CO₂ photoreduction by 4-ferrocenyl appended bipyridine coordinated Re(I) complexes

Abstract

4-Ferrocenyl-bipyridine ligands (2a–d) and their respective [Re(L)(CO)₃Br] (L = 3a–d) complexes were synthesized and investigated for CO₂ photoreduction. The single-crystal analysis showed the formation of a by-product, 2d′, and its synthesis by self-aldol condensation explains the low yield observed for ligands. The Re(I) complexes showed CO₂ photoreduction with >90% selectivity for CO.

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Rhenium tricarbonyl complexes ([Re(L)(CO)₃X])^1–13 are well known as effective electro- and photocatalysts for the conversion of carbon dioxide (CO₂) to carbon monoxide (CO), with CO/H₂ selectivity over 90%.¹⁴ With L = 2,2′-bipyridine (bpy) as a ligand (Rebpy: [Re(bpy)(CO)₃Cl], Fig. 1a), its electro^15,16 and photocatalytic^17,18 activity, as well as the mechanistic pathways^19–22 have been widely studied. The catalytically active Re(0) species is formed through one- or two-electron reduction, and undergoes dimerisation,^21,23 thus reducing the overall efficiency. This deactivation pathway via dimerisation can be mitigated by tuning the ligand's electronic properties, by incorporating substituents at the 4,4′-positions of bpy^24–27 or by introducing bulky or charged substituents at the 6,6′-positions of bpy (Fig. 1a).^28–30 Substituents at 4, 4′, 6, and 6′-positions on the bpy ligand, such as primary and secondary amines,^31,32 alcohols,³³ or imidazoles³⁴ can stabilise the intermediate via H-bonding or charge-stabilisation of the Re(0) species, thereby hindering dimerisation. Recent studies, including ours, explored the impact of substituted 2′;6′,2″-terpyridine (tpy) ligands on CO₂ reduction properties³⁵ of Re(tpy)(CO)₃Br complexes.³⁶ The CO₂ reduction activity of the complexes was comparable to that of Rebpy, but the pendant pyridine ring of κ²-NN tpy ligand displayed minimal direct engagement in the catalytic cycle.³⁶

Fig. 1 (a) Differently substituted for bipyridine (bpy) ligands for CO₂ reduction by Re(I) complexes. (b) Current work: design of the catalyst and the important role of different substituents for CO₂ reduction.

In a non-bonding type of secondary sphere modification, ferrocene is studied for its ability to modulate the electron density and provide the structural stability for catalysis.³⁷ Van Meervelt et al.,³⁸ reported a method for synthesising a ferrocene-appended bpy ligand at the 4-position via palladium-catalysed Sonogashira coupling of 4-bromobipyridine with ethynylferrocene, achieving a yield of 56%. Pt, Cu, and Mn-coordinated complexes of the bpy ligand with ferrocene group at the 5-position were investigated for their photophysical, electrochemical, and catalytic properties.^39,40 Ferrocene moiety at the 6-position of bpy has been investigated as an electrochemical marker.⁴¹ Although ferrocene-appended κ²-NN and κ³-NNN systems have been studied for their structural and electrochemical properties, their role in CO₂ reduction remains limited.^37,42,43 In our prior work,³⁶ Re(I) complex with 4′-ferrocene-tpy ligand (fctpy) was found to be unstable under potential and light, prompting modifications by replacing the aromatic amine at the 6-position with another group. Herein, we report a generalised synthetic methodology for the 4-ferrocene-bpy ligand and aim to introduce steric hindrance at the 6,6′ positions to prevent dimerisation of catalytically active Re(0) species. We further investigate the impact of ferrocene substituents at the 4-position on the photoreduction of CO₂ (Fig. 1b).

