Zbigniew Tomkowicz

Synthesis, Crystal Structure, and Magnetic Properties of the Coordination Polymer [Fe(NCS)2(1,2-bis(4-pyridyl)-ethylene)]n Showing a Two Step Metamagnetic Transition

Reaction of iron(II) thiocyanate with an excess of trans-1,2-bis(4-pyridyl)-ethylene (bpe) in acetonitrile at room temperature leads to the formation of [Fe(NCS)(2)(bpe)(2)·(bpe)] (1), which is isotypic to its Co(II) analogue. Using slightly different reaction conditions the literature known compound [Fe(NCS)(2)(bpe)(2)(H(2)O)(2)] (2) was obtained as a phase pure material. Simultaneous differential thermoanalysis and thermogravimetry prove that the hydrate 2 transforms into the anhydrate [Fe(NCS)(2)(bpe)(2)] (3), that decomposes on further heating into the new ligand-deficient 1:1 compound of composition [Fe(NCS)(2)(bpe)](n) (4), which can also be obtained directly by thermal decomposition of 1. Further investigations reveal that 4 can also be prepared under solvothermal conditions, and single crystal structure analysis shows that the iron(II) cations are linked via μ-1,3 bridging thiocyanato anions into chains, that are further connected into layers by the bpe ligands. Magnetic measurements, performed on powder samples, prove that 1 and 2 show only Curie-Weiss behavior, whereas in 4 antiferromagnetic ordering with a Néel temperature of 5.0 K is observed. At T < 4.0 K a two-step metamagnetic transition occurs at applied magnetic fields of 1300 and 1775 Oe. The magnetic properties are discussed and compared with those of related compounds.

Influence of the co-ligand on the magnetic and relaxation properties of layered cobalt(II) thiocyanato coordination polymers.

Reaction of Co(NCS)2 with 1,2-bis(4-pyridyl)-ethane (bpa) leads to the formation of [Co(NCS)2(bpa)2]n, which, on heating, transforms into the new layered coordination polymer [Co(NCS)2(bpa)]n. This compound can also be prepared in solution, but because no reasonable single crystals are available, its crystal structure was determined from X-ray powder data from scratch. In the crystal structure of [Co(NCS)2(bpa)]n, the cobalt(II) cations are coordinated by two S-bonded and two N-bonded thiocyanato anions and two N atoms of the bpa co-ligands in a distorted octahedral geometry. The cobalt(II) cations are linked into chains by pairs of μ-1,3 bridging thiocyanato anions. These chains are further connected into layers by the 1,2-bis(4-pyridyl)-ethane ligand. The compound was magnetically characterized, and, for comparative purposes, the complementary magnetic study of a known and very similar compound, [Co(NCS)2(bpe)]n (bpe = 1,2-bis(4-pyridyl)-ethylene), was also undertaken. The compounds differ in their interchain interactions, which are antiferromagnetic but significantly greater for [Co(NCS)2(bpe)]n. Magnetic measurements indicate that [Co(NCS)2(bpa)]n is a canted antiferromagnet with Néel temperature TN = 3.1 K and that Co(NCS)2(bpe) is an antiferromagnet with TN = 4.0 K. Both compounds show a metamagnetic transition with a critical field HC ∼ 40 Oe and ∼ 400 Oe, respectively. Magnetic relaxations were studied by means of dc and ac methods and analyzed using the Argand diagrams. Except for the thermally activated single chain and domain wall relaxations observed for both compounds, temperature-independent slow relaxations were observed for [Co(NCS)2(bpa)]n.

Structural and magnetic studies of a new Co(II) thiocyanato coordination polymer showing slow magnetic relaxations and a metamagnetic transition.

