J . Phys. Chem. 1989, 93, 4643-4645
4643
New Electronegativity Scale for the Correlation of Heats of Formation and Bond Dissociation Energies. 6. Alkylsilane Derivatives Yu-Ran Luo* and Sidney W. Benson Donald P. and Katherine B. Loker Hydrocarbon Research Institute, Department of Chemistry, University of Southern California, University Park, Los Angeles, California 90089- 1661 (Received: September 12, 1988)
We report two new linear relations between the differences in heats of formation of the compounds Me3SiX and CH3X and the new electronegativity measure Vx of the atom X in the group connected to Si or C: AAfHo(Me3SiX/CH3X)= 5.7 5.78Vx, X = H, CH,; AAfHo(Me3SiX/CH3X)= -27.8 - 5.35Vx, X = C1, Br, I, and OH. The reason for the two apparently independent relations is attributed to the p-d T back-bonding between Si and ligand halogen or oxygen atom. The back-bonding energy appears to be nearly independent of the type of ligand atoms and about 19.3 f 1.0 kcal mol-’ depending slightly on V x . New values are estimated for AfHo for Me,SiF, Me3SiNH2,Me3SiSH, and SiHJ.
Introduction The unshielded core potential Vx has been used to provide a new electronegativity scale for the bonding atom X in compounds RX. Vx has been shown earlier to quantitatively correlate the heats of formation of classes of organic compound^:^-^ V = nx/rx, where nx is the number of valence-shell electrons and rx is the covalent radius of X. We have found that differences in heats of formation AAfHo and differences in bond dissociation energies ADHO can be represented by AAfHo(RX/MeX) = a(C,m) + 6(C,m)Vx (1) AAfHo(MeX/Hd()/p = a(C) + 6 ( C ) V x (2) ADHo(X-CH3/X-R)
+ b’(C,m)Vx
= a‘(C,m)
AfHo[C-(X)(C)m(H)3-,,,] = a”(C,m) AAfHo(SiH3X/HX) = a(Si)
+ b”(C,m) Vx
+ b(Si)Vx
(3) (4)
(5)
By combination of these equations with each other, a number of new equations may be derived. For example AAfHo(SiH,X/CH3X) = a(Si,C)
+ 6(Si,C)Vx
(6)
In these equations R = CH3,(CH3),, carbon-centered groups, which are Et, i-Pr, and t-Bu when m = 1, 2, and 3, respectively. a(C,m), a(C), a’(C,m), a”(C,m), a(Si) or b(C,m), b(C), b’(C,m),b”(C,m), and b(Si) are constants dependent on m and the properties of either carbon or silicon as indicated; p is the number of hydrogen atoms attached to X in the molecule H,X. New linear relations between AAfHo(Me3SiX/CH3,X)and Vx are reported in this paper. The p d ?r back-bonding energy between Si and ligand halogen and oxygen atom has been inferred from these relations.
Relation between AAfHo(Me3SiX/CH3X) and V x The data on heats of formation of silicon-containing compounds have produced much controversy. Oxygen-bomb calorimetry, which is useful for hydrocarbons, has been shown to be difficult for organosilicon compound due to incomplete combustion. With other techniques, progress has been made in obtaining reliable data just in the past 10 years. These data have been reviewed in Walsh.6 The preferred values for AfHo of Me3SiX compounds, where X = OH, CI, Br, I, CH3, and H, are shown in Table I. Luo, Y. R.; Benson, S. W. J . Phys. Chem. 1988, 92, 5255. Luo, Y. R.; Benson, S. W. J . Am. Chem. SOC.,in press. Luo, Y. R.; Benson, S. W. J . Phys. Chem., in press. Luo, Y. R.; Benson, S. W. J . Am. Chem. SOC.,in press. Luo, Y. R.; Benson. S. W. J . Phys. Chem. 1989, 93, 1674. Walsh, R. The Chemistry of Organosilicon Compounds; Patai, S., RaDDort. Z.. Eds.: ChaDter 5. in Dress. r. (7) Baldwin, J.’C.; Lippert, M.‘F.; Pedley, J. B.; Treverton, J. A. J . Chem. SOC.A 1987, 1980. (8) Doncaster, A. M.; Walsh, R. J . Phys. Chem. 1979, 83, 3037. (9) Steele, W. V . J . Chem. Thermodyn. 1983, I S , 595.
