Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83–128) The determination of the molecular structure of 2-fluoro-3,5-di-tert-butyl-1,3,2-oxazaphospholene by means of electron diffraction and ab initio calculations Victor A. Naumov,*a Marwan Dakkuori,*b Rida N. Ziatdinovaa and Heinz Oberhammer*c a A. E. Arbuzov Institute of Organic and Physical Chemistry, Russian Academy of Sciences, 420088 Kazan, Russian Federation.E-mail: naumov@glass.ksu.ras.ru b Abteilung für Elektrochemie, University of Ulm, 89069 Ulm, Germany. Fax: +49 731 502 5409; e-mail: marwan.dakkuori@chemie.uni-ulm.de c Institut für Physikalische und Theoretische Chemie, University of Tuebingen, 72076 Tuebingen, Germany. Fax: +49 7071 29 6910; e-mail: heinz.oberhammer@uni-tuebingen.de The molecular structure of gaseous 2-fluoro-3,5-di-tert-butyl-1,3,2-oxazaphospholene has been determined by electron diffraction and ab initio calculations.The structural analysis has shown that the diheterophospholene ring possesses a P-envelope conformation with axial orientation of the P–F bond which is very long, 1.641(11) Å. It is well known1 that the P–Cl bond length depends strongly on the nature of the substituents at phosphorus.In compounds of the type 1–4: the P–Cl bond length changes from 2.04 Å (type 1) to 2.18 Å (type 4). It is interesting to note that P–Cl bonds in compounds of type 4 are considerably longer than the sum of the covalent radii of P and Cl (2.09 Å). One can expect that similar changes in P–F bond lengths occur in fluorine-containing compounds of phosphorus.With this aim we studied the structure of 2-fluoro- 3,5-di-tert-butyl-1,3,2-oxazaphospholene in the gas phase by electron diffraction. Diffraction photographs were recorded at 40 °C on Kazan EMR-100M ED-instruments at three camera distances which cover the s-ranges 3.00 < s < 10.50, 4.00 < s < 15.25 and 11.00 < s < 27.50 Å–1. Refinements of the structure were carried out by a least-squares method based on molecular intensities.The geometric structures were also optimized by ab initio calculations at the HF/6-31G** level.2 Vibrational amplitudes lij and perpendicular amplitudes Kij were derived from the theoretical force field (HF/3-21G*)2 and used in the experimental analysis. For describing the geometry of the five-membered ring, the bond lengths P–O, P–N, C=C and N–C(4), the bond angles O–P–N, C=C–N, and P–N–C(4), and the torsional angle C(4)–N–P–O were chosen as independent parameters (Figure 1).Planarity was assumed for the N–C=C–O moiety. This assumption is justified by the ab initio calculation which predicts this dihedral angle to be 0.4°. The substituents and their orientation were described by the P–F, N–C(7), C(5)–C(8), C–C(Me) and C–H bond lengths, the O–P–F, N–P–F, P–N–C(7), C(4)–N–C(7), C=C–C(8), N–C(7)–C(Me) and C(5)–C(8)–C(Me) bond angles and by the C(9)–C(7)–N–P and C(21)–C–C=C dihedral angles.Because of large correlations between some of the geometric parameters the following assumptions were made during the structure refinement: (1) C3v symmetry was assumed for the methyl and tert-butyl groups and all C–C bond lengths and C(Me)–C–C(Me) bond angles in the two groups were set as equal.These assumptions are justified by the ab initio calculations which predict deviations to be less than 0.007 Å and 0.8°. (2) The differences between bond lengths, d(PX), d(NC), d(CC), and between bond angles �(XNC), �(XPF) were constrained to the ab initio values (see Table 1).The bond length differences were applied to the ra structure. The refined bond lengths were converted to ra distances. PCl3 Cl2PR Cl–P O N O Cl–P Cl–P O N N and 1 2 3 4 aUncertainty values in parentheses are 3s values. bMean values for similar parameters. cd(PX) = r(P–O) – r(P–N). dDependent parameter. ed(NC) = = r[N–C(4)] – r[N–C(7)].fd(CC) = r[C(5)–C(8)] – r[C–C(Me)]. g�(XNC) = �[P–N–C(7)] – �(C–N–C). h�(XPF) = �(O–P–F) – �(N–P–F). Table 1 Geometric parameters for 2-fluoro-3,5-di-tert-butyl-1,3,2-oxazaphospholene as obtained from electron diffraction analysis and ab initio calculations. Bond lengths/Å Bond angles/° Parameters Experimental ab initio Parameters Experimental ab initio ra ra O–P–N 92.7(11) 91.2 P–F 1.641(11)a 1.638 1.610 P–N–C(4) 107.5(12) 108.7 P–O 1.645 (9) 1.644 1.628 C=C–N 112.4(16) 112.9 P–N 1.706 1.705 1.689 C=C–Od 113.0(18) 