Post-assembly Modification of Tetrazine-Edged Fe(II)4L6 Tetrahedra.

Post-assembly modification (PAM) is a powerful tool for the modular functionalization of self-assembled structures. We report a new family of tetrazine-edged Fe(II)4L6 tetrahedral cages, prepared using different aniline subcomponents, which undergo rapid and efficient PAM by inverse electron-demand Diels-Alder (IEDDA) reactions. Remarkably, the electron-donating or -withdrawing ability of the para-substituent on the aniline moiety influences the IEDDA reactivity of the tetrazine ring 11 bonds away.

Molecular containers in complex chemical systems.

Over the last decade molecular containers have been increasingly studied within the context of complex chemical systems. Herein we discuss selected examples from the literature concerning three aspects of this field: complex host-guest behaviour, adaptive transformations of molecular containers and reactivity modulation within them.

Predicting paramagnetic 1H NMR chemical shifts and state-energy separations in spin-crossover host-guest systems.

The behaviour of metal-organic cages upon guest encapsulation can be difficult to elucidate in solution. Paramagnetic metal centres introduce additional dispersion of signals that is useful for characterisation of host-guest complexes in solution using nuclear magnetic resonance (NMR). However, paramagnetic centres also complicate spectral assignment due to line broadening, signal integration error, and large changes in chemical shifts, which can be difficult to assign even for known compounds.

Selective encapsulation and sequential release of guests within a self-sorting mixture of three tetrahedral cages.

A mixture of two triamines, one diamine, 2-formylpyridine and a Zn(II) salt was found to self-sort, cleanly producing a mixture of three different tetrahedral cages. Each cage bound one of three guests selectively. These guests could be released in a specific sequence following the addition of 4-methoxyaniline, which reacted with the cages, opening each in turn and releasing its guest.

Aqueous anion receptors through reduction of subcomponent self-assembled structures.

To prepare new functional covalent architectures that are difficult to synthesize using conventional organic methods, we developed a strategy that employs metal-organic assemblies as precursors, which are then reduced and demetalated. The host-guest chemistry of the larger receptor thus prepared was studied using NMR spectroscopy and fluorescence experiments. This host was observed to strongly bind aromatic polyanions in water, including the fluorescent dye molecule pyranine with nanomolar affinity, thus allowing for the design of an indicator-displacement assay.

A self-organizing chemical assembly line.

Chemical syntheses generally involve a series of discrete transformations whereby a simple set of starting materials are progressively rendered more complex. In contrast, living systems accomplish their syntheses within complex chemical mixtures, wherein the self-organization of biomolecules allows them to form "assembly lines" that transform simple starting materials into more complex products. Here we demonstrate the functioning of an abiological chemical system whose simple parts self-organize into a complex system capable of directing the multistep transformation of the small molecules furan, dioxygen, and nitromethane into a more complex and information-rich product.

Guest binding subtly influences spin crossover in an FeII₄L₄ capsule.

How much should we switch? Two FeII₄L₄ tetrahedral capsules were shown to undergo thermally induced spin crossover (SCO). Guest binding to one of these capsules was observed to affect the thermodynamics of its SCO in solution, leading to different spin transition temperatures between the empty host (blue) and the host-guest complex (red). HS: high spin; LS: low spin.

Quantitative understanding of guest binding enables the design of complex host-guest behavior.

We report a detailed binding study addressing both the thermodynamics and kinetics of binding of a large set of guest molecules with widely varying properties to a water-soluble M4L6 metal-organic host. The effects of different guest properties upon the binding strength and kinetics were elucidated by a systematic analysis of the binding data through principal component analysis, thus allowing structure-property relationships to be determined. These insights enabled us to design more complex encapsulation sequences in which multiple guests that were added simultaneously were bound and released by the host in a time-dependent manner, thus allowing multiple states of the system to be accessed sequentially.

Chain-reaction anion exchange between metal-organic cages.

Differential binding affinities for a set of anions were observed between larger (1) and smaller (2) tetrahedral metal-organic capsules in solution. A chemical network could thus be designed wherein the addition of hexafluorophosphate could cause perchlorate to shift from capsule 2 to capsule 1 and triflimide to be ejected from capsule 1 into solution.

Metal-organic container molecules through subcomponent self-assembly.

A variety of different three-dimensional metal-organic container molecules have recently been prepared using subcomponent self-assembly, which relies upon metal template effects to generate complex structures from simple molecular precursors and metal salts. Many of these structures have well defined internal pockets, allowing guest species to be bound and the chemical reactivity of these guests to be modified. Such host molecules have potential applications ranging from the protection of sensitive chemical species to the separation and purification of substrates as diverse as gases, gold compounds, and fullerenes.

