Jour nal of the Chinese Chem i cal So ci ety, 2000, 47, 149-153 149 The Bonding Structure of Quadricyclanylidene Derivatives Tahsin J. Chow* a ( P j s ), Chia-Fu Lee b ( õ¹åºö ) and Kuan-Jiuh Lin a ( ªL ¼e ) a Institute of Chemistry, Academia Sinica, Taipei, Taiwan, R.O.C. b Department of Chemistry, National Chung Cheng University, Chia-Yi, Taiwan, R.O.C. The crys tal struc tures of bisquadricyclanylidene 1, 7-quadricyclanylidenenorbornadienes 2 and 3 are solved by X-ray dif frac tion anal y ses. The bond lengths of the cyclopropyl moi eties of 1, 2 and 3 are com pared with sev eral other quadricyclanylidene de riv a tives, and the dif fer ences are an a lyzed by com pu ta tional mod els. The re sults showed that the vari a tion of bond lengths in this se ries of com pounds is re lated to the elec tronic na - ture of sub stitu ents. INTRODUCTION It has been pro posed by Hoffmann and Stohrer that bond lengths of a cyclopropane can be af fected by cer tain sub stitu - ents. 1,2 In the pres ence of a π-elec tron with draw ing substitu - ent, for ex am ple, the bond to the substituent is length - ened and the one is short ened (Fig. 1). The the ory has been ver i fied sub se quently by sev eral groups of in ves ti ga - tors. 3-8 Our pre vi ous stud ies on the crys tal struc tures of quadricyclanone (QC, 4) and 3-(cyclopentadienylidene)- quidricyclane ( 5) sup ported such a geo met ri cal ef fect. 9 The struc tures of quadricyclane ( 6) and de riv a tives are of spe cial in ter est for this anal y sis be cause of their sym met ri cal ge om e - tries. All the struc tures of 1, 2 and 4-6 be long to point group C 2v, in which the π-bond at C(3) ( 2-5 ) is of proper sym me try to mix with the Walsh orbitals of the two three-membered rings. The ri gid ity of struc tures also pro vides an ad van tage for re - duc ing conformational con sid er ations. In this work the struc - tures of 1, 2 and 3 pos sess an other in ter est ing fea ture, i.e. the cen tral dou ble bond acts as an ef fec tive re lay for trans mit ting or bital in ter ac tions be tween QC and norbornadiene (NBD) moi eties on two sides. 4,10 The cen tral dou ble bonds of 2 and 3 are known to be highly po lar ized as re vealed by a large dif fer - ence on 13 C NMR chem i cal shifts. 11,12 By pro vid ing crys tal data of 1-3, the elec tronic na ture of this mol e cule can be ex - am ined in more de tail. RESULTS AND DISCUSSION Crystal structures of 1-5 Com pounds 1, 2 and 3 are pre pared as de scribed in the lit er a ture. 13,14 The crys tals of 1 and 2 are both monoclinic, Fig. 1. Or bital scheme show ing the in ter ac tion be tween the π Walsh or bital of cyclopropyl group and the π* or bital of car bonyl. Elec tron flow from π to π* in duces length en ing of the bond and short en ing of the one. Ded i cated to the mem ory of the late Pro fes sor Ta-shue Chou ( P j ÓV )
