2023 article
Origin of Ferromagnetic Exchange Coupling in Donor–Acceptor Biradical Analogues of Charge-Separated Excited States
Chen, J., Yang, J., Yadav, M., Shultz, D. A., & Kirk, M. L. (2023, January 4). Inorganic Chemistry, Vol. 1.
A new donor-acceptor biradical complex, Tp<sup>Cum,Me</sup>Zn(SQ-VD) (Tp<sup>Cum,Me</sup>Zn<sup>+</sup> = zinc(II) hydro-tris(3-cumenyl-5-methylpyrazolyl)borate complex cation; SQ = orthosemiquinone; VD = oxoverdazyl), which is a ground-state analogue of a charge-separated excited state, has been synthesized and structurally characterized. The magnetic exchange interaction between the <i>S</i> = 1/2 SQ and the <i>S</i> = 1/2 VD within the SQ-VD biradical ligand is observed to be ferromagnetic, with <i><b>J</b></i><sub><i><b>SQ-VD</b></i></sub> = +77 cm<sup>-1</sup> (<i>H</i> = -2<i>J</i><sub>SQ-VD</sub><i>S</i>̂<sub>SQ</sub>·<i>S</i>̂<sub>VD</sub>) determined from an analysis of the variable-temperature magnetic susceptibility data. The pairwise biradical exchange interaction in Tp<sup>Cum,Me</sup>Zn(SQ-VD) can be compared with that of the related donor-acceptor biradical complex Tp<sup>Cum,Me</sup>Zn(SQ-NN) (NN = nitronyl nitroxide, <i>S</i> = 1/2), where <i><b>J</b></i><sub><i><b>SQ-NN</b></i></sub> ≅ +550 cm<sup>-1</sup>. This represents a dramatic reduction in the biradical exchange by a factor of ∼7, despite the isolobal nature of the VD and NN acceptor radical SOMOs. Computations assessing the magnitude of the exchange were performed using a broken-symmetry density functional theory (DFT) approach. These computations are in good agreement with those computed at the CASSCF NEVPT2 level, which also reveals an <i>S</i> = 1 triplet ground state as observed in the magnetic susceptibility measurements. A combination of electronic absorption spectroscopy and CASSCF computations has been used to elucidate the electronic origin of the large difference in the magnitude of the biradical exchange coupling between Tp<sup>Cum,Me</sup>Zn(SQ-VD) and Tp<sup>Cum,Me</sup>Zn(SQ-NN). A Valence Bond Configuration Interaction (VBCI) model was previously employed to highlight the importance of mixing an SQ<sub>SOMO</sub> → NN<sub>LUMO</sub> charge transfer configuration into the electronic ground state to facilitate the stabilization of the high-spin triplet (<i>S</i> = 1) ground state in Tp<sup>Cum,Me</sup>Zn(SQ-NN). Here, CASSCF computations confirm the importance of mixing the pendant radical (e.g., VD, NN) LUMO (VD<sub>LUMO</sub> and NN<sub>LUMO</sub>) with the SOMO of the SQ radical (SQ<sub>SOMO</sub>) for stabilizing the triplet, in addition to spin polarization and charge transfer contributions to the exchange. An important electronic structure difference between Tp<sup>Cum,Me</sup>Zn(SQ-VD) and Tp<sup>Cum,Me</sup>Zn(SQ-NN), which leads to their different exchange couplings, is the reduced admixture of excited states that promote ferromagnetic exchange into the Tp<sup>Cum,Me</sup>Zn(SQ-VD) ground state, and the intrinsically weaker mixing between the VD<sub>LUMO</sub> and the SQ<sub>SOMO</sub> compared to that observed for Tp<sup>Cum,Me</sup>Zn(SQ-NN), where this orbital mixing is significant. The results of this comparative study contribute to a greater understanding of biradical exchange interactions, which are important to our understanding of excited-state singlet-triplet energy gaps, electron delocalization, and the generation of electron spin polarization in both the ground and excited states of (bpy)Pt(CAT-radical) complexes.