Tetranuclear Mixed-valence Research Articles

Influence of Substitution Effect on MMCT in Mixed‐Valence Cyanido‐Bridged FeII−CN−Ru2III,III−NC−FeII System

AbstractThis work reports the designed synthesis and characterization of nine tetranuclear mixed valence cyanido‐bridged complexes [CpMen(dppe)FeIICNRu2III,III(X‐DMBA)4NCFeII(dppe)CpMen][PF6]2 (1–9: CpMen=polymethylcyclopentadienyl, n=0, 4, 5; DMBA=N,N’‐dimethylbenzamidinate, X= H, MeO, CF3). The regular variation of crystallographic data, IR and UV/Vis/NIR spectra for complexes 1–9 demonstrates that the substitution effect of the ligand at the donor or acceptor fragment has explicit influence on the MMCT properties. Moreover, the electrochemical measurement might suggest the existence of long‐range electronic coupling between the two terminal FeII in this FeII−CN−Ru2III,III−NC−FeII array.

Field-induced slow magnetic relaxation from linear trinuclear CoIII-CoII-CoIII to grid [2 × 2] tetranuclear mixed-valence cobalt complexes.

By employing the ligand azotetrazolyl-2,7-dihydroxynaphthalene (H3ATD), two linear trinuclear mixed-valence cobalt complexes [CoIICoIII2(HATD)4(H2O)4]·4DMA·3H2O (1, DMA = N,N-dimethylacetamide) and [CoIICoIII2(HATD)4(DMF)2(H2O)2]·2DMF·2H2O (2, DMF = N,N-dimethylformamide) were synthesized. Two [2 × 2] grid-like tetranuclear ion-pair complexes [CoII2CoIII2(HATD)4(bpp)2(H2O)2][CoIII(HATD)2]2·8DMF·6H2O (3, bpp = 2,6-di(pyrazol-1-yl)pyridine) and [CoII2CoIII2(HATD)4(bpp)2(H2O)2][CoIII(HATD)2]2·8DMSO·4MeOH (4, DMSO = dimethyl sulphoxide) were obtained by the reaction of complex 1/2 with tridentate-chelating bpp in DMF and DMSO, respectively. The single-crystal X-ray diffraction analysis indicated that complexes 1 and 2 have a similar core, in which the DMA in 1 acts as a guest molecule, and the DMF in 2 acts as a coordinated molecule and guest molecule. Complexes 3 and 4 are isostructural. All the Co(ii) ions in 1-4 are present in a distorted octahedral geometry. The ac susceptibility measurements show that all complexes display frequency-dependent peaks in the out-of-phase (χm'') component of the alternating-current (ac) magnetic susceptibility data, which is the characteristic behavior of single molecule magnets (SMMs).

Magnetic Properties of End-to-End Azide-Bridged Tetranuclear Mixed-Valence Cobalt(III)/Cobalt(II) Complexes with Reduced Schiff Base Blocking Ligands and DFT Study.

Two tetranuclear mixed-valence cobalt(III/II) complexes having the general formula [(μ1,3-N3){CoII(Ln)(μ-O2CC6H4NO2)CoIII(N3)}2]PF6 (where H2L1 and H2L2 are two reduced Schiff base ligands) have been synthesized and characterized. The structures of both complexes show cobalt(II) and cobalt(III) centers with a distorted octahedral geometry with cobalt(III) and cobalt(II) centers located at the inner N2O2 and outer O4 cavities of the reduced Schiff base ligands, respectively. The oxidation states of both cobalt centers have been confirmed by bond valence sum (BVS) calculations. The magnetic properties show that both compounds behave as cobalt(II) dimers connected through an end-to-end azido bridging ligand and show moderate antiferromagnetic Co(II)–Co(II) couplings of −11.0 and −14.4 cm–1 for 1 and 2, respectively, as also corroborated by DFT calculations, Jtheo = −13.07 cm–1 for 1 and −12.49 cm–1 for 2. The calculated spin densities of both complexes at the cobalt(II) centers are −2.75 and +2.75, respectively, clearly supporting that they are the magnetic centers.

