Imidazolin-2-iminato Complexes of Rare Earth Metals with Very Short Metal−Nitrogen Bonds: Experimental and Theoretical Studies

The reactions of 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-imine (ImDippNH, 1-H) with trimethylsilylmethyl lithium (LiCH2SiMe3) and anhydrous rare earth metal trichlorides MCl3 afforded the imidazolin-2-iminato complexes [(1)MCl2(THF)3] (2a, M = Sc; 2b, M = Y; 2c, M = Lu) and [(1)GdCl2(THF)2]·[LiCl(THF)2] (2d). Treatment of complexes 2 with dipotassium cyclooctatetradienide, K2(C8H8) resulted in the formation of two- or three-legged piano-stool complexes of the type [(η8-C8H8)M(1)(THF)n] (3a, M = Sc, n = 1; 3b, M = Y, n = 2; 3c, M = Lu, n = 2; 3d, M = Gd, n = 2). X-ray diffraction analyses of all eight complexes 2 and 3 revealed the presence of very short metal−nitrogen bonds, which are among the shortest ever observed for these elements. [(η8-C8H8)Sc(1)(THF)] (3a) reacted with 2,6-dimethylphenyl isothiocyanate (Xy-NCS) to form the [2 + 2]-cycloaddition product 4, which contains a thioureato-N,N′ moiety. The related COT-titanium complex [(η8-C8H8)TiCl(1)] (6) could be obtained from [(1)TiCl3] (5) by reaction with K2(C8H8) and was structurally characterized. As a theoretical analysis of the nature of the metal−nitrogen bond, density functional theory (DFT) calculations have been carried out for complexes 3a and 6 and also for the model complexes [(η8-C8H8)Sc(NImMe)] (7), [(η8-C8H8)Ti(NImMe)]+ (8), and [(η8-C8H8)Ti(NXy)] (9), revealing a marked similarity of the bonding in imidazolin-2-iminato and conventional imido metal complexes.