Tuning the Fe(II/III) Redox Potential in Nonheme Fe(II)–Hydroxo Complexes through Primary and Secondary Coordination Sphere Modifications

Abstract

The derivatization of the imino-functionalized tris(pyrrolylmethyl)amine ligand framework, N(pi) (L; X = H, Br; R = cyclohexyl (Cy), phenyl (Ph), 2,6- diisopropylphenyl (DIPP)), is reported. Modular ligand synthesis allows for facile modification of both the primary and secondary coordination sphere electronics. The iron(II)-hydroxo complexes, N(pi)(afa)Fe(II)OH (LFeOH), are synthesized to establish the impact of the ligand modifications on the complexes' electronic properties, including their chemical and electrochemical oxidation. Cyclic voltammetry demonstrates that the Fe(II/III) redox couple spans a 400 mV range across the series. The origin of the shifted potential is explained based on spectroscopic, structural, and theoretical analyses of the iron(II) and iron(III) compounds.

Radical Rebound Hydroxylation Versus H-Atom Transfer in Non-Heme Iron(III)-Hydroxo Complexes: Reactivity and Structural Differentiation

“…Both azafulvene-amine arms of the secondary coordination sphere were engaged in hydrogen bonding to the hydroxo ligand with distances of 2.6666(18) and 2.7040(19) Å between the O and N afa hydrogen bond donor (Table ). The Fe–O distance of 2.0020(12) Å is similar to reported iron­(II)-hydroxo complexes. ,,,− The solution magnetic moment (μ eff ) of 5.21 μ B was determined by Evans’ method, corroborating the assignment of 2 as a high spin S = 2 iron­(II) species.…”

A Terminal FeIII–Oxo in a Tetranuclear Cluster: Effects of Distal Metal Centers on Structure and Reactivity

“…To gain further insights into these multimetallic effects, our group has examined well-defined tetranuclear clusters of Fe and Mn that facilitate intramolecular oxygen atom transfer (OAT) reactions; however, a terminal metal–oxo intermediate could not be observed . Inspired by reports of mononuclear terminal metal–oxo motifs stabilized by second-coordination-sphere hydrogen-bonding interactions, our group has previously used this strategy to access a terminal Mn III –OH moiety as part of a [Mn 4 ] cluster . Herein we describe the synthesis, structural characterization, and reactivity studies of clusters bearing a terminal Fe III –oxo motif stabilized by ( tert -butylamino)­pyrazolates to probe the significance of a multinuclear scaffold on structural and reactivity aspects of a terminal metal–oxo.…”

Tris(pyrazolyl)borate Copper Hydroxide Complexes Featuring Tunable Intramolecular H-bonding

“…Secondary sphere H-bond donor and/or acceptor groups are featured especially often in tripodal systems (Figure 1). [9][10][11][12][13][14][15] For instance, several synthetic systems showcase the utility of H-bonding interactions with oxygen-containing small molecules. The stabilization of the peroxide anion (O2 2-) via interactions with H-bond donors, first observed in Cummins' 16 metal-free cryptand, is underscored by Szymczak's 17 tripodal Zn(II) complex, where secondary coordination sphere H-bonding governs O2 2binding at Zn (Figure 1a).…”

“…H-bonding groups on the properties of the complex. 11,14,21,24 In a landmark report, Kitajima and Fujisawa established TpCu scaffolds (Tp = tris(pyrazolyl)borate) as important biorelevant models for O 2 reactivity at copperbased hemocyanin with [ iPr2 TpCu] 2 ( -O 2 ) and the related [ iPr2 TpCu] 2 ( -OH) 2 . 25 Molecular copper-oxygen complexes attract interest as intermediates in chemical transformations mediated by hemocyanin, [25][26][27] copper oxidases, [28][29][30] and other copper-based enzymes.…”