Caroline T. Saouma

Characterization of Structurally Unusual Diiron NxHy Complexes

A series of fascinating diiron complexes featuring bridging N(x)H(y) ligands stabilized by tris(phosphine)borate ([PhB(CH(2)PR(2))(3)] = [PhBP(R)(3)]) ligands have been characterized. Hydrazine activation by [PhBP(R)(3)]Fe-Me (R = Ph or cyclohexylmethyl) leads to diiron Fe(2)(mu-eta(1):eta(1)-N(2)H(4))(mu-eta(2):eta(2)-N(2)H(2)) complexes featuring both bridging hydrazine and hydrazido ligands. Thermolysis of {[PhBP(CH(2))(Cy)(3)]Fe}(2)(mu-eta(1):eta(1)-N(2)H(4))(mu-eta(2):eta(2)-N(2)H(2)) at 22 degrees C leads to a structurally unusual {[PhBP(CH(2))(Cy)]Fe}(2)(mu-eta(1):eta(1)-N(2)H(2))(mu-NH(2))(2) complex featuring bridging HN horizontal lineNH and NH(2)(-) ligands. This contrasts with {[PhBP(Ph)(3)]Fe}(2)(mu-eta(1):eta(1)-N(2)H(4))(mu-eta(2):eta(2)-N(2)H(2)), which can be chemically oxidized to produce either {[PhBP(Ph)(3)]Fe}(2)(mu-eta(1):eta(1)-N(2)H(2))(mu-eta(2):eta(2)-N(2)H(2)) or {[PhBP(Ph)(3)]Fe}(2)(mu-NH)(2), depending on the conditions. The former product is the only known complex to contain bridging N(2)H(2) ligands in each of their limiting states of oxidation (HN horizontal lineNH vs HN-NH(2-)). The latter product constitutes the first example of a diiron Fe(2)(mu-NH)(2) diamond-shaped core.

Fast proton-coupled electron transfer observed for a high-fidelity structural and functional [2Fe-2S] Rieske model.

Rieske cofactors have a [2Fe–2S] cluster with unique {His2Cys2} ligation and distinct Fe subsites. The histidine ligands are functionally relevant, since they allow for coupling of electron and proton transfer (PCET) during quinol oxidation in respiratory and photosynthetic ET chains. Here we present the highest fidelity synthetic analogue for the Rieske [2Fe–2S] cluster reported so far. This synthetic analogue 5x– emulates the heteroleptic {His2Cys2} ligation of the [2Fe–2S] core, and it also serves as a functional model that undergoes fast concerted proton and electron transfer (CPET) upon reaction of the mixed-valent (ferrous/ferric) protonated 5H2– with TEMPO. The thermodynamics of the PCET square scheme for 5x– have been determined, and three species (diferric 52–, protonated diferric 5H–, and mixed-valent 53–) have been characterized by X-ray diffraction. pKa values for 5H– and 5H2– differ by about 4 units, and the reduction potential of 5H– is shifted anodically by about +230 mV compared to that of 52–. While the N–H bond dissociation free energy of 5H2– (60.2 ± 0.5 kcal mol–1) and the free energy, ΔG°CPET, of its reaction with TEMPO (−6.3 kcal mol–1) are similar to values recently reported for a homoleptic {N2/N2}-coordinated [2Fe–2S] cluster, CPET is significantly faster for 5H2– with biomimetic {N2/S2} ligation (k = (9.5 ± 1.2) × 104 M–1 s–1, ΔH‡ = 8.7 ± 1.0 kJ mol–1, ΔS‡ = −120 ± 40 J mol–1 K–1, and ΔG‡ = 43.8 ± 0.3 kJ mol–1 at 293 K). These parameters, and the comparison with homoleptic analogues, provide important information and new perspectives for the mechanistic understanding of the biological Rieske cofactor.

A Five-Coordinate Phosphino/Acetate Iron(II) Scaffold That Binds N2, N2H2, N2H4, and NH3 in the Sixth Site

A family of iron(II) complexes that coordinate dinitrogen, diazene, hydrazine, and ammonia are presented. This series of complexes is unusual in that the complexes within it feature a common auxiliary ligand set and differ only by virtue of the nitrogenous N(x)H(y) ligand that occupies the sixth binding site. The ability of an iron center to bind N(2), N(2)H(2), N(2)H(4), and NH(3) is important to establish in the context of evaluating catalytic N(2) reduction schemes that invoke these nitrogenous species. Such a scenario has been proposed as an iron-mediated, alternating reduction scheme within the cofactor of nitrogenase enzymes.

