Photomagnetism of a Series of Dinuclear Iron(II) Complexes

This paper describes the photomagnetic properties of a series of binuclear iron (II) complexes belonging to the [{Fe(NCS)(py-X)}(2)(bpypz)(2)] family (NCS=thiocyanate; py=pyridine; bpypz=3,5bis(pyridine-2-yl)pyrazolate; and py-X=4-Mepy (1), py (3), 3-Mepy (4), 3-Clpy (5), and 3-Brpy (6)). All of these complexes display a complete thermal spin transition centred between 100 and 150 K, and undergo the light-induced excited-spin-state strapping (LIESST) effect at low temperatures. The T(LIESST) relaxation temperature of the photoinduced high-spin state for each compound has been determined. For all of the complexes, it has been found that the T-(LIESST) curves at low temperature are close to the antiferromagnetic response of the [{Fe(NCS)(3,5-dmpy)}(2)-(bpypz)(2)] (3,5-dmpy=3,5-dimethylpyridine) (7) complex that is characterised by two iron (II) metal ions in a HS electronic configuration, giving some evidence of a quantitative low spin-low spin -> high spin-high spin (LS-LS -> HS-HS) photoconversion process. Depending on the nature of the cooperativity, the kinetics have been treated with stretched exponential, simply exponential, or sigmoidal models. Interestingly, this series of dinuclear complexes follows a previously proposed linear relationship between the T(LIESST) and their thermal spin transition temperatures T-1/2: T(LIESST)=T-0-0.3T(1/2). T-0 for these compounds is equal to 100 K. Based on this, and by using the empirical linear relationship found between the thermal spin transition and the Hammett constant, the HS-HS properties of complex 7 have been understood as a reflection of the physical impossibility that the T(LIESST) was higher than T-1/2. The close vicinity of the thermal spin-crossover phenomenon of 7 has been successfully checked by applying hydrostatic pressure.