Cationic and neutral copper(I) iodide cluster MOFs derived from tridentate N-donor functionalized P(V) ligands: synthesis, structure and photophysical properties

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Formation of cationic and neutral CuI cluster MOFs have been reported starting from tridentate phosphoramide ligands, [ $$\hbox {(NHR)}_{3}\hbox {P}=\hbox {E}$$ ] ( $$\hbox {L}^{1}$$ : $$\hbox {R} = \hbox {3-aminoquinolinyl (AQ)}$$ , $$\hbox {E} = \hbox {S}$$ ; $$\hbox {L}^{2}$$ : $$\hbox {R} = \hbox {3-pyridyl (PY)}$$ , $$\hbox {E} = \hbox {S}$$ ; $$\hbox {L}^{3}$$ : $$\hbox {R} = \hbox {3-aminoquinolinyl (AQ)}$$ , $$\hbox {E} = \hbox {O}$$ ). By utilizing $$\hbox {L}^{1}$$ , a cationic 2D-MOF $$\{[(\hbox {L}^{1})_{2}\hbox {(Cu}_{6}\hbox {I}_{5})]\hbox {(OH)}\cdot \hbox {3DMF}\cdot \hbox {4MeOH}\}_{\mathrm{n}}$$ , 1 containing a rugby ball shaped discrete $$\hbox {Cu}_{6}\hbox {I}_{5}$$ cluster has been reported earlier. Formation of a new 3D-MOF $$\{[(\hbox {L}^{2})_{2}(\hbox {Cu}_{6}\hbox {I}_{4})]\hbox {(OH)}_{2}\cdot \hbox {2DMF}\}_{\mathrm{n}}$$ containing a Zintl type $$[(\hbox {Cu}_{6}\hbox {I}_{4})^{2+}]_{\mathrm{n}}$$ cluster chains is reported in this paper. A neutral cluster MOFs 3 with formula unit of $$\{[\hbox {Cu}_{4}\hbox {I}_{4}\hbox {L}^{3}(\hbox {CH}_{3}\hbox {CN})]\cdot \hbox {2DMF}\cdot 3\hbox {H}_{2}\hbox {O}\}_{\mathrm{n}}$$ has been prepared from the ligand $$\hbox {L}^{3}$$ . Formation of the smaller $$\hbox {Cu}_{4}\hbox {I}_{4}$$ clusters in the MOF 3 is due to the presence of a MeCN ligation at one of the Cu(I) atoms which not only precludes the extension of the assembly in three dimension but also reduces the size of the obtained cluster. Unlike 1 which showed a ligand-assisted thermochromism, photophysical studies on the 3D-MOF 2 exhibited green phosphorescence at both 298 K and 77 K. The occurrence of the phosphorescence at 77 K in 2 is due to triplet cluster centered ( $${}^3\hbox {CC}$$ ) excited state of the cluster as there is no ligand-centered transition observed at 298 K. The 2D-MOF 3 does not show any characteristic luminescence behavior as the presence of the acetonitrile coordination at one of the Cu(I) ion is believed to quench the emission by non-radiative pathways. Further, luminescence quenching experiments on 1 and 2 with aromatic nitro-analytes showed a very high sensing selectivity for picric acid (TNP) over other aromatic nitro-analytes. SYNOPSIS: Employing tridentate phosphoramide ligands containing 3-aminoquinolyl and 3-pyridyl moieties, new examples of cluster MOFs containing cationic and neutral copper(I) iodide clusters were prepared. The MOF 2 contains a rugby ball-shaped, edge-fused polymeric $$\{[\hbox {Cu}_{6}\hbox {I}_{4}]^{2+}\}_{\mathrm{n}}$$ clusters, whereas the MOF 3 contains a cubane-type discrete $$\hbox {Cu}_{4}\hbox {I}_{4}$$ neutral cluster. Photophysical studies on the cationic complexes showed interesting phosphorescence emission and selective detection of picric acid.