Synergism of Porphyrin-Core Saddling and Twisting of<i> meso</i>-Aryl Substituents

The structural chemistry of meso-aryl-substituted porhyrins has uncovered a bewildering variety of macrocycle distortions. Saddling angles range up to 40°, while the plane of the phenyl groups at the meso positions may be anywhere between perpendicular to the porphyrin plane (θ = 90°) and tilted to quite acute angles (θ = 30° or even less). These two distortions appear to be correlated. This has naturally been explained by steric hindrance:  when the phenyls rotate toward the porphyrin plane, for instance, coerced by packing forces, the pyrrole rings can alleviate the steric hindrance by tilting away to a saddled conformation. We demonstrate, however, that the two motions are intrinsically coupled by electronic factors and are correlated even in the absence of external forces. A saddling motion makes it sterically possible for the phenyl rings to rotate toward the porphyrin plane, which will always happen because of increasingly favorable π-conjugation interaction with smaller angles θ. The considerable energy lowering due to π conjugation counteracts the energy cost of the saddling, making the concerted saddling/rotation motion very soft. Unsubstituted meso-aryl porphyrins just do not distort, but an additional driving force may tip the balance in favor of the combined distortion motion. Internal forces having this effect are repulsion of the four hydrogens that occupy the central hole of the ring in porphyrin diacids but also steric repulsion in peripherally crowded porphyrins. These findings lead to a clarification and systematization of the observed structural variety, which indeed shows a remarkable correlation between saddling and phenyl ring tilting

Is [FeO]²⁺ the Active Center Also in Iron Containing Zeolites? A Density Functional Theory Study of Methane Hydroxylation Catalysis by Fe-ZSM-5 Zeolite

Arguments are put forward that the active α-oxygen site in the Fe-ZSM-5 catalyst consists of the FeO2+ moiety. It is demonstrated that this zeolite site for FeO2+ indeed obeys the design principles for high reactivity of the FeO2+ moiety proposed earlier: a ligand environment consisting of weak equatorial donors (rather oxygen based than nitrogen based) and very weak or absent trans axial donor. The α-oxygen site would then owe its high reactivity to the same electronic structure features that lends FeO2+ its high activity in biological systems, as well as in the classical Fenton chemistry

Synthesis, Structure, and Physicochemical Properties of ((Ethylsulfanyl)porphyrazinato)cobalt(II). Metal−Ligand Bonds in Co(OESPz) and in Related Cobalt(II) Tetrapyrroles: Insights from a Density Functional Study

The Co(OESPz) complex crystallizes in space group P21/n, with a = 10.260(5) Å, b = 22.650(5) Å, c = 16.720(5) Å, β = 91.680(5)°, and Z = 4 and forms, similarly to the isomorphous Mn(OESPz), solid-state extended one-dimensional aggregates where “ruffled” molecular units are held together by extra planar Co···S interactions. The complex, which contains a low-spin Co2+ (S = 1/2) ion, exhibits intermolecular ferromagnetic interactions (ϑ = 2.6 K, g = 2.44) propagated through a superexchange pathway by extra planar Co···S interactions, with a possible contribution of the π system. Co(OESPz) shows a Co−Np distance shorter than CoOEP and CoPc and a sensible electrochemical stabilization of the Co2+ vs Co3+ state. The bonding interactions between Co2+ and the macrocyclic ligands OMSPz2-, Pc2-, Pz2-, and P2- are analyzed in detail, within the density functional (DF) theory, to elucidate the effects of the ligand framework. It is concluded that the σ interactions, which are by far dominant in all members of the CoII tetrapyrrole series, in the aza-bridged complexes, particularly in CoPz and Co(OMSPz), are stronger than in CoP, due to the reduced “hole” size of the macrocycle. The π interactions, consisting of π back-donation from Co-3dπ into empty ring π orbitals and donation from the occupied ring π orbitals into the Co-4pz are rather weak, but there is a sizable contribution from polarization of the macrocyclic ligand. The aza bridges have little effect on the metal to ligand π back-donation which in Co(OMSPz) is completely absent; the peripheral substituents, which are responsible for large polarization effects, play a more relevant role in the metal−macrocycle π interactions. In all porphyrazines the total orbital interaction contribution (covalent component) prevails over the ionic component of the bond, the latter being identified as the sum of the Pauli repulsion and the attractive electrostatic interaction between Co2+ and the tetrapyrrole(2−)

Symmetrically Substituted <i>nido</i>-Carboranylphthalocyanines: Facile Synthesis, Characterization, and Solution Properties. Evidence for Intra- and Intermolecular H⁺/K⁺ Exchange

