Investigations by ¹³C NMR Spectroscopy of Ethene-Initiated Catalytic CO Hydrogenation

Abstract

13C NMR spectroscopy shows that the n-alkene and n-alkane products from the catalytic hydrogenation of CO in the presence of (13)C(2)H(4) probes over Ru/150 degrees C, Co/180 degrees C, Fe/220 degrees C, or Rh/190 degrees C (1 atm, CO:H(2) 1:1, "mild conditions") contain terminal (13)CH(3)(13)CH(2)- units. This is consistent with their formation by a regiospecific polymerization of C(1) species derived from CO and initiated by (13)C(2)H(4). Although the activities toward individual products differed somewhat, similar distributions and similar product labeling patterns were obtained over all the four catalysts. 1-Butene and the higher 1-n-alkenes from all the catalysts were largely (13)CH(3)(13)CH(2)(CH(2))(n)()CH=CH(2) (n = 0-3), propene formed over Ru or Co was (13)CH(3)(13)CH=CH(2), while both (13)CH(3)(13)CH=CH(2) and (13)CH(2)=(13)CHCH(3) were formed over Fe or Rh. Comparison of the conclusions from these probe experiments with those from isotope transient experiments by other workers indicates that the ethene initiator does not significantly modify the course of the CO hydrogenation. The reaction products are largely kinetically determined, and the primary products are mainly linear 1-n-alkenes, while the n-alkanes and 2-n-alkenes largely arise via secondary processes. Since the distribution of products and the labeling in them is so similar, it is concluded that one basic primary mechanism applies over all the four metals. Several different reaction paths involving a polymerization of surface methylene, [CH(2(ad))], have been proposed. Although the predictions based on several of these mechanisms agree with many of the results, the alkenyl + [CH(2(ad))] mechanism, initiated by a surface vinyl [CH(2)=CH((ad))], most easily accommodates the experimental evidence. An alternative path involving sequential addition of surface methylidyne and hydride either to a growing alkylidene chain (alkylidene + [CH(ad) + H(ad)]) or to an alkyl chain (alkyl + [CH((ad)) + H(ad)]) has recently been proposed by van Santen and Ciobica. The [CH(2(ad))] mechanism offers an easier explanation for the formation of the various alkenes, the distribution of products, and of the initiation, while the [CH(ad) + H(ad)] mechanism can explain any n-alkanes formed as primary products and not derived from alkenes. At higher reaction temperatures over Ru and Co, considerable (13)C(1) incorporation (from natural abundance in the CO and from cleavage of the (13)C(2)H(4) probe) was found in all the hydrocarbons. Thus, at higher temperatures (13)C(1(ad)) in addition to (13)C(2(ad)) species participate in both chain growth and initiation. In summary, adsorbed CO is transformed very easily into surface C(1(ad)), probably [CH(2(ad))] in equilibrium with [CH((ad))+H(ad)], which act as the propagating species.

Reactions of diiron mu-aminocarbyne complexes containing nitrile ligands

“…In fact, whereas the addition of cyanide occurs at the carbyne carbon of 1a-c to form the alkylidene compounds [Fe 2 {µ-C(CN)N(Me)R}(µ-CO)(CO) 2 The MeCN ligand in 2a-c seems therefore able to direct selectively the nuclephilic attack at the metal atom. In order to determine whether this would apply to carbon nucleophiles such as organolithium or organocopper reagents, the reactions of 2a-c with LiR were studied.…”

“…The formation of complex 8 is the result of a nucleophilic attack at the coordinated NCCMe 3 , which presumably forms the azavinylidene intermediate [Fe 2 {µ-CN(Me)Xyl}(µ-CO)(CO){N=C(C≡CC 6 H 4 Me-4)CMe 3 } (Cp 2 )] (9). In spite of the fact that several azavinylidene complexes are known, 17 the intermediate 9 appears too unstable in our hands to be characterized even by spectroscopy.…”

“…2 In recent years we have found that the formation of C-C bond in bridging aminocarbyne complexes [Fe 2 {µ-CN(Me)R}(µ-CO)(CO) 2 (Cp) 2 ][SO 3 CF 3 ] (R = Me, 1a; CH 2 Ph; 1b; Xyl, 1c) (Xyl = 2,6-Me 2 C 6 H 3 ) can be achieved by addition of carbon nucleophiles (R' -) which occurs selectively at different ligands: organocuprates and acetylides attack the carbonyl carbon to form the acyl complexes [Fe 2 {µ-CN(Me)R}(µ-CO)(CO)(COR')(Cp) 2 ], whereas organolithium or Grignard reagents give preferentially addition to the C 5 H 5 ring yielding the dienyl complexes [Fe 2 {µ-CN(Me)R}(µ-CO)(CO) 2 (Cp)(C 5 H 5 R')]. 3 Surprisingly, none of the above nucleophilic additions involve the µ-C of the aminocarbyne ligand, which appears rather unreactive, particularly if compared with the powerful electrophilic character of the µ-methylidyne carbon in the related complex [Fe 2 (µ-CH)(µ-CO)(CO) 2 (Cp) 2 ] + . 4 Only cyanide (from NBu 4 CN) has been found to react at the carbyne carbon to yield the aminocarbene 903 Reactions of Diiron µ-Aminocarbyne Complexes Containing Nitrile Ligands Vol.…”

C1 Chemistry for the Production of Ultra-Clean Liquid Transportation Fuels and Hydrogen

“…However, under our conditions, incorporation is the most important reaction on both catalysts, which contrasts with ethene addition. Hydrogenation of ethene has been found to be the predominant conversion on both iron and cobalt catalysts [18][19][20] . Albert 21 studied ethylene and acetylene chemisorptions on Co(0001) and observed that an acetylene monolayer blocks all surface sites for subsequent CO chemisorptions, while an ethylene monolayer allows the coadsorption of 25% of a CO monolayer.…”

“…SEM and EDX analysis will also be used to identify the Pt components. Other materials, such as Y-zeolite 17 and especially amorphous silica-alumina 18 have been reported to produce transportation fuels from long-chain paraffins. Combining these materials with PtWZr catalysts may result in a hybrid catalyst with enhanced yields of middle distillates.…”