Reaction of Co(NCS)2 with 4-ethylpyridine leads to the formation of three new compounds of composition Co(NCS)2(4-ethylpyridine)4 (1), [(Co(NCS)2]2(4-ethylpyridine)6 (2), and [Co(NCS)2(4-ethylpyridine)2]n (3). In all compounds the coordination of the Co(II) ions is distorted octahedral. 1 consists of discrete monomeric complexes and in 2 two Co(II) cations are linked by pairs of μ-1,3-bridging thiocyanato ligands into dimers. In the crystal structure of 3 the Co(II) cations are connected into chains by the same bridge as in 2. Magnetic studies show that 1 and 2 are paramagnets down to a temperature of 2 K, while compound 3, which is the main object of this study, is an antiferromagnet with the Néel temperature T(N) = 3.4 K. Its magnetic structure is built from ferromagnetic chains, which are weakly antiferromagnetically coupled. With increasing magnetic field a metamagnetic transition starts at ~175 Oe, as observed for a polycrystalline sample. Magnetic relaxations, which were observed in the antiferromagnetic state, are retained at the metamagnetic transition. With decreasing field 3 remains in a state, in which except of the faster magnetic relaxation process in single chains also a slower process coexists resulting in the appearance of a magnetic hysteresis loop.

Synthesis, structure, and physicochemical properties of dinuclear NiII complexes as highly efficient functional models of phosphohydrolases.

As metal ions are present in the catalytic sites of several enzymes, attention has been focused on the synthesis and characterization of metal complexes able to act as biomimetic functional and structural models for these systems. In this study, a novel dinuclear NiII complex was synthesized, [Ni2(L2)(OAc)2(CH3CN)]BPh4 (2) (HL2=2-[N-(2-(pyridyl-2-yl)ethyl)(1-methylimidazol-2-yl)amin omethyl]-4-methyl-6-[N-(2-(imidazol-4-yl)ethyl)amino methyl]phenol), employing a new unsymmetrical dinucleating ligand containing N,O-donor groups as a model for hydrolases. Complex 2 was characterized by a variety of techniques including: elemental analysis, infrared and UV-vis spectroscopies, molar conductivity, electrochemistry, potentiometric titration, magnetochemistry, and single-crystal X-ray diffractometry. The structural and magnetochemical data of 2 allow us to consider this complex as a structural model for the active site of the ureases, as previously reported for [Ni2(L1)(OAc)2(H2O)]ClO4.H2O (1) (HL1=2-[N-bis-(2-pyridylmethyl)aminomethyl]-4-methyl-6-[N-(2-pyridylmethyl)aminomethyl] phenol). The characterization of complexes 1 and 2 (mainly by X-ray diffraction and potentiometric titration) led us to study their reactivities toward the hydrolysis of the substrate bis(2,4-dinitrophenyl)phosphate (2,4-BDNPP). These studies revealed that complexes 1 and 2 show the best catalytic activity reported so far, with acceleration rates 8.8x10(4) and 9.95x10(5) times faster, respectively, than the uncatalyzed hydrolysis of 2,4-BDNPP. Catalytic activity of 2 on 2,4-DNPP showed that the monoester is hydrolyzed 27 times slower than the 2,4-BDNPP diester under identical experimental conditions. Therefore, 1 and 2 can undoubtedly be considered highly efficient functional models of the phosphohydrolases.

Unsymmetrical FeIIICoII and GaIIICoII Complexes as Chemical Hydrolases: Biomimetic Models for Purple Acid Phosphatases (PAPs)

