Carlos Kremer

Density functional study of technetium and rhenium compounds

Abstract Density functional calculations employing the B3LYP method and the LANL2DZ basis set have been performed on several Re dioxo complexes of simple polyamines. An extended test of the ability of the methodology to offer a good description of these complexes was performed, calculating the MO 2 , MO 2 + and MO 4 − metal oxides (both for Tc and Re) and the complex [MO 2 (NH 3 ) 4 ] + . It is shown that the results agree well with the experimental information available. The effect of the ligands is to stabilize the linear geometry of the dioxo metal core, much higher in energy than the bent geometry for the isolated species. The ligand-stabilized core exhibits a singlet electronic state, while the triplet is more stable for the isolated core. The consequence of these facts is that the metal–oxygen distance in the complexes is larger than that in any of the metal oxides. The calculated structures of the Re and Tc complexes with the amines exhibit a reasonable, although not perfect, agreement with the experimental data available. The B3LYP/LANL2DZ method seems able to describe with semiquantitative precision, the structure of Re and Tc complexes of the type, studied.

New family of thiocyanate-bridged Re(IV)-SCN-M(II) (M = Ni, Co, Fe, and Mn) heterobimetallic compounds: synthesis, crystal structure, and magnetic properties.

The heterobimetallic complexes of formula [(Me(2)phen)(2)M(μ-NCS)Re(NCS)(5)]·CH(3)CN [Me(2)phen = 2,9-dimethyl-1,10-phenanthroline and M = Ni (1), Co (2), Fe (3), and Mn (4)] have been prepared, and their crystal structures have been determined by X-ray diffraction on single crystals. Compounds 1-4 crystallize in the monoclinic C2/c space group, and their structure consists of neutral [(Me(2)phen)(2)M(μ-NCS)Re(NCS)(5)] heterodinuclear units with a Re-SCN-M bridge. Each Re(IV) ion in this series is six-coordinated with one sulfur and five nitrogen atoms from six thiocyanate groups building a somewhat distorted octahedral environment, whereas the M(II) metal ions are five-coordinated with four nitrogen atoms from two bidentate Me(2)phen molecules and a nitrogen atom from the bridging thiocyanate describing distorted trigonal bipyramidal surroundings. The values of the Re···M separation through the thiocyanate bridge in 1-4 vary in the range 5.903(1)-6.117(3) Å. The magnetic properties of 1-4 as well as those of the parent mononuclear Re(IV) compounds (NBu(4))(2)[Re(NCS)(6)] (A1) (NBu(4)(+) = tetra-n-butylammonium cation) and [Zn(NO(3))(Me(2)phen)(2)](2)[Re(NCS)(5)(SCN)] (A2) were investigated in the temperature range 1.9-300 K. Weak antiferromagnetic interactions between the Re(IV) and M(II) ions across the bridging thiocyanate were found in 1-4 [J = -4.3 (1), -2.4 (2), -1.8 (3), and -1.2 cm(-1) (4), the Hamiltonian being defined as Ĥ = -JŜ(Re)·Ŝ(M)]. The magnetic behavior of A2 is that of a magnetically diluted Re(IV) complex with a large and positive value of the zero-field splitting for the ground level (D(Re) = +37.0 cm(-1)). In the case of A1, although its magnetic behavior is similar to that of A2 in the high-temperature range (D(Re) being +19.0 cm(-1)), it exhibits a weak ferromagnetism below 3.0 K with a canting angle of 1.3°.

Re(V) complexes with amino acids based on the ‘3+2’ approach

Abstract Five cationic complexes containing the [Re(V)O] 3+ core have been prepared by substitution on the precursor [ReIO 2 (PPh 3 ) 2 ]. Complexes with general formula [ReO(dien-H)(aa)] + have been obtained by reaction of the precursor with 1 equiv. of dien (diethylenetriamine) and one equivalent of an amino acid (glycine, alanine, valine, leucine, proline). Complexes have been characterized by UV–Vis and IR spectrophotometry, elemental analyses, 1 H NMR spectra, HPLC and conductivity measurements. The molecular structures of [ReO(dien-H)(leu)]I·0.5H 2 O and [ReO(dien-H)(pro)]ReO 4 have been determined by means of single-crystal X-ray analyses. Cations consist of a six-coordinated rhenium atom bonded to two trans -oxo groups, one of the [ReO] 3+ core and one from the carboxilate. Four nitrogen atoms make the equatorial plane. Three N belong to the dien ligand and the other one to the amino acid. One of the secondary amine proton of the dien ligand is lost upon coordination. This arrangement corresponds to the so-called ‘3+2’ chelating framework, with the novel NNN/NO atom set. Complexes are not very stable in aqueous solution. However, solutions in MeOH and DMSO are stable enough to be studied by several techniques. The solid state structure seems to be retained in solution.

