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Journal of the American Chemical Society
/ I00:9 1 April 26, I978
Cyclotetraphosphanes: Geometry, lJ(PP) Nuclear Magnetic Resonance Coupling Constants, and Phosphorus Chemical Shift Anisotropy. A Nuclear Magnetic Resonance Study in Liquid Crystals J. P. Albrand, A. Cogne,* and J. B. Robert* Contribution from the Laboratoire de Chimie Organique Physique, Equipe de Recherche AssociCe au CNRS No. 674, DCpartement de Recherche Fondamentale, Centre d’Etudes NuclCaires de Grenoble, 85 X , F.38041 Grenoble Cedex. France. Received August 15, I977
Abstract: The 31P (‘H) or {I9F)N M R spectral analysis of five cyclotetraphosphanes (RP)4 (R = CH3, t-Bu, i-Pr, C2H5, cC6Hll) oriented by means of nematic phases is reported. The N M R spectrum in oriented phase allows us unambiguously to assign a tetrameric formula to the compounds under study. The ring puckering has been measured and shows a good agreement with x-ray data. The IJ(PP) values which are unobservable in isotropic phase have been measured. The phosphorus chemical shift shows a large anisotropy which ranges from 130 (R = CF3) to 172 ppm (R = i-Pr).
The structure and ring size of cyclopolyphosphanes (PR), from isotropic solution, and information concerning the has been, for a long time, a matter of dispute as illustrated by phosphorus chemical shift tensor, for which only the trace is the lasting uncertainty about the structure of the various forms obtained in the isotropic liquid phase. The data collected here, of “phosphobenzene”, (CsHsP),, 1-4 and by the controversy along with the ones already reported for (t-BuP)4 and (CF3P)4, over the first reported cyclotriphosphane, (C2F5P)3.5,6Since allow a more comprehensive discussion of the substituent efa few years, however, the advent of Fourier transform 31P fects on the ring structure and on the N M R parameters. N M R with large band decoupling of all other nuclei has proExperimental Section vided a very reliable tool for the identification of these cyclopolyphosphanes in s ~ l u t i o n . ~Thus, ~ ~ , ~the unambiguous Materials. Tetraisopropylcyclotetraphosphane, (i-C3H7P)4, te) ~ , tetra-tertanalysis, in terms of an AA’BB’C system, of the complex 31P tracyclohexylcyclotetraphosphane, ( C - C ~ H ~ I P and (other nuclei decoupled) spectra observed for ( C H ~ P ) S , ~ butylcyclotetraphosphane, (t-BuP)4,14 were prepared by the method of Henderson et aI.l5 Tetraethylcyclotetraphosphane was obtained (CF3P)5,I0 and (C2H5P)511 have established unequivocally by the same method; the crude reaction mixture distilled at a pot the five-membered ring structure of these compounds in sotemperature of 250-300 OC yields a 5050 mixture of the tetramer lution. There is also little doubt that the other cyclopolyphosand pentamer which was used without further fractionation. ( C ~ H S P ) ~ phanes ( C ~ H S P )(i-PrP)s, ~ , ~ and (n-BuP)5,’ which present also has been described previously by Baudler et Perfluorotetraa complex 31P(1H) spectrum between +10 and -30 ppm, have methylcyclotetraphosphane was prepared according to Mahler and a five-membered ring structure. On the other hand, the idenBurgI6 and the N M R study was performed on a 40:60 mixture of the tification and analysis of A2B systems in the 31Pchemical shift tetramer and pentamer. Spectra. 31P(1H)and 31P(19F) N M R spectra were obtained in the range 120-180 ppm elucidated the controversy about the exFourier transform mode on a Varian Associates XL. 100.15 specistence of the three-membered ring (C2F5P)312 and established trometer operating with a fluorine or proton external lock. The broad the existence of several other cyclotriph~sphanes.