1058
J. Phys. Chem. 1981, 85, 1058-1061
Vibrational Study of Ionic Association in Aprotic Solvents. 5. Formation and Structure of the Lithium Isothiocyanate Tetramer in Alkyl Ethers and in Tertiary Amines Martial Chabanel, * Maryvonne Lqon, and Danible Paoli Laboratoire de Spectrochimie des Ions, 44072 Nantes Cedex, France (Received: October 23, 1980)
Infrared and Raman spectrometry and vapor-pressure osmometry have been used to detect (LiNCS)4in ethers and in tertiary amines. The tetramerization of LiSCN is favored by steric hindrance close to the donor atom of the solvent. (LiNCS)4has a cubane-like structure and a Td symmetry. Important variations in frequencies relative to the free SCN- ion have been observed. They are negative for VCN and positive for vcs and’6SCN. As a consequence, it is shown that each nitrogen atom is bonded to three lithium atoms. Up to now this kind of bridging coordination of the SCN ligand does not seem to have been found in any other compound.
Introduction Vibrational spectroscopy has proved to be the most valuable tool for studying ionic association of 1-1 inorganic salts1 especially nitrates and thiocyanates. Up to now, most papers have been dedicated to contact ion pairs. This step is the first in the association process, and it is predominant in solvents of high or medium polarity such as dimethylformamide or pyridine. In weakly polar solvents lithium salts are mostly in the form of ion pairs, dimers, or higher aggregates. The lower the solvent polarity the higher the state of aggregation of the salt. The polarity of a solvent is measured by its dielectric constant for long-range solvation and by its donor ability for the short-range solvation of the Li+ cation. This ability can be measured by the Gutmann donor number or donicity. Steric hindrance is also an important factor because it limits the number of solvent molecules in the first coordination shell of an ion and therefore favors solute aggregation. Partial or complete dimerization of lithium salts is observed2in solvents of low dielectric constant and medium basicity (ethers, esters, carbonates) or of low donicity (nitromethane). Ion triplets are in too small concentration, a t least in low dielectric constant solvents, to be seen by vibrational spectrometry. Higher aggregates should be formed in still less polar solvents, but solubility imposes severe limitations. In fact lithium salts are completely insoluble in nonpolar solvents. They remain in the crystalline state, which is obviously the highest possible state of aggregation. It has seemed to us that the best way to produce aggregates beyond the dimer is to increase the steric hindrance of the solvent by using branched ethers or tertiary amines. Experimental Section Williamson’s method was used for the preparation of unsymmetrical ethers from ethyl bromide and 2-propanol or 2-butanol. Sodium alkoxide was prepared by adding sodium over an excess of secondary alcohol (-2.5 more than stoichiometric proportions). Then ethyl bromide was added slowly to the mixture under reflux. The ether was distilled from the mixture, dried over sodium for 1week, distilled on a 20-plate column, and stored over sodium for at least 2 weeks before use. Commercial triethylamine was (1) (a) D. E. Irish, “Ionic Interactions”, Vol. 2 S. Petrucci, Ed., Academic Press, 1971, Chapter 9; (b) D. E. Irish and M. H. Brooker, Adu. Infrared Raman Spectrosc., 2, Chapter 6 (1976). (2) (a) D. Paoli, M. Lucon, and M. Chabanel, Spectrochim. Acta, Part A , 34,1087 (1978); (b) ibid., 35,593 (1979); ( c ) M. Chabanel and D. Paoli, J. Chim. Phys. Phy~.-Chim.Bioi., 77, 913 (1980).
