Influence of the density functional and basis set on the relative stabilities of oxygenated isomers of diiron models for the active site of [FeFe]-hydrogenase.
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A series of different density functional theory (DFT) methodologies (24 functionals) in conjunction with a variety of six different basis sets (BSs) was employed to investigate the relative stabilities in the oxygenated isomers of diiron complexes that mimic the active site of [FeFe]-hydrogenase: (-pdt)[Fe(CO)2L][Fe(CO)2L'] (pdt = propane-1,3-dithiolate; L = L' = CO (1); L = PPh3, L' = CO (2); L = PMe3, L' = CO (3); L = L' = PMe3 (4). Although the enzyme may have a variety of possible sites for oxygenation, the model complexes would necessarily be oxygenated at either the diiron bridging site (-O) or at a sulfur (SO). Previous DFT studies with both B3LYP and TPSS functionals predicted a more stable -O isomer, whereas only the SO isomer was observed experimentally (J. Am. Chem. Soc. 2009, 131, 8296-8307). Here, further calculations reveal that the relative stabilities of the SO and -O isomers are extremely sensitive to the choice of the functional, moderately sensitive to the S basis set, but not to the Fe basis set. The relative free energies [Gsolv(-O) - Gsolv(SO)] range from +10 to -60 kcal/mol, a range much larger than what would have been expected on the basis of the previous DFT results. Benchmarking of these results against coupled cluster with single and double excitation calculations, which predict that the SO isomer is favored, shows that the best performing functionals are BP86 and PBE0, while B97-D, M05, and SVWN overestimate and B2PLYP, BH&HLYP, BMK, M06-HF, and M06-2X underestimate the energy differences. Most of the variation occurs with the -O isomer and appears to be associated with a functional's ability to predict the strength of the Fe-Fe bond in the reactant. With respect to the S basis set, it appears that the SO bond is sensitive to the nature of the d polarization functions available on the S atom. The S seems to need a d function more diffuse than the d orbital optimized to provide polarization for the S atom alone; that is, S seems to need a d orbital that has strong overlap with the O atom's valence 2p. Other basis functions and the relative position of the PR3 (R = Ph and Me) substituent groups have smaller influences on the free energy differences.
