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The Oxygen evolving complex of plant photosystem II is made of a manganese cluster that gives rise to a low temperature EPR multiline signal in the S2 oxidation state. The origin of this EPR signal has been addressed with respect to the question of the magnetic couplings between the electron and nuclear spins of the four possible Mn ions that make up this complex. Considering Mn(III) and Mn(IV) as the only possible oxidation states present in the S2 state, and no large anisotropy of the magnetic tensors, the breadths of the EPR spectra calculated for dimers and trimers with S = ½ have been compared with that of the biological site. It is concluded that neither a dinuclear nor a trinuclear complex made of Mn(III) and Mn(IV) can be responsible for the multiline signal; but that, by contrast, a tetranuclear Mn complex can be the origin of this signal. The general shape of the experimental spectrum, its particular hyperfine pattern, the positions of most of the hyperfine lines and their relative intensities can be fit by a tetramer model described by the following six fitting parameters: g ≈ 1.987, A1 ≈ 122.4 10-4 cm-1, A2 ≈ 87.2 10-4 cm-1, A3 ≈ 81.6 10-4 cm-1, A4 ≈ 19.1 10-4 cm-1 and δH = 24.5 G. A second model described by parameters very close to those given above except for A4 ≈ 77.5 10-4 cm-1 gives an equally good fit. However, no other set of parameters gives an EPR spectrum that reproduces the hyperfine pattern of the S2 multiline signal. This demonstrates that in the S2 state of the oxygen evolving complex, the four manganese ions are organized in a magnetic tetramer.

Synthesis or asymmetric heterodinuclear complexes (LM 1 (μ-O)(μ-CH 3 CO 2 ) 2 M 2 L'] 2+ , where L represents 1,4,7-triazacyclononane, L' is 1,4,7-trimethyl-1,4,7-triazacyclononane, and M 1 and M 2 are two different trivalent first-row transition metals, has been achieved by the hydrolytic reaction or an equimolar mixture orLM 1 Cl 3 and L'M 2 Cl 3 in aqueous sodium acetate solution

The new trinuclear cluster Mn II 3 (CH 3 CO 2 ) 6 (bpy) 2 has been synthesized, and its crystal structure was determined. It crystallizes in the triclinic system, space group P1, with a=12.849 (4) A, b=9.790 (3) A, c=8.187 (2) A, α=108.50 (2) o , β=96.69 (2) o , γ=106.41 (3) o , Z=1, and V=912.5 (5) A 3 . The structure was solved and refined by using 2071 reflections

International audience; The green complex S = 1 [(TPEN)FeO]2+ [TPEN = N,N,N',N'-tetrakis(2-pyridylmethyl)ethane-1,2-diamine] has been obtained by treating the [(TPEN)Fe]2+ precursor with meta-chloroperoxybenzoic acid (m-CPBA). This high-valent complex belongs to the emerging family of synthetic models of FeIV=O intermediates invoked during the catalytic cycle of biological systems. This complex exhibits spectroscopic characteristics that are similar to those of other models reported recently with a similar amine/pyridine environment. Thanks to its relative stability, vibrational data in solution have been obtained by Fourier transform infrared. A comparison of the Fe=O and Fe=18O wavenumbers reveals that the Fe-oxo vibration is not a pure one. The ability of the green complex to oxidize small organic molecules has been studied. Mixtures of oxygenated products derived from two- or four-electron oxidations are obtained. The reactivity of this [FeO]2+ complex is then not straightforward, and different mechanisms may be involved.

Two manganese complexes, (py2(NMe)2)MnIICl2 (1) and [(py2(NMe)2)MnIIIF2]+ (2), are here described with the macrocyclic ligand py2(NMe)2 (py2(NMe)2 = N,N'-dimethyl-2,11-diaza[3,3](2,6)pyridinophane). For both, the crystal structure is reported. The UV-visible spectrum of 2 exhibits a very broad near-infrared (NIR) band corresponding to the transition between the two e(g)-type orbitals split by the Jahn-Teller effect. A negative D value of ca. -4 cm(-1) was estimated by high-field and high-frequency electron paramagnetic resonance (HF-EPR) spectroscopy, which was consistent with symmetry considerations. Density functional theory (DFT) calculations on 2 support the 5B1 electronic ground state predicted from the X-ray structure. Moreover, to explain the large value of the D parameter, a spin triplet first excited spin state was postulated to occur at low energy. This was confirmed by the DFT calculations.

