Tricarbonyl(methylcyclopentadienyl)manganese

Stability: thermal, sunlight, metals

Link to relevant study record(s)

Reference

Endpoint:

stability: thermal, sunlight, metals, other

Remarks:

Migrated from section 'Stability: thermal, sunlight, metals'

Type of information:

experimental study

Adequacy of study:

key study

Study period:

Not mentioned

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Qualifier:

no guideline available

GLP compliance:

no

Remarks:

Study conducted prior to GLP guidelines

Test substance stable to sunlight:

no

Rate of decomposition in sun and ultraviolet light

Several series of samples were exposed to direct sunlight on different days. The results of these tests, shown in Figure 1, display the concentration of AP-3 manganese plotted against exposure time. Figure 1 also shows the decomposition rate for samples with initial concentrations of only 0.5 g Mn/gal. Each of these curves shows a characteristically steep decomposition rate for the first hour and then levels off. This decomposition pattern was observed for both commercial and highly purified AP-3.

A sample that was exposed to the light for ten minutes and then stored in the dark for three hours contained the same amount of AP-3 as one that was analyzed immediately after ten minutes of exposure. Therefore, partial decomposition does not catalyze further decomposition. The sample requires additional light energy to continue to decompose.

Ultraviolet light decomposes AP-3 (in isooctane) similar to sunlight. Analysis of samples exposed to a 250-watt UV reflector lamp are shown in Figure 2. The decomposition curve for samples at twenty-two inches distance is very similar to the average sunlight decomposition. At a closer distance (12 inches) the initial rate of decomposition is accelerated, indicating that light intensity is a controlling factor.

Effect of light wavelength on decomposition

A series of (Corning) optical glass filters was used to isolate specific wavelengths of light. These filters were sealed into the top of heavy cardboard boxes so that only light passing through the filter could reach the sample. Samples in one-ounce glass bottles were mounted with their long axes parallel to the filter at a distance of two inches below it. They were exposed to the bright sunlight for 30 minutes. After exposure, the samples were centrifuged and analyzed for AP-3.

Since sunlight was being used as the source, relative measurements were made by preparing five filter boxes and exposing five samples with their respective filters at the same time. Two series of tests were made at the same time of day on successive days and it is believed that all the measurements are comparable. Differences observed in the concentration of AP-3 were attributed solely to the effect of variations in the intensity of wavelengths of light.

The spectra of the filters used in the first test are shown in Figure 3 and those for the second test are shown in Figure 4. For comparison purposes, the absorption spectrum of AP-3 is shown in each figure for the spectral region which is transparent to glass. The concentration of AP-3 after exposure is expressed as grams of manganese per gallon (g Mn/gal) and is shown for each filter used. In every case the amount of sludge observed in the samples paralleled the losses indicated by quantitative analyzes.

An inspection of these data shows that light of wavelengths above 440 nm does not decomposed AP-3. A comparison of the effectiveness of filters, which transmit variable quantities of light in the spectral region where AP-3 absorbs, demonstrates that the light energy absorbed by AP-3 itself is what catalyzes its oxidation.

Effect of oxygen

In order to determine the effect of oxygen on the rate of decomposition , samples were exposed to sunlight in contact with air. The results of these tests are shown in Figure 5. One set of samples contained only 15 ml of solution in one-ounce bottles (about half full) with an atmosphere of air above the solution. These samples were shaken for 15 seconds to allow oxygen to saturate the solution. The control samples had no atmosphere and show the characteristic decomposition curve. The samples that contained an air atmosphere continued to decompose at the initial rate. These results suggest that the chemical reactions involve oxidation catalyzed by light activation.

