Intermolecular Exchange Processes Research Articles

DFT Study of Intermolecular Proton Exchange with Some Derivatives of Benzoic Acids

A theoretical study of proton transfer dynamics in complexes of some substituted benzoic acids with 3,6-di-tert-butyl-2-hydroxyphenoxyl (DTBPO) radical is presented. To elucidate the transfer mechanism, reaction pathways for various complexes of benzoic acid derivatives with DTBPO were modeled. The calculations were performed by the DFT method at the UB3LYP/6-31G+(d, p) level of theory using QST3, IRC procedures, in vacuum and toluene medium (PCM solvation model). Geometric and kinetic parameters of complexes with o-, p-, and m-isomers of nitrobenzoic and chlorobenzoic acids were calculated. Theoretically estimated activation barrier of 29-30 kJ/mol turned out to be approximately 30 % higher than the previously obtained experimental data. It was noted that in the case of o-isomers of aromatic acids the coplanarity of the transition state structure is violated, in contrast to the initial state. This leads to a change in the proton transfer mechanism. The dynamics of charge distribution, dipole moment and electrostatic potential in the studied complexes were analyzed. Based on the calculated data, it was shown that the studied intermolecular proton exchange process occurs throughout Hydrogen Atom Transfer (HAT), and not throughout Proton-Coupled Electron Transfer (PCET) mechanism

ESR-Study of the Proton Exchange with Aliphatic Amino Acids in Toluene

This paper presents the results of an ESR spectroscopic study of the intermolecular proton exchange kinetics with some isomers of aminocaproic acid, such as 2-aminohexanoic and 6-aminohexanoic acids in the toluene indifferent medium. A stable semiquinone radical, namely 3,6-di-tert-butyl-2-hydroxyphenoxyl, is used as a spin probe. The ESR spectra have been recorded on a RE-1306 spectrometer. The article provides the ob-tained dynamic spectra of the intermolecular proton exchange process. Modeling of the proton exchange ESR spectra has been carried out using a previously developed program, which is based on a four-jump model of this reaction and modified Bloch equations. The kinetic parameters of the process of acid-base interaction of the spin probe with amino acids and the activation barrier of the reaction have been estimated with a mini-mum error based on a comparison of the model spectra with the experimental ones. The obtained data have been analyzed and compared with previous studies. It has been found that the reaction rate of the intermo-lecular proton exchange between 3,6-di-tert-butyl-2-hydroxyphenoxy and studied aliphatic amino acids is comparable to the same with aliphatic carboxylic acids. Hoewever, in our case, there is an increase in the ac-tivation barrier, which apparently is associated with a negative effect on the intramolecular hydrogen bonding process between the amino and the carboxyl groups in the amino acid molecule.

Bisbiphenyl Phosphines: Structure and Synthesis of Gold(I) Alkene π-Complexes with Variable Phosphine Donicity and Enhanced Stability

This paper describes the synthesis and characterization of RP(o-biphenyl)2 phosphine ligands (where R = PhO, Ph, and t-Bu), corresponding gold(I) chloride precatalysts, gold(I) triflate catalysts, and gold(I) π-complexes. All ligands and gold chlorides and three π-complexes were characterized in the solid state. The most significant differences between complexes in the solid state were varying P–Au bond lengths, a consistent reflection of the different electronic character of each phosphine. NMR spectroscopic data on enol ether π-complexes is consistent with increasing cationic character in the alkene fragment, from R = t-Bu to R = Ph and OPh. The intermolecular alkene exchange process for [LAu(methoxypropene)]SbF6 (L = PhO(o-biphenyl)2P) is faster than that of the analogous complex with t-Bu2(o-biphenyl)P, presumably due to the higher electrophilicity of the former complex. Examination of a series of vinyl silane π-complexes with triaryl phosphine ligands reveals increasing stability and ease of preparation (Ph3P < Ph2P(o-biphenyl) < PhP(o-biphenyl)2). These results reveal that incorporation of a second biphenyl substituent into ligand architectures allows preparation of a series of gold π-complexes with increased stability over a range of phosphine donicity.

