Heme-like coordination chemistry within nanoporous molecular crystals

Crystal engineering of nanoporous structures has not yet exploited the heme motif so widely found in proteins. X-ray diffraction (XRD) analysis on Beamline I19 found that metal complexes of a phthalocyanine, a close analog of heme, forms molecular crystals that contain large interconnected voids (8 nm3), defined by a cubic assembly of six phthalocyanines. Rapid ligand exchange at the metal centres is achieved within these phthalocyanine nanoporous crystals (PNCs) by single-crystal-to-single-crystal (SCSC) transformations. Differentiation of the metal cation’s two axial 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 m2 g-1) and bind N2 at the equivalent of the heme distal site through a SCSC process characterized by X-ray crystallography.

Figure 1: (a) The molecular structures of the phthalocyanine complexes (M = metal cation Mg2+, Al3+, Ti4+, Mn2+, Fe2+, Co2+, Zn2+, Ru2+, In3+) and (b) a selection of axial ligands (L) that are compatible with the formation of the PNCs.

The recent progress in the development of novel nanoporous materials, such as the metal-organic frameworks (MOFs), has produced numerous examples incorporating a metalloporphyrin component. However, despite the obvious motivation to prepare nanoporous materials that contain the heme moiety, no such material containing an iron porphyrin has emerged, perhaps due to the highly reactive nature of the iron cation that may interfere with the coordination chemistry required for MOF formation. Several years ago we discovered that molecular crystals of zinc 2,3,9,10,16,17,23,24-octa(2’,6’-di-iso-propylphenoxy)phthalocyanine contain very large (8 nm3) solvent-filled voids [1]. Due to the close structural relationship between the phthalocyanine and porphyrin macrocycles and, importantly, the similar coordination chemistry of iron phthalocyanine to that of iron porphyrin, we thought that such phthalocyanine-based molecular crystals might provide alternative routes to nanoporous models of the heme reactive site. Hence, the analogous iron phthalocyanine (Fig. 1) was prepared from 4,5-(2’,6’-di-iso-propylphenoxy)phthalonitrile and ferrous acetate and, pleasingly, single crystal x-ray diffraction (XRD) analysis confirmed this novel complex gave a phthalocyanine nanoporous crystal (PNC), isomorphic with that of the Zn2+ complex (i.e. cubic symmetry, space group = Pn-3n, a = 3.74 ± 0.03 nm, Z = 12) on recrystallisation. It is well established that even minor structural modifications to molecules can alter markedly the subsequent packing arrangement within a molecular crystal. Therefore, it is remarkable that the PNC structure was obtained for many different metal complexes of this particular phthalocyanine derivative (Fig. 1; M = Mg2+, Al3+, Ti4+, Mn2+, Fe2+, Co2+, Zn2+, Ru2+, In3+), even with great variation in size, shape, type and number of axial ligand (L).

The nanoporous structure of the PNCs (Fig. 2a) resembles Schoen’s I-WP triply periodic minimal surface in which free volume is unequally partitioned between two interpenetrating labyrinths by a non-self-intersecting, two-sided surface. The larger labyrinth is here termed ‘void’ and is composed of the 8 nm3 voids inside the cubic assembly of six phthalocyanines and the interconnecting channels located at each corner of the assembly; the smaller labyrinth is termed ‘cavity’ and is composed of the narrow interconnecting channels that lie between the assemblies. For each phthalocyanine complex, the PNC structure differentiates the two axial binding sites of the metal cation so that one faces into the void and the other faces into the cavity.

Figure 2: (A) The nanoporous structure of a PNC as represented by Schoen’s I-WP triply periodic minimal surface with the transition metal cations (e.g. Fe2+) denoted as M. The void side of the surface is shaded yellow and the cavity side is shaded grey. (B) The crystal structure of a PUNC with the phenoxyl substituents removed and the carbon atoms of the pdic molecular wall-ties colored green for clarity. (C) A cross-section through a cavity brick wall showing the role of wall-ties in maintaining stability. The location of the analogous bidentate ligand molecular wall-tie within the cavity of the PNC is indicated in B.

