Iron-Mediated Hydrazine Reduction and the Formation of Iron-Arylimide Heterocubanes

[Cr(pincer2-)]2 as an electron shuttle for reductively promoted hydrazine disproportionation

We describe here delivery of hydrazine to a reducing, low oxidation state chromium bound to a proton responsive ligand which has already been deprotonated. Reaction of PhHNNH₂ at a 4 : 1 mole ratio with the bis-(pyrazolate)pyridyl pincer ligated reducing agent [Cr^IIL]₂ gives prompt conversion to [CrL₂(PhNH₂)₂(μ-PhHNN)], with release of NH₃ and C₆H₆, a new disproportionation of the hydrazine, with trapping of PhHNN as its dianion, bridging the two chromium centers. The redox balance of the reaction is discussed, and participation by Brønsted basic sites on the bis-(pyrazolate)pyridyl pincer ligand L^2- is suggested, but no hydrazine protons remain on the pincer in the product.

Covalent Metal-Metal-Bonded Mn4 Tetrahedron Inscribed within a Four-Coordinate Manganese Cubane Cluster, As Evidenced by Unexpected Temperature-Independent Diamagnetism

The electronic structures of the manganese(IV) cubane cluster Mn(μ₃-N^tBu)₄(N^tBu)₄ (1) and its one-electron-oxidized analogue, the 3:1 Mn^IV/Mn^V cluster [Mn(μ₃-N^tBu)₄(N^tBu)₄]⁺[PF₆]^- (1⁺[PF₆]), are described. The S = 0 spin quantum number of 1 is explained by a diamagnetic electronic structure where all metal-based d electrons are paired in Mn-Mn bonding orbitals. Temperature- and power-dependent studies of the S = ¹/₂ electron paramagnetic resonance signal of 1⁺ are consistent with an electronic structure described as a delocalized one-electron radical.

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.

Characterization of Iron-Imido Species Relevant for N-Group Transfer Chemistry

A sterically accessible tert-butyl-substituted dipyrrinato di-iron(II) complex [((tBu)L)FeCl]2 possessing two bridging chloride atoms was synthesized from the previously reported solvento adduct. Upon treatment with aryl azides, the formation of high-spin Fe(III) species was confirmed by (57)Fe Mössbauer spectroscopy. Crystallographic characterization revealed two possible oxidation products: (1) a terminal iron iminyl from aryl azides bearing ortho isopropyl substituents, ((tBu)L)FeCl((•)NC6H3-2,6-(i)Pr2); or (2) a bridging di-iron imido arising from reaction with 3,5-bis(trifluoromethyl)aryl azide, [((tBu)L)FeCl]2(μ-NC6H3-3,5-(CF3)2). Similar to the previously reported ((Ar)L)FeCl((•)NC6H4-4-(t)Bu), the monomeric iron imido is best described as a high-spin Fe(III) antiferromagnetically coupled to an iminyl radical, affording an S = 2 spin state as confirmed by SQUID magnetometry. The di-iron imido possesses an S = 0 ground state, arising from two high-spin Fe(III) centers weakly antiferromagnetically coupled through the bridging imido ligand. The terminal iron iminyl complex undergoes facile decomposition via intra- or intermolecular hydrogen-atom abstraction (HAA) from an imido aryl ortho isopropyl group, or from 1,4-cyclohexadiene, respectively. The bridging di-iron imido is a competent N-group transfer reagent to cyclic internal olefins as well as styrene. Although solid-state magnetometry indicates an antiferromagnetic interaction between the two iron centers (J = -108.7 cm(-1)) in [((tBu)L)FeCl]2(μ-NC6H3-3,5-(CF3)2), we demonstrate that in solution the bridging imido can facilitate HAA as well as dissociate into a terminal iminyl species, which then can promote HAA. In situ monitoring reveals the di-iron bridging imido is a catalytically competent intermediate, one of several iron complexes observed in the amination of C-H bond substrates or styrene aziridination.

A Low-Valent Iron Imido Heterocubane Cluster: Reversible Electron Transfer and Catalysis of Selective C-C Couplings

Enzymes and cofactors with iron-sulfur heterocubane core structures, [Fe4 S4 ], are often found in nature as electron transfer reagents in fundamental catalytic transformations. An artificial heterocubane with a [Fe4 N4 ] core is reported that can reversibly store up to four electrons at very negative potentials. The neutral [Fe4 N4 ] and the singly reduced low-valent [Fe4 N4 ](-) heterocubanes were isolated and fully characterized. The low-valent species bears one unpaired electron, which is localized predominantly at one iron center in the electronic ground state but fluctuates with increasing temperatures. The electrons stored or released by the [Fe4 N4 ]/[Fe4 N4 ](-) redox couple can be used in reductive or oxidative CC couplings and even allow catalytic one-pot reactions, which show a remarkably enhanced selectivity in the presence of the [Fe4 N4 ] heterocubanes.

