Dipole moment and helium(I) photoelectron spectroscopic studies of the conformation of di-2-pyridyl and diphenyl dichalcogenides R2X2(X = S, Se, or Te)

摘要

1981 281 Dipole Moment and Helium(i) Photoelectron Spectroscopic Studies of the Conformation of Di-2-pyridyl and Diphenyl Dichalcogenides R2X2 (X = S, Se, or Te) By Francesco P. Colonna and Giuseppe Distefano, Laboratorio dei Composti del Carbonio Contenenti Eteroatomi del CNR, Ozzano Emilia, Bologna, Italy Vinicio Galasso, lstituto di Chimich, Universiti di Trieste, 34127 Trieste, Italy Kurt J. Irgolic, Giuseppe C. Pappalardo,*t, and Leslie Pope, Department of Chemistry, Texas A amp; M University, College Station, Texas 77843, U.S.A. The dipole moments of di-2-pyridyl dichalcogenides in benzene solution at 25 and 45" and the He1 photoelectron spectra of di-2-pyridyl and diphenyl dichalcogenides R,X, (X = S, Se, or Te) were investigated to obtain inform- ation about the conformation of these molecules in solution and in the gas phase.The calculated C-X-X-C dihedral angles (100-1 1Oo) for the dipyridyl dichalcogenides, for which pCalc= pexp,are at least 10" larger than those for the diphenyl derivatives. These results indicate that the x-interactions between the X atoms and the pyridyl groups are more effective than between the X atoms and the phenyl rings, reducing, as a consequence, repul- sion between the lone electron pairs of the chalcogen (X) atoms and lowering the torsional barriers about the X-X bonds. The variation of the dihedral angles between the phenyl derivatives and the corresponding dipyridyl dichalcogenides as derived on the basis of photoelectron spectroscopic data, is in disagreement with the results from dipole moment data.This difference between the gas phase and solution results may be caused by solvent and temperature effects or by secondary interactions between the x and ~7orbitals. The dipole moment and photo- electron spectral data indicate that the diaryl dichalcogenides are conformationally flexible with a skew equilibrium conformation and very little interaction between the two aryl groups. THE conformation of the diphenyl dichalcogenides (C,H,),X, (X = S, Se, or Te), were studied earlier by dipole moment 1,2 and n.m.r. spectroscopic method^.^ In order to elucidate the dependence of the conform- ational characteristics of diaryl dichalcogenides on the aryl groups with special attention to the C-X-X-C torsion angles and the electronic structures of the di- chalcogenide groups, a study of the di-2-pyridyl di- chalcogenides, Py,X, (Py = 2-pyridyl, X = S, Se, or Te) has been undertaken.The conformations of di-2-pyridyl disulphide in solution 4-6 and in the solid state are known. Structural and conformational information about the analogous selenium and tellurium compounds is not available. In this study the electrical dipole moments of Py,Se, and Py,Te, were measured in benzene at 25 and 45" to elucidate the conformations of these molecules in solution. He1 Photoelectron spectroscopy is a useful tool to probe the conformational preferences of dichalcogenides in the gas phase. Several investigations of compounds of the general formula R-X-X-R (X = As, N, 0, P, S, or Se) *-11 established that the splitting of the first photo- electron band is determined by the dihedral angle between the lone electron pairs of the X atoms.The diphenyl dichalcogenides and di-2-p yridyl dichalcogenides were studied by He1 photoelectron spectroscopy. The differences between the conformations of the gaseous and dissolved molecules are briefly discussed. EXPERIMENTAL Diphenyl dichalcogenides and di-2-pyridyl disulphide were available from previous ~tuciies.l~~-~ Dz-Z-fi-yt4iyZ DiseZenzde.