~~ In the case band noise decoupling was performed using the spectrometer gyrocode of cyclotetraphosphanes, the phosphorus substituents can reChemical shift were measured relative to internal 85% alize a symmetrical trans alternation and accordingly the 31P system. phosphoric acid and by using the indirect referencing method.17 The N M R spectrum with decoupling of all other nuclei in the nematic solvents used are the commercially available phase IV and molecule shows a single line. The occurrence of the 31Psignal V (Merck) N-(p-methoxybenzi1idene)-p-butylaniline (MBBA) in the chemical shift range 50-80 pprn is indicative of a fourKodak) and N-(p-ethoxybenzi1idene)-p-butylaniline (EBBA) membered ring structureI2 but it does not establish unequiv(Kodak-Eastman). Samples were prepared using 5-mm NMR sample tubes and were degassed before sealing in vacuo. In the variable ocally this structure. Information from other sources should temperature experiment, the spectrometer dial is calibrated with a be sought before reaching a definitive conclusion. It must be thermocouple placed inside a solvent contained in a dummy N M R pointed out that a six-membered ring (e.g., (PC6H5)6) cyclotube. A precision of f 1 “C is expected. polyphosphane will also show a single line 31P(1HJN M R NMR Spectral Analysis. X-ray diffraction studies of several cyspectrum which can only be differentiated from that of a clotetraphosphanes (PR),, R = CF3,18 C6H11,I9 CgF5,20have shown four-membered ring by the 31Pchemical shift value. On the that these compounds adopt in the solid state a folded ring structure other hand, the single line phosphorus N M R spectrum does with pseudoequatorial substituents as depicted in Figure 1. The x-ray not contain any information concerning the J(PP) coupling structure reveals that in the solid state these molecules adopt a conconstants. formation of D2d symmetry (R = CF3, space group P42/nmc) or very In a recent paper,I4 we have shown that much more inforclose to it. This symmetry has been used to analyze the four-spin system displayed by the 31PN M R spectrum of the five tetracyclomation can be obtained by recording the 31Pspectra of cyclophosphanes under study when dissolved in a liquid crystal. tetraphosphanes oriented by the nematic phase of liquid The N M R spectrum of partially oriented molecules in the presence crystals. We report here the comparative study of several of a static magnetic field Ho is described by the spin Hamiltonian compounds of this family (PR)4 with R = C2H5, i-C3H7, t S.2’ C4H9, c-C6Hllr and CF3. Such a study not only establishes the existence of the four-membered ring in solution, but gives % = - E viI,i + X(Jij + 2Dij)IzjIzj geometrical information concerning the ring puckering, the i,,sin 4 cos 4 cos 4 + uzzsin2 4 upy = (ayy - u,,) sin 4 cos 4 + uyr(cos2 4 - sin2 4)
uop = uyy cos2 4
uyy = uyy sin2 4
+ 2uxycos 4 sin 4 + u,
cos2 4
The angle would be determined by canceling the upy term (tan 24 = -(uyy - u,,)/2uyz) but only u,, is known from the present experiments. However, it is interesting to note that the Au values referred to the molecular axes on Figure 1 are fairly large and very similar for the three compounds with an alkyl group attached to the phosphorus. The smaller Au value observed for R = CF3 is opposite to what is observed in phosphine molecules PR3 where the chemical shift anisotropy is higher for electronegative R groups.36
Acknowledgment. The authors are grateful to Professor J. Mills for a sample of ( C - C ~ H L ~ P ) ~ . References and Notes (1) L. Maier, Fortschr. Chem. Forsch., 8, l(1967). (2)A. H. Cowley, Top. phosphorus Chem., 4, 1 (1967). (3)T. J. Dupont and J. L. Mills, lnorg. Chem., 12, 2487 (1973). (4) P. R. Hoffman and K. G. Caulton, lnorg. Chem., 14, 1997 (1975). (5) A. H. Cowley, T. A. Furtsch, and D. S. Dierdorf, Chem. Commun., 523
(1970). (6)P. S.Elmes, M. E. Redwood, and B. 0. West, Chem. Commun., 1120 (1970). (7)L. R. Smith and J. L. Mills, J. Chem. SOC.,Chem. Commun., 808 (1974). (8)J. P. Albrand, m s e , no. d'ordre CNRS A 0 10414,Universitede Grenoble, 1974. (9)J. P. Albrand, D. Gagnaire, and J. B. Robert, J. Am. Chem. SOC.,05,6498 (1973). (10)J. P. Albrand and J. 8. Robert, J. Chem. SOC., Chem. Commun., 644 (1975). (11) Unpublished results from the author's laboratory. (12)L. R. Smith and J. L. Mills, J. Am. Chem. SOC.,08, 3852 (1976). (13)(a) M. Baudler, D. Koch, E. Toils, K. M. Diedrich, and B. Kloth, 2.A m g . Allg.
