Switching the Spin State of Diphenylcarbene via Halogen Bonding

Chemically Accurate Singlet-Triplet Gaps of Arylcarbenes from Local Hybrid Density Functionals

Singlet-triplet (ST) gaps are key descriptors of carbenes, because their properties and reactivity are strongly spin-dependent. However, the theoretical prediction of ST gaps is challenging and generally thought to require elaborate correlated wave function methods or double-hybrid density functionals. By evaluating two recent test sets of arylcarbenes (AC12 and AC18), we show that local hybrid functionals based on the "common t" local mixing function (LMF) model achieve mean absolute errors below 1 kcal/mol at a computational cost only slightly higher than that of global hybrid functionals. An analysis of correlation contributions to the ST gaps suggests that the accuracy of the common t-LMF model is mainly due to an improved description of nondynamical correlation which, unlike exchange, is not additive in each spin-channel. Although spin-nonadditivity can be achieved using the local spin polarization alone, using the "common", i.e., spin-unresolved, iso-orbital indicator t for constructing the LMF is found to be critical for consistent accuracy in ST gaps of arylcarbenes. The results support the view of LHs as vehicles to improve the description of nondynamical correlation rather than sophisticated exchange mixing approaches.

Photoinduced halogen-bonding enabled synthesis of oxindoles and isoindolinones from aryl iodides

We report the use of halogen bonding (XB) for the generation of aryl radicals from aryl halides under blue light irradiation and applied it in radical generation/1,5-hydrogen-atom transfer/radical cyclization cascade reactions for the synthesis of oxindoles and isoindolinones. On the basis of experimental studies, we propose that DBU can serve as a suitable XB acceptor with aryl halides for the formation of a photoactive electron donor and acceptor complex.

Experimental and Computational Mechanistic Study of Carbonazidate-Initiated Cascade Reactions

A variety of Huisgen cyclization or nitrene/carbene alkyne cascade reactions with different types of termination were investigated. Accessible nitrene precursors were assessed, and carbonazidates were found to be the only effective initiators. Solvents, terminal alkynyl substituents, and catalysts can all impact the reaction outcome. Study of the mechanism both computationally (by density functional theory) and experimentally revealed relevant intermediates and plausible reaction pathways.

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.

Theoretical study on the noncovalent interactions involving triplet diphenylcarbene

The properties of some types of noncovalent interactions formed by triplet diphenylcarbene (DPC³) have been investigated by means of density functional theory (DFT) calculations and quantum theory of atoms in molecule (QTAIM) studies. The DPC³···LA (LA = AlF₃, SiF₄, PF₅, SF₂, ClF) complexes have been analyzed from their equilibrium geometries, binding energies, and properties of electron density. The triel bond in the DPC³···AlF₃ complex exhibits a partially covalent nature, with the binding energy - 65.7 kJ/mol. The tetrel bond, pnicogen bond, chalcogen bond, and halogen bond in the DPC³···LA (LA = SiF₄, PF₅, SF₂, ClF) complexes show the character of a weak closed-shell noncovalent interaction. Polarization plays an important role in the formation of the studied complexes. The strength of intermolecular interaction decreases in the order LA = AlF₃ > ClF > SF₂ > SiF₄ > PF₅. The electron spin density transfers from the radical DPC³ to ClF and SF₂ in the formation of halogen bond and chalcogen bond, but for the DPC³···AlF₃/SiF₄/PF₅ complexes, the transfer of electron spin density is minimal.

Direct matrix isolation IR spectroscopic evidence of halogen bonding from a comparative study of complexes of CBr4 and CCl4with acetone and formic acid

Binary complexes of acetone and formic acid with tetrahalomethanes CBr₄ and CCl₄ have been isolated in argon matrix. Spectral shifts in the characteristic ν_C=O region of acetone, as well as in the fingerprint regions, are unambiguously assigned to the formation of halogen bond involving one of the halogen atoms on CBr₄/CCl₄ as donor, and the carbonyl oxygen of acetone as acceptor. The higher magnitude of shifts of ν_C=O and the fingerprint vibrations for the CBr₄ complex, as compared to the CCl₄ complex, is consistent with theoretical predictions of higher value of positive electrostatic potential in the "σ-hole" region of the former, and hence its higher susceptibility to halogen bonding. The formation of halogen bonded complexes involving formic acid as acceptor and CBr₄/CCl₄ as donors is also being reported for the first time. In this case too, distinct shifts are obtained in the ν_C=O as well as ν_C-O regions of formic acid, which again are significantly larger in magnitude for the CBr₄ complex, as compared to the CCl₄ complex. Electronic structure calculations have been carried out using different theoretical methods to identify the various possible structural isomers of the halogen bonded complexes, and to obtain relevant information regarding their energies and intermolecular geometrical parameters. In addition, NBO and AIM analysis have been carried out to understand the role of local interactions at the halogen bonded interface. Such predicted data are found to be consistent with experimental observations, and re-assert the stronger nature of CBr₄ as halogen bond donor, as compared to CCl₄.

Halogen bonding matters: visible light-induced photoredox catalyst-free aryl radical formation and its applications

Halogen bonding facilitated aryl halide activation for photoredox catalyst-free visible light-driven reactions.

