Chromium(II) complexes bearing 2,6-substituted N,N′-diarylformamidinate ligands

Abstract

The synthesis, isolation and characterisation of several new chromium(II) complexes of the N,N′-bis(2,6-xylyl)formamidinate ligand (DXylF) has been undertaken. The structure of three such complexes, namely Cr(DXylF)₂, 1, Cr₂(μ-Cl)₂(DXylF)₂(THF)₂, 2, and Cr₂(Ac)₂(DXylF)₂(THF)₂, 3, have been determined by X-ray crystallography. It has been shown that the steric demands of the 2,6-xylyl group effectively discourage the formation of the homoleptic paddlewheel structure Cr₂(DXylF)₄: instead, the mononuclear (but stoichiometrically equivalent) complex Cr(DXylF)₂, 1 was obtained. It is likely that the unusual dichromium dichloride-bridged intermediate 2 also owes its stability to the presence of such bulky substituents. Although this complex does not possess a significant metal–metal bonding interaction [Cr–Cr = 2.612(1) Å], it has however, proven to be a useful starting material for the preparation of the novel quadruply bonded dichromium mixed acetate–formamidinate complex 3 [Cr–Cr (avg) = 2.339(7) Å]. In both 2 and 3, the 2,6-xylyl groups force the formamidinate ligands to oppose one another contrary to the trans effect. In addition, it was found that compound 3 can be accessed from the chromium acetate starting material and, for comparison, an attempt was also made to prepare the N,N′-bis(o-anisyl)formamidinate (DPh^OMeF) analogue. With the formamidinates occupying positions cis to one another and with the absence of axially coordinated THF ligands, the molecular structure of the resultant Cr₂(Ac)₂(DPh^OMeF)₂, 4 [Cr–Cr = 2.037(1) Å], is different from that of 3.