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 1^2-, which possesses a quinoid-type structure.