Linear Hybrid Aminoborane/Phosphinoborane Chains: Synthesis, Proton-Hydride Interactions, and Thermolysis Behavior

The reaction of the lithiated phosphine−borane adducts Li[PPhR·BH3] or Li[CH2−PR2·BH3] with Me2NH·BH2Cl afforded
the hybrid linear species Me2NH−BH2−PPhR−BH3 (1, R = Ph; 2, R = H) or Me2NH−BH2−CH2−PR2−BH3 (3, R
= Ph; 4, R = Me). Single-crystal X-ray diffraction studies on 1 and 3, the first for linear hybrid aminoborane/phosphinoborane adducts, confirmed the expected four-coordinate N−B−P−B and N−B−C−P−B frameworks. In
addition, interactions between the protic N−H and hydridic B−H hydrogen atoms resulted in short intermolecular
H···H contacts for 1, whereas 3 was found to possess an exceptionally short intramolecular H···H distance of
1.95 Å. Solution and solid state infrared studies on 3 and 4 also suggest that these dihydrogen interactions were
maintained even in dilute solution. Hydrogen bond strengths in the range of 7.9 to 10.9 kJ mol-1 indicate the
presence of a relatively weak interaction. The thermal and catalytic dehydrocoupling reactivities of 1−4 were also
investigated. Chain cleavage reactions were observed for 1 and 2 upon thermolysis at 130 °C to afford species
such as Me2NH·BH3, [Me2N−BH2]2, PhPRH·BH3 (R = Ph, H), PhPRH (R = Ph, H), Ph2PH−BH2−PPh2−BH3, and
also the low molecular weight polyphosphinoborane [PhPH−BH2]n (Mw ∼ 5000). Similar products were observed
for the attempted catalytic dehydrocoupling reactions but under milder reaction conditions (50 °C). Thermolysis of
3 at 130 °C yielded the six-membered ring [BH2−CH2−PPh2]2 (5), which presumably results from the dissociation
of Me2NH·BH3 from 3. Thermolysis of 4 at 90 °C afforded Me2NH·BH3 and Me3P·BH3, in addition to a product
tentatively assigned as [BH2−CH2−PMe2]2 (6).