Preparation and Spectroscopic Characterization of Two Manganese(II)semiquinone Complexes

Summary. The complexes [CTA][Mn(II)(SQ)₃] were isolated in the solid state and purified. SQ is the o-semiquinone of L-dopa or dopamine and CTA is the cetyltrimethylammonium cation. These complexes were characterized by Raman, infrared, EPR and thermogravimetry (TG) techniques. The EPR spectra of the solids presented an intense signal characteristic of the o-semiquinone radical anion with g=2.0062 and g=2.0063 for L-dopa and dopamine, respectively. Six characteristic lines around the organic radical signal confirm the presence of the Mn²⁺ ion. The most intense Raman bands were observed at for dopamine and at 1356 cm^–1 for L-dopa and assigned to a C–O stretching with major C₁–C₂ character. The absence of an intense Raman band at ca. , characterizes the ligands as an o-semiquinone radical anion. Broad bands in the region can be assigned to deformations associated with the five-member ring chelate including the manganese ion, the oxygens, and the C₁–C₂ bonds. The more intense IR bands for the dopamine and the L-dopa-derived ligands at are assigned to CO. Mass loss mechanisms for the two complexes, based on the TG results, were proposed and confirm the formula proposed.     

Cationic Homoleptic Vanadium(II), (IV), and (V) Complexes Arising from Protonolysis of [V(NEt2)4]

At least three different cationic species arise in the classic protonolysis of [V^IV(NEt₂)₄] with a borate ammonium salt. The unexpected formation of the vanadium(V ) species [V(NEt₂)₄][B(C₆H₅)₄] (shown in the picture without its counterion) underlines the problem of deducing the true oxidation state of vanadium species in Ziegler–Natta reactions.     

Determination of the hydration number of gadolinium(III) complexes by high-field pulsed 17O ENDOR spectroscopy.

Pulsed 17O Mims electron-nuclear double resonance (ENDOR) spectroscopy at the W band (95 GHz) and D band (130 GHz) is used for the direct determination of the water coordination number (q) of gadolinium-based magnetic resonance imaging (MRI) contrast agents. Spectra of metal complexes in frozen aqueous solutions at approximately physiological concentrations can be obtained either in the presence or absence of protein targets. This method is an improvement over the 1H ENDOR method described previously, which involved the difference ENDOR spectrum of exchangeable protons from spectra taken in H2O and D2O. In addition to exchangeable water protons, the 1H ENDOR method is also sensitive to other exchangeable protons, and it is shown here that this method can overestimate hydration numbers for complexes with exchangeable protons at GdH distances similar to that of the coordinated water, for example, from NH groups. The 17O method does not suffer from this limitation. 17O ENDOR spectroscopy is applied to Gd(III) complexes containing zero, one, or two inner-sphere water molecules. In addition, 13C and 1H ENDOR studies were performed to assess the extent of methanol coordination, since methanol is used to produce a glass in these experiments. Under the experimental conditions used for the hydration number determination (30 mol % methanol), fewer than 15 % of the coordination sites were found to be occupied by methanol.     

Synthesis, Crystal Structure, UV-vis and EPR Studies of（ NH3CH2CH2NH2 ） 2.5 [ Mo0.5^（V）W0.5^（VI）O2 （ OC6H4O ） 2]

