Crystal structures of two bis-carbamoylmethylphosphine oxide (CMPO) compounds

The crystal structures of two multidentate CMPO-containing organic ligands are described. Both compounds feature N—H⋯O hydrogen bonds in the solid state.

Two bis-carbamoylmethylphosphine oxide compounds, namely { [(3-{[2-(diphenylphosphinoyl)ethanamido]methyl}benzyl)carbamoyl]methyl}diphenylphosphine oxide, C 36 H 34 N 2 O 4 P 2 , (I), and diethyl [({2-[2-(diethoxyphosphinoyl)ethanamido]ethyl}carbamoyl)methyl]phosphonate, C 14 H 30 N 2 O 8 P 2 , (II), were synthesized via nucleophilic acyl substitution reactions between an ester and a primary amine. Hydrogen-bonding interactions are present in both crystals, but these interactions are intramolecular in the case of compound (I) and intermolecular in compound (II). Intramolecularstacking interactions are also present in the crystal of compound (I) with a centroid-centroid distance of 3.9479 (12) Å and a dihedral angle of 9.56 (12) . Intermolecular C-HÁ Á Á interactions [CÁ Á Ácentroid distance of 3.622 (2) Å , C-HÁ Á Ácentroid angle of 146 ] give rise to supramolecular sheets that lie in the ab plane. Key geometric features for compound (I) involve a nearly planar, trans-amide group with a C-N-C-C torsion angle of 169.12 (17) , and a torsion angle of À108.39 (15) between the phosphine oxide phosphorus atom and the amide nitrogen atom. For compound (II), the electron density corresponding to the phosphoryl group was disordered, and was modeled as two parts with a 0.7387 (19):0.2613 (19) occupancy ratio. Compound (II) also boasts a trans-amide group that approaches planarity with a C-N-C-C torsion angle of À176.50 (16) . The hydrogen bonds in this structure are intermolecular, with a DÁ Á ÁA distance of 2.883 (2) Å and a D-HÁ Á ÁA angle of 175.0 (18) between the amide hydrogen atom and the P O oxygen atom. These non-covalent interactions create ribbons that run along the b-axis direction.

Chemical context
The carbamoylmethylphosphine oxide (CMPO) moiety has found use as the chelating portion of a ligand in the TRUEX process for the remediation of nuclear waste (Horwitz et al., 1985). It has been shown that the CMPO group binds lanthanide (Ln) and actinide (An) metals in a 1:2 or 1:3 metalligand ratio in solution, depending on the size of the metal ion. Many researchers have attempted to mimic this solution stoichiometry by tethering two, three or four CMPO groups together via an organic scaffold (Dam et al., 2007;Leoncini et al., 2017;Miyazaki et al., 2015;Sharova et al., 2014;Werner & Biros, 2019). In some cases, these multidentate ligands have demonstrated an increased binding affinity for certain Ln and An ions, as well as an increased ability to extract these metals out of aqueous solutions. To this end, we report here the synthesis of compounds (I) and (II) and their characterization by 1 H, 13 C, and 31 P NMR spectroscopy, and by X-ray crystallography.

Structural commentary
The structure of compound (I) was solved in the monoclinic space group C2/c. Since the entire molecule straddles a twofold symmetry axis, the asymmetric unit is composed of one half of the compound. The complete molecular structure of compound (I) is shown in Fig. 1 along with the atomlabeling scheme. The P O bond length is 1.4915 (13) Å , with P-C bond lengths that range from 1.7988 (18) to 1.8169 (19) Å . The 4 descriptor for fourfold coordination around the phosphorus atom P1 is 0.95, indicating a nearly perfect tetrahedral geometry of the phosphine oxide group (where 0.00 = square-planar, 0.85 = trigonal-pyramidal, and 1.00 = tetrahedral; Yang et al., 2007). The geometry between the amide nitrogen atom N1 and the -phosphine oxide phosphorus atom P1 is defined by a P1-C2-C1-N1 torsion angle of À108.39 (15) . The amide group adopts a nearly perfect trans geometry with a C3-N1-C1-C2 torsion angle of 169.12 (17) , and is staggered with respect to the plane of the C4-C7 aromatic ring with a H1-N1-C3-C4 torsion angle of 59.1 (17) .
Compound (II) crystallizes in the orthorhombic space group Pbca. Since the molecule lies on an inversion center (at 2 À x, 1 À y, 1 À z), the asymmetric unit comprises one half of the molecule. The electron density corresponding to the atoms of the phosphoryl group was disordered and was modeled over two positions with a 0.7387 (19):0.2613 (19) occupancy ratio (see the Refinement section for more details). The complete molecular structure of the major component of compound (II) is shown in Fig. 2 along with the labeling scheme. For the major component, the P O bond length is 1.474 (2) Å , with P-O bond lengths of 1.5791 (16) and 1.5619 (15) Å , and a P-C bond length of 1.801 (2) Å . The 4 descriptor for fourfold coordination around the phosphorus atom of the major component, P1, is 0.93, indicating that the geometry of the phosphoryl group is slightly distorted from an ideal tetrahedron. The geometry between the amide nitrogen atom N1 and the -phosphoryl group phosphorus atom P1 is defined by a N1-C1-C2-P1 torsion angle of À111.8 (2) . The amide group of this compound also adopts a nearly perfect trans geometry with a C3-N1-C1-C2 torsion angle of À176.50 (16) . The complete molecular structure of compound (I), with the atomlabeling scheme. Unlabeled atoms are related to labeled atoms by the crystallographic twofold axis. Displacement ellipsoids are drawn at the 50% probability level, and hydrogen atoms bonded to carbon atoms have been omitted for clarity.

