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Covalent post-assembly modification (PAM) reactions are useful synthetic tools for functionalizing and stabilizing self-assembled metal–organic complexes. Recently, PAM reactions have also been explored as stimuli for triggering supramolecular structural transformations. Herein we demonstrate the use of inverse electron-demand Diels–Alder (IEDDA) PAM reactions to induce supramolecular structural transformations starting from a tetrazine-edged FeII4L6 tetrahedral precursor. Following PAM, this tetrahedron rearranged to form three different architectures depending on the addition of other stimuli: an electron-rich aniline or a templating anion. By tracing the stimulus-response relationships within the system, we deciphered a network of transformations that mapped different combinations of stimuli onto specific transformation products. Given the many functions being developed for self-assembled three-dimensional architectures, this newly established ability to control the interconversion between structures u...

Meridional (mer) coordination promotes the generation of larger and lower-symmetry prismatic metallosupramolecular structures, in contrast with the facial (fac) coordination common to smaller and higher-symmetry polyhedra. Here, we describe a general route to the selective formation of large metallosupramolecular prisms that contain exclusively mer-coordinated metal vertices. The use of 2-formylpyridine subcomponents that contain perfluorophenylene substituents at their 5-positions resulted in stereoselective formation of the iron(II) complexes from these subcomponents. Only mer vertices were observed, as opposed to the statistical fac/mer mixture otherwise generated. This mer-selective self-assembly could be used to prepare tetragonal (M8L12), pentagonal (M10L15), and hexagonal (M12L18) prisms by taking advantage of the subtle selectivities imposed by the different anilines and counterions employed. The equilibrium between the tetragonal and pentagonal prism followed a linear free-energy relationship, with the ratio between structures correlating with the Hammett σp(+) parameter of the incorporated aniline. The contrasting preferences of the fluorinated and nonfluorinated ligands to generate prisms and tetrahedra, respectively, were quantified energetically, with the destabilization increasing linearly for each "incorrect ligand" incorporated into either structure.

Covalent post-assembly modification (PAM) reactions are useful synthetic tools for functionalizing and stabilizing self-assembled metal-organic complexes. Recently, PAM reactions have also been explored as stimuli for triggering supramolecular structural transformations. Herein we demonstrate the use of inverse electron-demand Diels-Alder (IEDDA) PAM reactions to induce supramolecular structural transformations starting from a tetrazine-edged Fe

Fe(II)4L6 tetrahedral cage 1 undergoes post-assembly modification (PAM) via a Diels-Alder cycloaddition of the anthracene panels of the cage with tetracyanoethylene (TCNE). The modified cage 2 possesses an enclosed cavity suitable for encapsulation of the fullerene C60, whereas original cage 1 forms a unique covalent adduct through a Diels-Alder cycloaddition of three of its anthracene ligands with C60. This adduct undergoes further PAM via reaction of the remaining three ligands with TCNE, enabling the isolation of two distinct products depending on the order of addition of C60 and TCNE. Modified cage 2 was also able to bind an anionic guest, [Co(C2B9H11)2](-), which was not encapsulated by the original cage, demonstrating the potential of PAM for tuning the binding properties of supramolecular hosts.

Post-assembly modification (PAM) is a powerful tool for the modular functionalization of self-assembled structures. We report a new family of tetrazine-edged Fe(II)4L6 tetrahedral cages, prepared using different aniline subcomponents, which undergo rapid and efficient PAM by inverse electron-demand Diels-Alder (IEDDA) reactions. Remarkably, the electron-donating or -withdrawing ability of the para-substituent on the aniline moiety influences the IEDDA reactivity of the tetrazine ring 11 bonds away. This effect manifests as a linear free energy relationship, quantified using the Hammett equation, between σ(para) and the rate of the IEDDA reaction. The rate of PAM can thus be adjusted by varying the aniline subcomponent.

