Binding of N 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 Fe through FeFe. One electron redox events result in small changes in the affinity for (at most ∼400-fold) and activation of CO (at most 28 cm for ν). 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 (10-10 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), it provides a conduit for CO binding at an Fe center. Indeed, internal electron transfer from these remote sites accommodates binding of CO at an Fe, with a small energetic penalty arising from redox reorganization (∼2.6 kcal·mol). The ease with which these clusters redistribute electrons in response to ligand binding highlights a potential pathway for coordination of N and CO by FeMoco, which may occur on an oxidized edge of the cofactor.
Experimental and Synthetic Details General Considerations All reactions were performed at room temperature in a nitrogen filled M. Braun glovebox or using standard Schlenk techniques unless otherwise specified. Glassware was oven dried at 140 o C for at least two hours prior to use, and allowed to cool under vacuum. Bis(odiisopropylphosphinophenyl)-chlorophosphine (P2P Cl) was prepared as described elsewhere. 1 All other reagents were obtained commercially unless otherwise noted and typically stored over activated 4 Å molecular sieves. Tetrahydrofuran, toluene-d8 and benzene-d6 were dried using sodium/benzophenone ketyl, degassed with three freeze-pump-thaw cycles, vacuum transferred, and stored over 3 Å molecular sieves prior to use. Diethyl ether, benzene, toluene, acetonitrile, hexanes, and pentane were dried by sparging with nitrogen for at least 15 minutes, then passing through a column of activated A2 alumina under positive nitrogen pressure. 1 H and 31 P NMR spectra were recorded on a Varian 300 or 400 MHz spectrometer. All chemical shifts (δ) are reported in ppm, and coupling constants (J) are in hertz. The 1 H-NMR spectra were referenced using residual H impurity in the deuterated solvent. UV-Vis spectra were recorded on a Varian Cary Bio 50 spectrophotometer. Infrared (ATR-IR) spectra were recorded on a Bruker ALPHA ATR-IR spectrometer. Elemental analyses were performed at Caltech. Physical Methods Mössbauer Measurements. Zero field 57 Fe Mössbauer spectra were recorded in constant acceleration on a spectrometer from See Co (Edina, MN) equipped with an SVT-400 cryostat (Janis, Wilmington, WA). The quoted isomer shifts are relative to the centroid of the spectrum of α-Fe foil at room temperature. Unless otherwise noted, samples were prepared by grinding polycrystalline (20-50 mg) into a fine powder and pressed into a homogenous pellet with boron nitride in a cup fitted with a screw cap. The data were fitted to Lorentzian lineshapes using the program WMOSS (www.wmoss.org). Magnetic Measurements. Magnetic measurements for 5 were conducted with a Quantum Design MPMS3 SQUID Magnetometer at the University of California, Los Angeles. A polycrystalline sample of 5 was wrapped in plastic film and placed in a gelatin capsule. The capsule was then inserted into a plastic straw. Magnetization data at 100 K from 0 to 4 T were collected to confirm the absence of ferromagnetic impurities. Direct current variable temperature magnetic susceptibility measurements were collected between 1.8 and 300 K with a 0.1 T field. Magnetic susceptibility data was corrected for diamagnetism of the sample, estimated using Pascal's constants. Magnetic susceptibility data was simulated with PHI. 2 X-ray Crystallography. For compounds 4-8, low-temperature (100 K) diffraction data (φand ω-scans) were collected on a Bruker AXS D8 VENTURE KAPPA diffractometer coupled to a PHOTON 100 CMOS detector with Mo Kα radiation (λ = 0.71073 Å) or with Cu Kα (λ = 1.54178 Å). All diffractometer manipulations, including data collection, integration, and ...
