Manganese(II) β-Diketonate Complexes with Pyridin-4-one, 3-Hydroxypyridin-2-one and 1-Fluoropyridine Ligands: Molecular Structures and Hydrogen-bonded Networks.

Manganese(II) bis(4,4,4-trifluoro-1-phenylbutane-1,3-dionate) complexes with pyridin-4-one (pyon), 3-hydroxypyridin-2-one (hpyon), 1-fluoropyridine (pyF) and methanol were prepared and the solid-state structures were determined by single-crystal X-ray analysis. The coordination of the metal center in all complexes was found to be octahedral. In compounds [Mn(tfpb)2(pyon)2] (1) and [Mn(tfpb)2(hpyon)2] (2) extended hydrogen bonding is present facilitating the formation of a three-dimensional supramolecular structure in 1 and a layered structure in 2 through N-H···O hydrogen bonding enhanced by C-H···O interactions as well as C-F···π interactions. In [Mn(tfpb)2(pyF)2] (3) a layered structure is formed through C-H···O and C-H···F interactions as well as π···π and C-F···π interactions. In [Mn(tfpb)2(MeOH)2] (4) a layered structure is formed through a combination of O-H···O and C-F···π interactions.


Introduction
Inorganic-organic hybrids, metal-organic coordination polymers and especially metal-organic frameworks (MOFs) are currently an extremely important topic and an active area of research because of their intriguing architectures and topologies, 1,2 as well as due to their potential applications in catalysis, chemical separation processes, wastewater treatment, gas storage, magnetism and as sensors. 3 Control of the solid-state arrangement of molecules within a crystal is the central challenge of materials chemistry. In metal-organic frameworks and coordination polymers, covalent bonding using bridging organic ligands for creation of robust polymeric structures is of prime importance. Various kinds of these materials have been designed with special attention dedicated to the geometry of the metal ions as well as flexibility, bridging potential and coordination preferences of different organic linkers. 1 On the other hand, in inorganic-organic hybrids non-covalent bonds adjust the dimensionality and enable new topologies to arise. Non-covalent forces, such as hydrogen bonding, C-H···π/F interactions, π···π stacking, and halogen bonding are much weaker compared to the covalent bonds, however, their multitude makes them a powerful tool in the crystal engineering. Also, a great variety of non-covalent donors-acceptors and their numbers, their unique directionality and simple introduction into structures make them a particularly good choice for the construction of self-assemblies.

1. Materials and Characterization
Reagents and chemicals were obtained as reagent grade from commercial sources and were used as purchased without any further purification. [Mn(tfpb) 2 (H 2 O) 2 ] was prepared according to the literature procedure. 5 Infrared (IR) spectra (4000-600 cm −1 ) of the samples were recorded using a Perkin-Elmer Spectrum 100, equipped with a Specac Golden Gate Diamond ATR as a solid sample support. Elemental (C, H, N) analyses were obtained using a Perkin-Elmer 2400 Series II CHNS/O Elemental Analyzer.

3. X-ray Crystallography
Single-crystal X-ray diffraction data were collected at room temperature (1, 2, 4) or 150 K (3) on a Nonius Kappa CCD diffractometer or an Agilent Technologies Su-perNova Dual diffractometer with an Atlas detector using monochromated Mo-K α radiation (λ = 0.71073 Å). The data were processed using DENZO 6 or CrysAlis Pro. 7 The structures were solved by direct methods implemented in SHELXS 8 and SIR-97 9 and refined by a full-matrix leastsquares procedure based on F 2 with SHELXL. 8 All non-hydrogen atoms were refined anisotropically. All H atoms were initially located in a difference Fourier maps. The hydrogen atoms on carbon atoms were treated as riding atoms in geometrically idealized positions. Hydrogen atoms attached to nitrogen and oxygen atoms were refined fixing the bond lengths and isotropic temperature factors as U iso (H) = kU eq (N,O), where k = 1.5 for OH groups, and 1.2 for NH groups. In 1 and 4 the CF 3 groups are disordered over two positions in 0.76(2):0.24 (2) and 0.71(3):0.29(3) (in 1) and 0.66(3):0.34(3) (in 4) ratio. In 1 a possible pseudo-translation was detected, however, no additional space group could be found using the Platon program. The crystallographic data are listed in Table 1.

