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Tuning the Electronic and Steric Parameters of a Redox-Active Tris(amido) Ligand

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journal contribution
posted on 18.02.2016, 17:42 authored by Rui F. Munhá, Ryan A. Zarkesh, Alan F. Heyduk
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(NNNcat)­H3, X = F, H, Me, tBu]). A general synthetic procedure for the X,iPr(NNNcat)­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(NNNcat)­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(NNNcat)­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(NNNcat)­TaCl3], whereas the neutral dichloride derivatives reacted with chlorine atom donors to produce the neutral trichloride derivatives X,R(NNNsq)­TaCl3, containing the one-electron-oxidized form of the redox-active ligand. Aryl azides reacted with the X,R(NNNcat)­TaCl2(L) derivatives, resulting in nitrene transfer to tantalum and two-electron oxidation of the ligand platform to give X,R(NNNq)­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.