The Fe2(NO)2 Diamond Core: A Unique Structural Motif In Non‐Heme Iron–NO Chemistry

Non‐heme high‐spin (hs) {FeNO}⁸ complexes have been proposed as important intermediates towards N₂O formation in flavodiiron NO reductases (FNORs). Many hs‐{FeNO}⁸ complexes disproportionate by forming dinitrosyl iron complexes (DNICs), but the mechanism of this reaction is not understood. While investigating this process, we isolated a new type of non‐heme iron nitrosyl complex that is stabilized by an unexpected spin‐state change. Upon reduction of the hs‐{FeNO}⁷ complex, [Fe(TPA)(NO)(OTf)](OTf) ( 1 ), the N‐O stretching band vanishes, but no sign of DNIC or N₂O formation is observed. Instead, the dimer, [Fe₂(TPA)₂(NO)₂](OTf)₂ ( 2 ) could be isolated and structurally characterized. We propose that 2 is formed from dimerization of the hs‐{FeNO}⁸ intermediate, followed by a spin state change of the iron centers to low‐spin (ls), and speculate that 2 models intermediates in hs‐{FeNO}⁸ complexes that precede the disproportionation reaction.     

Enhanced Reactivities of Iron(IV)‐Oxo Porphyrin π‐Cation Radicals in Oxygenation Reactions by Electron‐Donating Axial Ligands

The proximal axial ligand in heme iron enzymes plays an important role in tuning the reactivities of iron(IV)‐oxo porphyrin π‐cation radicals in oxidation reactions. The present study reports the effects of axial ligands in olefin epoxidation, aromatic hydroxylation, alcohol oxidation, and alkane hydroxylation, by [(tmp)^+. Fe^IV(O)(p‐Y‐PyO)]⁺ ( 1 ‐Y) (tmp=meso‐tetramesitylporphyrin, p‐Y‐PyO=para‐substituted pyridine N‐oxides, and Y=OCH₃, CH₃, H, Cl). In all of the oxidation reactions, the reactivities of 1 ‐Y are found to follow the order 1 ‐OCH₃ > 1 ‐CH₃ > 1 ‐H > 1 ‐Cl; negative Hammett ρ values of ?1.4 to ?2.7 were obtained by plotting the reaction rates against the σ_p values of the substituents of p‐Y‐PyO. These results, as well as previous ones on the effect of anionic nucleophiles, show that iron(IV)‐oxo porphyrin π‐cation radicals bearing electron‐donating axial ligands are more reactive in oxo‐transfer and hydrogen‐atom abstraction reactions. These results are counterintuitive since iron(IV)‐oxo porphyrin π‐cation radicals are electrophilic species. Theoretical calculations of anionic and neutral ligands reproduced the counterintuitive experimental findings and elucidated the root cause of the axial ligand effects. Thus, in the case of anionic ligands, as the ligand becomes a better electron donor, it strengthens the FeO? H bond and thereby enhances its H‐abstraction activity. In addition, it weakens the Fe?O bond and encourages oxo‐transfer reactivity. Both are Bell–Evans–Polanyi effects, however, in a series of neutral ligands like p‐Y‐PyO, there is a relatively weak trend that appears to originate in two‐state reactivity (TSR). This combination of experiment and theory enabled us to elucidate the factors that control the reactivity patterns of iron(IV)‐oxo porphyrin π‐cation radicals in oxidation reactions and to resolve an enigmatic and fundamental problem.     

Three Redox States of Nitrosyl: NO+, NO., and NO−/HNO Interconvert Reversibly on the Same Pentacyanoferrate(II) Platform

Not so elusive : [Fe^II(CN)₅(HNO)]^3? has been characterized spectroscopically after the two‐electron reduction of nitroprusside (see scheme). The complex is stable at pH 6, slowly decomposing to [Fe(CN)₆]^4? and N₂O. It is deprotonated at increasing pH value with oxidation of bound NO^? to [Fe^II(CN)₅(NO)]^3?. [Fe^II(CN)₅(HNO)]^3? is the first non‐heme iron–nitroxyl complex prepared in aqueous solution that is reversibly redox‐active under biologically relevant conditions.

