9213
J. Phys. Chem. 1992.96.9213-9215
Induced Rotatlonal Barrler In Benzocyclobutanechromlum Trlcarbonyl MingJyh Liang, San-Yen Chu,* Mmg-Chiu Ou, and Ming-Sung Tsai Department of Chemistry, National Tsing Hua University, Hsinchu. Taiwan, Republic of China (Received: March 31, 1992)
Rotational barriers in a series of centrally bound benzocyclobutanechromium tricarbonyl compounds have been calculated according to the extended Hackel method. The difference of bond populations (ABP) between the long and short C-C bonds in the distorted arene ring correlates well with the rotational barrier. ABP measures directly the decrease in the ability of arene to bond with chromium during rotation of Cr(CO)3;its magnitude depends on the extent of distortion of the arene and the strength of the antiaromatic effect from the ?r bond of annelated four-membered rings.
Introduction
The rotational barriers of organotransition-metal complexes with the general formula (CPJML,, in which a transition-metal fragment ML, is r bound to a cyclic polyene C,R,, have been investigated both experimentally and theoretically, especially Cr(CO)3 r bound to benzene and its derivatives.'-I0 Because of the highly symmetric nature of the g6-metal-benzene interaction, there is nearly free rotation about the C,axis and the lengths of the C-C bonds in the benzene ring are approximately equal.2'-'O Recently Siege1 and co-workers"-l2 however showed that the chromium tricarbonyl complex of angular-terphenylene (structure 2 in Figure 1) exhibits a particularly large barrier, of 8.3-9.4 kcal/mol. According to results from X-ray diffraction, the length of the average long C-C bond is 1.456 A, whereas that of the average short C-C bond is 1.374 A. The difference 0.082 A is about halfthe differencebetween the length of typical C-C double (1 -34 A) and single (1.54 A) bonds. The stable conformation corresponds to a pseudooctahedral structure (A) with the carbonyl groups bisecting the long bonds. The three short bonds with doublebond character are regarded as donors of three pairs of electrons. When the carbonyl groups bisect the short bonds, a less stable pseudotrigonalprismatic arrangement (B) occurs as a transition structure. 8
A
B
Cdculatioa, and Discussion Our main objective was to seek the basii of such a large barrier
to rotation in system 2 in relative to the nearly free rotation in 8. We made extended Hilckel calculations on (C6H&!r(cO),, the related model compounds.I3J4 The structures of the arenes in the chromium tricarbonyl complexes appear in Figure 1; the parameters are listed in Table I. All compounds 1-7 have the same distorted geometry of the central arene group as Siegel's compounds, 2. A and B are assumed to have local C3, symmetry. The two C-C bond lengths used are the average values for short bonds and long bonds from the X-ray diffraction. Although the values of the rotational barriers obtained from the present calculations are expected to be inaccurate,a comparison of the results of these compounds provides us some insight into the various contributions to the observed large rotational barrier of system 2. The rotational barriers range from 0.36 kcal/mol for the symmetric D6h benzene system (8) to 9.85 kcal/mol for 4 (Table 11). Although the distortion of the central arene ring is the same from 1 to 7, the rotational barriers vary greatly. The distorted D3h benzene 7 has a rotational barrier of 2.83 kcal/mol, much smaller than those of 1-6. The difference of length of the long bonds and short bonds is thus not the key parameter to control the rotational barrier. The bond population (BP) is expected to be a parameter more sensitive to the rotational barrier for the 0022-3654/92/2096-9213$03.00/0
TABLE I: Parameters of Extended Hikkel Cdculations orbital HJeV tl t, CI a C? -1 1.22 4.95 1.60 0.4876 0.7205 Cr 3d
Cr 4s Cr 4p
c 2s c 2P
0 2s 0 2P
-8.66 -5.24 -21.40 -1 1.40 -32.30 -14.80 -13.60
1.70 1.70 1.625 1.625 2.215 2.215 1.300
H 1s Contraction coefficients used in the double-f expansion. TABLE II: Rotntio~lBarrier (AE) and Difference of Bond Population (ABP) between Long Bonds and Short Bonds, for Compounds 1-8 (Figure 1)
complexed rings 1
2 3 4 5 6 7 8b
AE/(kcal/mol)
ABP
antiaromatic 7r bond no!
