Fundamental Characterization, Photophysics and Photocatalysis of a Base Metal Iron(II)‐Cobalt(III) Dyad

Abstract

A new base metal iron‐cobalt dyad has been obtained by connection between a heteroleptic tetra‐NHC iron(II) photosensitizer combining a 2,6‐bis[3‐(2,6‐diisopropylphenyl)imidazol‐2‐ylidene]pyridine with 2,6‐bis(3‐methyl‐imidazol‐2‐ylidene)‐4,4′‐bipyridine ligand, and a cobaloxime catalyst. This novel iron(II)‐cobalt(III) assembly has been extensively characterized by ground‐ and excited‐state methods like X‐ray crystallography, X‐ray absorption spectroscopy, (spectro‐)electrochemistry, and steady‐state and time‐resolved optical absorption spectroscopy, with a particular focus on the stability of the molecular assembly in solution and determination of the excited‐state landscape. NMR and UV/Vis spectroscopy reveal dissociation of the dyad in acetonitrile at concentrations below 1 mM and high photostability. Transient absorption spectroscopy after excitation into the metal‐to‐ligand charge transfer absorption band suggests a relaxation cascade originating from hot singlet and triplet MLCT states, leading to the population of the 3MLCT state that exhibits the longest lifetime. Finally, decay into the ground state involves a 3MC state. Attachment of cobaloxime to the iron photosensitizer increases the 3MLCT lifetime at the iron centre. Together with the directing effect of the linker, this potentially makes the dyad more active in photocatalytic proton reduction experiments than the analogous two‐component system, consisting of the iron photosensitizer and Co(dmgH)2(py)Cl. This work thus sheds new light on the functionality of base metal dyads, which are important for more efficient and sustainable future proton reduction systems.

Ultrafast Spectroscopy of Fe(II) Complexes Designed for Solar‐Energy Conversion: Current Status and Open Questions

“…In conclusion, there is still much ahead for this interdisciplinary research related to the chemistry and photo-physical properties of iron complexes, including the demonstration of their capacities as photosensitisers in prototype photocatalytic molecular assemblies. [116] Even though each metal is different, one should not forget to look across the borders and find inspiration in the photo-chemistry of other first row transition metals. [3][4][5][6][7][8][9][10][11][12][13]…”

High turnover photocatalytic hydrogen formation with an Fe(iii) N-heterocyclic carbene photosensitiser

“…1) as prototypical examples of heterogeneous and molecular PRCs, turn This journal is © The Royal Society of Chemistry 2022 over numbers (TONs) and turn over frequencies (TOFs) exceed those previously reported for Fe(II)NHC PSs by two orders of magnitude. 27,28 Hydrogen formation experiments were performed with monochromatic excitation (530 nm) close to the maximum of the lowest LMCT absorption band of [Fe(phtmeimb) 2 ] + directly populating the reactive 2 LMCT state (E 00 = 2.13 eV). 19 Concentrations of 0.5 or 1 mM of the PS ensured essentially complete light absorption in the sample volume (Abs 530 4 3) and high concentrations of the sacrificial electron donors triethylamine (Et 3 N) or triethanolamine (TEOA) enabled efficient quenching.…”

Photo-generation of H2 by heterometallic complexes

“…A few months later a new system for the photogeneration of H 2 was published in which a non-noble metal complex was used as a photocatalyst. 63 The dimetallic Fe-Co complex (17) (Fig. The photocatalytic activity of the Fe-Co complex 17 was studied in MeCN/water (1 : 1) with 5% triethanolamine, under argon and irradiated by simulated solar light using a Xe lamp with an AM1.5 global filter.…”

“…A few months later a new system for the photogeneration of H 2 was published in which a non-noble metal complex was used as a photocatalyst. 63 The dimetallic Fe–Co complex ( 17 ) (Fig. 5) was obtained by connection between an NHC-iron( ii ) photosensitizer (NHC = 2,6-bis[3-(2,6-diisopropylphenyl) imidazol-2-ylidene]pyridine) ( 15 ) and a cobaloxime ( 16 ) via a 2,6-bis(3-methyl-imidazol-2-ylidene)-4,4′-bipyridine bridge.…”