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An answer to the SPIRO versus ANSA dilemma in cyclophosphazenes Part XVIII. The structure of the simplest ANSA derivatives, N3P3Cl4[HN(CH2)2O] and N3P3Cl3(CH3)[HN(CH2)3O]: molecular modeling versus X-ray
Journal of
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 380 (1996) 157-165
An answer to the SPIRO versus A N S A dilemma in cyclophosphazenes Part XVIII. 1 The structure of the simplest A N S A derivatives, N3P3C14[HN-(CH2)2-O ] and N3P3C13(CH3)[HN-(CH2)3-O]: molecular modeling versus X-ray Franqois Crasnier, Marie-Christine Labarre, Franqois Sournies, Christiane Vidal, Jean-Francois Labarre* Institut de Chimie Mol~culaire Paul Sabatier, Laboratoire Structure & Vie, Universit~ Paul Sabatier, 118, Route de Narbonne, 31062 Toulouse Cedex, France
Received 23 October 1995;accepted 1 December 1995
Abstract
Molecular modeling is evidenced as a convenient tool for assigning the molecular structure of ANSA cyclophosphazenes. Keywords:
Molecularmodeling; Conformationalisomerism;Cyclophosphazenes
1. Introduction
During the last three decades, much literature has been written about the so-called SPIRO versus ANSA dilemma related to the molecular structure of products obtained upon reaction of hexachlorocyclotriphosphazene, N3P3C16, with bifunctional reagents. This contest was opposing partisans of the ANSA assumption rallied round Becke-Goehring and Boppel [1] to crusaders of the SPIRO hypothesis conducted by Shaw and co-workers [2] and the situation remaining toughly conflicting from 1963 to 1983, every party making * Corresponding author. l For Part XVII, see Ref. [39].
the best possible use of the most actual techniques available at that time. We accumulated from 1982 a lot of conclusive structural evidence in favor of the SPIRO configuration for the products of the reactions of N3P3C16 with natural diamines (1,3-diaminopropane and putrescine) and other biogene relatives such as spermidine and spermine. The whole structures (more than 20) were unambiguously assigned through X-ray determinations [3-10] and no ANSA moiety was ever observed in our Group until 1984, where Harris and Williams reported "the synthesis and spectral characteristics of the first real ANSA cyclotriphosphazene" [11] by the reaction of 3-amino-l-propanol with monomethylpentachlorocyclotriphosphazene [12].
0022-2860/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved S S D I 0022-2860(95)09203-X
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We were lucky enough in late 1984 to obtain the actual crystal and molecular structure of Harris compound (from his own single crystals) as a part of the general program of X-ray analyses we developed at that time with Ren~e Enjalbert and Jean Galy (CNRS Group of Crystallography, Toulouse, France) but the X-ray structure of this first real ANSA cyclotriphosphazene was never published. At that time, indeed, literature was not fond of such explosive contributions which would stir up the fire between ANSA and SPIRO partisans. However, we provided later crystal and molecular structures of several other ANSA cyclotriphosphazenes that we synthesized upon reaction of oxa-, dioxa- and trioxadiamines with N3P3C16 [13-26]. One decade later, our Group is dealing with dandelion dendrimers [27-31], i.e. the first spherical dendrimers built from a D3h core. The size of
these molecular monsters is so huge (to 228 x 10 6 relative .molecular mass) that molecular modeling appears actually to be among the very few suitable tools capable of approaching their structure. Thus, we had to choose the software (from the various ones available) that would allow us to reproduce conformations obtained from X-ray data for some simple molecules within the cyclophosphazenic series. An overall survey of the several comparisons we did in this way urged us to select the BIOSYM software [32], according to the promising results we got within the field of ANSA and/or SPIRO cyclophosphazenic cryptands. Then, this contribution reports on (i) the X-ray structure of the Harris ANSA (1984 fever seeming indeed actually abated), (ii) the efficiency of mOSYM software in reproducing, ab initio, the conformation of the ANSA arch in it and (iii) the BIOSVM approach to the unknown molecular structure of
Table 1 Crystallographically important data collection and data processing information for N3PsCI3 (CH3)[HN-(CH2)3-O]
Data collection Unit cell: orthorhombic, space group P212121, Z = 4 a = 8.033(2), b = 11.534(7), c = 13.450(4) A, V = 1246(1) ~3 at 298 K a = 7.984(3), b = 11.406(3), c = 13.380(3) ,~, V = 1218.5(6) ,~3 at 123 K Pexo = 1.74 g cm -3 P x = 1 . 7 6 g c m -3 Graphite monochromated MoKc~, A = 0.71069 ,~ Crystal size: 0.25 mm x 0.40 mm x 0.60 mm Linear absorption coefficient: # = 1.030 mm -I No absorption correction F(000) = 664 0 range of reflections: 1.5-26 ° 0/20 scan technique Controls of intensity: reflections 2 1 4, 1 2 7, 2 2 0, each 3600 s Take-off angle: 2.5 ° 25 reflections with 5° < 0 < 13° used for measuring lattice parameters Space group (identified by precession method) verified by rapid measurement of h01, 0kl, hk0 reflections implying P2t212 space group 0 - 20 scan with A0 scan = 1.0 + 0.35tan0, prescan speed = 10° min -I o ( l ) / l for final scan = 0.018 Maximum time for final scan: 80 s No significant variation during the whole data collection
Structure determh~ation and refinement 1413 measured reflections, 1246 unique reflections, 1226 utilized reflections with I > 3a(1) Use of F magnitudes in least-squares refinement Parameters refined: Reliability factor R = 0.0218 Rw = 0.0250 S = 0.948
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the simplest ANSA cyclotriphosphazene ever described, N3P3CI4[HN-(CH2)2-O ], we had just synthesized in a quite regiospecific way.
the.rmostatically controlled stream of cold nitrogen gas. All the data concerning the cell parameters and the conditions used for data collection at 123 K are listed in Table 1.