A variety of acyl protected phenols AcOAr participate in sp C-H etherification of substrates R-H to give alkyl aryl ethers R-OAr employing BuOOBu as oxidant with copper(I) β-diketiminato catalysts [Cu]. 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 β-diketiminato copper(II) phenolates [Cu]-OAr play a key role in this C-O bond forming reaction, formed via transesterification of AcOAr with [Cu]-OBu intermediates generated upon reaction of [Cu] with BuOOBu.
Experimental and Synthetic Details General Considerations All reactions were performed at room temperature in a nitrogen filled M. Braun glovebox or using standard Schlenk techniques unless otherwise specified. Glassware was oven dried at 140 o C for at least two hours prior to use, and allowed to cool under vacuum. The N-substituted aryl imidazoles pOMe ArIm and pNMe2 ArIm were synthesized from the corresponding anilines, glyoxal, formaldehyde and aqueous ammonia based on a literature procedure. 1 pCF3 ArIm was prepared from the corresponding aniline, thiophosgene and aminoacetylaldehyde diethyl acetal based on an adapted literature procedure. 1 All aryl imidazoles were further purified by sublimation at 100 o C under vacuum. Fe(OTf)2(MeCN)2, 2 [Fc][OTf] 3 and Na[BAr F 24] 4 were prepared according to literature procedures. [Fc * ][OTf] was prepared by oxidation of Fc * with [Fc][OTf] in dichloromethane followed by crystallization from dichloromethane/pentane. LFe3(OTf)3, [LFe3O(PhIm)3Fe][OTf]2 (1 H) and [LFe3O(PhIm)3Fe] were prepared as previously described. 5 All other reagents were obtained commercially unless otherwise noted and typically stored over activated 4 Å molecular sieves. Tetrahydrofuran was dried using sodium/benzophenone ketyl, degassed with three freeze-pump-thaw cycles, vacuum transferred, and stored over 3 Å molecular sieves prior to use. Dichloromethane, diethyl ether, benzene, acetonitrile, hexanes, and pentane were dried by sparging with nitrogen for at least 15 minutes, then passing through a column of activated A2 alumina under positive nitrogen pressure. Dichloromethane-d2 was dried over calcium hydride, degassed by three freeze-pump-thaw cycles, and vacuum transferred prior to use. 1 H and 19 F NMR spectra were recorded on a Varian 300 or 400 MHz spectrometer. All chemical shifts (δ) are reported in ppm, and coupling constants (J) are in hertz. The 1 H-NMR spectra were referenced using residual H impurity in the deuterated solvent, whereas the 19 F chemical shifts are reported relative to the internal lock signal. UV-Vis spectra were recorded on a Varian Cary Bio 50 spectrophotometer. Infrared (ATR-IR) spectra were recorded on a Bruker ALPHA ATR-IR spectrometer. Solution ATR-IR spectra were recorded on a Mettler Toledo iC10 ReactIR. Elemental analyses were performed at Caltech. Physical Methods Mössbauer Measurements. Zero field 57 Fe Mössbauer spectra were recorded in constant acceleration on a spectrometer from See Co (Edina, MN) equipped with an SVT-400 cryostat (Janis, Wilmington, WA). The quoted isomer shifts are relative to the centroid of the spectrum of α-Fe foil at room temperature. Unless otherwise noted, samples were prepared by grinding polycrystalline (20-50 mg) into a fine powder and pressed into a homogenous pellet with boron nitride in a cup fitted with a screw cap. The data were fitted to Lorentzian lineshapes using the program WMOSS (www.wmoss.org). EPR Spectroscopy. X-band EPR spectra were collected on a Bruker EMX spectrometer equipped with a He flow cryostat. Sample...
Binding of N2 by the FeMo-cofactor of nitrogenase is believed to occur after transfer of 4 eand 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 the diiron µcarbyne 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 etransfer which bear some resemblance to a pair of En and En+2 states of nitrogenase. Both species populate S = ½ 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 Fe III ) 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 ereduction), a conclusion further supported by X-ray absorption spectroscopy, 57 Fe 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 multi-electron transformations.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.