4294
G. R. WILLEY
Molar Refraction of Some Organosilicon Amines
by G. R. Willeyl Department of Chemistry, University of Reading, Berkshire, England Accepted and Transmitted by The Faraday Society
(April 19, 1967)
Experimental values of molar refractivity for the three series of organosilicon amines (A) MesSiNHaR, (B) MeaSiNRz,and (C) (n4e3Si)2NR,where R = Me to Ph, have been measured. For the n-alkyl homologs mean values of (A) 2.57 0.02, (B) 2.21 f 0.02, and (C) 2.05 f 0.02 have been calculated for Rs~--N. These results, together with those obtained including isomeric forms, are discussed using the proposed p,-d, bonding whereby the trimethylsilyl moiety acts as a "charge sink" for the nitrogen lone pair. Steric factors are seen to play a significant part in this over-all charge delocalization. In a similar fashion, Rsi is found to vary between these systems, and mean values calculated for the n-alkyl homologs are (A) 7.12 i 0.02, (B) 6.79 i 0.02, and (C) 6.61 0.02.
*
*
Introduction In an extensive study of molar refractivity of a wide variety of organic and organometallic compounds, V o g e P 4 has established a system of atomic, bond, and group refractions which is of general acceptance. Within the range of organosilicon compounds the bond refraction Rsi-d is quoted as 2.16. Other reports principally by WarwickGa and Sayre6b provide supplementary values of 2.00 and 2.10, respectively. Values for the atomic refraction Rsi have been reported7J in the range 3.87 < Rsi < 8.12. The much discussed acceptor role of 3d orbitals of silicon9 can lead to p,-d, bonding in the Si-N linkage, and with this delocalization of charge there should result accompanying variations in bond refraction according to the environment of both the silicon and nitrogen atoms. We have measured the molar refractions of the three series of organosilicon amines (A) Me,SiNH.R, (B) P\le3SiNRz,and (C) (Me3Si)zNR, and results indicate appreciable variations of refraction for these systems. To interpret these deviations a qualitative relationship is proposed linking the particular nitrogen atom environment with the extent of A bonding. Experimental Section The organosilicon amines were prepared and characterized as part of a study of the Si-N bond,l0 and all samples were freshly distilled prior to measurements. Values of nmD were obtained using a Standard Abbe The Journal of Physical Chemistry
"60" refractometer fitted with Amici prisms to give direct readings of n*D ( t = "C; D = 5893 A). The instrument was housed in a laboratory where roomtemperature fluctuation was small, and the water circulating around the prisms was drawn from a tank thermostated a t a temperature of 20.00 f 0.05". The short-stem thermometer attached to the prism block was calibrated over the range At = 10-30" to within 0.05". The instrument scale was checked by reference to a glass test piece with a certified n Z o ~ After . allowing the instrument to attain the temperature of the coolant water-a period of several days-and when the ambient temperature was within 1-2" (1) Extracted in part from a thesis submitted by the author to the University of Bristol in partial fulfillment of the requirements for the degree of Doctor of Philosophy, July 1963. Address all inquiries to Department of Chemistry, University of California, Riverside, Calif. (2) A. I. Vogel, W. T. Cresswell, G. H. Jeffrey, and J. Leicester, J . Chem. SOC.,514 (1952). (3) A. I. Vogel, W. T. Cresswell, and J. Leicester, J . Phys. Chem., 58, 174 (1954). (4) A. I. Vogel, J . Chem. SOC.,133 (1946). ( 5 ) All numerical values of refraction are expressed in cm3. R A - B signifies refraction of the A-B bond, etc., and R denotes substituted alkyl or aryl groups. (6) (a) E. L. Warwick, J . Am. Chem. Soc., 68, 2455 (1946); (b) R. Sayre, J . Chen. Eng. Data, 9, 146 (1964). (7) T.Sly, B.S. Thesis, Bristol, 1963. (8) D. B. Brady, Ph.D. Thesis, Bristol, 1965. (9) See, for example, D. P. Craig, A. Maccoll, R. S. Nyholm, L. E. Orgel, aad L. E. Sutton, J. Chem. SOC.,332 (1954). (10) E. W. Abel and G. R. Willey, ibid., 1528 (1964).
