The Fluorenyl Cation

Infrared Spectroscopy of Fluorenyl Cations at Cryogenic Temperatures

The notion of (anti)aromaticity is a successful concept in chemistry to explain the structure and stability of polycyclic hydrocarbons. Cyclopentadienyl and fluorenyl cations are among the most studied classical antiaromatic systems. In this work, fluorenyl cations are investigated by high-resolution gas-phase infrared spectroscopy in helium droplets. Bare fluorenyl cations are generated in the gas phase by electrospray ionization. After mass-to-charge selection, ions are captured in ultracold helium nanodroplets and probed by infrared spectroscopy using a widely tunable free-electron laser in the 600-1700 cm-1 range. The highly resolved cryogenic infrared spectra confirm, in combination with DFT computations, that all cations are present in their singlet states.

C-C Coupling of Carbene Molecules on a Metal Surface in the Presence of Water

A novel surface-confined C-C coupling reaction involving two carbene molecules and a water molecule was studied by scanning tunneling microscopy in real space. Carbene fluorenylidene was generated from diazofluorene in the presence of water on a silver surface. While in the absence of water, fluorenylidene covalently binds to the surface to form a surface metal carbene, and water can effectively compete with the silver surface in reacting with the carbene. Water molecules in direct contact with fluorenylidene protonate the carbene to form the fluorenyl cation before the carbene can bind to the surface. In contrast, the surface metal carbene does not react with water. The fluorenyl cation is highly electrophilic and draws electrons from the metal surface to generate the fluorenyl radical which is mobile on the surface at cryogenic temperatures. The final step in this reaction sequence is the reaction of the radical with a remaining fluorenylidene molecule or with diazofluorene to produce the C-C coupling product. Both a water molecule and the metal surface are essential for the consecutive proton and electron transfer followed by C-C coupling. This C-C coupling reaction is unprecedented in solution chemistry.

A long-lived fluorenyl cation: efficiency booster for uncaging and photobase properties

The photochemistry of fluorenols has been of special interest for many years. This is because both the fluorenol and the fluorenyl cation are antiaromatic in the ground state due to their 4n π-electrons according to the Hückel rule. The photolysis reaction of various fluorene derivatives takes place via a cation intermediate and is preferred due to its excited state aromaticity. Here we present an extremely long-lived fluorenyl cation and its effects on the uncaging of various leaving groups. We analyze the relationship between uncaging quantum yields of fluorene-based cages and the longevity of their fluorenyl cations with different spectroscopic methods in the steady state and on an ultrafast time scale and find that the uncaging quantum yield rises with the stability of the cation. In contrast to previous reports, the cation can be observed on a time scale of minutes, even in moderately protic solvents as methanol and ethanol. The stability of this cation depends on the dimethylamino-substituents on the fluorene scaffold and the properties of the solvent. In addition, with bis-dimethylamino fluorenol, a photobase is introduced that expands the small group of known photoinduced hydroxide emitters.

Lewis acid catalyzed heavy atom tunneling - the case of 1H-bicyclo[3.1.0]-hexa-3,5-dien-2-one

For many thermal reactions, the effects of catalysis or the influence of solvents on reaction rates can be rationalized by simple transition state models. This is not the case for reactions controlled by quantum tunneling, which do not proceed via transition states, and therefore lack the simple concept of transition state stabilization. 1H-Bicyclo[3.1.0]-hexa-3,5-dien-2-one is a highly strained cyclopropene that rearranges to 4-oxocyclohexa-2,5-dienylidene via heavy-atom tunneling. H₂O, CF₃I, or BF₃ form Lewis acid–base complexes with both reactant and product, and the influence of these intermolecular complexes on the tunneling rates for this rearrangement was studied. The tunneling rate increases by a factor of 11 for the H₂O complex, by 23 for the CF₃I complex, and is too fast to be measured for the BF₃ complex. These observations agree with quantum chemical calculations predicting a decrease in both barrier height and barrier width upon complexation with Lewis acids, resulting in the observed Lewis acid catalysis of the tunneling rearrangement.

