5002
Organometallics 1995,14, 5002-5004
Isolation of a Phosphenium Ion with a P-C
Bond
Robert W. Reed, Zuowei Xie, and Christopher A. Reed* Department of Chemistry, University of Southern California, Los Angeles, California 90089-0744 Received July 31, 1995@ Summary: The first example of a n X-ray crystallographically characterized two-coordinate phosphorus cation with a P-C single o bond is described. [P(mesityl)(N-i-Prdl[AlClalwas prepared by treatment of PCKmesityl)(N-i-Prd with AlzCls in dichloromethane. The phosphenium ion character of the cation is reflected i n the large downfield 31P chemical shift (500 ppm) and can be compared to 313 p p m in the corresponding bis(dialkylamido) cation [P(N-i-Prdd+. Phosphenium ions, R2P+,isoelectronicwith carbenes, are reactive species unless they are stabilized by the presence of strong n-donor substituents, classically two dialkylamino gr0ups.l Iminium ion character (I) in these compounds compromises their phosphenium ion character (11). When less compromising substituents
are used, decreased stability has typically meant that these species have been characterized only on the basis of a downfield 31Pchemical ~ h i f t . We ~ , ~now report the preparation and isolation of mesityl(diisopropy1amino)phosphenium tetrachloroaluminate, the first X-ray structurally characterized example of a phosphenium ion with a phosphorus-carbon single bond. Phosphoruscarbon n bonding is present in all other low-coordinate phosphorus cations that have been structurally characterized to date. These include the v2 n-bonded species [(C~M~~)(NH-~-BU)PI[A~C~~],~ the phosphaalkene [PhsPC(H)=P(N-i-Pr2)1[BF4],5 a heterocyclic diazaphospholium chloride,6and q6 n-arene complexes of iminophosphenium cation^.^
* To whom correspondence should be addressed: Fax, (213) 7400930; Tel, (213) 740-3337; email:
[email protected]. Abstract published in Advance ACS Abstracts, November 1,1995. (1)For reviews on phosphenium ions, see: Cowley, A. H.; Kemp, R. A. Chem. Rev. 1985,85,367. Sanchez, M.; Mazieres, M. R.; Lamande, L.; Wolf, R. In Multiple Bonds and Low Coordination in Phosphorus Chemistry; Regitz, M., Ed.; Thieme Verlag: Stuttgart, Germany, 1990; p 129. (2) Cowley, A. H.; Lattman, M.; Wilburn, J. C. Inorg. Chem. 1981, 20, 2916. (3) Attempts to generate phosphenium ions in the absence of at least one dialkylamino group have either failed ((a) Lambert, J. B.; So, Jeung-Ho, J. Org. Chem. 1991, 56, 5960) or led to Lewis acid-Lewis base adducts ((b) Burford, N.; Losier, P.; Sereda, S. V.; Cameron, T. S.; Wu, G. J . A m . Chem. Soc. 1994, 116, 6474). Even the most stable phosphenium ion, (i-PrzNjZP*,reacts slowly with CHZC12: (c) Burford, N.; Losier, P.; Bakshi, P. K.; Cameron, S. T. J. Chem. SOC.,Dalton Trans. 1993, 201. (4) Gudat, D.; Nieger, M.; Niecke, E. J . Chem. Soc., Dalton Trans. 1989, 693. (5)Gruetzmacher, H.; Pritzkow, H. Angew. Chem., Znt. Ed. Engl. 1989,28, 740. (6) Friedrich, P.; Huttner, G.; Luber, J.;Schmidpeter, A. Chem. Ber. 1978, 111, 1558. (71 Burford, N.; Clyburne, J. A. C.; Bakshi, P. K.; Cameron, T. S. Organometallics 1996, 14, 1578. @
