A novel base metal iron(II)-cobalt(III) dyad has been developed through the integration of a heteroleptic tetra-N-heterocyclic carbene (NHC) iron(II) photosensitizer with a cobaloxime catalyst. The photosensitizer features a 2,6-bis[3-(2,6-diisopropylphenyl)imidazol-2-ylidene]pyridine ligand combined with a 2,6-bis(3-methylimidazol-2-ylidene)-4,4′-bipyridine bridging ligand, enabling efficient electronic communication with the cobalt center. Comprehensive characterization was performed using X-ray crystallography, X-ray absorption spectroscopy, spectro-electrochemistry, and steady-state and time-resolved optical absorption techniques to elucidate both ground and excited state properties. The molecular integrity of the dyad in solution was assessed via NMR and UV-Vis spectroscopy, revealing dissociation at concentrations below 1 mM in acetonitrile, yet maintaining high photostability under irradiation. Transient absorption studies following excitation into the metal-to-ligand charge transfer (MLCT) band indicate a complex relaxation cascade originating from hot singlet and triplet MLCT states, culminating in the population of a long-lived 3MLCT state. Subsequent decay to the ground state proceeds through a 3MC state. Notably, the attachment of the cobaloxime moiety enhances the 3MLCT lifetime at the iron center compared to the isolated photosensitizer. This stabilization, coupled with the directional electron transfer facilitated by the covalent linker, suggests enhanced photocatalytic efficiency in proton reduction reactions relative to a non-covalently linked two-component system. The findings underscore the potential of such base metal dyads as sustainable alternatives in solar energy conversion systems.
Synthesis and Structural Analysis
The synthesis of the dyad begins with the preparation of the carbene precursor BL-Cl₂ via iridium-catalyzed C–H borylation followed by Suzuki coupling and nucleophilic aromatic substitution with 1-methylimidazole. Crystallization yields blue needles suitable for single-crystal X-ray diffraction. The iron(II) complex [Fe-BL] is formed by deprotonation of BL-Cl₂ and reaction with [FeL₁Br₂], followed by anion exchange with KPF₆. The bimetallic dyad [Fe-BL-Co] is then assembled by reacting [Fe-BL] with CoCl₂·6H₂O and dimethylglyoxime in ethanol under heating, leading to oxidation of Co(II) to Co(III) upon exposure to air. X-ray crystallography confirms the distorted octahedral geometry around Fe(II), with dihedral angles between L₁ and BL of 92.37° in [Fe-BL] and 94.06° in [Fe-BL-Co]. The 4,4′-bipyridine motif exhibits a twist angle of 45.45° in [Fe-BL] but reduces to 30.61° upon cobalt coordination, suggesting electronic interaction between the metal centers. EXAFS and XANES data validate the Fe(II) oxidation state and confirm structural consistency between the two complexes. Cobalt is coordinated equatorially by two dimethylglyoxime ligands, forming a planar pseudo-macrocycle with hydrogen bonding, consistent with known cobaloxime structures.
Solution Behavior and Stability
The pyridine–cobaloxime coordination bond represents the weakest link in the dyad. NMR studies reveal that at concentrations above 5 mM, more than 95% of the dyad remains intact in acetonitrile, while dilution to 0.18 mM results in 72% intact species. Below 0.04 mM, [Fe-BL] becomes dominant, indicating significant dissociation. In situ formation experiments show that only 33% of the dyad forms in concentrated solutions, dropping to 17% at 0.02 mM, highlighting kinetic limitations due to slow ligand exchange. Despite this, the dyad exhibits excellent photostability: after 22 hours of irradiation, 79% of the original structure remains intact, with no evidence of degradation beyond minor spectral changes attributed to bond distortion or conformational shifts. These results demonstrate that the dyad maintains its integrity during photochemical activation, making it suitable for transient spectroscopic and catalytic studies.
Optical and Electrochemical Features
UV-Vis absorption spectra reveal a redshifted MLCT band in [Fe-BL-Co] at 486 nm compared to 481 nm in [Fe-BL], accompanied by a distinct shoulder at 440 nm absent in the individual components. This feature is assigned to intermetallic electronic interaction. Concentration-dependent measurements confirm that the shoulder intensifies at higher concentrations, supporting the presence of the assembled dyad. Cyclic voltammetry shows reversible oxidation at +0.47 V for [Fe-BL] and +0.50 V for [Fe-BL-Co], indicating increased stability of the Fe(II)/Fe(III) couple upon cobalt coordination. Reduction potentials shift anodically for both the cobalt-centered processes and the ligand-based reduction, reflecting enhanced acceptor capability of the bridging ligand.DECR1 Antibody medchemexpress Spectroelectrochemical experiments confirm these assignments: oxidation generates LMCT bands in the 515–800 nm region, while reduction leads to spectral changes consistent with ligand reduction. The difference spectra provide clear signatures of oxidized and reduced species, enabling direct comparison with transient absorption data.
Excited State Dynamics and Electronic Structure
Ultrafast transient absorption spectroscopy reveals a multi-stage relaxation pathway following 515 nm excitation. A fast component (sub-100 fs) corresponds to internal conversion from hot 1MLCT* to 3MLCT*, followed by thermalization within the 3MLCT manifold. The longest-lived state is the 3MLCT, with a lifetime of 19.8 ps in [Fe-BL-Co], extended from 17.1 ps in [Fe-BL], demonstrating stabilization due to cobalt coordination. Global fitting analysis identifies four decay components, with the final recovery phase dominated by a 19.TRIM25 Antibody Cancer 8 ps process.PMID:34979149 The observed redshift in the GSB and ESA signals aligns with DFT-predicted reductions in the HOMO-LUMO gap. Computational modeling indicates that the average d-orbital energy is slightly stabilized in the dyad, while the Fe-ligand π-acceptor interaction increases. Although MC states are likely involved in the deactivation pathway, their spectral overlap prevents unambiguous identification. Nevertheless, the extended 3MLCT lifetime and directional electron transfer suggest a viable mechanism for intramolecular charge transfer to the cobalt center.
Photocatalytic Performance and Implications
In photocatalytic proton reduction experiments, the dyad [Fe-BL-Co] achieves a TON of 10 based on the photosensitizer and 5 based on the catalyst, outperforming the two-component system [Fe-BL]+[Co], which reaches TONs of 6 and 3, respectively. The activity gain is attributed to the conductive linker enabling directional electron transfer, minimizing diffusion-limited losses. While the absolute performance remains modest compared to noble metal systems, the result validates the design principle of base metal dyads. The study highlights the importance of stabilizing the MLCT state through ligand engineering and metal coordination, paving the way for next-generation sustainable photocatalysts. Future work will focus on enhancing excited state lifetimes and exploring new ligand architectures to improve efficiency.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com