Gold(II) Porphyrins as Key Intermediates in Novel Artificial Photosynthetic Systems
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Abstract
Over the past years, a promising approach to illuminate the first steps of photosynthesis was achieved by the design and preparation of artificial photosynthetic RCs, composed of donor-acceptor model compounds, mimicking the energy conversion process.
In this context, this work aims to elucidate the role of gold(III) porphyrins as a potent cationic electron acceptor in such artificial photosynthetic systems. Particularly, the site of gold(III) porphyrin reduction is discussed controversially in the literature, since it could be ligand centred or metal centred. The approach chosen here was the preparation and characterisation of a meso-tetraarylporphyrinato gold(III) cation reference system, bearing functional groups at the aryl substituents with variable electron-donating and electron-withdrawing behaviour (COOMe, COOH, NO2, NH2, NHAc, H, OnBu, CF3) to examine their influence on the site of gold(III) porphyrin reduction. The obtained gold(III) porphyrins were treated with the reducing agent cobaltocene and probed by electron paramagnetic resonance (EPR) spectroscopy to determine the preferred location of the spin density (AuII: 5d9 metallo radical; (P●−): organic -radical anion). Gratifyingly, the chemical one-electron reduction of the gold(III) porphyrins yielded the corresponding gold(II) porphyrin complexes with a characteristic EPR pattern revealing hyperfine coupling to 197Au and 14N and is therefore clearly preferred over the porphyrin pi-radical anion.
Encouraged by the unexpected high stability of the aforementioned gold(II) porphyrin complexes and to further investigate this usually elusive species, a mononuclear gold(II) porphyrin was successfully synthesized. This has been achieved via chemical reduction of a gold(III) porphyrin with stoichiometric amounts of KC8 or cobaltocene and by an excess of 1-benzyl-1,4-dihydronicotinamide (BNAH) in the presence of a base. The latter was employed as an NADH model compound to illustrate the potential mode-of-action of gold(III) porphyrins inside tumor cells, since they are used as potent anti-cancer drugs. Furthermore, it was possible to isolate and purify this mononuclear gold(II) complex by means of recrystallization or sublimation. This enabled the first thorough investigation of a thermodynamically stable mononuclear gold(II) species by using a combination of spectroscopic and theoretical methods.
Further, the knowledge obtained from the aforementioned studies has been exploited for the synthesis and examination of three novel amide-bridged donor-acceptor dyads designed to undergo ultrafast PET. For this purpose, zinc(II) porphyrin amino acid derivatives were utilised as chromophores and electron donors and gold(III) porphyrin amino acid derivatives as electron acceptors [Zn(P)-AuIII(P)][PF6]. The individual building blocks were equipped with electron-donating and electron-withdrawing meso-aryl groups (4-C6H4OnBu, 4-C6H4CF3) with the intent to influence the driving force of the forward PET and the backward electron transfer (BET). The PET processes were successfully investigated via time-resolved spectroscopic techniques and they revealed a charge-shifted (CSh) state [Zn(P● )-AuII(P)] featuring a gold(II) core for all the dyads. Since the transferred electron is located in a sigma-type orbital of gold (5dx2-y2) relative to the porphyrine plane, the direct back-electron transfer to the a2u SOMO of the zinc(II) porphyrin is hindered, resulting in a relatively long lifetime of the CSh state. As a consequence, the CSh states have been successfully exploited for bimolecular reactions with amines as sacrificial electron donors yielding a stable gold(II) species Zn(P)-AuII(P) as confirmed by using a combination of spectroscopic and theoretical methods. Furthermore, this stable gold(II) species was competent to reduce aromatic azides to amines during a photoredox experiment, whereby the initial gold(III) dyad was restored and thus a catalytic cycle was closed.