Dinuclear Metallacycles with Single M–O(H)–M Bridges [M = Fe(II), Co(II), Ni(II), Cu(II)]: Effects of Large Bridging Angles on Structure and Antiferromagnetic Superexchange Interactions

The reactions of M­(ClO4)2·xH2O and the ditopic
ligands m-bis­[bis­(1-pyrazolyl)­methyl]­benzene
(Lm) or m-bis­[bis­(3,5-dimethyl-1-pyrazolyl)­methyl]­benzene (Lm*) in the presence
of triethylamine lead to the formation of monohydroxide-bridged, dinuclear
metallacycles of the formula [M2(μ-OH)­(μ-Lm)2]­(ClO4)3 (M = Fe­(II), Co­(II), Cu­(II)) or [M2(μ-OH)­(μ-Lm*)2]­(ClO4)3 (M = Co­(II), Ni­(II),
Cu­(II)). With the exception of the complexes where the ligand is Lm and the metal
is copper­(II), all of these complexes have distorted trigonal bipyramidal
geometry around the metal centers and unusual linear (Lm*) or nearly linear (Lm) M–O–M
angles. For the two solvates of [Cu2(μ-OH)­(μ-Lm)2]­(ClO4)3, the Cu–O–Cu angles are significantly
bent and the geometry about the metal is distorted square pyramidal.
All of the copper­(II) complexes have structural distortions expected
for the pseudo-Jahn–Teller effect. The two cobalt­(II) complexes
show moderate antiferromagnetic coupling, −J = 48–56 cm–1, whereas the copper­(II) complexes
show very strong antiferromagnetic coupling, −J = 555–808 cm–1. The largest coupling is
observed for [Cu2(μ-OH)­(μ-Lm*)2]­(ClO4)3, the complex with a Cu–O–Cu angle of
180°, such that the exchange interaction is transmitted through
the dz2 and the oxygen s and
px orbitals. The interaction decreases,
but it is still significant, as the Cu–O–Cu angle decreases
and the character of the metal orbital becomes increasingly dx2–y2. These intermediate geometries and magnetic interactions lead
to spin Hamiltonian parameters for the copper­(II) complexes in the
EPR spectra that have large E/D ratios
and one g matrix component very close to 2. Density
functional theory calculations were performed using the hybrid B3LYP
functional in association with the TZVPP basis set, resulting in reasonable
agreement with the experiments.