Unsymmetrical Diborane(4) Derivatives (RO)2B-B(NR´R´´)2 as Precursors for Diamino Boryl Complexes: From modular Synthesis to tailored ligands

• Applicant: Privatdozent Dr. Christian Kleeberg
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2015 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 283860287

Diaminoboryl complexes of transition metals are currently intensively studied, both as reactive intermediates in borylation reactions as well as due to the coordination chemical properties of this remarkable class of ligands. In particular diamino boryl ligands have been described as building blocks of multidentate ligands, especially pincer ligands with an E-B-E donor atom set (E: P, N, etc.). In contrast to the great interest only rather limited synthetic methods are available to synthesise diamino boryl ligands. The established methods are all more or less limited either regarding the accessible metals or regarding the ligand framework. Especially diamino boryl pincer ligands are currently only accessible by oxidative addition of a B-H bond onto a metal precursor in a low oxidation state.The applicant recently reported on a new synthetic route to diamino boryl ligands that appears to overcome the present synthetic limitations. Here, the diamino boryl ligand is obtained by the selective B-B bond activation of an unsymmetrical diborane(4) derivative of the type pinB-B(NR)2C6H4 either by sigma bond metathesis or by oxidative addition. Moreover, the required diborane(4) derivatives appear synthetically well accessible and versatile with regard to the ligand substitution pattern; hence, also here synthetic limitations are overcome. This project aims to intensify the preliminary studies and develop the scope of this method further with regard to the transition metals as well as the ligand framework, but in particular towards the construction of boryl pincer ligands. Building on a detailed systematic study and optimisation of the synthesis of the unsymmetrical diborane(4) derivative new substitution patterns will be made available. Using these data diborane(4) derivatives (RO)2B-B(NR´R´´)2 exhibiting substituents R´ containing additional donor atoms that are suitable precursors for diamino boryl ligands should be accessible. In a second part the synthesised unsymmetrical diborane(4) derivatives will be reacted with selected transition metal complexes in order to obtain diamino boryl (pincer) complexes. The obtained diamino boryl (pincer) complexes will be characterised in detail spectroscopically as well as structurally but especially regarding their reactivity. In connection with this the impact of the ligands substitution pattern on the B-B bond activation process and the coordination chemical properties of the diamino boryl ligand will be studied systematically. Finally, certain of the obtained boryl (pincer) complexes will be tested as (pre-)catalysts in catalytically relevant transformations, e.g. iridium catalysed C-H borylation.

• DFG Programme: Research Grants