In order t o counter the electronic stabilization lost upon the replacement of a n-donor dialkylamino group at phosphorus, we chose a bulky mesityl substituent to provide kinetic stabilization close t o the phosphorus center. As a starting material, chloro(diisopropy1amino)mesitylphosphine (1) was prepared by the reaction of dichloromesitylphosphine with diisopropylamine.8In this phosphine the substituents are diastereotopic even at >60 "C as demonstrated by the doubling of the 'H and 13C NMR methyl group signals arising from both the aryl and isopropyl moieties. Substitution of chloride for triflate by treatment of 1 (~5(~lP) 132 ppm) with silver trifluoromethanesulfonate affords the new phosphine 2, P(OS02CF3)(N-i-Pra)(mesityl), whose 31Presonance is shifted 53 ppm downfield to 185 ppm. This compound also shows diastereotopic o-methyl groups to '60 "C for the mesityl substituent, but the diisopropylamino group is diastereotopic only below room temperature (AG* = 56 kJ mol-l). Clearly, rotation about the P-N bond is more rapid than rotation about the P-C bond and is favored by the more labile anion. Related observations (8)Synthesis adapted from: Schmidbaur, H.; Schnatterer, S. Chem. Ber. 1983, 116, 1947. The full synthetic procedures and identifying data for 1-3 are as follows. 1: To a cold toluene solution (0 "C, 250 mL) of (mesityl)PC1z3(6.20 g, 28.0 mmol) was slowly added i-Pr2NH (8.8 mL, 61 mmol). The mixture was then refluxed overnight. After it was cooled to 0 "C, the solution was filtered to remove i-PrzNHz+Cl-. Removal of the toluene in vacuo followed by recrystallization from dry acetonitrile (-30 "C) afforded large yellow blocks of 1.Yield 4.90 g, 17.1 mmol, 61%. Mp: 90-92 "C. Anal. Calcd: C, 63.04; H, 8.82; N, 4.90. Found: C. 62.87: H. 8.84 N. 4.93. 31PPHJ NMR(145 MHz. CDCL. 25 "C, 85% H3P04): b 132. lH NMR (360 MH'z (and 250 MHz),'CDCl;; 25 "C, TMS): b 6.70 (m, 2H), 3.46 (sept, 3J(HH) = 6.6 Hz, 1H), 3.42 (sept, 3J(HH) = 6.6 Hz, lH), 2.68 (s, 3H; o-CH~),2.67 (s,3H; o-CHd, 1.32 (d, 3J(HH) = 6.6 Hz. 6H), 0.88 (d, W H H ) = 2.04 (s. 3H. D-CH~), 6.6 Hz, 6H): 13C{iH} NMR (90 and 63 MHz, CDC13, 25 "C, TMS): 6 142.7, 142.4, 139.8 (para), 131.0, 130.9, 130.3 (ipso), 49.5, 49.4, 23.7, 23.6, 23.5, 23.3 (2C), 23.1, 20.8. 2: To a benzene solution of l(1.00 g, 3.5 mmol) was added Ag(CF3S03) (0.91 g, 3.6 mmol), and the mixture was stirred overnight. The solution was filtered to remove AgCl and concentrated in vacuo. Pentane was added until the onset of turbidity, at which point the solution was gently warmed before cooling to -30 "C to afford 2 as a crystalline mass. Yield 0.54 g, 1.4 mmol, 40%. Mp: 119-122 "C. Anal. Calcd: C, 48.12; H, 6.31; N, 3.51. Found: C, 47.86; H, 6.24; N, 3.42. 31P{1H}NMR (109 MHz, C6D6,25 "C, 85% H3P04): b 185. lH NMR (360 MHz (and 250 MHz), CDzClz, -40 "C, TMS): 6 6.98 (m, 2H), 3.74 (m, lH), 3.66 (m, lH), 2.46 (s, 3H; o-CH~),2.45 (s, 3H; o-CH3), 2.29 (8,3H;p-CH3), 1.48 (d, 3J(HH) = 6.7 Hz, 6H1, 1.03, (d, 3J(HH)= 6.7 Hz, 6H). 13C{lH)NMR (90 MHz, C&, 25 "C, TMS): 6 141.5, 141.3, 141.0, 130.9, 130.8, 128.2, 119.5 (CF3, WCF) = 319 Hz), 50 (br), 23 (br), 22.2, 22.0, 20.5. 3: To a cold (0 "C) stirred suspension of AlC13 (0.50 g, 3.8 mmol) in CHzClz was added a cold CH,C12 solution of 1 (1.0 g, 3.5 mmol). The solution quickly developed an intense yellow color. Filtration through a fine frit, layering of the filtrate with toluene, and storage at -30 "C for 1week produced large gemlike crystals of& Yield 1.12 g, 2.7 mmol, 76%. Mp: 133-135 "C. Anal. Calcd: C, 42.98; H, 6.01; N, 3.34. Found: C, 42.88; H, 6.02; N, 3.14. 3lP{lH} NMR (109 MHz, CDzC12, 25 "C, 85% HaP04): 6 500 (t (l:l:l), 'J(P,N) = 65 Hz). 14N{'H} NMR (19 MHz, CDzClz, 25 "C, 85% CH3N02): 6 -71.0 (d, 'J(P,N) = 65 Hz). 27AlNMR (70 MHz, CDzCIz, 25 "C, Al(HzO)63+):6 -105 (8, wln = 6 Hzj; lH NMR (360 MHz (and 250 MHz), CDzC12, -40 "C, TMS): 6 7.09 (8,2H), 4.67 (m, 2H), 2.36 (s, 3H), 2.31 ( 8 , 6H), 1.83 (d, 6H, J = 6.25 Hz), 1.38 (d, 6H, J = 6.44 Hz). 13C{lH) NMR (90 MHz, CDzClz, -40 "C, TMS): d 146.7, 139.6, 139.5, 130.5, 127.3 (lJ(PC)= 47.5 Hz), 64.5,64.3,58.2,58.0,29.4,29.3, 21.46, 21.40, 21.35, 21.25, 21.23.
