OTf Anion Research Articles

Screening of Ionic Liquid-Based Electrolytes for Al Dual-Ion Batteries: Thermodynamic Cycle and Combined MD-DFT Approaches

The electrochemical window (ECW) of the ionic liquid (IL) is the fundamental parameter to understand the stability of the electrolyte for a given electrode material. Here, we have considered room-temperature and molten salt-based 16 IL-based electrolytes, consisting of imidazolium-, pyrrolidinium-, urea-, and acetamide-based cations coupled with AlCl4 and OTf (trifluoromethanesulfonate) anions. Three different high-throughput computational screening techniques have been implemented to calculate the ECW of these electrolytes for dual-ion batteries (DIBs). The thermodynamic cycle method has been employed to calculate the oxidation and reduction potentials with respect to the Al3+/Al reference electrode for the individual ions of the IL. Classical molecular dynamics followed by DFT methods (MD + DFT) have been considered to calculate anodic and cathodic limiting potentials. From the MD + DFT, we have classified two methods, AIMD-min and AIMD-sp. The calculated ECWs for all considered ILs using the AIMD-min method are found to be close to experimental outcomes compared to other two methods. We show that the alkyl group of the imidazolium ring has a very limited effect on the ECW values. On the other hand, aromatic imidazolium rings are more prone to reduction compared to the nonaromatic pyrrolidinium ring. Moreover, we have investigated the contribution of cations and anions in cathodic and anodic potentials, from the study of density of states. Along with that, we have tested the robustness capability of the AIMD-min method and established a superior technique for other liquid systems to calculate the ECW. This work facilitates a step forward in selecting nonaqueous electrolytes for rechargeable DIBs.

Ab Initio Force Fields for Organic Anions: Properties of [BMIM][TFSI], [BMIM][FSI], and [BMIM][OTf] Ionic Liquids.

Room-temperature ionic liquids (ILs) composed of organic anions bis(trifluoromethanesulfonyl)imide (TFSI), bis(fluorosulfonyl)imide (FSI), and trifluoromethanesulfonate (OTf) exhibit interesting physical properties and are important for many electrochemical applications. TFSI and FSI form "hydrophobic" ILs, immiscible with water but miscible with many organic solvents and polymers; for computer simulation studies, it is thus essential to develop force fields for these anions that are transferable among this wide variety of chemical environments. In this work, we develop entirely ab initio force fields for the TFSI, FSI, and OTf anions and predict the properties of corresponding 1-butyl-3-methylimidazolium ILs. We discuss important subtleties in the force field development related to accurately modeling conformational flexibility, that is, relaxed torsional profiles and intramolecular electrostatic interactions. The TFSI anions have notable conformational flexibility in the IL, and we predict approximately 70% cisoid and 20% transoid conformations, which is largely driven by cation/anion ion-pair interactions and is opposite to the trend expected from the anion ab initio potential energy surface. The favorable interactions between the cation and cisoid TFSI conformations result in a shoulder in the cation/anion radial distribution function at short distances, whereas interconversion between cisoid and transoid conformations occurs on a commensurate time scale as ion diffusion processes. In addition to this physical insight on anion effects, we expect that these force fields will have important applications for studying a variety of complex electrolyte systems.

Electrochemical Evaluation of Titanocenes in Ionic Liquids with Non‐coordinating and Coordinating Anions and Application for NH3 Synthesis

AbstractRedox behaviors of monomeric and dimeric titanocenes, [Cp2TiCl2] (1) and [(Cp2TiCl)2] (2), were investigated in two different types of ionic liquid (IL) with non‐coordinating and coordinating anions, 1‐butyl‐1‐methylpyrrolidinium tris(pentafluoroethyl)‐ trifluorophosphate (Pyr4FAP) and 1‐butyl‐1‐methylpyrrolidinium triflate (Pyr4OTf), respectively. UV‐vis/NIR spectra obtained during the electrochemical reduction of 1 revealed that it was converted to 2 in the ILs through the formation of [Cp2TiCl] (3). In Pyr4OTf, 3 was coordinated with the OTf anion in the η1‐O mode, during the electrochemical reductions of Ti(IV/III) process, whereas 3 was not coordinated by the FAP anion in Pyr4FAP. Additionally, electrochemical NH3 synthesis was carried out in a solid polymer electrolyte cell by using a gas diffusion electrode coated with Pyr4FAP or Pyr4OTf containing titanocenes. The electrochemical reduction using 2 in Pyr4FAP at −1.5 V (vs. Ag/AgCl) gave a high yield (34 % at 29.9 C) of NH3 per Ti ion. The electrochemical reduction using 1 in Pyr4OTf gave a 6.8 % yield of NH3 per Ti ion at 20 °C. These findings indicate that the use of an IL with a non‐coordinating anion such as FAP contributes to the coordination and reduction of N2 to titanocenes.

