J. Am. Chem. SOC.1995, 117, 9972-9981
9972
Solution-State, Solid-state, and Calculated Structures of an a-Lithiated Monophosphazenet Fernando L6pez-Ortiz,*v*Elvira Pelhez-Arango,S Baudilio Tejerina," Enrique PCrez-Carreiio,*and Santiago Garcia-Granda5 Contribution from the Instituto Universitario de Quimica Organometcilica Enrique Moles, Unidad Asociada a1 CSIC, Universidad de Oviedo, 33071 Oviedo, Spain, and Departamento de Quimica Fisica y Analitica, Universidad de Oviedo, 33071 Oviedo, Spain Received April 3, 1 9 9 9
Abstract: The structure of lithiated P-diphenyl(methyl)(N-pheny1)phosphazene (Li+la-) has been determined. The crystal structure consists of monomeric units containing a four-membered ring with the lithium bonded to the nitrogen and methylene carbon atoms of the phosphazene. The coordination sphere of the metal is completed by two molecules of THF. The carbon bonded to lithium has a pyramidal configuration. Short intermolecular Li-phenyl-Pdistances (average 3.10 A) have been measured in the crystal packing. In THF at -1 11 "C, a 93:7 mixture of two compounds 6Li, I3C, 15N,and 31PNMR was found. The structure of the major component was determined by multinuclear 'H, spectroscopy of isotopically labeled 6Li+la-. From the 2JpLi-coupling constant measured and the NOES observed in 2D HOESY spectra, it was concluded that the major component retained a structure similar to that found in the solid state. I5N Characterization was carried out through 31P,15Ntriple resonance experiments. MNDO calculations reproduced well the main structural features of Li+la-. Two local minima interconnected by two transition states were located through MNDO calculations. The preference for the formation of the C-P-N-Li four-membered ring was considered to be of electrostatic nature. No P-Li bonding interaction was predicted, and the calculated dissociation energy for the C-Li was 30.8 kcalmol-I.
Introduction Heteroatom-stabilized anions are among the most useful synthetic intermediates for carbon-carbon bond formation.' Particularly, those derived from phosphorus compounds have played a dominant role in the construction of C-C single and double bonds. Phosphine oxides2 or pho~phonates,~ phosphondiamides," and their thio derivatives5 have been mainly used for that purpose. In recent years we have shown that anions of N-aryl(alky1)-p-diarylmonophosphazenes1 can also be added Ph2
RzYPb"Ph RI
1
la RI=R2=H
l e R1=Rt=CH3 I f Rl= CH3, R2= C2H5 Ig R'= C2H5, R2= CfjHJ
1b Rl= H, R2= CH3
IC RI= H. R2= C2H5 Id R1= H, R2= CH2CH=CH2
to different electrophiles with excellent regio- and stereochemical control of the reaction course.6 Furthermore, stepwise Dedicated to Prof. H. Giinther on the occasion of his 60th birthday.
* Instituto Universitario de Quimica Organomet6lica Enrique Moles.
