Molecular Complexes of Substituted Aryl DiphenylmethylSulphides with n-AcceptorsCharge Transfer Spectra and Ionization Potentials of the DonorsBY GUSTAVO REICHENBACH,” SERGIO SANTINI AND G. GAETANO ALOISIIstituto di Chimica Fisica, Universit; di Perugia, 06100 Perugia, ItalyReceived 29th June, 1976Charge transfer complexes of substituted aryl diphenylmethyl sulphides, X1C6H4(X2C6H4)CH-S-C6H4Y, with tetracyanoethylene, 2,3-dichloro-5,6-dicyano-p-benzoquinone and chloranilhave been studied spectrophotometrically. The energy of the charge transfer transition is influencedby the substituents in Y, but is not affected by the substituents in X. The ionization potentialscalculated from the energy of charge transfer transitions are in good accord with those measured byphotoelectron spectroscopy.The nature of the donor orbitals is also discussed.It is known that the ionization potentials (i.p.) of electron-donor molecules maybe correlated with the charge transfer transition energy (hv,,) of complexes of thesemolecules with electron acceptor~.l-~ The empirical relation utilized is of the type :where the coefficients a and b depend only on the acceptor. On the basis of chargetransfer (c.t.) theory, a non-linear relationship between ijct and i.p. is predicted.Nevertheless, for chemically similar donors having i.p. values in a restricted range, theexperimental data lie on a virtually linear part of the curve, justifying the applicationof eqn (l).lm3 Many authors have used equations such as (1) for several types ofc.t.complexes to calculate the i.p. values of donors from their transition energieswith various acceptor^.^ We also have recently applied eqn (1) to c.t. complexesbetween aromatic and heteroaromatic sulphur derivatives (donors) and the n-acceptorschloranil (CHL), tetracyanoethylene (TCNE) and 2,3-dichloro-5,6-dicyano-p-benzo-quinone (DDQ).”’ The i.p. values estimated in this way were in good agreementwith those available by photoelectron (p.e.) spectroscopy.This study is now extended to c.t. complexes formed by a series of aryl-alkylsulphides (donors) and TCNE, DDQ and CHL (acceptors). In a previous work itwas reported that these donors interact with iodine (a-acceptor) to give mixed(n, n) + o* complexes in which the donation centre is localized on the sulphur atom.8The aim of this work is to extend knowledge of the energy and nature of the valencedonor orbitals interacting with the n acceptors, using the i.p.values obtainedempirically.i.p. = a+bvCt (1)EXPERIMENTALThe c.t. absorption spectra were studied for complexes of TCNE (Fluka AG purumgrade, recrystallized from chorobenzene and sublimed), DDQ and CHL (Fluka puriss.grade) with a large number of substituted aryl-alkyl sulphide donors in dichloromethane(Erba RP, purified following Vogel,’ dried and distilled). All the donors were availablefrom previous work.’996 MOLECULAR COMPLEXES OF SULPHIDESMeasurements were performed with a double-beam Optica CF4-DR spectrophotoineterwith a thermostatted cell compartment.Samples were prepared immediately beforemeasurement, the donor and acceptor concentrations ranging from 0.1 to 0.5 rnol dm-3 and2 x to 5 x mol dm-3, respectively.The pe. spectra were obtained with a Perkin-Elmer PS 18 photoelectron spectrometer.Reproducibility was & 0.05 eV.RESULTS AND DISCUSSIONThe spectral data for c.t. complexes formed between aryl-alkyl sulphides and.n acceptors are reported in table 1. The complexes with the acceptors TCNE andDDQ show two bands, although that at higher energy is not well resolved in theTABLE 1 .-CHA4RGE TRANSFER ABSORPTION MAXIMA FOR COMPLEXES OF SUBSTITUTED ARYLDIPHENYLMETHYL SULPHIDES, X1CsH40(2C6H4)CHSCsH~Y, AND SIMILAR COMPOUNDS WITHTCNE, CHL and DDQ IN DICHLOROMETHANE AT 20°Cl/nmX1HHHHHH4-Me4-C14-Ph4-Me4-C14-0MeMeSPhPhCH2SPhPh3CSPhx2HHHHHHHHH4-Me4-Cl4-OMeYH4-OMe4-Me4-c13-C14-FHHHHHHacceptor TCNE555,385600,380583,390525,370515,390510,380550,388540,380540,383560,390535,410 sh558,380575,380565,385530,395CHL480490510440435425460445465450440 sh485515500450 shDDQ*615 (450)690 (450)660 (450)610 (450)590 (460)610 (460)610 (450)605 (450)610605 (470)605630623 (445)565597 (455)* The second band is not well resolved.DDQ complexes, because its maximum is partially superimposed on that of theacceptor.Double bands for the c.t. complexes