The Origin of Magnetic Anisotropy and Single-Molecule Magnet Behavior in Chromium(II)-Based Extended Metal Atom Chains

Chromium­(II)-based
extended metal atom chains have been the focus of considerable discussion
regarding their symmetric versus unsymmetric structure and magnetism.
We have now investigated four complexes of this class, namely, [Cr3(dpa)4X2] and [Cr5(tpda)4X2] with X = Cl– and SCN– [Hdpa = dipyridin-2-yl-amine; H2tpda = N2,N6-di­(pyridin-2-yl)­pyridine-2,6-diamine].
By dc/ac magnetic techniques and EPR spectroscopy, we found that all
these complexes have easy-axis anisotropies of comparable magnitude
in their S = 2 ground state (|D|
= 1.5–1.8 cm–1) and behave as single-molecule
magnets at low T. Ligand-field and DFT/CASSCF calculations
were used to explain the similar magnetic properties of tri- versus
pentachromium­(II) strings, in spite of their different geometrical
preferences and electronic structure. For both X ligands, the ground
structure is unsymmetric in the pentachromium­(II) species (i.e., with
an alternation of long and short Cr–Cr distances) but is symmetric
in their shorter congeners. Analysis of the electronic structure using
quasi-restricted molecular orbitals (QROs) showed that the four unpaired
electrons in Cr5 species are largely localized in four
3d-like QROs centered on the terminal, “isolated” Cr2+ ion. In Cr3 complexes, they occupy four nonbonding
combinations of 3d-like orbitals centered only on the two terminal
metals. In both cases, then, QRO eigenvalues closely mirror the 3d-level
pattern of the terminal ions, whose coordination environment remains
quite similar irrespective of chain length. We conclude that the extent
of unpaired-electron delocalization has little impact on the magnetic
anisotropy of these wire-like molecular species.