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639
Inorg. Phys. Theor.
Silicon-Nitrogen Compounds. Part V1.l The Preparation and Properties
of Disilazane
Published on 01 January 1969. Downloaded by University of Nottingham on 25/07/2016 09:27:36.
By
B. J.
Aylett * and M. J. Hakim, Chemistry Department, Westfield College, London N.W.3
Disilazane, a long-postulated intermediate in the preparation of trisilylamine, has been made in high yield from
diphenylaminosilane and ammonia at low temperatures. Its physical properties, including infrared and mass
spectra, are reported and discussed. Gaseous disilazane is stable either alone a t 150" or with ammonia a t room
temperature. Liquid disilazane a t 0" decomposes slowly to give trisilylamine and ammonia, while liquid ammonia
at -130" rapidly causes disproportionation about silicon. Although disilazane is a very weak Lewis base, it
reacts readily with iodosilane to give trisilylamine.
ALTHOUGHmany organo-substituted disilazanes are 10-cm. cell. Solvents were dried (CaH, or LiAlH,),
known,2 only two representatives of unsubstituted silyl fractionated i n vacuo, and stored in sealed glass tubes.
compounds of this type have been described. Emelkus Iodosilane was prepared by the literature method ; 13
diphenylaminosilane was made from iodosilane and diphenyland Miller 3 prepared N-methyl- and N-ethyl-disil- amine.l
azane by the reaction of chlorosilane with the approDisiZazaute.-Preparatioiz. Ammonia (0.0189 g., 1.1
priate primary amine, and the N-methyl derivative's mmoles) and toluene (1 ml.) were distilled into a reaction
pyroly~is,~
disprop~rtionation,~
ability to act as a tube containing diphenylaminosilane (0.06 g., 3.0 mmoles).
Lewis base,p and infrared spectrum7 have since been The mixture stood successively a t -85" (1 hr.), -64"
studied. Disilazane itself has proved elusive: Stock (Q hr.), and -46" (Q hr.). With the tube still a t -46", voland Somieski reported that when chlorosilane reacted atile products were pumped out into the fractionation line.
with an excess of ammonia, there was some evidence for Unreacted ammonia (0.0062 g. , 0-35 mmole) passed through
the formation of partly silylated products. These could a t -120". The fraction held at -120" was repeatedly
fractionated a t -112"; that just passing through was
not be isolated, but decomposed readily to give silane, disilazane (0.0450 g., 0.58 mmole, 75% based on ammonia
trisilylamine, and polymeric solids. It was suggested used) [Found: Si-H, 7.6; NH, 19.2; Si, 73.4%; M , 77.3.
that these products might arise via disproportionation (SiH,),NH requires Si-H, 7.8; NH, 19.4; Si, 72.8%;
about silicon and nitrogen [reactions (1)-(3)].
M , 77-21. Held a t - 112" were trisilylainine (-2 rng.) and
+
2SiH,NH, ---t (SiH,),NH
NH,
3(SiH,),NH -w 2(SiH,),N $- NH,
1
(SiH,),NH --+ rt (SiH,NH),
SiH,
+
(1)
(2)
(3)
In discussions of the formation of trisilylamine as a
stepwise p r o c e ~ s ,various
~
uncertainties have emerged.
Firstly, the silane observed may be formed by ammoniacatalysed disproportionation of trisilylamine ; Wells
and Scliaeffer in a thorough study1* have shown that
this is rapid in the liquid phase. Secondly, disilazane,
if formed, might like trisilylamine be insufficiently basic
to react with a halogenosilane. In order to resolve these
uncertainties, we have synthesised disilazane 11 by a
route that does not require the presence of an excess of
ammonia.
EXPERIMENTAL
Techniques were similar to those previously described.'. la
Infrared spectra were recorded with a Perkin-Elmer 337
spectrometer, with sample pressures of 8 and 30 mm. in a
Part V, B. J . Aylett and M. J . Hakim, precFding paper.
(a)C. Eaborn, Organosilicon Compounds, Butterworths,
London, 1960; (b) R. Fessenden and J . S. Fessenden, Chem.
Rev., 1961, 61, 361; (c) V. Baiant, V. Chvalovskf, and J.
Rathouskf, ' Organosilicon Compounds,' Czechoslovak Academy
of Sciences, Prague, 1965.
