Choice Of Ancillary Ligand Research Articles

Questions of Noninnocence and Ease of Azo Reduction in Diruthenium Frameworks with a 1,8-Bis(( E)-phenyldiazenyl)naphthalene-2,7-dioxido Bridge.

Ligands containing the azo group are often used in various metal complexes owing to their facile one-electron reduction, which in effect extends the means of degrading environmentally harmful azo dyes. In order to probe the idea of the generally accepted ease of reduction of azo-containing compounds, we present here three different diruthenium complexes [(acac)2RuIII(μ-L2-)RuIII(acac)2] (diastereomeric 1/2), [(bpy)2RuII(μ-L2-)RuII(bpy)2](ClO4)2 ([3](ClO4)2), and [(pap)2RuII(μ-L2-)RuII(pap)2](ClO4)2 ([4](ClO4)2 ) with a bridging ligand (L2- = 1,8-bis(( E)-phenyldiazenyl)naphthalene-2,7-dioxido) that contains azo groups in addition to phenoxide-type donors. The RuIII-RuIII complexes (1/2) display interesting one-dimensional-chain effects, as revealed by temperature-dependent magnetic studies. The stability of the RuIII oxidation state in 1/2 under ambient conditions correlates well with the σ-donating acetylacetonato (acac) coligands. However, with π-accepting 2,2/-bipyridine (bpy) or phenylazopyridine (pap) the RuII state is preferably stabilized in 32+ or 42+, respectively, but there are interesting differences in their oxidative chemistry. The moderately π accepting bpy allows for the RuII to RuIII oxidation at reasonably low anodic potentials. However, for the strongly π accepting pap, no RuII to RuIII oxidation is observed within the solvent window. Instead, a phenoxide to phenoxyl radical type of oxidation based on the bridging ligand is observed. Surprisingly, the reductive chemistry of all three complexes is dominated by either the ruthenium centers or the coligands (bpy or pap), with no reductions observed on the azo function associated with the central bridging ligand (L2-). All of the above conclusions were drawn from combined structural, electrochemical, magnetic, spectroelectrochemical, and DFT investigations. Our results thus conclusively establish that the ease of reduction of an azo group in a particular compound is critically dependent on its substituents and that the noninnocence of the bridging ligands (L2-) in the dinuclear complexes can be decisively tuned by the appropriate choice of ancillary ligands.

Direct carbocyclizations of benzoic acids: catalyst-controlled synthesis of cyclic ketones and the development of tandem aHH (acyl Heck-Heck) reactions.

The formation of exo-methylene indanones and indenones from simple ortho-allyl benzoic acid derivatives has been developed. Selective formation of the indanone or indenone products in these reactions is controlled by choice of ancillary ligand. This new process has a low environmental footprint as the products are formed in high yields using low catalyst loadings, while the only stoichiometric chemical waste generated from the reactants in the transformation is acetic acid. The conversion of the active cyclization catalyst into the Hermman-Beller palladacycle was exploited in a one-pot tandem acyl Heck-Heck (aHH) reaction, and utilized in the synthesis of donepezil.

Ancillary Ligand Effects upon Dithiolene Redox Noninnocence in Tungsten Bis(dithiolene) Complexes

An expanded set of compounds of the type [W(S2C2Me2)2L1L2](n) (n = 0: L1 = L2 = CO, 1; L1 = L2 = CN(t)Bu, 2; L1 = CO, L2 = carbene, 3; L1 = CO, L2 = phosphine, 4; L1 = L2 = phosphine, 5. n = 2-: L1 = L2 = CN(-), [6](2-)) has been synthesized and characterized. Despite isoelectronic formulations, the compound set reveals gradations in the dithiolene ligand redox level as revealed by intraligand bond lengths, υ(CCchelate), and rising edge energies in the sulfur K-edge X-ray absorption spectra (XAS). Differences among the terminal series members, 1 and [6](2-), are comparable to differences seen in homoleptic dithiolene complexes related by full electron transfer to/from a dithiolene-based MO. The key feature governing these differences is the favorable energy of the CO π* orbitals, which are suitably positioned to overlap with tungsten d orbitals and exert an oxidizing effect on both metal and dithiolene ligand via π-backbonding. The CN(-) π* orbitals are too high in energy to mix effectively with tungsten and thus leave the filled dithiolene π* orbitals unperturbed. This work shows how, and the degree to which, the redox level of a noninnocent ligand can be modulated by the choice of ancillary ligands(s).

Chelating Base Effects in Palladium-Mediated Activation of Molecular Oxygen

We elucidate here the mechanism for the reaction of molecular oxygen with palladium-hydride complexes in toluene using quantum mechanics (B3LYP/LACVP** with the PBF polarizable continuum solvent model) for ((−)sparteine)-PdII(H)OAc. Here we focus specifically on two classes of pathways: (1) those proceeding through Pd0 and (2) those involving direct insertion of oxygen into the Pd–H bond. In particular, we present calculated potential energy surfaces and structures of the ((−)-sparteine)PdH system in which the OAc ion is substituted for Cl. We find that the acetate ligand has the ability to act as a base while chelating the Pd, making an external base unnecessary and significantly lowering the energy barrier involved in the Pd0 pathway. This switches the calculated preference to the reductive elimination pathway proceeding through Pd0 (ΔΔH⧧ = 1.0 kcal/mol, ΔΔG⧧ = −7.7 kcal/mol). The results presented herein demonstrate the ability to steer the reaction pathway through the choice of ancillary ligands. We e...

