Tridentate Complexes of Group 10 Bearing Bis-Aryloxide N‑Heterocyclic Carbene Ligands: Synthesis, Structural, Spectroscopic, and Computational Characterization

A series
of group 10 complexes featuring chelating tridentate bis-aryloxide
N-heterocyclic carbenes were synthesized and characterized by using
different techniques. Ni­(II), Pd­(II), and Pt­(II) complexes were isolated
in good yields by straightforward direct metalation of the corresponding
benzimidazolium or imidazolium precursors in a one-pot procedure.
All of the compounds were fully characterized, including single-crystal
X-ray diffractometric determination for three of the derivatives.
In the solid state, the complexes adopt a typical square-planar coordination
geometry around the platinum atom, sizably distorted in order to comply
with the geometrical constraints imposed by the bis-aryloxide N-heterocyclic
carbene ligand. For platinum and palladium derivatives, a joint experimental
and theoretical characterization was performed in order to study the
optical properties of the newly prepared complexes by means of electronic
absorption and steady-state and time-resolved photophysical techniques
as well as density functional theory (DFT) and time-dependent DFT
in both vacuum and solvent. When the temperature was lowered to 77
K in frozen glassy matrix, three platinum complexes showed broad and
featureless, yet weak, photoluminescence in the green region of the
visible spectrum with excited-state lifetimes on the order of a few
microseconds. On the basis of joint experimental and computational
findings and literature on platinum complexes, such emission was assigned
to a triplet-manifold metal−ligand-to-ligand charge transfer
(3MLLCT) transition.