Syntheses of Square Planar Gallium Complexes and a Proton NMR Correlation Probing Metalloaromaticity
Thomas M. Bass, Cody R. Carr, Tobias J. Sherbow, James C. Fettinger, Louise A. Berben
Abstract
Syntheses of square planar (SP) coordination complexes of gallium(III) are reported herein. Using the pyridine diimine ligand (PDI), we prepared both (PDI2–)GaH (4) and (PDI2–)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 (PDI2–)AlH (1), (PDI2–)AlCl (2), (PDI2–)AlI (3), 4, and 5 are all discussed in this report, using aromaticity as a model for their electronic structure and reactivity properties.