     

Postsynthetic Improvement of the Physical Properties in a Metal–Organic Framework through a Single Crystal to Single Crystal Transmetallation

A single crystal to single crystal transmetallation process takes place in the three‐dimensional (3D) metal–organic framework (MOF) of formula Mg^II₂{Mg^II₄[Cu^II₂(Me₃mpba)₂]₃}?45 H₂O ( 1 ; Me₃mpba^4?=N,N′‐2,4,6‐trimethyl‐1,3‐phenylenebis(oxamate)). After complete replacement of the Mg^II ions within the coordination network and those hosted in the channels by either Co^II or Ni^II ions, 1 is transmetallated to yield two novel MOFs of formulae Co₂^II{Co^II₄[Cu^II₂(Me₃mpba)₂]₃}?56 H₂O ( 2 ) and Ni₂^II{Ni^II₄[Cu^II₂(Me₃mpba)₂]₃}? 54 H₂O ( 3 ). This unique postsynthetic metal substitution affords materials with higher structural stability leading to enhanced gas sorption and magnetic properties.     

The Reaction of Cs2[Tc(NO)F5] with BF3 in Acetonitrile: Formation and Structure of [{Tc(NO)(CH3CN)4}2(μ‐F)](BF4)3

The deep blue, paramagnetic Cs₂[Tc^II(NO)F₅] is formed during reactions of pertechnetate, acetohydroxamic acid, and CsF in aqueous HF. A reaction of Cs₂[Tc(NO)F₅] with BF₃ · MeOH in acetonitrile gives yellow blocks of the fluorido‐bridged dimer [{Tc^I(NO)(CH₃CN)₄}₂F](BF₄)₃. The compound is stable as solid and in acetonitrile solutions. The complex cation contains a bent μ‐F^– ligand and two linear nitrosyl groups.     

Synthesis and X‐ray Crystal Structure of Bis[oxamide dioximato‐κ2N,N′](oxamide dioxime‐κ2N,N′)cobalt(III) Perchlorate Hexahydrate

Co(OAc)₂ reacts with oxamide dioxime (H₂oxado) in water in the presence of ClO₄^– ions to produce [Co(Hoxado)₂(H₂oxado)]ClO₄ · 6H₂O ( 1 ), where Hoxado^– is the anion of H₂oxado, derived from the deprotonation of one of the two hydroximinic groups, and in which oxidation of Co^II to Co^III (in air) had occurred. 1 is the first example of a salt in which the cation, [Co(H₂oxado)₃]³⁺, is doubly deprotonated to generate the chiral cation, [Co(Hoxado)₂(H₂oxado)]⁺. The central cobalt cation is pseudo‐octahedrally coordinated by six nitrogen atoms. In the solid state, the complex cations form centro‐symmetric dimers via O–H ··· O bridges. The bulk structure is consolidated by an extended three‐dimensional network of O–H ··· O and N–H ··· O bridges that interconnect the ionic constituents and the water molecules.     

Direct Observation of Primary C−H Bond Oxidation by an Oxido‐Iron(IV) Porphyrin π‐Radical Cation Complex in a Fluorinated Carbon Solvent

Oxido‐iron(IV) porphyrin π‐radical cation species are involved in a variety of heme‐containing enzymes and have characteristic oxidation states consisting of a high‐valent iron center and a π‐conjugated macrocyclic ligand. However, the short lifetime of the complex has hampered detailed reactivity studies. Reported herein is a remarkable increase in the lifetime (80 s at 10 °C) of Fe^IV(TMP^+.)(O)(Cl) ( 2 ; TMP=5,10,15,20‐tetramesitylporphyrin dianion), produced by the oxidation of Fe^III(TMP)(Cl) ( 1 ) by ozone in α,α,α‐trifluorotoluene (TFT). The lifetime is 720 times longer compared to that of the currently most stable species reported to date. The increase in the lifetime improves the reaction efficiency of 2 toward inert alkane substrates, and allowed observation of the reaction of 2 with a primary C?H bond (BDE_C‐H=ca. 100 kcal mol^?1) directly. Activation parameters for cyclohexane hydroxylation were also obtained.     

