Nov., 1963
ELECTRONIC SPECTRA OF BIS-(CYCLOPEXTADIENYL)-METAL COMPOUKDS
in this work. These values may be compared with the C-C bond length values 1.337 and 1.483 A.16for the nonaromatic molecule butadiene and the completely aromatic benzene system where the C-C distances are probably all equal to 1.397 A.17 (16) A. Almenningen, 0. Bastiansen, and N. Traetteberg, Acta Chem. Scand., 12, 1221 (1958). (17) (a) I. 12. Karle, J . C k e n . Phys., 20, 65 (1952); (b) A. Almenningen, 0. Bastiansen, and L. Ferrtholt, KQZ. Norske Videnskab. Selskabs Skrzfter (1958).
2477
Acknowledgments.-The authors wish to thank Professor L. S. Bartell for the use of his electron diffraction unit and Professor L. 0. Brockway for the use of his microphotometer. We also wish to thank the Indiana University Research Computing Center for use of their equipment, Mrs. Connie Williams for her help in the preparation of the manuscript, and Mr. A. J. Atkinson for help in reading traces.
ELECTRONIC SPECTRA OF SOME BIS- (CYCLOPENTADIENYL)-METAL COMPOUNDS BY JAMES C. W. CHIEX The Hercules Research Center, Wilmington, Delaware Received J u n e 36, 1963 The electronic spectra of (C&)zTiC12, ( CSH5)2TiBrzJ ( CjHj)ZTiIa, and (C,€€~),VCLare described. The absorptionn are assigned to electronic transitions based on a LCAO-NO treatment. The metal-ligand bondings involve two bonding molecular orbitals per ligand; they are strongest in the chloride and weakest in the iodide. Two new forbidden bands have been found in the spectrum of (CiH&Fe. The band positions agree with Yamazaki’s SCF calculations
Introduction The electronic structure of bis-(cyclopentadieny1)iron has been a subject of continuing interest. Earlier treatmeiits1,2showed that the metal and the ligands are bonded by dr-pa bonds. Linnett3 theorized that there are three equivalent bonding orbitals between iron and eaclh cyclopentadienyl ring. The molecular orbital energy levels were calculated approximately by Dunitz and 01-gel.~ Recently results of more sophisticated calculations have Csing slightly different sets of basic functions but similar methods of calculation, these authors arrived at molecular orbital energy levels which differ in both the absolute values and the order of stabilities. The purpose of this communication is to present the electronic spectra of (C,H&TiX,, where X = C1, Br, I, and t o report weak new absorption bands in the spectrum of (C6H&Fe. The absorption bands have been assigned to appropriate electronic transitions. The relative merits of the (C6H6)*Femolecular orbital calculations are briefly discussed. Experimental The compounds used in this work were prepared by Dr. W. P. Long of these Laboratories. Bis-(cyclopentadieny1)-titaniumand -vanadium dihalides were prepared by the method of Wilkinson and Birmingham .* (CgM&Fewas prepared according to the procedure of Kealy and P a u ~ o n . ~ Analyses of all compounds were correct with the exception of (C5H&Ti12, which appeared to be contaminated with (C6H&Ti(OH)I. The electronic spectra were obtained in the absence of oxygen. A Cary Model 14 spectrophotometer was used in this work. (1) W. Moffitt, J . Am. Chem. Soe., 76, 3386 (1954). (2) A. D. Liehr and C . J. Ballhausen. Acta Chem. Scand., 11, 207 (19.57). (3) J. W-.Linnett, Trans. Faraday Soc., 52, 904 (1956). (4) J. D. Dunitz and L. E. Orgel, J . Chem. Phyu., 28, 964 (3865). (5) M. Pamaaaki, ibid., 24, 1260 (1956). (6) E. XI. Shustorovich and M. E. Dyatkina. Doh?,. A k a d . A’auk SSSR, 128,1234 (1959);Z h . Strukt. Khin., 3,345 (1962). (7) J. P. Dah1 and C. J Ballhausen, M a t . Fye. M e d d . Dan. Via. Selsk., 33,
I(1961).
