Cyclic Polyamidato Dianions as Bridges between Mo₂⁴⁺ Units: Synthesis, Crystal Structures, Electrochemistry, Absorption Spectra, and Electronic Structures

Compounds in which quadruply bonded Mo24+ units, Mo2(DAniF)3 (DAniF = N,N‘-di-p-anisylformamidinate), are linked by cyclic diamidate anions have been synthesized and characterized by X-ray crystallography and spectroscopic methods. As identified by the diamidate linker, these compounds are 4,6-dioxypyrimidinate (2), 2,3-dioxypyrazinate (3), 2,3-dioxyquinoxalinate (4), 2,3-dioxy-5,6-dicyanopyrazinate (5), and cyanurate (6). With uracilate, a dinuclear unlinked 1:1 adduct is formed, Mo2(DAniF)3(uracilate) (1). The cyclic voltammograms of 3−5 reveal significantly larger ΔE1/2 values (258 mV−308 mV) than that of the oxalate linked analogue (212 mV), which is indicative of greater charge delocalization in the mixed valent Mo24+/Mo25+ species and hence greater communication between the two Mo2 units. ΔE1/2 for 2 is substantially lower than those for 3−5. This difference is attributed to the meta disposition of the two amidate groups in 4,6-dioxypyrimidinate as compared to their ortho arrangement in the pyrazinate-type linkers. The absorption spectra of the linked compounds 3−5 are more complex than those of the analogous polyunsaturated dicarboxylate linked compounds and reveal at least two significant absorption bands within the region 420−550 nm. Compound 2 also has two bands but with significantly lower intensity. Time dependent DFT calculations upon 2 and 3 indicate rather different electronic structures for these two structural isomers. The two bands for 3 have δ → π* character, and the π* type orbitals have substantial contributions from the Mo2 units as well as from the diamidate linker. The excitations observed in 2 are mainly metal based. The differences between the electronic spectra of 2 and 3 are consistent with the electrochemistry in underscoring the profound physical effect of changing the symmetry of the diamidate linker.