We report an improved synthesis of the ferrocene-substituted unsymmetrical bipyridine ligands by modifying Housecroft's methodology from a one-step procedure to a two-step synthesis (Fig. 2a).⁴⁴ The yields from the reported method were consistently in the range of 20–30%,⁴⁴ and no improvement was observed with longer reaction times. We addressed the problem of low yields by purifying and isolating the intermediate chalcones (1a–d) obtained after cross-aldol condensation between an acetylpyridine derivative and ferrocene carboxaldehyde,⁴⁵ with yields of 60–96%. In the next step, condensation of the chalcone with the pyridinium methyl ketone salt in the presence of ammonium acetate yielded bpy ligands (2a–d) with yields of 25–65% (for characterisation data see Fig. S1). Ligand 2b consistently gave a lower yield (∼25%), and longer reflux (>24 h) led to decomposition of the product. Similarly, an analogue of ligand 2a was synthesised to probe the effect of the 6-substituent by replacing the methyl group at the 6-position with a phenyl group (2e), yielding 34% using the modified synthetic protocol.^46,47 Solid-state molecular structure of ligand 2c was determined by single crystal X-ray diffraction^48–51 (Fig. 2b), which showed that 2c crystallised in monoclinic space group P2₁/c. The angle between the ferrocene and bpy rings is ∼60° (Fig. S2), in line with the analogous compound reported by Van Meervelt et al.³⁸ Serendipitously, a hexasubstituted cyclohexyl derivative (2d′, Fig. 2c) was isolated while investigating 2d. To understand its formation, it was synthesised independently, as depicted in Fig. 2d. The by-product (2d′) results from the self-aldol condensation of three acetylpyridine derivatives, which likely contributes to the lower yield of ligand 2d and may explain the low yield in the earlier two-step process.⁴⁴ The successful synthesis of the ligands was further confirmed by NMR and mass spectrometry (Fig. S2). The UV-vis absorption spectra of ligands (2a–e), recorded in acetonitrile (Fig. S2), showed peaks in three distinct regions: the transitions below 300 nm are assigned to π → π* transition in the bpy ligand; transitions in the range of 300–400 nm might be due to intraligand charge transfer and the transition above 400 nm are assigned to the weak d–d transitions in the ferrocene moiety.⁵²

Fig. 2 (a) General scheme for the synthesis of 4-ferrocene-bpy ligands (2a–d), showing yields for both one-pot and two-step methodology. (b) ORTEP diagram of the ligand 2c at 50% of thermal ellipsoid. (c) ORTEP diagram of the by-product 2d′, at 50% of thermal ellipsoid. (d) Scheme for the synthesis of by-product (2d′).

The tricarbonyl rhenium(I) complexes (3a–e) were synthesised from Re(CO)₅Br and the corresponding bpy ligands (2a–e) with yields in the range of 50–78%. A single crystal was obtained for 3e (Fig. 3), where the phenyl ring is perpendicular to the two-pyridine ring coordinated to the Re(I) centre, with a bond length of Re–N as 2.187(2) Å (Table S5), similar to that in Rebpy (2.163(1) Å).⁵³ The bond angle between N1–Re1–N2 is 75°, whereas the rest of the bond angles with Re(I) centre are ∼90°, indicating a distorted octahedra with a facial geometry (Fig. 3). The thermogravimetric profile of 3a–e (Fig. S3) showed a consistent stepwise loss of the bromine atoms and the carbonyl moiety. However, for 3c, the complex decomposed in a single step at 338 °C, indicating that it is stable over a higher temperature range than the other derivatives. Complexes 3a–e were characterised by ATR-IR to probe the characteristic carbonyl stretching vibrations (ν_str(C=O)), which serve as an indicator of the electron density on Re and the extent of Re-to-carbonyl back-donation.⁵⁴ Complexes 3a–d exhibit only marginal shifts in the ν_str(C=O) bands compared to Rebpy, suggesting that the methyl substituents at the 6-position of bpy, as well as the ferrocene substituent, exert minimal electronic influence on the back-donating capability of the bpy ligand, and the complexes retain a similar coordination geometry to that of Rebpy. Introduction of phenyl at the 6-position in 3e led to an increase of ν_str(C=O) frequencies by ∼20 cm⁻¹, presumably due to steric hindrance from the 6-phenyl group, which distorts coordination geometry and reduces metal-to-carbonyl back-donation.⁵⁵ The NMR spectra of the rhenium complexes were recorded in DMSO-d₆, where a visible change was observed in the ¹H NMR spectrum compared to those of the ligands, further confirming complexation (Fig. S3). Three additional peaks in the ¹H NMR spectrum of the complex 3d at 7.25, 7.18 & 2.30 ppm related to toluene were observed,⁵⁶ consistent with the thermogravimetric profile of 3d (Fig. S3). Three distinct ¹³C resonances corresponding to the carbonyl were observed in the expected range³⁶ of 190–198 ppm for all complexes, downfield of the free ¹³CO (180 ppm).⁵⁷ The UV-vis spectra of all metal complexes in acetonitrile showed a ∼20 nm bathochromic shift in the 300–400 nm range compared to the pure ligands, attributed to metal coordination. Additionally, weak d–d transitions in the 400–450 nm range for free ligands shifted ∼50 nm for all metal complexes (Fig. S3).

