Charge-transfer spectra of organometallic complexes—I. Formation constants for complexes of trialkyltiniodides with iodine in carbon tetrachloride solutions

Sprclrochimica Ana, Vol.39A,No. I I. pp.959-963.1983

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Charge-transfer spectra of organometallic complexes-I. Formation constants for complexes of trialkyltiniodides with iodine in carbon tetrachloride solutions S. HOSTE, G. G. HERMAN, F. F. ROELANDT, W. LIPPENS, L. VERDONCK and G. P. VAN DERKELEN Department of General and Inorganic Chemistry-B, University of Ghent, Krijgslaan 271, B-9000 Ghent, Belgium (Received 5 April

1983)

Abstract-A

charge-transfer complex is formed as reaction intermediate in the iodinolysis of R,SnI (R = CH,, C2H5, tt-C,H,, iso-C,H,, n-C,H,, iso-C,H, and set-C,H,) in Ccl, solutions. By means of the spectrophotometric molar ratio method and by the detection of an isosbestic point at SlOnm, the complex species was identified as a 1: 1 molecular adduct between the alkyltiniodide and iodine. Formation constants of the complex were calculated using a non-linear regression analysis of absorbancy measurements on the perturbed iodine band. With the aid of these constants, the maxima of the charge-transfer peaks (u.v. region) and blue shifted iodine peaks (visible region) could be accurately determined.

INTRODUCTION Charge-transfer (CT) complexes between Group IVA organometallic compounds as donor and iodine, bromine or tetracyanoethylene (TCNE) as acceptors are gaining increasing interest [l-6]. Recently, transient CT complexes have been recognized as the most important intermediates in both the homolytic and electrophylic halogenolysis of tetra-alkyltin compounds [7-91. Nevertheless, information about the complex formation constants (KJ of the CT complexes with organotin compounds as donors is very scarce. Our current interest [l&14] in the kinetics and reaction mechanisms in halogenolysis reactions led us to the study of the interaction of a series of trialkyltinhalides with I, in CC14 solutions, by visible and U.V. absorption spectroscopy to obtain the apparent formation constants of the possible CT complexes. The stoichiometry of these complexes was derived by the molar spectrophotometric ratio conventional method [15] applied to the CT band. The apparent equilibrium constants were obtained from measurements on the perturbed iodine band [16]. Although the complexes are expected to be rather weak, a meaningful separation of K, and E seems possible under our experimental conditions since the criteria discussed by PERSON[ 171 and more recently by LA BUDDE and TAMRES[~~] were taken into account. Finally, the knowledge of the formation constants allowed an accurate determination of the CT absorption band maxima by appropriate subtraction of the different components in the electronic absorption spectra. EXPERIMENTAL Materials

The series of trialkyltiniodides used in this study has been prepared by gradual addition of stoichiometric amounts of iodine to the pure, predistilled tetra-alkyltin compounds. The 959

resulting monoiodides were then vacuum distilled and kept in the dark in N, Rushed vessels. On standing, these products eventually develop a yellowish to orange tinge. Therefore, they were all redistilled immediately before use and only the middle fractions were taken for the sample preparations. Gas chromatographic purity was at least 97 “A. All solutions were made in volumetric flasks of 10 or 25ml. To a weighed amount of R&I was subsequently added, in the dark or with a red safe-light, an appropriate amount of IJCCI, solution. All further manipulations also proceeded under reduced light conditions. Preliminary experiments with all these mixtures proved that no detectable iodinolysis took place, in the dark, in a time lapse comparable to that of the final experiments. Even prolonged exposure to the U.V. monitoring light of the spectrophotometer had no effect on the iodine absorbance at 520mm. In sunlight, however, most of the reaction mixtures discoloured within a few minutes except for the Me,SnI compound. Iodine (Merck, Analytical grade) was used without further purification. Carbon tetrachloride (Aldrich) was purified by successive washings with sulfuric acid, aqueous bicarbonate and distilled water followed by drying over molecular sieves (Perlform 4A, Merck) and distillation. Spectral

measurements

The spectra were recorded with a VarianCary 14 spectrophotometer with the cell compartment thermostated at 20.0 + O.l”C. Teflon stoppered quartz cells with optical pathlengths of 0.0220.1 and 1 cm were used thus providing a large flexibility in the choice of the concentration ranges. The pathlengths of the thinner cells were accurately determined using a known standardized I,/CCl, solution, The extinction coemcient of iodine in Ccl, at>_ = 517 nm was found to be 927 Imol- 1cm-’ differina less than 1 % from literature values [19, 201. After thermal equilibration of the solutions for IOmin in the dark cell compartment, spectral measurements were made relative to air. Blanc0 corrections were made afterwards using the same cell. This procedure is, especially in the case of strongly coloured substances like I,, to be preferred to spectral subtraction using an I, solution of equal concentration [7-91 in the reference beam. Exact compensation cannot be achieved in this way because the equilibrium concentration of I, in the reaction mixture is unknown. Furthermore, the total amount of light reaching the detector, in this contiguration, is much reduced thus forcing the slit to open beyond tolerable limits. Reproducibility and accuracy of the absorbance measurements was also improved by operating the I

