Photoinduced Electron Transfer in Inclusion Complexes of Carbon Nanohoops.

ConspectusPhotoinduced electron transfer (PET) in carbon materials is a process of great importance in light energy conversion. Carbon materials, such as fullerenes, graphene flakes, carbon nanotubes, and cycloparaphenylenes (CPPs), have unusual electronic properties that make them interesting objects for PET research. These materials can be used as electron-hole transport layers, electrode materials, or passivation additives in photovoltaic devices. Moreover, their appropriate combination opens up new possibilities for constructing photoactive supramolecular systems with efficient charge transfer between the donor and acceptor parts. CPPs build a class of molecules consisting of para-linked phenylene rings. CPPs and their numerous derivatives are appealing building blocks in supramolecular chemistry, acting as suitable concave receptors with strong host-guest interactions for the convex surfaces of fullerenes. Efficient PET in donor-acceptor systems can be observed when charge separation occurs faster than charge recombination. This Account focuses on selected inclusion complexes of carbon nanohoops studied by our group. We modeled charge separation and charge recombination in both previously synthesized and computationally designed complexes to identify how various modifications of host and guest molecules affect the PET efficiency in these systems. A consistent computational protocol we used includes a time-dependent density-functional theory (TD-DFT) formalism with the Tamm-Dancoff approximation (TDA) and CAM-B3LYP functional to carry out excited state calculations and the nonadiabatic electron transfer theory to estimate electron-transfer rates. We show how the photophysical properties of carbon nanohoops can be modified by incorporating additional π-conjugated fragments and antiaromatic units, multiple fluorine substitutions, and extending the overall π-electron system. Incorporating π-conjugated groups or linkers is accompanied by the appearance of new charge transfer states. Perfluorination of the nanohoops radically changes their role in charge separation from an electron donor to an electron acceptor. Vacancy defects in π-extended nanohoops are shown to hinder PET between host and guest molecules, while large fully conjugated π-systems improve the electron-donor properties of nanohoops. We also highlight the role of antiaromatic structural units in tuning the electronic properties of nanohoops. Depending on the aromaticity degree of monomeric units in nanohoops, the direction of electron transfer in their complexes with C60 fullerene can be altered. Nanohoops with aromatic units usually act as electron donors, while those with antiaromatic monomers serve as electron acceptors. Finally, we discuss why charged fullerenes are better electron acceptors than neutral C60 and how the charge location allows for the design of more efficient donor-acceptor systems with an unusual hypsochromic shift of the charge transfer band in polar solvents.

[K2(Bi@Pd12@Bi20)]4-: An Endohedral Inorganic Fullerene with Spherical Aromaticity.

Inorganic fullerene clusters have attracted widespread attention due to their highly symmetrical geometric structures and intrinsic electronic properties. However, cage-like clusters composed of heavy metal elements with high symmetry are rarely reported, and their synthesis is also highly challenging. In this study, we present the synthesis of a [K2(Bi@Pd12@Bi20)]4- cluster that incorporates a {Bi20} cage with pseudo-Ih symmetry, making it the largest main group metal cluster compound composed of the bismuth element to date. Magnetic characterization and theoretical calculations suggest that the spin state of the overall cluster is a quartet. Quantum chemical calculations reveal that the [Bi20]3- cluster has a similar electronic configuration to C606- and the [Bi@Pd12@Bi20]6- cluster exhibits a unique open-shell aromatic character.

Photoinduced Selective B-H Activation of nido-Carboranes.

The development of new synthetic methods for B-H bond activation has been an important research area in boron cluster chemistry, which may provide opportunities to broaden the application scope of boron clusters. Herein, we present a new reaction strategy for the direct site-selective B-H functionalization of nido-carboranes initiated by photoinduced cage activation via a noncovalent cage···π interaction. As a result, the nido-carborane cage radical is generated through a single electron transfer from the 3D nido-carborane cage to a 2D photocatalyst upon irradiation with green light. The resulting transient nido-carborane cage radical could be directly probed by an advanced time-resolved EPR technique. In air, the subsequent transformations of the active nido-carborane cage radical have led to efficient and selective B-N, B-S, and B-Se couplings in the presence of N-heterocycles, imines, thioethers, thioamides, and selenium ethers. This protocol also facilitates both the late-stage modification of drugs and the synthesis of nido-carborane-based drug candidates for boron neutron capture therapy (BNCT).

Aromaticity and Magnetic Behavior in Benzenoids: Unraveling Ring Current Combinations.

