690
Organometallics 1983, 2, 690-692
Me3M'MMe3+,which can be generated from their neutral diamagnetic precursors by a similar pr0~edure.l~Pertinent to the structure of the trimethyltin moiety in 11,the perpendicular component of the tin splitting in the ESR spectrum of the ditin species Me3SnSnMe3+was found to be 100 G, suggesting that the configuration about each tin center is nearly planar as in II.8 Similarly, we found the tin splitting in the heterobimetallic species Me3GeSnMe3+ to be of the same order of magnitude. The ESR parameters listed in Table I thus relate the tetraalkylmetal cations to the family of hexaalkyldimetal cations in a single consistent pattern.I5
Acknowledgment. We thank the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, and the National Science Foundation for financial support of the research carried out at Tennessee and Indiana, respectively, and Dr. K. Mochida for a sample of Me3SnGeMe3. Registry No. H3CPbMe3+,85080-92-8; H3CSnMe3+,8449488-2; (H3C)3CSnMe3+,85005-13-6; Me3SnSnMe3+,81419-26-3; Me3SnGeMe3+, 85005-14-7; Me3GeGeMe3+, 79644-92-1; Me3GeSiMe3+,85005-15-8;Me3SiSiMe3+,77958-47-5. (14)For the donor properties of these dimetallic systems, see: Szepes, L.; Korhyi, T.; N&ray-SzaM, G.; Modem, A.; Distefano, G. J: Organomet. Chem. 1981,217,35. (15)(a) For the related charge-transfer reactions see ref 4. (b) The sizeable configurational changes incurred during cation formation in these donor systems is no doubt related to the large reorganizational energies X observed during electron transfer. See: Klingler, R. J.; Kochi, J. K. J. Am. Chem. SOC. 1981,103,5839. (16)Wang, J. T.; Williams, F. J . Chem. SOC., Chem. Commun. 1981, 666. (17)Shida, T.; Kubodera, H.; Egawa, Y. Chem. Phys. Lett. 1981,79, 179.
Hydrocarbon-Hydrogen Interactlons wlth Metals. A Molecular Orbital Analysis of HFe,( CO),,( v2-CH) Catherine E. Housecrofl and Thomas P. Fehlner' Department of Chemistry, University of Notre Dame Notre Dame, Indiana 46556 Received November 3, 1982
Summary: The electronic structure of HFe,(C0),,(v2-CH) has been examined by using the Fenske-Hall quantum chemical approach with a fragment analysis in terms of the butterfly metal cluster HFe,(CO),,+ and the ligand CH-. The preference for the tilted (v2)orientation of the CHligand over a symmetric vertical orientation can be explained in terms of the unusual properties of the frontier orbitals of the butterfly fragment. The q2 orientation causes the CH bond to be weakened in the complex because of the mixing of an empty CH antibonding orbital with a filled metal cluster orbital.
While characterizing the electronic structure of HFe4(CO)12BH21using the Fenske-Hall quantum chemical approach,* we had cause to examine the isoelectronic compound HFe4(C0)&H, Ib.3 In doing so we observed (1)Wong, K.W.; Scheidt, W. R.; Fehlner, T. P. J . Am. Chem. SOC. 1982,104,1111.Fehlner, T. P.; Housecroft, C. E.; Scheidt, W. R.; Wong, K. S. Organometallics, in press.
0276-7333/83/2302-0690$01.50/0
:FelC013
0.c 0 .H
Figure 1. Generation of HFe4(C0)12(T2-CH) by the capping of an alkylidyne triiron complex, [HFe3(CO)&HI2-,with a Fe(C0):' fragment: X(ca1cd) = 1.82 A, X(measd) = 1.75 A.3 [HFe3(CO),CHI2-geometry was obtained from the known structure of H3Fe3(CO)SCCH3.8
the properties of a tetrametal "butterfly" fragment that facilitate the binding of a CH ligand in a tilted ($) geometry. Thus, not only does the nature of the ligand bonding generated by this multinuclear array of metal atoms provide a mechanism for B-H bond weakening,l but also it suggests one for CH as ell.^,^ Compounds containing transition-metal borane-hydrogen interactions are common;6however, those with metal hydrocarbon-hydrogen interactions are not. Ib has been proposed as a reasonable model for C-H bond activation on a metal surface.' A comparison of the bonding in the observed (tilted) structure with the hypothetical more symmetric (vertical) structure Ia reveals the orbital properties of a tetrametal "butterfly" fragment that permit the v2 binding of CH.