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Journal of the American Chemical Society
Chem., 420, 146 (1976); (b) M. Baudler, B. Carlsohn, W. BBhm, and C. Reuschebach, 2.Naturforsch. 6, 31, 558 (1976); (c) M. Beulder and C. Gruner, Ibld., 31, 1311 (1976); (d) M. Baudler, J. Hahn, H. Dietsch, and G. Furstenberg, ;bid., 31, 1305 (1976). J. P. Albrand. A. Cogne, D. Gagnaire, and J. B. Robert, Mol. Php., 31, 1021 (1976). W. A. Henderson Jr., M. Epstein, and F. S. Seichter, J. Am. Chem. Soc., 85, 2462 (1963). W. Mahler and A. B. Burg, J. Am. Chem. SOC.,80, 6161 (1958). R. K. Harris and 8. J. Kimber, J. Magn. Reson., 17, 174 (1975). G. J. Palenik and J. Donohue, Acta Crystallogr., 15, 564 (1962). J. C. J. Bart, Acta Crystallogr., Sect. 6, 25, 762 (1969). F. Sanz and J. J. Daly, J. Chem. SOC.A, 1083 (1971). P.Diehl and C. L. Khetrapal, in "NMR Basic Princlples and Progress", Vol. 1, Springer-Verlag, West Berlin, 1969. A. Saupe, 2.Naturforsch. A, 19, 161 (1964). J. W. Emsley and J. C. Lyndon, "NMR Spectroscopy Using Liquid Crystal Solvents", Pergamon Press, Oxford, 1975. A. Saupe and J. Nehring, J. Chem. Phys., 47, 5459 (1967). N. Zumbulyadis and B. P. Dailey, Chem. Phys. Left.,26, 273 (1974). J. P. Albrand, A. Cogne, and J. B. Robert, Chem. Phys. Lett., 42, 498 (1976). L. Maier, J. lnorg. Nucl. Chem., 24, 275 (1962); E. Fiuck and K. Issleib. Chem. 6er. 98,2674 (1965); J. B. Lambert and D. C. Mueller, J. Am. Chem. Soc., 88,3669 (1966); J. B. Lambert, G. F. Jackson ill, and D. C. Mueller. ibid., 90, 6401 (1968); 92, 3093 (1970).
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(28) J. P. Albrand and J. B. Robert, J. Chem. SOC.,Chem. Commun., 876 (1976). (29) P. Diehl, H. Bosiger and J. Vogt, J. Mol. Struct., 33, 249 (1976); P. Dlehl and H. Bosiger, Org. M a g . Reson., 9 , 9 8 (1977). (30) R. L. Amster, W. A. Henderson,and N. 8. Colthup, Can. J. Chem., 42, 2577 (1964); Spectrocbim. Acta, 19, 1841 (1964). (31) E. G. Finer and R. K. Harris, Prog. Nucl. Magn. Reson. Spectrosc., 6, 61 (1970). (32) H. C. E. McFariane and W. McFariane, J. Chem. SOC.,Chem. Commun., 582 (1975). (33) S. Aime, R. K. Harris, E. M. McVicker, and M. Fild, J. Chem. SOC.,Chem. Commun., 426 (1974); J. Chem. Soc., Dalton Trans.,2144 (1976). (34) H. C. E. McFarlane and W. McFariane, Chem. Commun., 1589 (1971). (35) J. J. Daly, J. Chem. SOC., 6147 (1964); 4789 (1965). (36) B. R. Appleman and B. P. Dailey, A&. Magn. Reson., 7 (1974). (37) P. K. Bhattachatyyaand B. P.Dailey, Mol. Pbys., 28, 209 (1974). (38) S.J. Kohier and M. P. Klein, Biochemistry, 15, 967 (1976). Kennedy and W. McFarlane, J. Chem. Soc., Chem. Commun., 666 (39) J. 13. (1976); I. J. Colquhoun and W. McFarlane, J. Chem. Soc., faraday Trans. 2, 722 (1977). (40) E. A. V. Ebsworth and G. M. Sheldrick, Trans. Faraday Soc., 63, 1071 (1967). (41) N. Zumbulyadis and B. P. Dailey, Mol. Phys., 27, 633 (1974). (42) M. D. Gordon and L. D. Quin, J. Magn. Reson., 22, 149 (1976). (43) A. J. Montana, B. R. Appleman, and B. P. Dailey, J. Chem. Phys.,66, 1850 (1977).