Cis-dioxo-metal complex ( NH3CH2CH2NH2 ) 2.5 [ Mo0.5^(V)W0.5^(VI)O2 ( OC6H4O ) 2] 1 was obtained by the reaction of tetra-butyl ammonium hexamolybdotungstate with 1, 2-dihydroxybenzene in the mixed solvent of CH3OH, CH3CN and ethylenediamine,and characterized by X-ray diffraction, UV-vis and EPR analysis. Compared with its analogous complexes (NH3CH2CH2NH2)3[Mo^(V)O2(OC6H40)2] 2 and (NH3CH2CH2NH2)2[W^(VI)O2(OC6H4O)2] 3, the results show that tungsten(VI) is less active in redox than molybdenum (VI) and that the change of the valence induced by substitution of W(VI) for Mo(V) in EMO2(OC6H40)2]n- does not influence the coordination geometry of the complex anion in which the metal center exhibits distorted octahedral coordination with cis-dioxo catechol. The responses to EPR of complexes 1 and 2 are active but complex 3 is silent,and the UV-vis spectra exhibited by the three complexes are obvious different because of the different electronic configuration between the central Mo(V) and W(VI) ions in the complexes.It is noteworthy that complexes 1 and 2 have the similar EPR signal to flavoenzyme, suggesting that the three complexes have the same coordination geometry feature with the co-factor of flavoenzyme.     

Synthesis and Reactivity of Molybdenum and Tungsten Alkyne Complexes Containing 6-Methylpyridine-2-thiolate Ligands

A series of M(II) and M(IV) (M=Mo, W) alkyne adducts employing two 6-methylpyridine-2-thiolate (6-MePyS) ligands was synthesized and investigated towards the nucleophilic attack of PMe₃ on the coordinated alkynes. For this approach, 2-butyne (C₂Me₂), phenylacetylene (HC₂Ph), and diphenylacetylene (C₂Ph₂) were used. For the exploration of an intramolecular attack, but-3-yn-1-ol (HCCCH₂CH₂OH) was coordinated to the metal centers. A nucleophilic attack of PMe₃ was observed in [W(CO)(HC₂Ph)(6-MePyS)₂] yielding an η²-vinyl compound. Reaction of [W(CO)(C₂Ph₂)(6-MePyS)₂] with excess PMe₃ resulted in the selective coordination of one molecule of PMe₃ concomitant with decoordination of the nitrogen atom of one 6-MePyS ligand. In contrast, the W(IV) complexes did not react with PMe₃. While no selectivity was observed in the reaction of the Mo(II) compounds with PMe₃, alkynes in the Mo(IV) compounds were replaced by PMe₃. Addition of Et₃N to the but-3-yn-1-ol complexes did not lead to the anticipated formation of 2,3-dihydrofuran.     

大环配体过渡金属配合物——Ⅵ.四(对-二硫杂环己烯)四氮杂卟啉钒(Ⅳ)、锰(Ⅱ)和钼(Ⅴ)配合物的合成与表征

d过渡金属含硫四氮杂卟啉配合物,是一类新型功能性配合物,因其具有比金属卟啉、金属酞菁更为优异的光电和催化特性,故近十多年来颇受重视。我们对该类金属配合物及其自由配体的合成、性质和某些功能,正在进行广泛和深入的研究。本文简要报道标题配合物的合成、元素分析,红外光谱、电子光谱和电子顺磁共振谱的表征。     

Identification and Spectroscopic Characterization of Nonheme Iron(III) Hypochlorite Intermediates

Fe^III–hypohalite complexes have been implicated in a wide range of important enzyme‐catalyzed halogenation reactions including the biosynthesis of natural products and antibiotics and post‐translational modification of proteins. The absence of spectroscopic data on such species precludes their identification. Herein, we report the generation and spectroscopic characterization of nonheme Fe^III–hypohalite intermediates of possible relevance to iron halogenases. We show that Fe^III‐OCl polypyridylamine complexes can be sufficiently stable at room temperature to be characterized by UV/Vis absorption, resonance Raman and EPR spectroscopies, and cryo‐ESIMS. DFT methods rationalize the pathways to the formation of the Fe^III‐OCl, and ultimately Fe^IV?O, species and provide indirect evidence for a short‐lived Fe^II‐OCl intermediate. The species observed and the pathways involved offer insight into and, importantly, a spectroscopic database for the investigation of iron halogenases.     

