Screening Hofmann Compounds as CO₂ Sorbents: Nontraditional Synthetic Route to Over 40 Different Pore-Functionalized and Flexible Pillared Cyanonickelates

Abstract

A simple reaction scheme based on the heterogeneous intercalation of pillaring ligands (HIPLs) provides a convenient method for systematically tuning pore size, pore functionality, and network flexibility in an extended series of pillared cyanonickelates (PICNICs), commonly referred to as Hofmann compounds. The versatility of the approach is demonstrated through the preparation of over 40 different PICNICs containing pillar ligands ranging from ∼4 to ∼15 Å in length and modified with a wide range of functional groups, including fluoro, aldehyde, alkylamine, alkyl, aryl, trifluoromethyl, ester, nitro, ether, and nonmetalated 4,4'-bipyrimidine. The HIPL method involves reaction of a suspension of preformed polymeric sheets of powdered anhydrous nickel cyanide with an appropriate pillar ligand in refluxing organic solvent, resulting in the conversion of the planar [Ni2(CN)4]n networks into polycrystalline three-dimensional porous frameworks containing the organic pillar ligand. Preliminary investigations indicate that the HIPL reaction is also amenable to forming Co(L)Ni(CN)4, Fe(L)Ni(CN)4, and Fe(L)Pd(CN)4 networks. The materials show variable adsorption behavior for CO2 depending on the pillar length and pillar functionalization. Several compounds show structurally flexible behavior during the adsorption and desorption of CO2. Interestingly, the newly discovered flexible compounds include two flexible Fe(L)Ni(CN)4 derivatives that are structurally related to previously reported porous spin-crossover compounds. The preparations of 20 pillar ligands based on ring-functionalized 4,4'-dipyridyls, 1,4-bis(4-pyridyl)benzenes, and N-(4-pyridyl)isonicotinamides are also described.

Crystallography of Representative MOFs Based on Pillared Cyanonickelate (PICNIC) Architecture

“…The gas adsorption properties of a large number of PICNIC derivatives were recently reported [61]. These materials were prepared by a heterogeneous ligand intercalation reaction which gave only powder products and hence detailed structural characterization was lacking.…”

“…The details of the powder syntheses of these MOF compounds using a different synthetic method are reported elsewhere [61].…”

“…In recent years, National Energy and Technology Laboratory (NETL) has been one of the leading laboratories in the area of science and engineering research of novel Ni(CN) 4 -based flexible MOFs [56,[61][62][63][64][65]. The precursors of these flexible MOFs are typically a cyanometallate complex that acts as a ligand and a transition metal complex with available coordination sites.…”

“…These compounds undergo cooperative spin transitions at high temperatures, and the host-guest interactions inside the pores have considerable effects on the spin crossover properties of these materials. Agusti et al [60] In recent years, National Energy and Technology Laboratory (NETL) has been one of the leading laboratories in the area of science and engineering research of novel Ni(CN)4-based flexible MOFs [56,[61][62][63][64][65]. The precursors of these flexible MOFs are typically a cyanometallate complex that acts as a ligand and a transition metal complex with available coordination sites.…”

“…The general synthetic strategy has been reported previously [84] and its adaptation to the synthesis of the materials in the current report is as follows. The reactants 3-methyl-4,4'-bipyridine; 3-amino-4,4'-bipyridine; nickel cyanide hydrate Ni[Ni(CN) 4 ] n (H 2 O) 3 were prepared as previously described [61]. The 2,6-naphthyridine was purchased from the Florida Center for Heterocyclic Compounds and used as received.…”