Magnetism in a homometallic square-planar Cu$({\mathrm{II})}$$_4$Cu$({\mathrm{II})}$ metallacrown

Abstract

The well-established homometallic metallacrown Cu(II)[12-MC$_\text{Cu(II)N(Shi)}$-4]$^{2-}$, comprising four square-planarly arranged Cu(II) ions surrounding a slightly off-plane central Cu(II) ion, provides an excellent platform for investigating magnetism in exchange-coupled Cu(II) clusters both by experimental and ab initio numerical methods. In the present work, sterically bulky substituents are introduced into the salicylhydroxamate ligand to obtain a modified derivative, Cu$({\mathrm{II})}$[12-MC$_\text{Cu$({\mathrm{II})}$N(L)}$-4]$^{2-}$ (Cu$_4$Cu), thereby effectively suppressing intermolecular magnetic interactions and enabling a detailed investigation of its intrinsic ground-state spin structure and exchange interactions. Static magnetization reveals competing antiferromagnetic exchange interactions of $J_1=-8.1(2)$ THz ($-33.5(8)$ meV) between the central and the peripheral sites and $J_2=-4.40(12)$ THz ($-18.2(5)$ meV) between neighboring sites on the ring of the metallacrown, which are corroborated by ab initio broken symmetry-based density functional theory (DFT). These couplings yield an $S=1/2$ ground state for Cu$_4$Cu at low temperatures, for which X-band and high-frequency/high-field electron paramagnetic resonance (EPR) precisely determine the $g$-factors $g_x = 2.028$, $g_y = 2.056$, and $g_z = 2.222$ close to axial anisotropy and in agreement with our DFT results. The ground-state spin-configuration is characterized by $d_{xy}$-character of the unoccupied states. Furthermore, our magnetization data evidence thermal transitions across a doublet-to-quartet gap of $Δ_{1/2\rightarrow3/2} = 4.8(4)$ THz which is also indicated by the emergence of an excited-state EPR signal.