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Design of higher valency in covalent organic frameworks

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Higher-valency ligands for COFs

Metal-organic frameworks (MOFs) have exhibited more extensive connectivity (valency) and topological diversity than covalent organic frameworks (COFs), mainly because MOF linkers can connect from 3 to 24 discrete units or even infinity for one-dimensional rods. For COFs, linkers generally have a valency of 3 or 4 that reflect the valency of organic carbon. Gropp et al. created cubane-like linkers from 1,4-boronophenylphosphonic acid that could condense to make COFs with a valency of 8 or, by adding acid, could form large, single crystals with an infinite-rod topology.

Science, this issue p. eabd6406

Structured Abstract

INTRODUCTION

Valency is the connectivity of building units in reticular frameworks. Although metal-organic frameworks (MOFs) are known to have valencies of 3 to 24, covalent organic frameworks (COFs) are limited to the lower valencies of 3 and 4, principally owing to the heavy reliance of organic chemistry on sp2 and sp3 hybridization. We show that the diversity of COFs can be increased by finding new ways of linking simple organic molecules into building units of higher and even infinite valencies.

RATIONALE

Now, COFs are made by connecting preformed trigonal-planar, square-planar, and tetrahedral organic building units with linkages of low valency. Our strategy is to implement higher valency in COFs by designing molecules capable of forming higher-valency linkages through clustering. This is difficult to achieve by conventional organic methods. We therefore relied on the isoelectronic replacement of carbon-group elements by boron and phosphorus and demonstrated the feasibility of this chemistry by using borophosphonate for the silicate cube motif. We designed simple organic BPA linkers (BPA-1 to 5) based on boron and phosphorus, which self-condensed into cubic units and polycubane COFs of valency 8. The versatility of this chemistry was further exploited by cleaving the cubes in the polycubane COFs, leading to structures with rod units of infinite valency.

RESULTS

The BPA-1 linker combines a boronic acid and a phosphonic acid functionality in a single molecule, which was converged into the boron-phosphorus (BP) cube of a reticulated polycubane BP-COF-1. Specifically, eight BPA-1 linkers self-condensed to form the BP cube with the elimination of eight water molecules per cube. The crystallization of BP-COF-1 was realized through microscopic reversibility: The polarized B–O–P linkage can dynamically form and break at the B–O bond. This constituted new chemistry whereby simple linkers converged into frameworks composed of higher-valency clusters. The versatility of this strategy was demonstrated by the successful crystallization of isoreticular polycubane structures, BP-COF-2 to 5 from the BPA-2 to 5 linkers. This series of functionalized and expanded polycubane structures exhibited permanent porosity. We found that upon addition of acid to BP-COF-1, eight B–O bonds per cube were cleaved and rearranged into BP-COF-6, which exhibits rods of infinite valency. The structure of BP-COF-6 was obtained from single-crystal x-ray diffraction, making it one of the few COFs grown as large crystals. It revealed infinite B–O–P rods linked by phenyl units to form layers, which in the crystal were joined by interstitial water molecules acting to stabilize an otherwise unusual rod-within-layer arrangement.

CONCLUSION

The chemistry of BPA linkers and their demonstrated ability to form cubes and higher-valency structures expand the scope of COFs. In essence, BPA linkers were indispensable in making linkages in the form of cubes and rods of higher valency and their corresponding extended BP-COF structures, thereby opening opportunities for the diversification of COFs through clustering of organic molecules based on sp2 and sp3-hybridized atoms. The extension of this approach to COFs containing combinations of different shapes promises to uncover what we anticipate is a large, untapped structure space.

Design of higher valency in COFs from single organic linkers.

(A) BPA linkers (BPA-1 to 5) combine a boronic and a phosphonic acid functionality in a single molecule. (B and C) Self-condensation of BPA-1 (B) afforded BP cubes of valency 8 (C) and their reticulated polycubane BP-COF-1. Isoreticular forms (BP-COF-2 to 5) demonstrated the versatility of this chemistry. (D) Upon addition of acid, BP-COF-1 rearranged into the layered BP-COF-6, which exhibits rods of infinite valency.

Abstract

The valency (connectivity) of building units in covalent organic frameworks (COFs) has been primarily 3 and 4, corresponding to triangles and squares or tetrahedrons, respectively. We report a strategy for making COFs with valency 8 (cubes) and “infinity” (rods). The linker 1,4-boronophenylphosphonic acid—designed to have boron and phosphorus as an isoelectronic combination of carbon-group elements—was condensed into a porous, polycubane structure (BP-COF-1) formulated as (–B4P4O12–)(–C6H4–)4. It was characterized by x-ray powder diffraction techniques, which revealed cubes linked with phenyls. The isoreticular forms (BP-COF-2 to 5) were similarly prepared and characterized. Large single crystals of a constitutionally isomeric COF (BP-COF-6), composed of rod units, were also synthesized using the same strategy, thus propelling COF chemistry into a new valency regime.

