Acid−Base Equilibria in Nonpolar Media. 2. 1 Self-Consistent Basicity Scale in THF Solution Ranging from 2-Methoxypyridine to EtP 1 (pyrr) Phosphazene

ArticleinThe Journal of Organic Chemistry 67(6):1873-81 · April 2002with 160 Reads
DOI: 10.1021/jo016185p · Source: PubMed
Abstract
Relative ion-pair basicities Delta(pK)(ip) of 25 substituted aryl and alkyl iminophosphoranes (phosphazenes) and 20 other N-bases (various pyridines, amines, amidines) have been measured in THF medium using the UV-Vis and/or (13)C NMR methods. The Delta(pK)(ip) values were corrected for ion pairing using the Fuoss equation to obtain relative ionic basicities Delta(pK)(alpha). Based on the measurements, a basicity scale ranging from 2-methoxypyridine to EtP(1)(pyrr) and having a total span over 18 pK units has been created. The scale has been anchored to the pK(alpha) value of triethylamine (pK(alpha) = 12.5). The results are compared to pK(a) values in various other solvents and in the gas phase. The pK(alpha) values give better correlations than the pK(ip) values, thus indirectly validating the procedure of correction for ion pairing. The predictability of the basicity together with suitable spectral properties in the UV range make the phenylphosphazenes convenient neutral indicators in the high basicity range where the choice of neutral indicators is very limited.

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    We have reported for the first time designed silylene superbases involving intramolecular H(+)π interaction using density functional theory (DFT) calculations. The non-covalent interactions augment the proton affinity values by ∼13 kcal mol(-1) in the designed superbase () compared to the acyclic silylene, [:Si(NMe2)2]. These divalent Si(ii) compounds can act as powerful neutral organic superbases in gas and solvent phases. The DFT calculations performed at the B3LYP/6-311+G**//B3LYP/6-31+G* level of theory showed that the gas phase proton affinity of the paracyclophane based silylene superbase () reaches up to ∼271.0 kcal mol(-1), which is the highest in paracyclophane Si(ii) compounds. In tetrahydrofuran solvent medium, the calculated proton affinity of was found to be 301.4 kcal mol(-1). The paracyclophane-based silylene systems are used for binding with small alkali metal ions. The calculated results showed that these systems selectively bind to lithium ions over sodium ions due to the small size of lithium ions which is well fitted in the space between the silicon atom and the phenyl ring. Furthermore, we have used the lithiated silylene and (which exhibits bis-protonation) for gas storage (CO and CO2). The calculated results showed both the lithiated silylene and bind preferentially to CO2 than CO. The calculated gravimetric density of CO2 is found to be 26.97 wt% for -Li2-(CO2)4. The energy decomposition analysis (EDA) has been performed to investigate the role of various contributing factors to the total binding strength of the CO2 or CO molecules with lithiated silylene superbases. EDA reveals that the electrostatic energy and polarization energy are the major driving force for higher total interaction energy of the lithiated-silylene-CO2 complex than the lithiated-silylene-CO complex. The lithiated silylene systems showed a higher binding energy with CO2 than the previously reported imidazopyridamine at the same level of theory. These results suggest that the lithiated silylene systems can be used as a more efficient CO2 storage material than the aforementioned system. The calculated desorption energies per CO2 and CO (ΔEDE) also indicate the recyclable property of the materials.
  • Article
    An inner-sphere synthetic cycle for the hydrophosphination of alkenes is proposed, based on observed [2 + 2] cycloaddition of a wide range of alkenes at a coordinatively unsaturated Ru=PR2 complex. Key intermediates in the cycle were prepared, and their reactions with various organic acid/base pairs were examined to identify both new ruthenium precursors and base cocatalysts that allow turnover of the proposed cycle. Two new cationic ruthenium indenyl phosphine complexes were isolated and structurally characterized. Although preliminary screening studies show the moderate activity of these and related neutral phosphido complexes for catalytic hydrophosphination of acrylonitrile by both HPPh2 and HPCy2, and comparable activity for the hydrophosphination of tert-butyl acrylate by HPPh2, no activity was observed for the analogous hydrophosphination of 1-hexene. This is attributed to strong binding of the substrate phosphine to the unsaturated, planar Ru - PR2 fragment generated in situ, which inhibits the inner-sphere, alkene cycloaddition mechanism. An alternative, outer-sphere Michael addition process, involving a saturated complex with a strongly nucleophilic pyramidal Ru-PR2 ligand, is proposed to rationalize the observed selectivity for catalytic hydrophosphination of activated, but not simple, alkenes. Implications for further catalyst development are discussed.
