Exploring the Photodeactivation Pathways of Pt[O^N^C^N] Complexes: A Theoretical Perspective

In this article, the influence of the tert‐butyl unit on the photodeactivation pathways of Pt[O^N^C^N] (O^N^C^N=2‐(4‐(3,5‐di‐tert‐butylphenyl)‐6‐(3‐(pyridin‐2‐l)phenyl) pyridin‐2‐yl)phenolate) is investigated by DFT/TDDFT calculations. To further explore the factors that determine the radiative processes, the transition dipole moments of the singlet excited states, spin–orbit coupling (SOC) matrix elements, and energy gaps between the lowest triplet excited states and singlet excited states are calculated. As demonstrated by the results, compared with Pt‐3 , Pt‐1 and Pt‐2 have larger SOC matrix elements between the lowest triplet excited states and singlet excited states, an indicator that they have faster radiative decay processes. In addition, the SOC matrix elements between the lowest triplet excited states and ground states are also computed to elucidate the temperature‐independent non‐radiative decay processes. Moreover, the temperature‐dependent non‐radiative decay mechanisms are also explored via the potential energy profiles.     

DFT/TDDFT insight into the impact of ring size of the NHC chelating unit of high effective phosphorescent Platinum (II) complexes

Uncovering the photodeactivation mechanisms of unique N‐heterocyclic carbene (NHC)‐based transition metal complexes is favorable for designing more high‐efficiency phosphorescent materials. In this work, four bidentate platinum (II) complexes with NHC‐chelate are investigated by the density functional theory (DFT) and time‐dependent density functional theory (TDDFT) to probe into how the ring size of NHC‐chelate unit influences on electronic structures and the phosphorescent properties. To illustrate the photodeactivation mechanisms clearly, three significant photodeactivation processes (radiative decay process, temperature‐independent and temperature‐dependent nonradiative decay processes) were taken into consideration. We stated that radiative decay rate constants k_r slightly increased with declined number of NHC‐chelate ring, owing to the gradually larger SOC matrix elements between the T₁ state and S_n states. Combining the temperature‐independent with temperature‐dependent nonradiative decay processes, the nonradiative decay rate k_nr is Pt‐4 (five‐membered) < Pt‐3 (six‐membered) < Pt‐2 (seven‐membered) < Pt‐1 (eight‐membered). The calculated results testify that the decrease of size of the NHC chelating unit is a reliable insurance to improve the quantum yield. The designed complex Pt‐4 with five‐membered NHC‐ring can serve as a highly efficient phosphorescent material in the future. The results indicated controlling the ring size of NHC‐chelate is a feasible method to tune phosphorescence properties of Pt (II) complexes.     

Potential strategy used for controlling the phosphorescent properties in tetradentate Pt(II) complexes: Effect of azole ligand

Designing deep‐blue phosphorescent materials is vital and essential in the construction of white organic light‐emitting diodes. Using density functional theory (DFT) and time‐dependent DFT, three tetradentate Pt(II) complexes were investigated in detail to reveal the influence of azole ligand with varying number of N atoms on the emission wavelengths and radiative and non‐radiative decay processes. The calculated results indicate that with an increase of N atoms in azole rings, the radiative decay process can be effectively facilitated. Moreover, an increase of N atoms in azole rings could lead to a distinct blue‐shift of emission wavelengths from 553 to 470 nm. Also, the non‐radiative decay processes, including temperature‐independent and temperature‐dependent ones, were taken into account. The results may provide some valuable and meaningful information for designing high‐performance phosphorescent Pt(II) complexes.     

