Aqueous Stability and Ligand Substitution of a Layered Cu(I)/Isocyanide-Based Organometallic Network Material with a Well-Defined Channel Structure

Abstract

Isocyanide coordination networks (^ISOCNs), which consist of multitopic isocyanide linker groups and transition-metal-based secondary building units (SBUs), are a promising class of organometallic framework materials for the inclusion of low-valent metal centers as primary structural components. Previously, it was demonstrated that the ditopic <i>m</i>-terphenyl isocyanide ligand, [CNAr^Mes2]₂ (Ar^Mes2 = 2,6-(2,4,6-Me₃C₆H₂)₂C₆H₃), could provide single-metal node frameworks based on Cu(I) and Ni(0) centers. However, the relatively short linker length in [CNAr^Mes2]₂ precluded the formation of networks with significant porosity. Here, it is shown that expansion of the [CNAr^Mes2]₂ scaffold with a central phenylene spacer allows for the formation of a robust Cu(I)-based framework with a distinct and solvent accessible channel structure. This new framework, denoted Cu-^ISOCN-4, is prepared as single-crystalline samples from a solvothermal reaction between [Cu(NCMe)₄]PF₆ and expanded linker 1,4-(CNAr^Mes2)₂C₆H₄. Crystallographic characterization of Cu-^ISOCN-4 revealed mononuclear [Cu(THF)(CNR)₃]⁺ structural nodes. The expanded diisocyanide linker results in fourfold interpenetrated (6,3) internal morphology. However, interpenetration in Cu-^ISOCN-4 results in discrete layer domains, each of which possesses well-defined 29 × 19 Å channels along the crystallographic <i>b</i> axis. Thermogravimetric analysis on Cu-^ISOCN-4 revealed THF solvent loss from the channels between 100-200 °C and dissociation of the Cu-coordinated THF ligand at 290 °C. The overall integrity of the network remains intact up to 400 °C, thereby signifying the robust nature of materials produced from metal-isocyanide M-C linkages. Aqueous stability studies revealed that Cu-^ISOCN-4 remains chemically resistant to exposure to liquid water for several days. In addition, ligand exchange studies in both THF and aqueous solution demonstrate that the Cu-coordinated THF group in Cu-^ISOCN-4 can be readily substituted with pyridine. This ligand exchange process occurs via single-crystal-to-single-crystal transformations and can also be readily monitored by infrared spectroscopy.