Borane B-C Bond Cleavage by a Low-Coordinate Iron Hydride Complex and N-N Bond Cleavage by the Hydridoborate Product

Photoinduced Cleavage of a Strained N-C Bond in an Iron Complex Supported by Super-Bulky Amidinate and Guanidinate Ligands

The reaction of Fe2(mes)4 with the super-bulky amidines and guanidines HLAr*-R (LAr*-R = [(Ar*N)2C(R)]-, Ar* = 2,6-bis(diphenylmethyl)-4-tert-butylphenyl), R = Me (LAr*-Me), tBu (LAr*-tBu), Ph (LAr*-Ph), NiPr2 (LAr*-iPr2N), and Pip (LAr*-Pip)) gives access to the three-coordinate iron-mesityl complexes (LAr*-R)Fe(mes) only where LAr*-R = LAr*-Me, LAr*-Ph, or LAr*-Pip. Subsequent protonolysis with the N-atom transfer reagent Hdbabh (Hdbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene) is limited in success, providing in one instance a few crystals of four-coordinate (LAr*-Me)Fe(dbabh)(Hdbabh), while three-coordinate (LAr*-Pip)Fe(dbabh) is synthesized reproducibly. Complexes (LAr*-Me)Fe(dbabh)(Hdbabh) and (LAr*-Pip)Fe(dbabh) are thermally insensitive in solution to temperatures of up to 100 °C. On the other hand, both (LAr*-Me)Fe(dbabh)(Hdbabh) and (LAr*-Pip)Fe(dbabh) show sensitivity to blue LED light (395 nm), undergoing photochemical transformations. For instance, the photolysis of (LAr*-Me)Fe(dbabh)(Hdbabh) leads to N-C bond scission and C-C bond coupling across the -dbabh moieties to give four-coordinate (LAr*-Me)Fe(N=dbabh-dbabhNH2). Photolyzing pyridine-d5 (py-d5) solutions of (LAr*-Pip)Fe(dbabh) at -5 °C produces a new paramagnetic photoproduct, [P]. Due to the thermal sensitivity of compound [P], it has eluded structural characterization; yet, Evans' method measurements suggest that the iron(II) oxidation state is maintained, thereby pointing to the -dbabh moiety as the locus of chemical change. In line with this assessment, addition of excess Me3SiCl to solutions of [P] produces the iron(II) complex (LAr*-Pip)FeCl(py-d5) as shown by 1H NMR spectroscopy. Gas chromatography/mass spectrometry analysis of the solutions of [P] shows a peak in the chromatogram with a molecular mass corresponding to a formulation of C14H11N that cannot be attributed to Hdbabh. This provides evidence for the photochemical-induced isomerization of the -dbabh ligand, revealing a heretofore unknown photochemical sensitivity of this N atom transfer reagent.

A thiolate-bridged ruthenium-molybdenum complex featuring terminal nitrido and bridging amido ligands derived from N−H and N−N bond cleavage of hydrazine

Biomimetic di- or multimetallic complexes featuring NxHy species in a sulfur-rich coordination sphere have attracted considerable attention in modelling the possible scenarios of biological nitrogen fixation by nitrogenases. Although the...

Investigation of iron-ammine and amido complexes within a C3-symmetrical phosphinic amido tripodal ligand

The primary and secondary coordination spheres can have large regulatory effects on the properties of metal complexes. To examine their influences on the properties of monomeric Fe complexes, the tripodal ligand containing phosphinic amido groups, N,N',N''-[nitrilotris(ethane-2,1-diyl)]tris(P,P-diphenylphosphinic amido) ([poat]^3-), was used to prepare [Fe^II/IIIpoat]^-/0 complexes. The Fe^II complex was four-coordinate with 4 N-atom donors comprising the primary coordination sphere. The Fe^III complex was six-coordinate with two additional ligands coming from coordination of O-atom donors on two of the phosphinic amido groups in [poat]^3-. In the crystalline phase, each complex was part of a cluster containing potassium ions in which KO[double bond, length as m-dash]P interactions served to connect two metal complexes. The [Fe^II/IIIpoat]^-/0 complexes bound an NH₃ molecule to form trigonal bipyramidal structures that also formed three intramolecular hydrogen bonds between the ammine ligand and the O[double bond, length as m-dash]P units of [poat]^3-. The relatively negative one-electron redox potential of -1.21 V vs. [Fe^III/IICp₂]^+/0 is attributed to the phosphinic amido group of the [poat]^3- ligand. Attempts to form the Fe^III-amido complex via deprotonation were not conclusive but isolation of [Fe^IIIpoat(NHtol)]^- using the p-toluidine anion was successful, allowing for the full characterization of this complex.

