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 (Im(DiPP)NH, 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)]center dot[LiCl(THF)(2)] (2d). Treatment of complexes 2 with dipotassium cyclooctatetradienide, K-2(C8H8) resulted in the formation of two- or three-legged pianostool complexes of the type [(eta(8)-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. [(eta(8)-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 [(eta(8) -C8H8)TiCl(1)] (6) could be obtained from [(1)TiCl3] (5) by reaction with K-2(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 [(eta(8)-C8H8)Sc(Nlm(Me))] (7), [(eta(8)-C8H8)Ti(Nlm(Me))](+) (8), and [(eta(8)-C8H8)Ti (NXy)] (9), revealing a marked similarity of the bonding in imidazolin-2-iminato and conventional imido metal complexes.