Linear hybrid aminoborane/phosphinoborane chains: Synthesis, proton-hydride interactions, and thermolysis behavior

The reaction of the lithiated phosphine-borane adducts Li[(PPhRBH3)-B-.] or Li[CH2-(PR2BH3)-B-.] with (Me2NHBH2Cl)-B-. 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 Angstrom. 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 degreesC to afford species such as (Me2NHBH3)-B-., [Me2N-BH2](2), (PhPRHBH3)-B-. (R = Ph, H), PhPRH (R = Ph, H), Ph2PH-BH2-PPh2-BH3, and also the low molecular weight polyphosphinoborane [PhPH-BH2](n) (M-w, similar to 5000). Similar products were observed for the attempted catalytic dehydrocoupling reactions but under milder reaction conditions (50 degreesC). Thermolysis of 3 at 130 degreesC yielded the six-membered ring [BH2-CH2-PPh2]2 (5), which presumably results from the dissociation of (Me2NHBH3)-B-. from 3. Thermolysis of 4 at 90 degreesC afforded (Me2NHBH3)-B-. and Me3P(.)BH3, in addition to a product tentatively assigned as [BH2-CH2-PMe2](2) (6).