To test this conclusion, dissolved oxygen was removed from an isooctane solution of AP-3 by bubbling nitrogen through the stock solution for 15 minutes. This solution was aliquoted to one-ounce bottles (clear glass), sealed, and exposed to sunlight for various lengths of time. The amount of undecomposed manganese and exposure times are shown in Figure 5. These data show that removing dissolved oxygen greatly reduced the amount of AP-3 decomposition. The initially rapid decomposition rate leveled off after 15minutes indicating that the residual oxygen had been consumed and further decomposition was arrested. The infrared spectra of samples, blown with nitrogen, showed two new bands in the carbonyl region which were not identified. These are probably representative of intermediate oxidation products.

Effect of Tetra ethyl Lead (TEL)

The effect of TEL on the light decomposition of AP-3 was studied by blending approximately 3.0 ml of neat TEL into isooctane solution of AP-3 (1.0 g Mn/gal) and exposing the samples to direct sunlight for various periods of time. The amount of AP-3 decomposition for samples in the presence and absence of an air atmosphere are shown in Figure 6. These data show that the amount of decomposition for these samples is identical with unleaded AP-3 isooctane solutions.

Effect of different fuel base stocks

Three fuels were selected from the premium fuel survey samples to determine the effect of different fuel base stocks on AP-3 stability. Each fuel was blended with AP-3 to the concentration of 1.10 g Mn/gal and aliquots were subjected to sunlight exposure with no atmosphere above the solutions. The exposure times and final concentration of manganese for each fuel are shown in Table 2. These data show that the AP-3 decomposition rate is the same for these 3 fuels, or for isooctane but continues to virtual completion.

Table 1 – Effect of base stock on AP-3 decomposition

Fuel^1	Time exposed to sunlight (min)
	0	22	90	175	259
SL 1522	1.10	0.99	0.51	0.22	0.10
SL 1526	1.10	0.90	0.49	0.20	0.07
SL 1527	1.10	0.97	0.47	0.17	0.08

¹Premium grade commercial leaded fuels, no atmosphere

Effect of fuel dyes and one UV absorber

Several commercial dyes and one UV absorber were added to isooctane solutions of AP-3in an effort to control the light catalyzed oxidation. Samples of isooctane and AP-3 (1.09 g Mn/gal) were prepared with various concentrations of fuel dyes and two sample contained the UV absorber, 2, 2’- dihydroxy-4-methoxybenzophenone. Clear flint 1-ounce glass bottles were filled to capacity and exposed to sunlight for 15 minutes. The bottles were sealed from atmospheric oxygen. Sludge was removed from treated samples by centrifuging and the supernatant solutions were analyzed for AP-3. The quantitative results fro the dyes and UV-absorber protection of AP-3 are shown in Table 2.

Table 2 – Protection of AP-3 in isooctane

Additive	Additive Conc., mg/gal	Final conc. of Mn, g/gal(1)
1-(4-(o-Tolylazo)-o(tolylazo)- 2-naphthol	10	0.69
N, N-Dimethyl-p-(phenylazo) aniline	10	0.66
1-Phenylazo-2-naphtol	20	0.67
(P-4; Patent Chemicals, Inc.)	20	0.66
1-Butylamino-4-methyl-aminoanthraquinone	20	0.63
(UV absorber) 2,2'-dihydroxy -4- methoxybenzophenone	400 2000	0.74 1.15

(1) Initial concentration of manganese is 1.09 g Mn/gal 

The fuel dyes provided a small amount of decomposition protection to AP-3. As expected from the glass filter studies, the red dye gave the greatest amount of protection and the blue dye none at all. Increasing the dye concentration from 10 to 20 mg/gal for the Ethyl yellow dye did not give any significant increase in AP-3 protection.

The UV absorbing compound blended at 400 mg/gal gave some protection to the AP-3 in isooctane solution. The concentration of manganese in a sample protected by 2000 mg absorber/gal was greater, but the protection was not complete. In this sample there was an unusual yellow sludge which indicated that the additive may have decomposed.

A UV absorber can be utilized if it is acceptable to merely reduce the rate of decomposition. When an absorber instead of a filter is used, the percentage of AP-3 decomposed will be directly proportional to the percent of the solution required to completely absorb the objectionable light. Migration of AP-3 into this volume will cause more material to be decomposed. Thus a filter is more attractive than the use of an additive.

