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Jürgen Ensling

Publications and source records attributed to Jürgen Ensling.

9 recordsLinked to original sources
PubMed2006

Decacyclene as complexation manifold: synthesis, structure and properties of its Fe2 and Fe4 slipped triple-decker complexes.

Reaction of [(eta(5)-Me4EtC5)Fe(II)Cl(tmeda)] (tmeda = N,N,N'N'-tetramethylethylenediamine) with a polyanion solution of decacyclene (1) results in the formation of the triple-deckers [{(eta(5)-Me4EtC5)Fe}2-mu2-(eta(6):eta(6)-decacyclene)] (3) and [{(eta(5)-Me4EtC5)Fe}4-mu4-(eta(6):eta(6):eta(6):eta(6)-decacyclene)] (4). Metal complexation in 3 and 4 occurs on opposite faces of the pi perimeter in an alternating mode. The decacyclene ring adopts a gently twisted molecular propeller geometry with twofold crystallographic symmetry (C2). Complex 4 crystallizes in the chiral space group C222(1); the investigated crystal only contains decacyclene rings with M chirality. The handedness can be assigned unambiguously to the presence of the iron atoms. Cyclovoltammetric studies revealed quasireversible behavior of the redox events and a strong interaction of the Fe atoms in 3 and 4, exemplified by potential differences deltaE of 660 and 770(780) mV between the first and the second individual oxidation processes. This corresponds to a high degree of metal-metal interaction for 3 and 4. The successful syntheses of 3 and 4 together with earlier results from our laboratory proves that all five- and six-membered pi subunit sets of 1 are prone to metal complexation. A clear site preference in 1 towards the complexation of [Cp(R)]iron, -cobalt, and -nickel fragments exists.

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PubMed2005

Electronic, vibrational, and structural properties of a spin-crossover catecholato-iron system in the solid state: theoretical study of the electronic nature of the doublet and sextet states.

As a functional model of the catechol dioxygenases, [(TPA)Fe(Cat)]BPh4 (TPA = tris(2-pyridylmethyl)amine and Cat = catecholate dianion) exhibits the purple-blue coloration indicative of some charge transfer within the ground state. In contrast to a number of high-spin bioinspired systems, it was previously shown that, in the solid state, [(TPA)Fe(Cat)]BPh4 undergoes a two-step S = 1/2 = S = 5/2 spin-crossover. Therefore, the electronic and vibrational characteristics of this compound were investigated in the solid state by UV/Vis absorption and resonance Raman spectroscopies over the temperature range of the transition. This allowed the charge-transfer transitions of the low-spin (LS) form to be identified. In addition, the vibrational progression observed in the NIR absorption of the LS form was assigned to a five-membered chelate ring mode. The X-ray crystal structure solved at two different temperatures, shows the presence of highly distorted pseudo-octahedral ferric complexes that occupy two nonequivalent crystalline sites. The variation of the molecular parameters as a function of temperature strongly suggests that the two-step transition proceeds by a successive transition of the species in the two nonequivalent sites. The thermal dependence of the high-spin fraction of metal ions determined by Mössbauer experiments is consistent with the magnetic data, except for slight deviations in the high temperature range. The optimized geometries, the electronic transitions, vibrational frequencies, and thermodynamic functions were calculated with the B3LYP density functional method for the doublet and the sextet states. The finding of a ground state that possesses a significant mixture of Fe(III)-catecholate and FeII-semiquinonate configurations is discussed with regard to the set of experimental and theoretical data.

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PubMed2005

Cyano-bridged structures based on [MnIIN3O2-macrocycle)]2+: a synthetic, structural, and magnetic study.