Nowadays, an active research topic is the connection between Clar's rule, aromaticity, and magnetic properties of polycyclic benzenoid hydrocarbons. In the present work, we employ a meticulous magnetically induced current density analysis to define the net current flowing through any cyclic circuit, connecting it to aromaticity based on the ring current concept. Our investigation reveals that the analyzed polycyclic systems display a prominent global ring current, contrasting with subdued semi-local and local ring currents. These patterns align with Clar's aromatic π-sextets only in cases where migrating π-sextet structures are invoked. The results of this study will enrich our comprehension of aromaticity and magnetic behavior in such systems, offering significant insights into coexisting ring current circuits in these systems.

Stabilization of Diborynes versus Destabilization of Diborenes by Coordination of Lewis Bases: Unravelling the Dichotomy.

We have quantum chemically investigated the boron-boron bonds in B2 , diborynes B2 L2 , and diborenes B2 H2 L2 (L=none, OH2 , NH3 ) using dispersion-corrected relativistic density functional theory at ZORA-BLYP-D3(BJ)/TZ2P. B2 has effectively a single B-B bond provided by two half π bonds, whereas B2 H2 has effectively a double B=B bond provided by two half π bonds and one σ 2p-2p bond. This different electronic structure causes B2 and B2 H2 to react differently to the addition of ligands. Thus, in B2 L2 , electron-donating ligands shorten and strengthen the boron-boron bond whereas, in B2 H2 L2 , they lengthen and weaken the boron-boron bond. The aforementioned variations in boron-boron bond length and strength become more pronounced as the Lewis basicity of the ligands L increases.

Unveiling the Reaction Mechanisms of the Synthesis and the Excited State Intramolecular Proton Transfer of 2-(2'-hydroxyphenyl)imidazo[1,2-a]pyridine.

Given its wide variety of applications in the pharmaceutical industry, the synthesis of imidazo[1,2-a]pyridines has been extensively studied since the beginning of the last century. Here, we disclose the mechanism for the synthesis of imidazo[1,2-a]pyridines by means of the Ortoleva-King reaction. We also reveal the reaction pathway leading to the formation of a iodinated byproduct, demonstrating the challenge of preventing the formation of such a byproduct because of the low energy barrier to access it. Moreover, quantum chemistry tools were employed to investigate the mechanism of intramolecular proton transfer in the excited state, and connections with aromaticity were explored.

Designing potentially singlet fission materials with an anti-Kasha behaviour.

Singlet fission (SF) compounds offer a promising avenue for improving the performance of solar cells. Using TD-DFT methods, anti-Kasha azulene derivatives that could carry out SF have been designed. For this purpose, substituted azulenes with a donor (-OH) and/or an acceptor group (-CN) have been systematically studied using the S2 ≥ 2T1 formula. We have found that -CN (-OH) substituents on electrophilic (nucleophilic) carbons result in improved SF properties when compared to azulene.

The n,π* States of Heteroaromatics: When are They the Lowest Excited States and in What Way Can They Be Aromatic or Antiaromatic?

Heteroaromatic molecules are found in areas ranging from biochemistry to photovoltaics. We analyze the n,π* excited states of 6π-electron heteroaromatics with in-plane lone pairs (nσ, herein n) and use qualitative theory and quantum chemical computations, starting at Mandado's 2n + 1 rule for aromaticity of separate spins. After excitation of an electron from n to π*, a (4n + 2)π-electron species has 2n + 2 πα-electrons and 2n + 1 πß-electrons (or vice versa) and becomes πα-antiaromatic and πß-aromatic. Yet, the antiaromatic πα- and aromatic πß-components seldom cancel, leading to residuals with aromatic or antiaromatic character. We explore vertically excited triplet n,π* states (3n,π*), which are most readily analyzed, but also singlet n,π* states (1n,π*), and explain which compounds have n,π* states with aromatic residuals as their lowest excited states (e.g., pyrazine and the phenyl anion). If the πß-electron population becomes more (less) uniformly distributed upon excitation, the system will have an (anti)aromatic residual. Among isomers, the one that has the most aromatic residual in 3n,π* is often of the lowest energy in this state. Five-membered ring heteroaromatics with one or two N, O, and/or S atoms never have n,π* states as their first excited states (T1 and S1), while this is nearly always the case for six-membered ring heteroaromatics with electropositive heteroatoms and/or highly symmetric (D2h) diheteroaromatics. For the complete compound set, there is a modest correlation between the (anti)aromatic character of the n,π* state and the energy gap between the lowest n,π* and π,π* states (R2 = 0.42), while it is stronger for monosubstituted pyrazines (R2 = 0.84).

