Mass spectrometry has made significant contributions to coordination and organometallic chem- istry.65–87 One of the most important applications of mass spectrometry is the determination of the molecular weight and elemental composition of metal compounds, and identification of their structure. Mass spectrometry was first applied to the analysis of relatively volatile metal com- plexes using electron impact and chemical ionization techniques. Further development of ioniza- tion techniques has made it possible to analyze and to study the structure of a wide range of organometallics, including nonvolatile, ionic, multiply charged, polymetallic, and high-molecular- weight derivatives. Some of the above ionization techniques allow identification of metal- containing intermediates directly from the condensed phase88–91(see also Chapter 2.30) providing beneficial information about reaction mechanisms.
A typical procedure for mass spectral characterization of an unknown metal complex by mass spectral methods includes several stages. The determination of the molecular weight of the analyte is a first step. The molecular weight can be obtained from the observation of ‘‘molecular species.’’
The latter may be positively or negatively, singly or multiply charged molecular ions, ions of protonated (MHþ) or deprotonated ([M–H]) molecules, or adducts of M with various positively or negatively charged ions. The type of molecular species produced depends on the ionization mode and can be predicted in most cases. The use of more than one ionization technique greatly benefits the correct identification of the molecular ion. The nonobservation of the (expected) molecular ions is often overlooked and considered as a failed identification of the (metal) compound. In many of these cases, however, the analysis of the whole set of mass spectral data may provide the necessary information for positive identification of the molecule without the observation of Mþor Mions.
Volatile and thermally stable complexes can be analyzed by EI, CI, APCI, and TSP methods. ESI is a likely first choice for analysis of nonvolatile and thermally unstable molecules. Most ionic com- pounds can be directly analyzed in the ESI. Several approaches can be employed for the ionization of neutral metal-containing complexes.88–91 They include either oxidation or reduction of the metal atom, which may be achieved in an electrochemical cell or using metal electrospray capillary as an electrode. Chemical derivatization of complexes, including ligand removal or substitution reactions, quaternization of nitrogen or phosphorus atoms in ligands, and metallation of complexes using metal ions, are common ways to make metal-containing molecules amenable for ESI analysis.
Desorption methods are also useful for the ionization of both ionic and neutral molecules. FD, LD, FAB, and SIMS have been used for the characterization of nonvolatile metal complexes.92,93 MALDI is probably the most convenient method for analysis of high-molecular-weight analytes, such as complex biomolecules, metal-containing polymers, etc.
After the molecular weight has been deduced, the elemental composition of the molecular ion can be determined. The isotope distribution of peaks of the molecular ion may provide a thorough preliminary evaluation of the quantity and type of metal atoms. This analysis is based on the fact that most metal atoms have specific isotope distributions. The next step towards the determination of the elemental composition is to measure accurate mass(es) of the molecular ion(s). These measurements require a mass analyzer with appropriate mass resolution. Sector instruments, TOF mass spectrometers, and ICR spectrometers are the most suitable for accurate mass measurement.
Detailed information about the structure of the metal complex can be obtained by using various mass spectral resources. Processes of electron attachment and removal, e.g., ionization and redox reactions, may give an insight about the oxidation state of the metal, its coordination saturation, and the number of ligands in the coordination sphere. However, the most convenient and direct way of elucidating structures of ions in mass spectrometry can be achieved by studying their reactivity. Two general types of reactions are used for this purpose. One of themis unimolecular dissociation of mass-selected ions of interest. Ions that are formed with a small excess of internal energy above their dissociation limit may fragment on the flight from the ion
source to detector. These transitions provide information about the weakest bonds in the ion. For stable ions, a variety of methods can be applied to increase the internal energy of the ions and to induce their dissociation. The other type of process that is used for studying atomconnectivities in gas-phase, metal-containing ions includes reactions of these ions with neutral molecules.
The most common question that is addressed in the elucidation of the structure of organo- metallic compounds is what the ligands are that surround the central metal atom(s). A common approach to this problem is to break the metal–ligand bond(s). This dissociation may occur upon ionization of molecular ions having an excess of internal energy. A sequential loss of metal–ligand bonds can also be achieved by activation of the ion. The observed mass loss leads to the knowledge of the elemental composition of the ligand. Similar information can be obtained from ligand substitution ion–molecule reactions. Coordinatively saturated molecular ions are usually inert to neutral reactants.
