Organometallics 1986, 5 , 118-120
118
Lewis Acidities of Trialkylhalostannanes J. N. Spencer, Robert B. Belser, Susan R. Moyer, Ronald E. Haines, Maria A. DiStravalo, and Claude H. Yoder” Department of Chemistry, Franklin and Marshall College, Lancaster, Pennsylvania 17604 Received May 29, 1985
T h e Lewis acidities of R3SnX (R = CH3, CzHj, C3H7, C4H9,C6H,; X = C1, Br, I) relative t o triphenylphosphine oxide in benzene solvent were determined calorimetrically and by 31PNMR. Equilibrium constants determined for the 1:l adducts by both methods were in generally good agreement. T h e enthalpies a n d entropies of adduct formation show that t h e acidities decrease slightly as the size of t h e alkyl group increases and increase as the size of the halogen increases. These trends are discussed in terms of a model that attributes t h e observed trends to polarizability effects in t h e adduct and changes in steric congestion upon a d d u c t formation.
Introduction Although the organotin halides are a m o n g the most studied g r o u p 14 halides, their complexes have been thoroughly characterized for only a few systems.’I2 In particular, for the t r i o r g a n o t i n halides, which form 1:l adducts with Lewis base^,^-^ only trimethyltin chloride seems to have been the object of several investigations. Two studies have been concerned with the effect of varying alkyl chain length on the acidity of trimethyltin chloride,3g4 one study has examined the acidity of trimethyltin iodide6 and investigation of t h e adducts of (CH,),SnCl with various bases has been reported.’ The present work was undertaken to provide a systematic investigation of the effects of alkyl and halo substituents on the Lewis acidity of triorganotin halides. Both calorimetric and NMR spectroscopic techniques were employed to determine the equilibrium constants and attendant t h e r m o d y n a m i c parameters for complexation of these acids with triphenylphosphine oxide (TPPO). TPPO was used as a reference base because of its moderate basicity and its N M R - a c t i v e 31Pnucleus.
Experimental Section ACS reagent grade benzene and cyclohexane were refluxed over
P20jand then distilled over nitrogen or argon and stored over activated Linde type 4A molecular sieves. ACS reagent grade MezSOwas dried over CaC12for 24 h and then fractionally distilled under reduced pressure and collected over activated molecular sieves. Fisher Scientific gold label pyridine was refluxed over KOH for several hours, distilled under nitrogen, and collected over activated molecular sieves. Aldrich reagent grade quinuclidine was sublimed under reduced pressure and stored in vacuo. Aldrich reagent grade T P P O was dried at 110 “C for 24 h and stored in vacuo (mp 156-157 “C). Alfa or Aldrich halostannanes were sublimed or distilled in vacuo before use. The melting or boiling points of all acids were in good agreement with literature values. Carbon-13 and tin-119 NMR spectra were also used as a criterion of purity. All reagents and solvents were handled in a glovebag or drybox under argon. Glassware was oven-dried a t 110 “C and cooled (1) (a) Satchell, D. P. N.; Satchell, R. S.Chem. Reu. 1969,69,251. (b) Ruidisch, I.; Schmidbaur, H.; Schumann, H. “Halogen Chemistry”; Gutmann, V., Ed.; Academic Press: New York, 1967; Vol. 2, p 233. (c) Poller, R. C. J. Organomet. Chem. 1965,3,321. (d) Davies, A. G.;Smith, P. J. “ComprehensiveOrganometallic Chemistry”,Vol. 2, Wilkinson, G., Stone, F. G.A., Abel, W., Eds.; Pergamon Press: New York, 1982; Vol. 2, p 519. (e) Zubiata, J. A,; Zuckerman, J. J. Prog. Inorg. Chem. 1978, 24. ~ 251.- -
-->
(2) Guryanova,E. N.; Goldshtein, J. P.; Romm, J. P. “Donor Acceptor Bond”;Wiley: New York, 1975. (3) Matwioff, N. A.; Drago, R. S. Inorg. Chem. 1964, 3, 337. (4) Graddon, D. P.; Rana, B. A. J . Organomet. Chem. 1976,105, 51. (5) Farhangi, Y.; Graddon, D. P. J . Organomet. Chem. 1975,87,67. (6) Bolles, T. F.; Drago, R. S. J . Am. Chem. SOC.1966, 88, 5730. ( 7 ) Bolles, T. F.; Drago, R. S. J . Am. Chem. SOC.1966, 88, 3921.
