Structural, spectroscopic and redox properties of uranyl complexes with a maleonitrile containing ligand

Abstract

The reaction of uranyl nitrate hexahydrate with the maleonitrile containing Schiff base 2,3-bis[(4-diethylamino-2-hydroxybenzylidene)amino]but-2-enedinitrile (salmnt^(Et₂N)₂H₂) in methanol produces [UO₂(salmnt^(Et2N)2)(H₂O)] (1) where the uranyl equatorial coordination plane is completed by the N₂O₂ tetradentate cavity of the (salmnt^(Et₂N)₂)²⁻ ligand and a water molecule. The coordinated water molecule readily undergoes exchange with pyridine (py), dimethylsulfoxide (DMSO), N,N-dimethylformamide (DMF) and triphenylphosphine oxide (TPPO) to give a series of [UO₂(salmnt^(Et₂N)₂)(L)] complexes (L = py, DMSO, DMF, TPPO; 2–5, respectively). X-Ray crystallography of 1–5 show that the (salmnt^(Et₂N)₂)²⁻ ligand is distorted when coordinated to the uranyl moiety, in contrast to the planar structure observed for the free protonated ligand (salmnt^(Et₂N)₂H₂). The Raman spectra of 1–5 only display extremely weak bands (819–828 cm⁻¹) that can be assigned to the typically symmetric OUO stretch. This stretching mode is also observed in the infrared spectra for all complexes 1–5 (818–826 cm⁻¹) predominantly caused by the distortion of the tetradentate (salmnt^(Et₂N)₂)²⁻ ligand about the uranyl equatorial plane resulting in a change in dipole for this bond stretch. The solution behaviour of 2–5 was studied using NMR, electronic absorption and emission spectroscopy, and cyclic voltammetry. Complexes 2–5 exhibit intense absorptions in the visible region of the spectrum due to intramolecular charge transfer (ICT) transitions and the luminescence lifetimes (< 5 ns) indicate the emission arises from ligand-centred excited states. Reversible redox processes assigned to the {UO₂}²⁺/{UO₂}⁺ couple are observed for complexes 2–5 (2: E_1/2 = −1.80 V; 3,5: E_1/2 = −1.78 V; 4: E_1/2 = −1.81 V : vs.ferrocenium/ferrocene {Fc⁺/Fc}, 0.1 M Bu₄NPF₆) in dichloromethane (DCM). These are some of the most negative half potentials for the {UO₂}²⁺/{UO₂}⁺ couple observed to date and indicate the strong electron donating nature of the (salmnt^(Et₂N)₂)²⁻ ligand. Multiple uranyl redox processes are clearly seen for [UO₂(salmnt^(Et₂N)₂)(L)] in L (L = py, DMSO, DMF; 2–4: 0.1 M Bu₄NPF₆) indicating the relative instability of these complexes when competing ligands are present, but the reversible {UO₂}²⁺/{UO₂}⁺ couple for the intact complexes can still be assigned and shows the position of this couple can be modulated by the solvation environment. Several redox processes were also observed between +0.2 and +1.2 V (vs. Fc⁺/Fc) that prove the redox active nature of the maleonitrile-containing ligand.