Magnetic investigations of a two‐dimensional coordination polymer with a three‐dimensional supramolecular framework: poly[[bis[μ2‐1,4‐bis(1,2,4‐triazol‐1‐yl)butane]bis(thiocyanato‐κN)cobalt(II)] dihydrate]

In the title mixed‐ligand metal–organic polymeric complex, {[Co(NCS)₂(C₈H₁₂N₆)₂]·2H₂O}_n, the asymmetric unit contains a divalent Co^II cation, which sits on an inversion centre, two halves of two crystallographically distinct and centrosymmetric 1,4‐bis(1,2,4‐triazol‐1‐yl)butane (BTB) ligands, one N‐bound thiocyanate ligand and one solvent water molecule. The Co^II atom possesses a distorted {CoN₆} octahedral geometry, with the equatorial positions taken up by triazole N atoms from four different BTB ligands. The axial positions are filled by thiocyanate N atoms. In the crystal, each Co^II atom is linked covalently to four others through the distal donors of the tethering BTB ligands, forming a neutral (4,4)‐topology two‐dimensional rhomboid grid layer motif, which is coincident with the (11) crystal planes. Magnetic investigations show that weak antiferromagnetic coupling exists between Co^II atoms in the complex.     

The Electronic Ground State of [Fe(CO)3(NO)]−: A Spectroscopic and Theoretical Study

During the past 10 years iron‐catalyzed reactions have become established in the field of organic synthesis. For example, the complex anion [Fe(CO)₃(NO)]^?, which was originally described by Hogsed and Hieber, shows catalytic activity in various organic reactions. This anion is commonly regarded as being isoelectronic with [Fe(CO)₄]^2?, which, however, shows poor catalytic activity. The spectroscopic and quantum chemical investigations presented herein reveal that the complex ferrate [Fe(CO)₃(NO)]^? cannot be regarded as a Fe^?II species, but rather is predominantly a Fe⁰ species, in which the metal is covalently bonded to NO^? by two π‐bonds. A metal–N σ‐bond is not observed.     

Two Heterometallic–Organic Frameworks Composed of Iron(III)‐Salen‐Based Ligands and d10 Metals: Gas Sorption and Visible‐Light Photocatalytic Degradation of 2‐Chlorophenol

Two examples of heterometallic–organic frameworks (HMOFs) composed of dicarboxyl‐functionalized Fe^III‐salen complexes and d¹⁰ metals (Zn, Cd), [Zn₂(Fe‐L)₂(μ₂‐O)(H₂O)₂] ? 4 DMF ? 4 H₂O ( 1 ) and [Cd₂(Fe‐L)₂(μ₂‐O)(H₂O)₂] ? 2 DMF ? H₂O ( 2 ) (H₄L=1,2‐cyclohexanediamino‐N,N′‐bis(3‐methyl‐5‐carboxysalicylidene), have been synthesized and structurally characterized. In 1 and 2 , each square‐pyramidal Fe^III atom is embedded in the [N₂O₂] pocket of an L^4? anion, and these units are further bridged by a μ₂‐O anion to give an (Fe‐L)₂(μ₂‐O) dimer. The two carboxylate groups of each L^4? anion bridge Zn^II or Cd^II atoms to afford a 3D porous HMOF. The gas sorption and magnetic properties of 1 and 2 have been studied. Remarkably, 1 and 2 show activity for the photocatalytic degradation of 2‐chlorophenol (2‐CP) under visible‐light irradiation, which, to the best of our knowledge, is the first time that this has been observed for Fe^III‐salen‐based HMOFs.     

