Di-μ-nitrato-bis(μ-octaethyl pyrophosphoramide)bis[aquadinitratocalcium(II)]

The structure of the first metal complex of octaethyl pyrophosphoramide is a dimer containing two calcium ions bridged by two nitrate and two octaethyl pyrophosphoramide ligands. Each calcium ion is further coordinated by a nitrate ion acting as a bidentate ligand and a water molecule and has a coordination number of 8.

[Ca 2 (NO 3 ) 4 -(C 16 H 40 N 4 O 3 P 2 ) 2 (H 2 O) 2 ] was obtained as a side product during the work up of the synthesis of octaethyl pyrophosphoramide and represents the first structurally characterized complex of this ligand. The compound crystallizes in the monoclinic space group P2 1 /n and the asymmetric unit contains one pyrophosphoramide molecule and one Ca 2+ ion coordinated to two nitrate ions and one water molecule. The complex exists as a dimer with a centre of inversion located between two eight-coordinate calcium(II) centres, which are bridged by two nitrate ions and two octaethyl pyrophosphoramide ligands. Each Ca 2+ cation is also coordinated to a further nitrate anion, acting as a bidentate ligand, and a water molecule. The complexes stack parallel to the a axis and are held in place by a network of intermolecular O-HÁ Á ÁO hydrogen bonds also running parallel to a.

Chemical context
The structures of octaethyl pyrophosphoramide, (O((Et 2 N) 2 PO) 2 ), and its complexes have not been determined to date. Given the structural similarity of O((Et 2 N) 2 PO) 2 to the more widely studied Schradan ligand, octamethyl pyrophosphoramide, O((Me 2 N) 2 PO) 2 (Goehring & Niedenzu, 1956), it might be expected that the complexes of these two ligands would have related structures. Schradan is known to complex with divalent transition metals and magnesium to form simple chelation complexes of formulae [M(O((Me 2 N) 2 PO) 2 ) 3 ][ClO 4 ] (where M = Mg 2+ , Cu 2+ and Co 2+ ), in which the metal(II) centre is octahedrally coordinated to three pyrophosphate chelate rings  and [Cu(O((Me 2 N) 2 PO) 2 ) 2 (ClO 4 ) 2 ], in which the Cu II atom is coordinated to two pyrophosphate chelate rings and two perchlorate oxygen atoms in an octahedral geometry . Schradan has also been reported as a bridging ligand in two dimeric Eu 3+ complexes (Chan et al., 2020). Here we report what we believe to be the first example of a metal-coordinated octaethyl pyrophosphoramide complex, which is dimeric and has the formula [Ca(O((Et 2 N) 2 PO) 2 (NO 3 ) 2 (H 2 O)] 2 .

Structural commentary
The asymmetric unit contains one pyrophosphoramide molecule together with one Ca 2+ ion coordinated to two nitrates and one water molecule. None of the atoms lie on special ISSN 2056-9890 positions. The content of one asymmetric unit makes up one half of the actual dimeric calcium complex, which has a centre of inversion midway between the two calcium atoms, bringing Z to 2.
In the title complex ( Fig. 1), the di-N-substituted pyrophosphoramide molecule acts as a bridging ligand, rather than a bidentate chelating ligand, unlike in the previously characterized transition-metal and alkaline-earth metal complexes of the Schradan ligand, O((Me 2 N) 2 PO) 2 Hussain et al., 1970).
The coordination number of the Ca 2+ cation in the title compound is eight, which is typical for Ca 2+ complexes. There are two Ca-O(P O) bond lengths per O((Et 2 N) 2 PO) 2 ligand, Ca1-O1 and Ca1-O3 i with 2.3054 (13) and 2.3324 (13) Å , respectively; (Table 1), both of which are rather longer than the average lengths for analogous bonds found in simple phosphoramide complexes of Ca 2+ (see Database Survey below). The corresponding P O bond lengths in the O((Et 2 N) 2 PO) 2 ligand, P1-O1 and P2-O3, are 1.4752 (13) and 1.4722 (13) Å , respectively (Table 1) and are comparable to values reported in other complexes where the ligands are also coordinated via the P O moiety.

Supramolecular features
The complexes pack to form chains running along the a-axis direction, where neighbouring complexes are bound by intermolecular hydrogen bonding of the type H-OÁ Á ÁO-N, as shown in Fig. 2, involving the aqua ligand and the nonbridging nitrate anion, namely O90-H90AÁ Á ÁO20 ( Table 2). The aqua ligand also forms a hydrogen-bonding motif with the bridging nitrate anion, namely O90-H90BÁ Á ÁO12 (Table 2).

