¹H NMR studies on strongly antiferromagnetically coupled dicopper(II) systems

Abstract

The ¹H NMR spectra of three well-characterized μ -phenoxo and μ -hydroxo spin coupled dicopper(II) complexes 1, 2, and 3 which are strongly antiferromagnetically coupled in the solid state have been studied in solution. The complexes studied were [(Cu₂(DAP)₂IPA)(OH)(H₂O)](ClO₄)₂·H₂O (1) (DAP = 1,3-diaminopropane; IPA = 2-hydroxy-5-methylisophthalaldehyde), [(Cu₂(DMDAP)₂IPA)(OH)(H₂O)](ClO₄)₂ (2) (DMDAP = N,N-dimethyl-1,3-diaminopropane), and [(Cu₂(AEP)₂IPA)(OH)(H₂O)](ClO₄)₂ (3) (AEP = 2-(2-aminoethyl)pyridine). All three complexes exhibit relatively sharp hyperfine shifted NMR signals. Signal assignments were based on intensity and T1 values. An analysis of the relaxation data shows that, for these binuclear copper(II) systems, the reorientational correlation time (τ_c) is dominated probably by a combination of electronic relaxation τ_s and rotational correlation time (τ_r) due to an exchange-modulated dipolar mechanism. The temperature dependence of the isotropic shifts has been interpreted in terms of the contact hyperfine interaction constant (A) and exchange coupling constant (-2J). The fitting of these shifts represents a good method for the evaluation of -2J in solution, which is compared to the solid state -2J value obtained by the SQUID method. The results indicate that the structures and magnetic properties of all three complexes (1-3) support a general correlation with the antiferromagnetic coupling constants as evidenced by both solid and solution studies. Our results show that ¹H NMR spectroscopy is an excellent tool to probe the solution structures of magnetically coupled binuclear Cu(II) centers in model complexes as well as biological systems. One of these complexes was crystallized from aqueous solution. The crystal and molecular structure of [(Cu₂(DMDAP)₂IPA)(OH)(H₂O)](ClO₄)₂ (2) has been determined. This crystallizes in the monoclinic system, space group Cc with formula weight = 692.48, a = 12.472(2) Å, b = 19.554(2) Å, c = 12.185(12) Å, β = 107.48 (9)°, Z = 4. The two Cu atoms in this copper(II) complex are bridged by the oxygen atoms of the phenolate and hydroxy groups. The axial position at one Cu atom is occupied by a water molecule, while another Cu has weak interaction with a perchlorate group. The coordination geometries around the two Cu atoms are distorted square pyramidal and square planar.