Abnormally Long-Range Diamagnetic Anisotropy Induced by Cyclic dδ–pπ π Conjugation within a Six-Membered Dimolybdenum/Chalcogen Ring

Incorporating two quadruply bonded dimolybdenum units [Mo2(DAniF)3]+ (ancillary ligand DAniF = N,N′-di-p-anisylformamidinate) with two hydroselenides (SeH–) gave rise to [Mo2(DAniF)3]2(μ-SeH)2 (1). With the molecular scaffold remaining unchanged, aerobic oxidation of 1, followed by autodeprotonation, generated [Mo2(DAniF)3]2(μ-Se)2 (2). The two complexes share a common cyclic six-membered Mo2/Se core, but compound 2 is distinct from 1 by having structural, electronic, and magnetic properties that correspond with aromaticity. Importantly, the aromatic behaviors for this non-carbon system are ascribable to the bonding analogy between the δ component in a Mo–Mo quadruple bond and the π component in a C–C double bond. Cyclic π delocalization via dδ–pπ conjugation within the central unit, which involves six π electrons with one electron from each of the Mo2 units and two electrons from each of the bridging atoms, has been confirmed in a previous work on the oxygen- and sulfur-bridged analogues (Fang, W.; et al. Chem.Eur. J. 2011, 17, 10288). Of the three members in this family, compound 2 exhibits an enhanced aromaticity because of the selenium bridges. The remote in-plane and out-of-plane methine (ArNCHNAr) protons resonate at chemical shifts (δ) 9.42 and 7.84 ppm, respectively. This NMR displacement, Δδ = 1.58 ppm, is larger than that for the oxygen-bridged (1.30 ppm) and sulfur-bridged (1.49 ppm) derivatives. The abnormally long-range shielding effects and the large diamagnetic anisotropy for this complex system can be rationalized by the induced ring currents circulating the Mo2/chalcogen core. By employment of the McConnell equation {Δσ = Δχ­[(l – 3 cos 2θ)/3R3N]}, the magnetic anisotropy (Δχ = χ⊥ – χ||) is estimated to be −414 ppm cgs, which is dramatically larger than −62.9 ppm cgs for benzene, the paradigm of aromaticity. In addition, it is found that the magnitude of Δχ is linearly related to the radius of the bridging atoms, with the selenium analogue having the largest value. This aromaticity sequence is in agreement with that for the chalcogen-containing aromatic family, e.g., furan < thiophene < selenophene.