Mechanistic insights into the electrochemical reduction of CO2 to CO on Ni(salphen) complexes

LA/P/0056/2020. The NMR spectrometers are part of the National NMR Network (PTNMR) and are partially supported by Infrastructure Project No 022161 (co-financed by FEDER through COMPETE 2020, POCI and PORL and FCT through PIDDAC). FCT is acknowledged for PTDCQUI-QIN0252_2021 (PNM). The CARISMA COST action CM1205 is acknowledged. MJC thanks N. A. G. Bandeira for technical assistance. The CATSUS doctoral programme is also acknowledged. Publisher Copyright: © 2023 The Royal Society of Chemistry.Cyclic voltammetry and bulk electrolysis showed that [Ni(ii)(salphen)] [1], [Ni(ii)(tBu-salphen)] [2], and a binuclear Ni(ii) compound combining salphen and tBu-salphen [3] react with CO2 to yield a metal-carbonyl species that is stable under an oxygen free atmosphere. Upon exposure to air, a stoichiometric amount of CO is released (detected by gas chromatography) and protonation regenerates the initial complex. To shed light on the mechanism of CO2 reduction and O2-dependent CO release by [1], UV-vis, EPR and SEC-IR spectroscopy studies complemented with DFT calculations were performed. It is proposed that the mono reduced [Ni(i)(salphen)]−, 2[1]−, formed a CO2 complex, 2[1(CO2)]−, which was then further reduced to 3[1(CO2)]2−. After addition of two protons, the coordinated CO2 was reduced to CO and released, regenerating 1[1]. Alternatively, 2[1(CO2)]− is protonated and then reduced to the same intermediate as before, continuing the same way. In the second cycle, the CO released competed with CO2 and coordinated to 2[1]− much more strongly, thereby deactivating the system. The new 2[1(CO)]− was reduced to 3[1(CO)]2− which was identified by comparison of experimental spectroscopic (UV-vis, EPR, SEC-IR) data with DFT calculated parameters.publishersversionpublishe

Click-Derived Triazoles and Triazolylidenes of Manganese for Electrocatalytic Reduction of CO2

022162, CEECIND/00509/2017, PTDC/QUI-QIN/28151/2017, RECI/BBB-BEP/0124/2012, SFRH/BD/131955/2017, UIDB/00100/2020, UIDB/04378/2020, UIDB/04612/2020, UIDP/00100/2020, UIDP/04046/2020, UIDP/04378/2020, UIDP/50006/2020, UIPD/04612/2020 Funding Information: This research was funded by FUNDA??O DE CI?NCIA E TECNOLOGIA, FCT, grant numbers: PTDC/QUI-QIN/28151/2017, SFRH/BD/131955/2017, CEECIND/00509/2017. The MOSTMICRO-ITQB was funded by UIDB/04612/2020 and UIPD/04612/2020. The NMR spectrome-ters at CERMAX were funded by 022162. LAQV and UCIBIO were funded by UIDB/50006/2020, UIDP/50006/2020, UIDB/04378/2020, and UIDP/04378/2020. The X-ray infrastructure was funded by RECI/BBB-BEP/0124/2012. The BioISI was funded by UIDB/04046/2020, UIDP/04046/2020, UIDB/00100/2020, and UIDP/00100/2020. We thank C. Almeida for elemental analysis at ITQB laboratories. Funding Information: Funding: This research was funded by FUNDAÇÃO DE CIÊNCIA E TECNOLOGIA, FCT, grant numbers: PTDC/QUI-QIN/28151/2017, SFRH/BD/131955/2017, CEECIND/00509/2017. The MOSTMICRO-ITQB was funded by UIDB/04612/2020 and UIPD/04612/2020. The NMR spectrometers at CERMAX were funded by 022162. LAQV and UCIBIO were funded by UIDB/50006/2020, UIDP/50006/2020, UIDB/04378/2020, and UIDP/04378/2020. The X-ray infrastructure was funded by RECI/BBB-BEP/0124/2012. The BioISI was funded by UIDB/04046/2020, UIDP/04046/2020, UIDB/00100/2020, and UIDP/00100/2020. Publisher Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland.A series of new fac-[Mn(L)(CO)3 Br] complexes where L is a bidentate chelating ligand containing mixed mesoionic triazolylidene-pyridine (MICˆpy, 1), triazolylidene-triazole (MICˆtrz, 2), and triazole-pyridine (trzˆpy, 3) ligands have been prepared and fully characterized, including the single crystal X-ray diffraction studies of 1 and 2. The abilities of 1–3 and complex fac-[Mn(MICˆMIC)(CO)3 Br] (4) to catalyze the electroreduction of CO2 has been assessed for the first time. It was found that all complexes displayed a current increase under CO2 atmosphere, being 3 and 4 the most active complexes. Complex 3, bearing a NˆN-based ligand exhibited a good efficiency and an excellent selectivity for reducing CO2 to CO in the presence of 1.0 M of water, at low overpotential. Interestingly, complex 4 containing the strongly electron donating di-imidazolylidene ligand exhibited comparable activity to 3, when the experiments were performed in neat acetonitrile at slightly higher overpotential (−1.86 vs. −2.14 V).publishersversionpublishe