(8) G . Wilkirisori and J . M. Birminghain, J . Ani. Chem. Suc., 76, 4 2 8 1 [ 1R54).
(9) T. J. Kealy arid P. L. Pauson, Xature. 168, 1039 (1951).
Results and Discussion Electronic Spectra of Bis- (cyclopentadieny1)-titanium and -vanadium Dihalides-In their study of (C&&Fe, Kaplan, Rester, and Katz’O found absorption a t 32.6 kK. in carbon tetrachloride. This absorption is absent in hexane, ethanol, and methanol. Brand and Snedden11 showed that the 32.6-kK. absorption is obtained in all halogenated solvents, and they attributed this absorption to the dissociative charge-transfer process hu
(C6H&Fe
+ CCh I_ (C&&F”+
-I- C1-
+ CC13 (1)
The possibility of dissociative charge-transfer absorptioil of (CsH6)2TiC12 in methylene chloridewasinvestigated by determining it,s spectra in methylene chloride, tetrahydrofuran, and diethyl ether (Fig. 1). The first two werefound to be identical. Apparently, dissociative chargetransfer processes of the type mentioned are unimportant for (C6H&TiC12. A slight decrease in short wave length absorption intensities was observed in diethyl ether. I n addition, there exists a long wave length tail which gives a pinkish tint to the diethyl ether solution. The nature of the long wave length tail absorption in diethyl ether is not understood. Another point which concerned us is the possibility of dimerization. Long12 furnished spectroscopic evidence for complex formation between (C5H&TiC1, and aluminum chloride and alkylaluminum chloride. ChienI3 also postulated these complexes as the active species in the low pressure polymerization of ethylene. We have determined the molecular weight of (C6H6)2TiCI2in ethylene dichloride and found it to be 250, 252. Furthermore, the methylene chloride solutions of (C6H6),TiC12follow Beer’s law from to M. (10) L. Kaplan,
W-.L. Kester, and J. J. Kate, J . Am. C h e n . Soc., 74, 5531
( 1952).
i l l ) J. C. D. B r a n d a n d W. Snedden, Trans. Faraday Soc., 53,894 (19571. (12) R-. P. Long, .1.A m . Chem. Soc., 81, 5312 (1959). (13) J. C. W. Chien, i b d . 8 1 , 86 (1959).
JAMESC. W. CHIEN
2478
Vol. 67
1 10,000
€. 1I 000
€. 100 45
35
40
25
-30
20
15
Y ,kK.
Fig. 3.-Electronio
spectrum of ( C6H6)2TiIzin methylene chloride.
-
L/ I k K . Fig. 1.-Electronic spectrum of ( C6Hs)zTiCl2in solvents: , methylene chloride and tetrahydrofuran; - - - -, diethyl ether.
--
10,000
1 ,000
€. 100
10 45
40
35
1/ J
30
25
20
~ K -
Fig. C.-Electronic~spectrum of (C6Hs)zVCls in-methylene chloride.
assumed to have Czvsymmetry in the following qualitative molecular orbital discussions. The group orbitals of interest belonging to the two cyclopentadienyl groups may be represented by c(al)
I kK. spectrum of (C&)zTiBrz chloride.
