Organometallics 1995, 14, 1292-1296
1292
Theoretical Study of the Pyramidal Geometry around the Sulfur in the S-BoundMode of Coordination of Thiophene to the [Cp(C0)2Fe]+Fragment Luis Rinc6n,*tt Joice Terra,$ Diana' Guenzburger,$and Roberto A. Sanchez-Delgadot Centro de Quimica, Instituto Venezolano de Investigaciones Cientificas (NE),Apartado 21827, Caracas 1020-A, Venezuela, and Centro Brasileiro de Pesquisas Fisicas (CBPF), Rua Dr. Xavier Sigaud 150, 22290 Rio de Janeiro, Rio de Janeiro, Brazil Received September 6, 1994@ Density functional calculations have been carried out to study the pyramidal coordination of the sulfur in the thiophene complex [Cp(C0)2Fe(q1-T)I+ (Cp = cyclopentadienyl; T = thiophene). Total energy calculations showed the optimal value of the angle between the Fe-S bond and the thiophene plane to be 120". An analysis of the changes in the orbitals brought about by the angular variation reveals that the mechanism by which this process take places is the reduction of the antibonding interaction between the occupied Fe d, orbitals and the S JC canonical lone pair in free thiophene. The mechanism found is consistent with the idea of sp2 sp3 rehybridization of the S atom in thiophene. Calculations performed with and without inclusion of the S 3d basis orbitals show a similar mechanism for the pyramidal distortion.
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Introduction In the last few years the coordination chemistry of thiophenes has developed rapidly;' several modes of coordination to discrete metal centers or metal clusters have been identified or proposed. Recent advances in this field are centered in the study of models at the molecular level for the hydrodesulfurization (HDS) reaction, which is one of the most important industrial applications of transition metal catalysk2 Of the known thiophene coordination modes, the S or r1 (1) and the q5 (2) forms are most frequently suggested for initial
thiophene adsorption to catalyst surface^.^ Therefore, the understanding of the nature of the thiophene-metal bonding in modes 1 and 2 is crucial for the future development of this field. The nature of the bonding MC. CBPF. Abstract published in Advance ACS Abstracts, February 1, 1995. (1)(a) Angelici, R. J. ACC.Chem. Res. 1988,21,387. (b) Angelici, R. J. Coord. Chem. Reu. 1990,105,61.(c) Rauchfuss, T.B. Prog. Inorg. Chem. 1991,39,259. (d) Sbnchez-Delgado, R.A. J . Mol. Catal. 1994, 86,287. (2) (a) Prins, R.; de Beer, V. H.; Somorjai, G. A. Catal. Rev.-Sci. Eng. 1989,31,1. (b) Grange, P. Catal. Rev.-Sci. Eng. 1980,21,135. (c) Gates, B. C.; Katzer, J . R.; Scuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill: New York, 1979. (d) Mitchell, P. C. H. Catalysis; Kemball, C., Ed.; The Chemical Society: London, 1977;Vol. 1. (e) Schuman, S. C . ; Shalit, H. Catal. Rev. 1970,4,245. ( 3 ) (a) Wiegand, B. C.; Friend, C. M. Chem. Rev. 1992,92,491.(b) Sauer, N. N.; Markel, E. J.; Schrader, G. L.; Angelici, R. J. J . Catal. 1989,117,295. (c) Markel, E.J.; Schrader, G. L.; Sauer, N. N.; Angelici, R. J . Catal. 1989,116,11. +
@
and geometricalpreferences in the +coordination mode of thiophene is the central topic of this paper. In examples of the S-bonding mode which have been well-characterized by X-ray diffraction, a pyramidal geometry around the sulfur has been found.' The same geometry around the sulfur was found for the adsorption of thiophene on the surface of Pt[11114and on C U [ ~ O O I , ~ in thiaporphyrin complexes6 and S-alkylthiophenes.' This result has been interpreted as a tendency of sulfur in the S-bound thiophene complexes to move toward a pyramidal sp3 hybridization. In terms