Syntheses of Square Planar Gallium Complexes and a Proton NMR Correlation Probing Metalloaromaticity

Syntheses of square planar (SP) coordination complexes of gallium(III) are reported herein. Using the pyridine diimine ligand (PDI), we prepared both (PDI^2-)GaH (4) and (PDI^2-)GaCl (5), which were spectroscopically and structurally characterized. Reduction of PDI using Na metal afforded "Na₂PDI", which reacts with in situ-prepared "GaHCl₂" or GaCl₃ to afford the SP 4 and 5. The planar geometry of these and previously reported SP Al(III) complexes is attributed to energetic stabilization derived from a ring-current effect, or metalloaromaticity. Typically, aromaticity in metal-containing ring systems can be difficult to characterize or confirm experimentally. An experimental approach employing proton NMR spectroscopy and described here provided an estimate of a downfield chemical shift promoted by a small ring-current associated with metalloaromaticity. Near infrared spectroscopic analyses display ligand-metal charge transfer bands which support the assignment of aromaticity. The SP complexes (PDI^2-)AlH (1), (PDI^2-)AlCl (2), (PDI^2-)AlI (3), 4, and 5 are all discussed in this report, using aromaticity as a model for their electronic structure and reactivity properties.