CO2 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, [PhBPR3]− ([PhBPR3]− = [PhB(CH2PR2)3]−; R = CH2Cy, 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 CO2 activation reaction. For instance, exposure of methylcyclohexane (MeCy) solutions of to CO2 yields the partial decarbonylation product . When the reaction is instead carried out in benzene or THF, reductive coupling of CO2 occurs to give the bridging oxalate species . 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., [PhBPPh3]− and [PhBPiPr3]−), complete decarbonylation is observed to afford structurally unusual diiron(II) products of the type {[PhBPR3]Fe}2(μ-O). A mechanistic hypothesis that is consistent with the collection of results described is offered, and suggests that reductive coupling of CO2 likely occurs from an electronically saturated “FeII–CO2˙−” species.

Protonation and concerted proton-electron transfer reactivity of a bis-benzimidazolate ligated [2Fe-2S] model for Rieske clusters.

A model system for biological Rieske clusters that incorporates bis-benzimidazolate ligands ((Pr)bbim)(2-) has been developed ((Pr)bbimH(2) = 4,4-bis(benzimidazol-2-yl)heptane). The diferric and mixed-valence clusters have been prepared and characterized in both their protonated and deprotonated states. The thermochemistry of interconversions of these species has been measured, and the effect of protonation on the reduction potential is in good agreement to that observed in the biological systems. The mixed-valence and protonated congener [Fe(2)S(2)((Pr)bbim)((Pr)bbimH)](Et(4)N)(2) (4) reacts rapidly with TEMPO or p-benzoquinones to generate diferric and deprotonated [Fe(2)S(2)((Pr)bbim)(2)](Et(4)N)(2) (1) and 1 equiv of TEMPOH or 0.5 equiv of p-benzohydroquinones, respectively. The reaction with TEMPO is the first well-defined example of concerted proton-electron transfer (CPET) at a synthetic ferric/ferrous [Fe-S] cluster.

Single-electron oxidation of N-heterocyclic carbene-supported nickel amides yielding benzylic C–H activation

The dimeric Ni(I)–Ni(I) N-heterocyclic carbene complex {(IPr)Ni(μ-Cl)}2 (3; IPr = 1,3-(2,6-iPr2C6H3)2imidazolin-2-ylidene)) reacts with the lithium terphenylamides LiNHdmp and LiNHdippp (dmp = 2,6-di(mesityl)phenyl; dippp = 2,6-bis(2,6-di-iso-propylphenyl)phenyl) to give the monomeric Ni(I) amides (IPr)Ni(NHdmp) (4) and (IPr)Ni(NHdippp) (5), respectively. These nickel amides are 1-electron paramagnets, and crystallographic characterization indicates both are stabilized by Ni–C(ipso) interactions with a flanking aryl group of the terphenyl fragment. This results in significant deviation from the linear CNHC–Ni–N geometry typical for a two-coordinate transition-metal complex (112.17(9)° in 4, 116.41(9)° in 5). One-electron oxidation of 4 by ferrocenium results in intramolecular deprotonation at a terphenyl benzylic position by the amide, giving the diamagnetic Ni(II) complex [(IPr)Ni(κ2-C,N:NH2C6H3(Mes)C10H9)][B(ArF)4] (7). DFT calculations on oxidized 4 (i.e., 4+) indicate short amide N⋯CH3 interactions. One-electron oxidation of 5 by ferrocenium gives a new high-spin Ni(II) amide complex salt, [(IPr)Ni(NHdippp)][B(ArF)4] (9). The solid-state structure of 9 indicates it maintains the bent CNHC–Ni–N core. Unlike three-coordinate cationic Ni(II) amides, 9 has not been observed to undergo smooth deprotonation (at N) to afford a two-coordinate imido complex.

Mononuclear five- and six-coordinate iron hydrazido and hydrazine species.