(1) (2) (3) (4) (5) (6)
0022-3654/89/2093-4643$01.50/0
TABLE I: AfHo(Me3SiX) in kcal mol-’ compound method (CH3)$iOH solution calorimetry (CH3),SiCI solution calorimetry (CH3)+3iBr solution calorimetry gaseous equilibrium study (CH,),SiI (CH3),Si rotating bomb calorimetry (CH3)3SiH equilibrium study
ArH02~s(gas) -119.4f1.0 -84.6 f 1.0 -70.1 f 1.0 -51.9 f 1.0
ref
-55.7 f 0.8 -39.1 f 1.0
7 7 7 8
9 10
TABLE I 1 Relation between AAfHo and V , in kcal mol-’ AAfHo-
X F O(0H) CI N(NH2) Br S(SH) I C(CH3) H
Vy 9.915 8.11 7.04 6.67 6.13 5.77 5.25 5.19 2.70
(Me3SiX/CH3X) AJfo(MelSiXY ArH”(CH,Xlb exDtl est -80.8 (-136.7)‘ -55.9 f l e -48.2f0.1 -71.2f1.1 -71.2 -119.4f1.0 -84.6f1.0 -19.6f0.1 -65.0fl.I -65.5 (-69.0)‘ -5.5 f 0.1 -63.5 -70.111.0 -8.5f0.3 - 6 1 . 6 f 1 . 1 -60.6 -58.7 (-64.2)‘ -5.5 f 0.2 3.5 f 0.3 -55.4 f 1.1 -55.9 -51.9 f 1.0 -20.0 f 0.1 -35.7 f 0.9 -55.7 f 0.8 -39.1 f 1.0 -17.8 f 0.1 -21.3 f 1.1
”The data are from Table I; values in parentheses are derived here; see text. bFrom ref 11, unless specified. cReference12. “Estimated values; see text.
Following ref 1, we can study the relation between AAfHO(Me3SiX/SiH3X) and V,. The data on AfHo(SiH3X),however, are unavailable for X = OH, and those for AfHo(SiH31)may have a large ~ n c e r t a i n t y .Data ~ on AfHo(CH3X)for X = CI and Br are available. Because of these uncertaintites and missing data, we would prefer to use AAfHo(Me3SiX/CH3X) instead of AAfHo(Me3SiX/SiH3X). Values for AAfHo(Me3SiX/CH3X) are listed in Table 11. The relation between AAfHo(Me3SiX/ SiH3X) and Vx will be discussed later. The six points for AAfHo(Me3SiX/CH3X)vs Vx are shown in Figure 1. The eight points for AAfHo(Me3CX/CH3X)vs Vx are also shown in this figure to compare the behaviors of carbon and silicon compounds. We observe a good linear relation for X = CI, Br, I, and O H between AAfHo(Me3SiX/CH3X)and Vx. The linear relation is given by AAfHo(Me3SiX/CH3X) = -27.8 - 5.35Vx, X = CI, Br, I, and O H (7) (10) Doncaster, A. M.; Walsh, R. J. Chem. Sac., Faraday Trans. 2 1986, 82, 707. (1 1) Pedley, J. B.; Naylor, R. D.; Kirby, S. P. Thermochemical Data of Organic Compounds, 2nd ed.; Chapman and Hall: London, 1986. ( 1 2) Kudchader, S. A,; Kudchader, A. P. J . Phys. Chem. Ref Data 1978, 7. 1285.
0 1989 American Chemical Society
4644
Luo and Benson
The Journal of Physical Chemistry, Vol. 93, No. 11, 1989
TABLE 111: p-d x Back-Bonding Energy in Si-X Bonds" X EJkcal OH 18.6 c1 19.1 Br 19.5 1 19.8 ( E M )= 19.3 f 1.0 kcal
"he uncertainty listed of fO.l kcal represents the uncertainties in absolute values of the compounds compared and not simply the deviations from the mean. TABLE IV: AAfHo(Me3SiX/SiH3X) in kcal mol-' AAfHoApYo( Me3SiX)
F
1201
17
2
i'
1111 x?
