Vitaly V. Pavlishchuk

Conversion constants for redox potentials measured versus different reference electrodes in acetonitrile solutions at 25°C

Abstract Caution should be exercised when one is comparing the redox potentials of complexes measured in CH 3 CN solutions versus different reference electrodes. A critical review of conversion constants amongst various reference electrodes reported in the literature reveals that in most cases the comparisons of redox potential values are far from accurate. From measurements of relationships amongst various common reference electrodes in CH 3 CN solutions, conversion constants for redox potentials measured versus different reference electrodes in acetonitrile solutions at 25°C are proposed, along with a convention for reporting redox potential values.

Delicate Crystal Structure Changes Govern the Magnetic Properties of 1D Coordination Polymers Based on 3d Metal Carboxylates

Homo- and heterometallic 1D coordination polymers of transition metals (Co II, Mn II, Zn II) have been synthesized by an in-situ ligand generation route. Carboxylato-based complexes [Co(PhCOO)2]n (1 a, 1 b), [Co(p-MePhCOO)2]n (2), [ZnMn(PhCOO)4]n (3), and [CoZn(PhCOO)4]n (4) (PhCOOH=benzoic acid, p-MePhCOOH=p-methylbenzoic acid) have been characterized by chemical analysis, single-crystal X-ray diffraction, and magnetization measurements. The new complexes 2 and 3 crystallize in orthorhombic space groups Pnab and Pcab respectively. Their crystal structures consist of zigzag chains, with alternating M(II) centers in octahedral and tetrahedral positions, which are similar to those of 1 a and 1 b. Compound 4 crystallizes in monoclinic space group P2 1/c and comprises zigzag chains of M II ions in a tetrahedral coordination environment. Magnetic investigations reveal the existence of antiferromagnetic interactions between magnetic centers in the heterometallic complexes 3 and 4, while ferromagnetic interactions operate in homometallic compounds (1 a, 1 b, and 2). Compound 1 b orders ferromagnetically at TC=3.7 K whereas 1 a does not show any magnetic ordering down to 330 mK and displays typical single-chain magnet (SCM) behavior with slowing down of magnetization relaxation below 0.6 K. Single-crystal measurements reveal that the system is easily magnetized in the chain direction for 1 a whereas the chain direction coincides with the hard magnetic axis in 1 b. Despite important similarities, small differences in the molecular and crystal structures of these two compounds lead to this dramatic change in properties.

Structural, magnetic and related attributes of some oximate-bridged tetranuclear nickel(ii) rhombs and a dinuclear congenerElectronic supplementary information (ESI) available: mass spectra, χT vs. T, response of magnetic properties, low-lying spin levels and UV-VIS data. See http://www.rsc.org/suppdata/dt/b3/b300539a/

New oximate-bridged tetranuclear nickel(II) complexes of compositions {Ni(Dien)}2(μ3-OH)2{Ni2(Moda)4}(ClO4)2·Solv (Solv = H2O, 1a; Solv = 2CH3NO2, 1b; Solv = 2H2O·2C4H8O2, 1c), {Ni(Sdien)}2(μ3-OH)2{Ni2(Moda)4}(ClO4)2·H2O (2), {Ni(Odien)}2(μ3-OH)2{Ni2(Moda)4}(ClO4)2·0.6H2O (3), {Ni(Dien)}2(μ3-OH)2{Ni2(Inaf)4}(ClO4)2·CH3NO2 (4) and {Ni(Odien)}2(μ3-OH)2{Ni2(Inaf)4}(ClO4)2·2NaClO4·2H2O (5) and the dinuclear complex (Ni{Odien})2(Moda)2(ClO4)2 (6) have been prepared (Dien = 1,5-diamino-3-azapentane, Odien = 1,5-diamino-3-oxapentane, Sdien = 1,5-diamino-3-thiapentane, ModaH = butane-2,3-dione monooxime, InafH = phenylglyoxaldoxime). X-ray examination revealed similar structures for 1a, 1b, 1c, 2 and 3, with a rhombic “chair” (out-of-phase “butterfly”) arrangement of the four nickel(II) atoms and two hydroxo-bridges. The complexes were examined by magnetochemistry, UV-VIS spectroscopy and voltammetry. 1–3 display antiferromagnetic coupling of the central with the terminal nickel(II) atoms and ferromagnetic spin alignment between the central nickel(II) atoms. In contrast, all the spin exchanges in 4 and 5 were found to be antiferromagnetic, as is the coupling in dinuclear 6.

