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

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.