Cyclic polyamidato dianions as bridges between Mo(2)(4+) units: synthesis, crystal structures, electrochemistry, absorption spectra, and electronic structures.
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abstract
Compounds in which quadruply bonded Mo(2)(4+) units, Mo(2)(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, Mo(2)(DAniF)(3)(uracilate) (1). The cyclic voltammograms of 3-5 reveal significantly larger DeltaE(1/2) 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 Mo(2)(4+)/Mo(2)(5+) species and hence greater communication between the two Mo(2) units. DeltaE(1/2) 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 delta --> pi character, and the pi type orbitals have substantial contributions from the Mo(2) 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.