The gas-phase reactions of Fe(CH2O)+ and Fe(CH2S)+ with a series of aliphatic alkanes were studied by Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. Like bare Fe+, C−C insertion, particularly terminal C−C insertion, is predominant for the reactions of Fe(CH2O)+, while C−H insertion is preferred for Fe(CH2S)+. About 90% of the Fe(CH2O)+ reaction products are formed by C−C insertion with small alkane loss. For Fe(CH2S)+, after initial C−H insertion, the proposed mechanism includes hydrogen transfer to sulfur, followed by migratory insertion of methylene into the metal−alkyl bond and formation of an activated H2S−Fe+−olefin complex, which dissociates by H2S elimination. The structures of the reaction products were probed by collision-induced dissociation, ion−molecule reactions, and use of labeled compounds, yielding information about the reaction mechanism. Collision-induced dissociation and ligand displacement reactions yield the brackets D0(Fe+−C3H6) = 37 ± 2 kcal/mol < D0(Fe+−CH2S) ...

The infrared multiphoton dissociation (IRMPD) of the two isomers, M(1,3-butadiene)+ and M(C2H2)(C2H4)+ (M = Fe, Co, or Ni), was investigated. For the butadiene system, FeC4H6+ was observed to give three products, CoC4H6+ four products, and NiC4H6+ only one product. The observation of four products has not been previously seen in IRMPD studies of ions and suggests a fortuitous balance of several factors. Interestingly, the IRMPD of M(butadiene-1,1,4,4-d4)+ was observed not only to give one product for M = Fe, Co, or Ni but also to proceed with a decreased photodissociation rate. For the M(C2H2)(C2H4)+ system, IRMPD suggests that D°(Co+−C2H2) ∼ D°(Co+−C2H4). Also, SORI and/or competitive CID indicate that D°(M+−C2H2) > D°(M+−C2H4) by 0.5−2 kcal/mol for M = Fe or Ni. On the basis of these observations, we assign D°(Fe+−C2H2) = 36 ± 2 kcal/mol, D°(Co+−C2H2) = 43 ± 2 kcal/mol, and D°(Ni+−C2H2) = 45 ± 2 kcal/mol. These results are compared to recent theoretical and experimental findings.

The infrared multiphoton dissociation (IRMPD) processes of groups 8−10 (Fe, Co, Ni) transition metal ion complexes containing acetylene and alkenes were investigated. Of the 30 ions studied, all but four were found to be photoactive at 944 cm-1, the wavelength of the CO2 infrared laser employed. Most of the ions were observed to yield one photodissociation product, as is typical for IRMPD. Ironacyclopentane, nickelacyclopentane, Co(isobutene)+, and Ni(isobutene)+, however, were observed to give multiple products. This observation is a result of kinetic and thermodynamic factors, and it gives information about the potential energy surface of the dissociating ion. More specifically, activation barrier heights leading to the product ions can be probed. Sustained off-resonance irradiation, an experiment designed to mimic infrared activation, was also performed on these ions, yielding results in good agreement with the infrared results.

The metallo−carbohedrene V8C12+ is studied using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer coupled to a compact supersonic source. Its reactions with water, ammonia, ROH (R = CH3, C2H5, C3H7, C4H9), and CH3X (X = Cl, Br, I) are reported. For polar molecules, sequential attachment reactions occur leading to the initial buildup of the product V8C12L4+. V8C12L5-7+ peaks for ammonia and alcohols are also observed, but they grow in slowly. These results provide evidence that the geometric structure of V8C12+ is the theoretically calculated more stable Td or D2d symmetry with two sets of metals, as opposed to the Th symmetry with all eight equivalent metals. In reactions with methyl halide, CH3X (X = Cl, Br, I), V8C12+ undergoes four sequential halide abstraction reactions, exclusively, yielding mass spectra that truncate sharply at V8C12X4+. Dehydrogenation is observed for the secondary reactions with water and alcohols, ROH (R = H, alkyl), yielding V8C12(OR)2+ intermediates. The r...

