Probing Redox Non-Innocence in Iron-Carbene Complexes {Fe=C(H)Ar}10-11 by 1,2 H and 13 C Pulse Electron Paramagnetic Resonance.

We report the synthesis and spectroscopic characterization of a series of iron-carbene complexes in redox states {Fe=C(H)Ar}10-11 . Pulse EPR studies of the 1,2 H and 13 C isotopologues of {Fe=C(H)Ar}11 reveal the high covalency of the Fe-carbene bonding, leading to a more even spin distribution than commonly observed for reduced Fischer carbenes.

Mixed-Valent Diiron µ-Carbyne, µ-Hydride Complexes: Implications for Nitrogenase.

Binding of N2 by the FeMo-cofactor of nitrogenase is believed to occur after transfer of 4 e- and 4 H+ equivalents to the active site. Although pulse EPR studies indicate the presence of two Fe-(µ-H)-Fe moieties, the structural and electronic features of this mixed valent intermediate remain poorly understood. Toward an improved understanding of this bioorganometallic cluster, we report herein that diiron µ-carbyne complex (P6ArC)Fe2(µ-H) can be oxidized and reduced, allowing for the first time spectral characterization of two EPR-active Fe(µ-C)(µ-H)Fe model complexes linked by a 2 e- transfer which bear some resemblance to a pair of En and En+2 states of nitrogenase. Both species populate S = 1/2 states at low temperatures, and the influence of valence (de)localization on the spectroscopic signature of the µ-hydride ligand was evaluated by pulse EPR studies. Compared to analogous data for the {Fe2(µ-H)}2 state of FeMoco (E4(4H)), the data and analysis presented herein suggest that the hydride ligands in E4(4H) bridge isovalent (most probably FeIII) metal centers. Although electron transfer involves metal-localized orbitals, investigations of [(P6ArC)Fe2(µ-H)]+1 and [(P6ArC)Fe2(µ-H)]-1 by pulse EPR revealed that redox chemistry induces significant changes in Fe-C covalency (-50% upon 2 e- reduction), a conclusion further supported by X-ray absorption spectroscopy, 57Fe Mössbauer studies, and DFT calculations. Combined, our studies demonstrate that changes in covalency buffer against the accumulation of excess charge density on the metals by partially redistributing it to the bridging carbon, thereby facilitating multielectron transformations.

Activation of an Open Shell, Carbyne-Bridged Diiron Complex Toward Binding of Dinitrogen.

Binding of N2 by nitrogenase requires a reductive activation of the FeMo-cofactor, but the precise structure and atomic composition of FeMoco in its activated form is not well understood. However, recent crystallographic studies suggest that N2 reduction may occur at a carbon-bridged diiron subunit of FeMoco. Toward modeling the activation of a Fe-(µ-C)-Fe site toward N2 binding, we synthesized a new dinucleating, hexaphosphine ligand derived from a 2,6-disubstituted toluene platform. Activation of the central methyl group of the ligand affords the diiron µ-carbyne complex (P6ArC)Fe2(µ-H) featuring a biologically relevant Fe(µ-carbyne)(µ-H)Fe motif. SQUID magnetometry, Mössbauer spectroscopy, and DFT calculations reveal that (P6ArC)Fe2(µ-H) has a well-isolated S = 1 ground state, distinguishing it from all other diiron µ-carbyne complexes which are diamagnetic. Upon the addition of sources of H+/e- (H2, TEMPO-H or HCl), (P6ArC)Fe2(µ-H) is activated toward N2 binding, with concomitant protonation of the carbyne ligand. Although reaction with H2 ultimately leads to complete protonation of the carbyne moiety, mechanistic investigations indicate that formation of a single C-H bond, with concomitant cleavage of one Fe-C bond, generates an iron-carbene intermediate capable of coordinating N2.

Remote Ligand Modifications Tune Electronic Distribution and Reactivity in Site-Differentiated, High-Spin Iron Clusters: Flipping Scaling Relationships.

We report the synthesis, characterization, and reactivity of [LFe3O(RArIm)3Fe][OTf]2, the first Hammett series of a site-differentiated cluster. The cluster reduction potentials and CO stretching frequencies shift as expected on the basis of the electronic properties of the ligand: electron-donating substituents result in more reducing clusters and weaker C-O bonds. However, unusual trends in the energetics of their two sequential CO binding events with the substituent σp parameters are observed. Specifically, introduction of electron-donating substituents suppresses the first CO binding event (ΔΔH by as much as 7.9 kcal mol-1) but enhances the second (ΔΔH by as much as 1.9 kcal mol-1). X-ray crystallography, including multiple-wavelength anomalous diffraction, Mössbauer spectroscopy, and SQUID magnetometry, reveal that these substituent effects result from changes in the energetic penalty associated with electronic redistribution within the cluster, which occurs during the CO binding event.

A Thermodynamic Model for Redox-Dependent Binding of Carbon Monoxide at Site-Differentiated, High Spin Iron Clusters.

Binding of N2 and CO by the FeMo-cofactor of nitrogenase depends on the redox level of the cluster, but the extent to which pure redox chemistry perturbs the affinity of high spin iron clusters for π-acids is not well understood. Here, we report a series of site-differentiated iron clusters that reversibly bind CO in redox states FeII4 through FeIIFeIII3. One electron redox events result in small changes in the affinity for (at most ∼400-fold) and activation of CO (at most 28 cm-1 for νCO). The small influence of redox chemistry on the affinity of these high spin, valence-localized clusters for CO is in stark contrast to the large enhancements (105-1022 fold) in π-acid affinity reported for monometallic and low spin, bimetallic iron complexes, where redox chemistry occurs exclusively at the ligand binding site. While electron-loading at metal centers remote from the substrate binding site has minimal influence on the CO binding energetics (∼1 kcal·mol-1), it provides a conduit for CO binding at an FeIII center. Indeed, internal electron transfer from these remote sites accommodates binding of CO at an FeIII, with a small energetic penalty arising from redox reorganization (∼2.6 kcal·mol-1). The ease with which these clusters redistribute electrons in response to ligand binding highlights a potential pathway for coordination of N2 and CO by FeMoco, which may occur on an oxidized edge of the cofactor.

Copper Catalyzed sp3 C-H Etherification with Acyl Protected Phenols.

A variety of acyl protected phenols AcOAr participate in sp3 C-H etherification of substrates R-H to give alkyl aryl ethers R-OAr employing tBuOOtBu as oxidant with copper(I) ß-diketiminato catalysts [CuI]. Although 1°, 2°, and 3° C-H bonds may be functionalized, selectivity studies reveal a preference for the construction of hindered, 3° C-OAr bonds. Mechanistic studies indicate that ß-diketiminato copper(II) phenolates [CuII]-OAr play a key role in this C-O bond forming reaction, formed via transesterification of AcOAr with [CuII]-OtBu intermediates generated upon reaction of [CuI] with tBuOOtBu.