Evidence for the Formation of Anionic Zerovalent Group 10 Complexes as Highly Reactive Intermediates

The
in situ generated CNC pincer lithium complex [Li­(bimca)] (2) (bimca = bis­(3-methyl­imidazolin-2-ylidene)­carbazolide)
reacts with M = Pt(0), Pd(0), and Ni(0) precursors under formal oxidative
addition to (bimca)­hydrido M­(II) complexes 3. This
unusual reaction involves proton abstraction that can derive from
various sources including the ligand itself. Mechanistic considerations
are given. The respective [M­(bmica)­Cl] complexes 4 have
been prepared from the hydrido complexes 3 and subjected
to reduction in order to identify a proposed zerovalent anionic [M­(bimca)]− complex. In the case of M = Pt, only the (bimca)­hydrido
Pt­(II) complex is observed, whereas, for [Pd­(bimca)­Cl], a dimeric
Pd(0) complex [Pd2(bimca)2]­K2 (6) bearing an anionic carbazole moiety is formed in the solid
state. NMR DOSY experiments show that, in solution, this dimer dissociates
to the monomeric anionic complex [Pd­(bimca)]− (5b). We conclude that such anionic zerovalent complexes are
plausible intermediates in the synthesis of group 10 metal­(II) complexes
from the respective metal(0) species with anionic CNC ligands.