The selective reactions of niobium pentachloride with two bulky NHC carbenes afforded NbCl5(NHC) complexes, bearing the highest oxidation state ever found for a metal centre in a transition metal halide-NHC adduct. The X-ray structure of 2a is the first one reported for a monodentate NHC-niobium species, and exhibits an abnormally long Nb-C bond.

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.

Two Co(III) complexes (1Py 2 and 2Py 2) of new corrole l i g a n d s H 3 L 1 (5 , 1 5-b i s (p-m e t h y l c a r b o x y p h e n y l)-1 0-(omethylcarboxyphenyl)corrole) and H 3 L2 (5,15-bis(p-nitrophenyl)-10-(o-methylcarboxyphenyl)corrole) with two apical pyridine ligands have been synthesized and thoroughly characterized by cyclic voltammetry, UV−vis−NIR, and EPR spectroscopy, spectroelectrochemistry, singlecrystal X-ray diffraction studies, and DFT methods. Complexes 1Py 2 and 2Py 2 possess much lower oxidation potentials than cobalt(III)-trispentafluorophenylcorrole (Co(tpfc)) and similar corroles containing pentafluorophenyl (C 6 F 5) substituents, thus allowing access to high oxidation states of the former metallocorroles using mild chemical oxidants. The spectroscopic (UV−vis−NIR and EPR) and electronic properties of several oxidation states of these complexes have been determined by a combination of the mentioned methods. Complexes 1Py 2 and 2Py 2 undergo three oxidations within 1.3 V vs FcH + /FcH in MeCN, and we show that both complexes catalyze water oxidation in an MeCN/H 2 O mixture upon the third oxidation, with k obs (TOF) values of 1.86 s −1 at 1.29 V (1Py 2) and 1.67 s −1 at 1.37 V (2Py 2). These values are five times higher than previously reported TOF values for C 6 F 5-substituted cobalt(III) corroles, a finding we ascribe to the additional charge in the corrole macrocycle due to the increased oxidation state. This work opens up new possibilities in the study of metallocorrole water oxidation catalysts, particularly by allowing spectroscopic probing of high-oxidation states and showing strong substituent-effects on catalytic activity of the corrole complexes.

The 1 : 1 molar reactions of niobium and tantalum pentahalides with the monodentate NHC ligand 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (Ipr), in toluene (or benzene) at ca. 80 °C, afforded the complexes NbX5(Ipr) (X = F, ; Br, ) and TaX5(Ipr) (X = F, ; Cl, ; Br, ), in generally good yields. Complexes represent uncommon cases of stable NHC adducts of metal halides with the metal in an oxidation state higher than +4, and also rare examples of Nb-NHC and Ta-NHC bonding systems. In particular, the X-ray molecular structure determined for provides the unprecedented crystallographic characterization of a tantalum compound with a monodentate NHC ligand. DFT results indicate that the metal-carbon bond in is a purely σ one. According to NMR studies ((1)H, (13)C, (93)Nb), the formation of , , , as well as the previously communicated NbCl5(Ipr), , proceeded with the intermediacy of [MX6](-) salts, presumably due to steric reasons. On the other hand, the intermediate formation of MF6(-) in the pathways to and was not observed, according to (19)F (and (93)Nb in the case of ) NMR. DFT calculations were carried out in order to shed light on structural and mechanistic aspects, and allowed to trace possible reaction routes.

The metal-mediated activation of PhNO represents an important starting point for understanding the reactivity patterns of nitrosoarenes in biological systems and catalysis. Here we report that the pyrazole-based dinickel­(II) dihydride complex [KL­(NiH)2] (1) reacts with PhNO to eliminate dihydrogen concomitant with binding of the doubly reduced substrate in μ-κ­(O):κ­(N) mode in the bimetallic pocket of [KLNi2(PhNO)] (2). The addition of [2,2,2]­cryptand leads to the ionic complex [K­(crypt)]­[LNi2(PhNO)] (3). Structural and spectroscopic analyses evidence that interaction with the Lewis acidic K+ in 2 causes significant elongation and weakening of the substrate’s N–O bond [d N–O = 1.487(12) Å in 2 vs 1.374(4) Å in 3]. Complex 2 (or 3) reacts with [FeCp*2]­[PF6] to give LNi2(PhNO) (4), which is shown by electron paramagnetic resonance and IR spectroscopies and density functional theory calculations to feature two low-spin d8 nickel­(II) ions and a bridging (PhNO)•– radical anion ligand, with the out-of-plane π*­(NO) being the singly occupied molecular orbital. Cyclic voltammetry and UV–vis spectroelectrochemical experiments show that 4 and the anion of 3 can be reversibly interconverted at very low potential (E 1/2 = −1.53 V vs Fc/Fc+). Protonation of 2 leads to the N-phenylhydroxylamine complex [LNi2(ONHPh)] (5) with a long N–O bond of 1.464(2) Å, and titration studies suggest a pK a of around 23–25 in tetrahydrofuran. This allows one to derive a bond dissociation energy of 62–65 kcal mol–1 for the N–H bond of 5. Accordingly, 5 readily reacts with the phenoxy radical 2,4,6- t Bu3C6H2O• to yield 4. This work demonstrates the reductive binding of PhNO without prior formation of unstable nickel­(I) species and the redox noninnocence of the PhNO ligand in the less common μ-κ­(O):κ­(N) bridging mode. Thermodynamic data for H-atom-abstraction chemistry at the activated PhNO may be valuable for understanding the reactivity patterns of the transient but biologically relevant nitroxyl (HNO) ligand.

