Elusive Reaction Intermediates Research Articles

Trapping an Elusive Fe(IV)-Superoxo Intermediate Inside a Self-Assembled Nanocage in Water at Room Temperature.

Molecular cavities that mimic natural metalloenzymes have shown the potential to trap elusive reaction intermediates. Here, we demonstrate the formation of a rare yet stable Fe(IV)-superoxo intermediate at room temperature subsequent to dioxygen binding at the Fe(III) site of a (Et4N)2[FeIII(Cl)(bTAML)] complex confined inside the hydrophobic interior of a water-soluble Pd6L412+ nanocage. Using a combination of electron paramagnetic resonance, Mössbauer, Raman/IR vibrational, X-ray absorption, and emission spectroscopies, we demonstrate that the cage-encapsulated complex has a Fe(IV) oxidation state characterized by a stable S = 1/2 spin state and a short Fe-O bond distance of ∼1.70 Å. We find that the O2 reaction in confinement is reversible, while the formed Fe(IV)-superoxo complex readily reacts when presented with substrates having weak C-H bonds, highlighting the lability of the O-O bond. We envision that such optimally trapped high-valent superoxos can show new classes of reactivities catalyzing both oxygen atom transfer and C-H bond activation reactions.

Direct Mechanistic Evidence for a Nonheme Complex Reaction through a Multivariate XAS Analysis.

In this work, we propose a method to directly determine the mechanism of the reaction between the nonheme complex FeII(tris(2-pyridylmethyl)amine) ([FeII(TPA)(CH3CN)2]2+) and peracetic acid (AcOOH) in CH3CN, working at room temperature. A multivariate analysis is applied to the time-resolved coupled energy-dispersive X-ray absorption spectroscopy (EDXAS) reaction data, from which a set of spectral and concentration profiles for the reaction key species is derived. These “pure” extracted EDXAS spectra are then quantitatively characterized by full multiple scattering (MS) calculations. As a result, structural information for the elusive reaction intermediates [FeIII(TPA)(κ2-OOAc)]2+ and [FeIV(TPA)(O)(X)]+/2+ is obtained, and it is suggested that X = AcO– in opposition to X = CH3CN. The employed strategy is promising both for the spectroscopic characterization of reaction intermediates that are labile or silent to the conventional spectroscopic techniques, as well as for the mechanistic understanding of complex redox reactions involving organic substrates.

ChemInform Abstract: Stabilization of Reactive Species by Supramolecular Encapsulation

AbstractReview: 93 refs.

ChemInform Abstract: Elusive Reaction Intermediates in Solution Explored by ESI‐MS: Reverse Periscope for Mechanistic Investigations

AbstractReview: [63 refs.

Elusive Reaction Intermediates in Solution Explored by ESI-MS: Reverse Periscope for Mechanistic Investigations.

Just as periscopes allow a submarine to visually search for objects above the surface of the sea, in a reversed periscope fashion electrospray mass spectrometry (ESI-MS) can analyze the compounds at the gas phase/liquid phase interface for chemical entities which may exist in solution. The challenge is the identification and structural characterization of key elusive reaction intermediates in chemical transformations, intermediates which are able to explain how chemical processes occur. This Minireview summarizes recent selected publications on the use of ESI-MS techniques for studying solution intermediates of homogeneous chemical reactions.

Stabilization of reactive species by supramolecular encapsulation.

Molecular containers have attracted the interest of supramolecular chemists since the early beginnings of the field. Cavitands' inner cavities were quickly exploited by Cram and Warmuth to construct covalent containers able to stabilize and assist the characterization of short-lived reactive species such as cyclobutadiene or o-benzyne. Since then, more complex molecular architectures have been prepared able to store and isolate a myriad of fleeting species (i.e. organometallic compounds, cationic species, radical initiators…). In this review we cover selected examples of the stabilization of reactive species by encapsulation in molecular containers from the first reports of covalent containers described by Cram et al. to the most recent examples of containers with self-assembled structure (metal coordination cages and hydrogen bonded capsules). Finally, we briefly review examples reported by Rebek et al. in which elusive reaction intermediates could be detected in the inner cavities of self-folding resorcin[4]arene cavitands by the formation of covalent host-guest complexes. The utilization of encapsulated reactive species in catalysis or synthesis is not covered.

Characterization of the Key Intermediates of Carbene‐Catalyzed Umpolung by NMR Spectroscopy and X‐Ray Diffraction: Breslow Intermediates, Homoenolates, and Azolium Enolates

Caught in the act: Diamino enols, diamino dienols, azolium enolates, and azolium enols are postulated intermediates of the N-heterocyclic carbene catalyzed umpolung of aldehydes and enals. Several of these elusive reaction intermediates were generated with the saturated imidazolidin-2-ylidene SIPr (R=2,6-bis(2-propyl)phenyl) and characterized by NMR spectroscopy and X-ray crystallography.

Ligand-Induced Tertiary Relaxations During the T-to-R Quaternary Transition in Hemoglobin

When human hemoglobin is encapsulated in nanoporous silica gels, tertiary and quaternary structural relaxations are dramatically slowed down, allowing the characterization of elusive reaction intermediates. In this work, the conformational and functional changes triggered by CO binding to human deoxyhemoglobin gels were determined in the absence and presence of allosteric effectors. CO rebinding kinetics to human deoxyhemoglobin gels after nanosecond laser photolysis were monitored as a function of time after CO saturation. A maximum entropy analysis of the CO rebinding kinetics shows that the T conformation slowly evolves toward R, with an associated redistribution of tertiary species. The tertiary species are characterized by different CO rebinding rates which are essentially unaffected by the protein quaternary state.