2. X-ray study of Harris ANSA, N3P3CI3(CH3) [HN-CH2 )3-0]
2.2. Structure determination and refinement
2.1. Crystal data The compound crystallized in the orthorhombic P212t21 space group; unit cell parameters at 298 K were as follows: a = 8.033(2) A, b = 11.534(7) and c = 13.450(4) A; V = 1246(1) ,~3, px = 1.76 g cm -3, Pexp = 1.74 g cm -3, Z = 4. Due to the slow decomposition of the compound at room temperature, the single crystal chosen for this study was rapidly mounted on a CAD-4 NONIUS diffractometer and cooled to 123 K using a Table 2 Fractional atomic coordinates and equivalent temperature factors (,/k2 x 100) with e.s.d.s in parentheses Atom
x/a
y/b
z/c
Ueq a
PI P2 P3 CII C12 C13 NI N2 N3 N4 O CI C2 C3 C4 HN4 H1CI H2CI H1C2 H2C2 H1C3 H2C3 HIC4 H2C4 H3C4
0.8795(1) 0.6588(1) 0.9114(1) 1.0145(1) 0.4137(1) 0.6397(I) 0.7222(4) 0.7530(4) 0.9992(4) 0.8586(4) 0.8115(3) 0.8852(5) 0.7716(5) 0.7188(5) 1.0588(6) 0.848(7) 0.995(7) 0.903(7) 0.825(7) 0.671(7) 0.650(7) 0.651(7) 1.143(7) 1.017(7) 1.094(8)
0.55665(8) 0.38767(9) 0.46432(8) 0.56922(9) 0.42316(9) 0.22683(8) 0.4747(3) 0.3798(3) 0.5159(3) 0.5752(3) 0.6855(2) 0.7612(3) 0.7690(3) 0.6515(4) 0.3822(4) 0.560(5) 0.734(5) 0.829(5) 0.813(5) 0.810(4) 0.613(4) 0.666(5) 0.426(5) 0.363(5) 0.327(5)
0.23743(6) 0.29586(7) 0.42392(7) 0.11168(7) 0.31996(7) 0.23422(7) 0.2110(2) 0.3976(2) 0.3245(2) 0.4968(2) 0.2524(2) 0.3297(3) 0.4198(3) 0.4651(3) 0.4958(3) 0.557(4) 0.347(4) 0.303(4) 0.477(4) 0.405(4) 0.417(4) 0.529(4) 0.510(4) 0.554(4) 0.462(4)
1.15(4) 1.27(4) 1.31(4) 1.99(5) 2.08(5) 1.97(5) 1.6(2) 1.8(2) 1.4(2) 1.6(2) 1.8(1) 1.7(2) 1.8(2) 1.9(2) 2.2(2) 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0
a U~q 1 / 3 ~
U,jara;a,'a.
=
i
j
Direct methods were used to determine the structure; groups of atoms whose geometry was known were introduced to help the procedures, i.e. the N3P3CI 2 moiety from gem-N3P3AzaC12 [33]. The best map yielded the positions of all the non-hydrogen atoms. Isotropic refinement in the space group P212121 gave a reliability index of 0.045. Introduction of anisotropic temperature parameters reduced the R index to 0.030. The difference Fourier synthesis, phased by the P, C1, N, C and O atoms, revealed the hydrogen atoms with their expected locations. Thus, the final refinement could be performed on the entire set of atoms including hydrogen atoms with a fixed isotropic thermal parameter factor, BH = 4 ~2. Final R and S values were 0.0218 and 0.948, respectively. Table 3 Selected intramolecular bond lengths and angles. C-H bond lengths are in the range 0.83(5)-1.02(5) ~,; H - C - H angles are in the range 104(5)-116(5) ° Bond
Length (,~)
Bonds
Angle (deg)
PI-NI PI-N3 P2-N1 P2-N2 P3-N2 P3-N3 P3-N4 PI-O PI-CII P2-CI2 P2-CI3 N4-C3 C3-C2 C2-C1 C1-O
1.606(4) 1.577(3) 1.591(3) 1.557(4) 1.629(4) 1.615(3) 1.651(4) 1.579(3) 2.003(2) 2.024(2) 2.017(2) 1.478(6) 1.529(6) 1.512(6) 1.470(5)
N1-PI-N3 PI-N3-P3 N3-P3-N2 P3-N2-P2 N2-P2-N1 P2-NI-P1 O-PI-CII C11-P2.-CI3 P3-N4-C3 N4-C3-C2 C3-C2-C1 C2-C1-O CI-O-P1 O-PI-N3 N3-P3-N4 N4-P3-C4
117.7(2) 116.9(2) 112.0(2) 122.0(2) 120.4(2) 117.1(2) 103.0(1) 100.0(I) 118.3(3) 114.9(3) 115.5(3) 110.8(3) 119.3(2) 112.9(2) 108.5(2) 104.6(2)
CI1...0 P I . . . P2 P I . . . P3 P2... P3 C12... C13
2.816(3) 2.726(2) 2.720(2) 2.787(2) 3.564(2)
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The last difference Fourier map showed no values to be greater than -t-0.3 e .~-3. Atomic scattering factors were corrected for anomalous dispersion from Cromer and Waber [34]. In addition tO local programs, the following programs were used on the CICT-CII-HB DPS8 MULTICS systems: Zalkin's FOURmR; Main and Germain's MULrArq; Busing, Martin and Levy's OaFFE; Sheldrick's SHELX;Johnson's oarEP. Final atomic coordinates and anisotropic temperature factors are listed in Table 2. Selected bond lengths and bond angles are listed in Table 3, along with the main intramolecular interatomic distances. 2.3. Results and discussion
The X-ray structure determination of the title compound clearly indicates the ANSA structure. The amino end of the arch is bound to the methylated phosphorus atom, while the oxygen end of the 3-amino-l-propanol group is bound to an adjacent phosphorus atom. The general structure of the molecule, the numbering system used for all nonhydrogen atoms, and selected bond angles and distances are shown in Fig. 1. A perspective view of the molecule, clearly displaying the ANSA structure, is shown in Fig. 2. Intermolecular hydrogen bonds observed are discussed below, as in the SPIRO relative N3P3Cla[HN-(CH2)3-NH ] and in another ANSA derivative, N3P3Cla[HN(CH2)3-O-(CH2)2-O-(CH2)3-NH], where two hydrogen bonds (2.08 A) per molecule are considered to be responsible for the crystal packing arrangement [5,18].
Fig. I. Numbering scheme and selected bond angles and distances for N3P3CI3(CH3)[HN-(CH2)3-OI.
Fig. 2. Perspective view of N3P3CI3(CH3)[HN-(CH2)3-O] from X-ray analysis. Several features of the molecule were revealed by the structure determination. (i) The spatial conformation o f the A N S A arch
The conformation of the 3-amino-l-propanol group is shown in Figs. 3 and 4. The central carbon atom in the methylene chain (C2) belongs both to the ~rv-like N3-C12-P2-C13 plane, and to the
Fig. 3. Conformationof the ANSAarch of N~P3C13(CH3)[HN(CH2)3-O] from X-ray analysis.
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N 4 - P 3 - P I - O plane. Carbon atoms C1 and C3 are +0.8 ,A,distant from the latter plane and the carbon atom C3 is situated over the mean plane of the phosphazene ring.
