Molecular complexes of cyclophanes—XIII. Charge-transfer complexes of cyclophanes with fluoranil

The charge-transfer (CT) complexes of some methylated [2.2]para-, multibridged cyclo- and [2.2]indenophanes as π-donors with fluoranil (TFQ) as π-acceptor have been studied spectrophotometrically. The role of the molecular structure of the donors on their Lewis basicities and stability of their CT complexes with TFQ are discussed. Eight pure crystalline complexes were prepared. Their i.r. spectra indicate an increase of the electron density in the TFQ part of the CT complex.     

Charge-transfer molecular complexes of 2-amino-1,3,4-thiadiazoles

Charge-transfer molecular complexes of 2-amino-5-X-1,3,4-thiadiazole (D) (X = H, I; = CH₃, II; = phenyl, III) with some π-electron acceptors (A) have been studied in methanol. It is concluded that these complexes are predominantly of the π-π type. Solid 1:1 CT complexes of the donors I–III with π-acceptors DDQ and TCNE have been synthesized and characterized.     

Electron-Rich Diruthenium Complexes with π-Extended Alkenyl Ligands and Their F4TCNQ Charge-Transfer Salts**

The synthesis of dinuclear ruthenium alkenyl complexes with {Ru(CO)(PⁱPr₃)₂(L)} entities (L=Cl⁻ in complexes Ru₂-3 and Ru₂-7 ; L=acetylacetonate (acac⁻) in complexes Ru₂-4 and Ru₂-8 ) and with π-conjugated 2,7-divinylphenanthrenediyl ( Ru₂-3 , Ru₂-4 ) or 5,8-divinylquinoxalinediyl ( Ru₂-7 , Ru₂-8 ) as bridging ligands are reported. The bridging ligands are laterally π-extended by anellating a pyrene ( Ru₂-7 , Ru₂-8 ) or a 6,7-benzoquinoxaline ( Ru₂-3 , Ru₂-4 ) π-perimeter. This was done with the hope that the open π-faces of the electron-rich complexes will foster association with planar electron acceptors via π-stacking. The dinuclear complexes were subjected to cyclic and square-wave voltammetry and were characterized in all accessible redox states by IR, UV/Vis/NIR and, where applicable, by EPR spectroscopy. These studies signified the one-electron oxidized forms of divinylphenylene-bridged complexes Ru₂-7 , Ru₂-8 as intrinsically delocalized mixed-valent species, and those of complexes Ru₂-3 and Ru₂-4 with the longer divinylphenanthrenediyl linker as partially localized on the IR, yet delocalized on the EPR timescale. The more electron-rich acac⁻ congeners formed non-conductive 1 : 1 charge-transfer (CT) salts on treatment with the F₄TCNQ electron acceptor. All spectroscopic techniques confirmed the presence of pairs of complex radical cations and F₄TCNQ^.− radical anions in these CT salts, but produced no firm evidence for the relevance of π-stacking to their formation and properties.     

Synthesis of cis and trans bis-alkynyl complexes of Cr(III) and Rh(III) supported by a tetradentate macrocyclic amine: a spectroscopic investigation of the M(III)-alkynyl interaction

Alkynyl complexes of the type [M(cyclam)(CCR)(2)]OTf (where cyclam = 1,4,8,11-tetraazacyclotetradecane; M = Rh(III) or Cr(III); and R = phenyl, 4-methylphenyl, 4-trifluoromethylphenyl, 4-fluorophenyl, 1-naphthalenyl, 9-phenanthrenyl, and cyclohexyl) were prepared in 49% to 93% yield using a one-pot synthesis involving the addition of 2 equiv of RCCH and 4 equiv of BuLi to the appropriate [M(cyclam)(OTf)(2)]OTf complex in THF. The cis and trans isomers of the alkynyl complexes were separated using solubility differences, and the stereochemistry was characterized using infrared spectroscopy of the CH(2) rocking and NH bending region. All of the trans-[M(cyclam)(CCR)(2)]OTf complexes exhibit strong Raman bands between 2071 and 2109 cm(-1), ascribed to ν(s)(C≡C). The stretching frequencies for the Cr(III) complexes are 21-28 cm(-1) lower than for the analogous Rh(III) complexes, a result that can be interpreted in terms of the alkynyl ligands acting as π-donors. UV-vis spectra of the Cr(III) and Rh(III) complexes are dominated by strong charge transfer (CT) transitions. In the case of the Rh(III) complexes, these CT transitions obscure the metal centered (MC) transitions, but in the case of the Cr(III) complexes the MC transitions are unobscured and appear between 320 and 500 nm, with extinction coefficients (170-700 L mol(-1) cm(-1)) indicative of intensity stealing from the proximal CT bands. The Cr(III) complexes show long-lived (240-327 μs), structureless, MC emission centered between 731 and 748 nm in degassed room temperature aqueous solution. Emission characteristics are also consistent with the arylalkynyl ligands acting as π-donors. The Rh(III) complexes also display long-lived (4-21 μs), structureless, metal centered emission centered between 524 and 548 nm in degassed room temperature solution (CH(3)CN).     