The ring-opening of a highly strained cyclopropene to a carbene proceeds via heavy-atom tunneling. This rearrangement is accelerated in the presence of H₂O, ICF₃ or BF₃, resulting in a novel Lewis-acid catalyzed tunneling reaction.

Isolation of an Antiaromatic 9-Hydroxy Fluorenyl Cation

Fluorenyl cations are textbook examples of 4π electron antiaromatic five-membered ring systems. So far, they were reported only as short-lived intermediates generated under superacidic conditions or by flash photolysis. Attempts to prepare a m-terphenyl acylium cation by fluoride abstraction from a benzoyl fluoride gave rise to an isolable 9-hydroxy fluorenyl cation that formed by an intramolecular electrophilic attack at a flanking mesityl group prior to a 1,2-methyl shift and proton transfer to oxygen.

Controlling Reactivity-Real-Space Imaging of a Surface Metal Carbene

Metal carbenes are key intermediates in a plethora of homogeneous and heterogeneous catalytic processes. However, despite their importance to heterogeneous catalysis, the influence of surface attachment on carbene reactivity has not yet been explored. Here, we reveal the interactions of fluorenylidene (FY), an archetypical aromatic carbene of extreme reactivity, with a Ag(111) surface. For the first time, the interaction of a highly reactive carbene with a metal surface could be studied by scanning tunneling microscopy (STM). FY chemisorbs on Ag(111) with an estimated desorption energy of 3 eV, forming a surface bound silver-carbene complex. The surface interaction leads to a switching of the electronic ground state of FY from triplet to singlet, and to controlled chemical reactivity. This atomistic understanding of the interplay between carbenes and metal surfaces opens the way for the development of novel classes of catalytic systems based on surface metal carbenes.

Conformational Spin Switching and Spin-Selective Hydrogenation of a Magnetically Bistable Carbene

The control of the spin states of molecules opens the path to tuning selectivity in chemical reactions and to developing novel magnetically switchable materials. 3-Methoxy-9-fluorenylidene is a carbene that is generated in cryogenic matrices both in its lowest energy singlet and triplet states, and the ratio of these states can be shifted by selective irradiation. The interconversion of the nearly degenerate spin states is induced by a conformational change of the methoxy group: switching the methoxy group into the "up" position results in the singlet state and switching into the "down" position in the triplet state. The spin control via a remote functional group makes this carbene unique for the study of spin-specific reactions, which is demonstrated for the hydrogenation reaction. Spin switching by switching the conformation of a remote functional group is a novel phenomenon with potential applications in the design of functional materials.

Persistent Organic High-Spin Trinitrenes

The septet ground state trinitrenes 1,3,5‐trichloro‐2,4,6‐trinitrenobenzene and 1,3,5‐tribromo‐2,4,6‐trinitrenobenzene were isolated in inert (Ar, Ne, and Xe) as well as reactive matrices (H₂, O₂, and H₂O) at cryogenic temperatures. These trinitrenes were obtained in high yields by UV photolysis of the corresponding triazides and characterized by IR and UV/Vis spectroscopy. The trinitrenes, despite bearing six unpaired electrons, are remarkably unreactive towards molecular oxygen and hydrogen and are persistent in water ice up to 160 K where the water matrix starts to sublime off.

Local Energy Decomposition of Open-Shell Molecular Systems in the Domain-Based Local Pair Natural Orbital Coupled Cluster Framework

Local energy decomposition (LED) analysis decomposes the interaction energy between two fragments calculated at the domain-based local pair natural orbital CCSD(T) (DLPNO-CCSD(T)) level of theory into a series of chemically meaningful contributions and has found widespread applications in the study of noncovalent interactions. Herein, an extension of this scheme that allows for the analysis of interaction energies of open-shell molecular systems calculated at the UHF-DLPNO-CCSD(T) level is presented. The new scheme is illustrated through applications to the CH₂···X (X = He, Ne, Ar, Kr, and water) and heme···CO interactions in the low-lying singlet and triplet spin states. The results are used to discuss the mechanism that governs the change in the singlet–triplet energy gap of methylene and heme upon adduct formation.

