Jörg Sutter

An Iron Nitride Complex

Coordination compounds of iron in high oxidation states have been invoked as reactive intermediates in biocatalyses. Iron(IV) ferryl species are examples of such highly reactive species that have long been known to be at the catalytic centers of oxygenases. Supported by X-ray diffraction studies on nitrogenase, the iron nitride moiety has recently been suggested to be present at the site of biological nitrogen reduction. As a result, well-characterized high-valent iron complexes have been sought as biomimetic models for transformations mediated by iron-containing enzymes. To gain understanding of iron nitride reactivity and the possible role of such species in biocatalysis, insight into the molecular structure of complexes stabilizing the [FeN] synthon is highly desirable. Whereas significant progress has been made in the synthesis and spectroscopic elucidation of Fe=NR and Fe N species, X-ray crystallographic characterization of a complex with a terminal Fe N functionality has not been accomplished. The first mononuclear Fe=O entity crystallographically characterized was stabilized in an octahedral environment provided by a macrocyclic tetra-N-methylated cyclam ligand. Similar cyclam derivatives also allow the stabilization and detailed spectroscopic characterization of octahedral Fe and Fe nitride complexes in unusually high oxidation states. 10] Recently, Peters and Betley developed a stunningly redox-rich iron system employing the tripodal tris(phosphino)borate ligand system (PhBP3 ), which stabilizes tetrahedral L3Fe=Nx species in oxidation states ranging from + I to + IV. Remarkably, this ligand system enabled the first room-temperature spectroscopic characterization of a terminal Fe nitride species. Concentration-dependent coupling to the Fe-N2-Fe I dinuclear product, however, prevents crystallization of this nitride species. We herein present the synthesis, spectroscopy, and most significantly, the X-ray diffraction analysis of a discrete iron nitride complex stabilized by the sterically encumbering Nanchored tris(carbene) ligand, tris[2-(3-aryl-imidazol-2-ylidene)ethyl]amine (TIMEN, R= xylyl (xyl), mesityl (mes)). Structurally and electronically related to the tetrahedral phosphinoborate ligand system by Peters and Betley, this tripodal N-heterocyclic carbene (NHC) system coordinates a high-spin Fe center in a trigonal-planar fashion, thus forming four-coordinate complexes with the metal ion in trigonalpyramidal environments. Under inert atmosphere, treatment of TIMEN with one equivalent of anhydrous ferrous chloride in pyridine at room temperature yields the four-coordinate Fe complexes [(TIMEN)Fe(Cl)]Cl (1, 1) as analytically pure, white powders in 80% yield (Scheme 1).

Transition metal complexes with sulfur ligands: Part CXLIV. Square planar nickel complexes with NiS4 cores in three different oxidation states: synthesis, X-ray structural and spectroscopic studies

Abstract The reaction of ‘buS2’2−=3,5-ditertiarybutyl-1,2-benzenedithiolate(2−) with Ni(ac)2·4H2O and subsequently AsPh4Cl, yielded (AsPh4)2[Ni(‘buS2’)2] (1). Complex 1 is extremely air sensitive and oxidized rapidly to give (AsPh4)[Ni(‘buS2’)2] (2). The anion of 2 could be further oxidized by iodine to give neutral [Ni(‘buS2’)2] (3). Complexes 1, 2 and 3 contain nickel in the formal oxidation states +2, +3 and +4, and could be completely characterized by X-ray structure determination and spectroscopic methods. The results indicate that the oxidation of 1→2→3 concerns electrons residing in molecular orbitals having [NiS4] character. No evidence could be obtained for an involvement of the benzene rings or an electron delocalization beyond the [NiS4] core. This conclusion corresponds with the reactivity of 3 versus 2 and 1 towards protons and CO. While 3 is inert to the attack of H+ and CO, 2 and 1 exhibit rapid reactions to give ‘buS2’H2 and either nickel–chloro complexes or Ni(CO)4.

Übergangsmetallkomplexe mit Schwefelliganden CXXIV. Koordination von C- und S-Liganden an Metall—Carben—Thiolat-Fragmente [M(‘S2C’)] [M = NiII, PdII, PtII; ‘S2C’2− = 1,3-Imidazolidinyl-N,N′-bis(2-benzolthiolat)(2 −)]. [M(L)(‘S2C’)]-Komplexe mit L = CN−, CH3−, COCH3−, CNBu, CR2, SH− und SR− (R = Et, Ph, o-C6H4NH2)