-Seleiiium (3.0g, 0.038 moll was suspended in ethylene glycol monomethyl ether (150 1111). Sodium boroliytlricle (1.0 g, 0.027 mol) was added to the suspension kept under nitrogen and chilled in an ice-water bath.12 After the initial reaction had subsided, the mixture was refluxed for 1.5 h.2-Bromopyridine (4.1 g, 0.026 mol) was then added. The mixture was refluxed (m.7 h) until t.1.c. showed that most of the bromopyridine had reacted. The solution was cooled to room temperature and then extracted with chloroform. The separated chloroform layer was washed with water. Evaporation of chloroform produced a residue which was dissolved in hot hexane. Ethanol was added until crystallization began. On cooling yellow crystals (2.8 g) separated and these were recrystal- lized from hot hexane. Yellow needle-like crystals of di-2-pyridyl diselenide (1.6 g, 40 based on bromopyridine) melted at 4'7.5-48.5" (lit.,13 47.5-48.0"). 2-Pyridylwzagnesiunz Bromide.-Magnesium (5.1 g, 0.21 mol) was added to a solution of ethyl bromide (1.2 g, 0.01 mol) in tetrahydrofuran (20 ml) kept under nitrogen.After the initial reaction had subsided, a solution of Z-bromo-pyridine (16.8 g, 0.10 mol) and ethyl bromide (5.0 g, 0.046 mol) in tetrahydrofuran (100 ml) was dropped on the magnesium turnings. After all the bromopyridine had been added, the mixture was refluxed for 4.5 h. Di-2-pyridyZ Dite1Zuride.-The ditelluride was prepared according to the procedure reported by Haller and Irg01ic.l~ The Grignard reagent described above was transferred into an Erlenmeyer flask. Tellurium (7.5 g, 0.059 mol) was added. The mixture was heated gently for 20 min and then cooled in an ice-water bath. The flask was then closed with a rubber stopper into which a glass tube provided with a stopcock and connected to a balloon containing 0.5 1 of oxygen was inserted.Oxygen was admitted to the flask every other minute by turning the stopcock. After all the oxygen had been consumed, the mixture was warmed to room temperature and stirred for 1.5 h. The dark red solution was poured into a large beaker kept in the hood to 4 PermaPzenl address : Istituto Dipartimentale di Chimica e Chimica Industriale, Universiti di Catania, Viale A. Doria 8, 95125 Catania, Italy. allow the solvent to evaporate. The residue was treated with a saturated, queous solution of arnrnonium chloride. The dipyridyl ditclluride was extracted with inethylene chloride (3 x 70 nil).The combined extracts were eva-porated to dryness. The residue was recrystallized from niethanol-water (12 : 2 v/v). The dark red crystals of di-2- pyridyl ditelluride (1.4g, 1 2yobased on Te) melted at 50.0--52.5'. This preparation of dipyridyl ditelluride is difficult to reproduce (Found: C, 29.45; H, 1.95. C,,H,N,Te, requires C, 29.2; H, 1.95). Dipole .Moment i2ileasztvements.-The electric dipole J.C.S. Perkin I1 t.L = + vy--x-2PPyPPy-XCOS 120")$ (2) pl, cr and a.17 The geometric parameter 8, the valency angle at the X atom, was taken from X-ray crystal structures of the diphenyl dichalcogenides Ph,X, (106" sin cc = pp,sin 12O0/pI (3) ~r)= 2 arcsin cos2(e -90)sin2+/2 +sin2(8-go)* (4) for X = Se; l8 99" for X = Te 19).The value of 2.20 D was used as the component group moment of pyridine, TABLE1 Polarization data * for and dipolc moments of di-2-pyridyl dichalcogenides, Py,X, (X = solution at 25 and 46" Cornpound t/T U P El0 *la P2m/cm3 Py2Se2 25 5.20 -0.582 2.2725 1.143 28 359.4 45 4.69 -0.629 2.2328 1.168 70 337.6 Py2Te2 25 3.45 -0.619 2.2723 1.142 66 330.6 45 3.05 -0.978 2.2325 1.168 49 268.3 Se,Te)t determined in benzene RD/Cm3 p/D 69.8 3.76 69.8 3.74 78.5 3.51 78.5 3.14 * For definition of the symbols see ref. 2. moments of Pv,Se, and Py,Te, were determined in benzene at 25 and 45" (amp;Q.Ol") using apparatus and techniques described earlier .I5 Experimental dipole moment values were calculated from the total solute polarization, obtained by extrapolation to infinite dilution (PZm)according to the Halverstadt-Kumler method ,I6 and from the molar re-fraction (RD,NaD line), which was considered to be equal to the electronic and atomic polarizations (Pe+ Pa).The dipole moment values ?(accuracy 40.02 11) and the polarization data are reported in Table 1.He1 Photoelectron Spectra.