The gas-phase ion chemistry between a series of Fe(arene)[sup +] ions and halobenzene and dihalobenzene species was studied. Fe[sup +] in the arene complex acts as a reaction-initiating center to insert oxidatively into the phenyl-X bond (X = Cl, Br, I). The arene ligand then undergoes migratory insertion into the Fe[sup +]-phenyl [sigma] bond. Subsequent hydrogen abstraction from the organic moiety and HX elimination generate Fe(biphenyl)[sup +] ions. This reaction cycle may be repeated up to five times to give Fe(polyphenyl)[sup +]. In contrast, Fe(fluorobenzene)[sup +] shows no reactivity with chlorobenzene, while CH[sub 3] groups on the arene ligand sterically hinder the migratory insertion into the Fe[sup +]-phenyl bond. 29 refs., 1 tab.

The inertness of SF[sub 6] is well-known to be associated with the protective layer of F atoms which obstructs the access to the reactive S center by an attacking species. Previous gas-phase studies of SF[sub 6] involved reactants having high ionization energies, such as He[sup +], Ar[sup +], and N[sub 2][sup +], which are observed to yield predominantly SF[sub 5][sup +] either via F[sup [minus]] abstraction or dissociative charge transfer. This study shows that early transition metal ions, despite their low ionization potentials, can react with SF[sub 6] yielding product distributions much different from those of the reactions studied previously. In contrast, the reduced reactivity and mainly absence of reactivity observed for the late transition metal ions indicates that an empty d orbital is required for reaction. Rate constants for some selected early transition metal ion reactions are determined. Sc[sup +] was chosen to be studied in detail because a complete set of thermodynamic data on ScF[sub n][sup +/0] were available in the literature. The reaction pathways are derived and a reaction mechanism involving oxidative addition is proposed. 44 refs., 3 figs., 7 tabs.

In the mature area of condensed-phase photochemistry, a limitless number of examples exist in which a species, activated by absorption of a photon, undergoes a reaction not observed in its ground state. In contrast, under the conditions used to study gas-phase ion-molecule chemistry at low pressures (10[sup [minus]5]-10[sup [minus]9] Torr), examples of photoinduced or photoenhanced reactions have been surprisingly sparse. This work more than doubles the number of reported examples in the literature, further demonstrating the potential of this new type of photochemistry. A survey is presented involving organometallic ions in which multiphoton infrared absorption is used to create long-lived vibrationally excited ground-state intermediates which exhibit some dramatic changes in reactivity. In two instances, sustained off-resonance (rf) irradiation, or SORI, which also adds energy in small increments, is shown to mimic the infrared results. The five general reaction types demonstrated are (1) deceleration of exothermic reactions; (2) photoinduced secondary fragmentation, whereby an internally excited product ion is generated which can undergo further fragmentation; (3) photoactivated ligand switching; (4) photoinduced isomerization; and (5) photoenhanced hydrogen/deuterium exchange. 49 refs., 7 figs., 2 tabs.

The gas-phase chemistry of Sc(CH{sub 3}){sub 2}{sup +} with alkenes was studied by Fourier transform mass spectrometry. The metal center on Sc(CH{sub 3}){sub 2}{sup +} is d{sup 0}, providing an opportunity to study alternative mechanisms of C-C or C-H activation other than the most common one involving oxidative addition. The elimination of H{sub 2} is observed in the reaction of Sc(CH{sub 3}){sub 2}{sup +} with ethylene, and the product ScC{sub 4}H{sub 8}{sup +} and ScC{sub 6}H{sub 10}{sup +} ions have a metal(methyl)(allyl) and metal-bisallyl structure, respectively, consistent with a proposed reaction mechanism involving the consecutive migratory insertion of ethylenes into the scandium-methyl bonds. In addition, theoretical calculations indicate that the metal(methyl)(allyl) structure is between 10 and 20 kcal/mol more stable than the metal(1-butene) isomer. Sc(CH{sub 3}){sub 2}{sup +} reacts with propene to form predominantly ScC{sub 4}H{sub 8}{sup +} by loss of CH{sub 4}, with minor amounts of ScC{sub 3}H{sub 4}{sup +} and ScC{sub 4}H{sub 6}{sup +} also observed. ScC{sub 4}H{sub 6}{sup +} is formed as either the exclusive or the predominant product ion in the reactions of Sc(CH{sub 3}){sub 2}{sup +} with butenes. Sc(CH{sub 3}){sub 2} reacts with cyclopentene to form predominantly ScC{sub 6}H{sub 8}{sup +} by losing CH{sub 4}more » and H{sub 2}. Isotope labeling studies with Sc(CD{sub 3}){sub 2}{sup +} and other structure studies indicate that all of the alkenes studied, with the exception of ethylene, react with Sc(CH{sub 3}){sub 2}{sup +} via a multicentered {sigma}-bond metathesis mechanism to activate allylic C-H bonds. Finally, the dehydrogenation reactions of Sc{sup +} with n-butane and neopentane were revisited, and a new mechanism is proposed for such chemistry in light of the new results from this study. 34 refs., 5 figs., 2 tabs.« less

