r V f .6 1 .*...... '=r t63 - t-1 f 3 - 1(

15. Reasilons of BecRiling Bteg with BBd Disilane 1 h -----=------=------=- =-=- =------=------= -=----=-

to (1) Financial support from the United States Atomic EnergyMASTERCommission 9K

t is gratefully acknowledged. This is · AEC report No. COO-1713-16. -40

M 1 -0..1 The formation of disilane and silane H331 SiSiH3 the fast-neutro'n +d 31 SiH4 from CD , irradiation of phosphine-silane mixtures has been rationalized by proposing a ia. 0.V' set of product-determing steps involving the intermediate silylene: 31 SiH2 - which is the product of the primary reactions of recoiling silicon atoms.2

31PH + ·n ;631Si + p + 3 H: :':· ' 1, 1- 3 gi ...:»,. : 3·1 •••••••• ) :31 SiH 2

44 U 1 9.: . ..'. X"" f .:31 SiH2 + SiH4 ·H-31.1 SiSiHi v

i - 6'' I b . 3 1 SiH · · , · :31 SiH2 + SiH4 4 ,4 31 SiH :31 SiH2 + PH3 4 ; involatile material :3 1 SiH2 ' ' PH3

., L 0 1 . 9. (2) P.P. Gaspar, S.A. B·ock and W.. C. Eckelman, J. Amer. Chem. Soc., 22,

6914 (1968). ..R'

The present report provides support,for the intermediacy of :31 SiH and suggests

the occurence of a novel reaction:-.the insertion of :31 SiH into silicon- 2 silicon bonds.

*$.An mechanism for the formation gf involving the ·1 . ternative H331SiSiH3 coupling of silyl radicals, '31 SiH3 with nonradioactive 'SiH3 formed from

. : A: silane by radiation damage „ had been ·rendered unlikely by the failure of ' scavengers to-affect product yields. 2, '.However; the reaction , :- (NO,.C2H4-).

r LEGAL NOTICE I u .. S ./ .,· »'.: I *- I .·6 - This report was prepared as an account of work sponsored by the United States Government. Neither ' the United States nor the United States Atomic Energy :08 r.r...... pdlrie: Commission, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, com- pleteness or usefulness of any information, apparatus, product or process disclosed, or represents that its use DISrBIBUTION OF THIS DOCUMENT IS UNLIMITIp would not infringe privately owned rights. .'...... 111'.77., . , / - 04 DISCLAIMER

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..

3 T / -2- 1 efficiencies of the scavengers employed toward silyl radicals is unknown,

and further evidence for the intermediacy of :31 SiH2 rather than '31 SiH3

/ is desirable. To this end the fast-neutron irradiation of phosphine-disilane

mixtures was undertaken, since in this system the insertion of :31 SiH2 into

Si-H bonds can be distinguished from the coupling of ' 31 SiH3 with the debris .. '. from radiation damage.

Radiation damage in the phosphine-disilane system would be expected to

produce 'SiH3 as well as 'SiH, SiH, '3 and thus coupling with·'31 would lead · .4 -, SiH3

(3) While the radiolysis of disilane has not been reported, it is well-

known that both C-C and C-H bond-rupture occur in the radiolysis of .

See S.C. Lind, Radiation Chemistry of Gases, Reinhold, N.Y., L961, pp. 103, 179;

also A.V. Topchiev (ed.),·Radiolysis of Hydrocarbons, ·Elsevier, Amsterdam, 1964.

to the formation of significant amounts of H331 SiSiH ds well as H331 SiSi SiH3.

In the event that :31 SiHi is the predominant intermediate the major expected

product is 31 Sisi2HB formed by Si-H insertion.

The results of fast-neutron irradiation of mixtures of phosphine with

, - .:.. 1 silane, disilane and both silane and disilane are given in Table I. The

product distributions suggest the following conclusions:

31 1. The small amount of H3 SiSiH3 and the large amount of 31 Sisi2H8 formed

from the reactions of recoiling silicon atoms in phosphine-disilane mixture

.. are in. accord with the intermediacy of :31 whi·ch SiH2 ·inserts into the Si-H (or

Si-Si, vide infra) bonds of disilane to form the major product. 31 31 2. The formation of small amounts of H3 SiSiH and SiH4 from phosphine-

disilane mixture suggests a mechanism for the formation of these products, and

for the formation of 31 SiH from phosphine-silane mixtures, consistent with a 4

dominant role for :31 SiH2 as reaction intermediate: I ' :7

-3-

7' ..., t. .'.. „4 1 -·· ,-, major H 31 SiSiH · >. 3 3 :31 SiH2 + .' SiH4 4-0 [H333SiSilj]* . D N minor 3 1 Si114 + . :SiH2 ·: : 91' ir

major 31 Sisi2H8 + minor H 31 SiSiH + :SiH :31 SiH2 H3SiSiH342 31SiSi 28H ]* >3 3 2

_mino#) 3 1 SiH4 + 2 : SiH2 Similar thermal decompositions of disilane and trisilane to silylene have

been recently suggested to explain the products4' 5, and reaction kineticss of

. . . , ; , 4. I ...... disilane and trisilane pyrolysis.

