Coordination Geometry Changes in Amorphous Cyanide-Bridged Metal–Organic Frameworks upon Water Adsorption

Abstract

Amorphous coordination polymers and metal–organic frameworks (MOFs) have attracted much attention owing to their various functionalities. Here, we demonstrate the tunable water adsorption behavior of a series of amorphous cyanide-bridged MOFs with different metals (M­[Ni­(CN)4]: MNi; M = Mn, Fe, and Co). All three compounds adsorb up to six water molecules at a certain vapor pressure (P ads) and undergo conversion to crystalline Hofmann-type MOFs, M­(H2O)2[Ni­(CN)4]·4H2O (MNi–H 2 O; M = Mn, Fe, and Co). The P ads of MnNi, FeNi, and CoNi for water adsorption is P/P 0 = 0.4, 0.6, and 0.9, respectively. Although the amorphous nature of these materials prevented structural elucidation using X-ray crystallography techniques, the local-scale structure around the N-coordinated M2+ centers was analyzed using L2,3-, K-edge X-ray absorption fine structure, and magnetic measurements. Upon hydration, the coordination geometry of these metal centers changed from tetrahedral to octahedral, resulting in significant reorganization of the MOF local structure. On the other hand, Ni­[Ni­(CN)4] (NiNi) containing square-planar Ni2+ centers did not undergo significant structural transformation and therefore abruptly adsorbed H2O in the low-pressure region. We could thus define how changes in the bond lengths and coordination geometry are related to the adsorption properties of amorphous MOF systems.

The ligand field in low-crystallinity metal–organic frameworks investigated by soft X-ray core-level absorption spectroscopy

“…Next, we present the L 2,3 -edge XAS spectra of MnNi, FeNi, and CoNi. As in previous XAS studies by Yoshino et al, 20 the asymmetric Mn, Fe, and Co L 2,3 -edge XAS spectra were clearly observed by scanning the hn from 630 to 665 eV, 690 to 740 eV, and 755 to 830 eV, respectively (Fig. 3(a-c)).…”

“…which differentiates it from a previous study 20 and adds to the previous results in a complementary manner. Further details can be found in the ESI.…”

“…Synthetic procedures, materials and methods, and crystallography M[Ni(CN) 4 ] (MNi, M = Mn, Fe, Co, and Ni) and Ni[Pd(CN) 4 ] (NiPd) were synthesized using a precipitation method. 20,25,26 The details of the sample preparation are provided in the ESI. † The infrared (IR) spectra were collected using a JASCO FT/IR-4200 spectrophotometer.…”

“…However, the previous XAS measurements were not analysed in detail for the LF of the N-coordinated Mn, Fe, and Co ions under T d symmetry because of the lower energy resolution. Additionally, spectroscopic investigation of N-coordinated Ni under square-planar (S p ) symmetry in low-crystallinity NiNi 20,24 (Fig. 1 In this study, we analysed the LF of the 3d electronic states of N-coordinated transition metal ions in low-crystallinity cyanide-bridged MOFs, MNi and NiPd, measured by 2p -3d XAS at the transition metal.…”

Photoluminescent coordination polymer bulk glasses and laser-induced crystallization

“…S7 †). 37,38 Only the broadness of the nCN bands is observed in AgCu-g and AgAu-g with a negligible shi (Fig. S8 and S9 †).…”