1434
Inorg. Chem. 1994, 33, 1434-1439
Anionic and Steric Factors Governing Coordinative Unsaturation at Carbenic Phosphenium Centers Neil Burford,’ Pierre Losier, Charles Macdonald, Vasiliky Kyrimis, Pradip K. Bakshi, and T. Stanley Cameron’ Department of Chemistry, Dalhousie University, Halifax, Nova Scotia B3H 453, Canada Received September 17, 1993’
I
Tetrachlorogallate and tetraphenylborate salts of the phosphenium cations [(iPr2N)2P]+(1) and [ M ~ N C H ~ C H Z N 1
(Me)P]+ (2) are examined. Spectroscopic characterization confirms the ionic formulation for all compounds except I
for 2[BPh4], which exists as thecovalent alternative phosphine-borane MeNCH2CH2N(Me)P(Ph)-BPh~(5) (crystal data: C28H3~BN2P,triclinic,Pi,a=9.710(1)A, b = 14.962(2)A,c=9.610(2)A,a = 105.45(1)O,@=113.59(1)O, y = 92.73(1)’, 2 = 2). The ionic borate 1[BPh4] represents the first isolated phosphenium salt containing a “noncoordinating anion”, and its stability with respect to the covalent alternative analogue 5 is postulated to rely upon the steric bulkiness of both cation and anion. Compound l[BPh4] reacts with chlorinated solvents within hours to give a number of products, one of which has been identified as the phosphonium salt [(‘Pr2N)2P(Cl)CH2Cl][BPhl] (8[BPh4]) (crystal data: C ~ ~ H ~ O B C ~monoclinic, ~ N Z P , P21/n, a = 10.049(3) A, b = 21.794(3) A, c = 16.391(3) A, @ = 92.78(2)O, 2 = 4), the result of an oxidative addition of CH2C12 to the phosphenium center. In contrast, tetrachlorogallate salts of 1 and 2 are indefinitely stable in CHzClz and CHC13 solutions. The crystal structures of 1[GaC14] (crystal data: Cl2H2&LGaN2P, tetragonal, 141cd, a = 20.014(4) A, 6 = 20.014(4) A, c = 21.466(6) A, 2 = 16) and 2[GaC14] (crystal data: C4H10C14GaN2P,monoclinic, El,a = 6.489(3) A, b = 13.893(5) A, c = 7.066(5) A, /3 = 92.58(5)O, 2 = 2) reveal similar anionic arrays around the cation despite different space groups and cation size. The closest contacts occur between the phosphorus and chlorine centers but are in excess of the van der Waals radii. Nevertheless, electrostatic interactions are believed to be maintained to some degree in solution and are responsible for protection of the cationic center. Such interactions are expected to be weaker in the tetraphenylborate salt, which renders the phosphorus center susceptible to attack by solvent molecules that are small enough to infiltrate the steric shield allowing for formation of 8 and other products.
Introduction Carbenes and their analogues represent important synthetic units in that they are small, simple molecules with coordinatively unsaturated sites and high reactivity.’ The phosphorus analogues, known as phosphenium cations, are perhaps the most abundant and have been known for over 30 years. The first isolable salts2-’ were reported long before the first stable carbenes,*+9over 40 cations have been identified in solution, and the chemistry of the phosphenium unit has been extensively investigated.10.l Nevertheless, few (less than 10) derivatives have been isolated and Abstract published in Advance ACS Abstracts, March 1, 1994. (1) See, for example: Clos, G. L.; Gaspar, P. P.; Hammond, G. S.;Hartzler, H. D.; Mackay, C.; Seyferth, D.; Trozzolo, A. M.; Wasserman, E. Carbenes; Moss,R. A., Jones, M., Us.; John Wiley & Sons: New York, 1975. Kirmse, W. Carbene Chemistry, 2nd ed.; Academic Press, Inc.: New York, 1971. Hine, J. Divalent Carbon; The Ronald Press Co.: New York, 1964. (2) Fleming, S.;Lupton, M. K.; Jekot, K. Inorg. Chem. 1972, 11, 2534. (3) Maryanoff, B. E.; Hutchins, R. 0. J . Org. Chem. 1972, 37, 3475. (4) Cowley, A. H.; Cushner, M.C.; Szobota, J. S. J. Am. Chem. SOC.1978, 100,7784. ( 5 ) Luber, J.; Schmidpeter, A. J . Chem. Soc., Chem. Commun. 1976,887. (6) Friedrich, P.; Huttner, G.; Luber, J.; Schmidpeter, A. Chem. Ber. 1978, 111, 1558. (7) Luber, J.; Schmidpeter, A. Angew. Chem., Inr. Ed. Engl. 1976,15,111. (8) Arduengo, A. J., 111; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991, 113, 361. Arduengo, A. J., 111; Kline, M.; Calabrese, J. C.; Davidson, F.J . Am. Chem. Soc. 1991,113,9704. Arduengo, A. J., 111; Dias, H. V. R.; Harlow, R. L.; Kline, M. J . Am. Chem. Soc. 1992,114, 5530. (9) Igau, A.; Grutzmacher, H.; Baceiredo, A.; Bertrand, G. J. Am. Chem. Soc. 1988,110,6463. Igau, A.; Baceiredo, A,; Trinquier, G.; Bertrand, G.Angew. Chetn..Int.Ed. Engl. 1989,28,621. Gil1ette.G. R.;Baceiredo, A.; Bertrand, G. Angew. Chem., Int. Ed. Engl. 1990, 29, 1429. (10) Sanchez, M.; Mazieres, M. R.; Lamande, L.; Wolf, R. In Multiple Bonds and Low Coordination in Phosphorus Chemistry; eds. Regitz, M., Scherer, 0. J., Eds.; Georg Thieme Verlag: Stuttgart, Germany, 1990; p 129.