  • Article
    The hydrogenolysis of titanium-nitrogen bonds in a series of bis(cyclopentadienyl) titanium amides, hydrazides and imides by proton coupled electron transfer (PCET) is described. Twelve different N-H bond dissociation free energies (BDFEs) among the various nitrogen-containing ligands were measured or calculated and effects of metal oxidation state and N-ligand substituent were determined. Two metal hydride complexes, (η5-C5Me5)(py-Ph)Rh-H (py-Ph = 2-pyridylphenyl, [Rh]-H) and (η5-C5R5)(CO)3CrH ([Cr]R-H, R= H, Me) were evaluated for formal H-atom transfer reactivity and were selected due to their relatively weak M-H bond strengths yet ability to activate and cleave molecular hydrogen. Despite comparable M-H BDFEs, disparate reactivity between the two compounds was observed and was traced to the vastly different acidities of the M-H bonds and overall redox potentials of the molecules. With [Rh-H], catalytic synthesis of ammonia, silylamine and N,N-dimethylhydrazine have been accomplished from the corresponding titanium(IV) complex using H2 as the stoichiometric H-atom source. The data provided in this study provides the thermochemical foundation for the synthesis of NH3 by proton coupled electron transfer at a well-defined transition metal center.
  • Article
    Experimental gas-phase superbasicity scale spanning 20 orders of magnitude and ranging from bicyclic guanidine 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene to triguanidinophosphazenes and P3 phosphazenes is presented together with solution basicity data in acetonitrile and tetrahydrofuran. The most basic compound in the scale-triguanidinophosphazene Et-N═P[N═C(NMe2)2]3-has the highest experimental gas-phase basicity of an organic base ever reported: 273.9 kcal mol(-1). The scale includes besides the higher homologues of classical superbasic phosphazenes and several guanidino-substituted phosphazenes also a number of recently introduced bisphosphazene and bis-guanidino proton sponges. This advancement was made possible by a newly designed Fourier transform ion cyclotron resonance (ICR) mass spectrometry setup with the unique ability to generate and control in the ICR cell sufficient vapor pressures of two delicate compounds having low volatility, which enables determining their basicity difference. The obtained experimental gas-phase and solution basicity data are analyzed in terms of structural and solvent effects and compared with data from theoretical calculations.
  • Chapter
    The fixation of atmospheric dinitrogen to ammonia using molecular catalysts has been a long-standing challenge in homogeneous catalysis and synthetic chemistry. New approaches to this problem may offer more energy-efficient and carbon-neutral routes to this important industrial compound. Despite the ubiquity of ammine, amide, imide and diazenide ligands in coordination chemistry, little thermodynamic data is available for understanding N-H bond strengths in molecules bearing these nitrogenous fragments. This article presents an overview of both computational and experimental approaches for the determination of N-H bond dissociation free energies in a variety of compounds relevant to nitrogen fixation to ammonia. The influence of metal oxidation state, ancillary ligand and identity of the nitrogen donor are highlighted. Implications for future design of molecular systems for the reduction of dinitrogen are discussed.
  • Article
    The endothermic reverse water‐gas shift reaction (rWGS) for direct CO2 hydrogenation to CO is an attractive approach to carbon utilization. However, direct CO2 hydrogenation with molecular catalysts generally gives formic acid instead of CO as a result of the selectivity of CO2 insertion into M‐H bonds. Based on the photochemical inversion of this selectivity, several synthetic pathways are presented for CO selective CO2 reduction with a nickel pincer platform including the first example of a photodriven rWGS cycle at ambient conditions.
  • Article
    The endothermic reverse water‐gas shift reaction (rWGS) for direct CO2 hydrogenation to CO is an attractive approach to carbon utilization. However, direct CO2 hydrogenation with molecular catalysts generally gives formic acid instead of CO as a result of the selectivity of CO2 insertion into M‐H bonds. Based on the photochemical inversion of this selectivity, several synthetic pathways are presented for CO selective CO2 reduction with a nickel pincer platform including the first example of a photodriven rWGS cycle at ambient conditions.
  • Article
    A reinvestigation of the experimental parameters used when polymerizing anionically a cyclopropane-1,1-dicarboxylate indicates that several key steps have to be considered, but that other steps—including some routinely used up to now—although not harmful, are nevertheless useless. A robust protocol is thus designed whose implementation allows us to routinely control the polymerization of di-n-propyl cyclopropane-1,1-dicarboxylate, used as a model monomer for the entire family of cyclopropyl monomers geminally activated by two ester groups. Degrees of polymerization of up to 200 can be routinely obtained, with polydispersity indexes lower than 1.05. In some cases, a small shoulder can be identified in the size exclusion chromatography (SEC) chromatogram, although not systematically. The close proximity between the two distributions affects the polydispersity index only marginally (Mw/Mn still in the 1.01–1.05 range), and highly resolutive SEC columns are needed in order to identify the issue. Finally, attempts to use organic superbases other than tBuP4 have failed thus far.
