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By Ed Perley
The material given below has been extracted from my Masters thesis. It has not appeared in any chemical journals. As such, it had not been scrutenized to the extent that a published article would be. The purpose of this page is to first provide information and ideas that others might find useful in their research. Secondly, it is to give readers a view of the complexity of what would appear on the surface to be a fairly simple system.
Because biological systems are extremely complex, biochemists often work with simpler systems, called model systems, that approximate these systems. Schrauzer(1) worked out a model system for fixation of nitrogen, using a cysteine-molybdenum(V) solution. It could not attack the nitrogen molecule, but it did quite sucessfully reduce hydrazine to ammonia, possibly the last step in nitrogen fixation.
The problem came in determining the structure of the redox active molybdenum(V) complex. Analysis and separation of compounds from the solutions indicated two major components. One was a momomeric oxomolybdenum(V)-cystiene complex. The other, obtained in less alkaline solution, was a dioxobridged dimeric oxomolybdenum(V)-cystiene complex. Unfortunately, neither compound showed significant redox behavior. It was concluded that there must be some intermediate structure or species that can reduce hydrazine.
In the hope of learning more about this elusive species, scientists looked at even simpler solutions. Molybdenum(V) solutions in hydrochloric and hydrobromic acids also show an equillibrium between monomers and dioxobridged dimers that is dependent on acid concentration. The basic system of Molybdenum(V) species is described in Figure 1, below. A number of workers (4,5,6,10,12) have proposed the presence of two monomeric molybdenum(V) species predominent at acid concentrations above 6M, designated here as Species A and B. As the acid concentration is reduced below 6M, a dioxobridged dimer, Species D, forms. Species C is the unknown intermediate.
There seems to be some question about what is located in the position opposite the terminal molybdenum oxygen bond. The majority of workers have concluded that it is occupied by a bromide ion. Observations I describe in another another page on this site lead me to conclude that in both cases it is probably a water molecule. If that is the case, Species A has a charge of -1, Species B is nonionic, and Species D has a charge of -2. It is possible that as the acid concentration is reduced further, water molecules could replace two of the coordinated bromide ions in Species D, resulting in a nonionic molecule. Changes in absorption in the ultraviolet as the acid concentration is reduced below 3 M suggest such a transformation.
Table I. Structures and Properties of Four Molybdenum(V) Species
The above system is not limited to aqueous solutions. Wendling(10) observed similar electronic absorption spectra from molybdenum(V) solutions in formic acid containing dissolved hydrogen bromide. My research demonstrated an analogous system also exists in ethanol-hydrogen bromide solutions of ammonium oxobromomolybdate(V).
The electron spin resonance (esr) spectra of dilute molybdenum(V) solutions in 13.4 M and a 6 M hydrobromic acid solution are shown in Figure 1. (The line labeled DPPH is the very narrow esr spectrum of an internal standard commonly used with these spectra. Esr spectra are commonly displayed as second derivative curves, to make them more distinct. What you are seeing are changes in the slope of the signal peak, rather than the peak itself. ) They have g values of about 1.993 and 1.988, respectively. (A completely free electron, would have a g value of 2.000.) The Species A signal, predominent in 13.4 M acid, is sharp and distinct. The Species B signal, predominent in 6M acid, appears less distinct and more assymetrical. The esr spectra of both solutions indicate that each species contains an unpaired electron. This is characteristic of a monomeric oxomolybdate(V) structure. As the acid concentration is reduced, changes in the relative concentrations of species A and B are indicated by a broadening and increased assymmetry of the esr signal. An increase in the concentration of the unknown Species C may also contribute to the change in the signal. Below an acid concentration of 5 M, the solutions show no esr signal, indicating the complete conversion to one or more dioxobridged dimers.
Figure 1. Esr Spectra of Molybdenum(V) Solutions in 6M to 13.5 M Hydrobromic Acid.
The visible absorption spectra of Species A and B in hydrobromic acid are quite distinctive (Figures 2A and 2B). There are two intense, sharp peaks at 375 nm and 414 nm, with a weaker band at 470 nm. There is a single weak broad band in the near infrared at 710 nm. The relative concentrations of Species A and B are indicated by changes in the relative intensities of the 375 nm and 414 nm bands. At higer acid concentrations (above 9.5 M), a single species is predominent, indicated by the 414 nm band being considerably more intense than the 375 nm band. At lower acid concentrations, the relative intensity of the 375 nm band surpasses that of the 414 nm band. These changes are consistant with the changes in the esr spectrum over the same range of acid concentration. They also indicate that, as does the esr spectra, that Species B increases in abundance at the expense of Species A as the acid concentration is reduced.
Figure 2A. Electronic Absorption Spectrum of a Molybdenum(V) Solution in 13.5 M Hydrobromic Acid.
Figure 2B. Electronic Absorption Spectra of Molybdenum(V) Solutions in 6.1 M Hydrobromic Acid.
At acid concentrations below 5 M, the Molydenum(V) species is generally believed to have the dioxobridged structure of Species D. Dioxybridged molybdenum(V) compounds are not paramagnetic because the two normally unpaired electrons are in close enough proximity to pair up with each other. Molybdenum(V) solutions at acid concentrations below 5 M show this kind of electronic absorption, they exhibit no paramagnetism, and they produce no esr signal at all.
As in the molybdenum(V)-cysteine solutions, there is evidence of an intermediate species that is most abundant at hydrobromic acid concentrations of 5.5 M to 6.5 M. This unidentified Species C indicates it's presence by a broad, somewhat weaker overlapping electronic absorption band centered at 450 nm. As the acid concentration is reduced to 6M, the 450 nm band increases in prominence at the expense of the 375 nm and 414 nm bands. The much weaker band at 710 nm at the same time, becomes more intense and shifts to about 750 nm.
Unlike the other Mo(V) species, the electronic absorption spectra indicate that Species C shows a very definite dependence on the Molybdenum(V) concentration. This is most clearly observed when the acid concentration is between 6.0 and 6.5 M. The bands attributible to Species C become much more prominent when the molybdenum(V) concentration is increased above .01 M. Figure 2B shows the dependence of this intermediate species on acid concentration. It should be noted that the 450 nm bands and 750 nm bands are also characteristic of the equivalant molybdenum(V) intermediate in hydrochloric acid solutions. Using changes in electronic absorption band intensities, workers(6,10) have concluded that Species C has a form that is intermediate between the dioxobridged dimers at low acid concentraions and the monomers at high acid concentrations.
More evidence concerning the identity of Species C was obtained from magnetic susceptability studies (4,14,15). It was concluded that Species C is paramagnetic, but not to the extent that the monomeric species are. A dimer with two hydroxo bridges was suggested. The idea was that this looser, more unstable structure would permit some paramagnetic behavior. This structure was proposed for both hydrobromic and hydrochloric acid solutions.
The results from electron spin resonance (esr) spectra were more ambiguous(12,16). It was concluded that if Species C does produce a signal, it is too weak to be seen clearly. Its presence was indicated by a decrease in the esr signal intensities of Species A and B as the acid concentration was reduced to 6M.
There was one more interesting characteristic of the intermediate species. Though the electronic absorption spectra of the monomeric species in hydrobromic acid are significantly different from those of their counterparts in hydrochoric acid, the spectrum of the intermediate species is similar in both solvents. Also, the electronic absorption spectra of the dioxobrided species believed to be present at acid concentrations below 5M are similar in both acid solutions. Apparently, when the number of halide ions coordinated to the molybdenum decrease, the corresponding species in hydrobromic and hydrochloric acid solutions become electronically more similar to each other.
