A seven-coordinate iron platform and its oxo and nitrene reactivity

We present a new structurally determined seven-coordinate iron platform supported by the tris(2-picolyl)amine ligand 6,6′-(pyridin-2-ylmethylazanediyl)bis(methylene)bis(N-tert-butylpicolinamide) (TPA^2C(O)NHtBu, 3) and its reactivity with oxo and nitrene transfer agents. Oxidation of the pentagonal bipyramidal, seven-coordinate iron(II)-triflate complex [TPA^2C(O)NHtBuFe^II(OTf)][OTf] (4) with PhIO produces the corresponding diiron(III) μ-oxo complex [(TPA^2C(O)NHtBuFe^III)₂(O)][OTf]₄ (5). Mössbauer and magnetic measurements on 5 in the solid-state establish antiferromagnetic coupling between its two Fe(III) centers. Reactions of 4 with the nitrene transfer agents PhINTs (Ts = p-MeC₆H₄SO₂) and PhINNs (Ns = p-NO₂C₆H₄SO₂) provide the corresponding iron(III)-amide congeners [TPA^2C(O)NHtBuFe^III(NHTs)][OTf]₂ (6) and [TPA^2C(O)NHtBuFe^III(NHNs)][OTf]₂ (7), respectively, affording a rare pair of isolable Fe(III)-amide compounds formed from nitrene transfer. By characterizing well-defined products in the crystalline form, derived from atom and group transfer to seven-coordinate iron, the collective data provide a starting point for the exploration of high-valent and metal-ligand multiply bonded species supported by approximate pentagonal-type ligand fields.     

Magneto-structural relationships for a mononuclear Co(II) complex with large zero-field splitting

A mononuclear cobalt(II) complex, [Co(ac)₂(H₂O)₂(MeIm)₂], with heteroleptic coordination sphere possessing the {CoO₂O′₂N₂} chromophore has been prepared and structurally characterized. The magnetic data down to 2 K show an enhanced magnetic anisotropy manifesting itself in a large zero-field splitting (ZFS) parameter. As a consequence, the magnetization deviates substantially from the Brillouin-function behavior. A fit to the zero-field splitting model gave the following set of magnetic parameters: D/hc = +95 cm⁻¹, g_x = 2.530, zj/hc = −0.078, χ_TIP = 16.7 × 10⁻⁹ m³ mol⁻¹, (g_z = 2.0). The Griffith-Figgis model and the Generalized Crystal-Field model lie beyond the spin-Hamiltonian formalism; they gave analogous, although not identical ZFS parameters: D/hc = 109 cm⁻¹, and D/hc = 77 cm⁻¹, respectively. The absorption spectrum taken in the FAR-IR region exhibits manifold absorption peaks referring to the transitions among the crystal-field multiplets of the parent ⁴A_2g + ⁴E_g terms (D_4h), originating in a crystal-field splitting of the octahedral ⁴T_1g ground term.     

Mixed-valency with cyanides as terminal ligands: Diruthenium(III,II) complexes with the 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine bridge and variable co-ligands (CN vs. bpy or NH3)

New diruthenium complexes (PPN)₄[(NC)₄Ru(μ-bptz)Ru(CN)₄], (PPN)₄1, and [(bpy)₂Ru(μ-bptz)Ru(CN)₄], 2, (PPN⁺ = bis(triphenylphospine)iminium; bptz = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine; bpy = 2,2′-bipyridine), were synthesised and characterised by spectroscopic and electrochemical techniques. The comproportionation constant K_c = 10^7.0 of the mixed-valent species [(NC)₄Ru(μ-bptz)Ru(CN)₄]³⁻ as obtained by oxidation of 1⁴⁻ in CH₃CN is much lower than the K_c = 10^15.0 previously detected for [(H₃N)₄Ru(bptz)Ru(NH₃)₄]⁵⁺, reflecting the competition between CN⁻ and bptz for the π-electron density of the metals. Comparison with several other bptz-bridged diruthenium(II,III) complexes reveals an approximate correlation between K_c and the diminishing effective π acceptor capacity of the ancillary terminal ligands. In addition to the intense MLCT absorption at λ_max = 624 nm, the main IVCT (intervalence charge transfer) band of 1³⁻ was detected by spectroelectrochemistry at λ_max = 1695 nm (in CH₃CN; ε = 3200 M⁻¹ cm⁻¹). The experimental band width at half-height, Δν_1/2 = 2700 cm⁻¹, is slightly smaller than the theoretical value Δν_1/2 = 3660 cm⁻¹, calculated from the Hush approximation for Class II mixed-valent species. In agreement with comparatively moderate metal-metal coupling, the mixed-valent intermediate 1³⁻ was found to be EPR silent even at 4 K. The unsymmetrical mixed-valent complex [(bpy)₂Ru^II(μ-bptz)Ru^III(CN)₄]⁺, obtained in situ by bromine oxidation of 2 in CH₃CN/H₂O, displays a broad NIR absorption originating from an IVCT transition at λ_max = 1075 nm (ε ≈ 1000 M⁻¹ cm⁻¹, Δν_1/2 ≈ 4000 cm⁻¹). In addition, the lifetime of the excited-state of the mononuclear precursor complex [Ru(bptz)(CN)₄]²⁻ was measured in H₂O by laser flash photolysis; the obtained value of τ = 19.6 ns reveals that bptz induces a metal-to-ligand electronic delocalisation effect intermediate between that induced by bpy and bpz (bpz = 2,2′-bipyrazine) in analogous tetracyanoruthenium complexes.     

