Coordinatively Unsaturated Nickel Nitroxyl Complex: Structure, Physicochemical Properties, and Reactivity toward Dioxygen.

For its important roles in biology, nitrogen monoxide (·NO) has become one of the most studied and fascinating molecules in chemistry. ·NO itself acts as a "noninnocent" or "redox active" ligand to transition metal ions to give metal-NO (M-NO) complexes. Because of this uncertainty due to redox chemistry, the real description of the electronic structure of the M-NO unit requires extensive spectroscopic and theoretical studies. We previously reported the Ni-NO complex with a hindered N3 type ligand [Ni(NO)(L3)] (L3- denotes hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate anion), which contains a high-spin (hs) nickel(II) center and a coordinated 3NO-. This complex is very stable toward dioxygen due to steric protection of the nickel(II) center. Here, we report the dioxygen reactivity of a new Ni-NO complex, [Ni(NO)(I)(L1″)], with a less hindered N2 type bis(pyrazolyl)methane ligand, which creates a coordinatively unsaturated ligand environment about the nickel center. Here, L1″ denotes bis(3,5-diisopropyl-1-pyrazolyl)methane. This complex is also described as a hs-nickel(II) center with a bound 3NO-, based on spectroscopic and theoretical studies. Unexpectedly, the reaction of [Ni(NO)(I)(L1″)] with O2 yielded [Ni(κ2-O2N)(L1″)2](I3), with the oxidation of both 3NO- and the I- ion to yield NO2- and I3-. Both complexes were characterized by X-ray crystallography, IR, and UV-Vis spectroscopy and theoretical calculations.

Electronic interactions between a quaternary pyridyl-ß-diketonate and anionic clay nanosheets facilitate intense photoluminescence.

Electrostatic interactions between a quaternary pyridyl-ß-diketonate and anionic charged nanosheets were observed to produce a highly emissive dispersion in a rich water solution. A greater fluorescence quantum yield of approximately 50% was obtained when a luminogenic ß-diketonate, 1-(4-methoxyphenyl)-3-(3-hydroxyethyl-pyridinium bromide)-1,3-propandione (prepared by the Claisen condensation reaction and subsequent quaternization), was molecularly dispersed and enclosed by a couple of atomically flat ultrathin (approximately 1.0 nm) silicate sheets of anionic layered clay. By accommodating ß-diketonate into a narrow interlamellar space (approximately 0.4 nm distance), the molecular motion was suppressed, as confirmed by a smaller non-radiative relaxation rate constant, which was obtained by time-resolved luminescence and quantum yield measurements. Because the dense packing of ß-diketonate quenched the excited state, the isolation of luminogens by the co-adsorption of photochemical inert cations (tetramethylammonium and benzylammonium) was prevented by concentration quenching. A lower quantum yield was obtained by expanding the interlayer distance above 1.0 nm by co-adsorbing a photo-inactive water-soluble polymer, poly(vinylpyrrolidone). Therefore, the fixation and spatial separation of ß-diketonate in the narrow interlayer space was determined to be essential for obtaining strong emission.

Stable Ferrous Mononitroxyl {FeNO}8 Complex with a Hindered Hydrotris(pyrazolyl)borate Coligand: Structure, Spectroscopic Characterization, and Reactivity Toward NO and O2.

The iron(II)-nitroxyl complex [Fe(NO)(L3)] (1) (with L3- = a hindered hydrotris(pyrazolyl)borate ligand), a high-spin (hs)-{FeNO}8 complex in the Enemark-Feltham notation, is surprisingly stable and is the first of its kind that could be structurally characterized. We further studied this compound using a variety of spectroscopic methods. These results indicate a hs iron(II) center with a bound 3NO- ligand where the spins are antiferromagnetic coupled ( St = 1). Vibrational data show that this complex has a very strong Fe-NO bond. DFT calculations support this result and link it to very strong π-donation from the 3NO- ligand to the iron(II) center. Furthermore, a very unusual equilibrium between the hs-{FeNO}8 complex and a dinitrosyl iron complex (DNIC) of {Fe(NO)2}9 type is observed. The O2 reactivity of the complex is finally reported.

A cobalt-nitrosyl complex with a hindered hydrotris(pyrazolyl)borate coligand: detailed electronic structure, and reactivity towards dioxygen.

The cobalt-nitrosyl complex [Co(NO)(L3)] is supported by a highly hindered tridentate nitrogen ligand, hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate (denoted as L3-), and shows a linear Co-N-O unit. This complex was prepared by the reaction of the potassium salt of L3- with the cobalt-nitrosyl precursor [Co(NO)2(tmeda)](BPh4) (tmeda = N,N,N,'N'-tetramethylethylenediamine). The obtained cobalt-nitrosyl complex as well as the corresponding products from the reaction with dioxygen, [Co(η2-O2N)(L3)] and [Co(η2-O2NO)(L3)], were characterised by X-ray crystallography and a number of spectroscopic methods including IR/far-IR, UV-Vis, and NMR spectroscopy. We also performed MCD measurements and DFT calculations to further elucidate the electronic structure of [Co(NO)(L3)] and the optical properties of the complex. The MCD spectra reveal two NO-to-Co charge-transfer transitions with strong excited state displacements that give rise to vibrational progressions in the MCD spectra, indicative of a very covalent Co-NO bond. These results provide new insight into the properties of the Co-NO bond and the electron distribution in the complex, which is best described as [CoII(NO-)(L3)].