Access to Metal Centers and Fluxional Hydride Coordination Integral for CO2 Insertion into [Fe3(μ-H)3]3+ Clusters

CO₂ insertion into tri(μ-hydrido)triiron(II) clusters ligated by a tris(β-diketiminate) cyclophane is demonstrated to be balanced by sterics for CO₂ approach and hydride accessibility. Time-resolved NMR and UV-vis spectra for this reaction for a complex in which methoxy groups border the pocket of the hydride donor (Fe₃H₃L², 4) result in a decreased activation barrier and increased kinetic isotope effect consistent with the reduced sterics. For the ethyl congener Fe₃H₃L¹ (2), no correlation is found between rate and reaction solvent or added Lewis acids, implying CO₂ coordination to an Fe center in the mechanism. The estimated hydricity (50 kcal/mol) based on observed H/D exchange with BD₃ requires Fe-O bond formation in the product to offset an endergonic CO₂ insertion. μ₃-hydride coordination is noted to lower the activation barrier for the first CO₂ insertion event in DFT calculations.

Relevance of Single-Transmetalated Resting States in Iron-Mediated Cross-Couplings: Unexpected Role of σ-Donating Additives

Control of the transmetalation degree of organoiron(II) species is a critical parameter in numerous Fe-catalyzed cross-couplings to ensure the success of the process. In this report, we however demonstrate that the selective formation of a monotransmetalated Fe^II species during the catalytic regime counterintuitively does not alone ensure an efficient suppression of the nucleophile homocoupling side reaction. It is conversely shown that a fine control of the transmetalation degree of the transient Fe^III intermediates obtained after the activation of alkyl electrophiles by a single-electron transfer (SET), achievable using σ-donating additives, accounts for the selectivity of the cross-coupling pathway. This report shows for the first time that both coordination spheres of Fe^II resting states and Fe^III short-lived intermediates must be efficiently tuned during the catalytic regime to ensure high coupling selectivities.

A Dimeric Chromium(II) Pincer as an Electron Shuttle for N=N Bond Scission

Reduction of the bis-pyrazolyl pyridine complex [CrL]₂ with 4 KC₈ , followed by addition of one azobenzene (overall mole ratio 1:4:1), PhNNPh, transfers reducing equivalents to three azobenzenes, to form [K₃ Cr(PhNNPh)₃ ]. This has three κ² PhNNPh^2- ligands and K⁺ bound to nitrogen atoms of azobenzene. When the stoichiometry is modified to 1:4:3, the product is changed to [K₂ CrL(PhNNPh)₂ ], which has C₂ symmetry except for the intimate ion pairing of two K⁺ ions to reduced azobenzene nitrogen atoms, and to pyrazolate and phenyl rings. The origin of the observed delivery of reducing equivalents to several, not to a single N=N bond, is traced to the resistance of the one-electron-reduced substrate to receiving a second electron, and is thus a general phenomenon. [CrL]₂ alone is shown to be a two-electron reductant towards benzo[c]cinnoline (BCC) resulting in a product of formula [Cr₂ L₂ (BCC)], in which the reducing equivalents originate purely from Cr^II . An analogous study of the reaction of [CrL]₂ with azobenzene yields [Cr₂ L₂ (PhNNPh)(THF)], an adduct in which one THF has displaced one of four hydrazide nitrogen/Cr bonds. Together these illustrate different modes for the Cr₂ L₂ unit to bind and reduce the N=N bond. Collectively, these results show that two divalent Cr, without added K⁰ , have the ability to reduce the N=N bond. Further KC₈ reduction of preformed Cr₂ L₂ (RNNR) inevitably gives products in which K⁺ stabilizes the charge in the increasingly electron-rich nitrogen atoms, in a phenomenon which mimics proton coupled electron transfer: K⁺ performs the role of H⁺ . A least-squares fit of the two singly reduced DFT structures shows that the only major change is a re-orientation of one of the two phenyl rings in order to avoid repulsion with potassium but to still allow interaction of that phenyl π system with K⁺ . This shows both the impact of K⁺ , being modest to nitrogen/chromium interactions, but nevertheless accommodating some π donation of phenyl to potassium. Finally, delivering increasing equivalents of KC₈ leads to complete cleavage of the N=N bond, and both N bind to three Cr^II . The varied impacts of the K⁺ electrophile on NN multiple bond reduction is discussed.