of molecular orbital theory, there is no clear understanding of the driving force that causes the sp2 sp3 rehybridization of sulfur in thiophene nor of the contribution of the 3d valence orbitals of S to this process. The aim of this paper was to elucidate the mechanisms leading to a pyramidal geometry in the S-bound coordination mode of thiophene to a metal center. To this purpose, we studied the well-known complex [CpFe(CO)2(+T)]+ (Cp = cyclopentadienyl,T = thiopheneXs The S-bound mode has been examined theoretically for the Mo(C0)s fragment using the CNDOI2 m e t h ~ dfor ,~ the complex using the scattered-wave method,1° and for [CpFe(CO)z(l;ll-Th)l+(Th = T, DBT) with the
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(4)(a) Stohr, J.; Gland, J.; Kollin, E. B.; Koestner, R. J.; Johnson, A. L.; Muetterties, E. L.; Sette, F. Phys. Reu. Lett. 1984,53,2161.(b) Lang, J. F.; Masel, R. I. Surf. Sci. 1986,183,44. (5)Sexton, B. A. Surf Sci. 1986,163,99. (6)(a) Latos-Grazynski, L.; Lisowski, L.; Olmstead, M. M.; Balch, A. L. J . Am. Chem. SOC.1987, 109,4428. (b) Latos-Grazynski, L.; Lisowski, L.; Olmstead, M. M.; Balch, A. L. Inorg. Chem. 1989,28, 1183.(c) Latos-Grazynski, L.; Lisowski, L.; Olmstead, M. M.; Balch, A. L. Inorg. Chem. 1989,28,4065.(d) Latos-Grazynski, L.; Lisowski, L.; Olmstead, M. M.; Balch, A. L. Inorg. Chem. 1989,28,3328. (7)Gillespie, R. J.; Murray-Rust, P.; Porter, A. E. A. J . Chem. Soc., Chem. Commun. 1978,83. ( 8 ) (a) Kuhn, N.; Schumman, H. J. Organomet. Chem. 1984,276, 55.(b) Goodrich, J. D.; Nickias, P. N.; Selegue, - J. P. Inorg. - Chem. 1987, 26,3424. (9)Ruette, F.; Valencia, N.; Sanchez-Delgado, R. A. J . Am. Chem. SOC.1989,11, 40. (10)Smit, T. S.; Johnson, K. H. Chem. Phys. Lett. 1993,212,525.
0276-7333/95/2314-1292$Q9.O0/0 0 1995 American Chemical Society
Organometallics, Vol. 14, No. 3, 1995 1293
Coordination of Thiophene to [Cp(CO)2Fe_7+ Fenske-Hall method,ll but a detailed study of the pyramidal coordination mechanism has not been presented previously.
is the LUMO low-lying acceptor metal-based orbital 19a’ ( 4 ~5%; , 4y,8%; 4 ~ , 2 %z2, ; 54%;Y Z , 3%; X’ - y2, 2%).
Computational Details Density functional calculations in the local density approximation12 were carried out by employing the discrete variational method (DVM) developed by Ellis and Painter.13 The local exchange-correlation potential of Hedin and Lundqvist was used.14 Symmetrized molecular orbitals were formed from numerical atomic orbitals, and the atomic configurations for atoms in the molecule (self-consistent within the framework of a Miilliken type population analysis15 ) were determined numerically.l6 Valence basis functions employed here include those for Fe (3s, 3p, 3d, 4s, and 4p), C and 0 (2s and 2p), H (Is), and S (3s and 3p). Calculations were also performed including the 3d function of S, as shall be discussed later. Core orbitals were kept frozen. Standard numerical integration methods were used. 1 7 9 1 8 The molecular charge density was fitted to a multipolar expansion.lg The method for calculating the total energy is described in detail in ref 20. The application of density functional theory to organometallic complexes has been reviewed recently.21The geometrical parameters for [CpFe(CO)2(q1-T)l+ were those given by Goodrich et aL8 for the dibenzothiophene analogue. These arameters are as follows: bond distances, Fe-S = 2.289 FeC(0) = 1.780 8,Fe-Cp(centroid) = 1.725 8,C-0 = 1.15 A; bond angles, S-Fe-C(0) = 94.3” (mean), S-Fe-Cp(centroid) = 121.3”,C(0)-Fe-C(0) = 94.8”, Fe-C-0 = 180”. Standard geometries were adopted for thiophene22and cyclopentadiene (bond distance C-C = 1.44 8,C-H = 1.09 8).
