Interaction of methylal and methylal-d6 with electron acceptors

Spec~ca

Acta,Vol.

29A, pp. 1471to 1477. Per2amon Press 1972.

Printed inNorthern Ireland

Interaction of methylal and methylal-& with electron acceptors K.

K. KALNINSH, V. N. ZQONNIE, N. S. D~T~IEVA

and N. I. NIKOLAEV

Institute for High Molecular Weight Compounds, Aced. Sci. USSR, Leningrad 199004, USSR (Received 27 September

1972; Rev&d

27 October 1972)

Ah&&-Infrared spectre of methylal and methyl&de complexes with butyl lithium and trichloroaceticacid have been obtained. Two states of interactingmethylal molecules (forms I and II) have been revealed. It was shown that form I correspondsto the interaction of one oxygen atom and form II to that of both methylal oxygen atoms. A stepwise character of associate solvation was established. The greatest number of methylal molecules in form I solvating the BuLi tetramer is 4. The enthalpy of bonding of the latter methylal moleculeto the BuLi tetramer is (4 f 1.5) kcal./mole. ETHERS as electron donors form complexes with organolithium compounds [l, 21, with metal salts [3], and other electron acceptors [4, 51,snd also take part in hydrogen bonding with alcohols and acids [6]. In the case of ethers of complex structure represented by several rotational isomers, the interaction with electron acceptors leads to a change in the isomer composition [7]. This work deals with complexes and H-bond of methylal (CH,OCH,OCHS) and methyl&Z, (CD,OCH,OCD,) with butyl lithium and triohloroacetic acid (TChAA). The peculiarity of methylal is that its molecules have two interaction centres-two oxygen atoms. In the vapour state methylal exhibits gauchegauche structure [8]. Butyl lithium in hydrocarbons is associated mainly in hexamers [9, lo], owing to the coordinative bonds C-Li. Associates of BuLi can interact with ethers and other strong electron donors. In the case of ether the degree of association of BuLi decreases to 4 [lO-121. The BuLi tetramer may be represented as a tetrahedron with Li atoms in its vertices. Not more than four molecules of dimethyl ether and not more than two molecules of diethyl ether may react with the BuLi tetramer [2, 131. [l] E. A. Ko ~RIZHNYCH and A. I. SHATENSETEIN, Usp. Khim. 38, 1836 (1969). [2] K. K. KALNINSH, V. N. ZUONNIK, N. I. NIKOLAEV and I. A. AJSTAMONOVA, Viieokomol. So&L 13A, 2121 (1971). [S] T. A. OLAN~ERand M. C. DAY, J. Am. Chem. Soo. 93, 3584 (1971). [4] T. LE CALVE,J. Oh&. phys. (Par&~) 67, 1987 (1970). [5] S. TA~EDA and R. TARAO, Bull. Chem. Soo. Japan 38, 1567 (1965). [6] I. M. GINZBURUand M. A. AB~AMOVICH,Opt. Spektroskopija,sbor. IIed., p. 230, Akad. Neuk SSSR, Moscow-Leningrad (1963). [7] A. WIESER, W. G. L~LOW, P. S. KROGERand H. FDERER, Spectrochirn. Acta 24A, 1066 (1968). [8] E. E. ASTRUP,Acta Chmn. Scami?. 25, 1494 (1971). [9] D. MA~GERISON and J. P. NEWPORT,Trans. Faraday Soo. 59, 2058 (1963). [lo] H. L. LEWIS and T. L. BRONX, J. Am. Chem. Soo. Q2,4664 (1970). [ll] P. WEST and R. WAACK, J. Am. Chem. Sot. 39,4396 (1967). [12] P. B. BA~TLET, C. V. GOEBELand W. P. WEBER, J. Am. Chmn. Sot. 91, 7426 (1969). [13] V. N. Zoo=, K. K. KALNINSH, N. I. NIEOLAEV and E. J. SHADRINA,12’~~.Akad. Nauk, SSSR, Otd. Khirn. Nauk 9, 1937 (1972). 1471

1472

K.

