Solvent-separated Ion Pair Complex Research Articles

Tritopic ion-pair receptors based on anion–π interactions for selective CaX2binding

Selective ion-pair binding of CaX2 (X = Br-, I-) was realized by a tritopic receptor containing homoditopic anion-π binding sites and a pentaethylene glycol site. The receptor formed an unusual solvent-separated ion-pair complex with CaI2 and a contact one with CaBr2, and can be applied to selectively dissolve calcium halides in organic solvents.

The synthesis and structures of rare earth 2-fluorophenyl- and 2,3,4,5-tetrafluorophenyl-N,N′-bis(aryl)formamidinate complexes

Redox transmetallation/protolysis (RTP) reactions between free rare-earth metals (La, Nd, or Yb), Hg(C6F5)2, and FFormH (FFormH=N,N′-bis(2-fluorophenyl)formamidine) in thf afforded four rare-earth complexes: trivalent [La(FForm)3(thf)2]·thf, [Nd(FForm)3(thf)x], [Yb(FForm)3(thf)] (when a slight excess of Yb was used), and divalent [Yb(FForm)2(thf)2] (when a large excess of Yb was used). With the exception of [Yb(FForm)3(thf)], the complexes did not readily crystallise from thf. However, crystallisation from either dme (La, Yb) or diglyme (Nd), produced single crystals of [La(FForm)3(dme)], [Yb(FForm)2(dme)2], and [Nd(FForm)3(diglyme)], respectively. A Nd complex of N,N′-bis(2,3,4,5-tetrafluorophenyl)formamidine (TFFormH): [Nd(TFForm)3(dme)], was also prepared by a similar RTP reaction, and was crystallised from dme. Heating the previously reported divalent TFForm complex: [Yb(TFForm)2(thf)3], in either PhMe or diglyme, separated out trace amounts of the trivalent complex [Yb(TFForm)3(thf)2] from PhMe, or the mixed oxidation state, solvent separated ion pair (SSIP) [Yb(TFForm)(diglyme)2][Yb(TFForm)4] from diglyme. The SSIP complex is the first crystallographically characterised trivalent ytterbium complex coordinating four discrete amidinate ligands.

Lithium Di- and trimethyl dimolybdenum(II) complexes with Mo-Mo quadruple bonds and bridging methyl groups.

New dimolybdenum complexes of composition [Mo2{μ-Me}2Li(S)}(μ-X)(μ-N^N)2] (3a-3c), where S = THF or Et2O and N^N represents a bidentate aminopyridinate or amidinate ligand that bridges the quadruply bonded molybdenum atoms, were prepared from the reaction of the appropriate [Mo2{μ-O2CMe}2(μ-N^N)2] precursors and LiMe. For complex 3a, X = MeCO2, while in 3b and 3c, X = Me. Solution NMR studies in C6D6 solvent support formulation of the complexes as contact ion pairs with weak agostic Mo-CH3···Li interactions, which were also evidenced by X-ray crystallography in the solid-state structures of the molecules of 3a and 3b. Samples of 3c enriched in (13)C (99%) at the metal-bonded methyl sites were also prepared and investigated by NMR spectroscopy employing C6D6 and THF-d8 solvents. Crystallization of 3c from toluene:tetrahydrofuran mixtures provided single crystals of the solvent separated ion pair complex [Li(THF)4] [Mo2(Me)2(μ-Me){μ-HC(NDipp)2}2] (4c), where Dipp stands for 2,6-iPr2C6H3. A computational analysis of the Mo2(μ-Me)2Li core of complexes 3a and 3b has been developed, which is consistent with a small but non-negligible electron-density sharing between the C and Li atoms of the mainly ionic CH3···Li interactions.