Kinetic and mechanistic investigation into the influence of chelate substituents on the substitution reactions of platinum(II) terpyridine complexes

The substitution kinetics of the complexes [Pt{4′‐(o‐CH₃‐Ph)‐terpy} Cl]SbF₆ (CH₃PhPtCl(Sb)), [Pt{4′‐(o‐CH₃‐Ph)‐terpy}Cl]CF₃SO₃ (CH₃PhPtCl(CF)), [Pt(4′‐Ph‐terpy)Cl]SbF₆ (PhPtCl), [Pt(terpy)Cl]Cl·2H₂O (PtCl), [Pt{4′‐(o‐Cl‐Ph)‐terpy}Cl]SbF₆ (ClPhPtCl), and [Pt{4′‐(o‐CF₃‐Ph)‐terpy}Cl]SbF₆ (CF₃PhPtCl), where terpy is 2,2′:6′,2″‐terpyridine, with the nucleophiles thiourea (TU), N,N′‐dimethylthiourea (DMTU), and N,N,N′,N′‐tetramethylthiourea (TMTU) were investigated in methanol as a solvent. The substitution reactions of the chloride displacement from the metal complexes by the nucleophiles were investigated as a function of nucleophile concentration and temperature under pseudo‐first‐order conditions using the stopped‐flow technique. The reactions followed the simple rate law k_obs = k₂[Nu]. The results indicate that the introduction of substituents in the ortho position of the phenyl group on the ancillary ring of the terpy unit does influence the extent of π‐backbonding in the terpy ring. This controls the electrophilicity of the platinum center, which in turn controls the lability of the chloro‐leaving group. The strength of the electron‐donating or ‐withdrawing ability of the substituents correlates with the reactivity of the complexes. Electron‐donating substituents decrease the rate of substitution, whereas electron‐withdrawing substituents increase the rate of substitution. This was supported by DFT calculations at the B3LYP/LACVP+** level of theory, which showed that most of the electron density of the HOMO is concentrated on the phenyl ligand rather than on the metal center in the case of the strongest electron‐withdrawing substituent in CF₃PhPtCl. The opposite was found to be true with the strongest electron‐donating substituent in CH₃PhPtCl. Thiourea was found to be the best nucleophile with N,N,N′,N′‐tetramethylthiourea being the weakest due to steric effects. The temperature dependence studies support an associative mode of activation. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 808–818, 2008     

Platinum trans‐Bis(borirene) Complexes Displaying Coplanarity and Communication Across a Platinum Metal Center

Ambient‐temperature photolysis of the aminoborylene complex [(OC)₅Cr?B?N(SiMe₃)₂] in the presence of a series of trans‐bis(alkynyl)platinum(II) precursors of the type trans‐[Pt(CCAr)₂(PEt₃)₂] (Ar=Ph, p‐C₆H₄OMe, and p‐C₆H₄CF₃) successfully leads to twofold transfer of the borylene moiety [ : B?N(SiMe₃)₂] onto the alkyne functionalities. The alkynyl precursors and resultant bis(borirene)platinum(II) complexes formed are of the type trans‐[Pt(B{?N(SiMe₃)₂}C?CAr)₂(PEt₃)₂] (Ar=Ph, p‐C₆H₄OMe, and p‐C₆H₄CF₃). These species have all been successfully characterized by NMR, IR, and UV/Vis spectroscopy as well as by elemental analysis. Single‐crystal X‐ray diffraction has verified that these trans‐bis(borirene)platinum(II) complexes display coplanarity between the twin three‐membered rings across the platinum core in the solid state and stand as the first examples of coplanar conformations of twin borirene systems. These complexes were modeled using density functional theory (DFT), providing information helpful in determining the ability of the transition metal core to interact with each individual borirene ring system and allowing for the observed coplanarity of these rings in the solid state. This proposed transition metal interaction with the twin borirene systems is manifested in the electronic characterization of these borirene species, which display divergent photophysical UV/Vis spectroscopic profiles compared to a previously published mono(borirene)platinum(II) complex.     

CNN Pincer Ruthenium Catalysts for Hydrogenation and Transfer Hydrogenation of Ketones: Experimental and Computational Studies