Synthesis and characterization of tetrakis(μ-hydroxo)tetrakis(2,2′-dipicolylamine)tetranickel perchlorate, a nickel-hydroxy cubane complex

A novel tetranuclear complex, [Ni(μ₃-OH)(DPA)]₄(ClO₄)₄ (where DPA = 2,2′-dipicolylamine) has been synthesized, with characterization including electronic and infrared spectroscopy, elemental analysis, mass spectrometry, crystal structure analysis, and variable-temperature and variable-field magnetic susceptibility measurements. The complex features a 4Ni-4OH cubane-type cluster, displaying both ferromagnetic and antiferromagnetic intracluster interactions in a 2J model (J₁ = −3.4 cm⁻¹, J₂ = 4.7 cm⁻¹, D = 2.0 cm⁻¹). Each nickel atom sits in a pseudooctahedral environment, with one DPA molecule facially coordinated and the remaining three coordination sites occupied by the bridging hydroxide anions that make up the cubane core.     

Synthesis, crystal structure and magnetic properties of a 1D mixed-metal-mixed-ligand Ni(II)/Fe(II) coordination polymer built on the nitroprusside anion

A new complex [Ni(L)Fe(CN)₅NO] · 2H₂O (L = 4,6,6-trimethyl-1,9-diamino-3,7-di-aza-nona-3-ene) has been obtained and characterized by means of X-ray crystallographic analysis and magnetochemistry. The Fe(CN)₅NO²⁻ anion links to the Ni(L)²⁺ cation through two bridging cyanide groups in a bent fashion. The intrachain Ni?Ni and interchain Ni?Ni distances are equal to 9.81(8) and 7.75(1) Å, respectively. The magnetic behaviour of the complex indicates the zero field splitting parameter D higher than 3 cm⁻¹ and the average exchange parameter (intra- and interchain) corresponding to direct Ni-Ni magnetic interaction in the crystal lattice equals ∼−0.2 cm⁻¹.     

Aminocarboxylate complexes of vanadium(III): Electronic structure investigation by high-frequency and -field electron paramagnetic resonance spectroscopy