Fluoride Binding and Crystal‐Field Analysis of Lanthanide Complexes of Tetrapicolyl‐Appended Cyclen

Lanthanide complexes of tetrapicolyl cyclen displayed remarkably high affinities for fluoride (log K≈5) in water, and were shown to form 1:1 complexes. The behaviour of these systems can be rationalised by changes to the magnitude of the crystal‐field parameter, . However, such changes are not invariably accompanied by a change in sign of this parameter: for early lanthanides, the N8 donor set with a coordinated axial water molecule ensures that the magnetic anisotropy has the opposite sense to that observed in the analogous dehydrated lanthanide complexes.     

Infrared Spectroscopy of Dioxouranium(V) Complexes with Solvent Molecules: Effect of Reduction

UO₂⁺–solvent complexes having the general formula [UO₂(ROH)]⁺ (R=H, CH₃, C₂H₅, and n‐C₃H₇) are formed using electrospray ionization and stored in a Fourier transform ion cyclotron resonance mass spectrometer, where they are isolated by mass‐to‐charge ratio, and then photofragmented using a free‐electron laser scanning through the 10 μm region of the infrared spectrum. Asymmetric O=U=O stretching frequencies (ν₃) are measured over a very small range [from ～953 cm^?1 for H₂O to ～944 cm^?1 for n‐propanol (n‐PrOH)] for all four complexes, indicating that the nature of the alkyl group does not greatly affect the metal centre. The ν₃ values generally decrease with increasing nucleophilicity of the solvent, except for the methanol (MeOH)‐containing complex, which has a measured ν₃ value equal to that of the n‐PrOH‐containing complex. The ν₃ frequency values for these U(V) complexes are about 20 cm^?1 lower than those measured for isoelectronic U(VI) ion‐pair species containing analogous alkoxides. ν₃ values for the U(V) complexes are comparable to those for the anionic [UO₂(NO₃)₃]^? complex, and 40–70 cm^?1 lower than previously reported values for ligated uranyl(VI) dication complexes. The lower frequency is attributed to weakening of the O?U?O bonds by repulsion related to reduction of the U metal centre, which increases electron density in the antibonding π* orbitals of the uranyl moiety. Computational modelling of the ν₃ frequencies using the B3LYP and PBE functionals is in good agreement with the IRMPD measurements, in that the calculated values fall in a very small range and are within a few cm^?1 of measurements. The values generated using the LDA functional are slightly higher and substantially overestimate the trends. Subtleties in the trend in ν₃ frequencies for the H₂O–MeOH–EtOH–n‐PrOH series are not reproduced by the calculations, specifically for the MeOH complex, which has a lower than expected value.     

Copper(II) interaction with prion peptide fragments encompassing histidine residues within and outside the octarepeat domain: speciation, stability constants and binding details

A 31-mer polypeptide, which encompasses residues 84-114 of human prion protein HuPrP(84-114) and contains three histidyl residues, namely one from the octarepeat (His85) and two histidyl residues from outside the octarepeat region (His96 and His111), and its mutants with two histidyl residues HuPrP(84-114)His85Ala, HuPrP(84-114) His96Ala, HuPrP(84-114)His111Ala and HuPrP(91-115) have been synthesised and their Cu2+ complexes studied by potentiometric and spectroscopic (UV/Vis, CD, EPR, ESI-MS) techniques. The results revealed a high Cu2+-binding affinity of all peptides, and the spectroscopic studies made it possible to clarify the coordination mode of the peptides in the different complex species. The imidazole nitrogen donor atoms of histidyl residues are the exclusive metal-binding sites below pH 5.5, and they have a preference for macrochelate structure formation. The deprotonation and metal-ion coordination of amide functions take place by increasing the pH; all of the histidines can be considered to be independent metal-binding sites in these species. As a consequence, di- and trinuclear complexes can be present even in equimolar samples of the metal ion and peptides, but the ratios of polynuclear species do not exceed the statistically expected ones; this excludes the possibility of cooperative Cu2+ binding. The species with a (N(im),N,N)-binding mode are favoured around pH 7, and their stability is enhanced by the macrochelation from another histidyl residue in the mononuclear complexes. The independence of the histidyl sites results in the existence of coordination isomers and the preference for metal binding follows the order of: His111>His96>His85. Deprotonation and metal-ion coordination of the third amide functions were detected in slightly alkaline solutions at each of the metal-binding sites; all had a (N(im),N,N,N)-coordination mode. Spectroscopic measurements also made it clear that the four lysyl amino groups of the peptides are not metal-binding sites in any cases.     

