Half-Sandwich Complexes of an Extremely Electron-Donating, Redox-Active η⁶‑Diborabenzene Ligand

The heteroarene 1,4-bis­(CAAC)-1,4-diborabenzene (1; CAAC = cyclic (alkyl)­(amino)­carbene) reacts with [(MeCN)3M­(CO)3] (M = Cr, Mo, W) to yield half-sandwich complexes of the form [(η6-diborabenzene)­M­(CO)3] (M = Cr (2), Mo (3), W (4)). Investigation of the new complexes with a combination of X-ray diffraction, spectroscopic methods and DFT calculations shows that ligand 1 is a remarkably strong electron donor. In particular, [(η6-arene)­M­(CO)3] complexes of this ligand display the lowest CO stretching frequencies yet observed for this class of complex. Cyclic voltammetry on complexes 2–4 revealed one reversible oxidation and two reversible reduction events in each case, with no evidence of ring-slippage of the arene to the η4 binding mode. Treatment of 4 with lithium metal in THF led to identification of the paramagnetic complex [(1)­W­(CO)3]­Li·2THF (5). Compound 1 can also be reduced in the absence of a transition metal to its dianion 12–, which possesses a quinoid-type structure.