An isolable pyridinium trifluoromethoxide salt is prepared from the reaction of 4-dimethylaminopyridine with the commercially available liquid 2,4-dinitro(trifluoromethoxy)benzene. The salt is an ...effective trifluoromethoxide source for SN2 reactions to form trifluoromethyl ethers.
This paper describes detailed organometallic studies of the aminoquinoline-directed Ni-catalyzed C–H functionalization of 2,3,4,5-tetrafluoro-N-(quinolin-8-yl)benzamide with diaryliodonium reagents. ...A combination of 19F NMR spectroscopy and X-ray crystallography is used to track and characterize diamagnetic and paramagnetic intermediates throughout this transformation. These provide key insights into both the cyclometalation and oxidative functionalization steps of the catalytic cycle. The reaction conditions (solvent, ligands, base, and stoichiometry) play a central role in the observation of a NiII precyclometalation intermediate as well as in the speciation of the NiII products of C–H activation. Both mono- and binuclear cyclometalated NiII species are observed and interconvert, depending on the reaction conditions. Cyclic voltammetry reveals that the NiII/III redox potentials for the cyclometalated intermediates vary by more than 700 mV depending on their coordination environments, and these differences are reflected in their relative reactivity with diaryliodonium oxidants. The oxidative functionalization reaction affords a mixture of arylated and solvent functionalization organic products, depending on the conditions and solvent. For example, conducting oxidation in toluene leads to the preferential formation of the benzylated product. A series of experiments implicate a NiII/III/IV pathway for this transformation.
This paper describes the one-electron interconversions of isolable NiIII and NiIV complexes through their reactions with carbon-centered radicals (R•). First, model NiIII complexes are shown to react ...with alkyl and aryl radicals to afford NiIV products. Preliminary mechanistic studies implicate a pathway involving direct addition of a carbon-centered radical to the NiIII center. This is directly analogous to the known reactivity of NiII complexes with R•, a step that is commonly implicated in catalysis. Second, a NiIV–CH3 complex is shown to react with aryl and alkyl radicals to afford C–C bonds via a proposed SH2-type mechanism. This pathway is leveraged to enable challenging H3C–CF3 bond formation under mild conditions. Overall, these investigations suggest that NiII/III/IV sequences may be viable redox pathways in high-oxidation-state nickel catalysis.
When subjected to arylboranes, anionic trifluoromethyl and difluorobenzyl palladium(II) complexes undergo fluoride abstraction followed by 1,1-migratory insertion. The resulting intermediate ...fluoroalkyl species can be induced to undergo a subsequent transmetalation and reductive elimination from either an in situ formed fluoroboronate (FB(Ar3)−) or an exogenous boronic acid/ester (ArB(OR)2) and nucleophilic activator, representing a net defluorinative arylation reaction. The latter method enabled a structurally diverse substrate scope to be prepared from either an isolated palladium-CF3 complex, or from Pd(PPh3)4 and other commercially available reagents.
Oscillators omitted: A self‐assembled inclusion complex between a LnIII12‐metallacrown‐423+ sandwich motif (green and bronze) and a 24‐metallacrown‐8 (purple) is stable in methanol. The YbIII complex ...has a large quantum yield (0.89 %) and luminescent lifetime (14 μs) in methanol, which are attributed to the exclusion of high‐energy oscillators from within 6.7 Å of the emitting YbIII ion by the metallacrown topology.
Co bis(benzenedithiolate) type complexes have captivated chemists for decades for their interesting geometric and electronic structures and more recently, for their impressive ability to mediate the ...hydrogen evolution reaction (HER) both photo- and electrocatalytically. However, these complexes have nearly exclusively been characterized in their air-stable Co(III) oxidation states. In this work, Co(II) bis(benzenedichlorodithiolate) was prepared by chemical and electrochemical one-electron reduction. This reduced Co(II) complex was characterized by X-ray crystallography and in-depth spectroscopic studies—including UV-Vis, magnetic circular dichroism, and electron paramagnetic resonance spectroscopy. Co(II)(Cl2bdt)22− is thereby shown to be a square planar complex, with a primarily metal-centered reduction, and an St = 1/2 spin state. This study informs our understanding of the first step in the HER catalytic cycle of Co bis(benzenedithiolate) type complexes and paves the way for future mechanistic studies on this catalyst family.