The primary expression of electronic structure is the geometrical relationship between the observed nuclear positions. A fragment analysis of Ib that is very revealing in this regard is shown in Figure 1. The observed geometry of Ib is quantitatively generated by capping a FezC face of a doubly deprotonated (k,-methylidyne)triiron nonacarbonyl complex8with a Fe(C0)32+fragment. The CH axis in the experimentalgeometry lies close to a C3axis of one metal triangle of the butterfly. Unless fortuitous, (2)Hall, M. B.; Fenske, R. F. Inorg. Chem. 1972,11,768.Hall, M. B. Ph.D. Thesis, University of Wisconsin, Madison, WI, 1971. Fenske, R. F. Pure Appl. Chem. 1971,27,61. (3)The synthesis and structural characterization of the compound HFe4(C0)&H has been reported in detail. Tachikawa, M.; Muetterties, E. L. J. Am. Chem. SOC. 1980,102,4541.Beno, M. A.;Williams, J. M.; Tachikawa, M.; Muetterties, E. L. Ibid. 1980,102,4542. Beno, M. A.; Williams, J. M.; Tachikawa, M.; Muetterties, E. L. Ibid. 1981,103,1485. (4)The C-H bond distance in Ia is significantly longer than the accepted value for hydrocarbons (1.19viz. 1.09 A).3 (5)M-H-C interactions in mononuclear complexes can lead to C-H bond weakening, but apparently to a lesser degree than is accomplished by binding to a multinuclear metal fragment: Goddard, R. J.; Hoffmann, R.; Jemmis, E. D. J . Am. Chem. SOC.1980,102,7667. A reported interaction between a 8-CH and a single metal center actually leads to C-H bond shortening: Dawoodi, Z.; Green, M. L. B.;Mtetwa, V. S. B.; Prout, K. J . Chem. SOC.,Chem. Commun. 1982,802. (6)Housecroft, C. E.;Fehlner, T. P. Adu. Organomet. Chem. 1982,21, 57. ( 7 ) Muetterties, E. L.; Rhodin, T. N.; Band, E.; Brucker, C. F.; Pretzer, W. R. Chem. Reu. 1979,79,91. Gavin, R.M., Jr.; Reutt, J.; Muetterties, E. L. Roc. Natl. Acad. Sci. U.S.A.1981,78, 3981. (8)Wong, K. W.; Haller, K. J.;Dutta, T. K.; Chipman, D. M.; Fehlner, T. P. Inorg. Chem. 1982,21,3197.
0 1983 American Chemical Society
Communications
Organometallics, Vol. 2, No. 5, 1983 691 15-33
75-3a
I
CHIverhcall
u la
HF~L(COI,;
Ib
CHltiltedl
Figure 2. Correlation of MO's of HFe4(C0)12+ and CH-. The center column shows a schematic representation of the frontier orbitals of the HFe4(C0)12+ fragment. The left-hand side of the figure shows how these fragment orbitals interact with those of CH- when the ligand is in a vertical orientation, Ia. (Note that although ~r, only interacts with 76, the interaction leads to two MOs in the complex Ia. In all there are six MOs of the complex with major metal ligand character.) The right-hand side of the figureshows additionalorbital interactionsthat are a consequence of tilting the ligand into geometry Ib. Orbital 76 is the HOMO of HFe4(C0)12. Orbitals 74,79, and 81 do not interact significantly with the CH- fragment. this geometrical analysis implies that the demonstrated affinity for C-R to be bound perpendicularly to a trimetal fragmentQis important in stabilizing Ib over Ia.'O The geometrical analysis reveals nothing concerning the nature of the Fe-H-C interaction (bonding, nonbonding, antibonding). Hence, molecular orbital (MO) calculations" on both Ia and Ib are compared below and provide a more detailed understanding of the preference for structure Ib. In contrast to the above geometrical analysis, the calculations are analyzed in terms of the HFe4(C0)12+and CH- fragments as this provides a straightforward method of exploring the differences between Ia and Ib. The orbitals of the CH- fragment are simple and may be found in standard texts; however, those of the HFe4(C0)12+ fragment are complex and are briefly described here. The (9) A large number of compounds of this type have been structurally characterized. See for example: Raithby, P. R. In 'Transition Metal Clusters"; Johnson, B. F. G., Ed.; John Wiley: New York, 1980; p 5. The fact that this system exists under metal fragment 'redistribution" conditions suggests considerable thermodynamic stability for the RCM3unit. Beurich, H.; Vahrenkamp, H. Angeur. Chem., Znt. Ed. Engl. 1981,93,128. (10)A recent report of the structure of HOs3(CO)&H demonstrates that preference of CH for a capping position on a trimetal fragment can be overridden. Shapley, J. R.; Cree-Uchiyama, M. E.; St. George, G. M.; Churchill, M. R.; Bueno, C.J. Am. Chem. SOC.1983,105, 140. (11)Calculationshave been carried out on a total of eight isoelectronic compounda; however, the results on Ia and Ib are sufficient to establish the qualitative points. The Fenske-Hall technique allows the solutions of the SCF problem in an atomic orbital basis set to be explicitly transformed into a basis set of the fragment orbitals.12 Not only does this simplify the development of a correlation between fragments and molecule but also allows the examination of Mulliken populations related to fragment-fragment bonding. The geometry of Ib was derived from that of HFe4(C0)12BH21 and the experimental structure? Calculations were carried out for structures with the CH carbon centered between the wing-tip irons as well as off center as found experimentally. The geometry of Ia was the same aa that of Ib except the CH hydrogen waa placed on the C2ads. The basis functions used have been described previ~usly.'~ There are no adjustable parameters in Fenske-Hall method. Results of extended Hiickel calculations on the same system may be found in ref 7. We have also completed extended Hiickel calculations on Ia and Ib with results that support the conclusions derived from the Fenske-Hall method. (12)See for example: Kostic, N. M.; Fenske, R. F. Organometallics 1982,1 , 974. (13)DeKock, R.L.;Wong, K. W.; Fehlner, T. P. Znorg. Chem. 1982, 21, 3203.
