Spin Transition of an Iron(II) Organoborate Complex in Different Polymorphs and in Vacuum-Deposited Thin Films: Influence of Cooperativity

Two polymorphic modifications ( 1-I and 1-II ) of the new spin crossover (SCO) complex [Fe{H 2 B(pz)(pypz)} 2 ] (pz = pyrazole, pypz = pyridylpyrazole; 1 ) were prepared and investigated by differential scanning calorimetry (DSC), magnetic measurements, Mößbauer, vibrational, and absorption spectroscopy as well as single-crystal and X-ray powder diffraction. DSC measurements reveal that upon heating the thermodynamically metastable form 1-II o ∼178 °C it transforms into 1-I in an exothermic reaction, which proves that these modifications are related by monotropism. Both forms show thermal SCO with T 1/2 values of 390 K ( 1-II ) and 270 K ( 1-I ). An analysis of the crystal structures of 1-II and the corresponding Zn(II) ( 2 ) and Co(II) ( 3 ) complexes that are isotypic with 1-I reveals that form II consists of dimers coupled by strong intramolecular π···π interactions, which is not the case for 1-I . In agreement with these findings, investigations of thin films of 1 , where significant π···π interactions should be absent, reveal SCO behavior similar to that of 1-I . These results underscore the importance of cooperativity for the spin-transition behavior of this class of complexes.