0012-5008/04/0001- © 2004

åÄIä “Nauka

/Interperiodica”0004

Doklady Chemistry, Vol. 394, Part 1, 2004, pp. 4–7. Translated from Doklady Akademii Nauk, Vol. 394, No. 2, 2004, pp. 203–206.Original Russian Text Copyright © 2004 by Voronkov, Chipanina, Shainyan, Bolgova, Trofimova, Chernov, Aksamentova, Turchaninov.

Previously, the synthesis of a new representative ofdragonoids,

N

-(trifluorosilylmethyl)phthalimide (

1

),and the results of its study by X-ray diffraction andNMR were described [1]. It was shown that, in crystaland in a low-polarity solvent, this compound contains aweak intramolecular

O

Si

bond. We carried out a

theoretical and experimental study of dragonoid

1

andstructurally similar model compounds—

N

-[(trifluoro-silyl)methyl]-2-pyrrolidone (

2‡

),

N

-methyl-2-pyrrolidone(

2

b

),

N

-methylphthalimide (

3a

),

N

-ethylphthalimide (

3b

),and

N

-(trimethoxysilylmethyl)phthalimide (

4

)—by quan-tum-chemical methods and IR spectroscopy.

N CH2

O SiF3

O

N CH2

O X

N R

O

O

N CH2

O

O

Si(OMe)3

1 2, X = SiF3 (a), H (b) 3, R = Me (a), Et (b) 4

In this work, we determined the structure of com-pound

1

in an isolated state, showed its distinctionsfrom the structures of compounds with a strong

O

Si

bond, analyzed the main factors stabilizing the con-former with the pentacoordinated silicon atom, and the-oretically estimated the barrier to internal rotationaround the N–C bond.

The geometry of molecule

1

calculated by theB3LYP/6-311++G(d,p) method (Fig. 1a) is in goodagreement with the experimental geometry [1]. Theenvironment of the Si atom has a configuration interme-diate between a trigonal bipyramid and a tetrahedron.As in the experiment, the calculated lengths of thebonds between the carbonyl carbon atoms and the ben-zene ring are different:

ë

2

–ë

5

is

1.484

Å and

C

3

–C

4

is1.495 Å. The calculated

F

1

SiO

1

bond angle (

177.1°

)coincides with the experimental one (

177.3°

). Theangles between the axial bond and the equatorial bonds(

C

1

SiF

1

100.4°, F

2

SiF

1

103.5°

) are close to the means of90

°

and

109.5°

. The sum of the angles subtended at the

silicon atom in the equatorial plane (

346.5°

; experi-ment,

344.4°

) is also close to the mean of the theoreticalvalues for a trigonal bipyramid (

360°

) and a tetrahedron(

328.5°

). Finally, the silicon atom is 0.36 Å out of theequatorial plane formed by the

ë

1

, F

2

, and

F

3

atoms.This value is roughly twice as small as the theoreticalvalue for a

SiX

4

tetrahedron:

0.6–0.7

Å, depending onthe Si–X bond length. The

Si–F

1

and

Si

−

F

2(3)

bondlengths, as in the experiment, differ by only

0.017

Å.The

Si

O

distance is rather large:

2.593

Å againstthe experimental value of

2.654

Å. Consequently, thecoordination of the silicon atom with oxygen is weak.In contrast to compound

1

, the configuration of theenvironment of the silicon atom in molecule

2‡

is muchcloser to a trigonal bipyramid, as predicted by theB3LYP/6-311++G(d,p) calculation (Fig. 1b). The

O

Si

coordination bond in this compound (2.11 Å)is

~0.5

Å shorter than in

1

, and the difference betweenthe SiF

eq

and SiF

ax

bond lengths is as much as

0.03

Å(against

0.017

Å in

1

). All these facts point to a ratherstrong

O

Si

coordination in molecule

2‡

.

Figure 2 shows the potentials of internal rotationaround the

N–CH

2

Si

bond in

1

and

2‡

calculated by theHF/6-31G(d) method on scanning of the dihedralSi

−

C

−

N–C angle with a step of

10°

and optimization of

CHEMISTRY

Intramolecular O

Si Coordination in

N

-(Trifluorosilylmethyl)phthalimide and Its Analogues

Academician

M. G. Voronkov, N. N. Chipanina, B. A. Shainyan, Yu. I. Bolgova, O. M. Trofimova, N. F. Chernov,

T. N. Aksamentova, and V. K. Turchaninov

Received July 28, 2003

Favorsky Institute of Chemistry, Siberian Division, Russian Academy of Sciences, ul. Favorskogo 1, Irkutsk, 664033 Russia

