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The polyfluorinated title compounds, [MBr 2 (C 18 H 16 F 8 N 2 O 2 )] or [4,4'-(HCF 2 CF 2 CH 2 OCH 2 ) 2 -2,2'-bpy]MBr 2 , (1) (M = Pd and bpy is bipyridine) and (2) (M = Pt), have -CH (α)2 OCH (β)2 CF 2 CF 2 H side chains with methylene H-atom donors at the α and β sites, and methine H-atom donors at the terminal sites, in addition to aromatic H-atom donors. In contrast to the original expectation of isomorphous structures, (1) crystallizes in the space group C2/c and (2) in P2 1 /n, with similar unit-cell volumes and Z = 4. The asymmetric unit of (1) is one half of the molecule, which resides on a crystallographic twofold axis. Both (1) and (2) display stacking of the molecules, indicating a planar (bpy)MBr 2 skeleton in each case. The structure of (1) exhibits columns with C-H (β) ...Br hydrogen bonds between consecutive layers which conforms to a static (β,β) linkage between layers. In the molecular plane, (1) shows double C-H (α) ...Br hydrogen bonds self-repeating along the b axis, the planar molecules being connected into infinite belts. Compound (2) has no crystallographic symmetry and forms π-dimer pairs as supermolecules, which then stack parallel to the a axis. The π-dimer-pair supermolecules exhibit (Pt-)Br...Br(-Pt) contacts [3.6937 (7) Å] to neighbouring π-dimer pairs crosslinking the columns. The structure of (2) reveals many C-H...F(-C) interactions between F atoms and aromatic C-H groups, in addition to those between F atoms and methylene C-H groups.

The polyfluorinated title compounds, [MCl 2 (C 16 H 16 F 4 N 2 O 2 )] or [4,4'-(HCF 2 CH 2 OCH 2 ) 2 -2,2'-bpy]MCl 2 [M = Pd, (1), and M = Pt, (2)], have -C(H α ) 2 OC(H β ) 2 CF 2 H side chains with H-atom donors at the α and β sites. The structures of (1) and (2) are isomorphous, with the nearly planar (bpy)MCl 2 molecules stacked in columns. Within one column, π-dimer pairs alternate between a π-dimer pair reinforced with C-H...Cl hydrogen bonds (α,α) and a π-dimer pair reinforced with C-H β ...F(-C) interactions (abbreviated as C-H β ...F-C,C-H β ...F-C). The compounds [4,4'-(CF 3 CH 2 OCH 2 ) 2 -2,2'-bpy]MCl 2 [M = Pd, (3), and M = Pt, (4)] have been reported to be isomorphous [Lu et al. (2012). J. Fluorine Chem. 137, 54-56], yet with disorder in the fluorous regions. The molecules of (3) [or (4)] also form similar stacks, but with alternating π-dimer pairs between the (α,β; α,β) and (β,β) forms. Through (C-)H...Cl hydrogen-bond interactions, one molecule of (1) [or (2)] is expanded into an aggregate of two inversion-related π-dimer pairs, one pair in the (α,α) form and the other pair in the (C-H β ...F-C,C-H β ...F-C) form, with the plane normals making an interplanar angle of 58.24 (3)°. Due to the demands of maintaining a high coordination number around the metal-bound Cl atoms in molecule (1) [or (2)], the ponytails of molecule (1) [or (2)] bend outward; in contrast, the ponytails of molecule (3) [or (4)] bend inward.

To enable a comparison between a C-H...X hydrogen bond and a halogen bond, the structures of two fluorous-substituted pyridinium iodide salts have been determined. 4-[(2,2-Difluoroethoxy)methyl]pyridinium iodide, C 8 H 10 F 2 NO + ·I - , (1), has a -CH 2 OCH 2 CF 2 H substituent at the para position of the pyridinium ring and 4-[(3-chloro-2,2,3,3-tetrafluoropropoxy)methyl]pyridinium iodide, C 9 H 9 ClF 4 NO + ·I - , (2), has a -CH 2 OCH 2 CF 2 CF 2 Cl substituent at the para position of the pyridinium ring. In salt (1), the iodide anion is involved in one N-H...I and three C-H...I hydrogen bonds, which, together with C-H...F hydrogen bonds, link the cations and anions into a three-dimensional network. For salt (2), the iodide anion is involved in one N-H...I hydrogen bond, two C-H...I hydrogen bonds and one C-Cl...I halogen bond; additional C-H...F and C-F...F interactions link the cations and anions into a three-dimensional arrangement.

