Ion molecule reactions between ligated transition metal cations with

JOURNAL OF CHEMICAL RESEARCH 2011

MARCH, 179–186

RESEARCH PAPER 179

Ion molecule reactions between ligated transition metal cations with methane, ethane and propane in a high pressure ion source Dag Ekeberg* and Carl Fredrik Naess-Andresen Department of Chemistry, Biotechnology and Food Science, Norwegian University of Life Sciences, PO Box 5003, N-1432 Ås, Norway

The reactions of CH4, C2H6 and C3H8 with [η5-C5H5M(CO)n]+ (M = Fe, Co and Ni; 0 ≤ n ≤ 2) in the gas phase have been studied using a double focusing mass spectrometer with a high pressure ion source (HPIS). The products observed are mainly described as weakly bound ion molecule complexes. Carbon–hydrogen bond insertions were observed with propane and only to a minor extent with ethane. Methane was found to form ion molecule complexes only with ligated transition metal cations. The reactivity was influenced by the number of carbonyl ligands and the metal cations. The main adducts formed were obtained when one or no CO ligands were bound to the cyclopentadienyl metal cation. No adducts between the alkanes and naked transition metal cations or protonated metal compounds were found.

Keywords: alkanes, C–C activation, C–H activation, mass spectrometry (MS), ion molecule reactions, transition metal cations Ion molecule reactions of transition metal cations with hydrocarbons is an area that allows study of intrinsic molecular interactions unperturbed by solvent effects. This makes gas phase reactions interesting both from an academic and industrial point of view.1–8 The possibility of investigating the reactivity of ligated metal cations in the gas phase, i.e. an environment with the absence of interfering solvents, gives us a great advantage. In this study, we have chosen metal cations with cyclopentadienyl complexes as a model for catalytic activity. This area of study can also provide better insight into the mechanisms of catalytic reactions. A brief review on these reactions has been published.9 Bohme et al. demonstrated that η5-C5H5Fe+ reacts at higher rates compared to that of naked Fe+.10–13 Alkanes like methane, ethane and propane are known to react with many first row transition metal cations in the gas phase.14–16 The reaction may either give direct C–H or C–C bond insertion products of the metal cation or proceed via an intermediate, which can be an ion molecule complex between the alkane and the transition metal cation. It is known that transition metal ions may react with alkanes, e.g. C2H6, to give products like M+(CH3)2 or (C2H5)M(H)+.17 Different experimental approaches can be used when studying ion molecule reactions in the gas phase depending on the pressure in the reaction chamber, e.g. single collision conditions or multiple collision conditions are obtained in some experiments. Single collision conditions are achieved in ion beam experiments,18–20 ion trap instruments21,22 and in Fourier transform ion cyclotron resonance (FT-ICR) experiments23–28 that are conducted at very low pressures. Multiple collision conditions are achieved using a high pressure ion source29–31 and in various flow reactors32. Single collision conditions make it possible to study endothermic products, and exothermic products are studied in a multiple collision environment. One of the main differences between the two approaches is that intermediates can be trapped in a multiple collision environment as the result of frequent stabilising collisions with gas molecules present. We have previously demonstrated that [(H)Fe(CO)n]+ (n = 1, 3 and 4, but not n = 2, 5) forms adducts with CH4 in a high pressure ion source.33 Tonkyn and Weisshaar studied adduct formation between transition metal cations and ethane in a fast flow reactor.34 Complexation energies between naked transition metal cations and CH4, C2H6 and C3H8 have been reported by Armentrout and Clemmer.35 They found an increase of bond dissociation energy with increasing size of the alkane and with decreasing ionic radius of the metal. *Correspondent. E-mail: [email protected]

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The main purpose of this study was to assess the possibilities for ligated transition metal cations to insert carbon–hydrogen and carbon–carbon bonds. We wanted to study the gas phase reactions between methane, ethane and propane and [η5C5H5M(CO)n]+ (M = Fe, Co or Ni; 0 ≤ n ≤ 2). As the experiments proceeded we observed formation of ion molecule complexes as the main products between the ions and the alkanes. Ion molecule complexes are species that are held together by an interaction of a charged species with a neutral reactant. Here the term adduct does not refer to a specific structure, only the molecular formula. Besides investigating the possible reaction of small alkanes with ligated transition metal cations we also wanted to study the dependence of reaction on the ligation number. If ligated transition metal cations, like [η5-C5H5M(CO)n]+, insert the C–H bond in either of the alkanes, rearrangement reactions may occur. A possible rearrangement is migration of H and alkyl ligand from the metal to the carbonyl. In the present study we have investigated the interaction and reactivity between [η5-C5H5M(CO)n]+ (M = Fe, Co or Ni; 0 ≤ n ≤ 2) and alkanes like methane, ethane and propane. The reaction products and intermediates were formed in a high pressure ion source (HPIS) and interpretation of the adduct structure has been performed by the B/E = constant linked scan technique.36 Results and discussion