150 J. Chin. Chem. Soc., Vol. 47, No. 1, 2000 Chow et al. Ta ble 1. Crys tal lo graphic and Re fine ment Data for Com pounds 1, 2 and 3 Com pound 1 2 3 Empirical for mula Crys tal sys tem Space group Cell di men sions a/å b/å c/å α/( ) β/( ) γ/( ) V/Å 3 Crys tal size/mm For mula weight Z F(000) D c/g cm -3 µ/mm -1 λ/å 2θ(max) Diffractometer Scan mode hkl mode No. unique reflns No. obs I o > 2 θ(i o) Trans mis sion fac tors Pa ram e ters R a R w Good ness of fit d D-map max i mum, e/å 3 D-map min i mum, e/å 3 C 7H 6 Monoclinic P21/c 6.9823(9) 8.3823(13) 7.9582(7) 103.512(9) 452.88(10) 0.25 0.25 0.30.12 4 192 1.322 0.07 44.9-7 < h < 7 0 < k <9 0 <l < 8 592 498 0.974-0.999 119 0.030 0.033 b 2.18 0.110-0.110 C 14 H 12 Monoclinic P21 8.263(1) 5.957(1) 10.116(1) 102.78(1) 485.60(11) 0.16 0.18 0.45 180.25 2 172 1.233 0.06 44.9-8 < h <8 0 <k <6 0 < 1 < 10 705 635 0.940-0.993 128 0.032 0.041 b 2.02 0.130-0.140 C 21 H 18 SO 2 Or thogo nal Pca21 16.964(3) 6.184(1) 15.936(3) 1671.8(5) 0.52 0.24 0.12 333.41 4 700 1.325 0.203 50-20 < h < 0-7 < k <7-18 < 1 < 0 2796 1535 0.883-0.954 218 0.0342 0.0922 c 1.037 0.285-0.176 a Σ F o - F c / Σ F o. b [ Σw( F o - F c ) 2 /( Σw( F o 2 )] 1/2. c [ Σ[w(F o 2 -F c 2 ) 2 ]/ Σ[w(F o 2 ) 2 ]] 1/2. d [ Σw( F o - F c ) 2 /(m-p] 1/2. whereas 3 is or thogo nal. The in ten sity data were col lected at am bi ent tem per a ture on a diffractometer us ing the scan method. A com plete list of re fine ment data is given in Ta ble 1. Car bon-carbon bond lengths are given in Ta bles 2-4, where the es ti mated stan dard de vi a tions (e.s.d.) are listed in pa ren the ses. The ORTEP draw ings of 1-3 are shown in Figs. 2-4, re spec tively. Ta ble 3. Car bon-carbon Bond Lengths for Com pounds 2 Ta ble 2. Car bon-carbon Bond Lengths for Com pound 1 C(1)-C(5) C(2)-C(7) C(3)-C(3 ) 1.519(3) 1.532(3) 1.493(3) 1.478(3) 1.515(3) 1.336(3) C(4)-C(6) C(6)-C(7) 1.475(3) 1.516(3) 1.518(3) 1.492(3) 1.530(3) C(1)-C(6) C(4)-C(7) C(1 )-C(2 ) C(1 )-C(5 ) 1.525(5) (6) 1.323(8) 1.527(6) 1.318(9) 1.513(6) (9) C(1 )-C(7 ) C(2 )-C(3 ) C(2 )-C(7 ) C(3 )-C(4 ) C(3 )-C(7) C(4 )-C(5 ) C(4 )-C(6 ) C(5 )-C(6 ) C(6 )-C(7 ) 1.487(5) 1.510(5) 1.471(5) 1.314(4) 1.507(5) 1.523(6) 1.521(6) 1.527(9)
Struc ture of Quadricyclanylidene J. Chin. Chem. Soc., Vol. 47, No. 1, 2000 151 Ta ble 4. Car bon-carbon Bond Lengths for Com pounds 3 C(1)-C(6) C(4)-C(7) C(9)-C(10) C(9)-C(14) C(10)-C(11) C(11)-C(12) C(12)-C(15) 1.533(5) 1.547(5) 1.528(4) 1.335(5) 1.539(5) 1.521(6) 1.535(5) 1.315(7) 1.381(5) 1.386(5) 1.389(5) 1.373(5) 1.503(5) C(1 )-C(2 ) C(1 )-C(5 ) C(1 )-C(7 ) C(2 )-C(3 ) C(2 )-C(7 ) C(3 )-C(4 ) C(3 )-C(7) C(4 )-C(5 ) C(4 )-C(6 ) C(5 )-C(6 ) C(6 )-C(7 ) C(12)-C(13) C(13)-C(14) 1.528(5) 1.540(6) 1.506(6) 1.485(5) 1.520(5) 1.480(5) 1.318(5) (5) 1.533(6) 1.379(5) 1.367(5) tal bond (1.524 Å av er aged) is found to be a lit tle lon ger than the one (1.514 Å on av er age). Be cause all crys tal data of com pounds 1-5 are col lected at room tem per a ture, ther mal mo tions of the nu clei caused sub stan tial un cer tainty. The struc ture of 2 is then com pared with that of 3, where the two skel e tons are sim i lar to each other. The (1.516 Å av er - In our pre vi ous stud ies on the crys tal struc tures of 4 and 5, it was shown that bond lengths in the cyclopropane moi eties change sub stan tially ac cord ing to substituent at C(3). 