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

Rodima, Toomas; Kaljurand, Ivari; Pihl, Aino; Mäemets, Vahur; Leito, Ivo; Koppel, Ilmar A.

doi:10.1021/jo016185p

Cited by 179 publications

(221 citation statements)

References 22 publications

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“…The N-H pK a of ( ( (1-NH 3 ) ) ) 2+ + was determined by measurement of the equilibrium concentration ratio with its conjugate base ( ( (1-NH 2 ) ) ) + + in the presence of 2-methoxy pyridine (pK a = 2.6 in THF) (36). Using this method, the average of three equilibration experiments established the pK a of ( ( (1-NH 3 ) ) ) 2 + + as 3.6 in THF solution (table S1 and fig.…”

Section: Downloaded Frommentioning

confidence: 99%

Coordination-induced weakening of ammonia, water, and hydrazine X–H bonds in a molybdenum complex

Bezdek

Guo

Chirik

2016

Science

195

192

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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.A mmonia and water are ubiquitous small molecules with strong bonds between hydrogen and the central atom (1). For over a century, transition metal-ammine (NH 3 ) and -aquo (H 2 O) compounds have defined bonding paradigms in chemistry (2), found application in cancer therapy (3), and promoted important fundamental chemical reactions such as electron transfer that rely on the inertness of the N-H or O-H bonds in the supporting ligands ( Fig. 1) (4).Common strategies for activation of ammonia and water include oxidative addition to a transition metal center (5-7), deprotonation by transition metal hydrides (8), reaction with bimetallic compounds (9), cooperative chemistry between a transition metal and a supporting ligand (10-12), and element-hydrogen (X-H) bond cleavage through reaction with main group compounds (13-16). Using most of these strategies, activation of the strong X-H bond is not typically coupled to H-H bond formation. One exception is the observation of H 2 elimination following oxidative addition of ammonia to a tantalum(III) complex (17).An alternative and less commonly explored strategy is homolytic cleavage of the X-H bond. Because of the high gas-phase bond dissociation free energies (BDFEs; 99.5 and 111.0 kcal/mol for NH 3 and H 2 O, respectively) (1), interaction with a transition metal or other appropriate reagent is necessary to induce bond weakening. As shown in Fig. 1, most classical transition metal compounds with ammine (aquo) ligands have N-H (O-H) bond strengths that are only slightly perturbed from the gas-phase value. Because experimental data are lacking, we used density functional theory (DFT) to compute N-H BDFEs.Coordination-induced bond weakening, whereby interaction of a ligand results in a considerable lowering of the X-H BDFE, has recently been identified or implicated in rare instances (18-23) and has been applied by Knowles's group (24) and others (25-27) in reactions of organic molecules involving N-H and O-H bonds, respectively. Cuerva's group (26, 27) and ours (28) However, this strategy has not yet been shown to be capable of ...

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Section: Downloaded Frommentioning

confidence: 99%

Coordination-induced weakening of ammonia, water, and hydrazine X–H bonds in a molybdenum complex

Bezdek

Guo

Chirik

2016

Science

195

192

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“…MeCN was chosen as a reference solvent because (1) reliable experimental pK a values of nearly all investigated bases are available from the literature, (2) there is a fairly good correlation between pK a values in water and acetonitrile as the equation 3 suggests and (3) the true ionic basicities can be measured in acetonitrile, unlike THF, where the actually measured values refer to ion-pair basicities. 52 Figure 4 shows that the basicity region investigated in this work is almost fully covered by measured pK a values in MeCN (except EtP 2 (dma) and t-BuP 4 (dma), which are too basic to be directly measured in MeCN and the MeCN pK a values have been estimated from THF data 49 ). The computed aqueous pK a values correlate quite well with the experimental data in MeCN, thereby indirectly confirming that the computed aqueous pK a values do not contain major errors.…”

