The synthesis and X-ray structural characterization of mer and fac isomers of the technetium(I) nitrosyl complex [TcCl2(NO)(PNPpr)]

The nitrosyl complex H[TcNOCl₄] reacts with the tridentate ligand bis[(2-diphenylphosphino)propyl]amine (PNPpr) to yield a mixture of the mer or fac isomers of [TcCl₂(NO)(PNPpr)]. In acetonitrile, where the ligand is freely soluble, reaction occurs at room temperature to yield mostly the mer isomer with the linear nitrosyl ligand cis to the amine ligand; and the phosphine ligands arranged in a mutually trans orientation. The reaction in methanol requires reflux to dissolve the lipophilic ligand and generates the fac isomer of [TcCl₂(NO)(PNPpr)] as the major product, with the tridentate ligand in a facial arrangement, leaving the chlorides and nitrosyl ligand in the remaining facial sites. The steric bulk of the tridentate ligand’s diphenylphophino-moieties results in a significant distortion from octahedral geometry, with the P-Tc-P bond angle expanded to 99.48(4)°.The infrared spectra display absorptions from these nitrosyl ligands in the 1700 and 1800 cm⁻¹ regions for the fac and mer isomers, respectively. The ESI(+) mass spectra each display the parent ion at 647 m/z.     

Cis-trans isomerism in cobalt(II) complexes with 3-hydroxypicolinic acid. Structural, DFT and thermal studies

Pyridine and 4-picoline cobalt(II) complexes with 3-hydroxypicolinic acid, [Co(3-OHpic)₂(py)₂], (2), and [Co(3-OHpic)₂(4-pic)₂], (3), were prepared, their molecular and crystal structures were determined by X-ray structure analysis and their thermal stability by TGA/DTA methods. Complex 2 appears only as trans isomer and 3 as cis isomer. Based on DFT calculations, the most significant effect on orientation of (un)substituted ligands around cobalt, i.e.cis-trans isomerism, comes from crystal packing. Theoretical calculations show that exchange of methyl group in pyridine does not affect relative stability of one monomer unit, i.e.cis isomer is for about 1 kcal mol⁻¹ more stable than trans isomer. Hydrogen bonds of the O-H···O type are present only as intramolecular ones in the crystal structures of 2 and 3, while intermolecular C-H···O hydrogen bonds and π-π stacking interactions (π-π interactions present only in 3) assemble molecules in 3D architecture. Interactions between two monomer units in crystal packing could be separated and theoretically investigated to calculate interaction energy. In our case, both non-hypothetical models, i.e.trans isomer of 2 and cis isomer of 3, show more favorable interaction energies than hypothetical ones, i.e.cis isomer of 2 and trans isomer of 3, for the same type of interaction.     

The solution structure of bis(acetylacetonato)oxovanadium(IV)

The behaviour of the bis-chelated oxovanadium(IV) complexes formed by acetylacetone (acac) and five of its derivatives was re-examined through a combination of spectroscopic methods in different solvents. It has been found that the complexes are penta-coordinated with a geometry close to the square pyramid and maintain in solution the same structure as in the solid state. The results rule out a cis-trans isomerism of the species VOL₂S (L = acac or derivative, S = solvent) in solution. Depending on the coordinating ability of the solvent a sixth molecule can be bound, more or less strongly, to the free axial position of the complexes. The combined application of the electronic absorption and IR spectroscopies allows to establish if the complexes in solution are penta- or hexa-coordinated.     

Synthesis and study of Pt(II)-aromatic amines complexes of the types cis- and trans-Pt(amine)2(NO3)2 and cis-Pt(amine)2(R(COO)2)