K.

KALNINSH, V. N. Zuom,

N. S. DMITRIEVAand N. I. NIKOLAEV

EXPERIMENTAL The infrared spectra over the range of 400-1300 cm-i were obtained with a Liquid cells with KBr windows 140 ,u thick were used, they UR-20 spectrometer. were thermostated over the range -60’ to f-40’%. The solutions were introduced into the cell through thin Teff on tubes glued to the cell window. The temperature was measured with a differential thermopair one end of which is inserted into the cell window. Owing to high reactivity of BuLi all the operations connected with the preparation of BuLi and its complexes were carried out in a special apparatus under vacuum (see Ref. [2]). The BuLi concentration was determined by double titration or from the 545 cm-1 band whose extinction is 75 1 mole-l cm-l at 25°C. In our experiments the concentrations of methylal and BuLi solutions in &octane were varied from 0.1 to 1.0 mole/l. The separation of overlapping bands was carried out with the assumption of dispersion contours. RESULTS AND DISCUSSION The interaction of the ROR ether with BuLi may be represented as the withdrawing of electrons from the oxygen atoms to vacant orbitals of Li. The most characteristic perturbations of the vibration spectra of this system should be expected in the region of the skeleton ether vibrations and stretching C-42 vibrations. And indeed, when (CH,),O interacts with BuLi, the frequencies of symmetric (yScoc) and antisymmetric ( vzoc) vibrations of COC groups decrease by S-10 cm-r and stretching C--Li vibrations decrease by 40 cm-l [13]. Strictly speaking, owing to uncharacteristic skeletal vibrations it is impossible to distinguish symmetric ( vboc) and antisymmetric (YTot) vibrations of COC groups as is done, for instance, for dimethyl ester [13, 141. Nevertheless it may be assumed that the vboc and vFoc vibrations give the main contributions to strong bands at 927 and 1045 cm-l occurring at approximately the same frequencies as the corresponding bands in the (CH,),O spectrum at 921 and 1094 cm-l [13]. The 1108 and 1138 cm-l bands in the methylal spectrum may be assigned to bending CH, vibrations (on the basis of comparison with the methylal-d, spectrum, Fig. 2). In the course of the phase transition vapour-liquid no qualitative changes occur in the methylal i.r. spectrum. This indicates that in liquid state methylal retains the gauche-gauche Over the low temperature range the bands in the i.r. methylal conformation. spectrum become more narrow and their maxima increase (Fig. 3); this is due to a decrease in their rotational mobility. The following assignment of bands may be suggested for methylal-d, (Fig. 2): 857 cm-l - vtoc, 1083 cm-l - vFoc. It is noteworthy that in deuterating (CH,),O the vcoc bands shift similarly [14]. The bending vibrations of or(DCD) CD, groupa should be expected at 1060-1080 cm- l, however, these bands are evidently overPresumably the lapped by strong ~,$~e absorption band (e = 715 1 mole-l cm-l). 925 cm-l band may be assigned to /3(COD). In the infrared spectrum of BuLi attention is drawn to a broad and strong [14]

L. M. SVERDLOV, M. A. KOVNER and E. P. mnyih molekyl p. 427, Nmka, Moskow (1970).

KRAINOV,Kolebatelnye

qvectry

wmogoato-

Interaction

of methylal and methylal-&

1473

with electron acceptors

(a)

cm-1 Fig. 1. (a) The i.r. spectra of methylal (1) and its complexes with BuLi; 2-form I (1.6 BuLi: 1M) and 3-form II (6 BuLi: 1M); -10’; solventiaooctsne; *-the bands of BuLi. (b) H-bond of trichloroacetic acid with methylal: I-methylal, 0.1 mole/l; 2-(1 TChAA : 1M); 3-(4 TChAA: 1M); -20’; solventCC1,; *-the bands of TChAA. 545 cm-l band due to CLi stretching vibrations of a complex associate of BuLi. ‘Ihe shape and position of this band are very sensitive to association of BuLi (the temperature and concentration effects) and to its interaction with ethers. Investigation of infrared spectra of the BuLi-methylal complexes in an isooctane solution enabled us to make the following conclusions. Firstly, since the interaction energy of Li . . . . 0 is not very high (of the order of the H-bond energy), in solution