A Tetrameric Nickel(II) “Chair” with both Antiferromagnetic Internal Coupling and Ferromagnetic Spin Alignment

Interest in polynuclear complexes of the 3d metals has been stimulated by the search for new magnetic materials[1, 2] and by demonstration of the occurrence of oligonuclear metal centers in proteins such as urease.[3] Of the relatively small number of reported tetranuclear complexes of S 1 nickel( ) of known structure, the majority have a hemicubaneor TMbutterfly-∫ rather than a squarelike core.[4] Of these molecules, just one is entirely antiferromagnetic,[5] while the remainder entail purely ferromagnetic interactions amongst the nickel( ) ions.[6±10] Oximes have shown promise as bridging ligands for the preparation of polynuclear complexes.[11, 12] The reaction of 1,4,7-triazaheptane (diethylenetriamine, Dien) with the monooxime of 2,3-butanedione (ModaH) in the presence of NiII ions, instead of yielding the anticipated Schiff base derivative, gave the tetranuclear NiII compound 1 (dark brown crystals; C4H8O2 1,4-dioxane), containing uncondensed but coordinated ketone and amine groups. Figure 1a shows the structure of the the cation of 1, while Figure 1b highlights its NiII core.

The First Example of a True “Turnbull’s Blue” Family Compound with Trapped Iron Oxidation States

The first compound of a true Turnbull′s Blue family [i.e. an iron(II) hexacyanoferrate(III)] complex with valence-trapped states of iron − [Fe([9]ane-S3)2]3[Fe(CN)6]2·2H2O (Ukrainian Red, UR) was synthesized. It crystallizes in the trigonal space group P3, a = 12.038(8) A, b = 12.038(8) A, c = 14.342(7) A, α = 90.00°, β = 90.00°, γ = 120.00°. The unit cell of UR consists of separate [Fe([9]ane-S3)2]2+ cations, [Fe(CN)6]3− anions and 5.3 molecules of water. Encapsulation of the FeII in the homoleptic thioether environment plays the dual role of stabilizing FeII and protecting the central metal ion from direct bridging with hexacyanoferrate(III) anions. The absence of such interactions and trapping of iron oxidation states in the cations and anions was demonstrated by X-ray diffraction and magnetic susceptometry. Magnetic measurements are consistent with diamagnetic FeII inside the macrocyclic moieties and S = 1/2 for FeIII in the hexacyanoferrate(III) anions.

Synthesis, structure and magnetic properties of Nd3+ and Pr3+ 2D polymers with tetrafluoro-p-phthalate

Two lanthanide tetrafluoro-p-phthalate (L(2-)) complexes, Ln(L)(1.5)·DMF·H(2)O (Ln = Pr(3+) (1), Nd(3+) (2)), were synthesized using pyridine as a base. The compounds were found to be isostructural, and the structure of 1 has been determined by single crystal X-ray diffraction (monoclinic, space group C2, a = 22.194(2) Å, b = 11.4347(12) Å, c = 11.7160(12) Å, β = 94.703(2)°, V = 2963.3(5) Å(3), Z = 4). The crystal structure of 1 consists of dinuclear Pr(3+) units, which are connected by tetrafluoro-p-phthalate, forming separate 2D polymeric layers. The Ln(3+) ions in the dinuclear Ln(2) units are linked by two μ-O atoms and by two bridging O-C-O groups. The structure is porous with DMF and water molecules located between layers. Non-coordinated DMF molecules occupy about 27% of the unit cell volume. A systematic analysis of reported structures of Ln(III) polymers with p-phthalate and its derivatives shows that the ca. known 60 structures can be divided into six possible structural types depending on the presence of certain structural motifs. The magnetic properties of compounds 1 and 2 were studied. The dependence of χ(M)T on T (where χ(M) is magnetic susceptibility per dinuclear lanthanide unit) for 1 and 2 was simulated using two different models, based on: (i) the Hamiltonian Ĥ = ΔĴ(z)(2)+ μ(B)g(J)HĴ, which utilises an axial splitting parameter Δ and temperature-independent paramagnetism (tip) and (ii) crystal field splitting. It was found that both models gave satisfactory fits, indicating that the Ln-Ln exchange interactions are small and the symmetry of the coordination environment is the main factor influencing the magnetic properties of these compounds.

