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.     

Cover: Journal of the Chinese Chemical Society 2/2020

This invited paper depicts that nitric oxide reductases have been recognized as NO denitrification to N₂O by utilizing transition metal ions cofactor. Biomimetic model study signifies that transition metal-mediated NO-to-[N₂O₂]²⁻-to-N₂O transformation via either the inter/intra-molecular [NO]⁻-[NO]⁻ coupling or metal-[NO]²⁻ radical coupling with exogenous ·NO. More details about this figure will be discussed by Prof. Wen-Feng Liaw and his co-worker on page 260–266 in this issue.

     

Conversion of Nitric Oxide into Nitrous Oxide as Triggered by the Polarization of Coordinated NO by Hydrogen Bonding

Reduction of the {Co(NO)}⁸ cobalt–nitrosyl N‐confused porphyrin (NCP) [Co(CTPPMe)(NO)] ( 1 ) produced electron‐rich {Co(NO)}⁹ [Co(CTPPMe)(NO)][Co(Cp*)₂] ( 2 ), which was necessary for NO‐to‐N₂O conversion. Complex 2 was NO‐reduction‐silent in neat THF, but was partially activated to a hydrogen‐bonded species 2 ??? MeOH in THF/MeOH (1:1, v/v). This species coupling with 2 transformed NO into N₂O, which was fragmented from an [N₂O₂]‐bridging intermediate. An intense IR peak at 1622 cm^?1 was ascribed to ν(NO) in an [N₂O₂]‐containing intermediate. Time–course ESI(?) mass spectra supported the presence of the dimeric [Co(NCP)]₂(N₂O₂) intermediate. Five complete NO‐to‐N₂O conversion cycles were possible without significant decay in the amount of N₂O produced.     

Lewis Acid Coordination Redirects S-Nitrosothiol Signaling Output

S-Nitrosothiols (RSNOs) serve as air-stable reservoirs for nitric oxide in biology. While copper enzymes promote NO release from RSNOs by serving as Lewis acids for intramolecular electron-transfer, redox-innocent Lewis acids separate these two functions to reveal the effect of coordination on structure and reactivity. The synthetic Lewis acid B(C₆F₅)₃ coordinates to the RSNO oxygen atom, leading to profound changes in the RSNO electronic structure and reactivity. Although RSNOs possess relatively negative reduction potentials, B(C₆F₅)₃ coordination increases their reduction potential by over 1 V into the physiologically accessible +0.1 V vs. NHE. Outer-sphere chemical reduction gives the Lewis acid stabilized hyponitrite dianion trans-[LA-O-N=N-O-LA]²⁻ [LA=B(C₆F₅)₃], which releases N₂O upon acidification. Mechanistic and computational studies support initial reduction to the [RSNO-B(C₆F₅)₃] radical anion, which is susceptible to N−N coupling prior to loss of RSSR.     

Two nickel(II) bis[(pyridin‐2‐yl)methyl]amine complexes with homophthalic and benzene‐1,2,4,5‐tetracarboxylic acids

Two new Ni^II complexes involving the ancillary ligand bis[(pyridin‐2‐yl)methyl]amine (bpma) and two different carboxylate ligands, i.e. homophthalate [hph; systematic name: 2‐(2‐carboxylatophenyl)acetate] and benzene‐1,2,4,5‐tetracarboxylate (btc), namely catena‐poly[[aqua{bis[(pyridin‐2‐yl)methyl]amine‐κ³N,N′,N′′}nickel(II)]‐μ‐2‐(2‐carboxylatophenyl)aceteto‐κ²O:O′], [Ni(C₉H₆O₄)(C₁₂H₁₃N₃)(H₂O)]_n, and (μ‐benzene‐1,2,4,5‐tetracarboxylato‐κ⁴O¹,O²:O⁴,O⁵)bis(aqua{bis[(pyridin‐2‐yl)methyl]amine‐κ³N,N′,N′′}nickel(II)) bis(triaqua{bis[(pyridin‐2‐yl)methyl]amine‐κ³N,N′,N′′}nickel(II)) benzene‐1,2,4,5‐tetracarboxylate hexahydrate, [Ni₂(C₁₀H₂O₈)(C₁₂H₁₃N₃)₂(H₂O)₂]·[Ni(C₁₂H₁₃N₃)(H₂O)₃]₂(C₁₀H₂O₈)·6H₂O, (II), are presented. Compound (I) is a one‐dimensional polymer with hph acting as a bridging ligand and with the chains linked by weak C—H...O interactions. The structure of compound (II) is much more complex, with two independent Ni^II centres having different environments, one of them as part of centrosymmetric [Ni(bpma)(H₂O)]₂(btc) dinuclear complexes and the other in mononuclear [Ni(bpma)(H₂O)₃]²⁺ cations which (in a 2:1 ratio) provide charge balance for btc⁴⁻ anions. A profuse hydrogen‐bonding scheme, where both coordinated and crystal water molecules play a crucial role, provides the supramolecular linkage of the different groups.     