Complexes of the types cis- and trans-Pt(amine)₂(NO₃)₂ with amines containing a phenyl group were synthesized and studied mainly by IR and multinuclear magnetic resonance spectroscopies. The cis complexes could be synthesized pure only with the amines of the type Ph-R-NH₂ (R = alkyl), while pure trans compounds were synthesized with all the studied amines. In ¹⁹⁵Pt NMR spectroscopy, the dinitrato complexes of the amines Ph-R-NH₂ were observed around −1700 ppm for the cis isomers and at about −1580 for the trans complexes. For the other amines, where a phenyl ring is directly attached to the amino group, the signals were observed at lower fields, −1528 ppm for cis-Pt(PhNH₂)(NO₃)₂ and around −1450 ppm for all the trans isomers. There is a linear relationship between the δ(Pt) of the Pt(amine)₂(NO₃)₂ complexes and the pK_a of the protonated amines. The coupling constants ²J(¹⁹⁵Pt-¹HN) are larger in the cis compounds (ave. 76 Hz) than in the trans isomers (ave. 63 Hz). The complexes cis-Pt(amine)₂(R(COO)₂) with bidentate dicarboxylato ligands were also synthesized and characterized mainly by IR spectroscopy. The compounds apparently decompose in DMF and are too insoluble in other solvents for solution studies.     

Nano-sized titanium(IV) ternary and quaternary complexes with electron-rich oxygen-based bidentate ligands

Some novel ternary and quaternary complexes of titanium(IV) of general formula [Ti(acac)Cl_3−n(OOCR)_n] (R = C₁₅H₃₁ or C₁₇H₃₅ and n = 1-3) have been synthesized by stepwise substitution of chloride ions of [Ti(acac)Cl₃] by straight chain carboxylic acid anions. The complexes are characterized by their elemental analyses, spectral (infrared, FAB mass, ¹H NMR and powder XRD) studies, molecular weight determination and molar conductance measurements. Infrared spectra suggested bidentate chelating nature of both acetylacetonate and carboxylate anions in the complexes. Monomeric nature of the complexes was confirmed by their molecular weight determination and FAB mass spectra. Molar conductance values indicated the complexes to be non-electrolytes in DMF. The complexes exhibited high resistance to hydrolysis. Their powder XRD data indicated the nano-size for the complexes. The coordination number of titanium(IV) in these complexes were found to be six, seven and eight which has been discussed in detail.     

Computational study on the mechanisms of action of the potential anticancer drug trans-isopropylaminedimethylaminedichloroplatinum (trans-IPADMADP) and its cis isomer with DNA purine bases

The monofunctional and bifunctional bindings of the potential anticancer drug trans-isopropylaminedimethylaminedichloroplatinum (trans-IPADMADP) and its cis isomer to purine base in DNA are explored by using density functional theory and IEF-PCM solvation models. The computed lowest free energy barrier in the aqueous solution is 14.0/11.6 kcal/mol (from trans-Pt-chloroaqua complex to trans-/cis-monoadduct) for guanine(G), and 11.7/13.3 kcal/mol (from trans-Pt-chloroaqua complex to trans-/cis-monoadduct) for adenine(A). Our calculations demonstrate that the trans reactant complexes (or isolated reactants) can generate trans- or cis-monoadducts via similar trigonal bipyramidal transition state structures, suggesting that the monoadducts can subsequently close to form the bifunctional intrastrand Pt-DNA adducts and simultaneously distort DNA in the similar way as cisplatin. Our calculations show that Pt(isopropylamine)(dimethylamine)G₂²⁺ head-to-head path has the lowest free energy of activation at 17.6 kcal/mol, closely followed by the Pt(isopropylamine)(dimethylamine)GA²⁺ head-to-head path at 19.6 kcal/mol when the monofunctional cis-Pt-G complex serves as the reactant; while the Pt(isopropylamine)(dimethylamine)G₂²⁺ head-to-tail adduct has the lowest barrier of 20.5 kcal/mol, closely followed by the Pt(isopropylamine)(dimethylamine)GA²⁺ head-to-tail adduct at 23.0 kcal/mol if the monofunctional trans-Pt-G complex is the reactant.The calculated relatively lower activation energy barrier than that of cisplatin theoretically confirm that trans-[PtCl₂(isopropylamine)(dimethylamine)] is a potential anticancer drug as described by experiment.     