Coupling dinitrogen and hydrocarbons through aryl migration

A persistent challenge in chemistry is to activate abundant, yet inert molecules such as hydrocarbons and atmospheric N₂. In particular, forming C–N bonds from N₂ typically requires a reactive organic precursor¹, which limits the ability to design catalytic cycles. Here, we report an diketiminate-supported iron system that is able to sequentially activate benzene and N₂ to form aniline derivatives. The key to this new coupling reaction is the partial silylation of a reduced iron-N₂ complex, which is followed by migratory insertion of a benzene-derived phenyl group to the nitrogen. Further reduction releases the nitrogen products, and the resulting iron species can re-enter the cyclic pathway. Using a mixture of sodium powder, crown ether, and trimethylsilyl bromide, an easily prepared diketiminate iron bromide complex² can mediate the one-pot conversion of several petroleum-derived compounds into the corresponding silylated aniline derivatives using N₂ as the nitrogen source. Numerous compounds along the cyclic pathway have been isolated and crystallographically characterized; their reactivity outlines the mechanism including the hydrocarbon activation step and the N₂ functionalization step. This strategy incorporates nitrogen atoms from N₂ directly into abundant hydrocarbons.

Reactions of hydrazones and hydrazides with Lewis acidic boranes

The reaction of (diphenylmethylene)hydrazone or 1,4-bis-hydrazone-ylidene(phenylmethyl)benzene with Lewis acidic boranes B(2,4,6-F₃C₆H₂)₃ or B(3,4,5-F₃C₆H₂)₃ generates the Lewis acid-base adducts. Alternatively, when (9H-fluoren-9-ylidene)hydrazone is employed several products were isolated including 1,2-di(9H-fluoren-9-ylidene)hydrazone, the 2 : 1 borane adduct of NH₂-NH₂ and the 1-(diarylboraneyl)-2-(9H-fluoren-9-ylidene)hydrazone in which one ArH group has been eliminated. The benzhydrazide starting material also initially gives an adduct when reacted with Lewis acidic boranes which upon heating eliminates ArH generating a CON₂B heterocycle.

Large-bite diboranes for the μ(1,2) complexation of hydrazine and cyanide

As part of our interest in the chemistry of polydentate Lewis acids as hosts for diatomic molecules, we have investigated the synthesis and coordination chemistry of bidentate boranes that feature a large boron-boron separation. In this paper, we describe the synthesis of a new example of such a diborane, namely 1,8-bis(dimesitylboryl)triptycene (2) and compare its properties to those of the recently reported 1,8-bis(dimesitylboryl)biphenylene (1). These comparative studies reveal that these two diboranes feature some important differences. As indicated by cyclic voltammetry, 1 is more electron deficient than 2; it also adopts a more compact and rigid structure with a boron-boron separation (4.566(5) Å) shorter by ∼1 Å than that in 2 (5.559(4) Å). These differences appear to dictate the coordination behaviour of these two compounds. While 2 remains inert toward hydrazine, we observed that 1 forms a very stable μ(1,2) hydrazine complex which can also be obtained by phase transfer upon layering a solution of 1 with a dilute aqueous hydrazine solution. The stability of this complex is further reflected by its lack of reaction with benzaldehyde at room temperature. We have also investigated the behaviour of 1 and 2 toward anions. In MeOH/CHCl3 (1/1 vol) both compounds selectively bind cyanide to form the corresponding μ(1,2) chelate complexes with a B-C[triple bond, length as m-dash]N-B bridge at their cores. Competition experiments in protic media show that the anionic cyanide complex formed by 1 is the most stable, with no evidence of decomplexation even in the presence of (C6F5)3B.