On the Molecular Structure of Beryllocene Ning-Shih Chiu and Lothar Schafer* Contribution from the Department of Chemistry, University of Arkansas, Fayetteville, Arkansas 72701. Received June 6,1977
Abstract: A conformational analysis of beryllocene, Be(CsH5)2, was executed based on ab initio standard single determinant MO theory with an STO-3G minimal basis. The study concentrated on two contradictory experimental models. One was the gas-phase electron diffraction model of Be(CsHs)z, an asymmetrical sandwich structure in which the metal atom can occupy two alternate positions on the fivefold rotation axis, hl = 1.472 A from one ring and h z = 1.903 A from the other. The second model was the x-ray crystallographic slip-sandwich structure, in which one of the two parallel rings has slipped sideways by about 1.2 A, apparently producing a u-bonded and a r-bonded ring system. The results of our calculations indicate that the gas-phase diffraction structure is 14-18 kcal/mol less stable in the STO-3G approximation than a symmetrical sandwich or a model which is similar to the x-ray structure, but has nonparallel rings. STO-3G calculations probably are too approximate to definitely rule out any conformation of a beryllium compound in a comparison of this kind. But it is possible to use these results as a basis for speculation that the vapors of Be(CsH5)Z consist of a complicated conformational equilibrium with u - r and u-u tautomerism which contains conformers in which the Be atom forms a T bond to one ring and a u bond to the other. It is not unlikely that the radial distribution of this equilibrium is compatible with the experimental radial distribution observed by gas electron diffraction. This study points in a striking way to the usefulness of hybrid experimental and theoretical procedures in structural studies of gaseous molecules.
Introduction The molecular structure which has been proposed for beryllocene (I), Be(CsH5)2, in order to explain its vapor phase electron diffraction data1,*is probably one of the most remarkable low-weight structural designs which are known. The Almenningen, Bastiansen, and Haaland (ABH) model of beryllocene1.2describes the compound as a sandwich complex with two planar, symmetrical, parallel, and staggered CsHs rings with a vertical ring-ring distance k = 3.375 A. The CH skeleton of this model belongs to point group D5d; the complete molecule has the lower group CsU,since the beryllium atom may occupy two alternative positions on the fivefold rotation axis, hl = 1.472 A from one ring and k 2 = 1.903 A from the other. Thus, the most characteristic feature of the ABH model for beryllocene is a double minimum potential energy curve for beryllium with the ability of the metal to tunnel between its two equilibrium positions. In subsequent investigations Fritz and Sellmann3 claimed that the ABH structure was also found for I by analysis of its IR spectrum. Similar conclusions were reported by McVicker 0002-7863/78/1500-2604%01.OO/O
and Morgan," who compared the IR condensed state spectrum of I to its vapor phase data. In direct contrast to this the earlier IR study of I by Fritz and Schneider5 seemed to favor an angular structure for this compound and inferred the presence of two equivalently bonded ligands. The semiempirical MO calculations by Sundbom6 indicated that the ABH structure might correspond to an energy minimum. But it was also warned6 that the shape of the energy curve depended strongly on the values chosen for some of the empirical parameters so that more refinements were needed to make a really conclusive statement. Lopatko, Klimenko, and Dyatkina' used a somewhat more advanced semiempirical M O LCAO S C F procedure to calculate the electronic structure of I in a PNDO approximation taking into account all the valence electrons of beryllium and the 2p, atomic orbitals of all the carbon atoms using a Slater-type basis set. The conclusion of these authors was that the asymmetrical conformation of I, CsU,was energetically more favored than its symmetric form, &d. In 1972 Wong, Lee, Chao, and Lee (WLCL)8 studied the crystal structure of I by x-ray diffraction at -120 OC and 0 1978 American Chemical Society
