Abnormally Long-Range
Diamagnetic Anisotropy Induced
by Cyclic d<sub>δ</sub>–p<sub>π</sub> π Conjugation
within a Six-Membered Dimolybdenum/Chalcogen Ring
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Abstract
Incorporating two quadruply bonded dimolybdenum units
[Mo<sub>2</sub>(DAniF)<sub><sup>3</sup></sub>]<sup>+</sup> (ancillary
ligand DAniF
= <i>N</i>,<i>N</i>′-di-<i>p</i>-anisylformamidinate) with two hydroselenides (SeH<sup>–</sup>) gave rise to [Mo<sub>2</sub>(DAniF)<sub>3</sub>]<sub>2</sub>(μ-SeH)<sub>2</sub> (<b>1</b>). With the molecular scaffold remaining unchanged,
aerobic oxidation of <b>1</b>, followed by autodeprotonation,
generated [Mo<sub>2</sub>(DAniF)<sub>3</sub>]<sub>2</sub>(μ-Se)<sub>2</sub> (<b>2</b>). The two complexes share a common cyclic
six-membered Mo<sub>2</sub>/Se core, but compound <b>2</b> is
distinct from <b>1</b> 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 d<sub>δ</sub>–p<sub>π</sub> conjugation within the central unit, which involves
six π electrons with one electron from each of the Mo<sub>2</sub> 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. <i>Chem.Eur. J.</i> <b>2011</b>, <i>17</i>, 10288). Of the three members in this family,
compound <b>2</b> exhibits an enhanced aromaticity because of
the selenium bridges. The remote in-plane and out-of-plane <i>methine</i> (ArNC<i>H</i>NAr) 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 Mo<sub>2</sub>/chalcogen core. By employment
of the McConnell equation {Δσ = Δχ[(l –
3 cos 2θ)/3<i>R</i><sup>3</sup><i>N</i>]},
the magnetic anisotropy (Δχ = χ<sub>⊥</sub> – χ<sub>||</sub>) 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