Cu4I4 clusters supported by P^N-type ligands: new structures with tunable emission colors.

A series of Cu(4)I(4) clusters (1-5) supported by two P^N-type ligands 2-[(diRphosphino)methyl]pyridine (1, R = phenyl; 2, R = cyclohexyl; 3, R = tert-butyl; 4, R = iso-propyl; 5, R = ethyl) have been synthesized. Single crystal X-ray analyses show that all five clusters adopt a rare "octahedral" geometry. The central core of the cluster consists of the copper atoms arranged in a parallelogram with μ(4)-iodides above and below the copper plane. The copper atoms on the two short edges of the parallelogram (Cu-Cu = 2.525(2)-2.630(1) Å) are bridged with μ(2)-iodides, whereas the long edges (Cu-Cu = 2.839(3)-3.035(2) Å) are bridged in an antiparallel fashion by the P^N ligands. This Cu(4)I(4) geometry differs significantly from the "cubane" and "stairstep" geometries reported for other Cu(4)I(4)L(4) clusters. Luminescence spectra of clusters 3 and 4 display a single emission around 460 nm at room temperature that is assigned to emission from a triplet halide-to-ligand charge-transfer ((3)XLCT) excited state, whereas clusters 1, 2, and 5 also have a second band around 570 nm that is assigned to a Cu(4)I(4) cluster-centered ((3)CC) excited state. The structural and photophysical properties of a dinuclear Cu(2)I(2)(P^N)(2) complex obtained during the sublimation of cluster 3 is also provided.