Investigating the Impact of Conformational Molecular Engineering on the Crystal Packing of Cavity Forming Porphyrins

Locating Guest Molecules inside Metal-Organic Framework Pores with a Multilevel Computational Approach

Molecular docking has traditionally mostly been employed in the field of protein-ligand binding. Here, we extend this method, in combination with DFT-level geometry optimizations, to locate guest molecules inside the pores of metal-organic frameworks. The position and nature of the guest molecules tune the physicochemical properties of the host-guest systems. Therefore, it is essential to be able to reliably locate them to rationally enhance the performance of the known metal-organic frameworks and facilitate new material discovery. The results obtained with this approach are compared to experimental data. We show that the presented method can, in general, accurately locate adsorption sites and structures of the host-guest complexes. We therefore propose our approach as a computational alternative when no experimental structures of guest-loaded MOFs are available. Additional information on the adsorption strength in the studied host-guest systems emerges from the computed interaction energies. Our findings provide the basis for other computational studies on MOF-guest systems and contribute to a better understanding of the structure-interaction-property interplay associated with them.

Nonplanar porphyrins: synthesis, properties, and unique functionalities

Porphyrins are variously substituted tetrapyrrolic macrocycles, with wide-ranging biological and chemical applications derived from metal chelation in the core and the 18π aromatic surface. Under suitable conditions, the porphyrin framework can deform significantly from regular planar shape, owing to steric overload on the porphyrin periphery or steric repulsion in the core, among other structure modulation strategies. Adopting this nonplanar porphyrin architecture allows guest molecules to interact directly with an exposed core, with guest-responsive and photoactive electronic states of the porphyrin allowing energy, information, atom and electron transfer within and between these species. This functionality can be incorporated and tuned by decoration of functional groups and electronic modifications, with individual deformation profiles adapted to specific key sensing and catalysis applications. Nonplanar porphyrins are assisting breakthroughs in molecular recognition, organo- and photoredox catalysis; simultaneously bio-inspired and distinctly synthetic, these molecules offer a new dimension in shape-responsive host-guest chemistry. In this review, we have summarized the synthetic methods and design aspects of nonplanar porphyrin formation, key properties, structure and functionality of the nonplanar aromatic framework, and the scope and utility of this emerging class towards outstanding scientific, industrial and environmental issues.

Steric Repulsion Induced Conformational Switch in Supramolecular Structures

Inspired by the rigidified architecture of ‘picket‐fence’ systems, we propose a strategy utilizing strain to impose intramolecular tension in already peripherally overcrowded structures leading to selective atropisomeric conversion. Employing this approach, tuneable shape‐persistent porphyrin conformations were acquired exhibiting distinctive supramolecular nanostructures based on the orientation of the peripheral groups. The intrinsic assemblies driven by non‐covalent bonding interactions form supramolecular polymers while encapsulating small molecules in parallel channels or solvent‐accessible voids. The developed molecular strain engineering methodologies combined with synthetic approaches have allowed the introduction of the pivalate units creating a highly strained molecular skeleton. Changes in the absorption spectrum indicated the presence of severe steric repulsions between the peripheral groups which were confirmed by single crystal X‐ray analysis. To release the steric strain introduced by the peripheral units, thermal equilibration strategies were used to selectively convert the most abundant atropisomer to the desirable minor one.

Structural effects of meso-halogenation on porphyrins

The use of halogens in the crystal engineering of supramolecular porphyrin assemblies has been a topic of strong interest over the past decades. With this in mind we have characterized a series of direct meso-halogenated porphyrins using single crystal X-ray crystallography. This is accompanied by a detailed conformational analysis of all deposited meso-halogenated porphyrins in the CSD. In this study we have used the Hirshfeld fingerprint plots together with normal-coordinate structural decomposition and determined crystal structures to elucidate the conformation, present intermolecular interactions, and compare respective contacts within the crystalline architectures. Additionally, we have used density functional theory calculations to determine the structure of several halogenated porphyrins. This contrasts conformational analysis with existing X-ray structures and gives a method to characterize samples that are difficult to crystallize. By using the methods outlined above we were able to deduce the impact a meso halogen has on a porphyrin, for example, meso-halogenation is dependent on the type of alternate substituents present when forming supramolecular assemblies. Furthermore, we have designed a method to predict the conformation of halogenated porphyrins, without need of crystallization, using DFT calculations with a high degree of accuracy.