distilled on a 20-plate column and then stored over sodium for at least 2 months before use. Raman degrees of defor a polarization p were measured as the ratio 11/1~~ vertically polarized incident light. Unless otherwise stated, Raman slit widths were equal to 2 cm-l. A Hewlett-Packard vapor-pressure osmometer was used for molecular-mass measurements which were performed at 50 “C. Other experimental conditions have been given in previous papers.2a Results and Discussion Formation of (~5iNcS)~. In normal ethers (Et,O, Bu20) LiNCS is completely dimerized even at 0.01 M. The dimer is a nonpolar quadrupole characterized by a vCN band at 2030 cm-l in infrared spectra and at 2043 cm-l in Raman spectra.2b In concentrated solutions (- 1-4 M) other weak infrared bands occur. One of them is on the low-frequency side of the main band of the dimer, at 1993 cm-l. Other bands are probably due to chain aggregate^.^ As the intensity of all of these bands increases with concentration, they are attributed to aggregates higher than the dimer. We have looked for solvents in which these aggregates would be predominant. Consequently, we have increased the steric hindrance around the donor oxygen atom of the solvent by using branched symmetrical ethers, but the solubility of LiSCN becomes very low. For instance it is only 0.03 M in isopropyl ether (i-Pr20). The spectrum of LiSCN in this solvent exhibits a prominent band at 1993 cm-l, while the band of the dimer almost completely disappears. As will be shown, this band is due to (LiNCS)@ In order to obtain a better solubility while preserving some steric hindrance, we prepared unsymmetrical ethers having only one branched alkyl group: isopropyl ethyl ether (iPrEtO) and sec-butyl ethyl ether (sec-BuEtO). In both cases, at -0.5 M, the predominant species are the dimer and the tetramer. As expected, the tetramer is most stable in sec-BuEtO (Figure 1). At 0.02 M the tetramer and the dimer are in about the same proportions in this solvent. Other weak IR bands are found at 2070,2079, and 2125 cm-l. They almost disappear in dilute solutions. They are also seen in the Raman spectrum of 0.5 M solutions, and they are presumably due to chain polymers. A weak spurious band at 2012 cm-l is due to traces of water. The donor atom of a solvent molecule can be bound to one group (nitriles, carbonyls, sulfoxides), two groups (ethers, pyridines), or three groups (tertiary amines). Steric hindrance is expected to be most important in this last (3) J. Vaea, M. Chabanel, and M. L. Martin, J. Phys. Chem., 82,2420 (1978).
0022-3654/81/2085-1058$01.25/00 1981 American Chemical Society
The Journal of Physical Chemistty, Vol, 85, No. 8, 198 1
Vibrational Study of Ionic Association
1059
7
~
A
1
L
-
2100
I
2000
2050
cm-1
Figure 1. Infrared and Raman spectra of LiSCN in sec-BuEt0 (t = tetramer; d = dimer; a = other aggregates).
TABLE I: Mean Association Number of of LiSCN in sec-BuEtO Measured by Vapor-Pressure Osmometry
molaritv _.
n
0.32 3.5
1
2050
0.50 3.6
0.53 3.9
0.58 4.0
class of solvents. Fortunately, amines are very basic. For instance, the donor number of primary and secondary amines, measured by an indirect method, has been estimated by Herlem and Popov4 to be 55. For that reason, LiSCN solubility in triethylamine (Et3N)is fairly high (0.6 M at 25 "C). A t this concentration tetramerization is almost complete. In short, the following sequence of the stability of (LiNCS)4 according to the solvent has been found E t 2 0 < BuzO < i-PrEtO < sec-BuEtO < Et3N When steric hindrance is lowered by replacing an n-alkyl group by a methyl group, the tetramer is no longer observed while dimerization is complete, for instance, in tert-butyl methyl ether. Also, in N-methylpiperidine the association process is limited to the dimer, while in Nethylpiperidine tetramerization is important as in Et3N. Vapor-Pressure Osmometry. The apparent molecular weight of LiSCN was measured by vapor-pressure osmometry (VPO), and the mean association number ri was deduced from it (Table I). As usual, all causes of nonideality were supposed to be due to association. The solvent sec-BuEtO was chosen because solutions in it are more stable than in Et3N. The value of ri could also be deduced from infrared measurements under the assumption of a dimer-tetramer equilibrium. Other aggregates, which are in small proportion, were neglected in this calculation. The dimerization of LiSCN is complete in some ethers and in N-methylpiperidine.2 The measured values of the extinction coefficient c of the dimer have been found to be between 4300 and 5100 M-' cm-l, but tAvlp is constant within experimental error. In BuzO, Avlj2 is exactly the same as in sec-BuEtO (9 cm-l). Therefore the E value in this solvent (4800 M-l cm-l) has been used to calculate the dimer concentration in sec-BuEtO. The infrared values of R were estimated by this method to be 3.6-3.8 in the same concentration range as for VPO measurements. The agreement is good, and consequently the predominant aggregate, characterized by vCN = 1993 cm-', is actually a tetramer. An approximate value of c in the tetramer (11 000 M-l cm-l) has also been deduced from these results. (4) M. Herlem and A. I. Popov, J. Am. Chem. Soc., 94, 1431 (1972).
