Synthesis,structure and magnetism of a new manganese carboxylate cluster: [Mn16O16(OMe)6(OAc)16(MeOH)3(H2O)3].6H2O

A new manganese(III/IV) carboxylate cluster [Mn16O16(OMe)6(OAc)16(MeOH)3(H2O)3].6H2O has been structurally characterised and shown to display net antiferromagnetic coupling with preliminary evidence for single-molecule magnetic behaviour.     

Crystal structure and heat capacity of the mixed-valence trinuclear iron monoiodoacetate complex, [Fe(III)2Fe(II)O(O2CCH2I)6(H2O)3][Fe(III)3O(O2CCH2I)6(H2O)3]I

The crystal structure of a trinuclear iron monoiodoacetate complex was determined. Although it has been incorrectly characterized as [Fe₃O(O₂CCH₂I)₆(H₂O)₃], the correct chemical formula turned out to be [Fe(III)₂Fe(II)O(O₂CCH₂I)₆(H₂O)₃]-[Fe(III)₃O(O₂CCH₂I)₆(H₂O)₃]I (1). The two kinds of Fe₃O molecules (Fe(III)₂Fe(II)O and Fe(III)₃O) are crystallographically indistinguishable. All the Fe atoms are crystallographically equivalent because of a crystallographic threefold symmetry. Heat capacities of 1 seem to exhibit no thermal anomaly in the temperature range 5.5–309 K, although the valence detrapping phenomenon has been observed in this temperature range. This fact indicates that the valence-detrapping phenomenon in 1 occurs without any phase transition, leading 1 to a glassy state, probably because the crystal of 1 is just like a solid solution of distorted mixed-valence Fe(III)₂Fe(II)O molecules and permanently undistorted Fe(III)₃O molecules which may act as an inhibitor for a cooperative valence-trapping.     

The properties of the [Mn12O12(O2CR)16(H2O)4] single-molecule magnets in truly axial symmetry: [Mn12O12(O2CCH2Br)16(H2O)4].4CH2Cl2

Detailed studies are reported of a Mn(12) single-molecule magnet (SMM) in truly axial (tetragonal) symmetry. The complex is [Mn(12)O(12)(O(2)CCH(2)Br)(16)(H(2)O)(4)].4CH(2)Cl(2) (2.4CH(2)Cl(2) or Mn(12)-BrAc), obtained by the standard carboxylate substitution method. The complex has an S = 10 ground state, typical of the Mn(12) family, and displays frequency-dependent out-of-phase AC susceptibility signals and hysteresis in single-crystal magnetization vs applied DC field sweeps. Single-crystal high-frequency EPR spectra in frequencies up to 360 GHz exhibit narrow signals that are not overlapping multiplets, in contrast to [Mn(12)O(12)(O(2)CMe)(16)(H(2)O)(4)].2MeCO(2)H.4H(2)O (1 or Mn(12)-Ac), which also crystallizes in an axial (tetragonal) space group but which now is recognized to consist of a mixture of six hydrogen-bonded isomers in the crystal and thus gives multiple, inhomogeneously broadened EPR signals. Similarly, single-crystal (55)Mn NMR spectra on Mn(12)-BrAc display much sharper signals than a single crystal of Mn(12)-Ac, and this allows one Mn(III) signal to show an almost baseline-resolved quintet from quadrupolar splitting ((55)Mn, I = 5/2, 100%), allowing quadrupole coupling parameters (e(2)qQ) to be determined. In addition, it was found that crushing crystals of Mn(12)-BrAc into a microcrystalline powder causes severe broadening and shifts of the NMR resonances, emphasizing the superiority of single-crystal studies. The combined results establish that Mn(12)-BrAc is far superior to Mn(12)-Ac for the study of the intrinsic properties of the Mn(12) family of SMMs in axial symmetry, and for the search for new phenomena such as quantum interference effects caused by higher-order (>2nd-order) transverse terms in the spin Hamiltonian.     

