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

The exchange-coupling constants () in a series of bimetallic complexes with an M(μ-OH)Fe core (M = Mn, Fe, Ni, and Cu; series 1), which were reported in a recent study ( Sano et al. 2017 , 56 , 14118 - 14128 ), have been analyzed with the help of density functional theory (DFT) calculations. The experimental 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 MFe cores reported by Chaudhuri and co-workers ( Biswas et al. 2010 , 49 , 626 - 641 ) (series 2) in which increases toward the upper end of the series. Broken symmetry DFT calculations for , which yielded values in good agreement for the MnFe and NiFe complexes of series 1, gave for the CuFe complex a 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 were not successful. The 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 , there are also 3e-pathways that contribute ferromagnetic terms and have the propensity to keep 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 value for the CuFe complex of series 1. The pathway analysis of series 2 reveals that the 3e-pathway contributions to are considerably smaller than those in series 1, resulting in values that increase toward the upper end of the series, in accordance with the experiment.