A Borane Lewis Acid in the Secondary Coordination Sphere of a Ni(II) Imido Imparts Distinct C-H Activation Selectivity

Abstract

Two borane-functionalized bidentate phosphine ligands that vary in tether length have been prepared to examine cooperative metal-substrate interactions. Ni(0) complexes react with aryl azides at low temperatures to form structurally unusual κ²-(N,N)-N₃Ar adducts. Warming these adducts affords products of N₂ extrusion and in one case, a Ni-imido compound that is capped by the appended borane. Reactions with 1-azidoadamantane (AdN₃) provide a distinct outcome, where a proposed nickel imido intermediate activates the sp² C-H bonds of arenes, even in the presence of benzylic C-H sites. Combined experimental and computational mechanistic studies demonstrate that the unique reactivity is a consequence of Lewis-acid-induced polarization of the Ni-NR bond, potentially providing a synthetic strategy for chemoselective reaction engineering.

Figure 1.

C-H bond activation reactivity triggered by previously reported Ni imido complexes as well as that reported in this work.

Figure 2.

Synthesis of 9-BBN functionalized bisphosphine ligands 1a and 1b.

Figure 3.

Synthesis of Ni(0)-COD complexes 3a and 3b based on ligands 1a and 1b.

Figure 4.

Generation of Ni(0)-azide monomer complexes, and the X-ray structures of 4b with 50% probability ellipsoids and 4c with 30%. The solvent (diethyl ether) and all H atoms were omitted and the 9-BBN is displayed in wireframe for clarity. Note that compounds 4 are only stable at temperatures <−35 °C.

Figure 5.

Generation of Ni(II) imido complex 5a and Ni(II) borylamide complex 5b, and X-ray structures (50% probability ellipsoids, selected H atoms omitted).

Figure 6.

Generation of Ni(II) benzyl amide complex 5c as well as its structure (50% probability ellipsoids, selected H atoms omitted).

Figure 7.

Arene activations by proposed Ni(II) imido complexes as well as the structures of 6a and 6b (only one isomer was shown) with 50% probability ellipsoids, with selected H atoms omitted.

Figure 8.

Hammett-type plot of observed initial rate of C-H activation on 1,3-disubstituted arenes against 2σ_m.

Figure 9.

²H NMR spectra of toluene-d₈, C-H activation experiments with 3a, 3c, 3c in conjunction with 9-octyl-9-BBN.

Figure 10.

NBO analysis of (dmpe)NiNtBu (7b) and 7a with a comparison in structural parameters such as natural charge difference between Ni and N (Δq_(Ni-N)), Ni-N bond length and order (BO_(Ni-N)).

Figure 11.

Base-induced reductive elimination of 6a.