18a‘
3d,?
3
The well-known frontier orbitals of thiophene25 are shown in the right side of Figure 1. For the q1 coordination the crucial orbitals of thiophene are the high-energy occupied sulfur “lone pairs”: the n lone pair 2bl (S(p,), 45%) and the cr lone pair 6a1 (S(3s),25%; S(3p2),58%)separated by 0.801 eV. In free thiophene, these canonical “lone pairs” of S come from different symmetries due to the sp2 hybridization. There are many possible conformations for the complex [CpFe(CO)2(q1-T)]+corresponding to different values of the angle 0 in 4. 0
i,
Results and Discussion The electronic structure of the coordinatively unsaturated complex d6-CpM(C0)2 has been studied by Schilling et al. using the extended Huckel method;23 therefore, only a brief description is given here. The more appropiate axis choice for CpM(C0)z is one in which the z axis is along the future Fe-thiophene bond. The mirror plane of this complex lies in the yz plane. The [CpFe(C0)2]+fragment of C, point group symmetry has at low energy three occupied metal-based orbitals (see left side of Figure 1): 17a’ (z2, 3%;yz, 18%;x2 - y2, 52%), 18a’ (z2, 18%;yz, 58%;x2 - y2, 6%) and 12a” (xz, 65%; xy, 5%).24In the 3dyzbased orbital 18a’, the small contribution of the 3d22 orbital causes a dissymetry in the nodal n character as is shown in 3. At higher energy (11)Harris, S.Organometallics 1994,13,2628. (12)Parr, R. G.; Yang, W. Density Functional Theory of Atoms and Molecules, Oxford University Press: New York, 1989. (13)Ellis, D. E.; Painter, G. S. Phys. Rev. 1970,B2, 2887. (14)Hedin, L.; Lundqvist, B. I. J. Phys. 1971,C4, 1971. (15)Umrigar, C.; Ellis, D. E. Phys. Rev. 1980,B21, 852. (16)Terra, J.; Guenzburger, D. Phys. Rev. 1991,B44,8584. (17)Stroud, A. H. Approximate Calculation of Multiple Integrals; Prentice-Hall: Englewood Cliffs, NJ, 1971. (18)Ellis, D.E. Int. J. Quantum Chem. 1968,S2,35. (19)Delley, B.; Ellis, D. E. J. Chem. Phys. 1982,76,1949. (20)(a) Delley, B.; Ellis, D. E.; Freeman, A. J.; Baerends, E. J.;Post, D. Phys. Rev. 1983,B27,2132.(b) Guenzburger, D.; Ellis, D. E. Phys. Rev. 1992,B45,205. (21)(a) Ziegler, T. Chem. Rev. 1991,91,651. (b) Ziegler, T.Pure Appl. Chem. 1991,63,873. (22)Harshbarger, W.R.;Bauer, S. H. Acta Crystallogr. 1970,B26, 1010. (23)Schilling, B. E. R.; Hoffmann, R.; Lichtenberger, D. L. J. Am. Chem. Soc. 1979,101,585. (24)In order to describe the molecular orbitals eigenvectors, s, x , y , 2, xz, y z , x2 - y2,and z2 are used to denote the correspondingns, np, and ( n - 1)d atomic orbitals of sulfur or transition metal.
