Structure, Stability, and Spin Resonance in Dicopper(II) Complexes

Designing molecular systems that access entangled spin states through spin-forbidden transitions is central to advancing molecular platforms for quantum sensing and information science, yet remains a fundamental challenge. Herein, we examine a family of dinuclear Cu(II) complexes [L₂Cu₂(μ-CA)]X₂, where systematic variation of cyclic (Me₃tacn and tmchd) and polypyridyl ligands (4'-Br/Cl-terpy and tpa), along with different counteranions (X = ClO₄^-, CF₃SO₃^-, and Cl^-), tunes local geometry, exchange coupling, and EPR spectra. Structural analysis reveals distorted octahedral to trigonal-bipyramidal Cu(II) environments, and magnetic susceptibility measurements establish moderate antiferromagnetic coupling (with singlet-to-triplet energy gaps spanning 2J = -1.26 to -30.68(3) cm^-1). Low-temperature EPR spectra display well-resolved half-field (ΔM_s = ±2) transitions within the S = 1 manifold, with frozen-solution EPR confirming persistence of the [L₂Cu₂(μ-CA)]²⁺ dinuclear core. Comparison to a mononuclear analogue isolates bridge-mediated magnetic effects, and multireference calculations capture trends in the g anisotropy. Overall, we reveal a counterintuitively robust magnetic core for the complexes to changes in counterions and ligand shells. These chemical changes have subtle influences on the intensities of the forbidden ΔM_S = ±2 transition in the excited triplet state. Importantly, our extensive investigation also reveals a remarkable absence of the singlet/triplet transition, which provides hints at future design strategies.