Mn12O12(OMe)2(O2CPh)16(H2O)2]2- single-molecule magnets and other manganese compounds from a reductive aggregation procedure

A new synthetic procedure has been developed in Mn cluster chemistry involving reductive aggregation of permanganate (MnO4-) ions in MeOH in the presence of benzoic acid, and the first products from its use are described. The reductive aggregation of NBu(n)4MnO4 in MeOH/benzoic acid gave the new 4Mn(IV), 8Mn(III) anion [Mn12O12(OMe)2(O2CPh)16(H2O)2]2-, which was isolated as a mixture of two crystal forms (NBu(n)4)2[Mn12O12(OMe)2(O2CPh)16(H2O)2].2H2O.4CH2Cl2 (1a) and (NBu(n)4)2[Mn12O12(OMe)2(O2CPh)16(H2O)2].2H2O.CH2Cl2 (1b). The anion of 1 contains a central [Mn(IV)4(mu3-O)2(mu-O)2(mu-OMe)2]6+ unit surrounded by a nonplanar ring of eight Mn(III) atoms that are connected to the central Mn4 unit by eight bridging mu3-O2- ions. This compound is very similar to the well-known [Mn12O12(O2CR)16(H2O)4] complexes (hereafter called "normal Mn12"), with the main difference being the structure of the central cores. Longer reaction times (approximately 2 weeks) led to isolation of polymeric [Mn(OMe)(O2CPh)2]n2, which contains a linear chain of repeating [Mn(III)(mu-O2CPh)2(mu-OMe)Mn(III)] units. The chains are parallel to each other and interact weakly through pi-stacking between the benzoate rings. When KMnO4 was used instead of NBu(n)4MnO4, two types of compounds were obtained, [Mn12O12(O2CPh)16(H2O)4] (3), a normal Mn12 complex, and [Mn4O2(O2CPh)8(MeOH)4].2MeOH (4.2MeOH), a new member of the Mn4 butterfly family. The cyclic voltammogram of 1 exhibits three irreversible processes, two reductions and one oxidation. One-electron reduction of 1 by treatment with 1 equiv of I- in CH2Cl2 gave (NBu(n)4[Mn12O12(O2CPh)16(H2O)3].6CH2Cl2 (5.6CH2Cl2), a normal Mn12 complex in a one-electron reduced state. The variable-temperature magnetic properties of 1, 2, and 5 were studied by both direct current (dc) and alternating current (ac) magnetic susceptibility measurements. Variable-temperature dc magnetic susceptibility studies revealed that (i) complex 1 possesses an S = 6 ground state, (ii) complex 2 contains antiferromagnetically coupled chains, and (iii) complex 5 is a typical [Mn12]- cluster with an S = 19/2 ground state. Variable-temperature ac susceptibility measurements suggested that 5 and both isomeric forms of 1 (1a,b) are single-molecule magnets (SMMs). This was confirmed by the observation of hysteresis loops in magnetization vs dc field scans. In addition, 1a,b, like normal Mn12 clusters, display both faster and slower relaxing magnetization dynamics that are assigned to the presence of Jahn-Teller isomerism.     

Polynuclear manganese complexes with the dicarboxylate ligand m-phenylenedipropionate: a hexanuclear mixed-valence (3Mn(III), 3Mn(IV)) complex

The dicarboxylate group m-phenylenedipropionate (mpdp(2)(-)) has been used for the synthesis of four new Mn compounds of different nuclearities and oxidation states: [Mn(2)O(mpdp)(bpy)(2)(H(2)O)(MeCN)](ClO(4))(2) (3), [Mn(3)O(mpdp)(3)(py)(3)](ClO(4)) (4), [Mn(3)O(mpdp)(3)(py)(3)] (5), and [Mn(6)O(7)(mpdp)(3)(bpy)(3)](ClO(4)) (6). Compound 3 (2Mn(III)) contains a [Mn(2)(micro-O)](4+) core, whereas 5 (Mn(II), 2Mn(III)) and 4 (3Mn(III)) contain the [Mn(3)(micro(3)-O)](6+,7+) core, respectively. In all three compounds, the mpdp(2)(-) ligand is flexible enough to adopt the sites occupied by two monocarboxylates in structurally related compounds, without noticeable distortion of the cores. Variable-temperature magnetic susceptibility studies establish that 3 and 5 have ground-state spin values of S = 0 and S = 1/2, respectively. Compound 6 is a highly unusual 3Mn(III), 3Mn(IV) trapped-valent compound, and it is also a new structural type, with six Mn atoms disposed in a distorted trigonal antiprismatic topology. Its electronic structure has been explored by variable-temperature measurements of its dc magnetic susceptibility, magnetization vs field response, and EPR spectrum. The magnetic data indicate that it possesses an S = 3/2 ground state with an axial zero-field splitting parameter of D = -0.79 cm(-)(1), and this conclusion is supported by the EPR data. The combined results demonstrate the ligating flexibility of the mpdp(2)(-) ligand and its usefulness in the synthesis of a variety of Mn(x) species.     

