Encapsulation and Stabilization of Reactive Aromatic Diazonium Ions and the Tropylium Ion Within a Supramolecular Host

Brumaghim, Julia L.; Michels, Martin; Pagliero, Daniela; Raymond, Kenneth N.

doi:10.1002/ejoc.200400533

Cited by 69 publications

(48 citation statements)

References 27 publications

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“…The examples shown here demonstrate that hostguest complexes containing both weak binding guests as well as sterically demanding guests can be analyzed using electrospray mass spectrometry. This opens up the possibility of using mass spectrometry to study reactive reaction intermediates stabilized via encapsulation [10,21,22], as well as giving insight into catalytic reaction pathways [23], both of which occur inside the cavity. It will also allow for using mass spectrometry as a screening technique to monitor the competitive binding of various guest molecules at lower concentrations than previously observable by NMR spectroscopy.…”

Section: Discussionmentioning

confidence: 99%

Characterization of self-assembled supramolecular [Ga₄L₆] host-guest complexes by electrospray ionization mass spectrometry

Andersen

Seeber

Fiedler

et al. 2006

J. Am. Soc. Mass Spectrom.

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Self-assembled supramolecular host-guest complexes have been characterized by electrospray ionization mass spectrometry. The spectra obtained by use of a Q-TOF instrument equipped with a Z-spray ion source show primarily the 3-and 4-charge states of the assemblies. The assemblies have the general formula [guest ʚ Ga 4 L 6 ] 11Ϫ where L represents the chelating bidentate catechol ligand 1,5-bis(2=,3=-dihydroxy-benzamido)naphthalene and guests are tetramethyl ammonium (Me 4 N ϩ ), tetraethyl ammonium (Et 4 N ϩ ), tetra-n-propyl ammonium (Pr 4 N ϩ ) and decamethylcobaltocenium (Cp* 2 Co ϩ ) cations. For the first time, the mass spectrum of the empty assembly [Ga 4 L 6 ] 12Ϫ is reported. This article also reports that provided the electrospray ion source is capable of preserving noncovalent interactions, it is possible to observe host-guest complexes containing both weak binding guests as well as sterically demanding guests in the mass spectra. The present data suggest that electrospray mass spectrometry is a powerful tool for characterization of supramolecular host-guest complexes. ( I n the area of supramolecular chemistry, metal-ligand interactions have been exploited to design highly symmetric closed shell structures that selfassemble from simple metal and ligand components. The formation of discrete molecular architectures relies on labile coordinative bonds that allow for self-correction and exclusive formation of the thermodynamic product. A great variety of self-assembled structures of different molecular sizes and shapes is present in the literature [1][2][3][4][5]. Here, we focus on contributions to this field made by Raymond and coworkers [2, 6 -10]. The common theme in these metal-ligand assemblies is the presence of chelating bidentate catechol amide ligands, which bear certain geometric constraints upon coordination to octahedral metal centers resulting in M 4 L 6 coordination tetrahedra (Figure 1). The complexes are highly charged, rendering them soluble in polar solvents such as water and methanol, yet contain hydrophobic cavities of variable sizes. The presence of these hydrophobic cavities affords rich host-guest chemistry, enabling encapsulation of monocationic guest molecules [2, 6 -8]. Several analytical methods are necessary to efficiently characterize these host-guest complexes. 1 H-NMR spectroscopy shows remarkable upfield shifts in the proton signals of encapsulated guests within the cavities [2]. However, since the structures exhibit high point symmetry and the metal components are often paramagnetic, complete characterization by NMR is not always possible. Although solid-state structures may, in the best of cases, be determined by single crystal X-ray crystallography, intrinsically labile metal-ligand interactions do not always allow for unambiguous characterization of solution state structures. Additionally, mass spectrometry is more sensitive than NMR spectroscopy and allows analysis of lower concentrations, thus observing species with solubility properties insufficient for NMR measureme...

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Section: Discussionmentioning

confidence: 99%

Characterization of self-assembled supramolecular [Ga₄L₆] host-guest complexes by electrospray ionization mass spectrometry

Andersen

Seeber

Fiedler

et al. 2006

J. Am. Soc. Mass Spectrom.

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“…The highly charged assembly is soluble in polar solvents such as H 2 O, MeOH, DMSO, and DMF, but the close proximity of the naphthalene rings provides a hydrophobic interior cavity. This hydrophobic interior is demonstrated by the ability of the M 4 L 6 host to stabilize species otherwise reactive toward water, such as the tropylium cation 30 and trialkylphosphine-acetone adducts. 31 Collaborative efforts between the Raymond and Bergman groups have explored the ability of the M 4 L 6 assembly to act as a medium for reactions taking place inside of the assembly; stoichiometric as well as catalytic reactivities have been accomplished inside of the assembly, as recently reviewed.…”

Section: Metal-ligand Frameworkmentioning

confidence: 99%

Reversible Guest Exchange Mechanisms in Supramolecular Host—Guest Assemblies

Pluth

Raymond

2007

ChemInform

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Synthetic chemists have provided a wide array of supramolecular assemblies able to encapsulate guest molecules. The scope of this tutorial review focuses on supramolecular host molecules capable of reversibly encapsulating polyatomic guests. Much work has been done to determine the mechanism of guest encapsulation and guest release. This review covers common methods of monitoring and characterizing guest exchange such as NMR, UV-VIS, mass spectroscopy, electrochemistry, and calorimetry and also presents representative examples of guest exchange mechanisms. The guest exchange mechanisms of hemicarcerands, cucurbiturils, hydrogen-bonded assemblies, and metal-ligand assemblies are discussed. Special attention is given to systems which exhibit constrictive binding, a motif common in supramolecular guest exchange systems.

