Cobalt-Catalyzed Bis-Borylation of Quinolines: The Importance of the Cobalt Triplet State.

We report herein a mild stereo- and regioselective dearomatization of quinolines using the simple low valent HCo(N2 )(PPh3 )3 complex that exhibits labile ligands. Conditions to form selectively, at room temperature, high-valued 1,4-bis-borylated tetrahydroquinolines from simple starting heteroaromatic compounds have been developed. The efficient and selective functionalization of a large scope of quinolines bearing various electron-donating or electron-withdrawing substituents is presented, as well as the post-modification of the resulting C-B bond. NMR and labelling studies are consistent with a cascade mechanism pathway, starting from an in situ generated paramagnetic bis-quinoline cobalt(I) hydride complex. A first quinoline dearomatization followed by a cobalt(I)-catalyzed Markovnikov hydroboration of the remaining double bond allows the introduction of the boronic ester group only at C4 position. DFT calculations particularly highlight the importance of the cobalt triplet state throughout the reaction pathway, and bring some rationalization for the observed C4 selective borylation.

Bond-Dissociation Energies to Probe Pyridine Electronic Effects on Organogold(III) Complexes: From Methodological Developments to Application in π-Backdonation Investigation and Catalysis.

In this work, we report on the synthesis of several organogold(III) complexes based on 4,4'-diterbutylbiphenyl (C^C) and 2,6-bis(4-terbutylphenyl)pyridine (C^N^C) ligands and bond with variously substituted pyridine ligands (pyrR). Altogether, 33 complexes have been prepared and studied with mass spectrometry using higher-energy collision dissociation (HCD) in an Orbitrap mass spectrometer. A complete methodology including the kinetic modeling of the dissociation process based on the Rice-Ramsperger-Kassel-Marcus (RRKM) statistical method is proposed to obtain critical energies E0 of the pyrR loss for all complexes. The capacity of these E0 values to describe the pyridine ligand effect is further explored, at the same time as more classical descriptors such as 1H pyridinic NMR shift variation upon coordination and Au-NpyrR bond length measured by X-ray diffraction. An extensive theoretical work, including density functional theory (DFT) and domain-based local pair natural orbital coupled-cluster theory (DLPNO-CCSD(T)) methods, is also carried out to provide bond-dissociation energies, which are compared to experimental results. Results show that dissociation energy outperforms other descriptors, in particular to describe ligand effects over a large electronic effect range as seen by confronting the results to the pyrR pKa values. Further insights into the Au-NpyrR bond are obtained through an energy decomposition analysis (EDA) study, which confirms the isolobal character of Au+ with H+. Finally, the correlation between the lability of the pyridine ligands toward the catalytic efficiency of the complexes could be demonstrated in an intramolecular hydroarylation reaction of alkyne. The results were rationalized considering both pre-catalyst activation and catalyst reactivity. This study establishes the possibility of correlating dissociation energy, which is a gas-phase descriptor, with condensed-phase parameters such as catalysis efficiency. It therefore holds great potential for inorganic and organometallic chemistry by opening a convenient and easy way to evaluate the electronic influence of a ligand toward a metallic center.

1,6-Naphthyridin-7(6H)-ones: synthesis and optical properties.

Efficient synthesis of original 1,6-naphthyridin-7(6H)-ones and their optical properties are described. Their powerful fluorescence properties including dual fluorescence, solvatochromism, acidochromism, large Stokes shifts and high quantum yields, suitable for biological applications or as luminescent devices in materials science, are evidenced.

N-Heterocyclic Carbene Copper (I) Complexes Incorporating Pyrene Chromophore: Synthesis, Crystal Structure, and Luminescent Properties.

Luminescent N-heterocyclic carbene chloride copper (I) complexes incorporating pyrene chromophore (1-Pyrenyl-NHC-R)-Cu-Cl, (3, 4) have been prepared and fully characterized. Two complexes were prepared with R = methyl (3) and R = naphthyl groups (4) at the nitrogen center of the carbene unit to tune their electronic properties. The molecular structures of 3 and 4 have been elucidated by X-ray diffraction and confirm the formation of the target compounds. Preliminary results reveal that all compounds including the imidazole-pyrenyl ligand 1 are emissive in the blue region at room temperature in solution and in solid-state. All complexes display quantum yields comparable or higher when compared to the parent pyrene molecule. Interestingly replacement of the methyl by naphthyl group increases the quantum yield by almost two-folds. These compounds might show promise for applications as optical displays.

