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

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].


Introduction
Compounds containing the P(O)(N) n (O) 3-n (n = 1, 2, 3), P(O)(N) m (O) 2-m X (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].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 .
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 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)NH 2 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.

Synthesis and purification of phosphoramidates and phosphoric acid esters
The reaction of phosphorus(V)-halogen compounds of the type P(O)X 3-n Y n (X = halide, Y = another group such as amide, alkoxide and so on, and n = 0, 1 and 2) with primary or secondary amines leads to the formation of phosphorus(V)-nitrogen compounds.The promotion of this reaction needs to the presence of an excess amount of amine as an HX scavenger or the presence of another acid scavenger such as tertiary amines [61] or pyridine [8] (Scheme 4).In this strategy, removing of the hydrohalide salt of the organic base is a challenging task in the purification process.
The purification may be performed by stirring the crude product in water to remove the amine hydrohalide or pyridinium halide and/or may be done by selecting the solvent 559 Scheme 3. The classification of compounds having a P(O)(N) n (O) 3-n (n = 1, 2, 3) skeleton 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 be performed by stirring the crude product in a diluted hydrochloric acid [65].
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 filtrate under reduced pressure [66].
or Another strategy for preparation of phosphoramidate compounds is the application of sodium amide salts which produces sodium halide as a by-product [82] (Scheme 4).
The P-N bond formation between an amide, of the type RC(O)NH 2 , and a phosphorus(V) site may be performed via a two-stages reaction, which is shown for PCl 5 in Scheme 5, showing the reaction of PCl 5 with an amide and then the treatment of HCOOH.Moreover, a few efforts have been devoted to the synthesis of RS(O) 2 NHP(O)Cl 2 by a similar procedure [23].The simple mentioned methods for the preparation of phosphoramidates from the reaction of phosphorus-chlorine compounds and amines may be extended to the diamines or amino alcohols to produce cyclic [83] or bridged compounds [61].

Crystallization of phosphoramidates
The convenient solvents for obtaining suitable single crystals for the studied compounds may be CH The crystal may be obtained at room temperature after slow evaporation of the solvent.

Compounds with formula RC(O)NHP(O)[NR 1 R 2 ] 2 and RC(O)NHP(O)[NHR 1 ] 2
The four different groups linked to the P atom result in a distorted tetrahedral configuration; as one instance, the bond angles around the P atom of 2 range from 102.67 (12)° to 117.60(12)° [87].In the C(O)NHP(O) moiety, however, the carbonyl and phosphoryl groups are separated from each other with one N-H unit, but the terms syn, gauche and anti were used for the description of the C(O) orientation versus P(O) in the literatures [10,26].Up to now, both gauche and anti orientations were found for phosphoric triamide compounds having a C(O)NHP(O) skeleton, respectively in 14 and 98 structures (from the 119 structurally reported compounds, 7 cifs are not available).Among them, for the acyclic compounds of the type RC(O)NHP(O)[NHR 1 ] 2 , merely, the anti orientation was reported, so far.In 1,3-diazaphosphorinane compounds, the P=O bond is placed in an equatorial position and the aliphatic six-membered rings adopt conformation between chair and envelope (Fig. 3).
www.intechopen.com1.In the reported compounds, the nitrogen atoms bonded to P don't involve in hydrogen bonding interaction as an acceptor (due to their low Lewis base characteristic).Scheme   If the remaining N amide -H unit is involved in an intramolecular hydrogen bond with the oxygen of carbonyl, the R 2 2 (10)/S 6 -graph-set is formed, in this case the oxygen atom of carbonyl acts as a double-H acceptor.In the case of involving this N-H unit in the H-bonding interaction with the oxygen of phosphoryl, the R 2 2 (10)/R 2 1 (6) is formed (Scheme 12). 7.In a solvated molecule [90], the hydrogen-bond pattern may not be predictable, but some previously mentioned rules may be beneficial, Scheme 13.
Scheme 17.The structurally investigated compounds of the formula [R 1 R 2 N] 3 P(O) or [R 1 R 2 N] 3 P(O).B, where B is a hydrogen-bonded species to phosphoric triamide (the related amido moieties and the CSD refcode are presented) In the crystal packing of molecules having a P(O)(NH) 3 skeleton, hydrogen bonded 1-D chain, 1-D ladder, 2-D layer and 3-D arrangements were found.Three different types of 1-D arrangement are formed respectively through a P=O…(H-N) 3 or P=O…(H-N) 2 groups or via the P=O…H-N hydrogen bond.In the two latter cases, respectively one and two N-H units don't cooperate in the hydrogen bond interaction.One example of a linear arrangement, involving the N-H•••O and N-H•••N HBs, is also found in the structure of P(O)(NH-C 5 H 4 N) 3 (LAFNAI) [100] in which the pyridine nitrogen atom is involving in the HB pattern as an acceptor, too.As, the phosphoryl oxygen atom may cooperate in H-bonding interaction as a double-or a triple-acceptor, some examples of 2-D and 3-D arrangements have also been found in this class of compounds.