The design and development of suitable biomimetic catalytic systems capable of mimicking the functional properties of enzymes continues to be a challenge for bioinorganic chemists. In this study, we report on the synthesis, X-ray structures, and physicochemical characterization of the novel isostructural [Fe(III)Co(II)(BPBPMP)(mu-OAc)(2)]ClO(4) (1) and [Ga(III)Co(II)(BPBPMP)(mu-OAc)(2)]ClO(4) (2) complexes with the unsymmetrical dinucleating ligand H(2)BPBPMP (2-bis[{(2-pyridyl-methyl)-aminomethyl}-6-{(2-hydroxy-benzyl)-(2-pyridyl-methyl)}-aminomethyl]-4-methylphenol). The previously reported complex [Fe(III)Zn(II)(BPBPMP)(mu-OAc)(2)]ClO(4) (3) was investigated here by electron paramagnetic resonance for comparison with such studies on 1 and 2. A magneto-structural correlation between the exchange parameter J (cm(-1)) and the average bond lengh d (A) of the [Fe(III)-O-M(II)] structural unit for 1 and for related isostructural Fe(III)M(II) complexes using the correlation J = -10(7) exp(-6.8d) reveals that this parameter is the major factor that determines the degree of antiferromagnetic coupling in the series [(BPBPMP)Fe(III)(mu-OAc)(2)M(II)](+) (M(II) = Mn, Fe, Co, Ni) of complexes. Potentiometric and spectrophotometric titrations along with electronic absorption studies show that, in aqueous solution, complexes 1 and 2 generate the [(HO)M(III)(mu-OH)Co(II)(H(2)O)] complex as the catalytically active species in diester hydrolysis reactions. Kinetic studies on the hydrolysis of the model substrate bis(2,4-dinitrophenyl)phosphate by 1 and 2 show Michaelis-Menten behavior, with 2 being 35% more active than 1. In combination with k(H)/k(D) isotope effects, the kinetic studies suggest a mechanism in which a terminal M(III)-bound hydroxide is the hydrolysis-initiating nucleophilic catalyst. In addition, the complexes show maximum catalytic activity in DNA hydrolysis near physiological pH. The modest reactivity difference between 1 and 2 is consistent with the slightly increased nucleophilic character of the Ga(III)-OH terminal group in comparison to Fe(III)-OH in the dinuclear M(III)Co(II) species.

Slow Magnetic Relaxations in Manganese(III) Tetra(meta-fluorophenyl)porphyrin-tetracyanoethenide. Comparison with the Relative Single Chain Magnet ortho Compound

Mn(III) tetra(meta-fluorophenyl)porphyrin-tetracyanoethenide coordination polymer (abbreviated meta-F) was synthesized and crystallographically and magnetically characterized. The compound crystallizes in the space group C2/c with four equivalent molecules in the unit cell arranged along two symmetry related nonparallel linear chain directions. Magnetic properties were studied by SQUID dc magnetization and ac susceptibility techniques and high field-high frequency electron spin resonance (HF-ESR). Glassy transition to a ferromagnetic-like state is observed at 10 K accompanied by slow magnetic relaxations. The glassiness is interpreted as due to 3D domain wall pinning. In a bias dc magnetic field the width of the relaxation time distribution decreases and the relaxations become similar to the relaxations of the single chain magnet Mn(III) tetra(ortho-fluorophenyl)porphyrin-tetracyanoethenide (abbreviated ortho-F), for which comparative HF-ESR studies were also conducted in this work. Magnetic properties of these two compounds are compared, and the nature of magnetic relaxations in meta-F is discussed.

Synthesis, characterization, hydrolase and catecholase activity of a dinuclear iron(III) complex: Catalytic promiscuity

Herein, we report the synthesis and characterization of the new di-iron(III) complex [(bbpmp)(H2O)(Cl)Fe(III)(μ-Ophenoxo)Fe(III)(H2O)Cl)]Cl (1), with the symmetrical ligand 2,6-bis{[(2-hydroxybenzyl)(pyridin-2-yl)methylamino]methyl}-4-methylphenol (H3bbpmp). Complexes 2 with the unsymmetrical ligand H2bpbpmp - {2-[[(2-hydroxybenzyl)(2-pyridylmethyl)]aminomethyl]-6-bis(pyridylmethyl) aminomethyl}-4-methylphenol and 3 with the ligand L(1)=4,11-dimethyl-1,8-bis{2-[N-(di-2-pyridylmethyl)amino]ethyl}cyclam were included for comparison purposes. Complex 1 was characterized through elemental analysis, X-ray crystallography, magnetochemistry, electronic spectroscopy, electrochemistry, mass spectrometry and potentiometric titration. The magnetic data show a very weak antiferromagnetic coupling between the two iron centers of the dinuclear complex 1 (J=-0.29cm(-1)). Due to the presence of labile coordination sites in both iron centers the hydrolysis of both the diester model substrate 2,4-BDNPP and DNA was studied in detail. Complex 1 was also able to catalyze the oxidation of the substrate 3,5-di-tert-butylcatechol (3,5-DTBC) to give the corresponding quinone, and thus it can be considered as a catalytically promiscuous system.

