Journal of Molecular Structure. 40 (1977) 65-75 @Elsepier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

PEROXO COMPOUNDS

Part XVI. Electron diffraction investigation of the molecular structures of di-t-butyl peroxide Meg COOCMe= and bis(trimethylsily1) peroxide Me&iOO!SiM&*

DIETER KASS and HEINZ OBERHAMMER

ImiCut itir Physihalische und Theoretische Chemie der Universith’t Tiibingen. 7400 TIibingen (Germany) DI&“MAR BRANDES and ARMAND BLASCHETI’E

Institut f@ An&ganische~Chemie der Technischen Universitci’t Braunschweig. 3300 Braunschweig (Germany) (Received 5 January 1977)

ABSTRACT

The mole&& geometries of Me,CbOCMe, and Me,SiOOSiMe, in the gas phase were det&mined hy electron diffraction. For the skeleton of di-t-butyl peroxide the following geometig parameters (r,%alues) were obtained: r(O-0) = 1.480 A (assumed), r(C-0) = 1.460 * 0.009 A, r(C-i&,) = 1.523 * 0.003 A, 4OOC = 103.9” 2 1.2”, M&cc, = 110.9” f 0.4.: and 6 (COW) = 165.8” * 2.4”. This dihedral angle is compared with the results of IR and Raman spectroscopy, dipole moment measurements and photoelectron spectroscopy. The main geometric parameters for bis(trimethyisily1) peroxide are (r,galks)~r(O-O) = 1;481 f 0.008 A. @i-O) = 1.681 * 0.003 A, r(Si*) = 1.855 ;t 0.002 A, &OOSi = 106.6 2 1.4”. 4CSiC = 110.6 f 0.4”. and a(SiOOSi) = 143.5 * 6.0”. For both peroxides SCF-MO calculations in the CNDOIB approximation do dot repro- duce the experimental results.

INTRODUCTION

In a previous ktudy [2] we found that even under mild conditions bis(trimethylsily1) peio&de undergoes a variety of nonradical reactions not encountered with the element-homologous di-t-butyl peroxide. A sharp point iif tioritrest is thit nucleophilic attack at peroxidic oxygen occurs much more re+l$Wi$h M&SiQ+Me3 than with Me,COOCMe,. In order to gain a , : . . . ~&er~tide.rstanding.pi-the. @chanistik differences between the two per- d@d&s, w6 have initiated a struqturai investigatidn of bis(trimethylsily1) p$roxi&&id ~h&&i’di&nq&i&o~ peroxides.

,Alligo@ ‘the-in&&$~ $ru&+es of -hydrogen peroxide and different _ :_of:.6/-~ _. .‘. . qpq+deS h&e .bee;i _&t&@vely studied by a variety of

EXPERIMENTALSECI’ION

ANALYSIS AND FtEsiJLls

TABLE 1

67

Fig. 1. Modified molecular intensities for Me,COOCMe, (0000 s-bfexm(s), - s-itPm cs)) and difference curve s. wxPt(s) - s - iWheo(s).

& .,+feXPL(,), -s-M *eo

I 2 3 4 5 6

Fig. 3. Final radial distribution curve for ~e,COOCMe~ (oooo RDexpr, - F&D*-) and difference curve RD-P’- RDtbeo .

(X = C or Si) and torsional angles 7 of the MeaX groups around the O-X bonds. If I = 0 one methyl carbon C, is in the 00X plane and in trarrs position to the O-O bond. r is measured clockwise looking from oxygen towards the X atom.

The preliminary models were then refined by a least squares refinement based on the modified molecular intensities. An s interval of As = 0.2 A” was chosen and a diagonal weight matrix was used. The weighting increases exponentially from 0.25 to 1.0 in the range 1.4 < s C 5 A-‘, is 1.0 for 5 f s G 30 A-’ and decreases exponentially to 0.1 for 30 < s =G 34 A-‘. The following assumptions were made fcr both peroxides: (1) The hydrogen atoms stagger the bonds around_ the central X atom of the MesX’grouM. ‘(2) The t-butyl and trimethylsilyl.groups respectively and all methyl groups have CW symmetry. (3) Effects of-perpendicular mean square amplitudes on the molecular geometry (shrinkage effects) were not considered; (4) Mean square amplitudes for some interatomic distarices were tied. together (-see Table 3). With these assumptions nine independent, geometric pa&neters, i.e. four bond distances, three bond angles, the dih&dral angle.6 tid the’- torsional angle r, &e needed tc.des&& the geometry &f th‘e plendxide’st.