Complexes of general formula [(TPA)Fe(R-Cat)]X·nS were synthesised with different catecholate derivatives and anions (TPA = tris(2-pyridylmethyl)amine, R-Cat2− = 4,5-(NO2)2-Cat2− denoted DNC2−; 3,4,5,6-Cl4-Cat2− denoted TCC2−; 3-OMe-Cat2−; 4-Me-Cat2− and X = BPh4−; NO3−; PF6−; ClO4−; S = solvent molecule). Their magnetic behaviours in the solid state show a general feature along the series, viz., the occurrence of a thermally-induced spin crossover process. The transition curves are continuous with transition temperatures ranging from ca. 84 to 257 K. The crystal structures of [(TPA)Fe(DNC)]X (X = PF6−; BPh4−) and [(TPA)Fe(TCC)]X·nS (X = PF6−; NO3− and n = 1, S = H2O; ClO4− and n = 1, S = H2O; BPh4− and n = 1, S = C3H6O) were solved at 100 (or 123 K) and 293 K. For those two systems, the characteristics of the [FeN4O2] coordination core and those of the dioxolene ligands appear to be consistent with a prevailing FeIII–catecholate formulation. This feature is in contrast with the large quantum mixing between FeIII–catecholate and FeII–semiquinonate forms recently observed with the more electron donating simple catecholate dianion. The thermal spin crossover process is accompanied by significant changes of the molecular structures as shown by the average variation of the metal–ligand bond distances which can be extrapolated for a complete spin conversion from ca. 0.123 to 0.156 A. The different space groups were retained in the low- and high-temperature phases.

As a functional model of the catechol dioxygenases, [(TPA)Fe(Cat)]BPh4 (TPA = tris(2-pyridylmethyl)amine and Cat = catecholate dianion) exhibits the purple-blue coloration indicative of some charge transfer within the ground state. In contrast to a number of high-spin bioinspired systems, it was previously shown that, in the solid state, [(TPA)Fe(Cat)]BPh4 undergoes a two-step S = 1/2 = S = 5/2 spin-crossover. Therefore, the electronic and vibrational characteristics of this compound were investigated in the solid state by UV/Vis absorption and resonance Raman spectroscopies over the temperature range of the transition. This allowed the charge-transfer transitions of the low-spin (LS) form to be identified. In addition, the vibrational progression observed in the NIR absorption of the LS form was assigned to a five-membered chelate ring mode. The X-ray crystal structure solved at two different temperatures, shows the presence of highly distorted pseudo-octahedral ferric complexes that occupy two nonequivalent crystalline sites. The variation of the molecular parameters as a function of temperature strongly suggests that the two-step transition proceeds by a successive transition of the species in the two nonequivalent sites. The thermal dependence of the high-spin fraction of metal ions determined by Mossbauer experiments is consistent with the magnetic data, except for slight deviations in the high temperature range. The optimized geometries, the electronic transitions, vibrational frequencies, and thermodynamic functions were calculated with the B3LYP density functional method for the doublet and the sextet states. The finding of a ground state that possesses a significant mixture of Fe(III)-catecholate and FeII-semiquinonate configurations is discussed with regard to the set of experimental and theoretical data.

The purpose of this commentary is to stress that in mixed-valence systems, electron localization and electron delocalization are competitive, the first being most often the stronger. In iron-sulfur clusters where a delocalized Fe(II)Fe(III) pair of maximal spin exists, a minimum value of 1200 cm–1 is proposed for the resonance integral β parametrizing the electron delocalization.