Ia
s -0
Ib
s>o
Figure 3. The three principal orbital interactions that lead to increased overlap, S, between the fragments when the CH- ligand is tilted with respect to the tetrairon fragment. The left-hand column emphasizes that these orbital combinationsare symmetry disallowed before tilting. set of eight frontier orbitals of this fragment are illustrated in Figure 2; however, only five of these (75,76,77,78, and 80) are of interest here since these are the principal orbitals involved in binding the CH- ligand (see the correlation diagram in Figure 2). Hence, only the changes in these interactions in going from Ia to Ib are examined. The major changes diagramatically illustrated in Figure 2 are described in the following. Complex Ia possesses six MO's containing the major fragment-fragment interactions. With respect to the Cz axis of the butterfly fragment, two have u symmetry, two have rxsymmetry, and two have r ysymmetry. In going to Ib the distinction between u and ry symmetry is lost and the four MO's having these symmetries in Ia are substantially altered in Ib. In terms of relative metalligand Mulliken overlap populations the u MO's of Ia become more bonding in going to Ib; i.e., tilting favors the Ib structure. In contrast the ryMO's of Ia experience a substantial loss of metal-ligand overlap population on going to Ib; i.e. tilting favors structure Ia. The balance between these two opposing interactions is a delicate however, the net fragment-fragment overlap populations do suggest a small preference for the observed structure Ib. In terms of qualitative understanding it is profitable to examine the fragment orbitals of HFe4(C0)12+ that experience significant perturbation on tilting the CH- ligand. As judged by changes in Mulliken populations between Ia and Ib, only orbitals 77 and 78 (Figure 2) qualify in this regard. Specifically the symmetry disallowed interaction (77-ry) in Ia becomes allowed in Ib (Figure 3) but takes place at the expense of the (78-ry) interaction. As shown in Figure 3, the ry orbital of CH interacts with one triangular array of iron atoms in Ib in the same fashion as the 2pr orbitals of CH interact with the cobalt atoms in CO~(CO)~CH Likewise .~~ the (78-a) interaction that is disallowed in Ia becomes important in Ib. Note that although tilting perturbs the (80-u) interaction, the symmetry of the fragment orbital suggests and the net overlaps confirm little change in the strength of the bonding interaction. Thus, the factor that favors the Ib structure is the strong CH u and r,, interactions with the iron atoms (14)In fact the total energy is only 3 kcal more negative for Ib vs. Ia in the extended Hiickel methods.