Adducts from propane and ligated transition metal cations can be a mixture of isomers. The main isomer corresponds to an ion molecule complex of propane and the ligated transition metal cations. Other isomers have been formed via C–H bond insertion followed by rearrangement reactions. The formation of ion molecule complexes can either be an association reaction between the alkane and an ion stabilised by collisions, or it can be formed by an ion molecule exchange reaction that subsequently liberates a neutral species. The main reaction in the experiments described in this article proceeded via an interaction between a collision-stabilised metal cation and the alkane. All results were reproduced when mixing 20–30% Ar and He as a buffer gas with the hydrocarbon. When the ion was a naked transition metal cation we observed an increased possibility of adduct formation compared to using a ligated metal cation. In addition, an increase of the formation of ion molecule complexes from methane, ethane and propane and [η5-C5H5M(CO)n]+ with decreasing number of carbonyl ligands was observed. Table 1 summarises the different adducts formed depending on the number of carbonyl ligands. These results support the model, that available space is a prerequisite for the formation of an ion molecule complex between methane,

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Table 1 ions.

Relative abundances of reactant ions and adduct

Ion [η5-C5H5Fe(CO)2]+ [η5-C5H5Fe(CO)]+ [η5-C5H5Fe]+ [η5-C5H5Co(CO)2]+ [η5-C5H5Co(CO)]+ [η5-C5H5Co]+ [η5-C5H5Ni(CO)]+ [η5-C5H5Ni]+

(m/z 177) (m/z 149) (m/z 121) (m/z 180) (m/z 152) (m/z 124) (m/z 151) (m/z 123)

H4

C2H6

C3H8

9:1 2:1 n.o. n.o. 1:2 n.o. 4:3 2:1

n.o. n.o. 3:1 10:1 1:2 n.o. n.o. 2:1

n.o. 2:1 4:1 n.o. 1:2 n.o. n.o. 2:1

n.o., not observed.

ethane and propane and [η5-C5H5M(CO)n]+, (M = Fe, Co or Ni; 0 ≤ n ≤ 2). Formation of ion molecule complexes might also be dependent on the transition metal, which becomes more coordinatively saturated upon bridging to the alkane. In the absence of lone pairs, the C–H bonding pairs are donated to the metal cation in a two electron, three-centre bond. In the reaction path, we assume that the C–H–M unit does not differ significantly from linearity when there is a long distance between the cation and the neutral molecule, but loses the linearity rapidly when the distance decreases, as discussed by Crabtree in 1985.37 Adducts from alkanes and protonated transition metal compounds were not found. Neither were adducts between methane, ethane and propane and a naked transition metal ion observed. An increasing tendency for adduct formation was observed with increasing size of the neutral alkane. This can be explained by the polarisability of the alkane, which increases from methane to propane. This leads to an increase in the ion-induced dipole interaction with the transition metal ion. Several reactions of [η5-C5H5M]+ ions have been examined, and it was found that positive activation entropy could be responsible for the low efficiency often found.38 In this study, the cyclopentadienyl ring is not observed to be involved directly in the reaction, but there are published similar experiments where the ring is involved.39 Our results support the formation of adducts with alkanes loosely bonded to ligated transition metal cations in the gas phase. Bond insertion reactions are observed when propane is used as the neutral reactant and only minor amounts were found when ethane was used. These results are in good agreement with results reported on naked iron-, cobalt- and nickel- cations in the gas phase.14 The results of the linked scan experiments were independent of the temperature of the ion source between 40 and 180 °C. Experimental The ligated transition metal cations [η5-C5H5M(CO)n]+ (M = Fe, Co or Ni) were produced in a high pressure ion source and reacted with CH4, C2H6 and C3H8 (Yara, Rjukan, NOR). The transition metal compounds used were [η5-C5H5Fe(CO)2I], [η5-C5H5Co(CO)2] and [{η5-C5H5NiCO}2] (Sigma Aldrich Steinheim, DE). The transition metal compounds were introduced into the ion source in small amounts with the direct inlet probe or mixed with the alkane (methane, ethane and propane) in a glass reservoir prior to the introduction. The experiments were mainly performed with a mixture of the alkane and the ligated transition metal compound. In some experiments He or Ar mixed with 20–30% of the alkane were used as buffer gases. The high pressure ion source used in these experiments was based on a design by van Koppen et al.46 and by Illies and Meisels.47 A characteristic feature of this source is that it has a circular ion exit with a diameter of 0.1 mm, small enough to obtain a pressure of 0.5–6.0 Torr (66.6– 799.8 Pa), and applying 350 eV electrons. The ion source temperatures were between 40 and 180 °C. Within the cylindrical ion source there are three concentric electric field rings, which are utilised to influence the residence time of the ions. The pressure in the ion source was measured with a capacitance manometer (MKS Instruments, Burlington, MA, USA). The ions were analysed using a double focusing mass spectrometer of EB geometry (model 7070H, VG, Fisons