Reactivity of an All-Ferrous Iron-Nitrogen Heterocubane under Reductive and Oxidative Conditions

The reactivity of the all-ferrous FeN heterocubane [Fe4 (Ntrop)4 ] (1) with i) Brønsted acids, ii) σ-donors, iii) σ-donors/π-acceptors, and iv) one-electron oxidants has been investigated (trop = 5H-dibenzo[a,d]cyclo-hepten-5-yl). 1 showed self-re-assembling after reactions with i) and proved surprisingly inert in reactions with ii) and iii), with the exception of CO. Reductive and oxidative cluster degradation was observed in reactions with CO and TEMPO, respectively. These reactions yielded new cluster compounds, namely a trinuclear bis(μ3 -imido) 48 electron complex in the former case and a tetranuclear all ferric μ-oxo-μ-imido species in the latter case. Characterization techniques include NMR and in situ IR spectroscopy, single crystal X-ray analysis, Mössbauer spectroscopy, cyclic voltammetry, magnetic susceptibility measurements, and DFT calculations.

Control of cerium oxidation state through metal complex secondary structures

A series of alkali metal cerium diphenylhydrazido complexes, M _x (py) _y [Ce(PhNNPh)₄], M = Li, Na, and K, x = 4 (Li and Na) or 5 (K), and y = 4 (Li), 8 (Na), or 7 (K), were synthesized to probe how a secondary coordination sphere would modulate electronic structures at a cerium cation. The resulting electronic structures of the heterobimetallic cerium diphenylhydrazido complexes were found to be strongly dependent on the identity of the alkali metal cations. When M = Li⁺ or Na⁺, the cerium(iii) starting material was oxidized with concomitant reduction of 1,2-diphenylhydrazine to aniline. Reduction of 1,2-diphenylhydrazine was not observed when M = K⁺, and the complex remained in the cerium(iii) oxidation state. Oxidation of the cerium(iii) diphenylhydrazido complex to the Ce(iv) diphenylhydrazido one was achieved through a simple cation exchange reaction of the alkali metals. UV-Vis spectroscopy, FTIR spectroscopy, electrochemistry, magnetic susceptibility, and DFT studies were used to probe the oxidation state and the electronic changes that occurred at the metal centre.

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.

Specific incorporation of chalcogenide bridge atoms in molybdenum/tungsten-iron-sulfur single cubane clusters

An extensive series of heterometal-iron-sulfur single cubane-type clusters with core oxidation levels [MFe(3)S(3)Q](3+,2+) (M = Mo, W; Q = S, Se) has been prepared by means of a new method of cluster self-assembly. The procedure utilizes the assembly system [((t)Bu(3)tach)M(VI)S(3)]/FeCl(2)/Na(2)Q/NaSR in acetonitrile/THF and affords product clusters in 30-50% yield. The trisulfido precursor acts as a template, binding Fe(II) under reducing conditions and supplying the MS(3) unit of the product. The system leads to specific incorporation of a μ(3)-chalcogenide from an external source (Na(2)Q) and affords the products [((t)Bu(3)tach)MFe(3)S(3)QL(3)](0/1-) (L = Cl(-), RS(-)), among which are the first MFe(3)S(3)Se clusters prepared. Some 16 clusters have been prepared, 13 of which have been characterized by X-ray structure determinations including the incomplete cubane [((t)Bu(3)tach)MoFe(2)S(3)Cl(2)(μ(2)-SPh)], a possible trapped intermediate in the assembly process. Comparisons of structural and electronic features of clusters differing only in atom Q at one cubane vertex are provided. In comparative pairs of complexes differing only in Q, placement of one selenide atom in the core increases core volumes by about 2% over the Q = S case, sets the order Q = Se > S in Fe-Q bond lengths and Q = S > Se in Fe-Q-Fe bond angles, causes small positive shifts in redox potentials, and has an essentially nil effect on (57)Fe isomer shifts. Iron mean oxidation states and charge distributions are assigned to most clusters from isomer shifts. ((t)Bu(3)tach = 1,3,5-tert-butyl-1,3,5-triazacyclohexane).