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, [C₄H ₂NH-(2,5-CH₂NH^tBu)₂] (1) with one equiv of n-BuLi in heptane generates lithiated 1 that turns to Ga[C ₄H₂N-(2,5-CH₂NH^tBu) ₂]Cl₂ (2) subjecting the addition of GaCl ₃/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-CH₂NH^tBu) 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-CH ₂NH^tBu)(5-CH₂NMe₂)] and n-BuLi initiates the lithiated 6 that undergoes the formation of a penta-coordinated gallium compound GaCl₂[C₄H₂N(2-CH ₂NH^tBu)(5-CH₂NMe₂)] (7) while reacting with GaCl₃. Furthermore, reacting 7 with two equivs of MeLi affords the gallium-alkyl compound, GaMe₂[C₄H ₂N(2-CH₂NH^tBu)(5-CH₂NMe ₂)] (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.