MOLARREFRACTION OF SOMEORGANOSILICON AMINES
4295
Table I : Molar Refraction Data for Series A, Mei3iNH.R [RI'
Me Et n-Pr i-Pr n-Bu ~-BLI see-Bu t-Bu Ph a
103.24 117.27 131.30 131.30 14Fj.33 145.33 145.33 145.33 165.31
0.739 0.730 0.747 0.733 0.759 0.758 0.756 0.754 0.926
1.3901 1.3929 1.4022 1.3950 1.4089 1.4073 1.4060 1.4076 1,5211
33.121 38.329 42.823 42.936 47.335 47.224 47.210 47.500 54.362
RSi-Nb
RBiC
2.266 2.828 2.670 2.790 2.532 2.422 2.412 2.702 2.636
6.810 7.373 7.216 7.336 7.079 6.969 6.959 7.249 12.073
[ R ]calculated from the Lorenz-Lorentz equation n2-1M n2+ld
--
[R] =
'Rsi-y obtained using the law of additivity of bond refraction: tion values used are given in Table 11'. Table IV.
K. Denbigh, Trans. Faraday Soc., 36, 936 (1940). Other bond refracRsi obtained using the additivity law. Other atomic refraction values used are given in
Table 11: hlolar Refraction Data for Series B, hIeaSiNRl R in Me3SiNRz
hlol n-t
d2b
nzoD
[Ela
Me Et n-Pr i-Pr n-Bu i-Bu sec-Bu Ph
117.27 145.32 173.37 173.37 201,42 201.42 201 .42 241.40
0.741 0.761 0.782 0.786 0.794 0.796 0.817 1.024
1 ,3962
38,051 47.463 56.400 56.285 65.482 65.462 64.805 79.441
See footnote a of Table I.
' See footnote b of Table I.
1.4113 1,4226 1.4241 1.4296 I ,4307 1.4385 1.5890
REi-sb
2.360 2.474 2.118 1.998 1.902 1,882 1.222 1.810
h i C
6.931 7.047 6.693 6.573 6.479 6.459 5.799 16.801
See footnote c of Table I.
Table 111: hlolar Refraction Data for Series C, (JlelSi)2NR R in (hlezSi)zKR
Mol at
a201
n aoD
IRla
H 3Ie Et n-Pr n-Bu i-B~i Ph
161.40 173.42 189.45 203 48 217 51 217.51 237.49
0.774 0.797 0.805 0.812 0.820 0.826 0.888
1,4081
51.431 55.957 60.644 64.921 69.079 69.130 76.731
1.4222 1.4289 1.4316 1.4342 1.4380 1.4856
Rsi- N~
2.340 2.187 2.203 2.019 1I77B 1,800 2.017
RBic
6.801 6.745 6.762 6.578 6.335 6.360 8.876
See footnote a of Table I. 'See footnote 6 of Table I. 'See footnote c of Table I.
of that of the instrument, measurements were made in duplicate for the liquid samples. We were able to obtain reproducible readings to *2 X 10-4. Measurements of dZo4were carried out using a 1Wpl pipet as a form of Sprerngels pycnometer. The
pycnometer in a stoppered tube, samples of organosilicon amines, and distilled water as reference were allowed to stand in a water bath maintained a t a temperature of 20.00 If: 0.05'. Using a suction piston, the pycnometer was filled with liquid (first with disVolume 7 1 , Number I S
December 1967
G. R. WILLEY
4296
tilled water as standardization and subsequently with liquid samples) and, after the final volume adjustment, was quickly replaced in the stoppered tube and weighed. A chamois cloth was employed in all wiping and handling operations. Consistent values for d2O4 were obtained on repeated runs to within f1 X
70
LI
40
Results The observed molar refractions are given in tabular form for series A (Table I), series B (Table 11)) and series C (Table 111).
Refraction
C-H N-H Si-Cal
1.676 1.762 2.470 1.296 2.688"
c.1-c,1
3
2
1
4
n.
Figure 1. Plot of molar refractivity against n for the three series of organosilicon amines: (A) MesSiNH.C,H2,+1; (B) MesSiN(C,H$,+1)2; (C) (Me&)PNCnHln+l.
Table IV : Auxiliary Refraction Data Bond
30
Atom
Refraction
C H N,, (secondary) N,, (tertiary) N,1 (secondary)
2.591 1.028 3.550 4.243 2.582 2.744
with breakdown to give the disilazane"--and no comparison with experimental values for (A) and (B) was possible. The equation provided relates to the breakdown of the unstable intermediate and is best inserted at this point. However, in the case of (C) (Me3%)*" was isolat,ed and the experimental value R = 51.43 is in very good agreement.
3.20, series C a Calculated from the benzene molecule. series where R = Ph.
' Used for three
In Figure 1 molar refraction has been plotted against n, the number of carbon atoms in the n-alkyl chain for the three series. From the graph, values of the respective gradients are (A) 4.55 f 0.05, (B) 4.55 f 0.05, and (C) 4.50 rt. 0.05. The ordinate intercepts obtained are 29.13 for series A and B, and 51.50 for series C.