Complexes of trimethylsilyl trifluoromethanesulfonate with nitrogen, oxygen, and phosphorus donors

The Lewis acceptor chemistry of Me3SiOTf with p-block Lewis bases has been explored and a library of complexes has been characterized by spectroscopic and, where possible, crystallographic methods. Compounds with the generic formula [Me3Si(L)][OTf] (L = 4-dmap, pyr, imz, OPMe3, OPCy3, OPPh3, OpyrMe, dmso, PMe3) were isolated from 1:1 mixtures of Me3SiOTf and the respective ligand in CH2Cl2. Characterization by NMR spectroscopy confirmed the solution stability of all but [Me3Si(PMe3)][OTf], with indications that the latter dissociates into PMe3 and Me3SiOTf. Solid-state structures of [Me3Si(4-dmap)][OTf], [Me3Si(pyr)][OTf], [Me3Si(OPCy3)][OTf], [Me3Si(OPPh3)][OTf], [Me3Si(OpyrMe)][OTf], and [Me3Si(PMe3)][OTf] were elucidated by single crystal X-ray diffraction, confirming the envisaged ionic formulations resulting from the displacement of the OTf anion from the silicon center of Me3SiOTf by the incoming ligand. Mixtures of Me3SiOTf with other related donors, including ChPPh3 (Ch = S or Se), NEt3, SMe2, PPh3, 2,2′-bipy, or Me2CO, show no evidence of reaction under ambient conditions, reflecting the lower basicity and (or) greater steric encumbrance of these ligands. Reactions of Me3SiOTf with bis-donor ligands yielded complexes of the generic formulae [Me3Si(L–L)SiMe3][OTf]2 (L–L = 4,4′-bipy, tmeda, dmpe) and [Me3Si(L–L)][OTf] (L–L = 4,4′-bipy, tmeda, dmpe). The tmeda and dmpe complexes, however, were found to dissociate in solution, with complexes only prevailing in the solid phase. X-ray diffraction studies of [Me3Si(4,4′-bipy)SiMe3][OTf]2 and [Me3Si(dmpe)SiMe3][OTf]2 confirmed the expected connectivities and ionic formulations, with Si–ligand bond lengths comparable to those observed in [Me3Si(pyr)][OTf] and [Me3Si(PMe3)][OTf], respectively.

Anion‐Exchange Properties of Trifluoroacetate and Triflate Salts of N‐Alkylammonium Resorcinarenes

The synthesis of N-benzyl- and N-cyclohexylammonium resorcinarene trifluoroacetate (TFA) and triflate (OTf) salt receptors was investigated. Solid-state analysis by single-crystal X-ray diffraction revealed that the N-alkylammonium resorcinarene salts (NARSs) with different upper substituents had different cavity sizes and different affinities for anions. Anion-exchange experiments by mixing equimolar amounts of N-benzylammonium resorcinarene trifluoroacetate and N-cyclohexylammonium resorcinarene triflate, as well as N-benzylammonium resorcinarene triflate and N-cyclohexylammonium resorcinarene trifluoroacetate showed that the NARS with flexible benzyl groups preferred the larger OTf anion, whereas the rigid cyclohexyl groups preferred the smaller TFA anions. The anion-exchange processes were confirmed in the solid state by single-crystal and powder X-ray diffraction experiments and in the gas phase by electrospray ionization mass spectrometry.