Section: Correlations Of Aqueous Pka Data With Mecn Pka Datamentioning

confidence: 84%

Basicities of Strong Bases in Water: A Computational Study

Kaupmees¹,

Trummal²,

Leito³

2014

Croat. Chem. Acta

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Abstract.Aqueous pKa values of strong organic bases -DBU, TBD, MTBD, different phosphazene bases, etc -were computed with CPCM, SMD and COSMO-RS approaches. Explicit solvent molecules were not used. Direct computations and computations with reference pKa values were used. The latter were of two types: (1) reliable experimental aqueous pKa value of a reference base with structure similar to the investigated base or (2) reliable experimental pKa value in acetonitrile of the investigated base itself.The correlations of experimental and computational values demonstrate that direct computations do not yield pKa predictions with useful accuracy: mean unsigned errors (MUE) of several pKa units were observed. Computations with reference bases lead to MUE below 1 pKa unit and are useful for predictions. Recommended aqueous pKa values are proposed for all investigated bases taking into account all available information: experimental pKa values in acetonitrile and water (if available), computational pKa values, common chemical knowledge.

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“…Under the same toluene reflux reaction conditions, the reaction between BA C H T U N G T R E N N U N G (NMe 2 ) 3 and methimazole in the absence of an added base provided the dimethylamine adduct 1 a, a (MeCN) [15] Reaction time + for pyridine, for example, and is therefore a measure of its Brønsted basicity. Also see ref.…”

Section: Resultsmentioning

confidence: 99%

A New Synthesis of Charge‐Neutral Tris‐Pyrazolyl and ‐Methimazolyl Borate Ligands

Bailey¹,

Budd²,

Cavaco³

et al. 2010

Chemistry A European J

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The dimethylamine in the adducts [(HNMe(2))B(azolyl)(3)] (azolyl=methimazolyl, pyrazolyl), obtained by reaction of the azole with B(NMe(2))(3), can readily be substituted with a range of nitrogen donors to provide new charge-neutral, tripodal ligands in high yield. This observation has led to a revision of an earlier interpretation of the mechanism of the formation of these species. The donor properties of the ligands [(nmi)B(azolyl)(3)] (nmi=N-methylimidazole) have been compared with their anionic analogues [HB(azolyl)(3)](-) by synthesis of their manganese(I)-tricarbonyl complexes and comparison of their infrared nu(CO) energies. This comparison indicates that the new neutral ligands are only marginally weaker donors than the corresponding anionic hydrotris(azolyl)borate ligands. This may be explained by the ability of the attached nmi ring to stabilize a positive charge remotely from the coordinated metal, which may also account for the fact that the [(nmi)B(pyrazolyl)(3)] ligand is a substantially stronger donor than the similarly neutral tris(pyrazolyl)methane ligand.

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Acid−Base Equilibria in Nonpolar Media. 2.¹ Self-Consistent Basicity Scale in THF Solution Ranging from 2-Methoxypyridine to EtP₁(pyrr) Phosphazene

Cited by 179 publications

References 22 publications

Coordination-induced weakening of ammonia, water, and hydrazine X–H bonds in a molybdenum complex

Coordination-induced weakening of ammonia, water, and hydrazine X–H bonds in a molybdenum complex

Basicities of Strong Bases in Water: A Computational Study

A New Synthesis of Charge‐Neutral Tris‐Pyrazolyl and ‐Methimazolyl Borate Ligands

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Acid−Base Equilibria in Nonpolar Media. 2.1 Self-Consistent Basicity Scale in THF Solution Ranging from 2-Methoxypyridine to EtP1(pyrr) Phosphazene

Cited by 179 publications

References 22 publications

Coordination-induced weakening of ammonia, water, and hydrazine X–H bonds in a molybdenum complex

Coordination-induced weakening of ammonia, water, and hydrazine X–H bonds in a molybdenum complex

Basicities of Strong Bases in Water: A Computational Study

A New Synthesis of Charge‐Neutral Tris‐Pyrazolyl and ‐Methimazolyl Borate Ligands

Contact Info

Product

Resources

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Acid−Base Equilibria in Nonpolar Media. 2.¹ Self-Consistent Basicity Scale in THF Solution Ranging from 2-Methoxypyridine to EtP₁(pyrr) Phosphazene