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

Accurate description of low-lying excited states in a series of photoreactive clusters [Os3(CO)10(α-diimine)] by DFT calculations

Density functional theory (DFT) calculations were performed on the clusters [Os3(CO)10(α-diimine)], for α-diimine = 2,2'-bipyridine (BPY), N-isopropyl 2-iminomethylpyridine (IMP), and N,N'-diisopropyl-l,4-diaza-1,3-butadiene (DAB), together with their spectroscopic study. This important family of clusters is known to convert upon irradiation with visible light into short-lived biradicals and long-lived zwitterions from a σ* (SBLCT) excited state that, however, has not been described accurately thus far by quantum mechanical calculations. Based on the combined DFT, UV-vis absorption and resonance Raman data, the lowest-lying visible absorption band is assigned to a σ(Os1–Os3)-to-*(α-diimine) CT transition for α-diimine = bpy and IMP, and to a strongly delocalized σ(Os1‒Os3)*-to-σ*(Os1‒Os3)* ¬transition for conjugated non-aromatic α-diimine = DAB. The DFT calculations rationalize the experimentally determined characteristics of this electronic transition in the studied series: (i) the corresponding absorption band is the dominant feature in the visible spectral region, (ii) the CT character of the electronic excitation declines from α-diimine = bpy to IMP and vanishes for DAB, (iii) the excitation energies decrease in the order α-diimine = DAB > BPY > IMP, (iv) the oscillator strength shrinks in the order α-diimine = DAB > IMP > BPY. Reference photoreaction quantum yields measured accurately for the formation of a cluster zwitterion from [Os3(CO)10(IMP)] in strongly coordinating pyridine, demonstrate that the optical population of the lowest-energy 1σ* and relaxed 3σ* excited states in the DFT model scheme is still capable of inducing the initial homolytic Os1‒Os3 σ-bond splitting, although less efficiently than the optical excitation into neighbor higher-lying electronic transitions due to a higher potential barrier for the reaction from a dissociative (σσ*) state

Group 6 metal complexes as electrocatalysts of CO2 reduction: strong substituent control of the reduction path of [Mo(η3-allyl)(CO)2(x,x′-dimethyl-2,2′-bipyridine)(NCS)] (x = 4–6)

A series of complexes [Mo(η3-allyl)(CO)2)(x,x′-dmbipy)(NCS)] (dmbipy = dimethyl-2,2′-bipyridine; x = 4–6) have been synthesized and their electrochemical reduction investigated using combined cyclic voltammetry (CV) and variable-temperature spectroelectrochemistry (IR/UV-vis SEC) in tetrahydrofuran (THF) and butyronitrile (PrCN), at gold and platinum electrodes. The experimental results, strongly supported by density functional theory (DFT) calculations, indicate that the general cathodic path of these Group 6 organometallic complexes is closely related to that of the intensively studied class of Mn tricarbonyl α-diimine complexes, which, themselves, have recently been identified as important smart materials for catalytic CO2 reduction. The dimethyl substitution on the 2,2′-bipyridine ligand backbone has presented new insights into this emerging class of catalysts. For the first time, the 2e– reduced 5-coordinate anions [Mo(η3-allyl)(CO)2)(x,x′-dmbipy)]− were directly observed with infrared spectroelectrochemistry (IR SEC). The role of steric and electronic effects in determining the reduction-induced reactivity was also investigated. For the 6,6′-dmbipy, the primary 1e– reduced radical anions exert unusual stability, radically changing the follow-up cathodic path. The 5-coordinate anion [Mo(η3-allyl)(CO)2)(6,6′-dmbipy)]− remains stable at low temperature in strongly coordinating butyronitrile and does not undergo dimerization at elevated temperature, in sharp contrast to reactive [Mo(η3-allyl)(CO)2)(4,4′-dmbipy)]− that tends to dimerize in a reaction with the parent complex. The complex with the 5,5′-dmbipy ligand combines both types of reactivity. Under aprotic conditions, the different properties of [Mo(η3-allyl)(CO)2)(x,x′-dmbipy)]− are also reflected in their reactivity toward CO2. Preliminary CV and IR SEC results reveal differences in the strength of CO2 coordination at the free axial position. Catalytic waves attributed to the generation of the 5-coordinate anions were observed using CV, but only a modest catalytic performance toward the production of formate was demonstrated by IR SEC. For 6,6′-dmbipy, a stronger catalytic effect was observed for the Au cathode, compared to Pt

