Silver carboxylate salts are widely used as additives in palladium-catalyzed C–H functionalization reactions. However, the role of these silver additives is often not fully understood. This paper ...describes an investigation of the role of AgOPiv in the stoichiometric activation of C6F5H at a well-defined PdII complex as well as in the PdII-catalyzed oxidative dimerization of 2-alkylthiophenes. Both in situ NMR spectroscopy and H/D exchange studies of the reactions of C6F5H implicate a role for AgOPiv in the C–H cleavage event, generating Ag–C6F5 as an intermediate. The catalytic studies show similar trends despite the different conditions and substrates, suggesting that AgOPiv promotes a similar metalation of the thiophene in the catalytic transformations. This proposal is supported by DFT calculations, which show energetically feasible pathways for concerted metalation–deprotonation of both 2-methylthiophene and pentafluorobenzene at Ag(OPiv)2. These studies suggest that initial metalation of C–H substrates at AgI carboxylates should be considered as a plausible pathway in C–H functionalization reactions involving mixtures of Ag and Pd salts.
The existence of a haematopoietic stem cell niche as a spatially confined regulatory entity relies on the notion that haematopoietic stem and progenitor cells (HSPCs) are strategically positioned in ...unique bone marrow microenvironments with defined anatomical and functional features. Here, we employ a powerful imaging cytometry platform to perform a comprehensive quantitative analysis of HSPC distribution in bone marrow cavities of femoral bones. We find that HSPCs preferentially localize in endosteal zones, where most closely interact with sinusoidal and non-sinusoidal bone marrow microvessels, which form a distinctive circulatory system. In situ tissue analysis reveals that HSPCs exhibit a hypoxic profile, defined by strong retention of pimonidazole and expression of HIF-1α, regardless of localization throughout the bone marrow, adjacency to vascular structures or cell-cycle status. These studies argue that the characteristic hypoxic state of HSPCs is not solely the result of a minimally oxygenated niche but may be partially regulated by cell-specific mechanisms.
This Article describes the development of a stable NiIV complex that mediates C(sp2)–H trifluoromethylation reactions. This reactivity is first demonstrated stoichiometrically and then successfully ...translated to a NiIV-catalyzed C–H trifluoromethylation of electron-rich arene and heteroarene substrates. Both experimental and computational mechanistic studies support a radical chain pathway involving NiIV, NiIII, and NiII intermediates.
Amongst the global decline of coral reefs, hope spots such as Cordelia Bank in Honduras, have been identified. This site contains dense, remnant thickets of the endangered species Acropora ...cervicornis, which local managers and conservation organizations view as a potential source population for coral restoration projects. The aim of this study was to determine the genetic diversity of colonies across three banks within the protected area. We identified low genetic diversity (F
= 0.02) across the three banks, and genetic similarity of colonies ranged from 91.3 to 95.8% between the banks. Clonality rates were approximately 30% across the three banks, however, each genotype identified was unique to each bank. Despite the low genetic diversity, subtle genetic differences within and among banks were demonstrated, and these dense thickets were shown not to be comprised of a single or a few genotypes. The presence of multiple genotypes suggests A. cervicornis colonies from these banks could be used to maintain and enhance genetic diversity in restoration projects. Management of hope spots, such as Cordelia Bank, and the incorporation of genetic information into restoration projects to ensure genetic diversity within out-planted populations, will be critical in the ongoing challenge of conserving and preserving coral reefs.
This manuscript describes the design, synthesis, characterization, and reactivity studies of organometallic NiIII complexes of general structure TpNiIII(R)(R1) (Tp = tris(pyrazolyl)borate). With ...appropriate selection of the R and R1 ligands, the complexes are stable at room temperature and can be characterized by cyclic voltammetry, EPR spectroscopy, and X-ray crystallography. Upon heating, many of these NiIII compounds undergo C(sp2)–C(sp2) or C(sp3)–C(sp2) bond-forming reactions that are challenging at lower oxidation states of nickel.
The treatment of pyridine- and pyrazole-ligated NiII σ-aryl complexes with Selectfluor results in C(sp2)–F bond formation under mild conditions. With appropriate design of supporting ligands, ...diamagnetic NiIV σ-aryl fluoride intermediates can be detected spectroscopically and/or isolated during these transformations. These studies demonstrate for the first time that NiIV σ-aryl fluoride complexes participate in challenging C(sp2)–F bond-forming reductive elimination to yield aryl fluoride products.
