Charge-Assisted Hydrogen-Bonded Networks of NH 4 + and [ Co ( NH 3 ) 6 ] 3 + with the New Linker Anion of 4-Phosphono-Biphenyl-4 1-Carboxylic Acid

The new linker molecule 4-phosphono-biphenyl-41-carboxylic acid (H2O3P-(C6H4)2-COOH, H3BPPA) has been structurally elucidated in hydrogen-bonded networks with the ammonium cation NH4(H2BPPA)(H3BPPA) (1) and the hexaamminecobalt(III) cation [Co(NH3)6](BPPA) ̈ 4H2O (2). The protic O-H and N-H hydrogen atoms were found and refined in the low-temperature single-crystal X-ray structures. The hydrogen bonds in both structures are so-called charge-assisted; that is, the H-bond donor and/or acceptor carry positive and/or negative ionic charges, respectively. The H-bonded network in 1 consists of one formally mono-deprotonated 4-phosphonato-biphenyl-41-carboxylic acid group; that is, a H2BPPA ́ anion and a neutral H3BPPA molecule, which together form a 3D hydrogen-bonded network. However, an almost symmetric resonance-assisted hydrogen bond (RAHB) bond [O ̈  ̈  ̈H = 1.17 (3) and 1.26 (3) Å, O ̈  ̈  ̈H ̈  ̈  ̈O = 180 (3) ̋] signals charge delocalization between the formal H2BPPA ́ anion and the formally neutral H3BPPA molecule. Hence, the anion in 1 is better formulated as [H2BPPA ̈  ̈  ̈H ̈  ̈ ̈H2BPPA] ́. In the H-bonded network of 2 the 4-phosphonato-biphenyl-41-carboxylic acid is triply deprotonated, BPPA3 ́. The [Co(NH3)6] cation is embedded between H-bond acceptor groups, –COO ́ and –PO3 ́ and H2O molecules. The incorporation of sixteen H2O molecules per unit cell makes 2 an analogue of the well-studied guanidinium sulfonate frameworks.