5.55 4.70 3.75 9.85 8.15 5.57 2.83 0.36
0.284 0.238 0.191 0.358 0.308 0.234 0.144
1.5 1 0.5 3 2 1 0 0
0.OOO
"Referring to the total r bond number from the annelated fourmembered ring. The contributiom from each arene ring is 0.5. The contribution of a double bond in the four-membered ring is 1.0. *Structure8 is arene with equal C-C lengths of 1.415 A, the average of short and long bonds. The ground state has carbonyl groups cclipsed to C C bonds but eclipsed to C atoms in the transition structure. following reason. In the transition structure Cr(CO)3is complexed octahedrally to the three long bonds (B); this conformation is much less stable than that in which Cr(CO)3 is complexed octahedrally to the three short bonds having double-bond character (A). Mainly the difference of bond population (ABP) between the long and the short bond hence determines the variable ability of arene to bond with Cr(C0)3 in the two conformations. This quantity is crucial for correlation with the rotational barrier. Furthermore, BP of a C-C bond of an arene depends not only on its length but also on the antiaromatic effect from the 7r bond in the annelated four-membered ring. According to Table 11, the values of ABP have a broad range for those model compounds. Figure 2 depicts a correlation between the rotational barriers AI3 and ABP. Two series of compounds with and without peripheral arene rings fall onto different lines. Each series has a variation of zero, one, two, and three annelated four-membered rings. BP of the long bonds in the central arene is effectively reduced by the antiaromaticeffect caused by the r bond in the annelated four-membered ring. We assign total T bond numbers 1.5, 1, and 0.5 from the four-membered rings for compounds 1,2, and 3, respectively, the last column in Table 11; each peripheral arene ring contributes 0.5 x bond to the annelated four-membered ring. These numbers also corralate 0 1992 American Chemical Society
9214 The Journal of Physical Chemistry, Vol. 96, No. 23, 1992
Liang et al.
TABLE Ilk Bond Popuhtioas (BP) of Long and Short Bondn and Their Merence (ABP) for Distorted Areaes 1-7
,
* . e
@ - -.
system 1
BP(short) 1.115 1.107 1.101 1.130 1.119 1.108 1.096
2 3 4 5
6 7
B
BP(1ong) 0.872 0.900 0.928 0.839 0.876 0.916 0.054
ABP 0.244 0.207 0.173 0.291 0.243 0.192 0.142
2
1
4
free ring
*.
A
BP(short) 1.102 1.089 1.079 1.127 1.107 1.089 1.07 1
3
5
6
Q 7
8
Figure 1. Arene derivatives complexed to Cr(CO),.
BP(1ong) 0.894 0.924 0.952 0.855 0.898 0.941 0.979
C
ABP 0.208 0.166 0.127 0.271 0.209 0.148 0.093
BP(short) 1.172 1.164 1.154 1.189 1.184 1.173 1.143
BPOong) 0.889 0.926 0.964 0.831 0.876 0.939 0.999
ABP 0.284 0.238 0.191 0.358 0.308 0.234 0.144 .- _
1. For comparison, the rotational barriers of distorted benzene 7 and symmetric benzene 8 are 2.83 and 0.36 kcal/mol, respectively. Table I11 provides insight into the rotational barrier. When Cr(CO), rotates to the transition structure B with the carbonyl groups bisecting the short C-C bonds, BP increases for the long C-C bonds. The interpretation for this effect is that Cr(CO), tends to coordinate to the three long C-C bonds octahedrally, thus enhancing their 7r bonding by a three-centered interaction. The importance of this increase of BP for the long C-C bonds and decrease for the short C-C bonds in the transition structure is demonstrated in the difference of the values of ABP for the two conformations, which we term AABP. The magnitude reflects the flexibility of T electrons to adjust to rotation of Cr(CO),. As expected, the order is 7 (0.049) > 3 (0.046) > 6 (0.044) > 2 (0.041) > 1 ((0.036) > 5 (0.034) > 4 (0.020); the corresponding M B P values appear in parentheses. This order is also consistent with an increasing antiaromatic effect indicated by the total T bond number in the annelated four-membered rings (in parentheses): 7 (0) < 3 (0.5) < 6 (1) < 2 (1) < 1 (1.5) < 5 (2) < 4 (3). Therefore, the larger the antiaromatic effect, the larger is ABP, and the smaller is AABP. According to entry 7 in Table I1 the distortion in geometry contributes ABP of 0.14. For compounds 1-6, the antiaromatic effect from each 7 bond in the annelated four-membered ring contributes about 0.07 to the ABP value. Therefore, compound 4 has the largest ABP, 0.358 (-0.14 + 3(0.07)) and the largest rotational bamer. For 2, ABP value is 0.238 (4.14 0.07) with a moderate rotational barrier; the antiaromatic effect is thus not fully utilized here to induce rotational barrier in 2 compared with 1, 4, and 5. Hoffmann and co-workers* studied a hypothetical benzene model with T BP deleted completely for long bonds; the rotational barrier reached 19.4 kcal/mol. We estimated the corresponding ABP for propene as a model system; the value 0.491 is a difference between the double bond and vinylmethyl single bond. Both ABP and hE in this case are much larger than the values of our model compounds. In conclusion, the value of ABP between the long and short bonds in the distorted arene correlates well with the values of the rotational barrier. This value represents a change of ability of arene to bond with Cr(CO), during rotation. ABP depends on both geometric distortion and the antiaromatic effect from annelated four-membered rings, but the latter is more effective to induce the rotational barrier. Acknowledgment. We thank the National Science Council, Taiwan, Republic of China, for support. We also thank Professor J. S.Siege1 for helpful comments.
