Infrared and collision-induced fragmentation of iron ethoxide cations

and Ion processes

ELSEVl E R

International Journal of Mass Spectrometry and Ion Processes 171 (1997) 39-49

Infrared and collision-induced fragmentation of iron ethoxide cations D.A. Kirkwood, A.J. Stace* School of Chemistry., Physics and Environmental Sciences, University of Sussex, Brighton, UK Received 13 January 1997; accepted 23 February 1997

Abstract

The multi-photon infrared photo-dissociation (MPD) of iron ethoxide cations of the general form Fe+(EtOH)m(EtO)n has been studied in an ion trap using a line tuneable CO2 laser. The ions exhibit very characteristic infrared absorption profiles which are shown to be quite different from those recorded for Fe+(EtOH)n cluster ions. From a comparison with solid state data, it is suggested that the mode responsible for absorption is a C - O stretch in the ethoxide group. To complement the interpretation of the MPD data, the collision-induced dissociation (CID) of the same series of ions was promoted by the application of a tickle voltage to the end caps of the ion trap. Both MPD and CID generate the same fragment ions, but the relative intensities are different. A detailed study of Fe*(EtOH)(EtO) using isotopes reveals fragmentation pathways leading primarily to the loss of H2, CH20 and CH3CHO, all of which can be accounted for via a series of insertion and radical transfer steps. Central to many of the reactions is an initial insertion step which results in the formation of HFe+(EtO)2. © 1997 Elsevier Science B.V. Keywords: Infrared; Photo-fragmentation; Ion trap; Iron ethoxide; Cations

1. Introduction

It is almost 2 decades now since Allison and Ridge [1] published the first of their seminal papers on the reactions of gas phase transition metal ions with alcohols and alkyl halides. In the intervening period, this field has undergone a remarkable expansion with development of the concept of metal ion mediated bond activation in organic substrates being a key goal [2]. There are now many examples in the literature of specific bond activation in organic molecules of industrial relevance by transition metal ions [3,4], with reaction specificity governed principally by the thermodynamics of the system, the valence electron configuration and spin state of the metal [5]. The study of iron cation mediated gas phase * Corresponding author.

chemistry has been of particular interest, with many examples prevalent in the literature subsequent to that of Allison and Ridge [1]. Schwarz and co-workers [6-9] have undertaken a range of studies on the reactivity of Fe ÷ with alcohols and related organic synthons. Alternatives to dehydration, such as dehydrogenation, were examined [6,7] in which the influence of cyclic transition states in determining product pathways was proposed, especially for long chain alcohols. In other works from the same group, the effects of secondary ligands on the iron mediated reactions were investigated for a range of small functional groups (such as C H 3 , NH2) [8] and for larger groups such as butadiene [9]. It was found that reaction specificity was highly ligand dependent with inter-ligand exchange processes an integral part of product formation. Most of these reactions are initiated by metal

0168-1176/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PH S 0 1 6 8 - 1 1 7 6 ( 9 7 ) 0 0 0 2 4 - 4

40

D.A. Kirkwood, A.J. Stace/lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 39-49

insertion into the organic molecule and subsequently followed by r-hydrogen transfer where such atoms exist. There are numerous examples of reactions where this is known to occur, notably the iron mediated dehydration of ethanol [1]. However, there have been few attempts to rationalise this transfer in industrially relevant processes [10], although a recent review article by Weldon and Friend [ 11 ] provides an overview on surface experiments where r-hydrogen transfer is known to occur upon adsorption of aliphatic alcohols on certain transition metal solids. It has been postulated [12] that a knowledge of the intermediates formed in such transfer reactions will facilitate a proper understanding of r-hydrogen transfer. Metal alkoxides [13] are predicted to be such an intermediate species, and are known to exist in many single crystal surface science experiments [11] in addition to being of importance in areas of bulk phase chemistry such as mixed inorganic/organic polymer production [14]. In this work the multi-photon infrared photochemistry of various gas phase iron ethoxide ions is discussed, and from a comparison with CID data the mechanisms of the fragmentation channels of these species elucidated. The ion trap enables single ion species to be studied in isolation with the detection and subsequent analysis of any or all products from a gas phase reaction to be examined separately. As shall be demonstrated below it also enables facile in situ photo-dissociation studies to be carried out, with the added advantage of an (MS) n capability for the analysis of photo-fragments. The use of multi-photon infrared radiation as a probe of photo-chemistry is desirable as it is a comparatively gentle technique which is widely believed to promote only the lowest energy chemical pathways in the chromophoric species [15]. The alternative, which is to use a single visible or u.v. photon, may lead to fluorescence of the electronically excited molecule. One of the first examples of the use of infrared MPD was by Beauchamp where the elimination reactions of a

proton bound alcohol dimer were characterised [15]. In this instance, the Vco stretch of the gas phase ion was excited using infrared radiation from a line tunable CO2 laser. The photo-induced fragmentation did not display an appreciable wavelength dependence, and indeed this was also found to be the case in subsequent infrared experiments, by Beauchamp and co-workers on cobalt/alkene cluster ions [16]. However, work by Kaldor and co-workers [17] on iron/methanol clusters did yield wavelength dependent photodissociation spectra, and similar behaviour in metal/alcohol complexes was observed recently both by Kirkwood et al. [18] and Knickelbein [ 19]. In the experiments described here the rate of photon absorption is such that molecule specific chemistry is possible, as the rate of intermolecular vibrational energy transfer is relatively slow in the low pressure regime under which the ion trap is operated. Intra-molecular transfer is, however, still sufficiently rapid to preclude the observation of multi-photon mode specific chemistry using an infrared laser.

2. Experimental The route taken to generate gas phase cluster cations of the form Fe+(EtOH)n(EtO)m was to promote controlled ion-molecule reactions between isolated fragments of ionised iron pentacarbonyl with neutral ethanol molecules within an ion trap mass spectrometer (ITMS). The desired product ion from these reactions was then isolated and irradiated in situ using a CO2 infrared pulsed laser. All photo-fragment ions thus produced were then ejected from the trap and detected using a channeltron electron multiplier. The Finnigan MAT Ion Trap Mass Spectrometer (ITMS) and the Edinburgh Instruments PL4 CO2 laser used in this work have been described previously [18]. In essence, the ITMS is a low pressure quadrupole mass spectrometer, which ionises gas phase molecules in situ via electron impact and then holds the ions thus

D.A. Kirkwood, A.J. Stace/International Journal of Mass Spectrometry and Ion Processes 171 (1997) 39-49

formed within a small (--1 cm -3) volume. The ion cloud is maintained by a fixed frequency, variable amplitude waveform which is capable of trapping ions for many seconds. Ions may be selectively isolated according to mass using either DC isolation or SWIFT filtered noise fields and these techniques have been discussed at length elsewhere [20,21]. For collision induced dissociation (CID) mass selected ions are given kinetic excitation by the application of a secondary AC waveform (tickle voltage). This excitation promotes collisions between ions and background helium bath gas, which in turn leads to fragmentation. The primary purpose of the bath gas, which is typically 5 × 10 -5 Torr of pure helium, is to quench excess energy imparted to ions from electron impact ionisation. For the purposes of multi-photon excitation, the ion cloud is maintained within the infrared laser fluence for a duration of 20 ms. The laser enters and exits the trap through two small (1 mm radii) holes machined in the ring electrode and the laser power is monitored beyond the photodissociation points using a Molectron J50 joulemeter probe connected to a JD500 energy meter. Integrity of the RF (Radio Frequency) field was maintained by placing two nickel mesh plugs in the holes in the ring electrodes. The wavelength of the CO2 laser is continually monitored by diverting a small portion of the laser beam into a Mackern Instruments spectrum analyser using a ZnSe beam splitter. Photo-fragments remain trapped upon production and are detected in the usual manner.

3. Results

In general, if an iron n-carbonyl ion, (Fe+(CO)n), reacts with n molecules of ethanol, the resultant product ion is of the form Fe+(EtOH)n [18]. This is a multi-step process, with sequential substitution of the carbonyl ligands by ethanol molecules to liberate carbon monoxide. For larger values of n, the timescale for complete substitution may be -> 1 s using

41

moderate ( -< 1 x 10 -5 Tort) pressures of ethanol. However, for small values of n an iron carbonyl ion may react further upon complete loss of all carbonyl ligands by a mechanism whereby a further ethanol molecule attaches itself to the iron with subsequent loss of a hydrogen radical to generate an ethoxide group. These ethoxide ions undergo wavelength dependent photo-dissociation in the infrared, and both the absorption profiles and photofragmentation pathways are almost always different to those seen for ions of the form Fem+(EtOH)n [18,22]. Ethoxide ions also undergo a wide range of CID fragmentation processes, which are similar but not identical to those seen for multi-photon dissociation (MPD). The MPD and CID fragmentation reactions of Fe+(EtOH)m(EtO)n cluster ions are presented here for m = (0, 1, 2) and n = 1. To enable accurate mechanistic interpretation of the processes involved, these ions have been generated using the following isotopomers: pure ethanol (CH3CHzOH), d2-ethanol (CH3CD2OH), d3ethanol (CD3CH2OH) and d6-ethanol (CD3CD2OD). In addition, due to the propensity for the last of these alcohols to undergo facile hydroxyl exchange with surface hydrogens within the apparatus, it was possible to generate appreciable amounts of ds-ethanol (CD3CD2OH).

3.1. Bare Fe+(dn-EtO) The bare iron ethoxide cations, Fe+(dn-EtO), are unique amongst all other iron/ethanol and iron/ethoxide cations investigated thusfar in that they exhibit no propensity to absorb infrared radiation at any wavelength accessible to the CO2 laser (i.e. 920 cm -1-1090 cm-l). However, all four isotopomeric Fe+(dn-EtO) ions fragmented readily upon the application of a CID tickle voltage, and the ions generated are given in Table 1. The relative intensities of the fragment ions in this table were heavily dependent on the tickle voltage, and this observation is in general true for all the systems examined here.

42

D.A. Kirkwood, A,J. Stace/lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 39-49

Table 1 CID loss channels from Fe+(dn-EtO) Parent ion

Ion fragment(s)

Neutral fragment(s)

Fe+(EtO) Fe+(da-EtO) Fe+(d3-EtO) Fe+(ds-EtO) Fe÷(H) Fe+(D) Fe+(CH3) Fe+(CD3)

Fe+(CH3), Fe+(H) Fe+(CH3), Fe÷(D) Fe+(CD3), Fe+(H) Fe+(CD3), Fe+(D) Fe + Fe + Fe + Fe +

CH20, CD20, CH20, CD20, H. D. CH3. CD3.

CH3CHO CH3CDO CD3CHO CD3CDO

From the dissociation of Fe+(EtO) it is obvious that there are two dominant loss channels, namely the loss of 30 mass units to leave Fe+(CH3) and loss of 44 mass units to yield the ion Fe+(H). Subsequent CID experiments on both of these product ions yield the bare iron cation Fe +, indicating that the formation of Fe+(H) is a separate process to that which generates Fe+(CH3). In the absence of additional information the neutral co-products are assigned as the aldehydes methanal and ethanal. The isotopomeric data clearly yields complementary results to the above. Making the assumption that the presence of the deuterium atom(s) will only affect the relative reaction rates of the fragmentation channels and not the actual channels themselves, then the results obtained from

the fragmentation of Fe+(d2-EtO) and Fe+(d3 EtO) indicate that the Fe+(H) ion is formed from/3-hydrogen transfer and that the Fe÷(CH3) species is generated as a result of simple C 1-C2 cleavage followed by expulsion of neutral CH20.

3.2. Mixed Fe+(EtOH)1,2(EtO) The ion Fe+(EtOH)(EtO), generated from the reaction of Fe+(CO) with ethanol, yields a complex range of fragments upon both MPD and CID. As such, a detailed examination of this system was entered into using isotopomeric species, wavelength dependent MPD, controlled CID and (MS/MS) n analysis of fragment ions. Table 2 shows the isotopomeric series of ions described by the general formula Fe+(da-EtOH)(da-EtO) for a = 0, 2, 3 and 5 and their associated fragments. Although MPD results in a considerable number of fragment ions, it is clear that the losses mirror very closely those of the bare iron ethoxide ions, with the additional loss of molecular hydrogen appearing both as an independent loss channel and in combination with the losses of either methanal or ethanal. CID of the same ions yields similar fragments, but with the loss of ethanal dominating and much smaller signals observed for the loss of H2 (or

Table 2 MPD/CID loss channels from Fe+(d.-EtOH)(d.-EtO) Parent ion

lon fragment

Neutral fragment

Fe +(EtOH)(EtO)

Fe+(MeCHO)(EtO), Fe+(EtOH)(CH 3) Fe+(MeCHO)(CH3), Fe+(EtOH)(H) Fe+(EtO) Fe*(MeCDO)(d2-EtO), Fe+(d2-EtOH)(CH3) Fe+(MeCDO)(CH3), Fe+(d2-EtOH)(D) Fe+(d2-EtO) Fe+(CD3CHO)(d3-EtO), Fe+(d3-EtOH)(CD3) Fe+(CD3CHO)(CD3), Fe+(d3-EtOH)(H) Fe+(d3-EtO) Fe+(CD3CDO)(ds-EtO), Fe+(ds-EtOH)(CD3) Fe+(CD3CDO)(CD3), Fe+(ds-EtOH)(D) Fe+(d2-EtO) Fe+(CD3CDO)(d5-EtO), Fe+(ds-EtOD)(CD3) Fe+(CD3CDO)(CD3), Fe+(ds-EtOH)(D) Fe+(d2-EtO)

H 2, CH 20 H2 + CH20, CH3CHO H 2 + CH 3CHO HD, CD20 HD + CD20, MeCDO HD + MeCDO H2, CH20 H2 + CH20, CD3CHO H2 + CD3CHO HD, CD20 HD + CD20, CD3CDO HD + CD3CDO D2, CD20 D2 + CD20, CD3CDO D2 + CD3CDO

Fe +(d2-EtOH)(d2-EtO)

Fe*(d3-EtOH)(d3-EtO)

Fe *(d5-EtOH)(d5-EtO)

Fe*(ds-EtOD)(d~-EtO)

D.A. Kirkwood, A.J. Stace/International Journal of Mass Spectrometry and Ion Processes 171 (1997) 39-49 CID of Fe + (CO)

43

CID of Fe+(EtOH)(EtO) I

I

I

F~+(CO)

I

Fe+ (EtOH)(EtO)

--

Fe+(MeCHO)(EtO)

- - -

II

#.

i/ i/ 0

I

# 0.`5

I~

/

11 /

I 1

I 1.`5

CID of Fe + (C0)2 I I

\

2.5

t 0.5

0

.----.

i

i

I

I

13

2

2.5

C I D of Fe + (EtOH)2 (EtO)

I

I

F~+(CO)~ - -

\/

=

_J

I 2

I

I

Fe+ (EtOH)2 (EtO)

Fe+(CO) - - -

Fe+ (EtOH)(EtO)

---

"...F~+....

3 _a

I

/I

III/L

It' ~" 0

0.5

/

~.°*

.-

~. . . . . . . .

/

I

I

I

1

1

1 .,5

2

2.5

0

Tickle Voltage [ m V

i/

I

0.5

I 1

I

I

1.5

2

2.5

Tickle Voltage / m V

Fig. 1. Appearance energies of ion fragments following the application of a tickle voltage to the end caps of the ion trap. In each case the voltage has been scaled by the mass of the parent ion in order to reflect a 'centre of mass' collision energy. The data for Fe+(CO). are used to calibrate the system.

their isotopomeric equivalents). What the isotopomeric data does directly indicate is that the loss of molecular hydrogen cannot occur from the same ligand as loss of methanal, i.e. a single dz-ethanol or dz-ethoxide species cannot yield both HD and CD20. In addition, the loss of HD from the ion Fe+(ds-EtOH)(ds-EtO) indicates that one of the hydrogen atoms in the H 2 fragment is the hydroxyl proton of the alcohol. None of these ions exhibits any propensity to dehydrate, and there is no evidence to suggest isotopic

scrambling between hydrogens on either the two carbon sites or between ligands. Both of these observations are very different from those made on Fe+(da-EtOH), cluster ions [22]. Controlled CID experiments give a value of approximately --1.3 eV for the appearance energy of H2. This value was obtained via a comparison between the tickle appearance voltage of Fe+(MeCHO)(EtO) from Fe+(EtOH)(EtO) with those of known standards, in this case Fe+(CO),. The actual spectra are depicted in

Table 3 CID loss channels from daughter ions of Fe+(EtOH)(EtO) Daughter mass

Daughter ion

Ion fragment

Neutral fragment

145

Fe+(MeCHO)(EtO)

117 115

Fe+(EtOH)(CH3) Fe+(MeCHO)(CH3)

103

Fe+(EtOH)(H)

101

Fe+(EtO)

Fe+(MeCHO)(CH3) Fe+(MeCHO)(H) Fe+(EtO), Fe+(CH3) Fe+(CH3) Fe + Fe+(EtO), Fe+(H)(H~O), Fe+(CH3)(H2), Fe+(CH3), Fe+(H) Fe+(CH3), Fe+(H)

CH20 MeCHO CH4, EtOH MeCHO MeCHO + CH3H2, C2H4 CH20, EtOH CH20, MeCHO

44

D.A. Kirkwood, A.J. Stace/lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 39-49 Loss of H2 + CHsO from Fe+(EtOH)(EtO) I

I

I P II II II

I

II *1 lit |11 Pl[ 1111 IIII iii I III I t I II I IIIII I IIII I

]

t

I II II II iI I ,

920

,

I

,

060

1000

qlt

',

1040

* 1080

Loss of HD + CD20 from Fe+(d2-EtOH)(d2-EtO) I

I

I

I II II II

] I I I

020

I

I

060

1000

l

I

1040

,

I

1080

Loss of H~ + CHsO from Fe+(d3-EtOH)(d3-EtO) I

I

I

I

I

I

I

I

960

1000

1040

1080

/I 920

Laser Wavelength / em -] Fig. 2. Relative photo-fragment yields for the loss of H2 + CH20 from Fe+(EtOH)(EtO), plotted as a function of laser wavelength. Data are also presented for two isotopomers.

Fig. 1, and the tickle voltages displayed are weighted by the mass of each parent ion to yield values proportional to the centre of mass collision energy. Also shown is the appearance of Fe+(EtOH)(EtO) from Fe+(EtOH)z(EtO), which is almost identical in behaviour to

Fe+(EtOH)(EtO). Multi-photon dissociation results indicate that the most facile loss channel is H2, which, from a comparison with the CID data, would suggest that the parent ions need to absorb of the order of 13 infrared photons to promote MPD. To enable the elucidation of the exact fragmentation mechanisms occurring via CID of Fe+(EtOH)(EtO), tickle experiments (MS 2) were carried out on each of the daughter ions generated from this ion. The immediate granddaughter ions were thus produced and their neutral co-products are listed in Table 3. In each case the ion formula has been assigned without proof, but based on the losses observed for Fe+(d,-EtO) above. The results shown in Table 3 show that once an ion yields H2, it may still undergo loss of methanal or ethanal which again supports the earlier conclusion that H2 originates from the alcohol ligand. If the ion loses methanal as the first fragment, however, hydrogen is no longer lost and instead methane and ethanol are generated upon subsequent CID. The daughter ion at m/z 115, which must have been formed by loss of H2 then CH20 loses primarily ethanal to yield Fe+(CH3), which is also formed by the daughter ion at m/z 117. The final daughter ion at m/z 103 has a complex fragmentation pattern in its own right, with losses of H2, ethene, ethanal and ethanol being observed. It is worth noting that other than the parent Fe+(EtOH)(EtO) ion, the only daughter ion which absorbs in the infrared is that obtained from the loss of molecular hydrogen at m/z 145, indicating that the chromophoric structure is still intact in this daughter ion but in none of the others. If the wavelength dependence of the combined loss channel of hydrogen and methanal is examined for the ions Fe+(EtOH)(EtO), Fe+(d2 EtOH)(d2-EtO) and Fe+(d3-EtOH)(d3-EtO) a very interesting picture emerges (see Fig. 2). All three profiles have a remarkable similarity to each other, exhibiting highly structured absorptions in the 9R and 9P branches of the

D.A. Kirkwood, A.J. Stace/lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 39-49 Photo-induced Loss of H20 from F e + ( E t O H ) 2 l

l

I

Photo-induced Loss of EtOH from F e + ( E t O H ) 2 ( E t O )

l

I

I

I

920

I

I

I

960

1000

1040

1080

Laser Wavelength / c m - t

, fi920

960

I

J ,, ,,

II Ii II ~, I i I

45

]~

:/

, 1000

: i, ,

,.,,,

1040

1080

Laser Wavelength / c m -l

Fig. 3. As for Fig. 2, but for the photo-induced loss of H20 from Fe*(EtOH)2.

Fig. 4. As for Fig. 2, but for the photo-induced loss of EtOH from Fe + (EtOH)2(EtO).

C O 2 laser (1030 cm -~ to 1090 cm-~). Unfortu-

of transition metal/alcohol cluster ions, [22] the laser power was always chosen to dissociate approximately one half of the parent ion signal so as not to distort the spectra with the effects of power broadening. While the required laser fluence from the data in Fig. 2 was independent of the isotopomeric alcohol species, this again is not the case when dealing with ions of the type Fe+(dn-EtOH)m. In particular iron/d2-ethanol ions appear to possess a comparatively small absorption cross-section while iron/d3-ethanol ions exhibit facile photo-fragmentation. When a further molecule of ethanol is added to the above system to produce an ion of the form Fe+(EtOH)z(EtO), the nature of both MPD and CID fragmentation changes dramatically. The sole loss channel is the liberation of an intact molecule of ethanol, with no evidence for the loss of molecular hydrogen or any aldehyde species. This is a similar situation to that encountered with Fe+(dn-EtOH)3 ions, which also fragment to yield ethanol. The wavelength dependence for the loss of a molecule of ethanol from Fe+(EtOH)2(EtO) is displayed in Fig. 4, where it can be seen that this ion exhibits a complex profile with absorptions in the 10R, 9P and 9R regions of the CO2 laser. While at first sight this has a rather haphazard appearance, closer examination reveals that this spectrum bears a strong similarity to a

nately, due to the characteristic 'holes' present in the output of the CO2 laser, it is not possible to obtain data on these absorptions between 1060 cm -1 and 1072 cm -j. There are several important points which must be raised regarding these spectra. Not least of these is the fact that all three absorptions are almost identical. If these absorptions are compared with MPD spectra of iron ions ligated to only intact ethanol units, and an example for the loss of H20 from Fe+(EtOH)2 is given in Fig. 3, [22] it is clear that the modes of absorption are significantly different. When only intact ethanols are ligated to Fe +, the absorptions are more gaussian in appearance and the centre of absorption is very sensitive to the isotopomer of ethanol (e.g. for Fe+(EtOH)2 the maximum lies at --980 cm -~ while for Fe+(d3-EtOH)2 this value is - 9 3 0 cm -~) [22]. It has been noted [13] that almost all first row transition metal tri-alkoxides in the solid phase possess an absorption in the region of 1000 cm -l and 1100 cm <, which has been assigned by Winter and co-workers [23] as a u(C-O)M stretching frequency. As a final point, it should be mentioned that each of the three scans depicted in Fig. 2 were obtained at the same laser fluence (in this case 0.7 mJ per pulse). In all of the experiments reported here and in other work on the photo-dissociation

46

D.A. Kirkwood, A.J. Stace/International Journal of Mass Spectrometry and Ion Processes 171 (1997) 39-49

Fe

0

% ÷

FeCH3

2

+

CH3

,,<, FdiO

Fe~? 3

C~

CH20

H

F~H

CHMe

+

MeCHO

Fig. 5. Reaction pathways from iron ethoxide. {I } and {II }: Elimination of methanal and ethanal, respectively, from the iron ethoxide cation via a four-membered 'cyclic' transition state.

composite spectrum which would be obtained if the spectra for Fe+(EtOH) and Fe+(EtOH)(EtO) were plotted on the same graph. This observation implies that absorption in the 10R region is due to the additional ethanol ligand, while other features are due solely to the Fe+(EtOH)(EtO) core. The same type of behaviour has been noted for the iron/1-propanol system [22] where the ion Fe+(1-PrOH)2(1-PrO) has absorption features which are clearly characteristic of both Fe+(1-PrOH) and Fe+(1-PrOH)(1-PrO) ions. From the CID spectra shown in Fig. 1 earlier, it is obvious that the binding energy of the third ethanol ligand is approximately equal to the energy required to liberate H2 from Fe+(EtOH)(EtO). The fact that H2 is not a competing channel in the fragmentation of the Fe+(EtOH)2(EtO) ion may aid the interpretation of the role which the electronic structure of the metal ion plays in these systems.

ethanol ligands present on each type of ion. The formation of an ethoxide ion is believed to be due to the valence s-electron on the metal ion (ground state 3d64sl), forming a covalent bond with the oxygen atom. This behaviour is in contrast with the bonding which is assumed to be responsible for ligating intact alcohol molecules [22]. In this latter case, bonding is probably via a 7r-d interaction between the lone pair of electrons on an sp 3 oxygen atom and partially occupied d-orbitals on the metal ion. This interpretation is supported by the fact that manganese ions also react with ethanol to form an ethoxide, and that, in both cases, no ethoxide ligand is formed when there are carbonyl ligands present (which are known to involve s-electrons in bond formation). However, ethoxides are formed in the presence of other intact ethanol groups.

4. Discussion

The collision-induced excitation of bare iron ethoxide ions leads to two fragmentation channels, involving the loss of the aldehydes methanal and ethanal, and these are depicted as proceeding through two separate four-centre transition states

As in Section 3, the interpretation of iron/ ethoxide cations will be sub-divided into categories based on the number of additional

4.1. Bare Fe+(d,-EtO) ions

D.A. Kirkwood, A.J. Stace/International Journal of Mass Spectrometry and Ion Processes 171 (1997) 39-49

47

4.2. The fragmentation of Fe +(EtOH)1,2(EtO)

in Fig. 5. In both cases it would appear that the pi-bonded ligand is lost as a neutral fragment in preference to the more strongly sigma-bound hydride and methyl groups. Evidence for the two proposed mechanisms comes primarily from the isotopomeric distribution of the fragment ions. All the isotopomeric data are mutually consistent and indicate that the hydrogen atoms in the Fe+(CH3) product all arise from the terminal carbon while the hydrogen in the Fe+(H) product arises from /3-transfer. The observed preference for C - C cleavage is thought to be largely due to the relative bond strengths of the C - C bond and C - H bonds on the methyl group [24]. The failure of bare iron ethoxide ions to undergo photo-dissociation upon irradiation is difficult to understand, given that Fe+(EtOH) (EtO) and Fe+(EtOH)2(EtO) both exhibit strong MPD signals for processes which have been observed following the CID of Fe+(EtO). The reasons for this failure are unclear, but may be due to a pronounced shift of the Vco stretching mode by the metal ion which results in the transition moving out of range of the CO2 laser.

