„Copper(I) boryl complexes: From Reactivity Studies on Catalytic Intermediates to Versatile Boryl Ligand Sources

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Copper(I) boryl complexes are well established reactive intermediates in copper catalysed borylation reactions with diborane(4) derivatives. Experimental mechanistic investigations on those reactions are, however, scarce. The few existing studies use the mononuclear and linear copper complex [(IDipp)Cu–Bpin] as catalyst model (IDipp = 1,3-bis(2,6-diisopropyl¬phenyl)imidazol-2-ylidene, pin = (OCMe2)2). In preliminary studies we showed that this complex with its sterically demanding IDipp ligand is not representative for the copper(I) boryl complexes present under typical catalytic conditions. With the typically employed, less sterically demanding NHC and phosphine ligands dinuclear bis-boryl bridged complexes are predominant. The structure of the relevant boryl complexes, however, depends on both the properties of the auxiliary ligand as well as the boryl ligand. The central aim of this research project is the systematic study of the reactivity of those copper boryl complexes as a function of the boryl as well as auxiliary ligand and, hence, the structure of the complex. As auxiliary ligands a variety of NHC and phospine ligands will be used, whereas different dialkoxido- and diaminoboryl ligands will be employed, as established in the preliminary studies. Central to this study are stoichiometric studies of the reactivity of well selected representative substrates with a focus on the NMR spectroscopic and structural characterisation of the present intermediates. The results will contribute to a fundamental understanding and consequently the rational further development of copper catalysed borylation reactions as synthetic tools. In addition comparative and analogous reactivity studies will be conducted on low-valent copper boryl complexes; species typically produced as decomposition products of copper(I) boryl complexes. The understanding of those species is crucial for a rational further development of the field. Furthermore, similar reactivity studies on the boryl complexes of the heavier coinage metals silver and gold will be conducted. As a first step, however, it is necessary to make silver and gold boryl complexes with different auxiliary ligands synthetically available. Those complexes will then be used to establish differences and similarities among the homologous coinage metal boryl complexes. Last, in a more explorative attempt, the use of copper(I) boryl complexs as a source of boryl ligands for transition metal complexes will be investigated. This should lead to novel, previously not accessible boryl complexes and will as such open the way to fundamentally new possibilities.