Preparation of (NH4)2MoOBr5 and [pyrHMoOBr4]n


Preparation of (NH4)2MoOBr5 and [pyrHMoOBr4]n

by Ed Perley

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The following preparations are unpublished documents from a Masters Thesis completed at South Dakota State University in May 1978. The structures of the compounds and how they are related to Mo(V) solutions are discussed on another page on this site. See the link at the bottom of this page. References to two Journals are listed at the bottom of this page.

Concentrated Molybdenum(V) Solutions in Hydrobromic Acid

Most of the oxobromomolybdate(V) compounds prepared in this study were precipitated from concentrated molybdenum(V) solutions in hydromic acid. The procedure for it and preparing molybdenum (V) solutions was a modification of one given by Saha and Bannerjee.

Approximately 40 ml of concentrated hydrobromic acid and was added to 5 g of molybdenum trioxide in the 250 ml beaker. A watch glass was put over the beaker and the mixture was refluxed gently on a hot plate in a chemical hood. After about two hours, the watch glass was removed and the thick dark brown solution was allowed to evaporate to a desired volume. The solution volume selected depended on the soluability of the compound to be precipitated. If necessary, the volume was adjusted by adding concentrated hydrobromic acid. The solution was allowed to cool and was suction filtered using a fine porosity sintered class crucible and a water aspirator. It was used immediately after filtration.

Ammonium Oxopentabromomolybdate(V) [ (NH4)2MoOBr5 ]

Molybdenum trioxide (10 gm, 69.4 mmole) was reacted with hydrobromic acid as described above to produce a molybdenum (V) solution with a volume of about 15 ml in a 150 ml beaker. Ammonium bromide (9.50 gm, 97.0 mmole) was added to the molybdenum (V) solution. Hydrogen bromide was bubbled through the solution. After it had been saturated with the gas, the beaker was cooled to about -20 degrees centigrade by holding it in the mouth of a Dewar flask containing liquid nitrogen.

The Brown, crystalline precipitate was isolated from the cold solution, using suction filtration in a closed system. Air sucked into the filter was first dried by passing it through anhydrous calcium chloride first and then potassium hydroxide. Filtration without a dry air flow through the filter cause the surface to become discolored, indicating possible decomposition. The product was washed twice with 5 ml of a cold and saturated ethanol-hydrogen bromide solution, and dried in a vacuum dessicator under reduced pressure over solid potassium hydroxide. A water aspirator was used to produce a partial vacuum in the dessicator.

Product weight was 22.21 g. ( % yield = 83.7% ).

A portion of the above product (6.00 gm, 10.96 mmole) was dissolved in 2 M hydrobromic acid in a 50 ml beaker. The solution was saturated with hydrogen bromide and cooled above liquid nitrogen. The product was separated, washed and dried as described above.

Product weight was 4.42 ( % yield = 73.8% ).

Theoretical: Mo 17.57%; Br 72.96%.
Found: Mo 17.52%; Br 72.87%

Pyridinium Catena-u-oxo-tretrabromomolybdate(V) [ (pyrhMoOBr4)n ]

Molybdenum trioxide 4.00 g, (27.8 mole) was reacted with concentrated hydrobromic acid to produce a 30 ml molybdenum(V) solution in a 150 ml Baker. Pyridinium hydrobromide was prepared by dissolving 40 ml of pyridine in 5 ml of ethanol, saturating the solution with hydrogen bromide, and heating it to evaporate the ethanol and excess hydrogen bromide. The resulting pyridinium hydrobromide (3.50 gm, 21.9 mmole) was dissolved in a minimum volume of 7 M hydrobromic acid. The solution was poured with stirring into the molybdenum(V) solution, resulting in a heavy brown precipitate.

The mixture was saturated with hydrogen bromide and the sealed beaker was cooled in an ice bath. The resulting red crystalline solid was isolated, washed and dried as described for the ammonium compound above.

Product weight was 9.34 g ( % yield = 78.1% ).

A portion of the product (6.92 gm, 12.7 mmole) was recrystallized by dissolving in 7M hydrobromic acid and saturating with hydrogen bromide. The product was isolated, washed and dried as described above.

Product weight was 5.46 gm (%yield = 78.8%).

Theoretical : Mo 18.7%; Br 62.4%; C 11.76%; H 1.18%
Found: Mo 18.66%; Br 62.08%; C 121.83%; H 1.25%

A different crystallization procedure was used to produce crystals of the compound suitable for an Xray crystallographic analysis. A sample of the compound (1.00 g, 1.83 mmole) was dissolved in a minimum volume of boiling 8.6 M hydrobromic acid. The solution was filtered, transferred to a thirty gm weighing bottle, and saturated with hydrogen bromide. The bottle was tightly capped and allowed to cool slowly to room temperature. While the solution was still hot, a few crystals of the compound were added to prevent super saturation, and the solution was allowed to stand for 48 hours. About 0.10 g of reddish brown crystalline product was separated, washed and dried as described above.

Product weight was 5.46 gm ( % yield = 78.8% )


James, R. G. and W. Wardlaw, J. Chem. Soc., 2145(1927)
Saha, H.K. and A. K. Banerjee, "J. Inorg. Nucl. Chem., 34, 697(1972)

Study of Mo(V) solutions in Hydrobromic Acid.

Kbr Pellet Spectra: How to do it

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Date last updated: March 18, 1999