J. Am. Chem. SOC.1981, 103, 169-176
169
Low-Temperature Neutron Diffraction Studies of C-H-Metal Interactions in Two Tantalum-Neopentylidene Complexes: [Ta(CHCMe3)(PMe3)C13]2[ T = 110 K] and the First Alkylidene/Olefin Complex, Ta(q5-C5Me5)(CHCMe3)(q2-C2H4)(PMe3) [ T = 20 K] Arthur J. Schultz,*'* Richard K. Brown,l1YbJack M. Williams," and Richard R. Schrock*la Contribution from the Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439, Department of Chemistry, Brookhaven National Laboratory, Upton. New York 1 1 973, and Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 021 39. Received May 5, 1980
Abstract: Neutron diffraction studies of the two title compounds, in which precise hydrogen atom location was essential, have established that the neopentylidene ligands are highly distorted. In [Ta(CHCMe3)(PMe3)C1Jz,the Ta=C,-CB angle is 161.2 (l)', the Ta=C,-H, angle is 84.8 ( 2 ) O , the Ta=C, bond is short (1.898 (2) A), the C,-H, distance is long (1.131 the T a = C , - C B angle is 170.0 (3) A), and H, is only 2.119 (4) A from Ta. In Ta(qS-CSMes)(CHCMe3)(~2-CzH4)(PMe3), (2)O, the Ta=C,-H, angle is 78.1 (3)O, the C,-H, bond is also long (1.135 (5) A), the T a x , bond length short (1.946 (3) A), and H, is only 2.042 ( 5 ) A from Ta. We believe the neopentylidene ligand is distorted in each case primarily because the metal is attracting electron density from the C,-H, bond. The tantalum-ethylene bonding in the second molecule is best described as that of a tantallacyclopropane ring on the basis of the long ethylene C - C bond (1.477 (4) A) and the large angle (68.5') between the normals to the two CHI planes. We believe the neopentylidene/ethylene complex does not decompose via a tantallacyclobutane complex because of the large separation between the neopentylidene and ethylene ligands.
Introduction For some time we have known that 'JcH. and V C H , in electron-deficient ( 0. The results unambiguously confirm the structure, previously deduced by spectroscopictechniques, for the general class of isobacteriochlorins (and for siroheme, the prosthetic group of nitrite and sulfite reductases) as porphyrins in which two adjacent pyrrole rings are reduced. In addition, the data indicate that successive saturation of the pyrrole rings in porphyrins, chlorins, and isobacteriochlorinsexerts no major effect on the stereochemistry of the macrocycles and suggest that the geometric details of high spin iron siroheme will not deviate substantially from those of iron(II1) porphyrins.
Introduction
The many-faceted roles which porphyrin derivatives fulfill biochemically are controlled by their metals, axial ligands, protein environment, and the nature of the porphyrin itself. Respiration, electron transport, peroxide decomposition, and hydroxylations are mediated by macromolecules which contain iron porphyrins (I), Figure 1, as prosthetic groups.*-* Photosynthetic reactions, on the other hand, are photocatalyzed by free base and magnesium compounds evolved from dihydroporphyrins (11), pheophytins, and chlorophylls, in algae and green plants, and from tetrahydroporphyrins (111), bacteriopheophytins and -chlorophylls, in purple Biologically active isomers of I11 in which two (1) (a) Department of Energy and Environment. (b) Department of Chemistry. (c) Address correspondence to L.D.S. at the Exxon Chemical Co., P.O. Box 536, Linden, NJ 07036. (2) Chance, B.; Estabrook, R. W.; Yonetani, T. “Hemes and Hemoproteins”; Academic Press: New York, 1966. (3) Antonini, E.; Brunori, M. “Hemoglobin and Myoglobin in their Reactions with Ligands”; Elsevier, North-Holland Publishing Co.: New York. (4) Lemberg, R.; Barrett, J. “Cytochromes”,Academic Press: New York, 1973. (5)
Hayaishi, O., Ed. “Molecular Mechanisms of Oxygen Activation”, Academic Press: New York, 1974. (6) Adler, A. D., Ed. Ann. N.Y.Acad. Sci. 1973, 206. (7) Smith, K. M., Ed. “Porphyrins and Metalloporphyrins”, Elsevier:
Amsterdam, 1975. (8) Dolphin, D., Ed. “The Porphyrins”,Academic Press: New York, 1979; VOl. 7. (9) Vernon, L. D.; Seely,G. R., Eds.“The Chlorophylls”; Academic Press: New York, 1966. (10) Fajer, J.; Davis, M. S.,ref 8, 1979; Vol. 4, pp 197-256.
adjacent pyrrole rings are reduced have been proposed, and a number of compounds assigned structure IV have been reported.12-24 Isobacteriochlorins (IV) have recently elicited considerable (1 1) Clayton, R. K., Sistrom, W. R., Eds. “The Photosynthetic Bacteria”, Plenum Press: New York, 1978. (12) Dorough, G. D.; Miller, J. R. J. Am. Chem. SOC.1952, 74, 6 106-6109. (13) Eisner, U. J. Chem. SOC.1957, 3461-3469. (14) Seely, G. R.; Calvin, M. J. Chem. Phys. 1955,23, 1068-1078. Seely, G. R. J. Am. Chem. Soc. 1966,88,3417-3422. Seely, G. R.; Talmadge, K. Photochem. Phorobiol. 1964, 3, 195-206. (15) Bonnett, R.;Dale, 1. A,; Stephenson, G. F. J . Chem. SOC.1967, 1168-1 172. (16) Inhoffen, H. H.; Buchler, J. W.; Thomas, R. TerrahedronLetr. 1969, 1141-1 144. (17) Whitlock, H. W.; Hanauer, R.;Oester, M. Y.; Bower, B. K. J. Am. Chem. SOC.1969, 91, 7485-7489. (18) Fuhrhop, J. H.; Lumbatobing,T. Terrahedron Lett. 1970,2815-2818. (19) Whitten, D. G.; Yau, J. C.; Carrol, F. A. J. Am. Chem. SOC.1971, 100,62284234. (20) Harel, Y.; Manassen, J. J. Am. Chem. SOC.1977, 99, 5817-5818. (21) Richardson, P. F.; Chang, C. K.; Spaulding, L. D.; Fajer, J. J. Am. Chem. Soc. 1979,101,7736-7738. Chang, C. K.;Fajer, J. Ibid. 1980,102, 848-851. Richardson, P. F.; Chang, C. K.;Hanson, L. K.; Spaulding, L. D.; Fajer, J. J. Phys. Chem. 1979,83, 3420-3424. (22) Stolzenberg, A. M.; Spreer, L. 0.; Holm, R.H. J. Chem. Soc., Chem. Commun. 1979. 1077-1078: J. Am. Chem. Soc. 1980.102, 364-370. (23) Montfok, F. P.; Ofner, S.;Rasetti, V.; Eschenmoser, A,; Waggon, W. D.; Jones, K.Battersby, A. R. Angew. Chem., Inr. Ed. Engl. 1979, 18, 675-677. (24) Angst, C.; Kajiwara, M.; Zass, E.; Eschenmoser, A. Angew Chem., Inr. Ed. Engl. 1980, 19, 140-141. Naab, P.; Lattman, R.; Angst, C.; Eschenmoser, A. Ibid. 1980, 19, 143-145.
0002-78631811 1503-176$01.OO/O 0 1980 American Chemical Society