  • Article
    Equilibrium constants for the proton transfer reaction between pyridines and trifluoroacetic acid have been measured in some room temperature ionic liquids (ILs) of different cation-anion compositions. The experimental equilibrium constant for ion pair formation have been corrected according to the Fuoss equation. The calculated equilibrium constant for the formation of free ions have been taken as a quantitative measure of the base strength in IL solutions and compared with the relative constants in water. The effect of IL composition has been discussed for a series of fixed IL anion and fixed IL cation. Finally, the sensitivity of the proton transfer reaction to the electronic effect of the substituent groups on the pyridine ring has been quantified by applying the Hammett equation. A more marked levelling effect on the base strength was observed in ILs than in water. The Hammett reaction constants, rho, were then correlated with some solvent parameters, according to a multiparametric analysis which shows that both specific hydrogen bond donor/acceptor and non-specific interactions play an important role, with alpha and permittivity being the main parameters affecting the IL ability to differentiate the strength of the base.
  • Article
    The nitrogen atom in cyclic hetero-oligophosphane cyclo-P4Mes4C(NCy) (1) can be protonated with HCl or HBF4. The resulting cation was isolated as the BF4-, AlCl4- and GaCl4- salts and characterised by NMR spectroscopy, mass spectrometry and X-ray crystallography. According to quantum mechanical calculations, the basicity of compound 1 is comparable to that of methyl-substituted monophospha(III)guanidine. The pKα value of 1 was also determined experimentally with indicator bases.
  • Article
    Efficient and versatile "click"-based post-polymerization tools for the straightforward modification of especially polymers with sophisticated architectures play a significant role in polymer chemistry. We here report on the synthesis of supramolecular three-arm star polymers by a combination of RAFT polymerization and the thio-bromo "click" reaction, consequently exploring the potential of this particular thiol-halogen "click" reactions as post-polymerization tool. Therefore, two new trivalent chain transfer agents (CTAs) bearing either trithiocarbonate or dithiobenzoate groups were designed and investigated for the RAFT polymerization of n-butyl acrylate (nBA) and styrene. Kinetic investigations of CTA/monomer combinations were performed to obtain star-shaped poly(n-butyl acrylate)s (PnBA) and poly(styrene)s (PS) with adjustable molecular weights and well controllable polydispersities in the range of 1.13–1.30. The capped thiol-groups were released by aminolysis and quantitatively converted into barbiturate or imidazolium end-groups, allowing to prepare functionalized supramolecular star polymers via this particular thio-bromo "click" reaction. Thereby, the best results for aminolysis were not obtained in a one-pot reaction as expected but in a two step process, while acetonitrile has been shown to be essential for the success of the reaction in contrast to the pKa-value of the investigated bases. As proven via NMR spectroscopy and mass spectrometry all desired supramolecular star polymers were obtained with an exceptional high extend of end-group functionality (>95%) underlying the potential of this specific "click"-based post-polymerization modification tool in combination with RAFT polymerization.
  • Article
    The conversion of metal nitride complexes to ammonia may be essential to dinitrogen fixation. We report a new reduction pathway that utilizes ligating acids and metal-ligand cooperation to effect this conversion without external reductants. Weak acids such as 4-methoxybenzoic acid and 2-pyridone react with nitride complex [(H-PNP)RuN](+) (H-PNP = HN(CH2CH2P(t)Bu2)2) to generate octahedral ammine complexes that are κ(2)-chelated by the conjugate base. Experimental and computational mechanistic studies reveal the important role of Lewis basic sites proximal to the acidic proton in facilitating protonation of the nitride. The subsequent reduction to ammonia is enabled by intramolecular 2H(+)/2e(-) proton-coupled electron transfer from the saturated pincer ligand backbone.
  • Article
    Wir präsentieren die erste Superbase, MHPN, mit zwei wechselwirkenden Phosphorylidfunktionalitäten. Anders als klassische Protonenschwämme weist diese neuartige Verbindungsklasse zwei Kohlenstoffatome als Basizitätszentren auf, die durch ein Naphthalinrückgrat in räumliche Nähe gezwungen werden. Das Bisylid zeigt einen experimentellen pKBH+-Wert von 33.3±0.2 auf der Acetonitrilskala und eine theoretische Protonenaffinität in der Gasphase von 277.9 kcal mol−1 (M062X/6-311+G**//M062X/6-31G*+ZPVE). Dieser Wert ist um 15 kcal mol−1 höher als für das entsprechende Monoylid. Der Ursprung der ungewöhnlich hohen Basizität von MHPN wurde mit NMR-spektroskopischen Methoden, Kristallstrukturanalysen und theoretischen Berechnungen untersucht, wobei ein schneller Austausch des “aciden” Protons zwischen den beiden basischen Kohlenstoffatomen eine große Rolle spielt.