Metal complexes of 2-acetylpyridine N(4)-dihexyl- and N(4)-dicyclohexylthiosemicarbazones

Summary 2-Acetylpyridine N(4)-dihexyl- and N(4)-dicyclohexylthiosemicarbazone, HAc4DHex and HAc4DCHex, respectively, and Fe^III, Co^II, Co^III, Ni^II, Cu^II and Zn^II complexes have been prepared and characterized by molar conductivities, magnetic susceptibilities and spectroscopic techniques. For many of the complexes, loss of the N(2)H hydrogen occurs, and the ligands coordinate to the metal centres as NNS monoanionic, tridentate ligands, e.g., [M(NNS)X] (M = Co^II, Ni^II, Cu^II, NNS = Ac4DHex or Ac4DCHex and X = Cl or Br), [Fe(NNS)₂]ClO₄, [Co(NNS)₂]BF₄, [Cu(NNS)NO₃] and [Zn(NNS)OAc]. Zn^II ion is also chelated by neutral ligands in [Zn(HNNS)X₂] (X = Cl, Br). In addition, [Ni(Ac4DHex)-(HAc4DHex)]X (X = BF₄, ClO₄) and [Ni(HAc4DCHex)₂]-(BF₄)₂ are reported where the neutral thiosemicarbazone is coordinated via the pyridyl nitrogen, azomethine nitrogen and thione sulfur. Crystal structure determinations of HAc4DCHex and [Cu(Ac4DHex)Br] show the former to contain the bifurcated hydrogen bonded form and the latter to be planar with no significant interaction between neighbouring centres.     

Origin of Nitric Oxide Reduction Activity in Flavo–Diiron NO Reductase: Key Roles of the Second Coordination Sphere

The second coordination sphere constitutes a distinguishing factor in the active site to modulate enzymatic reactivity. To unravel the origin of NO‐to‐N₂O reduction activity of non‐heme diiron enzymes, herein we report a strong second‐coordination‐sphere interaction between a conserved Tyr₁₉₇ and the key iron–nitrosyl intermediate of Tm FDP (flavo–diiron protein), which leads to decreased reaction barriers towards N–N formation and N–O cleavage in NO reduction. This finding supports the direct coupling of diiron dinitrosyl as the N–N formation mode in our QM/MM modeling, and reconciles the mechanistic controversy of external reduction between FDPs and synthetic biomimetics of the iron–nitrosyls. This work highlights the application of QM/MM ⁵⁷Fe Mössbauer modeling in elucidating the structural features of not only first, but also second coordination spheres of the key transient species involved in NO/O₂ activation by non‐heme diiron enzymes.     

Metal(II) complexes and metal(II) salts ofN-carbamoylpyrazole: N,O and N,N coordination

Summary Several new coordination compounds are reported withN-carbamoylpyrazole (Hcpz) as the ligand;viz. M(cpz)₂ where M = Cu^II and Ni^II; M(Hcpz)Cl₂ where M = Mn^II, Co^II, Cu^II, Zn^II and Cd^II; M(Hcpz)₂Cl₂ Where M = Fe^II, Co^II and Ni^II: M(Hcpz)₃(BF₄)₂ where M = Fe^II, Co^II, Ni^II, Zn^II and Cd^II; and Cu(Hcpz)₂(BF₄)₂. In the salts, Hcpz is coordinated through the nitrogen atoms of the pyrazole ring and the nitrogen atom of the carbamoyl group. In the Hcpz complexes, coordination takes place through the nitrogen atom of the pyrazole ring and the oxygen atom of the carbamoyl group.     

