Experimental and Theoretical Evidence of Aromatic Behavior in Heterobenzene-Like Molecules with Metal-Metal Multiple Bonds
Fang, Wenjuan（Tongji Univ, Dept Chem）
Tan, ZhongFei（Tongji Univ, Dept Chem）
Liu, Chun Y.（Tongji Univ, Dept Chem）
Murillo, Carlos A.（Texas A&M Univ, Dept Chem）
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Binding two quadruply bonded dimolybdenum units [Mo(2)(DAniF)(3)](+) (DAniF=N,N'-di-p-anisylformamidinate) with two chalcogen atoms generated two molecules with a central core composed of a cyclic six-membered [Mo(2)](2)(mu-EH)(2) species (E=S in 1 and O in 3, and [Mo(2)] is a quadruple-bonded [Mo(2)(formamidinate)(3)] unit). Aerobic oxidation of 1 and 3 followed by concomitant deprotonation gave rise to the corresponding [Mo(2)](2)(mu-E)(2) compounds 2 and 4. The latter show a striking coplanarity and near-bond equalization of the Mo/E cluster. The oxidized species 2 and 4 are diamagnetic in the measured temperature range of 5 to 300 K, which is somewhat unexpected for molecules that have di-metal units with a sigma(2)pi(4)delta(1) electronic configuration. This suggests there are strong interactions between the dimolybdenum units through the E atoms. The large electronic delocalization of the delta electrons over the entire Mo/E core is supported by the exceptionally large potential separation for the two successive one-electron reductions of the linked Mo(2)(5+) units from the oxidized species (Delta E(1/2) = 1.7 V for the sulfur analogue). This large electronic delocalization has an important effect on the NMR spectroscopic signals for the two sets of methine (N-(CH)-N) protons from the DAniF ligands. Those essentially parallel to the core, H(parallel to), and those essentially perpendicular to the core, H(vertical bar), exhibit downfield and upfield chemical shifts, respectively, that are separated by delta=1.32 ppm. The structural, electronic, magnetic, and chemical behaviors for 2 and 4 are consistent with aromaticity, with the [Mo(2)E(2)Mo(2)] cores that resemble the prototypical benzene molecule. Theoretical studies, including DFT calculations, natural bond orbital (NBO) analyses, and gauge-independent atomic orbital (GIAO) NMR spectroscopic calculations, are also consistent with the aromaticity of the [Mo(2)](2)(m-E)(2) units being promoted by d(delta)(Mo(2))-p(pi)(E) pi conjugation. The cyclic pi conjugation of the central moiety in 2 and 4 involves a total of six electrons with 2e from delta(Mo(2)) and 4e from p(pi)(E) orbitals, thereby conforming to Huckel's rule when electrons in the MOs with delta character are considered part of the delocalized system.
CitationCHEMISTRY-A EUROPEAN JOURNAL，2011,17（37）：10288-10296