Potassium Tetrakis l-pyrazolyl borate

POTASSIUM TETRAKIS(l-PYRAZOLYL)BORATE— ALKALI METAL POLY(1-PYRAZOLYL)BORATES 

A mixture of 54 g. (1 mole) of potassium borohydride pellets and 340 g. (5 moles) of pyrazole is stirred and heated, as in Sec. B, until about a 100-1. (4r-mole) volume of hydrogen has been evolved. The fourth mole of hydrogen is evolved slowly, and it takes over 24 hours for completion of the reaction. The melt, the temperature of which should be 220-230°C., is cooled to about 1B0°C. and poured carefully into 1200 ml. of stirred xylene. The solution is cooled to room temperature, and the solid is filtered. It is then stirred and boiled for a few minutes in 1 1. of heptane and filtered hot. The product is washed a few times with hot hexane to remove traces of pyrazole and is obtained, after air-drying, in about 270-g. (85%) yield. It is purified by recrystallizing from boiling water and drying in vacuo at 130-150°C. 

Potassium tetrakis(l-pyrazolyl)borate is a white solid, m.p. 253-254°C. Its infrared spectrum is devoid of BH bands. This salt is quite soluble in dimethylformamide (DMF) and dimethyl sulfoxide but less so in water and alcohols. Stable 0.1 M aqueous solutions may be prepared and used as needed. 

BIS[DIHYDROBIS(l-PYRAZOLYL)BORATO]NICKEL(II)— TRANSITION-METAL POLY (l-PYRAZOLYL)BORATES 

Neutral chelates derived from divalent transition-metal ions are prepared readily by metathetical reactions in water or, when the solubility of the ligand in water is low, in DMF-water mixtures. With the exception of some bidentate chelates of Mn(II) and Fe(II), which are air-sensitive, the transition-metal compounds are stable to air, and an inert atmosphere is not required during their synthesis or storage. The following procedures are applicable to the preparation of chelates derived from other transition-metal ions. 

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To a stirred solution of 31.8 g. (0.1 mole) of potassium tetrakis(l-pyrazolyl)borate in 300 ml. of dimethylformamide is added rapidly 100 ml. of 0.5 M manganese(II) sulfate solution. A precipitate forms immediately. The mixture is stirred for 10 minutes at room temperature and is then diluted with 300 ml. of water and filtered. The white solid is washed with copious amounts of water, then with methanol, and, finally, with a small amount of ether. After air-drying, there is obtained 29-30 g. (94-98%) of white solid. 

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