Copper ionic liquids: Examining the role of the anion in determining physical and electrochemical properties

Five new copper(II)-based ionic liquids and three crystalline compounds have been synthesized in order to further understand the role of the anion in determining their physicochemical properties. Materials were prepared with combinations of three different anions (2-ethylhexanoate (EHN), tetrafluoroborate (BF4), and triflate (OTf)) and six-coordinate Cu(II) cations. The complexes that contain at least one BF4 anion consistently displayed both the highest specific conductivity and electrochemical reversibility. The presence of one OTf anion (in combination with one EHN or BF4) facilitates the formation of crystalline materials. Single crystal X-ray diffraction studies were completed on two of the mixed anion compounds. The results show that both ethanolamine (EA) and diethanolamine (DEA) chelate to the Cu(II) centers in a highly distorted tetragonal geometry. The properties of the ionic liquid with the overall lowest viscosity and highest conductivity and electrochemical reversibility (Cu{NH2CH2CH2OH}6(BF4)2, 4) were studied as a function of temperature. The viscosity decreases with increasing temperature, but at 45°C (over three hours) there is a loss of two EA ligands to form a more viscous four-coordinate complex. Cyclic voltammetry of 4 reveals a quasi-reversible Cu(II)/Cu(I) reduction wave that shifts to more positive potentials with increasing temperature.

Mechanism of the N-protecting group dependent annulations of 3-aryloxy alkynyl indoles under gold catalysis: a computational study

The mechanism of the gold-catalyzed annulations of 3-aryloxy alkynyl indoles developed by Tu et al. was studied by DFT calculations. It was found that both indole derivatives of electron-donating and electron-withdrawing protective groups would first undergo the 5-exo-dig cyclization simultaneously upon activation by cationic [PR(3)Au(+)] species. However, divergent reactivity of the resulting spirocyclic intermediate in competitive 1,2-alkenyl migration and nucleophilic water addition reactions towards C3 was predicted. When protected by electron-donating group, the 1,2-alkenyl migration occurs to generate a tricyclic intermediate, from which an aromatic Claisen rearrangement/nucleophilic addition sequence results in the observed 1,2-phenoxy migration. In case of electron-withdrawing group, the 1,2-alkenyl migration would be unfavorable. Instead, the nucleophilic addition of water oxygen to C3 is more facile, and leads to the hemiketal intermediate. The possible roles of water-cluster and OTf anion as proton shuttles in both reactions were also evaluated.

Bis(di-2-pyridylamine-κ2N2,N2′)silver(I) trifluoromethanesulfonate: polar arrangement of trifluoromethanesulfonate anions in a pseudo-centrosymmetric framework of coordination cations

The asymmetric unit of the title compound, [Ag(C(10)H(9)N(2))(2)]CF(3)SO(3) or [Ag(dpa)(2)]OTf (dpa is di-2-pyridylamine and OTf is the trifluoromethanesulfonate anion), contains two [Ag(dpa)(2)](+) coordination cations and two OTf anions. The coordination geometry of the Ag(I) atom is intermediate between square-planar and tetrahedral, with similar deformations at the two symmetry-independent metal centres. The dpa ligands coordinate in a bidentate chelating mode. The OTf anions are in the outer coordination sphere and bridge the coordination cations via N-H...O interactions to form two symmetry-independent hydrogen-bonded chains. The [Ag(dpa)(2)](+) cations are arranged via interactions involving the aromatic groups into a pseudo-centrosymmetric three-dimensional framework with two types of channels, each confining congeners of one of the symmetry-independent anions. The most interesting feature of this structure is its bulk polarity resulting from an approximately parallel alignment of the anions in the channels.

Liquid−Liquid Equilibrium of Mixtures of Imidazolium-Based Ionic Liquids with Propanediols or Glycerol

In this work, liquid−liquid equilibria (LLE) of solutions of imidazolium-based ionic liquids—with either bistriflamide ([Cnmim][NTf2], n = 2 and n = 10) or triflate anions ([C2mim][OTf]), with 1-propanol, 1,2-propanediol, 1,3-propanediol, and glycerol (1,2,3-propanetriol)—were studied. The obtained phase diagrams show a remarkable difference in solvation properties between ionic liquids with NTf2 or OTf anions toward the aforementioned alcohols: although [C2mim][NTf2] constantly exhibited partial miscibility, [C2mim][OTf] was always completely miscible. This divergence is likely to be related to the distinct abilities of NTf2 and OTf anions toward hydrogen bonding. It was also found that the shorter cation alkyl chain in the [Cnmim][NTf2] ionic liquids provided better solubility with both propanediols and glycerol. In addition, the solubility of 1,3-propanediol in both [C2mim][NTf2] and [C10mim][NTf2] was much lower than that of 1,2-propanediol. From the application point of view, the present results show that mixtures of [C2mim][NTf2] and [C2mim][OTf] ionic liquids may achieve the separation of 1,3-propanediol from glycerol. The LLE experimental results were correlated using a model based on the Peng−Robinson equation of state (PR EoS) combined with the Mathias−Klotz−Prausnitz (MKP) mixing rule.

