Aminodiboranates: syntheses, structures, and applications for chemical vapor deposition

The reaction ThCl₄ with 4 equivalents of sodium N,N-dimethylaminodiboranate, Na(H₃BNMe₂BH₃), in tetrahydrofuran produces the new complex Th(H₃BNMe₂BH₃)₄. The thorium center forms bonds with fifteen hydrogen atoms; accordingly, this is the first example of a fifteen-coordinate atom of any kind. As determined by both single crystal X-ray and single crystal neutron diffraction studies, the eight boron atoms describe an approximate D₂d dodecahedral structure in which seven of the Th•••B distances lie between 2.88 and 2.95 Å, but the eighth is significantly longer at 3.19 Å. Two hydrogen atoms on each boron atom bridge each of the short Th•••B contacts, but only one bridges the long Th•••B contact. Quantum chemical calculations suggest that Th(H₃BNMe₂BH₃)₄ is 16-coordinate in the gas phase and that the 15-coordinate solid-state structure can be attributed to packing effects. Compound 1 reacts at elevated temperatures (80–110 °C) to produce (NMe₂BH₂)₂ and the mixed aminodiboranate/tetrahydroborate complex Th(H₃BNMe₂BH₃)₂(BH₄)₂; the reaction proceeds through the Th(H₃BNMe₂BH₃)₃(BH₄) intermediate. The structure of the fifteen-coordinate Th(H₃BNMe₂BH₃)₂(BH₄)₂(thf) is also described.