Pt(II), Pd(II), and Ni(II) Complexes with Tridentate Cyclometalating Five- and Six-Ring Chelating N^C*N, C^N*N, and N*C*N Quinolinyl Ligands

Nine cyclometalated Ni(II), Pd(II), and Pt(II) complexes
of the
type [M(NCN)Cl] and [M(CNN)X] (X = Cl for Pt and Pd, or Br for Ni)
containing 8-quinolinyl (Q), 2-pyridyl (Py), and phenyl (Ph) moieties
are presented. The introduction of Q leads to six-ring chelates (N*C
or N*N) compared with five-ring chelates (C^N or N^N) for 2-pyridyl-containing
systems in the PyPhQ– (N^C*N), QPhQ– (N*C*N), and PhPyQ– (C^N*N) ligands. Single-crystal
X-ray diffractometry showed the six-ring chelate units with angles
around 90° compared with 80° for the five-ring chelates.
Marked tilts of the planar Q systems toward the central Py or Ph core
(up to 37°) were observed. Cyclic voltammetry showed first reduction
potentials varying from −1.8 to −2.2 V vs ferrocene/ferrocenium
and oxidations at around 0.45 V for Pt, 0.75 V for Pd, and 0 V for
Ni complexes. DFT-calculated energies and compositions of the frontier
molecular orbitals allowed assigning reductions to ligand-centered
processes and oxidations to processes with mixed X(p)/M(d)/Ph(π)
contributions. Long-wavelength UV–vis absorption maxima ranging
from 400 to 430 nm were assigned to mixed ligand-centered(π–π*)/metal(d)-to-ligand(π*)
transitions. The new complex [Pt(PyPhQ)Cl] showed very long-wavelength
photoluminescence (>600 nm), similar to that of the previously
reported
[Pt(QPhQ)Cl] derivative.