Reaction of [RuCl(CNN)(dppb)] ( 1‐Cl ) (HCNN=2‐aminomethyl‐6‐(4‐methylphenyl)pyridine; dppb=Ph₂P(CH₂)₄PPh₂) with NaOCH₂CF₃ leads to the amine‐alkoxide [Ru(CNN)(OCH₂CF₃)(dppb)] ( 1‐OCH₂CF₃ ), whose neutron diffraction study reveals a short RuO ??? HN bond length. Treatment of 1‐Cl with NaOEt and EtOH affords the alkoxide [Ru(CNN)(OEt)(dppb)] ? (EtOH)_n ( 1‐OEt?n EtOH ), which equilibrates with the hydride [RuH(CNN)(dppb)] ( 1‐H ) and acetaldehyde. Compound 1‐OEt?n EtOH reacts reversibly with H₂ leading to 1‐H and EtOH through dihydrogen splitting. NMR spectroscopic studies on 1‐OEt?n EtOH and 1‐H reveal hydrogen bond interactions and exchange processes. The chloride 1‐Cl catalyzes the hydrogenation (5 atm of H₂) of ketones to alcohols (turnover frequency (TOF) up to 6.5×10⁴ h^?1, 40 °C). DFT calculations were performed on the reaction of [RuH(CNN′)(dmpb)] ( 2‐H ) (HCNN′=2‐aminomethyl‐6‐(phenyl)pyridine; dmpb=Me₂P(CH₂)₄PMe₂) with acetone and with one molecule of 2‐propanol, in alcohol, with the alkoxide complex being the most stable species. In the first step, the Ru‐hydride transfers one hydrogen atom to the carbon of the ketone, whereas the second hydrogen transfer from NH₂ is mediated by the alcohol and leads to the key “amide” intermediate. Regeneration of the hydride complex may occur by reaction with 2‐propanol or with H₂; both pathways have low barriers and are alcohol assisted.     

Methyloxy Substituted Heteroleptic Bis(phthalocyaninato) Yttrium Complexes: Density Functional Calculations

The effects of alkyloxy substituents attached to one phthalocyanine ligand of three heteroleptic bis(phthalocyaninato) yttrium complexes Y(Pc)[Pc(α‐OCH₃)₄] ( 1 ), Y(Pc)[Pc(α‐OCH₃)₈] ( 2 ), and Y(Pc)[Pc(β‐OCH₃)₈] ( 3 ), as well as their reduction products {Y(Pc)[Pc(α‐OCH₃)₄]}^? ( 4 ), {Y(Pc)[Pc(α‐OCH₃)₈]}^? ( 5 ), and {Y(Pc)[Pc(β‐OCH₃)₈]}^? ( 6 ) [H₂Pc(α‐OCH₃)₄=1,8,15,22‐tetrakis(methyloxy)phthalocyanine; H₂Pc(α‐OCH₃)₈=1,4,8,11,15,18,22,25‐octakis(methyloxy)phthalocyanine; H₂Pc(β‐OCH₃)₈=2,3,9,10,16,17,23,24‐octakis(methyloxy)phthalocyanine] are studied by DFT calculations. Good consistency is found between the calculated results and experimental data for the electronic absorption, IR, and Raman spectra of 1 and 3 . Introduction of electron‐donating methyloxy groups on one phthalocyanine ring of the heteroleptic double‐deckers induces structural deformation in both phthalocyanine ligands, electron transfer between the two phthalocyanine rings, changes in orbital energy and composition, shift of electronic absorption bands, and different vibrational modes of the unsubstituted and substituted phthalocyanine ligands in the IR and Raman spectra in comparison with the unsubstituted homoleptic counterpart Y(Pc)₂. The calculations reveal that incorporation of methyloxy substituents at the nonperipheral positions has greater influence on the structure and spectroscopic properties of bis(phthalocyaninato) yttrium double‐deckers than at the peripheral positions, which increases with increasing number of substituents. Nevertheless, the substituent effect of alkyloxy substituents at one phthalocyanine ligand of the double‐decker on the unsubstituted phthalocyanine ring and on the whole molecule and the importance of the position and number of alkyloxy substituents are discussed. In addition, the effect of reducing 1 – 3 to 4 – 6 on the structure and spectroscopic properties of the bis(phthalocyaninato) yttrium compounds is also discussed. This systemic DFT study is not only useful for understanding the structure and spectroscopic properties of bis(phthalocyaninato) rare earth metal complexes but also helpful in designing and preparing double‐deckers with tunable structure and properties.     