A New Thiolate-Bound Dimanganese Cluster as a Structural and Functional Model for Class Ib Ribonucleotide Reductases

In class Ib ribonucleotide reductases (RNRs) a dimanganese(II) cluster activates superoxide (O₂⋅⁻) rather than dioxygen (O₂), to access a high valent Mn^III−O₂−Mn^IV species, responsible for the oxidation of tyrosine to tyrosyl radical. In a biomimetic approach, we report the synthesis of a thiolate-bound dimanganese complex [Mn^II₂(BPMT)(OAc)₂](ClO)₄ (BPMT=(2,6-bis{[bis(2-pyridylmethyl)amino]methyl}-4-methylthiophenolate) ( 1 ) and its reaction with O₂⋅⁻ to form a [(BPMT)MnO₂Mn]²⁺ complex 2 . Resonance Raman investigation revealed the presence of an O−O bond in 2 , while EPR analysis displayed a 16-line S_t=1/2 signal at g=2 typically associated with a Mn^IIIMn^IV core, as detected in class Ib RNRs. Unlike all other previously reported Mn−O₂−Mn complexes, generated by O₂⋅⁻ activation at Mn₂ centers, 2 proved to be a capable electrophilic oxidant in aldehyde deformylation and phenol oxidation reactions, rendering it one of the best structural and functional models for class Ib RNRs.     

The Unprecedented [S4N2O10]2− Anion in the Molecular Gold Complexes [Au2Br2(S4N2O10)2] and [Au2Cl2(S4N2O10)2](S2O5Cl2)

The reaction of hexachlorophosphazene, P₃N₃Cl₆, with SO₃ and the gold halides AuCl₃ and AuBr₃, respectively, leads to the new cyclic anionic tetramer, [S₄N₂O₁₀]²⁻, which is coordinated to Au³⁺ in the dimeric complexes [Au₂X₂(S₄N₂O₁₀)₂] (X=Cl, Br). The [S₄N₂O₁₀]²⁻ anion can be seen as the condensation product of two sulfate anions, [SO₄]²⁻_, and two amidosulfate anions, [NH₂SO₃]⁻.     

New organic–inorganic frameworks incorporating iso‐ and heteropolymolybdate units and a 3,3′,5,5′‐tetramethyl‐4,4′‐bi‐1H‐pyrazole‐2,2′‐diium multiple hydrogen‐bond donor

