Azadipyrromethene Complexes of d⁸ Metal Centers: Rhodium(I), Iridium(I), Palladium(II), and Platinum(II)

Abstract

Azadipyrromethenes are blue pigments that chelate main-group and d-block Lewis acids. Reported here are azadipyrromethene complexes of d⁸ metal centers. The new compounds are prepared in salt metathesis reactions with chlorinated organometallic precursors. Sixteen new complexes are reported. The principal absorption features are an intense peak near 600 nm and transitions in the ultraviolet; all are characteristic of the azadipyrromethene chromophore. All compounds are dark solids that yield blue or blue-violet solutions. Ten complexes are crystallographically characterized. The structures uniformly show backbone strain, with a <i>meso</i>-nitrogen atom that dilates from pure sp²-hybridization. Structural comparisons are made to related dipyrromethene and tetra-azaporphyrin complexes. The electron-donating capacity of azadipyrromethene ligands is evaluated from CO stretching frequencies of three rhodium­(I) carbonyl complexes and from density-functional theory calculations. Frontier orbitals are confined to the azadipyrromethene ligand. HOMO–LUMO energy gaps are almost unperturbed from those of the free, anionic azadipyrromethene