Figure 2
The molecular structure of compound (II), with the atom-labeling scheme. Unlabeled atoms are related to labeled atoms by a crystallographic inversion center. Displacement ellipsoids are drawn at the 50% probability level, only the major component and hydrogen atoms bonded to nitrogen atoms have been included for clarity. Table 1 Hydrogen-bond geometry (Å , ) for (I).
Cg is the centroid of the C14-C19 ring.

Figure 4
A view down the c-axis of compound (I) showing the supramolecular sheets that are held together with intramolecular C-HÁ Á Á interactions using a ball-and-stick model with standard CPK colors. Hydrogen bonds are depicted with blue dashed lines, whileand C-HÁ Á Á interactions are shown with green dashed lines. Only (N)H1 and (C)H3A are shown for clarity.

Figure 5
Depiction of the hydrogen-bonding network present in the crystal of compound (II) using a ball-and-stick model with standard CPK colors. The minor component of the disordered phosphoryl group is omitted for clarity. Intermolecular hydrogen bonds are shown with blue dashed lines. Symmetry codes: hydrogen bond forms ribbons of compound (I) that run along the b-axis direction (Fig. 6).

Database survey
The (This count excludes metal complexes.) Of these, seven structures have two or more CMPO groups tethered to one another via an organic scaffold. The most similar structures to compound (I) are CIWFAR (Ouizem et al., 2014) and SISLIQ (Artyushin et al., 2006). Both structures use an aromatic ring as the scaffold to present two phenyl-substituted CMPO groups. In SISLIQ, a 1,2-disubstituted benzene ring is utilized to present the CMPO groups. In CIWFAR, the scaffold is a pyridine ring where the 2-and 6-positions bear CMPO groups, which makes it directly analogous to compound (I). The amide hydrogens of CIWFAR are engaged in intermolecular hydrogen bonds with the oxygen atoms of the phosphine oxide groups [rather than the intramolecular interaction observed for compound (I)], and the pyridine nitrogen is hydrogen bonded to the -OH group of a solvent methanol molecule. The hydrogen atoms of the pyridine scaffold interact with the phenyl rings of the phosphine oxide via intermolecular C-HÁ Á Á interactions. A structure closely related to compound (II) was reported by the Rebek group as OGIVIJ (Amrhein, et al., 2002). Here, a resorcin[4]arene scaffold presents two ethoxy-substituted CMPO units. We also note that the structure of compound (II) complexed with Sm(NO 3 ) 3 has been reported in this journal (Stoscup et al., 2014).

Synthesis and crystallization
Compound (I): 1,3-Bis(aminomethyl)benzene (128 mg, 0.124 mL, 0.785 mmol) and the p-nitrophenyl ester of diphenylphosphonoacetate (Arnaud-Neu et al., 1996) (1.0 g, 3.14 mmol) were dissolved in anhydrous, ethanol-free chloroform (30 mL). The solution was heated to 313 K and stirred for three days. The reaction mixture was then allowed to cool to room temperature, a small amount of 40% KOH was added (ca. 3 mL) and the solution was stirred for 3.5 h. The organic layer was separated, washed with brine (3 Â 10 mL), dried over solid magnesium sulfate and concentrated under reduced pressure. The crude product was triturated multiple times with ethyl acetate to give a white solid in 91% yield. X-ray quality crystals of compound (I) were grown by slow evaporation of a chloroform solution. was added dropwise. The reaction mixture was allowed to warm to room temperature and stirred overnight. The product precipitated from the solution, was isolated by vacuum filtration and rinsed with ethyl acetate. Some of this solid was crystalline and suitable for analysis by X-ray diffraction. The remainder of the isolated product was purified by silica gel chromatography

Refinement
Crystal data, data collection and structure refinement details for both compounds are summarized in Table 3. For compounds (I) and (II), all hydrogen atoms bonded to carbon atoms were placed in calculated positions and refined as riding: C-H = 0.95-1.00 Å with U iso (H) = 1.2U eq (C) for methylene groups and aromatic hydrogen atoms, and U iso (H) = 1.5U eq (C) for methyl groups. For both compounds (I) and (II), the hydrogen atoms bonded to nitrogen atoms were located using electron-density difference maps. The disordered electron density corresponding to the phosphoryl group of compound (II) was modeled over two positions with a relative occupancy ratio of 0.7387 (

{[(3-{[2-(Diphenylphosphinoyl)ethanamido]methyl}benzyl)carbamoyl]methyl}diphenylphosphine oxide (I)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (