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 m-terphenyl isocyanide ligand, [CNArMes2]2 (ArMes2 = 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 [CNArMes2]2 precluded the formation of networks with significant porosity. Here, it is shown that expansion of the [CNArMes2]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-(CNArMes2)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 b 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.

Crystalline Hydrogen-Bonding Networks and Mixed-Metal Framework Materials Enabled by an Electronically Differentiated Heteroditopic Isocyanide/Carboxylate Linker Group

“…Importantly, Fourier transform infrared (FTIR) analysis revealed a sharp ν CN band centered at 2155 cm −1 . As we have previously shown for both molecular copper(I) tris(isocyanide) complexes 42 and isocyanide networks featuring copper(I) tris(isocyanide) SBUs, 26,28 such a high-energy ν CN band is diagnostic of a cationic [(L)Cu(CNR) 3 ] + coordination environment, where the fourth L-type ligand is a weak σ donor. In the presence of stronger σ donors, a distinctive shift of ν CN to lower energy is observed because of greater Cu p-orbital participation in bonding.…”

“…These spectroscopic features agree well with previously reported mono-and ditopic mterphenyl isocyanides. 26,28,30 Crystallographic structural determination of TIB Mes2 H revealed that it is indeed an asymmetric rigid linear organoisocyanide containing a benzoic acid group. In the solid state, the benzoic acid group is canted ∼33°r elative to the Ar Mes2 unit (Figure S5.1).…”

“…In the presence of stronger σ donors, a distinctive shift of ν CN to lower energy is observed because of greater Cu p-orbital participation in bonding. 28 Unfortunately, despite multiple attempts, single crystals suitable for structural determination from this 3:1 TIB Mes2 H/[Cu(NCMe) 4 ]PF 6 reaction could not be obtained. However, circumstantial evidence for the formation of the tris(isocyanide) complex [(THF)Cu(TIB Mes2 H) 3 ]PF 6 comes from the reaction of TIB Mes2 and [Cu(NCMe) 4 ]PF 6 in a 4:1 ratio (Scheme 2).…”

“…Furthermore, the ATR-IR spectrum of crystalline Cu/Zn-ISO CN-5 revealed a sharp ν CN band at 2156 cm −1 , which provides additional evidence for the presence of a nonpyramidalized [Cu(CNR) 3 ] + node within Cu/Zn-ISO CN-5. 28 To this end, dissolution of the hydrogenbonding network [Cu(TIB Mes2 H) 4 ]PF 6 in THF, followed by the addition of NEt 3 and ZnCl 2 , also produces Cu/Zn-ISO CN-5 after solvothermal heating, as assayed by both single-crystal and powder X-ray diffraction (SCXRD and PXRD). As suggested from the solution-phase FTIR data of [Cu-(TIB Mes2 H) 4 ]PF 6 , dissociation of a TIB Mes2 ligand more than likely generates a tris(isocyanide) core that engages with other metal centers through carboxylate units after deprotonation.…”

“…Of these strategies, heteroditopic linkers offer unique advantages for the reliable formation of mixed-metal framework materials because two different binding groups can be specifically tuned to match the electronic requirements attendant between hard and soft metal centers. Recently, we introduced ditopic m -terphenyl isocyanides [CNAr Mes2 ] 2 and 1,4-(CNAr Mes2 ) 2 C 6 H 4 [Ar Mes2 = 2,6-(2,4,6-Me 3 C 6 H 2 ) 2 C 6 H 2 ] as linkers for MOF materials. − Isocyanides are electronically “soft” neutral ligands that function as good σ donors and effective π acids in a manner similar to that of CO. , While neutral in charge, isocyanides form robust M–L linkages on account of strong π-back-bonding interactions and are therefore adept at stabilizing low-valent metal centers. ,− As we have previously reported, ditopic m -terphenyl isocyanides can be employed for the formation of thermally robust and crystalline organometallic network materials featuring electron-rich metal centers [e.g., Cu­(I) and Ni(0)] as structural nodes. − However, isocyanides are particularly poor ligands for hard metals (e.g., Mg 2+ and Zn 2+ ) or metals in high formal oxidation states (e.g., Zr 4+ ). Seeking to exploit this disparate coordination behavior, we reasoned that heteroditopic ligands based on an isocyanide and an anionic binding group could offer a uniquely differentiated electronic environment for the formation of mixed-metal network materials.…”