1
2000
em-1
Flgure 2. Raman spectra of LiSCN (c = 0.6 M) in triethylamine.
Spectroscopic Determination of the Tetramer Structure. The infrared band at 1993 cm-' was also observed in Raman spectra (Figure 2) and is depolarized ( p = 0.75). For this band the rule of mutual exclusion is not followed, and consequently the aggregate does not possess a center of symmetry. Another band, which is completely polarized, ( p I 0.02), occurs in the Raman spectrum at 2022 cm-'. In any group with a noncubic symmetry, the depolarization ratio should be different from zero. The centrosymmetrical Oh group being excluded, the aggregate must have the Td symmetry. These results are sufficient to prove the existence of a tetrahedral tetramer, vapor-pressure measurements being only useful as confirmation. The internal vibrations of the SCN group are V C N , VCS, and 8 s ~ In ~ . the tetramer they belong to the following representations: r C N or res = Al Tz; ~ S C N= E + T1+ Tz. Therefore the two observed vibrations at 1993 and 2022 cm-' are attributed to V C N ( T ~ )and vCN(AJ. Moreover the Al Raman band observed with a slit width of 1 cm-' is exceptionally sharp ( A V ~=, ~4 cm-l), as is usually the case for breathing modes of highly symmetrical molecules like CC14. On the other hand, the T2 band is only slightly sharper (7 cm-l) than the VCN band of the dimer ( e 9 cm-I). Coordination of the SCN Group. Several organolithium compounds, for instance, (LiCH3)4 and (t-BuLi),, are known to form tetrahedral cubane-like structures.6 In the ion pair LiNCS and in the dimer (LiNCS)z, SCN is coordinated to lithium through the nitrogen atoma2We have proved that in the dimer each nitrogen atom is bidentate and coordinated to two lithium atomsa3 This new kind of coordination of SCN was later found by Cotton et al. in a rhenium compound.6 The most striking effect of this coordination is a lowering of the vCN frequency. In all other cases (N, S, or N- and S-bonded complexes) the vcN frequency is higher in the coordinated compound than in the free SCN- ion. In the tetramer the lowering of this frequency is still more important than in the dimer. The infrared frequency is situated 60 cm-' lower than for the free SCN- ion, in a region which is quite unusual for an SCN vibration. The vCN, vcs, and BSCN frequencies in SCN-, LiNCS, and (LiNCS)2have been given in previous papers.2 There is a quite regular variation in these frequencies with the number of lithium atoms (0, 1, or 2) bonded to each nitrogen atom (Figure 3). The frequencies of (LiNCS)4 follow the same trend if it is assumed that each nitrogen atom is bonded to three lithium atoms. This result is in
+
(5) E.Weiss and E. A. C. Lucken, J. Organomet. Chem.,2,197 (1964); G . E. Hartwell and T. L. Brown, J. Am. Chem. Soc., 88, 4625 (1966). (6) F. A. Cotton, A. Davison, W. N. Isley, and H. S. Trop, Inorg. Chem., 18, 2719 (1979).
1060
The Journal of Physical Chemistry, Vol. 85, No. 8, 1981 ION
DIPOLE
SCN-
SC H . . . L ,
PIIAIIHI,PL‘LE
OCTIIPOLE
Q=+ I
I I
I
e I+ I
QB2u
0
T2
I I
0
1 COORDINATION
QB3u
I
2
3
NUMBER OF S C N
Fi ure 3. Variation in SCN frequencies according to the number of Li fixed on each nitrogen atom.
