Extremely Bulky Amido d-Block Metal(II) Halide Complexes
It is perhaps surprising that bulky monodentate amide ligands (-NR2, R = an alkyl or aryl group), which have been utilized for decades to stabilize low coordinate d-block metal complexes, e.g. two-coordinate [M(NR2)2] complexes, have not been successfully
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Extremely Bulky Amido d-Block Metal(II) Halide Complexes
2.1
Introduction
It is perhaps surprising that bulky monodentate amide ligands (–NR2, R = an alkyl or aryl group), which have been utilized for decades to stabilize low coordinate d-block metal complexes [1], e.g. two-coordinate [M(NR2)2] complexes [2], have not been successfully employed in the preparation of related metal(I) dimers. This may result from the fact that reports of the amido d-block metal(II) halide precursor complexes are sparse, despite the many hundreds of structurally characterized amido d-block metal complexes that populate the literature [1]. Though monodentate amido d-block metal(II) halide complexes are limited in number, they are not unknown. However, this complex type usually requires either sterically demanding ligands, or strongly coordinating p-acid ligands such as phosphines or carbon monoxide to be isolated. Examples of amido d-block metal(II) halides with coordinating p-acid ligands include [{(dtma)Cu(PPh3)(µ-Cl)}2] [3] (dtma = di-p-tolylmethylamido) and [Ni(PMe3)2(pyr)Cl] [4] (pyr = pyrrolyl). However, in low-coordinate, low oxidation state chemistry, precursor complexes that do not possess strongly coordinating p-acid ligands are desired, as these ligands will likely be present in the final product, increasing the coordination number of the metal centre, thereby lowering its reactivity. Attempting to prepare a monodentate amido metal(II) halide complex without the use of sterically demanding or p-acid ligands will almost always lead to the formation of the homoleptic species [M(NR2)2] and the metal(II) halide, via redistribution. However, utilizing steric bulk on its own, without p-acid ligands, has led to a handful of monodentate amido d-block metal(II) halide complexes.
© Springer Nature Singapore Pte Ltd. 2017 J. Hicks, Preparation, Characterisation and Reactivity of Low Oxidation State d-Block Metal Complexes Stabilised by Extremely Bulky Amide Ligands, Springer Theses, DOI 10.1007/978-981-10-2905-9_2
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2 Extremely Bulky Amido d-Block Metal(II) Halide Complexes
2.1.1
Previously Reported Bulky Amido Transition Metal(II) Halide Complexes
In 1996, the group of Cummins reported the synthesis and isolation of the low valent amido iron(II) chloride complex [FeCl{N(R)(Ar#)}(tmeda)] [5] 2-01 (R = C (CD3)2CH3, Ar# = 2,5-FMeC6H3). The complex is stabilized by a bulky amide ligand, along with a molecule of chelating tetramethylethylenediamine (tmeda) coordinating to the iron centre. Complex 2-01 was prepared by a salt metathesis reaction between iron(II) bromide and the lithium amide salt [Ar#(R)NLi], in the presence of tmeda (Scheme 2.1). The four-coordinate iron(II) complex was found to be monomeric, with the iron(II) centre having a distorted tetrahedral geometry. The four coordination sites of the metal are occupied by the bulky amide ligand, the chloride, and a chelating molecule of tmeda. Complex 2-01 has an effective magnetic moment in solution of 5.29 µB at room temperature, determined by the Evans method, which
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