The rather complex fragmentation channels of Fe+(dn-EtOH)(dn-EtO) upon MPD and CID may be readily understood in terms of a few basic subprocesses. The ions generated through the initial loss of methanal and ethanal are clearly the same as for the bare ethoxide species, and as such it is believed that the same four membered transition states invoked to account for that reaction remain valid. From the isotopomeric data, the loss of molecular hydrogen arises from a combination of the hydroxyl proton from the alcohol with a /3-hydrogen from either the alcohol ligand or the alkoxide group on the iron ion. It is believed that this occurs through initial insertion by the iron ion into the O - H bond, followed by the abstraction of an/3-hydrogen. Fig. 6 gives a selection of possible fragmentation pathways based on this interpretation. The subsequent fragmentation of daughter ions may be interpreted in an analogous manner. The daughter ion at m/z 117 does not yield H2, instead C H 4 is predominantly formed. This species is generated through a combination of the methyl

YY . - L~lt ,l~l :O:--Fe--.O.---~---'~--H . Y Y

Yy

Fc--.O~--¢---H

/ -H2

o

/I

-- Fe--6--C

-OH#I Ill)

-~.~.o/{II

--.

nc

n

Fig. 6. Reaction pathways from Fe+(EtOH)(EtO). {I} and {II}: Proposed mechanism for the photo-induced dehydrogenation of Fe÷(EtOH) (EtO), followed by elimination of ethanal and methanal, respectively.

48

D.A. Kirkwood, A.J. Stace/lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 39-49

group on the iron (which is formed with methanal via the four-centre transition state shown in Fig. 5) and the hydroxyl hydrogen. Once this methane group has been lost, clearly further generation of H2 is no longer possible. This assertion is corroborated by the fact that the daughter ion at m/z 115, formed through the loss of H2 followed by the loss of CH20 does not lose methane, as there is now no longer a hydroxyl proton for the methyl group to combine with on the iron ion. It is of course not possible from these experiments to elucidate the origin of the 13-hydrogen in the product H2, as of course once the hydroxyl hydrogen is transferred to the iron ion both the alkoxide and alcohol ligands are effectively identical. To resolve this, a series of experiments was undertaken using mixtures of isotopomeric ethanol. What these experiments suggest [22] is that both ligands can contribute /3-hydrogens to the H2 molecule and, furthermore, that upon transfer of the hydroxyl proton to the iron ion either ligand could then behave as an alkoxide group and subsequently yield the aldehydes methanal and ethanai. In all cases, the only product ions which were infrared active were those which lost just molecular hydrogen. Turning to the ion Fe+(EtOH)z(EtO), the fact that this species loses an intact ethanol molecule may either be interpreted as the effect of the ethoxide ligand precluding the dehydration reaction of the Fe+(EtOH)2 ion or the effect of the additional ethanol ligand preventing the dehydrogenation and aldehyde formation reactions of Fe+(EtOH)(EtO) from taking place. It is likely that the further occupation of partially occupied 3d-orbitals on the iron ion by a lone pair of electrons on the oxygen of the third alcohol ligand acts as a block to insertion reactions by Fe +. Further reactivity involving the 4s-electron of the iron is also not possible as this is assumed to be taken up with the covalent bond to the ethoxide radical. The wavelength dependence of the loss of ethanol from Fe+(EtOH)2(EtO) is very informative, as

it clearly indicates the presence of both the ethoxide and ethanol groups as separate chromophores in the ion. The contribution from ethoxide is further confirmed by the fact that EtOcontaining fragments also undergo infrared photofragmentation. It is quite possible that HFe+(EtO)2 is formed from Fe+(EtOH)(EtO) at a very early stage in the reaction sequence, which is why the infrared absorption profiles the features shown in Fig. 2, rather than a contribution from an EtOH absorption. There is no indication of any mode dependent chemistry occurring upon irradiation (i.e. where irradiation at one wavelength leads to one photo-chemical process while irradiation at another yields a distinctly separate photo-chemistry). This is to be expected as other multi-photon studies [25] have shown that for mode dependent chemistry to occur the rate of photon uptake must be substantially in excess of the rate of intra-molecular energy transfer. In general, this situation is only true for ultra-short (pico-second) laser pulses. Due to the pressure regime under which the trap operates the rate of inter-molecular energy transfer is negligible with respect to the rate of photon absorption and so only molecule specific photochemistry is observed, rather than mode specific. 5. Conclusion