Infrared,Raman, 1H NMR,TG, and SEM properties of the charge-transfer interactions between tris(hydroxymethyl)methane with the acceptors picric acid,chloranilic acid,and 1,3-dinitrobenzene

Intermolecular charge-transfer complexes (CT) between the tris(hydroxymethyl)methane (THM) as a donor and picric acid (PA), chloranilic acid (CLA) and 1,3-dinitrobenzene (DNB) as a π-acceptor have been structurally, thermally and morphologically studied in methanol at room temperature. Based on elemental analyses (CHN), the stoichiometry of the obtained CT complexes (THM: acceptor molar ratios) was determined to be 1 : 1 for all three complexes. The CT complexes have been characterized via elemental analyses (CHN), IR, Raman and 1H NMR spectroscopy in order to predict the position of the CT interaction between the donating and accepting sites. Thermal decomposition behavior of these complexes was also investigated, and their kinetic thermodynamic parameters were calculated with Coats-Redfern and Horowitz-Metzger methods. Finally, the microstructure properties of these complexes were observed using scanning electron microscope (SEM).     

Charge-transfer complexes of two highly efficient drugs with σ- and π-acceptors: Spectroscopic,thermal, and surface morphology characteristics

Considerable attention has recently been devoted to the formation of stable charge-transfer (CT) complexes that result from the reaction of drugs with acceptors. Intermolecular CT interactions of two drugs, barbituratic acid (Bar) and ephedrine hydrochloride (Eph), with different σ- and π-acceptors have been studied stoichiometrically, structurally (IR, Raman, ¹H NMR, and UV-Vis spectroscopy), thermally, and morphologically (SEM).     

Effects of methyl substituent on the charge-transfer complexations of dicarbazolylalkanes with p-chloranil, tetracyanoethylene and tetracyanoquinodimethane

Series of 1,n-dicarbazolylalkanes and 1,n-di(3-methylcarbazolyl)alkanes (where n=1-5) were synthesized and the molar extinction coefficients, equilibrium constants, enthalpies, and entropies of their charge-transfer (CT) complexes with the π-acceptors p-chloranil, tetracyanoethylene, and tetracyanoquinodimethane were investigated. 1,n-Di(3-methylcarbazolyl)alkanes formed CT complexes with higher equilibrium constants, more negative enthalpies and entropies than 1,n-dicarbazolylalkanes. Vibrational spectra of CT complexes of one of the donor molecules (1,4-dicarbazolylbutane) with all three acceptors were compared.     

Anion-Coordination-Assisted Assembly of Supramolecular Charge-Transfer Complexes Based on Tris(urea) Ligands

Charge-transfer (CT) complexes, formed by noncovalent bonding between electron-rich (donor, D) and electron-deficient (acceptor, A) molecules (or moieties) have attracted considerable attention due to their fascinating structures and potential applications. Herein, we demonstrate that anion coordination is a promising strategy to promote CT complex formation between anion-binding, electron-rich tris(urea) donor ligands (D) and electron-deficient viologen cation acceptors (A), which form co-crystals featuring infinite ⋅⋅⋅DADA⋅⋅⋅ or discrete (circular DADA or three-decker DAD) π-stacking interactions. These CT complexes were studied by X-ray diffraction, UV/Vis spectroscopy, electric conductivity measurements, charge displacement curve (CDC) calculations, and DFT computations.     

Intermolecular charge-transfer complexes between chlorothiazide antihypertensive drug against iodine sigma and picric acid pi acceptors: DFT and molecular docking interaction study with Covid-19 protease

The interaction of chlorothiazide (CH) as donor (D) with picric acid (PA) and iodine (I₂) as π- and σ-acceptors (A), respectively, gives charge-transfer (CT) complexes as a final products. The reaction of donor and acceptors were studied spectrophotometrically. The complexes are generally of the n-π* and n-σ* types, with the ground state wave function primarily characterized by the non-bonding structure. For the micro determination of chlorothiazide using picric acid and iodine as acceptors, the ideal conditions encouraging the formation of complexes are thoroughly explored. It was discovered that the stoichiometry of the molecular structure is 1:1 (D:A). The equilibrium constant and the molar extinction coefficient were calculated using Benesi-Hildebrand and its modifications. DFT/TD-DFT calculations with B3LYP/LanL2DZ and 6-311G++ level of theory were used to provide comparable theoretical data along with electronic energy gap of HOMO→LUMO. Molecular docking calculations have been performed between CT complexes and Covid-19 protease (6LU7) to study the interaction between them and their inhibitory effect.     

