Influence of the ligand-field on EPR parameters of cis- and trans-isomers in MoV systems relevant to molybdenum enzymes: Experimental and density functional theory study.

The electron paramagnetic resonance (EPR) investigation of mononuclear cis- and trans-(L1O)MoOCl2 complexes [L1OH = bis(3,5-dimethylpyrazolyl)-3-tert-butyl-2-hydroxy-5-methylphenyl)methane] reveals a significant difference in their spin Hamiltonian parameters which reflect different equatorial and axial ligand fields created by the heteroscorpionate donor atoms. Density functional theory (DFT) was used to calculate the values of principal components and relative orientations of the g and A tensors, and the molecular framework in four pairs of isomeric mononuclear oxo­molybdenum(V) complexes (cis- and trans-(L1O)MoOCl2, cis,cis- and cis,trans-(L-N2S2)MoOCl [L-N2S2H2 = N,N'-dimethyl-N,N'-bis(mercaptophenyl)ethylenediamine], cis,cis- and cis,trans-(L-N2S2)MoO(SCN), and cis- and trans-[(dt)2MoO(OMe)]2- [dtH2 = 2,3-dimercapto-2-butene]). Scalar relativistic DFT calculations were conducted using three different exchange-correlation functionals. It was found that the use of hybrid exchange-correlation functional with 25% of the Hartree-Fock exchange leads to the best quantitative agreement between theory and experiment. A simplified ligand-field approach was used to analyze the influence of the ligand fields in all cis- and trans-isomers on energies and contributions of molybdenum d-orbital manifold to g and A tensors and relative orientations. Specifically, contributions that originated from the spin-orbit coupling of the dxz, dyz, and dx2-y2 orbitals into the ground state have been discussed. The new findings are discussed in the context of the experimental data of mononuclear molybdoenzyme, DMSO reductase.

Similar, Yet Different: Long-Range Metal-Metal Coupling and Electron-Transfer Processes in Metal-Free 5,10,15,20-Tetra(ruthenocenyl)porphyrin.

The electronic structure, redox properties, and long-range metal-metal coupling in metal-free 5,10,15,20-tetra(ruthenocenyl)porphyrin (H2TRcP) were probed by spectroscopic (NMR, UV-vis, magnetic circular dichroism (MCD), and atmospheric pressure chemical ionization (APCI)), electrochemical (cyclic voltammetry, CV, and differential pulse voltammetry, DPV), spectroelectrochemical, and chemical oxidation methods, as well as theoretical (density functional theory, DFT, and time-dependent DFT, TDDFT) approaches. It was demonstrated that the spectroscopic properties of H2TRcP are significantly different from those in H2TFcP (metal-free 5,10,15,20-tetra(ferrocenyl)porphyrin). Ruthenocenyl fragments in H2TRcP have higher oxidation potentials than the ferrocene groups in the H2TFcP complex. Similar to H2TFcP, we were able to access and spectroscopically characterize the one- and two-electron oxidized mixed-valence states in the H2TRcP system. DFT predicts that the porphyrin π-system stabilizes the [H2TRcP]+ mixed-valence cation and prevents its dimerization, which is characteristic for ruthenocenyl systems. However, formation of the mixed-valence [H2TRcP]2+ is significantly less reproducible than the formation of [H2TRcP]+. DFT and TDDFT calculations suggest the ruthenocenyl fragment dominance in the highest occupied molecular orbital (HOMO) energy region and the presence of the low-energy MLCT (Rc → porphyrin (π*)) transitions in the visible region with energies higher than the predominantly porphyrin-centered Q-bands.

Simultaneous Prediction of the Energies of Q x and Q y Bands and Intramolecular Charge-Transfer Transitions in Benzoannulated and Non-Peripherally Substituted Metal-Free Phthalocyanines and Their Analogues: No Standard TDDFT Silver Bullet Yet.