Magnetic properties and molecular structures of binuclear (2-pyrazinecarboxylate)-bridged complexes containing Re(IV) and M(II) (M = Co, Ni)

Three novel Re(iv) compounds, the mononuclear complex Bu(4)N[ReBr(5)(Hpyzc)] (1) and the heterobimetallic complexes [ReBr(5)(mu-pyzc)M(dmphen)(2)].2CH(3)CN [M = Co (2), Ni (3)] (Hpyzc = 2-pyrazinecarboxylic acid, dmphen = 2,9-dimethyl-1,10-phenanthroline), have been synthesized and their crystal structures determined by single-crystal X-ray diffraction. The structure of 1 consists of [ReBr(5)(Hpyzc)](-) complex anions and tetrabutylammonium cations, Bu(4)N(+). The Re(iv) is surrounded by five bromide anions and a N-donor Hpyzc monodentate ligand, in a distorted octahedral environment. The structures of 2 and 3 consist of dinuclear units [ReBr(5)(mu-pyzc)M(dmphen)(2)], with the metal ions linked by a pyzc bridge ligand, being bidentate toward M(II) and monodentate toward Re(IV). The environment of Re(IV) is the same as in 1, whereas M(II) is six-coordinate, being surrounded by four nitrogen atoms of two bidentate dmphen ligands and one oxygen atom and one nitrogen atom of the pyzc anion. The magnetic properties of 1-3 were investigated in the temperature range 2.0-300 K. 1 shows the expected magnetic behavior for a mononuclear Re(IV) complex with a weak intermolecular antiferromagnetic coupling at low temperatures. The bimetallic complexes exhibit an intramolecular ferromagnetic coupling between Re(IV) and the M(II) ion (Co, Ni).

Characterization of myo‐inositol hexakisphosphate deposits from larval Echinococcus granulosus

The abundant metabolite myo‐inositol hexakisphosphate (InsP6) can form vesicular deposits with cations, a widespread phenomenon in plants also found in the cestode parasite, Echinococcus granulosus. In this organism, the deposits are exocytosed, accumulating in a host‐exposed sheath of extracellular matrix termed the laminated layer. The formation and mobilization of InsP6 deposits, which involve precipitation and solubilization reactions, respectively, cannot yet be rationalized in quantitative chemical terms, as the solids involved have not been formally described. We report such a description for the InsP6 deposits from E. granulosus, purified as the solid residue left by mild alkaline digestion of the principal mucin component of the laminated layer. The deposits are largely composed of the compound Ca5H2L·16H2O (L representing fully deprotonated InsP6), and additionally contain Mg2+ (6–9% molar ratio with respect to Ca2+), but not K+. Calculations employing recently available chemical constants show that the precipitation of Ca5H2L·16H2O is predicted by thermodynamics in secretory vesicle‐like conditions. The deposits appear to be similar to microcrystalline solids when analysed under the electron microscope; we estimate that each crystal comprises around 200 InsP6 molecules. We calculate that the deposits increase, by three orders of magnitude, the surface area available for adsorption of host proteins, a salient ability of the laminated layer. The major inositol phosphate in the deposits, other than InsP6, is myo‐inositol (1,2,4,5,6) pentakisphosphate, or its enantiomer, inositol (2,3,4,5,6) pentakisphosphate. The compound appears to be a subproduct of the intracellular pathways leading to the synthesis and vesicular accumulation of InsP6, rather than arising from extracellular hydrolysis of InsP6.

Chelatable iron pool: inositol 1,2,3-trisphosphate fulfils the conditions required to be a safe cellular iron ligand.

Mammalian cells contain a pool of iron that is not strongly bound to proteins, which can be detected with fluorescent chelating probes. The cellular ligands of this biologically important “chelatable”, “labile” or “transit” iron are not known. Proposed ligands are problematic, because they are saturated by magnesium under cellular conditions and/or because they are not “safe”, i.e. they allow iron to catalyse hydroxyl radical formation. Among small cellular molecules, certain inositol phosphates (InsPs) excel at complexing Fe3+ in such a “safe” manner in vitro. However, we previously calculated that the most abundant InsP, inositol hexakisphosphate, cannot interact with Fe3+ in the presence of cellular concentrations of Mg2+. In this work, we study the metal complexation behaviour of inositol 1,2,3-trisphosphate [Ins(1,2,3)P3], a cellular constituent of unknown function and the simplest InsP to display high-affinity, “safe”, iron complexation. We report thermodynamic constants for the interaction of Ins(1,2,3)P3 with Na+, K+, Mg2+, Ca2+, Cu2+, Fe2+ and Fe3+. Our calculations indicate that Ins(1,2,3)P3 can be expected to complex all available Fe3+ in a quantitative, 1:1 reaction, both in cytosol/nucleus and in acidic compartments, in which an important labile iron subpool is thought to exist. In addition, we calculate that the fluorescent iron probe calcein would strip Fe3+ from Ins(1,2,3)P3 under cellular conditions, and hence labile iron detected using this probe may include iron bound to Ins(1,2,3)P3. Therefore Ins(1,2,3)P3 is the first viable proposal for a transit iron ligand.