Photochemical Dehalogenation of Aryl Halides: Importance of Halogen Bonding

Upon UVA irradiation, aryl halides can undergo dehalogenation in the presence of bases and methanol as a hydrogen donor. This catalyst-free photochemical dehalogenation is furnished through a facile radical chain reaction under mild conditions. The chain reaction follows UVA irradiation of the reaction mixture in a transition-metal-free environment. Mechanistic studies support a chain mechanism in which initiation involves absorption by a methoxide-bromoarene complex facilitated by halogen-bonding interactions. The methoxide-bromine interaction leads to a weakened Br-C bond that is prone to facile cleavage during the initiation and propagation steps.

Physical Nature of Differential Spin-State Stabilization of Carbenes by Hydrogen and Halogen Bonding: A Domain-Based Pair Natural Orbital Coupled Cluster Study

The variation in the singlet-triplet energy gap of diphenylcarbene (DPC) upon interaction with hydrogen (water and methanol) or halogen bond (XCF3, X = Cl, Br, I) donors to form van der Waals (vdW) complexes is investigated in relation to the electrostatic and dispersion components of such intermolecular interactions. The domain-based local pair natural orbital coupled cluster method, DLPNO-CCSD(T), is used for calculating accurate single-triplet energy gaps and interaction energies for both spin states. The local energy decomposition scheme is used to provide an accurate quantification to the various interaction energy components at the DLPNO-CCSD(T) level. It is shown that the formation of vdW adducts stabilizes the singlet state of DPC, and in the case of water, methanol, and ICF3, it reverses the ground state from triplet to singlet. Electrostatic interactions are significant in both spin states, but preferentially stabilize the singlet state. For methanol and ClCF3, London dispersion forces have the opposite effect, stabilizing preferentially the triplet state. The quantification of the energetic components of the interactions through the local energy decomposition analysis correlates well with experimental findings and provides the basis for more elaborate treatments of microsolvation in carbenes.

Spectroscopic Characterization of Nicotinoyl and Isonicotinoyl Nitrenes and the Photointerconversion of 4-Pyridylnitrene with Diazacycloheptatetraene

Recently, nicotinoyl nitrene (2) has been generated from the photodecomposition of nicotinoyl azide (1) and used as the key intermediate in probing nucleobase solvent accessibility inside cells. Following the 266 nm laser photolysis of nicotinoyl azide (1) and isonicotinoyl azide (5) in solid N₂ matrices at 15 K, nicotinoyl nitrene (2) and isonicotinoyl nitrene (6) have now been identified by matrix-isolation infrared (IR) spectroscopy. Both aroyl nitrenes 2 and 6 adopt closed-shell singlet ground states stabilized by significant N_nitrene···O interactions, which is consistent with the spectroscopic analysis and calculations at the CBS-QB3 level of theory. Upon subsequent visible light irradiations, 2 (400 ± 20 nm) and 6 (532 nm) undergo rearrangement to pyridyl isocyanates 3 and 7. Further dissociation of 3 and 7 under 193 nm laser irradiation results in CO elimination and formation of ketenimines 12 and 13 via the ring opening of elusive pyridyl nitrenes 4 and 8, respectively. In addition to the IR spectroscopic identification of 8 in the triplet ground state, its reversible photointerconversion with ring expansion to diazacycloheptatetraene 9 has been observed directly. The spectroscopic identification of the nitrene intermediates was aided by calculations at the B3LYP/6-311++G(3df,3pd) level, and the mechanism for their generation in stepwise decompositions of the azides is discussed in the light of CBS-QB3 calculations.

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.

Caged Nitric Oxide-Thiyl Radical Pairs

S-Nitrosothiols (RSNO) are exogenous and endogenous sources of nitric oxide in biological systems due to facile homolytic cleavage of the S-N bonds. By following the photolytic decomposition of prototypical RSNO (R = Me and Et) in Ne, Ar, and N₂ matrixes (<10 K), elusive caged radical pairs consisting of nitric oxide (NO•) and thiyl radicals (RS•), bridged by O···S and H···N connections, were identified with IR and UV/vis spectroscopy. Upon red-light irradiation, both caged radical pairs (RS•···•ON) vanish and reform RSNO. According to the calculation at the CASPT2(10,8)/cc-pVDZ level (298.15 K), the dissociation energy of MeS•···•ON amounts to 4.7 kcal mol^-1.

Comparison of pnicogen and tetrel bonds in complexes containing CX2 carbenes (X = F, Cl, Br, OH, OMe, NH2, and NMe2)

The similarities and differences of pnicogen and tetrel bonds formed by carbenes CX₂ with H₃AsO and H₃SiCN were investigated by carrying out ab initio calculations in association with topological analysis of electron density.

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.