4
We have carried out calculations changing the angle 0 of 4 from 100 to 260”. The energy profile for this process is shown in Figure 2. The calculation of Figure 2a does not include the 3d orbitals in S. Figure 2a shows two minima, one deeper well of -3.399 eV (-76.89 kcal mol-l) at 120”and one small well of -0.289 eV (-6.66 kcal mol-l) at 240”. The reference energy was taken as the coplanar geometry. The first well corresponds to the case where thiophene is oriented toward the cyclopentadiene and the second one where it is oriented toward the carbonyls. Our equilibrium theoretical angle 8 agrees reasonably well with the Cp(C0)2Fe(q1-DBT)(DBT = dibenzothiophene) complex8 which presents an angle of 119.5”and with the rhenium complex Cp*(C0)2Re(+T)26which presents an angle of 140” toward the Cp*. A dynamic nuclear magnetic resonance study on the benzothiophene (BT) complex, [Cp(C0)2Fe(q1-BT)]+,shows a clear double well potential as in Figure 2, and the AG* was calculated to be 0.404 eV (9.30 kcal mol-l); the barrier for the phenylmethyl sulfide complex [CpFe(C0)2(SMePh)l+ was 0.539 eV (12.41 kcal mol-l).lC Probably our theoretical higher values of the energy barrier for the isomerization processes are due to not relaxing other geometrical parameters besides the angle 0 and to the use of a local exchange-correlation potential. These results cannot be compared with other theoretical studies, because to the best of our knowledge ours is the first theoretical investigation of the pyramidal coordination of S in the q l-thiophene complexes. In the following paragraphs (25)Zonneville, M. C.; Hoffiann, R.; Harris, S. Sur,f Sci. 1988,199, 320. (26)Chi, M.-G.; Angelici, R. J. Organometallics 1991,10,2436.
1294 Organometallics,Vol. 14,No. 3, 1995
- 13.c
- 14.C n
> 0) Y
> (3
E
w
z w
-15.0
Rincbn et al. bonding and antibonding combinations of 18a' in the metal fragment and the 2bl n lone pair in thiophene. The ocurrence of this destabilizing interaction in the coplanar conformation provides an important clue for understanding why the S coordination in the v1geometry is pyramidal. The different interactions of a' symmetry are a consequence of the S lone pair orientation in the coplanar structure. We shall refer to the metal-based orbital 19a' as d, and the 18a' as d,. Now, as the angle 8 in 4 changes from 180", the overlap integral of the d, metal orbital and the 0 lone pair of thiophene decreases, but in compensation the overlap integral with the n lone pair increases. Similar arguments are applicable to the d, metal orbital. Then it is expected that the four orbitals d, and d, in the metal fragment and the two lone pairs of thiophene will mix together along the distortion. This mixing complicates the analysis. A simplification may be made by considering linear combinations of thiophene lone pairs, one of 0 symmetry (a,, 5) and the other one of n symmetry (a,, 6). These combinations are apropiate for a larger overlap with d, and d,.
r
1
2bl
5
r
1
- 16.0
Figure 1. Orbital interaction diagram for [CpFe(C0)2]+ and thiophene in the coplanar orientation. Figure 2a is rationalized by analyzing the fragment molecular orbitals.27 Our discussion will begin with a brief examination of the orbitals at the coplanar geometry. As shown in Figure 1the coplanar geometry of the complex [CpFe(C0)2(q1-T)I+has at low energy six occupied orbitals: the HOMO 26a' (Fe(yz), 11%;Sb), 40%), 17a", 1Ga" (Fe(b), 2%; Fe(xz), 60%; Fe(xy), 7%), 25a' (Fe(z2),16%;Fe(yz), 30%; Fe(x2 - y 2 ) , 14%),24a' (Fe(yz), 31%; Fe(x2 y2), 38%) and 23a' (Fe(z2),10%; Fe(x2 - y2), 3%; S(s), 15%; S(z), 57%). At somewhat higher energy is the unoccupied metal-based dz2orbital 27a' (Fe(4s), 1%;Fe(4y), 2%; Fe(z2),60%; Fe(yz), 4%; Fe(x2 - y 2 ) ,2%; S(s), 14%; S(z), 8%). Due to a small overlap integral the orbitals 12a" in [CpFe(C0)21+and la2 in thiophene are mainly nonbonding. The 23a' and 27a' orbitals come from the bonding and antibonding interaction of the unoccupied 19a' metal orbital with the 6a1 0 lone pair in thiophene. The 23a' orbital represents the 0 donation from the thiophene to the LUMO orbital in the metal fragment. The occupied 25a' and 26a' levels are the (27) (a) Fujimoto, H.; Hoffmann, R. J . Phys. Chem. 1974, 78,1167. (b) Fujimoto, H.; Inagaki, S. J. Am. Chem. SOC.1977, 99, 7424. (c) Fujimoto, H.; Osamura, Y.; Minato, T. J. Am. Chem. SOC.1978, 100, 2954. (d) Kato, S.; Fujimoto, H.; Yamabe, S.; Fukui, K. J. Am. Chem. SOC.1974,96,2024. (e) Fujimoto, H.; Kato, S.;Yamabe, S.; Fukui, K. J . Chem. Phys. 1974,60,572.
