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, ^tBu]). 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.