Two heptacoordinated Mn(II) complexes are isolated and X-ray characterized using the well-known tpen ligand (tpen = N,N,N',N'-tetrakis(2-pyridylmethyl)-1,2-ethanediamine): [(tpen)Mn(OH(2))](ClO(4))(2) (1(ClO(4))(2)) and [(tpen)Mn(micro-OAc)Mn(tpen)](ClO(4))(3).2H(2)O (2(ClO(4))(3).2H(2)O). Crystallographic data for 1(ClO(4))(2) at 110(2) K (respectively at 293(2) K): monoclinic, space group C2/c, a = 15.049(3) A (15.096(3) A), b = 9.932(2) A (10.105(2) A), c = 19.246(4) A (19.443(4) A), beta = 94.21(3) degrees (94.50(3) degrees ), Z = 4. Crystallographic data for 2(ClO(4))(3).0.5(C(2)H(5))(2)O at 123(2) K: triclinic, space group P, a = 12.707(3) A, b = 12.824(3) A, c = 19.052(4) A, alpha = 102.71(3) degrees, beta = 97.83(3) degrees, gamma = 98.15(3) degrees, Z = 2. Investigation of the variation upon temperature of the molar magnetic susceptibility of compound 2(ClO(4))(3).2H(2)O reveals a weak antiferromagnetic exchange interaction between the two high-spin Mn(II) ions (J = -0.65 +/- 0.05 cm(-)(1), H = -JS(1).S(2)). EPR spectra are recorded on powder samples and on frozen acetonitrile solutions, demonstrating the maintenance upon dissolution of the heptacoordination of Mn in complex 1 while complex 2 partially dissociates. Electrochemical responses of complexes 1 and 2 are investigated in acetonitrile, and bulk electrolyses are performed at oxidative potential in the presence of various amounts of 2,6-lutidine (0-2.65 equiv per Mn ion). The formation from either 1 or 2 of the mixed-valent complex [(tpen)Mn(III)(micro-O)(2)Mn(IV)(tpen)](3+) (3) is established from mass spectrometry and EPR and IR spectroscopy measurements. When reaction is started from 2, formation of [(tpen)Mn(IV)(micro-O)(2)(micro-OAc)Mn(IV)](3+) (4) is evidenced from cyclic voltammetry, EPR, and UV-vis data. The Mn vs tpen ratio in the electrogenerated complexes is accurately controlled by the quantity of additional 2,6-lutidine. The role of tpen as a base is discussed.

The electronic properties of the isostructural series of heterotrinuclear thiophenolate-bridged complexes of the general formula [LFeMFeL](n+) With M = Cr, Co and Fe where L represents the trianionic form of the ligand 1,4,7-tris(4-tertbutyl-2-mercaptobenzyl)-1,4,7-triazacyclononane, synthesized and investigated by a number of experimental techniques in the previous work(1), are subjected now to a theoretical analysis. The low-lying electronic excitations in these compounds are described within a minimal model supported by experiment and quantum chemistry calculations. It was found indeed that various experimental data concerning the magnetism and electron delocalization in the lowest states of all seven compounds are completely reproduced within a model which includes the electron transfer between magnetic orbitals at different metal centers and the electron repulsion in these orbitals (the Hubbard model). Moreover, due to the trigonal symmetry of the complexes, only the electron transfer between nondegenerate orbital, a(1), originating from the t(2g) shell of each metal ion in a pseudo-octahedral coordination, is relevant for the lowest states. An essential feature resulting from quantum chemistry calculations, allowing to explain the unusual magnetic properties of these compounds, is the surprisingly large value and, especially, the negative sign of the electron transfer between terminal iron ions, beta'. According to their electronic properties the series of complexes can be divided as follows: (1) The complexes [LFeFeFeL](3+) and [LFeCrFeL](3+) show localized valences in the ground electronic configuration. The strong antiferromagnetic exchange interaction and the resulting spin 1/2 of the ground-state arise from large values of the transfer parameters. (2) In the complex [LFeCrFeL](+), due to a higher energy of the magnetic orbital on the central Cr ion than on the terminal Fe ones, the spin 3/2 and the single unpaired a(1) electron are almost localized at the chromium center in the ground state. (3) The complex [LFeCoFeL](3+) has one ground electronic configuration in which two unpaired electrons are localized at terminal iron ions. The ground-state spin S = 1 arises from a kinetic mechanism involving the electron transfer between terminal iron ions as one of the steps. Such a mechanism, leading to a strong ferromagnetic interaction between distant spins, apparently has not been discussed before. (4) The complex [LFeFeFeL](2+) is characterized by both spin and charge degrees of freedom in the ground manifold. The stabilization of the total spin zero or one of the itinerant electrons depends on beta', i.e., corresponds to the observed S = 1 for its negative sign. This behavior does not fit into the double exchange model. (5) In [LFeCrFeL](2+) the delocalization of two itinerant holes in a(1) orbitals takes place over the magnetic core of chromium ion. Although the origin of the ground-state spin S = 2 is the spin dependent delocalization, the spectrum of the low-lying electronic states is again not of a double exchange type. (6) Finally, the complex [LFeCoFeL](2+) has the ground configuration corresponding to the electron delocalization between terminal iron atoms. The estimated magnitude of the corresponding electron transfer is smaller than the relaxation energy of the nuclear distortions induced by the electron localization at one of the centers, leading to vibronic valence trapping observed in this compound. ispartof: Journal of the American Chemical Society vol:125 issue:41 pages:12615-12630 ispartof: location:United States status: published