Strategic Synthesis of 'Picket Fence' Porphyrins Based on Nonplanar Macrocycles

Traditional 'picket fence' porphyrin systems have been a topic of interest for their capacity to direct steric shielding effects selectively to one side of the macrocycle. Sterically overcrowded porphyrin systems that adopt macrocycle deformations have recently drawn attention for their applications in organocatalysis and sensing. Here we explore the combined benefits of nonplanar porphyrins and the old molecular design to bring new concepts to the playing field. The challenging ortho-positions of meso-phenyl residues in dodecasubstituted porphyrin systems led us to transition to less hindered para- and meta-sites and develop selective demethylation based on the steric interplay. Isolation of the symmetrical target compound [2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetrakis(3,5-dipivaloyloxyphenyl)porphyrin] was investigated under two synthetic pathways. The obtained insight was used to isolate unsymmetrical [2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetrakis(2-nitro-5-pivaloyloxyphenyl)porphyrin]. Upon separation of the atropisomers, a detailed single-crystal X-ray crystallographic analysis highlighted intrinsic intermolecular interactions. The nonplanarity of these systems in combination with 'picket fence' motifs provides an important feature in the design of supramolecular ensembles.

Nucleophilic Aromatic Substitution (SNAr) and Related Reactions of Porphyrinoids: Mechanistic and Regiochemical Aspects

The nucleophilic substitution of aromatic moieties (SNAr) has been known for over 150 years and found wide use for the functionalization of (hetero)aromatic systems. Currently, several "types" of SNAr reactions have been established and notably the area of porphyrinoid macrocycles has seen many uses thereof. Herein, we detail the SNAr reactions of seven types of porphyrinoids with differing number and type of pyrrole units: subporphyrins, norcorroles, corroles, porphyrins, azuliporphyrins, N-confused porphyrins, and phthalocyanines. For each we analyze the substitution dependent upon: a) the type of nucleophile and b) the site of substitution (α, β, or meso). Along with this we evaluate this route as a synthetic strategy for the generation of unsymmetrical porphyrinoids. Distinct trends can be identified for each type of porphyrinoid discussed, regardless of nucleophile. The use of nucleophilic substitution on porphyrinoids is found to often be a cost-effective procedure with the ability to yield complex substituent patterns, which can be conducted in non-anhydrous solvents with easily accessible simple porphyrinoids.

Elucidating Atropisomerism in Nonplanar Porphyrins with Tunable Supramolecular Complexes

Atropisomerism is a fundamental feature of substituted biaryls resulting from rotation around the biaryl axis. Different stereoisomers are formed due to restricted rotation about the single bond, a situation often found in substituted porphyrins. Previously NMR determination of porphyrin atropisomers proved difficult, if not almost impossible to accomplish, due to low resolution or unresolvable resonance signals that predominantly overlapped. Here, we shed some light on this fundamental issue found in porphyrinoid stereochemistry. Using benzenesulfonic acid (BSA) for host‐guest interactions and performing 1D, 2D NMR spectroscopic analyses, we were able to characterize all four rotamers of the nonplanar 5,10,15,20‐tetrakis(2‐aminophenyl)‐2,3,7,8,12,13,17,18‐octaethylporphyirin as restricted H‐bonding complexes. Additionally, X‐ray structural analysis was used to investigate aspects of the weak host–guest interactions. A detailed assignment of the chemical signals suggests charge‐assisted complexation as a key to unravel the atropisomeric enigma. From a method development perspective, symmetry operations unique to porphyrin atropisomers offer an essential handle to accurately identify the rotamers using NMR techniques only.