The recognition of atomconnectivity in ligands is not always a straightforward task and a combination of experimental data should be considered. Reactions involving radical and neutral molecule losses, as well as migration of groups to the metal atom, are the most informative for the recognition of the structure and location of substituents in the ligand. The most abundant processes are those involving atoms and groups located in close proximity to the metal atom, for example, groups in- and-positions of the substituent in ligands. It is useful to compare the mass spectral behavior of the unknown compound with the reactivity of similar derivatives having a well-established structure.
Some isomeric organometallic complexes may be recognized using mass spectral methods.94 The difference in the mass spectra of isomers usually comes from the interaction of the metal atomwith electronegative groups or unsaturated bonds in the substituent. These interactions increase the bond strength between the metal atom and the ligand, resulting in a preferential loss of other ligands. If an electronegative group is involved in such an interaction, then its migration to the metal atom may occur. The extent of the latter reaction depends on how close the migrating group is to the metal atom.
Another effect that may result in differentiation of isomers is a metal-catalyzed loss of neutral molecules from a ligand(s). The isomer having an electronegative group in closer proximity to the metal atom usually easily loses this group as a part of a neutral molecule.
2.28.2.2 Interpretation ofMass Spectra ofMetal Compounds
The interpretation of mass spectra of positively charged metal-containing ions is based on two general concepts.
The concept of valence change95 has been applied to rationalize and predict the fragmentation of a variety of metal-complexes. It operates on the following assumptions:
(i) The metal atom contains an even number of electrons in the parent neutral molecule;
(ii) Stabilization of molecular ions occurs when the metal atom can increase its oxidation state (OS). Fragmentation of these complexes involves a loss of even-electron species (molecules);
(iii) If the metal atom has only one stable OS, then molecular ions are of low intensity and their fragmentation will likely involve loss of a radical, followed by losses of neutral molecules;
(iv) If the metal atom can be easily reduced, then the dissociation of molecular ions will involve a predominant loss of two radicals.
The valence-change effect is illustrated by the behavior of aluminum (one stable OS: III) and iron (two stable OSs: III and II) trisacetyacetonates, Mt(acac)3 (Scheme 1). Both complexes produced modest molecular ions in the EI mass spectra, with the dominant loss of one -diketonate ligand producing Mt(acac)2þ
ions in which metal atoms retained their stable OS III. The Fe derivative also underwent a significant loss of the second ligand, giving rise to ions having the metal atom in the OS II. The abundance of this process in the EI mass spectrum of Al(acac)3was very low, due to the low stability of AlII. The dissociation of complexes having two metals with two stable oxidation states follows the same rules.
The most successful interpretation of mass spectra of transition metal-complexes is based on the concept of charge localization on the metal atom.80,94 Ionization energies of the majority of organometallic molecules are lower than those of the ligands, but differ only slightly from the ionization energies of the free metal atoms. Accordingly, the ionization of transition metal
Mass Spectrometry 373
complexes most likely involves the removal of an electron from the metal atom. As a result, on the decomposition of molecular ions of transition metal hydrocarbon ions the positive charge usually remains on the metal-containing fragment. The other consequence of the charge localization on the metal atom is that the dissociations of organometallic ions become controlled by the central metal atom. Unlike ionized metal-free organic molecules, their metal complexes lose even-electron neu- trals rather than radicals. Also mechanisms of similar reactions in the coordinated and metal-free organic molecules are often different. A direct participation (‘‘catalysis’’) of the metal atom in transformations of the ligand regulates these mechanisms (Scheme 2).
NHCOCH3
NHCOCH3
Cr+ Cr+
NH2
H H
NH
+ +
- CH2CO CH2CO
-
Scheme 2
The formation of neutral, metal-containing species upon fragmentation of organometallic ions does not contradict the initial charge localization on the metal atom. The positively charged metal atomis easily attacked by electronegative groups in a ligand, forming stable, metal-containing molecules.