Ligand-displacement reactions in the gas phase using Fourier transform mass spectrometry yield the following metal-ligand bond energy order for Y 2+ :CH 4 , C 2 H 6

The gas-phase chemistry of YCH 3 + is studied, with particular emphasis on C-H activation mvia a multicentered intermediate. This activation pathway is observed for the benzyl-hydrogen bond and allylic C-H bonds but not for the H-H bond, C-H bonds in simple alkanes, vinylic C-H bonds, and phenyl-H bonds. The reaction of YCH 3 + with toluene generates (benzyl)yttrium ion and methane, exclusively. The reaction of YCH 3 + with ethylene occurs by migratory insertion of the C=C double bond minto Y + -CH 3 , followed by H 2 elimination to give Y + -allyl. Labeling experiments indicate facile reversible steps min the reaction mechanism, resulting in rapid isomerization of the Y + -propyl intermediate to Y + -isopropyl, which occurs prior to H 2 loss. In contrast, for propene, such C=C bond migratory insertion accounts for only a minor amount of the reaction and leads to the formation of YC 4 H 7 + consisting of about 80% (2-methylallyl) yttrium ion and 20% (1-methylallyl) yttrium ion. The majority of the reaction proceeds by methane elimination involving an allylic hydrogen and the methyl group originally attached to the yttrium center, leading to the formation of (allyl) yttrium ion

The reactions of gas-phase Fe + with n-pentanenitrile and several deuterium-labeled isotopomers have been studied together with low-energy collision-induced dissociation (CID) of Fe(C 4 H 9 CN) + complexes, employing Fourier transform ion cyclotron resonance (FTICR) spectrometry. While loss of H 2 is due to remote functionalization, ethylene is produced from internal positions, generating CH 3 -Fe + -CH 2 CN, an isomer of Fe(CH 3 CH 2 CN) + . Loss of propene is also observed in low abundance, producing an iron-acetonitrile complex. All three processes are in competition with the formation of a ferracyclobutane complex, which, upon reopening, brings about equilibration of the α and (ω-1) position, thus affording a specific, degenerate isomerization of the Fe(C 4 H 9 CN) + collision complex. Collision-induced dissociation experiments with isotopomeric reaction products were performed, which provided structural information and revealed the positions of the deuterium atoms after the degenerate isomerization

Kinetic and photochemical studies were performed on FeC{sub 5}H{sub 6}{sup +}. FeC{sub 5}H{sub 6}{sup +} was prepared by reacting Fe{sup +} with cyclopentene and subsequently with propane to give the cyclopentadienylallyiron ion. D{degree}(Fe{sup +}-cyclopentadiene) was determined by photodissociation threshold measurements to be 55 {plus minus} 5 kcal/mol, which in turn indicates that dehydrogenation of cyclopentene by Fe{sup +} is 33 {plus minus} 5 kcal/mol exothermic. Linear kinetics are observed by collisionally cooling the FeC{sub 5}H{sub 6}{sup +} ions in a high-background pressure of argon and yield a rate constant of 5 {plus minus} 2 {times} 10{sup {minus}10} cm{sup 3}/(molecule {times} s), corresponding to a reaction efficiency of about 0.4. Photodissociation of the cyclopentadienyallyliron ion yields two product ions, FeC{sub 8}H{sub 8}{sup +} and FeC{sub 6}H{sub 6}{sup +}, with the latter pathway, the higher energy process, prevailing when there is no background argon.