Aminocarboxylate complexes of vanadium(III) are of interest as models for biologically and medicinally relevant forms of this interesting and somewhat neglected ion. The V(III) ion is paramagnetic, but not readily suited to conventional EPR, due to its integer-spin ground state (S = 1) and associated large zero-field splitting (zfs). High-frequency and -field EPR (HFEPR), however, has the ability to study such systems effectively. Three complexes, all previously structurally characterized: Na[V(trdta)] · 3H₂O, Na[V(edta)(H₂O)] · 3H₂O, and [V(nta)(H₂O)₃] · 4H₂O (where trdta stands for trimethylenediamine-N,N,N′,N′-tetraacetate and nta stands for nitrilotriacetate) were studied by HFEPR. All the investigated complexes produced HFEPR responses both in the solid state, and in aqueous solution, but those of [V(nta)(H₂O)₃] · 4H₂O were poorly interpretable. Analysis of multi-frequency HFEPR spectra yielded a set of spin Hamiltonian parameters (including axial and rhombic zfs parameters: D and E, respectively) for these first two complexes as solids: Na[V(trdta)] · 3H₂O: D = 5.60 cm⁻¹, E = 0.85 cm⁻¹, g = 1.95; Na[V(edta)(H₂O)] · 3H₂O: D = 1.4 cm⁻¹, E = 0.14 cm⁻¹, g = 1.97. Spectra in frozen solution yielded similar parameters and showed multiple species in the case of the trdta complex, which are the consequence of the flexibility of this ligand. The EPR spectra obtained in frozen aqueous solution are the first, to our knowledge, of V(III) in solution in general and show the applicability of HFEPR to these systems. In combination with very insightful previous studies of the electronic absorption of these complexes which provided ligand-field parameters, it has been possible to describe the electronic structure of V(III) in [V(trdta)]⁻ and [V(edta)(H₂O)]⁻; the quality of data for [V(nta)(H₂O)₃] does not permit analysis. Qualitatively, six-coordinate V(III) complexes with O,N donor atoms show no electronic absorption band in the NIR region, and exhibit relatively large magnitude zfs (D ? 5 cm⁻¹), while analogous seven-coordinate complexes do have a NIR absorption band and show relatively small magnitude zfs (D < 2 cm⁻¹).     

Synthesis, structure, and magnetic properties of dinuclear nickel(II) complexes with a phenol-based dinucleating ligand with four methoxyethyl chelating arms

Dinuclear nickel(II) complexes [Ni₂(bomp)(MeCO₂)₂]BPh₄ (1) and [Ni₂(bomp)(PhCO₂)₂]BPh₄ (2) were synthesized with the dinucleating ligand 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-methylphenol [H(bomp)]. X-Ray analysis revealed that the complex 1 · 0.5CHCl₃ contains two nickel(II) ions bridged by phenolic oxygen and two acetate groups, forming a μ-phenoxo-bis(μ-acetato)dinickel(II) core. Electronic spectra were investigated for 1 and 2 in the range of 400-1800 nm, and the data were typical for the octahedral high-spin nickel(II) complexes. Obtained spectral components were well simulated based on the angular overlap model assuming the trigonally distorted octahedral geometry. Magnetic susceptibility was measured for 1 and 2 over a temperature range of 4.5-300 K. The optimized magnetic data were J = 1.75 cm⁻¹, zJ′ = −0.234 cm⁻¹, g = 2.21, D = 15.1 cm⁻¹, and TIP = 370 × 10⁻⁶ cm⁻¹ for complex 1 and J = 3.55 cm⁻¹, zJ′ = −0.238 cm⁻¹, g = 2.23, D = 21.8 cm⁻¹, and TIP = 470 × 10⁻⁶ cm⁻¹ for complex 2. The data revealed ferromagnetic interactions between the two nickel(II) ions.     

Formation and structural chemistry of the unusual cyanide-bridged dinuclear species [Ru2(NN)2(CN)7] (NN = 2,2′-bipyridine or 1,10-phenanthroline)

Crystallisation of simple cyanoruthenate complex anions [Ru(NN)(CN)₄]²⁻ (NN = 2,2′-bipyridine or 1,10-phenanthroline) in the presence of Lewis-acidic cations such as Ln(III) or guanidinium cations results, in addition to the expected [Ru(NN)(CN)₄]²⁻ salts, in the formation of small amounts of salts of the dinuclear species [Ru₂(NN)₂(CN)₇]³⁻. These cyanide-bridged anions have arisen from the combination of two monomer units [Ru(NN)(CN)₄]²⁻ following the loss of one cyanide, presumably as HCN. The crystal structures of [Nd(H₂O)_5.5][Ru₂(bipy)₂(CN)₇] · 11H₂O and [Pr(H₂O)₆][Ru₂(phen)₂(CN)₇] · 9H₂O show that the cyanoruthenate anions form Ru-CN-Ln bridges to the Ln(III) cations, resulting in infinite coordination polymers consisting of fused Ru₂Ln₂(μ-CN)₄ squares and Ru₄Ln₂(μ-CN)₆ hexagons, which alternate to form a one-dimensional chain. In [CH₆N₃]₃[Ru₂(bipy)₂(CN)₇] · 2H₂O in contrast the discrete complex anions are involved in an extensive network of hydrogen-bonding involving terminal cyanide ligands, water molecules, and guanidinium cations. In the [Ru₂(NN)₂(CN)₇]³⁻ anions themselves the two NN ligands are approximately eclipsed, lying on the same side of the central Ru-CN-Ru axis, such that their peripheries are in close contact. Consequently, when NN = 4,4′-^tBu₂-2,2′-bipyridine the steric bulk of the t-butyl groups prevents the formation of the dinuclear anions, and the only product is the simple salt of the monomer, [CH₆N₃]₂[Ru(^tBu₂bipy)(CN)₄] · 2H₂O. We demonstrated by electrospray mass spectrometry that the dinuclear by-product [Ru₂(phen)₂(CN)₇]³⁻ could be formed in significant amounts during the synthesis of monomeric [Ru(phen)(CN)₄]²⁻ if the reaction time was too long or the medium too acidic. In the solid state the luminescence properties of [Ru₂(bipy)₂(CN)₇]³⁻ (as its guanidinium salt) are comparable to those of monomeric [Ru(bipy)(CN)₄]²⁻, with a ³MLCT emission at 581 nm.     

