**Phosphoramidates: Molecular Packing and Hydrogen Bond Strength in Compounds Having a P(O)(N)n(O)3-n (n = 1, 2, 3) Skeleton**

Mehrdad Pourayoubi1, Fahimeh Sabbaghi2, Vladimir Divjakovic3 and Atekeh Tarahhomi1 *1Department of Chemistry, Ferdowsi University of Mashhad, Mashhad, 2Department of Chemistry, Zanjan Branch, Islamic Azad University, Zanjan, 3Department of Physics, Faculty of Sciences, University of Novi Sad, Novi Sad, 1,2Iran 3Serbia* 

## **1. Introduction**

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Sedimentation of intermetallic compounds in liquid aluminum alloys of type AlSiCu(Ti), In: *Nonferrous Metals – Science and Technology*, Bonderek Z., CCNS, pp. Compounds containing the P(O)(N)n(O)3-n (n = 1, 2, 3), P(O)(N)m(O)2-mX (m = 1, 2, X = C, Cl, F, S etc.) and P(O)(O)3 moieties are among the well-studied inorganic compounds [an interested reader may find many examples of compounds with the mentioned skeletons through a CSD search, [1]]. *N*,*N*,*N*',*N*',*N*",*N*"-hexamethyl phosphoric triamide (HMPA, Scheme 1) is an important polar aprotic solvent with a high-dielectric constant [2] and an excellent ligand for interaction with hard metal-cations [3].

Scheme 1. *N*,*N*,*N*',*N*',*N*",*N*"-hexamethyl phosphoric triamide

Tabun, NCP(O)[N(CH3)2][OCH2CH3] (Scheme 2), Sarin, FP(O)(CH3)[OCH(CH3)2] and Soman, CH3P(O)(F)[OCH(CH3)(C(CH3)3)] are among the well-known "nerve agents" that act as acetylcholinesterase enzyme (AChE) inhibitors in human body and mammals [4].

Scheme 2. Tabun, a nerve agent

Phosphoramidates: Molecular Packing and Hydrogen

Bond Strength in Compounds Having a P(O)(N)n(O)3-n (n = 1, 2, 3) Skeleton 567

Scheme 3. The classification of compounds having a P(O)(N)n(O)3-n (n = 1, 2, 3) skeleton

be performed by stirring the crude product in a diluted hydrochloric acid [65].

filtrate under reduced pressure [66].

which the salt is as precipitate (and the product is soluble) and then the filtering off the salt. Moreover, if more than twice mole ratio of amine relative to each P-X bond is used, removing the un-reacted amine should be done in the purification process, too, which may

Setzer and co-workers reported the synthesis of 1,3,2-oxazaphospholane from the reaction between (lR,2R)-(-)-pseudoephedrine, phenyl dichlorophosphate and triethylamine in ethyl acetate. Triethylamine hydrochloride was filtered off and the solvent removed from the

Some researchers focus on decontamination of such compounds under UV-irradiation or in the presence of nano-oxides or nano-photocatalysts under sun-light [5]. The flame retardancy of some phosphoric esters was studied [6] and some phosphoramidates have therapeutic applications in the treatments of HIV and cancer [7]. Some pure chemists have interested to the NMR consideration [8], chemical calculation [9] and crystallography [10] of such compounds. A few bio-inorganic chemists have worked on the prediction of the biological properties of compounds based on their structures, with the related software programs such as PASS [11], and the evaluation of some relationships between structural features and biological activities [12]. In our laboratory, we centralize on the synthesis of new phosphorus-nitrogen and phosphorus-oxygen compounds and on obtaining their suitable single crystals for the X-ray crystallography experiments [13-64].

A schematic classification for the compounds having a P(O)(N)n(O)3-n (n = 1, 2, 3) skeleton is shown in Scheme 3.

The numbers of the reported crystal structures in each family are presented in Scheme 3. The central box (blue) indicates the overall number of phosphoramidates having a P(O)(N)n(O)3-n (n = 1, 2, 3) skeleton; the more well-studied categories of phosphoramidates are shown as green boxes in the top and bottom of the central box namely: a) phosphoric triamides (having a P(O)(N)(N)(N) or P(O)(NHC(O))(N)(N) fragment), and b) amidophosphoric acid esters (containing a P(O)(O)(N)(N) or P(O)(O)(O)(N) skeleton).

As a nitrogen bonded H atom is very important in the H-bond pattern consideration, in the sub-categories, the presence or the absence of this H atom is clarified. In the applied notation, for example, the P(O)(NH)3 and P(O)(N)3 denote to the presence of secondary and tertiary nitrogen atoms, respectively. The phosphoramidates containing a P(O)NH2 moiety are distinguished in the left side box directly related to the central box.

The less-studied (so far) related compounds i.e. c) the proton-transfer and phosphate salts and the acids, and d) the anhydride compounds with a P(O)(O)P(O) skeleton are shown in the right and the left of the central box.

In this flowchart, the skeletons of 643 compounds -which their crystal structures were deposited- have been collected. In this classification, the phosphoramidates containing the phosphorus-carbon and the phosphorus-halogen bonds have not been considered.