B
a Li Figure 4. Structure of (LiNCS)+
complete agreement with the cubane structure of (LiNCW4 (Figure 4). In the ucs region it was only possible to find a polarized Raman band at 834 cm-l, which is therefore attributed to the ucs(Al) vibration of the tetramer. The reason that the ucS(T2)band has a very low intensity in Raman spectra will be explained later. It was also not found in the infrared spectrum, but ucs bands are weak (- 100 times less strong than uCN) and situated in a region where organic solvents absorb more or less. Moreover solvation bands, also observed with LiI solutions, are seen in this region. On the other hand, 6SCN bands are always too weak to appear in Raman spectra. In infrared spectra 6 s c ~ ( T Jis the only allowed transition, and it is observed at 515 cm-’. This band is not split, whereas in the dimer an important splitting occurs, due to the lower symmetry of this species. Force-Constant Variations. The uCN and ucs vibrators of SCN are almost independent so that the variation in F C N and Fcsroughly follows the same trend as the variation in ucNand UCS. However, in an aggregate, mechanical couplings occur between SCN and LiN vibrators, and
Chabanei et ai.
eventually between different SCN vibrators. In the tetramer these couplings induce a splitting of the ucs and ucN frequencies. A force-field calculation was performed on the tetramer by using a simplified model where it was assumed that the Li4N4unit has a cubic structure. As for the dimer, we took FL~N = 0.57 and F“ = 0.3 N/cm.2b Force constants F c N and Fcs were calculated to fit best the three experimental frequencies uCN(A~), uCN(TJ, and ucs(Al). The variations in F c N and FCS,relative to F O C N = 15.528 and to Foes = 4.769 N/cm in the free SCN- ion, are A F C N / F ° C N = -7.3% AFcs/Focs = +15.3% These results can be compared to those previously obtained for LiNCS and (LiNCS)2. Between SCN- and LiNCS there is almost no variation in FCN. Then the step variations in F c N are ca. -4% for the second and the third lithium fixed on the nitrogen site of SCN. In LiNCS, only the nonbonding nitrogen doublet is used in the bonding with lithium. Then, in (LiNCW2and (LiNCS)4,one or two p electron pairs of CN are also used. As a consequence, a weakening of the CN bond is observed. The variations in FCS are quite regular and equal to ca. +5% per lithium atom bound on each nitrogen. The same trend is also observed for FSCN. The increase of this force constant is -7% per lithium. A value of 20 cm-l (experimental, 29 cm-’) was calculated for the splitting uCN(A1) - UCN(T,). The agreement can be considered as good, owing to the fact that the mechanical coupling model of the tetramer is very rough. The frequency ucs(T2)would be expected between 750 and 780 crn-l. Raman Intensities. Absolute measurements of Raman intensities are difficult. On the contrary, the intensity ratio of uCN(A1) to uCN(T2) can be measured with a better accuracy. This ratio can also be calculated by transferring electrooptical parameters from the SCN group. These parameters can be calculated from measurements on the more simple species SCN-, LiNCS, and (LiNCS)2. The four SCN groups of the tetramer will be numbered by the subscript j (I’ = 1,2,3, or 4). For a given mode (ucN), qj is the normal coordinate associated to the group (SCN), considered as an isolated unit. In the different species SCN-, LiNCS, (LiNCQ2, and ( L i N W 4 ,the SCN groups are supposed to have identical mechanical properties. Coupling between them is also neglected. Then, normal coordinates Qk = Q(Al), Q ( T d Q(T2,), Q(Td in the tetramer are obtained from the q j coordinates by the projection operators method as symmetry coordinates 4
Qk
=
E Akjq] j=l
(1)
or
Coefficients Ajk are the same as those used to build up sp3 hybrids from atomic orbitals ( A k j = h1/2). If we consider an element a&) of the polarizability tensor of the tetramer (t),for instance, aJt), ax (t), ...,its derivative with respect to Qk can be expressed Y from derivatives with respect to qj as (3) (7) G. W. Chantry, “The Raman Effect”, Vol. 1, A. Anderson, Ed., Marcel Dekker, 1971, Chapter 2.
