Competitive Intramolecular Amination as a Clock for Iron-Catalyzed Nitrene Transfer.

Reaction of the complex [(TpPh,Me)FeII(NCMe)3]BF4, where TpPh,Me = hydrotris(3-phenyl,5-methyl-1-pyrazolyl)borate, with the iodonium heteroylide PhI═NTs (1.5 equiv) is proposed to result in the insertion of N-tosylnitrene into one C-H bond at the ortho ring position of a 3-pyrazole phenyl substituent; subsequent deprotonation of the nascent aniline and one-electron oxidation of iron forms TsNH2 (0.5 equiv) as a coproduct. The covalent ligand modification and oxidation results in an intense purple-brown anilinato-iron(III) LMCT chromophore. This intramolecular reaction is utilized as a consistent clock to determine relative rates of competitive intermolecular nitrene transfer to added substrates, specifically to para-substituted styrenes and thioanisoles. Prior addition of substrate to the reaction of PhI═NTs with the iron(II) complex attenuates the CT absorbance of the equilibrium solution. Fitting of the concentration-dependent absorption data gives the ratio of intra- versus intermolecular nitrene transfer. Because the former is independent of substrate, ratios for various substrates are directly comparable, and this approach enables acquisition of data for a single substrate under nearly stoichiometric, as opposed to competitive catalytic, conditions. Hammett analyses of such data are consistent with an electrophilic intermediate consistent with known or suspected imidoiron(IV) complexes. Because this intermediate was not observed directly, plausible geometric and electronic structures were modeled and assessed using density functional theory.

Steric and electronic effects on arylthiolate coordination in the pseudotetrahedral complexes [(Tp(Ph,Me))Ni-SAr] (Tp(Ph,Me) = hydrotris{3-phenyl-5-methyl-1-pyrazolyl}borate).

Synthesis and characterization of several new pseudotetrahedral arylthiolate complexes [(Tp(Ph,Me))Ni-SAr] (Tp(Ph,Me) = hydrotris{3-phenyl-5-methyl-1-pyrazolyl}borate; Ar = Ph, 2,4,6-(i)Pr3C6H2, C6H4-4-Cl, C6H4-4-Me, C6H4-4-OMe) are reported, including X-ray crystal structures of the first two complexes. With prior results, two series of complexes are spanned, [(Tp(Ph,Me))Ni-S-2,4,6-RC6H2] (R'' = H, Me, (i)Pr) plus the xylyl analogue [(Tp(Ph,Me))Ni-S-2,6-Me2C6H3], as well as [(Tp(Ph,Me))Ni-S-C6H4-4-Y] (Y = Cl, H, Me, OMe), intended to elucidate steric and/or electronic effects on arylthiolate coordination. In contrast to [(Tp(Me,Me))Ni-SAr] analogues that adopt a sawhorse conformation, the ortho-disubstituted complexes show enhanced trigonal and Ni-S-Ar bending, reflecting the size of the 3-pyrazole substituents. Moreover, weakened scorpionate ligation is implied by spectroscopic data. Little spectroscopic effect is observed in the series of para-substituted complexes, suggesting the observed effects are primarily steric in origin. The relatively electron-rich and encumbered complex [(Tp(Ph,Me))Ni-S-2,4,6-(i)Pr3C6H2] behaves uniquely when dissolved in CH3CN, forming a square planar solvent adduct with a bidentate scorpionate ligand, [(κ(2)-Tp(Ph,Me))Ni(NCMe)(S-2,4,6-(i)Pr3C6H2)]. This adduct was isolated and characterized by X-ray crystallography. Single-point DFT and TD-DFT calculations on a simplified [(κ(2)-Tp)Ni(NCMe)(SPh)] model were used to clarify the electronic spectrum of the adduct, and to elucidate differences between Ni-SAr bonding and spectroscopy between pseudotetrahedral and square planar geometries.