Titanium tetrachloride smoothly reacted with a selection of a-amino acids (aaH) in CH2Cl2 affording yellow\ud to orange solid coordination compounds, 1a–d, in 70–78% yields. The salts [NHEt3][TiCl4(aa)], 2a–b, were\ud obtained from TiCl4/aaH/NEt3 (aa ¼ L-phenylalanine, N,N-dimethylphenylalanine), in 60–65% yields. The\ud complex , 3, was isolated from the reaction of L-proline with\ud NbCl5/NHiPr2, performed in CH2Cl2 at room temperature. The X-ray structure of 3 features a bridging\ud (E)-1,2-bis(3,4-dihydro-2H-pyrrol-5-yl)ethene-1,2-diolate ligand, resulting from the unprecedented C–C\ud coupling between two proline units. Unusually stable a-ammonium acyl chlorides were prepared by the\ud reactions of PCl5/MCln (MCln ¼ NbCl5, WCl6) with L-proline, N,N-dimethylphenylalanine, sarcosine and Lmethionine.\ud MX5 (M ¼ Nb, Ta; X ¼ F, Cl) reacted with L-leucine methylester and L-proline ethylester to\ud give ionic coordination compounds, [MX4L2][MX6] (M ¼ Nb, L ¼ Me2CHCH2CH(NH2)CO2Me, X ¼ F, 9; Cl,\ud 11a; M ¼ Nb, X ¼ Cl, , 11c; Ta, 11d), in moderate to good yields.\ud [NbCl5(Me2CHCH2CHNH3CO2Me)][NbCl6], 12, was isolated as a co-product of the reaction of NbCl5\ud with L-leucine isopropylester, and crystallographically characterized. The reaction of NbCl5 with L-serine\ud isopropylester afforded NbCl3(OCH2CHNHCO2\ud iPr), 13, in 66% yield. The activation of the ester O–R bond\ud was observed in the reactions of L-leucine methyl ester with NbF5 and L-proline ethyl ester with MBr5 (M ¼\ud Nb, Ta), these reactions proceeding with the release of EtF and EtBr, respectively. All the metal products\ud were characterized by analytical and spectroscopic methods, while DFT calculations were carried out in\ud order to provide insight into the structural and mechanistic aspects

A series of diiron/tetrairon compounds containing a S- or a Se-function (2a–d, 4a–d, 5a–b, 6), and the monoiron [FeCp(CO){SeC1(NMe2)C2HC3(Me)}] (3) were prepared from the diiron μ-vinyliminium precursors [Fe2Cp2(CO)( μ-CO){μ-η1: η3-C3(R’)C2HC1N(Me)(R)}]CF3SO3 (R = R’ = Me, 1a; R = 2,6-C6H3Me2 = Xyl, R’ = Ph, 1b; R = Xyl, R’ = CH2OH, 1c), via treatment with S8 or gray selenium. The new compounds were characterized by elemental analysis, IR and multinuclear NMR spectroscopy, and structural aspects were further elucidated by DFT calculations. The unprecedented metallacyclic structure of 3 was ascertained by single crystal X-ray diffraction. The air-stable compounds (3, 4a–d, 5a–b, 6) display fair to good stability in aqueous media, and thus were assessed for their cytotoxic activity towards A2780, A2780cisR, and HEK-293 cell lines. Cyclic voltammetry, ROS production and NADH oxidation studies were carried out on selected compounds to give insights into their mode of action.

The reaction of pyrrolidine with a series of cationic diiron cyclopentadienyl complexes containing a bridging vinyliminium ligand gives access to piano stool monoiron complexes based on a five-membered metallacycle that includes a vinyl-aminoalkylidene moiety, in moderate to high yields. The resulting metallacyclic motif (aminoalkylidene-ferracyclopentenone) is unique in organometallic chemistry and is partially preconstructed on the dinuclear frame. The monoiron products were fully characterized by elemental analysis, IR and multi-nuclear NMR spectroscopy, and in a number of cases by X-ray diffraction and cyclic voltammetry. They are robust in aqueous solutions and generally unreactive towards alkylating agents in organic solvents. However, a cationic derivative was prepared in high yield by methylation of a 2-pyridyl group. The cytotoxicity of both neutral and ionic complexes was assessed on cancerous (A2780 and A280cisR) and non-cancerous (HEK293) cell lines, revealing the influence of local structural modifications on the antiproliferative activity and the selectivity of the compounds.