Co-ligand Exchange Research Articles

Mechanistic Insights into the Palladium-Catalyzed Perfluoroalkylative Carbonylation of Unactivated Alkenes to β-Perfluoroalkyl Esters: A DFT Study.

Transition metal-catalyzed multicomponent carbonylation is an efficient synthetic strategy to access multifunctional esters in high yields with broad functional group tolerance and good chemoselectivity. Considering the development of highly efficient synthetic methods for esters, it remains significant to grasp the mechanism of constructing multifunctional esters. Herein, density functional theoretical calculations were carried out to acquire mechanistic insight into the synthesis of β-perfluoroalkyl esters from a specific palladium-catalyzed perfluoroalkylative carbonylation of unactivated alkenes using carbon monoxide. A detailed mechanistic understanding of this reaction route includes (1) multistep radical reaction process, (2) C-C coupling and CO insertion, (3) ligand exchange, and (4) Pd-based intermediate oxidation and reductive elimination. The multistep radical process was fundamentally rationalized, including Rf· formation and radicals A and E from unactivated alkene and CO oxidation, respectively. The potential energy calculation indicated that the CO insertion into the perfluorinated alkyl radicals preceded Pd-catalyzed oxidation in the competitively multistep free radical reaction process. In addition, the I-/PhO- exchange step was predicted to be spontaneous to products. The IGMH analysis further attested to the reductive elimination process involved in the rate-determining step. Thus, a simple and valid density functional theory (DFT) approach was developed to reveal the multistep radical mechanism for the Pd-catalyzed perfluoroalkylative carbonylation of unactivated alkenes to access functional β-perfluoroalkyl esters.

Ligand Exchange at Carbon: Synthetic Entry to Elusive Species and Versatile Reagents.

How different is carbon compared to other elements in the periodic table? Can carbon compounds be regarded as coordination complexes with carbon as the central element undergoing a facile exchange of its ligands? Although carbon clearly plays a special role among the elements of the periodic table, recent studies have drawn parallels between the bonding situation and the reactivity of carbon compounds to transition metal complexes. This Perspective summarizes recent reports about ylidic and zwitterionic compounds that were shown to exhibit ambiguous bonding situations that can be interpreted as donor-acceptor interactions similar to the bond between a metal and a neutral ligand. Based on this conception, ligand exchange reactions prototypical of transition metal complexes were realized at carbon atoms, enabling new synthetic strategies for the synthesis of reactive species and building blocks. In particular, the exchange of N2, CO, and phosphine ligands led to the development of a mild method for accessing new compounds and reagents with unusual properties, such as vinylidene ketenes or stable ketenyl anions, that open up a diverse but still poorly explored follow-up chemistry.

Reversible Ligand Exchange in Atomically Dispersed Catalysts for Modulating the Activity and Selectivity of the Oxygen Reduction Reaction

AbstractRational control of the coordination environment of atomically dispersed catalysts is pivotal to achieve desirable catalytic reactivity. We report the reversible control of coordination structure in atomically dispersed electrocatalysts via ligand exchange reactions to reversibly modulate their reactivity for oxygen reduction reaction (ORR). The CO‐ligated atomically dispersed Rh catalyst exhibited ca. 30‐fold higher ORR activity than the NHx‐ligated catalyst, whereas the latter showed three times higher H2O2 selectivity than the former. Post‐treatments of the catalysts with CO or NH3 allowed the reversible exchange of CO and NHx ligands, which reversibly tuned oxidation state of metal centers and their ORR activity and selectivity. DFT calculations revealed that more reduced oxidation state of CO‐ligated Rh site could further stabilize the *OOH intermediate, facilitating the two‐ and four‐electron pathway ORR. The reversible ligand exchange reactions were generalized to Ir‐ and Pt‐based catalysts.

Enantiomeric Excess Dependent Splitting of NMR Signal through Dynamic Chiral Inversion and Coligand Exchange in a Coordination Complex.