Double layer structure and adsorption/desorption hysteresis of neat ionic liquid on Pt electrode surface — an in-situ IR-visible sum-frequency generation spectroscopic study

The electrochemical interface between a polycrystalline Pt electrode and the ionic liquid 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim][OTf]) has been studied by in-situ IR-visible sum-frequency generation (SFG) spectroscopy. Potential dependent adsorption/desorption processes of OTf anions has been monitored within the electrochemical window. SFG results indicate that the ions form a double layer structure at the interface. Significant adsorption/desorption hysteresis has been observed for the anions on the Pt surface.

Electrochemical Double-Layer Structure of Pt Electrode/Ionic Liquids Interface Studied by in situ IR-visible Sum Frequency Generation Spectroscopy

The electrochemical interface between a polycrystalline Pt electrode and an ionic liquid 1-butyl-3-methylimidazolium trifluoromethane sulfonate ([bmim]OTf) has been studied by in situ sum frequency generation (SFG) spectroscopy. Potential dependent adsorption and desorption processes of OTf anion as well as [bmim] cation have been observed. A double-layer model of the interface structure has been suggested based on the behavior of both the anion and the cation in the electrochemical potential window. Hysteresis effect has been observed during the desorption and the re-adsorption processes of both the anion and the cation on the Pt surface.

Anion Configuration at the Air/Liquid Interface of Ionic Liquid [bmim]OTf Studied by Sum-Frequency Generation Spectroscopy

The air/liquid interface of a room temperature ionic liquid, 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim]OTf), is investigated using infrared-visible sum frequency generation (SFG) spectroscopy. The SFG spectra clearly show low-frequency modes [CF3-symmetric stretching (ss) mode and SO3-symmetric stretching (ss) mode] of the OTf anion, demonstrating the existence of anions polar oriented at the interface. The amplitude of the CF3-ss peak of the OTf anion has the opposite sign with respect to that of the SO3-ss peak, indicating that OTf anions at the surface have polar ordering where the nonpolar CF3 group points away from the bulk into the air, whereas the SO3 group points toward the bulk liquid. The line width of the SFG peak from the submerged SO3 group is appreciably narrower than that from IR absorption, suggesting the environment of the surface OTf anions is much more homogeneous than that of the bulk. The vibrational calculations also suggest that the anions and the cations form a more specific aggregated configuration at the surface as compared to the bulk.

Trifluoromethanesulfonate (triflate) as a moderately coordinating anion: Studies from chemistry of the cationic coordinatively unsaturated mono- and diruthenium amidinates

Triflate complexes of mono- and diruthenium amidinates, (η 6-C 6R 6)Ru(κ 1-OTf){η 2-R′N C(R′′)NR′} ( 1: R = Me; 2: R = H) and (η 5-C 5Me 5)Ru(μ-η 2- i PrN C(Me)N i Pr)Ru(κ 1-OTf)(η 5-C 5R 5) ( 3: R = Me; 4: R = H), are synthesized, and coordination behavior of the triflate anion to the coordinatively unsaturated ruthenium species is investigated by crystallography and variable temperature (VT) NMR spectroscopy ( 19F, 1H). The monoruthenium amidinate complexes have three-legged piano-stool structures in single crystals, which include a κ 1-OTf ligand with the Ru–O bond of 2.15–2.20 Å. In contrast, reversible dissociation of OTf is observed in variable temperature 1H NMR spectroscopy in liquid states; the activation energy for the dissociation and recombination of the OTf ligand is varied with the substituents on the arene and amidinate ligand in the corresponding ruthenium cation and the solvent used. A typical example of moderately coordinating ability of the OTf ligand is seen in 19F NMR spectra of (η 6-C 6Me 6)Ru(κ 1-OTf){η 2- i PrN C(Me)N i Pr} ( 1a) and (η 6-C 6H 6)Ru(κ 1-OTf){η 2- i PrN C(Me)N i Pr} ( 2a) in CD 2Cl 2 at the temperature range from −90 to 20 °C, in which the OTf anion is dissociated in 1a, whereas 2a has a relatively robust Ru–OTf bond. Combination of crystallography and VT NMR contributes to understanding the difference in coordination behavior of the OTf ligand between two diruthenium amidinates, (η 5-C 5Me 5)Ru(μ-η 2- i PrN C(Me)N i Pr)Ru(κ 1-OTf)(η 5-C 5Me 5) ( 3) and (η 5-C 5Me 5)Ru(μ-η 2- i PrN C(Me)N i Pr)Ru(κ 1-OTf)(η 5-C 5H 5) ( 4); the results suggest that the electron-donating and sterically demanding η 5-C 5Me 5 helps for dissociation of the triflate ligand. Moderate coordinating ability of the triflate anion sometimes provides characteristic reactions of mono- and diruthenium amidinates which differ from the corresponding neutral halogeno-compounds or cationic coordinatively unsaturated homologues bearing fluorinated tetraarylborates; a typical example is given by inhibition of coordination of ethylene to the [(η 6-C 6H 6)Ru{η 2- t BuN C(Ph)N t Bu}] + species by the OTf ligand.