On the basis of only the values of iland A' for 5 and 6, we can distinguish the following cases:
(a)il > A'. In this case, the changes in the overlap integral of a, and d, orbitals is large compared with the overlap of a, and d, for angles different from 180". This situation rehybridizes the a, orbital away from d, and therefore reduces its antibonding character. In this case an energy decrease is expected a t angles far from 180" as a consequence of a decrease in the antibonding 26a' interaction. (b)il = A'. In this case the situation is similar to the case of coplanar geometry. A smooth energy profile is expected. This is the case in which the hybridization of the S along the whole process is sp2. (c) il < A'. This is the antithesis of case a. The overlap integrals of a, and d, are large compared with a, and d,. In this case the bonding interaction between the metal LUMO d, and the thiophene 0 lone pair is reduced for 8 f 180", and the energy profile displays a minimum at 8 = NO", in order to preserve the 0 bond and reduce the n antibonding interaction. As expected from the preceding discussion, the larger variation of the energy is for the orbital 26a' of antibonding character. With the angle change from 180" this orbital drops in energy and presents two minima of similar depth, 0.576 and 0.589 eV at 120 and 240," respectively. Bond order calculations for the Fe-S bond shows that the pyramidal distortion increases the Fe-S bond order from 0.344 at 180" to 0.356 at 140" and 0.354
Coordination of Thiophene to [Cp(CO)$e]+
Organometallics, Vol. 14, No. 3, 1995 1295
I 2 7 0.10
z v
*
F
c w
7
100
140
180
220
260
nnnJ v.vv
100
Figure 2. Variation of the calculated total energy as a function of 6, defined in 5: (a) whihtout 3d orbitals on the S; (b) with 3d orbitals on the S. The reference energy was taken as the coplanar geometry (6 = 180"). at 220". In other words, the Fe-S bond is equally strong in both extremes of 6 in Figure 2. On the basis of changes of the orbital 26a' or in the Fe-S bond order the difference in energy of the two minima in Figure 2a cannot be explained. The asymmetry of Figure 2a may be caused by the dissymmetry of d, shown in 3. Figure 3 shows that the overlap integral of d, (18a') with the 2b1 thiophene orbital is larger for angles near 180" and larger for the right part of the curve than for the left part. In the case of A'. The rehybridization of S was analyzed by inspection of the S(3s)/S(3p)Miilliken-typepopulation ratio shown in Figure 5. In this Figure we observe that the S(3s)/ S(3p) ratio increases from 0.40 at 180" to 0.45 a t 120" and 0.45 at 260". Formally if the Fe-S bond is perfectly
140
180
220
260
e
0
Figure 3. Variation of the overlap integral between the 18a' orbital of the [CpFe(CO)zl+fragment and the two lone pair orbitals in thiophene as a function of 8, defined in 5. covalent, a rehybridization on S from sp2 to sp3 represents a theoretical change from 0.29 to 0.33 in the S(3s)/ S(3p) ratio. Our calculations show a strong variation from this hypothetical case but essentially the same tendency. This result is in accord with the hypothesis of a rehybridization in the thiophene molecule in the direction sp2 sp3. This description can be easily extended t o other compounds because in a good number of known metalyl-S-thiophene complexes the metal fragment presents an unoccupied d, orbital of low energy and an occupied d, orbital; in such cases all the orbital interactions occurring during the pyramidal distortion are similar to the one herein described, and thus we expect as a general rule the pyramidal geometry around the S. Our analysis indicates that coordinatively unsaturated 16electron fragments in which, by an appropriate ligand field, both d, and d, metal orbitals are unnocupied can coordinate thiophene in a coplanar geometry. In the latter case, a formal four-electron double metalthiophene bond is to be expected. While this work was in progress, Fenske-Hall calculations by Harris on CpFe(CO)z(Th)(Th = T, DBT) were published.ll A number of features of those calculations are qualitatively similar to our findings, notably the importance of the antibonding interaction of the metal d, orbital with the n S lone pair of the ligand as a driving force for the pyramidal distortion. However, in contrast with our results, Harris concluded that tipping of the DBT ligand did not indicate rehybridization of the sulfur orbitals. No mention of the energy differences for the coplanar and pyramidal geometries
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1296 Organometallics, Vol. 14,No. 3, 1995
Rinc6n et al. 0.45
0.6
0.5
0.44
0.4
.-C0
0 -43
+.
