ABSTRACT
We present here a novel example of spin crossover phenomenon on a Fe(II) one-dimensional chain with unusual N5S coordination sphere. The [{Fe(tpc-OMe)(NCS)(µ-NCS)} n] (1) compound was prepared using the tridentate tpc-OMe ligand (tpc-OMe = tris(2-pyridyl)methoxymethane), FeCl2·4H2O, and the KSCN salt. Crystallographic investigations revealed that the Fe(II) ions are connected by a single bridging NCS- ligand (µ-κN:κS-SCN coordination mode) to afford a zigzag neutral chain running along the [010] direction, in which the thiocyanato bridging groups adopt a cis head-to-tail configuration. The (N5S) metal environment arises from one thiocyanato-κS and two thiocyanato-κN ligands and from three pyridine of the fac-tpc-OMe tripodal ligand. This compound presents a unique extension of Fe(II) binuclear complexes into linear chains built on similar tripodal ligands and bridging thiocyanate anions. Compound 1 shows a spin crossover (SCO) behavior which has been evidenced by magnetic, calorimetric, and structural investigations, revealing a sharp cooperative spin transition with a transition temperature of ca. 199 K. Temperature scan rate studies revealed a very narrow hysteresis loop (â¼1 K wide). Photoswitching of this compound was also performed, evidencing a very fast relaxation process at low temperature. Among other factors, the linearity of the N-bound terminal thiocyanato ligand appears as the main structural characteristic at the origin of the presence of the SCO transition in compound 1 and in the two others Fe(II) previous systems involving thiocyanato-bridges and tripodal tris(2-pyridyl)methane ligands.
ABSTRACT
New iron(II) complexes of formula [Fe(L1)](BF(4))(2) (1) and [Fe(L2)](BF(4))(2) x H(2)O (2) (L1 = 1,7-bis(2'-pyridylmethyl)-1,4,7,10-tetraazacyclododecane; L2 = 1,8-bis(2'-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane) have been synthesized and characterized by infrared spectroscopy, variable-temperature single-crystal X-ray diffraction, and variable-temperature magnetic susceptibility measurements. The crystal structure determinations of 1 and 2 reveal in both cases discrete iron(II) monomeric structures in which the two functionalized tetraazamacrocycles (L1 and L2) act as hexadentate ligands; the iron(II) ions are coordinated with six nitrogen atoms: four from the macrocycle and two from two pyridine groups occupying two cis positions around the metal ion. In 1, the N-Fe-N bond angles indicate that the Fe(II) ion adopts an unusual distorted trigonal prismatic geometry. In agreement with the observed paramagnetic behavior, the average of the six Fe-N distances at 293 K (2.218(6) A) and at 90 K (2.209(2) A) correspond well with distances observed for high-spin (HS) Fe(II) complexes with a coordination index of 6. For 2, the Fe(II) ion adopts a distorted octahedral geometry for which the six Fe-N distances (average 2.197(4) A) at room temperature are in the range expected for HS Fe(II) complexes. The crystal structure solved at 90 K showed a strong modification of the iron coordination sphere, suggesting the presence of a spin-crossover transition from HS to low spin (LS). Surprisingly, the averaged Fe-N value (2.077(4) A) at this temperature is not in agreement with the magnetic measurements since the chi(m)T product versus T showed a full LS state at 90 K. This may be explained by the presence of important distortions arising from the macrocycle constraints. To understand how the crystal and the lattice parameters were affected by the magnetic transition, the temperature dependence of the lattice parameters of 2 was determined in the range 293-90 K: the a and b parameters show essentially linear and gradual decreases, while the c and beta parameters show dramatic decreases nearly similar to that observed in the magnetic behavior.