Efficient hydroboration of carbonyls by an iron(ii) amide catalyst

An easily prepared iron(ii) amide precatalyst enables the selective hydroboration of carbonyls with HBpin (pinacolborane) in the absence of any additive. The reactions proceed with low catalytic loading (1-3 mol%) under mild reaction conditions and display wide functional group compatibility. Aldehydes are selectively hydroborated in the presence of other reducible functional groups, such as ketones, alkenes, nitriles, esters, amides, acids and halides.

One-Step Conversion of Potassium Organotrifluoroborates to Metal Organoborohydrides

This letter describes the one-step conversion of heteroatom-substituted potassium organotrifluoroborates (KRBF₃) to metal monoorganoborohydrides (MRBH₃) using alkali metal aluminum hydrides. The method tolerates a variety of functional groups, expanding MRBH₃ diversity. Hydride removal with Me₃SiCl in the presence of dimethylaminopyridine (DMAP) affords the organoborane·DMAP (RBH₂·DMAP) adducts.

Stable Boron Dithiolene Radicals

Whereas low-temperature (-78 °C) reaction of the lithium dithiolene radical 1^. with boron bromide gives the dibromoboron dithiolene radical 2^. , the parallel reaction of 1^. with (C₆ H₁₁ )₂ BCl (0 °C) affords the dicyclohexylboron dithiolene radical 3^. . Radicals 2^. and 3^. were characterized by single-crystal X-ray diffraction, UV/Vis, and EPR spectroscopy. The nature of these radicals was also probed computationally. Under mild conditions, 3^. undergoes unexpected thiourea-mediated B-C bond activation to give zwitterion 4, which may be regarded as an anionic dithiolene-modified carbene complex of the sulfenyl cation RS⁺ (R=cyclohexyl).

Hydrazine Capture and N-N Bond Cleavage at Iron Enabled by Flexible Appended Lewis Acids

Incorporation of two 9-borabicyclo[3.3.1]nonyl substituents within the secondary coordination sphere of a pincer-based Fe(II) complex provides Lewis acidic sites capable of binding 1 or 2 equiv of N₂H₄. Reduction of the 1:1 Fe:N₂H₄ species affords a rare Fe(NH₂)₂ complex in which the amido ligands are stabilized through interactions with the appended boranes. The NH₂ units can be released as NH₃ upon protonation and exchanged with exogenous N₂H₄.

Catalyst free boron carbon bond cleavage and facile formation of five-membered PNBCC heterocycles

The [3 + 2] cycloaddition reaction of phosphanyl aminoborane [N(2,6-iPr₂C₆H₃)(PPh₂)(BCy₂)] (1) with activated alkynes led to boron and phosphorus containing five-membered heterocycles [(2,6-iPr₂C₆H₃)NPPh₂(CO₂R)C-C(Cy)(CO₂R)(BCy)] [R = Me (2), Et (3) and H (4)] with facile cleavage of the B-C bond and concomitant formation of a P-C bond with an ylidic character. DFT calculations indicate that 1 can be considered as a non-conjugated 1,3-dipole having two reaction centers viz., a nucleophilic P-center and an electrophilic B-center. The reaction of 1 with the alkynes proceeds through a stepwise dipolar addition mechanism, followed by the migration of the cyclohexyl group from the B-atom to the adjacent C-atom.

Iron Catalyzed Hydroboration of Aldehydes and Ketones

We report an operationally convenient room temperature hydroboration of aldehydes and ketones employing Fe(acac)₃ as precatalyst. The hydroboration of aldehydes and ketones proceeded efficiently at room temperature to yield, after work up, 1° and 2° alcohols; chemoselective hydroboration of aldehydes over ketones is attained under these conditions. We propose a σ-bond metathesis mechanism in which an Fe-H intermediate is postulated to be a key reactive species.