Poly[bis(3,3′,5,5′‐tetramethyl‐4,4′‐bi‐1H‐pyrazole‐2,2′‐diium) γ‐octamolybdate(VI) dihydrate], {(C₁₀H₁₆N₄)₂[Mo₈O₂₆]·2H₂O}_n, (I), and bis(3,3′,5,5′‐tetramethyl‐4,4′‐bi‐1H‐pyrazole‐2,2′‐diium) α‐dodecamolybdo(VI)silicate tetrahydrate, (C₁₀H₁₆N₄)₂[SiMo₁₂O₄₀]·4H₂O, (II), display intense hydrogen bonding between the cationic pyrazolium species and the metal oxide anions. In (I), the asymmetric unit contains half a centrosymmetric γ‐type [Mo₈O₂₆]⁴⁻ anion, which produces a one‐dimensional polymeric chain by corner‐sharing, one cation and one water molecule. Three‐centre bonding with 3,3′,5,5′‐tetramethyl‐4,4′‐bi‐1H‐pyrazole‐2,2′‐diium, denoted [H₂Me₄bpz]²⁺ [N...O = 2.770 (4)–3.146 (4) Å], generates two‐dimensional layers that are further linked by hydrogen bonds involving water molecules [O...O = 2.902 (4) and 3.010 (4) Å]. In (II), each of the four independent [H₂Me₄bpz]²⁺ cations lies across a twofold axis. They link layers of [SiMo₁₂O₄₀]⁴⁻ anions into a three‐dimensional framework, and the preferred sites for pyrazolium/anion hydrogen bonding are the terminal oxide atoms [N...O = 2.866 (6)–2.999 (6) Å], while anion/aqua interactions occur preferentially viaμ₂‐O sites [O...O = 2.910 (6)–3.151 (6) Å].     

Aluminium‐Mediated Carbon Dioxide Reduction by an Isolated Monoalumoxane Anion

The deoxygenative conversion of carbon dioxide to carbon monoxide is promoted by the aluminyl anion [Al(NON^Ar)]^? (NON^Ar=[O(SiMe₂NAr)₂]^2?, Ar=2,6‐iPr₂C₆H₃). The reaction proceeds via the isolable monoalumoxane anion [Al(NON^Ar)(O)]^?, containing a terminal aluminum‐oxygen bond. This species reacts with a second equivalent of carbon dioxide to afford the carbonate [Al(NON^Ar)(CO₃)]^?, and with nitrous oxide to generate the hyponitrite anion, [Al(NON^Ar)(κ²O,O′‐N₂O₂)]^?.     

Evaluation of AM1-calculated radical cation ion-neutral complexes

AM1 semiempirical molecular orbital calculations are reported for 20 ion-neutral complexes, including hydrogen-bonded complexes, presumably involved in the gas-phase unimolecular decomposition of simple organic radical cations. The systems investigated are [C₂H₄O₂]^˙+, [C₂H₅NO]^˙+, [C₂H₆O]^˙+, [C₂H₆O₂]^˙+, [C₃H₆O]^˙+, [C₃H₆O₂]^˙+, [C₃H₈O]^˙+, and [C₃H₈O₂]^˙+. The AM1 results are compared with ab initio molecular orbital calculations at different levels of theory up to MP3/6-31G(d, p)//SCF/6-31G(d) + ZPVE and the available experimental data. AM1 fails to predict some local minima and the equilibrium geometries calculated for several complexes are found to be qualitatively different from those predicted by the ab initio calculations. However, reasonable agreement is generally found for the stabilization energies of the complexes toward dissociation into their loosely bound components. © John Wiley & Sons, Inc.     

Variation of hydrogen‐bonded networks in hexa­fluoro­phosphate salts of amidate‐bridged dirhodium(II,III) complexes with axial aqua ligands

In diaqua­tetra‐μ‐acetamidato‐κ⁴N:O;κ⁴O:N‐di­rhodium(II,III) hexa­fluoro­phosphate, [Rh₂(C₂H₄NO)₄(H₂O)₂]PF₆, and diaqua­tetra‐μ‐acetamidato‐κ⁴N:O;κ⁴O:N‐di­rho­dium(II,III)hexa­fluoro­phosphate dihydrate, [Rh₂(C₂H₄NO)₄(H₂O)₂]PF₆·2H₂O, the cations and anions lie on inversion centers. Diaqua­tetra‐μ‐propionamidato‐κ⁴N:O;κ⁴O:N‐dirhodium(II,III) hexa­fluoro­phosphate dihydrate, [Rh₂(C₃H₆NO)₄(H₂O)₂]PF₆·2H₂O, and diaqua­tetra‐μ‐butyramidato‐κ⁴N:O;κ⁴O:N‐dirhodium(II,III) hexa­fluoro­phosphate, [Rh₂(C₄H₈NO)₄(H₂O)₂]PF₆, crystallize with two crystallographically independent complexes that lie on inversion centers. In all of the structures, the dirhodium units are hydrogen bonded to one another. The hydrogen‐bonded networks vary with the alkyl substituents.     

