Phosphorus-nitrogen compounds- (Part 50): correlations between structural parameters for cylophosphazene derivatives containing ferrocenyl pendant arm(s)

The results of a systematic study of spiro -cyclotri/tetraphosphazenes with ferrocenyl pendant arm on the basis of correlation between structural parameters were presented. The main parameters were obtained from Xray crystallography and 31P NMR results in order to investigate the relationship between the δ Pspiro shift values and endocyclic and exocyclic NPN bond angles, and electron density transfer parameters. Structural parameters derived from 11 types of the ferrocenyl cyclophosphazene derivatives with 5- to 7-membered spiro -rings introduced to the literature from our research group were studied and compared with each other.


Introduction
The phosphazene chemistry has attracted much attention since 1960 [1,2]. Especially, hexachlorocyclotriphosphazene (N 3 P 3 Cl 6 , trimer) and octachlorocyclotetraphosphazene (N 4 P 4 Cl 8 , tetramer) are of particular interest to both theoretical and experimental researchers concerning phosphazene-based chemistry. Because of their tendency to react with the nucleophilic mono-, di-, or multi-functional groups [3][4][5][6], both of the cyclophosphazenes were used in the syntheses of a considerable range of organocyclotri/tetraphosphazene derivatives with diverse applications [7,8]. The substantial efforts have been performed on the nucleophilic substitution reactions, in which the 1-to 6-Cl-atoms on trimer and 1-to 8-Cl atoms on tetramer have been replaced by the NH and/or OH functioned reagents, forming isomeric products e.g., structural (spiro-, ansa-and bino-architectures or a mixed of the same or different architectures), geometrical (geminal, non-geminal cis/trans-), and optical isomers [9,10]. The nature of the products strongly depends on the various chemical factors which control the replacement reaction mechanisms such as chain lengths of nucleophilic groups, the polarity of solvents, and the reaction temperature [11]. So far only a limited number of published studies on cyclophosphazene derivatives with ferrocenyl pendant arm is present in the literature [12][13][14][15][16].
Besides, the chiral properties of mono Van-substituted dispiro-bis ferrocenyl cyclophosphazenes were investigated by 31 P NMR spectroscopy upon the addition of the chiral solvating agent [39].
On the other hand, we also succeeded in the preparation of ultrathin and highly ordered Langmuir-Blodgett films of tetrachloro-, and mono and gem DASD-substituted mono-ferrocenyl cyclotriphosphazenes [40,41]. These compounds are the first phosphazene derivatives prepared as thin films in the literature.
Shaw described the first systematic study of the relationship between the bond angles around the phosphorus atoms and 31 P NMR spectral data in phosphazene derivatives [42]. The changes in structural parameters for different kinds of structurally analogous cyclotriphosphazenes (cyclotriphosphazenes possessing 6-membered spiro ring/rings [43], monospiro-, dispiro-, spiro-ansa-spiro-and spiro-bino-spiro-cyclotriphosphazenes [44][45][46], spiro-cyclotriphosphazenic lariat (PNP-pivot) ether derivatives [47,48], monotopic and ditopic spiro-crypta cyclotriphosphazenes [49][50][51]) were investigated previously. It was found that the systematic variations in the 31 P NMR chemical shifts depend fundamentally on some electronic (electron-releasing and electron-withdrawing capacities of substituent groups), steric (the steric hindrance between the exocyclic groups) and conformational factors, and on the changes in bond lengths and bond angles around the phosphorus atoms [especially endocyclic ( α) and exocyclic ( α ′ ) bond angles] in cyclotriphosphazene derivatives. The current study deals with a number of correlations between structural parameters [e.g., 31 P NMR spectral data and X-ray crystallographic data (endocyclic and exocyclic NPN bond angles, and bond lengths)] in spirocyclic ferrocenyl cyclophosphazenes introduced to the literature from our research group (Table 1) [33][34][35][36][37][38][39][40][41]52]. Therefore the content of this report includes: (i) a brief description of the synthesis methods of 11 different structural types and a total of 28 spirocyclic ferrocenyl phosphazenes with 5-to 7 −membered spiro-rings used for the graph construction, (ii) the relationship between the δ P spiro shifts and the values of electron density transfer parameters ∆(P-N), and (iii) the correlation of δ P spiro shifts and endocyclic ( α) and exocyclic ( α ′ ) NPN bond angles of the compounds. Table 1. The endocyclic ( α) and exocyclic ( α ′ ) NPN bond angles and bond lengths (a, a ′ , b, and b ′ ) on the formulae of cyclophosphazenes.

