Comparative Analysis of Mononuclear 1:1 and 2:1 Tetravalent Actinide (U, Th, Np) Complexes: Crystal Structure, Spectroscopy, and Electrochemistry.

Six mononuclear tetravalent actinide complexes ( 1 - 6 ) have been synthesized using a new Schiff base ligand 2-methoxy-6-(((2-methyl-1-(pyridin-2-yl)propyl)imino)methyl)phenol ( HL pr ). The HL pr is treated with tetravalent actinide elements in varied stoichiometries to afford mononuclear 1:1 complexes [MCl 3 - L pr · n THF] ( 1 - 3 ) and 2:1 complexes [MCl 2 - L 2 pr ] ( 4 - 6 ) (M = Th 4+ ( 1 and 4 ), U 4+ ( 2 and 5 ), and Np 4+ ( 3 and 6 )). All complexes are characterized using different analytical techniques such as IR, NMR, and absorption spectroscopy as well as crystallography. UV-vis spectroscopy revealed more red-shifted absorption spectra for 2:1 complexes as compared to 1:1 complexes. 1 H NMR of Th(IV) complexes exhibit diamagnetic spectra, whereas U(IV) and Np(IV) complexes revealed paramagnetically shifted 1 H NMR. Interestingly, NMR signals are paramagnetically shifted between -70 and 40 ppm in 2 and 3 but are confined within -35 to 25 ppm in 2:1 complexes 5 and 6 . Single-crystal structures for 1:1 complexes revealed an eight-coordinated Th(IV) complex ( 1 ) and seven-coordinated U(IV) ( 2 ) and Np(IV) ( 3 ) complexes. However, all 2:1 complexes 4 - 6 were isolated as eight-coordinated isostructural molecules. The geometry around the Th 4+ center in 1 is found to be trigonal dodecahedral and capped trigonal prismatic around U(IV) and Np(IV) centers in 2 and 3 , respectively. However, An 4+ centers in 2:1 complexes are present in dodecahedral geometry. Importantly, 2:1 complexes exhibit increased bond distances in comparison to their 1:1 counterparts as well as interesting bond modulation with respect to ionic radii of An(IV) centers. Cyclic voltammetry displays an increased oxidation potential of the ligand by 300-500 mV, after coordination with An 4+ . CV studies indicate Th(IV)/Th(II) reduction beyond -2.3 V, whereas attempts were made to identify redox potentials for U(IV) and Np(IV) centers. Spectroscopic binding studies reveal that complex stability in 1:1 stoichiometry follows the order Th 4+ ≈ U 4+ > Np 4+ .