Two successive refinements w.ere perform& .for,each .comp&nd.-In the first run the geomet&.parameteti &erc refined .tog&hei..with. bonded. mean square amplitudes and i;? ~~e,fo~l~~~ Frin-tkelgeo~e~~_p~~e~*~~~~ ’ varied together wit;h eidt noi;~ldeii~~~,.s;;ri~~~~~~~~~~._~~~~~~I~~g

69

Me3 SiOOSiMe3

I 2 3 4 5 6 7 rIH1

Fig. 4. Final radial distribution curve for Me,SiOOSiMe, (0~00 RDeXP’, - RD*m) and difference curve RDeXP’ - RDtheO.

large correlation (0.98) between ~(0-0) and r(0-C) occurred in the case of di-t-butyl peroxide, the O-O distance and amplitude were held fixed to values of 1.480 A and 0.045 A, respectively_ The corresponding bond length in hydrogen peroxide is 1.475 + 0.005 A [9]. A similar value can be anticipated for dimethyl peroxide since the O-O force constants in CHJOOCH3 and HOOH are very similar [lo, 111 (4.07 and 4.01 mdyn a-‘). This assumption on the O-O bond distance has only a small influence on the dihedral angle S(COOC). Assuming ~(0-0) = 1.450 A leads to an increase of the dihedral angle by l-3”, i.e. this variation is smaller than the error limit of this parameter. With these constraints only the following five elements cii of the correlation matrix had values larger than 0.6 in the case of MeJCOOCMe3: c[r(C--O), I(C-C,)] = 0.92, c[r(C-O), F(C-&,)] = 0.69, c[r(C--CI), 4C, CC,] = 0.63, c[r(C-C,), 1(C--C,)] = 3.65, c[f(O - - - C,l), I(C,l - - - C,2)] = 0.64 (see Fig. 5 for atom numbers).

In the least squares refinement for the silylperoxide no further constraints were necessary. The following correlations had values > 0.6: c[&OOSi, 4HCH] = 0.62, c[IOOSi, I(Si - - - Hl)] = - 0.87, c[&C,SiC,, 4 HC,H] = 0.65, c[&HCH, I(0’ - - - Si)] = - 0.87, c[Z(C, 1 - - - C&2), I(0 - * - C,l)] = 0.74, c[1(0 * .- ‘C, l), I(Ol . - - C, 3) ] = - 0.63 (see Fig. 6 for atom numbers). Theresulk of the least squares analyses for both peroxides are summarized in Tables 2 and 3. All error limits are twice the standard deviations.

Fig. 5. Conformation of Me,COOCMe,.

DISCUSSION

Me,COOCMe3 Table 4 lists dihedral angles of three peroxides and the related molecule

FOOF. These valu& refer to structure determination? $I the gaS phase; Extensive ab initio ca.$ulations [lS-191 show that.the korifd*atid;n ‘of ’ Hz02 is mainly determined by a balance between varioq~ interactiqns, One

Fig. 6. Conformation of Mb;SiO~,Sih&,

TABLE 2

Molecular paramete& of Me3COOCMe3 and Me$jiOOSiMe:,

Me,COOCMe, Me,SiOOSiMe,

0-O. 1.480b C-O 1.460 C*m 1.523 Cm-H 1.091 4ooc 103.9 4c,cc, 110.9 %HC,H 107.0 6 165.6 T 10.4

(9) (3) (2) (1.2) (0.4) (O-8) (2.4) (1.6)

Si-0 Si-C C-H ZrOOSi 4CSiC &HCH 6 T

1.481 (8) 1.681 (3) 1.855 (2) 1.095 (4) 106.6 (1.4) 110.6 (0.4) 109.6 (1.2) 143.5 (6.0) 83.9 (3.0)

aDistances in A, angles in degrees; uncertainties in 10-l A or degrees, respectively. All distances are r,,-values. bAssume$ value, fixed during refinement.