Introduction
The organophosphonic acid function, which has a pK a1 of 2.0 for the first and a pK a2 of 6.59 for the second proton, is capable of forming strong metal-to-ligand coordinative bonds in thermodynamically stable complexes with high stability constants [1].Metal organophosphonate compounds are multifunctional organic-inorganic hybrid materials and as open frameworks can be regarded in between zeolite-like [1,2] and metal-organic framework materials [3,4], whereas phosphonate metal-organic frameworks (MOFs) are considerably rarer than MOFs with carboxylate linkers, with phosphonates forming stronger bonds to metals than carboxylate groups [3].Metal organophosphonates are stable in water or aqueous environment [5].The use of metal phosphonates in catalysis, luminescence [6], ion or proton exchange or conductivity [7,8] and in separation is discussed and investigated [9].Further, cobalt and iron organophosphonates are investigated for their magnetic properties [10][11][12][13].Metal phosphonates are also promising porous materials [14,15], and can be reversibly hydrated and dehydrated [16].
Neutralization of H 3 BPPA with one equivalent of ammonium acetate yielded colorless crystals of formula NH 4 (HO 3 P-(C 6 H 4 ) 2 -COOH)(H 2 O 3 P-C 6 H 4 -C 6 H 4 -COOH) (1).The ammonium monohydrogenphosphonato-biphenyl-carboxylic acid crystallized with one molecule of the free H 3 BPPA acid.The best results were obtained using a 1:1 ratio, though less ammonium acetate also led to product formation of lower quality.When the neutralization of H 3 BPPA was carried out with excess conc.aqueous NH 3 instead of stoichiometric ammonium acetate, the same product, 1, was formed, albeit of lower purity.Importantly, no complete or even twofold deprotonation of H 3 BPPA was achieved in that way.
The asymmetric unit of 1 consists of the ammonium-cation, and formally a H 2 BPPA ´anion and a neutral H 3 BPPA molecule (see below) (Figure 1a).The H 2 BPPA ´anion is derived by mono-deprotonation of the phosphonic acid group.The protic O-H and N-H hydrogen atoms were found and refined with U eq = 1.5 U eq (O,N).The three building blocks form a three-dimensional (3D) hydrogen bonded network.
The carboxylic acid groups are oriented towards each other with the typical tail-to-tail arrangement, also known as R 2 2 p8q-motif in the Etter-notation (Figure 1b) [33].The biphenyl systems of the BPPA molecules are in nearly planar geometry with 0.31 (14) and 2.79 (14) ˝for the dihedral angles between the aryl ring planes, and 1.7 (2) ˝and 3.3 (2) ˝for the dihedral angle between ´COOH and its aryl ring in the P1 and P2 molecule, respectively.The shape of the thermal ellipsoids of the carboxyl oxygen atoms O1, O2, O6, and O7 is indicative of some rotational movement (vibration) around the (carboxyl)C-C(aryl) bond (yet, no split refinement was suggested by SHELX from the principal mean square atomic displacements).Despite the presence of the biphenyl π-systems in 1, there are no π-π interactions [34] and only few intermolecular C-H¨¨¨π [35] are evident.The angle is 57 ˝for the plane formed by one biphenyl system to its neighbor.
The ammonium cation engages all of its four (found and refined) N-H bonds in the hydrogen network to four different phosphono groups.The ammonium cations and phosphono groups form hydrogen-bonded layers parallel to the ab-plane, separated by the biphenyl-carboxylic acid parts (Figure 1c).Ammonium benzenephosphonate, NH 4 (HO 3 PC 6 H 5 ) [36], consists of a layered structure due to hydrogen bonds, with a similar motif to that of 1.  + cation and the phosphonate and phosphonic acid groups.Details of the H-bonding interactions (orange dashed lines) are given in Table 1, selected non-hydrogen bonds and angles in Table 2. Symmetry transformations: i = ´1 ´x, ´y, 1 ´z; ii = ´x, 1 ´y, 1 ´z; iii = 1 + x, y, z; iv = x, ´1 + y, z; v = 1 + x, 1 + y, z; vi = 2 ´x, 2 ´y, ´z; vii = 1 ´x, 2 ´y, ´z.
The interpretation of delocalized anion charge over the two phosphono groups is in agreement with the P-O bond lengths (Scheme 2).In each phosphonato group, there is a longer P-O bond of ~1.Thermogravimetric analysis (TGA) of 1 shows a first a mass loss of ~4% up to 240 ˝C (Figure 2), which can be assigned to one molecule of ammonia (~3%), which is in agreement with literature values [42].In a second step decarboxylation of one mol CO 2 (44 g/mol) leads to a mass loss of ~8%.With a third step of ~24% rapid dephosphonation of one mol PO(OH) 2 and final decarboxylation of another mol CO 2 takes place, which is followed by steady decomposition up to 700 ˝C.
In the asymmetric unit of 2 there is one trivalent hexaamminecobalt cation, one completely deprotonated BPPA 3´t rianion and four water molecules (Figure 3a).The coordination sphere of Co 3+ with six crystallographically different ammine ligands results in the well-known [Co(NH 3 ) 6 ] 3+ octahedron [45].Despite its high symmetry, the Co(NH 3 ) 6 ] 3+ octahedron does not reside on a special position.The Co-N distances (Table 3) are comparable with that of [Co(NH 3 ) 6 ] 3+ in related complexes (Co-N = 1.951 (2) ´1.976 (2) Å, av.1.956 (2) Å) [43,44,46].3. The BPPA 3´a nions and the [Co(NH 3 ) 6 ] octahedra are connected to each other by hydrogen bonding (Table 4, Figure 4).The fully deprotonated phosphonate-carboxylate is solely an H-acceptor for the N-H and water O-H bonds.The carboxylate group is acceptor to O-H from water molecules.The four water molecules are held by hydrogen bonding from N-H and O-H-donors and -COO ´and ´P(O) 2 O 2´a cceptors.Finally, we note that in both structures, 1 and 2, the H-bonds to the phosphonato groups are so-called charge-assisted hydrogen bonds.The hydrogen bond donor and/or acceptor carry positive and negative ionic charges, respectively, hence are usually stronger and shorter than neutral H-bonds [12,46,47,[50][51][52][53][54].In 1 these are bonds NH 4 (+) ¨¨¨p ´qO-P and NH 4 (+) ¨¨¨(H)O-P, -P-OH¨¨¨p ´qO-P (Figure 1c).In 2 these are bonds Co-NH 3 (+) ¨¨¨p ´qO-P and HOH¨¨¨p ´qO-P (Figure 4).The large number of hydrogen-bonds in 2 results in a thermal stability that is higher than that of other supramolecular complexes of [Co(NH 3 ) 6 ] 3+ [55,56].The thermal stability of BPPA 3´i s reflected by the TGA measurement (Figure 5).The mass loss in 2 up to (~17%) is due to the evaporation of the four water molecules together with one ammine ligand (17.5%).The next five ammine ligands are removed along with decarboxylation of BPPA in the range from 220 ˝C to 500 ˝C, followed by decomposition of the biphenyl system (~42% in total).The remaining mass of ~40% can be assigned to cobalt phosphonate species (~35%).It has been observed that metal phosphonates are stable up to 650 ˝C and higher [57].Comparison of the experimental powder X-ray diffractogram for 2 with the simulation from the single-crystal X-ray dataset (Figure 6) shows that the investigated single crystal was representative of the bulk amount when one takes into account the preferential orientation of the column-or rod-shaped crystals of 2 (Figure S1 in Supplementary Material) on the flat sample holder.Due to the preferred orientation of the rod-shaped crystals on the flat sample holder during the powder X-ray diffraction (PXRD) measurement, and their small quantity, some reflections were not present in the experimental diffraction pattern or their intensity was strongly changed.Such a behavior is discussed in detail in the literature [58][59][60].

