Phosphorus-nitrogen compounds (Part 51): the relationship between spectroscopic and crystallographic data of mono- and di- spiro cyclophosphazene derivatives with 4-fluoro/nitrophenylmethyl pendant arm/arms

A great wealth of structural information about phosphazenes can be gleaned from the combined spectroscopic and crystallographic data. When data from 31P NMR spectroscopy and X-ray crystallography are put together like pieces in a puzzle, a number of correlations can be obtained for phosphazene derivatives. A systematic study concerning the correlations among the structural parameters (e.g., 31P NMR data, endocyclic/exocyclic NPN bond angles and bond lengths) revealed some characteristics of mono- and di- spiro cyclophosphazene derivatives bearing 4-fluoro/nitrophenylmethyl pendant arm/arms. These correlations include the relationship between the δ Pspiro shifts, the values of electron density transfer parameters Δ(P–N), and the endocyclic and exocyclic NPN bond angles of the cyclophosphazenes. The structural parameters were compared with each other for 19 compounds of 5 different architectural types of cyclophosphazenes with 5- to 7-membered spiro -rings.

Although a large number of papers published by our research group are available on cyclophosphazenes that provide information on their structures, synthesis, and biological activities; the present study focuses on correlation among the structural parameters of mono-and di-spirocyclophosphazene derivatives with 4fluoro/nitrophenylmethyl pendant arm/arms. In 1986, a systematic study on the relationship between the crystallographic and 31 P NMR spectral data on phosphazenes was described for the first time by Shaw [56]. Our group has published many studies on the correlations among the structural parameters of various types of cyclotriphosphazenes bearing structurally analogous motifs. It was found out that in cyclotriphosphazene derivatives, variations in the 31 P NMR shifts depend primarily on the electronic, steric and conformational factors (e.g., electron-releasing and withdrawing powers of substituents, the steric hindrance between the exocyclic groups), and on the differences in the bond lengths and bond angles around the phosphorus atoms, particularly endocyclic ( α) and on exocyclic ( α ′ ) bond angles. As a particular interest in our ongoing studies on phosphazene-based chemistry, the present study primarily focuses on a number of correlations established among the structural parameters in mono-and di-spirocyclophosphazene derivatives with 4-fluoro/nitrophenylmethyl pendant arm/arms of the compounds previously synthesized and published by our research group (Table 1) [49][50][51][52][57][58][59]. In this context, here we report our findings on the relationship among the δ P spiro shifts with endocyclic and exocyclic NPN bond angles, and electron density transfer parameters, and a brief description of the synthesis methods of 5 types and a total of 19 cyclotri/tetraphosphazenes containing 4-fluoro/nitrophenylmethyl pendant arm and 5-to 7-membered spiro-rings.

Correlation among the structural parameters
The endocyclic ( α) and exocyclic ( α ′ ) NPN bond angles, and the bond lengths (a, a ′ , b, and b ′ ) on the general formulae of cyclotri/tetraphosphazenes containing 4-fluoro/nitrophenylmethyl pendant arm/arms and 5-, 6-and 7-membered spiro-ring/rings are given in Table 1. The δ P spiro shifts, α and α ′ bond angles, and ∆ (P-N) values are listed in Table 2. The corresponding values for the δ P spiro shifts of the standard compounds trimer N 3 P 3 Cl 6 [60,61] and tetramer N 4 P 4 Cl 8 [62,63] were taken from the literature. Type I group members are partly and fully substituted mono(4-fluoro/nitrophenylmethyl)spiro-cyclotriphosphazenes.

∆(P-N)
The electron density transfer parameter ∆ (P-N) is the difference between the bond lengths of 2 adjacent endocyclic P-N bonds and is a measure of the electron-releasing and withdrawing powers of the substituents on cyclophosphazene ring. The ∆(P-N) values were calculated using the appropriate equations presented in Table   2 for values is given in Figure 1 for partly and fully pyrrolidine and benzylamine substituted spirocyclic phosphazenes. The linear correlation between δ P spiro shifts and ∆ (P-N) values observed in 3 groups of cyclophosphazenes are given in Figure 1. When comparing partly substituted types I, II, and III phosphazenes (a) with the fully pyrrolidine substituted type I phosphazenes (b), an inverse relation is observed in Figure 1.  [52]. As expected, the ∆ (P-N) value of benzylamine substituted t-Vc is larger than the value of ∆(P-N), which is zero, for the standard compound N 4 P 4 Cl 8 .