0020- 16691941 1333- 1434$04.50/0
comprehensivelycharacterized. Moreover, these compounds have characteristics that are common to most derivatives and are perhaps responsible for their stability: (1) Most salts contain the tetrachloroaluminate anion, and the remaining examples possess triflate anions.’ (2) It is generally thought that the coordinative unsaturation and electron deficiency at phosphorus is satisfied by r-donation from the immediate atomic neighbor. Indeed, most derivatives possess at least one nitrogen center adjacent to the phosphenium site or are incorporated into an efficiently *-delocalized framework.5.6JJ2 (3) Some derivatives possess sterically bulky substituent^,^ which may be responsible for kinetic stability.I3 In an attempt to evaluate the importance of these factors in the existence and chemistry of the carbenic phosphenium site we have examined some of the properties of compounds containing the two previously reported phosphenium cations 1and 2 (Chart 1) in the presence of anions GaCl4- and BPh4-. Our observations reveal the combined role of steric shielding and the “coordinating”I4 nature of the anion on the stability and reactivity of the phosphenium cationic center. Experimental Section General Procedures. N a B P 4 (Aldrich) was used without purification. Anhydrous GaC13and AlCl3 (Aldrich) were sublimed in uucuo, and PCl3, MeN(H)CHzCH*(H)NMe, and iPr2NH (Aldrich) were distilled before (1 1) Cowley, A. H.; Kemp, R. A. Chem. Rev.1985,85,367. Cow1ey.A. H.; Cushner, M. C.; Lattman, M.; McKee, M. L.; Szobota, J. S.;Wilburn, J. C. Pure Appl. Chem. 1980,52, 789. (12) Burford, N.; Dipchand, A. I., Royan, B. W.; White, P.S.Inorg. Chem. 1990,29,4938. Burford, N.; Royan, B. W.; Linden, A.; Cameron, T. S . Inorg. Chem. 1989, 28, 144. (13) See, for example: Weber, L. Chem. Rev. 1992, 92, 1839. (14) See, for example: Bochmann, M. Angew. Chem., Inr. Ed. Engl. 1992, 31, 1181. Seppelt, K.Angew. Chem., Inr. Ed. Engl. 1993, 32, 1025. 0 1994 American Chemical Society
Inorganic Chemistry, Vol. 33, No. 7, 1994 1435
Carbenic Phosphenium Centers
chart 1 I
I
1
4
I
2
CI
I
9
B
from solution. A layer of hexane ( N 10mL) was distilled ontothe mixture, and the solution was warmed (60 OC) to redissolve the solid. Removal of the volatiles in uacuo (static) within 3 h resulted in yellow crystalline plates, which were washed with fresh hexane and characterized as bis(diisopropy1amino)phosphenium tetraphenylborate: yield 0.89 g, 1.6 mmol, 81% decompsn pt above 114 OC. Anal. Calcd: C, 78.5; H, 8.8; N, 5.1. Found: C,78.4;H, 8.8;N, 5.0. IR(cm-1): 1937 (w), 1883 (w), 1815 (w),1759 (w), 1701 (w), 1673 (w), 1648 (w), 1582 (s), 1562 (w), 1481 (s), 1430 (s), 1400 (sh), 1390 (sh), 1347 (s), 1306 (m), 1268 (m), 1203(s), 1182(m), 1157(~),1132(s),1104(s), 1066(m),lo46(s),1028 (s), 947 (sh), 932 (s), 884 (w), 848 (m), 813 (w), 747 (s), 707 (s), 628 (m), 610 (s), 545 (m), 527 (m), 492 (w), 466 (m), 350 (w), 307 (w), 266 (w), 256 (w). NMR (ppm in CDzClZ): IlP, 308; I3C, 25.0 (d) (3Jpc = 8 Hz), 55.2 (s), 164.4 (q), 136.3 (s), 125.6 (s), 121.8 (s); 'H, 1.27 (d, 24H) ( 3 J =~6.8 ~ HZ), 3.92 (d Of Sept, 4H) ( 3 J p ~ 2.0 HZ, 3 J ~6.7 ~ Hz), 6.94 (t), 7.08 (t), 7.46 (s);"B, -7.0.
