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

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: E1/2 = -1.80 V; 3,5: E1/2 = -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+/{UO 2}+ 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 Bu 4NPF6) indicating the relative instability of these complexes when competing ligands are present, but the reversible {UO 2}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. © 2011 The Royal Society of Chemistry.