Thorium Triamidoamine Complexes: Synthesis of an Unusual Dinuclear Tuck-in–Tuck-over Thorium Metallacycle Featuring the Longest Known Thorium−σ-Alkyl Bond

We report the synthesis and characterization of a range of thorium(IV)–halide, −amide, and −alkyl complexes supported by a sterically demanding triamidoamine ligand. Reaction of [ThCl4(THF)3.5] with [Li3(TrenDMBS)(THF)3] (TrenDMBS = N(CH2CH2NSiMe2But)3) gave [Th(TrenDMBS)(Cl)(THF)] (1), which was converted to [Th(TrenDMBS)(I)] (2), with concomitant elimination of Me3SiCl, by treatment with Me3SiI. Treatment of 2 with [LiNEt2] afforded [Th(TrenDMBS)(NEt2)] (3), or treatment with [PhCH2K] produced the dimeric tuck-in–tuck-over alkyl complex [Th{N(CH2CH2NSiMe2But)2(CH2CH2NSiMeBut-μ-CH2)}]2 (4), which features the longest reported Th−σ-alkyl bond distance of 2.875(9) Å. Reaction of 4 with [Et3NH][BPh4] to give the putative separated ion pair complex [Th(TrenDMBS)][BPh4] resulted in an intractable product mixture. In order to furnish a greater understanding of the latter reaction, the solvent-separated ion pair complex [U(TrenDMBS)(NCMe)2][BPh4] (5) was prepared from the monomeric uranium analogue of 4, [U{N(CH2CH2NSiMe2But)2(CH2CH2NSiMeBut-μ-CH2)}] (I), to provide a related model complex and to examine potential routes to useful TrenDMBS–actinide precursors. The nascent reactivity of 4 is suggested by the isolation of a small quantity of the oxo-insertion product [Th{N(CH2CH2NSiMe2But)2(CH2CH2NSiMeButCH2-μ-O)}]2 (6) when 4 is stored in diethyl ether. The complexes have been variously characterized by single-crystal X-ray diffraction, NMR spectroscopy, FTIR spectroscopy, Evans method solution magnetic moment determination, and CHN elemental analyses.

Transition State Analysis of Acid-Catalyzed dAMP Hydrolysis

Multiple kinetic isotope effects (KIEs) on deoxyadenosine monophosphate (dAMP) hydrolysis in 0.1 M HCl were used to determine the transition state (TS) structure and probe its intrinsic reactivity. The experimental KIEs revealed a stepwise (SN1) mechanism, with a discrete oxacarbenium ion intermediate. This is the first direct evidence for the deoxyribosyl oxacarbenium ion in solution. In 50% methanol/0.1 M HCl the products were deoxyribose 5-phosphate (dRMP) and alpha- and beta-methyl dRMP. The alpha-Me-dRMP/beta-Me-dRMP ratio was 8.5:1. Assuming that a free oxacarbenium ion is equally susceptible to nucleophilic attack on either face, this indicated that approximately 20% proceeded through a solvent-separated ion pair complex, or free oxacarbenium ion, a DN+AN mechanism, while approximately 80% of the reaction proceeded through a contact ion pair complex. The oxacarbenium ion lifetime was estimated at 10(-11)-10(-10) s. Computational transition states were found for ANDN, DN*AN, DN*AN, and DN+AN mechanisms using hybrid density functional theory calculations. After taking into account 20% of DN+AN, there was an excellent match of calculated to experimental KIEs for 80% of the reaction having a DN*AN mechanism. That is, C-N bond cleavage is reversible, with dAMP and the {oxacarbenium ion*adenine} complex in equilibrium. The first irreversible step is water attack on the oxacarbenium ion. The calculated 1'-14C KIE for a stepwise mechanism with irreversible C-N bond cleavage (DN*AN) was 1.052, in the range previously associated only with ANDN transition states, and close to the calculated ANDN value, 1.059. The 1'-14C KIE was strongly dependent on the adenine protonation state.

