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Carbide Carbon Atom (Metal Cluster Metal Surface Analogy/X-Ray Crystallographic Study/Reactivity Ofmetal Carbide Carbon Atoms) JIMMY H

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Carbide Carbon Atom (Metal Cluster Metal Surface Analogy/X-Ray Crystallographic Study/Reactivity Ofmetal Carbide Carbon Atoms) JIMMY H Proc. NatL Acad. Sci. USA Vol. 78, No. 2, pp. 668-671, February 1981 Chemistry Structure and chemistry of a metal cluster with a four-coordinate carbide carbon atom (metal cluster metal surface analogy/x-ray crystallographic study/reactivity ofmetal carbide carbon atoms) JIMMY H. DAVIS*, M. A. BENO*, JACK M. WILLIAMS*, JOANN ZIMMIE*, M. TACHIKAWAt, AND E. L. MUETTERTIESt *Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439; and tDepartment ofChemistry, University ofCalifornia, Berkeley, California 94720 Contributed by Earl L. Muetterties, September 24, 1980 ABSTRACT Molecular metal clusters with carbide carbon at- days, large single crystals of[Zn(NH3)42+][Fe4C(CO)122-] formed. oms oflow coordination number have beenprepared; they are the The supernatant was removed by a siphon and the crystals were anionic [HFe4C(CO)521] and [Fe4C(CO)12 -1 clusters. An x-ray collected. Analysis: calculated for [Zn(NH3)42+][Fe4C(CO)122-] crystallographic analysis of a tetraaminozinc salt of the latter has established a butterfly array ofiron atoms with the carbide carbon [OH2]: C, 21.39; H, 1.95; N, 7.75. Found: C, 21.84; H, 1.95; N, atom centered above the wings ofthe Fe4 core. Each iron atom was 8.05. bonded to three peripheral carbonyl ligands. The distances from X-Ray Crystallographic Study. Single crystals of [Zn(NH3)42+] the carbide carbon to iron were relativelyy shor, fcularly those [Fe4C(CO)22-][OH2] were orthorhombic, space group Pnma to the apical iron atoms (1.80A average). Protonation ofthe anionic (DA, no. 62) with a = 15.502(4) A, b = 10.918(3) A, c = carbide clusters reversibly yielded HFe4(CHXCO) and methyl- 13.348(4) A, Vc = 2259(1) A, and Z = 4 at -1000C. With a Syn- ation of the dianion gave {Fe4(CC(O)CH3I(CO) 24. Oxidation of tex P21 automated diffractometer, a graphite monochromator, [Fe4C(CO)122-] yielded the coordinately unsaturated Fe4C(CO)12 = reflec- cluster, which was extremely reactive. Hydrogen addition to this and MoKa radiation (A 0.71073 A), 2746 independent iron cluster was rapid below 0C, and a C-H bond was formed in tions were collected (4.00 s 26 ' 50.00) at - 1000C by the 0-26 this transformation. scan technique; of these, 1801 had F2 : 3f(Fo). Data were treated for Lorentz and polarization effects, and individual ab- Carbon atoms chemisorbed on metal surfaces possess a high sorption corrections were applied (A, = 36.6 cm-', Tmin = 0.49, chemical reactivity (1). In further exploring the analogy be- and Tma, = 0.67). After the structure was solved by using Multan tween metal surfaces and metal clusters (2), we have attempted (7), the oxygen atom ofthe water molecule and all the hydrogen to demonstrate similar high reactivity for a carbide carbon atom atoms (except those in the water molecule) were found in dif- in a molecular metal carbide cluster. We anticipated two fea- ference Fourier maps. Full matrix least-squares refinement (an- tures to be critical to such high reactivity in a cluster: a low co- isotropic temperature factors for all atoms except the hydro- ordination numberfor the carbide carbon atom and coordination gens, which were fixed at B.,0 = 3.0 A3) yielded R(FO) = 0.065 unsaturation forthe molecular cluster (3, 4). These features have (all data) = 0.042 [for FP 2 3o(FP)]. Form factors for Zn, Fe, C, been realized (5, 6) in the following clusters with 4-coordinate and 0 (and anomalous dispersion corrections for Zn and Fe) carbide carbon atoms, [Fe4C(CO)122-] and [HFe4C(CO)12_1. were taken from the International Tables (8, 9); form factors for We describe here the crystal structure of [Zn(NH3)421] the H atoms were from Stewart et al. (10). [Fe4C(CO)122-] and the chemistry ofthese unique clusters with Oxidation of[Fe4C(CO)122-] with Ag+ in the Presence ofHy- low-coordinate carbide carbon atoms. drogen. Dichloromethane (5 ml) was vacuum transferred to a cooled (- 1960C) reaction tube (100 ml, Kontes stopcock) con- EXPERIMENTAL taining [(C6H5)3PNP(C6H5)3+]2[Fe4C(CO)122-] (1) (80 mg) and under argon AgBF4 (20 mg). Then hydrogen was admitted to a pressure of 1 Procedures. All manipulations were performed atm (1 atm = 1.013 X 105 Pa). The reaction mixture was warmed or nitrogen by using modified Schlenk techniques, a Vacuum to -780C with rapid stirring and then was allowed to warm to Atmospheres HE-43 DRI-LAB, or a glass vacuum system. All 200C, at which temperature deposition of silver metal was evi- solvents were dried by distillation from P4010, calcium hydride, dent. After 10 min at 