Modular construction of 3D coordination frameworks incorporating SiF62− links: Accessing the significance of [M(pyrazole)4{SiF6}] synthon

Abstract

Rational combination of Cd²⁺ cations, bitopic pyrazole ligands, bridging SiF₆²⁻ and terminal NCS⁻ anions provide generation of 3D frameworks and precise control over connectedness of the net nodes: two-fold interpenetrated 4-connected NbO-like net (nbo) in [Cd(Me₄bpz)₂{NCS}₂]·⅔CH₂Cl₂ (1), uninodal 5-connected noz framework in [Cd₂(Me₄bpz)₄{SiF₆}{NCS}₂]·6CHCl₃ (2), novel 5-connected binodal topology (with a point symbol of {4².5⁵.6².7}{4².5⁶.6²}) in [Cd₂(Me₄bpz)₄{SiF₆}{NCS}₂]·2CH₂Cl₂ (3) and 6-connected α-Po cubic nets (pcu) in [Cd(Me₄bpz)₂{SiF₆}]·6H₂O (4) and [Cd(Me₄bpz)₂{SiF₆}]·1.5CH₂Cl₂ (5) (Me₄bpz = 3,3′,5,5′-tetramethyl-4,4′-bipyrazole). Hexafluorosilicate anions act as bridges between Cd ions yielding further linkage of 4-connected [Cd(Me₄bpz)₂] subtopologies. Characteristic and specific interaction between SiF₆²⁻ and the metal–organic portion is conditioned by a synergy of coordination and multiple strong NH⋯F bonds, which suggests perfect compatibility of the bipyrazole and SiF₆²⁻ linkers for the construction of 3D structures, either by pillaring of 2D layers or cross-linking of 3D frameworks. Two observed motifs, 1D [M(pyrazole)₄{μ-SiF₆}]_n (in 4 and 5) and discrete [{M{NCS}(pyrazole)₄}₂{μ-SiF₆}] (in 2 and 3) are discussed as special supramolecular synthons for the framework solids. An improved large-scale and cost-effective procedure for the synthesis of the organic ligand Me₄bpz is also described.