Stereoselective Enolizations Mediated by Magnesium and Calcium Bisamides: Contrasting Aggregation Behavior in Solution and in the Solid State

The reactions of magnesium and calcium bis(hexamethyldisilazide) with propiophenone have been studied with a view to determine the utility of these bases in the stereoselective enolization of ketones and to uncover the nature of the metal enolate intermediates produced. Both base systems are highly Z-selective when the reactions are conducted in the presence of polar solvents. However, in situ monitoring of the magnesium system in arene solution revealed a preference for E-enolate formation, which was confirmed by silyl enol ether trapping studies. Solution NMR studies of the magnesium system in toluene-d8 show the presence of a monomer-dimer equilibrium for the intermediate amidomagnesium enolates. This assignment is supported by the characterization of a disolvated amidomagnesium enolate dimer by crystallographic analysis. Comparative studies of the calcium system show distinctly different behavior. This is exemplified by the characterization of a novel solvent-separated ion pair complex and a monomeric amidocalcium enolate in the solid state. Solution NMR studies of the calcium system in pyridine-d5 reveal the co-existence of the heteroleptic amidocalcium enolate, the bisamide, the bisenolate and the ion pair complex.

The effect of inert salts on the structure of the transition state in the SN2 reaction between thiophenoxide ion and butyl chloride.

The effect of inert salts on the structure of the transition state has been determined by measuring the secondary alpha deuterium and the chlorine leaving group kinetic isotope effects for the S(N)2 reaction between n-butyl chloride and thiophenoxide ion in both methanol and DMSO. The smaller secondary alpha deuterium isotope effects and very slightly larger chlorine isotope effects found in both solvents when the inert salt is present suggests that the S(N)2 transition state is tighter and more product-like, with a shorter S-C(alpha) and very a slightly longer C(alpha)-Cl bond when the added salt is present. The salt effect on the reaction in methanol where the reacting nucleophile is the solvent-separated ion-pair complex is much greater than the salt effect on the reaction in DMSO where the reacting nucleophile is the free ion. This greater change in transition-state structure found when the inert salt is present in methanol is consistent with the solvation rule for S(N)2 reactions. The greater change in the S-C(alpha) bond is predicted by the bond strength hypothesis. A rationale for the changes found in transition-state structure when the inert salt is present is suggested for both the free-ion and the ion-pair reactions.

Cation-induced structural variations in the alkali metal derivatives of 2-trimethylsilylaminopyridine: synthesis and X-ray structures of complexes for all five metals Li–Cs with 12-crown-4 †

The secondary amine 2-trimethylsilylaminopyridine [PyN(H)SiMe3] 1 was synthesised by mono-lithiation of 2-aminopyridine and subsequent reaction with Me3SiCl. The compound is readily metallated by BunLi in ethereal solvent, in the presence of the macrocyclic polyether 12-crown-4 (12C4), to afford the lithium secondary amide complex [Li(PyNSiMe3)(12C4)] 2, in which the amide ligand binds through both the amido and pyridyl nitrogen centres. Metathesis of 2 with ButONa yields the unusual ‘ate’ solvent-separated ion pair complex [Na(12C4)2][Na(PyNSiMe3)2(THF)]·(THF) 3. Metathesis of 2 with ROM [R = But, M = K; R = CH3CH2CH2CH2C(CH2CH3)HCH2, M = Rb] yields the two bridged dimers [{K(PyNSiMe3)(12C4)}2]·2PhMe 4 and [{Rb(PyNSiMe3)(12C4)}2] 5. In both 4 and 5 the amido and pyridyl nitrogens bridge between metal centres. Room temperature metathesis of 2 with CH3CH2CH2CH2C(CH2CH3)HCH2OCs yields the polymeric [{Cs(PyNH)(12C4)}∞] 6 containing tetranuclear cluster units, with concomitant cleavage of the N–Si bond. Such cleavage is prevented by low temperature synthesis, yielding the bridged dimer complex [{Cs(PyNSiMe3)(12C4)}2]·PhMe 7. The compounds have been characterised by multinuclear NMR spectroscopy, CHN microanalysis and (for 2–7) X-ray crystallography.