White Emitters by Tuning the Excited‐State Intramolecular Proton‐Transfer Fluorescence Emission in 2‐(2′‐Hydroxybenzofuran)benzoxazole Dyes

The synthesis, structural, and photophysical properties of a new series of original dyes based on 2‐(2′‐hydroxybenzofuran)benzoxazole (HBBO) is reported. Upon photoexcitation, these dyes exhibit intense dual fluorescence with contribution from the enol (E*) and the keto (K*) emission, with K* being formed through excited‐state intramolecular proton transfer (ESIPT). We show that the ratio of emission intensity E*/K* can be fine‐tuned by judiciously decorating the molecular core with electron‐donating or ‐attracting substituents. Push–pull dyes 9 and 10 functionalized by a strong donor (nNBu₂) and a strong acceptor group (CF₃ and CN, respectively) exhibit intense dual emission, particularly in apolar solvents such as cyclohexane in which the maximum wavelength of the two bands is the more strongly separated. Moreover, all dyes exhibit strong solid‐state dual emission in a KBr matrix and polymer films with enhanced quantum yields reaching up to 54 %. A wise selection of substituents led to white emission both in solution and in the solid state. Finally, these experimental results were analyzed by time‐dependent density functional theory (TD‐DFT) calculations, which confirm that, on the one hand, only E* and K* emission are present (no rotamer) and, on the other hand, the relative free energies of the two tautomers in the excited state guide the ratio of the E*/K* emission intensities.     

Fine‐Tuning of Saponification‐Triggered Gelation by Strategic Modification of Peripheral Substituents: Gelation Regulators

A pioneering approach towards controlling the efficiency of saponification assisted gelation in ethyl ester based Zn^II‐complexes have been described. Using four new ester containing bis‐salen Zn^II complexes ( C1–C4 ) involving different para‐azo phenyl substituted ligands it has been clearly shown that gelation efficiency is greatly influenced by the electronic effects of the substituents (‐H ( C1 ), ‐CH₃ ( C2 ), ‐NO₂ ( C3 ), and ‐OCH₃ ( C4 )). Morphological, photophysical, and rheological investigations corroborated the experimental observations well and established that gelation efficiency was enhanced with electron‐withdrawing characteristics of substituents ( C4 < C2 < C1 < C3 ). This conclusion was also supported by DFT studies.     

Direct ab initio dynamics calculations of the rate constants for the reaction of CHF2CF2OCH3 with Cl

A dual‐level direct dynamics method is employed to reveal the dynamical properties of the reaction of CHF₂CF₂OCH₃ (HFE‐254pc) with Cl atoms. The optimized geometries and frequencies of the stationary points and the minimum energy path (MEP) are calculated at the B3LYP/6‐311G(d,p) level by using GAUSSIAN 98 program package, and energetic information is further refined by the G3(MP2) method. Two H‐abstraction channels have been identified. For the reactant CHF₂CF₂OCH₃ and the two products, CHF₂CF₂OCH₂ and CF₂CF₂OCH₃, the standard enthalpies of formation are evaluated with the values of ?256.71 ± 0.88, ?207.79 ± 0.12, and ?233.43 ± 0.88 kcal/mol, respectively, via group‐balanced isodesmic reactions. The rate constants of the two reaction channels are evaluated by means of canonical variational transition‐state theory (CVT) including the small‐curvature tunneling (SCT) correction over a wide range of temperature from 200 to 2000 K. The calculated rate constants agree well with the experimental data, and the Arrhenius expressions for the title reaction are fitted and can be expressed as k₁ = 9.22 × 10^?19 T^2.06 exp(219/T), k₂ = 4.45 × 10^?14 T^0.90 exp(?2220/T), and k = 4.71 × 10^?22 T^3.20) exp(543/T) cm³ molecule^?1 s^?1. Our results indicate that H‐abstraction from ? CH₃ group is the main reaction pathway in the lower temperature range, while H‐abstraction from ? CHF₂ group becomes more competitive in the higher temperature range. © 2007 Wiley Periodicals, Inc. 39: 221–230, 2007     

Efficient structural modification of electron-withdrawing substituents on Pt(II) complexes for red emitters: A theoretical study