3
n
L L
c
0.3
8 0
e
0.42 0.2
0.41
0.1
0.0
260
0.40 1
U
Figure 4. (a) Variation of the contribution of the 6al orbital of thiophene in the 26a' orbital of the 11' complex as a function of 0. (b) Variation of the contribution of the 2bl orbital of thiophene in the 27a' molecular orbital of the 771 complex as a function of 8. was provided in that paper, and therefore no comparisons can be made with our data. Smit and Johnson,lousing a standard scattered-wave calculation with fractional occupation numbers and populating the orbitals by Fermi-Dirac distributions a t 673 K, found that for the CzU thiophene-R~S5~complex the orbital interaction between the Fermi level orbital 2b1 (thiophene n lone pair) and the metal fragment 7e orbitals is the dominant one at long distances. In this case, the d, metal orbital can interact in a bonding form with the thiophene n lone pair in a coplanar geometry ( 8 = 180"), in contrast with our organometallic Fe complex. Strongly pyramidal coordination was found by Calhorda et a1.28in a study of the coordination and reaction of ethylene sulfide and trimethylene sulfides with the fragments Mo(C015 and M o H ~ ~using the Extended Hiickel method. They found that the pyramidal geometry is related to the lowering of the weak four-electrondestabilizing interaction between the n-lone pair of sulfides and the d,(yz) orbital of the metal fragment. We have also investigated the role of the 3d basis orbitals on S in this process. The energy profile including 3d orbitals is presented in Figure 2b. This figure shows, with some numerical value variations, the same qualitative features of Figure 2a. A deeper minimum of -2.291 eV (-52.75 kcal mol-l) is obtained. With a detailed study of the Walsh diagram and population and (28)Calhorda, M. J.;Hoffmann, R.;Friend, C . M. J.Am. Chem. Soc. 1990, 112, 50.
0
140
180
220
260
e
n
Figure 5. Variation of the S(3s)/S(3p) population ratio as a function of 0, defined in 5. fragment orbital variations, we arrived a t the same conclusions as in the case in which S(3d) orbitals are excluded. Thus the inclusion of S 3d orbitals does not have any significant influence on the previous orbital analysis.
Conclusions Density functional calculations performed for the complex [Cp(C0)2Fe(y1-T)1+have provided insight into the pyramidal coordination of thiophene in the S-bound mode. Total energy calculations indicate a value of 120" for the dihedral angle between the Fe-S bond and the plane of thiophene; larger energies of stabilization are found as compared to the experimental values for the similar complexes [Cp(CO)zFe(yl-BT)I+and [Cp(COkFe(SMePh)l+. We have rationalized this process in terms of the reduction of the antibonding interaction between the occupied Fe d, orbital and the S n lone pair of thiophene. The nonsymmetrical form of the distortion process causes a preference of coordination with bending toward the cyclopentadienyl ligand. This preference is due in part to the dissymetry in the nodal x character of d,. Finally, the inclusion of S(3d)orbitals reveals the same features as the calculations without S(3d) orbitals.
Acknowledgment. We thank to the Brazilian National Supercomputer Center-CESUP UFRGS-for access to the Cray Y-MP2E. L.R. thanks the FUNDAYACUCHO (Venezuela) for making his travel to CBPF possible. The authors also thank Dr. D. E. Ellis (Northwestern University) and M. Boves ( M C ) for their interest and discussions. OM940698N