Isotope effects in nucleophilic substitution reactions. VIII. The effect of the form of the reacting nucleophile on the transition state structure of an SN2 reaction

A spectroscopic investigation indicated that lithium thiophenoxide exists as a contact ion pair complex in dry diglyme whereas the other alkali metal thiophenoxides exist as a solvent-separated ion pair complex in diglyme. The addition of small amounts of water converts the lithium thiophenoxide contact ion pair complex into a solvent-separated ion pair complex. A smaller secondary α-deuterium kinetic isotope effect and a larger Hammett p value are observed when the nucleophile is the contact ion pair complex in the SN2 reaction between n-butyl chloride and thiophenoxide ion in diglyme. This indicates that the transition state for the contact ion pair complex reaction is tighter with a shorter nucleophile–α-carbon bond than the transition state for the solvent-separated ion pair complex reaction. The secondary α-deuterium kinetic isotope effects for the free ion and the solvent-separated ion pair complex reactions between sodium thiophenoxide and n-butyl chloride in DMF suggest that the loosest transition state is found when the nucleophile is the free ion. Key words: transition state, SN2, isotope, deuterium, Hammett ρ.

Solvent Effect on Hydrogen Bonding Interaction between 5,6,7,8-Tetrahydro-2-naphthol in the Ground and Excited States and Triethylamine

Abstract In the ground state 5,6,7,8-tetrahydro-2-naphthol (ThOH) forms with triethylamine (TEA) simple hydrogen bond, hydrogen-bonded ion pair or solvent-separated ion pair complex depending on the polarity of solvents. Fluorescence emission of ThOH is quenched by TEA in all solvents. The quenching rate constant kq is large in weakly polar solvents than in highly polar solvents and there is no evidence of formation of solvent-separated ion pair in latter solvents. It is suggested that restricted electron delocalization in excited ThOH molecule lowers the tendency of deprotonation of the OH group. The likely pathway of energy depletion is through intermediate singlet contact CT exciplex of the type A−D+ between ThOH and TEA.

Solvent effects on SN2 transition state structure. II: The effect of ion pairing on the solvent effect on transition state structure

Identical secondary α-deuterium kinetic isotope effects (transition state structures) in the SN2 reaction between n-butyl chloride and a free thiophenoxide ion in aprotic and protic solvents confirm the validity of the Solvation Rule for SN2 Reactions. These isotope effects also suggest that hydrogen bonding from the solvent to the developing chloride ion in the SN2 transition state does not have a marked effect on the magnitude of the chlorine (leaving group) kinetic isotope effects. Unlike the free ion reactions, the secondary α-deuterium kinetic isotope effect (transition state structure) for the SN2 reaction between n-butyl chloride and the solvent-separated sodium thiophenoxide ion pair complex is strongly solvent dependent. These completely different responses to a change in solvent are rationalized by an extension to the Solvation Rule for SN2 Reactions. Finally, the loosest transition state in the reactions with the solvent-separated ion pair complex is found in the solvent with the smallest dielectric constant. Keywords: ion pairs, transition state, solvent effects, nucleophilic substitution, isotope effects.

Isotope effects in nucleophilic substitution reactions. VII. The effect of ion pairing on the substituent effects on SN2 transition state structure

The secondary α-deuterium kinetic isotope effects and substituent effect found in the SN2 reactions between a series of para-substituted sodium thiophenoxides and benzyldimethylphenylammonium ion are significantly larger when the reacting nucleophile is a free ion than when it is a solvent-separated ion pair complex. Tighter transition states are found when a poorer nucleophile is used in both the free ion and ion pair reactions. Also, the transition states for all but one substituent are tighter for the reactions with the solvent-separated ion pair complex than with the free ion. Hammett ρ values found by changing the substituent on the nucleophile do not appear to be useful for determining the length of the sulphur–α-carbon bond in the ion pair and free ion transition states. Keywords: Isotope effects, ion pairing, nucleophilic substitution, SN2 reactions, transition states.

Isotope effects in nucleophilic substitution reactions. VI. The effect of ion pairing on the transition state structure of SN2 reactions

Spectroscopic and conductivity studies of sodium thiophenoxide solutions in four different solvents and the secondary α-deuterium kinetic isotope effects found in the presence of 15-crown-5 ether demonstrate that the secondary α-deuterium kinetic isotope effect and transition state structure for the SN2 reaction between sodium thiophenoxide and n-butyl chloride are significantly different, depending on whether the ionic reactant is a solvent-separated ion-pair complex or a free ion. In all three solvents in which the form of the ionic reactant changes, a smaller isotope effect and tighter transition state are found for the reaction with the ion-pair complex.