In this study, the electronic structures and optical properties of a cyclometalated Pt(II) complex (M1) and a series of derivatives (M1–F, M1–CF₃, and M1–CN) with electron-withdrawing substituents (–F, –CF₃, and –CN) at the carbazole moiety were theoretically investigated by density functional theory and time-dependent density functional theory. The calculation results reveal that these Pt complexes display deep red phosphorescence emission above Λ = 640 nm. When the ³MLCT/π → π* to triplet metal-centered ³MC/d–d state decay mechanism is taken into consideration, the nonradiative decay rate constant (k_nr) decreased in the order M1 > M1–CF₃ > M1–F > M1–CN. The <T₁|H_SOC|S_m> and kr values of M1-F are similar with those of M1, however the Knr rate ofM1-F is larger than that of M1. M1–F is expected to have improved quantum yields. Moreover, through the analyses of the HOMO/LUMO level and triplet energy, it is found that the introduction of –F and –CN substituents in M1 results in efficient energy transfer from the host material 4,4′-N,N′-dicarbazole-biphenyl to these complexes. In view of the electroluminescent applications in organic light-emitting diodes, M1–F can serve as efficient deep-red guest materials with improved electron injection and transport ability.     

Enhanced Reactivities of Iron(IV)‐Oxo Porphyrin π‐Cation Radicals in Oxygenation Reactions by Electron‐Donating Axial Ligands

The proximal axial ligand in heme iron enzymes plays an important role in tuning the reactivities of iron(IV)‐oxo porphyrin π‐cation radicals in oxidation reactions. The present study reports the effects of axial ligands in olefin epoxidation, aromatic hydroxylation, alcohol oxidation, and alkane hydroxylation, by [(tmp)^+. Fe^IV(O)(p‐Y‐PyO)]⁺ ( 1 ‐Y) (tmp=meso‐tetramesitylporphyrin, p‐Y‐PyO=para‐substituted pyridine N‐oxides, and Y=OCH₃, CH₃, H, Cl). In all of the oxidation reactions, the reactivities of 1 ‐Y are found to follow the order 1 ‐OCH₃ > 1 ‐CH₃ > 1 ‐H > 1 ‐Cl; negative Hammett ρ values of ?1.4 to ?2.7 were obtained by plotting the reaction rates against the σ_p values of the substituents of p‐Y‐PyO. These results, as well as previous ones on the effect of anionic nucleophiles, show that iron(IV)‐oxo porphyrin π‐cation radicals bearing electron‐donating axial ligands are more reactive in oxo‐transfer and hydrogen‐atom abstraction reactions. These results are counterintuitive since iron(IV)‐oxo porphyrin π‐cation radicals are electrophilic species. Theoretical calculations of anionic and neutral ligands reproduced the counterintuitive experimental findings and elucidated the root cause of the axial ligand effects. Thus, in the case of anionic ligands, as the ligand becomes a better electron donor, it strengthens the FeO? H bond and thereby enhances its H‐abstraction activity. In addition, it weakens the Fe?O bond and encourages oxo‐transfer reactivity. Both are Bell–Evans–Polanyi effects, however, in a series of neutral ligands like p‐Y‐PyO, there is a relatively weak trend that appears to originate in two‐state reactivity (TSR). This combination of experiment and theory enabled us to elucidate the factors that control the reactivity patterns of iron(IV)‐oxo porphyrin π‐cation radicals in oxidation reactions and to resolve an enigmatic and fundamental problem.     

The Mechanism of Stereospecific C?H Oxidation by Fe(Pytacn) Complexes: Bioinspired Non‐Heme Iron Catalysts Containing cis‐Labile Exchangeable Sites