--The photoelectron spectra of diphenyl and di-2-pyridyl dichalcogenides were recorded 011 a Perkin-Elmer PS18 photoelectron spectrometer calibrated against the Xe and Ar lines. The reproducibility of the ionization energies (1,E.)was better than k0.05 eV. The samples were sublimed inside the reaction chamber, at the lowest temperatures at which sufficiently intense signals could be obtained (Ph2S, 60"; Ph,Se, 60"; Ph,Te, 110"; Py,S, 120"; I'y,Se, 120"; Py,Te, 130").The dipyridyl dichalcogenidc3s decomposed partially at these temperatures. ?'he bands caused by the decomposition products occurred at energies above 12 eV and did not influence the low energy bands on which the discussion is based. RESULTS AND DISCUSSION Dipole lWoments.-The experimental dipole moments (pexp)for Py2Se, and Py,Te, were analysed by the clas- sical vector addition method of component group mo- ments. Free rotation about the C-X bonds and a fixed skew conformation about the X-X bond characterized by a dihedral angle 4 was assumed (Figure 1). Free rotation implies that all possible rotational conform- ations about the C-X bonds are equally populated on the dimle moment time scale at 25 and 45".These assumn- tions are based on results of conformational studies Iof di-2-pyridyl disulphide "*3 The dihedral angles 4were obtained by matching calculated dipole moments, p,,~,, with experimental dipole moments, peXp. Relation (1) was employed to find pL,,lc. Equations (2)--(4) define pealc = (ap: + 2t.4 cos c( cos a)t t Data for Py2S2 are reported in ref. 4. p.~~.~~ bond moments, pry+ 1.25 D andThe C-X FPy-Te = 0.89 D,2were calculated earlier. The calculated dihedral angles + for which pealc == pexp 1FIGURE Geometrical parameters of di-(2-pyridyl) dichalcogenides are listed in Table 2, which also contains the corres- ponding angles for diphenyl dichalcogenides for com-parison purposes.These results indicate that the dihedral angles for the dipyridyl dichalcogenides are TABLE 2 Dihedral angles + for diary1 dichalcogenides Py2X2 and Ph,X, (X = S, Se, Te) in benzene at 25" as derived from dipole moment measurements C-X-X-C Dihedral angle 4 (")X Ph2X2 PY2x2 S 88.9. (-gob) * 100" (878) * Se 74.7 (97.1c) * 107 f Te 89.7 * (88.5d, * 1lOf * Dihedral angles determined bv X-rav structure analvsis. a Ref. 1. Rif. 21. Ref. 18. Kif. 19. CalculLted from data in ref. 4. f Present work. g Ref. 7. considerably larger than those found for the diphenyl derivatives. If one makes the assumption, justified by the values for the C-X-X bond angles, that the C-X-X-C c bonds are formed using? orbitals on the chalcogen atoms with their electron pairs in the ns orbitals stereo- 1981 283 chemically inactive, then the conformation with di-hedral angles of 98' between the two lone p electron pairs on the chalcogen atoms and the C-X groups would minimize lone pair-lone pair repulsion.The dihedral angles 4 in the pyridine derivatives (100-110") could be the result of more effective n-interaction between tlie lone electron pairs and the pyridine rings than possible in the diphenyl dichalcogenides resulting in reduced repulsion between the lone electron pairs1? The29536 dihedral angles for dipyridyl diclialcogenides increased in the sequence S Se Te pointing toward a decrease of lone electron pair repulsions with increasing X-X bond 8 12 16 8 12 16 lE/eV IE/eV 8 12 16 8 12 16 Ieuro;/ eV IE/eV N c4ol ww-mm'* Ph2Tez , m ,? =, I I-0 12 16 8 12 16 IE/eV I E/eV 2FIGURE He1 Photoelectron spectra of diphenyl and di-2-pyridyl dichalcogenides length and decreasing X-X bond strength.The tor- sional barriers about the X-X bonds are expected to decrease on going from tlie disulphick to the ditelluride. Although the dipole moments for Py,S, and Py,Se, are temperature-independent in the range 36---45", the dipole moment of dipyridyl ditelluride at 45"(3.14D) is smaller than the moment at 25" (3.51 D) (Table 1). This tem- perature dependence can be attributed to a low torsional barrier around the Te-Tc bond permitting the population of conformers having dihedral angles larger than the one observed at 25" to increase.Photoelectrovt (9.e.) Sjxctra.