Bis(acetylacetonato)ruthenium(II) complexes containing bulky tertiary phosphines. Formation and redox behaviour of Ru(acac)2 (PR3) (R = Pr, Cy) complexes with ethene, carbon monoxide, and bridging dinitrogen

Reaction of cis-[Ru(acac)₂(η²-C₈H₁₄)₂] (1) (acac = acetylacetonato) with two equivalents of PⁱPr₃ in THF at −25 °C gives trans-[Ru(acac)₂(PⁱPr₃)₂], trans-3, which rapidly isomerizes to cis-3 at room temperature. The poorly soluble complex [Ru(acac)₂(PCy₃)₂] (4), which is isolated similarly from cis-[Ru(acac)₂(η²-C₂H₄)₂] (2) and PCy₃, appears to exist in the cis-configuration in solution according to NMR data, although an X-ray diffraction study of a single crystal shows the presence of trans-4. In benzene or toluene 2 reacts with PⁱPr₃ or PCy₃ to give exclusively cis-[Ru(acac)₂(η²-C₂H₄)(L)] [L = PⁱPr₃ (5), PCy₃ (6)], whereas in THF species believed to be either square pyramidal [Ru(acac)₂L], with apical L, or the corresponding THF adducts, can be detected by ³¹P NMR spectroscopy. Complexes 3-6 react with CO (1 bar) giving trans-[Ru(acac)₂(CO)(L)] [L = PⁱPr₃ (trans-8), PCy₃ (trans-9)], which are converted irreversibly into the cis-isomers in refluxing benzene. Complex 5 scavenges traces of dinitrogen from industrial grade dihydrogen giving a bridging dinitrogen complex, cis-[{Ru(acac)₂(PⁱPr₃)} ₂(μ-N₂)] (10). The structures of cis-3, trans-4, 5, 6 and 10 · C₆H₁₄ have been determined by single-crystal X-ray diffraction. Complexes trans- and cis-3, 5, 6, cis-8, and trans- and cis-9 each show fully reversible one-electron oxidation by cyclic voltammetry in CH₂Cl₂ at −50 °C with E_1/2(Ru^3+/2+) values spanning −0.14 to +0.92 V (versus Ag/AgCl), whereas for the vinylidene complexes [Ru(acac)₂ (CCHR)(PⁱPr₃)] [R = SiMe₃ (11), Ph (12)] the process is irreversible at potentials of +0.75 and +0.62 V, respectively. The trend in potentials reflects the order of expected π-acceptor ability of the ligands: PⁱPr₃, PCy₃ <C ₂H₄ < CCHR < CO. The UV-Vis spectrum of the thermally unstable, electrogenerated Ru^III-ethene cation 6⁺ has been observed at −50 °C. Cyclic voltammetry of the μ-dinitrogen complex 10 shows two, fully reversible processes in CH₂Cl₂ at −50 °C at +0.30 and +0.90 V (versus Ag/AgCl) corresponding to the formation of 10⁺ (Ru^II,III) and 10²⁺ (Ru^III,III). The former, generated electrochemically at −50 °C, shows a band in the near IR at ca. 8900 cm⁻¹ (w_1/2 ca. 3700 cm⁻¹) consistent with the presence of a valence delocalized system. The comproportionation constant for the equilibrium 10 + 10²⁺ ? 2 10⁺ at 223 K is estimated as 10^13.6.     

Quantification of cis and trans influences in [PtX(PPh3)3] complexes. A P NMR study

In [PtX(PPh₃)₃]⁺ complexes (X = F, Cl, Br, I, AcO, NO₃, NO₂, H, Me) the mutual cis and trans influences of the PPh₃ groups can be considered constants in the first place, therefore the one bond Pt-P coupling constants of P_(cis) and P_(trans) reflect the cis and trans influences of X. The compounds [PtBr(PPh₃)₃](BF₄) (2), [PtI(PPh₃)₃](BF₄) (3), [Pt(AcO)(PPh₃)₃](BF₄) (4), [Pt(NO₃)(PPh₃)₃](BF₄) (5), and the two isomers [Pt(NO₂-O)(PPh₃)₃](BF₄) (6a) and [Pt(NO₂-N)(PPh₃)₃](BF₄) (6b) have been newly synthesised and the crystal structures of 2 and 4·CH₂Cl₂·0.25C₃H₆O have been determined. From the ¹J_PtP values of all compounds we have deduced the series: I > Br > Cl > NO₃ > ONO > F > AcO > NO₂ > H > Me (cis influence) and Me > H > NO₂ > AcO > I > ONO > Br > Cl > F > NO₃ (trans influence). These sequences are like those obtained for the (neutral) cis- and trans-[PtClX(PPh₃)₂] derivatives, showing that there is no dependence on the charge of the complexes. On the contrary, the weights of both influences, relative to those of X = Cl, were found to depend on the charge and nature of the complex.     

