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 * An Introduction to Matrix Isolation: A Brief Glimpse from an Inorganic Perspective

Drexel University Department of Chemistry 5 December 2009 **
 * A summary in the form of a powerpoint presentation is available on youtube in four parts:

Introduction to Matrix Isolation Presentation Part 1

I. Introduction**

Matrix isolation is a very unique technique that empowers chemists to essentially take a snapshot of molecular behavior. The number of desirable systems for study in matrix isolation grows more an more every year, as is evidenced by the increasing number of publications in the field. Even in review articles of Matrix Isolation, the amount of material covered is growing at an astonishing rate. 1,2,3,4,5 While at first glance it seems a strong member of the physical chemistry department, there are plenty of applications in analytical, organic, and inorganic chemistry; the latter of which will be the primary focus of this article.

It would be folly to begin an introduction to matrix isolation article without first acknowledging its creator, the late George C. Pimentel, so called "Father of Matrix Isolation". 6 Pimentel was a UCLA graduate in 1943, and was quickly swept away into the Manhattan Project at Berkley. It was not long before Pimentel realized the scope of the project, and decided that he would best serve his country by joining the armed forces (the U. S. Navy). 7 Once the war was over, Pimentel returned to Berkley to work on a Ph.D. under Kenneth Pitzer, and finished his degree in just three years! He immediately turned around and joined the faculty at Berkley in 1949. It was not long before Dr. Pimentel would publish his first paper on Matrix Isolation (1954). 8 He continued to teach and research as an active member of the Berkley faculty until his death in 1989. His life is commemorated by numerous awards, classrooms, publications, and conferences such as the biennial "Chemistry and Physics at Low Temperatures". 3 

There are several goals of matrix isolation. They can generally be summed up, however, as an attempt to study unstable molecules. By virtue of low pressures and temperatures, coupled with an inert "matrix", species can be prevented from reacting. Some examples of unstable species that are used for study are: Reactive intermediates, radicals, short-lived charged species (cations and anions), hydrogen bonding, high temperature vapor species, low energy molecules (conformers/tautomers/rotomers), species undergoing energy decay, relaxation, or transfer, and photochemically induced species.

There are some challenges to matrix isolation, however, that have required quite a bit work to get around. One of the earliest was obtaining reasonably sound bond length data. This is usually overcome by using two simultaneous experiments and detection methods, one of which is usually EXAFS. Another challenge facing matrix isolation chemists is obtaining data on excited state molecules. It is in the nature of the technique to prevent such excitations, and so creating excited states in a controlled environment requires the use of an excitation laser or light source within the matrix box. Also, traditionally work done under matrix isolation conditions was under much scrutiny, and frequently scientists would challenge them by saying that there is too much guesswork involved determining and assigning data. The advent of //ab initio// calculations using Density Functional Theory (DFT). These calculations using software such as //Gaussian// allow matrix isolation chemists to create accurate profiles of what their data should look like, and give them something in which to compare their experimental data. In the coming pages there will be different examples of how each challenge was overcome by some quite brilliant scientists.


 * II. Experimental Conditions**

Figure 1 shows a typical matrix isolation setup, with some ink annotations to help explain the key parts.

The **__Red Circle__** is the matrix box, where all the chemistry happens. The **__Green Circle__** is a vacuum pump, designed to keep the matrix box at near vacuum conditions. It is not uncommon for matrix boxes to have vacuum pumps left on for several minutes at a time to ensure that the box reaches pressures on the order of 10 -7 torr. The __**Blue Star**__ is the refrigeration unit. Matrix boxes (the deposition window in particular) must be kept at very low temperatures around 5 to 50 K. This is usually accomplished using liquid helium. The **__Black Star__** is the detection instrument (FTIR in this case). The real versatility of matrix isolation shines in its utility of multiple detection methods. Although IR is the staple for most matrix isolation labs, work has been done using (and this list is likely incomplete): Raman, NMR, EPR, ESR, ENDOR, UV-vis, Mössbauer, EXAFS, MLD, MCD, MCPL (Magnetic Circular Polarized Luminescence), Nuclear Quadrupole Resonance, and INS (Inelastic Neuron Scattering).



Figure 2 illustrates the deposition process. The **__sample (gray)__** may be just about anything in the gas state. The **__yellow-orange gas is the (typically inert) matrix gas__**. Argon is pictured here, but Neon, Xenon, Krypton, Methane, Carbon Dioxide, Nitrogen, Oxygen, and logically other gases can be used as well. The window is also variable, the frame usually made from copper, and the window of KBr, CsI, among others.

