Single-electron oxidation of N-heterocyclic carbene-supported nickel amides yielding benzylic C–H activation

The dimeric Ni(i)-Ni(i) N-heterocyclic carbene complex {(IPr)Ni( mu -Cl)} sub(2) (3; IPr = 1,3-(2,6- super(i)Pr sub(2)C sub(6)H sub(3)) sub(2)imidazolin-2-ylidene)) reacts with the lithium terphenylamides LiNHdmp and LiNHdippp (dmp = 2,6-di(mesityl)phenyl; dippp = 2,6-bis(2,6-di-iso-propylphenyl)phenyl) to give the monomeric Ni(i) amides (IPr)Ni(NHdmp) (4) and (IPr)Ni(NHdippp) (5), respectively. These nickel amides are 1-electron paramagnets, and crystallographic characterization indicates both are stabilized by Ni-C(ipso) interactions with a flanking aryl group of the terphenyl fragment. This results in significant deviation from the linear C sub(NHC)-Ni-N geometry typical for a two-coordinate transition-metal complex (112.17(9) degree in 4, 116.41(9) degree in 5). One-electron oxidation of 4 by ferrocenium results in intramolecular deprotonation at a terphenyl benzylic position by the amide, giving the diamagnetic Ni(ii) complex [(IPr)Ni( Kappa super(2)-C,N:NH sub(2)C sub(6) H sub(3)(Mes)C sub(10)H sub( 9))][B(Ar super(F)) sub(4)] (7). DFT calculations on oxidized 4 (i.e., 4 super(+)) indicate short amide N...CH sub(3) interactions. One-electron oxidation of 5 by ferrocenium gives a new high-spin Ni(ii) amide complex salt, [(IPr)Ni(NHdippp)][B(Ar super(F)) sub(4)] (9). The solid-state structure of 9 indicates it maintains the bent C sub(NHC)-Ni-N core. Unlike three-coordinate cationic Ni(ii) amides, 9 has not been observed to undergo smooth deprotonation (at N) to afford a two-coordinate imido complex.