TY - JOUR

T1 - Structural, spectroscopic and redox properties of uranyl complexes with a maleonitrile containing ligand

AU - Hardwick, Helen C.

AU - Royal, Drew S.

AU - Helliwell, Madeleine

AU - Pope, Simon J. A.

AU - Ashton, Lorna

AU - Goodacre, Roy

AU - Sharrad, Clint A.

PY - 2011

Y1 - 2011

N2 - 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)2)H(2)) 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)2)H(2)). The Raman spectra of 1-5 only display extremely weak bands (819-828 cm(-1)) that can be assigned to the typically symmetric O=U=O 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.

AB - 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)2)H(2)) 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)2)H(2)). The Raman spectra of 1-5 only display extremely weak bands (819-828 cm(-1)) that can be assigned to the typically symmetric O=U=O 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.

KW - TRANSITION-METAL COMPLEXES

KW - ELECTRON-SPIN-RESONANCE

KW - SCHIFF-BASE

KW - DIMETHYL-SULFOXIDE

KW - COORDINATION-COMPOUNDS

KW - CATECHOLATE LIGANDS

KW - MOLECULAR-STRUCTURE

KW - OPTICAL-PROPERTIES

KW - NEUTRAL MOLECULES

KW - CRYSTAL-STRUCTURE

U2 - 10.1039/c0dt01580f

DO - 10.1039/c0dt01580f

M3 - Journal article

VL - 40

SP - 5939

EP - 5952

JO - Dalton Transactions

JF - Dalton Transactions

SN - 1477-9226

IS - 22

ER -