Spin-crossover and LIESST Effect for Iron(III) Complex Based on π–π Stacking by Coordination Programming

Abstract

Spin-crossover (SCO) iron(III) compounds, [Fe(qnalOMe) 2 ]PF 6 ¢acetone (1) and [Fe(qnal-OMe) 2 ]BPh 4 ¢2MeOH (2) (qnal-OMe: 7-methoxy-1-[(8-quinolinylimino)methyl]-2-naphthalenol), were prepared and characterized by X-ray singlecrystal structure analysis, temperature-dependent magnetic susceptibility, and Mössbauer spectroscopy measurements. The compounds exhibited SCO behavior with thermal hysteresis loop (T 1/2 ↑ = 202 K, T 1/2 ↓ = 194 K for 1 and T 1/2 ↑ = 304 K, T 1/2 ↓ = 194 K for 2). Moreover, the light-induced excited spin-state trapping (LIESST) effect was observed for the compounds when the samples were illuminated (1000 nm) at 5 K. We suggest that the introduction of a strong intermolecular interaction, such as ππ stacking, can play an important role in the observed dynamic phase transition.Phase transitions induced by external stimuli (temperature, pressure, electric field, magnetic field, or light irradiation) have been investigated both for their intrinsic scientific interest and for their potential practical utilization. 13 Particularly, photoinduced phase transitions are very promising from the point of view of developing photoswitchable devices.46 Spin-crossover (SCO) compounds exhibiting conversion between high-spin (HS) and low-spin (LS) states are one category of phasetransition compounds with the SCO phenomenon very often observed in iron(II), iron(III), and cobalt(II) compounds. 7If intermolecular interactions in the solid state are sufficiently strong then cooperativity can result. Cooperativity normally induces abrupt spin transitions and hysteresis loops in SCO compounds, providing the rationale for the role of intermolecular interactions in influencing spin transitions having been extensively studied. 7,8 While the design of SCO compounds exhibiting large cooperativity remains a challenge, it has been proposed (and experimentally confirmed) that cooperativity can be increased by designing polymeric structures in which active sites are linked to each other by chemical bridges. Furthermore, SCO compounds with strong intermolecular interactions, such as ππ stacking or hydrogen bonding, can also exhibit abrupt transitions and hysteresis loops. 3,69Since Gütlich et al. observed a photoinduced LS ¼ HS transition of the iron(II) SCO compound [Fe(ptz) 6 ](BF 4 ) 2 in 1984, the phenomenon has been named light-induced excited spin-state trapping (LIESST). 5,7,8 It was demonstrated that reverse LIESST is also possible in some cases: illumination of the system with near-infrared light can also reverse the system to the LS state.7 The discovery of the LIESST effect suggested that such compounds could be used as optical switches, and a number of iron(II) LIESST compounds have been subsequently reported. 5,79As mentioned previously, we have suggested that the LIESST effect of iron(III) compounds with strong intermolecular interactions such as ππ stacking can be observed. In general, structural transition for typical iron(II) SCO compounds is considered by simple expanding and shrinking (stretching...

Unusual Magneto‐Structural Features of the Halo‐Substituted Materials [FeIII(5‐X‐salMeen)2]Y: a Cooperative [HS‐HS]↔[HS‐LS] Spin Transition

“…The room temperature values of χ M T for 1 – 4 , which occur in the range 4.06–4.28 cm 3 K mol −1 , are consistent with the spin‐only value for HS iron(III) (χ M T ≈4.38 cm 3 K mol −1 ). They also concur with literature values in the range 3.80–4.50 cm 3 K mol −1 for closely related ferric spin‐crossover materials; although some of these reported values are somewhat low . The spin conversion in 1 – 4 , evidenced by the decrease in the values of χ M T with temperature, culminates in a plateau with a width of 115 K (140‐25 K) for 2 – 4 and 85 K (110‐25 K) for 1 .…”

“…It was not until very recently that the literature witnessed two other salEen‐based examples of SCO materials exhibiting an abrupt spin transition with hysteresis, namely [Fe III (4‐OMe‐ salEen) 2 ]NO 3 and [Fe III (5‐Br‐salEen) 2 ]ClO 4 . Indeed, in general, there is a paucity of iron(III) spin‐crossover complexes that display the much sought‐after unique spin‐transition features such as abrupt and hysteretic, symmetry‐breaking, stepwise and wide [HS‐LS] plateau, and structural phase transition, attributable to cooperativity amongst the paramagnetic centres in the crystal lattice through noncovalent intermolecular forces. Within the [Fe III (R 1 ,R 2 ‐salRen) 2 ] + system, observation of cooperative spin crossover is even rarer.…”

“…[2] It was not until very recently that the literaturew itnessed two other salEen-based examples of SCO materials exhibiting an abrupt spin transition with hysteresis,n amely [Fe III (4-OMe-salEen) 2 ]NO 3 [3] and[ Fe III (5-Br-salEen) 2 ]ClO 4 . [4] Indeed, in general, there is ap aucity of iron(III) spin-crossover complexes that display the much sought-afteru nique spin-transition features such as abrupt and hysteretic, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] symmetry-breaking, [16][17][18] stepwisea nd wide [HS-LS] plateau, [5,6,[16][17][18][19][20] and structuralp hase transition, [21] attributable to cooperativity amongstt he paramagnetic centres in the crystal latticet hrough noncovalent intermolecular forces. Within the [Fe III (R 1 ,R 2 -salRen) 2 ] + system, observation of cooperative spin crossover is even rarer.T he impetusf or spin-crossover research derives from the potential technological applications [22,23] of magnetically bistable molecular materialsi nt he fabrication of electronic devices,a sw ell as the sheer scientific interest in designinga nd generating complexesw ith fascinating magneto-structuralp roperties.…”

Thermal and Light‐Activated Spin Crossover in Iron(III) qnal Complexes

“…As expected, the Fe III centre has an N 4 O 2 donor set with the two qnal ligands enforcing a cis ‐configuration. The Fe–O and Fe–N bond lengths at 123 K are 1.882 and 1.979 Å, respectively, in agreement with a LS electronic configuration (Table ) , , , . As the sample is heated to room temperature, these bond lengthen by 0.012 and 0.081 Å suggesting that while significant spin crossover has taken place it is incomplete at this temperature.…”

“…Interestingly, despite its smaller hysteresis the acetone solvate shows better LIESST properties being almost 100 % compared with 50 % for the MeOH solvate highlighting the fact that there seems to be no direct correlation between thermal hysteresis and LIESST . [Fe(qnal‐OMe) 2 ]PF 6 · acetone and [Fe(qnal‐OMe) 2 ]BPh 4 · 2MeOH {qnal‐OMe = 7‐methoxy‐1‐[(8‐quinolinylimino)methyl]‐2‐naphthalenolate}, also exhibit spin‐transition behavior with thermal hysteresis loops ( T 1/2 ↑ = 202 K, T 1/2 ↓ = 194 K for acetone and T 1/2 ↑ = 304 K, T 1/2 ↓ = 194 K for MeOH) . Moreover, a strong LIESST effect was observed in both compounds.…”