Synthesis and characterization of binuclear Pt(IV) complexes and tetranuclear mixed valence complexes of Platinum(II)-Platinum(IV)

The binuclear platinum(IV) complexes [NBu4]2[PtIV2(μ-Cl)2(C6F5)4Cl4] (2) and [PtIV2(μ-C8H6N4)2(C6F5)4Cl4] (4) have been synthesized by oxidative addition of chlorine to the binuclear platinum(II) species, but [NBu4]2[PtIV2(μ-OH)2(C6F5)4Cl4] (3) has been synthesized by substitution of the chloride bridges of the platinum(IV) complex. The square planar platinum environments of the Pt(II) starting materials change, as expected, to Pt(IV) octahedral environments with the chloride ligands in trans-position. Only minor changes in the Pt-X (X = Clbridge, OH) interatomic distances are produced because of the oxidation.Depending on the size of the bridging ligands and also probably the anionic character of the diplatinum(IV) complexes, the apical chloride ligands can still act as bridges toward other Pt centers. Thus, the organometallic tetra-nuclear mixed-valence platinum(IV)-platinum(II) complexes with formulas [NBu4]2[{PtIV(μ-X)(C6F5)2}2(μ-Cl)4{PtII(C6F5)2}2] [X = Cl (5), OH (6)] were obtained by reaction of 2 and 3 with cis-[PtII(C6F5)2(thf)2]. The structures of complexes 2, 4 and 6 have been determined by single-crystal X-ray diffraction studies. These crystal structures confirm that the oxidative addition of the chlorine molecule to the platinum(II) complexes 2 and 3 takes place on the axial positions of the platinum centers suggesting a SN2 mechanism. On the other hand, the central core of the anion of 6 has a chair like conformation and the long Pt⋯Pt distances (>3.2 Å) clearly exclude any Pt.···Pt interaction.On the other hand, cis-[(C6F5)2Pt(μ-C8H6N4)Pt(C6F5)2] (1) can be reduced either electrochemically or by reacting with [CoCp2] to the anion [Pt2(μ-C8H6N4)(C6F5)4]- which in the last case, can be isolated as complex 7. According to the EPR studies the anion is not a mixed Pt(II)/Pt(I) compound but a binuclear Pt(II) derivative with the anion [C8H6N4]- as bridging ligand.

Synthesis, Characterization and Magnetic Studies of a Tetranuclear Manganese(II/IV) Compound Incorporating an Amino-Alcohol Derived Schiff Base

A new tetranuclear mixed-valence manganese(II/IV) compound [MnIIMnIV3(μ-Cl)3(µ3-O)(L)3] (1) (where H3L = (3E)-3-((Z)-4-hydroxy-4-phenylbut-3-en-2-ylideneamino)propane-1,2-diol) has been synthesized and characterized by different physicochemical methods. Single crystal X-ray diffraction analysis reveals that 1 is a tetrahedral cluster consisting of a Mn4Cl3O4 core in which the only Mn(II) ion is joined through three μ2-O bridges to an equilateral triangle of Mn(IV) ions, which are connected by a μ3-O and three μ2-Cl bridges. The redox behavior of 1 was studied by cyclic voltammetry. Variable temperature magnetic susceptibility measurements of 1 revealed predominant antiferromagnetic coupling inside the Mn4Cl3O4 cluster.

Di- and tetranuclear transition metal complexes of a tetrakisguanidino-substituted phenazine dye by stepwise coordination.

2,3-Bis- and 2,3,7,8-tetrakisguanidino-substituted phenazines are intensely-coloured dyes that offer two different sites for metal coordination or benzylation, namely the imino nitrogen atoms of the guanidino groups and the phenazine nitrogen atoms. In this work, sequential coordination at both sites is studied, and the effects on the electronic structure and optical properties are analyzed. The step-wise coordination is used to synthesize tetranuclear mixed-valence (CuI)2(CuII)2 and heterobimetallic Cu2Ni2 complexes. Coordination at the guanidino groups switches on a ligand-metal charge-transfer character of electronic transitions in the visible region, leading to a massive intensity gain in the case of copper coordination. The second coordination step with CuI takes place at the phenazine nitrogen atoms and further decreases the ligand frontier orbitals. In the case of two tricoordinated CuI atoms in the product complex, the charge-transfer character of the electronic excitation vanishes, while their energies are very similar. On the other hand, for CuI atoms with a coordination number of only 2, the stronger Cu-N bond leads to a red-shift of the electronic transitions.

