Electronic Structure Studies and Photophysics of Luminescent Th(IV) Anilido and Imido Complexes.

A series of thorium anilide compounds [ThNHArR(TriNOx)] (R = para-OCH3 (1-ArOMe), para-H (1-ArH), para-Cl (1-ArCl), para-CF3 (1-Ar4-CF3), TriNOx3- = tris(2-tert-butylhydroxylaminato)benzylamine), and their corresponding imido compounds [Li(DME)][Th═NArR(TriNOx)] (2-ArR) as well as the alkyl congeners [ThNHAd(TriNOx)] (1-Ad) and [Li(DME)][Th═NAd(TriNOx)] (2-Ad), have been prepared. The para-substituents on the arylimido moiety were introduced for systematic variation of their electron-donating and withdrawing abilities, changes that were evident in measurements of the 13C{1H} NMR chemical shifts of the ipso-C atom of the ArR moiety. Room temperature, solution-state luminescence of the four new thorium imido compounds, along with the previously reported [Li(THF)2][Th═NAr3,5-CF3(TriNOx)] (2-Ar3,5-CF3) and [Li(THF)(Et2O)][Ce═NAr3,5-CF3(TriNOx)] (3-Ar3,5-CF3) have been described. Among these complexes, 2-Ar3,5-CF3 demonstrated the most intense luminescence feature with excitation at 398 nm and emission at 453 nm. The luminescence measurements, together with a time-dependent density functional theory (TD-DFT) study, helped uncover an intra-ligand n → π* transition that was assigned as the origin of the bright blue luminescence; 3-Ar3,5-CF3 has an 1.2 eV redshift in excitation energy compared with its proligand. The weak luminescence of other derivatives (2-ArR and 3-Ar3,5-CF3) was attributed to non-radiative decay from low-lying excited states originating from inter-ligand transitions (2-ArR) or ligand-to-metal charge transfer bands (3-Ar3,5-CF3). Overall, the results expand the range of the thorium imido organometallic compounds and demonstrate that thorium(IV) complexes can support strong ligand luminescence. The results also demonstrate the utility of applying a Th(IV) center for tuning the n → π* luminescence energy and intensity of an associated imido moiety.

Evaluating the photophysical and photochemical characteristics of green-emitting cerium(III) mono-cyclooctatetraenide complexes.

The photophysical and photochemical reactivity of the organometallic mono(cyclooctatetraenide)-Ce(III) complexes: [(C8H8)Ce(µ-X)(THF)2]2 X = O3SCF3- (1), Cl- (2), were studied. In the course of these studies, a new polymorph of [(C8H8)Ce(µ-O3SCF3)(THF)2]2 (1) was described. Photoluminescence (PL) studies of 1 and 2 showed characteristic excitation and emission bands in the visible region, with bright green light emission under light irradiation from states corresponding to the 5d → 4f transition. Complexes 1 and 2 exhibit relatively long lifetime of 205.4 ± 0.2 and 145.8 ± 0.4 ns respectively. And the quantum yields (Φ) for 1 and 2 are 0.180 and 0.068 respectively. Electrochemical studies were performed on complexes 1 and 2 with a reversible Ce(III)/Ce(IV) redox couple recorded at E1/2 = -1.53 V versus the Fc/Fc+ for 2. Complex 1 shows an irreversible Ce(III/IV) oxidation wave. Complexes 1 and 2 revealed strongly-reducing, estimated, excited state reduction potentials of -3.31 and -4.02 V versus the Fc/Fc+ respectively, with small Stokes shifts of 0.12 eV. With their associated relatively long lifetimes, small Stokes shifts and large, negative values, both complexes were evaluated as potential photosensitisers for halogen atom transfer (XAT) using a test reaction of the dehalogenation of benzyl chloride.

Electronic structure studies reveal 4f/5d mixing and its effect on bonding characteristics in Ce-imido and -oxo complexes.