Nuclear magnetic resonance (NMR) spectroscopy cannot be used to discriminate enantiomers, and NMR resonances of enantiomeric mixtures are generally not affected by enantiomeric excess (ee). Here, we report that a coordination complex (L·2Zn·3C), where L is a salen-like prochiral ligand and C is an exchangeable acetate coligand, exhibits symmetrical splitting of one of the 1H NMR resonances of L with the degree of splitting linearly proportional to ee of the chiral guest coligand C, 2-phenoxypropionic acid. Despite the well-defined chirality in the crystal structure of L·2Zn·3C, concurrent fast chiral inversion and coligand exchange in solution renders L·2Zn·3C the primary example of prochiral solvating agent (pro-CSA) based on a coordination complex. Notably, the NMR resonances remain split even in dilute solution due to the lack of chiral guest dissociation in the coligand exchange system. This work provides new insights into chiral transfer events in metal-ligand complexes.

Comparative equilibrium and structural studies of new pentamethylcyclopentadienyl rhodium complexes bearing (O,N) donor bidentate ligands

Complex formation processes of the (O,N) donor ligands 6-methylpicolinic acid (6-Mepic), quinoline-2-carboxylic acid (2-QA) and 3-isoquinolinecarboxylic acid (3-iQA) with the organometallic moiety (η5-pentamethylcyclopentadienyl)rhodium(III) (RhCp*) were studied in aqueous solution by the combined use of pH-potentiometry, 1H NMR spectroscopy and UV-Vis spectrophotometry. The solid phase structures of the [RhCp*(L)Cl] complexes bearing 6-Mepic and 2-QA were characterized by single-crystal X-ray diffraction analysis. Studies revealed the exclusive formation of mono complexes of the form [RhCp*(L)(H2O)]+ (L = deprotonated form of the ligands) and [RhCp*(L)(OH)]. The positively charged aqua species predominate at physiological pH even in the micromolar concentration range. The H2O/Cl− co-ligand exchange constants showed that all complexes preferably retain the chlorido ligand at the third coordination site at chloride ion concentrations present in the serum. In addition in vitro cytotoxicity of these [RhCp*(L)Cl] complexes was evaluated in three human cancer cell lines (A549, SW480 and CH1/PA-1) where they showed minor cytotoxic potency.

Post-synthetic modification of divalent nickel acetate cubanes with carboxylates

Starting from nickel(II) cubane [NiII4(HL)4(OAc)4] (2a), five different tetranuclear cubic complexes [NiII4(HL)4(O2CR)4] (3) were generated simply by applying a post-synthetic cubane modification strategy via a complete acetate-to-carboxylate co-ligand exchange. This was achieved by stirring the parent complex 2a with a large excess of the corresponding sodium carboxylate salt 4 for three days in THF. Single-crystal X-ray structure analyses of NiII cubanes 3a·2 Et2O and 3b·0.5 CH2Cl2·0.5 Et2O reveal unequivocally that both are isostructural in terms of the [Ni4(μ3-O4]4+ core and crystallize in the triclinic space group P-1 with two molecules in the unit cell. In the solid state, the cubic centers of 3a,b differ only slightly in bond lengths and angles mainly due to the different substituents in the carboxylate co-ligand and packing in the unit cell. The orientation of 3a,b and the formation of supramolecular aggregates in the crystal packing were controlled by π–π interactions as well as intra- and intermolecular O–H⋯O hydrogen bonds. Variable-temperature magnetic susceptibility measurements reveal that 3a–c show a ferrimagnetic coupling scheme that leads to a diamagnetic ground state for all investigated complexes.

Rational in situ tuning of a supramolecular photocatalyst for hydrogen evolution

In situ exchange of co-ligands in a molecular photocatalyst significantly boosts light driven hydrogen formation.