Yellowfin tuna, Thunnus albacares, is an important global fishery and of particular importance in the Eastern Pacific Ocean (EPO). According to the 2019 Inter-American Tropical Tuna Commission ...(IATTC) assessment, yellowfin tuna within the EPO is a single stock, and is being managed as one stock. However, previous studies indicate site fidelity, or limited home ranges, of yellowfin tuna which suggests the potential for multiple yellowfin tuna stocks within the EPO, which was supported by a population genetic study using microsatellites. If numerous stocks are present, management at the wrong spatial scales could cause the loss of minor yellowfin tuna populations in the EPO. In this study we used double digestion RADseq to assess the genetic structure of yellowfin tuna in the EPO. A total of 164 yellowfin tuna from Cabo San Lucas, México, and the Galápagos Islands and Santa Elena, Ecuador, were analysed using 18,011 single nucleotide polymorphisms. Limited genetic differentiation (F.sub.ST = 0.00058-0.00328) observed among the sampling locations (México, Ecuador, Peru, and within Ecuador) is consistent with presence of a single yellowfin tuna population within the EPO. Our findings are consistent with the IATTC assessment and provide further evidence of the need for transboundary cooperation for the successful management of this important fishery throughout the EPO.
This communication describes a series of oxidatively induced intramolecular arene C–H activation reactions of NiII model complexes to yield NiIV σ-aryl products. These reactions proceed within 10 min ...at room temperature, which represents among the mildest conditions reported for C–H cleavage at a Ni center. A combination of density functional theory and preliminary experimental mechanistic studies implicate a pathway involving initial 2e– oxidation of the NiII starting materials by the F+ transfer reagent N-fluoro-2,4,6-trimethylpyridinium triflate followed by triflate-assisted C–H cleavage at NiIV to yield the products.
Coastal eutrophication is an issue of serious global concern and although nutrient subsidies can enhance primary productivity of coastal wetlands, they can be detrimental to their long-term ...maintenance. By supplying nutrients to coastal ecosystems at levels comparable to intensive agriculture practices, roosting colonial waterbirds provide a natural experimental design to examine the impacts of anthropogenic nutrient enrichment in these systems. We tested the hypothesis that long-term nutrient enrichment from bird guano deposition is linked to declines in island size, which may subsequently decrease the stability and resilience of mangrove cays in Belize. We combined remote sensing analysis with field- and lab-based measurements of forest structure, sediment nutrients, and porewater nutrients on three pairs of rookery and control cays in northern, central, and southern Belize. Our results indicate that rookery cays are disappearing approximately 13 times faster than cays without seasonal or resident seabird populations. Rookery cays were associated with a significantly higher concentration of nitrogen (N) in mangrove leaves and greater aboveground biomass, suggesting that eutrophication from bird guano contributes to increased aboveground productivity. Sediments of rookery cays also had lower percentages of soil organic matter and total N and carbon (C) than control islands, which suggests that eutrophication accelerates organic matter decomposition resulting in lower total C stocks on rookery cays. Our results indicate that coastal eutrophication can reduce ecosystem stability by contributing to accelerated cay loss, with potential consequences for mangrove resilience to environmental variability under contemporary and future climatic scenarios.
This article describes a detailed comparison of the organometallic chemistry of high-valent nickel and palladium model complexes supported by tris(pyrazolyl)borate and cycloneophyl ligands. The ...accessibility of the MIII and MIV oxidation states with each metal is investigated through electrochemical and chemical oxidation of the MII precursors. These studies show that the NiII precursor readily undergoes both one- and two-electron oxidations to generate stable NiIII and NiIV products. In contrast, under the conditions examined, the PdII analogue undergoes exclusively two-electron-oxidation reactions to form PdIV. Reactivity studies of isolated NiIV and PdIV complexes show that both participate in C(sp3)–heteroatom coupling reactions and that the reactions at NiIV are approximately 2 orders of magnitude faster than those at PdIV. Experimental and computational mechanistic studies implicate outer-sphere SN2-type pathways for these processes. With most nucleophiles (e.g., phenoxide, acetate, thiophenoxide), the C(sp3)–heteroatom coupling reaction yields a TpMII(σ-aryl) product. However, with azide as the nucleophile, the NiII product of initial C(sp3)–N3 coupling undergoes a subsequent C(sp2)–N insertion reaction. Computations implicate an anionic NiIII–nitrene intermediate in this process and show that the Pd analogue of this species is a much higher energy species. Overall, the combined experimental and computational studies demonstrate remarkable similarities in the chemistry of NiIV and PdIV but an enhanced role for NiIII in enabling reactivity which is distinct from that of palladium.
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