Unveiling the regioselectivity of rhodium(I)-catalyzed [2 + 2 + 2] cycloaddition reactions for open-cage C70 production.

The regioselective functionalization of fullerenes holds significant promise for applications in the fields of medicinal chemistry, materials science, and photovoltaics. In this study, we investigate the regioselectivity of the rhodium(I)-catalyzed [2 + 2 + 2] cycloaddition reactions between diynes and C70 as a novel procedure for generating C70 bis(fulleroid) derivatives. The aim is to shed light on the regioselectivity of the process through both experimental and computational approaches. In addition, the photooxidation of one of the C-C double bonds in the synthesized bis(fulleroids) affords open-cage C70 derivatives having a 12-membered ring opening.

Facile Construction of New Hybrid Conjugation via Boron Cage Extension.

Aromatic polycyclic systems have been extensively utilized as structural subunits for the preparation of various functional molecules. Currently, aromatics-based polycyclic systems are predominantly generated from the extension of two-dimensional (2D) aromatic rings. In contrast, polycyclic compounds based on the extension of three-dimensional (3D) aromatics such as boron clusters are less studied. Here, we report three types of boron cluster-cored tricyclic molecular systems, which are constructed from a 2D aromatic ring, a 3D aromatic nido-carborane, and an alkyne. These new tricyclic compounds can be facilely accessed by Pd-catalyzed B-H activation and the subsequent cascade heteroannulation of carborane and pyridine with an alkyne in an isolated yield of up to 85% under mild conditions without any additives. Computational results indicate that the newly generated ring from the fusion of the 3D carborane, the 2D pyridyl ring, and an alkyne is non-aromatic. However, such fusion not only leads to a 1H chemical shift considerably downfield shifted owing to the strong diatropic ring current of the embedded carborane but also devotes to new/improved physicochemical properties including increased thermal stability, the emergence of a new absorption band, and a largely red-shifted emission band and enhanced emission efficiency. Besides, a number of bright, color-tunable solid emitters spanning over all visible light are obtained with absolute luminescence efficiency of up to 61%, in contrast to aggregation-caused emission quenching of, e.g., Rhodamine B containing a 2D-aromatics-fused structure. This work demonstrates that the new hybrid conjugated tricyclic systems might be promising structural scaffolds for the construction of functional molecules.

Single─Not Double─3D-Aromaticity in an Oxidized Closo Icosahedral Dodecaiodo-Dodecaborate Cluster.

3D-aromatic molecules with (distorted) tetrahedral, octahedral, or spherical structures are much less common than typical 2D-aromatic species or even 2D-aromatic-in-3D systems. Closo boranes, [BnHn]2- (5 ≤ n ≤ 14) and carboranes are examples of compounds that are singly 3D-aromatic, and we now explore if there are species that are doubly 3D-aromatic. The most widely known example of a species with double 2D-aromaticity is the hexaiodobenzene dication, [C6I6]2+. This species shows π-aromaticity in the benzene ring and σ-aromaticity in the outer ring formed by the iodine substituents. Inspired by the hexaiodobenzene dication example, in this work, we explore the potential for double 3D-aromaticity in [B12I12]0/2+. Our results based on magnetic and electronic descriptors of aromaticity together with 11B{1H} NMR experimental spectra of boron-iodinated o-carboranes suggest that these two oxidized forms of a closo icosahedral dodecaiodo-dodecaborate cluster, [B12I12] and [B12I12]2+, behave as doubly 3D-aromatic compounds. However, an evaluation of the energetic contribution of the potential double 3D-aromaticity through homodesmotic reactions shows that delocalization in the I12 shell, in contrast to the 10σ-electron I62+ ring in the hexaiodobenzene dication, does not contribute to any stabilization of the system. Therefore, the [B12I12]0/2+ species cannot be considered as doubly 3D-aromatic.

Cage-size effects on the encapsulation of P2 by fullerenes.

The classic pnictogen dichotomy stands for the great contrast between triply bonding very stable N2 molecules and its heavier congeners, which appear as dimers or oligomers. A banner example involves phosphorus as it occurs in nature as P4 instead of P2 , given its weak π-bonds or strong σ-bonds. The P2 synthetic value has brought Lewis bases and metal coordination stabilization strategies. Herein, we discuss the unrealized encapsulation alternative using the well-known fullerenes' capability to form endohedral and stabilize otherwise unstable molecules. We chose the most stable fullerene structures from Cn (n = 50, 60, 70, 80) and experimentally relevant from Cn (n = 90 and 100) to computationally study the thermodynamics and the geometrical consequences of encapsulating P2 inside the fullerene cages. Given the size differences between P2 and P4 , we show that the fullerenes C70 -C100 are suitable cages to side exclude P4 and host only one molecule of P2 with an intact triple bond. The thermodynamic analysis indicates that the process is favorable, overcoming the dimerization energy. Additionally, we have evaluated the host-guest interaction to explain the origins of their stability using energy decomposition analysis.