A spectroscopic study of the iodine complexes of donors—pyridines,phenanthrolines, bipyridines and diazines

Ultraviolet—visible spectral data of iodine complexes of n- and π-donors have been interpreted by considering that the repulsion energy responsible for the blue shift of the iodine band is also experienced by the donor partner which causes the blue shift of the original band of the donor. This reasoning explains the spectral data of iodine complexes of benzene, pyridine-N-oxide and stilbazoles. Ultraviolet—vis. spectra of the iodine complexes of pyridine, aminopyridines and diazines have been reinvestigated and discussed in the light of the above reasoning. The above reasoning is extended to the CT spectra of iodine complexes of twin-site donors such as 1,10-phenanthroline, its methyl and chloro derivatives, 1,7 and 4,7-phenanthrolines, 2,2′-bipyridine and 4,4′-bipyridine. Arguments are presented which indicate that the donors used in this study form only 1:1 complexes with iodine. The thermodynamic parameters were evaluated for iodine complexes of the above twin-site donors. The kinetics of transformation of outer CT complexes between the donors and iodine to inner complexes is presented and discussed. Two CT bands are observed for the iodine complexes of 1,10-phenanthroline and its substituents. These bands are explained by assuming a structure for the 1,10-phenanthroline complex in which iodine is in dynamic equilibrium between the two nitrogens.     

Long-lived and temperature-independent emission from a novel ruthenium(II) complex having an arylborane charge-transfer unit

We report the redox, absorption, and emission characteristics of the tris(1,10-phenanthroline)ruthenium(II) complexes [Ru(phen)(3)](2+) bearing a (dimesityl)boryldurylethynyl (DBDE) charge-transfer (CT) unit at the 4 (4BRu(2+)) or 5 (5BRu(2+)) position of one of the three phen ligands. In acetonitrile at 298 K, 4BRu(2+) showed absorption and emission maximum wavelengths at 473 and 681 nm, respectively, which were shifted to longer wavelengths by 25 and 74 nm, respectively, compared with the relevant value of 5BRu(2+), 448 and 607 nm, respectively. The effects of a fluoride ion on the absorption and emission spectra of the complexes demonstrated that the CT interaction between the π-electron system in the phen ligand (π(aryl)) and the vacant p orbital on the boron atom (p(B)) in the DBDE group (i.e., π(aryl)-p(B) CT) participated in the excited states of the complexes in addition to the Ru(II)-to-phen metal-to-ligand CT (MLCT) interaction. Reflecting such synergistic MLCT/π(aryl)-p(B) CT, both 4BRu(2+) and 5BRu(2+) exhibited intense emission at 298 K with a quantum yield of 0.11. Furthermore, the emission lifetime of 4BRu(2+) was as long as 12 μs and almost independent of the temperature (T = 280-330 K). The present study indicated that the nonemissive dd excited triplet state did not participate to nonradiative decay in the MLCT excited triplet state of 4BRu(2+). The effects of the synergistic MLCT/π(aryl)-p(B) CT interactions on the redox, absorption/emission, and photophysical characteristics of 4BRu(2+) and 5BRu(2+) are discussed in detail.     

Mechanism of NO photodissociation in photolabile manganese-NO complexes with pentadentate N5 ligands