Syntheses
Routes for the synthesis of spirocyclic ferrocenyl cyclophosphazenes clarified their solid-state structures using X-ray crystallography by our research group and investigated in this study are summarized in Scheme. The syntheses of mono and bisferrocenyl diamines, as the starting compounds, were carried out according to the published procedures, in which ferrocenecarboxaldehyde reacted with appropriate diamines and followed by reduction of the azomethine bonds in the intermediate products [53,54]. The reactions of trimer with mono and bisferrocenyl diamines gave partly substituted spiro-mono (I) [33] and spiro-bis (V) [33,52] ferrocenyl cyclotriphosphazenes, respectively. The substituted phosphazene derivatives were synthesized by stepwise substitutions of partly substituted spiro-mono (I) and spiro-bis (V) ferrocenyl cyclotriphosphazenes which consist of 4 reactive P-Cl units. The reactions of 1 equimolar amount of partly substituted spiro-bis (V) and spiromono (I) ferrocenyl cyclotriphosphazenes with 1 and 2 equimolar amounts of heterocyclic amines (DASD and Pyr) produced corresponding mono heterocyclic amine (DASD) substituted spiro-bis (VI) [35] and spiro-mono (II) [40] and geminal heterocyclic amine (DASD and Pyr) substituted spiro-bis (VII) [35] and spiro-mono (III) [35,40,41] ferrocenyl cyclotriphosphazenes in the presence of NEt 3 in refluxing dry THF. The fully heterocyclic amine [DASD, Pyr, and morpholine (Morp)] substituted spiro-bis (V) [33] and spiro-mono (I) [33,35,52] ferrocenyl cyclotriphosphazenes were prepared by replacing 4 Cl-atoms on partly substituted derivatives (I) and (V), respectively, with excess heterocyclic amines in boiling THF. The reactions of equimolar amounts of partly substituted spiro-mono ferrocenyl cyclotriphosphaze (I) and potassium vanillinate were found to yield the corresponding mono Van-substituted spiro-mono ferrocenyl cyclotriphosphaze (II) as a major product and geminal (III) [37] and nongeminal (cis) (IV) substituted spiro-mono ferrocenyl cyclotriphosphazenes as minor products.
Fully Van-substituted spiro-bisferrocenyl cyclotriphosphazene (V) was synthesized from the reaction carried out with excess potassium vanillinate [37]. The Cl-replacement reactions of trimer with 2 equimolar amounts of mono-ferrocenyldiamines resulted in the formation of the corresponding partly substituted cis-(meso) and trans-(racem) dispiro-bisferrocenyl cyclotriphosphazenes (VIII) as the major products and spiro-mono (I) ferrocenyl cyclotriphosphazenes as minor products [36]. Three products were separated performing column chromatography. The reactions of 1 equimolar amount of cis-and trans-dispiro-bisferrocenyl cyclotriphosphazenes (VIII) having 2 reactive Cl-atoms with 2 equimolar amounts of potassium vanillinate in refluxing THF afforded the mono (IX) and fully (VIII) Van-substituted cis-and trans-dispiro-bisferrocenyl cyclotriphosphazenes (IX) and (VIII) [39]. The mono and fully substituted derivatives were separated using column chromatography. On the other hand, the partly substituted spiro-mono (X) [34] and cis-and trans-dispiro-bis (XI) [38] ferrocenyl cyclotetraphosphazenes were obtained from the reactions of tetramer with 1 and 2 equimolar amounts of monoferrocenyl diamines in dry THF. The fully Pyr-substituted (X) and trans-(XI) were prepared by the reaction of partly substituted ones with excess Pyr in dry THF at ambient temperature.

