Effect of Ligand Modulation on Metal-to-Metal Electron Transfer in a Series of [Fe₂Co₂] Molecular Square Complexes

Abstract

A series of three new cyanide-bridged [FeCo] molecular square complexes, namely, {[Fe(Tp*)(CN) 3 ] 2 [Co-(L) 2 ] 2 }(BF 4 ) 2 •2DMF (L = bik (1), bik* (2), and vbik (3); Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate, bik = bis(1-methyl-1H-imidazol-2-yl)ketone, bik* = bis(1-ethyl-1H-imidazol-2-yl)ketone, and vbik = bis(1-vinyl-1H-imidazol-2-yl)ketone; DMF = dimethylformamide) were synthesized and characterized by singlecrystal X-ray diffraction analyses and by magnetic, electrochemical, and spectroscopic measurements. Magnetic studies reveal that all three complexes exhibit temperature-induced metal-to-metal electron transfer (MMET) from a high-spin Co(II) center to a low-spin Fe(III) center, transforming a high-temperature paramagnetic {Fe III LS −CN−Co II HS } ground state into a low-temperature diamagnetic {Fe II LS −CN−Co III LS } state with a decrease in the temperature from 300 to 100 K. Complexes 1 and 3 show the interconversion of the paramagnetic {Fe III LS −CN−Co II HS } ground state into a diamagnetic {Fe II LS -CN-Co III LS } state in a single-step transition with T 1/2 values of 180 and 186 K, respectively, while a two-step MMET with T 1/2 value of 214 and 178 K was observed for complex 2.

Tuning intramolecular electron transfer of cyanide bridged [Co2Fe2] squares through chemical modifications in solid and solution

“…18 In 2004, K. Dunbar et al reported a molecular trigonal pyramidal [Co 3 Fe 2 ] complex and its thermally induced ETCST at the molecular level for the first time. 19 Up to now, cyanide-bridged molecular complexes with [CoFe], 20 [CoFe 2 ], 21–23 [Co 2 Fe 2 ], 24–32 [Co 3 Fe 2 ], 19,33,34 [Co 4 Fe 4 ], 35 [Co 5 Fe 4 ], 36 and [Co 6 Fe 4 ] 37 cores were reported to show ETCST upon temperature changes and light irradiation in solids. Since thermal ETCST is an intramolecular redox process, the redox potentials of the donor and acceptor sites are important for designing molecular ETCST complexes.…”

“…29 Mondal and co-workers reported that in their [Co 2 Fe 2 ] squares, changing the capping ligands on the Co sites caused different crystal packings and bistable behavior. 30 The ETCST behavior in solid is also influenced by intermolecular interactions. For example, Liu and co-workers reported a series of {[Fe(PzTp)(CN) 3 ] 2 [Co(dpq) 2 ] 2 }X 2 (PzTp − = tetrakis(pyrazolyl)borate, dpq = dipyrido[3,2- d :2′,3′- f ]quinoxaline, and X = BF 4 − , PF 6 − , and OTf − ) that have different intermolecular interactions, leading to quite different ETCST behavior and T 1/2 values.…”

“…[CoFe], 20 [CoFe 2 ], [21][22][23] [Co 2 Fe 2 ], [24][25][26][27][28][29][30][31][32] [Co 3 Fe 2 ], 19,33,34 [Co 4 Fe 4 ], 35 [Co 5 Fe 4 ], 36 and [Co 6 Fe 4 ] 37 cores were reported to show ETCST upon temperature changes and light irradiation in solids. Since thermal ETCST is an intramolecular redox process, the redox potentials of the donor and acceptor sites are important for designing molecular ETCST complexes.…”

Impact of counter anions on spin-state switching of manganese(iii) complexes containing an azobenzene ligand

“…1,2 These materials have shown significant importance in a broad area of quantum science and technology. [3][4][5] Spin manipulation in bistable materials with spin crossover (SCO), [6][7][8][9][10][11][12][13][14][15] singlemolecule magnet (SMM), [16][17][18][19][20][21][22] single-chain magnets (SCM) [23][24][25] and metal-to-metal electron transfer [26][27][28][29][30][31][32] properties has been well investigated for decades, where SCO molecules represent the most important and exciting materials [33][34][35] due to their easy manipulation for application. [36][37][38][39][40] Various external stimuli, including temperature, light, and pressure, have been applied to tune their bistability precisely with significant alteration in photophysical properties (magnetic, conductive, optical, etc.).…”

Impact of Counteranion on Reversible Spin-State Switching in a Series of Cobalt(II) Complexes Containing a Redox-Active Ethylenedioxythiophene-Based Terpyridine Ligand

“…Among them, the molecular systems that can allow access to room temperature bistability and photomagnetic effects are the most fascinating. Over the last few decades, the switchable magnetic materials based on transition metals are gaining momentum with effective importance, particularly materials showing photo-, magnetic, or electric bistabilities such as spin crossover (SCO), − single-molecule magnets (SMMs), − single-chain magnets (SCMs), − metal-to-metal electron transfer (MMET), − etc . Especially, SCO materials represent a prominent field to provide an attractive and potential way to attain molecular spintronic, molecular sensors, and nanoscale devices over the past few decades. − In the SCO phenomenon, the spin states of the 3d transition-metal complexes with d 4 -d 7 electronic configuration in octahedral geometry can be reversibly switched between low-spin (LS) and high-spin (HS), which has been well known for almost nine decades and remains one of the most interesting electronic dynamics in coordination chemistry. , The spin-state switching can be originated by modification of external perturbation, e.g ., temperature, pressure, light irradiation, magnetic field, etc ., and is often accompanied by a physical change in color, volume, and in magnetic, conductive, optical, and other physicochemical properties.…”