Half-Sandwich Complexes of an Extremely Electron-Donating, Redox-Active η~6-Diborabenzene Ligand

摘要

The heteroarene 1,4-bis(CAAC)-1,4-diborabenzene ( 1 ; CAAC = cyclic (alkyl)(amino)carbene) reacts with [(MeCN)_(3)M(CO)_(3)] (M = Cr, Mo, W) to yield half-sandwich complexes of the form [(η~(6)-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, [(η~(6)-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 η~(4) 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.