(2)-’” [cA(a)
+ cB(a)I
c(bz) = (2)-’/* [cA(a) - c ~ ( a ) l
-Y
Fig. 2.-Electronic
=
c(ae) = c(bz’) in methylene
It is, therefore, established that (C5H5),TiC12is monomeric in these solvents. The electronic spectra of (C5H5)zTiC1~,(C,H5),TiBrz, (C6H5)2TiIz, and (C6H6),VCl2are shown in Fig. 1-4. Electronic Structure of Bis-(cyclopentadieny1)-titanium and -vanadium Dihalides.-These compounds are
=
(2) (3)
( 2 ) - ’ / * [cA(el) - cB(ei)l (4)
+
c(b1) = c(al’) = ( 2 ) - ’ / ’ [ c A ( e l ) c ~ ( e ~ )( l5 ) where c refers to cyclopentadienyl orbitals; the two rings are differentiated by subscripts A and B. The group orbital belonging to the two halogen atoms may be similarly represented by Cl(al)
= (2)-’12
[Clc(2pc)
+ Cl~(2pu)l
(6)
Cl(b1)
= (Z)-’”
[ClC(Zpu) - C l ~ ( 2 p a ) l
(7)
Nov., 1963 Cl(bz) =
(a)-'/' lClc(2pT) + C b ( 2 p ~ ) I
Cl(a2) = (2)-"2 lClC(2pT) - ClD(2pT) 1 Cl(al') = (2)-'" WClc(2pn') Cl(b1')
=
2479
ELECTRONIC SPECTRA OF BIS-(CYCLOPENTADIENYL)-M:ETAL COMPOUNDS
(?)-"* [Clc(2pn')
(8)
(9)
+ clD(2pT')I
(10)
- C~D(@T')]
(11)
The ligand orbitals are then formed by cornbiniiig group orbitals of the same symmetry. The 3d, 4s, and 4p orbitals of r,he metal are now introduced t o form the molecular orbitals of the compound. I n Fig. 5, lines connecting various orbits of appropriate symmetries are omitted to avoid cluttering the figure. The primes denote the orbitals and do not have symmetry significance. There are eight bonding orbitals. Of the six primarily nonbonding orbitals (ai*') is stabilized by combination with the metal (al) orbitals. (an*) and (bl*) are primarily the lone pair orbitals of the halogen (Qb; obz mal ' atoms. (bl"') and (bz*) are nearly degenerate in enANTI BOND INS Ob2 ergies; they are both bonding between one pair of the LONE PAIR METAL ATOMIC ORBITAL mal ?'-BONDING ligands but obherwise antibonding. The highest OCMOLECULAR ORBITAL 6-BoND1NG cupied molecular orbitals (az*) is antibonding among OF COMPOUND LIGAND ORBITAL all the ligands. With the exception of (&I*'),the other Fig. 5.-Schematic diagram of ligand orbitals involved in the starred molecular orbitals have no bonding characterismetal-ligand bondings. tic between the metal and the ligands. I n (C~HEJZVC~~ (al**) is also occupied. The compound is paramagnetic. The electron c,pin resonance spectra of (CsH5)zVClz and related compounds have been reported.14 I n the molecular orbital scheme above, the halogens and the cyclopentadienyls contributed toward the eight bonding molecular orbitals of the molecule. In contrast, the theory of Ballhausen and Dahlls called for three electronpair bonds to each of the cyclopentaa I** u' 30 Y dienyl ligands, and the other ligands were bonded to the metal by oiie electron-pair bonds. The present mheme is consistent with the chemistry of these compounds. !AI > Thus, (CsH6)TiClZ caii be readily prepared by the reacItion of (C5H&TiCI2and TiC14.l6 Band Assignments.-The dipolar vector in Czv sym20 & metry encompasses the &(ll), B z ( l ) ,and B l ( 1 ) reprev sentations. The possible traiisitioiis are
t
-
h
d
-
(core) (bl*1)2(bz*)2(a2Y')2 A, + (core) (bz*)1(bl*')z(a2*)1(al**)1 Az (12) -+ (core)(bz*)2(bl*'j2(a2*)1(bs"*)1 B1 (13)
-+ (core)(bz*)z(bl*')1(a2*)z(al**)1 B1 (14) ---j
(corej (b2*1)Z(bl*')1(a2*)2(b2**) 1 Az (15)
-+ (core) (bz*)1(bl*')2(az*)z(al**)' Bz (16)
--+ (core) (bz*)1(bl*')2(a2*)2(b2**)1 A1 (17)
Transitions 12 and 15 are symmetry forbidden, while the remainder axe allowed transitions. The longest wave length absorptions in ((>5&)2TiX2, band I, are found between 14.5 and 19.5 kK. They have extinction coefficients of about 200 [with the exception of ( C ~ H ~ ) Z T ~ These I Z ] . absorptions are attributed to the forbidden transition 12. The higher extinction coeficient in the case of (C5H&Ti12may be due to impurities or orbital mixings. As the (a,.**) (14) J. C. W. Chiein, J Am Chem Soc , 8 3 , 3767 (1961). (15) C. J. Ballhausen and J. P. Dahl, Acta Chem. Scand.. 15, 1333 (1961). (16) R. D.Gorsich, J. Am. ('hem. Soc., 80, 4744 (19583; C. C. Sioan and W. A. Barber, zbzd., 81, 1364 (1959).
