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

The reaction of the lithiated phosphine-borane adducts Li[PPhR.BH(3)] or Li[CH(2)-PR(2).BH(3)] with Me(2)NH.BH(2)Cl afforded the hybrid linear species Me(2)NH-BH(2)-PPhR-BH(3) (1, R = Ph; 2, R = H) or Me(2)NH-BH(2)-CH(2)-PR(2)-BH(3) (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 A. 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 degrees C to afford species such as Me(2)NH.BH(3), [Me(2)N-BH(2)](2), PhPRH.BH(3) (R = Ph, H), PhPRH (R = Ph, H), Ph(2)PH-BH(2)-PPh(2)-BH(3), and also the low molecular weight polyphosphinoborane [PhPH-BH(2)](n) (M(w) approximately 5000). Similar products were observed for the attempted catalytic dehydrocoupling reactions but under milder reaction conditions (50 degrees C). Thermolysis of 3 at 130 degrees C yielded the six-membered ring [BH(2)-CH(2)-PPh(2)](2) (5), which presumably results from the dissociation of Me(2)NH.BH(3) from 3. Thermolysis of 4 at 90 degrees C afforded Me(2)NH.BH(3) and Me(3)P.BH(3), in addition to a product tentatively assigned as [BH(2)-CH(2)-PMe(2)](2) (6).