Fig. 3 (a) General scheme for the synthesis of Re(I) complexes (3a–e). (b) ORTEP diagram of the complex 3e at 50% of thermal ellipsoid (hydrogens are omitted for clarity). (c) FTIR spectra of complexes 3a–e, in comparison with Rebpy.

The redox properties of the ligands were studied under non-catalytic conditions by cyclic voltammetry in acetonitrile under argon, and all potentials are reported relative to the ferrocene couple (Fc^+/0) (Fig. S4). All ligands displayed a small reversible peak ∼−1.60 V, assigned to bpy/bpy˙⁻, and an irreversible peak at ∼−2.10 V, assigned to additional ligand-based reductions.^58,59 The ferrocene peaks for the complexes are anodically shifted to more positive potentials by 0.30–0.45 V compared to unmodified ferrocene, which is consistent with the π-acceptor nature of the bpy ring (Fig. S5–S9).⁶⁰ As shown in Fig. 4a and Fig. S10–S13, CVs of 3a–e showed a reversible peak at ∼−1.75 V under argon, indicating the bpy/bpy˙⁻ reduction. Under CO₂-saturated conditions, complexes 3a, 3d, and 3e showed the disappearance of this reversible peak, and a new peak appeared at ∼−1.90 V,⁶¹ accompanied by an increase in current, indicating formation of a new product (Fig. 4a, S10 and S13). However, complexes 3b and 3c showed no significant increase in current at around −1.90 V (Fig. S11 and S12). The catalytic parameter i_cat/i_p is determined for all complexes under CO₂-standardised conditions, with values ranging from 1.10 to 4.70 (Table S7), suggesting CO₂ reduction activity depends on the substituent present on the ligand. Mechanistic investigations were carried out using IR spectroelectrochemistry (IR-SEC), following the methodology reported by Kubiak et al.²² In the potential range of −1.6 to −1.7 V, a broad band was observed at 1840–1895 cm⁻¹, consisting of multiple CO vibrations, and two more prominent bands at 1958–1967 cm⁻¹, and 1987–2005 cm⁻¹ (Fig. 4b and S14). These peaks are consistent with the formation of the singly reduced species, [Re(L)(CO)₃(NCMe)].^62–65 As the potential is further decreased to −2.1 V, the peaks become broader, suggesting presence of both [Re(L)(CO)₃(NCMe)] and the catalytically active double reduced [Re(L)(CO)₃]⁻ species in acetonitrile (Fig. S15).^22,61–65 However, the possibility of formation of the Re⁰–Re⁰ dimer could not be ruled out based on the IR-SEC data for 3a–d, as the dimer characteristic IR bands at 1950 and 1990 cm⁻¹,^21,66 are masked by signals from other species. Only for 3e, the formation of the dimer under reducing conditions can be conclusively excluded by the absence of any band in the 1950 cm⁻¹ region. This can be attributed to the steric hindrance of the phenyl substituent on the ligand in 3e, preventing dimerisation.

