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 κ 2 -(N,N)-N 3 Ar adducts. Warming these adducts affords products of N 2 extrusion and in one case, a Ni-imido compound that is capped by the appended borane. Reactions with 1-azidoadamantane (AdN 3 ) provide a distinct outcome, where a proposed nickel imido intermediate activates the sp 2 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.

Facile Synthesis of Uranium Complexes with a Pendant Borane Lewis Acid and 1,2‐Insertion of CO into a U−N Bond

“…Drover and co ‐workers demonstrated that the appended borane moiety has great impact on the ability of the hydride complexes [ B H] + in hydride transfer, as well as the oxidative addition behavior at the Ni 0 center in B [7] . Very recently, Chu, Szymczak and the co‐workers successfully applied a pendant 9‐BBN (borabicyclo[3.3.1]nonyl) group to stabilize the reactive Ni II imido intermediates (Ni=NR) thanks to the presence of the secondary B−N interactions ( C ), and thus achieved the intermolecular 1,2‐C Ar ‐H‐addition or intramolecular 1,2‐C benzylic ‐H‐addition across the Ni=N bonds [8] . It is noteworthy that the unique chemical behavior of the d ‐metal complexes bearing acidic groups in the secondary coordination sphere is commonly associated with the oxidative‐addition/reduction‐elimination process, [5d, 6, 7a,b, 8] which is nevertheless unfavorable for f ‐metals.…”

“…Very recently, Chu, Szymczak and the co‐workers successfully applied a pendant 9‐BBN (borabicyclo[3.3.1]nonyl) group to stabilize the reactive Ni II imido intermediates (Ni=NR) thanks to the presence of the secondary B−N interactions ( C ), and thus achieved the intermolecular 1,2‐C Ar ‐H‐addition or intramolecular 1,2‐C benzylic ‐H‐addition across the Ni=N bonds [8] . It is noteworthy that the unique chemical behavior of the d ‐metal complexes bearing acidic groups in the secondary coordination sphere is commonly associated with the oxidative‐addition/reduction‐elimination process, [5d, 6, 7a,b, 8] which is nevertheless unfavorable for f ‐metals. As such, uranium complexes with appended borane moieties should exhibit distinctive reactivity profiles, which led us to focus on this class of compounds.…”

“…Ni=N bonds. [8] It is noteworthy that the unique chemical behavior of the d-metal complexes bearing acidic groups in the secondary coordination sphere is commonly associated with the oxidative-addition/reduction-elimination process, [5d, 6, 7a,b, 8] which is nevertheless unfavorable for fmetals. As such, uranium complexes with appended borane moieties should exhibit distinctive reactivity profiles, which led us to focus on this class of compounds.…”

Secondary Coordination Sphere Alkylation Promotes Cyclometalation

“…[5][6][7] Furthermore, a role for such Lewis acidic SCSs has been exposed in iodoarene C-I bond oxidative addition using a [Ni(diphosphine)2] complex, and in producing divergent reaction pathways in the reaction of Ni(0) with organoazides. 8,9 Our group has contributed a number of diphosphine ligands where unsaturated R-groups serve as modifiable sites for the installation of Lewis-acids (boranes; -BR2) via hydroboration. 10,11,12,13 In most cases, installation is performed post-coordination, the success of which depends on the nature of metal starting material.…”

Facile Synthesis of Uranium Complexes with a Pendant Borane Lewis Acid and 1,2‐Insertion of CO into a U−N Bond

“…[7] Very recently, Chu, Szymczak and the co‐workers successfully applied a pendant 9‐BBN (borabicyclo[3.3.1]nonyl) group to stabilize the reactive Ni II imido intermediates (Ni=NR) thanks to the presence of the secondary B−N interactions ( C ), and thus achieved the intermolecular 1,2‐C Ar ‐H‐addition or intramolecular 1,2‐C benzylic ‐H‐addition across the Ni=N bonds. [8] It is noteworthy that the unique chemical behavior of the d ‐metal complexes bearing acidic groups in the secondary coordination sphere is commonly associated with the oxidative‐addition/reduction‐elimination process,[ 5d , 6 , 7a , 7b , 8 ] which is nevertheless unfavorable for f ‐metals. As such, uranium complexes with appended borane moieties should exhibit distinctive reactivity profiles, which led us to focus on this class of compounds.…”