Diversity of triflate coordination modes in neodymocene complexes containing bulky bis(trimethylsilyl)cyclopentadienyl ligands

The reaction of Nd(OTf) 3 (OTf = O 3SCF 3) with two molar equivalents of LiCp″ [Cp″ = C 5H 3(SiMe 3) 2-1,3] in thf solution generated the blue, tetrametallic dimer [{Nd(η 5-Cp″) 2(μ 2-κ 2-O 3SCF 3)(μ 3-κ 3-O 3SCF 3)Li(thf)} 2] ( 1a), which has been shown by X-ray crystallography to contain a tricyclic, ladder-like scaffold with Nd and Li cations and OTf anions at its core. Compound 1a was relatively labile, being readily cleaved by a variety of donor molecules. 18-Crown-6 efficiently encapsulated the lithium cation to yield the salt [Li(18-crown-6)][Nd(η 5-Cp″) 2(κ 1-O 3SCF 3)(κ 2-O 3SCF 3)] ( 2). Addition of N,N,N′ ,N′-tetramethylethylenediamine (tmeda) afforded the monomeric bimetallic [Nd(η 5-Cp″) 2(μ 2-κ 2-O 3SCF 3) 2Li(tmeda)] ( 3), while the more rigid donor 2,2′-bipyridine (bipy) produced the neutral, Li-free complex [Nd(η 5-Cp″) 2(κ 1-O 3SCF 3)(bipy)] ( 4). The molecular and crystal structures of 2– 4, as well as that of 1b, the unstable La analogue of 1a, have been determined by X-ray methods.

Hypervalent, Low-Coordinate Phosphorus(III) Centers in Complexes of the Phosphadiazonium Cation with Chelate Ligands

Trifluoromethylsulfonyloxy-(2,4,6-tri-tert-butylphenylimino)phosphine, Mes*NPOTf (Mes = 2,4,6-tri-tert-butylphenyl, OTf = trifluoromethanesulfonate, triflate) reacts quantitatively with the multifunctional ligands 2,2'-bipyridine (2,2'-BIPY), N,N,N',N'-tetramethylethylenediamine (TMEDA), 1,2-bis(diethylphosphino)ethane (DEPE), 1,2-bis(diphenylphosphino)ethane (DIPHOS), and N,N,N',N' ',N' '-pentamethyldiethylenetriamine (PMDETA) to give the Lewis acid-base complexes [Mes*NP(2,2'-BIPY)][OTf], [Mes*NP(TMEDA)][OTf], [Mes*NP(DIPHOS)][OTf], [Mes*NP(DEPE)][OTf], and [Mes*NP(PMDETA)][OTf], respectively. Single-crystal X-ray diffraction studies indicate that the closest contact of the ligand donor atoms occurs at phosphorus in all cases, affecting significant displacement of the OTf anion. The resulting cations [Mes*NP(L)]+ are best described as complexes of a neutral chelating ligand on a phosphadiazonium Lewis acceptor, and highlight the potential for electron-rich centers to behave as Lewis acids despite the presence of a lone pair of electrons at the acceptor site. More importantly, the new complexes represent rare examples of systems containing hypervalent, low-coordinate phosphorus(III) centers.