π–π Dimerization of a mono(pyridyl–imine) platinum(II) chelate

The cation of the title complex salt, chlorido{2,2‐dimethyl‐N‐[(E)‐1‐(pyridin‐2‐yl)ethylidene]propane‐1,3‐diamine}platinum(II) tetrafluoridoborate, [PtCl(C₁₂H₁₉N₃)]BF₄, exhibits a nominally square‐planar Pt^II ion coordinated to a chloride ion [Pt—Cl = 2.3046 (9) Å] and three unique N‐atom types, viz. pyridine, imine and amine, of the tridentate Schiff base ligand formed by the 1:1 condensation of 1‐(pyridin‐2‐yl)ethanone and 2,2‐dimethylpropane‐1,3‐diamine. The cations are π‐stacked in inversion‐related pairs (dimers), with a mean plane separation of 3.426 Å, an intradimer Pt...Pt separation of 5.0785 (6) Å and a lateral shift of 3.676 Å. The centroid (Cg) of the pyridine ring is positioned approximately over the Pt^II ion of the neighbouring cation (Pt...Cg = 3.503 Å).     

Vibration–vibration energy exchange between 15N14NO and CO

The temperature dependence of the vibration–vibration energy transfer between the v₃ mode of ¹⁵N¹⁴NO and the first vibrational level of CO was determined over a range of 680 to 1300°K using a shock tube. Several mixtures of ¹⁵N¹⁴NO? CO were tested, diluted in 95% Ar. The resulting exothermic transfer probabilities for the reaction, are compared to previous work on N₂O—CO. The results for ¹⁵N¹⁴NO? CO exhibit a more pronounced direct temperature dependence than for N₂O—CO even though the process has a closer resonance (ΔE = 59 cm^?1 for ¹⁵N¹⁴NO? CO and ΔE = 81 cm^?1 for N₂O? CO).     

μ‐Cyano‐1:2κ2C:N‐tetracyano‐1κ4C‐(5‐methyl‐5‐nitro‐3,7‐diazanonane‐1,9‐diamine‐2κ4N1,3,7,9)‐nitrosyl‐1κN‐copper(II)iron(II) dihydrate

The title binuclear complex, [CuFe(CN)₅(C₈H₂₁N₅O₂)(NO)]·2H₂O or [CuFe(nelin)(CN)₅(NO)]·2H₂O (nelin is 5‐methyl‐5‐nitro‐3,7‐di­aza­nonane‐1,9‐di­amine) consists of discrete binuclear mixed‐metal species, with a Cu centre linked to an Fe centre through a cyano bridge, and two water mol­ecules of crystallization. In the complex, the Cu^II ion is coordinated by five N atoms and has a distorted square‐pyramidal geometry. The Fe^II centre is in a distorted octahedral environment.     

Reductive Nitric Oxide Coupling at a Dinickel Core: Isolation of a Key cis‐Hyponitrite Intermediate en route to N2O Formation

Reductive coupling of nitric oxide (NO) to give N₂O is an important reaction in the global nitrogen cycle. Here, a dinickel(II) dihydride complex 1 that releases H₂ upon substrate binding and serves as a masked dinickel(I) scaffold is shown to reductively couple two molecules of NO within the bimetallic cleft. The resulting hyponitrite complex 2 features an unprecedented cis‐[N₂O₂]^2? binding mode that has been computationally proposed as a key intermediate in flavodiiron nitric oxide reductases (FNORs). NMR and DFT evidence indicate facile rotational fluxionality of the [N₂O₂]^2? unit, which allows to access an isomer that is prone to N₂O release. Protonation of 2 is now found to trigger rapid N₂O evolution and formation of a hydroxido bridged complex, reminiscent of FNOR reactivity. This work provides fundamental insight into the biologically relevant reductive coupling of two NO molecules and the subsequent trajectory towards N₂O formation at bimetallic sites.     