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“…As a host, 1 stoichiometrically mediates (19,20) as well as catalyzes (5,21) several important organic and organometallic reactions. In addition, it stabilizes reactive guests such as the tropylium cation, (22) phosphine-acetone adducts (23) and iminium cations (24), all of which rapidly decompose in water and are only stable under anhydrous or extremely acidic conditions.…”

mentioning

confidence: 99%

Acid Catalysis in Basic Solution: A Supramolecular Host Promotes Orthoformate Hydrolysis

Pluth

Bergman

Raymond

2007

Science

740

414

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Though many enzymes can promote chemical reactions by tuning substrate properties purely through the electrostatic environment of a docking cavity, this strategy has proven challenging to mimic in synthetic host-guest systems. Here we report a highly-charged, water soluble, metal-ligand assembly with a hydrophobic interior cavity that thermodynamically stabilizes protonated substrates and consequently catalyzes the normally acidic hydrolysis of orthoformates in basic solution, with rate accelerations of up to 890-fold. The catalysis reaction obeys Michaelis-Menten kinetics, exhibits competitive inhibition, and the substrate scope displays size selectivity consistent with the constrained binding environment of the molecular host.Synthetic chemists have long endeavored to design host molecules capable of selectively binding slow-reacting substrates and catalyzing their chemical reactions. While synthetic catalysts are often site-specific and require certain properties of the substrate to insure catalysis, enzymes are often able to modify basic properties of the bound substrate such as pK a in order to enhance reactivity. Two common motifs used by nature to activate otherwise unreactive compounds are the precise arrangement of hydrogen-bonding networks and electrostatic interactions between the substrate and adjacent residues of the protein.(1) Precise arrangement of hydrogen bonding networks near the active sites of proteins can lead to well-tuned pK a -matching,(2) and can result in pK a shifts of up to eight units, as shown in bacteriorhodopsin.(3) Similarly, purely electrostatic interactions can greatly favor charged states and have been responsible for pK a shifts of up to five units for acetoacetate decarboxylase.(4) Attempts have been made to isolate the contributions of electrostatic versus covalent interactions to such pK a shifts; however this remains a difficult challenge experimentally. This challenge emphasizes the importance of synthesizing host molecules that, like enzyme cavities, can enhance binding of small molecular guests and, in a few cases, catalyze chemical reactions. (5-7) Supramolecular assemblies with available functional groups have been used to generate solution-state pK a shifts of up to two pK a units (8-11) and to catalyze chemical reactions.(12, 13) Synthetic hosts often rely on hydrogen-bonding or ion-dipole interactions for guest inclusion, and numerous studies have investigated the effects of charge on guest binding affinities in supramolecular host-guest systems.(14, 15) We report here a synthetic supramolecular host assembly that relies exclusively on electrostatic and hydrophobic interactions for thermodynamic stabilization of protonated substrates. As nature has exploited pK a shifts to activate otherwise unreactive substrates toward catalysis, this stabilization is exploited to promote acid-catalyzed hydrolyses in strongly basic solution.During the past decade, the Raymond group has reported the formation and guesthosting properties of supramolecular assemblies of the...

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Encapsulation and Stabilization of Reactive Aromatic Diazonium Ions and the Tropylium Ion Within a Supramolecular Host

Cited by 69 publications

References 27 publications

Characterization of self-assembled supramolecular [Ga₄L₆] host-guest complexes by electrospray ionization mass spectrometry

Characterization of self-assembled supramolecular [Ga₄L₆] host-guest complexes by electrospray ionization mass spectrometry

Reversible Guest Exchange Mechanisms in Supramolecular Host—Guest Assemblies

Acid Catalysis in Basic Solution: A Supramolecular Host Promotes Orthoformate Hydrolysis

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Encapsulation and Stabilization of Reactive Aromatic Diazonium Ions and the Tropylium Ion Within a Supramolecular Host

Cited by 69 publications

References 27 publications

Characterization of self-assembled supramolecular [Ga4L6] host-guest complexes by electrospray ionization mass spectrometry

Characterization of self-assembled supramolecular [Ga4L6] host-guest complexes by electrospray ionization mass spectrometry

Reversible Guest Exchange Mechanisms in Supramolecular Host—Guest Assemblies

Acid Catalysis in Basic Solution: A Supramolecular Host Promotes Orthoformate Hydrolysis

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Characterization of self-assembled supramolecular [Ga₄L₆] host-guest complexes by electrospray ionization mass spectrometry

Characterization of self-assembled supramolecular [Ga₄L₆] host-guest complexes by electrospray ionization mass spectrometry