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

The reaction of M­(BF4)2·xH2O, where M is Fe­(II), Co­(II), Ni­(II), Cu­(II),
Zn­(II),
and Cd­(II), with the new ditopic ligand m-bis­[bis­(3,5-dimethyl-1-pyrazolyl)­methyl]­benzene
(Lm*) leads to the formation
of monofluoride-bridged dinuclear metallacycles of the formula [M2(μ-F)­(μ-Lm*)2]­(BF4)3. The analogous manganese­(II)
species, [Mn2(μ-F)­(μ-Lm*)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 gx = 2.150, gy = 2.329, gz = 2.010, D = 0.173
cm–1, and E = 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 gx = 2.26, gy = 2.29, gz =
1.99, J = −16.0 cm–1, D1 = −9.89 cm–1, and D12 = −0.065 cm–1 were
obtained for the iron­(II) complex and gx = gy = gz = 2.00, D1 = −0.3254 cm–1, E1 = −0.0153, J = −6.7
cm–1, and D12 = 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.