Decay of Iron(V) Nitride Complexes By a NN Bond‐Coupling Reaction in Solution: A Combined Spectroscopic and Theoretical Analysis

Cryogenically trapped Fe^V nitride complexes with cyclam‐based ligands were found to decay by bimolecular reactions, forming exclusively Fe^II compounds. Characterization of educts and products by Mössbauer spectroscopy, mass spectrometry, and spectroscopy‐oriented DFT calculations showed that the reaction mechanism is reductive nitride coupling and release of dinitrogen (2 Fe^V?N→Fe^II‐N?N‐Fe^II→2 Fe^II+N₂). The reaction pathways, representing an “inverse” of the Haber–Bosch reaction, were computationally explored in detail, also to judge the feasibility of yielding catalytically competent Fe^V(N). Implications for the photolytic cleavage of Fe^III azides used to generate high‐valent Fe nitrides are discussed.     

Effect of Organic Dicarboxylates with Different Rigidity on the Construction of Cobalt(II) Complexes with a Semi‐rigid Tripyridyl‐bis‐amide Ligand

Three coordination compounds with dimensions from 0D to 2D, namely, [Co(bppdca)₂(HL1)₂] ( 1 ) [Co(bppdca)(L2)(H₂O)] · 2H₂O ( 2 ) and [Co(bppdca)(L3)] · 3H₂O ( 3 ) [bppdca = N,N′‐bis(pyridine‐3‐yl)pyridine‐2,6‐dicarboxamide, H₂L1 = 2,5‐pyridinedicarboxylic acid, H₂L2 = 4,4′‐oxybisbenzoic acid, H₂L3 = 2‐carboxymethylsulfanyl nicotinic acid] were hydrothermally synthesized and structurally characterized. Single crystal X‐ray diffraction analysis reveals that complex 1 is a discrete 0D complex, in which the bppdca ligand and the H₂L1 act as the terminal groups to coordinate with the Co^II ions. In coordination polymer 2 , two bppdca ligands coordinate in anti configuration with two Co^II ions to generate a 28‐membered Co₂(bppdca)₂ loop, which is further extended into 1D ladder‐like double chain by pairs of L2 ligands. In 3 , the Co^II ions are linked by bppdca ligands to generate 1D wave‐like chain, which is further connected by the L3 to form a 2D network. Finally, the coordination compounds 1 – 3 are extended into 3D supramolecular frameworks through the hydrogen bonding interactions. The Co^II ions and the bppdca ligands in the title coordination compounds exhibit different coordination characters and conformations. The effect of organic dicarboxylates with different rigidity and length on the structures of Co^II coordination compounds was investigated. In addition, the fluorescence and electrochemical behaviors of coordination compounds 1 – 3 were reported.     

A novel two‐dimensional cobalt(II) complex composed of one‐dimensional twisted pair‐like chains: poly[[tetraaquabis(μ3‐pyridine‐2,5‐dicarboxylato‐κ4N,O2:O5:O5′)dicobalt(II)] dihydrate]

A novel neutral polymer, {[Co₂(C₇H₃NO₄)₂(H₂O)₄]·2H₂O}_n, was hydrothermally synthesized using pyridine‐2,5‐dicarboxylate (2,5‐PDC²⁻) as the organic linker. It features a two‐dimensional layer structure constructed from one‐dimensional {[Co(2,5‐PDC)₂]²⁻}_n chains interlinked by [Co(H₂O)₄]⁺ units. The two Co^II cations occupy special positions, sitting on inversion centres. Each 2,5‐PDC²⁻ anion chelates to one Co^II cation via the pyridine N atom and an O atom of the adjacent carboxylate group, and links to two other Co^II cations in a bridging mode via the O atoms of the other carboxylate group. In this way, the 2,5‐PDC²⁻ ligand connects three neighbouring Co^II centres to form a two‐dimensional network. The two‐dimensional undulating layers are linked by extensive hydrogen bonds to form a three‐dimensional supramolecular structure, with the uncoordinated solvent molecules occupying the interlamellar region.