Metal tetrakis-borohydrides

Four of the seven known metal tetrakis-borohydrides—Zr, Hf, Th, and U borohydrides (1,2)—were first synthesized about 30 years ago during the Manhattan project. They were found to be very volatile and reactive compounds. In recent years, much structural, spectroscopic, and chemical studies were done on these molecules. New tetrakis-borohydrides of the actinides Pa, Np, and Pu have recently been prepared by analogous reactions used in the syntheses of U and Th borohydrides (3). The Pa compound, Pa(BHi+K, is iso-morphous to and behaves like U(BHi+)i+ and Th(BHi+)i+ while x-ray studies on Np(BHi+)i+ and the isostructural Pu(BHi+)i+ have shown that they resemble the highly volatile Zr and Hf compounds both in structure and properties. All seven compounds contain triple hydrogen bridge bonds connecting the boron atom to the metal. 

Some of the physical properties of metal tetrakis-boro-hydrides, which are primarily determined by their solid-state structure, are listed in Table 1. The polymeric Th, Pa, and U borohydrides are of much lower volatility than the monomeric Zr, Hf, Np, and Pu compounds. The intermolecular bonds connecting molecules together decrease their volatility substantially since these bonds break when the solid vaporizes (12). A plot of log p(mmHg) vs 1/T yields the equation log p(mmHg) = -A/T + B, where T is in K. Values of A and B allow the calculation of the heats (AH) and entropies (AS) for phase-change processes as shown in Table 1. The actinide ions in the polymeric compounds are 14 coordinate however, in the gaseous state they are 12 coordinate (12). 

Methods (i) and (ii) require palladium(II) salts as reactants. Either palladium acetate, palladium chloride or lithium tetrachloropalladate(II) usually are used. These salts may also be used as catalysts in method (iii) but need to be reduced in situ to become active. The reduction usually occurs spontaneously in reactions carried out at 100 °C but may be slow or inefficient at lower temperatures. In these cases, zero valent complexes such as bis(dibenzylideneacetone)palladium(0) or tetrakis(triphenylphos-phine)palladium(O) may be used, or a reducing agent such as sodium borohydride, formic acid or hydrazine may be added to reaction mixtures containing palladium(II) salts to initiate the reactions. Triarylphosphines are usually added to the palladium catalysts in method (iii), but not in methods (i) or (ii). Normally, 2 equiv. of triphenylphosphine, or better, tri-o-tolylphosphine, are added per mol of the palladium compound. Larger amounts may be necessary in reactions where palladium metal tends to precipitate prematurely from the reaction mixtures. Large concentrations of phosphines are to be avoided, however, since they usually inhibit the reactions. 

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