Thermodynamically stable whilst kinetically labile coordination bonds lead to strong and tough self-healing polymers

There is often a trade-off between mechanical properties (modulus and toughness) and dynamic self-healing. Here we report the design and synthesis of a polymer containing thermodynamically stable whilst kinetically labile coordination complex to address this conundrum. The Zn-Hbimcp (Hbimcp = 2,6-bis((imino)methyl)-4-chlorophenol) coordination bond used in this work has a relatively large association constant (2.2 × 1011) but also undergoes fast and reversible intra- and inter-molecular ligand exchange processes. The as-prepared Zn(Hbimcp)2-PDMS polymer is highly stretchable (up to 2400% strain) with a high toughness of 29.3 MJ m−3, and can autonomously self-heal at room temperature. Control experiments showed that the optimal combination of its bond strength and bond dynamics is responsible for the material’s mechanical toughness and self-healing property. This molecular design concept points out a promising direction for the preparation of self-healing polymers with excellent mechanical properties. We further show this type of polymer can be potentially used as energy absorbing material.

Oxidation States, Stability, and Reactivity of Organoferrate Complexes

We have applied a combination of electrospray-ionization mass spectrometry, electrical conductivity measurements, and Mössbauer spectroscopy to identify and characterize the organoferrate species R nFe m- formed upon the transmetalation of iron precursors (Fe(acac)3, FeCl3, FeCl2, Fe(OAc)2) with Grignard reagents RMgX (R = Me, Et, Bu, Hex, Oct, Dec, Me3SiCH2, Bn, Ph, Mes, 3,5-(CF3)2-C6H3; X = Cl, Br) in tetrahydrofuran. The observed organoferrates show a large variety in their aggregation (1 ≤ m ≤ 8) and oxidation states (I to IV), which are chiefly determined by the nature of their organyl groups R. In numerous cases, the addition of a bidentate amine or phosphine changes the distributions of organoferrates and affects their stability. Besides undergoing efficient intermolecular exchange processes, several of the probed organoferrates react with organyl (pseudo)halides R'X (R' = Et, iPr, Bu, Ph, p-Tol; X = Cl, Br, I, OTf) to afford heteroleptic complexes of the type R3FeR'-. Gas-phase fragmentation of most of these complexes results in reductive eliminations of the coupling products RR' (or, alternatively, of R2). This finding indicates that iron-catalyzed cross-coupling reactions may proceed via such heteroleptic organoferrates R3FeR'- as intermediates. Gas-phase fragmentation of other organoferrate complexes leads to β-hydrogen eliminations, the loss of arenes, and the expulsion of organyl radicals. The operation of both one- and two-electron processes is consistent with previous observations and contributes to the formidable complexity of organoiron chemistry.

Hydrogen's isotopic exchange reaction in the C‐methyl sides in the medicinal agent xymedon: NMR spectroscopy and ab initio calculations

AbstractThe kinetics of intramolecular and intermolecular exchange processes in xymedon (1‐(2‐hydroxyethyl)‐4,6‐dimethyl‐1,2‐dihydropyrimidin‐2‐one, a regeneratory, wound‐healing drug) and its analogue were investigated in the solution. Hydrogen's mobility was detected in the C‐methyl sides of these compounds. This mobility was monitored via NMR in the hydrogen/deuterium exchange reaction in water. Two models were proposed as explanations for this hydrogen‐deuterium exchange. According to the main model, the key intermediates of these reactions are low‐energy tautomers of xymedon in which the N3 is protonated following which one proton leaves either 6‐Me or 4‐Me and thus its hybridization is changed. This hydrogen‐to‐deuterium exchange reaction is much faster under acidic conditions although it also occurs in alkaline conditions.Methylation via MeOTs or MeI leads to products with a quaternized ring N3 atom in which a hydrogen‐to‐deuterium exchange reaction also takes place, although the rates of the 6‐Me and 4‐Me hydrogens exchange are reversed.According to density functional theory calculations, the presence of methyl groups at the C4/C6 positions and of the C═O fragment is crucial to remarkably lower the energies of these “rare” tautomers. The exact position of the C═O in heterocycle is also very important in the tautomers' relative stability.

Association and Dissociation of Grignard Reagents RMgCl and Their Turbo Variant RMgCl⋅LiCl.

