Neil J. Meanwell

Decomposition reactions of dimethyl-di-µ-methylene-bis(η-pentamethylcyclopentadienyl)dirhodium and their relation to the mechanism of the Fischer-Tropsch reactions; the formation of propylene from three C1 ligands

Thermal decomposition of [{C5(CH3)5Rh}2(µ-CH2)2(CH3)2](1) at temperatures from 275 to 375 °C yielded methane, propylene, ethylene, and some ethane. Using (1) selectively labelled with 13C in the Rh–methylene and/or the Rh–methyl ligands showed that (a) the gases are formed in intramolecular decompositions not involving the C5(CH3)5 rings, (b) the methane arises from both the rhodium–methyls and the rhodium–methylenes, and (c) the C2 and C3 gases arise predominantly from the coupling of a Rh–methyl and one or two Rh–methylenes; direct coupling of two methylenes or of two methyls is not a favourable process here. Very similar gas mixtures are formed (but at 20–50 °C) on reaction of complex (1) with excess IrCl62–(or other one-electron oxidisers and electrophiles). Carbon-13 and deuterium labelling studies show that these reactions are again intramolecular and do not involve the C5(CH3)5 rings, or the coupling of two methyl or two methylene ligands. Methane arises mainly by combination of a Rh–CH3 and a methylene hydrogen, probably after a two-electron oxidation, leaving a transient species (A) formulated as [{C5(CH3)5Rh}2(µ-CH2)(µ-CH)(CH3)]2+. The C2 products must be formed from (A) by the coupling of the Rh–CH3 with the methylene, to give an Rh–ethyl intermediate which β-eliminates to give ethylene (or acquires a hydrogen to give ethane). The labelling shows propylene to arise from the coupling of one methyl and two methylenes. It can be formed via migration of the methyl onto the µ-methyne in (A), giving a µ-methylene-µ-ethylidene species which couples to give propylene directly. Implicit in the route is the need for two metal centres to allow three C1 fragments to couple together to form propylene. Labelling studies rule out appreciable ethylene formation from a Rh–ethylidene. Direct coupling of the methylenes does occur in the thermal decompositions of [{C5(CH3)5Rh}2(µ-CH2)2X2](X = halide, SCN, or N3), to give ethylene; the main product is again methane. The data are contrasted with results from the decomposition of the iridium analogue of (1) and of [C5(CH3)5Ir(CH3)4]. The relationships of the mechanisms proposed to current models for the mechanism of the Fischer-Tropsch reaction on metal surfaces are discussed.

Synthesis and characterisation of disubstituted di-µ-methylene-bis(η-pentamethylcyclopentadienyl)dirhodium(IV) complexes; X-ray structure of [{(C5Me5)Rh}2(µ-CH2)2(CO)2]2+

Reaction of either cis- or trans-[{(C5Me5)Rh}2(µ-CH2)2Me2] with HCl in pentane gave the trans-dichloro-complex [{(C5Me5)Rh}2(µ-CH2)2X2](4a; X = Cl), from which a variety of other complexes (4; X = Br, I, SCN, N3, CN, or NCO) were made by metathesis. Reaction of (4a; X = Cl) with neutral ligands (L) in the presence of non-co-ordinating anions gave first the monocations, [{(C5Me5)Rh}2(µ-CH2)2(L)Cl]+, and then the dications, [{(C5Me5)Rh}2(µ-CH2)2L2]2+(5; L = MeCN, CO, or H2O). The X-ray crystal structure of (5b; L = CO) shows the trans configuration predicted on the basis of the 1H n.m.r. spectra. Reaction of (5b) with methanol and base gave the bis(methoxycarbonyl) complex [{(C5Me5)Rh}2(µ-CH2)2(CO2Me)2]. cis Complexes [{(C5Me5)Rh}2(µ-CH2)2X]n+(X = O2CMe or O2CCF3, n= 1; X = pyridazine or Ph2PCH2PPh2, n= 2) were isolated and identified by their 1H n.m.r. spectra which showed the diastereotopic methylene protons as two resonances, only one of which was coupled to the rhodiums. The nitrate complex [{(C5Me5)Rh}2(µ-CH2)2(NO3)2] was found to exist as two isomers, one trans and the other cis with one bridging and one ionic nitrate, as shown by the n.m.r. spectra.