--Eight bands of formal n * In addition to the two ionizations from the nitrogen lone electron pair in Py,X, expected to occur between 9 and 10 eV, in both series some 0 ionizations are expected at ca. 9, 9.5, and 10 eV for the Te, Se, and S derivatives, respectively. origin are expected to be part of the low ionization energy (1.E.) region (13 eV) of the spectra of diphenyl and di-2-pyridyl dichalcogenides. Overlap of some of these bands and an intermingling of o-photoprocesses account for the apparent low resolution and complexity of the spectra* (Figure 2). This fact at present prevents an unambiguous assignment of the spectra of the di-2- pyridyl dichalcogenides, apart from the first partially split band.On the basis of qualitative steric and electronic arguments, the n-ionizations are anticipated to form four pairs of nearly degenerate components associated with possible combinations between the dichalcogen n-lone pairs and the x-ring orbitals of formal b, symmetry and the non-interacting x-ring orbitals of a2 symmetry. Partial assignment of the bands may be made on a cor- relative basis by reference to the spectra of methyl phenyl chalcogenide~,~~ anddimethyl dichalc~genides,~b pyridine.22 The assignments for the o-bands must be regarded as tentative (Table 3). The assignments summarized in Table 3 are in part supported by results of x-only semiempirical calculations.There exists a fairly good linear correlation (Y0.999; s.d. 0.08 eV) between the calculated SCF-YPP orbital energies Ej and the observed I.E. values of the PhXCH, compounds 9a equation (5). Because diphenyl di-chalcogenides are closely related to methyl phenyl 1.E.j = -1.010 6 Ej -1.543 4 (5) chalcogenides, equation (5) might also hold for the diphenyl derivatives. The theoretical 7 x-I.E. values for diphenyl dichalcogenides are reported in Table 3 to-gether with the observed I.E. values and the proposed assignments. Interesting features of the spectra of the diphenyl dichalcogenides are the modest splittings of the ~XX ionization bands (1)-(11) and (V)-(VI) (Table 3) and the similarity of their average values to the correspond- ing ionization energies in the PhXCH, conipo~nds.~~ Thus, very little interaction takes place between the two phenyl groups through the dichalcogen bridge.The splittings between the x-ring (asymmetric or non-inter- acting) MOs (111)-(IV) and the xxx (s) MOs (V)-(VI) are good indicators of conformational preferences in the gas phase. Other factors remaining constant, the intcr- action and thus the energy gap between two orbitals, depend crucially on their mutual orientation and reach a maximum in a coplanar arrangement. Because both the splittings of these 1.E.s A 0.80 (S),0.52 (Se), 0.28 eV (Te) and their absolute values arc very similar to those reported for the planar conformation of the PhXCH, systems9a A 0.85 (S), 0.60 (Se), 0.25 eV (Te), one can infer that the combination of two PhX moieties to give Ph2X, will cause only a trivial alteration in the inter- action between the chalcogen x-lone pair and n-ring -f The calculations were carried out using the Fabian para- metrization 23 with values of -1.8 and -0.3 eV for P(C-Te) and p(X-X), respectively.Experimentally determined structural parameters with the aryl groups in the C-X-X planes were used. 284 J.C.S. Perkin I1 orbitals and hence will not change the conformation of solvent and temperature effects (dipole moments 25"; each PhX fragment. p.e. spectra 60-130"). In addition, when deriving con- As a supplement of conformational information the formational information from p.e. data the possibility splittings of the 1.E.s (I) and (11) can be considered of secondary interactions between TC-and o-orbitals (Table 4).The near equality of A1.E. (1)-(11)for Ph,S, cannot be neglected. Accordingly, conformational and Py2S, in the gas phase can be regarded as evidence information based on p.e. data should be considered with TABLE3 Experimental and calculated ionization energies (eV) for diphenyl dichalcogenides Ph,X, x=s X = Se X == Te Band a Exp. Calc. Exp. Calc. Exp. Calc. Assignment b (1) 8.21 8.12 8.06 7.87 7.68 7.34 xxx(a)(W 8.45 8.37 8.54 8.23 8.08 7.80 8.8 } 9.14 CT ('re)9.14 9.14(111) 9.14 9.36 9.31 9.22 x-ring9.14 } 9.14(IV) (9.8) 9.53 (X) (1') 10.13 9.76 10.16 9.83 9.50 } XXX(S)10.42 } 9.97 9.57(VI) 