Bis(adamantan-1-aminium) hydrogen phosphate fumaric acid sesquisolvate

The asymmetric unit of the title compound, 2C10H18N+·HPO4 2−·1.5C4H4O4, contains two adamantan-1-aminium cations, one hydrogen phosphate anion, and one and a half molecules of fumaric acid, one of which exhibits crystallographic inversion symmetry. Each HPO4 2− anion is hydrogen bonded, via all of its O atoms, to four NH3 + groups of the adamantan-1-aminium cations, forming chains along [100]. These chains are, in turn, interconnected via a set of O—H⋯O hydrogen bonds involving the fumaric acid solvent molecules, forming layers parallel to (001). Weak C—H⋯O interactions lead to a consolidation of the three-dimensional set-up.

The asymmetric unit of the title compound, 2C 10 H 18 N + Á-HPO 4 2À Á1.5C 4 H 4 O 4 , contains two adamantan-1-aminium cations, one hydrogen phosphate anion, and one and a half molecules of fumaric acid, one of which exhibits crystallographic inversion symmetry. Each HPO 4 2À anion is hydrogen bonded, via all of its O atoms, to four NH 3 + groups of the adamantan-1-aminium cations, forming chains along [100]. These chains are, in turn, interconnected via a set of O-HÁ Á ÁO hydrogen bonds involving the fumaric acid solvent molecules, forming layers parallel to (001). Weak C-HÁ Á ÁO interactions lead to a consolidation of the three-dimensional set-up.
Here, we report the synthesis and the crystal structure of a new hydrogen phosphate with an organic cation, (C 10 H 18 N) 2 +. HPO 4 -. 1.5(C 4 H 4 O 4 ), (I), formed by the reaction of adamantan-1-aminium fumarate with orthophosphoric acid.
Reaction of the starting materials led to a partial proton transfer from the phosphoric acid to the fumarate anions, which are found to be fully protonated in the structure of compound (I). The asymmetric unit of compound (I) contains one hydrogen phosphate anion, two adamantan-1-aminium cations and one and a half molecules of fumaric acid (Fig. 1), one of which is located on a crystallographic inversion center. Arrangement of the ions and molecules in the crystal structure is governed by a series of strong O-H···O and one N-H···O hydrogen bonds, augmented by a few C-H···O interactions ( Table 1, Figures 2-4). In the structure, each HPO 4 2anion is hydrogen-bonded through all of its oxygen atoms to four NH 3 + groups of the adamantan-1-aminium cations to build infinite hydrogen bonded chains along [100] with an R 4 4 (12) graph set motif (Bernstein et al., 1995) (Fig. 2). These chains are in turn interconnected via hydrogen bonds involving the fumaric acid molecules, with two distinct types of hydrogen bonding patterns. For the centrosymmetric fumaric acid molecule both carboxyl groups undergo one O-H···O and one N-H···O hydrogen bond.
With respect to the O-H···O bonds, the furmaric acid is the H-bond donor and a phosphate O atom is the corresponding acceptor. With respect to the N-H···O bonds, the H donor is an adamantyl ammonium group and the corresponding acceptor the not-protonated fumaric acid oxygen atom. For the second fumaric acid molecule (located on a general position), the situation is different. One of the two carboxyl groups has a hydrogen bonding pattern similar to that of the centrosymmetric molecule. The second carboxyl group, on the other hand, undergoes two O-H···O hydrogen bonds with two oxygen atoms of the same phosphate anion, with an R 2 2 (8) graph set motif. While the two types of fumaric acid molecules are thus clearly distinct, their roles in the construction of the crystal structure are similar.
The two types of fumaric acid molecules are arranged in skewed stacks where they alternate with one another in an ABBA pattern, with A being the centrosymmetric fumaric acid molecule (Fig. 3). The stacks extend along [100], inbetween the chains of adamantan-1-aminium cations and monohydrogenophosphate anions. Pairs of B molecules in the ABBA pattern show π-π stacking interactions with an interplanar spacing of 3.108 Å, and a centroid-to-centroid distance of 4.144 Å, indicating substantial slippage of the molecules against one another. Neighboring A and B molecules are not parallel; the molecular planes are tilted by ca 10.8° against one another. Their centroid-to-centroid distance is 4.306 Å, and actual close contacts are limited to some O···O interactions. A and B molecules are not π-π-stacked with one another.

supplementary materials
Through their O-H···O and N-H···O hydrogen bonds to the hydrogen phosphate anions and adamantan-1-aminium cations, the fumaric acid molecules give rise to two-dimensional infinite layers parallel to (001) (Fig. 3). Fig. 4 shows that the adamantan-1-aminium cation and monohydrogenophosphate anion chains extend along [100] at (x, 0, 0) and (x, 1/2, 1/2), while the layers cross the unit cell at c = n/2. The adamantan-1-aminium cations of parallel layers interdigitate with one another and are located in alternate pairs on either side of the layers, leading to an extended three-dimensional structure (Fig. 4), which is further consolidated by a small number of weak C-H···O interactions ( Table 1).
The detailed geometry of the HPO 4 2group shows two kinds of P-O distances. The shorter ones (1.5252 (9), 1.5330 (8) and 1.5148 (8) Å correspond to the non-protonated oxygen atoms, while the largest one (1.5946 (8) Å) is associated with the P-OH bond. This is in agreement with literature data for the monohydrogenophosphate anion in similar arrangements (e.g. Chtioui & Jouini, 2006;Kaabi et al., 2004). The O-H···O interactions show elongated O-H distances typical for very strong hydrogen bonds and range from 0.902 (16) for H1 to 1.073 (18) for H8 (Table 1).

Experimental
Crystals of the title compound were prepared at room temperature by slow addition of a solution of orthophosphoric acid (3 mmol in 20 ml of water) to an alcoholic solution of adamantan-1-aminium fumarate (6 mmol in 20 ml of ethanol). The acid was added until the alcoholic solution became turbid. After filtration, the solution was allowed to slowly evaporate at room temperature over several days leading to formation of transparent prismatic crystals with suitable dimensions for single-crystal structural analysis (yield 55%). The crystals are stable for months under normal conditions of temperature and humidity.

Refinement
C, and N bound H atoms were placed in calculated positions riding on their respective carrier atom with C-H in the range 0.95-1.00 and N-H of 0.91 Å. Ammonium H atoms were allowed to rotate but not to tip to best fit the observed electron density distribution. U iso (H) values were constrained to be in the range of 1.2 U eq of the parent atom for C bound H atoms, and 1.5 times U eq for N and O bound H atoms. O bound H atoms were located in difference density Fourier maps, and their positons were freely refined.  The molecular components of compound (I), showing 50% probability displacement ellipsoids and spheres of arbitrary radius for the H atoms.   An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.