Electronic Structures of an Annulated meso-Tetraphenylchlorin and a Related Chlorin Analogue Incorporating an 8-Membered Ring through MCD Spectroscopy and DFT Calculations.

Herein, we compare the electronic structures of the metal-free and nickel(II) derivatives of an annulated meso-tetraphenyldihydroxychlorin with those of the (metallo)chlorin analogues derived by pyrroline ß,ß'-ring cleavage of the annulated (metallo)chlorins. These (metallo)chlorin analogues incorporate 8-membered heterocycles in place of the pyrroline, carry oxo-functionalities on the former pyrroline ß-carbon atoms, and were previously shown to possess drastically ruffled (twisted) nonplanar conformations. The magnetic circular dichroism spectra of all chromophores investigated feature chlorin-like UV-vis spectra and correspondingly reversed (positive-to-negative in ascending energy) sign sequences in the Q-band region, indicative of ΔHOMO < ΔLUMO relationships. Density functional theory (DFT) calculations indicate that the HOMOs in all compounds are a1u-type molecular orbitals (in traditional for the porphyrin spectroscopy D4h point group). Time-dependent DFT calculations correlate well with the experimental spectra and indicate that Gouterman's four-orbital model can be applied to these chromophores. This work highlights to which degree synthetic chlorin analogues can deviate from the structural parameters of natural chlorins without losing their electronic chlorin characteristics.

Pyrrole-Modified Porphyrins Containing Eight-Membered Heterocycles Using a Reversal of the "Breaking and Mending" Strategy.

The conversion of meso-aryl-porphyrins/chlorins to porphyrinoids containing nonpyrrolic heterocycles (so-called pyrrole-modified porphyrins, PMPs) along an approach we dubbed "the breaking and mending of porphyrins" is well known. However, examples are limited to the synthesis of PMPs containing up to six-membered heterocycles; the syntheses of larger rings failed. We report here hitherto unavailable eight-membered chlorin-type PMPs using an inverted "mending and breaking" approach. All examples are based on the addition of N,N'-dimethylurea derivatives to a meso-phenyl-ß,ß'-dioxoporphyrin, followed by oxidative cleavage of the intermediate diol adduct. We correlate the extremely nonplanar solid-state structures of three crystallographically characterized PMPs containing an eight-membered ring with their solution-state optical properties. The first examples of bis-modified, bacteriochlorin-type PMPs containing either two eight-membered rings or an eight-membered ring and an imidazolone ring are also detailed. Using other N,N'-nucleophiles failed to either generate chlorins containing a ß,ß'-dihydroxypyrroline, a prerequisite for the "breaking step," or the cleavage of those substrates that did generate a diol underwent subsequent reactions that thwarted the generation of the desired PMPs. This contribution adds novel PMPs containing eight-membered rings, highlights the effects these derivatizations have on the macrocycle conformation, and how that affects their optical properties.

Observations on the Mechanochemical Insertion of Zinc(II), Copper(II), Magnesium(II), and Select Other Metal(II) Ions into Porphyrins.

Building on a proof of concept study that showed the possibility of the mechanochemical insertion of some M(II) metals into meso-tetraphenylporphyrin using a ball mill as an alternative to traditional solution-based methods, we present here a detailed study of the influence of the many experimental variables on the reaction outcome performed in a planetary mill. Using primarily the mechanochemical zinc, copper, and magnesium insertion reactions, the scope and limits of the type of porphyrins (electron-rich or electron-poor meso-tetraarylporphyrins, synthetic or naturally occurring octaalkylporphyrins, and meso-triphenylcorrole) and metal ion sources suitable for this metal insertion modality were determined. We demonstrate the influence of the experimental metal insertion parameters, such as ball mill speed and reaction time, and investigated the often surprising roles of a variety of grinding agents. Also, the mechanochemical reaction conditions that remove zinc from a zinc porphyrin complex or exchange it for copper were studied. Using some standardized conditions, we also screened the feasibility of a number of other metal(II) insertion reactions (VO, Ni, Fe, Co, Ag, Cd, Pd, Pt, Pb). The underlying factors determining the rates of the insertion reactions were found to be complex and not always readily predictable. Some findings of fundamental significance for the mechanistic understanding of the mechanochemical insertion of metal ions into porphyrins are highlighted. Particularly the mechanochemical insertion of Mg(II) is a mild alternative to established solution methods. The work provides a baseline from which the practitioner may start to evaluate the mechanochemical metal insertion into porphyrins using a planetary ball mill.