A detailed mechanistic study of the hydroxylation of alkane C? H bonds using H₂O₂ by a family of mononuclear non heme iron catalysts with the formula [Fe^II(CF₃SO₃)₂(L)] is described, in which L is a tetradentate ligand containing a triazacyclononane tripod and a pyridine ring bearing different substituents at the α and γ positions, which tune the electronic or steric properties of the corresponding iron complexes. Two inequivalent cis‐labile exchangeable sites, occupied by triflate ions, complete the octahedral iron coordination sphere. The C? H hydroxylation mediated by this family of complexes takes place with retention of configuration. Oxygen atoms from water are incorporated into hydroxylated products and the extent of this incorporation depends in a systematic manner on the nature of the catalyst, and the substrate. Mechanistic probes and isotopic analyses, in combination with detailed density functional theory (DFT) calculations, provide strong evidence that C? H hydroxylation is performed by highly electrophilic [Fe^V(O)(OH)L] species through a concerted asynchronous mechanism, involving homolytic breakage of the C? H bond, followed by rebound of the hydroxyl ligand. The [Fe^V(O)(OH)L] species can exist in two tautomeric forms, differing in the position of oxo and hydroxide ligands. Isotopic‐labeling analysis shows that the relative reactivities of the two tautomeric forms are sensitively affected by the α substituent of the pyridine, and this reactivity behavior is rationalized by computational methods.     

Comparison of large basis set DFT and MP2 calculations in the study of the barrier for internal rotation of 2,3,5,6‐tetrafluoroanisole

The barrier for internal rotation around the ? OCH₃ bond in 2,3,5,6‐tetrafluoroanisole was calculated using the density functional theory (DFT) and second‐order Møller–Plesset (MP2) methods with Pople's basis sets up to 6‐311++G(3df,2p) and Jensen basis sets up to pc‐3. The results are converged only if fairly large basis sets are used (at least 6‐311++G(3df,2pd) or pc‐2). Both the DFT and MP2 potential energy curves show internal structure. Two minima and three maxima are observed on the curves, arising from the interplay between lone‐pair delocalization and changes in the hybridization around the oxygen atom, together with the attraction between the positively polarized hydrogens in the methyl group and the negatively polarized fluorine atom at the ortho position. These critical points are somehow ironed out by the addition of zero‐point and thermal corrections to the energy curves. At this level, the MP2 method can describe reasonably well the previously determined single‐well experimental rotational barrier, 2.7 ± 2.0 kcal/mol at 298 K, while all DFT methods yield a much smaller result. As observed experimentally, the ? OCH₃ group is perpendicular to the aryl ring in the equilibrium structure, although two very close minima with an intermediate hump at 90° are still observable. The theoretical free energy barrier of rotation at the MP2(full)/pc‐2 level is 2.0 ± 1.0 kcal/mol, in reasonable agreement with the experimental determination. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Magnetic Coupling and Optical Properties of the S6‐Dodecakis(trifluoromethyl)fullerene Radical Anions in the Layered Salt (PPN+)[C60(CF3)12.−]

Poly(trifluoromethyl)fullerene S₆‐C₆₀(CF₃)₁₂ was reduced by sodium fluorenone ketyl in the presence of (PPN)Cl (PPN=bis(triphenylphosphine)iminium) to afford the salt (PPN)[C₆₀(CF₃)₁₂] ( 1 ), which contains C₆₀(CF₃)₁₂^.? radical anions. In the crystal structure of 1 , C₆₀(CF₃)₁₂^.? layers alternate with the PPN⁺ cations. There are short F ??? F contacts between C₆₀(CF₃)₁₂^.? radical anions within the layers but no C ??? C contacts. DFT calculations revealed that the negative charge on C₆₀(CF₃)₁₂^.? is distributed mainly between sp² carbon and fluorine atoms, whereas spin density is localized mainly on the fullerene‐cage sp² carbon atoms. IR and UV/Vis/NIR spectra in the solid state and solution showed characteristic changes relative to those of neutral S₆‐C₆₀(CF₃)₁₂ due to the formation of radical anions. The solid‐state electronic spectrum of 1 exhibits a single broad band at 738 nm attributed to C₆₀(CF₃)₁₂^.?. Crystals of 1 show a narrow EPR signal with g=2.0025 (ΔH=0.45 mT) at 300 K. The temperature dependence of the integral intensity follows the Curie–Weiss law with a negative Weiss temperature of ?11.8 K (30–300 K) indicating antiferromagnetic interaction of spins. This dependence was approximated by the Heisenberg model for one‐dimensional chains of antiferromagnetically interacting spins with exchange interaction J/k_B=?9.1 K. It was assumed that magnetic interaction between the C₆₀(CF₃)₁₂^.? spins in the layers is mediated by short F ??? F contacts.     