Synthesis and reactivity of [Re(ClCH 2Cl)(CO) 3L][A] and [Re(OSO 2CF 3)(CO) 3L] (L=Ph 2PO(CH 2) 2OPPh 2; A=BF 4, [B{3,5-(CF 3) 2C 6H 3} 4]): the crystal structure of [Re(OSO 2CF 3)(CO) 3L] and [Re(NCCH 3)(CO) 3L][BF 4

The previously reported complex [ReBr(CO) 3L] (L=1,2-bis[diphenylphosphinite]ethane) was reacted with AgBF 4 in dichloromethane to yield fac-[Re(ClCH 2Cl)(CO) 3L][BF 4] ( 1). By contrast, reaction of the same starting material with AgOTf (OTf=OSO 2CF 3) afforded the neutral complex fac-[Re(OTf)(CO) 3L] ( 2). When 2 was treated with NaBAr F (BAr F=[B{3,5-(CF 3) 2C 6H 3} 4]) in the same solvent, the complex fac-[Re(ClCH 2Cl)(CO) 3L][BAr F] ( 3) was formed. Substitution of the loosely held ligand CH 2Cl 2 in 1 and 3 resulted in the formation of compounds of the type [Re(X)(CO) 3L][A] (X=H 2O, CH 3CN; A=BF 4 −, BAr F −), whereas the removal of OTf in 2 only was possible to form [Re(NCCH 3)(CO) 3L][OTf]. All the new complexes were characterized by 1H, 13C{ 1H}, 31P{ 1H} and 19F{ 1H} NMR as well as by IR spectroscopies. In the case of compounds 2 and [Re(NCCH 3)(CO) 3L][BF 4], X-ray suitable crystals were isolated. In both cases, the Re atom adopts six-coordination by bonding to the three CO ligands, the two P atoms of L and either the oxygen atom of the OTf anion or the N atom of CH 3CN.

Chalcogeno-urea ligands on a phosphadiazonium Lewis acceptor: a new synthetic approach to Ch-P bonds (Ch = O, S, Se).

The isolation and characterization of the first intermolecular chalcogeno-urea complexes of iminophosphines are described. Trifluoromethylsulfonyloxy(2,4,6-tri-tert-butylphenylimino)phosphine, MesNPOTf, reacts quantitatively with chalcogenoimidazolines (ChIm, Ch = O, S, Se) and 1,3-dimethyldiphenylurea (OU) to give Lewis acid-base complexes, [MesNP.ChIm]OTf and [MesNP.OU]OTf. Single crystal X-ray diffraction studies indicate that the closest contact of the chalcogeno-urea donor occurs at phosphorus in all cases, representing compounds that contain examples of O-P, S-P, and Se-P coordinate bonds. In all complexes, coordination of the ligand causes significant displacement of the OTf anion, and the resulting cations [MesNP.L](+) are best described as complexes of a neutral ligand on a phosphadiazonium Lewis acceptor. As such, the complex ions [MesNP.L](+) are novel examples of cationic systems containing dicoordinate phosphorus centers. The complexes highlight the potential for electron-rich centers to behave as Lewis acids despite the presence of a lone pair of electrons at the acceptor site.

1H and 19F PGSE diffusion studies on iridium PHOX complexes: counterion and solvent dependences.

1H and (19)F pulsed gradient spin-echo (PGSE) diffusion studies on cationic mono- and trinuclear iridium complexes containing the PHOX chiral P,N-auxiliary (S)-4-tert-butyl-2-[2-(di-o-tolylphosphino)phenyl]-4,5-dihydrooxazole with the anions BF(4)(-), PF(6)(-), OTf(-), B(C(6)F(5))(4)(-), and BArF(-) in methanol, chloroform, methylene chloride, and 1,2-dichloroethane are reported. In chloroform, the anion and cation within each salt afford almost the same, relatively small, diffusion constant (D-value) suggesting strong ion-pairing. In methanol, the D-value for the cation is the same in the five mononuclear salts, suggesting that the cation is moving independently of the anion (no ion-pairing). In methylene chloride and 1,2-dichloroethane the diffusion data suggest a mixed picture for the five anions. While the smaller BF(4)(-), PF(6)(-), and OTf(-) anions do not affect the translation of the cations, the larger boron-based anions B(C(6)F(5))(4)(-) and BArF(-) clearly slow the motions of the cations. However, it would seem that for all five anions there is some--but not complete--ion pairing in these two solvents.

Interaction of biotin with Mg-O bonds: bifunctional binding and recognition of biotin and related ligands by the Mg(15-crown-5)2+ unit.