+
0.00
0.10
0.20
0.30
0.40
AW-
Figure 2. Correlation between the rotational bamer ( M )and difference of bond populations (ABP) between long and short bonds. The numbers are labels of the compounds in Figure 1 .
with the increasing values of BP for the long bonds as the antiaromatic effect decreases; for example, the values 0,889.0.926, and 0.964for the free-ring systeap appear in Table 111. All values of BP reported in Tabks I1 and I11 are averaged for bonds of the two types. For 1-3, ABP values are 0.284,0.238, and 0.191 and the rotational barriers are 5.55, 4.70, and 3.75 kcal/mol, respectively. The rotational barrier of 2 is 4.70 kcal/mol, about half the experimentalresult of 8.3-9.4 kcal/mol. For the model compounds 4,s. and 6, ABP values are 0.358.0.308, and 0.234, and the rotational barriers are 9.85, 8.15, and 5.57 kcal/mol, respectively. The compounds exhibit a large antiaromatic effect with corresponding total antiaromatic 7 bond numbers 3, 2, and
References and Notes (1) Chinn, J. W., Jr.; Hall, M. B. J. Am. Chem. Soc. 1983, 105, 4930. (2) Albright, T. A,; Hoffmann, P.; Hoffmann, R. J. Am. Chem. Soc. 1977,
99, 7546.
(3) Albright, T. A. Acc. Chem. Res. 1982, 12, 149. (4) Rogers, R. D.; Atwood, J. L.; Albright, T. A.; Lee, W. A.; Rausch, M. D.Organometallics 1984, 3, 263. (5) Corradini, P.; Allegra, G. J. Am. Chem. Soc. 1959,81, 2271. (6) Muetterties, E. L.; Bleeke, J. R.; Wucherer, E. J.; Albright, T. A. Chem. Rev. 1982, 82, 499. (7) Bailey, M. F.; Dahl, L. F. Inorg. Chem. 1965, 4, 1314. (E) Cataliotti, R.; Poletti, A,; Santucci, A. J. Mol. Srrucr. 1970, 5, 215.
J. Phys. Chem. 1992,96,9215-9217 (9) Brunvoll, J.; Cyvin, S. J.; Schifer, L. J. J . Organomet. Chem. 1972, 36, 143. (10) SchBfer, L.; Begun, G. M.; Cyvin, S.J. Spectrochim. Acta, Part A 1972,28A, 803. (11) Nambu, M.; Hardcastle, K.; Baldridge, K. K.; Siegel, J. S.J . Am. Chem. Soc. 1992,114, 361.
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(12) (a) Nambu, M.; Siegel, J. S. J . Am. Chem. Soc. 1988,110,3675. (b) Kilway, K. V.; Siegel, J. S.J . Am. Chem. Soc. 1991,113,2332. (c) Kilway, K. V.; Siegel, J. S.Organometallics 1!392,11, 1426. (d) Nambu, M.;Mohler, D. L.; Baldridge, K. K.; Siegel, J. S. Submitted for publication. (13) Stanger, A. J. Am. Chem. Soc. 1992, 113, 8277. (14) Diercks, R.; Volhardt, K. P. C. J . Am. Chem. Soc. 1986,108, 3150.
Geometric Structure and Conformation of Bis(fiuorocarbonyl)disulfane, FC(0)S-SC(0)F Hans-Georg Mack," Carlos 0. Della Vedova,lb and Heinz Oberhammer*pla Institut fiir Physikalische und Theoretische Chemie, Universitdt Tiibingen. 7400 Tiibingen, Germany, and Facultad de Ciencias Exactas, Departamento de Quimica, Universidad Nacional de La Plata, 1900 La Plata, Argentina (Received: April 8, 1992)
The conformational composition and gas-phase structure of bis(fluorccarbonyl)dIfane, FC(0)S-SC(O)F, has been determined by matrix infrared spectroscopy,electron diffraction (GED), and ab initio calculations (HF/3-21G* and HF/6-31G*). The molecule possesses a skew structure, and both experimental methods result in a mixture of two conformers which differ by the orientation of the COF groups. The main conformer (86 (5)% from matrix infrared and 75 (15)% from GED) has both COF groups in the trans position (C-F trans to S-S),and the second form possesses a trans-cis structure. The experimental enthalpy differences AH = H(trans-cis) - H(trans-trans) are 1.5 (3) kcal/mol from matrix infrared and 1.1 ( 5 ) kcal/mol from GED. Ab initio calculations result in AE = 2.1 kcal/mol (HF/3-21G*) and 1.7 kcal/mol (HF/6-31GS). The following experimental geometric parameters (radistances in angstroms and La angles in degrees with 3u uncertainties) were obtained for the trans-trans conformer: C=O = 1.180 (3), C-F = 1.347 (4), S-C = 1.767 (4), S-C-0 = 130.4 (3), S-C-F = 105.9 (2), S-S-C = 100.5 (3), and S(CSSC) = 82.2 (19).