Molecular Magnetic Materials Based on {CoIII (Tp*)(CN)3 }- Cyanidometallate: Combined Magnetic, Structural and 59 Co NMR Study.

The cyanidocobaltate of formula fac-PPh4 [CoIII (Me2 Tp)(CN)3 ] ⋅ CH3 CN (1) has been used as a metalloligand to prepare polynuclear magnetic complexes (Me2 Tp=hydrotris(3,5-dimethylpyrazol-1-yl)borate). The association of 1 with in situ prepared [FeII (bik)2 (MeCN)2 ](OTf)2 (bik=bis(1-methylimidazol-2-yl)ketone) leads to a molecular square of formula {[CoIII {(Me2 Tp)}(CN)3 ]2 [FeII (bik)2 ]2 }(OTf)2 ⋅ 4MeCN ⋅ 2H2 O (2), whereas the self-assembly of 1 with preformed cluster [CoII 2 (OH2 )(piv)4 (Hpiv)4 ] in MeCN leads to the two-dimensional network of formula {[CoII 2 (piv)3 ]2 [CoIII (Me2 Tp)(CN)3 ]2 ⋅ 2CH3 CN}∞ (3). These compounds were structurally characterized via single crystal X-ray analysis and their spectroscopic (FTIR, UV-Vis and 59 Co NMR) properties and magnetic behaviours were also investigated. Bulk magnetic susceptibility measurements reveal that 1 is diamagnetic and 3 is paramagnetic throughout the explored temperature range, whereas 2 exhibits sharp spin transition centered at ca. 292 K. Compound 2 also exhibits photomagnetic effects at low temperature, selective light irradiations allowing to promote reversibly and repeatedly low-spin⇔high-spin conversion. Besides, the diamagnetic nature of the Co(III) building block allows us studying these compounds by means of 59 Co NMR spectroscopy. Herein, a 59 Co chemical shift has been used as a magnetic probe to corroborate experimental magnetic data obtained from bulk magnetic susceptibility measurements. An influence of the magnetic state of the neighbouring atoms is observed on the 59 Co NMR signals. Moreover, for the very first time, 59 Co NMR technique has been successfully introduced to investigate molecular materials with distinct magnetic properties.

Cyclometalated Rhodium and Iridium Complexes Containing Masked Catecholates: Synthesis, Structure, Electrochemistry, and Luminescence Properties.

Two neutral cyclometalated rhodium and iridium coordination assemblies [(F2ppy)2M(η-Cat)], M = Rh, (2) and M = Ir, (3) (F2ppy: 2,4-difluorophenylpyridine), displaying a masked catecholate (η-Cat = η-O∧O) are described. The catecholate ligand is π-bonded to an organometallic Cp*Ru(II) moiety. The latter brings stability to the whole system in solution and suppresses the formation of the related paramagnetic semiquinone complex. The determination of the molecular structure of the iridium complex [(F2ppy)2Ir(η-Cat)] (3) corroborates the formation of the target compound and reveals the generation of a rare two-dimensional (2D) honeycomb supramolecular architecture in the solid state, in which the Δ-enantiomer self-assembles with the Λ-enantiomer through encoded π-π interactions among individual units. The electrochemistry of complexes 2 and 3 was investigated and showed that reduction occurs at very negative potentials (∼-2.2 V versus saturated calomel electrode (SCE)), while oxidation of the cyclometalated Rh and Ir centers occurs at 0.8 and 0.86 V. In contrast to complexes with 1,2-dioxolene chelates, which are nonemissive, the heterodinuclear diamagnetic complexes 2 and 3 were found to be emissive at room temperature both in solution and in the solid state. Moreover, at 77 K in a solid state, both compounds display opposite emission behavior, for instance, complex 3 displays a blue-shifted emission, while rhodium compound 2 exhibits red-shifted emission to lower energy.

Rational Design of Mono- and Bi-Nuclear Cyclometalated Ir(III) Complexes Containing Di-Pyridylamine Motifs: Synthesis, Structure, and Luminescent Properties.