The direct, non ex post synthesis of a novel phthalocyanine decorated with eight thiohexyl-nido-carborane functions, nido-[H2MCHESPc]K8, where the anionic polyhedra are in the form of K+ salt, is reported and discussed. The solution properties of this compound, including the unprecedented exchange between the pyrrolic protons and the peripheral alkali-metal ions, are also analyzed

Hydrotropic Solubilization of Gold Nanoparticles Functionalized with Proto-Alkylthioporphyrazines

Hydrotropic anchoring of a prototype porphyrazine bearing eight ethylthio chains as peripheral substituents was shown to take place on gold nanoparticles (GNPs) by UV−vis, dynamic light scattering (DLS), Raman, resonance Raman (RR), and surface enhanced resonance Raman spectroscopy (SERRS) experiments. Density functional theory (DFT) calculations were used as a valuable help to make the hypothesis that ligation to the gold surface occurred through the concurrent contribution of the peripheral sulfur atoms and the π system of the macrocycle. The so functionalized GNPs proved to be stable in water solution for long time, thus providing a suitable chemical and structural basis to build up hierarchical structures with possible applications in technological and biomedical fields

Evidence for Tetraphenylporphyrin Monoacids

Upon dilution from a concentrated solution in dichloromethane, the diacid form of tetraphenylporphyrin {H4TPP(X)2} (X = Cl, PF6 and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, TFPB) affords eventually the unprotonated free base species H2TPP. At a difference of chloride, in the case of PF6- and TFPB- anions the conversion occurs with the intermediacy of a species, which has been assigned to a monoacid derivative on the basis of UV/vis absorption, fluorescence emission (static and dynamic), and resonance light scattering. Ground-state gas-phase geometries have been calculated both for the diacid {H4TPP(PF6)2} and monoacid {H3TPP(PF6)} and {H3TPP(Cl)} species at the DFT/BP86 level of theory. TDDFT calculations using different functionals (BP86, SAOP, and B3LYP) have been exploited to provide electronic vertical excitation energies and oscillator strengths, yielding a remarkably good description of the optical spectra for these compounds and supporting the identification of the monoacid species. Gas-phase thermodynamic calculations on the chloride species provide an estimate of the Gibbs free energy changes associated with the two protonation steps, supporting the observed different behavior of this anion with respect to PF6- and TFPB-

Effects of Porphyrin Core Saddling, <i>meso</i>-Phenyl Twisting, and Counterions on the Optical Properties of<i> meso</i>-Tetraphenylporphyrin Diacids: The [H₄TPP](X)₂ (X = F, Cl, Br, I) Series as a Case Study

The ground- and excited-state properties of a series of meso-tetraphenylporphyrin (H2TPP) diacids, [H4TPP](X)2 (X = F, Cl, Br, I), ad hoc synthesized and characterized by 1H NMR, RLS, and UV−vis spectroscopies, have been studied theoretically using density functional theory (DFT) and time-dependent density functional theory (TDDFT). Several conformations corresponding to different deformations of the porphyrin core have been explored. The nearly degenerate purely saddled (sad) and hybrid (saddled with a small superimposed ruffling:  sadruf) conformations are the preferred “gas phase” conformations. The type and degree of distortion of the macrocycle and the orientation of the phenyl rings compare well to X-ray data available for H2TPP diacids. Two electronic structure features are key to an understanding of the optical and photophysical properties. (1) Strong interaction of the π-system of the phenyls with the π-system of the porphyrin leads to an upshift of the G-a2u (G = Gouterman) orbital and, hence, to a significant splitting of the occupied pair of a2u/a1u Gouterman orbitals. The diminished G-a2u/G-eg* gap and the lifting of the a2u/a1u degeneracy explain the red shift of the Q and B bands and the hyperchromicity of the Q-band in the diacids. (2) The highest occupied orbitals of the diacids comprise the set of halide lone pair orbitals, which move from completely above the Gouterman orbitals (I- counterion) to below them (F-). The lowest halide to porphyrin charge-transfer (HPCT) transitions are therefore predicted at very low energy (to the red of the Q-band) for Cl-−I-, but with very low intensity. Weak measured absorptions to the red of the Q-band support these theoretical findings. Quenching of the S1 (Q) state via these low-lying singlet HPCT excited states accounts for the decrease of the fluorescence quantum yield and for the measured trend along the series

Effects of Porphyrin Core Saddling, <i>meso</i>-Phenyl Twisting, and Counterions on the Optical Properties of<i> meso</i>-Tetraphenylporphyrin Diacids: The [H₄TPP](X)₂ (X = F, Cl, Br, I) Series as a Case Study