Database survey
All searches were carried out using the Cambridge Structural Database (CSD Version 5.41, last update May 2020; Groom et al., 2016). A search for the structure of octaethyl pyro-phosphoramide and its complexes returned no hits. A search for the structure of octamethyl pyrophosphoramide (Schradan) and its complexes returned six hits in which the ligand was found to chelate with the metal cations. Of these, four were octahedral metal complexes of this ligand with magnesium:  Symmetry code: (i) Àx þ 1; Ày þ 1; Àz þ 1.

Figure 1
Molecular structure of (I  (Kepert et al., 1983). Two further hits were found in which the Schradan ligand formed a bridge between two seven-coordinate Eu 3+  A similar search for other di-N-substituted pyrophosphoramide complexes returned no hits, whilst a search for mono-N-substituted pyrophosphoramide complexes returned one hit, namely the octahedral complex, [Mn(O((tBuNH) 2 -PO) 2 ) 2 (DMF) 2 ][Cl] 2 Á2H 2 O (PEWRAM), in which the pyrophosphoramide ligand was found to chelate to a manganese(II) cation (Tarahhomi et al., 2013).
Although no pyrophosphoramide complexes of calcium were found, a search for di-5 4 -phosphorane species containing the fragment O P-X-P O-Ca yielded 17 hits. The complex tris Morales-Juá rez et al., 2005) was the only species found to contain the O P-X-P O-Ca fragment bridging two Ca 2+ cations that did not form part of a cluster or polymer. However in this case, both calcium centres have a coordination number of six, with distorted octahedral geometries, and bridging is achieved via one -oxygen atom per [N(Ph 2 PO) 2 ] À ligand. This is, however, unlike the bridging behaviour observed in the title complex.

Synthesis and crystallization
The title compound was obtained as a minor component on purification of octaethyl pyrophosphoramide through column chromatography. The synthesis of octaethyl pyrophosphoramide was undertaken using standard Schlenk line techniques. All solvents were dried over 4 Å molecular sieves. An excess amount of diethylamine (used as purchased), namely 7.6 ml (0.073 mol), was dissolved in 10 ml of chloroform. The solution was cooled to 195 K and 1 ml (0.007 mol) of pyrophosphoryl chloride (purified by short-path distillation) was added dropwise using a glass syringe with constant stirring. After the addition was complete, the cooling bath was removed and the mixture allowed to react at room temperature overnight with continuous stirring. Approximately 15 ml of n-pentane was then added to yield a deep-red-coloured suspension and this was left overnight to allow precipitation. The suspension was filtered using a series of cannula filtrations to remove the diethylammonium chloride by-product. Volatile products were removed under vacuum at 323 K. This yielded the crude octaethyl pyrophosphoramide as a viscous red liquid. This was subsequently purified by column chromatography using a dilute nitric-acid-activated Kieselgel 60 as the stationary phase and dichloromethane/acetonitrile as eluents. Octaethyl pyrophosphoramide was collected in acetonitrile as a dark-pink viscous liquid after removal of volatiles under vacuum at room temperature.
On storage of the liquid octaethyl pyrophosphoramide product over a number of weeks, single crystals of the title compound formed serendipitously. Introduction of Ca 2+ and NO 3 À ions most likely arose from either the use of dilute nitric acid in the activation process of the silica gel used for column chromatography or from impurities present in the molecular sieve. Both the Kieselgel 60 and the molecular sieve were not used as received from the supplier, but were reused following washing/cleaning partly with nitric acid. The Ca 2+ ions may have been introduced from previous use and remained inside the column or drying material.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms were positioned geometrically (O-H = 0.87, C-H = 0.98-0.99 Å ) and refined as riding with U iso (H) = 1.2U eq (C) or 1.5U eq (O, C-methyl).

Acknowledgements
The research work disclosed in this publication was partially funded by the Endeavour Scholarship Scheme (Malta). Scholarships are part-financed by the European Union -European Social Fund (ESF) -Operational Programme II -Cohesion Policy 2014-2020 'Investing in human capital to create more opportunities and promote the well being of society'. The authors would also like to acknowledge the project: Setting up of transdisciplinary research and knowledge exchange (TRAKE) complex at the University of Malta (ERDF.01.124), which is being co-financed through the European Union through the European Regional Development Fund 2014-2020.

Di-µ-nitrato-κ 3 O,O′:O;O:O,O′-bis(µ-octaethyl pyrophosphoramide-κ 2 O:O′)bis[aquabis(nitratoκ 2 O,O′)calcium(II)]
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.