An insight into the electronic structure of the metal-free, unsubstituted, and nonperipherally substituted with electron-donating groups tetraazaporphyrin (H2TAP), phthalocyanine (H2Pc), naphthalocyanine (H2Nc), anthracocyanine (H2Ac) platforms has been gained and discussed on the basis of experimental UV-vis and MCD spectra as well as density functional theory (DFT), time-dependent DFT (TDDFT), and semiempirical ZINDO/S calculations. Experimental data are suggestive of potential crossover behavior between the 1 1B2u and 1 1B3u excited states (in traditional D2h notation) around 800 nm. A large array of exchange-correlation functionals were tested to predict the vertical excitation energies in H2TAPs, H2Pcs, H2Ncs, and H2Acs both in gas phase and solution. In general, TDDFT-predicted energies of the Q x and Q y bands and the splitting between them correlate well with the amount of Hartree-Fock exchange present in a specific exchange-correlation functional with the long-range corrected LC-BP86 and LC-wPBE functionals providing the best agreement between theory and experiment. The pure GGA (BP86) exchange-correlation functional significantly underestimated, while long-range corrected LC-BP86 and LC-wPBE exchange-correlation functionals and semiempirical ZINDO/S method strongly overestimated the intramolecular charge-transfer (ICT) transitions experimentally observed for -OR, -SR, and -NR2 substituted at nonperipheral position phthalocyanines and their analogues in the 450-650 nm region. The hybrid CAM-B3LYP, PBE1PBE, and B3LYP exchange-correlation functionals were found to be much better in predicting energies of such ICT transitions. Overall, we did not find a single exchange-correlation functional that can accurately (MAD < 0.05 eV) and simultaneously predict the energies and the splittings of the Q x and Q y bands as well as energies of the ICT transitions in a large array of substituted and unsubstituted metal-free phthalocyanines and their benzoannulated analogues.

Magnetic Circular Dichroism Spectroscopy of meso-Tetraphenylporphyrin-Derived Hydroporphyrins and Pyrrole-Modified Porphyrins.

A large set of free-base and transition-metal 5,10,15,20-tetraphenyl-substituted chlorins, bacteriochlorins, and isobacteriochlorins and their pyrrole-modified analogues were investigated by combined UV-visible spectroscopy, magnetic circular dichroism (MCD), density functional theory (DFT), and time-dependent DFT (TDDFT) approaches and their spectral characteristics were compared to those of the parent compounds, free-base tetraphenylporphyrin 1H2 and chlorin 2H2. It was shown that the nature of the pyrroline substituents in the chlorin derivatives dictates their specific UV-vis and MCD spectroscopic signatures. In all hydroporphyrin-like cases, MCD spectroscopy suggests that the ΔHOMO is smaller than the ΔLUMO for the macrocycle-centered frontier molecular orbitals. DFT and TDDFT calculations were able to explain the large broadening of the UV-vis and MCD spectra of the chlorin diones and their derivatives compared to the other hydroporphyrins and hydroporphyrin analogues. This study contributes to the further understanding of the electronic effects of replacing a pyrrole in porphyrins by pyrrolines or other five-membered heterocycles (oxazoles and imidazoles).

Electronic properties of mono-substituted tetraferrocenyl porphyrins in solution and on a gold surface: assessment of the influencing factors for photoelectrochemical applications.

Two unsymmetric meso-tetraferrocenyl-containing porphyrins of general formula Fc3 (FcCOR)Por (Fc=ferrocenyl, R=CH3 or (CH2)5 Br, Por=porphyrin) were prepared and characterized by a variety of spectroscopic methods, whereas their redox properties were investigated using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) approaches. The mixed-valence [Fc3 (FcCOR)Por](n+) (n=1,3) were investigated using spectroelectrochemical as well as chemical oxidation methods and corroborated with density functional theory (DFT) calculations. Inter-valence charge-transfer (IVCT) transitions in [Fc3 (FcCOR)Por](+) were analyzed, and the resulting data matched closely previously reported complexes and were assigned as Robin-Day class II mixed-valence compounds. Self-assembled monolayers (SAMs) of a thioacetyl derivative (Fc3 (FcCO(CH2)5 SCOCH3 )Por) were also prepared and characterized. Photoelectrochemical properties of SAMs in different electrolyte systems were investigated by electrochemical techniques and photocurrent generation experiments, showing that the choice of electrolyte is critical for efficiency of redox-active SAMs.