Organometallics 1983, 2, 692-693
692
lation of a 1,P-silaoxetane are reinterpreted as most of one of the butterfly's triangular wings. But these (plus the P, interaction which is unchanged by tilting) are exconsistent with six-membered-ring ketene acetal. actly the ones that account for the bonding of the CH fragment to the Co3(CO)9fragment in C O ~ ( C O ) ~ C H . ' ~ In the decade since we first proposed' that silenes (1) Thus, the geometrical arguments are confirmed in the reacted with carbonyl compounds to afford olefins and calculations. silanones (3) through the intermediacy of 2-silaoxetanes The calculations show that a consequence of tilting the (2) (eq l ) , many unsuccessful attempts2have been made CH ligand is the involvement of the high-lying CH 3a to prepare isolable 2-silaoxetanes to test this often utilized3 antibonding orbital in the interaction with the metal mechanistic rational. fragment (Figure 2). In going from Ia to Ib, a previously symmetry-disallowed interaction develops between the filled fragment MO 7516 and the empty 3u orbital (Figure 3). A net bonding interaction develops between the carbon SI 0 R2Si-0 RZSi=O t atom and the same iron triangle emphasized above as well II + II R*' R" as between the hydrogen atom and the remaining wing-tip 3 R' R" iron. This further stabilizes the tilted geometry. The 3a R' R' R" R" n orbital is empty both in the free CH- and in Ia. Since the 1 1 3a orbital is C-H antibonding, acceptance of electronic charge from the metal fragment will necessarily weaken R',C=CR;' (1) the CH bond on forming Ib. Hence the appearance of such an interaction in the HOMO of Ib provides a satisfying Thus, it was of considerable interest to read the recent mechanism for the known lengthening of the CH bond in report4 that cothermolysis of ethyl pentamethyldiIb.3 It is the availability of a special cluster fragment silanyldiazoacetate (4) and 2-norbornanone produced orbital inherent in the butterfly fragment that permits this isolable 2-silaoxetane 7 in 38% yield. The suggested phenomenon to OCCUI.'~ mechanism was addition of silene 5 to generate zwitterion In summary, the CH- ligand prefers to be bound per6 which closes to 7 (eq 2). pendicularly to a triangle of iron atoms contained within the HFe,(CO),,+ butterfly. In attaining this orientation, Me3St an otherwise disallowed interaction between an empty, CH antibonding orbital and a filled metal cluster orbital occurs. I Me2St-C-COaEt a Me2St-C-C02Et Consequently, the carbon is bound more strongly to the - N2 triiron triangle, the hydrogen is bound to the unused 4 wing-tip metal, and the carbon-hydrogen bonding decreases; i.e., the butterfly becomes the "rack" upon which % ., Me, the CH bond is stretched. Me2Si=C,
A
A
-
-
..
-
-
CO2Et
Acknowledgment, The support of the National Science Foundation under Grant No. CHE 81-09503 is gratefully acknowledged. We also thank the Notre Dame Computing Center for computing time and Professor Roger DeKock for his help with the transformations. Registry No. Ib, 74792-06-6; Fe, 7439-89-6. (15) (a) Evans, J. J . Chem. SOC.,Dalton Trans. 1980, 1980. (b) Chesky, P. T.; Hall, M. B. Inorg. Chem. 1981, 20, 4419. (c) Granozzi, G.; Tondello, E.; Ajo, D.; Casarin, M.; Aime, S.; Osella, D. Ibid. 1982,21, 1081. (d) Xiang, S.F.; Bakke, A. A.; Chen, H.-W.; Eyermann, C. J.; Hoskins, J. L.; Lee, T. H.; Seyferth, D.; Withers, H. P., Jr.; Jolly, W. L.Organometallics, 1982,1, 699. (e) DeKock, R. L.; Deshmukh, P.; Dutta, T . K.; Fehlner, T. P.; Housecroft, C. E.; Hwang, J. L . 4 . Organometallrcs, in press. (16) Note that like 77 orbital 75 has neither u nor T symmetry with respect to the C2 axis of the metal butterfly. (17) The calculations on Ib with the carbon of the CH slipped towards a wingtip iron, as found in the known structure: support these conclusions and reveal a greater net CH-cluster overlap population as well as greater population of the CH 3a orbital in the complex.
1,2-Silaoxetane. An Alternative View Thomas J. Barton' and Gregory P. Hussmann Department of Chemistry, Iowa State University Ames, Iowa 50011 Received January 10, 1983
Summary: The NMR and I R spectral data recently reported by Ando in support of their claim of the first iso0276-7333/83/2302-0692$01,50/0
O)i+
C02Et
-
5 6
Me3
SiMeJ
,Si
o F C O , E +
7
Two strange spectroscopic features were noted for 7: "an unusual chemical shift of 13C a t C3 (113.8 ppm) and an abnormal stretching vibration of the ester carbonyl (1560 ~m-')".~In addition, although not commented upon, was the unusual low-field chemical shift (58.9 ppm) of Cq. We would submit that these spectral values are not abnormal for the correct structure of adduct 7, namely, ketene acetal &-not an unexpected cyclization product from zwitterion 6. (1) Barton, T.J.;Kline, E. A.; Garvey, P. M., 3rd International Symposium on Organosilicon Chemistry, Madison, WI, 1972. (2) (a) Gusel'nikov, L.E.; Nametkin, N. S. Chem. Rev. 1979, 79, 529. (b) Barton, T. J. Pure Appl. Chem. 1980,52, 615. (3) (a) Ando, W.; Sekiguchi, A.; Migita, T. J. Am. Chem. SOC.1975, 97,7159. (b)Barton, T.J.; Kilgour, J. A. Ibid. 1976,98, 7231. (c) Ando, W.; Ikeno, M.; Sekiguchi,A. Ibid. 1977,99,6447. (d) Golino, C. M.; Bush, R. D.; Sommer, L. H. Ibid. 1975, 97, 7371. (e) Reference 2. (4) Ando, W.; Sekiguchi, A.; Sato, T. J. Am. Chem. SOC.1982, 104, 6830.
0 1983 American Chemical Society