TABLE 3

Amplitudes of Me,COOCMe, and Me,SiOOSiMe,

Distance Amplitude (A) Distance Amplitude (A)

(a) Me,COOCMe,

C-Cm 0.058 0.047

o-o’ 0.045 Cm-H 0.053

Cm1 ’ .. e cm2 0.073 0

“‘F 0.090

0 _ _ . 0.122 C ---HI 0.135

-; **. Hl 0.110 .-- H2 0.141

(b) Me$iOOSiMe, Sii, 0.052 0-Si 0.048 o-o 0.050 CA-H 0.071

‘-C&l ---Cm2 -0.085 0 -: - cm.1 .0.105 0 -s-H _ 0.122 Si - y.- HJ ’ _ ko9;3 Hl ‘1.‘. Hl- b.iio Hl ---H2 0.110

(4) a

12; (6) (6)

US”, a a

(14) (2) (8)

02;

a

Cm1 m--H, 0.122 a C.--C’ 0.122 =

0.112 (26) 0.106 (20) 0.302 (82)

c . . . C, 0.182 (68) ___ l

Em.. HTm 0.171 (52) 0.141 a

C,--- H 0.141 a H . . . H’ 0.141 =

cm1 - - - H2 Si a a s Si’

:# ’ --- cm2

- ; - Cm3 01. - * Si Si “‘cm’

0.100 a 0.129 (38) 0.116 (50) 0.223 (74) 0.092 (14) 0.447 (154) .0.397 (454) 0.485 :0.141

(620) .s

0.141 a

~aAs&tkd~values;~fiied during refinemtitz.

TABLE 4

Dihedral angles of some peroxides

Methoda Ref.

FOOF 87.5” M 12 HOOH 119.8” i 3” IR 11 HOOH 12O”l’ f 15’ M 9 CF,-O-O-CF, 71.3 f 4.0”

K 13

CF,-0-O-CF, 123.3” f. 4.0” 14 F,S-O-O-SF, 107” + 5” ED 15

aIR, IR spectroscopy; M, microwave spectroscopy: ED, electron diffraction.

valence lone pair on each oxygen occupies a 2p orbital perpendicular to the OOH plane. The exchange repulsion betweeen these two pairs of electrons is minimum at S = 90”. The second oxygen lone pair lies in the OOH plane having a large amount of s character. Exchange repulsions between these lone pairs and between the O-H bonds favour the planar bans configuration (6 = 180”). For H,Oz the sum of these interactions leads to a dihedral angle of about 120°, with a tram barrier of only 1.1 kcal mol-‘. In Me3COOCMe3 the dihedral angle is increased considerably to S = 166” by additional steric interaction between the t-butyl groups. This value, obtained by the electron diffraction experiment, is an average angle due to the various molecular vibrations. The effect of the vibrations on this value as determined in the electron diffraction experiment (shrinkage effect) could not be calculated in the usual way [20], since no force field is known for this peroxide. However, a crude estimate of this effect, taking into account only the torsional vibration around the O-O bond, leads to a shrinkage effect of 7.7” for the dihedral angle. This value is obtained for a planar trans con- figuration of the skeleton and for a torsional frequency of u= 180 cm-‘; that is the lowest frequency observed in the Raman spectrum and was tentatively assigned by McKean et al. [21] to the twisting skeleton mode This estimate indicates that the experimental result for the dihedral angle (6 = 165.8” f 2.4”) is not compatible with a planar equilibrium configuration. This result is supported by IR and Raman spectra 121,221 which do not show the selection rules for a planar skeleton of C2h symmetry and by dipole moment measurements [23-251. From the dipole’,moment (I_( = 0.9 D) a dihedral angle of 6 = 123”. was estimated by Lobunei et al; 1241. Much caution however is necessary since the&pole. moment was measured in benzene solution. The photoelectroti spectra [26] w&uid also,sugge$t a near planar trans configukion in agreemerit with the elkron diffraction result.

On the other hand the total energy as cakulat&by CNDO/B.approximation attains its minimumat6= 180” (see Fig.- 8-&d.-Appei;dix)i;@in~ even ab initio calculation$ for H202 .Ie&i to a planarfmks-configuration 1171 .if

73

polarization functions are not included in the basis set, this CNDO/Z result has to. be taken with the respective caution. Westheimex-Hendrickson calculations [27] result in an equilibrium structure with 6 = 135”.

