New copper(II)-radical one dimensional chain: Synthesis, crystal structure, EPR, magnetic properties and DFT calculations

Abstract

The novel chain compound [Cu(Phtfac)₂(NITpPy)]_n (where NITpPy = 4-pyridyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and Phtfac = 4,4,4-trifluoro-1-phenylbutane-1,3-dione) was synthesized and characterized structurally, magnetically and by EPR. The compound contains two non equivalent Cu(II) ions, Cu1 and Cu2, located at inversion centers and bridged by a NITpPy ligand coordinating Cu1 through the pyridine donor atom, and Cu2 through a N–O group, resulting in a head-to-head chain structure. The chain exhibits an unusual spin topology with two alternating pairs of magnetic coupling constants. The magnetic behavior was modeled considering a 16-membered ring with alternating exchange couplings. The best fit parameters indicate a ferromagnetic (J₁ = 29.4 cm⁻¹), and antiferromagnetic (J₂ = −4.6 cm⁻¹) couplings and an average g = 2.05, corresponding to a ground state with three parallel and one anti-parallel spin for each Cu₂NITpPy₂ unit. DFT calculations allowed assigning the ferromagnetic coupling to Cu-O-NITpPy and the antiferromagnetic coupling to Cu-N_Py-NITpPy. Single crystal EPR spectra display only one resonance for most field orientations, as a consequence of the collapse of the signals of the different spins produced by the exchange interactions. The observed g-tensor of this resonance is related to those expected for the Cu(II) and radical ions. Comparison of this compound with other Cu-NIT radicals chains bearing different substituents in the organic radicals, highlights that the β-diketonate ligand plays an important role in determining the final architecture. Moreover, we show how a knowledge of the spin density distribution in the initial building blocks is essential to rationalize the magnetic behavior of the resulting product.