Figure 3 depicts the sample trapped in an inert matrix. Notice that the guest is completely surrounded, and it is unlikely that the guest will have any interaction with any other guest species, unless it is small enough to diffuse through the matrix. Water is almost always present inside the box, even under vacuum, though matrix isolation does a very good job at preventing interaction with the guest.



There are some additional intrinsic properties of a matrix box that provide advantages to scientists. By the nature of an inert matrix, there is generally no interference with the electronic ground state or molecular symmetry of the guest species. However, there are some cases in which the guest species does interact with the matrix in a unique way, and these are typically more surprising and notable results (one of which will be shown later in a study of metal dihalides). Also, the low temperature, low pressure conditions of the matrix box generally do not affect spectroscopic measurements (according to Perutz et al, less than 1% for neon matrices and 2% for argon matrices). 1 On the other hand, some characteristics are more greatly affected (and expectedly so), such as ionization energy and spin-orbit coupling constants.

There are a variety of ways in which materials can be deposited on the window within the matrix box. The deposition can be continuous or pulsed (although pulsing is more popular for more control over reaction conditions); and variations can be made for use of a cold quartz microbalance, or the use metal organic chemical vapors for deposition.

Although samples can be premixed before deposition, it is often preferable to wait and allow guest species in interact somewhat within the matrix, and trap them in a desired configuration. If energetically favorable, guest species can interact on the window if they come in through separate lines. Pyrolysis and Photolysis are some relatively primitive and straightforward means of sample preparation, though there are many others that used more advanced hardware like lasers and electron bombardment apparatus.


 * III. Experiments involving Inorganic Compounds**

//A. Nickel Ethylene: a Reinvestigation// 9,10,11

In 1992, Galan et al ran some experiments on various configurations of nickel ethylene of form Ni(C 2 H4) n, where n = 1, 2, or 3. They ran these experiments under matrix isolation conditions, and in most cases reported four distinct bands in the infrared region for these molecules. It all seemed like perfectly publishable material, and publish they did! However, this was before the major movement to perform //ab initio// calculations with matrix isolation experiments.

Lo and behold one year later, Papai publishes a paper on DFT calculations of the same molecule. It was certainly a great idea to double check his results against Galan et al's findings, seeing as how Papai predicted //__twelve bands__// would be present for these molecules.

It was not long before group headed by Lee investigated the compounds fully. By 1996, the technology was available to create even higher resolution spectra of molecules, and in this case it empowered Lee's group to confirm Papai's findings. Not only were they able to detect the vibrations of these molecules, but they also used deuterated forms of ethene to help them calculate the //force constants// of each bond! This brought some real closure to the study of matrix isolated nickel ethylene complexes.

Do not be so quick to criticize the work of Galan and colleagues - they did good work to be sure - but they simply were not looking for these other eight bands. They did not have any kind of baseline or way to compare their results to theory. So they made the best of the findings they had. This series of events really hi-lights the effectiveness of doing one's own //ab initio// calculations to help confirm results obtained from matrix isolation studies.

//B. Iron Oxide: Evidence of an FeVI species// 12,13,14

Iron Oxide studies were carried out in Ar and O 2 matrices, and three major infrared bands were noted when annealed to a CsI window at 10 K at 1204.5, 945.6, and 797.1 cm -1. When annealed at 30 and 50 K, a new band was observed at 956 cm -1. This band is attributed to an O-O stretching mode, which gives evidence for Fe-O-O and η 2 Fe(O 2 ) species. Both of these species are structurally similar to that of heme proteins, which gives hope to creating an enzyme mimic. The Fe molecules were laser ablated before insertion into the matrix, which possibly makes them responsible for such a strange oxidation state on iron (this is an example of Fe VI, though the coveted Fe VIII state remained out of reach). 3

Presumably, there must have been some energy barrier that prevented the extra bond from forming at low temperatures. By adjusting the temperature and amount of photolysis to just the right levels, these scientists were able to force an interesting conformation. Such is the power of matrix isolation!

//C. Mercury Methylation: Addition of a Methyl Group to d 10 Transition Metals// 15

Zinc, Cadmium, and Mercury are unique //d 10 // transition metals: their common oxidation states of I and II are (relatively) stable and long-lived. This makes them very prone to bonding to only one or two molecules to fill an electron shell. In particular it is important to observe how Hg adds to a methyl group to form H-Hg-CH 3 in an Ar matrix.

The stability of these metals actually worked against Greene's group, so they had to modify their setup accordingly, as seen in figure 4. There were only two real conceivable ways to activate Hg, and they chose to go the microwave route (photochemical reactions were the alternative, which can be touchy).