Synthesis, characterization and potentiometric studies of trans-dioxorhenium(V) complexes. X-ray crystal structure of [ReO2(tn)2]I·H2O

Abstract [ReO2(PPh3)2I] has been used as starting material to prepare [RevO2(tn)2]I (tn = trimethylenediamine) in high yield. The complex was characterized by elemental analysis and spectroscopic methods. The complex [ReO2(en)2]I (en = 1,2-diaminoethane) was prepared analogously for comparison. The crystal structure of [ReO2(tn)2]I·H2O was determined from X-ray data. It consists of a six-coordinate rhenium atom bonded to two trans-oxo groups. The amines lie in the equatorial plane, almost perpendicular to the ReO2 core. The average value for the ReO bond is 1.76 A, similar to that in other Re(V) dioxo complexes. The successive protonations of the oxo groups were determined for both complexes.

Synthesis, structure and magnetic properties of Mn(II) and Cu(II) complexes with the dicyano-acetic acid methyl ester anion

The preparation of the sodium salt of dicyano-acetic acid methyl ester (NaCH 3 OC(O)C(CN) 2 ) (NaL) is reported. The structure of this anion is related to the structure of the dicyanamide, whose chelating capability has been used to develop 2D networks. Two new complexes of formula [M(L) 2 (H 2 O) 2 ] (M=Mn 2+ ( 1 ) and Cu 2+ ( 2 )) have been synthesized and their crystal structures determined by single-crystal X-ray diffraction. The coordination geometry of Mn atom in 1 is approximately octahedral, and the Mn atoms are μ 2 -bridged by two pairs of L − resulting in an infinite one-dimensional chain. Two types of hydrogen bonds link the chains within a 3-dimensional infinite lattice. The coordination geometry of copper in 2 is tetragonal bipyramidal, showing considerable Jahn–Teller distortion. Each copper atom is connected by four ligands to four different neighboring copper atoms, resulting in sheets of an infinite two-dimensional network. The large separation between the paramagnetic centers in both compounds produces a weak antiferromagnetic interaction.

Oxalato complexes of Re(V). Synthesis and structural characterization of [ReO(OCH3)(ox)(L)] (L=bipy, dppe and dppee)

Three novel neutral complexes containing the [ReO]3+ core have been prepared starting from K2ReBr6. Complexes with general formula [ReO(OCH3)(ox)(L)] (ox=oxalate; L=2,2′-bipyridine (bipy), 1,2-bis(diphenylphosphino)ethane (dppe) or cis-1,2-bis(diphenylphosphino)ethylene (dppee)) have been obtained and characterized. The molecular structures of [ReO(OCH3)(ox)(bipy)] and [ReO(OCH3)(ox)(dppe)] have been determined of single-crystal X-ray analysis. The structure of the complexes consists of a six-coordinated rhenium atom bonded to two trans-oxygen atoms, one of the [ReO]3+ core and one from a methoxy group. One oxalate ligand and a neutral diamine or diphosphine make the equatorial plane.

Sm(III) complexation with α-amino acids: X-ray crystal structure of [Sm2(Hala)4(H2O)8](ClO4)4(Cl)2

Abstract Sm(III) coordination compounds are currently used as radiotherapeutic agents. Moreover, it is known that tumour cells show enhanced intake of α-amino acids (H n aa). With this in mind, the study of samarium complexes with these ligands has obvious interest. In this work, potentiometric studies of Sm(III) in the presence of glycine, alanine, proline, tryptophane, valine, glutamic acid and cysteine have been carried out. Experiments were performed in aqueous solution ([Sm 3+ ]=5.0–60.0 mM) at 37°C and 0.15 M NaClO 4 . Mono and dinuclear species were detected in the pH range 1.5–5.3. For higher pH values, Sm(OH) 3 competes with the formation of Sm complexes. In addition, Sm(III)–H n aa complexes were isolated and characterized at solid state. The structure of [Sm 2 (Hala) 4 (H 2 O) 8 ](ClO 4 ) 4 (Cl) 2 is reported. In this complex, two Sm centres are joined by four μ-COO bridges. Four molecules of water complete the positions of each eight-coordinate Sm atom.