11.4 11.14 10.22 0 10.50 11.7 10.96 d 12.1 d 11.80 I I12.29 12.20 , 12.15 12.7 12.5 12.2 x-ring 12.33 12.22 12.16 a The numbers refer to bands arising from x-photoprocesses.xxx(a) and xxK(s)denote orbitals of prevailing dichalcogen character corresponding to antibonding and bonding combinations with the ring orbitals, whereas x-ring denotes ring orbitals. e Broad band (see Figure 2) extending from the lower to the higher figure reported. for similar C-X-X-C dihedral angles as well as for caution. However, dipole moment analyses of the com- similar angles between the lone electron pairs on the two pounds in solution and p.e. data obtained in the gas phase chalcogen atoms. This close similarity of the dihedral agree that Ph,X, and Py,X, are conformationally angles is at variance with the results of dipole moment flexible molecules which assume a skew equilibrium con- measurements, which suggest a larger dihedral angle in formation.Very little information is transmitted by the Py,S, than in Ph,S,. The increase of A1.E. (1)-(11) X-X bridge between the two aromatic groups in a diary1 from Py,Se, to Ph,Se, is consistent with a decrease in the dichalcogenide molecule. TABLE4 Support of this investigation by a N.A.T.O. Research Grant Program is gratefully acknowledged. The synthetic part ofAverage values of the first two I.E.'s, =(I) -(11) and the investigation was supported by the Robert A. Welchtheir splittings AI.E.(I) -(11) (eV) for diphenyl and Foundation of Houston, Texas.cli-2-pyridyl dichalcogenides Compound -Ph2X2 PY2x2 0/589 Received, 22nd April, 19801 X l.E.(I) AI.E.(I) I.E.(I) A1.E.(I) REFERENCES -(11) -(11) -(111 -(11) G. C. Pappalardo, K. J. Irgolic and R. A. Grigsby, J,S 8.33 0.24 8.57 0.25 Se 8.30 0.48 8.35 0.26 Organometallic Chem., 1977, 133,311. G.C. Pappalardo, S. Gruttadauria, and K. J. Irgolic, J.sc 8.23 a 0.55 a 0.29 Organometallic Chem., 1975, 97,173.'re 7.88 0.44 7.85 M.Baldo, A. Forchioni, K. J. Irgolic, and G. C. Pappalardo, a Taken from ref. 9b. J. Amer. Chem. SOC., 1978, 100, 97. G. C. Pappalardo and G. Ronsisvalle, J.C.S. Perkirz 11,dihedral angles for these compounds in solution. For 1973, 701. the tellurium derivatives the p.e. data suggest that Py,- G. C. Pappalardo and S. Gruttadauris, Gazzetta, 1975, 195, Te, should have a dihedral angle closer to 90"than Ph,-427.G.C.Pappalardo and E. Tondello, Phosphorus and Sulfur,Te,. In solution the opposite trend was found. 1976, 1, 5. N. V. Raghavan and K. Seff, Acta Cryst., 1977, B33, 386.The conclusions concerning the dihedral angles in * M. F. Guimon, C. Guimon, and G. Pfister-Guillozo, Tetra-Ph,X, and Py,X, drawn from p.e. data obtained in the hedron Letters, 1975, 441,and references therein. gas phase and from dipole moment measurements on G. Tschmutova and 13. Bock, 2. Naturforsch., 1976, 31B, the dissolved dichalcogenides do not agree. The dif- (a).1611; (b) 1616. lo H. Bock and G. Wagner, Ang. Chem. Internat. Edn., 1972,ferences can be attributed to perturbation by the 11, 150; Ckem.Ber., 1974, 107,68. 1981 11 J. P.Snyder and L. Carlsen, J. Amer. Chem. SOC.,1977, 99, 2931. l2 D.L. Klayman and T. S. Griffin, J. Amer. Chem. SOC.,1973, 95, 197. 13 H. G. Mautner, S. H. Chu, and C. M. Lee, J. Org. Chem., 1962, 27,3671. 14 W. S. Haller and K. J. Irgolic, J. Organometallic Chem., 1972, 38,97. 15 G.C. Pappalarcio and S. Pistara, J. Chem. Eng. Data, 1972, 17,2. 16 I. I;. Halverstadt and W. D. Kumler, J. Amer. Chem. SOC., 1948, 64,2988. 1rsquo; G. C. Pappalardo and G. Ronsisvalle, J. Mol. Structure, 1973, 16, 167. 18 R.E.Marsh, Acta Cryst., 1952, 5, 458. lS G.Llabres, 0.Dideberg, and L. Dupont, Acta Cryst., 1972,B28,2433. A. L. McCleEan, lsquo; Tables of Experimental Dipole Moments,rsquo; Freeman, San Francisco, 1963. 21 J. D. Lee and M. W. Bryant, Acta Cryst. 1969, B25,2094; see also papers quoted in ref. 6. 22 E.Ileilbronner, V. Hornung, F. 1rsquo;. Pinkerton, and S. F. Thames, Helu. Chim. Ada, 1972, 55, 289. 23 J. Fabian, A. Mehlhorn, and R. Zahradnik, Theov. Chim. Ada, 1968, 12,247. 24 M. Klessinger and P. Rademaker, A$zgew. Chem. Internat. Edn., 1979, 18,826.