Structural and magnetic properties of manganese and nickel clusters with 9,10-phenanthrenedione-9-oxime ligands

Oximato-Based Ligands in 3 d/4 f-Metal Cluster Chemistry: A Family of {Cu3Ln} Complexes with a "Propeller"-like Topology and Single-Molecule Magnetic Behavior

The organic chelating and bridging ligands 9,10-phenanthrenedione-9-oxime (phenoxH) and 9,10-phenanthrenedione-9,10-dioxime (phendoxH₂) were synthesized and subsequently employed for the first time in heterometallic 3 d/4 f-metal cluster chemistry. The general reaction between CuCl₂·2H₂O, LnCl₃·6H₂O, phenoxH, and NEt₃ in a 1:2:2:4 molar ratio, in a solvent mixture comprising MeCN and MeOH, afforded brown crystals of a new family of [Cu₃LnCl₃(phenox)₆(MeOH)₃] clusters (Ln = Gd (1), Tb (2), Dy (3)) that possess an unprecedented [Cu₃Ln(μ-NO)₆]³⁺ "propeller"-like core. Complexes 1-3 are the first {Cu₃Ln} clusters in which the outer Cu^II and the central Ln^III atoms are solely bridged by diatomic oximato bridges. The {Cu-N-O-Ln} bridging units are very distorted with torsion angles spanning the range 35.5-48.9° and 25.2-55.6° in 1 and 2, respectively. As a result, complexes 1-3 are antiferromagnetically coupled, in agreement with previously reported magnetostructural criteria for oximato-bridged Cu/Ln complexes. The magnetic susceptibility data for all complexes were nicely fit to an isotropic spin Hamiltonian (for 1) or a Hamiltonian that accounts for the spin of the Cu^II atoms, the spin component of the Ln^III, the spin-orbit coupling (λ), an axial ligand-field component around the Ln^III atoms (Δ), and the Zeeman effect (for the anisotropic 2 and 3). The resulting fit parameters were J = -1.34 cm^-1 and g = 2.10 (1), J = -1.42 cm^-1, g_Cu = 2.10, and Δ = -26.3 cm^-1 (2), and J = -1.70 cm^-1, g_Cu = 2.05, and Δ = -38.1 cm^-1 (3). The reported fitting procedure, implemented in the PHI program, is here used for the first time. Even if this method is only valid in high-symmetry Ln environments, when it is properly used allows a very simple and efficient method to obtain the exchange parameters. In light of the negative anisotropy, compounds 2 and 3 were found to exhibit frequency-dependent tails of out-of-phase signals in the presence of a small external dc field, characteristic of the slow magnetization relaxation of a single-molecule magnet. By using the Kramers-Kronig equations, the effective energy barriers ( U_eff) were derived and reported as U_eff = 10.1 and 5.4 cm^-1 for 2 and 3, respectively.