C5H5FeC5H4COClỵ!C5H5FeIIClỵ C5H4COỵ ð1ị This effect is common for the interaction of metal ions with functionalized organic reactants:
Mgỵ ỵClCH2CH2Cl!MgIICl2 ỵ C2H4ỵ ð2ị
2.28.2.3 Thermochemistry ofOrganometallic Molecules and Ions
Mass spectrometry can be used to determine the fundamental thermochemical parameters of organometallic molecules (see also Chapter 2.12), such as ionization energies (IEs), electron and proton affinities. IEs can be obtained by determining the ionization thresholds of electron- induced ionization (vertical IE), or photoionization (adiabatic IE). The electron exchange between a positively charged ion and a neutral molecule (electron-transfer bracketing) allows the estima- tion of adiabatic IEs. In this approach, various reference compounds are introduced and the observation of (direct or reverse) electron-transfer reactions indicates which molecule has a lower IE. If it is possible to establish accurately a pressure for the neutral reactant, then the electron- transfer equilibriumcan be measured, giving IE values with high accuracy.
(acac)3Mt +
(acac)2Mt(III)+
(acac)
M = Fe 12% 59% 12%
Mt = Al 8% 88% <1%
- acac - acac
Mt(II) +
Scheme 1
Similar experiments, involving electron transfer between an anion and a neutral molecule, yield relative or absolute EAs. The method has been used to determine relative free energies for electron attachment for a variety of metallocenes and -diketonate molecules. Electron photo- detachment spectroscopy of negatively charged ions96 is another source for obtaining electron affinities of molecules. These data provide an important component of thermochemical cycles involving oxidation/reduction of metal complexes, and serve as a basis for obtaining other thermochemical values.
The understanding of substituent effects in metal complexes is one of the goals of electron- transfer equilibrium studies. For example, alkyl substitution in metallocenes predictably decreased their IEs. At the same time, it has been shown that the gas-phase EAs can be increased by a larger alkyl substituent in the hydrocarbon ligand.
Anion-transfer reactions to/from metal complexes are sources for the anion affinities of organometallic molecules.63,64 To illustrate: the hydroxide affinity of (CO)5Fe has been deter- mined by measurement of the equilibrium constant for hydroxide transfer exchange between (CO)5Fe and SO2. This value was used to estimate the heat of formation of (CO)4FeCOOH. Ion–molecule reactions of these ions and their collision-induced dissociation gave rise to a variety of negatively charged species having a coordinatively unsaturated metal atom. A study of their reactivity is a good source for obtaining the thermochemical characteristics of elusive metal complexes that cannot be produced otherwise.
Proton-transfer equilibrium measurements and proton-transfer bracketing methods are sources for proton affinity values of organometallic complexes. The determination of the site of proto- nation, i.e., metal atom vs. a ligand, is a fundamental dilemma of any study on the protonation of metal complexes. It was demonstrated, for example, that Fe(CO)5was protonated exclusively at the metal atom, whereas the results for the proton transfer to ferrocene can be explained by the formation of a metal-protonated form and a ring-protonated form, involving the agostic interac- tion of the proton with the metal atom.
Cation-exchange reactions are a common source for obtaining cation (CH3þ
, Naþ, NH4þ
, etc.) affinities of neutral molecules.
2.28.2.4 Chemistry ofMetal–Ligand Bonds
Knowledge of metal–ligand bond energies is fundamental information for organometallic chem- istry. It is essential for the understanding of catalytic reaction mechanisms, which often involve the cleavage or formation of these bonds. Mass spectrometry offers a series of experimental methods for determining absolute and relative bond strengths between a positively or negatively charged metal center and ligands69–71,76 (see also Chapter 2.12). Direct determination of metal–
ligand bond dissociation energies (BDE) can be performed by measuring appearance energies (AEs) of the molecular and fragment ions. The best results are obtained from AEs of ions produced by metastable dissociation of mass selected precursors. Kinetic energy release distribu- tion97 during metastable dissociation of ions is another source for the quantitative characteriza- tion of metal–ligand bonds. The experimental results obtained by this method require theoretical calculations in order to extract the information on the enthalpy change for the observed processes.
Appearance energy measurements and photodissociation of ions were used for measuring BDEs in anions.63 Energy-resolved, collision–induced dissociation is another source for obtaining infor- mation about bond stabilities. The general trend in metal–ligand bonds is that they decrease by going fromanions to neutrals and to positively charged ions. In LnMtXcomplexes, the ligands X having the highest electron affinity are usually those forming the strongest MtX bonds.
A large number of both relative and absolute bond energies in metal-containing ions have been measured by the kinetic method.98The method operates with an abundance of products formed by competitive ligand loss. The metal–ligand bond enthalpies can be determined from the metastable and collision-induced dissociation of LMtþL0 ions, where L and L0 are different molecules. The general trend in metal–ligand BDEs is that the larger alkyl derivatives are bound to the metal atom more strongly than their smaller homologs. A particular advantage of the kinetic method is that it can probe ions containing thermally unstable ligands—chemistry which is difficult to study in the condensed phase.
Ion–molecule ligand-exchange reactions are a convenient method for obtaining relative and absolute metal–ligand BDE. By using this approach, the affinities of molecules for a variety of
Mass Spectrometry 375
‘‘bare’’ and ligated metal ions have been determined. Two-ligand systems, D0(Mtþ2L), as well as negatively and doubly charged positive ions, have been characterized.
Monitoring the thresholds of endothermic reactions by the guided ion-beam technique is one of the methods used to obtain the most accurate bond energies (see also Chapter 2.12). Similar measurements can be performed employing the FTICR technique. General types of endothermic reactions studied to obtain BDEs are as follows (Mtþis a ‘‘bare’’ or ligated metal ion).
Mtỵ ỵ RX!MtXỵ ỵ R ð3ị
Mtỵ ỵ RX!MtXỵ Rỵ ð4ị
MtYỵ ỵ RX!MtXỵ ỵ RY ð5ị
If the thermochemistry of reactants (Mtþ, MtYþ, RX) and one of the reaction products (R, Rþ RY) is well established, then using the experimentally measured threshold energy of reaction, one can calculate MþX and MX bond strengths.
The results on metal–ligand BDEs obtained by different methods are usually in good agreement with each other. However, a critical analysis of the experimental techniques, understanding their limitations and sources of errors as well as the knowledge of trends in changing BDEs, is highly recommended before accepting a specific thermochemical value.
2.28.2.5 Transformations of Molecules on Charged Metal Centers
Mass spectrometry is widely used for studying reaction mechanisms involving metal-containing reaction intermediates. A great majority of these studies involve the investigation of transform- ations of organic molecules on ligated or ‘‘bare’’ metal ions.66–71,79,80,84
The gas-phase reactivity of charged metal clusters87 was the subject of multiple investigations in a search for mechanisms of reactions on metal surfaces. Reactions (1)–(5) provide a few examples from a large number of processes involving bond cleavage within ligands. Particular interest in this field of research is focused on intrinsic mechanisms of metal-ion-induced CH, CC, and C–heteroatombond activation reactions, being models for the elementary stages of important homogeneous and heterogeneous catalytic processes, metal ion biochemistry, synthesis of electronic and ceramic materials, etc. Unlike positively charged metal atoms, their negatively charged counterparts showed inertness toward saturated hydrocarbons and olefins.83 Other small molecules, including H2, CO, NO, N2, CO2, O2, NH3, and CH3OH, were subjected to reactions with positively and negatively charged metal-containing ions.
A series of experiments on the reactivity of metal ions with nitriles, RCN, led to the discovery of the remote functionalization mechanism.72,73 The initial interaction of the metal ion involves coordination at the nitrile group. The insertion of the metal atom into a CH or CC bond occurs only after the alkyl chain becomes long enough (at least three or four methylene groups) to interact with a remote bond. The dissociation of the metal-hydride or metal-alkyl intermediate results in a loss of H2alkene or alkane molecules, depending on the structure of the hydrocarbon group R.
Important reactive metal-containing intermediates (e.g., metal-benzyne complexes, MtCO2þ
) and processes (e.g., decarbonylation, oxidation, reductive addition) of practical interest have been characterized using various mass spectral methods, and provide insight as to the mechanisms of organometallic reactions.