Nature of halogen bonding involving π-systems, nitroxide radicals and carbenes: a highlight of the importance of charge transfer

The recently developed adiabatic absolutely localized molecular orbital energy decomposition analysis (ALMO-EDA) has proven to be useful in determining the effects of different energy components on the geometries of complexes bound by intermolecular interactions. The authors have applied it to systems such as the water dimer, water-ion complexes, metallocenes and lone-pair type halogen-bonded (XB) dimers. In this study, we have extended the second-generation ALMO-EDA method to 40 different XB complexes by benchmarking against its classical counterpart and symmetry-adapted perturbation theory (SAPT). In addition, we have examined the nature of halogen bonding involving less studied XB acceptors, namely π-systems, radicals and carbenes, using the adiabatic ALMO-EDA analyses, particularly to shed light on how each energy component affects the geometries of the XB complexes. Our results show that the second-generation ALMO-EDA predicts a higher electrostatic energy contribution in all XB complexes compared to SAPT and classical ALMO-EDA schemes. On the other hand, when comparing across different XB acceptors, all three partition schemes produced the same qualitative finding. The adiabatic ALMO-EDA analyses indicate that while the inclusion of a charge transfer contribution is important in achieving accurate XB bond lengths and interaction energies, as well as recovering the binding site specificity of XB involving benzene and naphthalene acceptors, it is sufficient to obtain the linearity of the XB complexes in the frozen approximation.

Reactions of Cyclopentadienylidenes with CF3I: Electron Bond Donation versus Halogen Bond Donation of the Iodine Atom

The interaction of cyclopentadienylidene and tetrachlorocyclopentadienylidene with the halogen bond donor CF₃I has been studied by matrix isolation spectroscopy. The carbenes were produced by photolysis of the corresponding diazo compounds, matrix-isolated in argon doped with 1% CF₃I at 3 K. Bimolecular reactions between the carbenes and CF₃I were induced by annealing these matrices to 25-30 K to allow for the diffusion of trapped species. Instead of classical halogen-bonded complexes, these carbenes form complexes in which the iodine atom is shared between the carbene center and the CF₃ group. Photolysis of the complexes at 3 K yields radical pairs, which reversibly react back to the complexes when the matrices are warmed to 25-30 K.

N, Deb S, Bhat V, Sasikumar D, Sebastian E, Hariharan M. Extending the scope of the carbonyl facilitated triplet excited state towards visible light excitation

A series of extended π-conjugated benzophenone analogs was synthesized through a facile Lewis-acid catalyzed Friedel–Crafts reaction in order to exploit the integral triplet state properties of benzophenone.

Halogen bond shortens and strengthens the bridge bond of [1.1.1]propellane and the open form of [2.2.2]propellane

The electron scavenger property of halogen bond donors is employed to stabilize the bridge-bond of [1.1.1]propellane and o-[2.2.2]propellane.

Sulfamoyl nitrenes: singlet or triplet ground state?

Two prototypical sulfamoyl nitrenes R₂NS(O)₂–N (R = H and Me) in the triplet state were generated via the closed-shell singlet state by passing a low-energy minimum energy crossing point (MECP).

Activation of the B-F Bond by Diphenylcarbene: A Reversible 1,2-Fluorine Migration between Boron and Carbon

Experiments in low-temperature matrices reveal that triplet diphenylcarbene inserts into the very strong B-F bond of BF₃ in a two-step reaction. The first step is the formation of a strongly bound Lewis acid-base complex between the singlet state of diphenylcarbene and BF₃ . This step involves an inversion of the spin state of the carbene from triplet to singlet. The second step requires visible-light photochemical activation to induce a 1,2-F migration from boron to the adjacent carbon atom under formation of the formal insertion product of the carbene center into BF₃ . The 1,2-F migration is reversible under short-wavelength UV irradiation, thus leading back to the Lewis acid-base adduct.

A halogen bond route to shorten the ultrashort sextuple bonds in Cr2 and Mo2

Selective extraction of destabilizing σ-electrons from the sextuple bond of Cr₂ and Mo₂via σ-hole on a halogen bond donor shortens and strengthens the ultra-short metal–metal bond.

Enhanced intersystem crossing in carbonylpyrenes

Photoexcited state relaxation of carbonylpyrenes displays ultrafast intersystem crossing to generate near-unity triplet formation.

Alkali Metal Carbenoids: A Case of Higher Stability of the Heavier Congeners

As a result of the increased polarity of the metal-carbon bond when going down the group of the periodic table, the heavier alkali metal organyl compounds are generally more reactive and less stable than their lithium congeners. We now report a reverse trend for alkali metal carbenoids. Simple substitution of lithium by the heavier metals (Na, K) results in a significant stabilization of these usually highly reactive compounds. This allows their isolation and handling at room temperature and the first structure elucidation of sodium and potassium carbenoids. The control of stability was used to control reactivity and selectivity. Hence, the Na and K carbenoids act as selective carbene-transfer reagents, whereas the more labile lithium systems give rise to product mixtures. Additional fine tuning of the M-C interaction by means of crown ether addition further allows for control of the stability and reactivity.

Cooperative halogen bonds in V-shaped H3N·X1X2·X3Y (X1, X2, X3 = Cl and Br; Y = F, Cl and Br) complexes

The dihalogen molecule can simultaneously interact with NH₃ and another dihalogen molecule, forming a V-shaped trimer via cooperative halogen bonds.