Half-sandwich manganese complexes Cp(CO)2Mn(NHC) as redox-active organometallic fragments.

Oxidation of the half-sandwich MnI complexes Cp(CO)2Mn(NHC) bearing dialkyl-, arylalkyl- and diarylsubstituted N-heterocyclic carbene ligands (NHC = IMe, IMeMes, IMes) affords the corresponding stable MnII radical cations [Cp(CO)2Mn(NHC)](BF4) isolated in 92-95% yield. Systematic X-ray diffraction studies of the series of MnI and MnII NHC complexes revealed the expected characteristic structural changes upon oxidation, namely the elongation of the Mn-CO and Mn-NHC bonds as well as the diminution of the OC-Mn-CO angle. ESR spectra of [Cp(CO)2Mn(IMes)](BF4) in frozen solution (CH2Cl2/toluene 1 : 1, 70 K) allowed the identification of two conformers for this complex and their structural assignment using DFT calculations. The stability of these NHC complexes in both metal oxidation states, moderate oxidation potentials and the ease of detection of MnII species by a variety of spectroscopic techniques (UV-Vis, IR, paramagnetic 1H NMR, and ESR) make these compounds promising objects for applications as redox-active organometallic fragments.

Small Iminocoumarin Derivatives as Red Emitters: From Biological Imaging to Highly Photoluminescent Non-doped Micro- and Nanofibres.

The fluorescence properties of four derivatives of 3-thienyl-2-(N-dicyanovinyl)iminocoumarin, bearing a diethylamino group in the 7-position or a methoxy group in the 6, 7 and 8 positions, were compared in solution and in the solid state. The 7-diethylamino derivative was strongly fluorescent in various solvents, with marked solvatochromism. Its fluorescence was quenched by aggregation. In contrast, the methoxy derivatives were only moderately or weakly fluorescent in solution, but two of them were strongly photoluminescent in the crystalline state, owing to favourable molecular packing. The 6-methoxy derivative even exhibited spectacular crystallization-enhanced emission, examples of which are particularly rare for this type of dyes. Dyes were tested for biological use. The 7-diethylamino derivative led to particularly strong fluorescence staining of the cytoplasm of HCT-116 colon cancer cells. No fading was observed over prolonged illumination by the microscope light beam, but a phototoxic effect was detected. The use of the dyes as red-emitting materials was also investigated. Using easy-to-implement preparation methods, the compounds self- assembled to give one-dimensional nano- and microsized particles, including millimeter-long microfibres that exhibited clear wave-guiding properties. This study shows the value of these low molecular-weight molecules for the preparation of new orange and red-emitting fluorescent materials based on totally pure dye.

A Stable but Highly Reactive Phosphine-Coordinated Borenium: Metal-free Dihydrogen Activation and Alkyne 1,2-Carboboration.

Borenium cations have been found to be valuable analogues of boranes as a result of their cationic character which imparts high electrophilicity. Herein, we report the synthesis, characterization, and reactivity of a new type of borenium cation employing a naphthyl bridge and a strong intramolecular P→B interaction. The cation reacts with H2 in the presence of PtBu3 (frustrated Lewis pair (FLP) approach) but also on its own. The mechanism of the reaction between the borenium cation and H2 in the absence of PtBu3 has been investigated using deuterium-labeling experiments and DFT calculations. Both experiments and calculations imply the side-on coordination of H2 to the B center, followed by heterolytic splitting and B-C bond cleavage. An uncommon syn 1,2-carboboration has also been observed upon reaction of the borenium ion with 3-hexyne.

Cationic gold(III) alkyl complexes: generation, trapping, and insertion of norbornene.

Migratory insertion of alkenes into gold-carbon bonds, a fundamental yet unprecedented organometallic transformation, has been investigated from a discrete (P,C) cyclometalated gold(III) dimethyl complex. Methide abstraction by B(C6F5)3 is shown to generate a highly reactive cationic Au(III) complex that evolves spontaneously by C6F5 transfer from boron. In the presence of norbornene, migratory insertion into the Au-C bond proceeds readily. The resulting norbornyl complex is efficiently trapped with pyridines or chloride to give stable four-coordinate adducts.

The ambivalent chemistry of a free anionic N-heterocyclic carbene decorated with a malonate backbone: the plus of a negative charge.

The anionic heterocycle "[maloNHC](-)", ([1](-)), is the archetype of a growing family of N-heterocyclic carbenes incorporating an anionic backbone; here, a malonate group. A comprehensive experimental exploration of its chemistry as a free entity (in the form of its lithium salt [1]·Li) is presented, and rationalized using DFT calculations at the B3LYP/6-31+G** level of theory. For the sake of comparison, similar computations were performed on other representative carbene types. Reactions of [1]·Li with a broad series of electrophilic reagents were used to ascertain its intrinsic nature as a nucleophilic carbene. Unexpectedly, [1]·Li was also seen to react with the nucleophilic tert-butylisocyanide, to give an anionic ketenimine, which could be subsequently derivatized, either into an imine by protonation of the ketenimine moiety, or into a neutral ketenimine by alkylation of the intracyclic malonate moiety. Further experiments on the electrophilic behavior of [1]·Li revealed its unexpected reactivity toward p-chlorobenzaldehyde, resulting in a formal C-H activation and the first structurally characterized keto-tautomer of the Breslow intermediate. Finally, [1]·Li remarkably activates polar E-H bonds, including N-H bonds from ammonia and amines, Si-H bonds, and B-H bonds. Importantly, DFT calculations indicate the importance of counterion effects. In particular, the key to the observed reactivity appears to be a modulation of energy levels associated with a dynamic variability of the Li-O distance between the remote malonate group and the counterion.

Anionic and zwitterionic copper(I) complexes incorporating an anionic N-heterocyclic carbene decorated with a malonate backbone: synthesis, structure and catalytic applications.

The anionic malonate-derived N-heterocyclic carbenes (maloNHCs) react cleanly and rapidly with copper chloride to generate the anionic complexes of type [(maloNHC)CuCl]·Li, which crystallize in the solid state either in an oligomeric trimer arrangement or in polymeric helixes depending on the substitution pattern and the solvent. Ten zwitterionic heteroleptic Cu(I) complexes combining the anionic maloNHC and a neutral imidazol-2-ylidene are also obtained in a very selective manner and fully characterized. Whereas the anionic complexes are relatively active catalysts for the hydrosilylation of carbonyl compounds, the zwitterionic complexes reveal to be efficient and extremely robust pre-catalysts for the intramolecular cyclopropanation reaction of a diazo ester and outperform the corresponding cationic Cu(i) complexes with classical imidazol-2-ylidenes.

On the incidence of non-covalent intramolecular interligand interactions on the conformation of carbene complexes: a case study.

The prevalence of a counterintuitive carbene conformation in a series of piano-stool Mn(I) alkylalkoxycarbene complexes was rationalized by means of DFT calculations and high-resolution XRD analysis in terms of stabilizing non-covalent C-H···C≡O interligand interactions.

Interplay between an elusive 4-(isopropylamino)imidazol-2-ylidene and its isolable mesoionic tautomer, and associated reactivities.

An elusive free 4-(isopropylamino)imidazol-2-ylidene is engaged in a tautomeric equilibrium with its mesoionic tautomer, 4-(isopropylamido)imidazolium, which displays the typical reactivity of a cyclic diaminocarbene; once coordinated to a Rh(I) centre, it undergoes a smooth 4e(-) oxidation of its backbone to yield an amido-amidino-carbene, a weak electron donor viable only in its complexed form.