This article describes the synthesis and characterization of several low-spin iron(II) complexes that coordinate hydrazine (N(2)H(4)), hydrazido (N(2)H(3)(-)), and ammonia. The sterically encumbered tris(di-meta-terphenylphosphino)borate ligand, [PhBP(mter)(3)](-), is introduced to provide access to species that cannot be stabilized with the [PhBP(Ph)(3)](-) ligand ([PhBP(R)(3)](-) = PhB(CH(2)PR(2))(3)(-)). Treatment of [PhBP(mter)(3)]FeMe with hydrazine generates the unusual 5-coordinate hydrazido complex [PhBP(mter)(3)]Fe(η(2)-N(2)H(3)) (1), in which the hydrazido serves as an L(2)X-type ligand. Upon coordination of an L-type ligand, the hydrazido shifts to an LX-type ligand, generating [PhBP(mter)(3)]Fe(L)(η(2)-N(2)H(3)) (L = N(2)H(4) (2) or NH(3) (3)). In contrast, treatment of [PhBP(Ph)(3)]FeMe with hydrazine forms the adduct [PhBP(Ph)(3)]Fe(Me)(η(2)-N(2)H(4)) (5). Complex 5 is thermally unstable to methane loss, generating intermediate [PhBP(Ph)(3)]Fe(η(2)-N(2)H(3)), which undergoes bimolecular coupling to produce {[PhBP(Ph)(3)]Fe}(2)(μ-η(1):η(1)-N(2)H(4))(μ-η(2):η(2)-N(2)H(2)). The oxidation of these and related hydrazine and hydrazido species is also presented. For example, oxidation of 1 or 5 with Pb(OAc)(4) results in disproportionation of the N(2)H(x) ligand (x = 3, 4), and formation of [PhBP(R)(3)]Fe(NH(3))(OAc) (R = Ph (9) and mter (11)).

Transformation of an [Fe(η2-N2H3)]+ Species to π-Delocalized [Fe2(μ-N2H2)]2+/+ Complexes†

A monomeric iron Fe(η-N2H3) species has been prepared, and exposure to oxygen yields a diiron complex that features five-coordinate iron centers and an activated bridging diazene ligand (NH=NH). Combined structural, theoretical, and spectroscopic data for the redox pair of complexes [Fe2(μ-N2H2)] are consistent with 4-center, 4-electron π-delocalized bonding picture across the Fe-NH-NH-Fe core that finds analogy in butadiene and the butadiene anion.

Electron Transfer and Proton-Coupled Electron Transfer Reactivity and Self-Exchange of Synthetic (2Fe−2S) Complexes: Models for Rieske and mitoNEET Clusters

This report describes the thermochemistry, proton-coupled electron transfer (PCET) reactions and self-exchange rate constants for a set of bis-benzimidazolate-ligated [2Fe–2S] clusters. These clusters serve as a model for the chemistry of biological Rieske and mitoNEET clusters. PCET from [Fe2S2(Prbbim)(PrbbimH)]2– (4) and [Fe2S2(Prbbim)(PrbbimH2)]1– (5) to TEMPO occurs via concerted proton–electron transfer (CPET) mechanisms (PrbbimH2 = 4,4-bis-(benzimidazol-2-yl)heptane). Intermolecular electron transfer (ET) self-exchange between [Fe2S2(Prbbim)2]2– (1) and [Fe2S2(Prbbim)2]3– (2) occurs with a rate constant of (1.20 ± 0.06) × 105 M–1 s–1 at 26 °C. A similar self-exchange rate constant is found for the related [2Fe–2S] cluster [Fe2S2(SArO)2]2–/3–, SArO2– = thiosalicylate. These are roughly an order of magnitude slower than that reported for larger [4Fe–4S] clusters and 1 order of magnitude faster than that reported for N-ligated high-spin iron complexes. These results suggest that the rate of intermolecular ET to/from [Fe–S] clusters is modulated by cluster size. The measured PCET self-exchange rate constant for 1 and 4 at −30 °C is (3.8 ± 0.7) × 104 M–1 s–1. Analysis of rate constants using the Marcus cross-relation suggests that this process likely occurs via a concerted proton–electron transfer (CPET) mechanism. The implications of these findings to biological systems are also discussed, including the conclusion that histidine-ligated [2Fe–2S] clusters should not have a strong bias to undergo concerted e–/H+ transfers.