DOKLADY CHEMISTRY

Vol. 394

Part 1

2004

INTRAMOLECULAR O Si COORDINATION

5

the other geometric parameters. The weak

O

Si

coordination in

1

is responsible for a low barrier of1.91 kcal/mol. In

2‡

, the internal rotation potential hastwo minima, corresponding to the synperiplanar (

sp

)form with the

O

Si

coordination bond and the anti-clinal (

ac

) uncoordinated form. The

sp

-

2‡

conformationis planar, whereas, in the

ac

conformation, the pyrroli-done fragment has the form of a slightly distorted enve-lope with an almost planar amide fragment and a flap,the C

4

atom, which is considerably out of the plane. Theenergy difference ∆Eac–sp is 4.76 kcal/mol, and the barrierto the sp-2‡ ac-2‡ transformation is 5.7 kcal/mol.This value can be taken approximately as a measure ofthe strength of the O Si coordination bond. Theexistence of compound 2‡ in the synperiplanar form isconfirmed by the fact that its dipole moment measuredin benzene at 25°ë (8.12 D) is close to that calculatedat the HF/6-31G(d) level of theory (8.70 D).

The formation of the O Si coordination bondcan be not only due to the interaction between the sili-con and oxygen atoms, but also due to the presence ofthe O–C–N–C–Si chain, holding the silicon and oxygenatoms at a distance favoring the coordination. In orderto elucidate the role of this factor, we performed ab ini-tio (HF/6-31G(d)) and DFT (B3LYP/6-311++G(d,p))calculations of two intermolecular complexes of meth-yltrifluorosilane MeSiF3, with N-methylphthalimide3a, modeling the O Si coordination in 1, and withN-methyl-2-pyrrolidone 2b, as a model of the O Sicoordination in 2‡. In both cases, the starting value ofthe intermolecular O···Si distance was set equal to2.1 Å. When the geometry is optimized, all anglesbetween the bonds of the silicon atom in the MeSiF3 · 3acomplex are strictly tetrahedral and the O···Si distance(3.52 Å) is virtually equal to the sum of their van derWaals radii. The calculated energy of complex forma-tion for MeSiF3 · 3a is 3.4 kcal/mol. It is contributed not

C C

O2

CC

N

é1

C

Si Feq

Fax

Feq

108.5°

105.0°112.2°

106.5°

107.7°

122.8°

122.4°

116.6°76.7°

101.6°

2.593 Å

1.600 Å

1.617 Å

OSiFax 177.1°

OSiFax

FeqSiFax

100.4°

103.5°

(a) (b)

OSiFax 176.5°

CSiFax

FeqSiFax

94.5°

97.9°

ëë

ëë

N

O

ëSi F

F

F

104.3°106.2°

103.8°

114.9°

110.8°

121.0°

121.0°

110.5° 82.0°

109.6°

1.244 Å

2.110 Å

1.645 Å

1.615 Å

Fig. 1. Molecular structures of (a) N-(trifluorosilylmethyl)phthalimide 1 and (b) N-[(trifluorosilyl)methyl]-2-pyrrolidone 2a calcu-lated by the B3LYP/6-311++G(d,p) method.

2.0

1.5

1.0

0.5

0

0 40 80 120 160 200

Ö, kcal/mol

012345678

0 50 100 150 200–1

SiCNC angle, deg

(a) (b)

Fig. 2. Potential functions of internal rotation around the N–CH2Si bond in molecules of (a) N-(trifluorosilylmethyl)phthalimide 1and (b) N-[(trifluorosilyl)methyl]-2-pyrrolidone 2a calculated by the HF/6-31G(d) method.

6

DOKLADY CHEMISTRY Vol. 394 Part 1 2004

VORONKOV et al.

so much by the O Si coordination as by noncova-lent intermolecular interactions due to the shortenedé···ç and F···H contacts, equal to 2.43 and 2.42 Å,respectively. Analogous characteristics were foundfor the MeSiF3 · 2b complex. Its stabilization(4.8 kcal/mol) is also determined by the existence ofshortened é···ç and F···H contacts. Thus, the O Sicoordination in 1 is mainly due to the favorable mutualorientation of the oxygen and silicon atoms. One canjudge the nature of the coordination by the change in thecharge distribution upon the formation of the O Sibond: the electron density on the é1 atom becomes0.05e higher than on é2, and that on the silicon atom in 1decreases by 0.12e compared to 4. Such a change in thecharges on the oxygen and silicon atoms points to theelectrostatic nature of the O Si bond.