f 8 i 03

P

..______.-.r. ‘______------

I

I

900

I 1000

I ,100

cm-l Fig. 2. The i.r. spectra of methylal-d, (1) and its complexes with BuLi : 2-form (1 BuLi: lM-d,); 3-form II (6 BuLi : IM-d,); -10”; solventisooctane, *-the bands of BuLi.

I

1474

K. K.

KALNINSH,

V. N. ZGONNIK,N. S. DMITRIEVAand N. I. NIKOLAEV

I

I

I

-50°

-40°

-3o-

I

-20

1

-IO0

I

I

o"

IO"

I

2on

1

30.

Fig. 3. The temperature dependence of intensity (l-4) snd width (I’-4’) of methylal bands: I-1045 cm-l, 2-1140 cm-l, 3-930 cm-l, P1112 cm-l. the reversible temperature-dependent equilibrium between the complex and the initial compounds is greatly shifted towards the latter. Secondly, when the composition of BuLi-methylal and the solution temperature are varied, it is possible to observe several states of interacting molecules. Methylal interacts in two forms. Form I [Fig. l(a)] appears when methylal is in excess or in equimolar ratio. Form II [Fig. l(a)] app ears when BuLi is in excess. Form I exhibits the following characteristics. The vroc band undergoes the greatest shift towards low frequencies which indicates that the COC bond participates in the interaction with BuLi. Nevertheless the 925 cm-l band (yFoc) remains practically in the same position. The 1108 cm-l band (CH, bending vibrations) shifts very little. It is interesting that a new band appears at 898-903 cm-l. When the experimental conditions (temperature, composition) change, the spectrum of form I remains qualitatively unchanged and only usual redistribution of the intensity of bands of form I, free ether and form II is observed according to equation: Methylal *

form I G=? form II

(I)

Measurements of the band intensities of free and bonded ethers have shown that the limiting composition of the complex BuLi-methylal is 1: 1, i.e. 4 methylal molecules may be bound to BuLi tetramer. As the BuLi tetramer forms only 4 coordination bonds, the above fact means that in form I only one oxygen methylal atom takes part in the interaction. This conclusion is confirmed by data obtained by investigating the H-bond of methylal with TChAA. Under conditions of equimolar ratio when TChAA interacts

Interaction of methylal and methylaM

with electron acceptors

1476

mainly with one oxygen atom in methylal, perturbations of methylal vibration spectrum are observed characteristic of form I (Fig. 1): low frequency shift of vroocby 13 cm-l, a slight shift of the 1108 cm-l band, no shift of vtoc at 925 cm-l. Completely solvated BuLi (great excessof methylal)absorbs at 504 cm-l Fig. 4(b)]

400

500

600

400

600

500

500

400

600

cm-l Fig. 4. (a) The i.r. spectra of complexes of BuLi-methylal 0’ (2) and +23O (3).

(1:l) at -29“

(b) The bands vcLIin the i.r. spectra of complexes of BuLi-methylal: 2-!i”MzlM” (1: l-1; -32’). (1:6*6; -60’): (c) The bands vCLI: l--TM2x

(1*9:1: -20’); 2--TM” bands of methylal.

(6.5:1;

-12’):

(l),

l--T&’ *-the

The redistribution of intensities of bands 526 and 504 cm-l [Fig. 4(a)] corresponds to attachment of the last (the fourth) methylal molecule to the BuLi associate (the tetramer). The following equilibrium is assumed: M,I.T+MsM&T

(2)

tetramer) The value of Kp is about 5 l/mole at O”C, ( T-BuLi

-M

= (4 f l-5) kcal./mole.