Solvent Role on Cobalt(II) Dioxygen Carriers Based on Simple Polyamine Ligands

The kinetics and thermodynamics of O₂ addition to Co^II complexes containing the simple triamine ligand (L) diethylenetriamine (=N‐(2‐aminoethyl)ethane‐1,2‐diamine; dien) or N,N″‐dimethyldiethylenetriamine (=N‐methyl‐N′‐[2‐(methylamino)ethyl]ethane‐1,2‐diamine; dmdien) in the aprotic solvent dimethyl sulfoxide (DMSO) were studied by UV/VIS spectrophotometry, potentiometry, and O₂ absorption measurements. A parallel investigation on the anaerobic formation of Co^II complexes with dmdien, as well as on their reactivity towards O₂, was carried out in aqueous 0.1M NaClO₄ solution. [CoL]²⁺ and [CoL₂]²⁺ were the common species formed under anaerobic conditions in both aqueous and DMSO solutions. Under aerobic conditions, O₂ adducts of different stoichiometry were formed: a superoxo complex [CoL₂O₂]²⁺ in DMSO and dimeric species in H₂O. The role of the reaction medium as well as effects of N‐alkylation of the triamine ligand in the formation and reactivity of the [Co^II(triamine)] complexes are discussed.     

A three‐dimensional framework in 2,2′‐biimidazolium bis(2,2′‐biimidazole‐κ2N,N′)bis(biphenyl‐2,4′‐dicarboxylato‐κO)manganese(II) hexahydrate

In the title salt, (C₆H₈N₄)[Mn(C₁₄H₈O₄)₂(C₆H₆N₄)₂]·6H₂O, the Mn^II atom lies on an inversion centre and is coordinated by four N atoms from two 2,2′‐biimidazole (biim) ligands and two O atoms from two biphenyl‐2,4′‐dicarboxylate (bpdc) anions to give a slightly distorted octahedral coordination, while the cation lies about another inversion centre. Adjacent [Mn(bpdc)₂(biim)₂]²⁻ anions are linked via two pairs of N—H...O hydrogen bonds, leading to an infinite chain along the [100] direction. The protonated [H₂biim]²⁺ moiety acts as a charge‐compensating cation and space‐filling structural subunit. It bridges two [Mn(bpdc)₂(biim)₂]²⁻ anions through two pairs of N—H...O hydrogen bonds, constructing two R₂²(9) rings, leading to a zigzag chain in the [2] direction, which gives rise to a ruffled set of [H₂biim]²⁺[Mn(bpdc)₂(biim)₂]²⁻ moieties in the [01] plane. The water molecules give rise to a chain structure in which O—H...O hydrogen bonds generate a chain of alternating four‐ and six‐membered water–oxygen R₄²(8) and R₆⁶(12) rings, each lying about independent inversion centres giving rise to a chain along the [100] direction. Within the water chain, the (H₂O)₆ water rings are hydrogen bonded to two O atoms from two [Mn(bpdc)₂(biim)₂]²⁻ anions, giving rise to a three‐dimensional framework.     

Poly[nickel(II)‐di‐μ‐4,4′‐bipyridyl‐κ4N:N′‐μ‐dichromato‐κ2O:O′] and poly[copper(II)‐di‐μ‐4,4'‐bipyridyl‐κ4N:N′‐μ‐dichromato‐κ2O:O′]

The novel title hybrid isomorphous organic–inorganic mixed‐metal dichromates, [Ni(Cr₂O₇)(C₁₀H₈N₂)₂] and [Cu(Cr₂O₇)(C₁₀H₈N₂)₂], have been synthesized. A non‐centrosymmetric three‐dimensional (4,6)‐net is formed from a linear chain of vertex‐linked [Cr₂O]²⁻ and [MN₄O]²⁺ (M = Ni and Cu) units, which in turn are linked by the planar bidentate 4,4′‐­bipyridine ligand through the four remaining vertices of the [MN₄O]²⁺ octahedra. There are two such three‐dimensional nets that interpenetrate with inversion symmetry.     