η6-(2-phenoxyethanol) ruthenium(II)-complexes of 2,2′-bipyridine and its derivatives: Solution speciation and kinetic behaviour

A novel family of RuII-arene compounds with the general formula of [RuII(η6-(2-phenoxyethanol))(L)Cl]+ (L: 2,2′-bipyridine (bpy) (3), 4,4′-dimethyl-2,2′-bipyridine (4) and 4,4′-diyldimethanol-2,2′-bipyridine (5)) was synthesized and characterized by standard spectroscopic and analytical methods. Complex 3 was further studied by single-crystal X-ray diffraction analysis, showing a pseudo octahedral geometry and strong π-π lateral stacking interactions in the crystal packing. Effect of the substituents on the electrochemical properties and on the aqueous solution stability was monitored by cyclic voltammetry, UV–Vis and 1H NMR spectroscopy. Complexes 3–5 presented multiple irreversible redox processes according to their cyclic voltammograms recorded in acetonitrile, and their RuII → RuIII oxidation peaks were found at ca. +1.6 V. Hydrolysis of the binuclear [RuII(η6-(2-phenoxyethanol))(μ2-Cl)Cl]2 precursor (1) resulted in binuclear hydroxido bridged species [(RuII(η6-(2-phenoxyethanol)))2(μ-OH)3]+ and [(RuII(η6-(2-phenoxyethanol)))2(μ-OH)2Z2] (Z = H2O/Cl−) in the presence of chloride ions in water. The hydrolytic behaviour of this RuII precursor is similar to that of the analogous species [RuII(η6-p-cymene)(μ2-Cl)Cl]2 regarding the hydrolysis products and their stability constants. Formation of complexes 3–5 by reaction of the RuII precursor with the (N,N) bidentate ligands was found to be relatively slow in aqueous solution. The complexation is complete already at pH 1 due to the formation of [RuII(η6-(2-phenoxyethanol))(L)Z] complexes of significantly high stability in all cases, which are predominant species up to pH 6. However, besides the formation of the mixed hydroxido species [RuII(η6-(2-phenoxyethanol))(L)(OH)]+ at neutral and basic pH values, the slow oxidation of the RuII centre takes place as well leading to the partial loss of the arene moiety. The rate of these processes depends on the pH and its maximum was found at pH 8–9. Additionally the chlorido/aqua co-ligand exchange processes of the [RuII(η6-(2-phenoxyethanol))(L)Cl]+ species were also monitored and only ∼5% of the chlorido ligand was found to be replaced by water in 0.1 M chloride ion containing aqueous solutions at pH 5.

Comparative solution equilibrium studies on pentamethylcyclopentadienyl rhodium complexes of 2,2ʹ-bipyridine and ethylenediamine and their interaction with human serum albumin

Complex formation equilibrium processes of the (N,N) donor containing 2,2ʹ-bipyridine (bpy) and ethylenediamine (en) with (η5-pentamethylcyclopentadienyl)rhodium(III) were investigated in aqueous solution via pH-potentiometry, 1H NMR spectroscopy, and UV–vis spectrophotometry in the absence and presence of chloride ions. The structure of [RhCp*(en)Cl]ClO4 (Cp*, pentamethylcyclopentadienyl) was also studied by single-crystal X-ray diffraction. pKa values of 8.56 and 9.58 were determined for [RhCp*(bpy)(H2O)]2+ and [RhCp*(en)(H2O)]2+, respectively resulting in the formation of negligible amount of mixed hydroxido complexes at pH 7.4. Stability and the H2O/Cl− co-ligand exchange constants of bpy and en complexes considerably exceed those of the bidentate O-donor deferiprone. The strong affinity of the bpy and en complexes to chloride ions most probably contributes to their low antiproliferative effect. Interactions between human serum albumin (HSA) and [RhCp*(H2O)3]2+, its complexes formed with deferiprone, bpy and en were also monitored by 1H NMR spectroscopy, ultrafiltration/UV–vis and spectrofluorometry. Numerous binding sites (≥8) are available for [RhCp*(H2O)3]2+; and the interaction takes place most probably via covalent bonds through the imidazole nitrogen of His. According to the various fluorescence studies [RhCp*(H2O)3]2+ binds on sites I and II, and coordination of surface side chain donor atoms of the protein is also feasible. The binding of the bpy and en complex is weaker and slower compared to that of [RhCp*(H2O)3]2+, and formation of ternary HSA-RhCp*-ligand adducts was proved. In the case of the deferiprone complex, the RhCp* fragment is cleaved off when HSA is loaded with low equivalents of the compound.