Synthesis, characterization, and electron-transfer processes in indium ferrocenyl-containing porphyrins and their fullerene adducts.

Three new indium(III) tetra- and penta(ferrocenyl)-substituted porphyrins of the general formula XInTFcP [X = Cl(-), OH(-), or Fc(-); TFcP = 5,10,15,20-tetraferrocenylporphyrin(2-); Fc = ferrocene] have been prepared and characterized by UV-vis, magnetic circular dichroism (MCD), (1)H, (13)C, 2D, and variable-temperature NMR spectroscopy, as well as elemental analysis. Molecular structures of the ClInTFcP, FcInTFcP, and FcInTFcP@4C60 complexes were determined by X-ray crystallography with the last compound being not only the first example of a C60 adduct to the organometallic porphyrins but also the first structure in which organometallic porphyrin antennas intercalated into four electron-transfer channels. The electronic structures and relative energies of individual atropisomers, as well as prospective electron-transfer properties of fullerene adducts of XInTFcP complexes, were investigated by the Density Functional Theory (DFT) approach. Redox properties of XInTFcP complexes were investigated using electrochemical (CV and DPV), spectroelectrochemical, and chemical oxidation approaches. Electrochemical experiments conducted in low-polarity solvent using noncoordinating electrolyte were crucial for the sequential oxidation of ferrocene substituents in XInTFcP compounds. In agreement with DFT calculations, the axial ferrocene ligand in FcInTFcP, with direct In-C σ-bond has a 240 mV lower oxidation potential compared to the first oxidation potential for equatorial ferrocene substituents connected to the porphyrin core. The first equatorial ferrocene oxidation process in all XInTFcP complexes is separated by at least 150 mV from the next three ferrocene based oxidations. The second, third, and fourth redox processes in the ferrocene region are more closely spaced. The addition of the bulky axial ferrocene ligand results in significantly larger rotational barriers for equatorial ferrocene substituents in FcInTFcP compared to the other complexes and leads to better defined redox waves in cyclic voltammetry (CV) and differential pulse voltammetry (DPV) experiments. Mixed-valence compounds of the general formula [XInTFcP](n+) (n = 1, 2) were observed and characterized by spectroelectrochemical and chemical oxidation approaches. In all cases, the presence of the intense intervalence charge transfer (IVCT) bands associated with the oxidation of a single equatorial ferrocene substituent were detected in the NIR region confirming the presence of the iron-based mixed-valence species and suggesting long-range metal-metal coupling in the target systems. The resulting data from the mixed-valence [XInTFcP](n+) (n = 1, 2) complexes matched very closely to the previously reported MTFcP and metal-free poly(ferrocenyl)porphyrins and were assigned as Robin and Day Class II mixed-valence compounds.

Unexpected formation of chiral pincer CNN nickel complexes with ß-diketiminato type ligands via C-H activation: synthesis, properties, structures, and computational studies.

Reaction of lithiated chiral, unsymmetric ß-diketimine type ligands HL(2a-e) containing oxazoline moiety (HL(2a-e) = 2-(2'-R(1)NH)-phenyl-4-R(2)-oxazoline) with trans-NiCl(Ph)(PPh(3))(2) afforded a series of new chiral CNN pincer type nickel complexes (3a-3e) via an unexpected cyclometalation at benzylic or aryl C-H positions. Single crystal X-ray diffraction analysis established the pincer coordination mode and the strained conformation. Chirality, and in one case, racemization of the target nickel complexes were confirmed by circular dichroism (CD) spectroscopy. Electronic structure and band assignments in experimental UV-vis and CD spectra were discussed on the basis of Density Functional Theory (DFT) and time-dependent (TD) DFT calculations.