Results and Discussion
Initial attempts to prepare 1 using methanol as a solvent gave 4 as the sole product. Thus, in the subsequent attempts of its synthesis other solvents were used instead. Compounds 1 and 2 were obtained by the reaction of [Mn(tfpb) 2 (H 2 O) 2 ] and the corresponding heteroaromatic ligands pyridine-4-on (pyon) and 3-hydroxypyridine-2-on (hpyon) in 1:2 molar ratio in warm ethanol or acetone, respectively. Compound 3 was prepared by the reaction of [Mn(tfpb) 2 (H 2 O) 2 ] in warm 1-fluoropyridine (pyF) acting as a solvent and as a ligand. Crystals suitable for X-ray analyses were obtained by slow evaporation of the solvent at room temperature over a few days. The IR spectrum of 1 shows two bands at 3244 and 3080 cm -1 and the spectrum of 2 two bands at 3251 and 3120 cm -1 that suggest the involvement of the O-H and N-H groups of pyridone ligands in strong hydrogen bonding. The spectrum of 4 shows one broad band at 3381 cm -1 that suggests the involvement of the O-H groups of methanol ligands in strong hydrogen bonding. In all four compounds, there are bands in the frequency range 1609-1527 cm -1 characteristic for the ν(C=O) and ν(C=C) stretching of the tfpb ligand.