Correlations between structural parameters
The endocyclic ( α) and exocyclic ( α ′ ) NPN bond angles, and the bond lengths (a, a ′ , b, and b ′ ) were defined in the generalized structures for the 11 types of cyclotri/tetraphosphazenes containing ferrocenyl pendant arm/arms and 5-, 6-and 7-membered spiro-ring/rings shown in Table 1. δ P s piro shifts, α , and α ′ bond angles, and ∆ (P-N) values that are needed to be used for graph construction are listed in Table 2. The corresponding values of the standard compounds trimer (N 3 P 3 Cl 6 ) [55,56] and tetramer (N 4 P 4 Cl 8 ) [57,58] were taken from the literature. Types I and V members are partly and fully substituted spiro-mono and spiro-bisferocenyl cyclotriphosphazenes, respectively. Mono and geminal substituted spiro-mono/bisferocenyl cyclotriphosphazenes are the types II and VI, and the types III and VII group members, respectively. Nongeminal (cis) substituted spiro-monoferocenyl cyclotriphosphazene constitutes the type IV. Members of types VIII and IX derivatives comprise partly and fully substituted and monosubstituted cis/trans-dispiro-bisferocenyl cyclotriphosphazenes, respectively. spiro-Mono and trans-dispiro-bisferocenyl cyclotetrahosphazenes constitute the types X and XI compounds.
The concept of the double-bond character of the P-N linkage in the cyclophosphazene derivatives is still not clearly understood. Negative hyperconjugation and ionic bonding alternatives are exclusive [59]. The natural bond orbital and topological electron-density analyses of phosphazenes have proved the crucial role of negative hyperconjugation in the description of the P-N bond.

The correlation of δ P spiro shifts and values of electron density transfer parameters ∆(P-N)
The electron density transfer parameter ∆ (P-N) is the difference between the bond lengths of 2 adjacent endocyclic P-N bonds as defined in Table 2 for spirocyclic ferrocenyl phosphazenes. It shows a measure of the electron releasing and withdrawing capacities of the substituent groups on cyclophosphazene ring.
The relationship between the δ P spiro shifts and ∆(P-N) values is illustrated in Figure 1   for (I-VII), (X) and (XI)  (7) 104.06 (7) 22.10  (7) 102.01 (7) 1.  (14) Moreover, there is no significant difference between the ∆(P-N) values of cis-and trans-structures of the same compound for types VIII and IX phosphazenes (0.00825 for VIIIb, 0.002475 for VIIIc, and 0.01095 for IXa). However, the difference between the ∆ (P-N) values of cis-and trans-structures of the phosphazenes with 5-membered spiro-rings (VIIIb and IXa) is slightly larger than that of the phosphazene with 6-membered spiro-rings (VIIIc). That could be significantly attributed to the fact that 5-membered spiro-rings of c-VIIIb, t-VIIIb, c-IX and t-IX are in envelope conformation and 6-membered spiro-rings of c-VIIIc and t-VIIIc are in the chair conformation [36,39]. It can be seen from Figure 1i that there are greater changes in ∆ (P-N) values for types II and VI with 1 heterocyclic amine substituent per P atom, types III and VII with 2 heterocyclic amine substituents per P atom and types I and V with 4 heterocyclic amine substituents. Therefore, the ∆(P-N) values of these types phosphazenes can be compared with each other according to the number of heterocyclic amine substituents. As expected, the ∆ (P-N) value of mono substituted compounds is between the ∆ (P-N) value of partly (cycle A) and fully (cycle E) substituted phosphazenes, while geminal substituted derivatives except for VIIa (cycle D) have the ∆ (P-N) value between those of mono (cycle C) and fully (cycle E) substituted ones. The ∆ (P-N) value of VIIa appears to the left more than other geminal substituted derivatives (IIIa-IIIc) (cycle D) or is greater than those of the fully substituted derivatives (cycle E). This situation may be related to the higher basicity of the DASD substituent in VIIa. A similar relationship was observed between the ∆(P-N) values of nongeminal cis-(IVa) and fully (Vd) Van substituted cyclophosphazenes and partly substituted Ia and Va, respectively ( Figure 1ii). Furthermore, the ∆(P-N) values of fully heterocyclic amine substituted types X (cycle F) and XI cyclotetraphosphazenes and types I and V cyclotriphosphazenes, respectively, are quite close together.