The solid-state structures of a series of bithiazole and thiophene oligomers, as well as a series of substituted pentacenes, are rationalized in terms of “pitch and roll” inclinations from an “ideal” ...cofacial π-stack. Pitch inclinations translate adjacent molecules relative to one another in the direction of the long molecular axis, whereas roll inclinations translate the molecules along the short molecular axis. Thus, moderately large pitch distortions preserve π−π interactions between adjacent molecules, whereas roll translations greater than 2.5 Å essentially destroy π−π overlap between adjacent molecules. The familiar herringbone packing is characterized by large roll distortions. It is shown that thiophenes tend to exhibit large roll translations, whereas thiazoles have small roll but large pitch translations. Substituted pentacenes tend to have both moderate pitch and roll distances. The relationship of molecular packing to transport properties is discussed.
A family of dimeric LnIII12‐MCGa(III)N(shi)‐4 metallacrowns (MCs) (LnIII=Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) was synthesized using the isophthalate group (ip2−) as a linker. The LnGa42 ...complexes exhibit remarkable photophysical properties, with large molar absorptivities of ≈4×104 m−1 cm−1, high quantum yields and long luminescence lifetimes with values of (i) 31.2(2)% and 1.410(1) ms, respectively for the visible‐emitting TbGa42 complex and (ii) 2.43(6)% and 30.5(1) μs for the near‐infrared (NIR) emitting YbGa42 in the solid state. The NIR emission was obtained not only from Yb, Nd, and Er complexes but also from the less frequently observed emitters such as Pr and Ho. In addition, emission in both visible and NIR domains could be detected for Dy and Sm MCs. ESI‐MS and UV/Vis data revealed that the complexes are highly stable in dimethylsulfoxide (DMSO) solution with the 1H‐ and COSY‐NMR spectra of the diamagnetic YGa42 analogue providing evidence for long‐term solution stability. This new approach allows one to construct a basis for highly luminescent MCs that may be further modified to be adapted for applications such as optical imaging.
Crowns of many colors: LnGa4 metallacrowns are superior Ln luminescent agents. The use of a bifunctional carboxylate bridging group allows for the isolation of (LnGa4)2 dimers that retain the high luminescent intensity of the parent but are solution soluble and stable.
The effect of ligands’ energy levels on thermal dependence of lanthanide emission was examined to create new molecular nanothermometers. A series of Ln2Ga8L8 ′L8 ″ metallacrowns (shorthand Ln2L8 ′), ...where Ln = Gd3+, Tb3+, or Sm3+ (H3L′ = salicylhydroxamic acid (H3shi), 5-methylsalicylhydroxamic acid (H3mshi), 5-methoxysalicylhydroxamic acid (H3moshi), and 3-hydroxy-2-naphthohydroxamic acid (H3nha)) and H2L″ = isophthalic acid (H2iph), was synthesized and characterized. Within the series, ligand-centered singlet state (S1) energy levels ranged from 23,300 to 27,800 cm–1, while triplet (T1) energy levels ranged from 18,150 to 21,980 cm–1. We demonstrated that the difference between T1 levels and relevant energies of the excited 4G5/2 level of Sm3+ (17,800 cm–1) and 5D4 level of Tb3+ (20,400 cm–1) is the major parameter controlling thermal dependence of the emission intensity via the back energy transfer mechanism. However, when the energy difference between S1 and T1 levels is small (below 3760 cm–1), the S1 → T1 intersystem crossing (and its reverse, S1 ← T1) mechanism contributes to the thermal behavior of metallacrowns. Both mechanisms affect Ln3+-centered room-temperature quantum yields with values ranging from 2.07(6)% to 31.2(2)% for Tb2L8 ′ and from 0.0267(7)% to 2.27(5)% for Sm2L8 ′. The maximal thermal dependence varies over a wide thermal range (ca. 150–350 K) based on energy gaps between relevant ligand-based and lanthanide-based electronic states. By mixing Tb2moshi8 ′ with Sm2moshi8 ′ in a 1:1 ratio, an optical thermometer with a relative thermal sensitivity larger than 3%/K at 225 K was created. Other temperature ranges are also accessible with this approach.
PDF