Some phosphoric triamides
where merely tertiary nitrogen atoms exist in the structure of molecule) have been reported (for example see: Scheme 22 [110]).Structures with a P(O)(N) 3 skeleton, where "N" is a tertiary nitrogen atom, do not show any classical (normal) hydrogen bonding in their crystal packing if the substituent involving the N atom doesn't contain the hydrogen linked to an electronegative atom.
www.intechopen.comA search on the CSD shows that the nitrogen atoms bound to phosphorus in phosphoramidate compounds aren't involved in normal H-bonding interaction as an acceptor due to the deviation of each N atom environment from pyramidality after binding to P and decreasing its Lewis base character with respect to the initial amine; so that, merely one example (refcode: HESCEO [111], Scheme 23), belonging to the diazaphosphorinane family, is observed so far with the donor…acceptor (N…N) distance of 3.258(8) Å in which it may be considered as a weak N-H…N-P hydrogen bond.
In compounds having a P(O)(NH)(N) 2 skeleton, only the P(O)NH unit cooperates in a HB interaction; so, two expectable HB patterns are the H-bonded dimer (with C i symmetry; a dimer with C 1 symmetry has not reported, so far) and the 1-D chain (Scheme 10).Usually, www.intechopen.comRefcode DIYMED [112] If the P(O) adopts an anti orientation versus NH, an extended 1-D chain arrangement is expectable through the intermolecular PO•••HN hydrogen bond which is found for the most of reported compounds.One example without any N-H•••O HB (BIFDUP [113], Scheme 25) and one example as H-bonded tetramer (XAVXEY, Scheme 26) were also found.
Scheme 29.Refcodes MIFYIJ (top) [114] and IKASAP (bottom) [47] www.intechopen.com Advances in Crystallization Processes 578 Scheme 30.Refcodes BIXFOE (top) [115] and GOMDOB (bottom) [116] Compounds with formula ( The tetrahedral configuration of phosphorus atom is significantly distorted as it has been noted for the other phosphoramides and their chalco-derivatives [117].For example, the bond angles around the P atom of (4-CH 3 -C 6 H 4 O)(C 6 H 11 NH) 2 P(O) [116]  Similar to the other phosphoramidates, each N atom bonded to phosphorus doesn't involve in any HB as an acceptor, showing its low Lewis-base character.In most cases, it has a nearly planar environment [26].Of course, the nitrogen atom' environment of some substituents, such as aziridinyl, like for example in the compound with refcode GOMDOB [118] (Scheme 30 (bottom)) shows some deviation from planarity, but such N atom doesn't cooperate in hydrogen bonding interaction, too.Moreover, the oxygen atom of the phenoxy or alkoxy groups in the 47 structures with an (O)P(=O)(NH)(N) skeleton (like for example in 4-CH 3 -C 6 H 4 OP(O)[N(CH 3 ) 2 ][NHC(CH 3 ) 3 ], GUDGIW: [119]) doesn't cooperate in the HB interaction, as it can not compete with the phosphoryl oxygen atom for H-accepting from the unique H-donor site in the molecule.Furthermore, among the 106 deposited structures with an (O)(O)P(=O)(NH) skeleton, only the structure of  1.In none of the reported structures, the nitrogen atom doesn't cooperate in HB interaction as an acceptor.2. Almost in all of the compounds having the (O) 2 P(=O)(NH) and (O)P(=O)(NH)(N) skeletons, the oxygen atom of the phenoxy (or alkoxy) group doesn't cooperate in the HB interaction, as it can't compete with the phosphoryl oxygen atom to H-accepting the unique H-donor site in the molecule.There is only one example of hydrogen bond of the type N-H...O(R) in this family of compounds in one compound containing some H-acceptor centers in addition to one phosphoryl group.3. The oxygen atom of OR moiety in some examples of compounds with a higher number of H-donor sites relative to the H-acceptor centers, such as compounds containing an (O)P(=O)(NH) 2 skeleton, however, it has a lower H-acceptability than the phosphoryl oxygen atom, is enforced to involve in the HB interaction.4. In compounds having an (O)P(=O)(NH) 2 skeleton, the better H-acceptability of the phosphoryl O atom than that of the RO moiety, in some cases, leads to act it as a double-H acceptor.

