Fe(I)-Mediated Reductive Cleavage and Coupling of CO_2: An Fe^(II)(μ-O,μ-CO)Fe^(II) Core

THF solutions of a new iron(I) source, [PhBP^(CH2_Cy_3)]Fe ([PhBP^(CH_2Cy_3)] = [PhBP(CH_2P(CH_2Cy)_2)_3]-), effect the reductive cleavage of CO_2 via O-atom transfer at ambient temperature. The dominant reaction pathway is bimetallic and leads to the formation of a structurally unprecedented diiron Fe^(II)(μ-O)(μ-CO)Fe^(II) core. X-ray data are also available to suggest that bimetallic reductive CO_2 coupling to generate oxalate occurs as a minor reaction pathway. These initial observations forecast a diverse reaction landscape between CO_2 and iron(I) synthons

CO_2 reduction by Fe(I): solvent control of C-O cleavage versus C-C coupling

This manuscript explores the product distribution of the reaction of carbon dioxide with reactive iron(I) complexes supported by tris(phosphino)borate ligands, [PhBP^R_3]- ([PhBP^R_3]- =[PhB(CH_2PR_2)_3]-; R = CH_2Cy,Ph, ^iPr, mter; mter = 3,5-meta-terphenyl). Our studies reveal an interesting and unexpected role for the solvent medium with respect to the course of the CO_2 activation reaction. For instance, exposure of methylcyclohexane (MeCy) solutions of [PhBP^(CH_2Cy)_3 ]Fe(PR’_3) to CO_2 yields the partial decarbonylation product {[PhBP^(CH_2Cy)_3 ]Fe}_2(µ-O)(µ-CO). When the reaction is instead carried out in benzene or THF, reductive coupling of CO_2 occurs to give the bridging oxalate species {[PhBP^(CH_2Cy_3 ]Fe}_2(µ- κOO’: κOO’-oxalato). Reaction studies aimed at understanding this solvent effect are presented, and suggest that the product profile is ultimately determined by the ability of the solvent to coordinate the iron center. When more sterically encumbering auxiliary ligands are employed to support the iron(I) center (i.e., [PhBP^(Ph)_3]- and [PhBP^(iPr)_3 ]-), complete decarbonylation is observed to afford structurally unusual diiron(II) products of the type {[PhBP^R_3]Fe}_2(µ-O). A mechanistic hypothesis that is consistent with the collection of results described is offered, and suggests that reductive coupling of CO_2 likely occurs from an electronically saturated “Fe^(II)–CO_2-” species

Characterization of Structurally Unusual Diiron N_xH_y Complexes

Mechanistic proposals concerning the pathway of N_2 reduction in biology at the MoFe-cofactor of nitrogenase continue to be advanced. In addition to nitrogen, hydrazine2 and diazene1a are nitrogenase substrates, and recent DFT calculations and spectroscopic studies suggest that whereas initial N_2 binding may occur at one iron center, diiron pathways may be involved at certain N_xH_y intermediate stages en route to ammonia formation. In this broad context, recent work has explored the synthesis and spectroscopic characterization of structurally unusual mono- and bimetallic iron complexes featuring nitrogenous ligand functionalities. The demand for such model complexes continues in light of recent ENDOR and ESEEM spectroscopic data that has been obtained under turnover conditions at the cofactor. To date, there are few synthetic iron systems that feature parent hydrazine (N_2H_4), hydrazido (N_2H_2^(2-)), diazene (N_2H_2), amide (NH_(2)^-), and imide (NH^2-) ligands. Herein we describe the synthesis and characterization of a series of structurally distinct diiron complexes that feature each of these ligand types

Modeling the Signatures of Hydrides in Metalloenzymes: ENDOR Analysis of a Di-iron Fe(μ-NH)(μ-H)Fe Core

The application of 35 GHz pulsed EPR and ENDOR spectroscopies has established that the biomimetic model complex L_3Fe(μ-NH)(μ-H)FeL_3 (L_3 = [PhB(CH_2PPh_2)_3]−) complex, 3, is a novel S = 1/2 type-III mixed-valence di-iron II/III species, in which the unpaired electron is shared equally between the two iron centers. ^(1,2)H and ^(14,15)N ENDOR measurements of the bridging imide are consistent with an allyl radical molecular orbital model for the two bridging ligands. Both the (μ-H) and the proton of the (μ-NH) of the crystallographically characterized 3 show the proposed signature of a ‘bridging’ hydride that is essentially equidistant between two ‘anchor’ metal ions: a rhombic dipolar interaction tensor, T ≈ [T, –T, 0]. The point-dipole model for describing the anisotropic interaction of a bridging H as the sum of the point-dipole couplings to the ‘anchor’ metal ions reproduces this signature with high accuracy, as well as the axial tensor of a terminal hydride, T ≈ [−T, –T, 2T], thus validating both the model and the signatures. This validation in turn lends strong support to the assignment, based on such a point-dipole analysis, that the molybdenum–iron cofactor of nitrogenase contains two [Fe–H––Fe] bridging-hydride fragments in the catalytic intermediate that has accumulated four reducing equivalents (E_4). Analysis further reveals a complementary similarity between the isotropic hyperfine couplings for the bridging hydrides in 3 and E_4. This study provides a foundation for spectroscopic study of hydrides in a variety of reducing metalloenzymes in addition to nitrogenase

XAS Characterization of a Nitridoiron(IV) Complex with a Very Short Fe-N Bond

X-ray absorption spectroscopy has been used to characterize the novel nitridoiron(IV) units in two [PhBP^R_3]Fe(N) complexes (R = iPr and CyCH_2) and obtain direct spectroscopic evidence for a very short Fe−N distance. The distance of 1.51−1.55 Å reflects the presence of an FeN triple bond in accord with the observed Fe_≡N vibration observed for one of these species (ν_(FeN) = 1034 cm^(-1)). This highly covalent bonding interaction results in the appearance of an unusually intense pre-edge peak, whose estimated area of 100(20) units is much larger than those of the related tetrahedral complexes with Fe^I−N_2−Fe^I, Fe^(II)−NPh_2, and Fe^(III)_≡NAd motifs, and those of recently described six-coordinate Fe^V≡N and Fe^V≡IN complexes. The observation that the Fe^(IV)−N distances of two [PhBPR_3]Fe(N) complexes are shorter than the Fe^(IV)−O bond lengths of oxoiron(IV) complexes may be rationalized on the basis of the greater π basicity of the nitrido ligand than the oxo ligand and a lower metal coordination number for the Fe(N) complex

Carbon dioxide activation by low-valent pseudo-tetrahedral iron

Upon exposure to CO_2, pseudo-tetrahedral iron(I) supported by 1;PhBP^R_33;- ligands (where 1;PhBP^R_33; =1;PhB(CH_2PR2)_33;-, and R = CH_2Cy, ^iPr, and Ph) effects either the reductive cleavage of CO_2 via O-atom transfer or reductive coupling of CO_2 to give an oxalate. THF solns. of 1;PhBP^(CH2Cy)_33;Fe(I) gives rise to the structurally unprecedented- Fe_2(μ-O)(μ-CO), whereas 1;PhBP^(CH2Cy)_33;FePPh_3 gives a bridging oxalate species. In situ sodium/amalgam redns. of 1;PhBP^R_33;FeCl under an atm. of CO_2 again gives Fe_2(μ-oxalate) for R =CH_2Cy, but gives a Fe_2O species as the major product for both R = -^iPr and Ph