  • Article
    A series of bis(phosphine) molybdenum(ii) diazenides [(dppe)2Mo(NNCy)(I)], [(dppe)2(CH3CN)Mo(NNCy)][BArF24] and [(dppe)2)(3,5-(CF3)2C6H3CN)Mo(NNCy)][BArF24] (dppe = 1,2-bis(diphenylphosphino)ethane; Cy = cyclohexyl; ArF24 = (3,5-(CF3)2C6H3)4) were synthesized and structurally characterized. Treatment of the diazenido complexes with a stoichiometric amount of [H(OEt2)2][BArF24] afforded the corresponding molybdenum(iv) hydrazido species [(dppe)2Mo(NNHCy)(I)][BArF24], [(dppe)2(CH3CN)Mo(NNHCy)][BArF24]2 and [(dppe)2(3,5-(CF3)2C6H3CN)Mo(NNHCy)][BArF24]2, enabling the study of N-H bond dissociation free energies (BDFEs) in the classical Chatt-type bis(phosphine) diazenide platform as a function of ligand (L) trans to the nitrogenous fragment. Deprotonation and electrochemical experiments established that the trans nitrile 3,5-(CF3)2C6H3CN afforded the least reducing molybdenum(iv) hydrazido complex in the series ( = -1.32 V vs. Fc/Fc(+)) with the most acidic N-H bond (pKa < 2.6, THF), whereas the ligands CH3CN ( = -1.60 V, pKa < 5.5) and I(-) ( = -2.03 V, pKa = 9.3) gave more reducing complexes with less acidic N-H bonds. Computational (DFT) studies confirm weak N-H bond strengths of 32.8 (L = I(-)), 35.4 (L = CH3CN) and 36.2 kcal mol(-1) (L = 3,5-(CF3)2C6H3CN) in the hydrazido series.
  • Article
    Herein we present the first superbase MHPN with two interacting P-ylide entities. Unlike classical proton sponges, this novel compound class has carbon atoms as basicity centers which are forced into close proximity by a naphthalene scaffold. The bisylide exhibits an experimental pKBH(+) value of 33.3±0.2 on the MeCN scale and a calculated gas-phase proton affinity of 277.9 kcal mol(-1) (M062X/6-311+G**//M062X/6-31G*+ZPVE method) exceeding that of the corresponding monoylide by nearly 15 kcal mol(-1) . The origin of the unexpectedly high basicity of the new bisylide was investigated by NMR spectroscopic methods, single-crystal X-ray diffraction as well as theoretical calculations and can be partly attributed to the rapid exchange of the "acidic" proton between the two basic carbon atoms after protonation.
  • Article
    Thiol-acetoacetate-acrylate ternary dual-curing thermosets were prepared by a sequential process consisting of thiol-Michael addition to acrylates at room temperature followed by Michael addition of acetoacetates to acrylates at moderately elevated temperature. The curing sequence can be controlled with the help of the different acidities of the protons on thiol and acetoacetate groups, the favorable pKa of the base used as catalyst and the self-limiting character of Michael additions. The latency of the curing steps can be regulated by selection of the right catalysts, temperature and curing conditions. The properties of the intermediate and final materials can be tuned by changing the structure of the monomers and the contribution of both Michael addition reactions.
  • Article
    Although scores of transition metal complexes incorporating ammonia or water ligands have been characterized over the past century, little is known about how coordination influences the strength of the nitrogen-hydrogen and oxygen-hydrogen bonds. Here we report the synthesis of a molybdenum ammonia complex supported by terpyridine and phosphine ligands that lowers the nitrogen-hydrogen bond dissociation free energy from 99.5 (gas phase) to an experimentally measured value of 45.8 kilocalories per mole (agreeing closely with a value of 45.1 kilocalories per mole calculated by density functional theory). This bond weakening enables spontaneous dihydrogen evolution upon gentle heating, as well as the hydrogenation of styrene. Analogous molybdenum complexes promote dihydrogen evolution from coordinated water and hydrazine. Electrochemical and theoretical studies elucidate the contributions of metal redox potential and ammonia acidity to this effect.
  • Article
    (POCOP)IrI(CO) [POCOP = κ³-C6H3-2,6-(OPR2)2 for R = tBu, ⁱPr] and (PCP)IrI(CO) [PCP = κ³-C6H3-2,6-(CH2PR2)2 for R = tBu and ⁱPr] complexes can add hydrogen via two distinct pathways. When R = tBu, (POCOP)Ir(CO) and (PCP)Ir(CO) complexes only add hydrogen via a proton-catalyzed pathway due to steric effects, yielding trans-dihydride complexes. For R = ⁱPr, both systems oxidatively add hydrogen to give cis-dihydride complexes which thermally isomerize to more thermodynamically favorable trans-dihydride species, consistent with previous reports. Proton-catalyzed hydrogen addition pathways are also observed for both ⁱPr-substituted (pincer)Ir(CO) complexes. (PCP)Ir(CO) complexes add hydrogen under milder conditions than the analogous POCOP species. Intermediate hydrido-pyridine Ir(III) carbonyl complexes from the proton-catalyzed pathway have been synthesized and characterized. This is the first report of a series of complexes shown to add hydrogen via concerted oxidative addition or a proton-catalyzed pathway to the same iridium center.