Departamento de Quimica Fisica y Analitica. Abstract published in Advance ACS Abstracts, August 15, 1995. (1) (a) Evans, D. A.; Andrews, G. C. Acc. Chem. Res. 1974, 7 , 147. (b) Magnus, P. D. Tetrahedron 1977,33,2019. (c) Beak, P.; Reitz, D. B. Chem. Rev. 1978, 78, 275. (d) Krief, A. Tetrahedron 1980,36, 2531. ( e )Solladie, G. Synthesis 1981, 185. (f) Werstiuk, N. H. Tetrahedron 1983,39,205. (g) Hoppe, D. Angew. Chem., Int. Ed. Engl. 1984,23,932. (h) Oae, S.; Uchida, Y. In The Chemistry of Sulfones and Sulfoxides; Sterling, C. J. M., Patai, S., Rappoport, Z., Eds.; Wiley-Interscience: New York, 1988; Chapter 12. (2) (a) Organophosphorus Reagents in Organic Synthesis; Cadogan, J. I. G., Ed.; Academic Press: New York, 1980. (b) Maryanoff, B. E.; Reitz, A. B. Chem. Rev. 1989, 89, 863. (c) Edmundson, R. S. In The Chemistv of Organophosphorus Compounds; Hartley, F. R., Ed.; Wiley-Interscience: New York, 1992; Vol. 2, Chapter 7. I
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0002-7863/95/1517-9972$09.00/0
processes combining the reactivity of the PIN linkage and the a-position of phosphazenes allowed several phosphoruscontaining heterocycles of previously unknown structure to be ~ b t a i n e d . In ~ order to understand the selectivity observed and to explore the applications of phosphazenyl anions in asymmetric synthesis in a rational way, an accurate model of their structure is required. Bxcluding ylides, structural studies on phosphorus-stabilized anions are scarce. The first attemm to elucidate the solution (3) (a) Wadsworth, W. S., Jr. Org. React. 1977, 25, 73. (b) Kirilov, M.; Petrov, G. Monatsh. Chem. 1968,99, 148; (c) Tetrahedron Lett. 1970, 129; (d) 1972,4487; (e) Phosphorus Sulfur 1980, 9, 87. (f) Amer, A,; Zimmer, H. In Handbook of Organophosphorus Chemistry; Engel, R., Ed.; Marcel Dekker: New York, 1992; Chapter 6. (g) Mastalerz, P. In ref 3f, Chapter 7. (h) Mikolajczyk, M.; Mikina, M. J. Org. Chem. 1994, 59, 6760. (4) (a) Denmark, S. E.; Dorow, R. L. J. Org. Chem. 1990, 55, 5926. (b) Denmark, S. E.; Stadler, H.; Dorow, R. L.; Kim, J. H. J. Org. Chem. 1991, 56,5063. ( c )Denmark, S. E.; Chatani, N.; Pansare, S. V. Tetrahedron 1992, 48, 2191. (d) Denmark, S. E.; Chen, C.-T. J. Am. Chem. SOC.1992, 114, 10674. (e) Denmark, S. E.; Amburgey, J. J. Am. Chem. SOC. 1993, 115, 10386. (f) Denmark, S. E.; Chen, C. T. J. Org. Chem. 1994, 59, 2922. (5) For thiophosphonates, see: (a) Corey, E. J.; Kwiatkowski, G. T. J. Am. Chem. SOC.1966,88,5654. For thiophosphinamides, see: (b) Johnson, C. R.; Elliott, R. C. J. Am. Chem. SOC.1982, 104, 7041. (c) Denmark, S. E.; Swiss, K. A. J. Am. Chem. SOC. 1993, 115, 12195. For bis(phosphine sulfides), see: (d) Goli, M. B.; Grim, S. 0. Tetrahedron Lett. 1991, 3631. (6) (a) For an early work on the metalation of N-arylmonophosphazenes, see: (a) Stuckwisch, C. G. J. Org. Chem. 1976, 41, 1173. (b) Barluenga, J.; Lopez, F.; Palacios, F. J. Chem Res. (S)1985, 211. (c) Barluenga, J.; Lopez, F.; Palacios, F.; Cano, F. H.; Foces-Foces, M. J. Chem. SOC.,Perkin Trans. I 1988, 2329. (d) Barluenga, J.; Lopez, F.; Palacios, F Synthesis 1988, 562; (e) 1989, 289. For metalated N-(trimethylsilyl) derivatives, see: (f) Schmidbaur H.; Jonas, G. Chem. Ber. 1967,100, 1120. (g) Wettermark, U. G.; Wisian-Neilson, P.; Scheide, G. M.; Neilson, R. H. Organometallics 1987,6,959 and references therein. Metalations of cyclotriphosphazenes6hs1 and polyphosphazenes6J have also been described: (h) Calhoun, H. P.; Lindstrom, R. H.; Oakley, R. T.; Paddock, N. L.; Todd, S. M. J. Chem. SOC., Chem. Commun. 1975, 343. (i) Gallicano, K. D.; Oakley, R. T.; Paddock, N. L.; Sharma, R. D. Can. J. Chem. 1981, 59, 2654. (j)Neilson, R. H.; Wisian-Neilson, P. Chem. Rev. 1988, 88, 541. (7) (a) Barluenga, J.; lope^, F.; Palacios, F. J. Chem. Soc., Chem. Commun. 1985, 1681; (b) 1986, 1574; (c) Tetrahedron Lett. 1987,28,2875; (d) J. Organomet. Chem. 1990, 382, 61.
