Why Iron? A Spin-Polarized Conceptual Density Functional Theory Study on Metal-Binding Specificity of Porphyrin

Density functional studies on photophysical properties and chemical reactivities of the triarylboranes: effect of the constraint of planarity

The geometric and electronic structures, absorption spectra, transporting properties, chemical reactivity indices and electrostatic potentials of the planar three-coordinate organoboron compounds 1-2 and twisted reference compound Mes(3)B, have been investigated by employing density functional theory (DFT) and conceptual DFT methods to shed light on the planarity effects on the photophysical properties and the chemical reactivity. The results show that the planar compounds 1-2 exhibit significantly lower HOMO level than Mes(3)B, owing to the stronger electronic induction effect of boron centers. This feature conspicuously induces a blue shifted absorption for 1, although 1 seemingly possesses more extended conjugation framework than Mes(3)B. Importantly, the reactivity strength of the boron atoms in 1-2 is much lower than that in Mes(3)B, despite the fact that the tri-coordinate boron centers of 1-2 are completely naked. The interesting and abnormal phenomenon is caused by the strong p-π electronic interactions, that is, the empty p-orbital of boron center is partly filled by π-electron of the neighbor carbon atoms in 1-2, which are confirmed by the analysis of Laplacian of the electron density and natural bond orbitals. Furthermore, the negative electrostatic potentials of the boron centers in 1-2 also interpret that they are not the most preferred sites for incoming nucleophiles. Moreover, it is also found that the planar compounds 1-2 can act as promising electron transporting materials since the internal reorganization energies for electron are really small.

On Ligand Binding Energies in Porphyrinic Systems

Energy decomposition analysis is a powerful tool to study the bond properties of molecules. Here, we used a recent implementation by P. Su and H. Li (J. Chem. Phys. 131 (2009) 014102) to test the method for investigations of ligand binding to medium sized molecules (30–40 atoms). We studied the properties of the bond between porphyrins or phthalocyanines with central metals without (Mg) or with a closed d shell (Zn and Cd) to ligands binding via various row II and III atoms (nitrogen, oxygen, phosphorus and sulphur) using various methods and basis sets. Several interesting results could be deduced: In the porphyrins, ligands binding via a nitrogen atom were preferred to those binding via oxygen and sixfold coordination was readily available for magnesium as central metal atom as opposed to zinc and cadmium derivatives which also showed a weaker bond strength. In phthalocyanines, a tendency to form fivefold coordination was also seen for magnesium compounds and bonds to nitrogen and oxygen had the same strength. Ligation to row III elements created only weak bonds that were prone to fluctuations.

Revealing substituent effects on the electronic structure and planarity of Ni-porphyrins

Using density functional theory, we have studied the effects on structural and electronic consequences (including HOMO-LUMO energy gaps, vertical ionization potentials (IP_v), and vertical electron affinities (EA_v)) of the following two factors: (a) meso- and β-substituents acting as inductive donors (CH₃), inductive acceptors that are electron-donating through resonance (Br), inductive electron acceptors (CF₃), and resonance enabled acceptors (NO₂); and (b) complete replacement of pyrrole nitrogens with P-atoms. The principal results of the study are: (1) For the bare Ni-porphyrin, the solvents were found not to affect the HOMO-LUMO gaps but to change the IP_v and EA_v noticeably. (2) In the series CH₃ → Br → CF₃ → NO₂ the HOMO-LUMO energy gaps, IP_v, and EA_v increase for both meso- and β-substituents. The ruffling distortion of the porphyrin core is retained, and becomes stronger for the two acceptor groups. In general, effects of meso-substituents on the ruffling distortion of the porphyrin core is more pronounced. (3) Most significantly, complete replacement of pyrrole nitrogens in the NiP with phosphorus atoms produces the species, NiP(P)₄, with the structural and electronic features drastically different from the original NiP. This implies that NiP(P)₄ can possess interesting and unusual novel properties, including aromaticity and reactivity, leading to its various beneficial potential applications. Furthermore, NiP(P)₄ high stability both in the gas phase and different solvents was shown, implying the feasibility of its synthesis.

Effects induced by axial ligands binding to tetrapyrrole-based aromatic metallomacrocycles

The axial positions of planar metallomacrocycles are unoccupied. The positively charged metal is thus a potential binding site for electron-donating groups. The binding strength is affected by the central metal, the ligand, and the macrocycle. One ligand leads to the out-of-plane displacement of the central metal, whereas two ligands from two sides structurally neutralize each other. The axial ligand donates charge to the central metal and the macrocycle when the lone pair orients along the interaction axis. The frontier orbital levels are elevated because of the charge donated to the macrocycle. Even though the singlet-triplet gap and the absorption maximum do not change significantly upon binding, the redox chemistry is considerably affected by the shifts of orbital levels. The macrocyclic M-N bonds are weakened by the binding, but their natures remain almost unchanged. Calcium phthalocyanine is a special case, as the central calcium is too large to fit the cavity. Accordingly, multiple ligands facilely bind to the calcium from one side. The aluminum phthalocyanine halogen is another special case, as it has a halogen ligand coordinating to the aluminum through a nondative bond. This leads to some effects different from those caused by dative binding. When there is no considerable steric demand, the lone pair points along the interaction axis to facilitate the donation. When in a stacked dimer, the electron-rich group is part of a large molecule, and the orientation of the lone pair is approximately perpendicular to the interaction axis. This induces the charge loss of the central metal. Because metallomacrocycles are widespread in the biological, medical, and material sciences, the results from this study are expected to bring useful insights to these fields.