Synthetic strategy towards halometallates with imidazo[1,5-a]pyridinium-based counterions

Abstract

Organic–inorganic halometallates are a very promising class of compounds within the more general domain of organic–inorganic hybrid materials. The efficient approach for creating new hybrid halometallate salts is the design and use of monovalent organic cations, where the size, shape and electronic structure can be varied over wide limits, for introducing useful properties into hybrid structures. Therefore, we have prepared two new chlorometallate salts [L′]₂[ZnCl₄] (1) and [L′]_n[PbCl₃]_n∞ (2) with 2-methyl-imidazo[1,5-a]pyridinium cations [L′]⁺ formed as a result of the oxidative cyclocondensation between formaldehyde, methylamine and 2-pyridinecarbaldehyde in water media. While the tetrahedral ZnCl₄²⁻ anions show no connectivity in 1, the hybrid exhibits a pseudo-layered structure with organic and inorganic sublattices arranged parallel to the bc plane. In the crystal of 2, stacks of the aromatic cations alternate with one-dimensional stepwise chloroplumbate wires along the a direction. The use of PdCl₂ as a source of metal ions in the same synthetic procedure did not result in a hybrid tetrachloropalladate but produced the cationic complex [PdClL′′]Cl (3) with trapped bis-hemiaminal L′′, an intermediate to imine. In the solid state, 3 with two stereogenic centres should be seen as a 50 : 50 racemic mixture of two optically-active compounds but it shows a meso structure in DMSO solution. Broad emission with a maximum at around 386 nm of 1 and 2 in solution (λ_ex = 275 nm) originates from the organic component. Upon excitation at 214 nm, crystalline powder samples of 1 and 2 exhibit intense sky blue-light photoluminescence.