(ii) Conformation of the phosphazene ring The phosphazene ring in the compound is clearly non-planar, as illustrated in Fig. 4. The nitrogen atom, N3, which is a part of both the ANSA ring and the phosphazene ring, is 0.435(3) ,~ below the mean P1-N1-P2-N2-P3 plane. The dihedral angle between the P1-P2-P3 and the P1-N3-P3 planes is 34.6°. However, despite the lack of planarity within the phosphazene ring, the transannular ( P . . . P ) distances are all approximately 2.7 A,, while the exocyclic P3-N4 and P1-O bond lengths are 1.651(4) and 1.579(3) A, respectively, all typical of cyclotriphosphazene bond lengths. The reason the phosphazene ring is puckered, therefore, appears to be as follows: In order for the 3-aminol-propanol group to span phosphorus P1-P3 without affecting the distance between these two atoms, a rotation about bonds P1-N1 and P3-N2 must occur. This rotation is clearly indicated in Fig. 2. The rotation brings atoms N4 and O to within 3.52 ,~, of each other, and forces Cll and C4 apart. The Cll-C4 distance in this
ANSA compound is 5.58 A while the CI-C distances in the aunpuckered N3P3Cla-i-PrH compound are 4.67 A and 4.88 A [35]. Thus, the rotation about P1-N1 and P3-N2, which is necessary to allow the ANSA arch to bridge these two atoms, must force nitrogen N3 down below the plane of the phosphazene ring if the pseudo tetrahedrai geometry around phosphorus P1 and P3 is to be maintained. This, indeed, appears to be the case. *
e
(iii) Bonding within the phosphazene ring The variations "observed in the P-N bond distances within the phosphazene ring are very interesting. As has been observed in other cases [35,36], the lengths of the P - N bonds vary with the electronegativity of the substituents bound to the phosphorus atoms at the ends of a P - N - P island. Consider the island P2-N2-P3 (Fig. 1). Phosphorus atom P2 clearly contains the most electronegative substituents in the ring, while phosphorus atom P3 contains the least electronegative groups. The P2-N2 length of 1.557 ,~,, the shortest in the molecule, and the P3-N2 length of 1.629 ,~,, the longest in the compound, clearly reflect these electronegativity differences. Similar changes can be observed in the islands P 1 - N I P2 and PI-N3-P3. The P1-N1-P2 island has the smallest difference in electronegativity between the phosphorus substituents and also has the smallest variation in P - N bond lengths. However, let us come back to the existence of intermolecular hydrogen bonds which do exist in the unit cell and which may be responsible for eventual significant alterations of the actual molecular structure with respect to the one it would have adopted if constraints-free. Two main questions arise from this point of view, namely about (i) the lack of planarity of the phosphazene ring and (ii) the real conformation of the ANSA arch. A definite answer to both questions is provided by molecular modeling, which is detailed below.
3. Molecular modeling study of Harris ANSA, N3P3CI3(CH3)[HN-(CH2)3-O]
3.1. Software and graphics station Fig. 4. Puckering of the N3i~3 ring of N3P3CI3(CH3)[HN(CH2)3-O] from X-ray analysis.
Results reported here were obtained by using the
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BIOSVM software [32] produced by Biosym Technologies. A home-made CVFF (Consistent Valence Force Field) force field was used because of the quite good reproducibility of the experimental molecular and conformational structures of small cyclophosphazenic moieties, mainly about the morphology of the cyclophosphazene ring, which was found to be strictly planar when the planarity in question had been observed by X-ray investigations. Incidentally, the ESFF (Extended Standard Force Field), which is so convenient for organometallics, is definitely not suitable in the case of genuine cyclophosphazenes, owing to the fact that it yields systematically to folded cyclophosphazene rings, even when a strict planarity was experimentally observed. Introduction of Monte-Carlo and simulated annealing techniques into the modeling were found to be unnecessary. The graphics station we used was 32 Mo Silicon Graphics, coupled with a NEC-DIGITAL color printer. 3.2. Unit cell and molecular packing
BIOSYM software allows the drawing of the unit cell from the X-ray data of Tables I and 2. Then, two strong intermolecular hydrogen bonds are made conspicuous between the N4-H group of arch A and the oxygen atom of the closest arch B ( d ( N 4 - H . . . O ) = 2 . 1 7 ,~, angle ( N 4 - H - O ) = 140°). What is the influence of these intermolecular contacts on the conformation of ANSA arches? Optimization of the four molecules of the unit cell as a whole (through the use of the common molecular mechanics processes included in the aIOSYM software) confirms both (i) the twisted conformation of the arches (as visualized on Fig. 3) and (ii) the lack of planarity of the cyclophosphazene rings. Conversely, a similar optimization (from the X-ray structure as the starting point) on one of these molecules assuming intermolecular contacts were canceled shows that the lack of planarity remains whereas the conformation of the arch becomes more symmetrical (Fig. 5, to be compared with Fig. 3). In other words, the crystallographically twisted conformation is definitely due to intermolecular hydrogen bonds in which oxygen
A
0
O
N
I
(3 Fig. 5. Actual conformation of the ANSA arch of
N3P3CI3(CH3)[HN-(CH2)3-O ] from molecularmodeling. atoms pull nitrogen atoms of related NH groups outside of the arch mean plane. In contrast, it seems that these contacts have no influence on the lack of planarity of cyclophosphazene rings which are actually non-planar. Thus, molecular modeling allows us to reach the real molecular structure of a simple molecular system (such as the one of the title molecule) as well as that of more intricate cyclophosphazenes [29-31]. We shall use this technique for assigning the structure and conformation of other ANSA derivatives and firstly of the simplest one described up to now, N3 P3C14[HN- (CH2)2-O].
4. Molecular modeling study of N3P3CI4 [HN- (CH 2)2-O] 4.1. Synthesis ofN3P3C14[HN-(CH2)2-O ]
This genuine ANSA derivative is obtained neat through a one'step reaction of N3P3CI6 with 2-amino-l-ethanol in Et20 as the solvent. This observation is really amazing considering the well-known difficulties that were encountered by chemists until 1984 in their attempts to produce
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ANSA moieties. Remember indeed that the synthesis of the Harris ANSA had to be performed through a two-step procedure (the second step requiring drastic conditions) [11], the cyclophosphazenic substrate having to be the monomethyl derivative, N3P3C15Me (the same reaction with N3P3C16 itself being definitely unsuccessful). The synthesis of N3P3C14[HN-(CH2)2-O] was achieved in Et20 according to the following pathway
GI
N3P3C16 + H2N-(CH2)2-OH --, N3P2C14[HN-(CH2)2-O ] + 2 HCI Triethylamine was used to pick off the hydrogen chloride. The reaction took 24 h and was considered complete when the 31p NMR singlet of N3P3C16 at 20.09 ppm (Bruker WM 200, CDCI 3, H3PO 4 85% as the standard) had disappeared. The hydrochloride was then filtered off and the solvent removed in vacuo at 25°C to give a white powder. The crude product was the expected ANSA moiety together with traces of N3P3Ci 6 which were eliminated by washing with n-heptane (N3P3CI6 being soluble in this solvent and N3P3CI4[HN-(CH2)2O] being insoluble). Pure ANSA derivative was obtained in 75% yield, Rr t.l.c. = 0.30 with Et20/ n-hexane (1:1) as the eluant (m.p. = 160°C). The 81.015 M Hz 31p NMR pattern comprised a singlet at 24.07 ppm, to be compared with the 21.38, 21.15, 21.14 and 20.97 ppm singlets for ANSA 20202, 30203, 30403 and 3020203, respectively [17,37]. Such a low-field shift may be related to a higher hindrance induced in N3P3CI4[HN-(CH2)2-O ] by the shortness of the methylenic bridge [38]. DCI/NH 3 mass spectrometry revealed the MH + ion at m/z 337 (relative molecular mass = 334 with 35C1) with a satellite distribution confirming the presence of four chlorine atoms in the molecule. No fragmentation peaks were observed, except at m/z 300, which would correspond to the loss of one chlorine atom. Incidentally, the [M, NH~-] molecular peak was also detected at m/z 354.