A Non Expected Alternative Ni(0) Species in the Ni-Catalytic Aldehyde and Alcohol Arylation Reactions Facilitated by a 1,5-Diaza-3,7-diphosphacyclooctane Ligand.

For decades there were many attempts to dispense with stoichiometric amounts of metal reagents for the synthesis of secondary alcohols. In 2021, the synthetic results of Newman and collaborators pioneered a synthesis still with metals, but not as reactants. Instead, they serverd as catalytic engines. Here we present a description by means of Density Functional Theory calculations of how this process can occur, and an attempt is made to shed light on the mechanism that facilitates the attainment of secondary alcohols, emphasizing the eternal cross-coupling debate of whether the catalytically active species is Ni(0) or they are really taking shortcuts following the course of Ni(II). Effective Orbital analyses give a clear picture. Furthermore, this paper provides insight not only into the nature of the ligands of the metal catalyst but also the role of the base.

Pioneering the Power of Twin Bonds in a Revolutionary Double Bond Formation. Unveiling the True Identity of o-Carboryne as o-Carborene.

The homolytic elimination of two H atoms from two adjacent carbons in benzene results in the aromatic product o-benzyne. In a similar way, the homolytic elimination of two H atoms from the two adjacent carbons in 1,2-C2 B10 H12 results in the aromatic product o-carboryne. In this work, we provide experimental and computational evidences that despite the similarity of o-carboryne and o-benzyne, the nature of the C-C bond generated between two adjacent carbons that lose H atoms is different. While in o-benzyne the C-C bond behaves as a triple bond, in o-carboryne the C-C bond is a double bond. Therefore, we must stop naming 1,2-dehydro-o-carboryne as o-carboryne but instead call it o-carborene.

Excited State Processes in Supramolecular Complexes of Cyclic Dibenzopyrrolopyrrole Isomers with C60 Fullerene.

An approach to modulating the properties of carbon nanorings by incorporating pyrrolo[3,2-b]pyrrole units is of particular interest due to the combined effect of heteroatom and antiaromatic character on electronic properties. The inclusion of units other than phenylene leads to the formation of stereoisomers. In this work, we computationally study how the spatial orientation of monomeric units in the ring affects the properties of cyclic dibenzopyrrolo[3,2-b]pyrroles and their complexes with C60 fullerene. For [4]PP and [4]DHPP, the most symmetrical AAAA isomer is the most stable and forms stronger interactions with fullerene than the isomers where one or two monomeric units are flipped, mostly due to less Pauli repulsion. π-Electron delocalization in the monomeric unit is crucial for directing the electron transfer (from or to nanoring). The energy of excited states with charge transfer depends on the HOMO-LUMO gap, which varies from one stereoisomer to another only for [4]DHPP⊃C60 with aromatic 1,4-dihydropyrrolo[3,2-b]pyrrole units. The rates of electron transfer and charge recombination reactions are relatively weakly dependent of the spatial isomerism of nanorings.

Redox Activity of IrIII Complexes with Multidentate Ligands Based on Dipyrido-Annulated N-Heterocyclic Carbenes: Access to High Valent and High Spin State with Carbon Donors.

Synthetic strategies to access high-valent iridium complexes usually require use of π donating ligands bearing electronegative atoms (e. g. amide or oxide) or σ donating electropositive atoms (e. g. boryl or hydride). Besides the η5 -(methyl)cyclopentadienyl derivatives, high-valent η1 carbon-ligated iridium complexes are challenging to synthesize. To meet this challenge, this work reports the oxidation behavior of an all-carbon-ligated anionic bis(CCC-pincer) IrIII complex. Being both σ and π donating, the diaryl dipyrido-annulated N-heterocyclic carbene (dpa-NHC) IrIII complex allowed a stepwise 4e- oxidation sequence. The first 2e- oxidation led to an oxidative coupling of two adjacent aryl groups, resulting in formation of a cationic chiral IrIII complex bearing a CCCC-tetradentate ligand. A further 2e- oxidation allowed isolation of a high-valent tricationic complex with a triplet ground state. These results close a synthetic gap for carbon-ligated iridium complexes and demonstrate the electronic tuning potential of organic π ligands for unusual electronic properties.

