A Cu(ii)-MOF based on a propargyl carbamate-functionalized isophthalate ligand

Abstract

A copper-based metal-organic framework (MOF) was prepared using a new linker, a 5-substituted isophthalic acid bearing a propargyl carbamate group, intended to provide a terminal alkyne function protruding from the material surface to generate supported gold species for potential catalytic applications. The novel material was fully characterized by spectroscopic analyses of different kinds: FTIR, Raman, EDX, and XPS, as well as by thermal and surface area measurements. Synchrotron X-ray diffraction data analysis, in particular, revealed that this MOF, labelled [Cu(1,3-YBDC)]·xH₂O (x ∼ 2), where Y stands for the pendant alkYne and BDC for benzene dicarboxylate, contains a complex network of 5-substituted isophthalate anions bound to Cu(ii) centers, arranged in pairs within paddlewheel (or "Chinese lantern") fragments of Cu₂(μ-COO)₄(D)₂ formulation (D being a neutral Lewis base), with a short Cu⋯Cu distance of 2.633(4) Å. Quite unexpectedly, the apical atom in the paddlewheel structure belongs to the carbamate carbonyl oxygen atom. Such extra coordination by the propargyl carbamate groups drastically reduces the MOF porosity, a feature that was also confirmed by BET measurements. However, the MOF functionality is retained at the external crystal surface where 2% of active terminal alkynes is located.

Scheme 1. Synthesis of 6 (1,3-H₂YBDC). Reagents and conditions: (a) NaHCO₃ (3 eq.), THF/H₂O, r.t., overnight; (b) NaI (1.5 eq.), Cs₂CO₃ (1.5 eq.), 2-butanone, 80 °C, overnight; (c) LiOH (2.0 M), THF/MeOH, r.t., 3 h.

Scheme 2. Synthesis of [Cu(1,3-YBDC)]·xH₂O and image of a pellet made by pressing at 100 bar for 2 min its polycrystals.

Fig. 1. TGA curves (continuous red lines) and their first derivatives (dashed lines) for 1,3-H₂YBDC (top) and [Cu(1,3-YBDC)]·xH₂O (bottom).

Fig. 2. (A and B) Asymmetric unit showing the distinct conformations (conformers a and b) of the flexible propargyl carbamate residues (with labeling scheme in A); (C and D) The paddlewheel moiety and the location of the O6 atoms (highlighted in yellow) completing the Cu(ii) coordination through CO⋯Cu bonds; (E and F) crystal packing, viewed down [001], for the two ordered and periodic models, highlighting extended channels, running in the c direction, and accounting, in both cases, for ≈20% of the crystal volume.

Fig. 3. Sketches of the weakly corrugated 2D layers in the crystal packing of Cu₂(μ-isophthalate)₄D₂, present also in [Cu(1,3-YBDC)]·xH₂O. (A) View down [001]. (B) View down [100]. Note that, in [Cu(1,3-YBDC)]·xH₂O, the layers are not separated, but are interlinked by the long propargyl carbamate residues (in both a and b conformations) completing the Cu₂(μ-COO)₄ coordination sphere (the apical D atom) with a truly 3D connectivity.

Fig. 4. (A) Representative FE-SEM image; (B) corresponding EDXS spectrum.

Fig. 5. XPS analysis for the target [Cu(1,3-YBDC)]·xH₂O specimen: (A) wide-scan spectrum; (B) C 1s, (C) O 1s, (D) N 1s, (E) Cu 2p photoelectron peaks.

Fig. 6. Adsorption and desorption isotherm of N₂ at 77 K on [Cu(1,3-YBDC)]·xH₂O.