Density functional study on geometrical features and electronic structures of di-mu-oxo-bridged [Mn2O2(H2O)8]q+ with Mn(II), Mn(III), and Mn(IV)

We report the geometrical features and electronic structures of di-mu-oxo-bridged Mn-Mn binuclear complexes with H2O ligands [Mn2O2(H2O)8]q+ in the iso- and mixed-valence oxidation states. All of the combinations among Mn(II), Mn(III), and Mn(IV) ions are considered the oxidation states of the Mn-Mn center, and the changes in molecular structure induced by the different electron configurations of Mn-based orbitals are investigated in relation to the oxygen-evolving complex (OEC) of photosystem II. The stable geometries of complexes are determined by using the hybrid-type density functional theory for both of the highest- and lowest-spin couplings between Mn sites, and the lowest-spin-coupled states are energetically more favorable than the highest-spin-coupled states except in the case of the complexes with the Mn(II) ion. The coordination positions of H2O ligands at the Mn(II) site tend to shift from the octahedral positions in contrast to those at the Mn(III) and Mn(IV) sites. The shape of the Mn2O2 core and the distances between the Mn ions and the H2O ligands vary depending on the electron occupations of the octahedral eg orbitals on the Mn site with an antibonding nature for the Mn-ligand interactions, indicating the trend as Mn(II)-O > Mn(III)-O and Mn(IV)-O, O-Mn(II)-O > O-Mn(III)-O > O-Mn(IV)-O among the iso-valence Mn2O2 cores, and O-Mn(lower)-O < O-Mn(higher)-O within the mixed-valence Mn2O2 core, and as Mn(II)-OH2 and Mn(III)-OH2 > Mn(IV)-OH2 for the axial H2O ligand. The optimized geometries of model complexes are compared with the X-ray structure of the OEC, and it is suggested that the cubane-like Mn cluster of the active site may not contain a Mn(II) ion. The effective exchange integrals are estimated by applying the approximate spin projection to clarify the magnetic coupling between Mn sites, and the superexchange pathways through the di-mu-oxo bridge are examined on the basis of the singly occupied magnetic orbitals derived from the singlet-coupled natural orbitals in the broken-symmetry state. The comparisons of the calculated results between [Mn2O2(H2O)8]q+ in this study and [Mn2O2(NH3)8]q+ reported by McGrady et al. suggest that the symmetric pathways are dominant to the exchange coupling constant, and the crossed pathway would be less important for the former than it would for the latter in the Mn(III)-Mn(III), Mn(IV)-Mn(IV), and Mn(III)-Mn(IV) oxidation states.     

Two mixed-valence vanadium(III,IV) phosphonoacetates with 16-ring channels: H2(DABCO)[V(IV)O(H2O)V(III)(OH)(O3PCH2CO2)2].2.5H2O and H2(PIP)[V(IVO)(H2O)V(III)(OH)(O3PCH2CO2)2].2.5H2O

