Phosphorescent, Cyclometalated Cinchophen-Derived Platinum Complexes: Syntheses, Structures, and Electronic Properties

The
syntheses of nine new monometallic heteroleptic platinum complexes
[Pt­(L1–4)­(acac)], [Pt­(L1)­(hmacac/hfacac)],
[PtCl­(L1)­(py)], [Pt­(L1)­(8-Q)], [Pt­(L1)­(bpy)]­(PF6) (where L1 = 2-phenyl-4-ethyl-quinolinecarboxylate;
L2/L3 = N-functionalization of 2-phenyl-N-aryl/alkyl-quinoline-4-carboxamides; L4 = 2-phenyl-4-quinolinecarboxylic
acid (cinchophen); acac = acetylacetonato; hmacac =2,2,6,6-tetramethyl-3,5-heptanedionate;
hfacac = hexafluoroacetylacetonate; py = pyridine; 8-Q = 8-quinolinato; bpy =2,2′-bipyridine)
are described from precursor dimeric Pt­(II) species via an intermediate
DMSO adduct of the general form [PtCl­(L1–4)­(DMSO)].
Single crystal X-ray diffraction studies were undertaken on three
complexes, [Pt­(L1)­(acac)], [PtCl­(L1)­(DMSO)],
and [Pt­(L1)­(bpy)]­(PF6). The structures show
that the complexes each adopt a distorted square planar geometry (most
severely in the case of [Pt­(L1)­(bpy)]­(PF6))
with indications of intermolecular Pt–Pt interactions in one
example. The complexes were investigated using 195Pt­{1H} NMR spectroscopy, revealing varied chemical shifts that
were strongly dependent upon the specific coordination environment
of Pt­(II). Luminescence studies showed the complexes possess a phosphorescent
character with tunable emission wavelengths between 605 and 641 nm
and luminescent lifetimes up to ∼450 ns. Supporting TD-DFT
studies provided descriptions of the HOMO and LUMO energy levels of
the key complex types, confirming an MLCT contribution to the lowest
energy absorption that generally correlated well with the experimental
spectra. The contribution of the Pt­(5d) center to the calculated HOMOs
was strongly ligand dependent, whereas the LUMOs are generally localized
over the quinoline component of the cyclometalated ligand.