Guest-induced pore breathing controls the spin state in a cyanido-bridged framework

Iron(ii) spin cross-over (SCO) compounds combine a thermally driven transition from the diamagnetic low-spin (LS) state to the paramagnetic high-spin (HS) state with a distinct change in the crystal lattice volume. Inversely, if the crystal lattice volume was modulated post-synthetically, the spin state of the compound could be tunable, resulting in the inverse effect for SCO. Herein, we demonstrate such a spin-state tuning in a breathing cyanido-bridged porous coordination polymer (PCP), where the volume change resulting from guest-induced gate-opening and -closing directly affects its spin state. We report the synthesis of a three-dimensional coordination framework {[Fe^II(4-CNpy)₄]₂[W^IV(CN)₈]·4H₂O} (1·4H₂O; 4-CNpy = 4-cyanopyridine), which demonstrates a SCO phenomenon characterized by strong elastic frustration. This leads to a 48 K wide hysteresis loop above 140 K, but below this temperature results in a very gradual and incomplete SCO transition. 1·4H₂O was activated under mild conditions, producing the nonporous {[Fe^II(4-CNpy)₄]₂[W^IV(CN)₈]} (1) via a single-crystal-to-single-crystal process involving a 7.3% volume decrease, which shows complete and nonhysteretic SCO at T_1/2 = 93 K. The low-temperature photoswitching behavior in 1 and 1·4H₂O manifested the characteristic elasticity of the frameworks; 1 can be quantitatively converted into a metastable HS state after 638 nm light irradiation, while the photoactivation of 1·4H₂O is only partial. Furthermore, nonporous 1 adsorbed CO₂ molecules in a gated process, leading to {[Fe^II(4-CNpy)₄]₂[W^IV(CN)₈]·4CO₂} (1·4CO₂), which resulted in a 15% volume increase and stabilization of the HS state in the whole temperature range down to 2 K. The demonstrated post-synthetic guest-exchange employing common gases is an efficient approach for tuning the spin state in breathing SCO-PCPs.