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

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.