  • Article
    Nitrogen bases containing one or more pushing amino-group(s) directly linked to a pulling cyano, imino, or phosphoimino group, as well as those in which the pushing and pulling moieties are separated by a conjugated spacer (C═X)n, where X is CH or N, display an exceptionally strong basicity. The n-π conjugation between the pushing and pulling groups in such systems lowers the basicity of the pushing amino-group(s) and increases the basicity of the pulling cyano, imino, or phosphoimino group. In the gas phase, most of the so-called push-pull nitrogen bases exhibit a very high basicity. This paper presents an analysis of the exceptional gas-phase basicity, mostly in terms of experimental data, in relation with structure and conjugation of various subfamilies of push-pull nitrogen bases: nitriles, azoles, azines, amidines, guanidines, vinamidines, biguanides, and phosphazenes. The strong basicity of biomolecules containing a push-pull nitrogen substructure, such as bioamines, amino acids, and peptides containing push-pull side chains, nucleobases, and their nucleosides and nucleotides, is also analyzed. Progress and perspectives of experimental determinations of GBs and PAs of highly basic compounds, termed as "superbases", are presented and benchmarked on the basis of theoretical calculations on existing or hypothetical molecules.
  • Article
    [Figure not available: see fulltext.] Air-stable and crystalline tetraaminophosphonium tetrafluoroborates possessing chiral, enantiomerically pure 1,2-diamine moiety have been synthesized by a three-step sequential one-pot approach. The tetrafluoroborate salts can be purified by recrystallization or chromatography and subsequently converted to the phosphazene bases by treatment with t-BuOK. Basicity values in tetrahydrofuran have been measured for the obtained phosphazene bases by means of spectrophotometric titration.
  • Article
    The gas phase proton affinities and basicities of a [1.1.1]‐(2,6)‐pyridinophane and its derivatives having one ‐CH2‐ bridge replaced by ‐CH=CH‐ and ‐CH2‐CH2‐ are examined by the DFT B3LYP/6‐311+G(2df,p)//B3LYP/6‐31G(d) method. The calculated proton affinities are within the range of 253.8 and 294.8 kcal mol⁻¹. The upper‐scale basicity is consequence of unfavorable steric interactions in neutral bases and diminishing of former in conjugate acids with forming of intramolecular hydrogen bonds (IMHB) being amplified by substitution in the para position of pyridine rings by electron‐releasing substituents. The inspection of the nature of IMHBs reveals that IMHB is of a bifurcated type with energy spanning a range between 5.7 and 7.4 kcal mol⁻¹. The basicities of pyridinophanes are examined in acetonitrile. It is found that their pKa values span a range between 18.9 and 31.3 units.
  • Article
    Full-text available
    Pyridine substituted with one and two bicyclic guanidine groups has been studied as a potential source of super-bases. 2-{hpp}C5H4N (I) and 2,6-{hpp}2C5H3N (II) (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) were protonated using [HNEt3][BPh4] to afford [I-H][BPh4] (1a), [II-H][BPh4] (2) and [II-H2][BPh4]2 (3). Solution-state 1H and 15N NMR spectroscopy shows a symmetrical cation in 2, indicating a facile proton exchange process in solution. Solid-state 15N NMR data differentiates between the two groups, consistent with a mixed guanidine/guanidinium. X-ray diffraction data are consistent with protonation at the imine nitrogen, confirmed for 1a by single crystal neutron diffraction. The crystal structure of 1a shows association of two [I-H]+ cations within a cage of [BPh4]– anions. Computational analysis performed in the gas-phase and in MeCN solution show that the free energy barrier to transfer a proton between imino centers in [II-H]+ is an order of magnitude lower in MeCN than in the gas-phase. The results provide evidence that linking hpp-groups with the pyridyl group stabilizes the protonation center, thereby increasing the intrinsic basicity in the gas phase, while the bulk prevents efficient cation solvation, resulting in diminished pKa(MeCN) values. Spectrophotometrically measured pKa values are in excellent agreement with calculated values and confirm that I and II are super-bases in solution.
  • Article
    The addition of an N-heterocyclic carbene to the carbonyl group of an α,β,γ,δ-unsaturated enol ester affords a hemiacetal azolium intermediate that enables a cascade olefin isomerization/Diels-Alder reaction, for which mechanistic studies implicate Lewis base catalysis. Preliminary studies into the utility of the products have been undertaken with reductive and oxidative cleavage, giving materials for potential use in complex-target synthesis. NHC-catalyzed cascade: The addition of an N-heterocyclic carbene (NHC) to the carbonyl group of an α,β,γ, δ-unsaturated enol ester affords a hemiacetal azolium intermediate that enables a cascade olefin isomerization/Diels-Alder reaction. Preliminary studies into the utility of the products using reductive and oxidative cleavage gave substrates for potential use in the synthesis of complex targets.