Trinuclear-based coordination compounds of Mn(II) and Co(II) with 4-amino-3,5-dimethyl-1,2,4-triazole and azide and thiocyanate anions: Synthesis, structure and magnetic properties

A 2D layer complex 1 and a linear trinuclear complex 2 with mixed ligands have been synthesized and characterized by elemental analyses, IR and single-crystal X-ray diffraction. In 1, the Mn(II) ions are six-coordinated and lie in distorted octahedron coordination environments. Complex 1 is connected into a 2D layer structure based on a linear trinuclear Mn₃(admtrz)₄(N₃)₆ (admtrz = 4-amino-3,5-dimethyl-1,2,4-triazole) building unit with either (6,3) topology when Mn1 cations as three-connected nodes or (4,4) network when the coordination trinuclear units being regarded as four connected nodes. In 2, the Co(II) ions are in slightly distorted octahedron coordination geometries. The magnetic behaviors are investigated in the temperature range 1.8-300 K. The magnetic susceptibility measurements show that the Mn(II) ions of complex 1 are weakly antiferromagnetically coupled with g = 1.98(1), J1 = −6.31(5) cm⁻¹ and J2 = −1.88(1) cm⁻¹. There is dominant zero field splitting (ZFS) effects with g values, g_// = 2.38(2) and g_⊥ = 4.96(4), indicated a significant presence of the spin-orbit coupling and magnetization experiment reveals large, uniaxial zero-field splitting parameters of D = −29.55 cm⁻¹ for 2.     

Selectivity of attack on a Si-C(sp) sigma bond coordinated to Ni

(PNP)Ni⁺ (as its ( salt) adds PhCN to Ni, but HX cleaves the Si-CH₂ bond to form Ni[η²⁻(^tBu₂PCH₂SiMe₂)N(H)(SiMe₂X)][η²-CH₂^tBu₂P]⁺, for X = OMe, piperidyl, N(H)CH₂Ph, N(H)Ph, morpholinyl. The diprotic reagent H₂O gives (η²-^tBu₂PCH₂SiMe₂OSiMe₂NH₂)(η²-^tBu₂CH₂P)Ni⁺. RCCH (R = Ph, SiMe₃, ^tBu) reacts, through three detected intermediates, to form (^tBu₂PCH₂SiMe₂)N(H)(SiMe₂CH₂^tBu₂PCCR)Ni⁺, a product where one P has been oxidized and Ni reduced, each by two electrons. This shows the dominant influence on reactivity of Si-C bond activation by its unconventional donation to nickel in the structure of (PNP)Ni⁺.     

Hydrothermal synthesis and structure of a Cu(I) bromide coordination polymer, [(tpyprz)3Cu10Br10] (tpyprz = tetra-2-pyridylpyrazine)

The hydrothermal reaction of CuBr₂ and tpyprz in the presence of NH₄VO₃ and HF for 72 h at 170 °C provided [(tpyprz)₃Cu₁₀Br₁₀] (1) in 20% yield. The two-dimensional structure of 1 may be described as Cu(I)-tpyprz chains, linked through {Cu₄Br₅}⁻ clusters in the ac-plane and decorated with {Cu₃Br₅}²⁻ clusters projecting from one face of the layer in the b-direction. The Cu(I) sites exhibit distorted trigonal coordination {CuBr₃} and distorted tetrahedral geometries, {CuBr₂N₂} and {CuN₄}. Crystal data for 1: monoclinic space group C2, a = 12.7561(8) Å, b = 19.359(1) Å, c = 15.860(1) Å, β = 97.178(1)°, V = 3885.8(4) Å³, Z = 2, D_calc = 2.222 g cm⁻³, μ(Mo Kα) = 78.75 cm⁻¹.     