Insight into Transmetalation Enables Cobalt-Catalyzed Suzuki-Miyaura Cross Coupling

Among the fundamental transformations that comprise a catalytic cycle for cross coupling, transmetalation from the nucleophile to the metal catalyst is perhaps the least understood. Optimizing this elementary step has enabled the first example of a cobalt-catalyzed Suzuki-Miyaura cross coupling between aryl triflate electrophiles and heteroaryl boron nucleophiles. Key to this discovery was the preparation and characterization of a new class of tetrahedral, high-spin bis(phosphino)pyridine cobalt(I) alkoxide and aryloxide complexes, (^iPrPNP)CoOR, and optimizing their reactivity with 2-benzofuranylBPin (Pin = pinacolate). Cobalt compounds with small alkoxide substituents such as R = methyl and ethyl underwent swift transmetalation at 23 °C but also proved kinetically unstable toward β-H elimination. Secondary alkoxides such as R = ⁱPr or CH(Ph)Me balanced stability and reactivity. Isolation and structural characterization of the product following transmetalation, (^iPrPNP)Co(2-benzofuranyl), established a planar, diamagnetic cobalt(I) complex, demonstrating the high- and low-spin states of cobalt(I) rapidly interconvert during this reaction. The insights from the studies in this elementary step guided selection of appropriate reaction conditions to enable the first examples of cobalt-catalyzed C-C bond formation between neutral boron nucleophiles and aryl triflate electrophiles, and a model for the successful transmetalation reactivity is proposed.

The Mechanism of N-N Double Bond Cleavage by an Iron(II) Hydride Complex

The use of hydride species for substrate reductions avoids strong reductants, and may enable nitrogenase to reduce multiple bonds without unreasonably low redox potentials. In this work, we explore the N═N bond cleaving ability of a high-spin iron(II) hydride dimer with concomitant release of H2. Specifically, this diiron(II) complex reacts with azobenzene (PhN═NPh) to perform a four-electron reduction, where two electrons come from H2 reductive elimination and the other two come from iron oxidation. The rate law of the H2 releasing reaction indicates that diazene binding occurs prior to H2 elimination, and the negative entropy of activation and inverse kinetic isotope effect indicate that H-H bond formation is the rate-limiting step. Thus, substrate binding causes reductive elimination of H2 that formally reduces the metals, and the metals use the additional two electrons to cleave the N-N multiple bond.

Iron-Catalyzed Hydroboration: Unlocking Reactivity through Ligand Modulation

Iron-catalyzed hydroboration (HB) of alkenes and alkynes is reported. A simple change in ligand structure leads to an extensive change in catalyst activity. Reactions proceed efficiently over a wide range of challenging substrates including activated, unactivated and sterically encumbered motifs. Conditions are mild and do not require the use of reducing agents or other additives. Large excesses of borating reagent are not required, allowing control of chemo- and regioselectivity in the presence of multiple double bonds. Mechanistic insight reveals that the reaction is likely to proceed via a highly reactive iron hydride intermediate.

Reactivity of Hydride Bridges in High-Spin [3M-3(μ-H)] Clusters (M = FeII, CoII)

The designed [3M-3(μ-H)] clusters (M = Fe(II), Co(II)) Fe3H3L (1-H) and Co3H3L (2-H) [where L(3-) is a tris(β-diketiminate) cyclophane] were synthesized by treating the corresponding M3Br3L complexes with KBEt3H. From single-crystal X-ray analysis, the hydride ligands are sterically protected by the cyclophane ligand, and these complexes selectively react with CO2 over other unsaturated substrates (e.g., CS2, Me3SiCCH, C2H2, and CH3CN). The reaction of 1-H or 2-H with CO2 at room temperature yielded Fe3(OCHO)(H)2L (1-CO2) or Co3(OCHO)(H)2L (2-CO2), respectively, which evidence the differential reactivity of the hydride ligands within these complexes. The analogous reactions at elevated temperatures revealed a distinct difference in the reactivity pattern for 2-H as compared to 1-H; Fe3(OCHO)3L (1-3CO2) was generated from 1-H, while 2-H afforded only 2-CO2.

Tuning steric and electronic effects in transition-metal β-diketiminate complexes

β-Diketiminates are widely used supporting ligands for building a range of metal complexes with different oxidation states, structures, and reactivities. This Perspective summarizes the steric and electronic influences of ligand substituents on these complexes, with an eye toward informing the design of new complexes with optimized properties. The backbone and N-aryl substituents can give significant steric effects on structure, reactivity and selectivity of reactions. The electron density on the metal can be tuned by installation of electron withdrawing or donating groups on the β-diketiminate ligand as well. Examples are shown from throughout the transition metal series to demonstrate different types of effects attributable to systematic variation of β-diketiminate ligands.