  • Article
    Amines form proton-transfer ion pairs of the type BH+ In- with acidic indicator hydrocarbons InH in THF. Relative equilibrium constants measured for a number of amines in THF differ substantially from ionic pKa values for BH+ in acetonitrile or DMSO. The results show that ion pair dissociation constants of the protonated amine ion pairs in THF vary by several orders of magnitude and point out that at the present time amines cannot be placed quantitatively on any of the ion pair acidity scales currently in use for neutral acids in THF.
  • Article
    The rate coefficients for proton transfer from the protonated forms of 1,8-bis(dimethylamino)-(I) and 1,8-bis-(diethylamino)-naphthalene (II) to hydroxide ion have been determined using the temperature-jump technique. For (I) in aqueous solution, the rate is five orders of magnitude below the diffusion limited rate expected for thermodynamically favourable proton transfer from a nitrogen acid. In 20% dioxan–water (v/v), the rate for (II) is ca. 102 times lower than for (I). Proton transfers involving (I) and (II) occur slowly largely because of a strong intramolecular hydrogen bond in the protonated amines. Steric strain in the transition states may also contribute and is partly responsible for the large rate difference observed for (I) and (II).
  • Article
    A series of RNP(Pyrr)3 iminophosphoranes (P1 phosphazenes), where R is amino-, α-naphthyl- or substituted phenyl group, is prepared by the Kirsanov reaction and characterized by FT NMR and other properties. The ΔpKa-values of the 12 different synthesized phosphazenes and C6H5NP(NMe2)3 are determined in acetonitrile relative to the reference bases using 13C NMR spectroscopy. The obtained pKa-values are compared with the corresponding values of RNH2 amines. The pKa-values of the synthesized phosphazenes in acetonitrile range from 14.6 to 26.8 pKa units.
  • Article
    A continuous self-consistent quantitative UV−vis spectrophotometric pKa scale of strong acids in acetonitrile has been created. The 36 compounds studied include phenols, phenylmalononitriles, sulfonimides, sulfonic acids, and sulfonimides modified with Yagupolskii's superacceptor substituents. The scale spans about 13 pKa units and consists of 74 independent equilibrium constant measurements, each describing the relative acidity of two acids. The method of measurements eliminates the need for the direct determination of the acidity of the medium. The most acidic compounds studied have pKa values around 4 in acetonitrile and can be regarded as true superacids. The scale is anchored to the pKa value 11.0 for picric acid. The status of 2,4,6-(SO2CF3)3C6H2OH as the most acidic phenol presently known (pKa = 4.9) is confirmed. It is shown that the replacement of an O fragment with N−SO2CF3 in a −SO2− group in aromatic sulfonimides results in an acidity increase of more than 105 times. The most acidic compound in the scale is 4-ClC6H4SO(NSO2CF3)NHSO2C6H4-4-NO2 (pKa = 3.75). The present results together with those from the other authors furnish a unified scale of acidity in acetonitrile ranging from 4 to 27 pKa units and set solid ground for pKa measurements of strong acids in acetonitrile.
  • Article
    A revised cesium ion pair acidity scale relative to fluorene at pK(a) = 22.90 is presented. The range of the scale is extended 3 pK units by including more acidic indicators and now encompasses pK(a)S from 15.62 to 38.73. The accuracy of the scale has been improved by multiple equilibria among indicators. Uncertainties between close lying indicators is generally less than +/- 0.01 pK unit. Bronsted plots are constructed for series of fluorenyl and arylmethyl hydrocarbons. From these plots, the acidity value for toluene in THF was extrapolated (pK(a) = 40.9). Comparisons are drawn to ionic acidities in dimethyl sulfoxide.