Although the compounds IIIa and VIIa both have geminal structure and 7-membered spiro-ring, and are close in δ Pspiro shifts, the major difference in their ∆(P-N) values and basicities is that the phosphazenes contain mono and bis ferrocenyl pendant arms, respectively. On the other hand, based on the electron-releasing capacity of the ferrocenyl pendant group for partly substituted phosphazenes (cycles A and B), it has been made the following order: Type VIII >type V >type I. Type I (Ia), and type V compounds (Va and Vb) are mono-spiro mono and bis structures, while type VIII (t-VIIIa, c/t-VIIIb, and c/t-VIIIc) phosphazenes are di-spiro bis structures. As expected, the electron releasing powers of 2 ferrocenyl pendant groups are greater than those of 1 ferrocenyl pendant group. Moreover, in partly substituted phosphazenes (cycle A), the δ P spiro shifts of 7-membered Ia and Vb are close to each other while 6-membered Va has a lower δ P spiro shift.
In the case of 5-membered spiro-ring geminal (IIIb and IIIc) and 6-membered spiro-ring fully (Ib and Ic) substituted phosphazenes, the electron releasing capacity of DASD group is much larger than that of Pip and Pyr, respectively.
Besides, when the number of atoms increases in the spiro-ring, the electron releasing capacity of the phosphazene decreases. In general, the electron releasing power of the rings is in the following order: spiro-rings with 5-membered >spiro-rings with 6-membered >spiro-rings with 7-membered.
As a result, electron −withdrawing substituents, like chlorine group, increase ∆ (P-N) values, pulling away electrons from spiro-ring/rings to the phosphorus atom bonded to the electron −withdrawing groups. Whereas the electron-releasing substituents, like heterocyclic amines, decrease ∆ (P-N) values, resulting in decreased the bond lengths a and a ′ and increased the bond lengths b and b ′ when compared bond lengths of partly substituted derivatives. Hence, the shortening of the endocyclic P-N bonds and decreased electron charge density at the exocyclic P-N bonds is likely to be a measure of the electron-releasing power of the substituent and the increase in negative hyperconjugation.
The relationship between the ∆(P-N) and δ P spiro shifts makes sense in the basicity of the ring nitrogen atoms in phosphazenes. The basicity of the chlorocyclophosphazene ring nitrogen atoms is quite low, and it may be improved by replacing Cl-atoms with electron-releasing substituents on phosphorus. Thus, the basicity of the phosphazene ring nitrogen atoms (N1-PX 2 and N2-P spiro ) in fully substituted cyclotriphosphazenes with those in partly substituted ones can be compared. The basicity of N1 atom/atoms in fully substituted phosphazenes appears to have increased due to electron-releasing power of the heterocyclic amine groups, while N2 atom/atoms in partly substituted phosphazenes due to electron-withdrawing power of the chloro groups.
As a result, an increase in the electron-releasing power of heterocyclic amine substituents seems to bring about an increase in the basicity of the nitrogen atom (N1) and the negative hyperconjugation.

The correlation of the δ P spiro shifts, endocyclic ( α ), and exocyclic ( α ′ ) NPN bond angles
A cluster of points between the δ P spiro shifts and the endocyclic NPN bond angles ( α) [A, B, C, D, E, and F given in Figure 2i)] and a trend of approximate linearity between the δ P spiro shifts and the exocyclic NPN bond angles ( α ′ ) [(a), (b), (c), and (d) given in Figure 2ii] were observed.