Grignard reagents RMgCl and their so-called turbo variant, the highly reactive RMgCl⋅LiCl, are of exceptional synthetic utility. Nevertheless, it is still not fully understood which species these compounds form in solution and, in particular, in which way LiCl exerts its reactivity-enhancing effect. A combination of electrospray-ionization mass spectrometry, electrical conductivity measurements, NMR spectroscopy (including diffusion-ordered spectroscopy), and quantum chemical calculations is used to analyze solutions of RMgCl (R=Me, Et, Bu, Hex, Oct, Dec, iPr, tBu, Ph) in tetrahydrofuran and other ethereal solvents in the absence and presence of stoichiometric amounts of LiCl. In tetrahydrofuran, RMgCl forms mononuclear species, which are converted into trinuclear anions as a result of the concentration increase experienced during the electrospray process. These trinuclear anions are theoretically predicted to adopt open cubic geometries, which remarkably resemble structural motifs previously found in the solid state. The molecular constituents of RMgCl and RMgCl⋅LiCl are interrelated via Schlenk equilibria and fast intermolecular exchange processes. A small portion of the Grignard reagent also forms anionic ate complexes in solution. The abundance of these more electron-rich and hence supposedly more nucleophilic ate complexes strongly increases upon the addition of LiCl, thus rationalizing its beneficial effect on the reactivity of Grignard reagents.

Facile Activation of Dihydrogen by a Phosphinito-Bridged Pt(I)−Pt(I) Complex

The phosphinito-bridged Pt(I) complex [(PHCy(2))Pt(mu-PCy(2)){kappa(2)P,O-mu-P(O)Cy(2)}Pt(PHCy(2))](Pt-Pt) (1) reversibly adds H(2) under ambient conditions, giving cis-[(H)(PHCy(2))Pt(1)(mu-PCy(2))(mu-H)Pt(2)(PHCy(2)){kappaP-P(O)Cy(2)}](Pt-Pt) (2). Complex 2 slowly isomerizes spontaneously into the corresponding more stable isomer trans-[(PHCy(2))(H)Pt(mu-PCy(2))(mu-H)Pt(PHCy(2)){kappaP-P(O)Cy(2)}](Pt-Pt) (3). DFT calculations indicate that the reaction of 1 with H(2) occurs through an initial heterolytic splitting of the H(2) molecule assisted by the phosphinito oxygen with breaking of the Pt-O bond and hydrogenation of the Pt and O atoms, leading to the formation of the intermediate [(PHCy(2))(H)Pt(mu-PCy(2))Pt(PHCy(2)){kappaP-P(OH)Cy(2)}](Pt-Pt) (D), where the two split hydrogen atoms interact within a six-membered Pt-H...H-O-P-Pt ring. Compound D is a labile intermediate which easily evolves into the final dihydride complex 2 through a facile (9-15 kcal mol(-1), depending on the solvent) hydrogen shift from the phosphinito oxygen to the Pt-Pt bond. Information obtained by addition of para-H(2) on 1 are in agreement with the presence of a heterolytic pathway in the 1 --> 2 transformation. NMR experiments and DFT calculations also gave evidence for the nonclassical dihydrogen complex [(PHCy(2))(eta(2)-H(2))Pt(mu-PCy(2))Pt(PHCy(2)){kappaP-P(O)Cy(2)}](Pt-Pt) (4), which is an intermediate in the dehydrogenation of 2 to 1 and is also involved in intramolecular and intermolecular exchange processes. Experimental and DFT studies showed that the isomerization 2 --> 3 occurs via an intramolecular mechanism essentially consisting of the opening of the Pt-Pt bond and of the hydrogen bridge followed by the rotation of the coordination plane of the Pt center with the terminal hydride ligand.