Two new iron complexes, [L(5)(3)Fe(II)Cl]PF(6) (1.PF(6)) and [(L(5)(3)H(+))Fe(III)Cl(3)]PF(6) (2.PF(6)), were synthesized (L(5)(3) = N-methyl-N,N',N'-tris(2-pyridylmethyl)propane-1,3-diamine), and their molecular structures were determined by X-ray crystallography. Their behavior in solution was studied by UV-vis spectroscopy and electrochemistry. Upon addition of a base to an acetonitrile solution of 2, the new unsymmetrical dinuclear complex [L(5)(3)Fe(III)OFe(III)Cl(3)](+) was detected. Treating 1 with hydrogen peroxide has allowed us to detect the low spin [L(5)(3)Fe(III)OOH](2+). Its spectroscopic properties (UV-vis, EPR and resonance Raman) are similar to those reported for related FeOOH complexes obtained with amine/pyridine ligands. Using stopped-flow absorption spectroscopy, the formation and degradation of [L(5)(3)Fe(III)OOH](2+) has been monitored, and a mechanism is proposed to reproduce the kinetic data.

The complexes [L(5)Fe(II)Cl]BPh(4) and [L(5)Fe(II)(H(2)O)](BPh(4))(2) (L(5) = N,N,N'-tris(2-pyridylmethyl)-N'-methyl-ethane-1,2-diamine) have been isolated. Bernal et al. (Bernal, J.; et al. J. Chem. Soc., Dalton Trans. 1995, 3667-3675) have prepared this ligand and the corresponding complex [L(5)Fe(II)Cl]PF(6). We obtained the structural data of [L(5)Fe(II)Cl]BPh(4) by X-ray diffraction. It crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a = 17.645(7) Å, b = 16.077(6) Å, c = 13.934(5) Å, V = 3953(3) Å(3), and Z = 4. It presents Fe(II)-N bond lengths close to 2.2 Å, typical of high-spin Fe(II). In solution the [L(5)Fe(II)(H(2)O)](BPh(4))(2) complex showed a dependence of spin state upon the nature of the solvent. It was high spin in acetone and changed to low spin in acetonitrile. This was detected by UV-vis spectroscopy and by (1)H NMR. Bernal et al. (ibidem) showed that these complexes in the presence of an excess of H(2)O(2) give a purple species, very likely the [L(5)Fe(III)(OOH)](2+) derivative, with spectroscopic signatures analogous to those of "activated bleomycin". The formation of [L(5)Fe(III)(OOH)](2+) is confirmed here by electrospray ionization mass spectrometry. We found that a L(5)/Fe system gave single-strand breaks on plasmid DNA in the presence of either a reducing agent (ascorbate) and air or oxidants (H(2)O(2), KHSO(5), MMPP) at 0.1mgr;M concentration. The methyl group in L(5) was substituted by a (CH(2))(5)N(CH(3))(3)(+) group in order to get higher affinity with DNA. The corresponding ligand L(5)(+) was used to prepare the complexes [L(5)(+)Fe(II)Cl]Y(2) (Y = BPh(4)(-), PF(6)(-), ClO(4)(-)) and [L(5)(+)Fe(II)Br](PF(6))(2). The crystal structure of [L(5)(+)Fe(II)Cl](ClO(4))(2) was solved. It crystallizes in the monoclinic space group P2(1)/a with a = 14.691(2) Å, b = 13.545(2) Å, c = 17.430(2) Å, beta = 93.43(1) degrees, V = 3462(1) Å(3), and Z = 4. The Fe(II)-ligand distances are similar to those of [L(5)Fe(II)Cl]BPh(4). At the relatively low concentration of 0.01mgr;M, [L(5)(+)Fe(II)Br](2+) promoted DNA breaks. The reaction was not inhibited by hydroxyl radical scavengers. The reaction might involve a nondiffusible oxygen reactive species, either a coordinated hydroperoxide or a high-valent metal-oxo entity.