[6,6]‐Open and [6,6]‐Closed Isomers of C70(CF2): Synthesis,Electrochemical and Quantum Chemical Investigation

Novel difluoromethylenated [70]fullerene derivatives, C₇₀(CF₂)_n (n=1–3), were obtained by the reaction of C₇₀ with sodium difluorochloroacetate. Two major products, isomeric C₇₀(CF₂) mono‐adducts with [6,6]‐open and [6,6]‐closed configurations, were isolated and their homofullerene and methanofullerene structures were reliably determined by a variety of methods that included X‐ray analysis and high‐level spectroscopic techniques. The [6,6]‐open isomer of C₇₀(CF₂) constitutes the first homofullerene example of a non‐hetero [70]fullerene derivative in which functionalisation involves the most reactive bond in the polar region of the cage. Voltammetric estimation of the electron affinity of the C₇₀(CF₂) isomers showed that it is substantially higher for the [6,6]‐open isomer (the 70‐electron π‐conjugated system is retained) than the [6,6]‐closed form, the latter being similar to the electron affinity of pristine C₇₀. In situ ESR spectroelectrochemical investigation of the C₇₀(CF₂) radical anions and DFT calculations of the hyperfine coupling constants provide evidence for the first example of an inter‐conversion between the [6,6]‐closed and [6,6]‐open forms of a cage‐modified fullerene driven by an electrochemical one‐electron transfer. Thus, [6,6]‐closed C₇₀(CF₂) constitutes an interesting example of a redox‐switchable fullerene derivative.     

Nature of the near‐IR band in the electronic absorption spectra of neutral bis(tetrapyrrole) rare earth(III) complexes: Time‐dependent density functional theory calculations

The nature of the near‐IR band in the electronic absorption spectra of bis(tetrapyrrole) rare earth(III) complexes Y(Pc)₂ (1), La(Pc)₂ (2), Y(Pc)(Por) (3), Y(Pc)[Pc(α‐OCH₃)₄] (4), Y(Pc)[Pc(α‐OCH₃)₈] (5), and Y(Pc)[Pc(β‐OCH₃)₈] (6) was studied on the basis of time‐dependent density functional theory (TD‐DFT) calculations. The electronic dipole moment along the z‐axis in the electronic transition of the near‐IR band in all the studied neutral bis(tetrapyrrole) yttrium(III) and lanthanum(III) double‐deckers is well explained on the basis of the composition analysis of the orbitals involved. The electronic transition in the near‐IR band causes the reversion of the orbital orientation of one tetrapyrrole ring in both homoleptic and heteroleptic bis(tetrapyrrole) rare earth complexes and induces electron transfer from the tetrapyrrole ring with lower orbital energy to the other ring in the heteroleptic bis(tetrapyrrole) rare earth(III) complexes. The near‐IR band can work as an ideal characteristic absorption band to reflect the π–π interaction between the two tetrapyrrole rings in bis(tetrapyrrole) rare earth(III) double‐decker complexes because of its peculiar electronic transition nature. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Stereoselective Access to Fluorinated and Non‐fluorinated Quaternary Piperidines: Synthesis of Pipecolic Acid and Iminosugar Derivatives

The preparation of optically pure quaternary piperidines, both fluorinated and non‐fluorinated, has been achieved from a chiral imino lactone derived from (R)‐phenylglycinol. In the case of the fluorinated derivatives, the addition of (trifluoromethyl)trimethylsilane (TMSCF₃) followed by iodoamination and migration of the CF₃ group allowed access to four derivatives of α‐(trifluoromethyl)pipecolic acid. A theoretical study of the CF₃‐group rearrangement has been carried out to help establish the reaction mechanism of this uncommon transformation. Moreover, a route to trifluoromethyl‐substituted iminosugars was also developed through the diastereoselective dihydroxylation of suitable synthetic intermediates. Conversely, alkylation of the starting substrate and subsequent cross‐metathesis and aza‐Michael reactions led to α‐alkyl derivatives of the target compounds.