Carbidocarbonyl clusters of iron

Abstract

Under reducing conditions, at elevated temperatures, coordinated carbon monoxide in transition metal carbonyls may disproportionate to CO 2 and a carbon atom. The carbon atom is trapped in a cage of metal atoms, shielded from further reaction in the core of the resulting carbidocarbonyl clusters. This class of compounds, which has been known for some time, bears some structural resemblance to the binary carbides and may therefore be relevant to Fischer-Tropsch catalysis, in which carbide phases and surface carbon atoms are implicated. The chemical, structural and physical properties of the iron carbidocarbonyIs have been investigated. Reactions are described that lead to C-H and C-C bond formation at the carbide carbon, and these are discussed with regard to the nature of this carbon atom. 13 C n.m.r. spectroscopy reveals large downfield chemical shifts for the carbide carbon, which may be interpreted as either a reflexion of low electron density at the carbon or the influence of paramagnetic contributions to the shift. Reactions of [Fe 4 (CO) 12 C CO 2 CH 3 ]-, [Fe 4 (CO) 12 C C(O)CH 3 ]- and [Fe 4 (CO) 12 C CHO]- with trimethyloxonium fluoborate yield the corresponding vinylidene clusters Fe 4 (CO) 12 - C = C (OCH 3 )R (R = OCH 3 , CH 3 , H).

Nearly Uniform MgO-Supported Pentaosmium Cluster Catalysts

“…This part of the spectrum is attributed on the basis of a T1 measurement to carbonyl ligands of osmium carbonyls such as the likely impurity [H 3 Os 4 (CO) 12 ] - and/or possibly to unidentified mononuclear osmium carbonyls. The large downfield shift of the peak at δ = 404 ppm is characteristic of an interstitial carbido carbon atom, as reported for several metal carbido carbonyl clusters, for example, [HOs 10 C(CO) 24 ] - , [Fe 4 C(CO) 12 ] 2- , [Ru 6 C(CO) 16 ] 2- , [Co 6 C(CO) 15 ] 2- , and [Rh 6 C(CO) 13 ] 2- , each of which has distinct carbido carbon chemical shifts greater than 400 ppm.…”

Multi-phosphine-chelated iron-carbide clusters via redox-promoted ligand exchange on an inert hexa-iron-carbide carbonyl cluster, [Fe6(μ6-C)(μ2-CO)4(CO)12]2−

“…The triply substituted Fe 4 (m 4 -C)(m 3 -CO)(CO) 9 (-PMe 3 ) 3 species contrasts with our chelated, triply substituted cluster 9. 21 The location of the phosphine donors on the 'wing-tip' iron sites (Fe 2 and Fe 3 ) is consistent with most literature studies (Fenitride clusters). However, the wing-tip motif is slightly different from the triply PMe 3 ligated Fe-carbide cluster reported by Bradley, 21 which exhibits only one wing-tip PMe 3 and two PMe 3 in the basal locations.…”

“…21 The location of the phosphine donors on the 'wing-tip' iron sites (Fe 2 and Fe 3 ) is consistent with most literature studies (Fenitride clusters). However, the wing-tip motif is slightly different from the triply PMe 3 ligated Fe-carbide cluster reported by Bradley, 21 which exhibits only one wing-tip PMe 3 and two PMe 3 in the basal locations. The orientation of the phosphine donors above the axial Fe 2 -C 1 -Fe 3 motif is also consistent with the reported Fe 4 structures (Table 2).…”

“…The orientation of the phosphine donors above the axial Fe 2 -C 1 -Fe 3 motif is also consistent with the reported Fe 4 structures (Table 2). 21,22,42,43 Finally, we speculate that the coordination of three phosphine ligands onto the two wing-tip irons in 9 thermodynamically destabilizes the cluster core, an effect primarily compensated for by multi-iron chelation. The 'cluster chelation' prevents (immediate) cluster disproportionation, but may not completely compensate for the unsymmetrical Fe electronics as evidenced by the eventual formation of the phenyl-transfer product [Fe 3 (m 3 -CPh){Triphos(PPh 2 ) 2 (PPh)}(CO) 7 ] (11) and mononuclear (Triphos)Fe(CO) 2 .…”

“…Analogous substitution using Fe 4 (m 4 -C)(CO) 13 with PR 3 variants by Bradley resulted in preliminary structural data of thricesubstituted [Fe 4 (m 4 -C)(m 3 -CO)(CO) 9 (PMe 3 ) 3 ], and a putative tetrasubstituted variant was spectroscopically characterized by 1 H/ 13 C/ 31 P NMR. 21 Relatedly, a singly substituted phosphine variant was explored by Wadepohl, who reported the X-ray structure of the protonated four-iron cluster [Fe 4 (m 4 -CH)(CO) 12 (PPh 3 )H] − ; 22 this was an expansion on Muetterties' extensive body of work with the four-iron cluster series. 4,5,23 The eventual aim of our research program is to systematically understand how to controllably insert a variety of multidentate ligands onto iron-carbide-carbonyl clusters pertaining to nitrogenase FeMoco.…”