Conclusions:

Samples of AP-3 in isooctane assume a brown color after exposure to light; as the color persists in the solution even after centrifuging, it indicates the presence of soluble decomposition product(s). Preliminary tests have shown that this brown color can be removed from the solution by acid extraction.
The data presented in this report support the following conclusions:
1. Light with wavelengths below 440 nm is most active in the decomposition process. This corresponds to the energy absorbed by the AP-3 molecule itself.
2. Oxygen is necessary for the decomposition to continue and its concentration is a limiting factor.
3. The rate of decomposition is dependent upon the quantity of light energy absorbed.
4. Commercial and highly purified AP-3 show the same decomposition rates.
5. An acid-soluble intermediate is formed in the photochemical decomposition of AP-3.
6. The presence of neat TEL shows no effect on photochemical decomposition of AP-3.
7. In three different premium fuels, AP-3 showed the same decomposition rate but there was more extended decomposition in the premium fuels than in isooctane.
8. Light catalyzed decomposition of AP-3 stops on removal of the light source, even though more oxygen is available (It should be noted, however, that even in dark storage, slow decomposition occurs and continues).
9. Protection with filter or amber bottles is more attractive than by the addition of dyes or UV absorbers.

Executive summary:

The light instability and subsequent decomposition of AP-3 is characteristic of the compound itself. This process involves the reaction of AP-3 with oxygen which is catalyzed by the absorption of light with wavelengths between 340 and 440 nm.

Exclusion of either atmospheric oxygen or light with these wavelengths prevents decomposition. Amber bottles are required for laboratory use. The significance of this reaction during the commercial handling of AP-3 has not been evaluated, but this study indicate that filters give a better protection from light activation than the addition of dyes or UV absorbers.

In the presence of direct sunlight and an excess of atmospheric oxygen, AP-3 in isooctane decomposes at a rate of 0.4 g manganese/gal/hr. In the absence of an oxygen atmosphere but with dissolved oxygen, AP-3 decomposition levels off when the oxygen is depleted. The rate of AP-3 decomposition is the same in three premium grade fuels as in isooctane, but the extent is greater, indicating that more oxidant is dissolved in the fuels. The addition of tetraethyllead does not change the rate of AP-3 decomposition.

Light accelerated decomposition ceases when the light source is removed. An apparent increase in sludging may occur as a result of the coagulation of decomposed AP-3 but the concentration of AP-3 in solution does not change at the accelerated rate observed under light catalysis.

Additional information

mmt is highly sensitive to light. Study records in Section 5.1.1 and 5.1.3 indicate that the half-life of mmt is less than 2 minutes.

The substance assumes a brown color after exposure to light, which persists in the solution even after centrifugation. The decomposition of mmt involves the reaction of mmt with oxygen and is catalyzed by the absorption of light with wavelengths between 340 and 440 nm. The decomposition of mmt stops upon removal of a light source, even in the presence of oxygen. Vapour-phase studies using Infra-Red spectroscopy (IR) to assess the decomposition products of mmt were conducted identified a mixture of manganese oxides, solid carboxylic acid, carboxylate ion, manganous carbonate, hydroxyl groups and both free and associated water. No metallo-carbonyl bands were found by the authors in the decomposition products. Decomposition of mmt with sunlight in the liquid-phase produced the same compounds described for the vapour phase, plus two manganese dicarbonyl complexes. These complexes were identified as olefin-stabilized methylcyclopentadienyl manganese dicarbonyl radicals. No traces of metallo-carbonyl complexes were found in vehicle exhaust particulates when fuel contained mmt, only a small percentage of inorganic manganese was detected, at approximately 0.04%.

Historical data on the use of mmt shows that it has a shelf-life of 120 months under ambient conditions.

Justification for classification or non-classification

Based on physical-chemical data and history of safe use, mmt is not corrosive to metals. No classification is required.