Syntheses,solid state electronic and infrared spectra of the Ni(II), Pd(II) and Cu(II) complexes of N-(2-hydroxyethyl) and N-(3-hydroxypropyl) oxamide

The new complexes K₂[Ni(Hheo)₂], K₂[Cu(Hheo)₂]·H₂O, K₂[Ni(Hhpo)₂]·H₂O, K₂[M(Hhpo)₂]·0.5H₂O (M = Cu, Pd) and K₂[Cu₂(hpo)₂·0.5H₂O, where H₃heo = N-(2-hydroxyethyl)oxamide and H₃hpo = N-(3-hydroxypropyl)oxamide, have been prepared. Several synthetic routes were investigated and the complexes were characterized by analyses, conductivity measurements, thermogravimetry, magnetic susceptibility and spectroscopy (i.r. and far i.r., diffuse reflectance u.v.). Monomeric square planar structures are found for the [M(Hheo)₂]²⁻ and [M(Hhpo)₂]²⁻ complex anions, while the hpo³⁻ Cu(II) complex appears to be a square planar dimer. The doubly deprotonated Hheo²⁻ and Hhpo²⁻ ions exhibit a bidentate N(secondary amide), N′(tertiary amide)-coordination with the OH-group remaining uncoordinated, while the triply deprotonated hpo³⁻ ion behaves as a bridging N(secondary amide), N′(tertiary amide), O(deprotonated) ligand, while two Cu(II) centres are bridged by two alkoxide-O atoms. The vibrational analysis of the dehydrated complexes is carried out, using NH/ND, OH/OD, ⁵⁸Ni/⁶²Ni and ⁶³Cu/⁶⁵Cu substitutions.     

Dilead(II) trimercury(II) tetraoxide chromate(VI), Pb2(Hg3O4)(CrO4)

Pb₂(Hg₃O₄)(CrO₄) consists of [CrO₄]²⁻ tetra­hedra, linear O—Hg—O dumbbells and divalent Pb atoms in [3+5]‐coordination. The HgO₂ dumbbells are condensed into [Hg₃O₄]²⁻ units and can be regarded as a section of the HgO structure. The [Hg₃O₄]²⁻ complex anions are connected by inter­stitial Pb²⁺ ions, while the [CrO₄]²⁻ tetra­hedra are isolated.     

Constructing mixed‐metal coordination polymers from copper(II)–pyridinedicarboxylate metalloligands

In the mixed‐metal complex catena‐poly[bis[diaquasilver(I)] [bis[aquacopper(II)]‐μ₃‐pyridine‐2,5‐dicarboxylato‐2′:1:1′κ⁵N,O²:O⁵:O⁵,O^5′‐μ‐pyridine‐2,5‐dicarboxylato‐2:1κ⁴N,O²:O⁵,O^5′‐disilver(I)‐μ₃‐pyridine‐2,5‐dicarboxylato‐1:1′:2′′κ⁵O⁵,O^5′:O⁵:N,O²‐μ‐pyridine‐2,5‐dicarboxylato‐1′:2′′′κ⁴O⁵,O^5′:N,O²] hexahydrate], {[Ag(H₂O)₂][AgCu(C₇H₃NO₄)₂(H₂O)]·3H₂O}_n, a square‐pyramidal Cu^II center is coordinated by two N atoms and two O atoms from two pyridine‐2,5‐dicarboxylate (2,5‐pydc) ligands and a water molecule, forming a [Cu(2,5‐pydc)₂(H₂O)]²⁻ metalloligand. One Ag^I center is coordinated by five O atoms from three 2,5‐pydc ligands and, as a result, the [Cu(2,5‐pydc)₂(H₂O)]²⁻ metalloligands act as linkers in a unique μ₃‐mode connecting Ag^I centers into a one‐dimensional anionic double chain along the [101] direction. The other Ag^I center is coordinated by two water molecules, forming an [Ag(H₂O)₂]⁺ cation. Four adjacent Ag^I centers are associated by Ag...Ag interactions [3.126 (1) and 3.118 (1) Å], producing a Z‐shaped Ag₄ unit along the [010] direction and connecting the anionic chains into a two‐dimensional layer structure. This study offers information for engineering mixed‐metal complexes based on copper(II)–pyridinedicarboxylate metalloligands.     

