Photochemical α-Aminonitrile Synthesis Using Zn-Phthalocyanines as Near-Infrared Photocatalysts

While photochemical transformations with sunlight almost exclusively utilize the UV-vis part of the solar spectrum, the majority of the photons emitted by the sun have frequencies in the near-infrared region. Phthalocyanines show high structural similarity to the naturally occurring light-harvesting porphyrins, chlorins, and mainly bacteriochlorins and are also known for being efficient and affordable near-infrared light absorbers as well as triplet sensitizers for the production of singlet oxygen. Although having been neglected for a long time in synthetic organic chemistry due to their low solubility and high tendency toward aggregation, their unique photophysical properties and chemical robustness make phthalocyanines attractive photocatalysts for the application in near-infrared-light-driven synthesis strategies. Herein, we report a cheap, simple, and efficient photocatalytic protocol, which is easily scalable under continuous-flow conditions. Various phthalocyanines were studied as near-infrared photosensitizers in oxidative cyanations of tertiary amines to generate α-aminonitriles, a synthetically versatile compound class.

Modular Synthesis of trans-A2 B2 -Porphyrins with Terminal Esters: Systematically Extending the Scope of Linear Linkers for Porphyrin-Based MOFs

Differently functionalized porphyrin linkers represent the key compounds for the syntheses of new porphyrin-based metal-organic frameworks (MOFs), which have gathered great interest within the last two decades. Herein we report the synthesis of a large range of 5,15-bis(4-ethoxycarbonylphenyl)porphyrin derivatives, through Suzuki and Sonogashira cross-coupling reactions of an easily accessible corresponding meso-dibrominated trans-A2 B2 -porphyrin with commercially available boronic acids or terminal alkynes. The resulting porphyrins were fully characterized through NMR, MS, and IR spectroscopy and systematically investigated through UV/Vis absorption. Finally, selected structures were saponified to the corresponding carboxylic acids and subsequently proven to be suitable for the synthesis of surface-anchored MOF thin films.

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.

Unexpected Interactions between Alkyl Straps and Pyridine Ligands in Sulfur-Strapped Porphyrin Nanorings

Strapped or “basket-handle” porphyrins have been investigated previously as hemoglobin mimics and catalysts. The facial selectivity of their interactions with axial ligands is a sensitive test for noncovalent bonding. Here the binding of pyridyl ligands to zinc porphyrins with thioester-linked alkyl straps is investigated in solution by NMR spectroscopy and UV–vis titration, and in the solid state by X-ray crystallography. We expected that coordination of the axial ligand would occur on the less hindered face of the porphyrin, away from the strap. Surprisingly, attractive interactions between the strap and the ligand direct axial coordination to the strapped face of the porphyrin, except when the strap is short and tight. The strapped porphyrins were incorporated into π-conjugated cyclic porphyrin hexamers using template-directed synthesis. The strap and the sulfur substituents are located either inside or outside the porphyrin nanoring, depending on the length of the strap. Six-porphyrin nanorings with outwardly pointing sulfur anchors were prepared for exploring quantum interference effects in single-molecule charge transport.

Metal-Organic Covalent Network Chemical Vapor Deposition for Gas Separation

The chemical vapor deposition (CVD) polymerization of metalloporphyrin building units is demonstrated to provide an easily up-scalable one-step method toward the deposition of a new class of dense and defect-free metal-organic covalent network (MOCN) layers. The resulting hyper-thin and flexible MOCN layers exhibit outstanding gas-separation performances for multiple gas pairs.

Gold(iii) tetraarylporphyrin amino acid derivatives: ligand or metal centred redox chemistry?

EPR spectroscopy and DFT calculations show that the site of reduction of porphyrinato gold(iii) complexes depends on the counterions X, the meso substituents R and the solvent.

Meso tetraarylporphyrinato gold(iii) cations bearing different substituents at the aryl substituents (COOMe, COOH, NO₂, NH₂, NHAc, H, OⁿBu, CF₃) were prepared and characterised. Their reversible one-electron reductions were studied by (spectro)electrochemical means as well as by selective chemical one-electron reduction using cobaltocene. The preferred location of the spin density, namely gold centred or porphyrin centred, was probed by electron paramagnetic resonance spectroscopy (g values, ¹⁹⁷Au hyperfine coupling) as well as by density functional theory calculations (spin densities). In all cases studied experimentally and theoretically, the gold(ii) valence isomer (5d⁹ electron configuration) is preferred over the porphyrin π radical anion. In the hexafluorophosphate salt of the nitro derivative a further nitro π radical anion valence isomeric species is significantly populated. In the presence of chloride ions this nitro π radical anion/Au^II valence isomeric equilibrium evolves towards the porphyrin π radical anion. The electronic structures of the nitro π radical and the Au^II σ radical valence isomers (5d_x²–y² orbital) could be calculated by DFT methods. The electron transfer pathway between the nitro π radical anion and the Au^II valence isomer is well described by the location of the hexfluorophosphate counterion, the Au–N distances (corresponding to the totally symmetric stretching vibration), the symmetric stretching mode of the NO₂ substituent and a meso-nitrophenyl rotation. The specific geometric and electronic properties of the favoured gold(ii) σ radical valence isomer, namely counterion dislocation and σ symmetry of the redox orbital, might stabilise charge-shifted states [(gold(ii) porphyrin)-donor˙⁺] by retarding the back electron transfer to give the ground state (gold(iii) porphyrin)-donor. This will guide the design of (photo-induced) electron transfer pathways with tetraarylporphyrinato gold(iii) complexes as electron acceptors.

Visual Recognition of Aliphatic and Aromatic Amines Using a Fluorescent Gel: Application of a Sonication-Triggered Organogel

A naphthalimide-based fluorescent gelator (N1) containing an alkenyl group has been designed and characterized. This material is able to gelate alcohols via a precipitate-to-gel transformation when triggered with ultrasound for less than 2 min (S-gel). The gelation process in n-propanol was studied by means of absorption, fluorescence, and IR spectra, scanning electron microscopy (SEM) images, and X-ray diffraction patterns. The fluorescence intensity of N1 decreased during the gelation process in a linear relationship with the sonication time. The S-gel of N1 could be used to sense aliphatic and aromatic amines by measuring the change in the signal output. For example, the addition of propylamine to the S-gel of N1 resulted in a dramatic enhancement of the fluorescence intensity, accompanied by a gel-to-sol transition. On the contrary, when the S-gel of N1 was treated with aromatic amines such as aniline, fluorescence was quenched and there was no gel collapse. The sensing mechanisms were studied by (1)H NMR, small-angle X-ray scattering, SEM and spectroscopic experiments. It is proposed that isomerization of the alkenyl group of N1 from the trans to cis form occurs when the S-gel is treated with propylamine, resulting in a gel-sol transition. However, the aromatic aniline molecules prefer to insert into the gel networks of N1 via hydrogen-bonding and charge-transfer interactions, maintaining the gel state. As potential applications, testing strips of N1 were prepared to detect aniline.