Table I. Enthalpies of Solution in Benzene (298 K ) hFI,, kcal f18,kcal mol-’ mol-’ (CH,),SnCl(s) +4.42 f 0.20 (C2Hj),SnBr(l) +0.88 f 0.21 (CH3),SnBr(l) +1.02 f 0.13 (C3H7),SnC1(1) +1.01 f 0.11 (CH3),SnI(1) +0.49 f 0.13 (C4H&3nC1(1) +1.27 f 0.02 (C2H&3nC1(1) +1.17 f 0.02 (C6H&3nCl(s) +5.21 f 0.44 either in a vacuum desiccator or in a glovebag. All calorimetric analysis was done with a Model 450 Tronac calorimeter with ampule assembly and 25-mL reaction vessel. Ampules were oven-dried for at least 24 h, filled and capped in a glovebag, and then heat sealed. The base concentrations varied from 0.05 to 0.3 M while the acid concentrations ranged from 0.004 to 0.04 M. Errors in concentrations are estimated to contribute less than 1%to the total experimental error. T h e calorimetric data were analyzed by a least-squares technqiue. The heat change for n reactions in the reaction vessel can be given by
Q=
h i ,
AHL
L
where n,is the number of moles of adduct produced for a given acid and base concentration. Q is the heat for a given determination, corrected for the enthalpy of solution of the acid. A minimum of 6 runs were used to determine Q for a given reaction. Enthalpies of solution for the acids are given in Table I. The equilibrium constant and enthalpy change are found from the error square sum over the data points.
WC,m,)= C(Q- Z:n,m,P The best values for K and AH are those which minimize U(KL,m,). The error analysis is that given by Rosseinsky and Kellawi.’ Phosphorus-31 spectra were obtained on a JEOL FX-9OQ at 25 “C in 10 mm tubes. Stock solutions of acid and base, both 0.2 M in benzene, were prepared and then mixed equally by volume in a 10-mL volumetric flask. A 5-mL aliquot was in turn diluted to 10 mL, and this was repeated until six solutions, ranging in concentrations from 0.1 to 0.001 M were prepared. A standard, trimethyl phosphate, dissolved in deuterioacetone in a 1:50 ratio was present in a coaxial tube. The equilibrium constant for formation of a 1:l adduct can be obtained from the exchangeaveraged chemical shift of some nucleus using the equationg Li = A, - (Lic/K)”2(Li/Co)’/2 where A is the difference between the chemical shift of the solution and that of the base, Ac is the difference between the chemical shift of the adduct and the free base, and Co is the initial concentration of the acid and base. The equilibrium constant is obtained from the slope of a plot of Li vs. (A/Co)”2. The standard (8) Rosseinsky, D. R.; Kellawi, H. J . Chem. Soc. A 1969, 1207. (9) Zeldin, M.; Mehta, P.; Vernon, W. P. Inorg. Chem. 1979, 76, 265.
0276-7333/&36/2305-0118$01.50/0 0 1986 American Chemical Society
Organometallics, Vol. 5, No. 1, 1986 119
Lewis Acidities of Trialkylhalostannanes Table 11. Thermodynamic Parameters for the Formation of Adducts of Triorganotin Halides with Triphenylphosphine Oxide" (298 K) -mo,
(CH3),Sn-C1 (CH3),Sn-Br (CH3),Sn-I (C2H5),Sn-Cl (C2H5),Sn-Br (C,H,),Sn-Cl (C4H,),Sn-C1 (C6H5),Sn-C1
kcal mol-' 8.1 f 0.1 8.3 f 0.1 9.4 f 0.3 8.3 f 0.2 8.7 f 0.2 7.3 f 0.2 7.6 f 0.4 8.3 f 0.1
KCAL' 21.4 f 0.5 19 f 1 11 f 1 9.1 f 0.4 7.6 0.5 5.0 f 0.2 5.5 f 0.6 12.6 f 0.3
*
-ASo, cal K-' mol-' 21 f 0.4 22 f 0.4 27 f 1.0 23 f 0.7 25 f 0.7 21 f 0.7 22 f 1.0 23 f 0.3
"Benzene solvent. bEstimated error is f20%. calorimetric analysis.