Metal complexes of N-o-chlorobenzamido-meso-tetraphenylporphyrin: cis-Tl(N-NCO(o-Cl)C6H4-tpp)(OAc) and trans-Cd(N-NHCO(o-Cl)C6H4-tpp)(OAc) (tpp=5, 10, 15, 20-tetraphenylporphyrinate)

Abstract The crystal structures of diamagnetic (cis-acetato) (N-o-chlorobenzamido-meso-tetraphenylporphyrinato)thallium(III)·0.5 water solvate [cis-Tl(N-NCO(o-Cl)C6H4-tpp)(OAc)·0.5 H2O; 3·0.5 H2O] and diamagnetic (trans-acetato) (N-o-chlorobenzamido-meso-tetraphenylporphyrinato)cadmium(II) methylene chloride solvate [trans-Cd(N-NHCO(o-Cl)C6H4-tpp)(OAc)·CH2Cl2; 4·CH2Cl2] were determined. The coordination sphere around the Tl3+ (or Cd2+) in 3 (or 4) is a distorted square-based pyramid in which the apical site is occupied by a chelating bidentate OAc− group. In 3, Tl3+ and N(5) are located on the same side at 1.18 and 1.26 A from it 3N plane, but in 4, Cd2+ and N(5) are located on different sides at 1.06 and −1.55 A from it 3N plane. The free energy of activation at the coalescence temperature Tc for the intermolecular acetate exchange process in 3 in CD2Cl2 solvent is found to be Δ G 198 ≠ = 42.1 kJ/mol through 1H NMR temperature-dependent measurements. Likewise, the free energy of activation Δ G 293 ≠ = 55.94 kJ/mol is determined for the intramolecular exchange of the ortho protons between o′-H (34, 40) and o′-H (38, 44) in 3 in CD2Cl2. VT NMR (1H and 13C) studies of 4 show that the acetate acts as a bidentate ligand and the OAc− exchange does not occur in CD2Cl2. Moreover, the NH proton [i.e., H(5)] of 4 in CD2Cl2 is observed as a singlet at δ −0.09 ppm with Δν1/2 = 13 Hz at 20 °C indicating that the NH protons undergo intermediate intermolecular proton exchange with water at this temperature.

Dynamics of the RING Domain from Human TRAF6 by 15N NMR Spectroscopy: Implications for Biological Function

Activation of transcription factor NF-kappaB requires Lys63-linked polyubiquitination of the E3 ubiquitin ligase TRAF6 via protein-protein interactions mediated by a RING domain. In this study, intra- and intermolecular chemical exchange processes of the TRAF6 RING domain were analyzed by (15)N NMR spectroscopy. Micro- to millisecond time scale motions were assessed through R 1, R 2, NOE, and cross-correlated relaxation measurements, and the kinetics of these motions were quantified with relaxation dispersion. The relaxation experiments indicate that the protein core is rigid, consistent with the functional requirement that RING domains form a binding scaffold for E2 ubiquitin conjugation enzymes. Chemical exchange is observed at the C-terminal end of the main alpha-helix of the RING domain. The C-terminal end of the main alpha-helix from the RING domain is involved in E2-E3 interactions, and modulation of slow motions for this region of the helix may be a general mechanism by which these interactions achieve ubiquitin transfer. Chemical shift mapping indicates that the TRAF6 RING domain does not self-associate in solution. Numerous RING domains are homo- or heterodimeric, and this is thought to be a functional necessity for recruitment of substrates for ubiquitination, or recruitment of multiple E2 enzymes for efficient substrate ubiquitination. However, lack of self-association for the RING domain from TRAF6, and the observation that the intact protein is a trimer, suggests that close association of RING domains within a homodimeric scaffold may not be a fundamental requirement for biological function.

Simultaneous Fluoride Binding to Ferrocene-Based Heteronuclear Bidentate Lewis Acids

A series of micro2-fluoro-bridged heteronuclear bidentate Lewis acid complexes [K(18-crown-6)THF]+ [Fc(BMeF)(SnMe2Cl)F]- (1-2F), [K(18-crown-6)THF]+ [Fc(BMeF)(SnMe2F)F]- (1-3F), [K(18-crown-6)THF]+ [Fc(BMePh)(SnMe2Cl)F]- (2-F), and [K(18-crown-6)THF]+ [Fc(BMePh)(SnMe2F)F]- (2-2F) (Fc=1,2-ferrocenediyl) was prepared. Compounds 2-F and 2-2F were obtained as a mixture of diastereomers, which arise due to the generation of a stereocenter at boron in addition to their inherent planar chirality. All compounds have been studied in the solid state by single-crystal X-ray diffraction analysis and by multinuclear NMR spectroscopy in solution. As a result of bridging-fluoride interactions, tetrahedral boron and distorted trigonal-bipyramidal tin centers are observed. Comparison with the corresponding monofunctional ferrocenylborates further supports the bridging nature of the fluoride anion. Two-dimensional exchange spectroscopy 19F NMR studies provide evidence for facile intermolecular and intramolecular fluorine exchange processes. All complexes display reversible one-electron oxidation events at lower potentials than those of the tricoordinate ferrocenylborane precursors, which is typical of ferrocenylborate complexes.