of TCNE and DDQ with t h i o p h e n ~ , ~ ~thioanisoles lo and diphenyl sulphides have recently been reported.They havebeen interpreted as due to transitions from two orbitals of the donor to the sameorbital of the acceptor. An analogous interpretation applies in the present case,in particular, the band at higher energy (second band) may be due to interactionbetween the acceptor molecule (TCNE or DDQ) and a more internal orbital of thethiyl group (see below). The lack of substituent effect in Y, in X1 and X" for thistransition (see table 1) is in agreement with the fact that the inner orbital which has u2symmetry is little affected by the substituent~.~~* l 1 For the CHL complexes, thesecond band has not been detected because, as expected from CHL electron affinity,6it falls at -300 nm, a region containing intense donor and acceptor absorption.With regard to the first band, some comments are in order. In fig.1 the energy ofthe first c.t. transition (vet) is plotted against the substituent constants opI2 accordingto a Hammett-type relation. The result shows that for donors substituted in Y theenergy of the transition is sensitive to the effect of substituents, and Vet increases withincrease in substituent constant 6, ( r = 0.982). The substituents in X1 and X2 haveno influence on the energy of the first band. This is in agreement with the suggestionG. REICHENBACH, S. SANTINI AND G. G. ALOISI 97reported previously,' that the band at lower energy is due to interaction between theTCNE (or other n-acceptor) and the outer anti-bonding orbital of the donor derivedfrom the sulphur-phenyl interaction, with a centre localized mainly on the sulphuratom.- 2.4 - 0.2 0 0 .2 0.4OPcircles represent derivatives substituted in Y, filled circles derivatives substituted in X.FIG. ].-Plot of the energy of the first c.t. transition against the op for TCNE complexes. OpenIn some sulphides, such as bis-4-methoxy-phenylmethyl phenyl sulphide, there is,indeed, evidence for a lower energy shoulder in the second band. This suggests thatit is a composite band. As for other examples in the literat~re,~ we suggest that theK N E interacts simultaneously with both parts of the molecule, i.e., with bothor,w C H - and thiyl part, S-giving rise to two isomeric complexes; consequently the band at higher energy isoriginated by two c.t.bands which overlap. This suggestion, which is also supportedby K,, measurement of complexes between TCNE and diphenylmethyl phenylsulphide, diphenyl methane and thioanisole,' may also be extended to the complexeswith the two other n-acceptors (CHL and DDQ).Table 2 lists the ionization potentials of the sulphides as calculated from Vct oftheir complexes with the various acceptors using eqn (1) in which a and b are asreported in ref. ( 5 ) and used in previous work. For some of the donors, the firsti.p. values were also determined experimentally by p.e. spectroscopy, the resultsbeing shown in the last column of table 2. The data in the table show that there isgood agreement between the experimental and calculated first ionization potentials.The exception is compound XV ; and the reason for this disagreement will be treatedbelow.Nevertheless, as also concluded elsewhere for other donors, it may beI--98 MOLECULAR COMPLEXES OF SULPHIDESaErmed that the empirically calculated i.p. values of table 2 represent reliable valuesand may even be considered good if compared with the experimental ones. Thesecond i.p. values reported in table 2 for TCNE were calculated from the maximumvalues of the second c.t. band. The separation between the two ionization energyvalues is of the same order of magnitude as that determined experimentally for thediphenyl ~u1phides.l~ This indicates that the second c.t. band originates from theinteraction between TCNE and a second inner n orbital of the thiyl group havingappropriate symmetry.TABLE 2.