Pyrazolate-bridged group 11 metal(I) complexes: Substituent effects on the supramolecular structures and physicochemical properties

Polynuclear homoleptic pyrazolate-bridged group 11 metal(I) complexes with three different alkyl substituted pyrazolate anions, 3,5-diisopropylpyrazolate (3,5-iPr₂pz⁻ = L1⁻), 3-tert-butyl-5-isopropylpyrazolate (3-tBu-5-iPrpz⁻ = L3⁻), and 3,5-di-tert-butylpyrazolate (3,5-tBu₂pz⁻ = L4⁻), i.e. [Cu(μ-3,5-iPr₂pz)]₃ (CuL1), [Ag(μ-3,5-iPr₂pz)]₃ (AgL1), [Au(μ-3,5-iPr₂pz)]₃ (AuL1), [Cu(μ-3-tBu-5-iPrpz)]₄ (CuL3), [Ag(μ-3-tBu-5-iPrpz)]₃ (AgL3), [Au(μ-3-tBu-5-iPrpz)]₄ (AuL3), [Cu(μ-3,5-tBu₂pz)]₄ (CuL4), [Ag(μ-3,5-tBu₂pz)]₄ (AgL4), and [Cu(μ-3,5-tBu₂pz)]₄ (AuL4), were systematically synthesized in order to investigate the influence of pyrazole bulkiness on their structures and physicochemical properties. The structural characterization indicates that the geometries are greatly influenced by the steric hindrance exerted by the substituent groups of the pyrazolyl rings and the differences of the central metal (I) ionic radius (Cu⁺ < Au⁺ < Ag⁺). These complexes were also characterized by spectroscopic techniques, namely, UV-Vis, IR/far-IR, Raman, and luminescence spectroscopy.     

Kinetics and mechanism of the reactions of ozone with superoxo, hydroperoxo, and hydrido complexes of rhodium(III)

Oxidation of the title complexes with ozone takes place by hydrogen atom, hydride, and electron transfer mechanisms. The reaction with (NH₃)₄(H₂O)RhH²⁺ is a two electron process, believed to involve hydride transfer with a rate constant k = (2.2 ± 0.2) × 10⁵ M⁻¹ s⁻¹ and an isotope effect k_H/k_D = 2. The oxidation of (NH₃)₄(H₂O)RhOOH²⁺ to (NH₃)₄(H₂O)RhOO²⁺ by an apparent hydrogen atom transfer is quantitative and fast, k = (6.9 ± 0.3) × 10³ M⁻¹ s⁻¹, and constitutes a useful route for the preparation of the superoxo complex. The latter is also oxidized by ozone, but more slowly, k = 480 ± 50 M⁻¹ s⁻¹.     

Three Co(III)-Fe(II) complexes based on hexacyanoferrates: Syntheses, spectroscopic and structural characterizations

Red or orange crystals of [Co(NH₃)₆]₂Cl₂[Fe(CN)₆] · 4H₂O (1), [Co(en)₃]₂Cl₂[Fe(CN)₆] · 2H₂O (2) and [Co(en)₃]₄[Fe(CN)₆]₃ · 21.6H₂O (3) were isolated from the aqueous systems Co³⁺-L_N-[Fe(CN)₆]⁴⁻ (L_N = NH₃, en = 1,2-diaminoethane). In all isolated samples the combination of Mössbauer (δ values were from the range −0.07 to −0.08 mm/s) and IR spectra (ν(CN) stretching vibrations in the range 2015-2047 cm⁻¹) confirms the presence of low spin Fe(II) in [Fe(CN)₆]⁴⁻ anions. X-ray structure analyses corroborate the ionic character of all studied compounds. These contain diamagnetic [Co(NH₃)₆]³⁺ (1) or [Co(en)₃]³⁺ (2 and 3) complex cations and diamagnetic [Fe(CN)₆]⁴⁻ complex anions. In compounds 1 and 2 chloride anions are present, too. All three compounds contain water of crystallization, in compound 3 as many as 21.6 molecules per formula unit.     

