Rational Design of a Photomagnetic Chain: Bridging Single-Molecule Magnets with a Spin-Crossover Complex

The spin-crossover complex [Fe­(LN5)­(CN)2]·H2O (1, LN5 = 2,13-dimethyl-3,6,9-12,18-pentaazabicyclo[12.3.1]­octadeca-1(18),2,12,14,16-pentaene),
reported previously by Nelson et al. in 1986, was reinvestigated,
and its structure determined by single crystal X-ray diffraction for
the first time. The reaction between [MnIII(saltmen)­(H2O)]+ and this photomagnetic linker yielded the
trinuclear molecular complex [{Mn­(saltmen)}2FeHS(LN5)­(CN)2]­(ClO4)2·0.5CH3OH (2) and the one-dimensional compound [{Mn­(saltmen)}2FeLS(LN5)­(CN)2]­(ClO4)2·0.5C4H10O·0.5H2O (3) depending on the addition order of the
reagents (HS: High-Spin; LS: Low-Spin). Compound 3 exhibits
a wave-shaped chain structure built from the assembly of the trinuclear
[MnIII-NC-FeII] motif found in 2. Static magnetic measurements revealed the existence of antiferromagnetic
MnIII···FeII (FeII HS, S = 2) interactions in the trinuclear entity
of 2 via the cyanido bridge leading to an ST = 2 ground state. In the case of 3, concomitant
ferromagnetic and antiferromagnetic exchange interactions are found
along the chain due to the presence of two crystallographically independent
{Mn2(saltmen)2} units, which behave differently
as shown by the magnetic susceptibility analysis, while the FeII (LS, S = 0) cyanido-bridging moiety is
isolating these dinuclear MnIII units. ac susceptibility
experiments indicated slow relaxation of the magnetization arising
from the ferromagnetically coupled [Mn2] units (τ0 = 1.1 × 10–7 s and Δeff/kB = 13.9 K). Optical reflectivity and
photomagnetic properties of 1 and 3 have
been investigated in detail. These studies reveal that the photomagnetic
properties of 1 are kept after its coordination to the
acceptor MnIII/saltmen complexes, allowing in 3 to switch “on” and “off” the magnetic
interaction between the photoinduced FeII HS unit (S = 2) and the Mn­(III) ions. To the best of our knowledge,
the compound 3 represents the first example of a coordination
network of single-molecule magnets linked by spin-crossover units
inducing thermally and photoreversible magnetic and optical properties.