The complex reactivity of β-diketiminato magnesium() dimers towards pinacolborane: implications for catalysis

Reactions between the borane, HBpin (pin = pinacolato), and three magnesium( i ) dimers, [{( Ar Nacnac)Mg-} 2 ] ( Ar Nacnac = [(ArNCMe) 2 CH] − ; Ar = xylyl (Xyl), mesityl (Mes) or 2,6-diethylphenyl (Dep)), have been carried out in 2 : 1, 5 : 1 and 20 : 1 ratios. In all cases, NMR spectroscopic studies have revealed complex mixtures of many known and novel products from these reactions. From extracts of the crude reaction mixtures, low yields of ten compounds, representing six different types of complexes between β-diketiminato magnesium fragments and boron containing ligands, have been isolated and crystallographically characterised. These include unprecedented examples of compounds in which the γ-carbon of the β-diketiminate ligand has been activated by boron hydride fragments. In addition, boryloxide (OBpin), borate ([B(pin) 2 ] − or [(pin)BH 2 ] − ), B-O bond ruptured [pinBH 2 ] − , a diborane(5) dianion, or BH 3 have been shown to be incorporated into the isolated complexes. The complexity of the products of the reported reactions are discussed in light of recently published patents and papers which report that magnesium( i ) dimers act as efficient catalysts, or as pre-catalysts to well defined catalysts, for the hydroboration of a variety of unsaturated substrates by HBpin. Our results strongly suggest that magnesium( i ) dimers are not catalysts in these reactions, and that there are many more potential (pre-)catalysts that are generated in these reactions, than have previously been reported. Reactions of magnesium( i ) dimers with pinacolborane are complex, yielding many products ( e.g. see picture). The results of this study bring into question mechanisms proposed for the magnesium( i ) catalysed hydroboration of unsaturated substrates.