Here's a tabular result:

Table 1: A list of peak frequencies and their assigned vibrations for the molecule H-Hg-CH 3.
 * = Vib. Freq. (cm -1 ) ||= Assignment ||
 * = 526.5 ||= δ(C-Hg-H) ||
 * = 534.0 ||= ν(Hg-C) ||
 * = 777.9 ||= CH3 Rocking (fund) ||
 * = 779.8 ||= CH3 Rocking (fund) ||
 * = 1191.8 ||= sym δ(CH3) ||
 * = 1424.7 ||= asym δ(CH3) ||
 * = 1955.3 ||= ν(Hg-H) (fund) ||
 * = 2921.2 ||= sym ν(C-H) ||

From table 1, the first note is the fundamental bands. These are diagnostic bands that tell scientists where they stand in terms of data collection... are these bands present? Are they intense? Do the shift uniformly in some way? All these parameters help process the data.

Once the fundamental bands are found and assigned, it becomes easier to assign the rest. The symmetric C-H stretch at 2921.2 cm is very characteristic of a methyl group, and fairly intense; it comes as no surprise that it is shown here. Some of the peaks involving Hg are a lot less common, and probably took the longest time to assign.

From the data, it was reasonably concluded that Hg adds to a C-H bond, thereby bonding the metal to the methyl group, and likely also bonding the hydrogen to the metal (while hydrogen may be able to diffuse through an Ar matrix, it is likely just as favorable to stay in the matrix and bind to Hg instead).

This experiment is interestingly repeated with ethane instead of methane to see if these metals would attack the C-C bond at all. The spectra came out roughly the same as in the methane-metal experiment. The absence of a C-Hg-C vibration indicates that dimethyl mercury is not formed. All things considered, it is fortunate that they did not form dimethyl mercury, due to its high toxicity.

//D. Metal Dihalide Studies: What Can Be Said About the Bond Angles of These Species?// 16,17

A question of bond angles in first row transition metal-dihalide compounds plagued scientists for a good period of time. The main factor here was that as conformations approach linearity, spectroscopic predictions become less and less accurate. Thus, attempts at infrared studies of intensity and isotopic shifts proved fruitless. It was not until a two-dimensional experiment running EXAFS and FTIR were used to solve this problem.

NiBr 2 species were studied in multiple matrices using these detection metrics. By causing the molecule to vibrate in a controlled manner, Young et al were able collect accurate EXAFS data that would illuminate that NiBr 2 is in fact linear... almost...

In all matrices except N 2, NiBr 2 proved to be linear with only minor isotopic shifts. In N 2, however, the bond angle of Br-Ni-Br was shown to be about 125°! Clearly, the NiBr 2 must have some kind of interaction with the N 2 matrix, but the papers here do not quantify the interaction or give any further answers on the subject. Still, it stands to reason that some of the most interesting results are those that are unexpected - in this case the interaction with the N 2 matrix.

//E.// //η// //5// //Heavy Metallocenes: Using Matrix Isolation to Monitor LMCT// 18,19,20

η 5 systems have always been extremely elegant. The chemistry is complicated, but rewarding. In this particular case, multiple heavy metallocenes were investigated under matrix isolation conditions in Ar to see if an LMCT was evident. LMCT stands for ligand to metal charge transfer, which essentially means that an electron must move from the ligand to the metal. Since bonds are comprised of electrons, it should be easy to see if one moves around in a compound by tracking molecular vibrations. This is especially so in an a delicate π system in metallocenes.

In particular Mo, W, and Re centered η 5 metallocenes were studied, in addition to decamethyl( η 5 -Cp 2 Re). The metallocenes had to be generated //in situ// by photolysis, and from there three primary vibrations were studied. They are noted as ν2 (ring breathing) & ν4 (ring-metal-ring stretch), and ν3 (symmetric C-H vibration). When the molecule is in its ground state, all of the observed vibrations are well within their normal characteristic parameters. When the authors threw in some laser-induced fluorescence by means of a XeCl laser, however, the metallocene changed dramatically. ν4 (ring-metal-ring stretch) increased by 5%, where ν2 (ring breathing) fell 3% and ν3 (symmetric C-H vibration) fell 9%. It is evident that an electron is moving from the cyclopentene rings (ligands) which logically causes a more intense ring-metal-ring vibration. The loss of the electron is shown equally well by the decay of energy in the rings alone.


 * IV. References**

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 * // 5. Almond, M. J. and Goldberg, N. Annu. Rep. Prog. Chem., Sect. C: Phys. Chem., 2007,  //****103 , ** 79-133, DOI: 10.1039/b605697k//