Synthesis and Characterization of Molybdenum(0) and Tungsten(0) Complexes of Tetramethylthiourea: Single-Source Precursors for MoS2 and WS2

The molybdenum(0) and tungsten(0) carbonyl complexes, [M(CO)n(TMTU)6–n] (M = Mo and W; TMTU = tetramethylthiourea; n = 4 and 5), have been prepared by addition of TMTU to the appropriate carbonyl precursors. In the solid state, the compounds are monomeric and feature octahedral coordination geometries about the metal. In the case of [M(CO)4(TMTU)2], the TMTU ligands are bound in a cis fashion. Dissolution of cis-[M(CO)4(TMTU)2] produces an equilibrium mixture of cis-[M(CO)4(TMTU)2], free TMTU, and the putative dimeric species, cis,cis-[M2(CO)8(μ-TMTU)2]. Attempts to prepare the tricarbonyl complex, [Mo(CO)3(TMTU)3], by addition of TMTU to fac-[Mo(CO)3(η6-cycloheptatriene)] results in isolation of cis-[Mo(CO)4(TMTU)2], demonstrating that exchange of CO and TMTU ligands is facile in these compounds. Each of the compounds decomposes readily in the solid state above 110 °C, ultimately forming the tetravalent metal disulfide. Thermolysis reactions of [M(CO)n(TMTU)6–n] complexes at 300 °C under an argon atmosphere produces amorphous MS2, establishing the utility of these compounds as precursors to molybdenum and tungsten disulfide materials under relatively mild conditions.

Antitumor pentamethylcyclopentadienyl rhodium complexes of maltol and allomaltol: Synthesis, solution speciation and bioactivity

The reaction of the dimer [RhIII(pentamethylcyclopentadienyl)(μ-Cl)Cl]2 ([RhIII(Cp*)(μ-Cl)Cl]2) with the hydroxypyrone ligands maltol and allomaltol affords complexes of the general formula [RhIII(Cp*)(L)Cl] under standard and microwave conditions. The organometallic compounds were characterized by standard analytical methods and in the case of the allomaltol derivative in the solid state by single-crystal X-ray diffraction analysis. The complexes showed similar cytotoxicity profiles and were proved to be moderately active against various human cancer cell lines. The stoichiometry and stability of these complexes were determined in aqueous solution by pH-potentiometry, 1H NMR spectroscopy and UV–visible spectrophotometry. Speciation was studied in the presence and in the absence of chloride ions. Hydrolysis of [RhIII(Cp*)(H2O)3]2+ gave dimeric mixed hydroxido species [(RhIII(Cp*))2(μ-OH)3]+ and [(RhIII(Cp*))2(μ-OH)2Z2] (Z=H2O/Cl−). Formation of the mononuclear complexes [RhIII(Cp*)(L)Z] of maltol and allomaltol with similar and moderate stability was found. These species predominate at physiological pH and decompose only partially at micromolar concentrations. In addition, hydrolysis of the aqua complex or a chlorido/hydroxido co-ligand exchange resulted in the formation of the mixed-hydroxido species [RhIII(Cp*)(L)(OH)] in the basic pH range. Replacement of the chlorido by an aqua ligand in the complex [RhIII(Cp*)(L)Cl] was monitored and with the help of the equilibrium constants the extent of aquation at various chloride concentrations of the extra- and intracellular milieu can be predicted. Complexation of these RhIII complexes was compared to analogous [RuII(η6-p-cymene)] species and higher conditional stabilities were found in the case of the RhIII compounds at pH7.4.

