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

Architecture, stability, and behavior with temperature of the hexafluorophosphate salts [(C<INF>5</INF>H<INF>5</INF>)<INF>2</INF>Co][PF<INF>6</INF>] (<BO>1</BO>), [(C<INF>5</INF>H<INF>5</INF>)<INF>2</INF>Fe][PF<INF>6</INF>] (<BO>2</BO>), [(C<INF>6</INF>H<INF>6</INF>)<INF>2</INF>Cr][PF<INF>6</INF>] (<BO>3</BO>), and [(C<INF>6</INF>H<INF>5</INF>Me)<INF>2</INF>Cr][PF<INF>6</INF>] (<BO>4</BO>) have been investigated. Crystals <BO>1</BO> and <BO>2</BO> have been measured by variable-temperature X-ray diffraction and differential scanning calorimetry. The phase-transitional behavior of [(C<INF>5</INF>H<INF>5</INF>)<INF>2</INF>Co][PF<INF>6</INF>], which undergoes two fully reversible crystal-to-crystal phase transitions, has been compared with the known behavior of [(C<INF>5</INF>H<INF>5</INF>)<INF>2</INF>Fe][PF<INF>6</INF>]. The room- and low-temperature phases (form I and form II) of [(C<INF>5</INF>H<INF>5</INF>)<INF>2</INF>Fe][PF<INF>6</INF>] and [(C<INF>5</INF>H<INF>5</INF>)<INF>2</INF>Co][PF<INF>6</INF>] are isostructural and isomorphous. The basic packing features of crystalline <BO>1</BO> and <BO>2</BO> are maintained on substituting the metallocene system with the bisbenzenechromium cation in <BO>3</BO>. The architecture common to the three salts is otherwise disrupted by substituting toluene for benzene in <BO>4</BO>. 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 <BO>1−4</BO> 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 PF<INF>6</INF>^- anion.