Synthesis and Characterization of Tris(oxazolinyl)borato Copper(II) and Copper(I) Complexes

The reaction of To^MTl (To^M=tris(4,4-dimethyl-2-oxazolinyl)phenylborate) and CuBr₂ in benzene at 60 °C provides To^MCuBr ( 1 ) as an entry-point into tris(oxazolinyl)phenylborato copper chemistry. To^MCuO^tBu ( 2 ) and To^MCuOAc ( 3 ) are prepared by the reactions of To^MCuBr with KO^tBu and NaOAc, respectively. To^MCuO^tBu is transformed into (To^MCuOH)₂ ( 4 ) through hydrolysis. NMR, FT-IR, and EPR spectroscopies are used to determine the electronic and structural properties of these copper(II) compounds, and the solid-state structures were characterized by X-ray crystallography. Reduction of copper is observed upon treatment of To^MCuO^tBu with phenylsilane in an attempt to synthesize monomeric copper(II) hydride. To^MCu ( 5 ) and To^M₂Cu ( 6 ) were independently synthesized and characterized for comparison.     

Synthesis and spectral characterization of dioxouranium (VI) complexes of salicylhydrazine and acetone salicylhydrazone

Dioxouranium(VI) complexes of the types UO₂LSO₄ and UO₂L₂SO₄ (where L=SH, ASH) have been prepared from reaction of uranyl sulphate with salicylhydrazine (SH) and acetone salicylhydrazone (ASH) and characterized by conventional chemical and physical measurements. Infrared and Raman spectra indicate thatmono- andbis-complexes contain six-and seven-coordinate uranium atom respectively with all the ligand atoms arranged in an equatorial plane around the linear uranyl group. The infrared spectra (4000-200 cm⁻¹) reveal that both SH and ASH act as neutral bidentate ligands coordinating through a carbonyl oxygen and primary amine/azomethine nitrogen atoms. The sulphato group coordinates to the uranyl ion as bidentate chelating ligand and terminal monodentate ligand in mono- and bis-complexes respectively.     

Synthesis and Structure of a Series of New Molybdenum(VI) Complexes with the Esters of 2‐Mercaptonicotinic Acid

The novel dioxomolybdenum(VI) complexes with methyl ( 1 ), ethyl ( 2 ), n‐propyl ( 3 ), i‐propyl ( 4 ), n‐butyl ( 5 ) and cyclohexyl ( 6 ) ester of 2‐mercaptonicotinic acid have been prepared in the reactions of MoO₂Cl₂ and MoO₂(acac)₂ (acac = 2,4‐pentandionate) with mercaptonicotinic acid in corresponding alcohol. The esterification reaction was catalyzed by Mo^V originated from the reduction of Mo^VI with mercaptonicotinic ‐SH group with simultaneous formation of S–S bond resulting from the condensation of two 2‐mercaptonicotinic molecules. The presence of Mo^V was proved by ESR spectra. The molecular and crystal structures of 1 , 2 , 3 and 4 as well as of the by‐products 1,1′‐dithio‐2,2′‐n‐butylnicotinoate ( 7 ) and tetramethylammonium hexachloromolybdate(V) ( 8 ) have been determined by a X‐ray single crystal diffraction. The complexes 1 – 4 contain MoO₂²⁺ core with octahedral coordination of each molybdenum atom complexed by two 2‐mercaptonicotinato N and S donor atoms.     