Two isostructural mixed-valence vanadium phosphonoacetates H2(DABCO)[V(IV)O(H2O)V(III)(OH)(O3PCH2CO2)2].2.5H2O (1) and H2(PIP)[V(IV)O(H2O)V(III)(OH)(O3PCH2CO2)2].2.5H2O (2) have been synthesized. They crystallize in the orthorhombic space group Pnna with a = 7.0479(10) A, b = 15.307(2) A, and c = 17.537(3) A for 1 and a = 7.0465(9) A, b = 15.646(2) A, and c = 17.396(2) A for 2. X-ray single-crystal diffraction reveals that 1 and 2 have a three-dimensional open framework featuring 16-ring ellipsoid channels that are filled with doubly protonated 1,4-diazabicyclo[2,2,2]octanium/piperazinium cations and water molecules. According to the classification in metal-organic frameworks, 1 and 2 contain infinite (-O-V-)(infinity) chains that are cross-linked by "metalloligand" [VO(H2O)(O3PCH2CO2)2](4-) into a 3-D net of the sra topology. The temperature dependence of the magnetic susceptibility of 1 shows that the chi(m)T value in the range of 60-320 K is constant of 1.105 cm3 K mol(-1)/V2 unit, and upon further cooling, the chi(m)T value rapidly increases to 1.81 cm3 K mol(-1) at 2 K. The corresponding effective magnetic moment (mu(eff))/V2 unit varies from 2.97 mu(B) at 320 K to 3.80 mu(B) at 2 K. The magnetic data in the range of 2-320 K follow the Curie-Weiss law with C = 1.074 cm3 K mol(-1) and Theta= -1.34 K.     

Single-molecule magnets: site-specific ligand abstraction from [Mn12O12(O2CR)16(H2O)4] and the preparation and properties of [Mn12O12(NO3)4(O2CCH2Bu(t))12(H2O)4

Site-selective carboxylate abstraction has been achieved from [Mn(12)O(12)(O(2)CR)(16)(H(2)O)(4)] complexes by treatment with HNO(3) in MeCN. The reaction of the R = Ph or CH(2)Bu(t)() complexes with 4 equiv of HNO(3) gives [Mn(12)O(12)(NO(3))(4)(O(2)CR)(12)(H(2)O)(4)] (R = CH(2)Bu(t) (6) or Ph (7)) in analytical purity. Complex 6.MeNO(2) crystallizes in monoclinic space group C2/c with the following cell parameters at -168 degrees C: a = 21.280(5), b = 34.430(8), c = 33.023(8) A, beta = 104.61(1) degrees, V = 23413 A, and Z = 8. The four NO(3)(-) groups are not disordered and are bound in bridging modes at axial positions formerly occupied by bridging carboxylate groups. (1)H NMR spectroscopy in CD(2)Cl(2) and CDCl(3) shows retention of the solid-state structure on dissolution in these solvents. DC magnetic susceptibility (chi(M)) and magnetization (M) studies have been carried out in the 2.00-300 K and 1.0-7.0 T ranges. Fits of M/Nmu(B) versus H/T plots gave S = 10, g = 1.92, and D = -0.40 cm(-1), where D is the axial zero-field splitting parameter. AC magnetic susceptibility studies on 6 have been performed in the 1.70-10.0 K range in a 3.5 Oe field oscillating at frequencies up to 1500 Hz. Out-of-phase magnetic susceptibility (chi(M)' ') signals were observed in the 4.00-8.00 K range which were frequency-dependent. Thus, 6 displays the slow magnetization relaxation diagnostic of a single-molecule magnet (SMM). The data were fit to the Arrhenius law, and this gave the effective barrier to relaxation (U(eff)) of 50.0 cm(-1) (72.0 K) and a pre-exponential (1/tau(0)) of 1.9 x 10(8) s(-1). Complex 6 also shows hysteresis in magnetization versus DC field scans, and the hysteresis loops show steps at regular intervals of magnetic field, the diagnostic evidence of field-tuned quantum tunneling of magnetization. High-frequency EPR (HFEPR) spectroscopy on oriented crystals of complex 6 shows resonances assigned to transitions between zero-field split M(s) states of the S = 10 ground state. Fitting of the data gave S = 10, g = 1.99, D = -0.46 cm(-1), and B(4)(0) = -2.0 x 10(-5), where B(4)(0) is the quartic zero-field coefficient. The combined results demonstrate that replacement of four carboxylate groups with NO(3)(-) groups leads to insignificant perturbation of the magnetic properties of the Mn(12) complex. Complex 6 should now be a useful starting point for further reactivity studies, taking advantage of the good leaving group properties of the NO(3)(-) ligands.     