A mixed-valence complex of cobalt based on 3-methoxysalicylaldoxime

A new tetranuclear mixed-valence cobalt complex, namely di-μ2-azido-diazidodiethanolbis{μ2-2-[(hydroxyimino)methyl]-6-methoxyphenolato}bis{μ3-6-methoxy-2-[(oxidoimino)methyl]phenolato}dicobalt(II)dicobalt(III) ethanol disolvate, [Co(II)2Co(III)2(C8H7NO3)2(C8H8NO3)2(N3)4(C2H5OH)2]·2C2H5OH, has been synthesized by the reaction of Co(OAc)2·4H2O (OAc is acetate) with 3-methoxysalicylaldoxime (H2mosao) in an ethanol solution. In the complex, the four Co cations all display distorted octahedral coordination environments and they are bridged by two κ(2),κ(1),κ(1);μ3-mosao(2-) ligands, two κ(2),κ(2);μ2-Hmosao(-) ligands and two μ2-N3(-) anions to form a tetranuclear [Co4N4O4] cluster. Adjacent clusters are connected through weak C-H···N and C-H···O interactions, resulting in a two-dimensional supramolecular network parallel to the ac plane. The magnetic properties of the complex have also been studied.

Iso-valence Co(II) and mixed-valence Co(II/III) tetranuclear complexes: Synthesis, structure, magnetic properties and DFT study

The reaction of Co(ClO4)2·6H2O with symmetrical N4O3 coordinating heptadentate ligand (H3L) in presence of triethylamine yielded cobalt(II) tetranuclear complexes [Co4(L)2](ClO4)2, 1. When the same reaction is carried out with sodium azide instead of triethylamine afforded tetranuclear mixed-valence Co(II/III) species, [Co4(L)2(μ1,3–N3)2](ClO4)2, 2. Here L3− is the deprotonated form of 2,6-bis[{{(1-hydroxy-2-methylpropan-2-yl)(pyridine-2-ylmethyl)}amino}methyl]-4-methylphenol. Complex 1 crystallizes in the cubic system, space group Fd-3c, with unit-cell parameters a=b=c=43.320(5)Å, Z=48, whereas complex 2 crystallizes in the space group I41/a in the tetragonal crystal system with Z=8 and unit-cell parameters a=b=16.3243(15)Å, c=50.093(4)Å. The X-ray structures of 1 and 2 have revealed a CoII4O6 core in 1 and a [CoII2CoIII2O2(μ1,3–N3)2] core in 2. Tetranuclear Co(II) compound shows interesting magnetic property and exhibit antiferromagnetic interaction between the paramagnetic centers. Density functional theory has been used to justify the exchange pathways for 1. Exchange coupling in the mixed valence cobalt complexes studied is insignificant, due to the large distances of separation of the Co(II) centers in tetranuclear units.

Synthesis, solid state and solution studies of cobalt(II) complexes with 2-hydroxyiminopropanoic acid

By using a synthetic approach alternative to the ones described up to now two tri- and tetranuclear mixed valence Co(II,III) complexes have been obtained ([Li(H2O)4]2[Co(CoL3)2(Li(H2O)3)2] and K[Co(CoL3)2Co(H2O)5]·H2O, respectively) in which the cobalt ions are linked by (N–O) oxime bridges. For both these compounds, a hydrazide derivative (2-hydroxyiminopropanehydrazide, HPH) has been used as starting ligand and then it underwent hydrolytical decomposition to 2-hydroxyiminopropanoic acid, H2L, ion upon coordination with cobalt. Cobalt(II) complexes formed by 2-hydroxyiminopropanoic acid in aqueous solution have been studied using independent pH-metric titrations as well as a combination of fibre-optics UV–Vis spectroscopy and pH glass-electrode potentiometry. The mononuclear CoL2H complex appeared to be the dominating equilibrium species within pH 6–9, most probably due to a hydrogen bond between the deprotonated and protonated oxime groups stabilizing planar cis-coordination.