, intermolecular C-H-F for t-Vb and intramolecular C-H-N for c-Vb
The Y group (F or NO 2 ) placed at the para position on the benzene ring is an electron-withdrawing substituent and does not cause a significant change in the ∆(P-N) values. However, the points of the NO 2containing compounds (Ib and Id) slightly deviate from the linear trend ( Figure 1).
Considering the electron-releasing capacity of the 4-fluorophenylmethyl pendant group for type I-III partly substituted cyclotriphosphazenes with 6-membered spiro-ring, the following order is established: IIIa >IIb >Ic. While the compounds Ia and IIb are mono-and bis-4-fluorophenylmethyl spiro-structures, respectively, compound IIIa is bis-4-fluorophenylmethyl di-spiro structure. As expected, the electron-releasing strength of 2 4-fluorophenylmethyl pendant groups is more than that of 1 4-fluorophenylmethyl pendant group. However, the same trend is not observed for 5-membered Ia and IIa. This is due to the fact that Ia has 2 independent molecules in the asymmetric unit [50].
There is no significant difference between the ∆ (P-N) values of type II phosphazenes containing the spirorings with 6-(IIb) and 7-(IIc) membered. However, the ∆(P-N) values of the phosphazene with 6-membered spiro-ring (IIa) is slightly larger than that of the phosphazene with 5-membered spiro-ring (IIb). That could be significantly attributed to the fact that 5-membered spiro-ring of IIa is in the twisted conformation and 6-membered spiro-ring of IIb is in the chair conformation [51].
The relationship between ∆ (P-N) and δ P spiro shifts strongly indicates the basicity of the nitrogen atoms in the phosphazene ring. The basicity of the chlorocyclophosphazene ring containing nitrogen atoms is quite low, and it can be improved by replacing Cl-atoms with electron-releasing substituents on phosphorus.
Therefore, the basicity of the nitrogen atoms on the cyclotriphosphazene ring, which is both adjacent (N2-P spiro ) and nonadjacent to the spiro-ring (N1-PX 2 ) in fully pyrrolidine substituted cyclotriphosphazenes can be compared with those in partly substituted ones. The basicity of the N1 atom/atoms in fully substituted phosphazenes appear(s) to have increased due to electron-releasing power of the heterocyclic amine groups.
However, N 2 atoms in partly substituted phosphazenes decreased due to electron-withdrawing power of the Clatoms. Nevertheless, protonation of type I heterocyclic amine substituted free cyclotriphosphazene bases with bulky organic acids (gentisic and γ -resorcylic acids) took place on the N2-atom [49] (type V) instead of N1atom [54,55] of the 4-fluorobenzylspirocyclotriphosphazenes. The H + ion may be exchanged between the N1and N2-atoms of the cyclotriphosphazene ring in the solution at ambient temperature. The 31 P NMR spectra recorded at low temperatures and the observed spin-systems in the 31 P NMR spectra of the PMOSs may also confirm that the H + ion can be displaced between the nitrogen atoms of the phosphazene ring. Although the number of type V group members is limited, they could be thought of as reference compounds. It appears that the δ Pspiro shifts and the basicity of the ring decrease after PMOS forms.
The double-bond character of the P-N linkage in the cyclophosphazene derivatives is not fully understood.
Negative hyperconjugation and ionic bonding alternatives are exclusive [64]. The natural-bond orbital and topological electron-density analyses of the phosphazenes have proved the crucial role of negative hyperconjugation in description of the P-N bond. An increase in the electron-releasing power of heterocyclic amine substituents seems to cause an increase in the negative hyperconjugation. The electron −withdrawing substituents such as Cl-atom increase the ∆(P-N) values since they attract electrons from spiro-ring/rings to the phosphorus atom.
However, the electron-releasing substituents such as pyrrolidine group decrease the ∆(P-N) values resulting in decreased bond lengths (a and a ′ ) and increased bond lengths (b and b ′ ) when the bond lengths of partly substituted derivatives are compared. Hence, the decrease in the length of the endocyclic P-N bonds and in electron charge density on the exocyclic P-N bonds are likely to be a measure of the electron-releasing power of the substituent and the increase in negative hyperconjugation.  bond angles [60] in the standard compound, N 3 P 3 Cl 6 . Besides, there is a decrease in δ Pspiro shifts with increasing number of members in the spiro-ring. For example, the α ′ bond angles of partly substituted Ia with 5-membered spiro-ring (δ Pspiro = 19.22 ppm, cycle A) and Ic with 6-membered spiro-ring (δ Pspiro = 14.34 ppm, cycle B) are respectively; 95.46 (15) and 94.97 (17), and 103.9 (2). This indicates that the electron-releasing power of 5-membered spiro-ring to the phosphazene ring is more than that of the 6-membered spiro-ring. When partly and fully pyrrolidine substituted type I phosphazenes with the same number of members in the spiro-ring (cycles A and C or cycles B and D) are compared, it is seen that the δ Pspiro shifts increase for fully substituted ones. While the α ′ bond angles decrease, the α bond angles increase. This indicates a change in the substituent groups leading to significant changes in both α and α ′ bond angles. When considering α bond angles, electrons are transferred from pyrrolidine groups to the cyclotriphosphazene ring in the fully substituted derivatives and from the cyclotriphosphazene ring to Cl-atoms in partly substituted counterparts. When taking into account the α ′ bond angles, pyrrolidine groups also release electrons to the phosphazene ring, but the Cl-atoms withdraw electrons not only from the phosphazene ring but also from the spiro-ring. The elongation of the 2 exocyclic P-N bonds of the spiro-ring is likely the best measure of the electron-withdrawing power of the Cl-atoms and the decrease in negative hyperconjugation.