3 H. J . EmelCus and N. Miller, J. Chem. Soc., 1939, 819.
B. J . Aylett, G. M. Burnett, L. K. Peterson, and N. C.
Ross, SOC.Chem. Ind. Monograph, 1961, No. 13, p. 5.
B. J. Aylett and M. J . Hakim, Chem. and Ind., 1965, 1626.
(u)A. B. Burg and E. S. Kuljian. J. Amer. Chem. Soc., 1950,
72, 3103; (b) S. Sujishi and S. Witz, ibid., 1954, 76, 4631; 1957,
79, 2447.
2
solvent.
The residue, consisting of unused SiH,NPh, and diphenylamine, was treated with further toluene and ammonia
a t -46". No more disilazane was produced, but 2.9 mg. of
an unknown compound were isolated, possessing the following i.r. spectrum (cm.-l): 2150s, 118Os, 980sh, 935s, 905s,
900sh. This, together with its volatility, suggest that i t
may be a condensed product such as SiH,(NHSiH,),.
Other experiments, in which the molar ratio of SiH,NPh,
to ammonia was (2, gave a residue shown (i.r., m.p.) to be
solely diphenylamine.
Mass spectvunz. Using an AEI model MS 10 with ionising energy of 70v, the following peaks and their relative
intensities were recorded: 136(0.2), 122(0-9), 121(0.3),
120(0-3), 105(0*5),102(0*4), 90(0.8), 88(0.1), 77(6-0), 76(55),
75(33), 74(37), 73(14), 72(28), 71(18), 66(0.2), 65(0.1),
64(0-1), 62(0-1), 61(0*4), 60(0*4), 59(1*2), 58(0*8), 57(1*0),
66(1.2), 54(0.3), 53(0-1), 52(0*1), 51(0-2), 50(0-2), 49(0.2),
48(0*2), 47(1-6), 46(5-2), 45(6.8), 44(7.6), 43(7*4), 42(4.6),
40(0-5), 38*5(0*5), 38(4*0), 37*5(4-2), 37(10*6), 36*5(7*2),
36(7-7), 35.5(5-2), 35(0*4),33(2*4),32(8.3), 31(84), 30(100),
29(40), 28(43), 27(3.9), 26(2-4), 25(0.5), 24(0.1), 18(7*2),
(a) M. J . Buttler, D. C. McKean, R. Taylor, and L. A.
Woodward, Spectrochim. Acta, 1965, 21, 1379; (b) T. D. Goldfarb
and B. N. Khare, J. Chem. Phys., 1967, 46, 3384.
8 A. Stock and C. Somieski, Bey., 1921, 54, B , 740.
(a) A. G. MacDiarmid. A d v . Inorg. Chem. Radiochem., 1961,
A. Stone, ' Hydrogen Compounds of the Group
IV Elements, Prentice-Hall, London, 1962; ( c ) E. A. V.
3, 207 ; (b) F.
9.
Ebsworth, ' Volatile Silicon Compounds,' Pergamon, Oxford,
1963.
lo R. L. Wells and R. Schaeffer, J. Amer. Chem. Soc., 1966,
88, 37.
l1 B. J . Aylett and M. J . Hakim, Inorg. Chem., 1966, 5, 167.
l2 B. J . Aylett and J . Emsley, J. Chem. Soc. ( A ) , 1967, 652,
1918.
l3 B. J. Aylett and I. A. Ellis, J . Chem. Soc., 1960, 3415.
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640
J. Chem. SOC. (A), 1969
17(88), 16(74), 15(7.4), 14*5(1-2),14(2-6), 13(0-2), 12(0.2),
2(38).
Vupour pressure. Measurements with a sensitive spoon
gauge in the range -96 to -23" gave the following values
(mm.): -96", 1.0; -84.5", 2.4; -64", 8.7; -44-5",
28.9; -23", 94.5; -44*5", 36.8; -66", 14.0; -84.5",
5.0; -96", 3.0 (the last four values refer to decreasing
temperatures). Some decomposition clearly occurred at
-223"; from -96 to -44.5", these values determine the
equation: log p = 6.832 - 1220/T, whence AH,, is 5-58
kcal. mole-I, AS,,, is 18.0 cal. mole-l deg.-l, and the extrapolated b.p. is 36". The Stock method gave a m.p. of
- 132"
1" (mean of three determinations).