2.28.2.6 Gas-phase Metal Negative Ion Chemistry
Negatively charged metal-containing complexes play an important role in inorganic and organo- metallic chemistry in solution. Various industrial processes involve anionic reagents, metal sub- strates, or else proceed via nucleophilic reactions. Mass spectrometry provides tools for the identification of anionic reaction products and intermediates, and for the study of their reactivity in the gas phase.83This method has been useful for the investigation of unimolecular dissociation
and ion–molecule reactions of metal-containing anions, in order to obtain information about their and their neutral counterparts’ reactivity and thermochemistry.
A variety of thermochemical data were obtained for negatively charged metal complexes. For example, appearance energy measurements; photodissociation, energy-resolved, collision-induced dissociation, electron-, proton- and anion-transfer reactions to and fromnegatively charged ions, were all used for obtaining a variety of thermochemical parameters for ions, molecules, and radicals.
Although the general approach to the study of gas-phase reactions of negatively charged ions by mass spectrometry is very similar to that for the chemistry of their positively charged counter- parts, some specifics can be mentioned. First, some ionization techniques (NICI, REC) are designed for the production of negatively charged ions in the gas phase.82Most of the work on negative ion chemistry83 has been carried out with the use of ICR (or FTICR), flowing afterglow techniques (FA)63and Knudcen cell mass spectrometry.64
The Knudcen cell method is used to characterize negative-ion equilibria taking place in the saturated vapor over binary mixtures containing metal compounds. This technique is especially valuable for measurements of the heats of formation of fluorides, MtFn
fluoride ion affinities, D[MtFnF], and high (>5 eV) electron affinities.
The main advantage of the FA method is that ions are carried through the flow tube to the mass spectrometer in the ‘‘stream’’ of a buffer gas at relatively high pressure (0.1–1.0 Torr).
During this time they become thermally equilibrated and achieve thermal energy distribution.
Ion–molecule reactions of the ions can be studied by the addition of small flows of neutral reagents along the length of the flow tube. Reliable kinetic measurements for ligand-attachment and ligand-substitution reactions have been performed for MtLn ions.
The overall contribution of negative metal ion chemistry to the gas-phase ion chemistry of metal complexes is relatively small, but it should not be overlooked.
2.28.2.7 Elusive Neutral Organometallics Generated from Ions
Neutralization-reionization mass spectrometry (NR MS) (see also Chapter 2.34) is a unique mass spectral technique that allows the generation of neutral species fromtheir charged counterparts.
The major application of NR MS is to produce unstable reaction intermediates that cannot be isolated or characterized by other means, to yield new, previously unknown molecules and radicals. The method has been used successfully to generate a variety of organometallic species.85,86 A wide range of elusive metal-containing molecules (AuF, PrF, C5H5FeF, C5H5Rhacac, etc.) and radicals (NiCCH, C5H5FeC5H4CO, (C5H5)2Zr and others) have been generated and characterized for the first time using the NR MS method. The observation of these species in the gas phase suggests their possible formation in other than gas-phase experimental conditions, at least as short-lived reaction intermediates. This information is used by chemists to confirm and evaluate the mechanisms of organometallic reactions, including elementary steps of catalytic transforma- tions on metal centers.
2.28.2.8 Contributions ofCoordination Chemistry to Mass Spectrometry
Complexes of metals have contributed to various fields of mass spectrometry. They may serve as standards for mass calibration. Molecules with well-established ionization energies, bond disso- ciation energies, proton affinities, etc., are used for the determination of the thermochemical characteristics of other molecules and ions. Metal complexes have played an important role in establishing concepts of interpretation of mass spectral data, and in understanding ionization and ionic fragmentation mechanisms. The presence of a metal atom often changes dramatically the dissociation pathways of organic molecules. For this reason, coordination with metal ions can be used for the generation of organic and inorganic molecules and radicals with unusual atom connectivities.
Metal-containing ions are useful reactants for the identification of organic compounds by mass spectrometry. The formation of metal adducts is especially advantageous when traditional meth- ods of ionization (EI, CI) do not result in stable molecular ions or protonated species. Chemical ionization with metals and metal-containing ions provides high selectivity and sensitivity to specific types of analytes (unsaturated and functionalized hydrocarbons, peptides, crown ethers,
Mass Spectrometry 377