Single molecule magnet: Heterodinuclear cyano-bridged cubic cluster [(Tp)8Fe4Ni4(CN)12] (Tp = hydrotris(1-pyrazolyl)borate)

The octanuclear cyano-bridged cluster [(Tp)₈Fe₄Ni₄(CN)₁₂] · H₂O · 24CH₃CN (1) (Tp = hydrotris(1-pyrazolyl)borate) showing magnetic properties of single-molecule magnet has been synthesized by reaction of [fac-Fe(Tp)(CN)₃]⁻ with {(Tp)Ni(NO₃)} species formed from an equimolar reaction mixture of Ni(NO₃)₂ · 6H₂O and KTp in MeCN. The X-ray analysis of 1 shows molecular cube structure in which Fe^III and Ni^II ions reside in alternate corners. The average intramolecular Fe?Ni distance is 5.124 Å. Out-of-phase ac susceptibility and reduce magnetization measurements show that 1 is a single molecule magnet with ground spin state S = 6 and spin reversal energy barrier U = 14 K. Magnetic hysteresis loops were also observed by applying fast sweeping field.     

Synthesis, crystal structure and magnetic properties of a mononuclear and a ferromagnetically coupled dinuclear nickel(II) complex derived from a hexadentate Schiff base ligand

Reactions with Ni^II of a hexadentate Schiff base ligand, L, prepared from the reaction of 2-benzoylpyridine with N,N′-bis-(3-aminopropyl) ethylenediamine in the 2:1 molar ratio are reported. Mixing of this ligand with Ni(NO₃)₂ affords the mononuclear complex [NiL](NO₃)₂ (1), whereas the presence of NaN₃ in the reaction system leads to the formation of the dinuclear complex [Ni₂(L)(N₃)₄] (2). Both adducts have been characterized by X-ray crystallography revealing distorted octahedral NiN₆ coordination environments around the Ni^II centers. The dinuclear complex contains terminal and end-on bridging azido ligands and displays Ni-N-Ni bridge angles of 101.1(2)° and 101.6(2)° and an intramolecular Ni?Ni separation of 3.330(1)Å. The bridges mediate ferromagnetic magnetic exchange interactions between the Ni^II centers of 2, leading to an S = 2 ground state. Fitting of bulk magnetization data provided the coupling constant J = +20.96 cm³ K mol⁻¹ (in the H = −2JS₁ · S₂ convention for the Heisenberg Spin Hamiltonian), as well as the parameters g = 2.17, D = 0.69 cm⁻¹ and TIP = 622 × 10⁻⁶ cm³ mol⁻¹.     

Ni(II) coordination compounds based on mixed phthalate and aromatic amine ligands: synthesis, crystal structures and magnetic properties

Three new coordination compounds, [Ni(Pht)(Py)₂(H₂O)₃] (1), [Ni(Pht)(β- Pic)₂(H₂O)₃] · H₂O (2) and [Ni(Pht)(1-MeIm)₂(H₂O)₃] (3) (where Pht²⁻ = dianion of o-phthalic acid; Py = pyridine, β-Pic = 3-methylpyridine, 1-MeIm = 1-methylimidazole), have been synthesized and characterized by IR spectroscopy and thermogravimetric analysis. Crystallographic studies 1-3 reveal that each Ni(II) center has a distorted octahedral geometry being coordinated by two nitrogen atoms of aromatic amines, one oxygen atom from a carboxylate group of a phthalate ligand and three water molecules. Pht²⁻ anions act as monodentate ligands, while the remaining uncoordinated carboxylate oxygen atoms participate in the formation of hydrogen bonding. The uncoordinated oxygen atoms form hydrogen bonds with the coordinated water molecules from adjacent complexes creating a centrosymmetric dimer unit. Further, these dimer units are connected by O-H?O hydrogen bonds in double-chains. Depending on the nature of aromatic amines, the arrangement of these double-chains differs. The double-chains are held together only by van der Waals interactions in 1. In contrast, in 2 these chains form layers by π-π interactions between antiparallel molecules of β-Pic as well as by π-π interactions between β-Pic and Pht aromatic rings. In complex 3, the double-chains are knitted together via C-H?O hydrogen bonds between the methyl group of 1-MeIm and the coordinated carboxylate oxygen atom of Pht, as well as π-π contacts involving antiparallel 1-MeIm cycles. The thermal dependence of the magnetic susceptibilities for compounds 1 and 2 shows a weak antiferromagnetic interaction between the two Ni²⁺ ions of the hydrogen bonded dimers. For compound 3, a ferromagnetic interaction could be observed. Modeling the experimental data with MAGPACK resulted in: g = 2.22, |D| = 4.11 cm⁻¹ and J = −0.29 cm⁻¹ for compound 1, g = 2.215, |D| = 3.85 cm⁻¹ and J = −0.1 cm⁻¹ for compound 2 and g = 2.23, |D| = 4.6 cm⁻¹ and J = 0.22 cm⁻¹ for compound 3.     