β-Hydrogen Kinetic Effect

A combined kinetic and DFT study of the uncatalyzed isomerization of cationic solvent complexes of the type cis-[Pt(R')(S)(PR3)2]+ (R' = linear and branched alkyls or aryls and S = solvents) to their trans isomers has shown that the reaction goes through the rate-determining dissociative loss of the weakly bonded molecule of the solvent and the interconversion of two geometrically distinct T-shaped 14-electron three-coordinate intermediates. The Pt-S dissociation energy is strongly dependent on the coordinating properties of S and independent of the nature of R'. The energy barrier for the fluxional motion of [Pt(R')(PR3)2]+ is comparatively much lower ( approximately 8-21 kJ mol-1). The presence of beta-hydrogens on the alkyl chain (R' = Et, Prn, and Bun) produces a great acceleration of the reaction rate. This accelerating effect has been defined as the beta-hydrogen kinetic effect, and it is a consequence of the stabilization of the transition state and of the cis-like three-coordinate [Pt(R')(PR3)2]+ intermediate through an incipient agostic interaction. The DFT optimization of [Pt(R')(PMe3)2]+ (R' = Et, Prn, and Bun) reproduces a classical dihapto Pt....eta2-HC agostic mode between the unsaturated metal and a dangling C-H bond. The value of the agostic stabilization energy (in the range of approximately 21-33 kJ mol-1) was estimated by both kinetic and computational data and resulted in being independent of the length of the hydrocarbon chain of the organic moiety. A better understanding of such interactions in elusive reaction intermediates is of primary importance in the control of reaction pathways, especially for alkane activation by metal complexes.

Atropisomerization, C-H activation, and dissociative substitution at some biphenyl platinum(II) complexes.

The reaction of 2,2'-dilithiumbiphenyl with cis-[PtCl(2)(SEt(2))(2)] at -10 degrees C in diethyl ether not only leads to the main product [Pt(2)(micro-SEt(2))(2)(bph)(2)], containing the planar 2,2'-biphenyl dianion (bph(2)(-)), but also forms a new dinuclear platinum(II) compound of formula [Pt(2)(micro-SEt(2))(2)(Hbph)(4)], 1a (Hbph(-) = eta(1)-biphenyl monoanion), in which each metal is in a square-planar environment. NMR spectroscopy and molecular mechanics (MMFF) calculations were used to characterize 1a. The results suggest that the favored conformation for the four Hbph biphenyl groups is alphabetabetaalpha. In chloroform solution, 1a undergoes atropisomerization to 1b (alphabetaalphabeta) (k(is) = 1.03 x 10(-)(4) s(-)(1), at 298 K) that subsequently cyclometalates (k(obs) = 4.48 x 10(-)(6) s(-)(1), at 298 K) to yield [Pt(2)(micro-SEt(2))(2)(bph)(2)] and biphenyl. Both processes, atropisomerization and C-H activation, presumably involve preliminary thioether bridge splitting. The dinuclear complex 1a has been shown to be a versatile and useful precursor to a variety of mononuclear eta(1)-biphenyl platinum(II) complexes. By reaction with diethyl sulfide, dimethyl sulfoxide, or with rigid dinitrogen containing ligands, such as 2,2'-bipyridine or 1,10-phenanthroline, complexes cis-[Pt(Hbph)(2)(dmso)(2)] 3, cis-[Pt(Hbph)(2)(SEt(2))(2)] 4, [Pt(Hbph)(2)(bpy)] 5, and [Pt(Hbph)(2)(phen)] 6 were obtained, respectively. The crystal structures of compounds 5 and 6 were determined. Only the head-to-tail isomer of these compounds was recognized in the solid state and in solution, where restricted rotation around the Pt-C bond prevents interconversion to the head-to-head form. A detailed kinetic study of ligand (dmso) exchange and substitution (by 2,2'-bipyridine and 1,10-phenanthroline) has been performed on complex 3 in CDCl(3) and toluene-d(8) by (1)H NMR magnetization transfer experiments, and in toluene by UV/vis spectroscopy, respectively. The rates of both processes show no dependence on ligand concentration, the rate of ligand substitution being in reasonable agreement with that of ligand exchange at the same temperature. The kinetics are characterized by largely positive entropies of activation. The results are consistent with a dissociative mode of activation analogous to the pattern already found for compounds with a similar [Pt(C,C)(S,S)] set of coordinating ligands. The role of ML(3) d(8) T-shaped 14-electron species, as elusive reaction intermediates or structurally characterized compounds, is discussed.

Competitive paths for methanol decomposition on Pt(111).

Periodic, self-consistent, Density Functional Theory (PW91-GGA) calculations are used to study competitive paths for the decomposition of methanol on Pt(111). Pathways proceeding through initial C-H and C-O bond scission events in methanol are considered, and the results are compared to data for a pathway proceeding through an initial O-H scission event [Greeley et al. J. Am. Chem. Soc. 2002, 124, 7193]. The DFT results suggest that methanol decomposition via CH(2)OH and either formaldehyde or HCOH intermediates is an energetically feasible pathway; O-H scission to CH(3)O, followed by sequential dehydrogenation, may be another realistic route. Microkinetic modeling based on the first-principles results shows that, under realistic reaction conditions, C-H scission in methanol is the initial decomposition step with the highest net rate. The elementary steps of all reaction pathways (with the exception of C-O scission) follow a linear correlation between the transition state and final state energies. Simulated HREELS spectra of the intermediates show good agreement with available experimental data, and HREELS spectra of experimentally elusive reaction intermediates are predicted.