Modulation of trans-to-cis photoisomerization and photoluminescence of 1,2-bis(4-pyridyl)ethylene or 4-styrylpyridine coordinated to fac-tricarbonyl(5-chloro-1,10-phenathroline)rhenium(I)

Photochemical and photophysical properties of fac-[Re(CO)₃(Clphen)(trans-L)]⁺ complexes, Clphen = 5-chloro-1,10-phenathroline and L = 1,2-bis(4-pyridyl)ethylene, bpe, or 4-styrylpyridine, stpy, were investigated to complement the understanding of intramolecular energy transfer process in tricarbonyl rhenium(I) complexes having an electron withdrawing group attached to polypyridyl ligands. These new compounds were synthesized, characterized and the photoisomerization quantum yields were accurately determined by ¹H NMR spectroscopy. The true quantum yields for fac-[Re(CO)₃(Clphen)(trans-bpe)]⁺ were constant (Φ = 0.55) at all investigated irradiation wavelengths. However, for fac-[Re(CO)₃(Clphen)(trans-stpy)]⁺, similar true quantum yields were observed only at higher energy irradiation (Φ_{313 nm} = 0.53 and Φ_{365 nm} = 0.57), but it decreased significantly at 404 nm (Φ = 0.41). These results indicated different deactivation pathways for the trans-stpy complex photoisomerization. Quantum yields decreased as the ³IL_trans-L and ³MLCT_Re→NN excited states become closer and the behavior was discussed in terms of the excited state energy gaps. Additionally, luminescence properties of photoproducts, fac-[Re(CO)₃(Clphen)(cis-L)]⁺, were also investigated in different environments to analyze the relative energy of the ³MLCT_Re→Clphen excited state for each compound.     

Transmembrane localization of cis-isomers of zeaxanthin in the host dimyristoylphosphatidylcholine bilayer membrane

The effects of the 9-cis and 13-cis isomers of zeaxanthin on the molecular organization and dynamics of dimyristoylphosphatidylcholine (DMPC) membranes were investigated using conventional and saturation recovery EPR observations of the 1-palmitoyl-2-(14-doxylstearoyl)phosphatidylcholine (14-PC) spin label. The results were compared with the effects caused by the all-trans isomer of zeaxanthin. Effects on membrane fluidity, order, hydrophobicity, and the oxygen transport parameter were monitored at the center of the fluid phase DMPC membrane. The local diffusion-solubility product of oxygen molecules (oxygen transport parameter) in the membrane center, studied by saturation-recovery EPR, decreased by 47% and 27% by including 10 mol% 13-cis and 9-cis zeaxanthin, respectively; whereas, incorporation of all-trans zeaxanthin decreased this parameter by only 11%. At a zeaxanthin-to-DMPC mole ratio of 1:9, all investigated isomers decreased the membrane fluidity and increased the alkyl chain order in the membrane center. They also increased the hydrophobicity of the membrane interior. The effects of these isomers of zeaxanthin on the membrane properties mentioned above increase as: all-trans < 9-cis ≤ 13-cis. Obtained results suggest that the investigated cis-isomers of zeaxanthin, similar to the all-trans isomer, are located in the membrane interior, adopting transmembrane orientation with the polar terminal hydroxyl groups located in the opposite leaflets of the bilayer. However, the existence of the second pool of cis-zeaxanthin molecules located in the one leaflet and anchored by the terminal hydroxyl groups in the same polar headgroup region cannot be completely ruled out.     

Structure and reactivity of [Ru(2,3-Medpp)2Cl2]

The structure and reactivity of the complex [Ru(2,3-Medpp)₂Cl₂](PF₆)₂ (2,3-Medpp⁺=2-[2-(1-methylpyridiniumyl)]-3-(2-pyridyl)pyrazine) was investigated by X-ray diffraction (XRD), ¹H NMR, redox, and UV-Vis absorption measurements. X-ray analysis shows that crystals obtained from an acetonitrile-toluene solution contain the trans-Cl₂, trans-pyrazine isomeric form, while ¹H NMR and redox measurements on the main product of the synthetic workup indicate the presence of the trans-Cl₂, cis-pyrazine isomer. In the dark at 70 °C, the complex [Ru(2,3-Medpp)₂Cl₂]²⁺ reacts slowly in acetonitrile isomerizing to the cis-[Ru(2,3-Medpp)₂(CH₃CN)Cl]³⁺ species. Under ambient light in the presence of excess AgNO₃ the cis-[Ru(2,3-Medpp)₂(CH₃CN)₂]⁴⁺ species is obtained.     