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Analytical, Manchester, UK). Collision-induced dissociation mass spectra were recorded using helium (99.9999%, Yara, Rjukan, NOR) in the collision cell located in the first field free region. The pressure in the collision region was increased until the precursor ion was attenuated by 70%. The organometallic compounds were of commercial grade (97%) and checked for purity prior use. To obtain mechanistic and structural information, experiments were performed using CD4, C2D6 and C3D8 (Yara, Rjukan, NOR). Reactions between methane and [η5-C5H5Fe(CO)n]+: The: mass spectrum of the mixture of methane and η5-C5H5Fe(CO)2I is shown in Fig. 1a. The main ionisation reaction for formation of molecular ions at m/z 304 is charge exchange, and to a minor amount electron ionisation. Protonated molecules like [η5-C5H5Fe(CO)2I]H+ (m/z 305) are formed by proton transfer due to chemical ionisation. The main fragment ion from the protonated molecule corresponds to [η5C5H5Fe(CO)2]+ (m/z 177) and is probably due to loss of HI. This assumption is supported by collision-induced dissociation (CID) experiments performed on the protonated molecular ion. We also observed successive CO loss from the molecular ion (m/z 276) and the protonated molecular ion (m/z 277), which then give peaks at m/z 248 and 249, respectively. The peak at m/z 186 corresponds to (η5C5H5)2Fe+and is probably due to thermal degradation. Fragments found at m/z 149 and m/z 121 correspond to successive CO losses from [η5-C5H5Fe(CO)2]+. The signal found at m/z 193 may be an adduct between CH4 and [η5-C5H5Fe(CO)2]+ (m/z 177). The most probable structure of this adduct is either an ion molecule complex [η5-C5H5Fe(CO)2(CH4)]+ or a product where the metal is inserted into the C–H bond, [η5-C5H5Fe(CO)2(CH3)(H)]+. Experiments using CD4 and detailed scrutiny of the CID spectra may lead to assignment of this structure.40 The CID spectrum of the product formed from CD4 and [η5-C5H5Fe(CO)2]+ at m/z 197 consists of only five product ions that are easily explained. The main product ion (base peak) corresponds to [η5-C5H5Fe(CO)2]+ (m/z 177) caused by loss of CD4. [η5-C5H5FeCO]+ and [η5-C5H5Fe]+ were found at m/z 149 (25%) and m/z 121 (50%), respectively. The signal caused by loss of CO prior to CD4 (m/z 169) is approximately 15% relative to the base peak. The peak found at m/z 56 (50%) corresponds to Fe+. Because loss of CD4 is found to be the dominating fragmentation reaction upon collision activation (CA), no hydrogen scrambling is observed. No loss of CD3 was observed, so we assume an ion molecule complex to be the most probable structure. Methane also forms adducts with [η5-C5H5FeCO]+ (m/z 149). The CID mass spectrum of this adduct ion has shown loss of methane followed by loss of CO as the main fragmentation path. Assuming the same mechanism as above, the structure of this adduct is also characterised as an ion molecule complex. In addition, experiments with CD4 also support our conclusion. The adduct formed by methane and [η5-C5H5Fe(CO)n]+ is assumed to be collision stabilised due to the high pressure of methane in the ion source. If the product was energetically rich on internal energy, we would expect to observe C–H bond insertion by iron. This would then result in an increased amount of CO loss upon CID. Reactions between methane and [η5-C5H5Co(CO)n]+: The mass spectrum of the mixture of CH4 and η5-C5H5Co(CO)2 shows that the molecular ion, m/z 180, constitutes the base peak, Fig. 1b. The protonated molecular ions are found at m/z 181. Peaks at m/z 152 and 153 correspond to loss of CO from the molecular ions and protonated moleculuar ions, respectively. Small amounts of η5-C5H5Co+ and η5-C5H5CoH+ are found at m/z 124 and 125. The signal at m/z 168 is an adduct of methane and [η5-C5H5Co(CO)]+ (m/z 152). This interpretation is supported by the finding of an expected mass increase, from m/z 168 to m/z 172, when CH4 was substituted with CD4. The main fragmentation reaction of CID of the adduct between CD4 and [η5-C5H5Co(CO)]+ (m/z 152, 95%) is loss of CD4 followed by CO loss (m/z 124, 100%). No CO loss prior to elimination of methane is observed. Collision-induced dissociation of the adduct between CD4 and [η5-C5H5Co(CO)]+ has mainly the same characteristics as discussed above for the ion molecule complex between CD4 and [η5-C5H5Fe(CO)2]+. However, there are small amounts (about 15%) of product ions in the CID spectrum that might indicate that some adducts have a structure where C–D bond insertion has taken place. The product ions supporting this assumption are found at m/z 154, 153 and 126, 125. We suggest that the most probable structure of the adduct in this case is an ion molecule complex, and that this product was formed analogous to the iron complex reaction.

