Through Bridge Spin Coupling in Homo‐ and Heterobimetallic Porphyrin Dimers upon Stepwise Oxidations: A Spectroscopic and Theoretical Investigation

Abstract

We have described copper(II)‐iron(III) and copper(II)‐manganese(III) heterobimetallic porphyrin dimers and compared them with the corresponding homobimetallic analogs. UV‐visible spectra are very distinct in the heterometallic species while electrochemical studies demonstrate that these species, as compared to the homobimetallic analog, are much easier to oxidize. Combined Mössbauer, EPR, NMR, magnetic and UV‐visible spectroscopic studies show that upon 2e‐oxidation of the heterobimetallic complexes only ring‐centered oxidation occurs. The energy differences between HOMO and LUMO are linearly dependent with the low‐energy NIR band obtained for the 2e‐oxidized complexes. Also, strong electronic communication between two porphyrin rings through the bridge facilitates coupling between various unpaired spins present while the coupling model depends on the nature of metal ions used. While unpaired spins of Fe(III) and the porphyrin π‐cation radical are strongly antiferromagnetically coupled, such coupling is rather weak between Mn(III) and a porphyrin π‐cation radical. Moreover, the coupling between two π‐cation radicals are much stronger in the 2e‐oxidized complexes of dimanganese(III) and copper(II)‐manganese(III) porphyrin dimers as compared to their diiron(III) and copper(II)‐iron(III) analogs. Furthermore, coupling between the unpaired spins of a π‐cation radical and copper(II) is much stronger in the 2e‐oxidized complex of copper(II)‐iron(III) porphyrin dimer as compared to its copper(II)‐manganese(III) analog. The Mulliken spin density distributions in 2e‐oxidized homo‐ and heterobimetallic complexes show symmetric and asymmetric spread between the two macrocycles, respectively. In both the 2e‐oxidized heterobimetallic complexes, the Cu(II) porphyrin center acts as a charge donor while Fe(III)/Mn(III) porphyrin center act as a charge acceptor. The experimental observations are also strongly supported by DFT calculations.

Electronic Coupling and Electrocatalysis in Redox Active Fused Iron Corroles

“…This is also reflected in the well-separated and localized spin density distribution for the complex (Figure S18). A similar symmetrical spin density distribution was found in a neutral homobimetallic porphyrin analogue, bridged by ethylene group . Computation shows the third and fourth reduction processes to occur at the macrocyclic ring, but with unfavorable electron addition energetics (Figure S17).…”

“…A similar symmetrical spin density distribution was found in a neutral homobimetallic porphyrin analogue, bridged by ethylene group. 37 Computation shows the third and fourth reduction processes to occur at the macrocyclic ring, but with unfavorable electron addition energetics ( Figure S17 ). Spectral changes recorded after applying a constant potential of −1.5 V on [Fe III tpfc(py)] 2 COT led to changes in the UV–Vis spectrum that are beyond our ability to analyze at this moment: formation of an absorption band at 750 nm with significant intensity and decrease in intensity of absorption band at 385 nm.…”

Cooperativity in Diiron(III)porphyrin Dication Diradical-Catalyzed Oxa-Diels–Alder Reactions: Spectroscopic and Mechanistic Insights

“…When pentafluorobenzaldehyde was added to the Cu catalyst, no observable change in the UV–visible spectra was obtained (Figure S60). Therefore, it is obvious that the aldehyde did not coordinate with the Lewis acidic Cu­(II) center since 4-coordinated square planar Cu­(II) porphyrins are very reluctant to form the 5-coordinated species , (Figures S61 and 62). Therefore, it is not only the Lewis acidity but also the coordination of the aldehyde to the metal center that is very much required for the reaction to proceed.…”