An octanuclear M8L12 coordination cage catalyses the Kemp elimination reaction of 5-nitro-1,2-benzisoxazole (NBI) with hydroxide to give 2-cyano-4-nitrophenolate (CNP) as the product. In contrast to the previously-reported very efficient catalysis...
A heterometallic
octanuclear coordination cage [Os4Zn4(Lnap)12]X16 (denoted Os•Zn; X = perchlorate or chloride) has been prepared (Lnap is a bis-bidentate bridging ligand containing two pyrazolyl–pyridine
chelating units separated by a 1,5-naphthalenediyl spacer group).
The {Os(NN)3}2+ units located at four of the
eight vertices of the cube have a long-lived, phosphorescent 3MLCT excited state which is a stronger electron donor than
[Ru(bipy)3]2+. The chloride form of Os•Zn is water-soluble and binds in its central cavity the hydrophobic
electron-accepting organic guests 1,2,4,5-tetracyanobenzene, 1,4-naphthoquinone
and 1-nitronaphthalene, with binding constants in the range 103–104 M–1, resulting in
quenching of the phosphorescence arising from the Os(II) units. A
crystal structure of an isostructural Co8 cage containing
one molecule of 1,2,4,5-tetracyanobenzene as a guest inside the cavity
has been determined. Ultrafast transient absorption measurements show
formation of a charge-separated Os(III)/guest•– state arising from cage-to-guest photoinduced electron transfer;
this state is formed within 13–21 ps, and decays on a time
scale of ca. 200 ps. In the presence of a competing guest with a large
binding constant (cycloundecanone) which displaces each electron-accepting
quencher from the cage cavity, the charge-separated state is no longer
observed. Further, a combination of mononuclear {Os(NN)3}2+ model complexes with the same electron-accepting species
showed no evidence for formation of charge-separated Os(III)/guest•– states. These two control experiments indicate
that the {Os(NN)3}2+ chromophores need to be
assembled into the cage structure to bind the electron-accepting guests,
and for PET to occur. These results help to pave the way for use of
photoactive coordination cages as hosts for photoredox catalysis reactions
on bound guests.
Coordination cages are well-known to act as molecular containers that can bind small-molecule guests in their cavity. Such cavity binding is associated with interactions of the guests with the surrounding set of surfaces that define the cavity; a guest that is a good fit for the cavity will have many favourable interactions with the interior surfaces of the host. As cages have exterior as well as interior surfaces, possibilities also exist for ‘guests’ that are not well-bound in the cavity to interact with the exterior surface of the cage where spatial constraints are fewer. In this paper, we report a combined solid-state and solution study using an octanuclear cubic M8L12 coordination cage which illustrates the occurrence of both types of interaction. Firstly, crystallographic studies show that a range of guests bind inside the cavity (either singly or in stacked pairs) and/or interact with the cage exterior surface, depending on their size. Secondly, fluorescence titrations in aqueous solution show how some flexible aromatic disulfides show two separate types of interaction with the cage, having different spectroscopic consequences; we ascribe this to separate interactions with the exterior surface and the interior surface of the host cage with the former having a higher binding constant. Overall, it is clear that the idea of host/guest interactions in molecular containers needs to take more account of external surface interactions as well as the obvious cavity-based binding.
<p>Water-dispersible porous polymeric dispersions (PPDs) have been
synthesised by reversible addition-fragmentation chain transfer mediated
polymerisation-induced self-assembly (RAFT-mediated PISA). The core-shell
particles posses a microporous core formed from divinylbenzene and
fumaronitrile while the outer polyethylene glycol shell enables the particles
to be dispersible in a wide range of solvents. The PPD was shown to have a
heirarchical structure of small primary nanoparticles within larger, well-defined
aggregates of 220 nm as measured by electron microscopy and small angle x-ray
scattering (SAXS) and exhibited a surface area of 274 m<sup>2</sup>/g.
Furthermore these samples were found to be fluoresent and demonstrate selective
detection of harmful nitroaramatics in solution with extremly low limits of
detection, 169 ppb for picric acid, as well as possessing a CO<sub>2</sub>
uptake of 1.1 mmol/g at 273 K.</p>
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