Two new diphenylphosphine-substituted benzimidazoles featuring an ethylene (2) or a methylene linker (5) between the benzimidazole and the phosphine have been prepared. These precursors for bidentate carbene/phosphine ligands react with [RuCp*( μ 3 -Cl)] 4 under tautomerization with formation of complexes of type [RuCp*Cl(L)] ([6] and [7], L = NR,NH-carbene/phosphine ligand). The crystal structures of [6] and [7] have been determined by X-ray diffraction. These studies revealed that the length of the bridge between the benzimidazol-2-ylidene and the phosphine determines the geometric parameters of the carbene/phosphine chelate ring and the donor properties of the phosphine. A 1 H NMR titration of complex [6] with DMPU revealed the formation of a (carbene)N-H 3 3 3 O(DMPU) hydrogen bond in solution.

A bidentate C^N donor set derived from an N-heterocyclic carbene (NHC) precursor linked to a trifluoromethyl (CF3) functionalized pyrazole ring is described for the first time. The ligands have been employed to prepare four new phosphorescent complexes by the coordination of platinum(II) centres bearing cyclometalated phenyl-pyridine/triazole-pyridine chelates. The electronic and steric environments of these complexes were tuned through the incorporation of suitable substituents in the phenyl-pyridine/triazole-pyridine ligands, wherein the position of the phenyl-ring substituent (a CF3 group) also directs the selective adoption of either a trans or a cis configuration between the C(NHC) and the C(phenyl) donor atoms. Molecular structures obtained by X-ray diffraction for three of the complexes confirm a distorted square-planar configuration around the platinum centre, and DFT calculations show that the substituents have a significant influence on the energies of the frontier orbitals. Moreover, a platinum(II) complex featuring the new bidentate NHC^pyrazolate ligand and a bulky adamantyl functionalized pyridine-triazole luminophore was observed to be highly emissive and exhibiting a sky-blue luminescence (λ(Em) = 470 nm) with photoluminescence quantum yields as high as 50% in doped PMMA matrices. A complete photophysical investigation of all of the complexes in solution as well as in the solid state is herein reported.

Platinum complexes featuring pyridine bis-N-heterocyclic-imidazol-2-ylidene/-mesoionic-triazol-5-ylidene donors as pincer ligands and chloro (-Cl), acetonitrile (-NCCH3) or cyano (-CN) groups as auxiliary ligands are prepared as highly strained organometallic phosphors. X-ray structures of four of these complexes confirm a distorted square planar geometry, where the pincer ligand and its mesityl wingtips occur in a twisted conformation to each other. Electrochemical and photophysical characterization have been carried out and the experimental results are interpreted with the aid of density functional theory calculations. Emission responses of complexes under exposure to different vapors and mechanical shear are reported. Notably, the platinum complex featuring pyridine bis-imidazol-2-ylidene and a weakly donating acetonitrile auxiliary ligand exhibited strong aquachromic and mechanochromic emission responses, showing color changes from sky blue to green or yellow-green.

Heteroleptic ruthenium (II) complexes featuring donor functionalized phenyl‐terpyridine (ph‐tpy) and a monocarboxylic‐(ph‐tpy)/(tpy) are synthesized and characterized. Reactions of ruthenium (II) precursors at 80 °C favored heteroleptic complexes formation over the homoleptic side products. Visible light excitation of these complexes resulted in the metal‐to‐ligand charge transfer (MLCT) transitions. The inter‐planar torsional angle between the atoms of donor functionalized phenyl ring and the central pyridine (py) of the tpy core strongly influences visible light absorption and photovoltaic properties. The lower inter‐ring py‐ph torsion in the acceptor end of the MLCT structures and its increase in the oxidized doublets could prevent the back electron transfer. The ruthenium atom and the acceptor functionalized tpy host the triplet‐MLCT spin density. Ambient temperature excited‐state decay followed the energy gap law and occurred in the order of a few nanoseconds. Herein, we evaluate the photosensitizing ability of these complexes via a combined experimental and computational approach.

N-Pyridyl, N'-amido functionalized imidazolium bromides were obtained in high yields as an N-heterocyclic carbene (NHC) precursor and used as bidentate or a pincer ligands to obtain ruthenium complexes via a silver NHC transmetallation route. The incorporation of a phenyl group as an amido-N substituent (R = Ph) results in a bidentate coordination mode through the C(NHC) and N(pyridyl) donors, whereas in its absence (R = H) a pincer coordination mode was observed through the N(pyridyl)^C(NHC)^O(amido) donors. The ruthenium complex featuring a pincer type NCO coordination mode with a protic NH function adjacent to the coordinating O(amido) atom was found to efficiently catalyse the oxidation of activated alcohols effecting quantitative conversions within 30 minutes. However the oxidation of deactivated alcohols required longer reaction times to effect the quantitative transformation.

Homoleptic ruthenium complexes [Ru(L n ) 2 ] 2+ bearing unsymmetric pyridine-based bis N-heterocyclic carbene (NHC) pincer ligands are synthesized and structurally characterized. These complexes feature the coexistence of a five-membered and a six-membered ruthenacycle within the same ligand framework, allowing an ideally octahedral C NHC − Ru−N Py bite angle close to 90°. In the solid state, impressive 3 MLCT lifetimes of microsecond regime were detected in all of the complexes, of which [Ru(L 4 ) 2 ] 2+ , featuring four benzimidazol-2-ylidene donors, exhibited the longest values. Electronic properties of all of the ruthenium complexes are investigated by cyclic voltammetry and spectroscopic techniques. Density functional theory results were used to support the experimental observations.

No abstract

No abstract