One might think that it should be possible to precipitate Species C from solution. Unfortunately, nothing with electronic spectra or magnetic properties similar to the species has been precipitated from 6M acid solutions. Most cations generally precipitate oxotetrabromo or oxopentrabromo molybdenum(V) compounds from 6M hydrobromic acid. The organic ligand 2,2 dipyridyl precipitates a nonionic monoxobridged diamagnetic compound from 5 M hydrobromic acid. But this insoluble product's electronic absorption and reflectance spectra are much different from the spectrum of the solution from which it was precipitated from. It also shows no paramagnetism, unlike Species C. At slightly higher acid concentrations, a mixture of the compound and the salt, 2, 2-dipyridyl oxopentabromomolybdate(V) precipitates out.
After duplicating the electronic absorption research on Molybdenum(V) hydrobromic acid solutions reported in the literature, I moved into some new areas that had not been reported before. I believe that I have come up with some more evidence concerning the identity of the intermediate molybdenum(V) species. I used ammonium oxopentabromomolybdate(V) as my solute in most of the solutions I describe below.
Investigations of electronic absorption spectra were extended to much higher molybdenum(V) concentrations. It should be noted that all of the oxobromomolybdate compounds precipitated from acid solutions were from ones with very high molybdenum(V) concentrations. I recorded the visible absorpion spectra of some of these solutions in 6M hydrobromic acid. The very high color densities of these solutions required the use of 0.03 mm and 0.07 mm cuvettes. (The making of these cuvettes from microscope slides will be discussed on another page on this site some time in the future.) At a molybdenum(V) concentration of 0.07 M, the 450 nm band (attributed to the intermediate Species C) becomes the most distinct and intense band in the visible spectrum (Figure 3).
A further increase in concentration up to 0.3 M does not have much further affect on the shape of the spectrum, but the band intensities (relative to the actual molybdenum(V) concentration) are only about a third as much. This may be attributable to axial polymerization of the terminal molybdenum oxygen bonds that has been proposed (15). I have prepared two oxobromomolybdate(V) compounds that appear to show an axially oxobridged structure. These are described elsewhere.
Figure 3. Visible Absorption Spectrum at a Higher Molybdenum(V) Concentration.
The esr spectra obtained in 5.9 M hydrobromic acid were quite weak when the molybdenum(V) concentration was low. Increasing the molybdenum(V) concentration above 0.1 M produced a significantly different esr signal. Two of these esr spectra are shown in Figure 1, above. The spectrum taken at a molybdenum(V) concentration of .29 M (c) was the most distinct. The symmetry of the signal was intermediate in position, width and symmetry between Species A and Species B, with an estimated g value of 1.991. It's intensity was much weaker than the signals obtained at lower molybdenum(V) and higher acid concentrations.
In the course of experimenting with solutions of molybdenum(V) solutions in organic solvents, I attempted to dissolve ammonium oxopentabromomolybdate(V) in ethyl ether. The result was very much unexpected. The ether was off the shelf, containing a low concentration of ethanol to stabilize it. It also probably contained some absorbed moisture. These impurities were essential to the results obtained below. In the purified solvent, the molybdenum(V) compounds were completely insoluble and unreactive.
When ammoniumoxopentabromomolybdate(V) was stirred with ethyl ether, a yellow solution and a white insoluble reside consisting almost entirely of ammonium bromide resulted. Elemental analysis of the precipitate was obtained and its weight was compared with that of the original compound. It was found that all of the ammonium ion had precipitated out as it's bromide salt, amd almost all of the molybdenum was still in solution. The ratio of molybdenum to bromide in the solution was therefore 1.0 to 3.0. Allen and Neumann(9) also described a molybdenum(V) solution of the same compound in ether that produced a similar spectrum. I must take issue with their conclusion that the spectrum was attributable to the oxopentabromomolybdate(V) ion, because of the composition of the salt that precipitated out.
This reaction was attempted with other oxobromomolybdate(V) compounds. The ammonium oxopentabromomolybdate salt was the only one that produced this seemingly pure solution. Some of the others reacted with etheyl ether to produce a spectrum resembling that of Species B. With all of the compounds tried, except for the ammonium one, most the sample remained undissolved, even after a very thorough stirring.
