Dehydrogenative Ga−Ga Coupling and Hydrogallation in Gallium Hydride Complexes of 3,5-Dimethylpyridine

Abstract

The 2:1 complex of 3,5-dimethylpyridine (L‘) and dichlorogallane, (L‘)2GaHCl2, undergoes dehydrogenative Ga−Ga coupling in boiling toluene to give high yields of (L‘)Cl2Ga−GaCl2(L‘). The structure of the dinuclear product (which has C2h symmetry) has been determined [Ga−Ga‘ 2.4000(8) Å]. By contrast, the 1:1 complex of L‘ and gallium trihydride, (L‘)GaH3, on prolonged standing at 20 °C or when treated with excess ligand L‘, is converted into the hydrogallation product, (L‘)(L‘H)GaH2. In this product, the gallium atom is attached to a pyridine donor ligand L‘ and a 4-hydropyridyl substituent, as proven by a crystal structure determination and by NMR and IR spectral studies. The Ga−N bond length is very significantly shorter for the hydropyridyl as compared to the pryridine group, and the carbon atom C4 of the former has become the center of an aliphatic CH2 group with its 1H and 13C resonances shifted into the corresponding upfield regions. The C3−C4 and C4−C5 linkages have been converted into single bonds, with double bonds localized at C2−C3 and C5−C6 of the (L‘H) heterocycle. While the N−Ga−N angle is very small at only 103(1)°, the H−Ga−H angle is exceedingly large at 127(2)°, indicating a strong steric pressure of the seemingly small hydride ligands and that hydride functions thus are to be considered as bulky substituents.