Solid-state structural transformations and photoreactivity of 1D-ladder coordination polymers of Pb^II

An attempt has been made to design double-stranded ladder-like coordination polymers (CPs) of hemidirected Pb^II. Four CPs, [Pb(μ-bpe)(O ₂C-C₆H₅)₂]×2H₂O (1), [Pb₂(μ-bpe)₂(μ-O₂C-C ₆H₅)₂(O₂C-C₆H ₅)₂] (2), [Pb₂(μ-bpe)₂(μ- O₂C-p-Tol)₂(O₂C-p-Tol)₂]× 1.5 H₂O (3) and [Pb₂(μ-bpe)₂(μ-O ₂C-m-Tol)₂(O₂C-m-Tol)₂] (4) (bpe=1,2-bis(4′-pyridyl)ethylene), have been synthesised and investigated for their solid-state photoreactivity. CPs 2-4, having a parallel orientation of bpe molecules in their ladder structures and being bridged by carboxylates, were found to be photoreactive, whereas CP 1 is a linear one-dimensional (1D) CP with guest water molecules aggregating to form a hydrogen-bonded 1D structure. The linear strands of 1 were found to pair up upon eliminating lattice water molecules by heating, which led to the solid-state structural transformation of photostable linear 1D CP 1 into photoreactive ladder CP 2. In the construction of the double-stranded ladder-like structures, the parallel alignment of Cï£C bonds in 2-4 is dictated by the chelating and μ₂-η²:η¹ bridging modes of the benzoate and toluate ligands. The role of solvents in the formation of such double-stranded ladder-like structures has also been investigated. A single-crystal-to-single-crystal transformation occurred when 4 was irradiated under UV light to form [Pb₂(rctt-tpcb)(μ-O₂C-m-Tol) ₂(O₂C-m-Tol)₂] (5). Dehydrate to be photoactive! The solid-state structural transformation of a linear coordination polymer of Pb^II into the corresponding ladder coordination polymer (see figure) and a single-crystal-to-single-crystal [2+2] cycloaddition reaction under UV light are reported.