Magnetooptical and structural investigations of five dimeric cobalt(II) complexes mimicking metalloenzyme active sites.

Four novel cobalt(II) complexes mimicking metalloenzyme active sites, novel C(14)H(22)Cl(12)Co(2)O(13)·2C(3)H(8)O (1), C(28)H(36)Cl(24)Co(4)O(28)·4C(4)H(8)O(2) (2), C(16)H(22)Cl(12)Co(2)O(13)·C(2)HCl(3)O(2) (3), C(16)H(22)Cl(12)Co(2)O(13) (4), and one known C(40)H(78)Cl(8)Co(2)O(17) (5) are composed of the same core of two high-spin cobalt(II) centers triply bridged by water and two trichloroacetato (1-4) or two pivalate (5) groups but differ in terminal ligands. The crystal structures of new compounds 1-4 belong to the space groups P ̅1, P2(1)/c, P ̅1, and Pbcn, respectively. All five investigated complexes contain Co atoms in distorted octahedral coordination. The complexes were characterized by magnetic susceptibility and magnetization measurements and by variable-temperature variable-field magnetic circular dichroism spectroscopy. Experimental data were analyzed in the frame of the theoretical model, which includes an unquenched orbital moment of the Co(II) ions. All investigated compounds are antiferromagnetically coupled with exchange constants in the range -1.5 to -2.1 cm(-1). However, there is a significant difference between the crystal-field-splitting parameters.

Design of a Dinuclear Nickel(II) Bioinspired Hydrolase to Bind Covalently to Silica Surfaces: Synthesis, Magnetism, and Reactivity Studies

Presented herein is the design of a dinuclear Ni(II) synthetic hydrolase [Ni(2)(HBPPAMFF)(μ-OAc)(2)(H(2)O)]BPh(4) (1) (H(2)BPPAMFF = 2-[(N-benzyl-N-2-pyridylmethylamine)]-4-methyl-6-[N-(2-pyridylmethyl)aminomethyl)])-4-methyl-6-formylphenol) to be covalently attached to silica surfaces, while maintaining its catalytic activity. An aldehyde-containing ligand (H(2)BPPAMFF) provides a reactive functional group that can serve as a cross-linking group to bind the complex to an organoalkoxysilane and later to the silica surfaces or directly to amino-modified surfaces. The dinuclear Ni(II) complex covalently attached to the silica surfaces was fully characterized by different techniques. The catalytic turnover number (k(cat)) of the immobilized Ni(II)Ni(II) catalyst in the hydrolysis of 2,4-bis(dinitrophenyl)phosphate is comparable to the homogeneous reaction; however, the catalyst interaction with the support enhanced the substrate to complex association constant, and consequently, the catalytic efficiency (E = k(cat)/K(M)) and the supported catalyst can be reused for subsequent diester hydrolysis reactions.

Three-dimensional magnetic ordering in manganese(III)–porphyrin–TCNE complexes

Abstract We have investigated Mn(III)–porphyrin–TCNE complexes showing peculiar magnetic properties at low temperatures. The tetraphenylporphyrin ligand is substituted on the periphery of the phenyl rings with different groups. Static and dynamic susceptibility measurements have been carried out on the samples.