Theoretical study of the electronic and redox properties of different metal-substituted Lindqvist-type polyanions

Calculations based on density functional theory have been carried out to investigate the electronic and redox properties of a series of Lindqvist-type polyanions: [W5O18Zr(H2O)3-n(DMSO)n]2- (n = 0, 1, and 2), [W5O18M(H2O)3]2- (M = Ti and Hf), and [Mo5O18M(H2O)3]2- (M = Ti, Zr, and Hf). The results show that fragments of {Zr(H2O)3-n(DMSO)n}4+ (n = 0, 1, and 2) modify the HOMO-LUMO energy gaps of [W5O18Zr(H2O)3-n(DMSO)n]2- (n = 0, 1, and 2), which are lower than that of the related parent [W6O19]2-. Therefore, they are more easily reduced than their parent species. The LUMO of [W5O18Zr(H2O)3]2- mainly localizes on the d-orbitals of W (61.71%), and LUMO + 1 mainly concentrates on the W atoms, the contribution of Zr being only 2.38%. Thus, W atoms will preferentially accept an electron when [W5O18Zr(H2O)3]2- is reduced as a catalysts, which is reinforced by the spin density of the monoreduced species. However, the electronic properties of [W5O18Ti(H2O)3]2- are different from those of [W5O18Zr(H2O)3]2- and [W5O18Hf(H2O)3]2-. The substituting metal Ti effectively modifies the electronic properties of [W5O18Ti(H2O)3]2-. The LUMO of [W5O18Ti(H2O)3]2- is mainly concentrated on the Ti atom. Thus, the Ti atom of [W5O18Ti(H2O)3]2- becomes the reduced center, which means that different metal substitutions can alter the reduced center. In addition, the replacement of five W atoms by five Mo atoms in [Mo5O18M(H2O)3]2- (M = Zr, Ti, and Hf) significantly reduces the HOMO-LUMO energy gap, which indicates that Ti/Zr/Hf-Mo polynuclear Lindqvist complexes may make promising catalysts. It is hoped that the results presented here will give some impetus to interrelated studies.