Tetrapodal iron complexes invoke observable intermediates in nitrate and nitrite reduction.

This study investigates the mechanistic pathways of nitrate and nitrite reduction by the tetrapodal iron complex [PyPy(afa)Fe]OTf, revealing key intermediates to elucidate the reaction process. Using UV-Vis, IR, mass and NMR spectroscopies, stable binding of oxyanions to the iron centre was observed, supporting the formation of the iron(iii)-hydroxide intermediate [PyPy(afa)Fe(OH)]OTf. This intermediate is less stable than in previous systems, providing insights into the behaviour of metalloenzymes.

Identity of the Silyl Ligand in an Iron Silyl Complex Influences Olefin Hydrogenation: An Experimental and Computational Study.

In this study, we explore the selective synthesis of iron silyl complexes using the reaction of an iron mesityl complex (CCC)FeMes(Py) with various hydrosilanes. These resulting iron silyl complexes, (CCC)Fe(SiHPh)(Py)(N), (CCC)Fe(SiMePh)(Py)(N), and (CCC)Fe[SiMe(OSiMe)](Py)(N), serve as effective precatalysts for olefin hydrogenation. The key to their efficiency in catalysis lies in the specific nature of the silyl ligand attached to the iron center.

Synthesis and characterization of tetrapodal nickel complexes with adaptable ligand binding geometries.

This study explores the versatile binding properties of a tetrapodal ligand framework with nickel, demonstrating significant ligand fluxionality through the interconversions of several complexes. Kinetic studies using UV-vis and NMR techniques underscore the pivotal role of solvent coordination in initiating these dynamic processes. A unique reverse-dative Ni → Ag interaction provides another approach in modifying nickel's geometry.

Synthesis and Characterization of Palladium Pincer Bis(carbene) CCC Complexes.

The metalation of the CCC (CCC = bis(diisopropylphenyl-imidazol-2-ylidene)phenyl) ligand platform with Pd was achieved under mild conditions by reacting [H(CCC)]Cl with Pd(OAc) at room temperature in the presence of 3.1 equiv of LiN(SiMe). The resulting complexes (CCC)PdX (X = Cl or Br) were oxidized by two-electron oxidants PhICl, Br, and BTMABr.

Formation of Red Elemental Selenium from Seleniferous Oxyanions: Deoxygenation by a Homogeneous Iron Catalyst.

Seleniferous oxyanions are groundwater contaminants from both anthropogenic and natural sources, while pure amorphous selenium nanoparticles have a variety of industrial applications. Biology can achieve the multicomponent 6 e/8 H reduction of selenate to amorphous selenium using multiple metalloenzymes, like selenate and selenite reductase. Inspired by biology, we developed a new homogeneous system that can generate pure elemental selenium with no caustic waste.

Iron-Catalyzed hydrogen Induced Polarization.

hydrogen induced polarization (PHIP) can address the low sensitivity problem intrinsic to nuclear magnetic resonance spectroscopy. Using a catalyst capable of reacting with hydrogen and substrate in either a hydrogenative or nonhydrogenative manner can result in signal enhancement of the substrate. This work describes the development of a rare example of an iron catalyst capable of reacting with hydrogen to hyperpolarize olefins.

γ-Agostic interactions in (CCC)Fe-Mes(L) complexes.

Agostic interactions were observed in the bound mesityl group in a series of iron compounds bearing a bis(NHC) pincer CCC ligand. The L-type ligand on [(CCC)FeMes(L)] complexes influences the strength of the agostic interaction and is manifested in the upfield shift of the H NMR resonance for the mesityl methyl resonances. The nature of the interaction was further investigated by density functional theory calculations, allowing rationalization of some unexpected trends and proving to be a powerful predictive tool.

Secondary Coordination Sphere Influences the Formation of Fe(III)-O or Fe(III)-OH in Nitrite Reduction: A Synthetic and Computational Study.

The reduction of nitrite (NO) to generate nitric oxide (NO) is a significant area of research due to their roles in the global nitrogen cycle. Here, we describe various modifications of the tris(5-cyclohexyliminopyrrol-2-ylmethyl)amine H[N(pi)] ligand where the steric bulk and acidity of the secondary coordination sphere were explored in the non-heme iron system for nitrite reduction. The cyclohexyl and 2,4,6-trimethylphenyl variants of the ligand were used to probe the mechanism of nitrite reduction.

Synthesis of a series of M(II) (M = Mn, Fe, Co) chloride complexes with both inter- and intra-ligand hydrogen bonding interactions.

Hydrogen bonding networks are vital for metallo-enzymes to function; however, modeling these systems is non-trivial. We report the synthesis of metal chloride (M = Mn, Fe, Co) complexes with intra- and inter-ligand hydrogen bonding interactions. The intra-ligand hydrogen bonds are shown to have a profound effect on the geometry of the metal center.

Effects of a Tridentate Pincer Ligand on Parahydrogen Induced Polarization.

