Charge-transfer spectra of organometallic complexes—VII. Electron—donor—acceptor complexation of trialkyltinisothiocyanates with iodine and tetracyanoethylene

SpectrochimicaAm, Vol. 46A. No. 7. pp. 1097-1100, 1990 Printed in Great Britain

05~8539190 s3.00+0.00 0 1990 Pergamon Press plc

Charge-transfer spectra of organometallic complexes-VII.* Electron-donor-acceptor complexation of trialkyltinisothiocyanates iodine and tetracyanoethylene P. VERBIEST,

L. VERDONCK

and G. P. VAN DER KELEN+

Laboratorium voor Algemene en Anorganische Chemie, Rijksuniversiteit Gent, Belgium (Received

14 November

with

Gent, Krijgslaan 281, B-9000

1989, in final form 2 January 1990; accepted 4 January 1990)

Abstract-An ultraviolet-visible study of the charge-transfer interaction between R, SnNCS (R = Me, Et, iPr and nBu) and iodine, respectively, tetracyanoethylene has revealed that the HOMO with dominant lone pair character located on S is the preferred donor site in the electron-donor-acceptor (EDA) complexes. Formation constants of the complexes were calculated using a non-linear regression analysis of the u.v.-vis. data.

PREVIOUS studies showed the direct halogenation of R,Sn [l] and R,SnI,_, [2,3] to proceed by the formation of an EDA complex as precursor. In the R,Sn and R,SnCl,_, the donor site is located in the alkyl-tin bond [l, 41 whereas in the R,SnX (X = I, Br) derivatives, the preferred donor sites are the lone pairs of the halide atoms [5,6]. In this

report, a study of the interaction of trialkyltinisothiocyanate donors with cr and z acceptors is presented. A photoelectron study and pseudopotential ab inito calculations [7] show that for the HOMO has dominant xs lone pair character with an (CH3)3@CSL d erivatives, ionization energy of almost 2 eV lower than the Sn-C bonding MO. In line with previous findings, the JQ lone pair is expected to be the preferred donor site since in this case, steric hindrance is unimportant. Experimental evidence for this assumption is presented and the relevant formation constants (ZQ are calculated from u.v.-vis. spectral data.

EXPERIMENTAL

Products The R3SnNCS (R = Me, Et, iPr) was prepared [8] by mixing ethanol solutions of the corresponding organotinchloride with an ethanolic solution of NaSCN. The products were purified by crystallization from benzene. A slightly different procedure was followed for BuSnNCS. The reaction mixture was refluxed for 2 h before removal of the precipitated NaCl. The residue after distillation of the ethanol was dissolved in ether, washed with water and dried. Fractionation gave the pure Bu,SnNCS. The identity and purity of the RSnNCS was checked by i.r. and NMR spectroscopy. CHQ, CH&12 (Janssens) and C2H4C12 (Alltech), spectrophotometric grade, were used after drying on molecular sieves (4 A) (Merck) and fractionation. Tetracyanoethylene (Janssens) was purified by two successive recrystallizations from chlorobenzene followed by vacuum sublimation. Resublimed I2 (Janssens) was used.

Measurements The u.v.-vis. spectra were recorded with a HP-8451 A diode array spectrophotometer, equipped with a thermostatted Rapid Kinetics Accessory (Hi Tech Scientific SFA 11) maintaining the temperature at 25 “C f O.l”C. The recording procedure is described elsewhere 191. * For part VI, see Ref. [4]. +Author to whom correspondence

should be addressed 1097

P. VERBIESTet al.

1098

Table 1. Formation constants Kcb of R$nX-I,

and -TCNE complexes

Acceptor TCNE

I2 Donor Me,SnX Et,SnX nBulSnX iPr,SnX

x = Ib(CCI,) 2.14 1.94 1.99 2.01

NCS(qH,CI,) 5.34 6.15 7.10 8.04

I’(CHC1,)

NCS(CH,CI,)

0.34 0.43 0.79 1.08

1.53 2.05 -

‘Formation constants in 1 mol-‘, obtained at 25 +O.l”C and 1 atm. ‘Ref. [lo]. ’ Ref [ll].

RESULTS AND

DISCUSSION

When a solution of trialkyltinisothiocyanate is mixed with an I2 or TCNE solution a new absorption band is observed in the U.V. region (300-320 nm) and the visible region (430-450 nm), respectively. The stochiometry of the complexes was determined as 1:l by the molar ratio method [12]. In the molecular EDA complexes [Eqn (l)], the new broad absorption bands are characteristic for a charge-transfer [Eqn (2)].

R,SnNCS + I 2 2 [R,SnNCS I21 [R,SnNCS I21 * [R,SnNCS+ I;].

