Investigation of cyanometallate coordination polymers with tetraamines for carbon capture

Research into carbon capture, utilisation, and storage (CCUS) from point sources and the atmosphere is essential for reducing greenhouse gas emissions and limiting the increase in global average temperature to well below 2 °C above pre-industrial levels. Cyanometallate (CM) coordination polymers (CPs) containing tetraamine ligands share structural similarities with some of the most effective metal-organic frameworks for carbon capture; however, their potential for CO₂adsorption remains largely unexplored. To address this gap, we synthesised a series of CM CPs using ferrocyanide and tetracyanonickelate (TCNi) building blocks with 1,2-bis(3-aminopropylamino)ethane (323) incorporated directly into the coordination network through a scalable, one-pot, room-temperature synthesis. Single-crystal x-ray diffraction of Ni-323-Fe^IIrevealed a new two-dimensional CP in which the 323 ligand coordinates to Ni centres in both facial (fac) and meridional (mer) configurations. Incorporation of the 323 ligand into the coordination network was further confirmed by infrared (IR) spectroscopy through characteristic vibrational bands. Under pure CO₂, the ferrocyanide materials Ni-323-Fe^IIand Zn-Fe^II-323 adsorbed 2.29 and 3.00 g CO₂per 100 g of material, respectively. In the TCNi series, Co-Ni-323 exhibited a higher CO₂uptake (2.36 g per 100 g) than Cu-Ni-323 (1.84 g per 100 g). Adsorption-desorption cycling of Cu-Ni-323 and Zn-Fe^II-323 demonstrated stable performance over ten cycles. Interestingly, under atmospheric conditions, Zn-Fe^II-323 consistently adsorbed 2.44-2.65 g of gas per 100 g of material over ten cycles. However, additional studies are required to determine the identity of the adsorbed gas. This work demonstrates a simple, scalable, and environmentally friendly route to CM CPs using aqueous, room-temperature synthesis while highlighting the challenges associated with CO₂adsorption when tetraamine ligands are coordinated to metal centres. These findings provide valuable insight into the design of cyanometallate CPs for carbon capture applications.