The original solvent of recrystallisation within the PNCs (methanol at 20% by mass) can be rapidly and reversibly exchanged with other solvents such as acetone, hexane or water without loss of crystalline order. In addition, rapid exchange of the axial ligands attached to the metal cation (Fig. 1b) is achieved by the simple addition of a drop of the new ligand to the solvent in contact with a crystal. These single-crystal-to-single-crystal transformations are readily monitored by XRD analysis on Beamline I19 where the high resolution afforded by synchrotron radiation allows characterisation of the axial sites of the iron cation. For example, the BuNC ligand of the iron complex can be replaced by N-methylimidazole with much the same ease as the solution-based reaction presumably due to the lack of structural reorganisation required to accommodate even large ligands within the nanoporous crystal. Previously, such direct observation of axial ligand exchange within a MOF or other nanoporous crystal has been elusive. The ease of ligand exchange suggested the possibility that bidentate ligands of an appropriate length (~1 nm) might bind simultaneously to two iron cations across the cavity between the hexa-phthalocyanine assemblies. This outcome was achieved (Fig. 2b) with surprising ease by the SCSC addition of either 4,4’-bipyridyl (bipy) or 1,4-phenylenediisocyanide (pdic).

As is usually observed for potential nanoporous molecular crystals most of the PNCs suffer structural collapse during the rapid loss of included solvent on exposure to the atmosphere. However, the XRD analysis of the PNCs that contain the rigid bidentate ligands, bipy or pdic, within the cavity confirmed the retention of the crystal structure on removal of included solvent. This example of crystal engineering to prevent the collapse of the PNC structure during the removal of solvent is closely analogous to the use of wall-ties to stabilise cavity brick walls in architectural engineering (Fig. 2c). It should be emphasised that the molecular wall-tie bridges two phthalocyanines to form a dimeric complex, rather than forming an extended framework; therefore, these Phthalocyanine Unsolvated Nanoporous Crystals (PUNC)s are still molecular crystals rather than MOFs [2].

The permanent nanoporosity of the PUNCs was confirmed by reversible nitrogen adsorption at 77 K following the application of a vacuum to remove adsorbed molecules. Brunauer, Emmett, Teller (BET) surface areas in the range 850 to 1000 m2 g-1 and nanopore volumes in the range of 0.40 to 0.46 ml g-1 can calculated from these Nitrogen adsorption isotherms. The surface area, total pore volume and pore size of the PUNCs all exceed those of other nanoporous molecular crystals and porphyrin-based MOFs.

These experiments show that the symmetry-driven self-assembly of the PNCs via crystallisation of a phthalocyanine complex provides a simple and readily scalable method of constructing sophisticated and versatile nanoporous structures. It has been demonstrated that the formation of the PNCs is compatible with a wide selection of transition metal cations, including those that impart useful catalytic activity.

The differentiation of the void and cavity axial binding sites of the phthalocyanine within the PNCs is analogous to the differentiation of the distal and proximal axial sites within hemoproteins. Thus, the molecular wall-tie within the cavity could perform a similar role to that of a heme proximal ligand (e.g. a histidine or cysteine residue) in the manipulation of the reactivity of the metallomacrocycle in addition to its task of maintaining the structural stability of the molecular crystal. An additional attractive feature of the PNCs for such studies is their stability following the exchange of included organic solvent for water, which will enhance the similarity with the natural environment of hemoproteins and may lead to practical heterogeneous catalysts for use in aqueous media.

References
[1] N. B. McKeown, S. Makhseed, K. J. Msayib, et al., A phthalocyanine clathrate of cubic symmetry containing interconnected solvent-filled voids of nanometer dimensions, Angewandte Chemie Intern. Edition, 44, 7546-7551 (2005).
[2] C. G. Bezzu, J. E. Warren, M. Helliwell, et al., Heme-Like Coordination Chemistry Within Nanoporous Molecular Crystals, Science, 327, 1627-1630 (2010).

Principal Publication and Authors
C. G. Bezzu, J. E. Warren, M. Helliwell, D. R. Allan, N. B. McKeown, Heme-Like Coordination Chemistry Within Nanoporous Molecular Crystals, Science, 327, 1627-1630 (2010).

Funding Acknowledgement
EPSRC Grant No. GR/F019114

Research carried out at Diamond Light Source and was started at Daresbury.