Computational Insights into Degenerate Ethylene Exchange with a Grubbs-Type Catalyst

Romero and Piers recently reported a mechanistic study in which they detailed the intramolecular exchange of adjacent methylene groups of a ruthenacyclobutane and the intermolecular degenerate exchange of free ethylene with a second-generation Grubbs-type catalyst, a 14-electron ruthenacyclobutane, (NHC)Cl2Ru(CH2CH2CH2). These authors measured activation parameters for the intramolecular and intermolecular exchange processes (ΔG⧧ = 12.3 kcal mol-1 and ΔH⧧ = 13.2(5) kcal mol-1 and ΔS⧧ = −15(2) eu, respectively). The current study applies density functional theory calculations to address their proposed exchange mechanisms. For intramolecular exchange, the formation of a structure with coordinated ethylene proceeds from the cyclobutane species via a direct rotational bond-breaking transition state, which produces an ethylene which is essentially perpendicular to the remaining methylene unit; and the computed free energy of activation including solvation for the pathway with trans-disposed chlorides (ΔGCH2Cl2...

Conformational Fluctuations Coupled to the Thiol-Disulfide Transfer between Thioredoxin and Arsenate Reductase in Bacillus subtilis

Arsenic compounds commonly exist in nature and are toxic to nearly all kinds of life forms, which directed the evolution of enzymes in many organisms for arsenic detoxification. In bacteria, the thioredoxin-coupled arsenate reductase catalyzes the reduction of arsenate to arsenite by intramolecular thiol-disulfide cascade. The oxidized arsenate reductase ArsC is subsequently regenerated by thioredoxin through an intermolecular thiol-disulfide exchange process. The solution structure of the Bacillus subtilis thioredoxin-arsenate reductase complex represents the transiently formed intermediate during the intermolecular thiol-disulfide exchange reaction. A comparison of the complex structure with that of thioredoxin and arsenate reductase proteins in redox states showed substantial conformational changes coupled to the reaction process, with arsenate reductase, especially, adopting an "intermediate" conformation in the complex. Our current studies provide novel insights into understanding the reaction mechanisms of the thioredoxin-arsenate reductase pathway.

Aggregation and Ionization Equilibria of Bis(pentafluorophenyl)borinic Acid Driven by Hydrogen-Bonding with Tetrahydrofuran

Bis(pentafluophenyl)borinic acid, Ar2BOH (1, Ar = C6F5), in dichloromethane solution is present as an equilibrium mixture of monomeric (1m) and trimeric (1t) forms. Previous studies showed that water affects both the position and the rate of this equilibrium. Here, the behavior of 1 in the presence of tetrahydrofuran (THF), a nucleophile able to behave as a Lewis base and H-bond acceptor only, has been studied, by monitoring with 1H and 19F NMR the course of titrations performed directly into NMR tubes. The addition, at 183 K, of 0.33 equiv of THF caused the instantaneous and quantitative formation of the hydrogen-bonded adduct between the trimer 1t and one molecule of THF. Homo- and heteronuclear 2D NMR correlation experiments led to a solution structure consistent with the C2-optimized geometry obtained by PM3 computations. The H-bonding of the THF molecule causes major deformations of the molecular geometry of the trimer, so that only one molecule of THF can interact with the trimer, in spite of its three OH groups. Intra- and intermolecular exchange processes involving this adduct have been investigated by 2D EXSY experiments, showing flopping of the cycle conformation, rotation of the aromatic rings around their B−C bonds, and exchange of THF among the three OH groups, in addition to the exchange between free 1t and the adduct. When the amount of added THF was higher than 0.33 equiv, an unexpected ionization process occurred, leading to the cation [Ar2B(OH2)2]+ and to deprotonated 1t, i.e., to the anion [Ar6B3O3H2]- of Cs symmetry. On increasing the temperature, progressive partial fragmentation of the trimeric species was observed. Both 11B NMR evidence and PM3 computations indicated that, at variance with what is observed in the interaction with H2O, the interaction between THF and 1m occurs preferentially via an H-bonded adduct, Ar2BO−H···THF, rather than a Lewis acid−base complex, Ar2B(OH)(THF). This confirms the poor Lewis acidity of the boron atom of 1m.