0 1995 American Chemical Society
Structure of an a-Lithiated Monophosphazene structure of phosphorus-containing anionic species were by the groups of Kirilov8 and Seyden-Penne? These works were based on the analysis of IR and NMR spectra of Homerlo* or Wadsworth-Emmons reagents,Iob i.e., a-lithiated phosphine oxides and phosphonic acid esters, respectively. More recently, Denmark et al. have contributed significantly to this area through detailed solid-state and solution studies on lithiated phosphonamides. A major difficulty concerning the structural elucidation of organolithium compounds in solution is their association behavior, which may vary depending on the reaction conditionsI2 and may even differ from that found in the solid state.I3 The classical methods of determination of the aggregation state in solution are based on colligative propertie~.'~However, NMR spectroscopy has additional advantages due to its simplicity and higher information ~0ntent.I~ The method relies on the existence of a scalar coupling between lithium and a directly bonded nucleus. For this coupling to be observed two criteria have to (8) (a) Kirilov, M.; Petrov, G. Chem. Ber. 1971,104,3073; (b) Monatsh. Chem. 1972, 103, 1651. (9) (a) Bottin-Strzalko, T.; Seyden-Penne, J.; Simonnin, M.-P. J. Chem. SOC., Chem. Commun. 1976, 905; (b) J. Org. Chem. 1978, 43, 4346. (c) Bottin-Strzalko, T.; Corset, J.; Froment, F.; Pouet, M.-J.; Seyden-Penne, J.; Simonnin, M.-P. J. Org. Chem. 1980, 45, 1270. (d) Seyden-Penne, J. Bull. SOC. Chim. Fr. 1987, 238. ( e ) Bottin-Strzalko, T.; Seyden-Penne, J.; Froment, F.; Corset, J.; Simonnin, M.-P. J. Chem. Soc., Perkin Trans. 2 1987, 783; (f) Can. J. Chem. 1988, 66, 391. See also: (g) Colquhoun, J.; Christina, H.; McFarlane, E.; McFarlane, W. J. Chem. SOC., Chem. Commun. 1982, 220. (h) Teulade, M.-P; Savignac, P.; About-Jaudet, E.; Collignon, N. Phosphorus Sulfur 1988, 40, 105. (i) Yuan, C.; Yao, J.; Li, S.; Mo, Y.; Zhong, X. Phosphorus Sulfur 1989, 46, 25. 0) Patios, C.; Ricard, L.; Savignac, P. J. J. Chem. Soc., Perkin Trans. 1 1990, 1577. (k) Yao, C. Y. J.; Li, S. Phosphorus Sulfur 1990, 53, 21. (10) (a) Homer, L.; Hoffmann, H.; Wippel, H. G. Chem. Ber. 1958, 91, 61. (b) Wadsworth, W. S.; Emmons, W. D. J. Am. Chem. SOC. 1961, 83, 1733. See also ref 2 and 3. (11) (a) Denmark, S. E.; Dorow, R. L. J. Am. Chem. SOC. 1990, 112, 864. (b) Denmark, S. E.; Miller, P. C.; Wilson, S. R. J. Am. Chem. SOC. 1991, 113, 1468. (c) Denmark, S. E.; Swiss, K. A,; Wilson, S. R. J. Am. Chem. Soc. 1993,115,3826. (d) Denmark, S. E.; Swiss, K. A. J. Am. Chem. SOC. 1993,115, 