Electrochemical formation of bi- versus tetranuclear μ-oxo terpyridine manganese complexes in CH3CN. Influence of the terpyridine substituents

To complete the elucidation of the electrochemical properties of Mn^II-bis(terpyridine) complexes in CH₃CN and evaluate the influence of the bulkiness of the terpy substituents, the oxidation processes of [Mn^II(L)₂]²⁺ (L = terpy for 2,2′:6′,2″-terpyridine, pTol-terpy for 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine and tBu₃-terpy for 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine) have been investigated in aqueous (1 M) CH₃CN solution. In this medium, exhaustive oxidations at 1.10-1.20 V versus Ag/Ag⁺ release two electrons per molecule of initial complex and lead to clean dimerization processes with the quantitative formation of the oxo-bridged binuclear [Mn₂^IVO₂(L)₂(H₂O)₂]⁴⁺ complex for L = tBu₃-terpy and of the tetranuclear [Mn₄^IVO₅(L)₄(H₂O)₂]⁶⁺ complexes for L = terpy and pTol-terpy. The formation of the tetranuclear complex with the tBu₃-terpy derivative is prevented by the steric hindrance induced by the bulkiness of the tert-butyl groups, as confirmed by molecular mechanics calculations, as well as by their strong electron-donating properties. All these electrogenerated multinuclear complexes have been fully characterized in solution by UV-vis and electron paramagnetic resonance (EPR) spectroscopy. A markedly improved chemical synthesis of [Mn₄^IVO₅(terpy)₄(H₂O)₂]⁶⁺ is also reported.     

Structure and magnetic properties of a tetranuclear Cu4O4 open-cubane in [Cu(L)]4·4H2O with L = (E)-N′-(2-oxy-3-methoxybenzylidene)benzohydrazide

The in-situ formed hydrazone Schiff base ligand (E)-N′-(2-oxy-3-methoxybenzylidene)benzohydrazide (L²⁻) reacts with copper(II) acetate to a tetranuclear open cubane [Cu(L)]₄ complex which crystallizes as two symmetry-independent (Z′ = 2) S₄-symmetrical molecules in different twofold special positions with a homodromic water tetramer. The two independent (A and B) open- or pseudo-cubanes with Cu₄O₄ cores of 4 + 2 class (Ruiz classification) each have three different magnetic exchange pathways leading to an overall antiferromagnetic coupling with J_1B = J_2B = −17.2 cm⁻¹, J_1A = −36.7 cm⁻¹, J_2A = −159 cm⁻¹, J_3A = J_3B = 33.5 cm⁻¹, g = 2.40 and ρ = 0.0687. The magnetic properties have been analysed using the H = −Σ_i,jJ_ij(S_iS_j) spin Hamiltonian.     

A new Cu-Mn heterobinuclear complex exhibiting magnetization relaxation

A new heterometallic complex [CuMn(5-Brsap)₂(MeOH)(Ac)] ⋅ CH₃OH (1) (5-Brsap = 5-bromo-2-salicylideneamino-1-propanol) has been synthesized and characterized structurally as well as magnetically. Complex 1 has an alkoxo-bridged Cu(II) and Mn(III) heterobinuclear core, where the Mn(III) and Cu(II) ions have elongated octahedral and square-pyramidal geometries, respectively. In dc magnetic susceptibility measurements reveal that there is strong ferromagnetic interaction between the Mn(III) and Cu(II) ions with an exchange coupling constant J = 67.64 cm⁻¹. The ac magnetic susceptibility measurements, frequency dependence in both the real and imaginary signals is observed, which indicates slow relaxation of magnetization. An Arrhenius plot gave the effective anisotropy barrier Δ/k_B = 11.58 K and the pre-exponential factor ι₀ = 1.28 × 10⁻⁶ s.