Materials and Methods
The chemicals used were obtained from commercial sources.No further purification has been carried out.The ligand has been synthesized starting from 4-biphenyl carboxylic acid in a four-step-synthesis.CHN analysis was performed with a Perkin Elmer CHN 2400.IR-spectra were recorded on a Bruker Tensor 37 IR spectrometer with ATR unit.Thermogravimetric analysis (TGA) was done with a Netzsch TG 209 F3 Tarsus in the range from 20 to 700 ˝C, equipped with Al-crucible and applying a heating rate of 10 K¨min ´1.The melting point was determined using a Büchi Melting Point apparatus B540.For powder X-ray diffraction patterns (PXRD), a Bruker D2 Phaser powder diffractometer was used with a flat silicon, low background sample holder, at 30 kV, 10 mA for Cu-Kα radiation (λ = 1.5418Å), with a scan speed of 0.2 s/step and a step size of 0.02 ˝(2θ).Diffractograms were obtained on flat layer sample holders with a beam scattering protection blade installed, which led to the low relative intensities measured at 2θ < 7 ˝.Details of the synthesis of 4-phosphono-biphenyl-4 1 -carboxylic acid (H 2 O 3 P-(C 6 H 4 ) 2 -COOH, H 3 BPPA) will be given elsewhere [62].

Single Crystal X-ray Structures
Suitable crystals were carefully selected under a polarizing microscope, covered in protective oil and mounted on a 0.05 mm cryo loop.Data collection.Bruker Kappa APEX2 CCD X-ray diffractometer with microfocus tube, Mo-Kα radiation (λ = 0.71073 Å), multi-layer mirror system, ωand θ-scans; data collection with APEX2, cell refinement and data reduction with SAINT [63], experimental absorption correction with SADABS [64].Structure analysis and refinement: All structures were solved by direct methods using SHELXL2014; refinement of 1 was done by full-matrix least squares on F 2 using the SHELX-97 program suite [65], of 2 with OLEX 2 [66,67].Non-hydrogen atoms were refined with anisotropic displacement parameters.Hydrogen atoms were positioned geometrically (C-H = 0.95 Å) and refined using riding models (AFIX 43 for aromatic CH with C-H = 0.93 Å and U iso (H) = 1.2U eq (C).In 1 the protic hydrogen atoms (O-H, N-H) were found and freely refined with U iso (H) = 1.5U eq (NH and OH).
In 2, NH 3 hydrogen atoms were found and freely refined.Water hydrogen atoms were also found and refined, except for O6, where they were positioned geometrically (O-H = 0.870 Å) and refined using a riding model (AFIX 6) with U iso (H) = 1.5U eq (O).
Crystal data and details on the structure refinement are given in Table 5. Graphics were drawn with DIAMOND [68].Analyses on the supramolecular C-H¨¨¨O, C-H¨¨¨π and π-π-stacking interactions were done with PLATON for Windows [69].CCDC 1450889 and 1450890 contain the supplementary crystallographic data for this paper.These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html.

Figure 1 .
Figure 1.(a) Asymmetric unit of 1 (50% thermal ellipsoids); (b) unit-cell packing diagram with tail-to-tail arrangement of the carboxylic acid groups (showing only the carboxyl and the O9-H-O5 H bonds for clarity); and (c) full hydrogen-bonding arrangement around the NH 4 + cation and the 56 Å and two shorter P-O bonds between 1.50-1.52Å.The P-O(H) bonds are 1.5583 (19) Å and 1.5553 (19) Å.One cannot clearly distinguish between a formally P=O double bond and a formally deprotonated P-O ´bond.The P-O bond lengths of the symmetric O9¨¨¨H9¨¨¨O5 hydrogen bridge are only slightly longer (~1.52 Å) then what should be P=O double bonds (~1.50 Å).The negative charge is delocalized over the P-O ´and P=O bonds, giving both of them a partial double bond character with P-O bond lengths between 1.50-1.52Å (Scheme 3).

Scheme 3 .
Scheme 3. Lewis valence structure for the bond order and charge-delocalization in the phosphonate groups in 1.

Figure 3 .
Figure 3. (a) Asymmetric unit of 2; and (b,c) projections of the unit-cell packing on different planes.The [Co(NH 3 ) 6 ] 3+ cations are illustrated as octahedra; hydrogen bonds are not shown in (a-c) for clarity.Selected bond distances and angles are given in Table3.

Figure 6 .
Figure 6.Comparison of the experimental PXRD pattern of 2 (black) with the unconstrained simulated pattern from the X-ray data (red) and simulated patterns with the preferred orientation of h, k, l = 1, 0, 1 and March-Dollase parameter = 4 (green) and = 10 (blue).The latter simulations try to take into account the rod-shaped crystal morphology of 2 with their non-random orientation on the flat sample holder.The Miller indices have been assigned to the reflections.Simulations were carried out with Mercury [61].

Table 4 .
Details of the hydrogen bonding interactions in 2 a .

Table 5 .
Crystal data and refinement details.Chemical formula C 26 H 21 O 10 P 2 ¨H4 N C 13 H 8 O 5 P¨CoH 18 N 6 ¨4(H 2 O)