12195. X-ray structures of phosphorus-stabilized anions of magnesium and copper salts of keto phosphonateslIesf and lithiophosphinomethanidesllg-k have been reported. For leading references, see, for example: (e) Weiss, E.; Kopf, J.; Gardein, T.; Comellin, S.; Schumann, U.; Kirilov, M.; Petrov, G. Chem. Ber. 1985, 118, 3529. ( f ) Macicek, J.; Angelova, 0.;Petrov., G.; Kirilov, M. Acta Crystallogr., Sect. C 1988, 44, 626. (g) Byme, L. T.; Engelhardt, L. M.; Jacobsen, G. E.; Leung, W.-P.; Papasergio, R. I.; Raston, C. L.; Skelton, B. W.; Twiss, P.; White, A. H. J. Chem. Soc., Dalton Trans. 1989, 105. (h) Fraenkel, G.; Winchester, W. R.; Willard, P. G. Organometallics 1989, 8, 2308. (i) Karsch, H. H.; Richter, R.; Paul, M.; Riede, J. J. Organomet. Chem. 1994,474, C1 and references therein. (i)Winkler, M.; Lutz, M.; Muller, G. Annew. Chem., Znt. Ed. End. 1994, 33; 2279. (k) Pape, A,; Lutz, M.; Muller G. Angew. Chem., Int. Ed. E n d . 1994, 33, 2281. (12) (a) Schlosser, M. In Organometallics in Synthesis, a Manual; Schlosser, M., Ed.; John Wiley: New York, 1994; Chapter 1. (b) Wardell, J. L. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A,, Abel, E. W., Eds.;%PergamonPress: Oxford, 1982; Vol. 1, Chapter 2. (13) Reviews: (a) Setzer, W. N.; Schleyer, P. v. R. Adv. Organomet. Chem. 1985, 24, 353. (b) Schade, C.; Schleyer, P. v. R. Adu. Organomet. Chem. 1987, 27, 169. (c) Williard, P. G. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, Y., Schreiber, S. L., Eds.; Pergamon Press: Oxford, 1991; Vol. 1, Chapter 1. (d) Seebach, D. Angew. Chem., In?. Ed. Engl. 1988.27, 1624. (e) Boche, G. Angew. Chem., Int. Ed. Engl. 1989,28,277. (f) Markies, P. R.; Akkerman, 0. S.; Bickelhaupt, F.; Smeets, W. J. J.; Spek, A. L. Adv. Organomet. Chem. 1991, 32, 147. (g) Gregory, K.; Schleyer, P. v. R.; Snaith, R. Adu. Inorg. Chem. 1991, 37, 47. (h) Mulvey, R. Chem. SOC. Rev. 1991, 20, 167. (i) Hanusa, T. P. Chem Rev. 1993, 93, 1023. (j)Weiss, E. Angew. Chem., Int. Ed. Engl. 1993,32, 1501. (14) (a) Bauer, W.; Seebach, D. Helv. Chim. Acta 1988, 67, 1972 and references therein. (b) Davidson, M. G.; Snaith, R.; Stalke, D.; Wright, D. S. J. Org. Chem. 1993, 58, 2810. (15) Reviews: (a) Gunther, H.; Moskau, D.; Bast, P.; Schmalz, D.Angew. Chem., Int. Ed. Engl. 1987, 26, 1212. (b) Akitt, J. W. In Multinuclear NMR; Mason, J., Ed.; Plenum Press: New York, 1987; Chapter 7, p 189. (c) Thomas, R. D.In Isotopes in the Physical and Biomedical Sciences; Buncel, E., Jones, J. R., Eds.; Elsevier: Amsterdam, 1991; Vol. 2, Chapter 7. (d) Bauer, W.; Schleyer, P. v. R. Adu. Carbanion Chem. 1992, I , 89.
