A New Class of Binuclear Gallium Hydrides: Synthesis and Properties of [{GaCl(hpp)H}2] (hpp=1,3,4,6,7,8-Hexahydro-2H-pyrimido[1,2-a]pyrimidate)

• Published in: Chemistry : a European journal 2007-01, Vol.13 (9), p.2648-2654
• Main Authors: Robinson, George, Tang, Christina Y., Köppe, Ralf, Cowley, Andrew R., Himmel, Hans-Jörg
• Format: Article
• Language: English
• Subjects: gallium; hydrides; quantum-chemical calculations; reaction mechanisms; X-ray diffraction
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200600807

Herein we report on the synthesis and characterization of the first representative of a new class of gallium hydrides, namely [{GaCl(hpp)H}2] (hpp=1,3,4,6,7,8‐hexahydro‐2H‐pyrimido[1,2‐a]pyrimidate). Our X‐ray diffraction data suggest the presence of weak HH contacts between adjacent [{GaCl(hpp)H}2] molecules in the crystal. With the aid of quantum‐chemical calculations, the pathway of its formation starting with the adduct H2ClGa⋅NMe3 and the pyrimidine hppH were analyzed. [{GaCl(hpp)H}2] slowly decomposes at 25 °C, a process which presumably leads under H2 elimination to [{GaCl(hpp)}2]. Although we were not yet able to characterize [{GaCl(hpp)}2] experimentally, DFT calculations provide information about its likely structure. According to these calculations, the molecule features a GaGa single bond. An eight‐membered ring is formed as the product of the reaction between H2GaNMe3 and the guanidine derivative hppH (see figure). The intermediately formed adduct loses dihydrogen and dimerizes. In the crystalline form, [{GaCl(hpp)H}2] is stabilized by intermolecular HH contacts. The experiments indicate that [{GaCl(hpp)H}2] eliminates an additional H2 at temperatures exceeding 25 °C.