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.

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. This effect manifests as a linear free energy relationship, quantified using the Hammett equation, between σ(para) and the rate of the IEDDA reaction. The rate of PAM can thus be adjusted by varying the aniline subcomponent.

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. Quantum chemical predictions can provide information that greatly facilitates the assignment of NMR signals and identification of species present. Here we explore how the prediction of paramagnetic NMR spectra may be used to gain insight into the spin crossover (SCO) properties of iron(II)-based metal organic coordination cages, specifically examining how the structure of the local metal coordination environment affects SCO. To represent the tetrahedral metal-organic cage, a model system is generated by considering an isolated metal-ion vertex: fac-ML3(2+) (M = Fe(II), Co(II); L = N-phenyl-2-pyridinaldimine). The sensitivity of the (1)H paramagnetic chemical shifts to local coordination environments is assessed and utilised to shed light on spin crossover behaviour in iron complexes. Our data indicate that expansion of the metal coordination sphere must precede any thermal SCO. An attempt to correlate experimental enthalpies of SCO with static properties of bound guests shows that no simple relationship exists, and that effects are likely due to nuanced dynamic response to encapsulation.

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.

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. The system here described thus behaved in an organized way in three distinct contexts: cage formation, guest encapsulation, and guest release. Such behavior could be used in the context of a more complex system, where released guests serve as signals to other chemical actors.

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. Moreover, by inclusion of the pH-sensitive guest pyridine, we were able to further extend our control over the binding by creating a reversible pH-controlled three-guest sequential binding cycle.

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. The novel use of a self-assembling container molecule to catalytically transform a high-energy intermediate is central to the system's functioning.

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.

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.

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.

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. An oversimplified and unifying approach to interpret their electron count is suggested.

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. In the TBP interactions, the metals use their vacant s and p orbitals using the electrons provided by Sn atoms, hence mimicking the electronic picture of main group analogues, which obey the Wade's rule. Other metal TBP clusters with the same total electron count (TEC) of 72 are different because the skeletal bonding is largely contributed by d-d interactions (e.g., [Os(5)(CO)(14)(PR(3))(µ-H)(n)](n-2), n = 0, 1, 2). In 1-4, fully occupied d shells at the Pt(ax) atoms exert a residual nucleophilicity toward the adjacent main group Sn(II) ions permitting their hypervalency through unsual metal donation.

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. The latter is probably isostructural with the [H(2)Ru(4)(CO)(12)](2-) congener. The nature of the first protonation product as a [HFe(4)(CO)(11)(CO-H)](2-) adduct, involving an oxygen-bound proton, has been corroborated by the preparation and spectroscopic characterization of the corresponding [HFe(4)(CO)(11)(CO-Me)](2-) dianion. The above findings demonstrate that protonation of a CO-shielded polynuclear metal anion initially occurs on one oxygen atom and then the oxygen-bound proton migrates to the metal cage. Finally, [HFe(4)(CO)(12)](3-) and its [H(2)Fe(4)(CO)(12)](2-) conjugate acid fill the previously existing gap between the chemistry of iron carbonyls and ruthenium and osmium congeners.

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.

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. The structures of these molecular clusters with diameters of 1.5-2 nm have been determined by X-ray crystallography. Both 1 and 2 are composed of icosahedral or bis-icosahedral Pt-CO cores decorated on the surface by Cd-Br motifs, whereas 3 and 4 display a cubic close packed Pt(26)Cd(12) metal frame decorated by CO and Br ligands. An oversimplified and unifying approach to interpret the electron count of these surface decorated platinum carbonyl clusters is suggested, and extended to other low-valent organometallic clusters and Au-thiolate nanoclusters.