A novel tetranuclear MnII2MnIII2 cluster [Mn4(hmp)6(N3)4]: Synthesis, crystal structure and magnetic properties

A novel tetranuclear mixed-valence manganese(II/III) cluster [Mn4(hmp)6(N3)4] (1) where hmp is the anion of 2-hydroxymethylpyridine, was obtained by the reaction of manganese perchlorate with hmpH and NaN3 in acetonitrile. Complex 1 was characterized by chemical analysis, IR spectroscopy, single crystal X-ray diffraction and variable-temperature magnetic susceptibility studies. Complex 1 crystallizes in triclinic system with space group Pī consisting of mixed-valence Mn ions whose coordination geometry can be described as distorted octahedron. The magnetic properties were studied in the temperature range of 1.9–300 K and the results show that mixed-valence Mn ions are ferromagnetically coupled in the cluster leading to an S = 9 ground state.

PH and molar ratio dependent formation of monomeric, dimeric and tetrameric cobalt malate complexes

The reactions of cobalt(II) chloride with racemic malic acid (H 3mal = C 4H 6O 5) result in the isolation of monomeric, dimeric and tetrameric cobalt malato complexes: (NH 4) 2[Co( R-Hmal)( S-Hmal)] · 2H 2O ( 1), [Co 2( R-Hmal)( S-Hmal)(H 2O) 4] n · 2 nH 2O ( 2), K 4[Co 4(OH) 2( R-mal) 2( S-mal) 2(H 2O) 4] · 10H 2O ( 3) and trans-[Co( R-H 2mal)( S-H 2mal)(H 2O) 2] · 2H 2O ( 4). The formations of the malato complexes are dependent on the pH value, the molar ratio of the solutions, the reaction temperature and the counterions. In the water-soluble compound 1, the Co II ion is octahedrally coordinated by two tridentate malates via their α-hydroxy, α-carboxy and β-carboxy groups. The malate ligands in 2 coordinate with the cobalt ion via their α-hydroxy and α-carboxy groups, while the β-carboxy group acts as a bridging ligand for the other two cobalt ions, forming a novel dimeric unit [Co 2( R-Hmal)( S-Hmal)(H 2O) 4], which further connects into a layered structure through links from the oxygen atoms of the β-carboxy groups. Complex 3 is a tetranuclear mixed-valence species. Both of the Co II ions exist in trans-[Co( R-mal)( S-mal)(H 2O) 2] units, which are linked by a Co III 2(OH) 2 unit with bridging α-alkoxy and β-carboxy groups. Compound 4 is the main product of reaction between cobalt chloride and excess malate under weakly acidic conditions.

A Tetranuclear Mixed-Valence Manganese Complex of a Diimine Ligand Derived from 1,4-Diformyl-2,3-dihydroxybenzene: Synthesis, Structure, and Magnetic Properties

An acyclic hexadentate diimine ligand, H4L1, was prepared in situ in methanol by the condensation of 1,4-diformyl-2,3-dihydroxybenzene (1) with 2-aminoethanol, and complexed directly with two equivalents of MnII(OAc)2·4H2O or MnIII(OAc)3·2H2O, or with one equivalent of each. All three of these one-pot reactions gave the mixed-valent tetrametallic complex [MnII2MnIII2(L1)2(OAc)2(solvents)4], [2(solvents)4]. An X-ray crystal structure determination on [2(MeOH)4]·2MeOH revealed a centre of inversion in the middle of the complex. The four metal ions are in an almost planar array, sandwiched by two offset ligands which provide all of the equatorial donors, with the axial sites occupied by acetate ions and methanol molecules. The two manganese(II) ions are seven coordinate and centrally located, whereas the two manganese(III) ions are Jahn–Teller elongated octahedra and are located in the outer sites. Magnetic analysis of an air-dried sample, [2(MeOH)2(H2O)2]·3H2O, showed that weak antiferromagnetic interactions between the manganese ions dominate, resulting in a low ground spin state.