6 As shown in Ta ble 5, the and bonds for 4 are 1.508(6) Å and 1.476(6) Å, re spec tively, and those for 5 are 1.55(5) Å and 1.44(5) Å. The car bonyl of 4 and the fulvenyl moi ety of 5 are gen er ally re garded as elec tron- withdrawing sub stitu ents so that elec tron den sity in the cyclopropyl Walsh orbitals may flow into their low-lying va cant π*-orbitals. Ab in itio cal cu la tions at three dif fer ent lev els (HF/STO-3G, HF/3-21G*, and HF/6-31G*) agree with the fact that the bonds are shorter than the ones for com pounds 1, 3, 4, and 5 (Ta ble 5). A re versed or der is ob served, how ever, for com pound 2, which is con trary to all pre dic tions, i.e., the dis - Fig. 3. ORTEP draw ing of com pound 2. Hy dro gens are Fig. 2. ORTEP draw ing of com pound 1. Hy dro gens are Fig. 4. ORTEP draw ing of com pound 3. Hy dro gens are
152 J. Chin. Chem. Soc., Vol. 47, No. 1, 2000 Chow et al. Ta ble 5. Com par i son of Se lected Bond Lengths (Å ) of Quadricyclanylidene Moi eties of 1-6 with Cal cu - lated Values. The Vic i nal and Dis tal Bonds Re fer to Fig. 1. Cal cu la tions Were Per formedo n SPARTAN 19 Soft ware In stalled in a RS 6000 Work sta tion mea sured PM3 STO-3G 3-21G* 6-31G* Cen tral C=C 1 2 3 4 5 6 (3) 1.493(3) 1.336(3) 1.525 1.502 1.508 1.514(6) 1.524(6) 1.314(4) 1.524 1.503 1.496 1.516(6) 1.318(5) 1.525 1.503 1.495 a Ref er ence 9. b Ref er ence 15. c Ref er ence 16. d Ref er ence 18. 1.508(6) a 1.476(6) a 1.520 a 1.525 a 1.51 3 1.500 1.527 1.512 1.487 1.55(5) a 1.44(5) a 1.33(2) a 1.525 a a 1.518 1.501 1.518 1.513 1.492 1.51(3) b 1.52(3) b 1.521 a 1.528 a 1.512 1.505 1.525 c c 1.505 d 1.500 d aged) and (1.523 Å av er aged) bond lengths of the lat - ter, how ever, are in a nor mal or der as pre dicted. A dif fer ence in 0.01 Å is well within the limit of ex per i men tal un cer tainty. It is worth men tion ing that the vari a tion of bond length does not seem to be re lated to ring strains be cause all bonds in the cyclopropyl moi eties of 6 are nearly equal (1.51 Å as shown in Ta ble 5). 15-18 Comparison among theoretical models It has been shown that semiempirical meth ods (PM3 and AM1) are not suit able for such type of anal y sis. 9 As shown in Table 5, es ti ma tions by PM3 for com pounds 1-6 sug gest about equal lengths for both and bonds. For in stance, bond lengths pre dicted for 1, 2 and 5 are all about the same, i.e. 1.525 Å. Ab in itio cal cu la tions based on HF/STO-3G are also in - sen si tive to struc tural dif fer ences, al though their pre dic tions for 1-6 con sis tently give a lon ger bond than. The im prove ment of 3-21G* ba sis set over STO-3G is mar ginal; how ever, it does be come more re spon sive to struc tural changes. 