KNmb 14 18 12 14 16 10 7 15
Determined by
state is molarity for both calorimetric and NMR analyses.
Results The AH", AS", and equilibrium constants found by calorimetry are given in Table 11. Also given in Table I1 are the equilibrium constants determined from NMR data. The equilibrium constants are given for the formation of 1:l adducts. The formation of 1:l adducts by the monhalotin derivatives is supported by previous isolation of 1:1 complexes,l X-ray structure determinationle of monohalo complexes and the linearity of the plot of A vs. (A/c)l12. Computer simulations of 1:2 adduct formation and simultaneous formation of both 1:l and 1:2 show that these possibilities produce curves with very marked curvature that bear no resemblance to the experimental plots. Although the linearity of the NMR plots for 1:l complexes was generally good as indicated by correlation coefficients of greater than 0.98, some runs produced plots with a slight concave curvature. Computer simulations of the effect of the formation of 1:2 adducts, 1:l and 1:2 adducts, self-associated base, and associated adduct all produced A vs. (A/c)ll2 curves of drastically different curvature. Only the assumption of the formation of 10-20% of protonated base resulted in similar curvatures. When the acid was repurified by sublimation and even more rigorous anhydrous conditions were used, the curvature was reduced. Thus, hydrolysis of the acid is presumably responsible for this phenomena. The thermodynamics of the formation of these 1:l adducts can be rationalized with the following thermodynamic cycle for the interaction of an acid A with a base B:
A
+ e-
-
--
A- -EA (1)
B: B- + e- IE (2) A* + B. A-B -BE (3)
A
+ B:
+
A-B
AHo = -EA
+ IE - BE
This gas-phase cycle has the advantage of using the homolytic bond energy (BE) as a measure of the strength of the adduct bond. The first step, the addition of an electron to the acid, is affected by the nature of the substituents on the acid moiety. Because the electron must be accommodated in an orbital eventually used to establish the A-B bond, any rehybridization and relief of steric congestion that occurs before (on) bonding to the base are also included in this step. The second step, removal of one electron from the base, can be ignored in a comparison of relative acidities. The third step is a function of the degree of overlap of A and B orbitals, a-interaction between A and B, ionic character in the A-B bond, steric repulsion between substituents on A and B, and other influences such
as charge- or dipole-induced dipole interactions between substituents and the acid or base centers. For the series of acids studied here, an increase in the size of the alkyl substituent should affect the electron affinity very little; a change from CH, to C6H5should slightly increase the electron affinity; and a change from chloro to bromo to iodo should decrease the electron affinity of A. These predictions are supported by aIvalues: CH3, -0.04; CzH5, -0.05; CSHT, -0.03; CdHg, -0.04; C1,0.47; Br, 0.44; I, 0.39; CsH5,0.10. The similarity of the l19Sn-lH coupling constants in a series of trimethylhalostannane adducts has been cited previously as evidence that the hybridization in a variety of adducts is the same.6 The effect of the substituents on the homolytic bond energy is somewhat more difficult to predict. An increase in electronegativity of the substituent should lead to a decrease in energy of the tin hybrid and a consequent increase in overlap. On the other hand, the more polarizable groups should stabilize the negative formal charge on tin, but because these are also the bulkiest, they should produce increased steric repulsions. Examination of the data in Table I1 shows that within experimental error AH" for the propyl and butyl derivatives is lower than AHofor the methyl and ethyl derivatives. For the halogens, there is a fairly consistent trend of increasing -AHo as the halogen gets larger. Within the framework of the thermodynamic model presented above, the first trend can only be explained by steric repulsions between the larger alkyl groups and the triphenylphosphine oxide and a consequent weakening of the adduct bond. Steric hindrance was also used to rationalize AHo for the interaction of (CH3),SnC1with a variety of bases.7 Rationalization of the second trend is more difficult because the decrease in electronegativity of the halogens from C1 to I should result in a decrease in electron affinity in step 1 and a decrease in bond strength due to less favorable overlap in step 2. On the other hand the larger halogens should produce more steric congestion between the alkyl groups in the tetrahedral molecule that can be relieved in the trigonal-bipyramidal adduct that probably contains the alkyl groups on the equatorial position.le Moreover, the larger and more polarizable halogens can also stabilize the formal negative charge on the Sn of the adduct through the charge-induced dipole interaction. The AHo for the (Ph),SnCl adduct can also be rationalized by the high polarizability of the phenyl group as well as its slight electron-withdrawing effect. The large AHo for the (CH3)3SnIadduct with several other bases has been previously attributed to a change in the Sn-X bond energy upon complexation. Because the Sn-I bond is weaker than the Sn-C1 bond, it was suggested that the Sn-I bond would be less affected than the Sn-C1 bond and that therefore the chloro adduct would have a lower AHo of complexation in spite of a greater tin-adduct bond energy.6 This analysis adds the additional complication of a change in energy of bonds other than the adduct bond. Indeed, the use of AHo as the indicator of relative acidities is problematic.2J0 Some review@ have focused mainly on equilibrium constants, partly because of the paucity of thermodynamic parameters. For many adducts, the sensitivity of the equilibrium constant to structural effects is moderated by a linear relationship between AHo and ASo. Because of this, ASo for adduct formation can also be used as a measure of the strength of the adduct. The change in entropy can be written as (10)Shepp, A.; Bauer, S. H.J.Am. Chem. SOC.1954, 76, 265.
