Metal ion influence on paddlewheel SBUs formed from a mixed donor multidentate ligand and the consequences for MOF structures

Abstract

In order to rationally design MOFs with predictable and reproducible structures, the effect of varying a metal ion on MOF topology while keeping the ligand constant is explored. In this study, a triptopic ligand with more than one type of coordination site is investigaed, namely 4′-(1H-imidazo[4,5-f]imidazo[1,10]phenanthrolin-2-yl)-[1.1′]biphenyl]-3,5-dicarboxylic acid) (H₂NCCPP = H₂L1). H₂L1 was synthesised and used to form HNCP type MOFs with non-default topologies revealing five new MOFs: Cu·L1(α) [Cu₃(L1)₂(H₂O)₃](NO₃)₂, Cu·L1(β) [Cu₃(L1)₂(H₂O)₃](NO₃)₂, Mg·L1 [Mg₂(L1)₂H₂O], Ca·L1 ([Ca₂(L1)₂(H₂O)₂], and Gd·L1 [Gd₄(L1)₆H₂O]. All five structures were based around at least one dinuclear SBU related to the well known paddlewheel geometry with the ‘cap’ showing increasing coordination as the size and malleability of the metal ion increased. Cu·L1(α) and Cu·L1(β) were polymorphic structures showing large square channels and based around both a traditional paddlewheel and a rarely seen semi-paddlewheel, both capped by a single solvent water. Mg·L1 and Ca·L1 each contained a single, dinuclear SBU capped by a phenanthroline, and a phenanthroline and water, respectively, due to the increase in size of the metal ion. Gd·L1 contained four distinct dinuclear SBUs, all capped by a phenanthroline, water and carboxylate as a result of the increased size and coordination of the lanthanide ion. Mg·L1 and Ca·L1 both showed adsorption of CO₂ peaking at 38.4 cm³ g⁻¹ and 26.1 cm³ g⁻¹, respectively, at 273 K.