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The first two mononuclear manganese citrate complexes, (NH4)4[MnII(C6H5O7)2] (1) and (NH4)5[MnIII(C6H4O7)2].2H2O (2) were synthesized in aqueous solutions near physiological pH values. They were isolated in their pure crystalline forms and characterized by elemental analyses and spectroscopic techniques, including UV/visible, electron paramagnetic resonance, Fourier transformed infrared, and magnetic susceptibility measurements. Compound 1 crystallizes in the monoclinic space group P2(1)/c, with a = 8.777(1) A, b = 13.656(3) A, c = 9.162(2) A, beta = 113.62(2) degrees, V = 1006.2(6) A3, and Z = 2. Compound 2 crystallizes in the triclinic space group P1, with a = 9.606(3) A, b = 9.914(3) A, c = 7.247(3) A, alpha = 91.05(1) degrees, beta = 105.60(1) degrees, gamma = 119.16(1) degrees, V = 571.3(3) A3, and Z = 1. The X-ray crystal structures of 1 and 2 revealed that, in both cases, the manganese ion is six-coordinate and is bound by two citrate ligands in a distorted octahedral fashion. In the case of complex 1, the citrate ion binds to Mn2+ as a triply deprotonated ligand, retaining the central carbon hydroxyl hydrogen, whereas, in the case of compound 2, the citrate ligand coordinates to Mn3+ as a fully deprotonated entity. Compound 2 contains water molecules of crystallization in the unit cell which, through extensive hydrogen-bonding interactions, bestow considerable stability upon the Mn(3+)-citrate assembly. There are significant contributions to the stabilities of the assembled lattices in 1 and 2 arising from the ammonium counterions neutralizing the high anionic charges of the complexes. The EPR spectra attest to the presence of paramagnetic Mn2+ and Mn3+ species in the solid state. Corroborative evidence is obtained from the magnetic susceptibility measurements in the range 5-300 K. Complexes 1 and 2 present clear cases of mononuclear manganese citrate species relevant to manganese speciation in biological media and potentially related to the beneficial as well as toxic effects of manganese on humans.

Several metalloproteins require manganese to perform their biological functions and have recently become important targets for synthetic modeling studies.1-13 Dinuclear manganese centers have been found in Lactobacillus plantarum pseudocatalase, which catalytically disproportionates H202,14-16and in the small subunit of Brevibacterium ammoniagenes and Micrococcus luteus ribonucleotide reductase (RR), which facilitates the reduction of ribonucleotides to deoxyribonucleotides17-19 A tetranuclear manganese center has been proposed as the active site in the oxygen evolving center (OEC) of photosystem II (PSII), where H20 is oxidized to 02.20-26 X-ray analyses performed on pseudocatalase from Thermus thermophilus revealed that the distance between the two manganese atoms is 3.6 Á,27 and an EPR investigation showed that the dinuclear center could achieve

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The novel nonairon(III) citrate complex [Fe9O(cit)8(H2O)3]7- (3) [cit = C(O-)(COO-)(CH2COO-)2] was prepared as its neocuproinium salt by reacting ferric nitrate, sodium citrate, and neocuproine (neo; 2,9-dimethyl-1,10-phenanthroline) in a 1:1:1 ratio in aqueous solution. The compound (Hneo)7·3·neo·61H2O crystallizes in the monoclinic space group P21/c with unit cell parameters of a = 21.910(3) Å, b = 25.788(5) Å, c = 39.653(6) Å, β = 99.47(1)°, V = 22 099(7) Å3, and Z = 4. The anionic complex consists of three parallel triangular Fe3 units, 5.6 Å apart, forming a slightly distorted trigonal prism. The outer two units are connected by three bridging tetradentate cit4- ligands to the central one, and each of the terminal Fe3 units is capped by another cit4- ligand. The central Fe3 unit has the oxo-bridged basic iron carboxylate structure, whereas the two terminal ones have a “voided cubane” structure with an {Fe3O4} core. Mössbauer spectra are composed of two subspectra (“a” and “b”) with intensity ratio 2:1, reflecting the existence of two inequivalent trivalent iron sites. Subspectra “a” are assigned to the three equivalent iron atoms of the terminal Fe3 units, and subspectra “b” to the three equivalent iron atoms of the central unit.

The reaction between ferric fluoride trihydrate and pyridine in hot methanol produced yellow-brown crystals of (PyH)5.[Fe13O4F24(OMe)12].4H2O.CH3OH. The anionic complex [Fe13O4F24(OMe)12]5- (1), which has a full Td symmetry, is the first example of an open-shell Keggin ion consisting of 13 high-spin d5 iron(III) atoms. The central, tetrahedral {FeO4} unit in 1 is surrounded by 12 octahedral iron atoms, which are bridged by methoxide oxygen atoms and fluoride ligands. In addition, each of the 12 iron atoms is coordinated to a terminal fluoride ligand with an Fe-F distance of 1.846(3) A. Magnetic susceptibility studies indicate strong exchange interactions between the iron atoms, the mueff value of 19.3 muB at 300 K being significantly lower than that expected for thirteen uncoupled S = 5/2 centers.

Data on the preparation, structure, and magnetism of the new complex [Cr 4 S(O 2 CCH 3 ) 8 (H 2 O) 4 ](BF 4 ) 2 ⋅H 2 O are presented. The metal-sulfur M 4 S core structure is similar to that found in M 4 S[S 2 As(CH 3 ) 2 ] 6 (where M is cobalt or zinc) compounds and in beryllium basic acetate. However, the six-coordinate metal ions in the new Cr 4 S system distinguish it from these structures. The tetranuclear chromium complex is the first example of a new structural type. Its magnetic spin S = 6 ground state, a striking example of intramolecular ferromagnetic coupling, was determined by variable-temperature magnetic susceptibility measurements. Long-range antiferromagnetic intercluster ordering was found below 170 millikelvin.