16,18 Pre dic tions by 3-21G* on 6, for ex am ple, are dis tinctly dif fer ent from those on 1-5 (Ta ble 5). At this level the es ti mated bond lengths tend to be lon ger than their real val ues. From these ob ser va tions, it may be re garded that both STO-3G and 3-21G* ba sis sets are still not of suf fi cient ac cu - racy for es ti ma tions on this se ries of com pounds. With a larger ba sis set, i.e., HF/6-31G*, the re sult is con - sid er ably better than in pre vi ous ones. 17 As shown in Ta ble 5, the es ti mated and bonds for 1, 4 and 6 agree rea - son ably well with the ex per i men tal val ues. The mea sured data for 5 was not of high ac cu racy; nev er the less the the o ret i - cal val ues fit well within the un cer tainty lim its. The cal cu - lated re sults for 2 and 3 are nearly iden ti cal in all cases, in di - cat ing the in flu ence in duced by the tosylate group of 3 is min i - mal. The crys tal data of 3 is pro vided as a good com par i son for re view ing the in verted or der of bond lengths in the crys tal data of 2. When the and bond lengths of 2 and 3 are av er aged, both bonds be come nearly equal in length. These val ues, how ever, still dif fer sig nif i cantly from those of cal cu lated ones. Orbital correlation in 2 and 3 Ac cord ing to Hoffmann s hy poth e sis, length en ing of bond may hap pen if an elec tron-donating substituent ap - pears. 1 The NBD moi ety of 2 and 3 may be re garded as an elec tron-donating group com par ing with the QC moi ety. The ir reg u lar bond lengths in 2 and 3 may be as cribed to an ab nor - mal flow of π-elec trons from NBD to QC. Such an ef fect of π-or bital po lar iza tion has not been taken into se ri ous con sid - er ation in HF cal cu la tions due to the large en ergy gap be tween π and π* orbitals. 10 There has been some ev i dence con cern ing the po lar iza - tion of the cen tral dou ble bond in mol e cules like 1 and 2. 11 The 13 C NMR sig nals of the olefinic car bons C(3) and C(7 ) of 2 ap pear at δ 110.3 and 160.4 ppm, com pared with value of 1 at δ 134.5. The sep a ra tion be tween the two ( = 50.1 ppm) is ex cep tion ally wide; this phe nom e non has been noted pre vi - ously by Knothe, et al. upon ex am in ing many re lated struc - tures. 11,12 A dif fer ence of ca. 50 ppm has been found con sis - tently be tween C(7) of 7-methylenenorbornadiene de riv a tives and C(3) of 3-methylenequadricyclane de riv a tives. The split - ting of sig nals in di cates that the π-elec tron den sity of the cen - tral dou ble bond of 2 is highly po lar ized. An up field shift of C(3) in di cates that the nu cleus is shielded by en hanced elec - tron den sity; whereas a downfield shift of C(7 ) in di cates it is deshielded. It tells that π-elec tron den sity in 2 is po lar ized
Struc ture of Quadricyclanylidene J. Chin. Chem. Soc., Vol. 47, No. 1, 2000 153 from the side of NBD to ward the side of QC. The rea son for the po lar ity of 2 may be ra tio nal ized by the sym me try na ture of mo lec u lar orbitals. The NBD moi ety of 2 is purely π-do nat ing be cause it can not with draw any π-elec trons from ei ther eth yl ene or QC to its va cant orbitals as re stricted by sym me try. 14 The sit u a tion is dif fer ent for QC, in which