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

Using an improved, chromatography-free dipyrrin synthesis, the alpha,beta- unsubstituted dipyrrins [ RC( C4H2N)(2)H] ( 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, 4R- 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 4R- 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 4mes displaying the greatest relative emission intensity.