Fig. 4 (a) CV of 3d under argon-saturated (blue) and CO₂-saturated (red) acetonitrile solution (1 mM 3d in 100 mM n-Bu₄NBF₄ in acetonitrile) at a scan rate of 0.1 V s⁻¹. The i_cat/i_p was calculated at −1.92 V vs. Fc^0/+. (b) Normalized FTIR spectra for 3d in absorbance mode (3 mM in 100 mM n-Bu₄NBF₄ in acetonitrile; 100 scans, 4 cm⁻¹ resolution); all potentials are reported against Fc^0/+. (c) CO₂ photoreduction by 3a–e and Rebpy showing the amount of CO and H₂ evolved after 1 h visible light irradiation (reaction conditions: 0.1 mM Re catalyst, 4 mL of MeCN/TEOA (9 : 1 v/v), 0.1 mM [Ru(bpy)₃]Cl₂, 10 mM BIH, visible light (100 mW cm², AM 1.5G, λ > 400 nm)).

The photocatalytic activity of the complexes towards CO₂ reduction was evaluated in CO₂-saturated acetonitrile using [Ru(bpy)₃]Cl₂ as a photosensitiser, triethanolamine (TEOA) and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as electron donors. Gas chromatography of the headspace confirmed the generation of CO and H₂ by 3a–e upon visible-light irradiation; results from 1 h of irradiation are shown in Fig. 4c and Table S8. Control experiments demonstrated that all components, including the catalyst, [Ru(bpy)₃]Cl₂, TEOA, visible light, and CO₂, were required for photocatalysis, and a negligible amount of CO was produced in the absence of any component. This confirms that the CO is produced from CO₂. The protons released from the decomposition pathway of oxidised TEOA serve as the source for H₂.⁶⁷3a and 3c displayed best activity amongst the series with evolution of ∼26 µmol CO after 1 h with >91% selectivity. However, the reference Rebpy complex outperformed 3a–e, yielding 35 µmol of CO at 100% selectivity. A control experiment with only ligand 2e, lacking the Re(CO)₃Br unit, showed negligible CO formation (2 µmol), which highlights the essential role of the Re catalytic site. These results indicate that incorporating ferrocene into the optimally substituted bpy framework can facilitate CO₂ photoreduction to CO, giving CO yields close to that of the reference Rebpy catalyst. However, this corroborates minimal direct influence of ferrocene substituent on the catalytic property of the complexes, which is in line with the IR data of the complexes.

The photocatalysis results demonstrate the effect of substituents on bpy. 3a, with a methyl substituent, and 3d, with methyl and bromo substituents, displayed the highest CO evolution among the series. In contrast, 3b and 3c, which contain methyl groups at different positions (4′ and 6′, respectively), showed slightly lower CO yields (21 and 19 µmol, respectively) and product selectivity (86% and 85%, respectively). We speculate that this may be due to the electron-donating nature of the methyl groups, as reported previously for Re-complexes with para-substituted bipyridine ligands.²⁵ The positioning of the methyl substituents in 3c can introduce additional steric hindrance, potentially restricting the access of substrates to the Re centre. Increasing the irradiation time to 3 h shows that for all complexes, CO evolution largely plateaued after ∼1 h, including the reference Rebpy catalyst (Fig. S16). For example, 3a showed only a small increase in CO amount from ∼26 µmol after 1 h to ∼28 µmol after 3 h. Interestingly, the CO/H₂ ratio remained unchanged over 3 h of irradiation, suggesting that the catalyst decomposition products do not mediate hydrogen evolution. Overall, these findings highlight that the nature and position of substituents on Fc-bpy ligands can critically influence CO₂ photoreduction performance through electronic and steric modulation.⁶⁸

In conclusion, a two-step synthesis method for ferrocene-substituted unsymmetrical bipyridines (2a–d) was developed, enhancing synthetic efficiency over the one-pot approach. The by-product, an unusual hexasubstituted cyclohexane derivative (2d′), accounts for the lower yield of the ligands. Re(I) tricarbonyl complexes (3a–e) showed ligand-controlled photoreduction of CO₂ to CO with more than 90% selectivity, which is comparable to the activity of the well-known Rebpy catalyst.

Author contributions

RM conceived and supervised the study. SS synthesised, characterised, and performed electrochemical studies on all ligand and Re(I) complexes. DGG-G and SR performed and analysed the photocatalytic experiments. MJG and DM provided the crystallographic data. The manuscript was written with contributions from all authors. All authors approved the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article are included in the supplementary information (SI). Supplementary information: materials and methods; detailed synthesis and characterisation of ligand and Re complexes; electrochemistry data; IRSEC; photochemical studies; copies of NMR, HRMS, LC, TGA, ATR-IR, etc. See DOI: https://doi.org/10.1039/d6dt00728g.