The interaction between biotin and the macrocyclic magnesium complex Mg(15-crown-5)(Otf)2 (15-crown-5 is 1,4,7,10,13-pentaoxacyclopentadecane, Otf(-) is trifluoromethanesulfonate anion) in solution was studied as a model for metal-biotin interactions that may be important in its speciation and function. Shifts in the solution IR spectrum establish that the interaction is dominated by ligation between the carbonyl oxygen of the ureido ring of biotin and the Mg2+ cation. However, comparative binding studies using NMR spectroscopy and conductivity reveal a substantial enthalpic contribution to binding that arises from interactions between the ureido -NH moiety and the macrocyclic ring. This is interpreted in terms of a weak-to-moderate hydrogen bond formed between the -NH group and an oxygen from the crown, which is simultaneously coordinated to Mg2+. This hypothesis is reinforced by quantitative examination of the binding of N-methylated derivatives of 2-imidazolidone, which shows that N,N'-dimethylation decreases the affinity of Mg(15-crown-5)(Otf)2 for the ligand by 2 orders of magnitude. This can be understood in terms of the structure of Mg(15-crown-5)(Otf)2. It shows a pentagonal bipyramidal coordination geometry where the five equatorial positions are occupied by the macrocyclic oxygen donors. The axial positions are occupied by weakly coordinating Otf(-) anions, which are readily displaced by biotin and related derivatives. The M-O(crown) bond distance ranges from 2.1 to 2.3 A, providing structural complementarity for the 2.2 A C=O...HN- bite distance in the ureido group, which leads to strong interaction. The contribution from hydrogen bonding illustrates the importance of second-shell interactions in the biocoordination chemistry of Mg2+. These can serve to organize cofactor interactions with biomolecules, as was recently demonstrated for a biotin-selective RNA aptamer that depends on a direct biotin-magnesium interaction for recognition of biotin (Nix, J.; Sussman, D.; Wilson, C. J. Mol. Biol. 2000, 296, 1235-1244). These results are significant in the context of the observed magnesium requirement in biotin-dependent carboxylase enzymes, where noncovalent interactions with biotin may be important in its activation toward carboxylation in the first step of biotin-dependent CO2 transfer. The synthetic system presented here also suggests that the Mg-O bond may be considered a constituent design element in the rational preparation of complexes to bind and recognize biotin.

Electronic and medium effects on the rate of arene C [bond] H activation by cationic Ir(III) complexes.

A detailed mechanistic study of arene C [bond] H activation in CH(2)Cl(2) solution by Cp(L)IrMe(X) [L = PMe(3), P(OMe)(3); X = OTf, (CH(2)Cl(2))BAr(f); (BAr(f) = B[3,5-C(6)H(3)(CF(3))(2)](4))(-)] is presented. It was determined that triflate dissociation in Cp(L)IrMe(OTf), to generate tight and/or solvent-separated ion pairs containing a cationic iridium complex, precedes C [bond] H activation. Consistent with the ion-pair hypothesis, the rate of arene activation by Cp(L)IrMe(OTf) is unaffected by added external triflate salts, but the rate is strongly dependent upon the medium. Thus the reactivity of Cp(PMe(3))IrMe(OTf) can be increased by almost 3 orders of magnitude by addition of (n-Hex)(4)NBAr(f), presumably because the added BAr(f) anion exchanges with the OTf anion in the initially formed ion pair, transiently forming a cation/borate ion pair in solution (special salt effect). In contrast, addition of (n-Hex)(4)NBAr(f) to [CpPMe(3)Ir(Me)CH(2)Cl(2)][BAr(f)] does not affect the rate of benzene activation; here there is no initial covalent/ionic pre-equilibrium that can be perturbed with added (n-Hex)(4)NBAr(f). An analysis of the reaction between Cp(PMe(3))IrMe(OTf) and various substituted arenes demonstrated that electron-donating substituents on the arene increase the rate of the C [bond] H activation reaction. The rate of C(6)H(6) activation by [Cp(PMe(3))Ir(Me)CH(2)Cl(2)][BAr(f)] is substantially faster than [Cp(P(OMe)(3))Ir(Me)CH(2)Cl(2)][BAr(f)]. Density functional theory computations suggest that this is due to a less favorable pre-equilibrium for dissociation of the dichloromethane ligand in the trimethyl phosphite complex, rather than to a large electronic effect on the C [bond] H oxidative addition transition state. Because of these combined effects, the overall rate of arene activation is increased by electron-donating substituents on both the substrate and the iridium complex.