Through Bridge Spin Coupling in Homo- and Hetero-bimetallic Porphyrin Dimers upon Stepwise Oxidations: A Spectroscopic and Theoretical Investigation.

Abstract

We have described Cu(II)-Fe(III) and Cu(II)-Mn(III) heterobimetallic porphyrin dimers and compared with their homo-bimetallic analogs. UV-visible spectra are very distinct while electrochemical studies demonstrate that these species, as compared to the homobimetallic analog, are much easier to oxidize. Combined Mössbauer, EPR, NMR, magnetic, UV-vis and DFT studies show only ring-centered oxidation after 2e-oxidations. The HOMO-LUMO gaps are linearly dependent with the low-energy NIR band of 2e-oxidized complexes. Strong electronic communication between two rings through the bridge facilitates coupling between various unpaired spins while the coupling model depends on the nature of metal ions used. Unpaired spins of Fe(III) and porphyrin π-cation radical are strongly anti-ferromagnetically coupled, while such coupling is rather weak between Mn(III) and porphyrin π-cation radical. Moreover, the coupling between two π-cation radicals are much stronger in the 2e-oxidized complexes of Mn2(III) and Cu(II)-Mn(III) complexes as compared to their Fe2(III) and Cu(II)-Fe(III) analogs. Furthermore, coupling between the unpaired spins of π-cation radical and Cu(II) ion is much stronger in the 2e-oxidized complex of Cu(II)-Fe(III) complex as compared to its Cu(II)-Mn(III) analog. The Mulliken spin density distributions in 2e-oxidized homo- and hetero-bimetallic complexes show symmetric and asymmetric spread between the two macrocycles, respectively, however, the Cu(II) center acts as a charge donor in the heterobimetallic complexes while Fe(III)/Mn(III) center as a charge acceptor.