Crystal Forms of Hexafluorophosphate Organometallic Salts and the Importance of Charge-Assisted C−H---F Hydrogen Bonds

Abstract

Architecture, stability, and behavior with temperature of the hexafluorophosphate salts [(C5H5)2Co][PF6] (1), [(C5H5)2Fe][PF6] (2), [(C6H6)2Cr][PF6] (3), and [(C6H5Me)2Cr][PF6] (4) have been investigated. Crystals 1 and 2 have been measured by variable-temperature X-ray diffraction and differential scanning calorimetry. The phase-transitional behavior of [(C5H5)2Co][PF6], which undergoes two fully reversible crystal-to-crystal phase transitions, has been compared with the known behavior of [(C5H5)2Fe][PF6]. The room- and low-temperature phases (form I and form II) of [(C5H5)2Fe][PF6] and [(C5H5)2Co][PF6] are isostructural and isomorphous. The basic packing features of crystalline 1 and 2 are maintained on substituting the metallocene system with the bisbenzenechromium cation in 3. The architecture common to the three salts is otherwise disrupted by substituting toluene for benzene in 4. It has been shown that, when assisted by the difference in charge between anions and cations, the C−H---F interactions play a significant role. The structural parameters obtained for C−H---F interactions in 1−4 have been compared with data retrieved from the Cambridge Crystallographic Database on interactions of the X−H---F(δ-) type (X = C, N, O) in organometallic salts of the PF6- anion.