Heteroleptic cyclometalated iridium (III) complexes (1-3) containing di-pyridylamine motifs were prepared in a stepwise fashion. The presence of the di-pyridylamine ligands tunes their electronic and optical properties, generating blue phosphorescent emitters at room temperature. Herein we describe the synthesis of the mononuclear iridium complexes [Ir(ppy)2(DPA)][OTf] (1), (ppy = phenylpyridine; DPA = Dipyridylamine) and [Ir(ppy)2(DPA-PhI)][OTf] (2), (DPA-PhI = Dipyridylamino-phenyliodide). Moreover, the dinuclear iridium complex [Ir(ppy)2(L)Ir(ppy)2][OTf]2 (3) containing a rigid angular ligand "L = 3,5-bis[4-(2,2'-dipyridylamino)phenylacetylenyl]toluene" and displaying two di-pyridylamino groups was also prepared. For comparison purposes, the related dinuclear rhodium complex [Rh (ppy)2(L)Rh(ppy)2][OTf]2 (4) was also synthesized. The x-ray molecular structure of complex 2 was reported and confirmed the formation of the target molecule. The rhodium complex 4 was found to be emissive only at low temperature; in contrast, all iridium complexes 1-3 were found to be phosphorescent in solution at 77 K and room temperature, displaying blue emissions in the range of 478-481 nm.

Novel luminescent benzopyranothiophene- and BODIPY-derived aroylhydrazonic ligands and their dicopper(II) complexes: syntheses, antiproliferative activity and cellular uptake studies.

Two novel unsymmetrical binucleating aroylhydrazonic ligands and four dicopper(II) complexes carrying fluorescent benzopyranothiophene (BPT) or boron dipyrromethene (BODIPY) entities were synthesized and fully characterized. Complex 1, derived from the BPT-containing ligand H3L1, had its crystal structure elucidated through X-ray diffraction measurements. The absorption and fluorescence profiles of all the compounds obtained were discussed. Additionally, the stability of the ligands and complexes was monitored by UV-vis spectroscopy in DMSO and biologically relevant media. All the compounds showed moderate to high cytotoxicity towards the triple negative human breast cancer cell line MDA-MB-231. BPT derivatives were the most cytotoxic, specially H3L1, reaching an IC50 value up to the nanomolar range. Finally, fluorescence microscopy imaging studies employing mitochondria- and nucleus-staining dyes showed that the BODIPY-carrying ligand H3L2 was highly cell permeant and suggested that the compound preferentially accumulates in the mitochondria.

Phenyl Silicates with Substituted Catecholate Ligands: Synthesis, Structural Studies and Reactivity.

While the generation of aryl radicals by photoredox catalysis under reductive conditions is well documented, it has remained challenging under an oxidative pathway. Because of the easy photo-oxidation of alkyl bis-catecholato silicates, a general study of phenyl silicates bearing substituted catecholate ligands has been achieved. The newly synthesized phenyl silicates have been fully characterized, and their reactivity has been explored. It was found that, thanks to the substitution of the catecholate moiety, notably with the 4-cyanocatecholato ligand, the phenyl radical could be generated and trapped. Computational studies provided a rationale for these findings.

[(C C)Au(N N)]+ Complexes as a New Family of Anticancer Candidates: Synthesis, Characterization and Exploration of the Antiproliferative Properties.

A library of eleven cationic gold(III) complexes of the general formula [(C C)Au(N N)]+ when C C is either biphenyl or 4,4'-ditertbutyldiphenyl and N N is a bipyridine, phenanthroline or dipyridylamine derivative have been synthesized and characterized. Contrasting effects on the viability of the triple negative breast cancer cells MDA-MB-231 was observed from a preliminary screening. The antiproliferative activity of the seven most active complexes were further assayed on a larger panel of human cancer cells as well as on non-cancerous cells for comparison. Two complexes stood out for being either highly active or highly selective. Eventually, reactivity studies with biologically meaningful amino acids, glutathione, higher order DNA structures and thioredoxin reductase (TrxR) revealed a markedly different behavior from that of the well-known coordinatively isomeric [(C N C)Au(NHC)]+ structure. This makes the [(C C)Au(N N)]+ complexes a new class of organogold compounds with an original mode of action.

Indolizy Carbene Ligand. Evaluation of Electronic Properties and Applications in Asymmetric Gold(I) Catalysis.

We report herein a new family of carbene ligands based on an indolizine-ylidene (Indolizy) moiety. The corresponding gold(I) complexes are easily obtained from the gold(I)-promoted cyclization of allenylpyridine precursors. Evaluation of the electronic properties by experimental methods and also by DFT calculations confirms strong σ-donating and π-accepting properties of these ligands. Cationization of the gold(I) complexes generates catalytic species that trigger diverse reactions of (poly)unsaturated precursors. When armed with a methylene phosphine oxide moiety on the stereogenic center adjacent to the nitrogen atom, the corresponding bifunctional carbene ligands give rise to highly enantioselective heterocyclizations. DFT calculations brought some rationalization and highlighted the critical roles played by the phosphine oxide group and the tosylate anion in the asymmetric cyclization of γ-allenols.

