Nickel(0)-Induced ß-H Elimination of Magnesium Alkyls: Formation and Reactivity of Heterometallic Hydrides.

We report the synthesis and reactivity of heterometallic Mg-Ni complexes with bridging hydrides. Treatment of magnesium monoalkyl complexes, which are supported by a tridentate ß-diketiminato ligand bearing a pendent phosphine group, with nickel(0) reagent Ni(COD)2 (COD: 1,5-cyclooctadiene) at a molar ratio of 2:1 resulted in the formation of a heterotrimetallic hydride-bridged [Mg-Ni-Mg] complex via facile elimination of the corresponding alkenes. A heterobimetallic hydride-bridged [Mg-Ni] complex served as an intermediate species for the formation of the [Mg-Ni-Mg] complex. Computational studies revealed that the reaction was initiated by coordination of nickel to magnesium followed by an alkyl group transfer. ß-H elimination at the nickel center subsequently occurred to give the heterometallic hydride-bridged complex. Density functional theory analysis also highlighted a three-center two-electron interaction for the Mg-H-Ni unit. The hydride-bridged [Mg-Ni-Mg] complex showed diverse reactivity toward unsaturated small molecules. For instance, reactions with isocyanides provided heterometallic species by coordination of isocyanides to the nickel center, with no subsequent reduction detected. Isocyanides could also be dissociated at 80 °C. In contrast, hydromagnesiation occurred upon treatment of the heterotrimetallic hydride with carbodiimides, affording C3-symmetric complexes with three heteroleptic magnesium mixed ß-diketiminate/amidinate moieties. The hydride-bridged heterotrimetallic complex underwent dehydrogenation reaction with phenyl acetylene to produce an acetylide-bridged [Mg-Ni-Mg] complex.

Stoichiometric reactions and catalytic dehydrogenations of amine-boranes with calcium aryloxide.

Catalysis with the main-group metal calcium conventionally relies on σ-bond metathesis to initiate the reactions. Here, we report a calcium aryloxide based on the tridentate ß-diketiminato ligand, which shows transition-metal-like reactivity with dialkylamine-boranes H3B·NHR2 (R = CH3, Cy, C6H5CH2, and iPr) to afford unprecedented amine-borane coordinated complexes through CaH interactions. This calcium aryloxide was also an active catalyst in dehydrogenation of amine-boranes, and gave aminoboranes H2B[double bond, length as m-dash]NR2, as the sole product. The initiating species was a calcium amine-borane complex rather than the traditional calcium amidoborane complex formed by σ-bond metathesis.