Low-valent first row transition metal complexes stabilised by bulky amidinates and guanidinates: synthesis and reactivity studies

2017-01-24T00:44:52Z (GMT) by Fohlmeister, Lea
This thesis describes the synthesis and reactivity of low-valent and low-coordinate amidinato and guanidinato first row transition metal complexes, in particular of the elements iron and manganese. The work presented in this thesis is divided into five chapters and a short summary of each chapter will be given here. Chapter 1 gives a general introduction into the d-block elements, including commonly used models to explain bonding in transition metal compounds and some of the properties that arise from having partly filled d orbitals, e.g. magnetism and electronic transitions. It includes an overview of important low oxidation-state and metal-metal bonded complexes of the d-block elements and will familiarise the reader with amidinate and guanidinate ligands ([(RN)2CR’]-, R = aliphatic/aromatic group, R’ = aliphatic/aromatic group (amidinate) or amino group (guanidinate)), which were mainly used to stabilise the target compounds described throughout this thesis. Chapter 2 focuses on the preparation of low-valent amidinato and guanidinato iron complexes, especially the synthesis of a guanidinato iron(I) dimer, [{(µ-N,N’-Pipiso)Fe}2] (Pipiso- = [(2,6-iPr2C6H3N)2C(cis-2,6-Me2NC5H8)]-), which has an extremely short Fe‒Fe multiple bond. Its properties and characterisation using magnetic, Mössbauer and theoretical studies are also detailed. The reactivity of [{(µ-N,N’-Pipiso)Fe}2] towards a variety of small molecules is presented in Chapter 3. Of special interest were the reactions of the Fe dimer with group 16 elements, which led to the isolation of rare examples of neutral bis(µ-chalcogenido)diiron(III) complexes with the general formula [{(κ2-N,N’-Pipiso)Fe(µ-E)}2] (E = O, S, Se). UV/vis and Mössbauer spectroscopy, as well as theoretical calculations, were amongst the methods employed to gain an understanding of the properties and electronic structure of these compounds. A short discussion on the feasibility of using the dinuclear bis(µ-sulfido)-diiron(III) compound as a model system for Rieske-type [2Fe-2S] clusters is included. Chapter 4 summarises the synthesis of manganese(II) halides supported by amidinate and guanidinate ligands, which were intended as potential precursors for low-valent manganese complexes. The reaction of the amidinato Mn bromide [{(κ2-N,N’-Piso)Mn(µ-Br)}3(THF)2] (Piso- = [(2,6-iPr2C6H3N)2C(tBu)]-) with the magnesium(I) reagent [{(Mesnacnac)Mg}2] (Mesnacnac- = [{(2,4,6-Me3C6H2N)C(Me)}2CH]-) resulted in the isolation of a manganese(I) dimer, [{(N,arene-Piso)Mn}2], with an unsupported Mn‒Mn bond, while treatment of the same precursor with K[BHEt3] yielded its hydride analogue [{(N,arene-Piso)Mn(µ-H)}2]. Initial reactivity studies of both compounds are described, which show that the hydride complex can be used as a source of manganese(I). Several miscellaneous results are presented in Chapter 5. These include the syntheses of homoleptic amidinato and phosphaguanidinato transition metal complexes, the formation of which competes with that of heteroleptic compounds, [{LMX}n] (L = amidinate/guanidinate, M = transition element, X = halide) in salt metathesis reactions between alkali metal salts of the ligands and MX2. The attempted preparation of amidinato vanadium halide precursors is also discussed, which instead led to the isolation of a tetranuclear vanadium cluster and a vanadium(II) adduct. Finally, the use of bulky bis(aryl)amide ligands for the synthesis of two new transition metal halide complexes is portrayed.