OH CI NH, Br SH I
C o I S B r NI C1 I 1
4
6
vx
0
IO
-75
Figure 1. Relation between AAfHo(Me,CX/CH3X),AAfHo(Me3SiX/ CH3X) or ADHo(Me3Si-X/Ch-X), and Vx: (1) -8.1 - 2.30Vx; (2a) -5.7 - 5.78Vx; (2b) -27.8 - 5.35Vx; (3) -8.1 + 5.35Vx.
We have assumed that the two points for X = H and CH3 are also part of a general but different linear relation: AAfHo(Me3SiX/CH3X) = -5.7 - 5.78VX, X = H and CH3 (8) By way of contrast, a single equation describes AAfHo(Me3CX/CH3X) vs Vx:' AAfHo(Me3CX/CH3X)= -8.1 - 2.30Vx
AfHo(SiH3X)
(Me,SiX/SiH,X) obsd est
-51.5 -50.2 -52.1 f 2.6 -53.0 -49.4 -54.8 f 2.4 -53.5 -70.1 f 1.0 -15.3 f 2.2 -49.0 -51.9 f 1.0 -0.5 f 2.0 (2.2 f 2.5)" -51.6 f 2.4 -53.9
(-136.5)" -84.8 f 0.5 -1 19.4 f 1.0 -84.6 f 1 0 -32.5 f 2.4
Estimated values; see text. For example, the bond length shortens, the dipole moments and the ionization potentials decrease, the heats of formation of the compounds are more negative, and the bond dissociation energy is larger.26 As shown in Figure 1, the intercept of the line 2b is more negative than that of line 2a. It implies that heats of formation for silicon compounds are very low or more negative. It is very easy to determine these decrements. The decrements are given by EN = 22.1 - 0.43Vx
(10)
(9)
Discussion p-d x-Back-Bonding. Pauling13 has pointed out that the Si-F bond possesses from 35 to 65% double-bond character judged by the shortening of the bond length. Later, the simple idea of d-orbital utilization in Si compounds became a universal tool for interpretation of anomalies in the properties and structure of organosilicon compound^.'^-^^ A x-back-bonding is presumed to take place between a p-orbital electrons on the atom attached to Si by a bond and a vacant 3d orbital of Si. Such bonding is expected for groups V, VI, and VI1 elements while elements of groups I, 11, 11, and IV as illustrated by Si-C, Si-Si, and Si-H bonds will not show this effect. (One may speculate that Si-vinyl bonds may show some delocalization energy arising from electron donation from the C=C x bond.) There are lone-pair electrons in the atoms of the groups V, VI, and VI1 elements but not in the C, Si, and H atoms. The back-bonding has been inferred in a variety of Si-containing compounds from studies of photoelectronic spectra, by structural studies (using infrared, UV, Raman, NMR, ESR,microwave and electron, and X-ray diffraction techniques) and by studies of chemical reactions. These properties of silicon compounds change in directions opposite to that found for similar carbon com( I 3) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1960; p 313. (14) Ebsworth, E. A. V. Chem. Commun. 1966, 530. (15) Randall, E W.; Zuckerman, J. J. Chem. Commun. 1966, 732. (16) Perkins, P. G. Chem. Commun. 1967, 268. (17) Hogben. M. G.;Oliver, A. J.; Graham, W. A . G.Chem. Commun. 1967, 1183. (18) Band, S. J.; Davidson, I. M. T.; Lambert, C. A. J . Organomet. Chem. 1968, 12, 3. (19) Gownelock, B. G.; Stevenson, J . J . Organomet. Chem. 1968 13, 13. (20) Mitchell. K . A . R. Chem. Reo. 1969, 69, 157.