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.

Two structurally homologous Mn compounds in different oxidation states were studied to investigate the relative influence of oxidation state and ligand environment on Mn K-edge X-ray absorption near-edge structure (XANES) and Mn Kbeta X-ray emission spectroscopy (Kbeta XES). The two manganese compounds are the di-mu-oxo compound [L'2Mn(III)O2Mn(IV)L'2](ClO4)3, where L' is 1,10-phenanthroline (Cooper, S. R.; Calvin, M. J. Am. Chem. Soc. 1977, 99, 6623-6630) and the linear mono-mu-oxo compound [LMn(III)OMn(III)L](ClO4)2, where L- is the monoanionic N,N-bis(2-pyridylmethyl)-N'-salicylidene-1,2-diaminoethane ligand (Horner, O.; Anxolabéhère-Mallart, E.; Charlot, M. F.; Tchertanov, L.; Guilhem, J.; Mattioli, T. A.; Boussac, A.; Girerd, J.-J. Inorg. Chem. 1999, 38, 1222-1232). Preparative bulk electrolysis in acetonitrile was used to obtain higher oxidation states of the compounds: the Mn(IV)Mn(IV) species for the di-mu-oxo compound and the Mn(III)Mn(IV) and Mn(IV)Mn(IV) species for the mono-mu-oxo compound. IR, UV/vis, EPR, and EXAFS spectra were used to determine the purity and integrity of the various sample solutions. The Mn K-edge XANES spectra shift to higher energy upon oxidation when the ligand environment remains similar. However, shifts in energy are also observed when only the ligand environment is altered. This is achieved by comparing the di-mu-oxo and linear mono-mu-oxo Mn-Mn moieties in equivalent oxidation states, which represent major structural changes. The magnitude of an energy shift due to major changes in ligand environment can be as large as that of an oxidation-state change. Therefore, care must be exercised when correlating the Mn K-edge energies to manganese oxidation states without taking into account the nature of the ligand environment and the overall structure of the compound. In contrast to Mn K-edge XANES, Kbeta XES spectra show less dependence on ligand environment. The Kbeta1,3 peak energies are comparable for the di-mu-oxo and mono-mu-oxo compounds in equivalent oxidation states. The energy shifts observed due to oxidation are also similar for the two different compounds. The study of the different behavior of the XANES pre-edge and main-edge features in conjunction with Kbeta XES provides significant information about the oxidation state and character of the ligand environment of manganese atoms.

A biologically relevant dinuclear manganese mono-mu-oxo complex with a bound phenolate ligand in three oxidation states, (III,III), (III,IV) and (IV,IV), was studied using resonance Raman spectroscopy. Depending upon the excitation frequency, phenolate vibrations or mu-oxo vibrations were enhanced, which allowed us to assign the UV-visible absorption spectra. In the case of the mixed valence species (III,IV), the mu-oxo vibration at 854 cm-1 has been assigned by isotopic substitution (H2(18)O) to nu as(Mn-O-Mn). This preferential enhancement of the asymmetric vibration stresses the asymmetric character of the bridge.