9-Borafluoren-9-yl and diphenylboron tetracoordinate complexes of F- and Cl-substituted 8-quinolinolato ligands: synthesis, molecular and electronic structures, fluorescence and application in OLED devices

Six new four-coordinate tetrahedral boron complexes, containing 9-borafluoren-9-yl and diphenylboron cores attached to orthogonal fluorine- and chlorine-substituted 8-quinolinolato ligand chromophores, have been synthesised, characterised, and applied as emitters in organic light-emitting diodes (OLEDs). An extensive steady-state and time-resolved photophysical study, in solution and in the solid state, resulted in the first-time report of delayed fluorescence (DF) in solid films of 8-quinolinolato boron complexes. The DF intensity dependence on excitation dose suggests that this emission originates from triplet–triplet annihilation (TTA). Density functional theory (DFT) and time-dependent density functional theory (TDDFT) studies give insight into the ground and excited state geometries, electronic structures, absorption energies, and singlet–triplet gaps in these new organoboron luminophores. Finally, given their highly luminescent behaviour, organic light-emitting diode (OLED) devices were produced using the synthesised organoboron compounds as emissive fluorescent dopants. The best OLED displays green-blue (λmaxEL = 489 nm) electroluminescence with an external quantum efficiency (EQE) of 3.3% and a maximum luminance of 6300 cd m−2

Electron-Transfer-Induced Side-Chain Cleavage in Tryptophan Facilitated through Potassium-Induced Transition-State Stabilization in the Gas Phase

Fragmentation of transient negative ions of tryptophan molecules formed through electron transfer in collisions with potassium atoms is presented for the first time in the laboratory collision energy range of 20 up to 100 eV. In the unimolecular decomposition process, the dominating side-chain fragmentation channel is assigned to the dehydrogenated indoline anion, in contrast to dissociative electron attachment of free low-energy electrons to tryptophan. The role of the collision complex formed by the potassium cation and tryptophan negative ion in the electron transfer process is significant for the mechanisms that operate at lower collision energies. At those collision times, on the order of a few tens of fs, the collision complex may not only influence the lifetime of the anion but also stabilize specific transition states and thus alter the fragmentation patterns considerably. DFT calculations, at the BHandHLYP/6-311++G(3df,2pd) level of theory, are used to explore potential reaction pathways and the evolvement of the charge distribution along those.F.F.d.S., T.C., and A.R. acknowledge the Portuguese National Funding Agency FCT-MCTES for IF-FCT IF/00380/2014, SFRH/BD/52538/2014, and PD/BD/114449/2016 and together with P.L.-V. the research grants PTDC/FIS-AQM/31215/2017 and PTDC/FIS-AQM/31281/2017. This work was also supported by Radiation Biology and Biophysics Doctoral Training Programme (RaBBiT, PD/00193/2012); UIDB/00068/2020 (CEFITEC) and UIDB/04378/2020 (UCIBIO). M.J.C. and A.G. also thank FCT-MCTES UIDB/04046/2020 and UIDP/04046/2020, and A.G. thanks the SFRH/BPD/89722/2012 grant. G.G. is partially funded by the Spanish Ministerio de Ciencia, Innovacion y Universidades (project no. PID2019-104727RB-C21) and CSIC (Project LINKA20085). O.I. acknowledges the Icelandic Center of Research (RANNIS) and the University of Iceland Research Fund for financial support. The authors thank Ragnar Bjornsson for fruitful discussions while preparing this manuscript.Pre-prin