ctl
ct
Fig. 6. Conformationof the ANSA arch of N3P3CI4[HN(CH2)2-O] frommolecularmodeling. BIOSYM software with the CVFF subroutine. The ab initio optimized structure is visualized in Figs. 6 and 7 which reveals two main features. (i) The endocyclic nitrogen atom which lies between the two arch-bearing phosphorus atoms is pulled down away from the phosphazenic plane in the same way as in N3P3C13(CH3)[HN-(CH2)3O]. The distance between this peculiar N atom and the mean plane of the ring is 0.45 ,~, (to be
0
,
N
Cl
4.2. Molecular modeling o f
N3P3C14 [HN- (CH2)2-O] Results reported here were obtained by using the
Fig. 7. Puckeringof the N3P3 ring of N3P3CI4[HN-(CH2)2-O] from molecularmodeling.
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c o m p a r e d with 0.435 ,~ in N 3 P 3 C I 3 ( C H 3 ) [ H N (CH2)3-O], see above). In other words, the n u m b e r o f methylenic links into the arch does not alter significantly the lack o f planarity o f the cyclophosphazenic ring. (ii) A twisted c o n f o r m a t i o n is evidenced for the constraints-free A N S A arch (Fig. 6), in constrast to what was observed in the constraints-free Harris A N S A (Fig. 5), but in agreement with what was m a d e conspicuous for the Harris A N S A in the solid state (Fig. 3), Thus, 3 - a m i n o - l - p r o p a n o l yields a symmetrical A N S A arch whereas 2 - a m i n o - l - e t h a n o l leads to a twisted one. The confrontation between such molecular modeling approaches and the results provided b y X - r a y investigations o f other A N S A moieties confirms the u n a m b i g u o u s efficiency o f molecular modeling for assigning molecular structures within the cyclophosphazenic series in a time-and-money n o n - c o n s u m i n g way.
5. Conclusions In 1996, A N S A cyclophosphazenes are no m o r e Nessies. We contributed from 1984, t h r o u g h a lot o f X-ray analyses, all published in this Journal, to demonstrate that these,peculiar cryptands (according to Lehn's terminology) do really exist. N o w , we hope that it will become possible to replace step by step crystallography by molecular modeling, essentially for saving time and money. If successful, the new a p p r o a c h will be a convenient tool for assigning configurations and c o n f o r m a t i o n s in cyclophosphazenes whatever their size. Indeed, day-after-day improvements in the power o f computers for molecular modeling [40] will allow in the very near future enhancement o f the limits o f the technique in terms o f molecular size. This is essential when the world o f huge dendrimers is currently coming [29-31].
Acknowledgments A u t h o r s are greatly indebted to Drs Ren6e Enjalbert and Jean Galy ( C E M E S Center o f C N R S , Toulouse, France) who achieved to our
privilege more than 20 crystal and molecular structures quoted in the references o f the present paper.
References [1] M. Becke-Goehring and B. Boppel, Z. Anorg. Allg. Chem., 322 (1963) 239. [2] S.S. Krishnamurthy, K. Ramachandran, A.R. Vasudevamurthy, R.A. Shaw and M. Woods., Inorg. Nucl. Chem. Lett., 13 (1977) 407 and references therein. [3] G. Guerch, J.-F. Labarre, R. Roques and F. Sournies, J. Mol. Struct., 96 (1982) 113. [4] G. Guerch, M. Graffeuil, J.-F. Labarre, R. Enjalbert, R. Lahana and F. Sournies, J. Mol. Struct., 95 (1982) 237. [5] R. Enjalbert, G. Guerch, J.-F. Labarre and J. Galy, Z. Kristallogr., 160 (1982) 249. [6] G. Guerch, J.-F. Labarre, R. Lahana, R. Roques and F. Sournies, J. Mol. Struct., 99 (1983) 275. [7] G. Guerch, J.-F. Labarre, R.. Lahana, F. Sournies, R. Enjalbert, J. Galy and J.-P. Declercq, Inorg. Chim. Acta, 83 (1984) L33. [8] N. El Murr, R. Lahana, J.-F. Labarre and J.-P. Declercq, J. Mol. Struct., 117 (1984) 73. [9] J.-F. Labarre, G. Guerch, F. Sournies, R. Lahana, R. Enjalbert and J. Galy, J. Mol. Struct., 116 (1984) 75. [10] J.-F. Labarre, Top. Curr. Chem., 129 (1985) 173, and 88 references therein. [11] P.J. Harris and K.B. Williams, Vlth Proc. Int. Conf. on Phosphorus Chemistry, Nice, France, 1983, Gordon and Breach, Langhorne, p. 169. [12] H.R. Allcock and P.J. Harris, Inorg. Chem., 20 (1981) 2844. [13] R. Enjalbert, J. Galy, P.J. Harris, K.B. Williams, R. Lahana and J.-F. Labarre, J. Mol. Struct., 133 (1986) 150. [14] P. Castera, J.-P. Faucher, J.-F. Labarre and B. Perly, J. Mol. Struct., 160 (1987) 365. [15] P. Castera, J.-P. Faucher, M. Graffeuil and J.-F. Labarre, J. Mol. Struct., 176 (1988) 295. [16] R. Enjalbert, J. Galy, P. Castera and J.-F. Labarre, Acta Crystallogr., Sect. C, 44 (1988) 1813. [17] A. El Bakili, P. Castera, J.-P. Faucher, F. Sournies and J.-F. Labarre, J. Mol. Struct., 195 (1989) 21. [18] R. Enjalbert, J. Galy, A. E1 Bakili, P. Castera, J.-P. Faucher, F. Sournies and J.-F. Labarre, J. Mol. Struct., 196 (1989) 207. [19] T.S. Cameron, A. Linden, A. El Bakili, P. Castera, J.-P. Faucher, M. Graffeuil, F. Sournies and J.-F. Labarre, J. Mol. Struct., 212 (1989) 281. [20] J. Jaud, F. Sournies and J.-F. Labarre, J. Mol. Struct., 212 (1989) 305. [21] F. Sournies, A. El Bakili, B. Zanin, J.-F. Labarre and J. Jaud, J. Mol. Struct., 220 (1990) 43. [22] R. Enjalbert, J. Galy, F. Sournies and J.-F. Labarre, J. Mol. Struct., 221 (1990) 253.
