A [4+2] Condensation Strategy to Imine-Linked Single-Crystalline Zeolite-Like Zinc Phosphate Frameworks

Double-4-ring zinc phosphate (D4R), [Zn(dipp)(4-Py-CHO)]₄ (2) (dipp=diiminopyridine), bearing four formyl groups, has been utilized as a building block (SBU) for the synthesis of a new class of imine-linked [4+2] COF-like polycrystalline zinc phosphate frameworks. Reactions of 2 with a series of linear aromatic diamines results in the formation of polycrystalline frameworks [Zn₄ (dipp)₄ (L)₂ ]_n (3-6) (L=L¹ to L⁴ , diimines formed by condensation of 4-pyridine carboxaldehyde with diamines). Employing an alternative synthetic strategy, through a diffusion-controlled slow reaction of 2 with the pre-synthesized 4,4'-bispyridyl bisimine (L³ ), [Zn₄ (dipp)₄ (L³ )₂ ]_n (5') has been obtained as single crystals. Complex 5' is a 3D-framework, exhibiting a rare eightfold interpenetrated diamondoid network. The long spacer length (19.6 Å) results in extensive entanglement in 5'. Powder diffraction data suggest that these compounds are isoreticular 3D-frameworks. To study the effect of the relative position of pyridyl donors with respect to the central benzidine moiety, 3,3'-bispyridyl bisimine (L⁵ ) was investigated as the spacer. A slow reaction of 1 b with L⁵ leads to the isolation of a 2D-boxed-sheet coordination polymer [Zn₄ (dipp)₄ (L⁵ )₂ ]_n (7). Selective formation of 3D-framework 5' from L³ and the 2D-framework 7 from L⁵ is due to the angles created by the coordination of para- and meta-pyridyl nitrogen centers at the zinc centers of the D4R cubane. Compound 5' has been utilized as a catalyst for Knoevenagel condensation.