Role of the Meso Substituent in Defining the Reduction of Uranyl Dipyrrin Complexes

Abstract

The uranyl complex U^VIO₂Cl(L^Mes) of the redox-active, acyclic dipyrrin-diimine anion L^Mes- [HL^Mes = 1,9-di-tert-butyl-imine-5-(mesityl)dipyrrin] is reported, and its redox property is explored and compared with that of the previously reported U^VIO₂Cl(L^F) [HL^F = 1,9-di-tert-butyl-imine-5-(pentafluorophenyl)dipyrrin] to understand the influence of the meso substituent. Cyclic voltammetry, electron paramagnetic resonance spectroscopy, and density functional theory studies show that the alteration from an electron-withdrawing meso substituent to an electron-donating meso substituent on the dipyrrin ligand significantly modifies the stability of the products formed after reduction. For U^VIO₂Cl(L^Mes), the formation of a diamond-shaped, oxo-bridged uranyl(V) dimer, [U^VO₂(L^Mes)]₂ is seen, whereas in contrast, for U^VIO₂Cl(L^F), only ligand reduction occurs. Computational modeling of these reactions shows that while ligand reduction followed by chloride dissociation occurs in both cases, ligand-to-metal electron transfer is favorable for U^VIO₂Cl(L^Mes) only, which subsequently facilitates uranyl(V) dimerization.

Scheme 1. Previous Work Carried out on U^VIO₂Cl(L^F), (1)

The molecular orbital plot of 1. The ISO value is 0.02 au. Positive is blue; negative is red.

Scheme 2. Synthesis of Ligand HL^Mes

Figure 1

Solid-state structure of HL^Mes. For clarity, all hydrogen atoms except that of NH are omitted (where shown, displacement ellipsoids are drawn at 50% probability). Carbon atoms are gray.

Figure 2

Solid-state structure of K(L^Mes)·(THF)₂ viewed from the top (left) and side (right). For clarity, all hydrogen atoms and one molecule are omitted (displacement ellipsoids are drawn at 50% probability). Selected bonds (Å) and angles (deg): K1–N1, 2.897(3); K1–N2, 2.732(4); K1–N3, 2.715(3); K1–N4, 2.916(3); K1–O1, 2.761(3); K1–O2, 2.799(3); N1–K1–N2, 61.65(9); N2–K1–N3, 66.40(9); N3–K1–N4, 62.28(9); N4–K1–N1, 169.56(9); O1–K1–O2, 171.36(9); C20–C10–C9, 128.2(4); C9–C10–C11, 116.5(4); and C11–C10–C20, 115.3(4).

Scheme 3. Synthesis of U^VIO₂Cl(L^Mes) by Transmetalation with K(L^Mes) (Method A) and Directly from HL^Mes (Method B)

Figure 3

Solid-state structure of U^VIO₂Cl(L^Mes) viewed from the top (left) and side (right). For clarity, all hydrogen atoms are omitted (displacement ellipsoids are drawn at 50% probability). Selected bonds distances (Å) and angles (deg): U1–N1, 2.676(2); U1–N2, 2.469(2); U1–N3, 2.477(2); U1–N4, 2.675(2); U1–O1, 1.765(2); U1–O2, 1.768(2); U1–Cl1, 2.6882(7); N1–U1–N2, 65.85(6); N2–U1–N3, 70.30(6); N3–U1–N4, 65.30(6); N4–U1–N1, 152.03(6); N4–U1–Cl1, 79.22(4); N1–U1–Cl1, 87.84(4); and O1–U1–O2, 176.15(8).

Figure 4

UV–vis spectra of HL^Mes, K(L^Mes), and U^VIO₂Cl(L^Mes) in anhydrous THF.

Figure 5

Solid-state structure of [U^VO₂(L^Mes)]₂ viewed from the side (top) and top (bottom). For clarity, one molecule, one benzene solvate molecule, and all hydrogen atoms are omitted (displacement ellipsoids are drawn at 50% probability). Carbon atoms are gray. Selected bond distances (Å) and angles (deg): U1–U1′, 3.5299(4); U1–N1, 2.694(3); U1–N2, 2.495(3); U1–N3, 2.502(3); U1–N4, 2.665(4); U1–O1, 1.933(3); U1–O2, 1.833(3); U1–O1′, 2.395(3); N1–U1–N2, 65.4(1); N2–U1–N3, 70.6(1); N3–U1–N4, 65.2(1); N4–U1–N1, 150.6(1); O1–U1–O2, 175.5(1); O1–U1–O1′, 70.7(1); O1′–U1–O2, 113.3(1); U1–O1′–U1′, 109.3(1); and U1–O1–U1′, 109.3(1).

Figure 6

Stacked CVs of HL^Mes, K(L^Mes), and U^VIO₂Cl(L^Mes). All were measured as 1 mM anhydrous CH₂Cl₂ solutions (a 1.0 M [ⁿBu₄N][PF₆] supporting electrolyte, a glassy carbon working electrode, a Pt gauze counter electrode, and a silver wire quasi-reference electrode). Potentials are referenced against Fc/Fc⁺ couple recorded under identical conditions.

Figure 7

X-Band EPR spectra of [U^VIO₂Cl(L^Mes•)]⁻ (a) and [U^VIO₂Cl(L^F•)]⁻ (b) generated in anhydrous CH₂Cl₂ solution at ambient temperature. The measured spectra are shown in black solid lines, and the simulated spectra are shown in dashed black and solid red lines.

Figure 8

Molecular orbital plots of U^VIO₂Cl(L^Mes) and [U^VIO₂Cl(L^Mes•)]⁻ (a,b) and U^VIO₂Cl(L^F) and [U^VIO₂Cl(L^F•)]⁻ (c,d) and spin density plots of the singly reduced complexes [U^VIO₂Cl(L^•)]⁻ (e,f). The ISO value is 0.02 au. Positive is blue; negative is red.

Scheme 4. Reduction Processes for U^VIO₂Cl(L^Mes) and U^VIO₂Cl(L^F) Resulting in [U^VO₂(L^Mes)]₂ and [Cp₂Co][U^VIO₂CL(L^F•)], Respectively