Reactions between the complex [MnII(L)]2+, where L is a N3O2 macrocyclic ligand, and different cyanometalate precursors such as [M(CN)n]m- (M(III) = Cr, Fe; M(II) = Fe, Ni, Pd, Pt) lead to cyano-bridged molecular assemblies exhibiting a variety of structural topologies. The reaction between [MnII(L)]2+ and [FeII(CN)6]4- forms a trinuclear complex with formula [(MnII(L)(H2O))2(FeII(micro-CN)2(CN)4)] x 2MeOH x 10H2O (1) which crystallizes in the triclinic space group P1. The reaction between [MnII(L)]2+ and [M(II)(CN)4]2-, where M(II) = Ni (2), Pd (3), Pt (4), gives rise to three isostructural linear chain compounds with stoichiometry [(MnII(L))(M(II)(micro-CN)2(CN)2)]n and which crystallize in the monoclinic space group C2/c. The self-assembly between [MnII(L)]2+ with [M(III)(CN)6]3-, where M(III) = Cr (5), Fe (6, 7, 8), forms three types of compounds. Compounds 5 and 6 are isostructural (monoclinic, space group P2(1)/n), and the structures comprise anionic linear chains [(MnII(L))(M(III)(micro-CN)2(CN)4)]n(n-) with cationic trinuclear complexes [(MnII(L)(H2O))2(M(III)(micro-CN)2(CN)4)]+ as counterions. Using an excess of K3[FeIII(CN)6], an analogous compound to 6 but with K+ as counterion is obtained (7), which crystallizes in the triclinic space group P1. Compound 8 consists of 2-D layers with formula [(MnII(L))3(FeIII(micro-CN)4(CN)2)(FeIII(micro-CN)2(CN)4)]n x 2nMeOH; it crystallizes in the monoclinic space group P2(1)/n. The magnetic properties were investigated for all samples. In particular, compound 5, which shows antiferromagnetic exchange interactions between Mn(II) and Cr(III) ions through cyanide bridging ligands, has been studied in detail; the magnetic exchange parameter amounts to J = -7.5(7) cm(-1). Compound 8 shows a magnetically ordered phase below 6.4 K which is confirmed by Mössbauer spectroscopy; two hyperfine split spectra were observed below Tc from which IJI values of 2.1 and 1.6 cm(-1) could be deduced.

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PubMed2004

Synthesis and characterization of a new family of bi-, tri-, tetra-, and pentanuclear ferric complexes.

Nine members of a new family of polynuclear ferric complexes have been synthesized and characterized. The reaction of Fe(O(2)CMe)(2) with polydentate Schiff base proligands (H(2)L) derived from salicylidene-2-ethanolamine, followed in some cases by reaction with carboxylic acids, has afforded new complexes of general formulas [Fe(2)(pic)(2)(L)(2)] (where pic(-) is the anion of 2-picolinic acid), [Fe(3)(O(2)CMe)(3)(L)(3)], [Fe(4)(OR)(2)(O(2)CMe)(2)(L)(4)], and [Fe(5)O(OH)(O(2)CR)(4)(L)(4)]. The tri-, tetra-, and pentanuclear complexes all possess unusual structures and novel core topologies. Mössbauer spectroscopy confirms the presence of high-spin ferric centers in the tri- and pentanuclear complexes. Variable-temperature magnetic measurements suggest spin ground states of S = 0, 1/2, 0, and 5/2 for the bi-, tri-, tetra-, and pentanuclear complexes, respectively. Fits of the magnetic susceptibility data have provided the magnitude of the exclusively antiferromagnetic exchange interactions. In addition, an easy-axis-type magnetic anisotropy has been observed for the pentanuclear complexes, with D values of approximately -0.4 cm(-)(1) determined from modeling the low-temperature magnetization data. A low-temperature micro-SQUID study of one of the pentanuclear complexes reveals magnetization hysteresis at nonzero field. This is attributed to an anisotropy-induced energy barrier to magnetization reversal that is of molecular origin. Finally, an inelastic neutron scattering study of one of the trinuclear complexes has revealed that the magnetic behavior arises from two distinct species.

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PubMed2003

Iron-promoted nucleophilic additions to diimine-type ligands: a synthetic and structural study.

