Synthesis, Characterization, and Magnetism of Divalent Aryl Transition-metal Complexes of the Simplest Dialkylamide, Nme₂: Rare T-shaped Coordination at Chromium


The synthesis and characterization of a series of first-row aryl transition metal derivatives of the simplest dialkylamido ligand NMe2 are reported. The complexes CrAr'Cr(μ-NMe2)22 (1) and Ar'M(μ-NMe2)2 (M = Mn (2), Fe (3); Ar' = C6H3-2,6-(C6H3-2,6- iPr2)2) were obtained by reaction of the aryl metal halides Ar'M(μ-X)2 (M = Cr, X = Cl; M = Fe, X = Br) or Li(THF)Ar'MnI22 with LiNMe2 in a 1:2 ratio. A similar reaction of Ar#Co(μ-I)2 (Ar#= C6H3-2,6-(C6H2-2,4,6-Me 3)2) and LiNMe2 in hexanes gave the unusual complex Ar#Co(μ-I)(η1-CH2=NCH 3)2 (4), in which the NMe2 ligand is dehydrogenated to afford a complexed imine. Complexes 1-4 were characterized by X-ray crystallography, UV-vis spectroscopy, and magnetic measurements. In the unique trinuclear complex 1, the central chromium(II) ion is bound to four NMe2 groups in a square planar fashion. The NMe2 groups also bridge to the two outer chromium(II) ions, which are bound to a terminal Ar' group to yield a rare example of three-coordinate T-shaped geometry at these atoms. In the dimers 2 and 3, each metal center is coordinated to a terminal terphenyl ligand and two bridging NMe2 groups to give a distorted trigonal planar geometry. In contrast, the reaction of LiNMe2 with Ar#Co(μ-I)2 in a 2:1 ratio did not yield an amido product; instead, the NMe2 ligand underwent hydrogen elimination. As a result, in the dimeric structure of 4, each cobalt ion is coordinated to a terphenyl ligand, two bridging iodides, and a neutral methylimine ligand, CH2=NCH3, to yield a very distorted tetrahedral cobalt(II) coordination environment. The magnetic propertiesof1-4 revealed antiferromagnetic exchange coupling between the metal ions with J= -47(1) cm-1 and J13= -25(1) cm-1 for 1, J = -38(1) cm-1 for 2, J = -75(3) cm-1 for 3, and J = -32(4) cm -1 for 4; the latter compound exhibited an unusually large temperature independent contribution to its molar magnetic susceptibility. © 2009 American Chemical Society.



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