Introduction Geometric gas-phase structures of noncyclic disulfanes XSSX are characterizedby dihedral angles S(XSSX) close to 90': e.g., H2S2 90.6 (5)': F2S287.7 (4)',3 C12S285.2 (2)": (CH3)2S2,85.3 (37)015and (CF3)2S2104.4 (4O)Om6In this configuration the pshapcd lone pairs of the sulfur atoms are perpendicular to each other and their mutual repulsion is minimized Furthermore, such a structure is favored by the anomerk effect, Le., electron donation from the sulfur lone pairs into the empty u* orbitals of the opposite S-X bonds.' This latter effect depends strongly on the relative energies of the two orbitals involved and can explain the short SS and long S-F bonds in S2F2?Only disulfanes with very bulky substituents, such as (S(CSSC) = 128 (3)' *) have dihedral angles which are considerably larger than 90'. Regardless of the dihedral angle in bis(fluorocarbonyl)disulfane, FC(0)S-SC(O)F, three conformations are conceivable for this compound. Depending on the torsional position of the fluoro-
.
s-s
.s-s F-C'
F-C' \'O trans-trans
F
F)c=o
O C F;-
\'O trans-cis
o=c(
s-s
)c=o
F cis-cis
carbonyl group, trans-trans, trans-cis, and cis-cis structures can occuf (trans/& refers to the relative positions of the C-F and SS single bonds). Recently, infrared and Raman spectra for this disulfane have been r e p ~ r t e d .These ~ spectra indicate that in the gas phase the most stable conformer possesses a center of symmetry, Le., a planar structure (S(CSSC) = 180') of C, symmetry which has been assigned to the trans-trans form. The proposed planarity of this compound is explained by extended *-electron interactions involving the Mr-bonds and the sulfur lone pairs. The gas-phase and matrix infrared spectra suggest the presence of a small amount of a second non-centrosymmetricconformer for which the planar trans-cis structure is proposed as a possible candidate. In the present study we report the determination of the geometric structure of this disulfane by gas-phase electron diffraction (GED). The conformational composition is also derived from
matrix infrared spectra. These experimental studies are supplemented by ab initio calculations at the HF/3-21G* and HF/631G* level, using the GAUSSIAN 86 program package.1°
Experimental Section The sample of FC(0)S-SC(0)F was prepared by photochemical reaction of FC(0)SCl with CS2.11 The purity of the compound was checked by infrared, 19F-NMR, and "C-NMR spectroscopy and by gas chromatography. The N2-matrix infrared spectra (Figure 1) were measured with a Bruker IFS 66 spectrometer. The electron diffraction intensities were recorded with a Gasdiffractograph KD-G212 at two camera distances (25 and 50 cm)and with an accelerating voltage of ca.60 kV. The electron wavelength was calibrated with ZnO diffraction patterns. The sample reservoir was kept at 0 *C, and the nozzle was at room temperature. The camera pressure during the experiment was ca. lV5mbar. Two photographic plates for each camera distance were evaluated by the usual methods.13 Averaged molecular intensities in the s - ranges (s = (4r/A) sin(9/2); A, electron wavelength; 9, scattering angle) of 2-18 and 8-35 A-' in intervals of As = 0.2 A-' are presented in Figure 2.
Vibrational Analysis Figure 1A shows the N2-matrix infrared spectrum of FC(0)S-SC(0)F in the C - 0 stretching fundamental region. Four bands are observed at 1847, 1839, 1836, and 1815 cm-l. If the matrix is irradiated for 5 min with UV light of A < 300 nm, a drastic change occurs in this spectral region due to photolytic interconversion of the two conformers (Figure 1B). This irradiation leads also to decomposition of the disulfane in the matrix, but after 5 min the intensity ratio of the c-0 bands remains constant with further irradiation. Comparison of Figure 1A and 1B demonstrates that the intensities of the bands at 1847 and 1836 cm-' decrease, whereas those of the bands at 1839 and 1815 cm-' increase. Thus, the former two bands are assigned to the more stable trans-trans conformer (thii assignment is confirmed by the GED analysis; see below); the latter two bands comspond to the less stable trans-cis structure. The above sequence of C-0 vibrations is reproduced correctly by ab initio calculations which predict the following scaled frequencies (HF/6-31GZ values with
0022-3654/92/2096-921 5$03.00/0 0 1992 American Chemical Society