We report here three examples of the reactivity of protic nucleophiles with diimine-type ligands in the presence of Fe(II) salts. In the first case, the iron-promoted alcoholysis reaction of one nitrile group of the ligand 2,3-dicyano-5,6-bis(2-pyridyl)-pyrazine (L1) permitted the isolation of an stable E-imido-ester, [Fe(L1')(2)](CF(3)SO(3))(2) (1), which has been characterized by spectroscopic studies (IR, ES-MS, Mössbauer), elemental analysis, and crystallographically. Compound 1 consists of mononuclear octahedrally coordinated Fe(II) complexes where the Fe(II) ion is in its low-spin state. The iron-mediated nucleophilic attack of water to the asymmetric ligand 2,3-bis(2-pyridyl)pyrido[3,4-b]pyrazine (L2) has also been studied. In this context, the crystal structures of two hydration-oxidation Fe(III) products, [Fe(L2')(2)](ClO(4))(3).3CH(3)CN (2) and trans-[FeL2"Cl(2)] (3), are described. Compounds 2 and 3 are both mononuclear Fe(III) complexes where the metals occupy octahedral positions. In principle, L2 is expected to coordinate to metal ions through its bipyridine-type units to form a five-membered ring; however, this is not the case in compounds 2 and 3. In 2, the ligand coordinates through its pyridines and through the hydroxyl group attached to the pyrazine imino carbon after hydration, that is, in an N,O,N tridentate manner. In compound 3, the ligand has suffered further transformations leading to a very stable diamido complex. In this case, the metal ion achieves its octahedral geometry by means of two pyridines, two amido N atoms, and two axial chlorine atoms. Magnetic susceptibility measurements confirmed the spin state of these two Fe(III) species: compounds 2 and 3 are low-spin and high-spin, respectively.

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PubMed1999

An Iron-Based Molecular Redox Switch as a Model for Iron Release from Enterobactin via the Salicylate Binding Mode.

The iron release mechanism from protonated ferric enterobactin [Fe(III)(enterobactinH(3))] via the salicylate binding mode was probed. For this purpose, a tripodal dodecadentate ligand incorporating three salicylamide (OO) and three bipyridine (NN) binding sites was synthesized as well as iron complexes thereof. It was shown that a ferric ion coordinates selectively to the hard salicylamides and a ferrous ion binds to the softer bipyridines. Upon reduction or oxidation, the iron translocates reversibly and intramolecularly from one site to the other, thus displaying switchlike properties. Both states were characterized by cyclic voltammetry and visible and Mössbauer spectroscopy. The Mössbauer spectrum for the ferric complex is fully consistent with that obtained by Pecoraro et al. upon lowering the pH of [Fe(III)(enterobactin)](3)(-) solutions (Pecoraro, V. L., et al. J. Am. Chem. Soc. 1983, 105, 4617), thus supporting the alternative iron release mechanism from enterobactin via the salicylate binding mode.

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PubMed1999

Bis-macrocyclic Ligands with Two Ferrocenyl End Groups, and Their Tetranuclear Dicopper(I) Compounds.

A series of bismacrocyclic ligands with two ferrocenyl groups, exo/endo-1,1':1' ',1' "-[1,2,4,5-tetrakis(5-aza-2-thiahexa-5-enyl)benzene]bisferrocene (exo/endo-FeBeFe), 1,1':1' ',1' "-[1,2:1',2'-tetrakis(5-aza-2-thiahexa-5-enyl)ethene]bisferrocene (1,2-FeEnFe), 1,1':1' ',1' "-[1,1':2,2'-tetrakis(5-aza-2-thiahexa-5-enyl)ethene]bisferrocene (1,1-FeEnFe), 1,1':1' ',1' "-[tetrakis(5-aza-2-thiahexa-5-enyl)methane]bisferrocene (FeMeFe), and their dicopper(I) compounds have been synthesized and characterized (electrochemistry, IR, NMR and Mössbauer spectroscopy). The molecular structure of endo-FeBeFe has been determined by X-ray structure analysis and the copper(I)-induced discrimination of the exo- and endo-isomers of FeBeFe has been investigated by (1)H NMR spectroscopy. The interaction between copper and iron in the tetranuclear compounds is discussed on the basis of the electrochemical and spectroscopic data.

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