The ground- and excited-state properties of a series of meso-tetraphenylporphyrin (H2TPP) diacids, [H4TPP](X)2 (X = F, Cl, Br, I), ad hoc synthesized and characterized by 1H NMR, RLS, and UV−vis spectroscopies, have been studied theoretically using density functional theory (DFT) and time-dependent density functional theory (TDDFT). Several conformations corresponding to different deformations of the porphyrin core have been explored. The nearly degenerate purely saddled (sad) and hybrid (saddled with a small superimposed ruffling:  sadruf) conformations are the preferred “gas phase” conformations. The type and degree of distortion of the macrocycle and the orientation of the phenyl rings compare well to X-ray data available for H2TPP diacids. Two electronic structure features are key to an understanding of the optical and photophysical properties. (1) Strong interaction of the π-system of the phenyls with the π-system of the porphyrin leads to an upshift of the G-a2u (G = Gouterman) orbital and, hence, to a significant splitting of the occupied pair of a2u/a1u Gouterman orbitals. The diminished G-a2u/G-eg* gap and the lifting of the a2u/a1u degeneracy explain the red shift of the Q and B bands and the hyperchromicity of the Q-band in the diacids. (2) The highest occupied orbitals of the diacids comprise the set of halide lone pair orbitals, which move from completely above the Gouterman orbitals (I- counterion) to below them (F-). The lowest halide to porphyrin charge-transfer (HPCT) transitions are therefore predicted at very low energy (to the red of the Q-band) for Cl-−I-, but with very low intensity. Weak measured absorptions to the red of the Q-band support these theoretical findings. Quenching of the S1 (Q) state via these low-lying singlet HPCT excited states accounts for the decrease of the fluorescence quantum yield and for the measured trend along the series

Sitting-Atop Metallo-Porphyrin Complexes: Experimental and Theoretical Investigations on Such Elusive Species

The interaction between the sodium cation and two meso-aryl porphyrins (tetraphenylporphyrin, TPP, and tetra(4-methoxyphenyl)porphyrin, TMPP) leads to the formation of new species that have been identified as Sitting-Atop (SAT) complexes, where the metal ion interacts with the N atoms of the porphyrin core without the concomitant deprotonation of the N−H groups. These species have been attained in low polarity solvent through the interaction of the porphyrin free bases with sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaTFPB), and investigated in situ through a combination of spectroscopic techniques, such as UV/vis absorption and fluorescence (static and time-resolved), resonance light scattering, FT-IR, and 1H NMR. All spectroscopic evidence points to the occurrence of a single equilibrium between each parent compound and its SAT complex, ruling out the presence of other metallo-, protonated, or aggregated porphyrins in solution. The 1:1 stoichiometry of the adducts has been determined via continuous variation method (Job’s plot), and an estimate of the corresponding association constants in CH2Cl2 at 298 K have been obtained by UV/vis titration (Keq = (9 ± 4) × 105 L mol−1 and (5 ± 2) × 106 L mol−1 for TPP and TMPP, respectively). Density-functional theory (DFT) calculations on SAT model complexes, [NaTPP(PF6)] and [NaTMPP(PF6)], have provided information on the molecular structure of these elusive species and on the nature and strength of the sodium−porphyrin interaction. It is found that the sodium ion is bound to the four nitrogen atoms of the porphyrin core. The involvement of the pyrrolic N atoms results in a modest but not negligible elongation of the N−H bonds, pyramidalization of the hydrogen atoms, and blue shift of the N−H stretching frequencies. Electronic structure and energy decomposition analysis reveal that covalent interactions, mainly consisting of porphyrin to sodium charge transfer interactions, are an important component of the sodium-porphyrin bond. Time-dependent DFT (TDDFT) calculations of the lowest excited states of the model systems provide an unambiguous interpretation of the absorption and emission properties of the experimentally identified SAT complexes

Evidence for Tetraphenylporphyrin Monoacids

Upon dilution from a concentrated solution in dichloromethane, the diacid form of tetraphenylporphyrin {H4TPP(X)2} (X = Cl, PF6 and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, TFPB) affords eventually the unprotonated free base species H2TPP. At a difference of chloride, in the case of PF6- and TFPB- anions the conversion occurs with the intermediacy of a species, which has been assigned to a monoacid derivative on the basis of UV/vis absorption, fluorescence emission (static and dynamic), and resonance light scattering. Ground-state gas-phase geometries have been calculated both for the diacid {H4TPP(PF6)2} and monoacid {H3TPP(PF6)} and {H3TPP(Cl)} species at the DFT/BP86 level of theory. TDDFT calculations using different functionals (BP86, SAOP, and B3LYP) have been exploited to provide electronic vertical excitation energies and oscillator strengths, yielding a remarkably good description of the optical spectra for these compounds and supporting the identification of the monoacid species. Gas-phase thermodynamic calculations on the chloride species provide an estimate of the Gibbs free energy changes associated with the two protonation steps, supporting the observed different behavior of this anion with respect to PF6- and TFPB-