Characterization of "Free Base" and Metal Complex Thioalkyl Porphyrazines by Magnetic Circular Dichroism and TDDFT Calculations

Abstract

UV-vis absorption and magnetic circular dichroism (MCD) spectra of octakis thioethyl "free base" porphyrazine H₂OESPz and its metal complexes MOESPz (M = Mg, Zn, Ni, Pd, Cu), as well as of [MnOESPz(SH)] were recorded. In the last case, MCD proved to have quite good sensitivity to the coordination of this complex with 1-methylimidazole (1-mim) in benzene. Time-dependent density functional theory (TDDFT) calculations were carried out for the considered porphyrazine complexes and showed good performance on comparing with MCD and UV-vis experimental spectra, even in the open-shell Cu and Mn cases. Calculations accounted for the red shift observed in the thioalkyl compounds and allowed us to reveal the role of sulfur atoms in spectroscopically relevant molecular orbitals and to highlight the importance of the conformations of the thioethyl external groups. Calculated MCD spectra of [MnOESPz(SH)] confirm the Mn(III) → Mn(II) redox process, which leads to the [Mn(OESPz)(1-mim)₂] species, and the relevance of the spin state for MCD is revealed.

Scheme 1. Structures of the Studied Compounds: “Free Base” H₂OESPz, Octakis Thioethyl MOESPz Complexes, the Octakis Ethyl MgOEPz Complex, and Axially Coordinated Mn(II) and Mn(III) Complexes

Figure 1

Experimental absorption and MCD spectra of “free base” thioethyl porphyrazine H₂OESPz (A), MgOESPz (B) and NiOESPz (C) thioethyl porphyrazine complexes, and Mg ethylporphyrazine MgOEPz (D); the last data have been redrawn from ref (59).

Figure 2

Calculated energy levels of H₂OESPz, NiOESPz, and MgOESPz complexes, and Mg ethylporphyrazine MgOEPz (occupied Gouterman orbitals written in red).

Figure 3

Comparison of experimental (black lines) and calculated (colored lines) UV–vis and MCD spectra. (A) Calculated and experimental MCD spectra of H₂OESPz. (B) Calculated and experimental UV–vis spectra of H₂OESPz. (C) Calculated and experimental MCD spectra of NiOESPz. (D) Calculated and experimental UV–vis spectra of NiOESPz.

Figure 4

Comparison of experimental (black lines) and calculated (colored lines) UV–vis and MCD spectra. (A) Calculated and experimental MCD spectra of MgOESPz. (B) Calculated and experimental UV–vis spectra of MgOESPz. (C) Calculated and experimental MCD spectra of MgOEPz. (D) Calculated and experimental UV–vis spectra of MgOEPz.

Figure 5

Calculated energy levels CuOESPz (Gouterman orbitals written in red).

Figure 6

Comparison of experimental (black lines) and calculated (colored lines) UV–vis and MCD spectra of CuOESPz.

Figure 7

Comparison of experimental (black lines) and calculated (blue lines) UV and MCD spectra of the manganese complex. (A) Calculated and experimental MCD spectra of [Mn(OESPz)(SH)]. (B) Calculated and experimental UV–vis spectra of [Mn(OESPz)(SH)]. (C) Calculated and experimental MCD spectra of [Mn(OESPz)(1-mim)₂]. (D) Calculated and experimental UV–vis spectra of [Mn(OESPz)(1-mim)₂].

Figure 8

Spectroscopic titration of 0.012 mM solution of [Mn(OESPz)(SH)] with 1-mim (0–0.294 M) in benzene. Black = no 1-mim and red = highest 1-mim concentration.