Discussion The plots illustrated in Figure 1 show linearity, and from the respective gradients, which are almost identical, the mean value of 4.53(3) is obtained over-all. This represents the >CH2 increment of refraction within the homologous series, and since it is consistent, then #CH2 # f(n) which would preclude any association through >N-H. . . ' bonding. Comparison with RCHl values found in organic systems,2 viz., aliphatic hydrocarbons where R C H= ~ 4.64, shows our value to be slightly lower. For the systems studied, the ordinate intercept represents the parent molecule, i.e., (A) R!te3SiNH.H(CH2)o (B) Me3SiN.H(CH2),H(CH&, and (C) (Me3Si),N.H(CH&, and the extrapolated refraction values obtained were R = 29.13 for (A) and (B) and R = 51.50 for (C). The compound Me3SiNHz was not isolated-all attempts so far indicate possible existence as an unstable intermediate The Journal
of
Physical Chemistry
Electronegativity considerations12 applied to C-N and Si-N bonds predict partial ionicity leading to enhanced basicity of the order Si-N > C-N. Comparison of base strengths from heats of mixing measurements with chloroform13 and from observed shifts in the C-D stretching frequency in mixtures of base and de~teriochloroform,~~J~ however, shows the reverse order in that these organosilicon amines are noticeably weaker bases. In addition, the silicon atom would seem to have a pronounced effect with regard to removal of charge, when lone-pair atoms are close-by, over and above that of its group IV congeners. In relation to our skeletonal system, uiz., the mes-
R' Me3-Si--N
/ \
R"
(A), R' (B), R' (C), R'
= H ; R" = = R" = R =
R
SiMe3; R" = R
(11) R. Fessenden and J. S. Fessenden, Chem. Rev., 61, 361 (1961). (12) R. T.Sanderson, J . Inorg. Nucl. Chem., 28, 1563 (1966). (13) A. W. Jarvie and D. Lewis, J . Chem. Soc., 1073 (1963). (14) E.W. Abel, D. A. Armitage, and G. R. Willey, Trans. Faradag Sac., 60, 1257 (1964). (15) E. W. Abel, D. A. Armitage, and D. B. Brady, ibid., 62, 3459 (1966).
MOLARREFRACTION OF SOMEORGANOSILICON AMINES
4297
omeric contribution from the electron-releasing hiledigroupingls should be a t a maximum for (C) especially in the case R” = Me. Transmission of this charge through a u Si-N bond should lead to a relative order RSi-N(C) > Rs~-N(B) > Rsi-s(A) allowing for the greater inductive effect of the two n-alkyl chains in (B). The results obtained give Rs~-N(A) = 2.57, Rs~-N(B)= 2.21, and RSi--N(C) = 2.05 which is in fact the complete reverse. To the extent that Rsi-N(A) > RSi-N(B) > R s i - ~ ( c )we suggest this represents charge delocalization through a ir Si-N bond with concomitant depression of refraction. An increase of formation of ir bonding gives increased bond order with an allied increase in stretching frequency v S i - ~ (assuming nonmixing of other symmetrical modes) and decrease in bond length.” 18 All of the organosilicon amines gave a strong band at 835-843 cm-I assigned to the vsi-s stretch,lg but, over this range AV = 8 cm-l, no serious differentiation could be made between the three series. Inclusion of results obtained for isomers gives Rs,-r;(A) = 2.58, Rsi-s(B) = 1.99, and Rs~-N(C) = 2.05. Clearly we should not expect the additivity law to be consistent for isomeric forms and this is well demonstrated for (B). I n this system the two alkyl chains are adjacent and we consider the anomalous value to be a direct result of steric interference possibly chain “wagging” interaction. The complications arising in the case of R = Ph where charge delocalization can arise through involvement of the benzene ir system and where steric factors present a problem, especially for (B), leads us to consider the three compounds (a) lIe3SiNH.Ph, (b) Me3SiNPhz, and (c) (Me3Si)zNPhas a separate series. Although the benzene molecule itself is reported20 to be an example where refraction exaltation does not occur even in the presence of conjugation, substitution to nitrogen will give rise to extensive delocalization of the typez1
As a complementary approach it is interesting to consider variations of the atomic refraction of the silicon atom in the three series. Measurements of the diamagnetic susceptibilityz2 of a representative group of these organosilicon amines give a value of xsi much lower than that found in systems where pW-d, bonding involving silicon atoms is improbable, e.g., n-RSiMe;. I n a similar fashion this type of bonding should affect values of Rsi, and the mean values obtained for the three series are given in Table V.
This is borne out by the results obtained, RSL-N(a) = 2.63, RSi-N(b) = 1.81, and Rs~-N(c) = 2.01, indicating that such P bonding is a t a maximum for (b) as expected. Whether or not steric factors are destructive, especially for (b), will depend largely on the barrier to rotation of the N-C,, bond.
