Inorganic Chemistry

A cobalt complex bearing a κ-N₃P₂ ligand is presented (1⁺ or Co^I(L), where L is (1E,1′E)-1,1′-(pyridine-2,6-diyl)bis(N-(3-(diphenylphosphanyl)propyl)ethan-1-imine). Complex 1⁺ is stable under air at oxidation state Co^I thanks to the π-acceptor character of the phosphine groups. Electrochemical behavior of 1⁺ reveals a two-electron Co^I/Co^III oxidation process and an additional one-electron reduction, which leads to an enhancement in the current due to hydrogen evolution reaction (HER) at E_onset=−1.6 V vs Fc/Fc⁺. In the presence of 1 equiv of bis(trifluoromethane)sulfonimide, 1⁺ forms the cobalt hydride derivative Co^III(L)-H (2²⁺), which has been fully characterized. Further addition of 1 equiv of CoCp*₂ (Cp* is pentamethylcyclopentadienyl) affords the reduced Co^II(L)-H (2⁺) species, which rapidly forms hydrogen and regenerates the initial Co^I(L) (1⁺). The spectroscopic characterization of catalytic intermediates together with DFT calculations support an unusual bimolecular homolytic mechanism in the catalytic HER with 1⁺.

The synthesis, full characterization, photochemical properties, and cytotoxic activity toward cisplatin-resistant cancer cell lines of new semisquaraine-type Pt(II) complexes are presented. The synthesis of eight semisquaraine-type ligands has been carried out by means of an innovative, straightforward methodology. A thorough structural NMR and X-ray diffraction analysis of the new ligands and complexes has been done. Density functional theory calculations have allowed to assign the trans configuration of the platinum center. Through the structural modification of the ligands, it has been possible to synthesize some complexes, which have turned out to be photoactive at wavelengths that allow their activation in cell cultures and, importantly, two of them show remarkable solubility in biological media. Photodegradation processes have been studied in depth, including the structural identification of photoproducts, thus justifying the changes observed after irradiation. From biological assessment, complexes C7 and C8 have been demonstrated to behave as promising photoactivatable compounds in the assayed cancer cell lines. Upon photoactivation, both complexes are capable of inducing a higher cytotoxic effect on the tested cells compared with nonphotoactivated compounds. Among the observed results, it is remarkable to note that C7 showed a PI > 50 in HeLa cells, and C8 showed a PI > 40 in A2780 cells, being also effective over cisplatin-resistant A2780cis cells (PI = 7 and PI = 4, respectively). The mechanism of action of these complexes has been studied, revealing that these photoactivated platinum complexes would actually present a combined mode of action, a therapeutically potential advantage.

A new CNNC carbene-phthalazine tetradentate ligand has been synthesised. Interestingly, regulation of the stability regions of different Ru oxidation states is obtained by different ligand combinations, going from 6²⁺, where Ru(III) is clearly stable and mono-electronic transfers are favoured, to 2a²⁺/2b²⁺, where Ru(III) is almost unstable with regard to its disproportionation. The catalytic performance of the Ru–OH₂complexes in chemical water oxidation at pH 1.0 points to poor stability (ligand oxidation), with subsequent evolution of CO₂ together with O₂, especially for 4²⁺and 6²⁺. In electrochemically driven water oxidation, the highest TOF values are obtained for 2a²⁺at pH 1.0. In alkene epoxidation, complexes favouring bi-electronic transfer processes show better performances and selectivities than those favouring mono-electronic transfers, while alkenes containing electron-donor groups show better performances than those bearing electron-withdrawing groups. Finally, when cis-β-methylstyrene is employed as the substrate, no cis/trans isomerization takes place, thus indicating the existence of a stereospecific process.

In this work, we describe the synthesis of a new N-heterocyclic carbene (NHC) ligand, derived from a hybrid pyrazole-imidazolium scaffold, namely 1-[2-(3,5-dimethylpyrazol-1-yl)ethyl]-3-((S)-1-phenylethyl)-3H-imidazol-2-ylidene (L). This ligand has been used as a stabilizer for the organometallic synthesis of palladium(0) nanoparticles (Pd NPs). L presents a better stabilizing effect than its pre-carbenic HLCl counterpart, allowing the formation of isolated Pd NPs while HLCl yields aggregated ones. Additionally, molecular Pd(II) coordination compounds of L and HLCl were synthesized and characterized to better understand the coordination modes of these ligands. Both molecular and colloidal Pd systems have been further tested in catalytic C–C coupling processes. Three different types of reactions have been observed depending on the catalytic system: (i) the Suzuki–Miyaura reaction takes place with Pd molecular complexes; (ii) a secondary reaction, the dehalogenation of the substrate, is always detected and (iii) the C–C homocoupling between two molecules of bromoarenes is observed with colloidal catalysts.

The pincer d⁸-monohydride complex RhH{xant(PⁱPr₂)₂} (xant(PⁱPr₂)₂ = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) promotes the release of 1 equiv of hydrogen from H₃BNH₃ and H₃BNHMe₂ with TOF_50% values of 3150 and 1725 h^–1, to afford [BH₂NH₂]_n and [BH₂NMe₂]₂ and the tandem ammonia borane dehydrogenation–cyclohexene hydrogenation. DFT calculations on the ammonia borane dehydrogenation suggest that the process takes place by means of cis-κ²-PP-species, through four stages including: (i) Shimoi-type coordination of ammonia borane, (ii) homolytic addition of the coordinated H–B bond to afford a five-coordinate dihydride-boryl-rhodium(III) intermediate, (iii) reductive intramolecular proton transfer from the NH₃ group to one of the hydride ligands, and (iv) release of H₂ from the resulting square-planar hydride dihydrogen rhodium(I) intermediate.

Several new nitrilotriacetic acid (NTA) based C₃‐symmetric tripodal amides have been synthesized. The NTA branches are alkyl chains or esters derived from amino acids of different length, namely glycine, β‐alanine and γ‐aminobutyric acid. The behavior of these compounds to entrap different monoanions has been tested revealing that they are good ligands able to form host‐guest complexes with the following affinity order: H₂PO₄^– > CH₃CO₂^– > PhCO₂^– > Cl^–, in DMSO. It was found that, while the end‐group effect (CH₃, methyl or benzyl ester) was irrelevant, the length of the branches played a role in the selectivity of the receptors considered and, in special, the glycine derivative has shown remarkably high recognition of H₂PO₄^–. The 1:1 stoichiometry of the complexes has been experimentally determined based on NMR titration and justified by theoretical calculations that also allowed their structure to be predicted for selected instances. Otherwise, formation of bifluoride anion (HF₂^–) has been experimentally observed by NMR and this fact rends difficult the reliability of the affinity constants determined for fluoride complexes.