Heme-Like Coordination Chemistry Within Nanoporous Molecular Crystals

Abstract

Crystal engineering of nanoporous structures has not yet exploited the heme motif so widely found in proteins. Here, we report that a derivative of iron phthalocyanine, a close analog of heme, forms millimeter-scale molecular crystals that contain large interconnected voids (8 cubic nanometers), defined by a cubic assembly of six phthalocyanines. Rapid ligand exchange is achieved within these phthalocyanine nanoporous crystals by single-crystal-to-single-crystal (SCSC) transformations. Differentiation of the binding sites, similar to that which occurs in hemoproteins, is achieved so that monodentate ligands add preferentially to the axial binding site within the cubic assembly, whereas bidentate ligands selectively bind to the opposite axial site to link the cubic assemblies. These bidentate ligands act as molecular wall ties to prevent the collapse of the molecular crystal during the removal of solvent. The resulting crystals possess high surface areas (850 to 1000 square meters per gram) and bind N2 at the equivalent of the heme distal site through a SCSC process characterized by x-ray crystallography.

Porous organic molecular solids by dynamic covalent scrambling

“…Unlike porous molecular crystals [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] , these amorphous solids comprise a distribution of molecular species, which can be controlled by the diamine feed ratio (Fig. 3a).…”

“…That is, pores are synthetically prefabricated in the molecules and can be identified by viewing the structure of an isolated molecule. Metal organic polyhedra 25 and heme-like coordination crystals 26 also exhibit intrinsic porosity of this type. Other systems have 'extrinsic' porosity [15][16][17]19 and the pore structure arises purely from the molecular packing.…”

“…There has been less focus here in comparison with the preparation of ordered porous solids using crystal engineering [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] . Polymers of intrinsic microporosity (PIMs), for example, form amorphous microporous solids-that is, amorphous solids with pore dimensions smaller than 2 nm-as a result of their rigid and contorted chain structures 30,31 .…”

Modular and predictable assembly of porous organic molecular crystals

“…This is a potential limitation for 'one-pot' chemical syntheses of porous frameworks from simple starting materials. An alternative strategy is to prepare porous solids from synthetically preorganised molecular pores [9][10][11][12][13][14][15] . In principle, functional organic pore modules could be covalently prefabricated and then assembled to produce materials with specific properties.…”

Porphyrin-Based Metal Organic Frameworks: Unusual examples of Mn(II) carboxylate frameworks containing free-base porphyrins.

“…The incorporation of porphyrins into porous materials [5] and MOFs [6] has received attention over a number of years because of the potential exploitation of the interesting properties, photochemical and electrochemical, that porphyrins and related compounds possess. We have previously studied the use of tetrapyridylporphyrin species, in particular zinc[tetra(4-pyridyl)porphyrin] [7], in forming coordination polymers, as have others [8], and have now broadened our studies to carboxylic acid functionalised porphyrin species.…”