The role of ligands in rhodium- and iridium-catalyzed Parahydrogen Induced Polarization (PHIP) and SABRE (signal amplification by reversible exchange) chemistry has been studied in the benchmark systems, [Rh(diene)(diphos)] and [Ir(NHC)(sub) (H) ] , and shown to have a great impact on the degree of hyperpolarization observed. Here, we examine the role of the flanking moieties in the electron-rich monoanionic bis(carbene) aryl pincer ligand, CCC (Ar=Dipp, 2,6-diisopropyl or Mes, 2,4,6-trimethylphenyl) on the cobalt-catalyzed PHIP and PHIP-IE (PHIP via Insertion and Elimination) chemistry that we have previously reported. The mesityl groups were exchanged for diisopropylphenyl groups to generate the ( CCC)Co(N ) catalyst, which resulted in faster hydrogenation and up to 390-fold H signal enhancements, larger than that of the ( CCC)Co-py (py=pyridine) catalyst.

Characterization of Terminal Iron(III)-Oxo and Iron(III)-Hydroxo Complexes Derived from O Activation.

O activation at nonheme iron centers is a common motif in biological systems. While synthetic models have provided numerous insights into the reactivity of high-valent iron-oxo complexes related to biological processes, the majority of these complexes are synthesized using alternative oxidants. This report describes O activation by an iron(II)-triflate complex of the imino-functionalized tris(pyrrol-2-ylmethyl)amine ligand framework, H[N(pi)].

Radical Rebound Hydroxylation Versus H-Atom Transfer in Non-Heme Iron(III)-Hydroxo Complexes: Reactivity and Structural Differentiation.

The characterization of high-valent iron centers in enzymes has been aided by synthetic model systems that mimic their reactivity or structural and spectral features. For example, the cleavage of dioxygen often produces an iron(IV)-oxo that has been characterized in a number of enzymatic and synthetic systems. In non-heme 2-oxogluterate dependent (iron-2OG) enzymes, the ferryl species abstracts an H-atom from bound substrate to produce the proposed iron(III)-hydroxo and caged substrate radical.

C NMR Signal Enhancement Using Parahydrogen-Induced Polarization Mediated by a Cobalt Hydrogenation Catalyst.

The use of a cobalt-based catalyst for the generation of hyperpolarized C NMR resonances by parahydrogenation of ethyl acrylate is presented herein. Comparisons of the carboxylate C NMR signal enhancement factor of ethyl propionate between using (CCC)Co-py and a commonly utilized cationic diphosphine rhodium complex demonstrates that the cobalt system is a viable PHIP catalyst alternative. Furthermore, the operative hydrogenation mechanism of the cobalt system was examined by using H, C, and parahydrogen-induced polarization NMR spectroscopies to elucidate reaction intermediates affiliated with the observed H and C NMR signal enhancements in ethyl propionate.

Cobalt-Catalyzed and Lewis Acid-Assisted Nitrile Hydrogenation to Primary Amines: A Combined Effort.

The selective hydrogenation of nitriles to primary amines using a bench-stable cobalt precatalyst under 4 atm of H is reported herein. The catalyst precursor was reduced in situ using NaHBEt, and the resulting Lewis acid formed, BEt, was found to be integral to the observed catalysis. Mechanistic insights gleaned from para-hydrogen induced polarization (PHIP) transfer NMR studies revealed that the pairwise hydrogenation of nitriles proceeded through a Co(I/III) redox process.

Tuning the Fe(II/III) Redox Potential in Nonheme Fe(II)-Hydroxo Complexes through Primary and Secondary Coordination Sphere Modifications.

The derivatization of the imino-functionalized tris(pyrrolylmethyl)amine ligand framework, N(pi) (L; X = H, Br; R = cyclohexyl (Cy), phenyl (Ph), 2,6- diisopropylphenyl (DIPP)), is reported. Modular ligand synthesis allows for facile modification of both the primary and secondary coordination sphere electronics. The iron(II)-hydroxo complexes, N(pi)(afa)Fe(II)OH (LFeOH), are synthesized to establish the impact of the ligand modifications on the complexes' electronic properties, including their chemical and electrochemical oxidation.

A bioinspired iron catalyst for nitrate and perchlorate reduction.

Nitrate and perchlorate have considerable use in technology, synthetic materials, and agriculture; as a result, they have become pervasive water pollutants. Industrial strategies to chemically reduce these oxyanions often require the use of harsh conditions, but microorganisms can efficiently reduce them enzymatically. We developed an iron catalyst inspired by the active sites of nitrate reductase and (per)chlorate reductase enzymes.

Alkyne Semihydrogenation with a Well-Defined Nonclassical Co-H Catalyst: A H Spin on Isomerization and E-Selectivity.

The reactivity of a Co-H complex was extended toward the semihydrogenation of internal alkynes. Under ambient temperatures and moderate pressures of H, a broad scope of alkynes were semihydrogenated using a Co-N precatalyst, resulting in the formation of trans-alkene products. Furthermore, mechanistic studies using H, H, and para-hydrogen induced polarization (PHIP) transfer NMR spectroscopy revealed cis-hydrogenation of the alkyne occurs first.

