Analysis of the Puzzling Exchange-Coupling Constants in a Series of Heterobimetallic Complexes

The exchange-coupling constants (J) in a series of bimetallic complexes with an M2+(μ-OH)Fe3+ core (M = Mn, Fe, Ni, and Cu; series 1), which were reported in a recent study ( Sano et al. Inorg. Chem. 2017 , 56 , 14118 - 14128 ), have been analyzed with the help of density functional theory (DFT) calculations. The experimental J values of series 1 showed the remarkable property that they were virtually independent of metal M. This behavior contrasts with that observed for a related series of complexes with M2+Fe3+ cores reported by Chaudhuri and co-workers ( Biswas et al. Inorg. Chem. 2010 , 49 , 626 - 641 ) (series 2) in which J increases toward the upper end of the series. Broken symmetry DFT calculations for J, which yielded values in good agreement for the MnFe and NiFe complexes of series 1, gave for the CuFe complex a J value that was persistently much larger than that obtained from the experiment. Attempts to bridge the discrepancy by invoking various basis sets and corrections for hydrogen-bonding effects on J were not successful. The J values for series 1 were subsequently analyzed in the context of an exchange pathway model. From this analysis, it emerged that, in addition to the regular 2e-pathways, which contribute antiferromagnetic terms to J, there are also 3e-pathways that contribute ferromagnetic terms and have the propensity to keep J constant along series 1. It is shown that, while DFT evaluates the 2e-pathway terms reliably, this method seriously underestimates the 3e-pathway contributions, resulting in a too high J value for the CuFe complex of series 1. The pathway analysis of series 2 reveals that the 3e-pathway contributions to J are considerably smaller than those in series 1, resulting in J values that increase toward the upper end of the series, in accordance with the experiment

Nonheme oxoiron(IV) complexes of tris(2-pyridylmethyl) amine with cis-monoanionic ligands

Treatment of [Fe-IV(O)(TPA)(NCMe)](CF3SO3)(2) [TPA, N, N, N- tris(2-pyridylmethyl) amine] with 3 equiv of NR4X (X = CF3CO2, Cl, or Br) in MeCN at -40 degrees C affords a series of metastable [FeIV(O)(TPA)(X)](+) complexes. Some characteristic features of the S = 1 oxoiron(IV) unit are quite insensitive to the ligand substitution in the equatorial plane, namely, the Fe-O distances (1.65-1.66 angstrom), the energy (similar to 7114.5 eV) and intensity [25(2) units] of the 1s-to-3d transition in the X-ray absorption spectra, and the Mossbauer isomer shifts (0.01-0.06 mm center dot s(-1)) and quadrupole splittings (0.92-0.95 mm, s(-1)). The coordination of the anionic X ligand, however, is evidenced by red shifts of the characteristic near-IR ligand-field bands (720-800 nm) and spectroscopic observation of the bound anion by F-19 NMR for X = CF3CO2 and by EXAFS analysis for X = Cl (r(Fe-Cl) = 2.29 angstrom) and Br (r(Fe-Br) = 2.43 angstrom). Density functional theory calculations yield Mossbauer parameters and bond lengths in good agreement with the experimental data and produce excited-state energies that follow the trend observed in the ligand-field bands. Despite mitigating the high effective charge of the iron(IV) center, the substitution of the MeCN ligand with monoanionic ligands X- decreases the thermal stability of [FeIV(O)(TPA)](2+) complexes. These anion-substituted complexes model the cis-X-Fe-IV = O units proposed in the mechanisms of oxygen-activating nonheme iron enzymesclose495