111.2 d(PX)c –0.061(20) –0.061 P–O–Cd 112.1(20) 112.8 C–Od 1.365(9) 1.362 1.387 P–N–C(7) 125.1 (19) 127.1 C=C 1.344(ass.) 1.342 1.318 C–N–C 119.9 121.9 N–C(4) 1.435 (9) 1.431 1.414 �(XNC)g 5.2 5.2 N–C(7) 1.494 1.490 1.473 O–P–F 102.5 (20) 98.8 d(NC)e –0.059 –0.059 N–P–F 105.1 101.4 C–C(Me) 1.542 (3) 1.535 1.535b �(XPF)h –2.6(11) –2.6 C(5)–C(8) 1.513 1.506 1.506 C=C–C 131.4(42) 132.9 d(CC)f –0.029 –0.029 N(C5)–C–C(Me) 110.6 (11) 109.5b C–H 1.095(4) 1.079 1.085 =C–C–C(Me) 110.6 109.5b C(Me)–C(7,8)–C(Me)d 108.4(11) 109.5b R-factor: 3.44 Torsional angles/° C(4)–N–P–O 13.2(54) 15.5 C(9)–C(7)–N–P 144.7(60) 139.5 C(Me)–C–C=C 126.1(98) 120.0Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83-128) With these assumptions the least-squares refinement resulted in a C=C bond [1.344(29) Å] and a C–O bond [1.366(24) Å] with very large error limits. The C=C bond length correlates strongly with other ring bonds and angles.This value is in good agreement with the experimental C=C bond length in methyl vinyl ether, 1.343(6) Å.3 On the other hand, HF/6-31G** values for C=C double bond lengths are known to be too short.On the basis of the experimental C=C bond length in methyl vinyl ether and HF/6-31G** values for this compound (1.320 Å) and for the phospholene (1.318 Å), an ra value of 1.341 Å is estimated in the oxazaphospholene. Therefore the C=C distance was constrained to 1.344 Å (Table 1).The structure analysis showed that the diheterophospholene ring possesses a P-envelope conformation with axial orientation of the P–F bond. The tert-butyl group bonded to N is pseudoequatorial. The sum of angles at the N atom is 354.2(15)° in good agreement with the ab initio result of 357.7°. As expected from trends in P–Cl bond lengths, the P–F bond in oxazaphospholene [1.641(12) Å] is very long.It is about 0.08 Å longer than that in PF3 [1.565(1) Å].4 Intermediate values have been reported for compounds of the type F2PX, e.g. F2PBut [1.589(4) Å],5 F2PNH2 [1.587(4) Å]6 and F2POCH3 [1.591(10) Å].7 A very long P–F bond has been observed in FPBu2 t 1.619(7) Å.5 The variations in the P–F bond lengths, however, are considerably smaller than those for the P–Cl bond lengths.The C–O and C–N bonds in the ring are shorter than normal single bonds and this indicates conjugation in the O–C=C–N part of the ring. The P–O and P–N bonds in the ring, however, are longer than or equal to those in non-cyclic compounds such as P(OCMe)3 [r(P–O) = 1.620(2) Å],1 F2POMe [r(P–O) = = 1.56(2) Å], P(NMe2)3 [r(P–N) = 1.70(1) Å],1 ClP(NMe2)2 [r(P–N) = 1.730(5) Å]1 and F2PNH2 [r(P–N) = 1.650(4) Å].In cyclic compounds such as 2-chloro-3-methyl-1,3,2-oxazaphospholane or in 1,3,4,2-oxadiazaphospholene P–O and P–N bonds are 1.62–1.63 and 1.70 Å respectively.1 We are grateful for financial supn Foundation for Basic Research (grant no. 96-03-00008G) and to the Deutsche Forschungsgemeinschaft which made this cooperative research possible. We are grateful to Dr.Yu. V. Balitzkii for providing us with the sample of oxazaphospholene. References 1 V. A. Naumov and L. V. Vilkov, Molekulyarnye struktury phosphororganicheskikh soedinenii (Molecular structures of organophosphorus compounds), Nauka, Moscow, 1986, p. 319 (in Russian). 2 (a) M. J. Frisch, G.W. Trucks, H. B. Schlegel, P.M.W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Petersson, J. A. Montgomery, K. Raghavachari, M. A. Al-Laham, V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D.J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzalez, and J. A. Pople, Gaussian-94, Revision D.4, Gaussian, Inc., Pittsburgh PA, 1995. (b) SPARTAN version 5.0, Wavefunction, Inc., 18401 Von Karman Avenue, Suite 370, Irvine, CA 92612, USA. 3 Van den Enden and H. J. Geise, J. Mol. Struct., 1983, 97, 139. 4 Y. Kawashima and A. P. Cox, J. Mol. Struct., 1977, 65, 318. 5 H. Oberhammer, R. Schmutzler and O. Stelzer, Inorg. Chem., 1978, 17, 1254. 6 H. Britton, J. E. Smoth, P. L. Lee, K. Cohn and R. H. Schwendeman, J. Am. Chem. Soc., 1971, 93, 6772. 7 E. G. Godding, C. E. Jones and R. H. Schwendeman, Inorg. Chem., 1974, 13, 178. Figure 1 Atomic numbering of 2-fluoro-3,5-di-tert-butyl-1,3,2-oxazaphospholene. C(25) C(8) C(21) C(29) C(5) C(4) O(3) N(2) P(1) F(6) C(7) C(13) C(9) C(17) Received: Cambridge, 27th January 1998 Moscow, 14th April 1998; Com. 8/00718G