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[23] R. Enjalbert, J. Galy, B. Zanin, J.-P. Bonnet, F. Sournies and J.-F. Labarre, J. Mol. Struct., 246 (1991) 123. [24] R. Enjalbert, J. Galy, C. Galliot, S. Scheidecker, J.-P. Bonnet and J.-F. Labarre, J. Mol. Struct., 271 (1992) 95. [25] F. Sournies, P. Castera, A. El Bakili and J.-F. Labarre, J. Mol. Struct., 221 (1990) 239. [26] F. Sournies, P. Castera, A. E1 Bakili, J.-P. Faucher, M. Graffeuil and J.-F. Labarre, J. Mol. Struct., 221 (1990) 245. [27] F. Sournies, L. Labrousse, M. Graffeuil, F. Crasnier, J.-P. Faucher, M.-C. Labarre and J.-F. Labarre, Phosphorus Sulfur Silicon Relat. Elem., 89 (1994) 47. [28] F. Sournies, C. Fouga, M. Graffeuil, F. Crasnier, J.-P. Faucher, M.-C. Labarre and J.-F. Labarre, Phosphorus Sulfur Silicon Relat. Elem., 90 (1994) 159. [29] F. Sournies, F. Crasnier, M. Graffeuil, J.-P. Faucher, R. Lahana, M.-C. Labarre and J.-F. Labarre, Angew. Chem., 107 (1995) 610; Angew. Chem. Int. Ed. Engl., 34 (1995) 578. [30] J.-F. Labarre, F. Sournies, F. Crasnier, M.-C. Labarre, C. Vidal, J.-P. Faucher and M. Graffeuil, Main Group Chem. News, 3 (1995) 4. [31] F. Sournies, F. Crasnier, C. Vidal, M.-C. Labarre and J.-F. Labarre, Main Group Chem., 2 (1996) in press.
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[32] BIOSYM software (Version 2.3, 1994) is available .from Biosym Technologies, 9685 Scranton Road, San Diego, CA 92121-2777, USA. ° [33] R. Enjalbert, G. Guerch, F. Sournies, J.-F. Labarre and J. Galy, Z. Kristallogr., 164 (1983) 1. [34] D.T. Cromer and J.T. Waber, International Tables for X-Ray Crystallography, Vol. 4, Kynoch Press, Birmingham, 1974, Table 2.2A. [35] R.J. Ritchie, P.J. Harris and H.R. Allcock, Inorg. Chem., 19 (1980) 2483. [36] R. Keat and R.A. Shaw, Organic Phosphorus Compounds, Vol. 6, John Wiley, New York, 1973, pp. 833 and 519 and references therein. [37] F. Sournies, A. E1 Bakili, J.-F. Labarre and B. Perly, J. Mol. Struct., 196 (1989) 201. [38] M.-C. Labarre and J.-F. Labarre, J. Mol. Struct., 300 (1993) 593. [39] J. Jaud, B. Raynaud, S. Scheidecker and J.-F. Labarre, J. Mol. Struct., 271 (1992) 289. [40] Silicon Graphics International Magazine, 8 0995) 6.
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 380 (1996) 157-165
An answer to the SPIRO versus A N S A dilemma in cyclophosphazenes Part XVIII. 1 The structure of the simplest A N S A derivatives, N3P3C14[HN-(CH2)2-O ] and N3P3C13(CH3)[HN-(CH2)3-O]: molecular modeling versus X-ray Franqois Crasnier, Marie-Christine Labarre, Franqois Sournies, Christiane Vidal, Jean-Francois Labarre* Institut de Chimie Mol~culaire Paul Sabatier, Laboratoire Structure & Vie, Universit~ Paul Sabatier, 118, Route de Narbonne, 31062 Toulouse Cedex, France
Received 23 October 1995;accepted 1 December 1995
Abstract
Molecular modeling is evidenced as a convenient tool for assigning the molecular structure of ANSA cyclophosphazenes. Keywords:
Molecularmodeling; Conformationalisomerism;Cyclophosphazenes
1. Introduction
During the last three decades, much literature has been written about the so-called SPIRO versus ANSA dilemma related to the molecular structure of products obtained upon reaction of hexachlorocyclotriphosphazene, N3P3C16, with bifunctional reagents. This contest was opposing partisans of the ANSA assumption rallied round Becke-Goehring and Boppel [1] to crusaders of the SPIRO hypothesis conducted by Shaw and co-workers [2] and the situation remaining toughly conflicting from 1963 to 1983, every party making * Corresponding author. l For Part XVII, see Ref. [39].
the best possible use of the most actual techniques available at that time. We accumulated from 1982 a lot of conclusive structural evidence in favor of the SPIRO configuration for the products of the reactions of N3P3C16 with natural diamines (1,3-diaminopropane and putrescine) and other biogene relatives such as spermidine and spermine. The whole structures (more than 20) were unambiguously assigned through X-ray determinations [3-10] and no ANSA moiety was ever observed in our Group until 1984, where Harris and Williams reported "the synthesis and spectral characteristics of the first real ANSA cyclotriphosphazene" [11] by the reaction of 3-amino-l-propanol with monomethylpentachlorocyclotriphosphazene [12].
