In this work, six porous hexacyanometallate complexes (Ni3[Co(CN)6]2, Co3[Co(CN)6]2, Fe3[Co(CN)6]2, Ni3[Fe(CN)6]2, Co3[Fe(CN)6]2, Fe4[Fe(CN)6]2) were synthesized by a complexing agent assisted coprecipitation method and thoroughly characterized via X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), in situ high-temperature X-ray diffraction (HT-XRD), elemental analysis (EA), X-ray fluorescence (XRF), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 physisorption, and gas–solid phase titration with NH3. The thermal stability, chemical composition, pore size and volume, crystallite size and density of surface acid sites were strongly sensitive to both the transition metal and the cyanometallate anion employed. On that basis, transition metal hexacyanometallates must be perceived as an adaptable class of zeolite-like microporous materials. The catalytic properties of these compounds were tested by copolymerization of propylene oxide and CO2, a green route to obtain biodegradable aliphatic polycarbonates. All compounds under study showed moderate activity in the target reaction. The obtained copolymers were characterized by modest CO2 content (carbonate units ranging from 16 to 33%), random structure (RPEC ≈ 70%), and moderate molecular weight (Mw = 6000–85,400 g/mol) with broad dispersity values (ĐM = 4.1–15.8).

The catalytic activity of four transition metal hexacyanoferrate(II) complexes (Ni2[Fe(CN)6], Co2[Fe(CN)6], KFe[Fe(CN)6] and Zn2[Fe(CN)6]) in the ring-opening copolymerization (ROCOP) of CO2 and propylene oxide (PO) is reported here for the first time and compared with that of other hexacyanometallate compounds. Complexes were prepared by coprecipitation employing tert-butanol as complexing agent. X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis, elemental analysis, X-ray fluorescence, scanning electron microscopy, transmission electron microscopy and N2 physisorption were used to confirm the identity of the obtained materials. Except for Zn2[Fe(CN)6], which showed an amorphous nature, the complexes were constituted by aggregates of cubic nanocrystals with intra-crystalline micropores and inter-crystalline mesopores. Gas–solid phase titration with NH3 revealed the high potential of hexacyanoferrates as Lewis acid catalysts. In the case of Zn2[Fe(CN)6], the lack of structural organization led to an extremely high density of acid sites (43 μmol m−2). The resulting copolymers were analyzed via nuclear magnetic resonance spectroscopy and gel permeation chromatography. The studied transition metal hexacyanoferrate(II) catalysts showed mild activity in the target reaction, giving rise to polyethercarbonates with moderate CO2 content (9.3–18.1 wt%), random configuration (67.0–92.4% of polyethercarbonate linkages), modest molecular weights (MW, g mol−1 = 3400–20,200) and high dispersity (ĐM = 4.0–5.4). Cyclic propylene carbonate (PC) was also produced (1.4–19.8 wt%). Among all, the Co2[Fe(CN)6] complex stands as a potential catalyst for CO2/PO ROCOP due to its high CO2 uptake, selectivity and molecular weight of the obtained copolymer.

Although zinc-cobalt double metal cyanide (DMC) complex is a popular catalyst for the copolymerization of CO2 and epoxides, it faces important challenges, such as poor CO2 uptake, high cyclic carbonate...

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