Abstract A series of [M(L)(‘S 2 C’)] complexes (M = Ni II , Pd II , Pt II ) containing the carbene dithiolate ligand ‘S 2 C’ 2− = 1,3-imidazolidinyl- N,N ′-bis(2-benzenethiolate)(2 −) and various C and S co-ligands have been synthesized. When treated with KCN, n -butylisonitrile, the electron-rich olefin R 2 C=CR 2 [=bis(1,3-diphenylimidazolidine-2-ylidene)], LiMe, thiolates such as SEt − , SPh − or o -SC 6 H 4 NH 2 , and hydrogen sulfide, the parent complex [Ni(‘S 2 C’)] 2 · DMF ( 1 ) yields the corresponding anionic or neutral [Ni(L)(‘S 2 C’)] complexes which were isolated as [K 2 (Kryptofix5)(THF) 2 (μ-OH 2 )][Ni(CN)(‘S 2 C’)] ( 2 ), [Ni(CNBu)(‘S 2 C’)] ( 3 ), [Ni(CR 2 )(‘S 2 C’)] ( 4 ), [Li(12-crown-4) 2 ][Ni(CH 3 )(‘S 2 C’)] ( 5 ), [NBu 4 ][Ni(SEt)(‘S 2 C’)] · THF ( 6 ), [Na(15-crown-5)][Ni(SPh)(‘S 2 C’)] ( 7 ), [Na(15-crown-5)][Ni( o -SC 6 H 4 NH 2 )(‘S 2 C’)] · 0.5THF ( 8 ), and [Na(15-crown-5)][Ni(SH)(‘S 2 C’)] ( 9 ). Analogous complexes of 9 have also been obtained with Pd ( 10 ) and Pt ( 11 ). The complexes were characterized by elemental analyses and the usual spectroscopic methods. X-ray structure determinations of 2 , 3 , 4 , 6 , 9 and 10 revealed that the [M(‘S 2 C’)] fragments are stereochemically very rigid, being little influenced by the different co-ligands L. The four-coordinate metal centers exhibit an approximately square-planar coordination geometry. The ‘S 2 C’ 2− ligands show a characteristic propeller-like twist resulting from positioning the C 6 rings of the ‘S 2 C’ unit above and below the coordination plane. As evidenced by the molecular structures of 9 vs. 10 , this twist can vary, and the ‘S 2 C’ 2− ligand is flexible enough to accommodate also larger metal ions than Ni II . Complexes 9–11 belong to the rare cases of isolable SH complexes. The complexes 2–11 exhibit a remarkable thermal stability ( 4 is stable up to 220°C), and the [M(‘S 2 C’)] fragments so far proved inert towards decomposition. When treated with Bronsted acids, the general reactivity feature of all anionic [M(L)(‘S 2 C’)] complexes is release of HL and regeneration of the parent complexes [M(‘S 2 C’)] 2 . The methyl complex [Li(4-crown-4) 2 ][Ni(CH 3 )(‘S 2 C’)] ( 5 ) is one of the rare examples in which a methyl ligand binds to an [NiS] center. While the parent complex [Ni(‘S 2 C’)] 2 · DMF ( 1 ) proved unreactive towards CO, 5 readily inserts CO yielding the highly reactive acetyl derivative [Ni(COCH 3 )(‘S 2 C’)] − . This complex could not be isolated, but its formation was established by spectroscopic methods and by its subsequent reaction with PhSH yielding, among other products, the thioester CH 3 COSPh. The model character of this reaction sequence for acetyl—CoA synthesis catalyzed by CO dehydrogenases is discussed.

The nitrogenase catalyzed N2 dependent HD formation: a model reaction and its significance for the FeMoco function

Abstract The first completely characterized model for the nitrogenase catalyzed ‘N 2 dependent HD formation’ from D 2 and protons is described. This key reaction of nitrogenases is most plausibly rationalized by the ‘open-side’ FeMoco model, which enables us to explain the severe constraints imposed on the N 2 dependent HD formation as well as the noncompetitive inhibition of N 2 fixation by CO.

Low-valent iron(i) amido olefin complexes as promotors for dehydrogenation reactions.

Fe(I) compounds including hydrogenases show remarkable properties and reactivities. Several iron(I) complexes have been established in stoichiometric reactions as model compounds for N2 or CO2 activation. The development of well-defined iron(I) complexes for catalytic transformations remains a challenge. The few examples include cross-coupling reactions, hydrogenations of terminal olefins, and azide functionalizations. Here the syntheses and properties of bimetallic complexes [MFe(I) (trop2 dae)(solv)] (M=Na, solv=3 thf; M=Li, solv=2 Et2 O; trop=5H-dibenzo[a,d]cyclo-hepten-5-yl, dae=(N-CH2 -CH2 -N) with a d(7) Fe low-spin valence-electron configuration are reported. Both compounds promote the dehydrogenation of N,N-dimethylaminoborane, and the former is a precatalyst for the dehydrogenative alcoholysis of silanes. No indications for heterogeneous catalyses were found. High activities and complete conversions were observed particularly with [NaFe(I) (trop2 dae)(thf)3 ].