Monoalkyl-di-µ-methylene-bis[(η-pentamethylcyclopentadienyl)-rhodium(IV)] complexes and the intramolecular migration of alkyl groups between two metal atoms

Reaction of the trans dimethyl complex [{(C5Me5)Rh}2(µ-CH2)2Me2](1) with one equivalent of acid in the presence of acetonitrile gave the methyl–acetonitrile complex [{(C5Me5)Rh}2(µ-CH2)2(Me)(MeCN)]PF6(2a); the acetonitrile could be replaced by other ligands to give [{(C5Me5)Rh}2-(µ-CH2)2(Me)(L)]PF6[L = ButCN (2b), PhCN (2c), pyridine (2d), 2-methylpyridine (2e), or CO (2f)]. Reaction of (2a) with halide gave [{(C5Me5)Rh}2(µ-CH2)2(Me)X][X = Cl (3) or I (4)]. The other monoalkyl complexes [{(C5Me5)Rh}2(µ-CH2)2(R)(MeCN)]PF6[R = Et (9), Prn(10), or Bun(11)] were obtained analogously from reaction of the appropriate dialkyl complexes [{(C5Me5)Rh}2(µ-CH2)2R2] which were in turn synthesised from the trans dichloro-complex [{(C5Me5)Rh}2(µ-CH2)2Cl2]. The n.m.r. spectra of (2a) showed the presence of cis and trans isomers (ratio ca. 1 : 2) at –80 °C and of dynamic behaviour at higher temperatures. The dynamic behaviour arises from loss of the MeCN, movement of the methyl into a bridging position in a transition state, followed by readdition of the MeCN. Overall this corresponds to an intramolecular migration of the methyl from one rhodium to the other. The other complexes (2) behave similarly but (2d) and (2f) show the ‘frozen-out’ spectra even at +22 °C. Under identical conditions the complexes (9)–(11) exhibited similar behaviour to (2a), but the rates of alkyl migration were ca. 10 times faster. Complex (2a) also disproportionated to give (1) on reaction with base; this involves an intermolecular methyl migration. The other alkyl complexes did not undergo this reaction. The halide complexes (3) and (4) were rigid and of cis configuration in benzene but showed more complex behaviour in dichloromethane.

Further studies relating to the formation of propene in the Fischer–Tropsch reaction

The Fischer–Tropsch model complexes [(C5Me5Rh)2(Me)2(µ-CH2)2](Me =13CH3, CD3) react with Na2IrCl6 or FeCl3 hydrate to give largely 13CH212CH–12CH3 and CD2CH–CH2D respectively; a mechanism is proposed and it is suggested that charged organometallic species may play a vital role also in C–C coupling reactions on surfaces.

The reactions of di-µ-methylene-bis(pentamethylcyclopentadienyl)-dirhodium(IV) complexes with benzylmagnesium chloride and with alkynylmagnesium chloride; X-ray structure of [(η5-C5Me5)Rh(CH2Ph)(µ-CH2)2Rh(η4-C5Me5CH2Ph)]

Reaction of trans-[(C5Me5)2Rh2(µ-CH2)2Cl2](1) with RCCMgCl gave trans-[(C5Me5)2Rh2(-µ-CH2)2(RC2)2](R = Ph or But). However attack on (1) by benzylmagnesium chloride occurred at one C5Me5 ring as well as at one rhodium to give [(η5-C5Me5)Rh(CH2Ph)(µ-CH2)2Rh(η4-C5Me5CH2Ph)](6). The structure of (6) has been deduced by n.m.r. spectroscopy and confirmed by a single-crystal X-ray structure determination which showed the two rhodiums [2.559(3)A apart] to be linked by two µ-methylene bridges. One rhodium (oxidation state formally +4) bears a σ-benzyl and an η5-C5Me5 ring; the other rhodium (formally +2) is η4-bound to C5Me5CH2Ph where the benzyl on the C5 ring is exo to the metal. Reaction of [(C5Me5)2Rh2(η-CH2)2(MeCN)2][PF6]2 with R′CCH gave a complex [(C5Me5)2Rh2(µ-CH2CHCR′CHCR′CH)]PF6(7)(R′=p-chlorophenyl); on the basis of the n.m.r. spectra it is proposed that the two rhodiums are linked by a σ,σ,η5-CH2CHCR′CHCR′CH bridge. The mode of formation of these complexes is discussed.

The syntheses and characterisation of, and the determination of 1J(103Rh–103Rh) in [(η5-C5Me5)2Rh2(µ-CH2)2{µ-CH2CR(CH2CRCH2)CH2}](R = H or Me) and [(η5-C5Me5)2Rh2(µ-CH2)2{µ-Ph2P(CH2)nPPh2}]2+(n= 1 or 2)

Reaction of [(η5-C5Me5)2Rh2(µ-CH2)2Cl2] with CH2CRCH2MgCl gave [(η5-C5Me5)2Rh2(µ-CH2)2{µ-CH2CR(CH2CRCH2)CH2}](3, R = H; 4, R = Me). Reaction of [(η5-C5Me5)2Rh2(µ-CH2)2(MeCN)2]2+ with Ph2P(CH2)nPPh2gave [(η5-C5Me5)2Rh2(µ-CH2)2{µ- Ph2P(CH2)nPPh2}]2+(5, n= 1; 6, n= 2). The complexes (3)–(6) were characterised largely by their n.m.r. spectra which also allowed the determination of 1J(103Rh–103Rh) as 13.5 for (3), 13.2 for (4), 12.4 for (5), and 11.9 Hz for (6). After allowance for gyromagnetic ratios and s-electron densities at the nuclei, these coupling constants are comparable with a 1J(l95Pt–195Pt) coupling of ca. 8 000 Hz.