C27-H27···O4 ii interaction between pyon and tfpb ligand.
The NH groups of the pyon ligands of both independent complexes act as hydrogen-bond donors interacting with the tfpb carbonyl oxygens of the adjacent complexes, facilitating the formation of a hydrogen-bonded tree-dimensional supramolecular structure (Fig. 3). Complex A inter-acts with two complexes B through N1-H1···O6 iii bonding enabling the formation of an ABAB chain. Complex B interacts with two complexes A through N2-H2···O3 bonding enhanced by C26-H26···O1 i interaction with R 2 2 (7) ring motif 11 enabling the formation of an ABAB chain in the second dimension. Furthermore, complex B interacts with two adjacent complexes B through the centrosymmetric C29-H29···F6a iv interactions with R 2 2 (18) ring motif forming a BBB chain in the third dimension (Table 3).   The supramolecular structure is further stabilized also by C11-F4a···π interaction between -CF 3 group of complex B and pyon ring of complex A with F···Cg3 distance of 3.806(11) Å and C-F···Cg3 angle of 139.1(5)°. Compound 2 crystallizes in the triclinic P-1 space group. Selected bond distances and angles are summarized in Table 4. The asymmetric unit contains one half of the complex, with the Mn II atom sitting on the inversion center. Octahedrally coordinated manganese(II) atom is surrounded by four oxygen atoms positioned in the equatorial plane, stemming from two chelating tfpb ligands in a trans arrangement, with Mn-O distances 2.1132(10) and 2.1218(9) Å (Fig. 4). The Mn(tfpb) 2 fragment deviates from planarity, the angle between the mean plane formed by the equatorial MnO 4 core and that of the tfpb chelate C 3 O 2 moiety being 15.91(4)°. The axial positions are occupied by two hpyon ligands bonded to the metal center through the O3 atom, with Mn1-O3 distance of 2.2768(10) Å and Mn1-O3-C11 angle of 128.19(9)°. The hpyon ligand is inclined toward the tfpb moiety, with the angle between the plane of the hpyon ring and that of the equatorial MnO 4 core being 43.25(6)°. The hydroxy group of the hpyon ligand is involved in intramolecular hydrogen bonding with the tfpb ligand through O4-H4···O1 i interaction (Table 3). The NH group of the hpyon ligand acts as a hydrogen bond donor, facilitating the formation of a centrosymmetric hydrogen-bonded motif via N1-H1···O3 ii interactions with the ligated carbonyl O3 atom enhanced by C15-H15···O2 iv interactions with the graphset motifs R 2 2 (8) and R 2 2 (7), respectively ( Fig. 5 and Table  3). This interaction is further stabilized by C1-F3···π interaction between CF 3 group and the hpyon ring with d(F3···Cg3) = 3.2278(17) Å and <(C1-F3···Cg3) = 135.64(11)°, where Cg3 is N1/C11-C15 ring centroid. Consequently, a chain is formed along the a axis. The chains are further connected into layers along the ac plane via centrosymmetric C13-H13···O4 iii hydrogen bonding between hpyon CH moiety and the hydroxy group of the adjacent molecule (Fig. 5). There are no significant π···π interactions.   (2) 168 (4) Symmetry codes for 1: In the solid state, pyridin-4-one and 3-hydroxypyridin-2-one are in the lactam form. 12,13 Also in metal complexes the lactam form of both predominates. A search of the Cambridge Structural Database (CSD, Version 5.41, plus updates) 14 has revealed 26 entries 15 of metal complexes where pyridin-4-one, in its lactam form, is bonded via O atom also observed in complex 1. However, 9 entries with the lactim form (as 4-hydroxypyridine) bonded via N atom were found in the CSD with Re, Ir, Pt, and Ag 16 as well as Cu and Fe. 17 This observation can be explained by the Pearson HSAB (hard-soft acid-base) concept 18 since soft acids, such as Re, Ir, Pt, and Ag, show a preference for bonding via pyridine N atom (an intermediate base) as opposed to the -OH group (a hard base). Additionally, 3 entries with the lactim form bonded via OH group were also found with Nd,Tb,Dy. 19 In metal complexes with 3-hydroxypyridin-2-on lactam form with monodentate ligation via O atom 20,21 was found in 9 entries; the same type  was also observed in complex 2. Additionally, 3 entries were found with O,O'-chelating ligation. 21,22 However, no entries were found with lactim form (as 2,3-dihydroxypyridine) bonded to the metal center. For comparison, metal complexes with pyridine-2-one were more often investigated than complexes with pyridin-4-one and 3-hydroxypyridin-2-one and a variety of coordination modes has been observed. 21,23,24 Compound 3 crystallizes in the monoclinic P2 1 /c space group. Selected bond distances and angles of 3 are summarized in Table 5. Initial attempts to collect XRD data at room temperature failed due to slow decomposition of the crystal when exposed to the air. Most probably 1-fluoropyridine molecule is eliminated from the complex and the crystal lattice is being thus destroyed. Similar loss of pyridine bonded in zinc picolinato complexes has been previously observed. 61,62 The asymmetric unit contains one half of the complex, with the Mn II atom sitting on the inversion center. The manganese(II) atom in compound 3 is octahedrally coordinated (Fig. 6). In the equatorial plane, Mn II atom is surrounded by four oxygen atoms stemming from the two chelating tfpb ligands, being in a trans arrangement, with Mn-O distances 2.1415(14) and 2.1337 (14) Å. The Mn(tfpb) 2 fragment deviates from planarity, the angle between the mean plane formed by the MnO 4 core and that of the tfpb chelate C 3 O 2 moiety being 18.00 (7)°. The axial positions are occupied by two pyF ligands bonded to the metal center through the N1 atom, with Mn1-N1 distance of 2.3425(17) Å. PyF ligand plays the main role in the formation of a layered structure due to the absence of the competing strong hydrogen bond donors. As a hydrogen bond donor PyF is involved in C14-H14···O1 iii interaction with carbonyl oxygen atom and in centrosymmetric C13-H13···F2 ii interactions with fluorine atom of -CF 3 group of tfpb ligands of the adjacent complexes ( Fig. 7 and Table 3). Thus, each complex is involved in eight hydrogen bonds with six adjacent complexes forming a layered structure. 2D structure is supported by centrosymmetric π···π interactions between adjacent pyF rings with centroid-to-centroid distance of 3.9403 (14) Å, perpendicular distance between rings of 3.2624(10) Å and ring slippage of 2.210 Å. Layered structure is further supported also by C-F···π interactions between pyF fluorine atom and pyF aromatic ring with d(F4···Cg3) = 3.6714(19) Å and <(C11-F4···Cg3) = 75.63(13)° as well as by interactions between -CF 3 group and benzene ring of tfpb ligand with d(F2···Cg4) = 3.715(2) Å and <(C1-F2···Cg4) = 126.72(15)°, where Cg3 and Cg4 are N1/C11-C15 and C5-C10 ring centroids, respectively (Fig. 7).
The inclination of pyon and hpyon ligands toward the tfpb moiety in 1 and 2 is best compared with the compound 3 since the ligation of pyF via N atom cannot enable much deviation in comparison to the pyon and hpyon ligands bonded via O atom. Superposition of both crystallographically independent molecules in 1 as well as mole- Compound 4 crystallizes in the triclinic P-1 space group. Selected bond distances and angles of 4 are summarized in Table 6. The asymmetric unit contains one complex molecule with cis-octahedral arrangement of methanol ligands on the manganese(II) central atom (Fig. 9). Two methanol ligands are bonded to the metal center with Mn1-O5 and Mn1-O6 distances of 2.1714(18) and 2.173(2) Å, respectively, and O5-Mn1-O6 angle of 88.54(8)°. The Mn1-O bond lengths with four oxygen atoms of the two chelating tfpb ligands are asymmetric with the longer ones of 2.1821 (18) and 2.1751(17) Å at the trifluoromethyl substituent and the shorter ones of 2.1266 (18) and 2.1315(18) Å at the phenyl substituent. The Mn(tfpb) fragments deviate from planarity, the tfpb ligands being in- Figure 7. a) Hydrogen-bonded layer along the ac plane in 3 is formed by C13-H13···O1 iii and centrosymmetric C14-H14···F2 ii interactions as well as centrosymmetric π···π interactions and C-F···π interactions; b) packing of layers (arbitrary colors). Blue and green dashed lines indicate hydrogen bonds and π···π and C-F···π interactions, respectively. For the sake of clarity, H atoms not involved in the motif shown have been omitted. For symmetry codes see Table 3.  (6)° (2) representing a substantial deviation from 90°. However, in the   clined by 25.94(8) and 23.51(8)°. Each methanol ligand is involved in a centrosymmetric hydrogen-bonded motif via O5-H5···O1 i and O6-H6···O3 ii interactions with the carbonyl oxygen atom at the trifluoromethyl substituent of the adjacent complex. Both centrosymmetric hydrogen bonds have the graph-set motif R 2 2 (8) (Fig. 10 and Table 3) and enable the formation of a hydrogen-bonded chain along the b axis. A centrosymmetric C1-F3···π interaction between CF 3 group and the benzene ring of tfpb ligand of the adjacent molecule is present with d(F3···Cg3) = 3.661(4) Å and <(C1-F3···Cg3) = 121.9(3)°, where Cg3 is C5-C10 ring centroid, connecting chains into a layer along the bc plane (Fig. 10). There are no significant π···π interactions.
is formed through C-H···O and C-H···F interactions as well as π···π and C-F···π interactions. In 4 a layered structure is formed through a combination of O-H···O and C-F···π interactions. CCDC 2024368-2024371 (1-4) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif.