Well-Defined Cobalt(I) Dihydrogen Catalyst: Experimental Evidence for a Co(I)/Co(III) Redox Process in Olefin Hydrogenation.

The synthesis of a cobalt dihydrogen Co(I)-(H2) complex prepared from a Co(I)-(N2) precursor supported by a monoanionic pincer bis(carbene) ligand, (Mes)CCC ((Mes)CCC = bis(mesityl-benzimidazol-2-ylidene)phenyl), is described. This species is capable of H2/D2 scrambling and hydrogenating alkenes at room temperature. Stoichiometric addition of HCl to the Co(I)-(N2) cleanly affords the Co(III) hydridochloride complex, which, upon the addition of Cp2ZrHCl, evolves hydrogen gas and regenerates the Co(I)-(N2) complex.

Accessing Pincer Bis(carbene) Ni(IV) Complexes from Ni(II) via Halogen and Halogen Surrogates.

This communication describes the two-electron oxidation of ((DIPP)CCC)NiX ((DIPP)CCC = bis(diisopropylphenyl-benzimidazol-2-ylidene)phenyl); X = Cl or Br) with halogen and halogen surrogates to form ((DIPP)CCC)NiX3. These complexes represent a rare oxidation state of nickel, as well as an unprecedented reaction pathway to access these species through Br2 and halogen surrogate (benzyltrimethylammonium tribromide). The Ni(IV) complexes have been characterized by a suite of spectroscopic techniques and can readily reduce to the Ni(II) counterpart, allowing for cycling between the Ni(II)/Ni(IV) oxidation states.

Monoanionic bis(carbene) pincer complexes featuring cobalt(I-III) oxidation states.

The synthesis and characterization of a series of cobalt complexes featuring a pincer bis(carbene) ligand of the meta-phenylene-bridged bis-N-heterocyclic carbene ((Ar)CCC, Ar = 2,6-diispropylphenyl or mesityl) are reported. Cleavage of the aryl C-H bond of the ligand was achieved in a one-pot metalation procedure using Co(N(SiMe3)2)2(py)2, an equivalent of exogenous base, and trityl chloride to form the ((DIPP)CCC)CoCl2py complex. This species could be reduced to the Co(ii) and Co(i)-N2 molecules with the appropriate equivalents of reductant.

Synthesis and characterization of M(II) (M = Mn, Fe and Co) azafulvene complexes and their X3(-) derivatives.

The syntheses of M(ii) (M = Mn, Fe, Co) complexes bearing the tris(5-cycloaminoazafulvene-2-methyl)amine (H3N(afa(Cy))3) ligand in its datively coordinated, tautomeric form is reported. The metal-azafulvene complexes [N(afa(Cy))3M](OTf)2 are generated in high yields, featuring a secondary coordination sphere composed of amino moieties from the ligand platform. To investigate the ability of the hydrogen bonding network to support hydrogen-bond accepting, coordinating anions, pseudohalide derivatives, [N(afa(Cy))3MX](OTf) (X = NCS(-), NCO(-), N3(-)) were synthesized by exposure of [N(afa(Cy))3M](OTf)2 to an equivalent of the corresponding salt, [((n)Bu)4N](X).

Exploring Mn-O bonding in the context of an electronically flexible secondary coordination sphere: synthesis of a Mn(III)-oxo.

Complexes containing manganese-oxygen bonds have been implicated in a variety of biological and synthetic processes. Herein, we describe the synthesis of a family of stable, high-spin trigonal bipyramidal manganese complexes of the electronically flexible ligand tris(5-cyclohexylimino-pyrrol-2-ylmethyl)amine [H3N(pi(Cy))3] featuring apical water, hydroxyl, and oxo ligands. Terminal Mn(III)-O complexes are rare and the formation of this species was achieved from a variety of reagents including O2, PhIO and NO2(-).

Facile nitrite reduction in a non-heme iron system: formation of an iron(III)-oxo.

Reaction of tetrabutylammonium nitrite with [N(afa(Cy))3Fe(OTf)](OTf) cleanly resulted in the formation of an iron(III)-oxo species, [N(afa(Cy))3Fe(O)](OTf), and NO(g). Formation of NO(g) as a byproduct was confirmed by reaction of the iron(II) starting material with half an equivalent of nitrite, resulting in a mixture of two products, the iron-oxo and an iron-NO species, [N(afa(Cy))3Fe(NO)](OTf)2. Formation of the latter was confirmed through independent synthesis.

Meridional vs. facial coordination geometries of a dipodal ligand framework featuring a secondary coordination sphere.

The synthesis of a novel dipodal ligand framework, H2[(Me)N(pi(Cy))2], is summarized. Upon metalation with MCl2 salts (M = Fe, Cu), the ligand undergoes a conformational change, resulting in the formation of a trigonal bipyramidal metal center with a pseudoplanar, meridionally-bound ligand framework. This tautomerization positions pendant amines in the metal's secondary coordination sphere.