N–O Bond Homolysis of an Iron(II) TEMPO Complex Yields an Iron(III) Oxo Intermediate

The reaction of TEMPO with the iron(I) synthon PhB(MesIm)(3)Fe(COE) leads to formation of the κ(1)-TEMPO complex PhB(MesIm)(3)Fe(TEMPO). Structural and spectroscopic data establish the complex contains divalent iron bound to a nitroxido anion and is isoelectronic to an iron(II) peroxo complex. Thermolysis of the complex results in N-O bond homolysis, leading to the formation of an iron(III) oxo intermediate. The oxo intermediate is active in oxygen atom transfer reactions and can be trapped by the triphenylmethyl radical to give the iron(II) alkoxo complex PhB(MesIm)(3)Fe(OCPh(3)).

Ferritin-Mediated Iron Sequestration Stabilizes Hypoxia-Inducible Factor-1α upon LPS Activation in the Presence of Ample Oxygen

Both hypoxic and inflammatory conditions activate transcription factors such as hypoxia-inducible factor (HIF)-1α and nuclear factor (NF)-κB, which play a crucial role in adaptive responses to these challenges. In dendritic cells (DC), lipopolysaccharide (LPS)-induced HIF1α accumulation requires NF-κB signaling and promotes inflammatory DC function. The mechanisms that drive LPS-induced HIF1α accumulation under normoxia are unclear. Here, we demonstrate that LPS inhibits prolyl hydroxylase domain enzyme (PHD) activity and thereby blocks HIF1α degradation. Of note, LPS-induced PHD inhibition was neither due to cosubstrate depletion (oxygen or α-ketoglutarate) nor due to increased levels of reactive oxygen species, fumarate, and succinate. Instead, LPS inhibited PHD activity through NF-κB-mediated induction of the iron storage protein ferritin and subsequent decrease of intracellular available iron, a critical cofactor of PHD. Thus, hypoxia and LPS both induce HIF1α accumulation via PHD inhibition but deploy distinct molecular mechanisms (lack of cosubstrate oxygen versus deprivation of co-factor iron).

One-Pot Synthesis of an Fe(II) Bis-Terpyridine Complex with Allosterically Regulated Electronic Properties

Herein we report the one-pot synthesis of Fe(II) bis-terpyridine complexes with two peripheral square-planar Pt(II) bis-phosphinoalkylthioether moieties. These novel structures, which exhibit allosterically controllable electronic properties, are made by taking advantage of two orthogonal and high-yielding reactions. The prototypical complex can be structurally regulated through the reversible abstraction and introduction of chloride ions to the Pt(II) centers. This moves the Fe(II) center and two Pt(II) metal centers into and out of communication with each other, causing changes in the electronic structure of the complex and its corresponding optical and redox properties. The start and end points of the allosterically regulated system have been characterized by single-crystal X-ray diffraction and NMR, UV-vis, and (57)Fe Mößbauer spectroscopy.

A Neutral Tetraphosphacyclobutadiene Ligand in Cobalt(I) Complexes

The unusual reactivity of the newly synthesized β-diketiminato cobalt(I) complexes, [(L(Dep)Co)2] (2 a, L(Dep)=CH[C(Me)N(2,6-Et2C6H3)]2) and [L(Dipp)Co⋅toluene] (2 b, L(Dipp)=CH[CHN(2,6-(i)Pr2C6H3)]2), toward white phosphorus was investigated, affording the first cobalt(I) complexes [(L(Dep)Co)2(μ2:η(4),η(4)-P4)] (3 a) and [(L(Dipp)Co)2(μ2:η(4),η(4)-P4)] (3 b) bearing the neutral cyclo-P4 ligand with a rectangular-planar structure. The redox chemistry of 3 a and 3 b was studied by cyclic voltammetry and their chemical reduction with one molar equivalent of potassium graphite led to the isolation of [(L(Dep)Co)2(μ2:η(4),η(4)-P4)][K(dme)4] (4 a) and [(L(Dipp)Co)2(μ2:η(4),η(4)-P4)][K(dme)4] (4 b). Unexpectedly, the monoanionic Co2P4 core in 4 a and 4 b, respectively, contains the two-electron-reduced cyclo-P4(2-) ligand with a square-planar structure and mixed-valent cobalt(I,II) sites. The electronic structures of 3 a, 3 b, 4 a, and 4 b were elucidated by NMR and EPR spectroscopy as well as magnetic measurements and are in agreement with results of broken-symmetry DFT calculations.

Giant Spherical Cluster with I-C140 Fullerene Topology.

We report on an effective cluster expansion of CuBr-linked aggregates by the increase of the steric bulk of the CpR ligand in the pentatopic molecules [CpRFe(η5-P5)]. Using [CpBIGFe(η5-P5)] (CpBIG=C5(4-nBuC6H4)5), the novel multishell aggregate [{CpBIGFe(η5:2:1:1:1:1:1-P5)}12(CuBr)92] is obtained. It shows topological analogy to the theoretically predicted I-C140 fullerene molecule. The spherical cluster was comprehensively characterized by various methods in solution and in the solid state.