200C, the solvent was removed under vac- or sodium/benzophenone. NMR spectra were recorded on a uum. Extraction ofthe residue yielded HFe4(CH)(CO)12 (5), as modified Bruker 42-kG multinuclear, pulse-Fourier transform with NMR spectrometer equipped with Nicolet Technology Corpo- shown by the infrared spectrum. The analogous reaction ration software. All chemical shifts were referenced to tetra- 2H2 yielded the same product but with a 45% deuterium enrich- methylsilane. Microanalyses were performed by Vazenken ment. Tashinian (University of California, Berkeley, Department of Oxidation of[Fe4C(CO)1221] by Ag+ Under CO Atmosphere. Chemistry Microanalytical Laboratory). The above procedure was followed except that carbon monoxide [Zn(NH3)42+J[Fe4C(CO)1221][OH2]. A methanolic solution of was substituted for hydrogen. In this reaction, silver metal de- HFe4(CH)(CO)12(5) (200 mg, 15 ml) was mixed with an aqueous position was accompanied by a color change of the solution to ammonia solution of Zn(NH3)4Br2 (120 mg, 20 ml). Water(50 ml) dark green. Fe4C(CO)13 was obtained from a hexane extract of was layered over the water/methanol reaction system and al- the reaction residue. Infrared v(CO) (hexane): 2063s, 2050vs, into the lower aqueous layer. After 2 2039m, 2031m, 1989w, 1898w, br. Mass spectrum: 600 for the lowed to diffuse slowly parent ion (56Fe) and ions formed from successive losses of 13 The publication costs ofthis article were defrayed in part by page charge carbonyls. Analysis: calculated for Fe4C14O13: C, 28.04; H, 0.00; payment. This article must therefore be hereby marked "advertise- N, 0.00; Fe, 37.3. Found: C, 28.30; H, 0.05; N, 0.01; Fe, 38.4. ment" in accordance with 18 U. S. C. §1734 solely to indicate this fact. In the absence ofa CO atmosphere, this compound was also ob- 668 Downloaded by guest on September 24, 2021 Chemistry: Davis et al. Proc. Natl. Acad. Sci. USA 78 (1981) 669 tained when more than 2 equivalents ofAgBF4 were added, pre- - C) (CO)a2-] sumably due to generation offree carbon monoxide by oxidative [Fe4 (p4 decomposition ofiron carbonyl clusters. Oxidative Degradation of [Fe4C(CO)121 by Ag'. C (5) [(C6H5)3PNP(C6H5)3+]2[Fe4C(CO)122i] (100 mg) and 2 equiva- Fe (I) 4 ,Fe (4) lents of AgBF4 (23 mg) were placed in a 50-ml reaction tube; dichloromethane (-3 ml) was vacuum condensed into the reaction tube at - 1960C. The reaction tube was first warmed to -780C with stirring; then it was warmed to room temperature. Removal of the solvent under vacuum was followed by hexane extraction of the residue. The extract was evaporated to give Fe3C(CO),,, which was then recrystallized from chloroform at -250C. Infrared v(CO) (dichloromethane): 2110w, 2053s, 2012w, 1720vw. Mass spectrum: 488 parent (5'Fe) and ions from successive losses of 11 carbonyls. Preparation of [(C6H5)3PNP(C6H5)3+]{Fe4[CC(O)CH3] (CO)12-}. [(C6H5)3PNP(C6H5)3+]2[Fe4C(CO)12] (150 mg) was placed in a 50-ml Schlenk flask equipped with a Kontes stopcock and a Dry dichloromethane (10 ml) was vacuum trans- FIG. 1. Fe4C core cluster structure for [Fe4C(CO)122-1 in the hy- ball-joint. drated tetraaminozinc salt is presented with values for all Fe-Fe and ferred into the flask followed by =0.2 ml ofiodomethane. The Fe-C (carbide) distances; a stereoscopic view of this cluster anion, solution was stirred at room temperature for 2 days, during shown in Fig. 2, depicts the stereochemistry of carbonyl ligand place- which the solution turned from deep brown to green-black. The ment in the cluster. There is a crystallographic mirror plane that volume ofthe solvent was reduced to =1 ml; then 10 ml ofethyl passes through Fe(1), C(5), and Fe(4) and bisects the Fe(2)-Fe(3) bond; acetate was added to precipitate white crystals. The solvent was the atom labeling scheme for the butterfly core structure is that used removed from the supernatant, and the solid thus obtained was by Manassero et al. (12) and by us for related butterfly clusters (6) ex- crystallized from ethanol at -25C. Infrared v(CO) (dichloro- cept that Fe(3) in the present structure above becomes Fe(2)' because methane): 2021m, 1987vs, 1027sh, 1628w. 1H NMR (8 ofthe crystallographic mirror plane. CH2C12, +20°C and -90°C): 2.0 (s, 3H), 7.2-7.8 (m, '=32H). connectivity context to avoid ambiguities associated with elec- CO `3C NMR (ppm, tetrahydrofuran, -80°C): 196.7 (s, acetyl), tronic or reactivity features (3, 5). A carbide carbon atom is one 210.7 (s, 6CO), 219.1 (s, 3CO), 219.5 (s, 3CO). `3C NMR (ppm, that is within bonding distance of only metal atoms. A carbide tetrahydrofuran, +27°C): 196.0 (s, acetyl), 214.6 (br.s, 12CO). carbon atom that lies within a polyhedron of metal atoms is a Analysis: calculated for C51H33NFe4P203: C, 53.12; H, 2.88; N, cage (or interstitial) carbide, and those that lie in polyhedral 1.21.
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