Pseudorotat ion in XPF , '

The temperature dependence of the nmr spectra of (CH&NPF4 (l), ClPF4, and CH3PF4 has been examined. The interchange of axial and equatorial fluorines in (CH&NPF4 and ClPF, follows the permutational scheme expected for Berry pseudorotation; uiz., both axial and equatorial fluorine atoms interchange at the same time. The rate of pseudorotation of 1 is independent of the concentration of 1 but is catalyzed by THF and (CH&O. The free energy of activation for pseudorotation of 1 in CHC12F is AG * = 8.8 + 0.2 kcal/mol at 85 O , and for ClPF, in CHC12F is AG * N 4.2 i 0.3 kcal/mol at 177”. Combination of these rates with data obtained by others indicates that the relative rates of pseudorotation of compounds having the composition XPF4 increases in the order (CH&N < SR, H < C1 < CH3, F. A new procedure has been developed which permits calculation of the influence of an intermediate present in low concentration along the pseudorotation coordinate for 1 on its spectrum. This procedure establishes that the influence of a single intermediate in equilibrium with two ground states can in general be described exactly by incorporation of only one further operator, called the “transfer operator” A , into the usual Kaplan-Alexander equation of motion of the nuclear spin system. Application of this procedure to the spectrum of 1 indicates that even if a square pyramid were an intermediate along the pseudorotation coordinate, it would not be possible to detect it by nmr line shape analysis at presently accessible spectral resolution. he polytopalj exchange of ligands around pentaT coordinate phosphorus has been extensively studied,6-8 both because this class of intramolecular interchange reactions provides experimentally tractable examples of the fluxional behavior that is an important characteristic of many pentacoordinate inorganic complexesg~lO and because pentacoordinate phosphorus compounds are believed to be intermediates in many biological processes involving phosphate esters. l 1 Substances having the composition XPF, ha\.e been subjected to particularly detailed study for four reasons. First, these compounds are easily synthesized, purified, and manipulated. Second, the combination of 31P and I9F nmr spectroscopy with other physical techniques makes it possible to define the ground state (1) This work was supported by the National Institutes of Health (Grants G M 16020 and HL 150291, the National Science Foundation (Grants G P 28586X and G P 31930), and the donors of the Petroleum Research Fund, administered by the American Chemical Society (Grant 4032). (2) DAAD (NATO) Postdoctoral Fellow, 1972-1973. (3) National Institutes of Health Predoctoral Fellow, 1966-1969. (4) National Science Foundation Predoctoral Fellow, 1971-1973. (5) E. L. Muetterties, J . Amer. Chem. Soc., 91, 1636 (1969). (6) Rcviews: R. Schniutzler in “Halogen Chemistry,” Vol. 11, V. Gutmann, Ed., Academic Press, New York, N. Y., 1967, p 31 f f ; J. C. Tebbe in “Organophosphorus Chemistry,” Vol. I, The Chemical Society, London, 1970, Chapter 11; Vol. 11, 1971, Chapter 1 1 ; Vol. 111, 1972, Chapter 1 1 ; R . Schmutzler, Adcan. Fluorine Chem., 5, 31 (1965). (7) J. B. Florey and L. C. Cusachs, J . Amer. Chem. SOC., 94, 3040 (1972); R . Hoffmann, J. M. Howell, and E. L. Muetterties, ibid., 94, 3047 (1972); A. Rauk, L. C. Allan, and I<. Mislow, ibid., 94, 3035 (1972); L. S . Bartell and V. Plato, ibid., 95,3097 (1973). (8) W. G. Klemperer, J . A m e r . Chem. SOC., 94, 8360, 6940 (1972); Inorg. Chem., 11, 2668 (1972); J . Chem. Phys., 56, 5478 (1972); E. L. Muetterties, J . A m e r . Chem. SOC., 91,4115 (1969). (9) B. F. Hoskins and F. D. Whillans, Coord. Chem. Rec., 9, 365 (1973); J. S. Wood, Progr. Iiiorg. Chem., 16,227 (1972). (10) (a) P. Meakin, E. L. Muetterties, and J. P. Jesson, J . Amer. Chem. SOC., 94, 5271 (1972); (b) P. Meakin, E. L. Muetterties, F. N. Tebbe, and J. P. Jesson, ibid., 93,4701 (1971). (11) T. C. Bruice and S. Benkovic “Bioorganic Mechanisms,” Vol. 2, W. A. Benjamin, New York, N. Y . , 1966, pp 1-109; F. H. Westheimer, Accorriits Chem. Res., 1, 70 (1968); F. Ratnirez, ibid., 1, 168 (1968); S. J. Benkovic atid I<. J. Schray in “The Enzymes,” 3rd ed, Vol. VII, P. D. Boyer, Ed., Academic Press, New York, N. Y., 1973, Chapter 6 ; F. H. Richards and H. W. Wycotf, ibid., Vol. IV, 1971, Chapter 4; J. H. Young, J. McLick, add E. F. Korman, Bioorg. Chem., 3, 1 (1974). geometry of these substances as trigonal bipyramidal, with the substituent X in an equatorial p ~ s i t i o n . ~ , ~ ~ . ~ ~ Third, the polytopal exchange reactions of many of these compounds occur a t rates that are convenient for study by magnetic resonance techniques. Finally, the observation that 19F-3 ‘P coupling is preserved during these exchange reactions guarantees their intramolecularity (although not their unimolecularity, vide infra). Recent interest in these polytopal exchanges has centered on questions of the details of their mechanisms. One elementary problem in establishing a mechanism is that of defining the permutational scheme that describes the process by which the four fluorine atoms interchange; the influence of the fifth ligand, X, and of the solvent on the energetics of this process is also of obvious interest. For XPF, compounds having trigonal bipyramidal ground state geometry, there are only two classes of permutational isomerizations that are distinguishable by nmr: (13)(24) and ( 13)(2)(4).8