Iron(II) hexafluorophosphate complexes with N,N,N-tridentate ligands, 2-(pyridin-2-yl)-4-(1 H -pyrazol-1-yl)-6-methylpyrimidine (L (H,H) ) and 2-(pyridin-2-yl)-4-(3,5-dimethyl-1 H -pyrazol-1-yl)-6-methylpyrimidine (L (Me,Me) ) were synthesized by reacting FeCl 2 ·4H 2 O, (NBu 4 )PF 6 and L (H,H) or L (Me,Me) in alcohol media. Supramolecular 3D structure of [FeL (H,H) 2 ](PF 6 ) 2 ·nH 2 O is organized by CH…π interactions, π…π-stacking and CH…F hydrogen bonds. In the stricture of [FeL (Me,Me) 2 ](PF 6 ) 2 ·nH 2 O the cations and anions are assembled into a 3D network via CH…F hydrogen bonds and lone pair…π interactions. Both complexes demonstrate high thermal stability. Under vacuum, the complex [FeL (Me,Me) 2 ](PF 6 ) 2 ·nH 2 O is predominantly in the low spin state below 400 K, but at higher temperatures it shows gradual spin state switching which remains incomplete even at 495 K. Under vacuum or in sealed ampoule in the first heating run the complex [FeL (H,H) 2 ](PF 6 ) 2 ·nH 2 O shows abrupt spin state switching related to dehydration near 400 K. On further thermal cycling under vacuum the complex demonstrates gradual spin crossover curve centered at ca . 250 K, whereas in sealed ampoules this compound demonstrates a very abrupt transition near 250 K. [ABSTRACT FROM AUTHOR]