Cooperative Spin Crossover above Room Temperature in the Iron(II) Cyanoborohydride–Pyrazine Complex

Abstract

Hysteretic spin crossover in coordination complexes of 3d-metal ions represents one of the most spectacular phenomena of molecular bistability. In this paper we describe a self-assembly of pyrazine (pz) and Fe­(BH3CN)2 that afforded the new 2D coordination polymer [Fe­(pz)2(BH3CN)2]∞. It undergoes an abrupt, hysteretic spin crossover (SCO) with a T 1/2 of 338 K (heating) and 326 K (cooling) according to magnetic susceptibility measurements. Mössbauer spectroscopy revealed a complete transition between the low-spin (LS) and the high-spin (HS) states of the iron centers. This LS-to-HS transition induced an increase of the unit cell volume by 10.6%. Meanwhile, a modulation of multiple [C–Hδ+···Hδ−–B] dihydrogen bonds stimulates a contraction in direction c (2.2%). The simplicity of the synthesis, mild temperatures of transition, a pronounced thermochromism, stability upon thermal cycling, a striking volume expansion upon SCO, and an easy processability to composite films make this new complex an attractive material for switchable components of diverse applications.

Bifunctional Role of Methyl Viologen in UV and X-ray Sensitive Switchable Organophosphonate Single Crystal

“…Besides, the high temperature SCXRD experiment was also conducted to confirm the absence of a thermochromic effect into the present system (Table S2). − Importantly, the enhancements of two closer O···N distance were alleged throughout the framework after longer time irradiation of X-ray, and the highest enlargement of O···N distance was found to be 3.5109 (7) Å for templated MV 2+ ion and 3.5525(5) Å for coordinated MV + ion in NIT1B (Figure S9, Supporting Information). The major enlargement occurred along the c axis after X-ray irradiation on NIT1 crystal, that can be seen by comparing the N–C bond length (pyridinium N and methyl C) of coordinated methyl viologen moiety (from 1.4809(1) to 1.4856(1) Å) (Figure S9, S11 and Table S4–S7 in ESI).…”

Two-Dimensional Coordination Polymer Showing Spin-Crossover Behavior with a 64 K Wide Hysteresis Loop

“…Lastly, the interlayer C py –H δ+ ···H δ− –B dihydrogen bonds (DHBs, Figure S7) may also be key to the synergetic SCO. The H δ+ ···H δ− contacts in 1·4CHCl 3 are 2.44(9)–2.74(2) Å at 250 K and 2.02(0)–2.84(4) Å at 110 K (Table S5), being typical values for the C–H δ+ ···H δ− –B DHBs. , These DHBs have resulted in a dense and stable packing of the 2D layers and the shortest Fe···Fe distances through space, i.e., 9.60(2) and 9.40(7) Å at 250 and 110 K. The communication between SCO centers through such an Fe–NCBH 3 ···Hpy–Fe pathway (Figure S7) may be more effective than that through bridging ligands.…”

“…Simulation with eq 1 gives a corresponding Γ value of 6.70 kJ mol −1 , which indicates a high SCO cooperativity compared with 2RT 1/2 = 3.2 kJ mol −1 . 34,35 On the other hand, in order to verify the complete SCO behavior of compound 1•4CHCl 3 , 57 Fe Mossbauer spectra were measured at 250 and 70 K, respectively, and the hyperfine parameters are summarized in Table S1. As shown in Figure 2, the Fe(II) ions in 1•4CHCl 3 are in 100% HS state at 250 K, featuring a δ value of 0.9637 mm s −1 and a ΔE Q value of 0.7441 mm s −1 .…”

“…The H δ+ •••H δ− contacts in 1•4CHCl 3 are 2.44(9)−2.74(2) Å at 250 K and 2.02(0)−2.84(4) Å at 110 K (Table S5), being typical values for the C−H δ+ •••H δ− −B DHBs. 35,42 These DHBs have resulted in a dense and stable packing of the 2D layers S7) may be more effective than that through bridging ligands.…”

“…The hysteretic and abrupt SCO behavior confirms cooperativity in compound 1·4CHCl 3 , which can be expressed by the parameter of cooperativity (Γ) that is derived from the Slichter–Drickamer equation (eq ), where γ HS is the HS fraction, Δ H and Δ S are the enthalpy and entropy changes, respectively, extracted from the DSC data (Figure S2). Simulation with eq gives a corresponding Γ value of 6.70 kJ mol –1 , which indicates a high SCO cooperativity compared with 2 RT 1/2 = 3.2 kJ mol –1 . , On the other hand, in order to verify the complete SCO behavior of compound 1·4CHCl 3 , 57 Fe Mössbauer spectra were measured at 250 and 70 K, respectively, and the hyperfine parameters are summarized in Table S1. As shown in Figure , the Fe­(II) ions in 1·4CHCl 3 are in 100% HS state at 250 K, featuring a δ value of 0.9637 mm s –1 and a Δ E Q value of 0.7441 mm s –1 . , While at 70 K all Fe­(II) ions are in the LS state that can be deduced from the doublet with δ value of 0.3383 mm s –1 and Δ E Q value of 0.1740 mm s –1 . , The exclusion of LS and HS population at high- and low-temperature range, respectively, confirms the complete HS-LS conversion for 1·4CHCl 3 .…”