The C--O bond distance (r(C~0) .= 1.460 + 0.009 A) and the OOC bond angle (4OOC F.lO3.9 f 1.2”) agree within the error limits with the corre- sponding values in dimethylperoxide [28] (r(C-0) = 1.445 i 0.02 A and 4OOC = 105 f: 3”.) These C-O distances in the peroxides are considerably longer than the C-O distance in dimethylether [29] (r(C-0) = 1.416 f 0.003 a). This trend cannot be observed between hydrogen peroxide and water. In both molecules [9,30,31] the O-H distances agree within the error limits.

Me3SiOOSiMe3 The’dihedral angle (6 = 143.5 + 6”) for the silylperoxide is again much

larger than.for H202 but still considerably smaller than for the di-f-butyl peroxide. .This can be explained by reduced steric interactions between the XMe3 group due to larger O-X and X-Cm distances in the case of the silylperoxide. The CNDO/B approximation is unable to reproduce this experimental result for the dihedral angle. Figure 7 shows the effect of substitution of methyl groups by silyl groups bonded to an oxygen atom. Substituting one methyl group in dimethylether by a silyl group leads to an increase of the oxygen bond angle of about 10” in methylsilylether. Further substitution of one methyl group leads to an increase of more than 20” in the bond angle in disiloxane. This effect is usually explained by partial delocalization of the oxygen lone pairs [32]. A consequence of the delocali- zation is a strengthening of the Si-0 bond which leads to Si-0 bond lengths of about 1.63-1.64 A. Similar Si-0 distances were determined for Me$iOSiMe~ 1331 (r(Si-0) = 1.631 f O.O03A, 4SiOSi = 148.0 -+ 3”) and methylcyclos,iIoxanes [34]. The substitution effect is much smaller for the peroxides. In this case the substitution of a t-butyl group by a trimethylsilyl group causes an increase of only 3.3 + 1.8” in the bond angle of the oxygen atom. only slight delocahzation of the oxygen lone pairs has to be assumed. This is in agreement with the long Si-0 bond length (r(Si-0) = 1.68 f 0.003 A: in the peroxide. This is the longest Si-0 distance determined for any molecule in the gas phase.

Fi& 7, .Variation of tlie oxy&n-bond.angle with substitution of methyl groups by silyl groups or hbut$l-gr&ps by trimetb$Isilyl groups. respectively.

These results suggest that .the ~enhance$react@ty ‘of,th&peioxogroup~ in Me,SiOOSiMe3 tdWard:nucleophilic:at&ck. is$rimarily cati:by .the lower basicity of the Me$iO--leaving group as -comp&d. tith-:M&COT;&d’.by, the lesser steric shieldbig df thk-peioz& &&p :ik the $IyI .p;iii.oxide;.O&thk other hand, the difference-b .ele&@on de~-~~,_orrt~~.~~~~~~‘.~~i~~~~tl;ie’ two compounds Seems to .dispIay tinIy.:a miikr role zisa.‘rea&ivity. go&&g factor.

APPENDIX

CNDO/2 calculations Semiempirical-molecular .orb~t..$~+xlations were peiformed4xT the

CNDO/B approximation [ 351: fok. both p&oxideS~ thereby:only: the dihedral angle S(XOOX) was varied. The~erpe~~ta+zsu@ were.used for all other, geometric parameters;:‘J’he to&l energy as a~funct@n_ofthe dihe&%l-&gle for &_f_butyl pePodde ~~v&.&,-~~~~;~~~o~ a,ir&w.&j& &&j&k .&

KS = 180°, i-e_ for a +nar.tr&+ &Gg&at~o~~~Sp&tinitting the tot+ -e&rgy’~,-~ into electronic and.Coulorinb~re’~ulsiolr’~~er~~ betw~en.~~e:cojiei.(Fig_‘g). 1 demonstrates that .the kirge clihec@, angleof. thisperoxida’is a +is?xjuence: of strong steric int+ra;ction.:N&&ting Coulom,b~re~uJ&onbettii&n the cores would lead to a dihedralaingle.of-about S =..1,05”, whjch-‘&.&se to the value for H202. The result obt.ai.ned for. the. silylperoxide:de~~ds,s~~~y on the assumption mide for theAlicon~3d orbitals. +ss@ning contracted 3d

. . . c&f.t7eGY/A.U.