0276-733319512314-5002$09.00/0 0 1995 American Chemical Society
Communications
Organometallics, Vol. 14, No. 11, 1995 5003 A
I
Figure 1. Proton-decoupled 31P NMR spectrum of the [(mesityl)(i-Pr2N)P]+cation in 3 (referenced vs H3P04).
Figure 2. Perspective view of [(mesityl)(i-Pr2N)Pl+[AlC141-.
on triflate lability have been made by Dahlg and by N i e ~ k eThe . ~ triflate compound shows greatly increased sensitivity toward moisture. Abstraction of chloride from 1 with aluminum trichloride affords the new phosphenium ion mesityl(diisopropy1amino)phosphenium tetrachloroaluminate (3). The 31PNMR resonance of 3 is now dramatically shifted downfield to 500 ppm (i.e. 368 ppm downfield from that of 1). In addition, the signal has been transformed into a 1:l:ltriplet due to coupling to the quadrupolar 14N nucleus (J = 65 Hz; see Figure 1). This is a very rare example of observable 31P-14N coupling.1° The 27AlNMR spectrum in CD2C12 shows an extremely narrow singlet (width at half height 6 Hz,6 105 ppm) comparable to that of free AlC14- ions (8 Hz).ll This indicates that 3 is ionic or is loosely ion-paired in solution. The methyl groups of the diisopropylamino substituent are diastereotopic below 50 "C, corresponding to a AG* for P-N bond rotation of 63 kJ mol-l. This barrier is the same as that in (Me2N)2P+ (63 kJ mol-l)12 but higher than that of (iPr2N)2P+ (45 kJ mol-l).13 The 31Pchemical shift and the 31P-14N coupling constant of 3 are essentially unaffected by temperature. The o-methyl groups of the mesityl substituent are equivalent at all temperatures, indicating either unrestricted rotation about the P-C bond or, much more likely, orientation above and below a plane of symmetry. The large downfield chemical shift of 3 is matched only by P(N(SiMe3)2)2+(450 ppm) and P(t-Bu)(NMe2)+(513 ppmh2 Steric bulk in the former compound twists the N( SiMe& groups out of conjugation, whereas replacement of a P-N bond by a P-C u bond in the latter must be the cause of the downfield shift. The effect of the mesityl group in 3 is similar to that of the tert-butyl group in P(t-Bu)(NMe2)+,presumably because the aryl group is rotated out of conjugation (see the X-ray structure below).