Through quantum tunneling to dipolar order: the effect of varying magnetic anisotropy in three structurally related Mn4 molecular clusters

By means of time-dependent specific heat experiments, we have studied the long-time electron spin-lattice relaxation mechanisms of three tetranuclear mixed-valence clusters of manganese ions (Mn4Cl, Mn4Ac and Mn4Me), which have the same magnetic core but different ligand molecules, and thus varying anisotropy. The relaxation of the spins towards equilibrium takes place, below 1 K, via an incoherent tunneling mechanism in the magnetic ground doublet, in which process energy is exchanged with the phonon bath. It is shown that the spin-lattice relaxation rate increases when the tunneling probabilities increase, thus indicating that quantum tunneling brings the spins towards equilibrium. For Mn4Me, this fast relaxation enables the magnetic system to undergo a transition to a long-range ordered state below TC=0.21 K.

Isolation of a new mixed valence Pt molecular oxide using phosphine as protecting group.

A novel tetranuclear mixed valence cationic Pt molecular oxide [((Me3P)2Pt)3Pt(OH)6]4+ is obtained by reacting H2Pt(OH)6 and (Me3P)2Pt2+, whose NMR spectra suggest the existence of another species in solution that may serve as a starting point for further synthesis of Pt molecular oxides.

Syntheses, crystal structures, and physical properties of dinuclear copper(I) and tetranuclear mixed-valence copper(I,II) complexes with hydroxylated bipyridyl-like ligands.

Four copper complexes with hydroxylated bipyridyl-like ligands, namely [Cu(2)(ophen)(2)] (1), [Cu(4)(ophen)(4)(tp)] (2), [Cu(4)(obpy)(4)(tp)] (3), and [Cu(4)(obpy)(4)(dpdc)].2H(2)O (4), (Hophen=2-hydroxy-1,10-phenanthroline, Hobpy=6-hydroxy-2,2'-bipyridine, tp=terephthalate, dpdc=diphenyl-4,4'-dicarboxylate) have been synthesized hydrothermally. X-ray single-crystal structural analyses of these complexes reveal that 1,10-phenanthroline (phen) or 2,2'-bipyridine (bpy) ligands are hydroxylated into ophen or obpy during the reaction, which provides structural evidence for the long-time argued Gillard mechanism. The dinuclear copper(I) complex 1 has three supramolecular isomers in the solid state, in which short copper-copper distances (2.66-2.68 A) indicate weak metal-metal bonding interactions. Each of the mixed-valence copper(i,ii) complexes 2-4 consists of a pair of [Cu(2)(ophen)(2)](+) or [Cu(2)(obpy)(2)](+) fragments bridged by a dicarboxylate ligand into a neutral tetranuclear dumbbell structure. Dinuclear 1 is an intermediate in the formation of 2 and can be converted into 2 in the presence of additional copper(II) salt and tp ligands under hydrothermal conditions. In addition to the ophen-centered pi-->pi* excited-state emission, 1 shows strong emissions at ambient temperature, which may be tentatively assigned as an admixture of copper-centered d-->s,p and MLCT excited states.

Synthesis, structure and properties of a new unsymmetric tetranuclear mixed-valence vanadium(IV/V) complex containing distinct V 2O 33+ cores

The synthesis, X-ray crystal structure, spectroscopic, and electrochemical properties of a new tetranuclear vanadium(IV,V,IV,V) complex [ ( VO) 4( btppnol) 2( μ-O) 2]( ClO 4) 2·2 DMF·2 H 2 O ( 1 ) utilizing the previously described unsymmetrical dinucleating ligand N-(2-hydroxybenzyl)- N, N ′, N ′-tris(2-pyridylmethyl)-1,3-diaminopropan-2-ol (H 2btppnol) are reported. The complex comprises two different [V 2O 3] 3+ core structures: one [V 2O 3] 3+ core in a syn-dioxo-μ-O(alkoxo) configurarion and another [V 2O 3] 3+ core in an anti-dioxo-μ-oxo configuration, as well as being composed of non-equivalent vanadium centers as a consequence of the use of an unsymmetrical ligand. This complex represents the first example in the literature of a vanadium complex with this particular structure and its interesting electrochemical and spectroscopic properties were investigated.