CCDC 2443100 (3e), 2443118 (2c) and 2532926 (2d′) contain the supplementary crystallographic data for this paper.^69a–c

Acknowledgements

SS and RM thank IIT Goa for infrastructure and teaching assistant fellowships, and the ANRF for funding (CRG/2023/004767). MJG acknowledges access to the single-crystal X-ray facility at the Materials Characterisation Laboratory at the ISIS Facility. DGG-G acknowledges funding from the Ramón Areces Foundation for a postdoctoral research fellowship (BEVP37S21376). SR acknowledges funding from EPSRC (grant EP/Y002911/1).

References

1. J. M. Savéant, Chem. Rev., 2008, 108, 2348–2378.
2. H. Takeda, K. Koike, H. Inoue and O. Ishitani, J. Am. Chem. Soc., 2008, 130, 2023–2031.
3. D. A. Popov, J. M. Luna, N. M. Orchanian, R. Haiges, C. A. Downes and S. C. Marinescu, Dalton Trans., 2018, 47, 17450–17460.
4. R. Chafin, M. I. Sujan, S. Parkin, J. W. Jurss and A. J. Huckaba, RSC Adv., 2025, 15, 12547–12556.
5. S. Kar and E. Reisner, J. Am. Chem. Soc., 2026, 148, 10267–10285.
6. S. Sinha, E. K. Berdichevsky and J. J. Warren, Inorg. Chim. Acta, 2017, 460, 63–68.
7. L. Suntrup, F. Stein, J. Klein, A. Wilting, F. G. L. Parlane, C. M. Brown, J. Fiedler, C. P. Berlinguette, I. Siewert and B. Sarkar, Inorg. Chem., 2020, 59, 4215–4227.
8. S. A. Chabolla, C. W. Machan, J. Yin, E. A. Dellamary, S. Sahu, N. C. Gianneschi, M. K. Gilson, F. A. Tezcan and C. P. Kubiak, Faraday Discuss., 2017, 198, 279–300.
9. G. A. Andrade, A. J. Pistner, G. P. A. Yap, D. A. Lutterman and J. Rosenthal, ACS Catal., 2013, 3, 1685–1692.
10. W. Liang, T. L. Church, S. Zheng, C. Zhou, B. S. Haynes and D. M. D'Alessandro, Chem. – Eur. J., 2015, 21, 18576–18579.
11. N. Elgrishi, M. B. Chambers, X. Wang and M. Fontecave, Chem. Soc. Rev., 2017, 46, 761–796.
12. J. Hawecker, J.-M. Lehn and R. Ziessel, J. Chem. Soc., Chem. Commun., 1983, 536–538.
13. J. Hawecker, J.-M. Lehn and R. Ziessel, J. Chem. Soc., Chem. Commun., 1984, 328–330.
14. A. Barbero, L. Rotundo, C. Reviglio, R. Gobetto, R. Sokolova, J. Fiedler and C. Nervi, Molecules, 2023, 28, 7535.
15. T. Yoshida, K. Tsutsumida, S. Teratani, K. Yasufuku and M. Kaneko, J. Chem. Soc., Chem. Commun., 1993, 631–633.
16. C. Jiang, A. W. Nichols and C. W. Machan, Dalton Trans., 2019, 48, 9454–9468.
17. H. Takeda and O. Ishitani, Coord. Chem. Rev., 2010, 254, 346–354.
18. P. Lorwongkamol, T. Watanabe, M. Kitada, Y. Uetake, Y. Saga, T. Kambe, M. Kondo and S. Masaoka, JACS Au, 2025, 5, 4170–4177.
19. C. Riplinger, M. D. Sampson, A. M. Ritzmann, C. P. Kubiak and E. A. Carter, J. Am. Chem. Soc., 2014, 136, 16285–16298.
20. J. Agarwal, E. Fujita, H. F. I. Schaefer and J. T. Muckerman, J. Am. Chem. Soc., 2012, 134, 5180–5186.