Amidophosphoric acid and amido phosphate compounds
Compounds with refcodes PHOXBP (Scheme 32, left) [124] and TMPMET (Scheme 32, right) [125] are respectively the examples of an acid and a solvated acidic-salt belonging to the www.intechopen.comIn the hydrated zwitterionic compound shown in Scheme 33 (refcode: GAHFUS) [126], the N + which is lack of the lone electron pair doesn't involve in H-bonding interaction; whereas, the [P(O)(O)] -unit cooperates in some N-H…O and O-H…O hydrogen bonds (Fig. 6).Compounds with refcodes IGASUF (Scheme 34) [127] and WIYFAL (Scheme 35) [128] are respectively a simple phosphate salt and an HCl-water absorbed phosphorus-nitrogen compound in which the Cl -ion is hydrogen-bonded to the N-H units of two-neighboring phosphoramidates.

Crystallographically independent molecules and ions
Different orientations resulting from non-rigid units in some molecules and ions and the presence of different H-bonds or the other short contacts may result in two or more conformers (or symmetrically independent molecules (or ions)) in solid state.Compound C 6 H 5 C(O)NHP(O)[NH(tert-C 4 H 9 )] 2 , exists as two conformers in crystalline lattice (which are detectable in solution, too by NMR experiment) [27].They are due to different spatial orientations of tert-butyl amido groups.One of the two conformers has two NH units (of tert-butyl amido moieties) which are syn, but not in the other.Another example is the presence of disorder in the cyclic amido moiety.For example, C 6 H 5 C(O)NHP(O)[NC 4 H 8 ] 2 appears as two crystallographically independent molecules [55].This is based on the conformational forms of the pyrrolidinyl groups and the orientation of the phenyl ring.The dimmeric aggregate in this case, between two independent molecules, is not centrosymmetric.The structure of [NH 2 (C 6 H 11 )(tert-C 4 H 9 )][PO 2 Cl 2 ] consists of two symmetrically independent dichlorophosphate anions as well as cyclohexyl-tertbutylammonium cations [75].[79], there are three symmetrically independent trifluoroacetyl-N-(tert-butylamino) phosphate anions and three independent cations of tert-butyl-ammonium; one of the anion indicates disorder in the tert-C 4 H 9 moiety.There are some other examples of disordered components for the groups such as tert-C 4 H 9 , cyclopentyl and cyclohexyl etc. in the deposited cifs.

Some future aims and proposals
Phosphorus has a very deep and widespread chemistry and understanding its nature in the compounds is noticeable interest.This may be achieved through the study on collective behaviors of phosphorus compounds in point of view of their different aspects.
The structural investigations and the study on the hydrogen-bond patterns may help to predict the molecular packing from the molecular structure.Moreover, as the biological activity of phosphorus compounds is very important, finding a relationship between the structure and a biological property is beneficial.For example, as well-known, a biological property may be related to the three important factors: lipophilicity, electronic and steric parameters.Probably, part of these factors could be well-understand by considering the crystal structure' study of such compounds; the electronic parameters may be related to the nature of chemical bonds or to the electron density or valence bond in different parts of the molecule.It is believed that, in the first step of interaction with acetylcholinesterase, the phosphoryl group is involving with the enzyme active site through a non-covalent bond; so, considering the non-covalent interaction of phosphoryl group with different atoms such as hydrogen helps to understand that the molecule how much could close to the enzyme active site.The steric parameters may be elucidated from the volume of molecule or considering the V(volume of unit cell)/Z(number of molecules in the unit cell).This may be in fact the practical volume which the molecule has, as the molecule usually cannot be closer to the neighboring molecules from this frontier boundary.
The solubility of molecules in different solvents must be checked in the crystal growth process; so, elucidation of lipophilicity is easy; however, the best method for measuring of this parameter is using of the spectrophotometer.And finally, the steric parameters can simply accessible through a structural study.
Preparation of phosphorus acids of the formula RP(O)(OH) 2 may develop the synthesis of the functionalized nano-phosphate materials and also polyoxometalate-based organic/inorganic hybrid compounds in which the phosphorus atom is trapping between the R group from one side and a cluster containing metal-oxygen framework in the other side.These acids may develop the extraction process of cations, too.
Application of phosphoryl donor ligands in preparation of oxo-centered clusters, in which their terminal ligands are replaced by phosphorus compounds, may be interesting for consideration.Preparation of single-enantiomer phosphoramidates by using a chiral primary or secondary amine is easy; it may extend the strategies for the synthesis of chiral phosphoramidates, phosphoric acids, nano-phosphates and so on.Synthesis of phosphoramidate-based hybrid compounds by using polyoxoanions may be valuable for spending the time on its consideration and experiment.Some of the wellknown hybrids contain the molecule-cation components of the type [B-H…B] + , where the B may be a base such as amide.Designing of such molecule-cation pairs with phosphoramidates, [PO-H…OP] + , may extend the experimental data about the 31 P-31 P coupling constant through the hydrogen-bond.
The NMR experiments on phosphoramidate-based compounds may develop the study on coupling constants of phosphorus and the other atoms, such as 2 J( 31 P-127 Tl) or 2 J( 31 P-39 K).These values may apply to evaluate the strengths of P=O-Tl or P=O-K bonds in their complexes.
We wish to develop the spectroscopic features and chemical calculations on phosphorus compounds, preparation of N-deuterated compounds in order to a good assignment of IR and Raman absorption bands, collecting the NMR data such as chemical shifts and short and long-range coupling constants, and finally chemical calculations on hydrogen-bonded molecules in the crystals.