I,

S. HOSTEet al.

960

spectrophotometer in a static mode and allowing the detector to stabilize at fixed, pre-set wavelength values. The estimated uncertainty in the measurement of absorbance is 0.002 units.

species formed under our experimental conditions seems to be a, presumably weak, 1: 1 molecular complex and the determination of its apparent formation constant was attempted.

RESULTS AND DISCUSSION Formation Stoichiometry

of the CT complexes

Recent kinetic experiments in our laboratory and GC-MS analysis revealed that R,SnI/I, in Ccl, yielded R,SnI,, with R = Me, Et, n-Prop., i-Prop., nBu., i-Bu., tert-Bu. upon U.V. illumination [21]. The dependence of the initial reaction rate on the light intensity suggests that this reaction may be very similar to that described by FUKUZUMI and KocHI[~], the absolute value of the homolytic reaction rate however being smaller in the monoiodides as compared to the tetra-alkyl compounds. Furthermore the contribution of electrophilic substitution seems quite negligible. Similarly, evidence was found for the formation of a complex between the trialkyltiniodide donor and I, acceptor. A new broad band in the 270-340nm region of the U.V. absorption spectrum of the mixture is supposed to stem from a CT transition in the molecular complex formed as a reaction intermediate in iodinolysis: R,SnI+I,&R,SnI.

IJ%[R,Sn;.Il]

constants

A first estimate of the magnitude of the formation constants of these CT complexes can, in principle, be established directly from the sudden drop in absorbance of iodine in the visible region as the reaction mixtures are made. The quantity of iodine consumed in complex formation derived in this way is however subject to very large errors. First, the absolute value of the decrease in absorbance is relatively small except in extremely concentrated solutions, i.e. large excess of the alkyltin ligand since the I, solubility in Ccl, at room temperature is limited to f 0.08 M. Secondly there is some interference with the blue shifted peak of the [R,SnI.I,] complex. Therefore the measurements are preferentially made at the low energy side of the I, absorption peak in the visible region (550-580nm). The so-obtained values (K,,J thus are to be considered as lower limits; they are included in the first column of Table 1. A rigorous determination of the formation constants K is based on the general relation [18]

-+ R, SnI,

+ RI. Moreover, the visible I, absorption band (J.,,,, 517 nm) is blue shifted upon addition of the various organotiniodides and the occurrence of an isosbestic point at - 510 nm proves that only one other absorbing species next to iodine is present (Fig. 1) [22]. To determine the stoichiometry of this second species the molar ratio method [15], using 0.03 M solutions, was applied at different wavelengths evenly distributed over the u.v.-CT peak. For all the compounds studied here a maximum in the absorbance us. mole fraction (y) diagram was obtained at y = 0.5. So the only new

where the spectroscopic IUPAC nomenclature is used and where A, and D, represent initial acceptor and donor concentrations. The absorbance A’ of the complex species c is derived from the measured absorbance A with A’ = A - e,A,b and its molar absorptivity El =6,---E

- Q&b

(2)

E, is defined as A -ED

(3)

The relation (1) is extensively discussed in the literature[17-19, 231 and is most commonly used in the linear approximation:

4,Dob A’

=&+;(A,,+D,).

(4)

Table 1. Apparent formation constants obtained by different methods at 20.0 + O.l”C for the complexes of some trialkyltiniodides with iodine in Ccl, solutions

440

460

520 WAVELENGTH

560 (nm)

Fig. 1. Isosbestic point detected at 510nm for Et&I = 3.6 x 10-l M, 2.5 x 10-l M, 1.1 x 10-l M and I, = 6.4 x lo-* M in Ccl, at 20°C.

Alkyl group

K,,,

Kr

KkL

Methyl Ethyl n-Propyl iso-Propyl n-Butyl iso-Butyl see-Butyl

1.1 1.2 1.1 0.8 1.2 1.0 0.8

2.4 1.9 2.1 2.0 2.5 1.7 1.5

2.14 1.94 2.03 2.01 1.99 1.87 1.45

*Obtained with equation (4). tobtained with equation (1).