Crystal structures of Pt(II) and Pd(II) complexes with the free radical 4-aminoTEMPO

Pt(II) and Pd(II) compounds containing the free radical 4-aminoTEMPO (4amTEMPO) were synthesized and characterised by X-ray diffraction methods. The disubstituted complexes cis- and trans-Pt(4amTEMPO)₂I₂ were studied. The trans isomer was prepared from the isomerisation of the cis analogue. The two Pd(II) compounds trans-Pd(4amTEMPO)₂X₂ (X = Cl and I) were also characterised by crystallographic methods. A mixed-ligand complex cis-Pt(DMSO)(4amTEMPO)Cl₂ was synthesized from the isomerisation of the trans isomer in hot water. Its crystal structure was also determined. In all the complexes, the 4amTEMPO ligand is bonded to the metal through the -NH₂ group, since the nitroxide O atom is not a good donor atom for the soft Pt(II) and Pd(II) metals. The conformation of the 4-aminoTEMPO ligand was compared to those of the few reported structures in the literature.     

H NMR spectroscopy as a tool to determine accurate photoisomerization quantum yields of stilbene-like ligands coordinated to rhenium(I) polypyridyl complexes

In this work, the use of proton nuclear magnetic resonance, ¹H NMR, was fully described as a powerful tool to follow a photoreaction and to determine accurate quantum yields, so called true quantum yields (Φ_true), when a reactant and photoproduct absorption overlap. For this, Φ_true for the trans-cis photoisomerization process were determined for rhenium(I) polypyridyl complexes, fac-[Re(CO)₃(NN)(trans-L)]⁺ (NN = 1,10-phenanthroline, phen, or 4,7-diphenyl-1,10-phenanthroline, ph₂phen, and L = 1,2-bis(4-pyridyl)ethylene, bpe, or 4-styrylpyridine, stpy). The true values determined at 365 nm irradiation (e.g. Φ_NMR = 0.80 for fac-[Re(CO)₃(phen)(trans-bpe)]⁺) were much higher than those determined by absorption spectral changes (Φ_UV-Vis = 0.39 for fac-[Re(CO)₃(phen)(trans-bpe)]⁺). Φ_NMR are more accurate in these cases due to the distinct proton signals of trans and cis-isomers, which allow the actual determination of each component concentration under given irradiation time. Nevertheless when the photoproduct or reactant contribution at the probe wavelength is negligible, one can determine Φ_true by regular absorption spectral changes. For instance, Φ_313 nm for free ligand photoisomerization determined both by absorption and ¹H NMR variation are equal within the experimental error (bpe: Φ_UV-Vis = 0.27, Φ_NMR = 0.26; stpy: Φ_UV-Vis = 0.49, Φ_NMR = 0.49). Moreover, ¹H NMR data combined with electronic spectra allowed molar absorptivity determination of difficult to isolate cis-complexes.     

Reaction of cis and trans isomers of platinum(II) diamminedichloride with purine,adenine and its derivatives in dilute solutions

Reactions of the cytostatic and antitumour agent, cis-platinum(II) diamminedichloride, and its trans isomer with purine, adenine and its N-9 derivatives were followed spectrophotometrically in dilute aqueous solutions. While the cis isomer formed with all compounds studied complexes with the metal-to-ligand ratios ML, ML₂ and M₂L, it was found that the trans isomer did not form the complexes ML₂ with adenosine and AMP. The values of dissociation constants, reaction rate constants and activation energies of the reactions of the cis and trans isomers with purine, adenine and its derivatives were in most cases comparable. Lower stability was exhibited only by the complexes of trans-platinum(II) diamminedichloride (trans-Pt(II)) with AMP as well as by its complexes M₂L with adenine and adenosine. The ability of cis-platinum(II) diamminedichloride (cis-Pt(II)) to react with two adenine residues can explain the greater tendency of the cis isomer to form intrastrand cross-links in nucleic acids as compared with the trans isomer.     