The Mn-nitrosyl complexes [Mn(PaPy(3))(NO)](ClO(4)) (1; PaPy(3)(-) = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide) and [Mn(PaPy(2)Q)(NO)](ClO(4)) (2, PaPy(2)Q(-) = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-quinoline-2-carboxamide) show a remarkable photolability of the NO ligand upon irradiation of the complexes with UV-vis-NIR light [Eroy-Reveles, A. A.; Leung, Y.; Beavers, C. M.; Olmstead, M. M.; Mascharak, P. K. J. Am. Chem. Soc. 2008, 130, 4447]. Here we report detailed spectroscopic and theoretical studies on complexes 1 and 2 that provide key insight into the mechanism of NO photolabilization in these compounds. IR- and FT-Raman spectroscopy show N-O and Mn-NO stretching frequencies in the 1720-1750 and 630-650 cm(-1) range, respectively, for these Mn-nitrosyls. The latter value for ν(Mn-NO) is one of the highest transition-metal-NO stretching frequencies reported to this date, indicating that the Mn-NO bond is very strong in these complexes. The electronic structure of 1 and 2 is best described as Mn(I)-NO(+), where the Mn(I) center is in the diamagnetic low-spin state and the NO(+) ligand forms two very strong π backbonds with the d(xz) and d(yz) orbitals of the metal. This explains the very strong Mn-NO bonds observed in these complexes, which even supersede the strengths of the Fe- and Ru-NO bonds in analogous (isoelectronic) Fe/Ru(II)-NO(+) complexes. Using time-dependent density functional theory (TD-DFT) calculations, we were able to assign the electronic spectra of 1 and 2, and to gain key insight into the mechanism of NO photorelease in these complexes. Upon irradiation in the UV region, NO is released because of the direct excitation of d(π)_π* → π*_d(π) charge transfer (CT) states (direct mechanism), which is similar to analogous NO adducts of Ru(III) and Fe(III) complexes. These are transitions from the Mn-NO bonding (d(π)_π*) into the Mn-NO antibonding (π*_d(π)) orbitals within the Mn-NO π backbond. Since these transitions lead to the population of Mn-NO antibonding orbitals, they promote the photorelease of NO. In the case of 1 and 2, further transitions with distinct d(π)_π* → π*_d(π) CT character are observed in the 450-500 nm spectral range, again promoting photorelease of NO. This is confirmed by resonance Raman spectroscopy, showing strong resonance enhancement of the Mn-NO stretch at 450-500 nm excitation. The extraordinary photolability of the Mn-nitrosyls upon irradiation in the vis-NIR region is due to the presence of low-lying d(xy) → π*_d(π) singlet and triplet excited states. These have zero oscillator strengths, but can be populated by initial excitation into d(xy) → L(Py/Q_π*) CT transitions between Mn and the coligand, followed by interconversion into the d(xy) → π*_d(π) singlet excited states. These show strong spin-orbit coupling with the analogous d(xy) → π*_d(π) triplet excited states, which promotes intersystem crossing. TD-DFT shows that the d(xy) → π*_d(π) triplet excited states are indeed found at very low energy. These states are strongly Mn-NO antibonding in nature, and hence, promote dissociation of the NO ligand (indirect mechanism). The Mn-nitrosyls therefore show the long sought-after potential for easy tunability of the NO photorelease properties by simple changes in the coligand.     

Pronounced solvatochromism of charge-transfer complexes in strongly hydrogen bonding solvents

A large bathochromic shift is reported for the charge-transfer (CT) transition in complexes of π-electron donors with electron acceptors containing CO groups in strongly H-bonding solvents such as trifluoroacetic acid and hexafluoroisopropanol. The results of ¹³C-NMR measurements and SCF-LCAOMO calculations indicate that this effect can be attributed to an increased electronegativity of the acceptor upon hydrogen bond formation with the solvent.     

Gold(III) π-Allyl Complexes

Gold(III) π-complexes have been authenticated recently with alkenes, alkynes, and arenes. The key importance of Pd^II π-allyl complexes in organometallic chemistry (Tsuji–Trost reaction) prompted us to explore gold(III) π-allyl complexes, which have remained elusive so far. The (P,C)Au^III(allyl) and (methallyl) complexes 3 and 3′ were readily prepared and isolated as thermally and air-stable solids. Spectroscopic and crystallographic analyses combined with detailed DFT calculations support tight quasi-symmetric η³-coordination of the allyl moiety. The π-allyl gold(III) complexes are activated towards nucleophilic additions, as substantiated with β-diketo enolates.     

Design,synthesis and electronic states of an iminonitroxide radical with excited quartet high-spin state and photo-induced electron transfer in its CT complex

In this paper, we report the design, synthesis and electronic state of a π-conjugated stable iminonitroxide radical, 1, the electron donor property of which is improved by an attachment of dimethylamino group. The photo-excited quartet high-spin state was observed by a time-resolved ESR experiment. CV measurement has clarified that the π-HOMO of the ground state is located higher ca. 0.35 eV in energy than 2 reported previously. These results show that the electron donor property is improved and the nature of the photo-excited spin alignment is conserved. Two CT complexes (1-TCNQ and 1-BQF₄) were synthesized using 1 as an electron donor. Their complexes have shown strong CT bands. The time-resolved ESR signal without the fine-structure splitting was observed for 1-TCNQ CT complex. The signal may arise from the photo-induced electron transfer from the quartet excited state of 1 to the TCNQ acceptor.     