Isoxazole-Derived Aroylhydrazones and Their Dinuclear Copper(II) Complexes Show Antiproliferative Activity on Breast Cancer Cells with a Potentially Alternative Mechanism Of Action.

This paper reports the design, synthesis and cytotoxicity studies of two new isoxazole-derived aroylhydrazone ligands and their dinuclear copper(II) complexes. Compounds were fully characterized by various spectroscopic and analytical techniques. The molecular structures of four derivatives were confirmed by X-ray crystallography. The stability of the ligands and the complexes in aqueous medium was monitored spectroscopically. Both the ligands and the complexes were shown to interact with calf thymus DNA (ct-DNA). Additionally, structures containing a phenol pendant arm were significantly more cytotoxic than those carrying a pendant pyridine substituent, reaching sub-micromolar IC50 values on the triple-negative human breast cancer cell line MDA-MB-231. The metal chelation and transchelation ability of the compounds towards FeII , FeIII and ZnII ions was explored as a possible mechanism of action of these compounds.

Electron Transfer in the Cs⊂{Mn4 Fe4 } Cubic Switch: A Soluble Molecular Model of the MnFe Prussian-Blue Analogues.

A mixed-valence {MnII 3 MnIII FeII 2 FeIII 2 } cyanide-bridged molecular cube hosting a caesium cation, Cs⊂{Mn4 Fe4 }, was synthesized and structurally characterized by X-ray diffraction. Cyclic-voltammetry measurements show that its electronic state can be switched between five different redox states, which results in a remarkable electrochromic effect. Magnetic measurements on fresh samples point to the occurrence of a spin-state change near room temperature, which could be ascribed to a metal-to-metal electron transfer converting the {FeII -CN-MnIII } pair into a {FeIII -CN-MnII } pair. This feature was only previously observed in the polymeric MnFe Prussian-blue analogues (PBAs). Moreover, this novel switchable molecule proved to be soluble and stable in organic solvents, paving the way for its integration into advanced materials.

Capturing a Square Planar Gold(III) Complex Inside a Platinum Nanocage: A Combined Experimental and Theoretical Study.

A novel synthetic procedure was set up to gain access to platinum coordination cages Pt2L4, which are less investigated compared to their palladium counterparts. This Pt2L4 nanocage exhibits an adequate cavity for guest encapsulation. Indeed, the Au(III) metal complex [Au(bdt)2]- (bdt = benzene-1,2-dithiolate) was successfully captured inside the cavity, in contrast to the analogous palladium cage which failed to host the gold complex. This result represents a rare example where a metal complex with thio-ligands can be encapsulated in a coordination cage. Moreover, it highlights the role of the metal center and the robustness of the platinum cage for host-guest chemistry. This discovery will inspire researchers in this area to pay more attention to Pt-cages. The host-guest system was fully characterized by NMR techniques and X-ray crystallographic analysis. Moreover, the nature of the host-guest interaction in this unique example was investigated and rationalized by DFT computational studies.

Unique Class of Enantiopure N-Heterocyclic Carbene Half-Sandwich Iridium(III) Complexes with Stable Configurations: Probing Five-Membered versus Six-Membered Iridacycles.

A unique class of enantiopure N-heterocyclic carbene half-sandwich iridium complexes is reported. These compounds display stable configurations at the metal center, as demonstrated by their chiroptical properties. Remarkably, because of the nature of the naphthalimide molecule, two regioisomers containing five-membered [( R)-2a and ( S)-2a] and six-membered [( R)-2b and ( S)-2b] iridacycles were obtained. Density functional theory calculations are advanced to rationalize their relative stability.

Radical Germylzincation of α-Heteroatom-Substituted Alkynes.

The regio- and stereoselective addition of germanium and zinc across the C-C triple bond of nitrogen-, sulfur-, oxygen-, and phosphorus-substituted terminal and internal alkynes is achieved by reaction with a combination of R3GeH and Et2Zn. Diagnostic experiments support a radical-chain mechanism and the ß-zincated vinylgermanes that show exceptional stability are characterized by NMR spectroscopy and X-ray crystallography. The unique feature of this new radical germylzincation reaction is that the C(sp2)-Zn bond formed remains available for subsequent in situ Cu(I)- or Pd(0)-mediated C-C or C-heteroatom bond formation with retention of the double bond geometry. These protocols offer modular access to elaborated tri- and tetrasubstituted vinylgermanes decorated with heteroatom substituents ß to germanium that are useful for the preparation of stereodefined alkenes.