J. Am. Chem. Soc., Vol. 117, No. 40, 1995 9913 be met: (i) slow interaggregateexchange in the NMR time scale and (ii) slow relaxation of the lithium nucleus. The first condition can generally be fulfilled by working at very low temperatures. The second suggests a preference for the 6Li over the 7Li isotope because of its low quadrupolar moment,i5b although coupling constants to 6Li are lower than those to 'Li by a factor of 2.64 ( ~ 7 ~ i l y 6 ~ i ) . A common feature of organolithium phosphorus(V) compounds containing P - X (X = N, 0, S) groups is the lack of a carbon-lithium bond, a situation similar to that found in monometalated sulfoxidesI6and s u l f o n e ~ . In ~ ~this ~ , paper ~ ~ we report the first single-crystal X-ray and solution structure of a lithium monophosphazenyl derivative showing the existence of a nitrogen- and a carbon-lithium bond. These results are compared with the theoretical structure obtained by MNDO calculations. The X-ray analysis has been carried out on crystals formed in a THF solution. The solution structure has been derived from a multinuclear NMR study (IH, 6Li, I3C, 15N,and 3'P), including several combinations of triple resonance experiments. The effect of metalation of l a on several NMR parameters is discussed in terms of changes in geometry and charge distribution versus the parent compound, and the results are in good agreement with those expected from the computed structure. The stability of the ring formed between the bidentate phosphazenyl ligand and the lithium was analyzed according to the MNDO method.
Experimental Section General. All manipulations were carried out using inert atmosphere techniques or in a MBraun MB 150-GI drybox. Isotopically labeled [6Li]-n-butyllithium-hexane solutions were prepared according to the Gilman method.17 6Li 96% (Aldrich) was used, and the concentration was determined by double titration with 0.1 N HCI. Solvents (THF and THF-da) were freshly distilled from potassium benzophenone ketyl and degassed three times prior to use. NMR samples were flame sealed and stored at -30 "C. X-ray Crystallography. Suitable single crystals were selected, mounted at room temperature, and immediately placed in the lowtemperature nitrogen stream at 200 K. X-ray data were collected with an Enraf-Nonius CAD4 diffractometer (Mo K a radiation, graphite monochromator, 1 = 0.710 73 A) equipped with a low-temperature device. The structure was solved by Patterson interpretation and phase expansion (DIRDIF),'* and the refinement was performed by full-matrix least-squares procedures (SHELX76).I9 Hydrogen atoms were refined isotropically riding on their parent atom with a common thermal parameter. All calculations were made at the University of Oviedo on the Scientific Computer Center and X-Ray Group Vax computers. Crystal data: C ~ T H ~ ~ L ~ M NW O ~=P441.48, , monoclinic, space group C2/c, a = 31.76(6) A, b = 9.69(1) A, c = 17.8(1) A, 8 , = 115.8(4)', V = 4930(47) A3, Z = 8, D, = 1.19 g/cm3, p = 1.29 cm-I, F(000) = 1888, T = 200(2) K, R = 0.048, and R, = 0.044 for 1438 observed reflections and 281 variables. Preparation of the Lithium N-Phenylmethylenediphenylphosphazene Complex. Methyldiphenylphosphazene (5 g, 17.2 mmol) in 10 mL of dry THF were added dropwise to an n-butyllithium-hexane solution (7.2 mL, 18.1 mmol) at -30 "C. The yellow solution was (16) C-Li bonds have been observed in [(2,2-diphenyl-l-(phenylsulfonyl)cyclopropyl)lithium]~dimethoxetane16aand dilithio[(trimethylphenyl)sulf~nyl]methylsilane:~~~ (a) Hollstein, W.; Harms, K.; Marsch, M.; Boche, G. Anaew. Chem.. Int. Ed. Enal. 1988, 27. 846. (b) Gais, H. J.; Vollhardt, H.; Gikher, H.; Moskau, D.; iindner, H. J.; Braun, S. J. Am. Chem. Soc.
IPAR -_ _ _ ,-110 - -, 918 - . _. (17) Gilman, H.; Beel, J. A.; Brannen, C. G.; Bullock, M. W.; Dum, G. E.; Miller, L. S. J. Am. Chem. SOC. 1949, 71, 1499. (18) Beurskens, P. T.; Admiraal, G.; Behm, H.; Beurskens, G.; Bosman, W. P.; Garcia-Granda, S.; Gould, R. 0.;Smykalla, C. The DIRDIF Program System. Technical Report of the Crystallography Laboratory, University of Nijmegen, The Netherlands, 1992. (19) Sheldrick, G . M. SHELX, A Program for Crystal Structure Determination; University Chemical Laboratory, Cambridge, England, 1976.