Theyma1 stability. (a) Gas phase. Disilazane at a pressure of about 10 cm. was heated a t 150" for 3 hr. Its recovery was quantitative and the i.r. spectrum was unaltered.
(b) Liquid phase. Disilazane (0.0200 g., 0.26 mmole)
was held a t 0" for 72 hr. in a small tube. Fractionation
yielded silane (<Om5 mg.), ammonia (0.06mmole), unreacted
disilazane (0-05 mmole) and trisilylamine (0-11 mmole).
Hydrogen was absent. A little white solid remained; i t
was attacked slowly by cold water and rapidly by alkali,
giving hydrogen and ammonia.
Reaction with iodosilane. In a preliminary experiment,
there appeared to be no rapid reaction in the gas phase
between disilazane and iodosilane. However, on condensing the two together a t low temperatures, a white solid
quickly formed. Disilazane (0.0178 g., 0-23 mmole) and
iodosilane (0.0123 g., 0.08 mmole) were allowed to react for
15 min. successively at - 96", - 64", and room temperature.
A white solid formed a t -64". Fractionation of the
volatile products gave trisilylamine (0.0162 g., 0.15 mmole),
identified by mol. wt. (Found: 1064; calc., 107.3) and i.r.
spectrum. An i.r. spectrum of the solid residue (mulls)
corresponded to that of ammonium iodide, with no Si-H
absorptions. The solid effervesced, however, on treatment
with alkali; this was possibly due t o traces of sorbed
(SiH,),N or its degradation products.
1Peactio.Pc with ammonia. Disilazane (0.03 rnmole) and
ammonia (0.3mmole) were condensed into separate cold
fingers attached to a 10-cm. gas cell, and allowed to
volatilise. The i.r. spectrum of the gaseous mixture was
a superposition of the separate spectra. After 72 hr. at
room temperature, the spectrum was unchanged. The two
components were then brought together in the liquid phase
by keeping one cold finger at - 130" for 2 min. On warming to room temperature, white solid was observed in the
cold finger, and the gaseous mixture consisted only of
silane and ammonia.
Reaclim with di~henylamlnosilme. DisiIazane (0.10
mmole) was distilled into a reaction tube containing diphenylaminosilane (0.20 mmole). The mixture was then
held for 1 hr. in turn a t -120, -885, and -46". No reaction
occurred. The reactants were re-sealed together and
alternatively frozen in liquid nitrogen and warmed to
room temperature several times. Disilazane (0.10 mmole)
was the only volatile product.
RESULTS AND DISCUSSION
The amine exchange reaction (4) gave good yields
of disilazane when a deficit of ammonia was used
2Ph2NSiH, NH, _t (SiHJ2NH 2Ph2NH (4)
+
+
D. W. Aubrey and M. F. Lappert, Proc. Chem. Soc., 1960,
148; D. W. Aubrey, M. F. Lappert, and M. K. Majumdar, J .
Chem. SOC.,1962, 4088.
14
and the temperature did not rise above -46".
Toluene was the preferred solvent; hexane was difficult
to separate from the product, and yieIds were erratic with
no solvent at all. Although carefully sought, no evidence for silylamine, SiH,NH,, was found.
It is not known whether diphenylaminosilane reacts
first with ammonia and then again with silylamine to
give the observed product, or if reaction (1) occurs
very rapidly [reactions designed to produce M%SiNH,
have yielded only (Me,Si),NH]. It was established,
however, that there is no rapid reaction between
SiH,NPh, and disiliazane, even at room temperature.
Thus the small amount of trisilylamine formed in the
preparation ((5%)
probably arose from reaction (2).
A four-centre intermediate has been proposed for amine
exchange rea~ti0ns.l~The intermediates for the silylation of ammonia (I; R1= R2 = H) and silylamine
(I; R1= SiH,, RZ= H) appear sterically feasible,
but that for the next stage (I; R1= R2 = SiH,) is
appreciably hindered. Furthermore the low basicity of
SiH,NPh, and disilazane (see later) will not encourage
its formation.
Physical properties of disilazane are given in Table 1,
together with those of its carbon analogue, dimethylamine. These values suggest generally that intermoleTABLE1
Some physical properties of disilazane and dimethylamine
(SiH,),NH
- 132'
M.p. ("c)...........................
36"
B.p. ("c) ...........................
A H , (kcal.mole-') ............
5.68
AS,,, (cal. mole-' deg-1) ......
