Stereoelectronic Effects in C–H Bond Oxidation Reactions of Ni(I) N‑Heterocyclic Carbene Complexes

Activation
of O2 by the three-coordinate Ni­(I) ring-expanded
N-heterocyclic carbene complexes Ni­(RE-NHC)­(PPh3)Br (RE-NHC
= 6-Mes, 1; 7-Mes, 2) produced the structurally
characterized dimeric Ni­(II) complexes Ni­(6-Mes)­(Br)­(μ-OH)­(μ-O-6-Mes′)­NiBr
(3) and Ni­(7-Mes)­(Br)­(μ-OH)­(μ-O-7-Mes′)­NiBr
(4) containing oxidized ortho-mesityl
groups from one of the carbene ligands. NMR and mass spectrometry
provided evidence for further oxidation in solution to afford bis-μ-aryloxy
compounds; the 6-Mes derivative was isolated, and its structure was
verified. Low-temperature UV–visible spectroscopy showed that
the reaction between 1 and O2 was too fast
even at ca. −80 °C to yield any observable intermediates
and also supported the formation of more than one oxidation product.
Addition of O2 to Ni­(I) precursors containing a less electron-donating
diamidocarbene (6-MesDAC, 7) or less bulky 6- or 7-membered
ring diaminocarbene ligands (6- or 7-o-Tol; 8 and 9) proceeded quite differently, affording
phosphine and carbene oxidation products (Ni­(OPPh3)2Br2 and (6-MesDAC)O) and the mononuclear
Ni­(II) dibromide complexes (Ni­(6-o-Tol)­(PPh3)­Br2 (10) and (Ni­(7-o-Tol)­(PPh3)­Br2 (11)) respectively. Electrochemical
measurements on the five Ni­(I) precursors show significantly higher
redox potentials for 1 and 2, the complexes
that undergo oxygen atom transfer from O2.