(4) E.N. Tebben and M. A. Ring, Inorg. Chem.,.8, 1787 (1969).

(5) M. Bowrey and J.H.Purnell,''J..Amer. Chem. 'Soc., 22,· 2594 (1970).

3:' .The previously overlooked2 formation a minor product Of31 Sisi.2118 as

from phosphine-silane mixtures cannot be due to a reaction ·of :3 1 SiH2 . The formation of trisilane from silane may be understood, however, if a primordial ., step toward the production of :3 1 SiH, is the insertion 9f a silicon atom into 1 11 i it .1..1, 1 ,1.,1 .1 KI j . , 11 . , .1 1 1 '. , an Si-H bond of silane. The resulting divalent species H-31 may occasionally ...... ,1 Si-siH3. survive to take part in a second insertion forming trisilane, although its normal 1 ... '' , 1 1 1 1 1 1 .1, ...... fate seems to be Si-Si bond scission.

.'.. .A.. , 1. ..t... L .. , 1.... , 1

11 1 r 4. A remarkably high ratio of 31 Sisi2HB to H331 SiSiH3 is obtained from 1 & 1 1. . : ."... 1 / ' . ,1 Y phosphine-silane-disilane mixtures. A reactivity ratio, disilane to silane,

of 4.9+1.9 has been obtained from a series of experiments in which the silane-

I...... 1 1.1 / , disilane ratio varied from 0.25 to 4.0. Since the ratio of Si-H bonds in the .,1. il'.. . 1 two molecules is 1 6 4, : Jit 11is .,clear that either the Si-H bonds o f disilane

·more reactive toward insertion by :31 SiH2 than are the bonds ., , .,1., 1.... , t.' 1 11 , are considerably I I . . • i i . . . .

... & 1.. 1 1.,i . l .. ,.,

1.'. . / 1 i i.... lilli. 1.., y ..l.

...1 L 1... .,It 10„«, ..1 1 1,• '

' It.. ../,1 , i,l ..1. 11 Ll 1/.t' & -

-4-

or undergoing of silane, : 31 SiH2 is insertion into the Si-Si bond of disilane as to form H)Si 31 SiH-SiH.,LJ as well H331 SiSiH2Sili by Si-H insertion. 1 p It has recently been suggested that the Si-H bonds of disilane may be weaker

than those in silane,6 and this may explain the differential reactivity toward

(6) R.P. Hollandsworth and·M.A. Ring, Inorg. Chem., 2, 1635 (1968). :1

1 silylene. On the other hand, Si-Si bond insertion haa been noted as a

possible explanation for a slightly greater than statistical ratio of

B-tetrasilane to iso-tetrasilane believed to have resulted from reaction

of :SiH2 with trisilane.5 We are continuing to investigate the possible insertion of :31 SiH2 into Si-Si bonds by appropriate product degradation studies.

, Peter P. Gaspar, Peter Markus ch.· 4-

9. ' · · Department of Chemistry . 111

Washington University

11 i . . , C. .: ' -7 -. c, i..- Saint Louis, Missouri 63130

M E 1 11

1

•. 11 - I 1 ./ 4 ... ': 0. , . I : "T b.1 :. 11 .„ " 11 - f .. ., r. . 4

.

..4 (, '.,..-.

3 .

I ..5 , 0 , 4 J t 6

.

, i "

.

3 ./

31 Table-==== I.a Yields of Si-Labeled Products from Reactions of 31 Si Atoms

Expt SUbstrate 31 sili4 H3-31 SiSiH3b 31 Sisi28 H k

A p 1 PH3 , SiH4 18,3005.,700 .61,100 + 9.200 -.. 8,900 + 1,300

A 1:1 PH : SiH ..3 4 · 18,300 + 2,700 - - - 67,300 + 10,100 CLO,000 + 1,500 A 1:1 PH : H SiSiH 3 3 3 6,800 + 1,000 - - 4,400 + 700 58,300 + 8,700 SiSiH A 1:1:1 PH3 : SiH 433 : H 7,100 + 1,100 11,800 + - 1,800 44,400 + 6,700 A 1:1:1 PH : SiH : H SiSiH 5,700 + 900 10,600 + 3 4 39 3 4 1,600 47,800 i 7,200 B 5:1:4 PH : SiH : H Si SiH 3 433 7,900 -+ 1,200 7,800 + 1,200 60,700 + 9,100 B 5:4:1 PH : SiH H Si SiH 6,400 + 1,000 3 43 3 26,500 + 4,000 17,900 + 2,300

... a -All samples at total pressure 2.0 + 0.1 atm and, within an experiment, irradiated simultaneously at equal neutron flux as described in ref 2. k:Yields given as counts above background extrapolated to a common time for each experiment to correct for decay. Absolute combined yields of produdts as % of total 31 Si- produced are 40-60%. I ,-. 2

...

.-,