Synthesis, magnetochemistry, and spectroscopy of heterometallic trinuclear basic trifluoroacetates [Fe2M(μ3-O)(CF3COO)6 (H2O)3]·H2O (M = Mn, Co, Ni)

Three new µ3-oxo(trifluoroacetato) complexes [FeIII2MII(μ3-O)(CF3COO)6(H2O)3]·H2O [M = Mn (1), Co (2), Ni (3)] have been prepared. Compounds 1 and 2 crystallize in the monoclinic space groups C2/c [a = 22.002(5), b = 13.647(3), c = 24.767(4) A, β = 98.23(3)°] and C2/m [a = 21.426(4), b = 15.100(2), c = 14.815(3) A, β = 117.99(2)°], respectively. The coordination spheres of the metal ions are essentially octahedral, with the Fe−O distances [1.870(5) A] falling in the usual range for these systems. Magnetochemical studies reveal the presence of antiferromagnetic exchange in the isosceles triangular skeletons of the polynuclear species. Application of the isotropic spin Hamiltonian H = −2JFeM[SFe1SM + SMSFe2] − 2JFeFe[SFe1SFe2] gives the fitting parameters: gFe = gMn = 2.00, JFe-Fe = −56.50(7) and JFe-Mn = −16.23(4) cm−1 (1), gmol = 2.09(1), JFe-Fe = −42.8(3.5) cm−1, JFe-Co = −17.8(1.4) cm−1 (2) and gFe = 2.00, gNi = 2.215(2), JFe-Fe = −45.60(1) and JFe-Ni = −16.96(2) cm−1 (3). A Mossbauer investigation confirms that no electron transfer from MnII or CoII to FeIII occurs during the syntheses of these complexes. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Manifestation of π–π Stacking Interactions in Luminescence Properties and Energy Transfer in Aromatically-Derived Tb, Eu and Gd Tris(pyrazolyl)borate Complexes

The three new complexes Tp(Py)Ln(CH3CO2)2(H2O) (Ln = Eu (1), Gd(2), or Tb (3)) were prepared and characterized crystallographically. In the crystal lattices of these complexes, separate molecules are connected in infinite chains by π-stacking interactions. Complexes 1 and 3 display intense photoluminescence and triboluminescence (red and green respectively), while compound 3 exhibits electroluminescence commencing at 9 V in an ITO/PVK/3/Al device (ITO = indium-tin oxide, PVK = poly(N-vinylcarbazole)). A series of Eu/Tb-doped Gd compounds was prepared by cocrystallization from mixtures of 1 and 2 or 2 and 3, respectively. It was shown that π-stacking interactions are involved in increasing the efficiency of energy transfer from the gadolinium complex to emitting [Tp(Py)Eu](2+) or [Tp(Py)Tb](2+) centers, and this energy transfer occurs through hundreds of molecules, resembling the process of energy harvesting in chloroplast stacks.

Effect of solvent and strong base on electrochemical and chemical behaviour of copper tetraaza macrocyclic complex

Abstract Reactions of copper(II) complex with tetrabenzo- [b,f,j,n] [1,3,9,13]tetraazacyclohexadecine (TAAB) as a Lewis acid have been studied. Coordination of solvent molecules and stabilization of the copper(II) oxidation state according to solvent donor properties has been confirmed. The product of hydroxyl ion nucleophilic addition has been isolated and characterized. Differences in electrochemical as well as chemical reduction and reoxidation of the complex in acidic and basic medium have been investigated and the reaction mechanism is discussed.

Synthesis, spectroscopic and redox behaviour of some copper(II) and copper(I) biomimetic complexes

Abstract Copper complexes with acyclic ligands have been synthesized with a chelate unit of the composition CuN x S y . In the electronic spectra of copper(II) complexes a highly intensive charge-transfer band n S→d x 2 − y 2 (Cu 2+ ) occurs within the range 620–720 nm. Corresponding copper(I) complexes were prepared by electrochemical reduction and characterized by electron spectroscopy. Values of the redox potential ( E 1/2 ) of the CuL 2+/+ pair as well as those of the rate constant of the electrode reaction ( k sh ) were obtained by cyclic voltammetry on a platinum electrode in acetonitrile. The behaviour of the complexes as new models of ‘blue’ copper proteins is discussed.