9974 J. Am. Chem. SOC.,Vol. I 1 7, No. 40, I995 stirred for 2 h and then evaporated to dryness under reduced pressure. On addition of 5 mL of dry THF at -30 "C. a precipitate formed. The yellow powder was filtered and redissolved in 10 mL of THF. On the solution standing at -30 "C for I week, colorless crystals were isolated and used for the X-ray and NMR studies. N M R Spectroscopic Analysis. NMR spectra were NI?on a Bluker AMX400 spectrometer equipped with a third radio frequency channel. A 5 mm reverse triple probe head was used. The inner coil was doubly tuned for 'H and "P. and the outer coil was tunable in the frequency range 18- 160 MHc. The pulse widths for the 90" pulses and operating frequencieswere 10.4 (IH, 400 MHc). 17.1("N, 40.5 MHz). 16.7 (hLi, 58. MHz), 14.5 ("C, 100.6 MHz), and 14.5 ps ("P. 161.98 MHz). The attenuation levels used were 5 d B for the proton channel and 3 dB for the heteronuclei. The spectral references used were tetramethylsilane for 'H and "C. 85% H3PO, for "P, nest nitromethane for 'IN, and I M LiBr in D 2 0 for fiLi. Selected spectral parameters were as follows. One-dimensional IH NMR: 32K data points: spectral width, 4000 Hr. "C NMR: 32K data point: spectral width, 18 000; exponential multiplication with a line broadening factor of I Hr. "P NMR: 32K data points; spectral width. 6000 Hz: exponential line broadening of I Hz. "Li NMR: 16K data points; spectral width, 600 Hz; exponential line broadening of 0.5 Hc. 'IN,"P INEPT: 32K data points: spectral width, 4000 Hz; exponential line broadening of 2 Hz. The experiment was conducted overnight because our prohe head was designed for 'W"P observation,and about 50% loss of sensitivity is estimated when a heteronucleus is directly ohserved through the broad band channel. hLi.'H2D HOESY: spectral width, 200 Hz in F2 and 4000 Hc in FI: 48 increments recorded: final matrix after zero filling, 512 x 256; mixing time, 2.0 s; 40 scans1 increment i n FI: qsine multiplication of R/2 in both dimensions prior to transformation. "P."C 2D HMQC: spectral width, 800 Hz in F2 and 1000 HZ in FI: 128 increments recorded: final matrix after zero 5.6 ms: 40 scanslincrement filling. 1024 x 256; evolution delay of 'Jr3 in FI: qsine multiplication of x12 in both dimensions prior to transformation. "P,'sN 2D HMQC: spectral width, 800 Hz in F2 and 1000 Hz in FI; 96 increments recorded: final matrix after zero filling. 1024 x 256: evolution delay of I J p ~18.7 . ms; 200 scanslincrement in FI: qsine multiplication of x12 in both dimensions prior to transformation. Computational Methods. Considering the large size of the mnlecules under issue, we have restricted our computational study to a semiempirical level of theory. MNDO calculations were carried out using the MOPAC'" and GAMESS" programs. The programs used depended upon the computational problem being treated. The stationary points located on the potential energy surface(PES) were characterized by harmonic vibrational frequencies analysis. The X-ray geometry of Li'la- was used as a starting point for the calculations. The fourmembered ring was forced to break by increasing the torsion angle (see Figure 4). CI-PI-NI-LiI
(THF)i iO-2i
Li+la-
Results X-ray Crystallographic Analysis of Li+la-. The lithiated anion of l a (abbreviated as Li+la-) crystallizes from THF as a monomer with two molecules of solvent coordinated to the (20) (a) MOPAC: Stewart. I. J. P. QCPE &I/. 1990. NO.445. (b) MNDO Dewar, M. J. S.: Thiel. W. J. A m Chem. Soc. 1977, 9Y. 4899.
4907.
(21) GAMESS: Schmidt, M. W.; Baldridge. K. K.: Boatr. I. A,: Elbert, S. T.: Gordon. M. S.: Jensen. J. H.; Koseki. S . : Mataunaga. N.: Nguyen. K. A.: Su. S. J.; Windur, T. L.; Duouis. M.: Montgomery, J. A. J. Comput. Chem. 1993. 14. 1347.