Mechanistic Studies on 14-Electron Ruthenacyclobutanes: Degenerate Exchange with Free Ethylene

The phosphonium alkylidene [(NHC)Cl2Ru=CH(PCy3)]+[B(C6F5)4]-, 1, (NHC = N-heterocyclic carbene, Cy = cyclohexyl, C6H11) reacts with 2.2 equiv of ethylene at -50 degrees C to form the 14-electron ruthenacyclobutane (NHC)Cl2Ru(CH2CH2CH2), 2. NMR spectroscopic data indicates that 2 has a C2v symmetric structure with a flat, kite shaped ruthenacyclobutane ring with significant Calpha-Cbeta agostic interactions with the Ru center. Intramolecular exchange of Calpha and Cbeta is fast (14(2) s-1 at 223 K) as measured by EXSY spectroscopy. Intermolecular exchange of Calpha and Cbeta with the methylene groups of free ethylene is much slower and first order in both [Ru] and [H2C=CH2] (4.8(3) x 10-4 M-1 s-1). Activation parameters for this process are DeltaH++ = 13.2(5) kcal mol-1 and DeltaS++ = -15(2) cal mol-1 K-1, also consistent with a rate limiting associative substitution as the key step in this exchange process. On the basis of this observation, mechanisms for the intermolecular exchange process are proposed and the implications for the mechanism of the propagation steps in catalytic olefin metathesis as mediated by Grubbs catalysts are discussed.

Thallium complexes of tetraphenylporphyrin: Tl(tpp- N-O)(OAc) and cis-acetato-N- p- tert-butylbenzensulfonylimido- meso-tetraphenylporphyrinatothallium(III) Tl(N- p-NSO 2C 6H 4tBu-tpp)(OAc)

The crystal structures of Tl(tpp- N-O)(OAc) ( 3) and acetato-N- p- tert-butylbenzensulfonylimido- meso-tetraphenylporphyrinatothallium(III) Tl(N- p-NSO 2C 6H 4 t Bu-tpp)(OAc) ( 5) have been determined. The coordination sphere around Tl 3+ is a distorted square-based pyramid in which the apical site is occupied by a bidentate chelating OAc − group for 3 and 5. The plane of three pyrrole nitrogen atoms (i.e., N(1), N(2) and N(3)), strongly bonded to Tl 3+ in 3 and 5, is adopted as a reference plane, 3 N. The porphyrin ring is severely distorted and the pyrrole ring N(4) bonding to the oxygen and NSO 2C 6H 4 t Bu group makes a dihedral angle of 46.5° and 46.7° with the 3 N plane for 3 and 5, respectively. In 3, Tl 3+ and O(1) are located on the same side at 1.11 and 1.34 Å from its 3 N plane, and in 5, Tl 3+ and N(5) are also located on the same side at 1.15 and 1.30 Å from its 3 N plane. The free energy of activation at the coalescence T c for the intermolecular acetate exchange process of 3 and 5 in CD 2Cl 2 is found to be Δ G 184 ‡ = 39.3 and Δ G 208 ‡ = 44.1 kJ / mol , respectively, through 1H NMR variable temperature measurements. An electronegative substituent, O(1), bonded to Tl in 3 causes a significant negative contribution up to 145 Hz for Δ 3 J(Tl–C, O) and 79 Hz for Δ 2 J(Tl–C, O) for OAc − in 3.