18.0
a Ref. 15.
(CH,) ,NH
-96"
7.4"
6-49 a
23.2 0
cular forces in disilazane are rather weak. No solid
complex could be isolated between disilazane and trimethylborane, although the vapour pressures of mixtures showed negative deviations from Raoult's law
below -goo, and such mixtures could not .be separated
by fractionation at low temperatures. Thus disilazane
is only a weak Lewis base. A recent investigation of
disilazane's structure by electron diffraction has shown
that the SiNSi bond angle is about 128". All these
facts are consistent with marked intramolecular fin--dT
dative bonding in disilazane, and with considerable s
character in the Si-N bond. Nevertheless, disilazane
is sufficiently basic to react rapidly with iodosilane at
low temperatures: reaction (5) gave more than 85%
E. Wiberg and W. Suttetlin, 2.Elektrochem., 1935, 41, 151.
A. G. Robiette, G . M. Sheldrick, W. S. Sheldrick, B.
Beagley, D. W. J. Cruickshank, J. T. Monaghan, B. J. Aylett,
and I. A. Ellis, Chenz. Comm., 1968, 909.
l8
16
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641
Inorg. Phys. Theor.
conversion into trisilylamine when a small excess of
iodosilane was used.
Published on 01 January 1969. Downloaded by University of Nottingham on 25/07/2016 09:27:36.
4(SiH,),NH
+ SiH,I +3(SiH,),N + NH,I
(5)
A study of the thermal decomposition of disilazane
presented some difficulty: if reaction (2) did occur, then
the ammonia produced might be expected to catalyse
efficiently reaction (3). It was first established that
disilazane in the gas phase was unchanged after 3 hr. at
150". This suggests that intramolecular processes such
as reaction (6) can. be discounted.* Samples held at
-80" decomposed in a complex way, yielding silane,
(SiH,),NH
_t
1
(SiH,NSiH,),
+ H,
(6)
ammonia, and polymeric solids, as might be expected
from simultaneous reactions (2) and (3). However, a
sample kept at 0" decomposed almost quantitatively
according to equation (2); it appears that, under these
conditions, the ammonia produced was almost all in
the gas phase, being insufficiently soluble in disilazane or
trisilylamine to promote disproportionation.
The effect of ammonia on disilazane was then deliberately studied. Results exactly paralleled those found
by Wells and Schaeffer lofor trisilyIamine and ammonia:
there was no reaction in the gas phase at room temperature, but very rapid disproportionation took place
according to equation (3) when the mixture was cooled
to -130" and then allowed to warm to room temperature.
In summary, disilazane alone decomposes by one of
Stock's routes [equation (2)], and with ammonia decomposes by the other (equation (3)]. Both these
reactions proceed only in a condensed phase. It still
cannot be said that disilazane definitely occurs in the
reaction between monohalogenmilanes and ammonia,
but its properties are entirely consistent with the view
that it does. Any disilazane that may have been produced in the past would have been efficiently destroyed
either by fractionation in the presence of ammonia or by
rapid reaction with more halogenosilane.
Infrared Spectrum.-Disilazane has a rather simple
spectrum, which may be compared with those of the
isoelectronic molecule disiloxane l7and of trisilylamine l8
(Table 2). It appears reasonable that disilazane possesses Czvsymmetry; it will then have 24 normal vibrational
modes, which can be subdivided into the following
4A,
7B1 5B2. The A ,
symmetry classes: 8A,
vibrations are inactive in the infrared. Most of the
observed bands can be assigned with some confidence
by drawing analogies from related compounds.
The N-H stretching vibration appears characteristically at 3425 cm.-l, while the five Si-H stretching vibrations are closely grouped around 2165 cm.-l. The
+
+
+
* Although early workers reported hydrogen as a decomposition product of systems which may have contained disilazane,
we have not found more than a minute trace. We believe that
hydrogen arose from the action of relatively strong bases and
their salts on silyl compounds.
l7 R. C . Lord, D. W. Robinson, and W. C . Schumb, J . Anzev.
Chem. SOC.,1956, 78, 1327.
TABLE2
Infrared spectra of gaseous (SiH,),Y (Y = NH,0)and
(SiH,),N (cm.-l)
(SiH,),NH
34251x1
2165s
1520w
::EhI
975s
930vs
926s,br
740s,br
(645?)
,
.