  • Article
    More than 70 equilibrium constants K between acids and bases, mainly phosphine derivatives, have been measured in tetrahydrofuran (THF) at 20 °C by 1H and/or 31P NMR. The acids were chosen or newly synthesized in order to cover the wide pKαTHF range of 5−41 versus the anchor compound [HPCy3]BPh4 at 9.7. These pKαTHF values are approximations to absolute, free ion pKaTHF and are obtained by crudely correcting the observed K for 1:1 ion-pairing effects by use of the Fuoss equation. The acid/base compounds include 14 phosphonium/phosphine couples, 17 cationic hydride/neutral hydride couples, 9 neutral polyhydride/anionic hydride couples, 14 dihydrogen/hydride couples, and 4 other nitrogen- and phosphorus-based acids. The effects on pKα of the counterions BAr‘4- and BF4- vs BPh4- and [K(2,2,2-crypt)]+ versus [K(18-crown-6)]+ are found to be minor after correcting for differences in inter-ion distances in the ion-pairs involved. Correlations with ν(M−H) noted here for the first time suggest that destabilization of M−H bonding in the conjugate base hydride is an important contributor to hydride acidity. It appears that Re−H bonding in the anions [ReH6(PR3)2]- is greatly weakened by small increases in the basicity of PR3, resulting in a large increase in the pKα of the conjugate acid ReH7(PR3)2. Correlations with other scales allow an estimate of the pKαTHF values of more than 1000 inorganic and organic acids, 20 carbonyl hydride complexes, 46 cationic hydrides complexes, and dihydrogen gas. Therefore, many new acid−base reactions can be predicted and known reactions explained. THF, with its low dielectric constant, disfavors the ionization of neutral acids HA over HB+, and therefore separate lines are found for pKαTHF(HA) and pKαTHF(HB+) when plotted against pKaDMSO or pKaMeCN. The crystal structure of [Re(H)2(PMe3)5]BPh4 is reported.
  • Article
    Application of a method devised by Boltzmann to the equilibrium between ion pairs and free ions leads directly to the mass action equation (1 - γ) = KACγ2f2 where c is concentration, f is the Debye-Hückel activity coefficient, γ is the fraction of 1-1 electrolyte present as free ions and KA is the association constant. For the latter, the explicit value KA = 4π-Na3eb/3000 is found; N = Avogadro's number, a = ionic diameter, b = e2/aDkT, D = dielectric constant, k = Boltzmann's constant and T = absolute temperature. The functional form of KA is the same as that derived by Denison and Ramsey by a thermodynamic approach. The model used for the solvent was a continuum; therefore no factor appears which allows for solvent-solute interaction.
  • Article
    Dissociation constants in dimethyl sulfoxide (DMSO) of various uncharged acids, monovalent cation acids, and some sulfonephthaleins have been determined. For uncharged acids, the anions of which have a localized charge, the difference between pKdHA in acetonitrile (AN) and DMSO is 9.7 to 10. For acids, the undissociated forms of which have intramolecular hydrogen bonding in AN, the difference is of the order of 11.5. This larger difference is attributed to breaking of these bonds by the strongly protophylic aprotic solvent DMSO. For ammonium and aliphatic amine cations, ΔpKdBH + in AN and DMSO varies between 6 and 10, whereas ΔpKdBH + in AN and water is 7.5 ± 0.3. Taking the logarithm of the medium activity coefficient log γH + in DMSO with reference to water equal to -1.5 ± 0.5 yields log γH + in AN with reference to water of +7.9 as derived from ΔpKdHA of carboxylic acids. Based on these values a medium activity coefficient of the order of 1 of the picrate ion is found between AN and water. Using solubility products of potassium picrate in water and AN, a value of log γK + of 1.3 ± 0.5 in AN with reference to water was obtained. Because of hydrogen bonding of DMSO with acids, values of homoconjugation constants KfHA2-in DMSO are some 100 times smaller than in AN. In isodielectric aprotic solvents the difference between pKdHA gives a better indication of the difference in basicity of the solvents than the difference between PKdBH.
  • Article
    The dissociation constants of the conjugate acids of 31 amines of various structural types were measured in anhydrous acetonitrile as solvent by means of a glass electrode shown to respond reversibly to hydrogen ion activity in this solvent. The electrode was calibrated in a series of buffers containing picric acid and tetraethylammonium picrate, using the dissociation constant found for picric acid in a careful study by Kolthoff and Chantooni. All measurements were carried out at a constant and sufficiently low ionic strength to allow extrapolation of equilibrium constants to their limiting values. No simple, quantitative correlation was found between base strengths in acetonitrile and water. Although the pKa values of the majority of the ammonium ions studied are from 7.2 to 7.9 units greater in acetonitrile than in water, several ions differ significantly from this "norm." Aromatic ions are considerably stronger acids in acetonitrile than would be predicted from the strengths of aliphatic ions. In the series of monoprotonated diamines, H2N(CH2)nNH3+, the acids with n = 3 and 4 derive considerable stability from intramolecular hydrogen bonding. Although acetonitrile is much more inert than water, its salvation of ammonium ions still exerts an important influence. Steric inhibition of salvation appears to be more important in acetonitrile than in water. A correlation exists between the base.
  • Article
    Equilibrium cesium ion pair acidities of acetophenone, propiophenone, isobutyrophenone, and o-methoxyacetophenone and the lithium ion pair acidity of o-methoxyacetophenone in tetrahydrofuran have been determined by an indicator method. For all of these compounds, the observed pK a values decrease as the equilibrium enolate concentration is increased. It is proposed that this concentration dependence is a consequence of the aggregation of the enolates, and a method is described whereby average aggregation numbers can be determined from the acidity data. The results indicate that the extent of aggregation of enolate ions is influenced by both electronic and steric factors. In addition, internal solvation is found to be important for the lithium, but not the cesium enolate of o-methoxyacetophenone.