Conclusion
In this chapter, the common methods for the synthesis and crystallization of phosphoramidates, their molecular structural features and the hydrogen-bond patterns and strengths were reviewed; the important structural aspects may be classified as follows: 1.The four different groups linked to the P atom result in a distorted tetrahedral configuration.-fragment, similar P-N bond are shorter than the two others.3. The nitrogen atom of the P(O)N unit has a sp 2 character and virtually doesn't involve in hydrogen-bond pattern as an H-acceptor.4. In the C-O-P(=O) fragment, the oxygen of phosphoryl is a better H-acceptor than the other oxygen atom; the C-O-P angle is about 120º. 5.In the diazaphosphorinane ring, the P=O bond is placed in an equatorial position.6.The hydrogen bond in a neutral phosphoramidate is of the type polarization-assisted hydrogen bond; whereas, in the proton-transfer and phosphate compounds two factors help to strength of hydrogen bond: polarization-assisted and charge-assisted.

Fig. 1 .
Fig. 1. indicates a general view of compounds with formula CF 3 C(O)NHP(O)[NHR] 2 .In the C(O)NHP(O) moiety, the P-N bond is longer and the O-P-N angle is contracted compared with the respective values in the [P(O)NHR] 2 section.For the phosphoramidate compounds, each N atom bonded to phosphorus has a sp 2 character which is reflected in the C-N-P angles of the C(O)NHP(O) or C-NH-P moiety or sum of the surrounding angles around the tertiary nitrogen atom (C-N-C + C-N-P + P-N-C).The deviation of this summation from 360º (to a lower value) has been used to show the deviation of nitrogen atom environment from planarity.This may be also illustrated with the distance between the position of N atom from the plane crossing from the directly attached atoms to nitrogen, i.e.C, C and P. In [C 6 H 5 O] 2 P(O)[NC 4 H 8 N]P(O)[OC 6 H 5 ] 2 (Fig.2) which belongs to the amidophosphoric acid ester family, the N atom shows some deviation from planarity and it is 0.25(1) Å above (or below) the CCP plane[61].For the phosphoramidate compounds, the P-N bonds are shorter than the P-N single bond and the P=O bond are longer than the normal P=O bond[88].

Fig. 1 .
Fig. 1.A typical view for a compound with formula CF 3 C(O)NHP(O)[NHR] 2 [Color key: O atoms are red, the N atom of C(O)NHP(O) is light blue, the other amido N atoms are dark blue, F atoms are yellowish green., C and H atoms are light grey and P atom is orange]; the R substituents are shown as big balls.

Fig. 2 .
Fig. 2. A view of the PCC mean plane which is crossed from the phosphorus and carbon atoms shown as balls in the right-side [C 6 H 5 O] 2 P(O)(NC 2 H 4 ) moiety of [C 6 H 5 O] 2 P(O)[NC 4 H 8 N]P(O)[OC 6 H 5 ] 2 (the molecule is organized around an inversion center located at the centre of the piperazine ring), the N atom environment shows some deviation from planarity.The balls representation denote to the P (orange), N (blue) and C (grey) atoms.