Charge-transfer spectra of organometahic complexes since E’ is expected to be large for CT complexes. Application of this equation to extract K and E’ from the charge-transfer data in the U.V. region is however difficult mainly for two reasons: (1) interference with absorption of reactants and products and (2) failure to obey the Beer-Lambert law for iodine [20,24,25]. So, at first glance, it seems advisable to use the visible absorption peak as an alternative. In this case however molar absorptivities for the complexes are expected to be of a magnitude comparable to that of I, and a breakdown of the linear approximation might result. Therefore both the linear approximation (4) and the more rigorous quadratic (1) regression analysis were simultanously used. In the last case, formation constants and molar absorptivities were obtained through unconstrained least squares fitting of observed and calculated absorbance with a non-linear optimization method using conjugate directions [26]. Optimal values were determined with a precision of at least 1 %. The wave length range over which acceptable values for K and E’were obtained was about twice as large in the non-linear approximation as compared to the linear case: typically a 60nm wavelength range symmetrically distributed with respect to the blue shifted peak maximum located at 478 k 1 nm for all compounds studied. No such information can of course be obtained at the isosbestic point (510nm) and at higher wavelengths since the absorbance of the complex is too low to yield accurate constants. It is therefore advisable to use absorbance measurements at 510nm only to determine the rate constants for the photochemically induced iodinolysis reaction, which is presently under study. For each reaction studied, data were obtained from 7 solutions with alkyltiniodide concentrations ranging from 0.35 to 0.05 M and constant I, concentration (0.07 M). Under these conditions from 10 to 40 Y,, of the component with the smaller concentration is consumed in complexation. A meaningful separation of K and E’ is then possible according to the reliability criteria discussed by PERSON [ 171 and others [ 18, 231. The data are summarized in Table 1. It can be seen from Table 1 that the differences induced in the series by varying the alkyl group are rather small. The only noticeable effect is a slight decrease in K for the bulkier

961

CT

Band

s-

I 270

290

I

310

I

and branched substituents. It is of interest to note here that these formation constants are at least an order of magnitude larger than those estimated for the corresponding tetra-alkyltin compounds [7-91. CT spectra and blue shifted bands Knowledge of the formation constants allowed the determination of the CT absorbance maxima by appropriate correction for the absorbances of free donor and acceptor in the U.V.region between 350 and 250nm. The final spectra were all obtained using equimolar solutions (0.02 M) of alkyltiniodide and iodine. Peak maxima were determined using the bisection of chords technique and are believed to be accurate within 1 nm. These maxima cover a smaller range as a function of R as compared with the corresponding tetra-alkyl compounds [9]; nevertheless a similar increase in the ,%g is observed as the steric effects of alkyl subatituents grow. As expected for CT complexes the molar absorptivites are quite high (see Table 2). The U.V. spectrum for [Et,SnI.IJ is given in Fig. 2.

Ecrxlo-3 A,, (nm)

hv&eV)

Ess X 10-2 &(nm)

(1 mol-‘cm-l) Methyl Ethyl n-Propyl iso-Propyl n-Butyl iso-Butyl set-Butyl * Subscript band.

297 302 304 305 304 305 308

4.17 4.10 4.08 4.06 4.08 4.06 4.02

CT refers to charge-transfer

I 350 lnm)

Fig. 2. Molar absorptivity of the CT complex formed between Et,SnI and I, in Ccl, solution in the U.V. region.

Table 2. Spectral parameters for some trialkyltiniodide complexes with iodine in Ccl, solution*

Alkyl group

I

330 WAVELENGTH

16.9 18.1 17.5 12.9 17.6 17.6 25.0 U.V.data, subscript

(lmol~‘cm~l) 478 478 478 478 478 477 477

12.9 13.2 13.3 11.0 13.8 12.8 14.5

BS refers to the blue shifted iodine

S.

962

HOSTE

el a/.

Acknowledgement~The authors wish to thank MOUTON for the synthesis and gas chromatographic

of the compounds

Mr.

R.

analysis

used in this study.

REFERENCES [‘I

PI

M. A. LOPATIN and A. N. EGOROCHKIN, Zh. obshch. Khim. 51, 1086 (1981). S. I. BOBROVSKII, Deposited document, Viniti 3782, 145

[31 c41 Lb0

Fig. 3. Molar absorptivity of [Et&I. 12] (-)

460

520 WAVELENGTH

of iodine (---) in CCll in the

560 (nm)

[51 161

and CT complex

visible region.

171

(1979). Y. G. BUNDEL, S. I. BOBROVSKII, I. A. NovIKovAand 0. A. RENTOV, Izl;. Akud. Nauk. SSSR 925 (1979). 1. G. GAZIZOV~~~ G. A. CHMUTOVA, Zh. obshch. Khim.