This study presents the role of 5d orbitals in the bonding, and electronic and magnetic structure of Ce imido and oxo complexes synthesized with a tris(hydroxylaminato) [((2- t BuNO)C6H4CH2)3N]3- (TriNO x 3-) ligand framework, including the reported synthesis and characterization of two new alkali metal-capped Ce oxo species. X-ray spectroscopy measurements reveal that the imido and oxo materials exhibit an intermediate valent ground state of the Ce, displaying hallmark features in the Ce LIII absorption of partial f-orbital occupancy that are relatively constant for all measured compounds. These spectra feature a double peak consistent with other formal Ce(iv) compounds. Magnetic susceptibility measurements reveal enhanced levels of temperature-independent paramagnetism (TIP). In contrast to systems with direct bonding to an aromatic ligand, no clear correlation between the level of TIP and f-orbital occupancy is observed. CASSCF calculations defy a conventional van Vleck explanation of the TIP, indicating a single-reference ground state with no low-lying triplet excited state, despite accurately predicting the measured values of f-orbital occupancy. The calculations do, however, predict strong 4f/5d hybridization. In fact, within these complexes, despite having similar f-orbital occupancies and therefore levels of 4f/5d hybridization, the d-state distributions vary depending on the bonding motif (Ce[double bond, length as m-dash]O vs. Ce[double bond, length as m-dash]N) of the complex, and can also be fine-tuned based on varying alkali metal cation capping species. This system therefore provides a platform for understanding the characteristic nature of Ce multiple bonds and potential impact that the associated d-state distribution may have on resulting reactivity.

A Metal-Free, Photocatalytic Method for Aerobic Alkane Iodination.

Halogenation is an important alkane functionalization strategy, but O2 is widely considered the most desirable terminal oxidant. Here, the aerobic iodination of alkanes, including methane, was performed using catalytic [nBu4N]Cl and light irradiation (390 nm). Up to 10 turnovers of CH3I were obtained from CH4 and air, using a stop-flow microtubing system. Mechanistic studies using cyclohexane as the substrate revealed important details about the iodination reaction. Iodine (I2) serves multiple roles in the catalysis: (1) as the alkyl radical trap, (2) as a precursor for the light absorber, and (3) as a mediator of aerobic oxidation. The alkane activation is attributed to Cl• derived from photofragmentation of the electron donor-acceptor complex of I2 and Cl-. The kinetic profile of cyclohexane iodination showed that aerobic oxidation of I3- to produce I2 in CH3CN is turnover-limiting.

MMP-7 affects peritoneal ultrafiltration associated with elevated aquaporin-1 expression via MAPK/ERK pathway in peritoneal mesothelial cells.

Peritoneal membrane dysfunction and the resulting ultrafiltration failure are the major disadvantages of long-term peritoneal dialysis (PD). It becomes increasingly clear that mesothelial cells play a vital role in the pathophysiological changes of the peritoneal membrane. Matrix metalloproteinases (MMPs) function in the extracellular environment of cells and mediate extracellular matrix turnover during peritoneal membrane homeostasis. We showed here that dialysate MMP-7 levels markedly increased in the patients with PD, and the elevated MMP-7 level was negatively associated with peritoneal ultrafiltration volume. Interestingly, MMP-7 could regulate the cell osmotic pressure and volume of human peritoneal mesothelial cells. Moreover, we provided the evidence that MMP-7 activated mitogen-activated protein kinases (MAPKs)-extracellular signal-regulated kinase 1/2 (ERK) pathway and subsequently promoted the expression of aquaporin-1 (AQP-1) resulting in the change of cell osmotic pressure. Using a specific inhibitor of ERK pathway abrogated the MMP-7-mediating AQP-1 up-regulation and cellular homeostasis. In summary, all the findings indicate that MMP-7 could modulate the activity of peritoneal cavity during PD, and dialysate MMP-7 might be a non-invasive biomarker and an alternative therapeutic target for PD patients with ultrafiltration failure.

Photocatalytic C-H activation and the subtle role of chlorine radical complexation in reactivity.

