Metalla-macro-tricyclic cryptands: Anion encapsulation and selective separation of sulfate via in situ crystallization

Abstract

A conformationally flexible bis-pyridyl-bis-amide ligand, namely N,N′-bis-(3-pyridyl)isophthalamide (L1), has been exploited to synthesize metalla-macro-tricyclic cryptands by reacting it with various Cu(II) salts having different counteranions. Out of the five coordination compounds, namely [{Cu(μ-L1)₂(H₂O)₂}·SO₄·2H₂O·X] (1a), [{Cu(μ-L1)(H₂O)₄}·SO₄·3H₂O]_∝ (1b), [{Cu(μ-L1)₂(H₂O)₂}·SiF₆·CH₃OH·4H₂O]_∝ (2), [{Cu(μ-L1)₂ (μ-Cl)}·ClO₄]_∝ (3) and [{Cu(μ-L1) ₂(H₂O)}·(NO₃)2·2H₂O·X]_∝ (4a) (X = disordered solvents), compounds such as 1a, 2 and 3 are metallacryptands, of which 3 is the first example of a polymeric metalla-macro-tricyclic cryptand. The effect of the conformation-dependent ligating topology and hydrogen bonding backbone of ligand L1, and counteranions on the formation of metallacryptands is discussed. Interestingly, an important anion, namely SO₄^2-, has been separated by concomitantly crystallizing 1a and 1b from a complex mixture of anions, such as SO₄^2-, NO₃^- and ClO₄^-, by following an in situ crystallization technique. Magnetic interactions in 3 have been investigated as a typical example. Weak antiferromagnetic coupling is observed in 3, as expected given the topology of the networks.