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

The spin crossover complex [Fe(LN5)(CN)(2)]center dot 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 [Mn-III(saltmen)(H2O)](+) and this photomagnetic linker yielded the trinuclear molecular complex {Mn(saltmen)}(2)Fe-HS(LN5)(CN)(2)](ClO4)(2)center dot 0.5CH(3)OH (2) and the one-dimensional compound [{Mn(saltmen)}(2)Fe-LS(LN5)(CN)(2)](ClO4)(2)center dot 0.5C(4)H(10)O center dot 0.5H(2)O (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 [Mn-III-NC-Fe-II] motif found in 2. Static magnetic measurements revealed the existence of antiferromagnetic Mn-III center dot center dot center dot Fe-II (Fe-II HS, S = 2) interactions in the trinuclear entity of 2 via the cyanido bridge leading to an S-T = 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 {Mn-2(saltmen)(2)} units, which behave differently as shown by the magnetic susceptibility analysis, while the Fe-II (LS, S = 0) cyanido-bridging moiety is isolating these dinuclear Mn-III units. ac susceptibility experiments indicated slow relaxation of the magnetization arising from the ferromagnetically coupled [Mn-2] units (tau(0) = 1.1 x 10(7) s and Delta(eff)/k(B) = 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 Mn-III/saltmen complexes, allowing in 3 to switch on and off the magnetic interaction between the photoinduced Fe-II 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.