posted on 2023-01-04, 16:04authored byJu Chen, Jing Yang, Munendra Yadav, David A. Shultz, Martin L. Kirk
A new donor–acceptor biradical complex, TpCum,MeZn(SQ-VD) (TpCum,MeZn+ = 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 S = 1/2 SQ and the S = 1/2 VD within the SQ-VD biradical ligand is observed
to be ferromagnetic, with JSQ‑VD = +77 cm–1 (H = −2JSQ‑VDŜSQ·ŜVD) determined from an analysis of the variable-temperature
magnetic susceptibility data. The pairwise biradical exchange interaction
in TpCum,MeZn(SQ-VD) can be compared with that of the related
donor–acceptor biradical complex TpCum,MeZn(SQ-NN)
(NN = nitronyl nitroxide, S = 1/2), where JSQ‑NN ≅ +550 cm–1. 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 S = 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 TpCum,MeZn(SQ-VD)
and TpCum,MeZn(SQ-NN). A Valence Bond Configuration Interaction
(VBCI) model was previously employed to highlight the importance of
mixing an SQSOMO → NNLUMO charge transfer
configuration into the electronic ground state to facilitate the stabilization
of the high-spin triplet (S = 1) ground state in
TpCum,MeZn(SQ-NN). Here, CASSCF computations confirm the
importance of mixing the pendant radical (e.g., VD, NN) LUMO (VDLUMO and NNLUMO) with the SOMO of the SQ radical
(SQSOMO) for stabilizing the triplet, in addition to spin
polarization and charge transfer contributions to the exchange. An
important electronic structure difference between TpCum,MeZn(SQ-VD) and TpCum,MeZn(SQ-NN), which leads to their
different exchange couplings, is the reduced admixture of excited
states that promote ferromagnetic exchange into the TpCum,MeZn(SQ-VD) ground state, and the intrinsically weaker mixing between
the VDLUMO and the SQSOMO compared to that observed
for TpCum,MeZn(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.