The relationship among the
On the other hand, in tetrameric phosphazenes, the α bond angle of fully benzylamine substituted 6membered phosphazene (Vc) is larger with respect to the value of partly substituted counterpart (Vb). But, the α ′ angle of Vc is larger than the α ′ angle of Vb. This situation may be attributed to the basicity or electronreleasing power of benzylamine substituent, which is a secondary aliphatic amine group after the substitution, in Vc not as high as pyrrolidine substituent, a tertiary heterocyclic amine group after the substitution. When compared α and α ′ bond angles of type I free phosphazene bases (cycle C) and type IV PMOSs (cycle E) with 5-membered spiro-ring, it is observed that the formation of PMOSs of free phosphazene bases results in a decrease in the α bond angles, and increase in the α ′ bond angles. In fact, the α ′ bond angles of PMOSs (IVa and IVb) are even larger than the corresponding angles in partly substituted cyclotriphosphazenes (cycle A), and the standard compound N 3 P 3 Cl 6 , indicating that the positive charge on the N2-atom withdraws electrons from the 5-membered spiro-ring in PMOSs.
Besides, the α and α ′ angles of cis-and trans-structures of the type V cyclotetraphosphazenes with 2 6-membered spiro-rings (c-Vb and t-Vb) can be compared with each other. The α and α ′ angles of c-Vb are considerably and slightly larger than those of t-Vb, respectively. That could be significantly attributed to the fact that the N 4 P 4 ring of t -Vb has a twisted conformation and the N 4 P 4 ring of c-Vb has a boat conformation [52].

Conclusions
The results of a systematic study of spiro-cyclotri/tetraphosphazenes with 4-fluoro/nitrophenylmethyl pendant arm on the basis of correlation between the structural parameters were presented. The main parameters were obtained from X-ray crystallography and 31 P NMR results in order to investigate the relationship between the δ P spiro shift values and endocyclic and exocyclic NPN bond angles, and electron density transfer parameters. The correlations obtained from the present study ought to be considered as highly informative. Although there are visual comparisons for assessing the accuracy of the relationships, more values are required to learn more about the correlations for cyclophosphazenes. In this approach, our research group or one can plot on the same relationships the new values of the other members of mono-and di-spirocyclophosphazene derivatives Zeynel Kılıç thanks Turkish Academy of Sciences (TÜBA) for partial support of this work.