Role of Redox-Inactive Metal Ions in Modulating the Reduction Potential of Uranyl Schiff Base Complexes: Detailed Experimental and Theoretical Studies.

A mononuclear uranyl complex, [UO 2 L] ( 1 ), has been synthesized with the ligand N , N '-bis(3-methoxy-2-hydroxybenzylidene)-1,6-diamino-3-azahexane (H 2 L). The complex showed a reversible U(VI)/U(V) redox couple in cyclic voltammetric measurements. The reduction potential of this couple showed a positive shift upon the addition of redox-inactive alkali- and alkaline-earth Lewis acidic metal ions (Li + , Na + , K + , Ca 2+ , Sr 2+ , and Ba 2+ ) to an acetonitrile solution of complex 1 . The positive shift of the reduction potential has been explained on the basis of the Lewis acidity and internal electric-field effect of the respective metal ions. The bimetallic complexes [UO 2 LLi(NO 3 )] ( 2 ), [UO 2 LNa(BF 4 )] 2 ( 3 ), [UO 2 LK(PF 6 )] 2 ( 4 ), [(UO 2 L) 2 Ca]·(ClO 4 ) 2 ·CH 3 CN ( 5 ), [(UO 2 L) 2 Sr(H 2 O) 2 ]·(ClO 4 ) 2 ·CH 3 CN ( 6 ), and [(UO 2 L) 2 Ba(ClO 4 )]·(ClO 4 ) ( 7 ) have also been isolated in the solid state by reacting complex 1 with the corresponding metal ions and characterized by single-crystal X-ray diffraction. Density functional theory calculations of the optimized [UO 2 LM] n + complexes have been used to rationalize the experimental reduction and electric-field potentials imposed by the non-redox-active cations.