  • Article
    Equilibrium constants for the dissociation of the lithium and cesium salts of eleven hydrocarbons have been determined in tetrahydrofuran solution by a conductimetric technique. The dissociation constants for the fluorenyllithium derivatives are found to be remarkably insensitive to the structure of the carbanion; this is consistent with these compounds existing as solvent separated ion pairs. The K d's of these lithium salts may be approximated by the primitive Bjerrum model of dissociation. The dissociation constants of the organocesium compounds are generally 10 2-10 3 times lower than those of the corresponding lithium salts, and are consistent with the cesium compounds forming contact ion pairs. The variation of dissociation constants with molecular structure for organocesium compounds is complex, and apparently it is dependent on the charge distribution and steric bulk of the carbanion. The conductimetric dissociation constants can be used to construct a scale of relative free ion acidities in tetrahydrofuran. Comparison of the resulting ionic pK values with those measured in the ion pairing solvent dimethoxyethane and the ionizing solvent dimethyl sulfoxide reveals that the relative acidities of hydrocarbons yielding delocalized anions are almost identical in each solvent. The present data are used to revise the previously published cesium ion pair scale in THF.
  • Article
    [2.1.1]Cryptated lithium salts of a wide range of CH-acids were studied spectrophotometrically and conductimetrically in solvents of low polarity. Such salts exist in media of different dielectric constants in the form of cryptand-separated ion pairs which are identical in their spectral and conductance properties with common solvent-separated ion pairs. No formation of higher ionic clusters was observed for the studied ions at concentrations below 10(-3) mol-1 even in the benzene. The relative equilibrium acidities in benzene, N-methylmorpholine and tetrahydrofuran for a number of CH-adds are reported. Solvent polarity does not influence the cryptate ion-pair acidity of the investigated compounds. The relative free ion acidities were calculated for some of the studied acids in different solvents by combination of the ion-pair acidity values with the conductimetric data on dissociation constants of the cryptate ion pairs. A linear relationship between the free ion acidity and the reciprocal of the dielectric constant was found. An extrapolation method is suggested for the calculation of gas-phase acidity from the data obtained in solution.
  • Article
    A self-consistent spectrophotometric basicity scale in acetonitrile, including DBU, ten (arylimino)tris(1-pyrrolidinyl)phosphoranes, two (arylimino)tris(dimethylamino)phosphoranes, 2-phenyl-1,1,3, 3-tetramethylguanidine, 1-(2-tolyl)biguanide, benzylamine, two substituted benzimidazoles, pyridine, and ten substituted pyridines, has been created. The span of the scale is almost 12 pK(a) units. Altogether, 29 different bases were studied and 53 independent equilibrium constant measurements were carried out, each describing the relative basicity of two bases. The scale is anchored to the pK(a) value of pyridine of 12.33 that has been measured by Coetzee et al. Comparison of the basicity data of phenyliminophosphoranes and phenyltetramethylguanidines implies that the P=N bond in the (arylimino)tris(1-pyrrolidinyl)phosphoranes involves contribution from the ylidic (zwitterionic) structure analogous to that found in phosphorus ylides.
  • Article
    A UV-vis spectrophotometric method for measurements of relative acidities in heptane has been developed. The phosphazene base t-BuP(4) is used as the deprotonating agent. Its protonated form is a good counterion for the anions of the acids because it is bulky, has delocalized charge, and therefore does not have specific interactions with the anions. A self-consistent scale of relative acidities in heptane spanning for 3 pK(a) units has been constructed.
  • Part 1 of this series
    • I Leito
    • T Rodima
    • I A Koppel
    • R Schwesinger
    • V M Vlasov
    Part 1 of this series: Leito, I.; Rodima, T.; Koppel, I. A.; Schwesinger, R.; Vlasov, V. M. J . Org. Chem. 1997, 62, 8479-8483.
  • 13 C NMR (50 MHz, THF) δ 26
    • Mhz
    MHz, THF) δ 1.8 (overlapped by solvent, 12H), 3.05 (dt, 12H, JH-H ) 6.6, JP-H ) 3.8), 3.35 (d, 2H, JP-H ) 10.4), 5.50 (d, 1H, JP-H ) 35.6), 6.73 (t, 4H, JH-H ) 7.2), 6.87 (t, 8H, JH-H ) 7.2), 7.28 (m, 8H). 13 C NMR (50 MHz, THF) δ 26.9 (d, JC-P ) 8.1), 47.9 (d, JC-P ) 4.0), 121.9, 125.8 (m, JC-B ) 2.8), 137.1, 165.2 (m, JC-B ) 49.7).