21. E. E. Benson, M. D. Sampson, K. A. Grice, J. M. Smieja, J. D. Froehlich, D. Friebel, J. A. Keith, E. A. Carter, A. Nilsson and C. P. Kubiak, Angew. Chem., Int. Ed., 2013, 52, 4841–4844.
22. M. D. Sampson, J. D. Froehlich, J. M. Smieja, E. E. Benson, I. D. Sharp and C. P. Kubiak, Energy Environ. Sci., 2013, 6, 3748–3755.
23. A. V. Müller, L. A. Faustino, K. T. de Oliveira, A. O. T. Patrocinio and A. S. Polo, ACS Catal., 2023, 13, 633–646.
24. D. H. Apaydin, S. Schlager, E. Portenkirchner and N. S. Sariciftci, ChemPhysChem, 2017, 18, 3094–3116.
25. A. V. Müller, W. M. Wierzba, L. G. A. do Nascimento, J. J. Concepcion, S. Nikolaou, D. E. Polyansky and A. S. Polo, Artif. Photosynth., 2025, 1, 214–225.
26. W. W. Kramer, T. R. Sheridan, J. Pérez-Vázquez, C. A. Boivin, A. M. Browne, C. G. Logan and B. C. Sanders, Dalton Trans., 2026, 55, 5197–5209.
27. M. L. Clark, P. L. Cheung, M. Lessio, E. A. Carter and C. P. Kubiak, ACS Catal., 2018, 8, 2021–2029.
28. S. P. de Visser, Chem. – Eur. J., 2020, 26, 5308–5327.
29. A. M. Abudayyeh, S. De Kreijger, S. Grau, E. Eryılmaz, K. Robeyns, M. Z. Ertem, A. Llobet, B. Elias and L. Troian-Gautier, ACS Catal., 2026, 16, 542–554.
30. S. Amanullah, P. Saha, A. Nayek, M. E. Ahmed and A. Dey, Chem. Soc. Rev., 2021, 50, 3755–3823.
31. A. N. Hellman, R. Haiges and S. C. Marinescu, Dalton Trans., 2019, 48, 14251–14255.
32. S. Sinha Roy, K. Talukdar, S. T. Sahil and J. W. Jurss, Polyhedron, 2022, 224, 115976.
33. G. F. Manbeck, J. T. Muckerman, D. J. Szalda, Y. Himeda and E. Fujita, J. Phys. Chem. B, 2015, 119, 7457–7466.
34. A. W. Nichols and C. W. Machan, Front. Chem., 2019, 7, 1–19.
35. S. Sk. Akhter, K. Makhal, D. Raj, M. T. Natarajan, P. K. Jaiswal, B. S. Mallik, P. Kumar, S. Kumar Padhi and S. K. Padhi, Catal. Sci. Technol., 2025, 15, 7043–7058.
36. S. Saha, T. Doughty, D. Banerjee, S. K. Patel, D. Mallick, E. S. S. Iyer, S. Roy and R. Mitra, Dalton Trans., 2023, 52, 15394–15411.
37. A. A. J. Torriero, Inorganics, 2025, 13, 244.
38. T. T. Luong Thi, N. Nguyen Bich, H. Nguyen and L. Van Meervelt, Crystallogr. Rep., 2015, 60, 1100–1105.
39. S.Ø Scottwell, K. J. Shaffer, C. J. McAdam and J. D. Crowley, RSC Adv., 2014, 4, 35726–35734.
40. P. B. Ghanwat, D. Bora, R. Pandya, K. Vanka, B. Saha and S. S. Sen, Dalton Trans., 2025, 54, 10426–10432.
41. P. Edinç, A. M. Önal and S. Özkar, J. Organomet. Chem., 2007, 692, 1983–1989.
42. S. Saha, I. Tarannum, R. Rojas-Luna, J. Sannigrahi, S. K. Singh, S. Roy and R. Mitra, Inorg. Chem., 2026, 65, 1884–1899.
43. S. Saha, R. Rojas-Luna, P. Robinson, S. Roy and R. Mitra, ChemistrySelect, 2026, 11, e06941.
44. A. Büttner, S. Y. Brauchli, R. Vogt, E. C. Constable and C. E. Housecroft, RSC Adv., 2016, 6, 5205–5213.