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Fig. 1 Mass spectra recorded in 1% mixtures of (a) η5-C5H5Fe(CO)2I, (b) η5-C5H5Co(CO)2, and 9c) [η5-C5H5NiCO]2 in methane at 39.9 Pa.

Reactions between methane and [η5-C5H5Ni(CO)n]+: The mass spectrum of the mixture of CH4 and [η5-C5H5Ni(CO)]2 is shown in Fig. 1c. The protonated molecular ions are found at m/z 303. Two peaks corresponding to an adduct formed by methane and [η5C5H5Ni(CO)n]+ (n = 0, 1) can be seen. The expected mass shifts using CD4, from m/z 139 to m/z 143 for n = 0 and from m/z 167 to m/z 171 for n = 1 were observed.

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The most probable structure of the adduct between CD4 and [η5-C5H5Ni(CO)n]+ is either [η5-C5H5Ni(CO)n(CD4)]+ or [η5-C5H5 Ni(CO)n(CD3)(D)]+. The adduct between CD4 and [η5-C5H5Ni(CO)]+ has the same CID fragmentation pattern as the ion molecule complexes as discussed above. The base peak in the CID spectrum corresponds to Ni+, and [η5-C5H5Ni(CO)]+ at m/z 123 has an intensity of 30%. Loss of CD4 from these adducts together with the CID fragmentation

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pattern support the assumption that an ion molecule complex is formed by a thermolecular association reaction. Reactions between ethane and [η5-C5H5Fe(CO)n]+: Figure 2a shows the mass spectrum of the mixture of ethane and η5-C5H5Fe(CO)2I is shown. The peak found at m/z 151 is proposed to correspond to adducts between ethane and [η5-C5H5Fe]+ (m/z 121). The expected

mass shift from m/z 151 to m/z 157 was observed when C2D6 was used. If iron was inserted into the C–H bond we would expect migration of the β-hydrogen from the alkyl ligand to the metal, as illustrated with propane in Scheme 1. This reaction pattern gives a mixture of at least two different isomers implying that fragment ions caused by CID have two different parent ions with the same m/z ratio. Dehydrogenation

Fig. 2 Mass spectra recorded in 1% mixtures of (a) η5-C5H5Fe(CO)2I, (b) η5-C5H5Co(CO)2, and (c) [η5-C5H5NiCO]2 in ethane at 39.9 Pa.

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183

Scheme 1

is expected to be a significant fragmentation on CID of an ion where the metal has inserted the C–H bond in ethane. If iron was inserted into the C–C bond, the resulting CID spectrum would be expected to consist of peaks caused by loss of CH3 from the precursor ion. However, if the adduct between ethane and [η5-C5H5Fe]+ was a weakly bonded ion molecule complex loss of ethane should be one of the main fragmentation products. The CID mass spectrum obtained from adducts of C2D6 and [η5-C5H5Fe]+ consists of two signals; [η5-C5H5Fe]+ (m/z 121) as the main product ion and Fe+ (m/z 56, 10%). The former is the product ion caused by loss of C2D6 from the precursor ion, and the latter is due to successive loss of C2D6 and the cyclopentadienyl ligand. We did not observe any peaks in the CID spectrum of m/z 157 that could indicate a structure of the adduct where C–H and/or C–C bond insertion has occurred; nor any peaks corresponding to [η5C5H5Fe(D)]+ or [η5-C5H5Fe(CD3)]+. These results are consistent with FT-ICR-MS experiments on a mixture of ethane and [η5-C5H5Fe]+.19