The Halogen Effect on the Magnetic Behaviour of Dimethylformamide Solvates in [Fe(halide-salEen)2]BPh4

Funding Research was funded by Fundação para a Ciência e a Tecnologia (FCT): projects UIDB/00100/2020, UIDP/00100/2020, LA/P/0056/2020, UIDB/04046/2020, UIDP/04046/2020, UIDB/50006/2020, UIDP/50006/2020 and LA/P/0008/2020, UIDB/04378/2020, UIDP/04378/2020, and LA/P/0140/2020, PTDC/QUI-QFI/29236/2017, PTDCQUI-QIN0252_2021, CEECIND/00509/2017; Fonds de la Recherche Scientifique (FNRS): PDR T.0095.21); Portugal2020: CENTRO-01-0145-FEDER-000018; Royal Society of Chemistry (RSC): R21-7511142525. Acknowledgments Centro de Química Estrutural (CQE) and Institute of Molecular Sciences (IMS) acknowledge the financial support of Fundação para a Ciência e a Tecnologia (FCT): Projects UIDB/00100/2020, UIDP/00100/2020, and LA/P/0056/2020, respectively. BioISI acknowledges FCT for financial support (UIDB/04046/2020, UIDP/04046/2020). This work was supported by the FNRS (PDR T.0095.21). Clara S. B. Gomes acknowledges the Associate Laboratory for Green Chemistry—LAQV, the Applied Molecular Biosciences Unit—UCIBIO and Associated Laboratory i4HB, which are financed by national funds from FCT (UIDB/50006/2020, UIDP/50006/2020 and LA/P/0008/2020, UIDB/04378/2020 and UIDP/04378/2020, and LA/P/0140/2020, respectively). Sónia Barroso thanks project SmartBioR for financial support (CENTRO-01-0145-FEDER-000018)and Centro de Química Estrutural for the access to crystallography facilities. Nuno A. G. Bandeira gratefully acknowledges the NanoBioSolutions FCT grant PTDC/QUI-QFI/29236/2017 for the computational infrastructure. Paulo N. Martinho thanks FCT and RSC for financial support (grants PTDCQUI-QIN0252_2021 and R21-7511142525). Paulo N. Martinho also thanks FCT for the contract CEECIND/00509/2017.Complexes [Fe(X-salEen)2]BPh4·DMF, with X = Br (1), Cl (2), and F (3), were crystallised from N,N′-dimethylformamide with the aim of understanding the role of a high boiling point N,N′-dimethylformamide solvate in the spin crossover phenomenon. The counter ion was chosen for only being able to participate in weak intermolecular interactions. The compounds were structurally characterised by single crystal X-ray diffraction. Complex 1 crystallised in the orthorhombic space group P212121, and complexes 2 and 3 in the monoclinic space group P21/n. Even at room temperature, low spin was the predominant form, although complex 2 exhibited the largest proportion of the high-spin species according to both the magnetisation measurements and the Mössbauer spectra. Density Functional Theory calculations were performed both on the periodic solids and on molecular models for complexes 1–3 and the iodide analogue 4. While all approaches reproduced the experimental structures very well, the energy balance between the high-spin and low-spin forms was harder to reproduce, though some calculations pointed to the easier spin crossover of complex 2, as observed. Periodic calculations with the functional PBE led to very similar ΔEHS-LS values for all complexes but showed a preference for the low-spin form. However, the single-point calculations with B3LYP* showed, for the model without solvate, that the Cl complex should undergo spin crossover more easily. The molecular calculations also reflected this fact, which was more clearly defined when the cation–anion–solvate model was used. In the other models there was not much difference between the Cl, Br, and I complexes.publishersversionpublishe

Mo(II) complexes: A new family of cytotoxic agents?

respectively. These results suggest that they interact with DNA changing its conformation and possibly inducing cell death, and may therefore provide a valuable tool in cancer chemotherapy