The Journal of Physical Chemistry, Vol. 85, No. 8, 198 1
Vibrational Study of Ionic Association
TABLE 11: Degrees of Depolarization
p
for SCN Species
in Various Solventsa SCN-
LiNCS
0.27
0.37
( LiNCS),
(LiNCS),
~
PCN
0.36
G0.02 ( A l ) ,
(DMF, (DMF, (DEM) 0.75 (T,) Me,SO) CH,CN) (Et,N) pcs GO.02 G0.02 ~0.03 G0.05 (DMF, (DMF) (DMM) (Et,N) H,O) a Solvents are indicated in parentheses: DMF = dimethylformamide; Me,SO = dimethyl sulfoxide; DEM = diethoxymethane; DMM = dimethoxymethane; Et,N = triethylamine.
where a,,G) is the element of the polarizability tensor of the (SCN), unit. This relation can be written in a simpler manner as 4
ak’(t) =
E Ajkaj’ j=1
(4)
where the primes indicate the derivative with respect to the normal coordinate j or h. The elements ai are calculated from the longitudinal and the transverse polarizabilities a1 and cyp of the SCN unit, by using suitable rotations of the axis ~ y s t e m .The ~ tensor invariants of SCN are
+ 2aP)/3
L?CN = (a1
YCN
=
-ap
(5)
For the vCN(AJ mode of the tetramer the tensor a[(t) is isotropic, with elements al’(t) =
2L?’CN
(6)
If, for instance, the component Th of the vCN(Tp) mode is considered, its tensor a z i ( t )is diagonal with elements ‘IQY‘CN, - 2 / 3 ~ ’ ~and N , 0. Hence the value of the anisotropy T i ( t ) in this mode is Y{(t) = (2/31’z)Y’CN
(7)
The ratio ap‘/al/has been deduced from polarization measurements in other species containing the SCN unit. In fact, the degrees of depolarization pCN (Table 11) are not very different for SCN-, LiNCS, and (LiNCS)2. This result ensures a good transferability of ci;/apl between different species.
1081
The degree of depolarization pcNis close to lI3. For this particular value apl = 0, so that the transverse polarizability derivative of SCN can be neglected. The integrated intensity of a Raman line has the expression I = A(45d’ 77”) (8)
+
where A is a proportionality factor. For the Al mode y’ = 0 so that the intensity I(Al) can be deduced from eq 5 and 6 I(A1) = 180ACi”c~’= 2 0 A ~ 1 ‘ ~ (9) For the triply degenerate Tz mode a’ = 0, and by use of eq 5 and 7 I(T,) = 21Ayi2(t) = ~ ~ A C Y ~ ’ ~(10) By comparison of eq 9 and 10, we obtain I(T2)/I(Al)= 1.4. The experimental value is 1.5 f 0.2. The same method can be applied to vcs vibrations. The measured values of pcs for the SCN group are very close to zero, so that apl a{ and y ’ c ~ 0. From eq 10, it is shown that I(T2)N 0, so that vcs(T2)must be very weak in Fhnan spectra. This is why it is not seen in the Raman spectrum.
Conclusion In previous papers8 we have shown, by colligative and by dielectric methods, that, in weakly polar solvents, LiBr could exist in the form of a nonpolar dimer (quadrupole) or of a tetramer (octupole). By the use of LiSCN, spectroscopic methods could be applied to the investigation of this process. After having investigated ion pairing and dimerization,2we have now studied tetramerization, which seems to be the last association step in solution before crystallization. This research has led to the discovery of two new coordination modes of the thiocyanate ion: bidentate through the nitrogen atom in the dimer, and tridentate through the same atom in the tetramer. These results have shown that is is possible to identify these novel kinds of coordination by spectroscopy in solution, before their discovery in the solid state by X-ray structure determination. (8)(a) M.Chabanel, J.Chin. Phys. Phys.-Chin. Eiol., 62,678,1965; 63,1143 (1966); (b) D.Menard and M. Chabanel, J. Phys. Chem., 79, 1081 (1975).