-
Preparation of ~ e N C H ~ z N ( M e ) P I G n & ] .A solution of Me-
NCHzCHzN(Me)PCl(1.7 g, 11 mmol) in CHzClz (5 mL) was added to a solution of GaCl3 (2.0 g, 11 mmol) in CHZC12 (8 mL), and the mixture was stirred for 5 h. The ,IP NMR spectrum of the reaction mixture showed a single signal at 269 ppm. Slow removal of solvent in uacuo (static) resulted in the precipitation of a white crystalline solid which was characterized as 1,3-dimethyl-l,3-diaza-2-phospholidinium tetrachlorogallate: yield 2.4 g, 7.1 mmol, 62%, mp 95-97 OC. Anal. use. CHzCIz, CHCl3, CDzClz, and CDCl3 were dried over PZOSand Calcd: C, 14.6; H, 3.1; N, 8.5. Found C, 14.7; H, 3.1; N, 8.4. IR I (cm-I): 1356 (s), 1310 (m), 1253 (s), 1204 (s), 1143 (s), 1077 (m), 1036 CaHz and stored in evacuated bulbs. ('Pr2N)ZPCl and MeNCHzCHzN(s), 1013 (s), 964 (s), 849 (m), 742 (s), 593 (s), 492 (m), 382 (s), 325 1 (Me)PCl were prepared by literature methods.1sJ6 Solids were handled (s). NMR (ppm in CDzClZ): I3P, 269; 13C, 35.3 (d) (zJpc = 19 Hz), in a VAC Vacuum/Atmospheresnitrogen-filled glove-box, while liquids 56.0 (d) (2Jpc = 9 Hz); 'H, 3.25 (d, 6H) ( 3 J p = ~ 11.6 Hz), 4.03 (d, 4H) were manipulated in a nitrogen-filledglovebag. Reactions were performed (3JHH = 4.4 HZ). in an evacuated (10-3 Torr, ca. 0.133 Pa) reactor," and all glass Preparation of MeNCH&Hfi(Me)P(Ph)-BPh% A solution of apparatuses were flame-dried before use. Melting points were recorded on a Fisher-Johns apparatus and are uncorrected. Elemental analyses MeNCH2CHzN(Me)PCl(O.l6g, 1.0mmol) inCHzClzwasaddedquickly were performed by Beller Laboratories, Gottingen, Germany. Infrared to a suspension of NaBP4 (0.37 g, 1.1 mmol) in CHZC12 (total CHzClz spectra were recorded at Nujol mulls on CsI plates using a Nicolet 510P of 7 mL), and the mixture was stirred for 12 h at room temperature. The FT-IR spectrometer. 3LP{1H),IH, and 13C(lH)NMR spectra were resulting precipitate was separated from the solution by decantation. A recorded on a Bruker AC-250 MHz, and IlB NMR spectra were recorded single signal at 80 ppm was observed in the OIP NMR spectrum of the on a Bruker AMX-400 MHz. NMRsamples were flame-sealedin 5-mm reaction mixture after 10 min. Slow removal of the solvent in uacuo Pyrex tubes. All chemical shifts are reported in ppm relative to external (static) yielded crystals of 2-phenyl-1,3-dimethyl-l,3-diazastandards, 85% H3PO, for 3lP, BHs-EtzO for llB, and TMS for IH and phospholidine-triphenylboron: yield 0.23 g, 0.52 mmol, 49%; mp 168L3C. Crystalline samples were obtained by slow removal of solvent from 173 OC. Chemical analyses were not obtained. IR (cm-1): 1960 (w), the reaction mixture within the reaction vessel by passing a cool stream 1891 (w), 1816 (w), 1588 (w), 1335 (m), 1304 (w), 1255 (m), 1229 (w), of water over the empty adjacent compartment (or placing it over liquid 1206 (m), 1198 (m), 1164 (m), 1128 (s), 1100 (s), 1069 (w), 1035 (s), nitrogen) and distilling the volatiles in uacuo (static). Crystals were 1002 (w), 976 (w), 937 (s), 862 (m), 849 (sh), 803 (w), 751 (sh), 743 washed with cool solvent by cold spot back-distil1ati0n.l~ (s), 715 (s), 704 (s), 697 (sh), 686 (sh), 635 (m), 619 (m), 609 (m), 526 Preparation of [(iPrfi)#)IGaCL]. A solution of (iPrzN)zPCl(l .02 (s), 487 (m), 462 (m), 438 (m), 283 (w), 229 (w). NMR (ppm in CDzg, 3.75 mmol) in CHZClz was added to a solution of GaCl3 (0.64 g, 3.6 Clz): 31P,82; 13C,34.9 (d) (zJpc= 7 Hz), 52.1 (s), 125.9 (s), 126.9 (s), mmol) in CHZC12 (total 30 mL) giving a clear yellow solution within a 128.3 (d), 131.0 (s), 131.8 (d), 136.7 (s); 'H, 2.07 (d, 6H), ( 3 J p ~ 10.8 few seconds. After the solutions were stirred for approximately 3 h, Hz), 2.98 (m, 2H), 3.15 (m, 2H), 7.30 (m, 20H); IlB, 0.2. solvent was removed in uucuo (dynamic) to half the volume, and the Isolation of [(iPrfl)#(Cl)CH&lIBPb].18 In one experiment, a resulting precipitate was recrystallizedfrom the warmed solution. Pale solution of ('PrzN)zPCl(O.27 g, 1.0 mmol) in CHzClz was added quickly yellow crystals (two crops) were characterized as bis(diisopropy1amino)to a mixture of NaBP4 (0.34 g, 1.0 mmol) in CHClp (1:l CHzC12: phosphenium tetrachlorogallate:yield 1.22g, 2.75 mmol, 76%; mp 127.5CHCl3). After 10 days slow removal of solvent from the orange solution 128.5 OC. Anal. Calcd: C, 32.5; H, 6.4; N, 6.3. Found: C, 32.7, H, in U ~ C U O(static) gave 20-30 small rectangularly shaped crystals, which 6.4; N, 6.3. IR (cm-l): 1399 (s), 1377 (s), 1337 (m), 1300 (m), 1209 were characterized by X-ray crystallographyas [(iPrzN)2P(Cl)CHzCl](s), 1172 (sh), 1163 (sh), 1135 (br), 1060 (s), 1038 (s), 925 (s), 884 (w), [BPhd] (31P(1H)57 ppm). Other attempts ( 5 ) to isolate this material 859 (w), 835 (w), 620 (w), 549 (m), 529 (m), 375 (s), 331 (sh), 305 (w), have been unsuccessful yielding only ['PrzNHz] [BP4].19 281 (w),256 (w), 248 (w). NMR (ppm in CDzClz): 31P,313; 13C,25.2 (d) (3Jpc = 8 Hz), 55.2 (9); 'H, 1.52 (d, 24H) ( 3 J =~6.7 ~ Hz), 4.20 (d 3'P NMR Study of [(*hfl)#mPl4] in and Equimolar of sept, 4H) (3JpH = 2.6 Hz, 3 J = ~6.7 Hz). Approximate solubility mixture (0.3 mmol) of ('Pr2N)zPCl and NaBPb in three differentsolvent in CHzClZ: 0.03 g/mL. A single signal at 313 ppm was observed in the systems (1 mL) were treated in an ultrasonic bath and examined by 3IP 3IP NMR spectrum of the reaction mixture. An NMR study has shown NMR spectroscopy at 2-h intervals. Observations are presented as 3lP that [(iPr2N)2P][GaC14] is stable in a CHzClz solution at room NMR chemical shifts with relative signal heights in parentheses. CHztemperature for a period exceeding 1 year. Clz: 2 h, 312; 4 h, 312 (loo), 338 (5), and more than 10 minor peaks 100-0; 6 h, 312 (loo), 58 (20), 45 (25). and 13 (20); 8 h, 13 and minor Preparation of [(iPrfl)#IBP4]. A solution of ('Pr2N)ZPCI (0.54 g, 2.0 mmol) in CHzC12 (- 10 mL) was added to a suspension of NaBPh4 peaks 100-0. "BNMR: -7,Ocharacteristicof BP4-. CHC13: 2 h, 150 (0.68 g, 2.0 mmol) in CHzClZ (- 10 mL), and the reaction mixture was (loo), 132 ( < 5 ) , 122 (