Spectral and structural studies of a new oxalato-bridged dinuclear copper(II) complex having two 3-(thiophen-2-yl)-1,10-phenanthroline ligands in a trans configuration

In this paper, spectral and structural characterizations of a new dinuclear copper(II) complex (1), formulated as [Cu₂(3-(thiophen-2-yl)-1,10-phenanthroline)₂(μ-oxalate)(DMF)₂](ClO₄)₂ (DMF = N,N′-dimethylformamide), have been described. Two five-coordinate copper(II) centers are bridged by a four-dentate oxalate dianion forming a planar molecular geometry with the Cu-Cu separation of 5.117(4) Å. The two ligands in 1 adopt a trans configuration to each other and two monodentate DMF molecules are positioned at each side of the molecular plane. In addition, typical π-π stacking interactions are found between adjacent phenanthroline and thiophene rings forming a layered packing structure. A compressed pyramidal configurational alteration is observed for each copper(II) center when the temperature is decreased from 291(2) to 100(2) K.     

Synthesis and structures of mono and binuclear nickel(II) thiolate complexes of a dicompartmental pseudo-macrocycle with N(imine)2S2 and N(oxime)2S2 metal-binding sites

The reaction of [Ni(pftp)] [pftp = N,N-propane-1,3-diyl-(6-formyl-4-methyliminatothiophenolato)] with hydroxylamine hydrochloride in the presence potassium acetate in MeOH resulted in the formation of the complex [Ni(LH₂)] [L = N,N-propane-1,3-diyl-(4-methyl-2-methyliminato-6-methyloxime-thiophenolato)] in good yield. A single crystal X-ray diffraction structural determination showed a mononuclear nickel(II) complex with the new acyclic ligand LH₂ that had been functionalised with two oxime groups containing an empty N(oxime)₂S₂ pocket to which another metal ion could be added. A further reaction of [Ni(LH₂)] with NiCl₂·6H₂O, triethylamine and ammonium hexafluorophosphate in MeOH gave a dark red product that yielded red crystals of [Ni₂(LH)]PF₆·DMF via slow recrystallisation from a DMF/PrⁱOH solvent mixture. A single crystal X-ray diffraction study of these crystals confirmed the presence of a dinuclear nickel(II) complex linked by a dithiolato-bridge. Both nickel(II) ions exhibited square-planar geometry where the metal centres are coordinated in two distinct cis-S₂N(imine)₂ and cis-S₂N(oxime)₂ binding sites provided by the new dicompartmental oxime/thiolate-containing ligand LH.     

Solvent exchange, solvent interchange, aquation and isomerisation reactions of cis- and trans-[Co(tmen)2(NCMe)2] in water, Me2SO and MeCN: kinetics and stereochemistry

The synthesis and characterisation of cis- and trans-[Co(tmen)₂(NCCH₃)₂](ClO₄)₃ are described. Solvolysis rates have been measured by both ¹H NMR spectroscopy and UV-Vis spectrophotometry in dimethyl sulfoxide at 298.2 K. The cis isomer undergoes solvolysis by consecutive first-order reactions, k₁=5.61 × 10⁻⁴ and k₂=5.35 × 10⁻⁴ s⁻¹, each with steric retention. The measured solvolysis rate (single step reaction) for the trans isomer is k=1.54 × 10⁻⁵ s⁻¹. The solvent exchange rates have been measured by ¹H NMR spectroscopy in CD₃CN at 298.2 K: k_ex(cis)=k_ct + k_cc=2.0 × 10⁻⁵ and k_ex(trans)=k_tc + k_tt=4.56 × 10⁻⁶ s⁻¹. From these data, the measured cis-trans isomerisation rate (1.71 × 10⁻⁶ s⁻¹) and equilibrium position in CH₃CN (17% trans), the steric course for substitution in the exchange processes has been determined: trans reactant - 69% trans product; cis reactant - 99% cis product. Aquation rates for cis- and trans-[Co(tmen)₂(NCCH₃)₂](ClO₄)₃ have also been determined spectrophotometrically and by NMR; k_cis=1.3 × 10⁻⁴ and k_trans=2.7 × 10⁻⁵ s⁻¹. In both cases the steric course for the primary aquation step is indeterminate because the subsequent steps are faster. Where data are available, the [Co(tmen)₂X₂]ⁿ⁺ complexes are found to be consistently much more reactive than their [Co(en)₂X₂]ⁿ⁺ analogues.     