a low-lying empty MO with proper sym me try can with - draw π elec trons from other part of the mol e cule, i.e., both the eth yl ene and the NBD moi eties. CONCLUSION The crys tal struc tures of com pounds 1-6 were com pared in or der to ex am ine the substituent ef fect on the bond lengths of quadricyclane moi eties. The length en ing of bonds and short en ing of ones was ev i denced on the struc tures of 3-6. Ab in itio cal cu la tions with HF/6-31G* ba sis set gave a rea son able es ti ma tion on the struc tures of 1, 4, 5 and 6. How - ever, an un ex pected elon ga tion of bonds of 2 and 3 was ob served, which may be as cribed to an en hanced elec tron den - sity in the quadricyclanyl moi eties. Ac cord ing to 13 C chem i - cal shifts, the cen tral dou ble bonds of 2 and 3 are po lar ized so that π-elec trons flow from the side of NBD to ward that of QC. How ever, ow ing to the lim i ta tion of X-ray dif frac tion data, as well as an in her ent de fi ciency on HF cal cu la tions, some am bi - gu ities can not be com pletely clar i fied at this stage. ACKNOWLEDGMENT This work was sup ported by the Na tional Sci ence Coun - cil of the Re pub lic of China. Re ceived Sep tem ber 15, 1999. Key Words Quadricyclane; Norbornadiene; Cyclopropyl ketone; Bond length elon ga tion; Dou ble bond po - lar iza tion. REFERENCES 1. Hoffmann, R. Tet ra he dron Lett. 1970, 17. 2. Hoffmann, R.; Stohrer, W.-D. J. Am. Chem. Soc. 1971, 93, 6941. 3. Durmaz, S.; Kollmar, H. J. Am. Chem. Soc. 1980, 102, 6942. 4. Mar tin, H.-D.; Heller, C.; Haselbach, E.; Lanyjova, Z. Hel. Chim. Acta 1974, 57, 465. 5. Paquette, L. A.; Volz, W. E. J. Am. Chem. Soc. 1976, 98, 2910. 6. Irngartinger, H.; John, R.; Rodewald, H.; Kiers, C. T.; Schenk, H. Acta. Crystallogr. 1986, C42, 847. 7. Wat son, W. H.; Tavanaiepour, I.; Marchand, A. P.; Dave, P. R. Acta. Crystallogr. 1987, C43, 1356. 8. Matijasic, I.; Ugozzoli, F.; Bocelli, G.; Vancik, H.; Sunko, D. E. J. Chem. Soc. Chem. Commun. 1986, 1134. 9. Lee, C.-F.; Lin, K.-J.; Wang, Y.-W.; Cheng, U.-L.; Chow, T. J. J. Chin. Chem. Soc. 1995, 42, 943. 10. Kovac, B.; Heilbronner, E.; Prinzbach, H.; Wiedmann, K. Hel. Chim. Acta 1979, 62, 2841. 11. Knothe, L.; Werp, J.; Babsch, H.; Prinzbach, H. Liebigs Ann. Chem. 1977, 709. 12. Knothe, L.; Prinzbach, H. Liebigs Ann. Chem. 1977, 687. 13. Sauter, H.; Horster, H.-G.; Prinzbach, H. Angew. Chem. 1973, 85, 1106. 14. Lee, C.-F.; Lin, K.-J.; Chow, T. J. Bull. Inst. Chem. Acad. Sin. 1996, 43, 17. 15. Mizuno, K.; Fukuyama, T.; Kuchitsu, K. Chem. Lett. 1972, 249. 16. Greenberg, A.; Chen, H.-T. E.; Lyu, P.-C. J. Mol. Str. (Theochem.) 1988, 163, 89. 17. Alsenoy, C. V.; Scars dale, J. N.; Schaefer, L. J. Comp. Chem. 1982, 3, 53. 18. Raghavachari, K.; Had don, R. C.; Roth, H. D. J. Am. Chem. Soc. 1983, 105, 3110. 19. Hehre, W. J.; Nel son, J. E.; Huang, W. W. A Guide to Graph i cal Models and Graph i cal Medeling in SPARTAN, Wavefunction, Inc., Irvine, CA (1997); SPARTAN ver sion 4.1 and PC SPARTAN Plus were dis trib uted by Wavefunction, Inc., Irvine, CA, USA.