Metal–organic framework structures of fused hexagonal motifs with cuprophilic interactions of a triangular Cu(i)3(pyrazolate-benzoate) metallo-linker

Abstract

The reaction of the N,O-heteroditopic bifunctional ligand 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoic acid (H₂mpba) with Cu(NO₃)₂·2.5H₂O and Zn(NO₃)₂·4H₂O or Zn(CH₃COO)₂·2H₂O in N,N-dimethylformamide (DMF) results in concomitant formation of three bimetallic metal–organic frameworks (MOFs) with open structures, [Me₂NH₂][Zn₄{Cu^I₃(mpba)₃}₃(Me₂NH)-(DMF)₂] (1), [Zn₆{Cu^I₃(mpba)₃}₄(DMF)₅] (2) and [Zn₃{Cu^I₃(mpba)₃}₂(DMF)₃(H₂O)][Zn₄(μ₄-O){Cu^I₃(mpba)₃}₂(H₂O)₄] (3). Compounds 1 and 3 are isolable in phase pure form under specified conditions. The HSAB hard/soft metal–ligand selectivity (Cu^I–N and Zn–O) allows the structure assembly based on the 3-c trigonal planar {Cu^I₃(pz)₃} moiety (pz = pyrazolate) and various 4-6-c {Zn_x(O)_y(COO)_z} (x = 2–4, y = 0–1, z = 4–6) secondary building units to be realized neatly. The in situ formed softer Cu^I atoms coordinate with the softer pyrazolate nitrogen donor atoms of the pyrazolate–carboxylate ligand, leaving the carboxylate groups for linking to the Zn atoms. The triangular and trinuclear {Cu^I₃(mpba)₃}³⁻ metallacycle acts as a tricarboxylate linker between the Zn_x nodes and can be regarded as an expanded version of the benzene-1,3,5-tricaboxylate linker. In all structures the {Cu^I₃(pz)₃} moiety is stacked in infinite columns by cuprophilic interactions suggesting the initial formation of the Cu₃(mpba) metallo-ligand. The variable geometry Zn clusters, which are paddle-wheel type, pyramidal and trigonal-prismatic, adapt to the stacked orientation of the carboxylate groups from the {Cu^I₃(mpba)₃}³⁻ metallacycles. As a result, the formed structures could be viewed as derived from stacked hexagonal honeycomb hcb nets, via ‘fusion’ of alternating stacked nodes. The resulting nets retain the geometric similarity to the hcb net when viewed along the normal direction to the stack, and the individual topologies represent open structures. The minority phase 2 features 2D layers with a ‘dual-strand’ hcb net, {4³}₂{4³·6¹²}. 1 consists of parallel polycatenation, 2D + 2D → 3D, of triple layers with a point symbol of {4·6²}₂{4²·6⁶·8²}{6³}{6⁵·8}, while 3 presents a pair of interpenetrated 3D + 3D {4·6·8}{4²·6⁵·8⁵·10³} nets as a rare case of hetero-interpenetration. The binodal net in 3 is a rare sqc-3,6-Fdd2-2 type sqc-subnet and the first example in a MOF. In spite of the interpenetration, 3 retains significant porosity and could be activated to demonstrate permanent porosity, thereby classifiable as a MOF with an Ar-based BET surface area of 762 m² g⁻¹ (87 K) and a CO₂ adsorption capacity at 1 bar of 78 cm³ g⁻¹ (273 K) and 46 cm³ g⁻¹ (293 K). The structure organization similarities of 1–3, stipulated by cuprophilic interactions, could have a general importance for linear heteroditopic ligands analogous to H₂mpba offering a ‘fused-hcb’ design approach towards open structures.