Lhpez-Orriz et a/.
Figure I. Molecular structure of compound Liila- determined by X-ray including the numbering scheme used. The H atoms are omitted for clarity. lithium atom. The molecular structure and the adopted numbering scheme are shown in Figure I . The unit cell and a detail of the crystal packing are included in the supporting information. Selected bond distances and angles are given in Table I . The unit cell is formed by eight molecules arranged in pairs, where each lithium atom is close to one of the P-phenyl rings of a molecule belonging to a neighbor unit cell. The packing of the crystal consist of pairs of molecules affording a tridimensional figure resembling a helical structure. The shortest Li-H intermolecular distances were found for the meta protons of the corresponding aromatic rings (LiI-HIO', average 3.1 8,). The most important structural characteristic of Li+la- is the four-membered ring formed by the P1-NI-Lil-CI atoms, which implies that the lithium is bonded to both the nitrogen, 2.00(1) 8,. and CI, 2.23(1) A, atoms. The P-Li distance of 2.65(3) 8, has a value appropriate for the existence of a bonding interaction,'tx.22but it must be bome in mind that the rigidity of the molecule would force these atoms to be close, anyway. The ring slightly deviates from planarity due to a displacement of 8.0(1)" of the lithium atom above the plane defined by the CI-PI-NI atoms. The lithium is solvated by two molecules of THF (average Li-0 bond lengths 1.94 8,) and has a distorted tetrahedral coordination geometry, with a very small NI-LiIC1 angle of 77.2(4)", a constraint imposed by the chelating phosphazenyl anion. The other bond angles at lithium lie between 106.3(5)" and l19.9(5)". The P atom shows a tetrahedral coordination, with bonding angles in the range of 105.0(2)-115.5(3)". It is interesting to point out the large angle of 105.6(3)" formed by the CI-PIN1 framework of the four-membered ring. The P-N bond distance of 1.614(4) 8, is significantly shorter than a formal single bond (cf. values of 1.ll" and 1.57 8,24for the formal P-N single and double bonds, respectively), which suggests a bond order larger than unity. Accordingly, the N1-C2 bond length of 1.381(5) 8, is typical of planar N sp2 bonded to a C sp2carbon.25 The exocyclic bond angles involving the nitrogen are close to 120". whereas a value of 93.4(4)" is found for the
-
(22) Compare (a) P-Li [ ( T M E D A ) L I C I A I M ~ ~ C ( S ~ M ~= ~ )2.67 ~PM~~] 8, (Karah. H. H.: Zellner. K.: Muller. G. Orpnometallic.? 1991. IO, 2884). (b) P-Li(mean) [[LiP(SiMei)ilsl= 2.51 A (Hey-Hawkins.E.: Sattler. E. 3. Chem. Soc.. Chern. Comnrun. 1992. 775). (23)Allcock. H. R. Chrm. Re". 1972. 72. 315. 124) Hewlinr, M. J. E. J. Chem. Snc. ( E ) 1971. 942. ( 2 5 ) Allen. F. H.; Kennard. 0.; Watson. D.G.: Brammer, L.: Orpen. A. G.: Taylor. R. J. Clwrn. Sot., Pvrkin Troni. 2 1987. SI.
