-visible, NMR, Mössbauer) and electrochemistry. The substitution of acetonitrile by water is clearly evidenced by Mössbauer spectroscopy by a reduction of the quadrupole splitting value from 3.14 to 2.41 mm/s. In addition, it causes a 210 mV downshift of the oxidation potential of the ferrous site and a similar reduction of the stability domain of the mixed-valence state. Exhaustive electrolysis of a solution of [Fe2(L-Bn)(mpdp)(H2O)]2+ shows that the aqua diferric species is not stable and undergoes a chemical reaction which can be partly reversed by reduction to the mixedvalent state. This and other electrochemical observations suggest that upon oxidation of the diiron center to the diferric state the aqua ligand is deprotonated to a hydroxo. This hypothesis is supported by Mössbauer spectroscopy. Indeed, this species possesses a large quadrupole splitting value (ΔEQ ≥ 1.0 mm·s-1) similar to that of analogous complexes with a terminal phenolate ligand. This study illustrates the drastic effects of aqua ligand exchange and deprotonation on the electronic structure and redox potentials of diiron centers. [ABSTRACT FROM AUTHOR]

The coupling of electron and proton transfers is currently under intense scrutiny. This Communication reports a new kind of proton-coupled electron transfer within a homodinuclear first-row transition-metal complex. The triply-bridged complex [FeIII(μ-OPh)(μ2-mpdp)FeII(NH2Bn)] (1; mpdp2- = m-phenylenedipropionate) bearing a terminal aminobenzyl ligand can be reversibly deprotonated to the anilinate complex 2 whose core [FeII(μ-OPh)(μ2-mpdp)FeIII(NHBn)] features an inversion of the iron valences. This observation is supported by a combination of UV-visible, 1H NMR, and Mössbauer spectroscopic studies. [ABSTRACT FROM AUTHOR]