Crystals of the phosphenium species 3 were obtained from dichloromethane/toluene. The X-ray structure15 (Figure 2) reveals several notable features. Somewhat surprisingly, the P-N bond length of 1.617(5) A is not significantly shorter than those found in [(i-Pr2N)2PI+[AlC14]- (1.611(4) and 1.615(4) 81),13 [(i-Pr2N)2P]+[GaClJ (1.587(12) and 1.601(13)&,3c and [(MesSihNP-N(SiMes)(GaCl3)] (1.590(5) and 1.637(5)A),14where PN double-bond character is distributed over two dialkylamido substituents. The P-C bond length of 1.787(6) is relatively short for phosphorus(III), presumably because of low coordination number and positive charge, but is nevertheless that of a single u bond. Comparisons can be made to 1.848(5) A for the same bond in the neutral chlorophosphine 116 and 1.684(14) A for the PC double bond in a cationic ph~sphaalkene.~ The mesityl group is rotated 69" out of the plane defined by the P, N, C(1), and C(4) atoms of the dialkylamido group, effectively preventing any conjugation of the mesityl n system with the empty 3p orbital on phosphorus. Of further interest, the C-P-N angle of 107.0(3)" is substantially smaller than the N-P-N angles in [(i-Pr2N)2P]+[AlC14]- ( 114.8")13 and [(i-Pr2N)2P]+[GaClJ (117.0°),3cdespite the closer proximity of the tetrachloroaluminate counterion. The shortest P-C1 contacts in 3 are 3.296(3) and 3.471(3) A to Cl(3) and C1(4),respectively, compared to 3.867(6)A for the closest approach in [(i-Pr2N)2P]+[GaCl&.3c This suggests that a true phosphenium ion, R2P+, will have a more acute bond angle. The closer approach to a true &Pt type phosphenium ion represented in the present work enhances the prospects of isolating and exploring the chemistry of even more reactive species. There will probably be a role in this chemistry for anions less reactive and less coordinating than Ac14-. We note that at a certain threshold of leaving group basicity (Lewis), there is an abrupt switch from covalentlpyramidal to ionic/ planar coordination. Thus, for bis(dialkylamino) species
(9)Dahl, 0.Tetrahedron Lett. 1982,23,1493. (10)Gudat, D.;Schniffner, H. M.; Nieger, M.; Stalke, D.; Blake, A. J.; Grodey, H.; Niecke, E. J.Am. Chem. Chem. 1992,114,8857.Niecke, E.;Gudat, D. In Phosphorw-31 NMR Spectral Properties in Compound Characterization and Structural Analysis; Quin, L. D., Verkade, J. G., Eds.; VCH: New York, 1994,p 159.The asymmetry of the triplet has its origin in differential relaxation rates: Harris, R. K. In Nuclear Magnetic Resonance Spectroscopy;Longman: Essex, U.K, 1986;p 139. (11) Akitt, J. W.Annu. Rep. NMR Spectrosc. 1972,5,489. (12)Thomas, M.G.; Schultz, C. W.; Parry, R. W.Znorg. Chem. 1977, 16, 994. (13)Cowlev. A.H.:Cushner. M. C.:Szobota. J. S. J.Am. Chem. Soc. l978,100,7~€i4. . (14)Oberdoerfer, R.;Nieger, M.; Niecke, E. Chem.Ber. 1994,127, 2397.
(15)Crystal data for 3: M,= 419.1,monoclinic, P2&, a = 8.175(1) = 95.78(1)",V = 2149.4(3)A3, Z = 4, palc = 1.295 Mg m-3. Data collection: Cu Ka, 173 K, 28 = 2.0105.0", 3205 measured reflections, 2318 independent reflections. Refinement: full-matrix least-squares refinement on IF1 with 2106 reflections having F > 4a(F), direct methods (SHELXTL),299 parameters, calculated H atom positions, Lorentzian polarization, absorption max 0.9587,min 0.3991;residual electron correction p = 6.066 "-1, density max 1.00,min -0.70 e A-3, R = 0.0594. (16)Unpublished results.
A, b = 16.015(1)A,c = 16.501(1)A,
5004 Organometallics, Vol. 14,No.11, 1995
of theP(NR&X type, the X = C1- compound is covalent but the X = CF3SO3-, AlCL-, and GaC14- compounds are ionic. However, for mono(dialky1amino) species of the PAr(NR2)X type, the X = C1- and CF3S03- compounds are covalent and the species is ionic. This is in notable contrast to R3Si+ type silylium ions, where there is an apparent continuum of cationlanion association and a gradual change from covalent to ionic.l 7
Communications
Acknowledgment. This work was supported by
NSF Grant No. CHE 9407284. Supporting Information Available: For 3, tables giving a summary of X-ray collection and refinement data, atomic coordinates, bond lengths, bond angles, and anisotropic displacement factors and an atom-numbering diagram (9 pages). Ordering information is given on any current masthead page.
OM9505928 (17) Reed, C. A,; Xie, Z.; Bau, R.; Benesi, A. Science 1993,262,402.