0 1b,;lb2*
M .o. Fig. 6.-Relative
--
- -
1-
(C5 H g W C 12 (c5 H5)2Ti Br2 (Cg-H5)2TiCI2 (CgHghTi 12 energies of the molecular orbitals involved in the electronic transitions.
orbital is occupied in (C5H5)2VC12,band I is absent in this compound. Band I1 appears between 20 and 26 kK. in all four compounds; the extinction coefficients are 5000-4000. This band is assigned to transition 13. Band I11 is found between 27.5 and 31.5 kK. It is absent in (C~H&VCIZ. This is attributed to traiisi-. tions 14 and 16. The remaining band IV is assigned to transitions 15 and 17. From these transitions the relative energies of the molecular orbitals involved are
2480
JAMES C. W. CHIEX BANDASSIGNXENTS FOR
THE
TABLE I ELECTROXIC SPECTRUM OF ( CSH&Fe
Assignments 'AIg +. (
'Aig
--
core (alg'1 z( e e 13( &Ig* )
Intensity lEpg
(core)(alp')2(e~g*)3(ai,*)i lEgu
iA41g (c~re)(a~,')'(e~,*)~((al,*)~ 'A1, 'Alg ( c o r e ) ( a ~ ~ ' ) ~ ( e ~ , * )IEzu ~(e*~,~~)~ lAlg +. ( c ~ r e ) ( a ~ ~ ' ) ~ ( e ~ ~ * ) ~]El, (e*l,i)i
Symmetry and parity forbidden Symmetry and parity forbidden Symmetry forbidden Symmetry forbidden Allowed
1, 000
E. 100
10
45
40
35
-30
25
20
Vol. 67
15
,kK. Fig. 7.-Electronic spectrum of (C6H&Fe in solvents: --, ethanol-diethyl ether; - - - -, methylene chloride.
deduced and shown in Fig. 6. If the energies of the noiibonding molecular orbitals are assumed to be the same for the various halides, then the antibonding molecular orbitals decrease in energies in the series C1, Br, I. It is concluded that the metal-ligand bonding orbitals have decreasing stability in the order C1 > Br > I. Other experimeiital results support these band assignments. I n particular, when (C5H&VC12 is airoxidized, presumably to (CaH&VOCl, its electronic spectrum has, in addition to the absorption bands I1 and IT7, new absorptions a t 17.4 and 32.9 kK. corresponding to bands I and 111, respectively. The spectrum is not included here because of the uncertainties iu the composition and the concentration of the oxidation product. Electronic Spectrum of Bis-(cyclopentadieny1)-iron. -Fjgure 7 contains the previously unreported spectra of (C5H&Fe in two solvents. The weak absorption a t 0.56 kK. (E 0.04) is not shown. These spectra are compared with the spectrum in ethanol. The spectra in ethanol and in diethyl ether are identical. In the CH2C12,the absorption intensity a t 32 kK. is almost the
__-_-
Band position kK .