Solution equilibrium studies of anticancer ruthenium(II)-η6-p-cymene complexes of pyridinecarboxylic acids

Stoichiometry and stability of antitumor ruthenium(II)-η6-p-cymene complexes of picolinic acid and its 6-methyl and 6-carboxylic acid derivatives were determined by pH-potentiometry, 1H NMR spectroscopy and UV–Vis spectrophotometry in aqueous solution in the presence or absence of coordinating chloride ions. The picolinates form exclusively mono-ligand complexes in which they can coordinate via the bidentate (O,N) mode and a chloride or a water molecule is found at the third binding site of the ruthenium(II)-η6-p-cymene moiety depending on the conditions. [Ru(η6-p-cymene)(L)(H2O/Cl)] species are predominant at physiological pH in all studied cases. Hydrolysis of the aqua complex or the chlorido/hydroxido co-ligand exchange results in the formation of the mixed-hydroxido species [Ru(η6-p-cymene)(L)(OH)] in the basic pH range. There is no indication for the decomposition of the mono-ligand complexes during 24h in the ruthenium(II)-η6-p-cymene-picolinic acid system between pH 3 and 11; however, a slight dissociation with a low reaction rate was found in the other two systems leading to the appearance of the dinuclear trihydroxido-bridged species [Ru2(η6-p-cymene)2(OH)3]+ and free ligands at pH>10. The replacement of the chlorido by an aqua ligand in [Ru(η6-p-cymene)(L)Cl] was also monitored and equilibrium constants for the exchange process were determined.

Solution equilibrium studies on anticancer ruthenium(II)–η6-p-cymene complexes of 3-hydroxy-2(1H)-pyridones

RuII(η6-p-cymene) complexes of bidentate (O,O) alkoxycarbonylmethyl-3-hydroxy-2(1H)-pyridone ligands exhibit in vitro antitumor activity. We determined the stoichiometry and stability in aqueous solution of two examples by pH-potentiometry, 1H NMR spectroscopy and UV–vis spectrophotometry and also characterized the proton dissociation processes of the ligands. Formation of mono-ligand complexes with moderate stability was found to predominate in the physiological pH range. Moreover, the chlorido/aqua co-ligand exchange processes of the [RuII(η6-p-cymene)(L)(H2O)]+ species were also monitored and 55–65% of the aqua ligand was found to be replaced by chloride in 0.2 M KCl-containing aqueous solutions. Under basic conditions, the complexes decompose to dinuclear tri-hydroxido-bridged [Ru2(η6-p-cymene)2(OH)3]+ and metal-free ligand and also a hydroxido species [RuII(η6-p-cymene)(L)(OH)] was found. Furthermore, the ligands contain an ester functional group, which may hydrolyze at basic pH, which is however negligible at acidic or neutral pH.

“In-house” preparation of 99mTc-EDDA/HYNIC-TOC, a specific targeting agent for somatostatin receptor scintigraphy

The use of radiolabeled peptide ligands as diagnostics and therapeutics in nuclear oncology has increased recently. One of the most frequently used radiopharmaceutical is 99mTc-EDDA/HYNIC-TOC, a somatostatin analog with affinity for certain types of somatostatin receptors, overexpressed in tumors of neuroendocrine origin. The radiopharmaceutical is not readily available; therefore we introduced its “in house” preparation within project activities supported by the International Atomic Energy Agency (IAEA). We optimized the radiolabeling protocol, prepared a small batch of frozen kits, performed ITLC quality control and animal biodistribution during the preclinical evaluation procedures. The co-ligand exchange labeling procedure was carried out at 100°C during 10 min, resulting in radiochemical purity >90%. The biodistribution scintigrams in normal Wistar rats showed rapid blood clearance after 15 min and predominant kidney accumulation after 4 h, in accordance with the data reported by other authors. Storage stability of the formulated small batch frozen kit (-20°C) was evaluated within 6 months, with radiolabeling yield ranging between 94,3% and 96,9%. We conclude that frozen kit can be a safe alternative to the freeze-dried for small batch in house production, and after the satisfactory preclinical evaluation, the “in house” prepared 99mTc-EDDA/ HYNIC-TOC can be introduced in clinical practice as specific targeting agent for somatostatin receptor scintigraphy.

Determining Transition-State Geometries in Liquids Using 2D-IR

Many properties of chemical reactions are determined by the transition state connecting reactant and product, yet it is difficult to directly obtain any information about these short-lived structures in liquids. We show that two-dimensional infrared (2D-IR) spectroscopy can provide direct information about transition states by tracking the transformation of vibrational modes as a molecule crossed a transition state. We successfully monitored a simple chemical reaction, the fluxional rearrangement of Fe(CO)5, in which the exchange of axial and equatorial CO ligands causes an exchange of vibrational energy between the normal modes of the molecule. This energy transfer provides direct evidence regarding the time scale, transition state, and mechanism of the reaction.

