Alexander J. Metherell

A tetrameric hetero-octanuclear cyclic helicate formed from a bridging ligand with two inequivalent binding sites

A bis-bidentate bridging ligand H2L with inequivalent hard and soft binding sites (catecholate and pyrazolyl-pyridine, respectively) reacts with a mixture of Ti(IV) and Zn(II) ions to afford an octanuclear heterometallic Ti4Zn4 cyclic helicate formed from four dinuclear {TiZn(μ-L)2} units connected in a ring via methoxide ions.

Stepwise synthesis of mixed-metal assemblies using pre-formed Ru( ii ) ‘complex ligands’ as building blocks

Two families of heteronuclear coordination complexes have been prepared in a stepwise manner using pre-formed, kinetically inert [RuL3]2+ building blocks, in which L is a bis-bidentate bridging ligand with two pyrazole–pyridyl termini, coordinated at one end to the Ru(II) centre. These pre-formed ‘complex ligands’ – with three pendant binding sites – react with additional labile transition metal dications to complete the stepwise assembly of mixed-metal arrays in which labile [Co(II)/Cd(II)] or inert [Ru(II)] ions strictly alternate around the framework. When L = the thiophene-2,5-diyl spaced ligand Lth, the complex [Ru(Lth)3]2+ is formed in the expected 3:1 mer:fac ratio: reaction with labile Co(II) or Cd(II) ions completes formation of a heteronuclear square [Ru2Co2(Lth)6]8+ or one-dimensional coordination polymer {[CdRu(Lth)3]4+}∞, respectively. In these only the mer isomer of [Ru(Lth)3]2+ is selected by the self-assembly process, whereas the fac isomer is not used. When L = a 1,3-benzene-diyl spaced ligand (Lph), the complex ligand [Ru(Lph)3]2+ formed in the initial step is enriched in mer isomer (80–87% mer, depending on reaction conditions). Two quite different products were isolated from reaction of [Ru(Lph)3]2+ with Co(II) depending on the conditions. These are the rectangular, hexanuclear ‘open-book’ array [Ru3Co3(Lph)9]12+ which contains a 2:1 proportion of fac/mer Ru(II) metal centres; and the octanuclear cubic [Ru4Co4(Lph)12{Na(BF4)4}]13+ cage which is a new structural type containing all mer Ru(II) vertices and all fac Co(II) vertices. The cavity of this cubic cage contains a tetrahedral array of fluoroborate anions which in turn coordinate to a central Na(I) ion – an unusual example of a metal complex [Na(BF4)4]3− acting as the guest inside the cage-like metal complex [Ru4Co4(Lph)12]16+.

Converting an intensity-based sensor to a ratiometric sensor: luminescence colour switching of an Ir/Eu dyad upon binding of a V-series chemical warfare agent simulant

Interaction of the V-series chemical warfare agent simulant ‘VO’ with Eu(III) results in selective quenching of the red Eu-based emission component from a dual-luminescent (blue/red) Ir(III)/Eu(III) dyad, resulting in a colour change in the luminescence from red to blue in the presence of the simulant.

Catalysis in a Cationic Coordination Cage Using a Cavity-Bound Guest and Surface-Bound Anions: Inhibition, Activation, and Autocatalysis

The Kemp elimination (reaction of benzisoxazole with base to give 2-cyanophenolate) is catalyzed in the cavity of a cubic M8L12 coordination cage because of a combination of (i) benzisoxazole binding in the cage cavity driven by the hydrophobic effect, and (ii) accumulation of hydroxide ions around the 16+ cage surface driven by ion-pairing. Here we show how reaction of the cavity-bound guest is modified by the presence of other anions which can also accumulate around the cage surface and displace hydroxide, inhibiting catalysis of the cage-based reaction. Addition of chloride or fluoride inhibits the reaction with hydroxide to the extent that a new autocatalytic pathway becomes apparent, resulting in a sigmoidal reaction profile. In this pathway the product 2-cyanophenolate itself accumulates around the cationic cage surface, acting as the base for the next reaction cycle. The affinity of different anions for the cage surface is therefore 2-cyanophenolate (generating autocatalysis) > chloride > fluoride (which both inhibit the reaction with hydroxide but cannot deprotonate the benzisoxazole guest) > hydroxide (default reaction pathway). The presence of this autocatalytic pathway demonstrates that a reaction of a cavity-bound guest can be induced with different anions around the cage surface in a controllable way; this was confirmed by adding different phenolates to the reaction, which accelerate the Kemp elimination to different extents depending on their basicity. This represents a significant step toward the goal of using the cage as a catalyst for bimolecular reactions between a cavity-bound guest and anions accumulated around the surface.

Coordination chemistry of an amine-substituted bis(pyrazolyl)-pyridine ligand: interaction of a peripheral functional group on a coordination cage with the internal contents of the cavity

Abstract The ligand L an contains two bidentate chelating pyrazolyl-pyridine termini connected to a central aminophenyl ring. In [Ag3(L an )2](ClO4)3 each ligand coordinates in a pentadentate 2+1+2 manner, connecting all three Ag(I) ions. In contrast, in the octanuclear cubic coordination cage [Co8(L an )12][BF4]16 each L an coordinates as a bis(pyrazolyl-pyridine) tetradentate chelate, with the externally-directed amine groups not coordinated. From its 1H NMR spectrum this cage has the S 6 symmetric structure containing two fac and six mer tris-chelate Co(II) centres of other octanuclear cages. Slow crystallisation afforded [Co8(L an )12Na][BF4]17, with S 4 symmetry arising from four fac and four mer tris-chelate Co(II) centres which alternate; the central cavity accommodates an Na+ cation and four fluoroborate anions which form a {Na(BF4)4}3– guest. The {Na(BF4)4}3– guest forms F•••HN hydrogen bonds through the cage windows with exterior amine groups. Rearrangement of the cage structure from S 6 to S 4 symmetry appears to facilitate inclusion of the fourfold symmetric guest.