Insight into the electronic structure, optical properties, and redox behavior of the hybrid phthalocyaninoclathrochelates from experimental and density functional theory approaches.

An insight into the electronic structure of several hafnium(IV), zirconium(IV), and lutetium(III) phthalocyaninoclathrochelates has been discussed on the basis of experimental UV-vis, MCD, electro- and spectroelectrochemical data as well as density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. On the basis of UV-vis and MCD spectroscopy as well as theoretical predictions, it was concluded that the electronic structure of the phthalocyninoclathrochelates can be described in the first approximation as a superposition of the weakly interacting phthalocyanine and clathrochelate substituents. Spectroelectrochemical data and DFT calculations clearly confirm that the highest occupied molecular orbital (HOMO) in all tested complexes is localized on the phthalocyanine ligand. X-ray crystallography on zirconium(IV) and earlier reported hafnium(IV) phthalocyaninoclathrochelate complexes revealed a slightly distorted phthalocyanine conformation with seven-coordinated metal center positioned ∼1 Šabove macrocyclic cavity. The geometry of the encapsulated iron(II) ion in the clathrochelate fragment was found to be between trigonal-prismatic and trigonal-antiprismatic.

Profiling energetics and spectroscopic signatures in prototropic tautomers of asymmetric phthalocyanine analogues.

Density functional theory (DFT) and time-dependent density functional theory (TDDFT) were used to explain discrepancies in UV-vis and MCD spectra of the metal-free tribenzo[b,g,l]thiopheno[3,4-q]porphyrazine (1), substituted tribenzo[b,g,l]porphyrazine (2), and 2,3-bis(methylcarboxyl)phthalocyanine (3). On the basis of gas-phase and polarized continuum solvation model (PCM) DFT and TDDFT calculations, it was suggested that both NH tautomers contribute to the spectroscopic signature of 1, whereas the Q-band region of 2 and 3 is dominated by a single NH tautomer. For all tested compounds, it was found that the combination of the BP86 exchange-correlation functional, 6-31G(d) basis set, and TDDFT-PCM approach provides the best accuracy in energies of the Q(x)- and Q(y)-bands of the individual NH tautomers as well as correctly describes their relative energy differences, which are important in understanding of experimental spectroscopy of the target systems.

Photoinduced charge transfer in short-distance ferrocenylsubphthalocyanine dyads.

Two new ferrocenylsubphthalocyanine dyads with ferrocenylmethoxide (2) and ferrocenecarboxylate (3) substituents directly attached to the subphthalocyanine ligand via the axial position have been prepared and characterized using NMR, UV-vis, and magnetic circular dichroism (MCD) spectroscopies as well as X-ray crystallography. The redox properties of the ferrocenyl-containing dyads 2 and 3 were investigated using the cyclic voltammetry (CV) approach and compared to those of the parent subphthalocyanine 1. CV data reveal that the first reversible oxidation is ferrocene-centered, while the second oxidation and the first reduction are localized on the subphthalocyanine ligand. The electronic structures and nature of the optical bands observed in the UV-vis and MCD spectra of all target compounds were investigated by a density functional theory polarized continuum model (DFT-PCM) and time-dependent (TD)DFT-PCM approaches. It has been found that in both dyads the highest occupied molecular orbital (HOMO) to HOMO-2 are ferrocene-centered molecular orbitals, while HOMO-3 as well as lowest unoccupied molecular orbital (LUMO) and LUMO+1 are localized on the subphthalocyanine ligand. TDDFT-PCM data on complexes 1-3 are consistent with the experimental observations, which indicate the dominance of π-π* transitions in the UV-vis spectra of 1-3. The excited-state dynamics of the dyads 2 and 3 were investigated using time-correlated single photon counting, which indicates that fluorescence quenching is more efficient in dyad 3 compared to dyad 2. These fluorescence lifetime measurements were interpreted on the basis of DFT-PCM calculations.