Open-Source, Python-Based Redevelopment of the ChemShell Multiscale QM/MM Environment

ChemShell is a scriptable computational chemistry environment with an emphasis on multiscale simulation of complex systems using combined quantum mechanical and molecular mechanical (QM/MM) methods. Motivated by a scientific need to efficiently and accurately model chemical reactions on surfaces and within microporous solids on massively parallel computing systems, we present a major redevelopment of the ChemShell code, which provides a modern platform for advanced QM/MM embedding models. The new version of ChemShell has been re-engineered from the ground up with a new QM/MM driver module, an improved parallelization framework, new interfaces to high performance QM and MM programs, and a user interface written in the Python programming language. The redeveloped package is capable of performing QM/MM calculations on systems of significantly increased size, which we illustrate with benchmarks on zirconium dioxide nanoparticles of over 160000 atoms.

Reactions of Arylcarbenes with Lewis Acids

The reactions of the three triplet ground state arylcarbenes diphenylcarbene 1, fluorenylidene 2, and dibenzocycloheptadienylidene 3 with the Lewis acids H₂ O, ICF₃ , and BF₃ were studied under the conditions of matrix isolation. H₂ O was selected as typical hydrogen bond donor, ICF₃ as halogen bond donor, and BF₃ as strong Lewis acid. H₂ O forms hydrogen-bonded complexes of the singlet carbenes with 1 and 2, but not with 3. This is rationalized by the larger singlet-triplet gap of 3, which does not allow to stabilize the singlet state below the triplet state by hydrogen bonding. With ICF₃ , both 1 and 3 form halogen-bonded complexes of the singlet states of the carbenes. This indicates that halogen bonding stabilizes singlet carbenes more than hydrogen bonding. Carbene 2 reacts differently from 1 and 3 by forming an iodonium ylide, thus avoiding antiaromatic destabilization of the fluorenyl unit. With BF₃ , all three carbenes form zwitterionic Lewis acid/base complexes.

Use of Charge-Charge Repulsion to Enhance π-Electron Delocalization into Anti-Aromatic and Aromatic Systems

A series of 9-fluorenyl cations has been studied and it is shown that increasing charge on a heterocyclic substituent group enhances the anti-aromatic character of the carbocation system. Similarly, a series of dibenzosuberenyl cations has been studied and increasing charge on a substituent group is shown to enhance aromatic character in the carbocation system. These studies include the direct observations of dicationic and tricationic species using stable-ion conditions and low temperature NMR. The structures of these ions were further characterized using DFT calculations, confirming that highly charged organic ions may exhibit unusual distributions of π-electrons and delocalization of electrons in 4n or 4n+2 π-systems.

Structures, Lewis Acidities, Electrophilicities, and Protecting Group Abilities of Phenylfluorenylium and Tritylium Ions

The isolation, characterization, and the first X-ray structures of a fluorenylium ion and its Lewis adducts with nitrogen- and phosphorus-centered Lewis bases are reported. Kinetics of the reactions of a series of fluorenylium ions with reference π-, σ-, and n-nucleophiles of various sizes and nucleophilicities allowed the interplay between electronic and structural parameters on the electrophilicities of these planarized tertiary carbenium ions to be elucidated. Structure-reactivity correlations and extensive comparisons of their reactivities with those of di- and triarylcarbenium ions are described. Quantitative determination of the electrofugalities of fluorenylium ions revealed to which extent they are complementing tritylium ions as protecting groups and how their tuning is possible. Determination of the equilibrium constants of the Lewis adducts formation between pyridines of calibrated Lewis basicities and phenylfluorenylium and tritylium ions allowed the determination of their Lewis acidities and to showcase the potential of these carbon-centered Lewis acids in catalysis.