Pyridine-2-Carbaldehyde Thiosemicarbazonecopper System: Extending Some Findings to Other Thiosemicarbazone and Coordination Compounds

The study of thiosemicarbazones and their metal complexes has become a fruitful area of the coordination chemistry due to the interesting properties exhibited by these compounds, in particular those dealing with the biological activity. Our efforts have focused on the structure-properties relationship in copper(II) derivatives of pyridine-2- carbaldehyde thiosemicarbazone. Usually, this molecule acts as a NNS terdentate ligand and gives rise to [Cu(HL)]2+ / [CuL]+ rigid planar blocks, which can form dimers by bonds through the thiosemicarbazone S atom (S-bridged systems) or the X atom belonging to different coligands (X-bridged systems). Such structural features, together with the high stability of these compounds, allow the synthesis of complexes with small structural modifications. This fact has prompted us to carry out systematic studies in which variations of physic and chemical parameters (as pH, temperature or reactants present in the reaction medium) induce changes in the ligand or modify the structural, spectroscopic and magnetic properties of the complexes. In this sense, an appropriate choice of ancillary ligands (halides, oxalate, carboxylates, polyoxometalate or biomolecules, among others) is essential to attain different behaviors. Oxalato derivatives with relatively high ferromagnetism and unusually twisted carboxylato groups, structures of thiosemicarbazone-polyoxometalate hybrid systems and nucleic acid base-thiosemicarbazone complexes, reactivity with glutathione, interaction and cleavage of DNA and biological activity against tumoral cell lines are some of the aspects discussed here. Some conclusions of our research can be extrapolated to a broad range of compounds. Keywords: Carboxylato complexes, coordination compounds, copper, magnetic properties, spectroscopic properties, structure, polyoxometalate, thiosemicarbazone, Pyridine-2-Carbaldehyde, Thiosemicarbazone copper(II) System

Synthesis and Structural Diversity of ZnCl2 and Zn(C6F5)2 Complexes of a Phosphinimine Ligand

AbstractZinc complexes of the potentially bidentate neutral monophosphinimine ligand 4‐(ArN=PPh2)dibenzofuran were prepared. Two derivatives of this framework were studied, which differ in the steric demand of the N‐aryl group (Ar = Dipp, Mes). The ligand interacts with diethylzinc in solution but does not form a tightly bound complex, whereby the degree of association is found to be dependent on temperature. However, isolable complexes are formed upon reaction with the more Lewis acidic precursors ZnCl2 and Zn(C6F5)2. In this way, the ZnCl2 complexes 1 and 2 and Zn(C6F5)2 complexes 3 and 4 were prepared and structurally characterized. The nuclearity of 1 and 2 was shown to depend on the bulk of the ligand. Furthermore, the strength of the bonding interaction between the zinc atoms and the dibenzofuran oxygen atoms in 3 and 4 also depends on the choice of ancillary ligand. Attempts to further derivatize these complexes were unsuccessful, and thus, functionalization of the complex LZnEt(OTf) was undertaken instead, resulting in formation of the novel linear trinuclear complex 5.

22.1: Invited Paper: Color Tuning Dopants for Electrophosphorescent Devices: Toward Efficient Blue Phosphorescence from Metal Complexes

In this paper, we describe different strategies for achieving efficient blue electrophosphorescence. The first approach involves the use of ancillary ligand tuning of emission. The emissive unit is an organometallic Ir fragment, whose triplet energy is tuned by the choice of ancillary ligand. This approach is useful for making blue emitters, but luminance efficiency drops when the energy is shifted to saturated blue. The second approach described here is to shift the cyclometallated ligand from phenyl-pyridine to either a phenyl-pyrrazole or phenyl-imidazole. Both complexes emit strongly in the near UV, however, only the imidazole complex (carbene ligand) emits strongly at room temperature.

Design of Long-Lived Ru(II) Terpyridine MLCT States. Tricyano Terpyridine Complexes.

The photophysics of Ru(tpy)(CN)(3)(-) and Ru(ttpy)(CN)(3)(-) (where tpy = 2,2':6',2"-terpyridine and ttpy = 4'-(p-tolyl)-2,2':6,2"-terpyridine) has been studied in detail. The complexes exhibit strongly solvatochromic behavior. Emission energies correlate linearly with solvent acceptor number, consistent with the metal-to-ligand charge transfer (MLCT) nature of the emission and the occurrence of second-sphere donor-acceptor interactions at the cyanide ligands. On the other hand, the correlation of emission lifetimes is clearly biphasic, with a sharp maximum found for solvents of intermediate acceptor number. Such a behavior is explained in terms of competition between a direct deactivation channel and an indirect, thermally activated decay pathway, with relative efficiencies depending strongly on MLCT state energy. A gain of 2 orders of magnitude in lifetime is obtained using the tricyano complexes (Ru(tpy)(CN)(3)(-), tau = 48 ns in DMSO; Ru(ttpy)(CN)(3)(-), tau = 40 ns in CH(3)CN) instead of the corresponding bis-terpyridine species (Ru(tpy)(2)(2+), tau = 250 ps in CH(3)CN; Ru(ttpy)(2)(2+), tau = 860 ps in CH(3)CN). This shows how an appropriate choice of ancillary ligands can be used to improve the properties of photosensitizers containing the Ru(II)-terpyridine chromophoric unit.