Synthesis and structure of two tetranuclear osmium carbonyl isotopomers: a crystallographic isotope effect

A synthetic route to [Os4(mu-H)(mu-OH)(mu-CO)(CO)12] ( 1) has been devised through the activation of [Os4(CO)14] with Me3NO. The pyrolysis and photolysis of the reactant in the presence of a trace amount of water produces 1 in low yield. The solid-state structure of [Os4(mu-H)(mu-OH)(mu-CO)(CO)12 x H2O] (1 x H2O) reveals a butterfly Os4 skeleton with bridging H, OH and CO ligands as well as hydrogen-bonded molecules of water in the crystal lattice. A low-temperature 13C{1H} NMR spectroscopic study revealed a merry-go-round exchange of CO ligands around the Os3 plane containing the asymmetric bridging CO. The exposure of 1 x H2O to D2O yielded [Os4(mu-H)(mu-OD)(mu-CO)(CO)12]2. Although the solid-state, intramolecular structure of 2 closely matched that of 1 x H2O, the intermolecular structure did not: its crystal lattice contained no water of crystallization, a previously unreported crystallographic isotope effect.

The active site of the [FeFe]-hydrogenase from Desulfovibrio desulfuricans. II. Redox properties, light sensitivity and CO-ligand exchange as observed by infrared spectroscopy

In [FeFe]-hydrogenases, the H cluster (hydrogen-activating cluster) contains a di-iron centre ([2Fe]H subcluster, a (L)(CO)(CN)Fe(mu-RS2)(mu-CO)Fe(CysS)(CO)(CN) group) covalently attached to a cubane iron-sulphur cluster ([4Fe-4S]H subcluster). The Cys-thiol functions as the link between one iron (called Fe1) of the [2Fe]H subcluster and one iron of the cubane subcluster. The other iron in the [2Fe]H subcluster is called Fe2. The light sensitivity of the Desulfovibrio desulfuricans enzyme in a variety of states has been studied with infrared (IR) spectroscopy. The aerobic inactive enzyme (H(inact) state) and the CO-inhibited active form (H(ox)-CO state) were stable in light. Illumination of the H(ox) state led to a kind of cannibalization; in some enzyme molecules the H cluster was destroyed and the released CO was captured by the H clusters in other molecules to form the light-stable H(ox)-CO state. Illumination of active enzyme under 13CO resulted in the complete exchange of the two intrinsic COs bound to Fe2. At cryogenic temperatures, light induced the photodissociation of the extrinsic CO and the bridging CO of the enzyme in the H(ox)-CO state. Electrochemical redox titrations showed that the enzyme in the H(inact) state converts to the transition state (H(trans)) in a reversible one-electron redox step (E (m, pH 7) = -75 mV). IR spectra demonstrate that the added redox equivalent not only affects the [4Fe-4S]H subcluster, but also the di-iron centre. Enzyme in the H(trans) state reacts with extrinsic CO, which binds to Fe2. The H(trans) state converts irreversibly into the H(ox) state in a redox-dependent reaction most likely involving two electrons (E (m, pH 7) = -261 mV). These electrons do not end up on any of the six Fe atoms of the H cluster; the possible destiny of the two redox equivalents is discussed. An additional reversible one-electron redox reaction leads to the H(red) state (E (m, pH 7) = -354 mV), where both Fe atoms of the [2Fe]H subcluster have the same formal oxidation state. The possible oxidation states of Fe1 and Fe2 in the various enzyme states are discussed. Low redox potentials (below -500 mV) lead to destruction of the [2Fe]H subcluster.