Assembly of a Tetranuclear Mixed-Valence Manganese(II,III) Complex from a Binuclear Manganese(II) Complex by Photo-Irradiation

Abstract A tetranuclear mixed-valence manganese(II,III) complex, [Mn2(O)(Ph2MeCCOO)2(phen)2]2(PF6)2·10H2O (2), has been assembled from a binuclear manganese(II) complex, [Mn(Ph2MeCCOO)(phen)2]2(PF6)2·2MeCN (1) by use of a photo-induced oxidation reaction.

Preparation and Structure of a Novel Tetranuclear Mixed-Valence Vanadium(IV/V) Complexes Having Alkoxo and Oxo Groups

Abstract Novel tetranuclear complexes, M4[V4O4(μ-O)2(dptaO)2]•nH2O (M = Na, n = 4; M = Cs, n = 10) (dptaO = 1,3-diamino-2-propanoltetraacetato) having a mixed-valence (V(IV) and V(V)) V4O84+ core, are first synthesized and characterized by X-ray cystallography, EPR and UV-Vis spectroscopy. The four vanadium atoms reveal octahedral geometry, very close to each other, and afford a nearly square-planar nuclear core.

Unusually stable peroxocopper complexes. Stoichiometry, products and kinetics of oxidation of the dimeric copper(I) complex [LCuBr] 2 (L= N, N′-diethylethylenediamine) by dioxygen in methylene chloride from −51 to 30 °C

Copper(I) bromide dissolves in deoxygenated methylene chloride and nitrobenzene solutions of equimolar N, N′- diethylethylenediamine (DEED) to give the colorless copper(I) dimer [(DEED)CuBr] 2 ( D). Dioxygen uptake, analytical, cryoscopic, spectral and kinetic data show that D is oxidized to the blue tetranuclear mixed valence peroxocomplex [(DEED)CuBr] 4O 2 ( A) at temperatures from -51 to 30 °C. The rate law is d[ A]/d t=koL[O 2][ D] 2 with activation parameters δ H ≠ DL=-1.8±0.4 kcal mol −1 and δSYL=-38±5 cal deg −1 mol −1 at 25 °C. These parameters resemble those for third-order oxidation of the copper(I) dimer [(TEED)CuCI] 2 (TEED is N, N, N′, N′- tetraethylethylenediamine) to the peroxocopper product [(TEED)CuCI] 4O 2 at low temperatures. They are completely different from those for direct, third-order oxidation of [LCuX] 2 dimers to the oxocopper(II) products [LCuX] 2O at ambient temperatures because of slow transfer of the third electron from copper(I) to O 2 in ambient [(DEED)CuBr] 2/O 2 and low temperature [(TEED)CuCI] 2/O2 reactions. As observed in the [(TEED)CuCI] 2/O 2 system, primary product A relaxes to a different tetranuclear copper complex B, with A thermodynamically favored at higher temperatures up to - 17 °C. First-order decomposition of A to give 2 mol of the oxocopper(II) product [(DEED)CuBr] 2O ( C) has an exceptionally long half-life of 3.2 ± 0.1 h at 25 °C. The resistance of A to intramolecular copper(I)→ peroxide electron transfer with L=DEED and X=Br is attributed to (i) hydrogen bonding between the NH groups of DEED and bound peroxide and (ii) stabilization of copper(I) by Br.

Electron transfer in tetranuclear mixed-valence iron clusters. Role of the topology on the magnetic properties

The energy levels and magnetic properties of mixed-valence d 5d 5d 5d 6 clusters are studied from a model that takes into account the electron transfers between orbitally nondegenerate orbitals of the metal ions, and superexchange interactions. The influence of the electron transfer on the spin state energies and magnetic properties is examined as a function of the geometry of the tetranuclear cluster, including the limiting cases with T d and D 4h symmetries. For geometries close to the tetrahedral, effects of antiferromagnetic superexchange interactions are also discussed.