120
Organometallics 1986,5, 120-127 A S = AS,,
+ AS,
where AS,,, the change in translational and rotational entropy, is very similar for a series of acids and AS,, the change in vibrational entropy, is due mainly to the new adduct bond. Upon complex formation three degrees of translational and three degrees of rotational freedom are lost and AS,, must be negative. It is this loss of entropy which accounts for the negative entropy changes of Table 11. However, the vibrational entropy change must be positive because six new vibrational degrees of freedom result from the association. If the bonding between the acid and base is strong, the new vibrations associated with the Sn-B bond will have a higher frequency and the entropy gain will be small. If the bond is weak the entropy gain will be correspondingly larger. Hence AS" may be a more unambiguous indicator of bond strength than AH".The linear relationship that has been shown to exist between AH' and AS" is a result of the fact that a tighter bond, as evidenced
by a more negative AH", will produce a corresponding larger loss of entropy. The entropy decrease &om (Cy H,),Sn-Cl to (CH,),Sn-Br to (CH,),Sn-I results from larger vibrational entropy loss due to increased enthalpy of bond formation. This relationship would not be expected to hold if the ligand bonds changed in less than a regular way for similar adducts or if the rehybridization enthalpy differed for each compound. This requires that the enthalpy data of Table I1 be interpreted as being due to a stronger bond between the tin atom and the base and not as being due to differences in the Sn-X bond energies in the complex or to rehybridization differences.
Acknowledgment. We are indebted to the National Science Foundation for support of this work and to Marjorie Samples and Scott Penfil for their assistance. Registry No. (CH3),SnC1,1066-45-1;(CH,),SnBr, 1066-44-0; 2767(CHJ3SnI,811-73-4;(C2H5),SnC1,994-31-0;(C2H5)3SnBr, 54-6;(c3H7),SnC1,2279-76-7;(C4H9)3SnCl, 1461-22-9;(C6H,)3SnC1, 639-58-7; (C6H5)3PO,791-28-6.
Approaches to the Synthesis and Detection of a Transient Palladium(0) Alkylidene Robert A. Wanat and David B. Collum" Baker Laboratoty, Department of Chemistry, Cornell University, Ithaca, New York 14853 Received June 25, 1985
Treatment of palladium enolate (PPh3)zBrPdCH2C(0)-t-Bu with t-BuOK in tetrahydrofuran at -63 "C affords Ph,P=CHC(O)-t-Bu (2) in good yield. Kinetics demonstrated the reaction to involve rate-determining dissociation of phosphine. In the presence of added phosphine, the reaction exhibited fractional direct dependence on the concentration of t-BuOK and fractional inverse dependence on the concentration of added phosphine. Isotopic labeling studies showed that the t-BuOK did not function as a Brmsted base up to the rate-determining step. Crossover experiments demonstrated that the post-rate-determining step involving phosphorus-carbon bond formation occurred by an intramolecular mechanism. Reactions of (2)-BrCH=C(OSiMe,)-t-Bu, Br,CHC(O)-t-Bu, (2)-BrCH=C(OLi)-t-Bu, and N,CHC(O)-t-Bu with Pd(PPh3)4each provided phosphorane 2. The mechanism for formation of 2 and the possible intermediacy of low-valent palladium alkylidenes are discussed.