Spectroscopic characterization of charge-transfer complexes of morpholine with chloranilic and picric acids in organic media: Crystal structure of bis(morpholinium 2,4,6-trinitrocyclohexanolate)

Electron donor–acceptor interaction of morpholine (morp) with chloranilic acid (cla) and picric acid (pa) as π-acceptors was investigated spectrophotometrically and found to form stable charge-transfer (CT) complexes (n–π*) of [(Hmorp)₂(cla)] and [(Hmorp)(pa)]₂. The donor site involved in CT interaction is morpholine nitrogen. These complexes are easily synthesized from the reaction of morp with cla and pa within MeOH and CHCl₃ solvents, respectively. ¹HNMR, IR, elemental analyses, and UV–vis techniques characterize the two morpholinium charge-transfer complexes. Benesi–Hildebrand and its modification methods were applied to the determination of association constant (K), molar extinction coefficient (?). The X-ray crystal structure was carried out for the interpretation the predict structure of the [(Hmorp)(pa)]₂ complex.     

Estimation of σ- and π-donor properties of heterocyclic thioamides by spectroscopic and magnetic resonance methods

The charge-transfer complexes (CTC) of few thioamide: 1-methylimidazoline-2-thione (MMI), 3-methyl-1-ethoxycarbonilimidazoline-2-thione (Carb), 5-methylbenzimidazoline-2-thione (BIZ), benzothiazoline-2-thione (BTZ), benzoxazoline-2-thione (BOZ) as σ-donors and diiodine as σ-acceptor were studied by spectroscopic methods (UV/Vis, (1)H NMR). CTC formation constants of thioamides with diiodine were determined using the function of the average-iodine number. The charge-transfer complexes of thioamides as π-donors with tetracyanoethylene (TCNE) as π-electron acceptor, were studied by UV-spectroscopy in dichloromethane and chloroform solutions. The mechanism of interaction MMI and Carb with TCNE have been studied by EPR spectroscopy. Spectral characteristics and formation constants are discussed in the terms of electron donor affinity of thioamides and the nature of the organic solvent used. The ionization potentials of donors were estimated from the CT transition energies of their complexes. The photolytic equilibrium constants of five thioamides are determined using pH-metric titrations.     

Importance of CH/π hydrogen bonds in recognition of the core motif in proline-recognition domains: an ab initio fragment molecular orbital study

We examined CH/π hydrogen bonds in protein/ligand complexes involving at least one proline residue using the ab initio fragment molecular orbital (FMO) method and the program CHPI. FMO calculations were carried out at the Hartree–Fock (HF)/6‐31G*, HF/6‐31G**, second‐order Møller–Plesset perturbation (MP2)/6‐31G*, and MP2/6‐31G** levels for three Src homology 3 (SH3) domains and five proline‐recognition domains (PRDs) complexed with their corresponding ligand peptides. PRDs use a conserved set of aromatic residues to recognize proline‐rich sequences of specific ligands. Many CH/π hydrogen bonds were identified in these complexes. CH/π hydrogen bonds occurred, in particular, in the central part of the proline‐rich motifs. Our results suggest that CH/π hydrogen bonds are important in the recognition of SH3 and PRDs by their ligand peptides and play a vital role in the signal transduction system. Combined use of the FMO method and CHPI analysis is a valuable tool for the study of protein/protein and protein/ligand interactions and may be useful in rational drug design. © 2011 Wiley Periodicals, Inc. J Comput Chem 2011     

2-alkyl-1,3-butadiene polymerization under the influence of π-allylic complexes of transition metals

Stereoregulation in the polymerization of 2-alkyl-1,3-butadienes with transition metal π-allylic complexes has been studied. The direction of isoprene polymerization is shown to be a function of the nature of the metal and ligands in the allylic compound. The presence of acidic ligands in π-allylic complexes of Zr, Cr, Mo, and Co contributes to 1,4-addition and increases the selectivity of π-allylic nickel complexes, favoring cis-1,4-structure formation. Investigation of the model reaction of 2-alkyl-1,3-butadienes with bis(π-perdeuterocrotyl nickel iodide) revealed that active sites have an π-allylic type structure. The mechanism of formation of π-allylic adducts and the main factors which determine the dependence of direction and rate of polymerization on the nature of a monomer in the diene series: 2-methyl-1,3-butadiene(isoprene), 2-ethyl-1,3-butadiene, 2-isopropyl-1,3-butadiene, and 2-tert-butyl-1,3-butadiene, are discussed.