Table V : Values of the Atomic Refraction of Silicon Found in the Three Systems
Series
Rsi (excluding isomers and R Ph)
Rsi (excluding R = Ph)
(total)
( A ) Me3SiNH.R (B) MerSiNRz (C) (Me3Si),NR
7.12 6.79 6.61
7.12 6.57 6.56
7.67 7.85 6.92
-
RSi
These results are treated only but lightly in the sense that the inadequacies of the additivit’y law of at,omic refraction prevent serious correlations to be made especially in those cases where large structural changes occur, For the n-alkyl homologs Rsi decreases in the order &(A) > Rsi(B) > Rsi(C), and within each series Rsi decreases in a linear fashion except where methyl groups are substituted. The same over-all order is found on inclusion of results for isomeric forms but within each series anomalous values of Rsi are found for the bulkier substituted groups. Since the polarizability of the silicon atom must vary with the extent of pl,-d, bonding, it is significant that these trends of Rsi are in unison with those of Rs~-N.The most reliable mean values of the atomic (16) R. W.Bott, C. Eaborn, and D. R. M. Walton, J . Organmetal. Chem. (Amsterdam), 2 , 154 (1964). (17) Compare with the treatment18 of the (SiHa)aN molecule where the proposed p A r bonding gives rise to a flattened planar molecule of low basicity and with shortened Si-N bond lengths. (18) K. Hedburg, J . Am. Chem. SOC.,7 7 , 6491 (1955). (19) E.W. Randall, J. J. Ellner, and J. J. Zuckermann, Inorg. Nuel. Chem. Letters, 1, 109 (1965). (20)Y. K. Syrkin and M. E. Dyatkina, “Structure of Molecules,” Butterworth and Co. Ltd., London, 1950,p 203 ff. (21) J. A. Bedford, J. R. Bolton. A. Carrington, and R. H. Prince, Trans. Faraday Soc., 59, 53 (1963). (22) E. W.Abel, R. P. Bush, C. R. Jenkins, and T. Zobel, ibid., 60, 1214 (1964).
Volume 71 Number 13 December 1967
L. E. ALEXANDER,M. G. NORTHOLT, AND R. ENGMANN
4298
refraction of silicon are taken as &(A) Rsi(B) = 6.79, and Rsi(C) = 6.61.
= 7.12,
Acknowledgment. The author wishes to acknowl-
edge the award of a research grant from the Department of Scientific and Industrial Research (England) (1960-1963) during which time most of the experimental data were collected.
Crystal Structure of pBis(dimethylhydroxysily1) benzene, a Monomeric Model of Polytetramethyl-psilphenylenesiloxanel
by Leroy E. Alexander, Maurits G. Northolt, and Rolf Engmann Mellon Institute of CarnepieMeUon University, Pittsburgh, Pennsylvania
16815 (Received April 80, 1967)
The crystal structure of p-bis(dimethylhydroxysily1)benzene (DMHSB) has been determined from intensity data measured with an X-ray single-crystal diff ractometer and refined by the methods of full-matrix least squares and difference synthesis. The triclinic unit cell contains three molecules and has space-group symmetry Pi. The structure is a three-dimensional network of molecular chains held together by cross links consisting of single molecules. Approximately linear 0-Ha * .O hydrogen bridges connect the molecules into chains and tie the cross links between them.
Introduction This investigation is the second in a series of X-ray crystallographic studies of oligomeric models of linear polymers.2 The formal molecular configuration of DMHSB is
structure of the corresponding cyclic dimer was solved in this laboratory by Smith.4
Experimental Sectioh The sample as supplied by R. L. Merker of Mellon Institute had been recrystallized from a hexane-tetm hydrofuran solution and contained lath-shaped crystals suitable for X-ray analysis, the long dimension corresponding to the c axis. Weissenberg and precession CH, CH, photographs showed the crystals to be triclinic, space group P1 or Pi, and gave preliminary values of the from which it is seen that it may be regarded as the protomonomer of polytetramethyl-p-silphenylenesilox- unit-cell dimensions. A piezoelectric test made with a sensitive apparatus similar to one described by Pepinsky ane
The preparation and unusual properties of this polymer have been described by Merker and Scott.8 The The Journal of Physical Chemistry
(1) This investigation was supported in part by Public Health Service Grant GM-12396 from the National Institutes of Health. (2) First investigation: L. E. Alexander, R. Engmann, and H. G. Clark, J. Phys. Chem., 70, 252 (1966). (3) R. L. Merker and M. J. Scott, J . Polyner Sci., A2, 15 (1964). (4) G. S. Smith, Program and Abstracts, American Crystallographic Association, Bozeman, Mont., July 26-31, 1964, No. J-9.