Malonate-containing manganese(III) complexes: synthesis, crystal structure, and magnetic properties of AsPh4[Mn(mal)2(H2O)2

The novel manganese(III) complexes PPh4[Mn(mal)2(H2O)2] (1) and AsPh4[Mn(mal)2(H2O)2] (2) (PPh4+ = tetraphenylphosphonium cation, AsPh4+ = tetraphenylarsonium cation, and H2mal = malonic acid) have been prepared, and the structure of 2 was determined by X-ray diffraction analysis. 2 is a mononuclear complex whose structure is made up of trans-diaquabis(malonato)manganate(III) units and tetraphenylarsonium cations. Two crystallographically independent manganese(III) ions (Mn(1) and Mn(2)) occur in 2 that exhibit elongated octahedral surroundings with four oxygen atoms from two bidentate malonate groups in equatorial positions (Mn(1)-O = 1.923(6) and 1.9328(6) A and Mn(2)-O = 1.894(6) and 1.925(6) A) and two trans-coordinated water molecules in the axial sites (Mn(1)-Ow = 2.245(6) A and Mn(2)-Ow = 2.268(6) A). The [Mn(mal)2(H2O)2]- units are linked through hydrogen bonds involving the free malonate-oxygen atoms and the coordinated water molecules to yield a quasi-square-type anionic layer growing in the ab plane. The shortest intralayer metal-metal separations are 7.1557(7) and 7.1526(7) A (through the edges of the square). The anionic sheets are separated from each other by layers of AsPh4+ where sextuple- and double-phenyl embraces occur. The magnetic behavior of 1 and 2 in the temperature range 1.9-290 K reveals the occurrence of weak intralayer ferromagnetic interactions (J = +0.081(1) (1) and +0.072(2) cm(-1) (2)). These values are compared to those of the weak antiferromagnetic coupling [J = -0.19(1) cm(-1)], which is observed in the chain compound K2[Mn(mal)2(MeOH)2][Mn(mal)2] (3), where the exchange pathway involves the carboxyate-malonate bridge in the anti-syn conformation. The structure of 3 was reported elsewhere. Theoretical calculations on fragment models of 2 and 3 were performed to analyze and substantiate both the nature and magnitude of the magnetic couplings observed.     

Electrochemical and chemical formation of [Mn4(IV)O5(terpy)4(H2O)2]6+, in relation with the photosystem II oxygen-evolving center model [Mn2(III,IV)O2(terpy)2(H2O)2]3+

To examine the real ability of the binuclear di-mu-oxo complex [Mn2(III,IV)O2(terpy)2(H2O)2]3+ (2) to act as a catalyst for water oxidation, we have investigated in detail its redox properties and that of its mononuclear precursor complex [Mn(II)(terpy)2]2+ (1) in aqueous solution. It appears that electrochemical oxidation of 1 allows the quantitative formation of 2 and, most importantly, that electrochemical oxidation of 2 quantitatively yields the stable tetranuclear Mn(IV) complex, [Mn4(IV)O5(terpy)4(H2O)2]6+ (4), having a linear mono-mu-oxo{Mn2(mu-oxo)2}2 core. Therefore, these results show that the electrochemical oxidation of 2 in aqueous solution is only a one-electron process leading to 4 via the formation of a mono-mu-oxo bridge between two oxidized [Mn2(IV,IV)O2(terpy)2(H2O)2]4+ species. 4 is also quantitatively formed by dissolution of the binuclear complex [Mn2(IV,IV)O2(terpy)2(SO4)2] (3) in aqueous solutions. Evidence of this work is that 4 is stable in aqueous solutions, and even if it is a good synthetic analogue of the "dimers-of-dimers" model compound of the OEC in PSII, this complex is not able to oxidize water. As a consequence, since 4 results from an one-electron oxidation of 2, 2 cannot act as an efficient homogeneous electrocatalyst for water oxidation. This work demonstrates that a simple oxidation of 2 cannot produce molecular oxygen without the help of an oxygen donor.     