Guanidinium binding modulates guest exchange within an [M4L6] capsule.

Take it slow! A metal-organic container molecule has been shown to bind guanidinium cations (blue) between the sulfonate groups on its periphery, as well as accommodating guests such as cyclopentane and cyclohexane in its internal cavity (red). Kinetic studies on the system demonstrated a linear relationship between the amount of bound guanidinium ions and the rate of guest exchange.

Surface decorated platinum carbonyl clusters.

Four molecular Pt-carbonyl clusters decorated by Cd-Br fragments, i.e., [Pt(13)(CO)(12){Cd(5)(μ-Br)(5)Br(2)(dmf)(3)}(2)](2-) (1), [Pt(19)(CO)(17){Cd(5)(μ-Br)(5)Br(3)(Me(2)CO)(2)}{Cd(5)(μ-Br)(5)Br(Me(2)CO)(4)}](2-) (2), [H(2)Pt(26)(CO)(20)(CdBr)(12)](8-) (3) and [H(4)Pt(26)(CO)(20)(CdBr)(12)(PtBr)(x)](6-) (4) (x = 0-2), have been obtained from the reactions between [Pt(3n)(CO)(6n)](2-) (n = 2-6) and CdBr(2)·H(2)O in dmf at 120 °C.

Electronic stabilization of trigonal bipyramidal clusters: the role of the Sn(II) ions in [Pt5(CO)5{Cl2Sn(μ-OR)SnCl2}3]3- (R = H, Me, Et, iPr).

The new [Pt(5)(CO)(5){Cl(2)Sn(μ-OR)SnCl(2)}(3)](3-) (R = H, Me, Et, (i)Pr; 1-4) clusters contain trigonal bipyramidal (TBP) Pt(5)(CO)(5) cores, as certified by the X-ray structures of [Na(CH(3)CN)(5)][NBu(4)](2)[1]·2CH(3)CN and [PPh(4)](3)[4]·3CH(3)COCH(3). The TBP geometry, which is rare for group 10 metals, is supported by an unprecedented interpenetration with a nonbonded trigonal prism of tin atoms. By capping all the Pt(3) faces, the Sn(II) lone pairs account for both Sn-Pt and Pt-Pt bonding, as indicated by DFT and topological wave function studies.

Icosahedral Pt-centered Pt13 and Pt19 carbonyl clusters decorated by [Cd5(μ-Br)5Br(5-x)(solvent)x]x+ rings reminiscent of the decoration of Au-Fe-CO and Au-thiolate nanoclusters: a unifying approach to their electron counts.

The new [Pt(13)(CO)(12){Cd(5)(μ-Br)(5)Br(2)(dmf)(3)}(2)](2-) and [Pt(19)(CO)(17){Cd(5)(μ-Br)(5)Br(3)(Me(2)CO)(2)}{Cd(5)(μ-Br)(5)Br(Me(2)CO)(4)}](2-) clusters have been obtained in good yields by reaction of [Pt(12)(CO)(24)](2-) with CdBr(2)·H(2)O in dmf at 90 °C and structurally characterized by X-ray diffraction. Their structures consist of a Pt-centered Pt(13)(CO)(12) icosahedron and a Pt(19)(CO)(17) interpenetrated double icosahedron, respectively, decorated by two Cd(5)(μ-Br)(5)Br(5-x)(solvent)(x) rings. Their surface decoration may be related to that of Au-Fe-CO clusters as well as to the staple motifs stabilizing gold-thiolates nanoclusters.

New findings in the chemistry of iron carbonyls: the previously unreported [H(4-n)Fe4(CO)12]n- (n = 1, 2) series of clusters, which fills the gap with ruthenium and osmium.

The new [HFe(4)(CO)(12)](3-) cluster anion has been obtained in high yields by reduction of [Fe(4)(CO)(13)](2-) or [HFe(3)(CO)(11)](-) with a 6 M methylalcoholic KOH solution under a nitrogen atmosphere and isolated with miscellaneous tetrasubstituted ammonium salts. The [NEt(4)](3)[HFe(4)(CO)(12)] salt has been characterized by IR, (1)H and (13)C NMR, electrospray ionization mass spectrometry, and X-ray studies. Investigation of its protonation reaction afforded spectroscopic proof for the existence of its unstable isomeric [HFe(4)(CO)(11)(CO-H)](2-) and [H(2)Fe(4)(CO)(12)](2-) conjugated acids.