Trapping and Reactivity of a Molecular Aluminium Oxide Ion

Aluminium oxides constitute an important class of inorganic compound that are widely exploited in the chemical industry as catalysts and catalyst supports. Due to the tendency for such systems to aggregate via Al‐O‐Al bridges, the synthesis of well‐defined, soluble, molecular models for these materials is challenging. Here we show that reactions of the potassium aluminyl complex K₂[( NON )Al]₂ ( NON =4,5‐bis(2,6‐diiso‐propylanilido)‐2,7‐di‐tert‐butyl‐9,9‐dimethylxanthene) with CO₂, PhNCO and N₂O all proceed via a common aluminium oxide intermediate. This highly reactive species can be trapped by coordination of a THF molecule as the anionic oxide complex [( NON )AlO(THF)]^?, which features discrete Al?O bonds and dimerizes in the solid state via weak O???K interactions. This species reacts with a range of small molecules including N₂O (to give a hyponitrite ([N₂O₂]^2?) complex) and H₂, the latter offering an unequivocal example of heterolytic E?H bond cleavage across a main group M?O bond.     

Metal‐Free,Room‐Temperature Oxygen‐Atom Transfer in the N2O/CO Redox Couple as Catalyzed by [Si2Ox].+ (x=2–5)

The thermal reduction of N₂O by CO mediated by the metal‐free cluster cations [Si₂O_x]^.+ (x =2–5) has been examined in the gas phase using Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometry in conjunction with quantum chemical calculations. Three successive oxidation/reduction steps occur starting from [Si₂O₂]^.+ and N₂O to form eventually [Si₂O₅]^.+; the latter as well as the intermediate oxide cluster ions react sequentially with CO molecules to regenerate [Si₂O₂]^.+. Thus, full catalytic cycles occur at ambient conditions in the gas phase. Mechanistic aspects of these sequential redox processes have been addressed to reveal the electronic origins of these unparalleled reactions.     

Unusual Magneto-Structural Features of the Halo-Substituted Materials [FeIII(5-X-salMeen)2]Y: a Cooperative [HS-HS]↔[HS-LS] Spin Transition

X-ray structures of the halo-substituted complexes [Fe^III(5-X-salMeen)₂]ClO₄ (X=F, Cl, Br, I) [salMeen=N-methyl-N-(2-aminoethyl)salicylaldiminate]at RT have revealed the presence of two discrete HS complex cations in the crystallographic asymmetric unit with two perchlorate counter ions linking them by N−H_amine⋅⋅⋅O_perchlorate interactions. At 90 K, the two complex cations are distinctly HS and LS, a rare crystallographic observation of this coexistence in the Fe^III-salRen (R=alkyl) spin-crossover (SCO) system. At both temperatures, crystal packing shows dimerization through C−H_imine⋅⋅⋅O_phenolate interactions, a key feature for SCO cooperativity. Moreover, there are noncovalent contacts between the complex cations through type-II halogen-halogen bonds, which are novel in this system. The magnetic profiles and Mössbauer spectra concur with the structural analyses and reveal 50 % SCO of the type [HS-HS]↔[HS-LS] with a broad plateau. In contrast, [Fe^III(5-Cl-salMeen)₂]BPh₄⋅2MeOH is LS and exhibits a temperature-dependent crystallographic phase transition, exemplifying the influence of lattice solvents and counter ions on SCO.