The electronic absorption spectrum of the molybdenum(V) solution is shown in Figure 4. It shows strong absorption at 452 nm, where the band attributed to Species C is found. The weak band at 680 nm also matches absorption attributed to the species. The spectrum is different from the spectrum of Species C in aqueous solutions in one respect. There is a single band at 367 nm that seems to be atturbutable to the compound. An equivalent band was not identified in the hydrobromic acid solutions. If it is present there, it must be considerably weaker. The intensities of all of the visible and near infrared bands decreased significantly within an hour, indicating the solution was rather unstable. As in hydrobromic acid, the concentration of this species was clearly dependent on the molybdenum(V) concentration. It should be noted that some of this dependence could be caused by irreversable decomposition during the dilution process, resulting from a decrease in the acidity of the solution. Addition of hydrogen bromide gas to the solution produced a spectrum resembling that of a 10 (-4) M Molybdenum(V) solution in 6.5 hydrobromic acid. It is likely that this could indicate a structure similar to Species B.
Figure 4. Electronic Absorption Spectrum of Mo(V) Solutions in Ethyl Ether.
The ethyl ether solution produced a very distinct esr signal that resembled those shown by the concentrated molybdenum(V) solutions in hydrobromic acid, even though the molybdenum(V) concentration in the ether was considerably lower. The band showed a g value of 1.986, slightly lower than that observed in hydrobromic acid. The esr spectrum of one of the ethyl ether solutions is shown in Figure 4. Also shown is the spectrum of a solution to which hydrogen bromide gas was added. As does the electronic spectra described above, it suggests to me the possible presence of a compound similar to Species B. The presence of a compound similar to Species C is also indicated by the esr spectrum.
Attempts to isolate the Molybdenum(V) compound from ether solution were unsuccessful. If a solution was evaporated below a certain point, it would rapidly decompose, producing molybdenum blue, a complex polymer containing molybdenum in both the V and VI oxidation states.
The final attempt to isolate the species was done under the most extremely inert conditions this worker had available to him. A previously prepared solution was evaporated in a 6 inch test tube under a flow of nitrogen gas. During the course of the procedure, the test tube was chilled by suspending it above a dewar flask containing liquid nitrogen.
The evaporation process demonstrated that the compound was very soluble in ethyl ether. A precipitate only appeared just as the solution was going to dryness. It formed a very fine bright yellow powdery film on the bottom of the test tube. The dry precipitate remained there for a few seconds before decomposing into molybdenum blue. I concluded that the compound was too unstable for me to separate using the equipment I had available.
The limited information obtained seem to indicate it is likely that the species observed in etheyl ether and 6M hydrobromic acid could have the same structure. Though it could not be isolated from either solution, there is some evidence concerning the structure of the compound. The following is based on the assumption that the basic structures are equivalent, if not exactly the alike.
As described above, ammonium oxopentabromomolybdate(V) reacts with ethyl ether to produce a molybdenum(V) solution with a molybdenum to oxygen ratio of 1 to 3. Since all of the cation has precipitated out, and the remaining solute is highly soluble in a very slightly polar solvent, it would seem likely that the molybdenum(V) species is nonionic. It's dependence on concentration and it's weak magnetic susceptablility suggest that it is probably a dimer. Also, my experiences in trying to isolate it and the short lifetime of it's solutions indicate that the compound it is quite unstable and capable of redox behavior. It's structure, therefore, would be a good candidate for being equivalent to that of the redox active species in the cysteine model systems. The dihydroxobridged structure, shown below, is suggested. This is similar to structures proposed by Jezowska-Trzebiatowska, B. and M. Rudolf (14,15) for hydrochloric and hydrobromic acid solutions. They have also proposed that in hydrobromic acid, the species are axially bridged into tetramers through molybdenum-terminal oxygen-molybdenum bonds.
Figure 5. A Possible Structure for Species C.
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