The series of ions Fe+(EtOH)n(EtO)l, formed by the reactions of iron carbonyl cations with excess gas phase ethanol, exhibit unique fragmentation products when photo- and collisionexcited within an ion trap mass spectrometer. In addition, the ions have highly characteristic absorption profiles in the infrared, which are very different to those observed for either iron ions or small iron clusters when ligated with intact ethanol molecules. These ions readily undergo a variety of dehydrogenation and oxidation reactions under the application of both MPD and CID to yield molecular hydrogen, methane and the first two primary aldehydes. The ion by-products of these reactions revolve around the ubiquitous

D.A. Kirkwood, A.J. Stace/International Journal of Mass Spectrometry and Ion Processes 171 (1997) 39-49

formation of the sigma-bonded species Fe+(H) and Fe+(CH3). Data obtained from the use of isotopomerically distinct ethanol molecules have enabled mechanisms involved in the formation of these ions to be deduced. Experiments on ions containing mixed isotopomers of ethanol have led to the proposal of a hydride switching mechanism being a key step in the above reaction sequence, where upon excitation the ion inserts into the O - H bond of the ethanol to create two equivalent ethoxide groups. This hydride switch influences a large number of the fragmentation pathways (for example, it can probably be assumed that ions such as Fe+(EtOH)(CH3) are in fact of the form (H)Fe+(OEt)(CH3)). The product distribution is then governed principally by the relative stabilities of the species formed. Acknowledgements The authors would like to thank the EPSRC for financial support and for the award of a research studentship to DAK. References [1] J. Allison, D.P. Ridge, J. Am. Chem. Soc. 98 (1976) 7445. [2] P.B. Armentrout, T. Baer, J. Phys. Chem. 100 (1996) 12866. [3] K. Seemeyer, D. Schrfder, M. Kemp/ O. Lettau, J. Muller, H. Schwarz, Organometallics 14 (1995) 4465.

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[4] V.B. Goncharov, A.V. Kikhtenko, A.N. Startsev, Kinet. Catal. 37 (1996) 4774. [5] E.R. Fisher, P.B. Armentrout, J. Am. Chem. Soc. 114 (1992) 2039. [6] T. Priisse, H. Schwarz, Organometallics 8 (1989) 2856. [7] S. Karral3, T. Priisse, K. Eller, H. Schwarz, J. Am. Chem. Soc. 111 (1989) 9018. [8] D. Schrrder, J. Hrug~tk, H. Schwarz, Ber. Buns. Ges. Phys. Chem. 97 (1993) 1085. [9] S. Karral3,D. Schrrder, H. Schwarz, Chem. Ber. 125 (1992) 751. [10] O. Blum, D. Milstein, J. Am. Chem. Soc. 117 (1995) 4582. [11] M.K. Weldon, C.M. Friend, Chem. Rev. 96 (1996) 1391. [12] A. Fiedler, D. Schrrder, H. Schwarz, B.L. Tjeha, P.B. Armentrout, J. Am. Chem. Soc. 118 (1996) 5047. [13] D.C. Bradley, R.C. Mehrotra, D.P. Gaur, Metal Alkoxides, Academic Press, 1978. [14] U. Schubert, N. Husing, A. Lorenz, Chem. Mater. 7 (1995) 2010. [15] D.S. Bomse, J.L. Beauchamp, J. Am. Chem. Soc. 103 (1981) 3292. [16] M.A. Hanratty, C.M. Paulsen, J.L. Beauchamp, J. Am. Chem. Soc. 107 (1985) 5074. [17] M.R. Zakin, R.O. Brickman, D.M. Cox, K.C. Reichmann, D.J. Trevor, A. Kaldor, J. Chem. Phys. 85 (1986) 1198. [18] D.A. Kirkwood, J.F. Winkel, A.J. Stace, Chem. Phys. Lett. 247 (1995) 332. [19] M.B. Knickelbein, J. Chem. Phys. 104 (1996) 3517. [20] J.N. Louris, J.S. Brodbeltlustig, R.G. Cooks, G.L. Glish, G.J. Vanberkel, S.A. McLuckey, Int. J. Mass Spectrom. Ion Processes 96 (1990) 117. [21] M.H. Soni, R.G. Cooks, Anal. Chem. 66 (1994) 2488. [22] D.A. Kirkwood, DPhil Thesis, University of Sussex, 1996. [23] L. Dubicki, G.A. Kakos, G. Winter, Aust. J. Chem. 21 (1968) 1461. [24] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 74th ed., 1994. [25] V.N. Bagratashvili, M.V. Kuz'min, V.S. Letokhov, A.A. Shibanov, JETP Lett. 37 (1983) 112.