Coordination self-assembly of tetranuclear Pt(ii) macrocycles with an organometallic backbone for sensing of acyclic dicarboxylic acids

Abstract

Coordination self-assembly of a series of tetranuclear Pt(II) macrocycles containing an organometallic backbone incorporating ethynyl functionality is presented. The 1 : 1 combination of a linear acceptor 1,4-bis[trans-Pt(PEt₃)₂(NO₃)(ethynyl)]benzene (1) with three different dipyridyl donor ‘clips’ (L_aa–L_cc) afforded three [2 + 2] self-assembled Pt^II₄ macrocycles (2a–2c) in quantitative yields, respectively [L_aa = 1,3-bis(3-pyridyl)isothalamide; L_bb = 1,3-bis(3-pyridyl)ethynylbenzene; L_cc = 1,8-bis(4-pyridyl)ethynylanthracene]. These macrocycles were characterized by multinuclear NMR (¹H and ³¹P); ESI-MS spectroscopy and the molecular structures of 2a and 2b were established by single crystal X-ray diffraction analysis. These macrocycles (2a–2c) are fluorescent in nature. The amide functionalized macrocycle 2a is used as a receptor to check the binding affinity of aliphatic acyclic dicarboxylic acids. Such binding affinity is examined using fluorescence and UV-Vis spectroscopic methods. A solution state fluorescence study showed that macrocycle 2a selectively binds (K_SV = 1.4 × 10⁴ M⁻¹) maleic acid by subsequent enhancement in emission intensity. Other aliphatic dicarboxylic acids such as fumaric, succinic, adipic, mesaconic and itaconic acids caused no change in the emission spectra; thereby demonstrating its potential use as a macrocyclic receptor in distinction of maleic acid from other aliphatic dicarboxylic acids.