Water-Soluble Fe(II)–H₂O Complex with a Weak O–H Bond Transfers a Hydrogen Atom via an Observable Monomeric Fe(III)–OH

Abstract

Understanding the metal ion properties that favor O−H bond formation versus cleavage should facilitate the development of catalysts tailored to promote a specific reaction, e.g., C−H activation or H2O oxidation. The first step in H2O oxidation involves the endothermic cleavage of a strong O−H bond (BDFE = 122.7 kcal/mol), promoted by binding the H2O to a metal ion, and by coupling electron transfer to proton transfer (PCET). This study focuses on details regarding how a metal ion’s electronic structure and ligand environment can tune the energetics of M(HO−H) bond cleavage. The synthesis and characterization of an Fe(II)−H2O complex, 1, that undergoes PCET in H2O to afford a rare example of a monomeric Fe(III)−OH, 7, is described. High-spin 7 is also reproducibly generated via the addition of H2O to {[FeIII(OMe2N4(tren))]2-(µ-O)}2+ (8). The O−H bond BDFE of Fe(II)−H2O (1) (68.6 kcal/mol) is calculated using linear fits to its Pourbaix diagram and shown to be 54.1 kcal/mol less than that of H2O and 10.9 kcal/mol less than that of [Fe(II)(H2O)6]2+. The O−H bond of 1 is noticeably weaker than the majority of reported Mn+(HxO−H) (M = Mn, Fe; n+ = 2+, 3+; x = 0, 1) complexes. Consistent with their relative BDFEs, Fe(II)−H2O (1) is found to donate a H atom to TEMPO•, whereas the majority of previously reported Mn+−O(H) complexes, including [MnIII(SMe2N4(tren))(OH)]+ (2), have been shown to abstract H atoms from TEMPOH. Factors responsible for the weaker O−H bond of 1, such as differences in the electron-donating properties of the ligand, metal ion Lewis acidity, and electronic structure, are discussed.

Structure of 3-mercaptopropionic acid dioxygenase with a substrate analog reveals bidentate substrate binding at the iron center

“…The pKa of the 3HPA hydroxyl (14.4–15.1) is expected to be much higher than the 3MPA-thiol (∼8) ( 43 ). While it is unlikely that the Fe-bound 3HPA-hydroxyl is deprotonated, pKa values for metal-coordinated water can decrease by several pH units ( 44 , 45 ). To corroborate the ionization state of the 3HPA inhibitor, optimized DFT models for the deprotonated (3HPA 2− )- and protonated (3HPA 1− )-hydroxyl group were made with ferric and ferrous iron for comparison to the crystallographic results ( Table 1 ).…”

Synthetic High‐Valent M–O–X Oxidants

“…A BDFE was determined for Mn III –OH 2 (74 kcal/mol). Conversely, an iron analogue [Fe II (OH 2 ){O Me2 N 4 (tren)}] ( 14 , Figure ) was found to have a weaker BDFE (69 kcal/mol), and consequently its reaction was spontaneous in the direction [Fe II (OH 2 ){O Me2 N 4 (tren)}] + TEMPO → [Fe III (OH){O Me2 N 4 (tren)}] + TEMPOH. In this case the change from Mn to Fe, and the switch from the thiolate to a less electron‐donating alkoxide donor, contributed to the lowering of both E 0 and p K a in 14 in comparison to 13aH , thus weakening the M[O(H)–H] bond.…”

Fast Hydrogen Atom Abstraction by a Hydroxo Iron(III) Porphyrazine

“…11,18 Further, the E pa – E pc difference at pH 2.2 decreased as the scan rate decreased, with a Δ E of 68 mV at 2 mV/s (Figure S6 and Table S1). This behavior is indicative of the transfer one proton per electron and that the ferric complex 1 has one hydroxo and one aqua axial ligand over this entire pH range.…”