Surface-Mediated Ring-Opening and Porphyrin Deconstruction via Conformational Distortion

Abstract

The breakdown of macrocyclic compounds is of utmost importance in manifold biological and chemical processes, usually proceeding via oxygenation-induced ring-opening reactions. Here, we introduce a surface chemical route to selectively break a prototypical porphyrin species, cleaving off one pyrrole unit and affording a tripyrrin derivative. This pathway, operational in an ultrahigh vacuum environment at moderate temperature is enabled by a distinct molecular conformation achieved via the specific interaction between the porphyrin and its copper support. We provide an atomic-level characterization of the surface-anchored tripyrrin, its reaction intermediates, and byproducts by bond-resolved atomic force microscopy, unequivocally identifying the molecular skeletons. The ring-opening is rationalized by the distortion reducing the macrocycle’s stability. Our findings open a route to steer ring-opening reactions by conformational design and to study intriguing tetrapyrrole catabolite analogues on surfaces.

On-Surface Synthesis of Square-Type Porphyrin Tetramers with Central Antiaromatic Cyclooctatetraene Moiety

“…Our data provide no indication for an organometallic intermediate (C−Ag−C) in the coupling of the porphyrin macrocycles, in contrast to reports on Cu(111) 33 and Cu(110) 39,40 describing an organometallic (C−Cu−C) stage, which may not transition to a C−C bond. Furthermore, no peripheral dehydrogenation, which is expected to modify the STM and AFM contrast, 41,42 is discernible in the observed reaction products.…”

Zinc-Porphine on Coinage Metal Surfaces: Adsorption Configuration and Ligand-Induced Central Atom Displacement

“…This lack of change in the N adsorption height may suggest that it is more dictated by interactions with the underlying surface, rather than “pulled down” into the surface by the metal center. Indeed, Cu has a strong affinity to molecular N, which has previously been seen to result in the significant distortion of tetraphenyl porphyrins on this surface. ,− …”

“…This finding is reminiscent of bond-resolved AFM data on selected metalated tetrapyrroles, featuring a central depression (e.g., planarized Zn-porphyrin, 77 square-type Zn-porphyrin tetramers, 78 Zn-Pc/NaCl 79 Ag-Pc/ Ag(111), 80 and CuPorA2 21 ), whereas other metalated, as well as free-base, tetrapyrroles show faint cross-like signatures at the macrocycle center. 18,21,23,26,60 Generally, the contrast is influenced by electrostatic interaction between the COfunctionalized tip and the central ion (expected to be attractive for Zn 2+ ) and by Pauli repulsion affected by the d-orbital occupancy and the vertical position of the atom/ion. AFM image simulations based on the DFT-optimized geometry (see Scheme 1) indeed reflect these competing contributions.…”

Polymerization of Epoxides via Ring-Opening Coupling on Surfaces

“…In the reported on-surface coupling reactions, the R–X functional group is activated and subsequently cleaved, forming a monoradical intermediate R· via the dissociation of X·. , In contrast, the ring-opening of a functional group could produce a diradical intermediate through the cleavage of the ring, which may bring new reaction modes that result in the formation of novel polymers. For example, the heterocyclic ring-opening reactions, such as on-surface desulfurization of thiophene, − deconstruction of porphyrin and N -doped nanographene, and the ring-opening polymerization of melamine, have been reported to generate a variety of novel nanostructures. Recently, Gao et al reported a debromination ring-opening coordination reaction of hexabromobenzene to synthesize C6–Ag organometallic polyyne …”