50, 2063 (1980). H. SAKUROU, M. KIRA and M. OCHIAI, Chem. Left. 87 (1972). P.G. SENNIKOV,S. E. SKOBELEVA,V. A. KUZNETSOV, A. N. EGOROCHKIN, P. RIVIERE, J. SATGE and S. RICHELME, J. orgunomet. Chem. 201, 213 (1980). S. FUKUZUMI and J. K. KOCHI, J. org. Chem. 45, 2654

(1980).

PI S.

FUKUZUMI and J. K. KOCHI, J. phys. Chem. 84, 608

(1980).

Characterization of the blue shifted iodine band depends on subtracting the contributions of free iodine from the total absorbance. The spectral separation of this peak from the CT peak is so large that we can safely discard the possibility of significant overlap in this case. The subtraction however requires a reliable value for K because the determination of band position and intensity is quite sensitive to the magnitude of K [27]. The visible blue shifted band obtained in this way for the [Et,SnI.I,] complex is shown in Fig. 3 and the data for all the productsare summarized in Table 2. All the blue shifted peak maxima cluster around 478nm. This fits nicely in the range determined by Vorcr[28] for the maxima of the visible absorption band of iodine in iodine-containing solvents such as methyliodide (is, = 48 1 nm) and see-Butyl-iodide (,I,, = 472 nm). Although the good correlations [27] between Ia, and the stability of the complex or the donor strength, obtained earlier [27] has been questioned (see [16], p. 247), the La, values and the formation constants determined in this work fit the general relationship [29, 301 reasonably well. The increase in intensity of the blue shifted band relative to free iodine relative to &sS in (e 1,,520nm= 9.2 x 10’ 1 mol-‘cm-’ Table 2) has been attributed to intensity borrowing from the CT band due to mixing of the upper level of the CT band with that of the iodine band [30]. Both aCT and &as also show an increasing trend towards the bulkier substituents. Extension of these CT studies to unsaturated alkyltiniodides and bromides in different solvents and to other thermodynamic aspects of CT complex formation are presently under study.

[91 S.

FUKUZUMI and J. K. KOCHI, J. Am. them. Sot. 102, 2141 (1980). II101 G. P. VAN DER KELEN, E. V. VAN DEN BERGHE and L. VERDONCK, Organotin Compounds, Vol. 1, pp. 81-151 (edited by A. K. SAWYER). Marcel Dekker, New York (1971). I1 ‘I L. VERDONCK and G. P. VAN DER KELEN, Spectrochim.

Acta 28, 433 (1972).

[121 [I31 [I41 [E] [E] iI91 PO1 [ii]

L. VERooNcK and G. P. VAN DERKELEN, J. organomet. Chem. 40, 135 (1972). D. F. VAN DE VONDEL, H. WILLEMEN and G. P. VAN DER KELEN, J. organomet. Chem. 63, 205 (1973). S. HOSTE, H. WILLEMEN, D. F. VAN DE VONDEL~~~ G. P. VAN DER KELEN, J. electron Sjectrosc. 5,227 (1974). P. JOB, Ann. Chim. 6, 97 (1935): M. TAMRES, Molecular Complexes, Vol. 1, Ch. 3. Paul Elec Ltd., London (1973). W. B. PERSON, J. Am. them. Sot. 87, 167 (1965). R. A. LA BUDDEand M. TAMRES, J. phys. Chem. 744009 (1970). H. A. BENESI~~~ J. H. HILDEBRAND, J. Am. them. Sot.

71, 2703 (1949). R. M. KEEFER and T. L. ALLEN, J. them. Phys. 25,1059 (1959). P. DE RYCK, unpublished results. T. NOWICKA-JANKOWSKA, J. inorg. nucl. Chem. 33,2043

(1971).

[ii]

D. A. DERANLEAU, J. Am. them. Sot. 91, 4044 (1969). M. TAMRES, W. K. DUERKSEN~~~ J. M. GOODENOW, J.

phys. Chem. 72, 966 (1968).

P51 PI

[:i; c291 [301

A. A. PASSCHIERand N. W. GREGORY, J. phys. Chem. 72.

2697 (1968). D.

CARTON

and

B. T. SUTCLIFFE, in Theoretical 1 (edited bv R. N. DIXON). The Society, London (1974).

Chemistrv. Vol.

Chemical J. HAM, J. Am. them. Sot. 76, 3675 (1954). E. M. VOIGT, J. phys. Chem. 72, 3300 (1968). SR. M. BRANDON, M. TAMRES and S. SEARLES, JR., J.

Am. them. Sot. 82, 2129 (1960). R. P. LANG, J. Am. them. Sot. 84, 1185 (1962).