The C=O and SiF stretching vibration frequencies inthe compounds of penta- and tetracoordinate silicon areconsiderably different. The C=O stretches are rathersensitive to the state of the coordination bond and canbe used for assessing its strength in various media.Thus, in the IR spectrum of solid N-[2-(trifluorosi-lyl)ethyl]-2-pyrrolidone (5), containing a strongintramolecular O Si bond, the low-frequency shiftof the carbonyl stretching vibration band amounts to upto ~70 cm–1 compared to its analogues without anO Si bond [2]. Stretching vibrations of the SiF3group in the IR spectrum of 5 are represented by threebands characterizing the trigonal-bipyramidal environ-ment of the silicon atom. These are the stretching vibra-tion bands of the equatorial (νasSiFeq, ~ 890 cm–1;νsSiFeq, ~ 840 cm–1) and axial (νSiFax, ~ 700 cm–1) SiFbonds. For the tetracoordinate silicon compounds con-taining the SiF3 group, two bands are observed in thisspectral region. One of them arises from a quasi-degen-erate set of antisymmetric vibrations.

Carbonyl stretching vibrations in the IR spectrum ofsolid compound 1 give rise to bands with maxima at1775 (νsë=é) and 1700 (νasC=O) cm–1, which differonly slightly from the corresponding bands in the spec-trum of crystalline imide 3b (1770 and 1710 cm–1,respectively) or liquid imide 4 (1765 and 1711 cm–1,respectively) (the synthesis of 4 was described in [3]).The SiF stretching vibration bands in the IR spectrumof compound 1 are observed at 957 (νasSiFeq),918 (νsSiFeq), and 866 (νSiFax) cm–1. They are consid-erably shifted toward higher frequencies as comparedto the bands observed in the spectrum of 5. This is espe-cially true of the SiFax vibration band, the most sensi-tive to the strength of the O Si coordination bond.Thus, IR spectroscopy, as well as X-ray diffraction [1],shows that molecule 1 in the solid phase has a weakintramolecular coordination interaction between theoxygen and silicon atoms.

The IR spectrum (4000–400 cm–1) of compound 1 inthe gas phase was recorded on a Specord 75 IR spectro-photometer at 440 K with the use of a thermostatically

controlled cell with a path length of 1 cm. The assign-ment of the observed bands to C=O and SiF stretchingvibrations was made with the use of its vibrationalspectrum simulated at the HF/6-31G(d) level of theory.

According to the calculation, the stretching vibra-tions of the SiF3 group in the molecule of 1 are charac-terized by three frequencies: νasSiFeq = 1089, νsSiFeq =1029, and νSiFax = 932 cm–1. This is due to the above-mentioned nonequivalence of the fluorine atoms and isthe main criterion for pentacoordination of the siliconatom in 1. Carbonyl stretching vibrations are character-ized by two frequencies: νsC=O = 2062 cm–1 andνasC=O = 1989 cm–1. They slightly differ from thosecalculated for compound 4 (2059 and 1979 cm–1,respectively), which is an extra piece of evidence forthe weak O Si interaction in the molecule of 1. TheνC=O frequencies in the synperiplanar (with the O Sibond) and antiperiplanar (with the tetrahedral siliconatom) forms of 2a calculated by the HF/6-31G(d)method differ by 112 cm–1.

The IR spectrum of gaseous compound 1 in theregion of Si–F stretching vibrations (1000–800 cm–1)shows, as the calculations predict, three absorptionbands with νmax at 974, 945, and 870 cm–1. These bandsarise from the νasSiFeq, νsSiFeq, and νSiFax vibrations,respectively. In contrast to compound 2a, for which theνSiFax frequency considerably depends on the proper-ties of the medium, the transformation of solid com-pound 1 to the gas state is accompanied by a high-fre-quency shift of stretching vibration bands caused by theequatorial rather than axial SiF bonds. The antisymmet-ric and symmetric stretching vibrations of the carbonylgroups are characterized by absorption bands centeredat 1779 (νsC=O) and 1718 (νasC=O) cm–1. In the spec-trum of the vapor of 1, the νasC=O band is shifted tohigher frequencies by 18 cm–1 compared to its positionin the spectrum of a pellet with KBr. Thus, it followsfrom the IR spectroscopy data that the O Si coor-dination bond in compound 1 also persists in the gas-eous state at 400 K. This distinguishes N-(trifluorosilyl-methyl)phthalimide 1 from the well-studied (aroyloxy-methyl)trifluorosilanes, whose molecules exist, underthe same conditions, as an ensemble of two rotationalisomers: with the O Si bond and without it [4].

Quantum-chemical calculations were performedwith the Gaussian 98 program package [5].

ACKNOWLEDGMENTS

We are grateful to V.A. Lopyrev for supplyingsoftware.

REFERENCES

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DOKLADY CHEMISTRY Vol. 394 Part 1 2004

INTRAMOLECULAR O Si COORDINATION 7

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