Structure and Ferromagnetic Interactions in Open-Shell Supramolecular Assemblies Constructed from Radical Cations and Hexacyanometallate Anions

The goal of our work was to design in a controlled
manner open-shell supramolecular assemblies with
dominant intermolecular ferromagnetic interactions. Along this
line, the compounds of formula
rad6M(CN)6I3·
2H2O with M = Cr, Fe, and Co have been synthesized;
rad+ stands for the
2-(3-N-methylpyridinium)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazo-1-oxyl
3-N-oxide radical cation. The crystal structures have
been solved. The
three compounds are isomorphous. They crystallize in the trigonal
space group P3 (a = 13.178(3) Å,
c = 10.763(6)
Å for M = Fe). The structure may be viewed as a two-dimensional
honeycomb lattice, with three kinds of hexagons
and three kinds of rad- - -rad magnetic interactions. In
addition, for M = Cr and Fe, there is a fourth kind of
magnetic interaction, occurring between the
M(CN)63- anion and the radical core. The
magnetic properties of the
three compounds have been investigated. They are characteristic of
dominant ferromagnetic interactions to which
weak antiferromagnetic interactions are superimposed. A
quantitative interpretation of the magnetic data has been
developed. In particular, for the chromium derivative, three out
of four interactions have been found to be
ferromagnetic, including the rad- - -Cr(CN)6
interaction. The spin topologies of our supramolecular assemblies
have
been discussed in the context of the spin frustration phenomenon.
The potentialities of the supramolecular chemistry
of open-shell units have been emphasized.