Regioselectivity in the Controversial Scholl Reaction of 1-Benzoylpyrene: Formation of a Five-Member Ring Is Not Unexpected

Intramolecular Scholl reaction of 1-benzoylpyrene (1) gave 8H-dibenzo[def,qr]chrysen-8-one (2) and 11H-indeno[2,1-a]pyren-11-one (3) in a 1:2 ratio. The structures of 2 and 3 were determined, using ¹H NMR, ¹³C NMR, and IR spectroscopies. A DFT B3LYP/6-311G(d,p) study of the reaction's arenium-cation mechanism of (E)-1 and (Z)-1 giving 2 and 3, respectively, indicated the reaction's regioselectivity and kinetic control. The analogous reaction of 1-(1'-naphthoyl)pyrene gave exclusively 13H-benz[4,5]indeno[2,1-a]pyren-13-one. Contrary to previous claims, the preferred formation of five-member rings in Scholl reactions is not unexpected.

Electronic Characterization of Reaction Intermediates: The Fluorenylium, Phenalenylium, and Benz[f]indenylium Cations and Their Radicals

Three vibrationally resolved absorption systems commencing at 538, 518, and 392 nm were detected in a 6 K neon matrix after mass-selected deposition of C13 H9 (+) ions (m/z=165) produced from fluorene in a hot-cathode discharge ion source. The benz[f]indenylium (BfI(+) : 538 nm), fluorenylium (FL9(+) : 518 nm), and phenalenylium (PHL(+) : 392 nm) cations are the absorbing molecules. Two electronic systems corresponding to neutral species are apparent at 490 and 546 nm after irradiation of the matrix with λ<260 nm photons and were assigned to the FL9 and BfI radicals, respectively. The strongest peak at 518 nm is the origin of the 2 (1) B2 ←X̃ (1) A1 absorption of FL9(+) , and the 490 nm band is the 2 (2) A2 ←X̃ (2) B1 origin of FL9. The electronic systems commencing at 538 nm and 546 nm were assigned to the 1 (1) A1 ←X̃ (1) A1 and 1 (2) A2 ←X̃ (2) A2 transitions of BfI(+) and BfI. The 392 nm band is the 1 (1) E'←X̃ (1) A1 ' transition of PHL(+). The electronic spectra of C13 H9 (+) /C13 H9 were assigned on the basis of the vertical excitation energies calculated with SAC-CI and MS-CASPT2 methods.

Switching the Spin State of Diphenylcarbene via Halogen Bonding

The interactions between diphenylcarbene DPC and the halogen bond donors CF3I and CF3Br were investigated using matrix isolation spectroscopy (IR, UV-vis, and EPR) in combination with QM and QM/MM calculations. Both halogen bond donors CF3X form very strong complexes with the singlet state of DPC, but only weakly interact with triplet DPC. This results in a switching of the spin state of DPC, the singlet complexes becoming more stable than the triplet complexes. CF3I forms a second complex (type II) with DPC that is thermodynamically slightly more stable. Calculations predict that in this second complex the DPC···I distance is shorter than the F3C···I distance, whereas in the first (type I) complex the DPC···I distance is, as expected, longer. CF3Br only forms the type I complex. Upon irradiation I or Br, respectively, are transferred to the DPC carbene center and radical pairs are formed. Finally, on annealing, the formal C-X insertion product of DPC is observed. Thus, halogen bonding is a powerful new principle to control the spin state of reactive carbenes.

Computational Predictions of the Beryllium Analogue of Borole, Cp(+), and the Fluorenyl Cation: Highly Stabilized, non-Lewis Acidic Antiaromatic Ring Systems

A computational study of a set of synthetically unknown beryllium-containing rings, anionic analogues of antiaromatic boroles, has been carried out to investigate their structure, stability, and potential reactivity. The results indicate that these compounds should be electronically viable (as assessed from HOMO-LUMO and singlet-triplet gaps) and therefore potential targets for synthesis. In strong contrast with boroles, these beryllium species are predicted to be not Lewis acidic but rather Lewis basic, with reactivity centered on the endocyclic Be-C bond.