-IONIZATION POTENTIALS OF ARYL DIPHENYLMETHYL SULPHIDES, X1 CsH4(X2CsH4)CHSC6H4Y, AND SIMILAR COMPOUNDS AS OBTAINED FROM TRANSITION ENERGIES OF c.t.COMPLEXES WITH TCNE, CHL AND DDQ, AND FROM p.e.SPECTRAi .p . /eVdonor X1 X2 Y acceptor TCNE CHL DDQ p.e.HHHHHH4-Me4-Cl4-Ph4-Me4-OMe4-C1MeSPhPhCHzSPhPh3CSPhHHHHHHHH€34-Me4-C14-OMeH4-OMe4-Me4-C13 -C14-FHHHHHH8.18, 9.50 8.19 8.23 8.127.97,9.55 8.06 7.968.04,9.44 8.00 8.06 8.098.35,9.67 8.48 8.25 8.268.41,9.44 8.52 8.318.40, 9.55 8.60 8.338.21,9.46 8.36 8.258.27, 9.55 8.44 8.258.27, 9.52 8.29 8.278.21, 9.44 8.40 8.25 7.998.29,9.23 sh 8.48 sh 8.278.17, 9.55 8.15 8.278.09, 9.55 7.97 8.17 8.078.13, 9.50 8.06 8.20 8.008.32, 9.39 8.40 sh 8.45 7.97Fig.2 shows a plot of the p.e. ionization potentials of several diary1 and alkyl-arylsulphides as a function of the transition energy, vet, of their c.t. complexes with TCNE.The data, some of which have been taken from the literature, were obtained in thesame experimental conditions. Excluding the derivative Ph,C-S-Ph, the linearcorrelation obtained is reasonably good (Y = 0.991) and confirms that the donorcomplexes with TCNE are similar in nature. The deviation of Ph,C-S-Ph maybe ascribed to steric hindrance between the three phenyl groups and the reactioncentre, which alters the geometry of the complex since the TCNE is forced to lie farfrom the reaction centre.The presence of steric factors as a cause of substratedeviation from the reported relationship has been suggested previously. l4The linear correlation obtained fits the equation :i.p. = 1.63 x vCt + 5.19,which is in very good agreement with that obtained for mono- and poly-cyclicsubstituted benzenes, utilized in this and previous works to calculate the i.p. valuesof several aromatic sulphur derivatives, i.e. : i.p. = 1.65 x vCt + 5.21. Thisagreement confirms the correctness of the i.p. data obtained using the latter equationand also allows some conclusions to be drawn regarding the nature of the c.t. complexesbetween aromatic sulphides and n acceptors. It has been mentioned that, for agiven acceptor, the correlation is the same for donors of the same type.It is knownthat the outer donor orbital in aromatic sulphides is a R: orbital originating fromanti-bonding interaction between the sulphur atom and the adjacent phenyl groupG . REICHENBACH, S . SANTINI AND G. G. ALOISI9608.5->+ 3.I 8.07.599---I ! I I15 17 19 21 231 0-3 x &/cm-lFIG. 2.-Plot of the p.e. ionization potentials of diary1 and alkyl-aryl sulphides againstthe transition energy of their c.t. complexes with TCNE. The roman numerals refer to donors ofSMetable 2. The other compounds are : 1, 4a * 2, 4a psMe * 3,4=g5JgJSMe . 4, 4a MeSCHZPh. 5, PhSPh. 6, thiophen.'Consequently, when TCNE interacts with this orbital, a complex is formed which islittle different from that expected for typical n donors such as benzene and its deriva-tives.A difference between the two types of complexes may consist in the fact that,whilst for benzene the TCNE gives rise to a complex delocalized over the whole/\--5;.\molecule (03 , for aromatic sulphides, as the charge density is largely con-centrated on the S-C bond, TCNE interacts mainly with this zone, giving rise tocomplexes of the typesulphur atom., with the donor centre shifted towards theThe authors thank Dr. G. Distefano for the p.e. measurements100 MOLECULAR COMPLEXES OF SULPHIDESR. Foster, Nature, 1959, 183, 1253.R. Foster, Organic Charge Transfer Complexes (Academic Press, London, 1969).R. S. Mulliken and W. B. Person, Molecular Complexes: A Lecture and Reprint Volunte(Wiley, New York, 1969).See for instance (a) H. Bock, G . Wagner and J. Kroner, Chem. Ber., 1972, 105, 3850 ; (b) G.Wagner and H. Bock, Chem. Ber., 1974, 107, 68 ; (c) H. Bock and G. Wagner, TetrahedronLetters, 1971, 3713 ; (d) R. Locht, R. Cahay, J. Momigny and L. D’or, BulZ. Acad. roy. Belg.,1972, 58, 821.G. G. Aloisi and S. Pignataro, J.C.S. Faraday I, 1973, 69, 534.G. G. Aloisi, S. Santini and S. Sorriso, J.C.S. Faraday Z, 1974,70, 1908. ’ G. G. Aloisi, S. Santini and G. Savelli, J.C.S. Favaday Z, 1975, 71, 2045. * S. Santini, G. Reichenbach, S. Sorriso and A. Ceccon, J.C.S. Perkin IZ, 1974, 1056.A. I. Vogel, A Textbook of Practical Organic Chemistry (Longmans, London, 3rd edn., 1957),p. 173.A. D. Baker, D. P. May and D. W. Turner, J. Chem. SOC. B, 1968, 22 ; D. C. Frost, F. G.Herring, A. Katrib, C. A, McDowell and R. A. N. McLean, J. Phys. Chenz., 1972,76, 1030.l2 H. H. J a E , Chem. Rev., 1953, 53, 222.l3 F. P. Colonna, G . Distefano, G. Reichenbach and S. Santini, Z. Naturforsch., 1975,30a, 1213.i4 E. M. Voigt and C. Reid, J, Amer. Clrem. SOC., 1964, 86, 3930.lo A. Zweig, Tetrahedron Letters, 1964, 89.(PAPER 6/1259