Silver(I) and copper(I) complexes with ferrocenyl ligands bearing imidazole or pyridyl substituents

The reactions between five ferrocenyl derivatives containing both a CO and at least an imidazole or pyridine nitrogen atom and AgPF6, AgOTf, or [Cu(NCCH3)4]PF6 precursors were studied. The ligand {[bis(2-pyridyl)amino]carbonyl}ferrocene (L3), derived from (2-pyridyl)amine, favored tetrahedral coordination of Ag+ (with two ligands) and of Cu+ (with two acetonitrile ligands left from the precursor). In all the other ligands, both metal centers coordinated linearly to two ligands, preferring the imidazole or pyridinic nitrogen to other nitrogen atoms (amine) or oxygen donors. When the counter anions were triflate, the crystal structure showed a dimerization of the complex, with the ferrocenyl moieties occupying cis positions, by means of a weak Ag⋯Ag interaction. This was shown experimentally in the crystal structure of complex [Ag(L1)2]OTf (L1 = ferrocenylimidazole) and in the presence of peaks corresponding to {Ag2(L2)3(OTf)}+ and {Ag2(L2)4(OTf)}+ in the mass spectra of [Ag(L2)2]OTf (L2 = ferrocenyl benzimidazole). In all complexes containing PF6, there was no evidence for dimerization. Indeed, in the crystal structure of [Ag(L2)2]PF6, the ferrocenyl moieties occupy trans positions and the metal centers are far from each other. DFT calculations showed that the energy of the cis and trans conformers is practically the same and the balance of crystal packing forces leads to dimerization when triflate is present.S.Q. thanks FCT for a postdoctoral fellowship (SFRH/BPD/11463/ 2002) and M.J.C. thanks FCT, POCI, and FEDER (project PPCDT/QUI/ 58925/2004). M.C.G. and A.L. thank the Dirección General de Investigación Científica y Técnica (CTQ2007-67273-C02-01) for financial support.Peer Reviewe

Chemoselective Sulfide and Sulfoxide Oxidations by CpMo(CO)3Cl/HOOR: a DFT Mechanistic Study. Organometallics

The mechanism of sulfide and sulfoxide oxidation with peroxides (ROOH, R = H, Me), catalyzed by Mo(VI) complexes, was investigated by means of DFT/PBE1PBE calculations. Two different catalytic systems were considered: the first is based on the dioxocyclopentadienyl (Cp) complex CpMoO2Cl (Cp = η5-C5H5), also active as a catalyst for olefin epoxidation, and the second based on MoO2Cl2. The most favorable mechanism in the Cp system is initiated by the O−H activation of the HOOR oxidant, which in the presence of CpMoO2Cl leads to formation of CpMoO(OH)(OOR)Cl. Although this is the active species for olefin epoxidation, an alternative pathway with lower energy is available. With the crucial H-bond assistance of another oxidant molecule, the oxoperoxo complex CpMoO(O2)Cl forms, with release of alcohol ROH as byproduct and a calculated energy barrier below 25 kcal mol−1. The mechanisms unveiled for sulfide to sulfoxide oxidation and for sulfoxide to sulfone oxidation are equivalent in their general features and follow outer-sphere mechanisms with S-nucleophilic attack from a free molecule of substrate (sulfide or sulfoxide) to the peroxide which is activated through Mo−O coordination. The MoO2Cl2 catalyst follows a similar course, calculated from MoO2Cl2(H2O)(H2O2). Again, explicit consideration of one molecule of solvent (water) proved essential to facilitate the H-transfer processes involved in the mechanism. The highest energy barrier calculated (ca. 25 kcal mol−1) corresponds to a H shift from the Oα to the Oβ atom of the coordinated H2O2 molecule, activating Oα for the oxidation reaction and preparing water (H2Oβ) as the future leaving group. The outer-sphere mechanism ends with coordination of the oxidation product.info:eu-repo/semantics/publishedVersio