The functionalization of methane, ethane, and other alkanes derived from fossil fuels is a central goal in the chemical enterprise. Recently, a photocatalytic system comprising [CeIVCl5(OR)]2- [CeIV, cerium(IV); OR, -OCH3 or -OCCl2CH3] was disclosed. The system was reportedly capable of alkane activation by alkoxy radicals (RO•) formed by CeIV-OR bond photolysis. In this work, we present evidence that the reported carbon-hydrogen (C-H) activation of alkanes is instead mediated by the photocatalyst [NEt4]2[CeCl6] (NEt4 +, tetraethylammonium), and RO• are not intermediates. Spectroscopic analyses and kinetics were investigated for C-H activation to identify chlorine radical (Cl•) generation as the rate-limiting step. Density functional theory calculations support the formation of [Cl•][alcohol] adducts when alcohols are present, which can manifest a masked RO• character. This result serves as an important cautionary note for interpretation of radical trapping experiments.

Exploration of the Solid- and Solution-State Structures and Electrochemical Properties of CeIV(atrane) Complexes.

Chemical oxidation of cerium complexes can be unpredictable because of labile metal-ligand bonds leading to ligand redistribution. The use of tripodal frameworks such as silyl-substituted tren ligands (NN'3 = [N(CH2CH2N(SiMe2tBu))3]3-) and a tris(hydroxylaminato) ligand, [((2- tBuNO)C6H4CH2)3N]3- (TriNOx3-), has been shown to mitigate ligand redistribution effects to allow access to tetravalent cerium complexes with different apical ligands. In the current work, the coordination chemistry of CeIV with the related tripodal atrane3- (H3atrane = [N(CH2C(CH3)2OH)3]) ligand framework was examined. A series of CeIV(atrane) complexes with supporting chloride, silylamide, and aryloxide ligands were synthesized and characterized by X-ray crystallography. The solution-state behaviors of these complexes were studied using 1H and diffusion-ordered (DOSY) NMR spectroscopies. The electrochemical stabilization of the CeIV cation within the atrane3- framework was examined. Our results showed that a combination of suitable apical ligands and the atrane framework provided excellent stabilization for the CeIV cation.

Photoinduced Miyaura Borylation by a Rare-Earth-Metal Photoreductant: The Hexachlorocerate(III) Anion.

The first photoinduced carbon(sp2 )-heteroatom bond forming reaction by a rare-earth-metal photoreductant, a Miyaura borylation, has been achieved. This simple, scalable, and novel borylation method that makes use of the hexachlorocerate(III) anion ([CeIII Cl6 ]3- , derived from CeCl3 ) has a broad substrate scope and functional-group tolerance and can be conducted at room temperature. Combined with Suzuki-Miyaura cross-coupling, the method is applicable to the synthesis of various biaryl products, including through the use of aryl chloride substrates.

Controlling the Accumulation of Water at Oil-Solid Interfaces with Gradient Coating.

In this work, we demonstrate a strategy to control the accumulation of water in the oil-solid interface using a gradient coating. Gradient chemistry on glass surface is created by vapor diffusion of organosilanes, leading to a range of contact angles from 110 to 20°. Hexadecane is placed on the gradient substrate as an oil layer, forming a "water/hexadecane/gradient solid substrate" sandwich structure. During incubation, water molecules spontaneously migrate through the micrometer-thick oil layer and result in the formation of micrometer-sized water droplets at the oil-solid interface. It turns out that water droplets at more hydrophobic regions tend to be closer to a regular spherical shape, which is attributed to their higher contact angle with the hydrophobic substrate. However, along the gradient from hydrophobic to hydrophilic, the water droplets gradually form more irregular shapes, as hydrophilic surfaces pin the edges of droplets to form a distorted morphology. It indicates that more hydrophilic surfaces containing more Si-OH groups lead to a higher electrostatic interaction with water and a higher growth rate of interfacial water droplets. This work provides further insights into the mechanism of spontaneous water accumulation at oil-solid interfaces and assists in the rational design for controlling such interfacial phenomenon.