Fig. 3 .
Fig. 3.A general view of a 1,3-diazaphosphorinane, a six-membered ring heterocyclic phosphorus compound (the carbon-bonded H atoms were omitted for clarity).The grey big ball in the figure may be RC(O)NH, RNH or the other moieties.The hydrogen bond pattern of compounds having the C(O)NHP(O)(N) 2 and C(O)NHP(O)(NH) 2 skeletons may be predictable with considering the following "empirical rules": Scheme 9.The possible H-donor sites and H-acceptor centers in the C(O)NHP(O)(N) 2 and C(O)NHP(O)(NH) 2 skeletons (the R, X or Y groups may also be contained the additional Hdonor or H-acceptor sites in their structures which may be involved in the H-bond pattern, the curved arrow shows that the orientation of C=O versus P=O may change)

R 1 =
Scheme 10.The observed hydrogen bond patterns in compounds having a P(O)NH skeleton (such as C(O)NHP(O)[N] 2 )

Scheme 23 .
Scheme 23.One N atom of the diazaphosphorinane ring cooperates in hydrogen bonding interaction as an H-acceptor[111]

Fig. 6 .Scheme 34 . 2 O
Fig. 6.Fragment of the crystal packing of the hydrated zwitterionic compound with refcode GAHFUS [126] showing the involvement of [P(O)(O)] -units in N-H…O (blue dotted lines) and O-H…O (black dotted lines) hydrogen bonds, two symmetrically independent zwitterionic compounds in the structure are shown as blue and green apart the oxygen atoms of [P(O)(O)] -units which are shown as red balls, the water molecules are represented with grey color.The cyclohexyl groups are shown as balls (green and blue).

2 .
In the C(O)NHP(O) unit, the P=O is a better H-acceptor than the C=O counterpart, moreover, the anti orientation of P=O versus C=O is more common than the gauche orientation; in acyclic compounds with formula RC(O)NHP(O)[NHR'] 2 , a gauche situation has not been reported, so far.In the C(O)NHP(O)[N] 2 fragment, the P-N bond of the C(O)NHP(O) moiety is longer than the two other P-N bonds; whereas, in the [C(O)NP(O)[N] 2 ] For a bulky amine such as iso-propylbenzyl amine or di-cyclohexyl amine as a nucleophile, it seems that a totally-dried solvent is better; of course, it needs to approve with further experiments.
[69]me 4. A common route for the synthesis of phosphoramidatesThis method may be developed to the reaction between phenols and phosphorus-chlorine compounds.Selecting of a suitable solvent, which triethylamine hydrochloride or the other salt is low-soluble, develops the synthesis of some initial phosphorus-chlorine compounds such as [(CH 3 )(C 6 H 11 )N]P(O)Cl 2[67], [4-CH 3 C 6 H 4 NH]P(O)Cl 2[68], [C 6 H 5 O][4-CH 3 C 6 H 4 NH]P(O)Cl[69]and so on.For example, para-toluidine hydrochloride is relatively insoluble in CH 3 CN; so, the reaction of P(O)Cl 3 or [C 6 H 5 O]P(O)Cl 2 with 4-CH 3 -C 6 H 4 NH 2 (1:2 mole ratio) respectively leads to the formation of [4-CH 3 C 6 H 4 NH]P(O)Cl 2[68]and[C 6 H 5 O][4-CH 3 C 6 H 4 NH]P(O)Cl[69]which are soluble in acetonitrile, whereas para-toluidine hydrochloride is simply filtered off.starting material (Y = amide, alkoxy, phenoxy and so on) is very reduced to the moisture and the solvent which was dried with a moderate desiccant (such as CaCl 2 ) is good for the www.intechopen.com [36]hows N-H...O(CH 3 ) not N-H...O(P) hydrogen bond (in this consideration, some structures with unavailable cifs were not enumerated).So, such compounds [if the substituents linked to the N or/and O atoms don't contain any H-acceptor or H-donor centers] may be almost always considered as compounds with "one H-acceptor (the oxygen of phosphoryl) and one H-donor sites", both in the P(O)NH group.The oxygen atom of OR moiety in some examples of compounds with a higher H-donor sites, such as compounds containing an (O)P(=O)(NH)(NH) skeleton, however, it has a lower H-acceptability than the phosphoryl oxygen atom, is enforced to involve in the HB interaction ([4-CH 3 -C 6 H 4 O]P(O)[NHC 6 H 4 (4-CH 3 )] 2 : MUBPIJ,[36]).The better Hacceptability of the phosphoryl O atom than that of the RO moiety, in some cases for example in[C 6 H 5 O]P(O)[NHC 6 H 11 ][NHC 6 H 4 (4-CH 3 )] (ERUFIH: [69]), leads to act it as a double-H acceptor.In the molecular packing of [C 6 H 5 O]P(O)[NHC 6 H 11 ] 2 .CH 3 OH (HIVLOO, [