X-ray analysis on the nanogram to microgram scale using porous complexes.

X-ray single-crystal diffraction (SCD) analysis has the intrinsic limitation that the target molecules must be obtained as single crystals. Here we report a protocol for SCD analysis that does not require the crystallization of the sample. In our method, tiny crystals of porous complexes are soaked in a solution of the target, such that the complexes can absorb the target molecules. Crystallographic analysis clearly determines the absorbed guest structures along with the host frameworks. Because the SCD analysis is carried out on only one tiny crystal of the complex, the required sample mass is of the nanogram-microgram order. We demonstrate that as little as about 80 nanograms of a sample is enough for the SCD analysis. In combination with high-performance liquid chromatography, our protocol allows the direct characterization of multiple fractions, establishing a prototypical means of liquid chromatography SCD analysis. Furthermore, we unambiguously determined the structure of a scarce marine natural product using only 5 micrograms of the compound.

Chiral Crystalline Sponges for the Absolute Structure Determination of Chiral Guests.

Chiral crystalline sponges with preinstalled chiral references were synthesized. On the basis of the known configurations of the chiral references, the absolute structures of guest compounds absorbed in the pores of the crystalline sponges can be reliably determined without crystallization or chemical modification.

Networked-cage microcrystals for evaluation of host-guest interactions.

We have developed a new synthetic protocol for the preparation of a microcrystalline powder (median size: X50 = 25 µm) of networked M6L4 cages 1a for the stationary phase of an affinity column on a greater than 50 g scale. Analogously to large single crystals 1b (X50 ≈ 0.5 mm), microcrystals 1a accommodate guest molecules tetrathiafulvalene (TTF) and fullerene (C60) at up to 32 and 35 wt %, respectively. Importantly, the host-guest interactions within networked cages could be evaluated in terms of the retention time from HPLC analysis by using microcrystals 1a as the stationary phase. In this way, favorable guests for networked cages 1 and even solution M6L4 cage 2 could easily be assessed by HPLC.

Bimolecular reaction via the successive introduction of two substrates into the crystals of networked molecular cages.

Two substrates, 4-hydroxydiphenylamine (3) and ethyl isocyanate (4), were successively introduced into the crystals of networked M(6)L(4) cages 1. Because of the encapsulation effect, most of the initially introduced substrate 3 remained within the crystals during immersion in a solution of 4. X-ray analysis revealed that before the reaction, the nucleophilic NH group of 3 is effectively protected by tight packing within the cage units while the OH group is exposed to the incoming second substrate. Successive introduction of 4 into the crystal results in the chemoselective acylation of 3 at the less nucleophilic OH group. The observed chemoselectivity is consistent with that exhibited by discrete M(6)L(4) cage 2 in solution.

Preparation and guest-uptake protocol for a porous complex useful for 'crystal-free' crystallography.

We recently reported a new method for single-crystal X-ray diffraction (SCD) analysis that does not require the crystallization of the target compound. In this 'crystal-free' crystallography, a tiny crystal of a porous complex is soaked in the solution of the target guest. The guest molecules are absorbed and oriented in the crystal pores and can be analyzed by X-ray diffraction. We describe here a detailed synthetic protocol for the preparation of uniform single crystals of the porous host complex and for the subsequent guest uptake. The protocol describes our most versatile porous complex, which is prepared from commercially available ZnI2 and 2,4,6-tri(4-pyridyl)-1,3,5-triazine. The host complex has large pores with a cross-section of 8 × 5 Å(2). Single crystals of the complex are grown from layered solutions of the two components. The pores of the as-synthesized complex are filled with nitrobenzene, which is replaced with the inert solvent cyclohexane. This solvent exchange is essential for the rapid and effective inclusion of target compounds. The most crucial and delicate step is the selection of high-quality single crystals from the mixture of crystals of various shapes and sizes. We suggest using the facial indices of the single crystals as a criterion for crystal selection. Single-crystal samples for X-ray analysis can be prepared by immersing the selected crystals in a cyclohexane/dichloromethane solution of target compound. After a very slow evaporation of the solvent, typically over 2 d, the final crystal can be picked and directly subjected to SCD analysis. The protocol can be completed within ∼16 d.

Networked molecular cages as crystalline sponges for fullerenes and other guests.

Many molecular cages selectively bind guests in solution, but in the solid state close packing often prevents guest entry, which renders the cages inactive. We envisioned that coordination networks constructed from well-known molecular cages could transfer the richness of solution-state host-guest chemistry into the solid state. We report a crystalline coordination network generated from an infinite array of octahedral M(6)L(4) cage subunits (M = metal, L = ligand). This coordination network is a 'crystalline molecular sponge' engineered on the molecular level and retains similar guest recognition properties to those found in solution. The network crystallinity is robust and thus X-ray diffraction analysis can be used to unambiguously observe single-crystal to single-crystal guest inclusion. The void spaces define alternating M(12)L(8) and M(12)L(24) cuboctahedral molecular cages and these large cages absorb up to 35 weight per cent of C(60) or C(70) by simply soaking the crystals in a toluene solution of the fullerene. When the crystals are immersed in fullerene mixtures, C(70) is preferentially absorbed.