where EN is the,so-called "p-d K back-bonding energy". The estimated back-bonding energies are listed in Table 111. It is noteworthy that the estimated back-bonding energies are nearly independent of Vx or the types of bonding or of the donor atom X, halogen or oxygen. This would appear to contradict the popular assumption25that p-d K back-bonding is stronger for the more electronegative atoms. The greater strength of silicon bonds to more electronegative elements would appear from our correlations to be attributable to the c bonding. The slopes of both lines of Figure 2a,b, we can see, are nearly parallel. This implies that the effect of the types of p-orbital electrons ( n = 2, 3, 4, and 5 ) in the donor atoms is very small. Neglecting this small effect, we can take an average value (EN) = 19.3 f 1.0 kcal. Relation between AA,H'' (Me3SiX/SiHJ) and V., Using the observed data on heats of formation, we can calculate AAfHo(Me3SiX/SiH3X). There are only three values (Table IV), and the range of these three values is very small, from -49 to -55 kcal. Recombining eq 6 and 7, we have AAfHo(Me3SiX/SiH3X)= -56.6
+ 0.51 Vx
(1 1 )
showing very weak dependence on Vx. The estimated values of AAfHo(Me3SiX/SiH3X)are listed in the last column of Table (21) Kwart, H.; King, K. d-Orbitals in the Chemistry of Silicon, Phosphorus, and Sulfur; Spring-Verlag: New York, 1911. (22) Colvin, E. Silicon in Organic Synthesis; Butterworths: London, 198 I . (23) Weber, W. P. Silicon Reagents for Organic Synthesis: Reactivity and Structure; Spring: Berlin, 1982; Vol. 14. (24) Organosilicon and Bioorganosilicon Chemistry, Sakurai, H., Ed.; Ellis Horwood: New York, 1985. (25) Ponec, R. Carbon-Functional Organsilicon Compounds; Chvalosky, V . , Bellama, J. M., Eds.; Plenum Press: New York, 1984. (26) Mortimer, C. T. Reaction Heats and Bond Strengths; Pergamon Press: New York, 1962.
Alkylsilane Derivatives
The Journal of Physical Chemistry, Vol. 93, No. 1 I , I989 4645
TABLE V: ADH"(MeSi-X/Me-X) in kcal mol-' P D H ' (Me3Si-X/CH3-X) X
obsd
est
OH CI NH2
35.3 f 1.5 29.1 i 1.5
Br SH I
25.7 f 1.5
F
44.9 35.3 29.6 27.6 24.7 22.8 20.0
19.5 f 1.5
1V. They agree with those observed within the experimental uncertainties. This agreement gives further and independent support to the conclusion that Ed is nearly independent of the donor atom X and, interestingly, equally of the H or CH3 substitution on Si. Relation between ADHo(Me3Si-X/Me-X) and V,. Following ref 3, we have for the differences in bond dissociation energies of Me3Si-X and CH3-X Me + Me3SiX Me3Si MeX
+
ADHo(Me3Si-X/Me-X) DHO(Me,Si-X) - DHo(Me-X) = AAfHo(Me3Si/Me)- AAfHo(Me3SiX/MeX) (12) where AAfHo(Me,Si/Me) = AfHo(Me3Si)- AfHo(Me), which is a constant independent of the nature of X. Therefore, there must be a linear relation between ADHo(Me3Si-X/Me-X) and VX .
Using AfHo(Me3Si) = -0.8 f 1.0 kcal AfHo(Me) = 35.1 f 0.1 kcal mol-',28-30and eq 7, we have 5.35Vx, X = ADHo(Me3Si-X/Me-X),,, = -81 C1, Br, I, or O H (1 3)
+
These estimated values of ADHo(Me3Si-X/Me-X) have been listed in the last column of Table V. Of course, we can also obtain the observed ADHo(Me3SiX/Me-X) from Table 11. Observed values have also been listed in Table V. As shown in Figure 1, there is a good linear relation between ADHo(Me3Si-X/Me-X),b,d and Vx. Twenty years ago, the relation between ADHo(Me3Si-X/ Me-X) and a few molecular parameters was discussed by a few author^.'^.'^ They proposed that there might be a linear relation between ADHo(Me3Si-X/Me-X) and the differences in ionization potentials (IP). This is not supported if we use the recent, preferred data on AfHo (Tables I and 11). This further implies that Vx and the IP are not linearly related either. On ADHO(SiH,-X/CH,-X). Following the previous discussion, we consider the differences in bond dissociation energies of SiH3X and CH3X. These are represented by the exchange reaction CH, + SiH3X CH,X SiH,
-
+
for which A,H = ADHO(SiH,-X/CH,-X) and ADH O (SiH,-X /CH,-X) = AAfHo(SiH3/CH3)- AAfHo(SiH,X/CH,X) (14) substituting eq 6 into eq 14 and using AfHo(SiH,) = 46.4 f 1.0 kcal mol-' (ref 21) and AfHo(CH3)= 35.1 f 0.1 kcal mol-', we find ADHo(SiH3-X/CH,-X),,, = 11.3 - (28.8 - 5.86Vx)p (15) (27) Walsh, R.Ace. Chem. Res. 1981, 14, 246. (28) Baghal-Vayjooee, M. H.; Colussi, A. J.; Benson, S. W. Int. J . Chem. Kiner. 1979, 1 1 , 147. (29) Heneghan, S. P.; Knott, P. A,; Benson, S. W. Int. J . Chem. Kinet. 1981, 13, 677. (30) Dobis, 0.; Benson, S.W Int. J . Chem. Kinet. 1987, 19, 691.