Over a certain range of conditions form II coexists with form I and with free methylal; it can be observed in a pure form when BuLi is present in a 4-fold or greater excess. Form II was shown to crystallize from solution at -IO’; this permitted to establish that the stoichiometry of this complex is eq 4 : I. Hence, in the 4 : 1 complex each BuLi tetramer interacts with only one methylal molecule in form II. On the basis of all these data it was concluded that form II corresponds to methylal interacting with both oxygen atoms.

1476

K. K. KALNINSH, V. N. ZUONNIE, N. S. D MITBIEVA and N. I.

NIKOLAEW

Thus, these methylal forms may be represented as follows: Li

H 3C’“\&o’~H Form I

2

Li

Li

:

:

H 3c/“\cH/o\CH 2 Form II

3

3

In the infrared spectra form II (Fig. 1) is revealed by a further low frequency shift of the bands, in this case all the methylal bands shift over the range of SOO1200 cm-l. It is interesting to note that in passing to form II the ytoc band shifts markedly. Of particular interest is the case when BuLi is in a great (6-s-fold) excess. Under these conditions free methylal is practically absent in solution and the complex is respresented by forms I and II the proportions of which are determined by temperature. This case may be described by equation: M,T.T+TzBM?T

(3)

or

where q is the designation of the BuLi tetramer and o-o

methylal.

[MI1 . T-j8 K1-ll = [M,I . T] . [T] ’

Assuming that [T] = const. which is valid when BuLi is in great excess, we determined K,_n and then -AH from three pairs of bands: 1131 and 1138 cm-l, 1096 and 1108 cm-l, 898 and 915 cm-l which correspond to forms I and II respecis (12 & 1) kcal. mole-l and, according to tively. The results coincided : -AH scheme (3), characterizes the attachment of two oxygen atoms to the BuLi tetramer. In our opinion the assumption about the tetramer state of free BuLi made in equation (3) has little influence upon above results. Different bands correspond to form II in the range of rcLi. Probably, the value of %Li is determined by the amount of oxygen atoms bonded to BuLi. Thus, similar frequencies are observed in the following cases:

CD

x>

-535

cm-l [Fig. 4(c)]

-504

cm-1 [Fig. 4(b)]

Interaction of methylal and methyld-ds with electron acceptors

1477

Table 1. The vibrational spectra of methylal, methylal-ds and their complexes with BuLi; cm-l Methylal

Form I

Form II

Methylal-d,

1138

1138

1131

1134

1108

1105

1096

1109

1045

1030

1027

1083

1065 1054

1139 1115 1108 1095 1065 1054

923 900

923 900

923 900

857

863 847

865 851 830

927

927 900

915

Form I-d, 1137 1113 i 1108

Form II-&

It is also shown here that the BuLi associate interacts simultaneously with methylal molecules in two forms. Perturbations of infrared methylal spectra resembling form II can be induced by interaction with TChAA. Under conditions of considerable TChAA excess when the formation of two H-bonds by methylal molecules becomes probable, the infrared spectrum exhibits the shifted bands at 906 and 1095 cm-l instead of 927 cm-i(Ytoc) and 1105cm-l which resembles form II [Fig. l(a) and (b)]. Finally, we shall consider the infrared spectra of methylal-d, complexes. In this case under the same experimental conditions as for methylal two forms of interacting molecules of methylal-d, are found (Fig. 2). However, the spectra of these forms are somewhat more complex. As a rule, the shifted bands are split into doublets. Thus, in the spectrum of form I a$& is split into two bands and in passing to from II the doublet remains practically unchanged. The vtoc band at 857 cm-l is shifted and split even for form I and in the spectrum of form II this doublet is replaced by three bands (Table 1). It should be said that in the range of 850-870cm-i one of the COD) bands occurs which possibly appears in the complex spectra. Comparatively complex changes in the frequencies and intensities of the CH, bending vibrations at 1109 and 1134cm-l are observed.