Metal complexes of 21-(4- tert-butyl-benzenesulfonamido)-5,10,15,20-tetraphenyl-porphyrin: Ga( N- p-NSO 2C 6H 4tBu-tpp)(OH), Tl( N- p-NSO 2C 6H 4tBu-tpp)(O 2CCF 3) and Zn( N- p-NSO 2C 6H 4tBu-tpp) [tpp = 5,10,15,20-tetraphenylporphyrinate

The crystal structures of trans-hydroxo- N- p- tert-butylbenzenesulfonylimido- meso-tetraphenylporphyrinatogallium(III) 0.5 aqua and 0.5 methanol solvate [Ga( N- p-NSO 2C 6H 4 t Bu-tpp)(OH) · 0.5MeOH · 0.5H 2O; 2 · 0.5MeOH · 0.5H 2O], cis-trifluoroacetato- N- p- tert-butylbenzenesulfonylimido- meso-tetraphenylporphyrinatothallium(III) [Tl( N- p-NSO 2C 6H 4 t Bu-tpp)(O 2CCF 3); 3] and N- p- tert-butylbenzenesulfonylimido- meso-tetraphenylporphyrinatozinc(II) 0.7 methanol solvate [Zn( N- p-NSO 2C 6H 4 t Bu-tpp) · 0.7MeOH; 4 · 0.7MeOH], were determined. The coordination sphere around Tl 3+ ion in 3 is a distorted square-based pyramid in which the apical site is occupied by a chelating bidentate CF 3 CO 2 - group, whereas for the Ga 3+ ion in 2, it is a distorted trigonal bipyramid with N(2), N(4), and O(1) lying in the equatorial plane. The geometry around Zn 2+ in 4 is a distorted square planar. The free energy of activation at the coalescence temperature T c for the intermolecular trifluoroacetate exchange process for 3 in CD 2Cl 2 is found to be Δ G 223 ‡ = 46.3 kJ / mol through 19F NMR variable temperature measurements. In the slow-exchange region, the CF 3 and carbonyl (CO) carbons of the CF 3 CO 2 - group of 3 in CD 2Cl 2 are separately located at δ 115.9 [ 3 J(Tl– 13C) = 155 Hz] and 160.2 [ 2 J(Tl– 13C) = 81 Hz] at −90 °C.

Dynamic NMR properties of DOTA ligand: variable pH and temperature 1H NMR study on [K(HxDOTA)](3-x)- species.

The (1)H NMR spectra of [H(x)DOTA]((4-x)-) species are reported as a function of pH and temperature in aqueous solution. The spectra show line broadening both in ligand proton signals and also in the water proton signal by titration with KOH solution. The formation of different [K(H(x)DOTA)]((3-x)-) complexes is found to be responsible for this behaviour. At high pH the usual fluxional motions, i.e. the ring inversion and the change in the acetate arms' helicity, which are also characteristic for other but inert metal-DOTA complexes, have been detected. However, because of the kinetic lability of K(+)-O and K(+)-N coordinative bonds a new type of rearrangement appears. This new motion requires breaking of coordinative bonds in the complex and can be described as a certain type of "ring slewing" around the ring C-C bonds. At low temperature (about 270 K) the ring slewing slows down and becomes negligible compared with the ring inversion and the change in the arms' helicity. These two latter processes have the same rate. When the temperature is higher (about 320 K) the ring slewing accelerates and its rate exceeds the rate of ring inversion. At this temperature the change in the acetate arms' helicity has the same rate as the ring slewing. Additionally, in the pH range 4-5 a slow intermolecular proton exchange process has been observed between the water and the dissociable protons of [K(H(x)DOTA)]((3-x)-). A water-assisted proton exchange mechanism is proposed on the basis of the activation parameters. This finding supports the previously suggested slow proton motion hypothesis for the formation of DOTA complexes.

Monodentate and bidentate trifluoroacetato ligands in bis(trifluoroacetato)-( N-methyl- meso-tetraphenylporphyrinato)thallium(III)—a new dynamic 4:3 piano stool seven-coordinate geometry