TABLE VI: ADH'(SiH3-X/CH3-X) in kcal mol-' and 0-Bond Contributions, ADH," ADH"(SiH,-X/CH,-X)
X F
CI Br I H
40.2 24.1 18.1 15.3 -14.7
obsd i 1.2 f 2.7 f 2.4 f 2.4" (12.6) f 1.1
est 40.6 23.7 18.4 13.3 -14.6
ADH,' 22.8 4.7 -1 .o -6.6
"Taking AfHo(SiH31) = -0.5 f 2.0 kcal mol-'.6 If we use our estimated AfHo(SiH31) = 2.2 f 2.5 kcal mol-' (see Table VII), then it is 12.6 kcal mol-'.
TABLE VU: Some Estimated AiHo in kcal mol-' ~~
compd' (unknown AfH') Me3SiF (11) Me3SiF (IV) Me3SiNH2 (11) Me,SiSH (11) S i H J (IV)"
ArH",,
ref compd (known AfHo)
-136.7 -136.6 (av -136.5) -69.0 -64.2 2.3 2.1 (av 2.2)
CH3Fb SiH,Fc CH3NH2b CH3SHb Me3SiIC HId
"Table I in ref 5. W e would prefer to take the averaged AfHo(SiH31)e8, = 2.2 f 2.5 kcal mol-' and use it for estimating ADHO(SiH,-X/CH,-X) in Table VI. bSee eq 7. 'See eq 11. dSee eq 5. e Roman numerals are the tables in which the estimated values are listed.
where p is the number of hydrogen atoms attached to X in the H,X molecule (see ref 2 and 5 ) . For X = halogen, p = 1, for X = H, p = 2, and for X = N , p = 3. Values estimated by eq 15 are listed in Table VI. The observed values are also listed in the same table, and they are in agreement. As shown in Table VI DHo(SiH3-X)
> DHo(CH3-X),
for X = halogen
while DHo(SiH3-H) C DHO(CH,-H),
for X = H
Here, the p i a back-bonding between Si and the halogen atom is a very important effect. Subtracting out the back-bonding contribution, the cr-bond contribution difference between Sihalogen and C-halogen is given by ADH,'
= ADHo(SiH3-X/CH3-X)
-39.6
- EM =
+ 6.29Vx,
X = F, C1, Br, I (16)
Values are listed in Table VI. Consistency of Estimated AfHo of Silicon-Containing Compounds. The heats of formation for compound Me3SiF, Me3SiNH2,Me3SiSH, and SiH,I have been estimated as described (Table VII). We have also listed the measured reference compounds. Notice that the twin values for AfHo, of Me3SiF and SiH31 come from two independent sources but are in excellent agreement with each other. The averaged values of AfHo(Me,SiF),,, and AfHo(SiH31),,, are -136.5 f 2.0 and 2.2 f 2.5 kcal mol-', respectively. Acknowledgment. This work has been supported by a grant from the National Science Foundation (CHE-87-14647). Registry No. (CH,),SiOH, 1066-40-6; (CH3)3SiCI, 75-77-4; (CH,),SiBr, 2857-97-8; (CH,),SiI, 16029-98-4; (CH,),Si, 75-76-3; (CH3),SiH, 993-07-7; Si, 7440-21-3; 0 2 , 7782-44-7; F2, 7782-41 -4; I,, 7553-56-2; Br2, 7726-95-6; CI2, 7782-50-5; O H , 14280-30-9.