A novel aggregate of [Mn2(mu-O)2] units: [Mn8O10(O2CMe)6(H2O)2(bpy)6]4+ with a serpentine core

The synthesis, characterization and initial reactivity studies are reported of the mixed-valence (Mn(IV)6Mn(III)2) title compound, which possesses an unusual serpentine-like core and is the highest average oxidation state (+3.75) Mn(x) (x > 4) cluster to date.     

Octanuclearity and tetradecanuclearity in manganese chemistry: an octanuclear manganese(II)/(III) complex featuring the novel [Mn8(mu4-O)2(mu3-OH)2]14+ core and [Mn10(II)Mn4(III)O4(O2CMe)20[(2-py)2C(OH)O]4] (2-py = 2-pyridyl)

Reactions of Mn sources with di-2-pyridyl ketone, (2-py)2CO, and phenyl 2-pyridyl ketone oxime, (ph)(2-py)CNOH, give the novel clusters [Mn10(II)Mn4(III)O4(O2CMe)20[(2-py)2C(OH)O]4] 1 and [Mn4(II)Mn4(III)O2(OH)2(O2CPh)10[(ph)(2-py)CNO]4] 2, respectively, which possess low-spin ground states; the observed tetradecanuclearity in 1 is extremely rare in 3d-metal chemistry, while the core of 2 has a unique topology consisting of two linked [Mn2(II)Mn2(III)O(OH)] units.     

Structural and physical characterization of tetranuclear [Mn(II)3Mn(IV)] and [Mn(II)2Mn(III)2] valence-isomer manganese complexes

Two tetranuclear Mn complexes with an average Mn oxidation state of +2.5 have been prepared. These valence isomers have been characterized by a combination of X-ray crystallography, X-ray absorption spectroscopy, and magnetic susceptibility. The Mn(II)3Mn(IV) tetramer has the Mn ions arranged in a distorted tetrahedron, with an S = 6 ground spin state, dominated by ferromagnetic exchange among the manganese ions. The Mn(II)2Mn(III)2 tetramer also has a distorted tetrahedral arrangement of Mn ions but shows magnetic behavior, suggesting that it is a single-molecule magnet. The X-ray absorption near-edge structure (XANES) spectra for the two complexes are similar, suggesting that, while Mn XANES has sufficient sensitivity to distinguish between trinuclear valence isomers (Alexiou et al. Inorg. Chem. 2003, 42, 2185), similar distinctions are difficult for tetranuclear complexes such as that found in the photosynthetic oxygen-evolving complex.     

New insights into the thermal stability of Mn12 clusters: the case of complex [Mn12O12(O2CC[triple bond]-CH)16(H2O)4] x 3H2O and its thermolysis derived [Mn3(O2CC[triple bond]CH)6(H2O)4] x 2H2O complex

Two novel Mn12 derivatives [Mn12O12(O2CC[triple bond]CH)16(H2O)4] x 3H2O (1) and [Mn12(O2CC[triple bond]CC6H5)16(H2O)4] x 3H2O (2) have been prepared and characterized. Magnetic measurements confirm that both function as single-molecule magnets (SMM), showing frequency-dependent out-of-phase AC susceptibility signals and magnetization hysteresis curves. Thermal stability studies of both complexes were first conducted in the solid state. While complex 1 undergoes a sudden exothermal decomposition at T(onset) = 118 degrees C, complex 2 exhibits a higher stability. Thermolysis reaction of 1 was hence assessed in solution to yield dark red crystals of a two-dimensional Mn(II)-based co-ordination polymer [Mn3(O2CC[triple bond]CH)6(H2O)4] x 2H2O (3), which corresponds to an extended sheet-like structure that crystallizes in the monoclinic space group P2(1)/n; a = 9.2800(2) angstroms, b = 9.4132(2) angstroms, c = 14.9675(3) angstroms, beta = 99.630(1) degrees, and Z = 2. Finally, the magnetic properties of complex 3 have been studied on an oriented single crystal over two different orientations of the reciprocal vector versus the external field.