Electronic Transitions in Different Redox States of Trinuclear 5,6,11,12,17,18-Hexaazatrinaphthylene-Bridged Titanium Complexes: Spectroelectrochemistry and Quantum Chemistry

Multinuclear transition metal complexes bridged by ligands with extended π-electronic systems show a variety of complex electronic transitions and electron transfer reactions. While a systematic understanding of the photochemistry and electrochemistry has been attained for binuclear complexes, much less is known about trinuclear complexes such as hexaphenyl-5,6,11,12,17,18-hexaazatrinaphthylene-tristitanocene [(Cp2 Ti)3 HATN(Ph)6 ]. The voltammogram of [(Cp2 Ti)3 HATN(Ph)6 ] shows six oxidation and three reduction waves. Solution spectra of [(Cp2 Ti)3 HATN(Ph)6 ] and of the electrochemically formed oxidation products show electronic transitions in the UV, visible and the NIR ranges. Density functional theory (DFT) and linear response time-dependent DFT show that the three formally titanium(II) centers transfer an electron to the HATN ligand in the ground state. The optically excited transitions occur exclusively between ligand-centered orbitals. The charged titanium centers only provide an electrostatic frame to the extended π-electronic system. Complete active self-consistent field (CASSCF) calculation on a structurally simplified model compound, which considers the multi-reference character imposed by the three titanium centers, can provide an interpretation of the experimentally observed temperature-dependent magnetic behavior of the different redox states of the title compound in full consistency with the interpretation of the electronic spectra.

Acid Responsive Hydrogen-Bonded Organic Frameworks

A porous hydrogen-bonded organic framework (HOF) responsive to acid was constructed from a hexaazatrinaphthylene derivative with carboxyphenyl groups (CPHATN). Precise structures of both 1,2,4-trichlorobenzene solvate [CPHATN-1(TCB)] and activated HOF with permanent porosity (CPHATN-1a) were successfully determined by single-crystalline X-ray diffraction analysis. Permanent porosity of CPHATN-1a was evaluated by gas sorption experiments at low temperature. CPHATN-1a also shows significant thermal stability up to 633 K. Its crystals exhibit a rich photochemistry thanks to intramolecular charge-transfer and interunit proton-transfer reactions. Femtosecond (fs) experiments on crystals demonstrate that these events occur in ≤200 fs and 1.2 ps, respectively. Moreover, single-crystal fluorescence microscopy reveals a shift of the emission spectra most probably as a result of defects and a high anisotropic behavior, reflecting an ordered crystalline structure with a preferential orientation of the molecular dipole moments. Remarkably, CPHATN-1a, as a result of the protonation of pyradyl nitrogen atoms embedded in its π-conjugated core, shows reversible vapor acid-induced color changes from yellow to reddish-brown, which can be also followed by an ON/OFF of its emission. To the best of our knowledge, this is the first HOF that exhibits acid-responsive color changes. The present work provides new findings for developing stimuli responsive HOFs.

Synthesis, Crystal Structures, and Magnetic and Electrochemical Properties of Highly Phenyl Substituted Trinuclear 5,6,11,12,17,18-Hexaazatrinaphthylene (HATNPh6)-Bridged Titanium Complexes

Trinuclear titanium complexes coordinated by one ligand with three coordination sites have shown properties of mixed valency and a high number of reversible redox steps. Herein we report on the hexaphenyl-substituted derivative (Cp₂Ti)₃(μ₃-HATNPh₆) (2). On reaction of 2 with the ferrocenium salt [Cp₂Fe]BF₄, the cationic complexes [(Cp₂Ti)₃(μ₃-HATNPh₆)] ⁿ⁺ ( n = 1-3; 3-5) become available in a selective way. Cyclic voltammograms show 10 reversible redox states of the trinuclear species 2 without decomposition. In order to classify the degree of electronic communication between the titanium centers, comproportionation constants K_c, IVCT bands in NIR spectra, and magnetic measurements were analyzed. These parameters show strong coupling effects between the titanium centers but no full delocalization. In addition, single-crystal X-ray analysis of the neutral complex 2 and its oxidation products (1+ (3), 2+ (4), and 3+ (5)) revealed the geometric structure of the molecule in the solid state. For the cationic species anion-π interactions between the electron-deficient central ring of the HATNPh₆ ligand and BF₄^- counterions were found.

Crystal structures of 2,3,8,9,14,15-hexa-methyl-5,6,11,12,17,18-hexa-aza-tri-naphthyl-ene and 2,3,8,9,14,15-hexa-phenyl-5,6,11,12,17,18-hexa-za-tri-naphthyl-ene di-chloro-methane disolvate

The crystal structures of two substituted HATN (hexa-aza-tri-naphthyl-ene) derivatives, namely 2,3,8,9,14,15-hexa-methyl- and 2,3,8,9,14,15-hexa-phenyl-5,6,11,12,17,18- hexa-zatri-naphthyl-ene (HATNMe6 and HATNPh6), are reported. Whereas the structure of the methyl-substituted derivative (HATNMe6) contains no solvent mol-ecules (C30H24N6), the hexa-phenyl-substituted structure (HATNPh6) contains two mol-ecules of di-chloro-methane (C60H36N6·2CH2Cl2). This class of planar bridging ligands is known for its electron-deficient systems and its ability to form π-π stacking inter-actions. Indeed, in both crystal structures strong π-π stacking inter-actions are observed, but with different packing features. The di-chloro-methane mol-ecules in the crystal structure of HATNPh6 are situated in the voids and are involved in C-H⋯N contacts to the nitro-gen atoms of the pyrazine units.

Synthesis, photophysical properties and application in organic light emitting devices of rhenium(i) carbonyls incorporating functionalized 2,2′:6′,2′′-terpyridines

Photophysics of [ReCl(CO)₃(4′-R-terpy-κ²N)].

A pair of dinuclear Re(i) enantiomers: synthesis, crystal structures, chiroptical and ferroelectric properties

A pair of dinuclear Re(i) enantiomers have been successfully synthesized, representing the first example of polynuclear Re(i) complexes with ferroelectric properties and possessing a multifunctional feature combining chirality, NLO-activity and ferroelectricity within one molecule.

Reactivity of a fac-ReCl(α-diimine)(CO)3 complex with an NAD+ model ligand toward CO2 reduction

The reactivity of a rhenium complex containing an NAD(+) model ligand was examined toward photochemical formation of the corresponding NADH-like dihydro form of the complex and electrochemical CO2 reduction. The hydricity of the NADH-like complex was estimated by a thermodynamic cycle and reaction with Ph3C(+).