Obsd
0.56
Calod
0.65
16.5
16.1
23.0 31.0 38.0
21.7 31.3 37.0
same as those in nonhalogenated solvents. However, absorptions a t wave lengths shorter than 32 kK. become very intense. This observation is consistent with the dissociative charge transfer absorption postulated by Brand and Snedden.ll The shorter wave length observed here may be attributed to the less stable chloromethyl radical which is produced from methylene chloride. Let us consider the electronic transitions in (CbHE)*Fe by assuming the ground state electronic structure to be (alg)2 ( a d2(elg*)4(a1g')2(el,*)4(e2g*)* 'A1, where the orbital notation is that of Linnett3 and Shustorovich and Dyatkina.6 By drawing appropriate correspondence between the molecular orbitals in (C5H&TiX2 and in ( C L H S ) ~ Fand ~ , taking into account that the dipolar vectors in D5d point group have Az,(//) and El,(I) representations, we arrive a t the band assignments summarized in Table I. The calculated band position is that derived from the molecular orbital energy levels calculated by Y a m a ~ a k i . ~ Because of the close-lying positions of the elu* and the eZg*orbitals, those transitions involving the ezgJ' orbital are probably also accompanied by the other. The results are not appreciably altered. The results showed that band assignments in the electronic spectrum of (CsHs)zFecan be made which agree with the calculations of Yamazaki5 and which are qualitatively compatible with the results of Dunitz and Orgel.4 Both these treatments found that there is no appreciable admixing between the elu* orbitals of the ligands and the pel, orbitals of ??e and that the metalligand boiidings involve essentially two bonding molecular orbitals per ligand. Similarly, the metal-ligand boiidings in (C5H5)2;\IX2involved two boiiding molecular orbitals per ligand (uicle supra). A point of interest is the calculated charge density distribution in (C5H&Fe. Shustorovich and Dyatkinae found a positive charge of 0.7 on Fe and argued that electrophilic substitution reactions of (C5H&Fe supported the results. Dah1 and Ballhausen7 found a negative charge of 0.7 on Fe and cited nucleophilic substitution reactions for support. They attributed the differences to the use by them of Watson's more COIF tracted SCF iron atomic 0rbita1s.l~ We question the merits of these comparisons. For instance, benzene derivatives containing either electron-donating groups or electron-withdrawing groups (such as the nitro group) undergo electrophilic substitution reactions. The donor-acceptor properties of these groups can be deduced only by comparing the relative rates of reactions of the substituted and unsubstituted benzenes. For the case in point, the comparisoii would have to be between the ligand without the metal and the or(17) R. E. Watson, Phya. REV.,119, 1934 (1960).
FLUORESCENCE SPECTRUM OF ACRIDINE
Nov., 1963
ganometallic compound. Whereas this comparison is not possible when the ligand is cyclopentadienyl, it is possible when the ligand is benzene (such as dibeiizenechromium). The electronic spectra of other organometallic com-
ACRIDINE : A
Lon'
2481
pounds and their complexes with aluminum halides ndl be the subject of another communication. Acknowledgment.-The author is grateful to Drs. D. C. Lincoln and H. G. Tennent of these Laboratories for stimulating discussions.
TEMPERATURE IXT'ESTIGATION OF ITS ENIGMATIC SPECTRAL CHARACTERISTICS'" BY S. JULES LADNER AND ~ ~RALPHS. BECKER
Department of Chemistry, L-niversity of Houston, Houston, Texas Received M a y 11, 1963 The fluorescence spectrum of acridine, the K-heterocyclic analog of anthracene, is presented. The positions of the band maxima do not agree with those previously reported. Justification of the assignment of this emission as the intrinsic r + T * fluorescence of acridine is given. This shows that acridine does not exhibit a lowest lying singlet excited state of n, T * character. Evidence is presented which suggests the possibility that a three component complex, ethanol-acridine-solvent impurity, exists involving two different kinds of weak interactions. The complexes are dirjcussed with regard to their bearing on the problem of acridine's intrinsic fluorescence