0022-2860/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved S S D I 0022-2860(95)09203-X
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We were lucky enough in late 1984 to obtain the actual crystal and molecular structure of Harris compound (from his own single crystals) as a part of the general program of X-ray analyses we developed at that time with Ren~e Enjalbert and Jean Galy (CNRS Group of Crystallography, Toulouse, France) but the X-ray structure of this first real ANSA cyclotriphosphazene was never published. At that time, indeed, literature was not fond of such explosive contributions which would stir up the fire between ANSA and SPIRO partisans. However, we provided later crystal and molecular structures of several other ANSA cyclotriphosphazenes that we synthesized upon reaction of oxa-, dioxa- and trioxadiamines with N3P3C16 [13-26]. One decade later, our Group is dealing with dandelion dendrimers [27-31], i.e. the first spherical dendrimers built from a D3h core. The size of
these molecular monsters is so huge (to 228 x 10 6 relative .molecular mass) that molecular modeling appears actually to be among the very few suitable tools capable of approaching their structure. Thus, we had to choose the software (from the various ones available) that would allow us to reproduce conformations obtained from X-ray data for some simple molecules within the cyclophosphazenic series. An overall survey of the several comparisons we did in this way urged us to select the BIOSYM software [32], according to the promising results we got within the field of ANSA and/or SPIRO cyclophosphazenic cryptands. Then, this contribution reports on (i) the X-ray structure of the Harris ANSA (1984 fever seeming indeed actually abated), (ii) the efficiency of mOSYM software in reproducing, ab initio, the conformation of the ANSA arch in it and (iii) the BIOSVM approach to the unknown molecular structure of
Table 1 Crystallographically important data collection and data processing information for N3PsCI3 (CH3)[HN-(CH2)3-O]
Data collection Unit cell: orthorhombic, space group P212121, Z = 4 a = 8.033(2), b = 11.534(7), c = 13.450(4) A, V = 1246(1) ~3 at 298 K a = 7.984(3), b = 11.406(3), c = 13.380(3) ,~, V = 1218.5(6) ,~3 at 123 K Pexo = 1.74 g cm -3 P x = 1 . 7 6 g c m -3 Graphite monochromated MoKc~, A = 0.71069 ,~ Crystal size: 0.25 mm x 0.40 mm x 0.60 mm Linear absorption coefficient: # = 1.030 mm -I No absorption correction F(000) = 664 0 range of reflections: 1.5-26 ° 0/20 scan technique Controls of intensity: reflections 2 1 4, 1 2 7, 2 2 0, each 3600 s Take-off angle: 2.5 ° 25 reflections with 5° < 0 < 13° used for measuring lattice parameters Space group (identified by precession method) verified by rapid measurement of h01, 0kl, hk0 reflections implying P2t212 space group 0 - 20 scan with A0 scan = 1.0 + 0.35tan0, prescan speed = 10° min -I o ( l ) / l for final scan = 0.018 Maximum time for final scan: 80 s No significant variation during the whole data collection
Structure determh~ation and refinement 1413 measured reflections, 1246 unique reflections, 1226 utilized reflections with I > 3a(1) Use of F magnitudes in least-squares refinement Parameters refined: Reliability factor R = 0.0218 Rw = 0.0250 S = 0.948
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the simplest ANSA cyclotriphosphazene ever described, N3P3CI4[HN-(CH2)2-O ], we had just synthesized in a quite regiospecific way.
the.rmostatically controlled stream of cold nitrogen gas. All the data concerning the cell parameters and the conditions used for data collection at 123 K are listed in Table 1.
2. X-ray study of Harris ANSA, N3P3CI3(CH3) [HN-CH2 )3-0]
2.2. Structure determination and refinement
2.1. Crystal data The compound crystallized in the orthorhombic P212t21 space group; unit cell parameters at 298 K were as follows: a = 8.033(2) A, b = 11.534(7) and c = 13.450(4) A; V = 1246(1) ,~3, px = 1.76 g cm -3, Pexp = 1.74 g cm -3, Z = 4. Due to the slow decomposition of the compound at room temperature, the single crystal chosen for this study was rapidly mounted on a CAD-4 NONIUS diffractometer and cooled to 123 K using a Table 2 Fractional atomic coordinates and equivalent temperature factors (,/k2 x 100) with e.s.d.s in parentheses Atom
x/a
y/b
z/c
Ueq a
PI P2 P3 CII C12 C13 NI N2 N3 N4 O CI C2 C3 C4 HN4 H1CI H2CI H1C2 H2C2 H1C3 H2C3 HIC4 H2C4 H3C4
0.8795(1) 0.6588(1) 0.9114(1) 1.0145(1) 0.4137(1) 0.6397(I) 0.7222(4) 0.7530(4) 0.9992(4) 0.8586(4) 0.8115(3) 0.8852(5) 0.7716(5) 0.7188(5) 1.0588(6) 0.848(7) 0.995(7) 0.903(7) 0.825(7) 0.671(7) 0.650(7) 0.651(7) 1.143(7) 1.017(7) 1.094(8)
0.55665(8) 0.38767(9) 0.46432(8) 0.56922(9) 0.42316(9) 0.22683(8) 0.4747(3) 0.3798(3) 0.5159(3) 0.5752(3) 0.6855(2) 0.7612(3) 0.7690(3) 0.6515(4) 0.3822(4) 0.560(5) 0.734(5) 0.829(5) 0.813(5) 0.810(4) 0.613(4) 0.666(5) 0.426(5) 0.363(5) 0.327(5)
0.23743(6) 0.29586(7) 0.42392(7) 0.11168(7) 0.31996(7) 0.23422(7) 0.2110(2) 0.3976(2) 0.3245(2) 0.4968(2) 0.2524(2) 0.3297(3) 0.4198(3) 0.4651(3) 0.4958(3) 0.557(4) 0.347(4) 0.303(4) 0.477(4) 0.405(4) 0.417(4) 0.529(4) 0.510(4) 0.554(4) 0.462(4)
1.15(4) 1.27(4) 1.31(4) 1.99(5) 2.08(5) 1.97(5) 1.6(2) 1.8(2) 1.4(2) 1.6(2) 1.8(1) 1.7(2) 1.8(2) 1.9(2) 2.2(2) 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0
a U~q 1 / 3 ~
U,jara;a,'a.
=
i
j
Direct methods were used to determine the structure; groups of atoms whose geometry was known were introduced to help the procedures, i.e. the N3P3CI 2 moiety from gem-N3P3AzaC12 [33]. The best map yielded the positions of all the non-hydrogen atoms. Isotropic refinement in the space group P212121 gave a reliability index of 0.045. Introduction of anisotropic temperature parameters reduced the R index to 0.030. The difference Fourier synthesis, phased by the P, C1, N, C and O atoms, revealed the hydrogen atoms with their expected locations. Thus, the final refinement could be performed on the entire set of atoms including hydrogen atoms with a fixed isotropic thermal parameter factor, BH = 4 ~2. Final R and S values were 0.0218 and 0.948, respectively. Table 3 Selected intramolecular bond lengths and angles. C-H bond lengths are in the range 0.83(5)-1.02(5) ~,; H - C - H angles are in the range 104(5)-116(5) ° Bond
Length (,~)
Bonds
Angle (deg)
PI-NI PI-N3 P2-N1 P2-N2 P3-N2 P3-N3 P3-N4 PI-O PI-CII P2-CI2 P2-CI3 N4-C3 C3-C2 C2-C1 C1-O
1.606(4) 1.577(3) 1.591(3) 1.557(4) 1.629(4) 1.615(3) 1.651(4) 1.579(3) 2.003(2) 2.024(2) 2.017(2) 1.478(6) 1.529(6) 1.512(6) 1.470(5)
N1-PI-N3 PI-N3-P3 N3-P3-N2 P3-N2-P2 N2-P2-N1 P2-NI-P1 O-PI-CII C11-P2.-CI3 P3-N4-C3 N4-C3-C2 C3-C2-C1 C2-C1-O CI-O-P1 O-PI-N3 N3-P3-N4 N4-P3-C4
117.7(2) 116.9(2) 112.0(2) 122.0(2) 120.4(2) 117.1(2) 103.0(1) 100.0(I) 118.3(3) 114.9(3) 115.5(3) 110.8(3) 119.3(2) 112.9(2) 108.5(2) 104.6(2)
CI1...0 P I . . . P2 P I . . . P3 P2... P3 C12... C13
2.816(3) 2.726(2) 2.720(2) 2.787(2) 3.564(2)
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The last difference Fourier map showed no values to be greater than -t-0.3 e .~-3. Atomic scattering factors were corrected for anomalous dispersion from Cromer and Waber [34]. In addition tO local programs, the following programs were used on the CICT-CII-HB DPS8 MULTICS systems: Zalkin's FOURmR; Main and Germain's MULrArq; Busing, Martin and Levy's OaFFE; Sheldrick's SHELX;Johnson's oarEP. Final atomic coordinates and anisotropic temperature factors are listed in Table 2. Selected bond lengths and bond angles are listed in Table 3, along with the main intramolecular interatomic distances. 2.3. Results and discussion
The X-ray structure determination of the title compound clearly indicates the ANSA structure. The amino end of the arch is bound to the methylated phosphorus atom, while the oxygen end of the 3-amino-l-propanol group is bound to an adjacent phosphorus atom. The general structure of the molecule, the numbering system used for all nonhydrogen atoms, and selected bond angles and distances are shown in Fig. 1. A perspective view of the molecule, clearly displaying the ANSA structure, is shown in Fig. 2. Intermolecular hydrogen bonds observed are discussed below, as in the SPIRO relative N3P3Cla[HN-(CH2)3-NH ] and in another ANSA derivative, N3P3Cla[HN(CH2)3-O-(CH2)2-O-(CH2)3-NH], where two hydrogen bonds (2.08 A) per molecule are considered to be responsible for the crystal packing arrangement [5,18].