Phenoxy Derivatives of Dicyclopentadienyl Tungsten(VI) Oxydichloride and Bisindenyl Tungsten (VI) Oxydichloride

Phenoxy complexes containing π-bonded cyclopentadienyl and indenyl groups attached to metals ytterbium¹⁾, titanium¹⁾, and cerium¹⁾. are reported in the literature. These phenoxide derivatives were prepared by the actious of various phenols on the cyclopentadienyl or indenyl metal halide complexes in benzene or tetrahydrofuran. There is no mention the literature about the prepartion and characterization of phenolic derivatives of π-cyclopentadienyl and π-indenyl oxotungsten (VI). The effect of electron releasing or electron attracting substiuents in the phenols on the nature of the complexes and the metal-phenoxide bond has not been reported. The possibility of formation of hydrogen bond between hydrogen atom of the free hydroxyl group of a polyhydric phenoxide group and the oxygen atom attached to the metal atom has not also been explored. This communication deals with such an attempt and reports the synthesis and characterization of 26 phenoxy derivatives of dicyclopentadienyl and bisindenyltungsten (VI) oxydichlorides and conclusions derived from their infrared spectral study.     

Synthese und Charakterisierung von Tetralithiumpentaoxoselenat(VI)

Synthesis and Characterization of Tetralithiumpentaoxoselenate(VI) Pure Li₄SeO₅ was prepared by solid state reaction at 500 °C from a mixture of Li₂O and Li₂SeO₄ in silver crucibles. The crystal structure was solved and refined with x‐ray powder methods (profile matching, C2/c, a = 873.3(1), b = 572.5(1), c = 783.6(1) pm, β = 98.29(1)°, R_p = 0.052, R_wp = 0.066). Li₄SeO₅ contains novel SeO₅^4– anions, which form slightly distorted trigonal bipyramids. All ions are coordinated by 5 ligands in the shape of trigonal bipyramidal polyhedra, according to the formula Li₄^[5]Se^[5]O₅^[5]. From the empirical formula and the coordinaton environments, it is clear that this is an order variant of the A^[5]B^[5] structure type, that was found in the system NaCl by global optimisation methods. The crystal structure is consistent with spectroscopic data (IR, Raman, NMR). The ionic conductivity (σ = 3.34 10^–5 Ω^–1 cm^–1 at 340 °C) of the compound was determined with impedance measurements.     

Affinity,Speciation, and Molecular Features of Copper(II) Complexes with a Prion Tetraoctarepeat Domain in Aqueous Solution: Insights into Old and New Results

Characterization of the copper(II) complexes formed with the tetraoctarepeat peptide at low and high metal‐to‐ligand ratios and in a large pH range, would provide a breakthrough in the interpretation of biological relevance of the different metal complexes of copper(II)‐tetraoctarepeat system. In the present work, the potentiometric, UV/Vis, circular dichroism (CD), and electron paramagnetic resonance (EPR) studies were carried out on copper(II) complexes with a PEG‐ylated derivative of the tetraoctarepeats peptide sequence (Ac‐PEG₂₇‐(PHGGGWGQ)₄‐NH₂) and the peptide Ac‐(PHGGGWGQ)₂‐NH₂. Conjugation of tetraoctarepeat peptide sequence with polyethyleneglycol improved the solubility of the copper(II) complexes. The results enable a straightforward explanation of the conflicting results originated from the underestimation of all metal–ligand equilibria and the ensuing speciation. A complete and reliable speciation is therefore obtained with the released affinity and binding details of the main complexes species formed in aqueous solution. The results contribute to clarify the discrepancies of several studies in which the authors ascribe the redox activity of copper(II)‐tetraoctarepeat system considering only the average effects of several coexisting species with very different stoichiometries and binding modes.