Synthesis and Characterization of the Homoleptic Octahydrotriborate Complex Cr(B₃H₈)₂ and Its Lewis Base Adducts

Solvate-free sodium octahydrotriborate, NaB₃H₈, is prepared on a 20 gram scale from sodium amalgam and diborane in diethyl ether. This substance, which is chemically related to borohydride-based compounds being investigated as hydrogen storage materials, is also useful for the preparation of transition-metal complexes bearing B₃H₈ ligands. Treatment of CrCl₃ with NaB₃H₈ affords a thermally unstable purple liquid thought to be a chromium(III) hydride of stoichiometry CrH(B₃H₈)₂. This hydride converts rapidly at room temperature to the chromium(II) complex Cr(B₃H₈)₂, which adopts a square-planar structure in which four hydrogen atoms form the coordination sphere of the chromium atom. This chromium(II) species forms six-coordinate Lewis base adducts Cr(B₃H₈)₂L₂, where L is Et₂O, THF, or PMe₃; the first two of these adopt trans geometries, whereas the latter is cis. Volatile Cr(B₃H₈)₂ is the first homoleptic transition-metal complex of the octahydrotriborate anion, and it is an excellent single-source precursor for the chemical vapor deposition of thin films of CrB₂ at temperatures as low as 200 °C. Crystal structures of the new complexes are reported.