Four-Coordinate Iron(II) Diaryl Compounds with Monodentate N-Heterocyclic Carbene Ligation: Synthesis, Characterization, and Their Tetrahedral-Square Planar Isomerization in Solution

The salt elimination reactions of (IPr2Me2)2FeCl2 (IPr2Me2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) with the corresponding aryl Grignard reagents afford [(IPr2Me2)2FeAr2] (Ar = Ph, 3; C6H4-p-Me, 4; C6H4-p-(t)Bu, 5; C6H3-3,5-(CF3)2, 6) in good yields. X-ray crystallographic studies revealed the presence of both tetrahedral and trans square planar isomers for 3 and 6 and the tetrahedral structures for 4 and 5. Magnetic susceptibility and (57)Fe Mössbauer spectrum measurements on the solid samples indicated the high-spin (S = 2) and intermediate-spin (S = 1) nature of the tetrahedral and square planar structures, respectively. Solution property studies, including solution magnetic susceptibility measurement, variable-temperature (1)H and (19)F NMR, and absorption spectroscopy, on 3-6, as well as an (57)Fe Mössbauer spectrum study on a frozen tetrahydrofuran solution of tetrahedral [(IPr2Me2)2(57)FePh2] suggest the coexistence of tetrahedral and trans square planar structures in solution phase. Density functional theory calculations on (IPr2Me2)2FePh2 disclosed that the tetrahedral and trans square planar isomers are close in energy and that the geometry isomerization can occur by spin-change-coupled geometric transformation on four-coordinate iron(II) center.

Highly nucleophilic dipropanolamine chelated boron reagents for aryl-transmetallation to iron complexes

New arylborates chelated by dipropanolamine are readily synthesised from boronic acids and demonstrated to be highly nucleophilic reagents.

Synthesis, spectroscopy, and hydrogen/deuterium exchange in high-spin iron(II) hydride complexes

Very few hydride complexes are known in which the metals have a high-spin electronic configuration. We describe the characterization of several high-spin iron(II) hydride/deuteride isotopologues and their exchange reactions with one another and with H2/D2. Though the hydride/deuteride signal is not observable in NMR spectra, the choice of isotope has an influence on the chemical shifts of distant protons in the dimers through the paramagnetic isotope effect on chemical shift. This provides the first way to monitor the exchange of H and D in the bridging positions of these hydride complexes. The rate of exchange depends on the size of the supporting ligand, and this is consistent with the idea that H2/D2 exchange into the hydrides occurs through the dimeric complexes rather than through a transient monomer. The understanding of H/D exchange mechanisms in these high-spin iron hydride complexes may be relevant to postulated nitrogenase mechanisms.

Testing the polynuclear hypothesis: multielectron reduction of small molecules by triiron reaction sites

