Tuning the electronic and steric parameters of a redox-active tris(amido) ligand

A family of tantalum compounds was prepared to probe the electronic effects engendered by the addition of electron-donating or electron-withdrawing groups to the 4/4' positions of the redox-active ligand derived from bis(2-isopropylamino-4-X-phenyl)amine [(X,iPr)(NNN(cat))H3, X = F, H, Me, (t)Bu]). A general synthetic procedure for the (X,iPr)(NNN(cat))H3 ligand family was developed starting from the 4/4' disubstituted diphenylamine derivative. A second ligand modification, incorporation of aromatic substituents at the flanking nitrogen moieties, was achieved via palladium-catalyzed cross-coupling to afford bis(2-3,5-dimethylphenylamino-4-methoxy-phenyl)amine (OMe,DMP)(NNN(cat))H3 (DMP = 3,5-C6H3Me2), allowing a comparative study to the less sterically hindered isopropyl derivative. Treatment of the triamines with 1 equiv of TaMe3Cl2 generated the corresponding dichloro complexes (X,R)(NNN(cat))TaCl2(L) (L = empty or Et2O) in high yields. These neutral dichloride derivatives reacted with [NBnEt3][Cl] to produce the anionic trichloride derivatives [NBnEt3][(X,R)(NNN(cat))TaCl3], whereas the neutral dichloride derivatives reacted with chlorine atom donors to produce the neutral trichloride derivatives (X,R)(NNN(sq))TaCl3, containing the one-electron-oxidized form of the redox-active ligand. Aryl azides reacted with the (X,R)(NNN(cat))TaCl2(L) derivatives, resulting in nitrene transfer to tantalum and two-electron oxidation of the ligand platform to give (X,R)(NNN(q))TaCl2(═NR') (R = iPr; X = OMe, F, H, Me; R' = p-C6H4tBu, p-C6H4CF3; and R = 3,5-C6H3Me2; X = OMe; R' = p-C6H4CH3). Electrochemistry, UV-vis-NIR, IR, and EPR spectroscopies along with X-ray diffraction methods were used to characterize and compare complexes with different redox-active ligand derivatives in each oxidation state. This study demonstrates that while the ligand redox potentials can be adjusted over a 270 mV range through substitutions at the 4/4' ring positions, the coordination chemistry and reactivity patterns at the bound tantalum center remain unchanged, suggesting that such ligand modifications can be used to tune the redox potentials of a complex for a particular substrate of interest.