The salts 3-[(2,2,3,3-tetrafluoropropoxy)methyl]pyridinium saccharinate, C 9 H 10 F 4 NO + ·C 7 H 4 NO 3 S - , (1), and 3-[(2,2,3,3,3-pentafluoropropoxy)methyl]pyridinium saccharinate, C 9 H 9 F 5 NO + ·C 7 H 4 NO 3 S - , (2), i.e. saccharinate (or 1,1-dioxo-1λ 6 ,2-benzothiazol-3-olate) salts of pyridinium with -CH 2 OCH 2 CF 2 CF 2 H and -CH 2 OCH 2 CF 2 CF 3 meta substituents, respectively, were investigated crystallographically in order to compare their fluorine-related weak interactions in the solid state. Both salts demonstrate a stable synthon formed by the pyridinium cation and the saccharinate anion, in which a seven-membered ring reveals a double hydrogen-bonding pattern. The twist between the pyridinium plane and the saccharinate plane in (2) is 21.26 (8)° and that in (1) is 8.03 (6)°. Both salts also show stacks of alternating cation-anion π-interactions. The layer distances, calculated from the centroid of the saccharinate plane to the neighbouring pyridinium planes, above and below, are 3.406 (2) and 3.517 (2) Å in (1), and 3.409 (3) and 3.458 (3) Å in (2).

Neutralization of 4-[(2,2,3,3-tetrafluoropropoxy)methyl]pyridine with hydrohalo acids HX (X = Cl and Br) yielded the pyridinium salts 4-[(2,2,3,3-tetrafluoropropoxy)methyl]pyridinium chloride, C 9 H 10 F 4 NO + ·Cl - , (1), and 4-[(2,2,3,3-tetrafluoropropoxy)methyl]pyridinium bromide, C 9 H 10 F 4 NO + ·Br - , (2), both carrying a fluorous side chain at the para position of the pyridinium ring. Single-crystal X-ray diffraction techniques revealed that (1) and (2) are isomorphous. The halide anions accept four hydrogen bonds from N-H, ortho-C-H and CF 2 -H groups. Two cations and two anions form a centrosymmetric dimeric building block, utilizing complimentary N-H...X...H-Csp 3 connections. These dimers are further crosslinked, utilizing another complimentary Csp 2 -H...X...H-Csp 2 connection. The pyridinium rings are π-stacked, forming columns running parallel to the a axis that make angles of ca 44-45° with the normal to the pyridinium plane. There are also supramolecular C-H...F-C interactions, namely bifurcated C-H...F and bifurcated C-F...H interactions; additionally, one type II C-F...F-C halogen bond has been observed.

It is possible that fluorous compounds could be utilized as directing forces in crystal engineering for applications in materials chemistry or catalysis. Although numerous fluorous compounds have been used for various applications, their structures in the solid state remains a lively matter for debate. The reaction of 4-[(2,2,2-trifluoroethoxy)methyl]pyridine with HX (X = I or Cl) yielded new fluorous ponytailed pyridinium halide salts, namely 4-[(2,2,2-trifluoroethoxy)methyl]pyridinium iodide, C 8 H 9 F 3 NO + ·I - , (1), and 4-[(2,2,2-trifluoroethoxy)methyl]pyridinium chloride, C 8 H 9 F 3 NO + ·Cl - , (2), which were characterized by IR spectroscopy, multinuclei ( 1 H, 13 C and 19 F) NMR spectroscopy and single-crystal X-ray diffraction. Structure analysis showed that there are two types of hydrogen bonds, namely N-H...X and C-H...X. The iodide anion in salt (1) is hydrogen bonded to three 4-[(2,2,2-trifluoroethoxy)methyl]pyridinium cations in the crystal packing, while the chloride ion in salt (2) is involved in six hydrogen bonds to five 4-[(2,2,2-trifluoroethoxy)methyl]pyridinium cations, which is attributed to the smaller size and reduced polarizability of the chloride ion compared to the iodide ion. In the IR spectra, the pyridinium N-H stretching band for salt (1) exhibited a blue shift compared with that of salt (2).