A smart post-synthetic route towards [Fe2Co2] molecular capsules with electron transfer and bidirectional switching behaviors

Anion-Dependent Electron Transfer in the Cyanide-Bridged [Fe2Co2] Capsules

“…The average Co–N bond lengths of 2.119, 2.120, and 2.114 Å for 1 , 3 , and 4 , respectively, at 260 K are in good agreement with HS Co­(II) ions in the paramagnetic [Fe III LS2 Co II HS2 ] configuration (Tables S4–S6). − Upon cooling to 100 K, the average Co–N bond lengths of 1 and 3 undergo a significant decrease to 1.920 and 1.918 Å, respectively, which are characteristic of LS Co­(III) ions in the diamagnetic [Fe II LS2 Co III LS2 ] phase. − These results suggest the occurrence of a complete ETCST event for 1 and 3 from the high temperature (HT) [Fe III LS2 Co II HS2 ] phase to the low temperature (LT) [Fe II LS2 Co III LS2 ] phase. However, for 4 , the average Co–N bond length of 2.103 Å at 100 K indicates that the HT phase remains unchanged.…”

“…With this work as a backdrop, here we prepared a family of cyanide-bridged [Fe 2 Co 2 ] capsules balanced with different anions: {[(Tp*)­Fe­(CN) 3 Co­(bpy CN(CH 2 ) 7 NC bpy)] 2 [X] 2 }·sol ( 1 , X = ClO 4 , sol = 6DMF; 2 , X = PF 6 , sol = 6DMF; 3 , X = OTf, sol = 6DMF; 4 , X = BPh 4 , sol = 2DMF; Tp* = hydrotris­(3,5-dimethylpyrazol-1-yl) borate, bpy = 2,2′-bipyridine) (Scheme ). Note that complex 2 has been reported in our previous work . All the complexes contain the same cationic [{(Tp*)­Fe­(CN) 3 } 2 {Co­(bpy CN(CH 2 ) 7 NC bpy)} 2 ] 2+ units.…”

“…is an important topic associated with the emerging technologies like switches, sensors, and storage devices. − Among the well-studied systems are those mixed-valence Fe-Co cyanometallate complexes, which may exhibit thermo- and/or photo- induced electron-transfer-coupled spin transition (ETCST) between the diamagnetic {Fe II LS (μ-CN)­Co III LS } and paramagnetic {Fe III LS (μ-CN)­Co II HS } motifs (LS = low spin; HS = high spin), and the photo-responsive behavior was first reported in 1996 by Hashimoto and co-workers in a bulk Fe/Co Prussian Blue analogues (PBAs), K 0.2 Co 1.4 [Fe­(CN) 6 ]·6.9H 2 O . Since then, great efforts have been focused on the miniaturization of the three-dimensional PBAs, in order to better understand the key factors correlated with this particular behavior and ultimately satisfy the technological demand for molecule-based electronic devices like solubility, stability, and manipulability. − …”

“…Their ETCST behavior may be systematically manipulated through modifying the coordination environment of metal centers with selected chelating ligands, or regulating intermolecular interactions (hydrogen bonds, or π–π stacking, etc.) by introducing functionalized groups, solvent polarity, or co-crystallization. − Specifically, (1) methylating the Tp (tris­(pyrazolyl)­borate) ligand of the [Fe 2 Co 2 ] squares may lead to a more negative reduction potential for the Fe III/II redox couple and, thus, shift the electron transfer process toward lower temperature. , (2) On the contrary, incorporating stronger electron-donating R groups on the chelating bipyridine ligands (bpy R ) would effectively increase the charge distribution around the Co centers, driving the transition occurring at higher temperature . (3) Introducing hydrogen bonds on the terminal cyanide groups of the [Fe 2 Co 2 ] cores may pull down the charge density around Fe centers and stabilize Fe­(II) ions, thus leading to a higher transition temperature.…”

“…As a result, the grafted metal complex units, as effective electron acceptors, modulated the ETCST behavior at near room temperature . Furthermore, we successfully decorated the [Fe 2 Co 2 ] squares with chemical-active aldehyde-substituted ligands (bpy CHO ) and prepared two cyanide-bridged [Fe 2 Co 2 ] capsules via an efficient Schiff-base condensation between bpy CHO and alkyl diamines . Interestingly, both the capsules exhibited abrupt and complete thermo-induced ETCST as well as photo-responsive behavior.…”