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Our results support the assumption that the mechanism of the reaction between ethane and [η5-C5H5Fe]+ is analogous to the reaction between methane and [η5-C5H5Fe(CO)2]+. We therefore propose that the adduct between ethane and [η5-C5H5Fe]+ is an ion molecule complex, formed by a thermolecular association reaction. Reactions between ethane and η5-C5H5Co(CO)n+: The mass spectrum of the mixture of ethane and η5-C5H5Co(CO)2 is shown in Fig. 2b. The peaks at m/z 182 and m/z 210 correspond to adducts between ethane and [η5-C5H5CoCO]+ (m/z 152) and [η5-C5H5Co(CO)2]+ (m/z 180), this interpretation is supported by the expected mass increase by six mass units when using C2D6 instead of C2H6. The main product ions formed by CID of the adducts corresponds to loss of C2D6 from the precursor ion. About 5% of the base peak corresponds to [η5-C5H5Co]+ (m/z 124). No dehydrogenation products were found in the CID spectra that could indicate structures where bond insertion had occurred. The ions observed at m/z 209 in Fig. 2 correspond to

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Fig. 3 Mass spectra recorded in 1% mixtures of (a) η5-C5H5Fe(CO)2I, (b) η5-C5H5Co(CO)2, and (c) [η5-C5H5NiCO]2 with propane as reagent gas at 39.9 Pa.