Fig. I. Numbering scheme and selected bond angles and distances for N3P3CI3(CH3)[HN-(CH2)3-OI.
Fig. 2. Perspective view of N3P3CI3(CH3)[HN-(CH2)3-O] from X-ray analysis. Several features of the molecule were revealed by the structure determination. (i) The spatial conformation o f the A N S A arch
The conformation of the 3-amino-l-propanol group is shown in Figs. 3 and 4. The central carbon atom in the methylene chain (C2) belongs both to the ~rv-like N3-C12-P2-C13 plane, and to the
Fig. 3. Conformationof the ANSAarch of N~P3C13(CH3)[HN(CH2)3-O] from X-ray analysis.
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N 4 - P 3 - P I - O plane. Carbon atoms C1 and C3 are +0.8 ,A,distant from the latter plane and the carbon atom C3 is situated over the mean plane of the phosphazene ring.
(ii) Conformation of the phosphazene ring The phosphazene ring in the compound is clearly non-planar, as illustrated in Fig. 4. The nitrogen atom, N3, which is a part of both the ANSA ring and the phosphazene ring, is 0.435(3) ,~ below the mean P1-N1-P2-N2-P3 plane. The dihedral angle between the P1-P2-P3 and the P1-N3-P3 planes is 34.6°. However, despite the lack of planarity within the phosphazene ring, the transannular ( P . . . P ) distances are all approximately 2.7 A,, while the exocyclic P3-N4 and P1-O bond lengths are 1.651(4) and 1.579(3) A, respectively, all typical of cyclotriphosphazene bond lengths. The reason the phosphazene ring is puckered, therefore, appears to be as follows: In order for the 3-aminol-propanol group to span phosphorus P1-P3 without affecting the distance between these two atoms, a rotation about bonds P1-N1 and P3-N2 must occur. This rotation is clearly indicated in Fig. 2. The rotation brings atoms N4 and O to within 3.52 ,~, of each other, and forces Cll and C4 apart. The Cll-C4 distance in this
ANSA compound is 5.58 A while the CI-C distances in the aunpuckered N3P3Cla-i-PrH compound are 4.67 A and 4.88 A [35]. Thus, the rotation about P1-N1 and P3-N2, which is necessary to allow the ANSA arch to bridge these two atoms, must force nitrogen N3 down below the plane of the phosphazene ring if the pseudo tetrahedrai geometry around phosphorus P1 and P3 is to be maintained. This, indeed, appears to be the case. *
e
(iii) Bonding within the phosphazene ring The variations "observed in the P-N bond distances within the phosphazene ring are very interesting. As has been observed in other cases [35,36], the lengths of the P - N bonds vary with the electronegativity of the substituents bound to the phosphorus atoms at the ends of a P - N - P island. Consider the island P2-N2-P3 (Fig. 1). Phosphorus atom P2 clearly contains the most electronegative substituents in the ring, while phosphorus atom P3 contains the least electronegative groups. The P2-N2 length of 1.557 ,~,, the shortest in the molecule, and the P3-N2 length of 1.629 ,~,, the longest in the compound, clearly reflect these electronegativity differences. Similar changes can be observed in the islands P 1 - N I P2 and PI-N3-P3. The P1-N1-P2 island has the smallest difference in electronegativity between the phosphorus substituents and also has the smallest variation in P - N bond lengths. However, let us come back to the existence of intermolecular hydrogen bonds which do exist in the unit cell and which may be responsible for eventual significant alterations of the actual molecular structure with respect to the one it would have adopted if constraints-free. Two main questions arise from this point of view, namely about (i) the lack of planarity of the phosphazene ring and (ii) the real conformation of the ANSA arch. A definite answer to both questions is provided by molecular modeling, which is detailed below.
