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

The heteroarene 1,4-bis­(CAAC)-1,4-diborabenzene (<b>1</b>; 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 (<b>2</b>), Mo (<b>3</b>), W (<b>4</b>)). Investigation of the new complexes with a combination of X-ray diffraction, spectroscopic methods and DFT calculations shows that ligand <b>1</b> 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 <b>2</b>–<b>4</b> 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 <b>4</b> with lithium metal in THF led to identification of the paramagnetic complex [(<b>1</b>)­W­(CO)₃]­Li·2THF (<b>5</b>). Compound <b>1</b> can also be reduced in the absence of a transition metal to its dianion <b>1</b>^2–, which possesses a quinoid-type structure