Conclusion
We have prepared and structurally characterized four manganese(II) bis(4,4,4-trifluoro-1-phenylbutane-1,3-dionate) complexes with pyon, hpyon, pyF and methanol ligands. In all prepared compounds the coordination of the metal center is octahedral. Complexes 1-3 possess trans arrangement of ligands while in complex 4 the arrangement is cis. In 1-3 the Mn(tfpb) 2 fragments deviate from planarity, the angles between the mean planes formed by the equatorial MnO 4 cores and that of the tfpb chelate C 3 O 2 moieties being in the range 14.48(6)-18.00 (7)°. In 1 and 2 the axial positions are occupied by two pyon and hpyon ligands, respectively, bonded to the metal center through the O atom. Pyon and hpyon ligands are inclined toward the tfpb moiety by 78.60(5)° (molecule A in 1), 44.51(5)° (molecule B in 1) and 43.25(6)° (2) representing a substantial deviation from 90°. Extended hydrogen bonding is present in 1 and 2 facilitating the formation of a three-dimensional supramolecular structure in 1 and a layered structure in 2 through N-H···O hydrogen bonding enhanced by C-H···O interactions as well as C-F···π interactions. In 3 pyF ligand plays the main role in the formation of crystal aggregation due to the absence of the competing strong hydrogen bond donors. A layered structure