3. Molecular modeling study of Harris ANSA, N3P3CI3(CH3)[HN-(CH2)3-O]
3.1. Software and graphics station Fig. 4. Puckering of the N3i~3 ring of N3P3CI3(CH3)[HN(CH2)3-O] from X-ray analysis.
Results reported here were obtained by using the
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BIOSVM software [32] produced by Biosym Technologies. A home-made CVFF (Consistent Valence Force Field) force field was used because of the quite good reproducibility of the experimental molecular and conformational structures of small cyclophosphazenic moieties, mainly about the morphology of the cyclophosphazene ring, which was found to be strictly planar when the planarity in question had been observed by X-ray investigations. Incidentally, the ESFF (Extended Standard Force Field), which is so convenient for organometallics, is definitely not suitable in the case of genuine cyclophosphazenes, owing to the fact that it yields systematically to folded cyclophosphazene rings, even when a strict planarity was experimentally observed. Introduction of Monte-Carlo and simulated annealing techniques into the modeling were found to be unnecessary. The graphics station we used was 32 Mo Silicon Graphics, coupled with a NEC-DIGITAL color printer. 3.2. Unit cell and molecular packing
BIOSYM software allows the drawing of the unit cell from the X-ray data of Tables I and 2. Then, two strong intermolecular hydrogen bonds are made conspicuous between the N4-H group of arch A and the oxygen atom of the closest arch B ( d ( N 4 - H . . . O ) = 2 . 1 7 ,~, angle ( N 4 - H - O ) = 140°). What is the influence of these intermolecular contacts on the conformation of ANSA arches? Optimization of the four molecules of the unit cell as a whole (through the use of the common molecular mechanics processes included in the aIOSYM software) confirms both (i) the twisted conformation of the arches (as visualized on Fig. 3) and (ii) the lack of planarity of the cyclophosphazene rings. Conversely, a similar optimization (from the X-ray structure as the starting point) on one of these molecules assuming intermolecular contacts were canceled shows that the lack of planarity remains whereas the conformation of the arch becomes more symmetrical (Fig. 5, to be compared with Fig. 3). In other words, the crystallographically twisted conformation is definitely due to intermolecular hydrogen bonds in which oxygen
A
0
O
N
I
(3 Fig. 5. Actual conformation of the ANSA arch of
N3P3CI3(CH3)[HN-(CH2)3-O ] from molecularmodeling. atoms pull nitrogen atoms of related NH groups outside of the arch mean plane. In contrast, it seems that these contacts have no influence on the lack of planarity of cyclophosphazene rings which are actually non-planar. Thus, molecular modeling allows us to reach the real molecular structure of a simple molecular system (such as the one of the title molecule) as well as that of more intricate cyclophosphazenes [29-31]. We shall use this technique for assigning the structure and conformation of other ANSA derivatives and firstly of the simplest one described up to now, N3 P3C14[HN- (CH2)2-O].
4. Molecular modeling study of N3P3CI4 [HN- (CH 2)2-O] 4.1. Synthesis ofN3P3C14[HN-(CH2)2-O ]
This genuine ANSA derivative is obtained neat through a one'step reaction of N3P3CI6 with 2-amino-l-ethanol in Et20 as the solvent. This observation is really amazing considering the well-known difficulties that were encountered by chemists until 1984 in their attempts to produce
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ANSA moieties. Remember indeed that the synthesis of the Harris ANSA had to be performed through a two-step procedure (the second step requiring drastic conditions) [11], the cyclophosphazenic substrate having to be the monomethyl derivative, N3P3C15Me (the same reaction with N3P3C16 itself being definitely unsuccessful). The synthesis of N3P3C14[HN-(CH2)2-O] was achieved in Et20 according to the following pathway
GI
N3P3C16 + H2N-(CH2)2-OH --, N3P2C14[HN-(CH2)2-O ] + 2 HCI Triethylamine was used to pick off the hydrogen chloride. The reaction took 24 h and was considered complete when the 31p NMR singlet of N3P3C16 at 20.09 ppm (Bruker WM 200, CDCI 3, H3PO 4 85% as the standard) had disappeared. The hydrochloride was then filtered off and the solvent removed in vacuo at 25°C to give a white powder. The crude product was the expected ANSA moiety together with traces of N3P3Ci 6 which were eliminated by washing with n-heptane (N3P3CI6 being soluble in this solvent and N3P3CI4[HN-(CH2)2O] being insoluble). Pure ANSA derivative was obtained in 75% yield, Rr t.l.c. = 0.30 with Et20/ n-hexane (1:1) as the eluant (m.p. = 160°C). The 81.015 M Hz 31p NMR pattern comprised a singlet at 24.07 ppm, to be compared with the 21.38, 21.15, 21.14 and 20.97 ppm singlets for ANSA 20202, 30203, 30403 and 3020203, respectively [17,37]. Such a low-field shift may be related to a higher hindrance induced in N3P3CI4[HN-(CH2)2-O ] by the shortness of the methylenic bridge [38]. DCI/NH 3 mass spectrometry revealed the MH + ion at m/z 337 (relative molecular mass = 334 with 35C1) with a satellite distribution confirming the presence of four chlorine atoms in the molecule. No fragmentation peaks were observed, except at m/z 300, which would correspond to the loss of one chlorine atom. Incidentally, the [M, NH~-] molecular peak was also detected at m/z 354.
ctl
ct
Fig. 6. Conformationof the ANSA arch of N3P3CI4[HN(CH2)2-O] frommolecularmodeling. BIOSYM software with the CVFF subroutine. The ab initio optimized structure is visualized in Figs. 6 and 7 which reveals two main features. (i) The endocyclic nitrogen atom which lies between the two arch-bearing phosphorus atoms is pulled down away from the phosphazenic plane in the same way as in N3P3C13(CH3)[HN-(CH2)3O]. The distance between this peculiar N atom and the mean plane of the ring is 0.45 ,~, (to be
0
,
N
Cl
4.2. Molecular modeling o f
N3P3C14 [HN- (CH2)2-O] Results reported here were obtained by using the
Fig. 7. Puckeringof the N3P3 ring of N3P3CI4[HN-(CH2)2-O] from molecularmodeling.
164
F. Crasnier et al./Journal o f Molecular Structure 380 (1996) 157-165
c o m p a r e d with 0.435 ,~ in N 3 P 3 C I 3 ( C H 3 ) [ H N (CH2)3-O], see above). In other words, the n u m b e r o f methylenic links into the arch does not alter significantly the lack o f planarity o f the cyclophosphazenic ring. (ii) A twisted c o n f o r m a t i o n is evidenced for the constraints-free A N S A arch (Fig. 6), in constrast to what was observed in the constraints-free Harris A N S A (Fig. 5), but in agreement with what was m a d e conspicuous for the Harris A N S A in the solid state (Fig. 3), Thus, 3 - a m i n o - l - p r o p a n o l yields a symmetrical A N S A arch whereas 2 - a m i n o - l - e t h a n o l leads to a twisted one. The confrontation between such molecular modeling approaches and the results provided b y X - r a y investigations o f other A N S A moieties confirms the u n a m b i g u o u s efficiency o f molecular modeling for assigning molecular structures within the cyclophosphazenic series in a time-and-money n o n - c o n s u m i n g way.
5. Conclusions In 1996, A N S A cyclophosphazenes are no m o r e Nessies. We contributed from 1984, t h r o u g h a lot o f X-ray analyses, all published in this Journal, to demonstrate that these,peculiar cryptands (according to Lehn's terminology) do really exist. N o w , we hope that it will become possible to replace step by step crystallography by molecular modeling, essentially for saving time and money. If successful, the new a p p r o a c h will be a convenient tool for assigning configurations and c o n f o r m a t i o n s in cyclophosphazenes whatever their size. Indeed, day-after-day improvements in the power o f computers for molecular modeling [40] will allow in the very near future enhancement o f the limits o f the technique in terms o f molecular size. This is essential when the world o f huge dendrimers is currently coming [29-31].
Acknowledgments A u t h o r s are greatly indebted to Drs Ren6e Enjalbert and Jean Galy ( C E M E S Center o f C N R S , Toulouse, France) who achieved to our
privilege more than 20 crystal and molecular structures quoted in the references o f the present paper.
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