The crystal structure of bis(trifluoroacetato)-( N-methyl- meso-tetraphenylporphyrinato) thallium(III), Tl(NMetpp)(CF 3CO 2) 2 ( 2), was established and the coordination sphere around the Tl 3+ ion is described as 4:3 tetragonal base–trigonal base piano stool seven-coordinate geometry in which the two cis CF 3CO 2 − groups occupy two apical sites. The plane of the three pyrrole nitrogen atoms [i.e. N(2), N(3) and N(4)] strongly bonded to Tl 3+ is adopted as the reference plane 3N. The pyrrole N(1) ring bearing the methyl group [i.e. C(45)H 3] is the most deviated one from the 3N plane making a dihedral angle of 23.3° whereas smaller angles of 9.9, 2.7 and 4.7° occur with pyrroles N(2), N(3), and N(4), respectively. Because of the larger size of the thallium(III) ion, Tl is considerably out of the 3N plane; its displacement of 1.02 Å is in the same direction as that of the two apical CF 3CO 2 − ligands. The intermolecular trifluoroacetate exchange process for 2 in CD 2Cl 2 solvent is examined through 19F and 13C NMR temperature-dependent measurements. In the slow-exchange region, the CF 3 and carbonyl (CO) carbons of the CF 3CO 2 − groups in 2 are separately located at δ 114.3 [ 1 J(C–F)=290 Hz, 3 J(Tl–C)=411 Hz] and 155.1 [ 2 J(C–F)=37 Hz, 2 J(Tl–C)=204 Hz], respectively, at −106 °C. In the same slow-exchange region, the fluorine atoms of 2, Tl(NMetpp)(CF 3CO 2) + and the free CF 3CO 2 − are located at δ −73.76 [ 4 J(Tl–F)=44 Hz], −73.30 [ 4 J(Tl–F)=22 Hz], and −76.15 ppm at −97 °C, respectively.

Single-molecule magnets: preparation and properties of mixed-carboxylate complexes.

Methods are reported for the preparation of mixed-carboxylate versions of the [Mn(12)O(12)(O(2)CR)(16)(H(2)O)(4)] family of single-molecule magnets (SMMs). [Mn(12)O(12)(O(2)CCHCl(2))(8)(O(2)CCH(2)Bu(t))(8)(H(2)O)(3)] (5) and [Mn(12)O(12)(O(2)CHCl(2))(8)(O(2)CEt)(8)(H(2)O)(3)] (6) have been obtained from the 1:1 reaction of the corresponding homocarboxylate species. Complex 5.CH(2)Cl(2).H(2)O crystallizes in the triclinic space group P1 with, at -165 degrees C, a = 15.762(1), b = 16.246(1), c = 23.822(1) A, alpha = 103.92(1), beta = 104.50(1), gamma = 94.23(1) degrees, Z = 2, and V = 5674(2) A(3). Complex 6.CH(2)Cl(2) crystallizes in the triclinic space group P1 with, at -158 degrees C, a = 13.4635(3), b = 13.5162(3), c = 23.2609(5) A, alpha = 84.9796(6), beta = 89.0063(8), gamma = 86.2375(6) degrees, Z = 2, and V = 4207.3(3) A(3). Complexes 5 and 6 both contain a [Mn(12)O(12)] core with the CHCl(2)CO(2-) ligands ordered in the axial positions and the RCO(2-) ligands (R = CH(2)Bu(t) (5) or Et (6)) in equatorial positions. There is, thus, a preference for the CHCl(2)CO(2-) to occupy the sites lying on the Mn(III) Jahn-Teller axes, and this is rationalized on the basis of the relative basicities of the carboxylate groups. Direct current magnetic susceptibility studies in a 10.0 kG field in the 2.00-300 K range indicate a large ground-state spin, and fitting of magnetization data collected in the 10.0-70.0 kG field and 1.80-4.00 K temperature range gave S = 10, g = 1.89, and D = -0.65 K for 5, and S = 10, g = 1.83, and D = -0.60 K for 6. These values are typical of [Mn(12)O(12)(O(2)CR)(16)(H(2)O)(4)] complexes. Alternating current susceptibility studies show the out-of-phase susceptibility (chi(M)' ') signals characteristic of the slow relaxation in the millisecond time scale of single-molecule magnets. Arrhenius plots obtained from chi(M)' ' versus T data gave effective barriers to relaxation (U(eff)) of 71 and 72 K for 5 and 6, respectively. (1)H NMR spectra in CD(2)Cl(2) show that 5 and 6 are the main species present on dissolution, but there is evidence for some ligand distribution between axial and equatorial sites, by intra- and/or intermolecular exchange processes.