Iron(II) hexafluorophosphate complexes with N,N,N-tridentate ligands, 2-(pyridin-2-yl)-4-(1H-pyrazol-1-yl)-6-methylpyrimidine (L(H,H)) and 2-(pyridin-2-yl)-4-(3,5-dimethyl-1H-pyrazol-1-yl)-6-methylpyrimidine (L(Me,Me)) were synthesized by reacting FeCl2·4H2O, (NBu4)PF6 and L(H,H) or L(Me,Me) in alcohol media. Supramolecular 3D structure of [FeL(H,H)2](PF6)2·nH2O is organized by CH…π interactions, π…π-stacking and CH…F hydrogen bonds. In the stricture of [FeL(Me,Me)2](PF6)2·nH2O 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](PF6)2·nH2O 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](PF6)2·nH2O 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.