Supramolecular self-assembly for designing non-centrosymmetric crystals based on Keggin polyoxometallates and crown ether

Abstract

We report the synthesis of five novel crystals [(4-BrAni⁺)(DB[18]crown-6)]₂[SMo₁₂O₄₀²⁻]·2CH₃CN (1), [(4-BrAni⁺)(B[18]crown-6)]₂[SMo₁₂O₄₀²⁻]·CH₃CN (2), [(4-BrAni⁺)(B[18]crown-6)]₃[PMo₁₂O₄₀²⁻]·2CH₃CN (3), [(3-AP⁺)₃(B[18]crown-6)₂][PMo₁₂O₄₀³⁻] (4) and [NBu₄⁺][(3-AP²⁺)(DB[30]crown-10)][PMo₁₂O₄₀³⁻]·CH₃CN (5) (4-BrAni⁺ = 4-bromoanilinium; B[18]crown-6 = benzo[18]-crown-6; DB[18]crown-6 = dibenzo[18]-crown-6; DB[30]crown-10 = dibenzo[30]-crown-10; 3-AP⁺ = 3-aminopyridinium; 3-AP²⁺ = 3-ammoniumpyridinium; NBu₄⁺ = tetrabutylammonium). In order to construct non-centrosymmetric crystals, the five crystals were designed using a method to gradually introduce asymmetry into the building units. Crystal 1 was constructed with a symmetric supramolecular cation (SPC) [(4-BrAni⁺)(DB[18]crown-6)], resulting in a P2₁/n space group. The asymmetric SPC [(4-BrAni⁺)(B[18]crown-6)] was introduced into [SMo₁₂O₄₀²⁻] to obtain crystal 2, which belongs to the symmetric P space group. Introducing trivalent [PMo₁₂O₄₀³⁻], [(4-BrAni⁺)(B[18]crown-6)] produced crystal 3 with a non-centrosymmetric Pc space group. The asymmetric sandwich SPC [(3-AP⁺)₃(B[18]crown-6)₂] was designed with multiple hydrogen bonding sites on the 3-AP⁺ cation, and crystal 4 was obtained with trivalent [PMo₁₂O₄₀³⁻]. Crystal 4 has the properties of the chiral P1 space group. The distorted SPC (3-AP²⁺)(DB[30]crown-10) was constructed using flexible DB[30]crown-10, resulting in crystal 5 which matched the chiral P2₁ space group with trivalent [PMo₁₂O₄₀³⁻]. This work focuses on strategies for the rational design of novel non-centrosymmetric crystals without a chiral synthon.