Link between Affinity and Cu(II) Binding Sites to Amyloid-ß Peptides Evaluated by a New Water-Soluble UV-Visible Ratiometric Dye with a Moderate Cu(II) Affinity.

Being able to easily determine the Cu(II) affinity for biomolecules of moderate affinity is important. Such biomolecules include amyloidogenic peptides, such as the well-known amyloid-ß peptide involved in Alzheimer's disease. Here, we report the synthesis of a new water-soluble ratiometric Cu(II) dye with a moderate affinity (109 M-1 at pH 7.1) and the characterizations of the Cu(II) corresponding complex by X-ray crystallography, EPR, and XAS spectroscopic methods. UV-vis competition was performed on the Aß peptide as well as on a wide series of modified peptides, leading to an affinity value of 1.6 × 109 M-1 at pH 7.1 for the Aß peptide and to a coordination model for the Cu(II) site within the Aß peptide that agrees with the one mostly accepted currently.

Highly Phosphorescent Crystals of Square-Planar Platinum Complexes with Chiral Organometallic Linkers: Homochiral versus Heterochiral Arrangements, Induced Circular Dichroism, and TD-DFT Calculations.

A novel class of chiral luminescent square-planar platinum complexes with a π-bonded chiral thioquinonoid ligand is described. Remarkably the presence of this chiral organometallic ligand controls the aggregation of this square planar luminophor and imposes a homo- or hetero-chiral arrangement at the supramolecular level, displaying non-covalent Pt-Pt and π-π interactions. Interestingly these complexes are highly luminescent in the crystalline state and their photophysical properties can be traced to their aggregation in the solid state. A TD-DFT calculation is obtained to rationalize this unique behavior.

C-N Bond Formation from a Masked High-Valent Copper Complex Stabilized by Redox Non-Innocent Ligands.

The reactivity of a stable copper(II) complex bearing fully oxidized iminobenzoquinone redox ligands towards nucleophiles is described. In sharp contrast with its genuine low-valent counterpart bearing reduced ligands, this complex performs high-yielding C-N bond formations. Mechanistic studies suggest that this behavior could stem from a mechanism akin to reductive elimination occurring at the metal center but facilitated by the ligand: it is proposed that a masked high oxidation state of the metal can be stabilized as a lower copper(II) oxidation state by the redox ligands without forfeiting its ability to behave as a high-valent copper(III) center. These observations are substantiated by a combination of advanced EPR spectroscopy techniques with DFT studies. This work sheds light on the potential of redox ligands as promoters of unusual reactivities at metal centers and illustrates the concept of masked high-valent metallic species.

Selenoquinones stabilized by ruthenium(II) arene complexes: synthesis, structure, and cytotoxicity.

A new series of monoselenoquinone and diselenoquinone π complexes, [(η(6) -p-cymene)Ru(η(4) -C6 R4 SeE)] (R=H, E=Se (6); R=CH3 , E=Se (7); R=H, E=O (8)), as well as selenolate π complexes [(η(6) -p-cymene)Ru(η(5) -C6 H3 R2 Se)][SbF6 ] (R=H (9); R=CH3 (10)), stabilized by arene ruthenium moieties were prepared in good yields through nucleophilic substitution reactions from dichlorinated-arene and hydroxymonochlorinated-arene ruthenium complexes [(η(6) -p-cymene)Ru(C6 R4 XCl)][SbF6 ]2 (R=H, X=Cl (1); R=CH3 , X=Cl (2); R=H, X=OH (3)) as well as the monochlorinated π complexes [(η(6) -p-cymene)Ru(η(5) -C6 H3 R2 Cl)][SbF6 ]2 (R=H (4); R=CH3 (5)). The X-ray crystallographic structures of two of the compounds, [(η(6) -p-cymene)Ru(η(4) -C6 Me4 Se2 )] (7) and [(η(6) -p-cymene)Ru(η(4) -C6 H4 SeO)] (8), were determined. The structures confirm the identity of the target compounds and ascertain the coordination mode of these unprecedented ruthenium π complexes of selenoquinones. Furthermore, these new compounds display relevant cytotoxic properties towards human ovarian cancer cells.