45. X. Wu, E. R. T. Tiekink, I. Kostetski, N. Kocherginsky, A. L. C. Tan, S. B. Khoo, P. Wilairat and M.-L. Go, Eur. J. Pharm. Sci., 2006, 27, 175–187.
46. T. N. Francisco, H. M. T. Albuquerque and A. M. S. Silva, Chem. – Eur. J., 2024, 30, e202401672.
47. Z. Meng, C.-L. Ho, H.-F. Wong, Z.-Q. Yu, N. Zhu, G. Li, C.-W. Leung and W.-Y. Wong, Sci. China Mater., 2019, 62, 566–576.
48. O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. a. K. Howard and H. Puschmann, J. Appl. Crystallogr., 2009, 42, 339–341.
49. L. J. Farrugia, J. Appl. Crystallogr., 2012, 45, 849–854.
50. C. F. Macrae, I. Sovago, S. J. Cottrell, P. T. A. Galek, P. McCabe, E. Pidcock, M. Platings, G. P. Shields, J. S. Stevens, M. Towler and P. A. Wood, J. Appl. Crystallogr., 2020, 53, 226–235.
51. A. L. Spek, Acta Crystallogr., Sect. D: Biol. Crystallogr., 2009, 65, 148–155.
52. T. Sørensen and M. Nielsen, Open Chem., 2011, 9, 610–618.
53. R. Kia and F. Safari, Inorg. Chim. Acta, 2016, 453, 357–368.
54. T. Auvray, B. Del Secco, A. Dubreuil, N. Zaccheroni and G. S. Hanan, Inorg. Chem., 2021, 60, 70–79.
55. L. Rotundo, E. Azzi, A. Deagostino, C. Garino, L. Nencini, E. Priola, P. Quagliotto, R. Rocca, R. Gobetto and C. Nervi, Front. Chem., 2019, 7, 1–13.
56. G. R. Fulmer, A. J. M. Miller, N. H. Sherden, H. E. Gottlieb, A. Nudelman, B. M. Stoltz, J. E. Bercaw and K. I. Goldberg, Organometallics, 2010, 29, 2176–2179.
57. J. Seravalli and S. W. Ragsdale, Biochemistry, 2008, 47, 6770–6781.
58. R. Wang, X. Wang, E. B. Sundberg, P. Nguyen, G. P. G. Grant, C. Sheth, Q. Zhao, S. Herron, K. A. Kantardjieff and L. Li, Inorg. Chem., 2009, 48, 9779–9785.
59. B. Li, B. L. Geoghegan, C. Wölper, G. E. I. Cutsail and S. Schulz, ACS Omega, 2021, 6, 18325–18332.
60. P. Edinç, A. M. Önal and S. Özkar, J. Organomet. Chem., 2007, 692, 1983–1989.
61. J. M. Smieja and C. P. Kubiak, Inorg. Chem., 2010, 49, 9283–9289.
62. S. Chattopadhyay, M. H. Cheah, R. Lomoth and L. Hammarström, ACS Catal., 2024, 14, 16324–16334.
63. L. M. Kiefer, L. B. Michocki and K. J. Kubarych, J. Phys. Chem. Lett., 2021, 12, 3712–3717.
64. F. P. A. Johnson, M. W. George, F. Hartl and J. J. Turner, Organometallics, 1996, 15, 3374–3387.
65. E. E. Benson and C. P. Kubiak, Chem. Commun., 2012, 48, 7374–7376.
66. K. T. Oppelt and P. Hamm, J. Phys. Chem. C, 2024, 128, 16040–16049.
67. Y. Pellegrin and F. Odobel, C. R. Chim., 2016, 20, 283–295.
68. L. Rotundo, D. C. Grills, R. Gobetto, E. Priola, C. Nervi, D. E. Polyansky and E. Fujita, ChemPhotoChem, 2021, 5, 526–537.
69. (a) CCDC 2443100: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2n07pm; (b) CCDC 2443118: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2n0887; (c) CCDC 2532926 : Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r0q9t.

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