Introduction The existence of an intrinsic fluorescence for acridine, the S-heterocyclic analog of anthracene, has always been a subject of some uacertainty. Bertrand2* reported fluorescence from both acridine and its N-hydrochloride salt. Later, HarrellZbpresented the fluorescence of acridine obtained from a solution in methylcyclohexane-ether, frozen a t 77 OK., presumably verifying the r,r* nature of the lowest singlet excited state of acridine. Xevertheless, it has been suggested that acridine may not exhibit an intrinsic fluorescence. 14ccording to Bowen and Sahu13acridine is nonfluorescent in most organic!liquids, and any fluorescencewhich is observed is due to water impurity. Recently, McGlynn and c o - w o r k e r ~in , ~ a~ ~study of photoconductivity, observed no fluorescence for crystalline acridine or for its solutions in benzene. They suggested that acridine may not possess a T,T* lowest singlet excited state but rather an n,7r" lowest singlet excited state. The fluorescence obtained by Harrellzb was attributed to the reversal of the n , r * and next lowest n-,r* singlet excited state by the presence of water as an impurity, Presumably, this wou1.d be similar to the situation in chlorophyll reported by Fer:nandez and Becker.6 The quantum yield of fluorescence of -lop3 for acridine, reported by Sangster and Irvine,' was also attributed to the presence of impurities. The present Atudy involves a careful investigation of the electronic spectral properties of acridine. This work consists primarily of low temperature (77 OK.) (1) (a) Partially supported b y a grant from the Department of Health, Education, and Welfare, No. 3133 BBC, held by Ralph 9. Beaker; (b) National Science Foun'iation Cooperative Graduate Fellow, June, 1960, t o June, 1963. P a r t of a thesis t o be submitted for partial fulfillment of the r~riuirenientsfor the P h . D . degree, University of Houston. (2) (a) D. Bertrand, Bull. S a c . Chim., 1 2 , 1019 (1946): (b) R. W. Harrell, Dissertation Abstr., 2.L, 2476 (1961). ( 3 ) E. J. B o a e n and J. Sahu, J . Chem. Soc., 3716 (1958); E. J. Bowen, N. J. Holder, and G. B. Woodger, J . Phus. Chem., 66, 2491 (1962). (4) S. P. McGlynn, J . Chem. Phys., 37, 1825 (1962). ( 6 ) M. Kleinerman, L. Azarraga, a n d S. P. RTcGlynn, "The Photoconductive a n d Emission Spectroscopic Properties of Organic R/Ioleoular Materials" i n "Luminescence of Organic and Inorganio Materials," ed. b y H. P. Kallman a n d G. M. Spruch, John Wiley a n d Sons. Ino., New York, N. Y . , 1962, p. 196. (6) J. Fernandez and R. S. Beaker, J. Chem. Phys., 3 1 , 467 (1959). (7) R. C. Sangster a n d J. W. Irvine, Jr., i b i d . , 24, 670 (1956).
electronic absorption and emission measurements on acridine, the acridine-ethanol hydrogen bonded systern, and the acridine hydrochloride salt. Special attention has been given to the elimination of water and other impurities from the solvents and chemicals employed in this work. Data will be presented which will establish the fact that acridine has an inherent fluorescence. This wid1 be ascertained by consideration of the new experimentd data coupled with correlative considerations among the different solvent systems noted in the previous paragraph. 41~0,comparisons of the results of our work with those of other investigations will be considered. In addition, evidence is presented for the formation of unusual complexes at low temperature. Though this phenomenon is interesting in its own regard, the precicie nature of the complex has not been established. Hovrever, the phenomenon has been studied to the extent of establishing the exiistence of these complexes and the circumstances under which they arise. Comment will be made concerning their nature and also their bearing on the confusion which has existed with regard to the acridine fluorescence. Experimental The emission and most of the absorption spectra were obtained a t liquid nitrogen temperature (77'K.) in either of two solvent systems which form clear glasses a t this temperature: (1) EPA or (2) EI.8 The isopentane (Phillips instrument grade) and tile ether (Baker's A.R. grade), for the majority of the work, were purified by fractional distillation over calcium hydride. In addition, the isopentane was chromatographed by paseing it through an activated silica gel column after distillation. Specially purified ether was prepared in the following manner. -42 5 7 , by volume middle-cut from the distillation described above was rcLdistilled over sodium shavings. From the second distillatiorl, only a 2 5 7 , by volume middle portion was taken and it was used immediately. The pure, absolute ethanol (U.S.1.) was used without further purification. The low temperature absorption spectra were recorded on a Baush and Lomb Spectronic 505 or on a Beckman Model DK-1 spectrophotometer. The technique employed the use of 5 Pyrex dewar with an unsilvered portion to admit the light beam. A Beckman Model DU was used to perform a Beer's law study of acridine. Emission spectra were photographed on Kodak 103a-B and ( 8 ) D R. Scott a n d J. B illison, J Phys. Chenz
, 66, 561 (1962).