Our continuing investigation of gas phase ion-molecule chemistry involving metal ions has led us to to examine the nature and behavior of organometallic ion radicals of a sort that would represent a new class of distonic ions. As a starting point, we generated a radical cation, {sup +}FeC{sub 6}H{sub 4}{sup *}. All the experiments were performed on an Extrel FTMS-2000 dual cell Fourier transform ion cyclotron resonance mass spectrometer equipped with 3T superconducting magnet. Fe{sup +} was generated by laser desorption-ionization of the pure metal target using a Nd:YAG laser. The results reported here represent the first evidence of an organometallic distonic ion in the gas phase. 18 refs., 1 fig.

In this paper we report the gas-phase reactions of C[sub 60] with Fe(benzyne)[sup +] in which metallacyclic C[sub 80] derivatives are formed. All experiments were performed on an Extrel FTMS-2000 dual cell Fourier transform ion cyclotron resonance mass spectrometer equipped with a 3 T superconducting magnet. The ion-trapping potential was set at 2 V. A Bayard-Alpert ion gauge was used to monitor pressure. Fe[sup +] was generated by laser desorption of the pure iron metal target using a Quanta-Ray Nd:YAG laser operated at its fundamental output (1064 nm). 43 refs., 1 fig.

We report the reactions of CoC[sub 60][sup +] with some simple C[sub 2]-C[sub 4] hydrocarbons and, in particular, the reactions with cyclopropane (c-C[sub 3]H[sub 6]) in which C[sub 60] derivatives are formed. All of the experiments were performed in an Extrel FTMS-2000 Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR-MS) equipped with a 3 T superconducting magnet. C[sub 60] was introduced into the source side of the dual FT-ICR cell using a solids probe at 350[degree]C. The temperature of the ion source was kept at 230[degree]C to maintain a static background C[sub 60] pressure of [approximately] 1.0 x 10[sup [minus]8] Torr. Background argon pressure at [approximately] 5.0 x 10[sup [minus]6] Torr was used as the collision gas for thermalization and for collision-induced dissociation (CID) experiments. CoC[sub 60][sup +] was generated by first forming Co[sup +] by laser desorption of the pure metal target using a Nd:YAG laser and then allowing it to react with background C[sub 60], yielding both the charge transfer product, C[sub 60][sup +], and the condensation product, CoC[sub 60][sup +]. The latter was then isolated using FT-ICR ion ejection techniques and allowed to react with a neutral organic molecule which was introduced into the ion trap viamore » either a leak valve or a pulsed valve. 31 refs., 1 fig.« less

A new class of transition metal-carbon clusters named [open quotes]metallocarbohedrenes[close quotes] or [open quotes]met-cars[close quotes] with the stoichiometry M[sub 8]C[sub 12](M = Ti, V, Zr and Hf) has become the focus of intense investigation. In this communication, we report preliminary results on the reactivity of V[sub 8]C[sub 12][sup +] with O[sub 2] and provide evidence for the existence of two different metal atom sites consistent with the D[sub 2d] or T[sub d] structures. All of the experiments were performed on an Extrel FTMS-2000 dual-cell Fourier transform ion cyclotron resonance (FTICR) mass spectrometer combined with a compact supersonic source developed by Smalley and co-workers. The laser vaporization technique is used to generate metal-carbon clusters in a manner similar to that of Castleman and co-workers. The results presented here represent the first evidence of the high combustibility of V[sub 8]C[sub 12][sup +] in the presence of oxygen and lend further support to the T[sub d] or D[sub 2d] structure predicted by theory. The reactivity observed is also consistent with the predicted electronic structure indicating that V[sub 8]C[sub 12][sup +] is a high-spin species. 34 refs., 2 figs.