The first TDDFT and MCD studies of free base triarylcorroles: a closer look into solvent-dependent UV-visible absorption.

Absorption spectra of several free base triarylcorroles were investigated by MCD spectroscopy. The MCD spectra exhibit unusual sign-reverse (positive-to-negative intensities in ascending energy) features in the Soret- and Q-type band regions, suggesting a rare ΔHOMO < ΔLUMO relationship between π and π* MOs in the corrole core.

Long-range electronic communication in free-base meso-poly(ferrocenyl)-containing porphyrins.

H(2)FcPh(3)P [FcPh(3)P = 5-ferrocenyl-10,15,20-triphenyl porphyrin(2-)], cis-H(2)Fc(2)Ph(2)P [cis-Fc(2)Ph(2)P = 5,10-bisferrocenyl-15,20-diphenyl porphyrin(2-)], trans-H(2)Fc(2)Ph(2)P [trans-Fc(2)Ph(2)P = 5,15-bisferrocenyl-10,20-diphenyl porphyrin(2-)], and H(2)Fc(3)PhP [Fc(3)PhP = 5,10,15-trisferrocenyl-20-phenyl porphyrin(2-)] along with H(2)TPP [TPP = 5,10,15,20-tetraphenylporphyrin] and H(2)TFcP [TFcP = 5,10,15,20-tetraferrocenyl porphyrin(2-)] were isolated from the direct cross-condensation reaction between pyrrole, benzaldehyde, and ferrocene carboxaldehyde or from the reaction between ferrocenyl-2,2'-dipyrromethane and benzaldehyde, suggesting a scrambling reaction mechanism for the last approach. All compounds were characterized by UV-vis, MCD, and NMR spectroscopy; APCI MS and MS/MS methods; as well as high-resolution ESI MS spectrometry. The conformational flexibility of ferrocene substituents in all compounds was confirmed using variable-temperature NMR and computational methods. DFT calculations were employed to understand the degree of nonplanarity of the porphyrin core as well as the electronic structure of ferrocene-containing porphyrins. In all cases, a set of occupied, predominantly ferrocene-based molecular orbitals was found between the highest occupied and the lowest unoccupied, predominantly porphyrin-based molecular pi orbitals. The redox properties of all ferrocene-containing porphyrins were investigated in a CH(2)Cl(2)/TFAB [TFAB = tetrabutylammonium tetrakis(perfluorophenyl)borate] system using cyclic voltammetry, differential pulse voltammetry, and square wave voltammetry methods. In all cases, oxidations of individual ferrocene substituent(s) along with porphyrin core oxidation(s) and reductions have been observed. Mixed-valence [cis-H(2)Fc(2)Ph(2)P](+), [trans-H(2)Fc(2)Ph(2)P](+), [H(2)Fc(3)PhP](+), and [H(2)Fc(3)PhP](2+) complexes were formed in situ under spectroelectrochemical and chemical oxidation conditions and were characterized using UV-vis and MCD approaches. Analysis of intervalence charge-transfer bands observed in the NIR region for all mixed-valence complexes suggests electron localization and thus class II behavior in the Robin-Day classification.

Unexpected fluorescence properties in an axially sigma-bonded ferrocenyl-containing porphyrin.

Molecular structure, redox, and unexpected fluorescence properties of a tetraphenylporphyrin tin(iv) complex axially sigma-bonded with two ferrocene substituents were investigated using UV-vis, MCD, electro- and spectroelectrochemical methods as well as DFT calculations and X-ray crystallography.