Tuning Spin–Spin Coupling in Quinonoid-Bridged Dicopper(II) Complexes through Rational Bridge Variation

Bridged
metal complexes [{Cu­(tmpa)}2­(μ-L1–2H)]­(ClO4)2 (1), [{Cu­(tmpa)}2­(μ-L2–2H)]­(ClO4)2 (2), [{Cu­(tmpa)}2­(μ-L3–2H)]­(BPh4)2 (3), and [{Cu­(tmpa)}2­(μ-L4–2H)]­(ClO4)2 (4) (tmpa = tris­(2-pyridyl­methyl)­amine,
L1 = chloranilic acid, L2 = 2,5-dihydroxy-1,4-benzoquinone,
L3 = (2,5-di-[2-(methoxy)-anilino]-1,4-benzoquinone, L4 = azophenine) were synthesized from copper­(II) salts, tmpa,
and the bridging quinonoid ligands in the presence of a base. X-ray
structural characterization of the complexes showed a distorted octahedral
environment around the copper­(II) centers for the complexes 1–3, the donors being the nitrogen atoms
of tmpa, and the nitrogen or oxygen donors of the bridging quinones.
In contrast, the copper­(II) centers in 4 display a distorted
square-pyramidal coordination, where one of the pyridine arms of each
tmpa remains uncoordinated. Bond-length analyses within the bridging
ligand exhibit localization of the double bonds inside the bridge
for 1–3. In contrast, complete delocalization
of double bonds within the bridging ligand is observed for 4. Temperature-dependent magnetic susceptibility measurements on the
complexes reveal an antiferromagnetic coupling between the copper­(II)
ions. The strength of antiferromagnetic coupling was observed to depend
on the energy of the HOMO of the bridging quinone ligands, with exchange
coupling constants J in the range between −23.2
and −0.6 cm–1 and the strength of antiferromagnetic
coupling of 4 > 3 > 2 > 1. Broken-symmetry density functional theory calculations
(DFT) revealed that the orientation of magnetic orbitals in 1 and 2 is different than that in 3 and 4, and this results in two different exchange pathways.
These results demonstrate how bridge-mediated spin–spin coupling
in quinone-bridged metal complexes can be strongly tuned by a rational
design of the bridging ligand employing the [O] for [NR] isoelectronic
analogy.