Synthesis and structural aspects of gallium compounds containing tridentate pincer type pyrrolyl ligands: Intramolecular hydrogen bonding of gallium aryloxides

A series of gallium derivatives incorporating with pincer type tridentate symmetric and asymmetric pyrrolyl ligands are conveniently synthesized. Compilation of a symmetrical tridentate pyrrole ligand, [C4H2NH-(2,5-CH2NHtBu)2] (1) with one equiv of n-BuLi in heptane generates lithiated 1 that turns to Ga[C4H2N-(2,5-CH2NHtBu)2]Cl2 (2) subjecting the addition of GaCl3/ether suspension into diethyl ether. Reacting 2 with two equiv of lithium reagents (LiR) in same solvent originates a series of gallium compounds Ga[C4H2N-(2,5-CH2NHtBu)2]R2 (3, R = Me; 4, R = O–C6H4–4-Me; 5, R = O–C6H3–2,6-Me2) in moderate yield through metathesis reactions. Furthermore, the combination of a tridentate asymmetrical pyrrole ligand, [C4H2NH(2-CH2NHtBu)(5-CH2NMe2)] and n-BuLi initiates the lithiated 6 that undergoes the formation of a penta-coordinated gallium compound GaCl2[C4H2N(2-CH2NHtBu)(5-CH2NMe2)] (7) while reacting with GaCl3. Furthermore, reacting 7 with two equivs of MeLi affords the gallium-alkyl compound, GaMe2[C4H2N(2-CH2NHtBu)(5-CH2NMe2)] (8) in high yield. The Ga-derivatives are characterized by 1H and 13C NMR spectroscopy and the molecular structures of 2, 3, 4, 5 and 7 are determined by single crystal X-ray diffractometry in solid state.