Dinuclear Complexes Containing Linear M–F–M [M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II)] Bridges: Trends in Structures, Antiferromagnetic Superexchange Interactions, and Spectroscopic Properties

Abstract

The reaction of M­(BF₄)₂·<i>x</i>H₂O, where M is Fe­(II), Co­(II), Ni­(II), Cu­(II), Zn­(II), and Cd­(II), with the new ditopic ligand <i>m</i>-bis­[bis­(3,5-dimethyl-1-pyrazolyl)­methyl]­benzene (<b>L_<i>m</i>*</b>) leads to the formation of monofluoride-bridged dinuclear metallacycles of the formula [M₂(μ-F)­(μ-<b>L_<i>m</i>*</b>)₂]­(BF₄)₃. The analogous manganese­(II) species, [Mn₂(μ-F)­(μ-<b>L_<i>m</i>*</b>)₂]­(ClO₄)₃, was isolated starting with Mn­(ClO₄)₂·6H₂O using NaBF₄ as the source of the bridging fluoride. In all of these complexes, the geometry around the metal centers is trigonal bipyramidal, and the fluoride bridges are linear. The ¹H, ¹³C, and ¹⁹F NMR spectra of the zinc­(II) and cadmium­(II) compounds and the ¹¹³Cd NMR of the cadmium­(II) compound indicate that the metallacycles retain their structure in acetonitrile and acetone solution. The compounds with M = Mn­(II), Fe­(II), Co­(II), Ni­(II), and Cu­(II) are antiferromagnetically coupled, although the magnitude of the coupling increases dramatically with the metal as one moves to the right across the periodic table: Mn­(II) (−6.7 cm^–1) < Fe­(II) (−16.3 cm^–1) < Co­(II) (−24.1 cm^–1) < Ni­(II) (−39.0 cm^–1) ≪ Cu­(II) (−322 cm^–1). High-field EPR spectra of the copper­(II) complexes were interpreted using the coupled-spin Hamiltonian with <i>g</i>_<i>x</i> = 2.150, <i>g</i>_<i>y</i> = 2.329, <i>g</i>_<i>z</i> = 2.010, <i>D</i> = 0.173 cm^–1, and <i>E</i> = 0.089 cm^–1. Interpretation of the EPR spectra of the iron­(II) and manganese­(II) complexes required the spin Hamiltonian using the noncoupled spin operators of two metal ions. The values <i>g</i>_<i>x</i> = 2.26, <i>g</i>_<i>y</i> = 2.29, <i>g</i>_<i>z</i> = 1.99, <i>J</i> = −16.0 cm^–1, <i>D</i>₁ = −9.89 cm^–1, and <i>D</i>₁₂ = −0.065 cm^–1 were obtained for the iron­(II) complex and <i>g</i>_<i>x</i> = <i>g</i>_<i>y</i> = <i>g</i>_<i>z</i> = 2.00, <i>D</i>₁ = −0.3254 cm^–1, <i>E</i>₁ = −0.0153, <i>J</i> = −6.7 cm^–1, and <i>D</i>₁₂ = 0.0302 cm^–1 were found for the manganese­(II) complex. Density functional theory (DFT) calculations of the exchange integrals and the zero-field splitting on manganese­(II) and iron­(II) ions were performed using the hybrid B3LYP functional in association with the TZVPP basis set, resulting in reasonable agreement with experiment