Density-Functional Theory Study of the Stereochemistry of Chloroiron(III) and Chloromanganese(III) Complexes of a Bridled Chiroporphyrin

Abstract

Transition metal complexes of chiroporphyrins, in which two adjacent meso substituents are linked by a strap of eight methylene groups, [M(BCP8)], can exist as either an RRRR or RβRβ atropisomer depending on the nature of the coordinated metal cation. This remarkable conformational versatility was investigated by density-functional theory calculations for the d 5 chloroiron(III) complex in the low-spin and high-spin states and for the d 4 high-spin chloromanganese(III) complex. The lowest-lying electronic state of all of the conformers of the chloroiron(III) bridled chiroporphyrin is found to be the high-spin state. For the chloroiron(III) complex in the low-spin or the high-spin state and for the high-spin chloromanganese(III) complex, the most stable form is predicted to be the RRRR conformer in which the chloride axial ligand is located within the cavity provided by the bridles. The predicted stereochemistries are compared with those similarly obtained (i) for the chloroiron(III) and chloromanganese(III) complexes of the tetramethylchiroporphyrin, which is devoid of straps, and (ii) for the d 10 zinc(II) and low-spin d 8 nickel(II) BCP8 complexes, on the basis of the effects tied to the occupancy of the stereochemically active d x 2 -y 2-type antibonding orbital level, to the restraints imposed by the straps, and to the presence of the axial chloride ligand.

Toward Reliable DFT Investigations of Mn-Porphyrins through CASPT2/DFT Comparison

“…In order to check their consistency and convergence, the CASPT2 results were calibrated using several active spaces on the experimental structure a (see Table 1). In agreement with experimental observations, the three active spaces CAS [5,5], CAS [14,13], and CAS [15,14] give rise to a sextet (S = 5/2) ground state. As seen in Table 1, the spin gap between the S = 5/2 (HS) and S = 3/2 (IS) states is almost not affected by the active space enlargement.…”

“…2. CAS [14,13] and CAS [13,13] add to the previous one a set of bonding and antibonding porphyrin-localized (π/π*) orbitals (see Figure 2) within each irreducible representation. 3.…”

“…3. CAS [15,14] and CAS [14,14] finally consider a supplementary σ-type orbital (see Figure 2) representing the ionÀ porphyrin σ bond.Whereas the enlargement from the fiveorbital active spaces to the 13-orbital ones is rather natural in light of the extended π system over the porphyrin ring, the inclusion of an additional σ-type orbital deserves some explanation. The importance of ligand-to-metal charge transfers (LMCT), in particular along the σ channel, has been stressed in the ground state from previous DFT-based calculations.…”

“…Such remarkable interplay has also led to intense efforts in order to take advantage of these features in widespread areas of interest such as health, catalysis, − and molecular materials . In particular, metalloporphyrins have been recently investigated as possible information storage devices taking advantage of charge transfer effects. − The association of a redox metal center such as manganese with smart functionalization of the porphyrin has turned out to be a promising route in the design of molecular switches . From this perspective, we got interested in gaining a better understanding of the particular relationships between the electronic structures of Mn(II) and Mn(III) porphyrins and their structural features through theoretical approaches.…”

“…10À13 The association of a redox metal center such as manganese with smart functionalization of the porphyrin has turned out to be a promising route in the design of molecular switches. 14 From this perspective, we got interested in gaining a better understanding of the particular relationships between the electronic structures of Mn(II) and Mn(III) porphyrins and their structural features through theoretical approaches.…”