¹H‑ENDOR Evidence for a Hydrogen-Bonding Interaction That Modulates the Reactivity of a Nonheme Fe^IVO Unit

We report that a novel use of 35 GHz ¹H-ENDOR spectroscopy establishes the presence in <b>1</b> of an Fe^IVO···H–O–Fe^III hydrogen bond predicted by density functional theory computations to generate a six-membered-ring core for <b>1</b>. The hydrogen bond rationalizes the difference in the C–H bond cleavage reactivity between <b>1</b> and <b>4</b>(OCH₃) (where a CH₃O group has replaced the HO on the Fe^III site). This result substantiates the seemingly paradoxical conclusion that the nonheme Fe^IVO unit of <b>1</b> not only has the electrophilic character required for H-atom abstraction but also retains sufficient nucleophilic character to accept a hydrogen bond from the Fe^III–OH unit

Evaluating the Identity and Diiron Core Transformations of a (μ-Oxo)diiron(III) Complex Supported by Electron-Rich Tris(pyridyl-2-methyl)amine Ligands

The composition of a (μ-oxo)­diiron­(III) complex coordinated by tris­[(3,5-dimethyl-4-methoxy)­pyridyl-2-methyl]­amine (R₃TPA) ligands was investigated. Characterization using a variety of spectroscopic methods and X-ray crystallography indicated that the reaction of iron­(III) perchlorate, sodium hydroxide, and R₃TPA affords [Fe₂(μ-O)­(μ-OH)­(R₃TPA)₂]­(ClO₄)₃ (<b>2</b>) rather than the previously reported species [Fe₂(μ-O)­(OH)­(H₂O)­(R₃TPA)₂]­(ClO₄)₃ (<b>1</b>). Facile conversion of the (μ-oxo)­(μ-hydroxo)­diiron­(III) core of <b>2</b> to the (μ-oxo)­(hydroxo)­(aqua)­diiron­(III) core of <b>1</b> occurs in the presence of water and at low temperature. When <b>2</b> is exposed to wet acetonitrile at room temperature, the CH₃CN adduct is hydrolyzed to CH₃COO^–, which forms the compound [Fe₂(μ-O)­(μ-CH₃COO)­(R₃TPA)₂]­(ClO₄)₃ (<b>10</b>). The identity of <b>10</b> was confirmed by comparison of its spectroscopic properties with those of an independently prepared sample. To evaluate whether or not <b>1</b> and <b>2</b> are capable of generating the diiron­(IV) species [Fe₂(μ-O)­(OH)­(O)­(R₃TPA)₂]³⁺ (<b>4</b>), which has previously been generated as a synthetic model for high-valent diiron protein oxygenated intermediates, studies were performed to investigate their reactivity with hydrogen peroxide. Because <b>2</b> reacts rapidly with hydrogen peroxide in CH₃CN but not in CH₃CN/H₂O, conditions that favor conversion to <b>1</b>, complex <b>1</b> is not a likely precursor to <b>4</b>. Compound <b>4</b> also forms in the reaction of <b>2</b> with H₂O₂ in solvents lacking a nitrile, suggesting that hydrolysis of CH₃CN is not involved in the H₂O₂ activation reaction. These findings shed light on the formation of several diiron complexes of electron-rich R₃TPA ligands and elaborate on conditions required to generate synthetic models of diiron­(IV) protein intermediates with this ligand framework

Spectroscopic and Theoretical Investigation of a Complex with an [OFe^IV–O–Fe^IVO] Core Related to Methane Monooxygenase Intermediate <b>Q</b>

Previous efforts to model the diiron­(IV) intermediate <b>Q</b> of soluble methane monooxygenase have led to the synthesis of a diiron­(IV) TPA complex, <b>2</b>, with an O=Fe^IV–O–Fe^IV–OH core that has two ferromagnetically coupled S_loc = 1 sites. Addition of base to <b>2</b> at −85 °C elicits its conjugate base <b>6</b> with a novel OFe^IV–O–Fe^IVO core. In frozen solution, <b>6</b> exists in two forms, <b>6a</b> and <b>6b</b>, that we have characterized extensively using Mössbauer and parallel mode EPR spectroscopy. The conversion between <b>2</b> and <b>6</b> is quantitative, but the relative proportions of <b>6a</b> and <b>6b</b> are solvent dependent. <b>6a</b> has two equivalent high-spin (<i>S</i>_loc = 2) sites, which are antiferromagnetically coupled; its quadrupole splitting (0.52 mm/s) and isomer shift (0.14 mm/s) match those of intermediate <b>Q</b>. DFT calculations suggest that <b>6a</b> assumes an anti conformation with a dihedral OFe–FeO angle of 180°. Mössbauer and EPR analyses show that <b>6b</b> is a diiron­(IV) complex with ferromagnetically coupled <i>S</i>_loc = 1 and <i>S</i>_loc = 2 sites to give total spin <i>S</i>_t = 3. Analysis of the zero-field splittings and magnetic hyperfine tensors suggests that the dihedral OFe–FeO angle of <b>6b</b> is ∼90°. DFT calculations indicate that this angle is enforced by hydrogen bonding to both terminal oxo groups from a shared water molecule. The water molecule preorganizes <b>6b</b>, facilitating protonation of one oxo group to regenerate <b>2</b>, a protonation step difficult to achieve for mononuclear Fe^IVO complexes. Complex <b>6</b> represents an intriguing addition to the handful of diiron­(IV) complexes that have been characterized

Hydrogen-Bonding Effects on the Reactivity of [X–Fe^III–O–Fe^IVO] (X = OH, F) Complexes toward C–H Bond Cleavage

Complexes <b>1</b>–OH and <b>1</b>–F are related complexes that share similar [X–Fe^III–O–Fe^IVO]³⁺ core structures with a total spin <i>S</i> of ¹/₂, which arises from antiferromagnetic coupling of an <i>S</i> = ⁵/₂ Fe^III–X site and an <i>S</i> = 2 Fe^IVO site. EXAFS analysis shows that <b>1</b>–F has a nearly linear Fe^III–O–Fe^IV core compared to that of <b>1</b>–OH, which has an Fe–O–Fe angle of ∼130° due to the presence of a hydrogen bond between the hydroxo and oxo groups. Both complexes are at least 1000-fold more reactive at C–H bond cleavage than <b>2</b>, a related complex with a [OH–Fe^IV–O–Fe^IVO]⁴⁺ core having individual <i>S</i> = 1 Fe^IV units. Interestingly, <b>1</b>–F is 10-fold more reactive than <b>1</b>–OH. This raises an interesting question about what gives rise to the reactivity difference. DFT calculations comparing <b>1</b>–OH and <b>1</b>–F strongly suggest that the H-bond in <b>1</b>–OH does not significantly change the electrophilicity of the reactive Fe^IVO unit and that the lower reactivity of <b>1</b>–OH arises from the additional activation barrier required to break its H-bond in the course of H-atom transfer by the oxoiron­(IV) moiety