Synthesis of a 2‑Isocyanophenolate Ligand, (2-CNC6H4O)1–, by Ring-Opening of Benzoxazole with Rare-Earth Metal Complexes

The allyl complexes (C5Me5)2Ln­(η3-C3H5) (Ln = Y, Dy, Tb) react with benzoxazole, C7H5NO, by deprotonation and ring-opening to yield dark red complexes identified by X-ray crystallography as the 2-isocyanophenolate-bridged bimetallic species, [(C5Me5)2Ln­(μ-2-CNC6H4O-κC:κO)]2, 1-Ln. Each lanthanide center is bound to the carbon of one isocyanophenolate ligand and the oxygen of another to form a 12-membered (Ln–O–C–C–N–C)2 ring which is coplanar to within 0.024 Å. The four C5Me5 rings have a square planar rather than tetrahedral arrangement with an angle of 89.4–89.5° between the plane of the four ring centroids and the 12-membered ring. Time-dependent density functional theory calculations on 1-Y indicated that the red color originated from transitions between filled C5Me5 π orbitals to empty (CNC6H4O) π* orbitals which have no significant metal character. DFT calculations on the product of one-electron reduction of 1-Y indicated that the ring, rather than the Y­(III) center, would be reduced, and the reaction of 1-Y with potassium graphite confirmed this.