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

Abstract

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 "Na 2 PDI", which reacts with in situ-prepared "GaHCl 2 " or GaCl 3 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 ringcurrent effect, or metalloaromaticity. Typically, aromaticity in metalcontaining 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.

Synthesis of Unsupported Primary Phosphido Complexes of Aluminum(III)

“…In contrast, SP complexes containing a halide ligand have smaller angles that range from 167.01(4) to 168.78 (19)°across the three compounds (I 2 P 2À )AlCl, (I 2 P 2À )AlI, and (I 2 P 2À )GaCl. [11,15] In the case of the halides and in the current case of the phosphide ligands there appears to be no steric driving force for the deviation from 180°. These smaller angles are therefore believed to derive from π-donation from the π-donor halide ligands, and from a π-donating contribution from the phosphido which is more effective when the πsymmetry orbital on the P atom approaches the empty orbital on Al more closely.…”

“…To test this hypothesis we synthesized an analog of the I 2 P ligand where the 2,6-diisopropylphenyl substituents are replaced with the smaller 2,6-dimethylphenyl substituents, and we sought out the analogous complex ( Xyl I 2 P 2À )AlI following a synthetic procedure identical to the one that was used to obtain ( Xyl I 2 P 2À )AlI. 11 Unfortunately, we were not able to reliably isolate ( Xyl I 2 P 2À )AlI in good yields and with a clean proton NMR spectrum. We did obtain confirmation of the identity of that compound from proton NMR and from the crystallization of the compound from a concentrated toluene solution (Figure S5, Tables S1, S2).…”

“…[10][11][12] Analogous SP Ga complexes display similar electronic and reactivity properties. [11] In the present work we explore ligand substitution reactions at the Al center in SP complexes, and we also explored the nature of the empty orbital on Al and whether it can be exploited to access Al-ligand bonds with πsymmetry that contribute to Al-ligand multiple bond character (Chart 1).…”

“…The square planar (SP) bis(imino)pyridine Al complexes, denoted as (I 2 P 2À )AlX, where X=H, Cl, or I, were previously reported, and present an unusual geometry for a molecular main group species. [10,11] The chemical reactivity properties and theoretical work that has been communicated regarding these complexes suggests that the Al atom has an empty molecular orbital which is perpendicular to the square plane. [10][11][12] Analogous SP Ga complexes display similar electronic and reactivity properties.…”

“…[10,11] The chemical reactivity properties and theoretical work that has been communicated regarding these complexes suggests that the Al atom has an empty molecular orbital which is perpendicular to the square plane. [10][11][12] Analogous SP Ga complexes display similar electronic and reactivity properties. [11] In the present work we explore ligand substitution reactions at the Al center in SP complexes, and we also explored the nature of the empty orbital on Al and whether it can be exploited to access Al-ligand bonds with πsymmetry that contribute to Al-ligand multiple bond character (Chart 1).…”