Oxidation States and CO Ligand Exchange Kinetics in a Self‐Assembled Monolayer of a Triruthenium Cluster Studied by In Situ Infrared Spectroscopy

Oxidation states and CO ligand exchange kinetics in a self-assembled monolayer (SAM) of an oxo-centered triruthenium cluster [Ru(3)(mu3-O)(mu-CH3COO)6(CO)(L1)(L2)] (L1 = [(NC5H4)CH2NHC(O)(CH2)10S-]2, L2 = 4-methylpyridine) have been extensively investigated on the surface of a gold electrode in aqueous and nonaqueous solutions. The SAM exhibits three consecutive one-electron transfers and four oxidation states, which have been characterized by electrochemistry, in situ infrared spectroscopy, and in situ sum frequency generation (SFG) vibrational spectroscopy measurements. The original electron-localized state of the Ru cluster center was changed to electron delocalization states by oxidation or reduction of the central Ru ions. These changes are revealed by the IR absorptions of the CO ligand and the bridging acetate ligands of the triruthenium cluster in the SAM. The IR absorptions of the two kinds of ligands are strongly dependent on the oxidation state of the Ru cluster center. One-electron oxidation of the central Ru ion in the SAM triggers a CO ligand liberation process. Solvent molecules may then occupy the CO site to result in a CO-free SAM. One-electron reduction of this CO-free SAM in a CO-saturated solution leads to re-coordination of the CO ligand into the SAM. Both processes can be precisely controlled by tuning the electrode potential. The kinetics of the CO exchange cycle in the SAM, including liberation and coordination, has been investigated by in situ IR and SFG measurements for the first time. The CO exchange cycle is significantly dependent on the temperature. The reaction rate greatly decreases with decreasing solution temperature, which is an important factor in the CO ligand exchange process. The activation energies of both CO liberation and coordination have been evaluated from the reaction rate constants obtained at various temperatures.

Trinuclear Oxo‐Centered Iron and Iron/Nickel Clusters – Ligand‐Controlled Homo/Hetero Valency

AbstractCleavage of the all‐iron(III) μ2‐O2–‐linked dimer [FeIII3O(L)2‐(OAc)4]2O (3) with lithium chloride yields [FeIII3O(L)2(OAc)4]Cl (4) and with sodium benzoate, in combination with complete acetate‐to‐benzoate coligand exchange, gives [FeIII3O(L)2‐(OBz)4]OBz (5). Reduction of the all‐FeIII complex 5 [Fe/L/OBz = 3:2:(4+1)] with sodium iodide affords the homonuclear mixed‐valent cluster [FeIIFeIII2O(L)3(OBz)3] (6) (Fe/L/OBz = 1:1:1) and requires fundamental coligand‐to‐ligand exchange. Likewise, when the all‐FeIII dimer [FeIII3O(L)2(OAc)4]2O (3) (Fe/L/OAc = 3:2:4) is cleaved with nickel acetate, the mixed‐valent heteronuclear complex [NiIIFeIII2O(L)3(OAc)3] (7) (M/L/OAc = 1:1:1) is formed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Preparation via coligand exchange and characterization of [99mTc‐EDDA‐HYNIC‐D‐Phe1,Tyr3]Octreotide (99mTc–EDDA/HYNIC–TOC)

Abstract[99mTc‐EDDA–HYNIC‐D‐Phe1,Tyr3]octreotide (99mTc‐EDDA/HYNIC–TOC) is a promising new agent with the potential to replace [111In‐DTPA‐D‐Phe1]‐octreotide in somatostatin receptor scintigraphy. This hydrazinonicotinic acid derivatized somatostatin complex contains ethylenediamine N,N′ diacetic acid (EDDA) as a coligand resulting in a high in vitro and in vivo stability. Since direct 99mTc‐labelling of HYNIC–TOC with EDDA results in low labelling yields, in this study we describe the preparation of 99mTc‐EDDA/HYNIC‐TOC via coligand exchange from Tricine for EDDA. Exchange of coligands is achieved at elevated temperature and under optimized conditions of pH, EDDA concentration and stannous ion. High labelling yields (mean 95.9%) were achieved at high specific activities (>37GBq/µmol). Characterization via HPLC, receptor binding and LC–MS of the resulting complex is described. The formulation developed enables rapid and simple labelling of 99mTc‐EDDA/HYNIC–TOC in a manner suitable for a clinical setting. Copyright © 2003 John Wiley & Sons, Ltd.