Iron-peroxo Fe(III)O2 and hydroperoxo Fe(III)OOH systems are important intermediates between the initial Fe(II)-dioxygen adduct and the “activated” form of the catalytic site in many mono-iron biomolecules. To the same peroxidic level correspond, in diiron enzymes, bridged peroxo Fe(III)-O-O-Fe(III) intermediates. This review is concerned with the preparation and spectroscopic characterization of such intermediates in non-heme chemical systems, the properties of the natural systems being quoted as references. Although none have been crystallized, it seems very likely that Fe(III)OOH systems present a η 1 - coordination mode for the hydroperoxo group. These Fe(III)OOH units have given clear signatures in UV-vis, resonance Raman and mass spectrometry. By EPR it was found that in Fe(III)OOH, the Fe(III) is low-spin (S = 1/2) and we propose here a simple rationalization of the characteristics of the EPR g-tensor. The electronic properties of the Fe(III)(η1-OOH) known so far, point toward a strong Fe-O bond and a weak O-O bond, in total agreement with the reactivity scheme implying a cleavage of the O-O bond to lead formally to a Fe(V)O unit. Alkylperoxo systems are also included in this review. Fe(III)-peroxo systems Fe(III)O2 have been prepared and described. They contain high-spin Fe(III) and those identified seem to be of the η 2 type. The Fe-O bond is weaker and the O-O one is stronger than in the Fe(III)OOH systems. The implication of these Fe(III)O2 units in catalysis is unclear. “Complementary” systems, such as Fe(III)(η 1-OO) or Fe(III)(η 2-OOH) have been evoked in publications but not identified spectroscopically. These systems certainly deserve to be actively looked for.

Reaction of pink [L2Cr2(µ-OH)(µ-MeCO2)2][ClO4]3 in MeCN with NEt3 and addition of Na[BPh4] affords deep blue crystals of [L2Cr2(µ-O)(µ-MeCO2)2][BPh4]2(L = 1,4,7-trimethyl-1,4,7-triazacyclononane) the crystal structure of which reveals the presence of the µ-oxo-di(µ-acetato)-dichromium(III) core; weak intramolecular antiferromagnetic exchange coupling between the CrIII ions has been established.

The structure of the oxygen evolving center (OEC) in photosystem II of plants and the mechanism of water oxidation are still unknown. Recent biological results have been reviewed in ref 1. Chemists can contribute to this study by synthesizing new manganese clusters and studying the electronic properties of those complexes; then useful comparisons can be made with the spectroscopic properties of the natural system. This approach led, for instance, to the synthesis of (Mn{sub 4}O{sub 3}Cl){sup 6+} cores and of adamantane-like (Mn{sub 4}O{sub 6}){sup 4+} cores. Chemical results have been reviewed in ref 6. The authors report here the synthesis and magnetic studies of a new type of manganese cluster made of a triangular (Mn{sub 3}O{sub 4}){sup 4+} core with two mono-{mu}-oxo bridges and one di-{mu}-oxo bridge.

During the past decades, spectroscopic characterization of exchange interactions and electron delocalization has developed into a powerful tool for the recognition of metal clusters in metalloproteins. By contrast, the biological relevance of these interactions has received little attention thus far. This paper presents a theoretical study in which this problem is addressed. The rate constant for intermolecular electron-transfer reactions which are essential in many biological processes is investigated. An expression is derived for the dependence of the rate constant for self-exchange on the delocalization degree of the mixed-valence species. This result allows us to rationalize published kinetic data. In the simplest case of electron transfer from an exchange-coupled binuclear mixed-valence donor to a diamagnetic acceptor, the rate constant is evaluated, taking into account spin factors and exchange energies in the initial and final state. The theoretical analysis indicates that intramolecular spin-dependent electron delocalization (double exchange) and Heisenberg-Dirac-van Vleck (HDvV) exchange have an important impact on the rate constant for intermolecular electron transfer. This correlation reveals a novel relationship between magnetochemistry and electrochemistry. Contributions to the electron transfer from the ground and excited states of the exchange-coupled dimer have been evaluated. For clusters in which these states have different degrees of delocalization, the excited-state contributions to electron transfer may become dominant at potentials which are less reductive than the potential at which the rate constant for the transfer from the ground state is maximum. The rate constant shows a steep dependence on HDvV exchange, which suggests that an exchange-coupled cluster can act as a molecular switch for exchange-controlled electron gating. The relevance of this result is discussed in the context of substrate specificity of electron-transfer reactions in biology. Our theoretical analysis points toward a possible biological role of the spin-state variability in iron-sulfur clusters depending on cluster environment.

Novel manganese (III) μ-oxo complexes with the formula Mn2IIIO(C3F7CO2)4(bipy)2 ( 1 ) were synthesized from the oxidation of

Das vollstandige Manuskript dieser Zuschrift erscheint in: Angew. Chem. Suppl. 1982, 953. DOI: 10.1002/ange.198209530