High-spin trinuclear iron complex ((tbs)L)Fe3(thf) ([(tbs)L](6-) = [1,3,5-C6H9(NC6H4-o-NSi(t)BuMe2)3](6-)) (S = 6) facilitates 2 and 4e(-) reduction of NxHy type substrates to yield imido and nitrido products. Reaction of hydrazine or phenylhydrazine with ((tbs)L)Fe3(thf) yields triiron μ(3)-imido cluster ((tbs)L)Fe3(μ(3)-NH) and ammonia or aniline, respectively. ((tbs)L)Fe3(μ(3)-NH) has a similar zero-field (57)Fe Mössbauer spectrum compared to previously reported [((tbs)L)Fe3(μ(3)-N)]NBu4, and can be directly synthesized by protonation of the anionic triiron nitrido with lutidinium tetraphenylborate. Deprotonation of the triiron parent imido ((tbs)L)Fe3(μ(3)-NH) with lithium bis(trimethylsilyl)amide results in regeneration of the triiron nitrido complex capped with a thf-solvated Li cation [((tbs)L)Fe3(μ(3)-N)]Li(thf)3. The lithium capped nitrido, structurally similar to the pseudo C3-symmetric triiron nitride with a tetrabutylammonium countercation, is rigorously C3-symmetric featuring intracore distances of Fe-Fe 2.4802(5) Å, Fe-N(nitride) 1.877(2) Å, and N(nitride)-Li 1.990(6) Å. A similar 2e(-) reduction of 1,2-diphenylhydrazine by ((tbs)L)Fe3(thf) affords ((tbs)L)Fe3(μ(3)-NPh) and aniline. The solid state structure of ((tbs)L)Fe3(μ(3)-NPh) is similar to the series of μ(3)-nitrido and -imido triiron complexes synthesized in this work with average Fe-Nimido and Fe-Fe bond lengths of 1.941(6) and 2.530(1) Å, respectively. Reductive N═N bond cleavage of azobenzene is also achieved in the presence of ((tbs)L)Fe3(thf) to yield triiron bis-imido complex ((tbs)L)Fe3(μ(3)-NPh)(μ(2)-NPh), which has been structurally characterized. Ligand redox participation has been ruled out, and therefore, charge balance indicates that the bis-imido cluster has undergone a 4e(-) metal based oxidation resulting in an (Fe(IV))(Fe(III))2 formulation. Cyclic voltammograms of the series of triiron clusters presented herein demonstrate that oxidation states up to (Fe(IV))(Fe(III))2 (in the case of [((tbs)L)Fe3(μ(3)-N)]NBu4) are electrochemically accessible. These results highlight the efficacy of high-spin, polynuclear reaction sites to cooperatively mediate small molecule activation.

A Sulfide-Bridged Diiron(II) Complex with a cis-N2H4Ligand

A sulfide-bridged diiron(II) complex bearing a cis-N2H4 (hydrazine) ligand has been prepared by reaction of LFeII(μ-S)FeIIL (1; L = sterically encumbered βdiketiminate ligand) with 2 molar equivalents of N2H4. The metastable diiron(II) hydrazine complex LFeII(μ-S)(μH N-NH2)FeII (3) is formed, as shown by crystallography, and NMR, vibrational, and electronic absorption spectroscopies. Compound 3 has been crystallographically characterized as its DBU (1,8-diazabicyclo[5.4.0]undec-7$ene) adduct, which exhibits weak N-H···DBU hydrogen bonding. The synthetic process evolves roughly 2 equivalents of NH3. The cis-N2H4 bridge in 3 may be relevant to the structure and function of intermediates on the FeMoco of nitrogenase.

Synthesis and characterization of iron(II) and ruthenium(II) hydrido hydrazine complexes

Treatment of trans-[MHCl(dmpe)(2)] (M = Fe, Ru) with hydrazine afforded the hydrido hydrazine complexes cis- and trans-[MH(N(2)H(4))(dmpe)(2)](+) which have been characterized by NMR spectroscopy ((1)H, (31)P, and (15)N). Both cis and trans isomers of the Fe complex and the trans isomer of the Ru complex were characterized by X-ray crystallography. Reactions with acid and base afforded a range of N(2)H(x) complexes, including several unstable hydrido hydrazido complexes.

Triggering N(2) uptake via redox-induced expulsion of coordinated NH(3) and N(2) silylation at trigonal bipyramidal iron

The biological reduction of nitrogen to ammonia may occur via one of two predominant pathways in which nitrogenous N_xH_y intermediates including hydrazine (N₂H₄), diazene (N₂H₂), nitride (N^3-) and imide (NH^2-) may be involved. To test the validity of hypotheses concerning iron’s direct role in the stepwise reduction of N₂, iron model systems are needed. Such systems can test the chemical compatibility of iron with various proposed N_xH_y intermediates, and the reactivity patterns of such species. Here we describe a TBP (SiP^R₃)Fe-L scaffold (SiP^R₃ represents [Si(o-C₆H₄PR₂)₃]⁻; R = Ph and iPr) where the apical site is occupied by nitrogenous ligands such as N₂, N₂H₄, NH₃ and N₂R. The system accommodates terminally bound N₂ in the three formal oxidation states (iron(0), +1, and +2). N₂ uptake is demonstrated via displacement of its reduction partners NH₃ and N₂H₄, and N₂ functionalizaton is illustrated via electrophilic silylation.