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Science

Too bright to breed

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Night light from coastal cities overpowers natural signals for coral spawning from neighboring reefs.

PHOTO: NOKURO/ALAMY STOCK PHOTO

Most coral species reproduce through broadcast spawning. For such a strategy to be successful, coordination has had to evolve such that gametes across clones are released simultaneously. Over millennia, lunar cycles have facilitated this coordination, but the recent development of bright artificial light has led to an overpowering of these natural signals. Ayalon et al. tested for the direct impact of different kinds of artificial light on different species of corals. The authors found that multiple lighting types, including cold and warm light-emitting diode (LED) lamps, led to loss of synchrony and spawning failure. Further, coastal maps of artificial lighting globally suggest that it threatens to interfere with coral reproduction worldwide and that the deployment of LED lights, the blue light of which penetrates deeper into the water column, is likely to make the situation even worse.

Curr. Biol. 10.1016/j.cub.2020.10.039 (2020).

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SpaceX launches Starlink app and provides pricing and service info to early beta testers

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SpaceX has debuted an official app for its Starlink satellite broadband internet service, for both iOS and Android devices. The Starlink app allows users to manage their connection – but to take part you’ll have to be part of the official beta program, and the initial public rollout of that is only just about to begin, according to emails SpaceX sent to potential beta testers this week.

The Starlink app provides guidance on how to install the Starlink receiver dish, as well as connection status (including signal quality), a device overview for seeing what’s connected to your network, and a speed test tool. It’s similar to other mobile apps for managing home wifi connections and routers. Meanwhile, the emails to potential testers that CNBC obtained detail what users can expect in terms of pricing, speeds and latency.

The initial Starlink public beta test is called the “Better than Nothing Beta Program,” SpaceX confirms in their app description, and will be rolled out across the U.S. and Canada before the end of the year – which matches up with earlier stated timelines. As per the name, SpaceX is hoping to set expectations for early customers, with speeds users can expect ranging from between 50Mb/s to 150Mb/s, and latency of 20ms to 40ms according to the customer emails, with some periods including no connectivity at all. Even with expectations set low, if those values prove accurate, it should be a big improvement for users in some hard-to-reach areas where service is currently costly, unreliable and operating at roughly dial-up equivalent speeds.

Image Credits: SpaceX

In terms of pricing, SpaceX says in the emails that the cost for participants in this beta program will be $99 per moth, plus a one-time cost of $499 initially to pay for the hardware, which includes the mounting kit and receiver dish, as well as a router with wifi networking capabilities.

The goal eventually is offer reliably, low-latency broadband that provides consistent connection by handing off connectivity between a large constellation of small satellites circling the globe in low Earth orbit. Already, SpaceX has nearly 1,000 of those launched, but it hopes to launch many thousands more before it reaches global coverage and offers general availability of its services.

SpaceX has already announced some initial commercial partnerships and pilot programs for Starlink, too, including a team-up with Microsoft to connect that company’s mobile Azure data centers, and a project with an East Texas school board to connect the local community.

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Erratum for the Report “Meta-analysis reveals declines in terrestrial but increases in freshwater insect abundances” by R. Van Klink, D. E. Bowler, K. B. Gongalsky, A. B. Swengel, A. Gentile, J. M. Chase

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S. Rennie, J. Adamson, R. Anderson, C. Andrews, J. Bater, N. Bayfield, K. Beaton, D. Beaumont, S. Benham, V. Bowmaker, C. Britt, R. Brooker, D. Brooks, J. Brunt, G. Common, R. Cooper, S. Corbett, N. Critchley, P. Dennis, J. Dick, B. Dodd, N. Dodd, N. Donovan, J. Easter, M. Flexen, A. Gardiner, D. Hamilton, P. Hargreaves, M. Hatton-Ellis, M. Howe, J. Kahl, M. Lane, S. Langan, D. Lloyd, B. McCarney, Y. McElarney, C. McKenna, S. McMillan, F. Milne, L. Milne, M. Morecroft, M. Murphy, A. Nelson, H. Nicholson, D. Pallett, D. Parry, I. Pearce, G. Pozsgai, A. Riley, R. Rose, S. Schafer, T. Scott, L. Sherrin, C. Shortall, R. Smith, P. Smith, R. Tait, C. Taylor, M. Taylor, M. Thurlow, A. Turner, K. Tyson, H. Watson, M. Whittaker, I. Woiwod, C. Wood, UK Environmental Change Network (ECN) Moth Data: 1992-2015, NERC Environmental Information Data Centre (2018); .

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