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JOURNAL OF CHEMICAL RESEARCH 2011 [η5-C5H5Co(CO)2C2H5]+ caused by addition of the ethyl ions to η5C5H5Co(CO)2. However, using the same argument as previously, our results support the assumption that the adduct formed between ethane and [η5-C5H5Co(CO)n]+, under multi collision conditions, is an ion molecule complex formed by a thermolecular association reaction. Reactions between ethane and [η5-C5H5Ni(CO)n]+: In the mass spectrum, Fig. 2c, of a mixture of ethane and [η5-C5H5Ni(CO)]2 the signal found at m/z 153 corresponds to a complex between C2H6 and [η5-C5H5Ni]+. This is confirmed by the expected mass increase (m/z 153 to m/z 159) when using C2D6. In addition to these ions we also observed [η5-C5H5NiCH2]+ and [η5-C5H5Ni(CO)CH2]+, at m/z 137 and 165 respectively. Collision-induced dissociation of adducts between C2D6/C2H6 and [η5-C5H5Ni]+ gave two product ions. The main peak observed in these CID spectra is [η5-C5H5Ni]+ due to loss of C2D6/ C2H6. About 20% of the base peak is [η5-C5H5NiH2]+ observed at m/z 125 and [η5-C5H5NiD2]+ at m/z 127. This fragmentation behaviour supports the conclusion that the main structure corresponds to an ion molecule complex. In addition to this are some ions where nickel has inserted the C–H bond in ethane. Reactions between propane and [η5-C5H5Fe(CO)n]+: Figure 3a shows the mass spectrum of the mixture between propane and η5-C5H5Fe(CO)2I. The signals at m/z 165 and m/z 193 correspond to adducts of propane and [η5-C5H5Fe(CO)n]+ for n = 0 and 1, respectively. This is supported by a mass shift of 8 amu (to m/z 173 and m/z 201 respectively) when C3D8 was used. The most abundant adduct observed was between propane and [η5-C5H5Fe]+, when no CO was ligated to iron. The mechanisms of formation of these adducts are mainly the same as discussed for methane and ethane, except for some differences. The main difference between propane and ethane is the secondary hydrogen atoms, (CH3)2CH–H, which are more loosely bound to the carbon than the primary hydrogen atoms.1 Insertion of an iron cation into a secondary C–H bond generates an intermediate that may rearrange by β-hydrogen shift,41 from which reductive elimination of dihydrogen may occur42–44. This is illustrated in Scheme 1. If bond insertion takes place, we would expect to observe peaks in the CID mass spectrum caused by rearrangement reactions followed by loss of a neutral molecule, e.g. C2D4 or CD4. In the CID mass spectrum of adducts between C3D8 and [η5-C5H5Fe]+, the main peak observed was [η5-C5H5Fe]+ (m/z 121), formed by loss of C3D8 from the parent ion. Loss of deuterium or an alkyl group was not observed. This would have been expected if the parent ion had been [η5-C5H5 Fe(C3D7)(D)]+. The CID spectrum of the adduct between C3D8 and [η5-C5H5Fe(CO)]+ showed successive loss of propane and CO as the main reactions. These results strongly support a reaction pattern analogous to the former results; formation of ion molecule complexes in the high pressure ion source by interactions between propane and [η5-C5H5Fe(CO)n]+, for n = 0 and 1. Reactions between propane and [η5-C5H5Co(CO)n]+: In the mass spectrum, Fig. 3b, of the mixture of propane and η5-C5H5Co(CO)2 the peak seen at m/z 196 corresponds to an adduct between [η5C5H5Co(CO)]+ and propane. The expected mass increase from m/z 196 to m/z 204 was observed using C3D8. The CID spectrum of the adduct of C3D8 and [η5-C5H5Co(CO)]+ showed that loss of C3D8 was the main fragmentation reaction (m/z 152, 100%). In addition, peaks corresponding to [η5-C5H5Co(CO)(CD3)(D)]+ (m/z 172, 5%) and [η5C5H5Co(CO)(CD3)]+ (m/z 170, 10%) were observed. The expected mass shifts, m/z 168 and m/z 167, were observed in the CID spectrum of the adduct between C3H8 and [η5-C5H5Co(CO)]+. The high degree of C3D8 elimination supports the assumption that the main product is an ion molecule complex between C3D8 and [η5-C5H5Co(CO)]+. However, the results also show that bond insertion has occurred to some extent. Reactions between propane and [η5-C5H5Ni(CO)n]+: Electron ionisation of the mixture between C3H8 and [η5-C5H5Ni(CO)]2, Fig. 3c, produced adducts of propane and [η5-C5H5Ni]+, m/z 167. The expected mass increase was observed when C3D8 was used. The CID spectrum of the adduct ion between C3D8 and [η5-C5H5Ni]+ showed loss of C3D8 as the main fragmentation path, giving a base peak at m/z 123. In addition, significant amounts of product ions were formed by rearrangement reactions followed by elimination of a neutral molecule. The peak at m/z 125 (30%) corresponds to [η5-C5H5Ni(D)]+ formed by loss of C3D7 from [η5-C5H5Ni(C3D7)(D)]+. The product ion at m/z 127 (30%) corresponds to η5-C5H5Ni(D)2+, which was formed by loss of C3D6 from [η5-C5H5Ni(η2-C3D6)(D)2]+. Formation of these ions is due to migration of the β-deuterium in [η5-C5H5Ni(C3D7)(D)]+ to nickel,

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as illustrated in Scheme 1. Isotope scrambling between deuterium atoms in C3D8 and the hydrogen atoms in the cyclopentadienyl ring followed by loss of propyl, is also assumed to contribute to the peaks found in the mass interval from m/z 124 to m/z 130 in the CID spectrum (30–40%). Reductive elimination of CD4 from the adduct between C3D8 and [η5-C5H5Ni]+ was not observed by CA. Absence of methane loss can be due to insertion of β-C–D bond rather than to α-C–D and the C–C bond.45 In addition, due to the weaker secondary C–D bond relative to the primary, these results support the assumption that nickel inserts into the secondary C–D bonds. The main conclusion to be drawn from these experiments is that the adduct between propane and [η5-C5H5Ni]+ consists of a mixture of an ion-molecule complex in addition to some [η5-C5H5Ni(C3H7)(H)]+ and [η5-C5H5 Ni(η2-C3H6)(H)2]+ ions.

Conclusion

From this study we have seen that the reactivity of [η5C5H5M(CO)n]+ towards alkanes depends both on the cluster size, the size of the alkane and the metal ion. The main conclusions from these experiments support first that we have mainly loosely bound alkanes to the ligated transition metal cations and secondly that bond insertion reactions are observed when propane and ethane are used as the neutral reactant. The authors wish to thank The Norwegian Academy for Science and Letters, and Statoil for financial support and Professor Einar Uggerud for helpful discussions. Received 15 December 2010; accepted 28 February 2011 Paper 1000481 doi: 10.3184/174751911X12992388104926 Published online: 00 March 2011 References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

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