J. Am. Chem. SOC., Vol. 117, No. 40, 1995 9975
Structure of an a-Lithiated Monophosphazene
Table 1. Selected Bond Lengths (A), Bond Angles (deg), and Torsion Angles (deg) of Li+la' bond lengths bond angles C1-P1-N1 = 105.6(3) P1-N1 = 1.614(4) Lil-N1-P1 = 93.4(4) P1-C1 = 1.707(7) C1-Li-N1 = 77.2(4) Pl-C8 = 1.817(3) P1-C14 = 1.813(3) Lil-C1-P1 = 83.0(4) Cl-Lil = 2.23(1) C2-N1-P1 = 129.5(4) N1-Lil = 2.00(1) C8-P1,-Cl = 115.5(3) C8-Pl-Cl4 = 105.0(2) Nl-C2 = 1.381(5) Lil-01 = 1.94(1) 01-Lil-02 = 106.3(5) Lil-02 = 1.95(1) Hl-Cl-H2 = 114.0(6) H1-C1-P1 = 115.0(4)
torsion angles C1-P1-N1-Lil = 8.(1) C1 -P1 -N1 -C2 = - 16242) Cl-Pl-C8-C13 = 55.(2) Cl-Pl-Cl4-Cl9 = -69.(2) Pl-Nl-C2-C3 = 172.(2) P1-N1-Lil-01 = 113.(1) Lil-Nl-C2-C3 = 6.(3) C8-Pl-Nl-C2 = -37.(2) C8-P1-N1-Lil = 133.(1) C8-Pl-Cl4-Cl9 = 106.(2)
Table 2. Selected NMR Data for 6Li+la- Recorded at - 111 "C in THF-ds" 6 'H(As) ( P P ~ ) 6 I3C (Ad) ( P P 4 " J (AJ) (Hz) 6 3'P 6 I5N H1 H3 c1 C8/C14 (A61 (ppm) (Ad) (ppm) 'JPCI JPCS/CM 'JPN 'JPH -0.08 (-2.44) 6.45 (0.25) 7.39 (-6.23) 140.36 (5.83) 23.73 (21.69) -274.56 (24.6) 90.1 (27.7) 69.5 ( -29.0) 26.8 ( -9.8) 0.8 ( -12.1)
'
Ad or AJ is referenced to the difference between the metalated and neutral la. Positive Ad numbers are downfield shifts.
metalation extends to C2, which is observed as a clear doublet inner P1-N1-Lil angle. The P1-C1 bond distance of 1.707than to a of 2 J=~5.1 Hz. However, lJcH= 132.9 Hz for C1 increases (7) 8, is closer to a formal single bond (1.80 formal double bond (1.57 8,). The sum of bond angles of C1 only moderately (AJ = 6.5 Hz), indicating a very small increase is only 343" on average, thus indicating a pyramidal configuin s character, Le., C1 remains sp3 hybridized in metalated la. ration for C1. However, a P1-C1-Lil angle of 83.0(4)" is On the other hand, C1 shows a half-width of 20 Hz, and no found as consequence of the four-membered ring to which C1 6Li,'3C-couplingis resolved. Among other causes, broad signals belongs. may be related to intermolecular aggregation, which in tum are The aromatic rings at phosphorus are arranged nearly favored by high concentrations. Therefore, a dilute sample of perpendicular to each other (the angle between the planes formed 6Li+la- (0.1 M) was prepared and the I3C NMR spectrum by the rings is 101.7(6)"), whereas the phenyl ring bonded to measured. At this concentration, the measured line width of nitrogen lies slightly rotated from the plane defined by C116 Hz for C1 is still too large to resolve any I3C,6Li-coupling, P1-N1 (torsion angle Pl-Nl-C2-C3 of 172.0(2)") in the even with resolution enhancement processing of the FID. same sense as the lithium atom does. As a result, H3 approaches 31PNMR Spectra. The 31P NMR spectrum of 6Li+laLil at a distance as short as 2.89(1)8,. shows two resonances at 23.73 and 30.76 ppm in an approximate Solution Structure of Li+la- in THF. Crystals of 6Li+laratio of 93:7 and with half-widths of 13 and 3 1 Hz, respectively were dissolved in THF-ds, and the NMR spectra were recorded (Figure 2a). Line narrowing was observed when a more diluted at different temperatures. Samples with lithium in natural sample was used (0.1 M, 11 Hz for the major and 25 Hz for abundance afforded very broad signals, which prevented a the minor components), but no 3'P,6Li-coupling could be detailed analysis. The following discussion refers to 0.2 M detected in any case. These two signals merged at -72 "C into samples 95% enriched in the 6Li isotope and measured at - 111 a singlet at 23.78 ppm. Some other very small signals that could "C, unless otherwise stated. be distinguished in the spectrum were assigned to impurities 'H and 13CNMR Spectra. Metalation of l a is accompanied because their temperature line widening and coalescence were by marked changes in chemical shifts and coupling constants not related to major signals. They amount to