75

orbita& with a Slater coefficient equal to the 3s and 3p orbitals, total energy becomes min+um at about S = 90”. This startling configuration is probably a consequence of overestimating the population of the 3d orbitals, leading tti~.a..ting bondizig int&raction between the two trimethylsilyl groups. Neglecting-the silicon 3ti orbitals compl&ely results in total energy with its ininimum at 6 = 18OS Neither calculation reproduces the experimental r&ult.

REFERENCES

1 D. Brandes and A. Biaschette, J. Organomet. Chem., 99 (1975) C 33. 2 D. Brandes and A. Biaschette. J. Orgauomet. Chem., 73 (1974) 217.

‘3 N. A. Midas and D. M. Surgenor, J. Am. C&em. Sot., 66 (1946) 205,643. 4 W. Hahn and L. Metzinger, 2. Makromoi. Cbem., 21 (1956) 113. 5 B. Bressei and k Blascbette, 2. Auorg. Ailg. Cbem., 377 (1970) 182. 6 W. Zeil, J. Haase and L. Wegmann, Z. Instrumentenkd., 74 (1966) 84. 7 H. Oberhammer and J. Sttihle. Z. Naturforsch., Teii A, 30 (1975) 296. 8 J. Haase, Z. Naturforsch., Teii A, 23 (1968) 1000. 9 W. C. Celfke and W. Cordy, J. Chem. Phys., 51 (1969) 5336.

10 M. E. Butwili Beii and J. Laane. Spectrochim. Acta. Part A, 28 (1972) 2239. 11 R. L. Redington, W. B. Olson’and P. C. Cross, J. Chem. Phys., 36 (1962) 1311. 12 R. H. Jackson, J. Chem. Sot., (1962) 4585. 13 A. H. Lowrey, G. George and P. D’Antonio, private communication. 14 C. J. Marsden, IVth European Microwave Spectroscopy Conference, Tiibingen,

Germany, 1977. 15 R. B. Harvey and S. H. Bauer, J. Am. Chem. Sot., 76 (1954) 859. 16 C. Guidotti, U. Lamanna, M. Maestro and R. Moccia, Tbeor. Chim. Acta, 27 (1972)

55. 17 R. B. Davidson and L. C. Aiien, J. Chem. Phys., 55 (1971) 519. 18 A. Veiikard, Thizor. Chim. Acta, 18 (1970) 21. 19 T. H. Dunning and N. W. Winter, Chem. Phys. Lett., ll(l971) 194. 20 S. J. Cyvin, Molecular Vibrations and Mean Square Amplitudes. Eisevier. Amsterdam,

1968. 21 D. C. McKean. J. L. Duncanand R. K. M. Hay, Spectrochim. Acta, Part A, 23 (1967)

605. 22 k Simon and H.~Arnold. J_ Prakt. Chem.. 8 (1959) 241. 2.3 M. T. Rogers and T. W. Campbell, J. Am. Chem. Sot., 74 (1952) 4742. 24 W. &bu&z. J_ R. Rittaqhouse and J_ G. Miller. J. Am. Clam. Sot., 80 (1958) 3505. 26 M. J..Aroney, R. J. W. k P+re ami R. K. Pierens, Aust. J. Chem., 20 (1967) 2251. 26 C. B&ich and W. Adam, Tetrahedron Lett., (1974) 1467.

27 R. Rademacher, private communication. 28 Tables of Interatomic Distances and Conformation in Molecules and Ions, Chemical

S&&y. L&ion. 1958. 29 K. Kimura and M. Kubo,, J. Chem. Phys.. 30 (1959) 151. 30 D; W. P&e&r .urd M:W:St&ndberg, Phys. Rev., 95 (1954) 374. 31 W.~C.Xirqand.W. G&dy,-Phys; Rev., 93 (1954) 407. 32 R/ J. @B&spie.,Molecular Cjeometry, Van Nostrand Reinhold Company, London, 1972.

p.,>20. ‘$3 B;C&kv&$& Wajner; P. G&&y, F, C. Mijihoff, B. Rozsandai and I. Hargittai. _. J&&&&,&~ &.&jilm (1996) 287.

.‘3;i Hr’O)ier ;: m; W- Zaii a&i-G; Fog&a& .L’MOI. Struct.. 18 (1973) 309.

.3@.J;-;& Ponl&&d.G..&SegJ, J, Chem. Phys.; 44(1966) 3289.