F-element metalated dipyrrins: synthesis and characterization of a family of uranyl bis(dipyrrinate) complexes

Abstract

Using an improved, chromatography-free dipyrrin synthesis, the α,β-unsubstituted dipyrrins [RC(C₄H₂N)₂H] (2) (R = tolyl (2^tolyl), p-OMe-C₆H₄ (2^anis), mesityl (2^mes), ferrocenyl (2^Fc)) were isolated in good to excellent yields. Deprotonation of 2 with Na[N(SiMe₃)₂] gives the alkali metal salts [Na(DME)_n][RC(C₄H₂N)₂] (3) which reacts with UO₂Cl₂(THF)₃ to give the uranyl bis(dipyrrinates) UO₂[RC(C₄H₂N)₂]₂(L) (L = THF (4^R-THF); DMAP (4^R-DMAP)) (R = tolyl, p-OMe-C₆H₄, mesityl, ferrocenyl). The THF adducts, 4^R-THF, are unstable in aromatic and nonpolar solvents and rapidly decompose to 2 and an intractable uranium-containing solid. On the other hand, the DMAP adducts, 4^R-DMAP, are indefinitely stable in solution. The solid-state structures of 4^R-THF and 4^R-DMAP reveal distorted trigonal bipyramidal geometries. In the solid-state, the dipyrrinate ligands exhibit significant distortions including bowing and, in some instances, out-of-plane equatorial N-atom coordination, likely as a consequence of steric crowding and interligand repulsion. The complexes, 4^R-DMAP, have been fully characterized by NMR, UV/Vis, and fluorescence spectroscopies, and their electrochemical properties have been investigated through cyclic voltammetry. The cyclic voltammograms of 4^R-DMAP display several redox features but present a reversible wave at ca. −1.9 V (vs. Fc^0/+) attributable to a ligand centred reduction. Fluorescence measurements of all compounds reveal that only the mesityl derivatives 2^mes, 3^mes, and 4^mes fluoresce with modest Stokes shift that ranges from ca. 30–70 nm, with 4^mes displaying the greatest relative emission intensity.