Cryptand-Functionalized Highly Oriented Pyrolytic Graphite Electrodes

Reproducible materials that have detection properties towards a certain molecule are very important for applications in the fabrication of devices. Among all the substrates that are used, highly oriented pyrolytic graphite allows to clearly image a monolayer. On the other hand, cryptand molecules are versatile because they can sense certain analytes with high selectivity. The highly oriented pyrolytic graphite electrode was first functionalized with an aryl bearing a bromine or an alkyne group to further attach cryptand molecules to its surface. The functionalization was performed through the electroreduction of aryl diazonium salts. While functionalization with an aryl-bromine produced a 20 nm-thick dendritic layer, functionalization of the surface with an aryl bearing a terminal alkyne produced a 9.7 nm-thick multilayer. However, if the diazonium salt is prepared in situ, a 0.9 nm monolayer with aryl–alkyne groups is formed. The alkyne functionalized electrode reacted with a bromo-cryptand through a Sonogashira C–C coupling reaction yielding electrodes functionalized with cryptands. These were immersed in a solution of a Co(II) salt resulting in Co(II)-cryptate modified electrodes, highlighting the ability of the cryptands’ modified electrode to sense metal ions. The electrode surface was analyzed by X-ray photoelectron spectroscopy after each modification step, which confirmed the successful functionalization of the substrate with both the cryptand and the cryptate. Cyclic voltammetry studies showed stable current response after approximately six cycles. Different reduction processes were detected for both cryptand (−1.40 V vs. SCE) and cryptate (−1.22 V vs. SCE) modified highly oriented pyrolytic graphite

Structural and electronic properties in asymmetric binuclear Zn(II) amphiphilic compounds

A new type of asymmetric binuclear Zn(II) complex was synthesized from a suitable mononuclear precursor. One Zn(II) was coordinated to a salphen ligand (salphen = N,N′-disalicylidene-1,2-phenylenediamine) and the other to a modified salphen bearing OC12H25 chain in the 4,4’ positions, the two being joined by phenylene rings. The molecular structure, determined by DFT calculations (ADF/BP86(COSMO:THF)/TZ2P), showed the non-planarity of each of the two complexes, the dihedral angle at the junction being ∼36°. The absorption spectra of 2 and its precursor 1 in DMSO showed a strong band in the visible at 427 and 408 nm, respectively, assigned based on TDDFT calculations mainly to intra- and interligand π→π* transitions. Complex 2 displayed emission at 531 nm in DMSO and 534 nm in other solvents. The long alkyl chain of its substituents promoted self-assembly of these amphiphilic molecules. No gels were formed in DMSO, but several bands (420, 418 and 480 nm) and shoulders appeared in other solvents, and gels were detected when increasing concentration. The formation of aggregates was studied by scanning electron microscopy and atomic force microscopy, and the images found in the three solvents reflected different supramolecular arrangements. These studies revealed that the binuclear compound formed stable gels above 8.88 mM for tetrahydrofuran and 13.3 mM for toluene and chloroform.We thank the Fundação para a Ciência e a Tecnologia, Portugal, for financial support (UIDB/04046/2020 and UIDP/04046/2020) and project PTDC/QEQ­QIN/3414/2014. SQR thank FCT for fellowship (PD/BD/52368/2013) under the CATSUS doctoral programme. PNM thanks FTC for the programme CEECIND/00509/2017. We also thank Fundação Calouste Gulbenkian for financial support under the Estímulo à Investigação 2013 programme.Peer reviewe

Solution and solid state properties of Fe(III) complexes bearing N-ethyl-N-(2-aminoethyl)salicylaldiminate ligands

The effect of the phenolate ring derivatisation on the magnetic properties of Fe(III) complexes bearing Nethyl- N-(2-aminoethyl) salicylaldiminate ligands both in solid state and solution have been investigated. Two new complexes [Fe(3,5-Br-salEen)(2)]ClO4 center dot EtOH (5) and [Fe(3,5-Br-salEen)(2)]BPh4 center dot DMF (6) have been synthesised. SQUID magnetometry studies on these complexes showed that while complex 5 is in the low-spin (LS) state, complex 6 displays a gradual and incomplete spin crossover (SCO) transition over the temperature measured. Solution measurements on a series of six complexes e [Fe(salEen)2]ClO4 (1), [Fe(salEen)(2)]BPh4 center dot 0.5H(2)O (2), [Fe(5-Br-salEen)(2)]ClO4 (3), [Fe(5-Br-salEen)(2)]BPh4 center dot DMF (4), [Fe(3,5-Br-salEen)(2)]ClO4 center dot EtOH (5) and [Fe(3,5-Br-salEen)(2)]BPh4 center dot DMF (6) - were performed by UVevis and NMR spectroscopies and cyclic voltammetry. Solution studies show that the presence of electron withdrawing groups (bromine atoms) affect the electronic density at the phenolate ring, thus influencing the ligand field strength and the separation between the t(2g) and e(g)* energy levels. The presence of two bromide substituents at the phenolate ring has a more pronounced effect on the magnetic behaviour in solution than in the solid state, with both complexes 5 and 6 adopting preferentially the LS state. Electrochemical studies of complexes 1-6 reveal that the reduction of the metallic centres in the complexes with electron withdrawing groups is easier, with E-1/2 values of iron moving to more positive potentials with the number of bromide substituents at the phenolate ring. (C) 2013 Elsevier B. V. All rights reserved