The changes in α and α ′ bond angles show parallelism except for a contrasting trend observed for partly substituted types I and V cyclotriphosphazenes (cycle A) and fully substitute type X cyclotetraphosphazenes (cycle F). Small changes in α ′ bond angles lead to significant changes in δ P spiro shifts. The number of members in the spiro-ring seems to be effective on α ′ bond angles. In fact, the α ′ bond angles of cyclotriphosphazenes with 5-membered spiro-ring are narrower than those with larger 6-and 7-membered ones and even narrower  (Figure 2ii) due to its 6-membered spiro-ring, and the α ′ value of IIa close to the α ′ angle of the standard N 3 P 3 Cl 6 [101.2(1)°] [55]. As mentioned before, there is a difference between ∆ (P-N) values of the phosphazenes IIIa and VIIa having geminal structure and 7membered spiro-ring and nearly the same δ Pspiro shift values. The difference between the α ′ and α bond angles of both compounds is~3 and 1°, and this explains that the α bond angle is less sensitive to the electronic changes. When spiro-bisferocenyl Va and VIa cyclotriphosphazenes are compared, it is seen that the δ Pspiro shift value increases from 6.20 to 14.41 ppm by mono substitution, while the α ′ bond angle decreases from 105.0(2) to 102.85(11)°and the α bond angle increases from 110.0(2) to 114.42(13)°, respectively, indicating a change in substituent groups causes a major change in both α and α ′ bond angles. In fact, the values of α and α ′ bond angles of 7-membered partly substituted cyclotriphosphazene (Vb) are larger and smaller than those of the 7-membered heterocyclic amine substituted cyclotriphosphazene (Vc). Based on the electron-releasing capacities of the substituents for Vb and Vc, electrons are transferred from heterocyclic amine groups to the cyclotriphosphazene ring in Vc and from the cyclotriphosphazene ring to Cl-atoms in Vb. The α and α ′ bond angles of fully pyrolidine substituted cyclotetraphosphazenes (Xa and Xb) are close to each other, and the angles have the values to be expected for cyclotetraphosphazenes with 5-membered spiro-ring. In addition, the α ′ angle of the 5-membered DASD substituted IIIc is larger than that of the 5-membered Pip substituted IIIb, which once again confirms that the DASD substituent has a greater electron-releasing power than the Pip substituent and shows that the electron transferred from the DASD group to the phosphazene ring does not remain only in the phosphazene ring but also transfers towards the spiro-ring. In case of partly and fully substituted type XI cyclotetraphosphazenes, α angle is much affected by the substitution, but, α ′ angle is less affected. Moreover, the correlations between the δ P spiro shifts and α ( Figure 3i) and α′ (Figure 3ii) NPN bond angles show contrasting trends. For example, the α and α ′ angles of 6-(Va) and 4-(Ia) membered partly substituted phosphazenes are smaller than 6-membered fully Van-(Vd) and 4-membered nongeminal cis-(IVa) substituted phosphazenes, respectively.

Conclusions
A systematic study concerning the correlations between structural parameters [e.g., 31 P NMR spectral data and X-ray crystallographic data (endocyclic and exocyclic NPN bond angles, and bond lengths)] displayed some characteristic results for mono-and di-spirocyclophosphazene derivatives bearing ferrocenyl pendant arm/arms. Naturally, these results become more reliable when more cyclic phosphazenes from this series are synthesized and the 31 P NMR spectroscopic and X-ray crystallographic data of these molecules are taken into account. It is necessary to extend the study for other members of the spirocyclic ferrocenyl cyclophosphazene family to get a more general and including views about the correlations between structural parameters of these molecules. Research along these lines is actually under development in our laboratory and results will be presented elsewhere in the forthcoming future.

Acknowledgments
Selen Bilge Koçak is grateful to the Scientific and Technological Research Council of Turkey (TÜBİTAK) (Project No. 113Z861) and Ankara University Scientific Research Projects (BAP) (Project No. 10B4240008).
Zeynel Kılıç thanks Turkish Academy of Sciences (TÜBA) for partial support of this work.