Introduction We are interested in the elucidation of the organic chemistry of highly reactive transition-metal alkylidenes.' Specifically, we are intrigued by the class of alkylidenes that bear potentially destabilizing, and thus highly activating, electron-withdrawing group^.^,^ These species are frequently implicated as reactive intermediates derived (1)Selected reviews of metal carbene/alkylidene complexes: (a) Cardin, D. J.; Cetinkaya, B.; Lappert, M. F. Chem. Rev. 1972,72,545.(b) Schubert, U.Coord. Chem. Reu. 1984,55,261.Grubbs, R. H. h o g . Inorg. Chem. 1978,24,1.(c) Brown-Wensley,K. A.; Buchwald, S. L.; Cannizzo, L.; Clawson, L.; Ho, D.; Stille, J. R.; Straua, D.; Grubbs, R. H. f i r e Appl. Chem. 1983, 55, 1733. (d) Casey, C. P. In "Transition Metal Organometallics in Organic Synthesis"; Alper, H., Ed.; Academic Press: New York, 1976;Vol. 1, pp 189-233. (2)In those systems in which the metal carbene moieties are stabilized by electron-donating groups (Fisher carbenes), electron-withdrawingacyl substituents should be destabilizing. However, such substituent effects will be dependent upon the electronic configuration at the metal center. For recent theoretical treatments that address the question of M=C stabilities and philicities, see ref 3. (3) (a) Taylor, T. E.; Hall, M. B. J . Am. Chem. SOC.1984,106,1576. (b) Ushio, J.; Nakatsuji, H.; Yonezawa, T. J . Am. Chem. SOC.1984,106,
5892.
0276-7333/86/2305-0120$01.50/0
from diazo ketones and diazo esters en route to C=C,4 C=C,5 C=N,6 C-H,7 C-0,8a-c C-S,8c 0-14,~S-H,1° (4)Leading references: Doyle, M. P.; Dorow, R. L.; Buhro, W. E.; Griffin, J. H.; Tamblyn, W. H.; Trudell, M. L. Organometallics 1984,3, 44. Doyle, M.P.;Griffin, J. H.; Bogheri, V.; Dorow, R. L. Ibid. 1984,3, 55. Anciaux, A. J.; Hubert, A. J.; Noels, A. F.; Petiniot, N.; TeyssiB, P. J . Org. Chem. 1980,45, 695. Doyle, M. P.; Wang, L. C.; Loh, K. L. Tetrahedron Lett. 1984,25,4087. Salomon, R.G.; Kochi, J. K. J . Am. Chem. SOC.1973,95,3300.Anciaux, A. J.; Demonceau, A.; Noels, A. F.; Warin, R.; Hubert, A. J.; Teyssi6, P. Tetrahedron 1983,39,2169.Peace, Wulfman, D. S. Synthesis 1973,137. B. W.; (5)Petiniot, N.; Anciaux, A. J.; Noels, A. F.; Hubert, A. J.; Teyssi6, P. Tetrahedron Lett. 1978,1239. (6)Paulissen, R.;Moniotte, P.; Hubert, A. J.; Teyssi6,P. Tetrahedron Lett. 1974,3311. (7)Demonceau, A,; Noels, A. F.; Hubert, A. J.; Teyssi6, P. J. Chem. SOC., Perkin Trans. I 1981,688.Cane, D. E.;Thomas, P. J. J. Am. Chem. SOC. 1984,106,5295. Taber, D. F.;Raman, K. Ibid. 1983,105, 5935. Taber, D. F.; Petty, E. H. J . Org. Chem. 1982,47,4808. Callot, H.J.; Metz, F. Tetrahedron Lett. 1982,23,4321. Taylor, E. C.; Davies, H. M. L. Ibid. 1983,24,5453. (8)(a) Martin, M. G.; Ganem, B. Tetrahedron Lett. 1984,25,251. (b) Doyle, M.P.; Griffin, J. H.; Chinn, M. S.; van Leusen, D. J . Org. Chem. 1984,49,1917. (c) Kametani, T.; Kanaya, N.; Mochizuki, T.; Honda, T. Heterocycles 1982,19,1023. (d) Kirmse, W.; Chien, P. V. Tetrahedron Lett. 1985,26, 197.
0 1986 American Chemical Society