(1) (2)

A rigorous determination of the formation constants, Kc (Table 1) based on the general quadratic LABUDDE [13] equation, was performed through unconstrained least squares fitting of observed and calculated absorbance with a non-linear optimization method using conjugate directions [ 141. Experimental conditions conforming to the reliability criteria discussed by PERSON [15] and others [13,16] are used, allowing a meaningful separation of Kc and ser. In a first approximation, the charge-transfer energy for weak complexes [17] is given by Eqn (3); (3) with Zn= first ionisation energy of R,SnNCS, E, = vertical electron affinity of I2 (TCNE) and rnA = mean distance between RSnNCS and I2 (TCNE) in the complex. In a series of R$nNCS with a common acceptor and constant e*/r DAterm, the lcr is inversely related to Zn. From this, one can conclude that Kc should be related to the donor strength (-1lZn) of the R,SnNCS and a linear relationship is expected between K, and rZcrfor the [R,SnNCS I21complexes. The observed relationship (Fig. 1) can be considered as a proof for the invariability of the e*/rnA term for these complexes. This means that steric effects are negligible. It is a first indication that the donor site is located on a terminal atom i.e. the S atom. The effect of the donor strength is also reflected in the increase of K, comparing R3SnI and R,SnNCS, in accordance with the ionization energy of the HOMO with the dominant I lone pair and S lone pair character, respectively (Table 2). Although the vertical electron affinity of I2 (1.6 eV) and TCNE (1.7 eV) are very

Charge-transfer

spectra of organometallic

Fig. 1 Kc vs A,,

complexes-VII

1099

for [R,SnNCS I21complexes.

similar and both are considered as covalent acceptors (EA= C,) [18,19] the K, data for the TCNE complexes are almost an order of magnitude lower compared to the I2 complexes (Table 1). This has to be ascribed to the different nature [20] of the interaction which is n - o for [R,SnX I21 and n - n for [R,SnX TCNE]. Another argument in the discussion about the preferential donor site is the dependency of Icr on the nature of the alkyl group. EDA interaction at the Sri--- bond is seriously influenced by the size of the alkyl group as evidenced for the R.,Sn [l, 191, whereas donation from a terminal atom as for the R3SnI [lo, 111 shows only a minor dependency of &. The lzcr data in Table 2 can be considered to reveal negligible steric influence of the alkyl group. So obviously the donor site in the R,SnNCS is located on the S atom. In combination with PES and pseudopotential ab initio results [7] donation most probably occurs from the S lone pairs. In such systems, the e2/rDA term can be considered as constant and a linear relation is observed between hvcr and Zn. Thus, in combination with the measured I,, value of Me,SnNCS [7], the Z, values of the R,SnNCS can be calculated from the hv, data. They are listed in Table 2. Both the I2 and TCNE complexes are seen to yield the same ID values which is a strong support for the validity of this approach, certainly when keeping in mind that TCNE complexes are very sensitive to steric effects [21]. The solvent effects on the Lcr and K, are straightforward for the [R,SnNCS I21 complexes. EDA interactions of the n - u type produce complexes which are polar and

Table 2. Spectral parameters of R$nX-I2

and -TCNE complexes

Donor

Acceptor TCNE

I2 UeV) Me$nNCS” Et,SnNCS nBu,SnNCS iPrsSnNCS MeSnI’ Et,SnI nBusSn1 iPr$nI

8.73 8.706 8.69 8.67’ 8.95 8.64 8.51 8.48

‘Solvent CH&lr. b Calculated values, see text. ’ Ref. [6], solvent CCh.

HOMO (dominant character) S lone S lone S lone S lone I lone I lone I lone I lone

pair pair pair pair pair pair pair pair

Wnm) 308.9 310.5 311.4 312 297 302 304 305

h a&eV) 4.04 4.01 4.00 3.98 4.17 4.10 4.08 4.06

k(nm)

k-r(eV)

435 440 442 444 3% 415 423 422

2.86 2.83 2.81 2.80 3.13 2.99 2.93 2.94

1100

P. VERBIESTet al. Table 3. Solvent effect on 1,

and K,”

Acceptor 12 CHCIa (E = 4.8) DOnOr

Mnm)

Me$nNCS

Et,SnNCS nBu&NCS iPr+nNCS

307.1 309.7 310.1 311.7

TCNE

CH>Cl> (8.9)

K,

&-I

K,

h 4.60 5.22 6.30

308.9 310.5 311.4 312.0

5.34 6.15 7.10 8.04

CICH>CH&I (10.4) &-I K, 309.0 310.6 311.8 312.6

7.10 8.66 10.16 10.24

CHCI,

CHzCIZ

CICH>CHZCI

&l

K,

In

K,

I.m

K,

434.8 439.4 441.7 444.2

* 1.59 1.51 h

428.2 436.7 438.0 440.0

b 1.53 2.05 h

429.4 436.7 438.8 439.6

h 0.60 1.20 h

a Formation constants in 1 mol-‘, obtained at 25 ?r 0.1 “C and 1 atm. b Too low a solubility.

which are likely to have a unique geometry [20]. These complexes can be stabilized to a greater extent by using solvents with a high dielectric constant. This results in larger Kc and A(Jr(Table 3). A different approach for handling solvent effects is to consider the acceptor (donor) strength of the solvent as measured by their acceptor (donor) number, A(D)N [21]. The solvent can be an effective competitor for the acceptor (donor), at least for the less favourable EDA orientations [22]. With an AN sequence CHCl,> CH,Cl,> ClCH2CH2Cl, the Kc and Icr have to increase in the same direction. For the corresponding TCNE complexes no meaningful solvent effects on Kc have been observed. Acknowledgements-We ance.

thank the Nationaal Fonds voor Wetenschappelijk

Onderzoek

for financial assist-

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