Stereoretentive Formation of Cyclobutanes from Pyrrolidines: Lessons Learned from DFT Studies of the Reaction Mechanism.

The stereoselective synthesis of cyclobutanes that possess an array of stereocenters in a contiguous fashion has attracted the wide interest of the synthetic community. Cyclobutanes can be generated from the contraction of pyrrolidines through the formation of 1,4-biradical intermediates. Little else is known about the reaction mechanism of this reaction. Here, we unveil the mechanism for this stereospecific synthesis of cyclobutanes by means of density functional theory (DFT) calculations. The rate-determining step of this transformation corresponds to the release of N2 from the 1,1-diazene intermediate to form an open-shell singlet 1,4-biradical. The formation of the stereoretentive product is explained by the barrierless collapse of this open-shell singlet 1,4-biradical. The knowledge of the reaction mechanism is used to predict that the methodology could be amenable to the synthesis of [2]-ladderanes and bicyclic cyclobutanes.

Aromaticity of Osmaacenes in Their Lowest-Lying Singlet and Triplet States.

The aromatic character of a series of osmaacenes in their lowest-lying singlet and triplet states was thoroughly examined by means of the magnetically induced current densities and multicentre delocalization indices (MCIs). Both employed approaches agree that the osmabenzene molecule (OsB) in the S0 state exhibits dominant π-Hückel-type aromatic character, with a small but nonnegligible amount of π-Craig-Möbius aromaticity. Contrary to benzene, which is antiaromatic in the T1 state, OsB preserves some of its aromaticity in the T1 state. In higher members of the osmaacene series in the S0 and T1 states, the central Os-containing ring becomes nonaromatic, acting as a barrier between the two side polyacenic units which, on the other hand, exhibit a significant extent of π-electron delocalization.

Mechanistic Studies of Transition-Metal-Catalyzed [2 + 2 + 2] Cycloaddition Reactions.

The development of catalytic methodologies involving the formation of C-C bonds to enable the generation of cyclic systems constitutes a field of great relevance in synthetic organic chemistry. One paradigmatic process to accomplish this goal efficiently is the transition-metal-catalyzed [2 + 2 + 2] cycloaddition reaction, since it permits the formation of a wide range of highly functionalized 6-membered carbo- and heterocyclic molecules in a single step with high efficiency and perfect atom economy. A key feature of these transformations is the mechanistic pathway that they follow, since a deep knowledge of this mechanism may enable us to understand and improve the efficiency of the reaction. This review covers the mechanistic aspects, studied both from theoretical and experimental points of view, of the transition-metal-catalyzed [2 + 2 + 2] cycloaddition reaction involving all kinds of unsaturated substrates with metals such as Co, Ni, Ru, Rh, Ir, Pd, Zr, Ti, Ta, and Nb. A thorough overview is undertaken, from the seminal studies until the present day, of the key mechanistic aspects that influence the reactivity and selectivity of the reaction, comparing the involvement of different unsaturated substrates as well as the different transition metals used.

The peptide bond rupture mechanism in the serine proteases: an in silico study based on sequential scale models.

Given the importance of serine proteases for biochemical processes, we have studied the peptide bond rupture mechanism using three sequential scale models as representations of the KLK5 enzyme (a protein overexpressed in ovarian cancer). The first model contains the basic functional groups of the residues that conform to the catalytic triad present in serine proteases; the second model contains some additional residues and, finally, the last representation includes all atoms of the KLK5 protein together with 10.000 explicit water molecules. This separation into three scale models allows us to separate the intrinsic reactivity of the catalytic triad from the process taking place in the enzyme. The methodologies employed in this work include full DFT calculations with a dielectric continuum in the first two models and a multi-level setup with a Quantum Mechanics/Molecular Mechanics (QM/MM) partition in the whole protein system. Our results show that the peptide-bond rupture mechanism is a stepwise process involving two proton transfer reactions. The rate-determining step is the second proton transfer from the imidazole group to the amidic nitrogen of the substrate. In addition, we find that the simplest model does not provide accurate results compared to the full protein system. This can be attributed to the electronic stabilization conferred by the residues around the reaction site. Interestingly, the energy profile obtained with the second scale model with additional residues shows the same trends as the full system and could therefore be considered an appropriate model system. It could be used for studying the peptide bond rupture mechanism in case full QM/MM calculations cannot be performed, or as a rapid tool for screening purposes.