(SiH,),O
-
{ i:}
1700m
(SiH,),N
-
2167s
1490w
1107vs
957vvs
996s
944vs
764s
748m
606 *
*
490 *
Raman frequency.
Probable assignment
v(N-H)
v(SiH)
See text
8(N-H)
v(Si0Si) asym.
v(SiNSi) asym.
G(SiH,)
See text
6(SiH,) rock.
v(Si0Si) sym.
v(SiNSi) sym.
peak at 1183 cm.-l may be assigned to the N-H bending
mode [cf. (Me3Si),NH,l9 1177 cm.-l], while silyl deformation modes are responsible for the very intense absorption at 930 cm.-l. The peak a t 975 cm.-l is almost
certainly due to the asymmetric SiNSi stretch; this
vibration occurs at 996 cm.-l in (SiH,),N and at 1026
cm.-l in (SiH,),"
(SiH,),.m The possibility remains,
however, that one or more of the asymmetric SiH,
deformation modes may produce absorption in this region
and obscu-e the SiNSi peak.
The strong band at 825 cm.-l parallels that observed in
(SiH,),NN(SiH,), at 803 cm.-l and may be discussed in
similar terms.20 It is considered that there is strong
coupling between the SiNSi symmetrical stretching
vibration and one of the silyl rocking modes of the same
symmetry (Al), which increases the frequency of the
latter. The remaining strong peak at 740 cm.-l is then
assigned to the other two silyl rocking modes in the B,
and 13, species; similar values are found in (SiH,),O
(764 cm.-l), (SiH,),N (748 cm.-l), and (SiH,),NN(SiH,),
(709 cm.-l). Although the symmetrical SiNSi stretching mode is active in the infrared, it is expected to give
only weak absorption, and has not yet been observed.
A tentative assignment of the weak band at 1520 cm.?
would make it a combination band of asymmetric and
symmetric v(SiNSi), placing the latter at about 545
cm.-l. A similar conclusion could be made more firmly
in the case of (SiH,),O and (SiH,),N, where similar
combination bands occurred, at 1700 and 1490 cm.-l
respectively.
It is emphasized that modes are only approximately
described by such terms as ' SiNSi stretching,' and mixing
may be important in cases other than the one mentioned
above. The bands below 1150cm.-l are rather broad and
featureless, as seen with disiloxane. In the Iatter compound, this effect was related to the easy deformability
of the SiOSi skeleton and accessibility of its low-lying
vibrational l e ~ e l s . l ~ ,Disilazane
~l
may share this flexibility, although it seems unlikely that it will vibrate with
D. 14.7.Robinson, J . Amer. Chem. SOC.,1968, 80, 6924.
H. Kriegsmann, 2. EZeRtrochem., 1957, 61, 1088.
2o B. J. Aylett, J. R. Hall, D. C. McKean, R. Taylor, and
L. A. Woodward, Spectvochim. Acta, 1960, 16,747.
21 J. K. Aronson, D. W. Robinson, W. J. Lafferty, J. R. Durig,
and R . C. Lord, J . Chem. Phys., 1961, 35, 2245, and references
quoted therein.
l8
l9
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642
sufficient amplitude to pass through a linear SiNSi configuration.
Mass Spectrum.-As well as a molecular ion, disilazane showed peaks with m/e values of M - 1 to M - 6,
but not M - 7, suggesting that only the silyl hydrogens
are successively removed. Doubly charged ions corresponding to these species were also abundant. A range
of m/e values between 42 and 47 attributed to (SiNH,)+
(n = 0-5) indicated that Si-N bond cleavage was
important, while significant concentrations of Si2Hn+
(a = 0-5) were also present, presumably having been
formed by radical-ion combinations. The most abundant species were SiH,+, SiH,+, NH,+, and NH,+,
J. Chem. SOC. (A), 1969
which suggests that some at least of the positive ions
with Si-N bonds have a relatively short life.
Definite evidence of ions corresponding to longer Si-N
chains was obtained: the most abundant were Si,N,H6-'(go), Si2N,H,+ (105), and Si,N,H,,+ (122). The last
formula corresponds to the molecular ion of the compound SiH,(NHSiH,)2, an obvious disproportionation
product of disilazane.
We are grateful to Dr. I. A. Ellis for recording the infrared spectrum of disilazane, and to the Ministry of Technology
for their support of this work.
[8/1409 Received, September 27th, 19681