Resonance Raman and crystallographic studies on the complex [Fe2(bbpnol)2]·2DMF (bbpnol=N,N′-bis(2-hydroxybenzyl)-2-ol-1,3-propanediamine)

The ligand N,N′-bis(2-hydroxybenzyl)-2-ol-1,3-propanediamine (H₃bbpnol) reacts with iron(III) perchlorate forming two dinuclear bis-μ-alkoxo complexes, a ‘cis-trans’ isomer (complex 1) previously reported and a ‘cis-cis’ isomer (complex 2) characterized in this work. The main differences found in complex 2 structure are, (a) the four phenolic oxygens are trans to the alkoxo bridges; (b) each ligand is shared by the two Fe(III) ions occupying two coordination positions at each center. The Fe(III) centers in the resulting centrosymmetric structure in complex 2 have a distorted-octahedral geometry with the equatorial plane occupied by the phenolic and alcoholic oxygen atoms and the apical positions are filled by the aminic nitrogen atoms. The resonance Raman (RR) spectra of these two isomeric complexes are somewhat different with the intensity of some low-frequency modes increasing in the less symmetric core. The electronic spectra of both complexes are similar, but the molar absorptivities are substantially increased in complex 2, indicating the presence of an electronic coupling between the phenolate moieties trans in relation to the alkoxo bridge, and that phenolates coordinated cis to the alkoxo bridge do not seem to contribute to LMCT oscillator strength. This is directly reflected in the Raman excitation profiles (REP) of the chromophore modes, with the vibrational modes of the ‘cis-cis’ isomer showing a greater intensification compared with the ‘cis-trans’ isomer.     

Combining coordination chemistry with hydrogen bonds: perturbation of the structures by interaction of an organotin(IV) complex with O-donor solvent molecules

We have examined the role of different solvents in the crystallisation process of cis-octahedral, diphenyltin(IV)-bis-cupferronato complex, Ph₂Sn(cupf)₂ (1), where . The Mössbauer spectra of frozen chloroform solution of 1 revealed the presence of cis and trans isomers. This cis-trans isomerisation was investigated by Mössbauer spectroscopy and the results inspired the synthesis of two new heptacoordinated derivatives: Ph₂Sn(cupf)₂(H₂O) (2) and Ph₂Sn(cupf)₂(EtOH) · EtOH (3). In both compounds, the O-donor solvent molecules (H₂O, EtOH) form novel Sn-O bonds with the Ph₂Sn(IV) centre of 1, consequently the phenyl groups attached to tin undergo an intramolecular rearrangement. Compound 2 contains O-H ? O hydrogen bonded infinite chains. In compound 3, O-H ? O hydrogen-bonds and short O ? O contacts assemble the complexes and uncoordinated solvent molecules into dimeric supramolecules. These solvents have structure-determining roles at both molecular and supramolecular levels: at molecular level the coordination of solvent determines intramolecular rearrangement by changing the conformation of the parent unsolvated complex, whilst at supramolecular level they control the association of solvated molecules via hydrogen bonds.     

ATP-triggered ADP release from the asymmetric chaperonin GroEL/GroES/ADP7 is not the rate-limiting step of the GroEL/GroES reaction cycle

The GroEL/GroES protein folding chamber is formed and dissociated by ATP binding and hydrolysis. ATP hydrolysis in the GroES-bound (cis) ring gates entry of ATP into the opposite unoccupied trans ring, which allosterically ejects cis ligands. While earlier studies suggested that hydrolysis of cis ATP is the rate-limiting step of the cycle (t_½ ∼ 10 s), a recent study suggested that ADP release from the cis ring may be rate-limiting (t_½ ∼ 15-20 s). Here we have measured ADP release using a coupled enzyme assay and observed a t_½ for release of ?4-5 s, indicating that this is not the rate-limiting step of the reaction cycle.