Reactivitat del fragment {Pt2(℗æ-S)2} en compostos de fórmula {C2Pt(℗æ-S)2PtC2}

Consultable des del TDXTítol obtingut de la portada digitalitzadaEls compostos de fórmula general [L2Pt(m-S)2PtL2] han estat usats ampliament com a lligands degut a l'elevada densitat electrónica del anions sulfur de l'anell {Pt2(m-S)2}. Així, durant anys, s'ha obtingut una llarga serie de compostos polimetàl·lics amb diverses geometries de coortdinació en els que un o varis centres metàl·lics es coordinen als sofres del fragment {Pt2(m-S)2}. Malgrat l'intens estudi al que ha estat sotmesa aquesta família de compostos, varis aspectes interessants de la seva reactivitat romanien inexplorats. Alguns d'ells han estat estudiats a aquesta tesi doctoral utilitzant diverses tècniques experimentals (RMN, ESI-MS, MALDI-TOF-MS difracció de raigs X, voltametria cíclica), així com cálculs teórics. En primer lloc, s'ha determinat un mètode d'obtenció d'espècies macromoleculars de nuclearitat controlada, utilizant el complex [(dppp)Pt(m-S)2Pt(dppp)] com unitat estructural bàsica. Aquest mètode implica, inicialment, l'obtenció del compost trimetàl·lic [{Pt2(m-S)2(dppp)2}PtCl2] en el que la labilitat dels lligands clorur permet la seva substitució per ligands sulfur, els quals poden actuar como pont entre dos centres metàl·lics, donant lloc a l'autoensamblatge de dos unitats de fórmula {Pt3(m3-S)2(dppp)2}2+. Per altre part, s'ha analizat l'efecte en la reactivitat del fragment metál·lic {Pt(cod)}2+ de la coordinació a un metal·loligand de fórmula general [L2Pt(m-S)2PtL2]. La reactivitat d'aquest fragment organometàl·lic queda significativament afectada en els compostos de fórmula [{L2Pt(m-S)2PtL2}Pt(cod)] de tal manera que la seva reacció amb l'anió metóxid consisteix en la desprotonació del ligand olefínic en lloc de la reactivitat habitual descrita per aquests sistemes, la qual consisteix en l'atac nucleòfil de la base sobre un doble enlaç. Un altre aspecte de la reactivitat dels compostos [L2Pt(m-S)2PtL2] que ha estat explorat a aquesta tesi doctoral són els processos químics que es deriven de la reacció d'aquests complexos amb electròfils no metàl·lics, en concret amb CH2Cl2 i àcids pròtics. Aquests processos impliquen múltiples etapas i inclouen el trancament i posterior regeneració de l'estructura bimetál·lica {Pt2(m-S)2}. Finalment, les propietats redox de l'anell {Pt2(m-S)2} han estat determinades mitjantçant la combinació de mesures electroquímiques y de càlculs teòrics. Els resultats obtinguts indiquen que aquest fragment pot donar lloc a dues oxidacions monoelectròniques que resulten en la formació d'un enlaç S-S.The compounds of general formula [L2Pt(m-S)2PtL2] have been intensively studied on its role of metalloligands as a result of the high electron density of the sulfide anions of the {Pt2(m-S)2} ring. Thus, during the last years, a large series of polymetallic complexes has been obtained where one or several metallic centers are coordinated to the sulfide anions in the fragment {Pt2(m-S)2}. Despite the large number of publications devoted to this family of compounds, some interesting aspects of its chemical behaviour remained unexplored. Some of them have been studied in this thesis using diverse experimental techniques (NMR, ESI-MS, MALDI-TOF-MS, X-ray diffraction, cyclic voltametry), as well as theoretical calculations. On the one hand, a new method to obtain multimetallic nuclearity-controlled compounds has been described, using the complex [(dppp)Pt(m-S)2Pt(dppp)] as building block. This method entails, initially, the synthesis of [{Pt2(m-S)2(dppp)2}PtCl2] where the lability of cloride anions allows its substitution by bridging sulfide ligands, and thus, giving the self-assembly of two trimetallic {Pt3(m3-S)2(dppp)2}2+ units. On the other hand, the effect of the coordination to a [L2Pt(m-S)2PtL2] metalloligand on the reactivity of the organometallic fragment {Pt(cod)}2+ has been investigated. This fragment exhibits an unusual chemical behavior in the complexes of formula [{L2Pt(m-S)2PtL2}Pt(cod)], which consist on the deprotonation of the olefinic ligand by reacting with methoxide anion, instead the nucleophilic attack of the base to the double bond, usually reported. An additional aspect of the reactivity of the complexes of formula [L2Pt(m-S)2PtL2] studied in this thesis is the whole set of reactions exhibited by these compounds in presence of two different non-metallic electrophiles: CH2Cl2 and protic acids. These processes imply the disintegration and subsequent regeneration of the bimetallic {Pt2(m-S)2} structure. Finally, the redox properties of the {Pt2(m-S)2} ring have been determined by combination of electrochemical measurements and theoretical calculations. The results obtained indicate that this fragment exhibits two monoelectronic consecutive oxidations which result on the formation of a S-S bond

Photoredox heterobimetallic dual catalysis using engineered covalent organic frameworks

The functionalization of an imine-based layered covalent organic framework (COF), containing phenanthroline units as ligands, has allowed the obtention of a heterobimetallated material. Photoactive Ir and Ni fragments were immobilized within the porous structure of the COF, enabling heterogeneous light-mediated Csp 3 -Csp 2 cross-couplings. As radical precursors, potassium benzyl- and alkoxy-trifluoroborates, organic silicates, and proline derivatives were employed, which brings out the good versatility of Ir,Ni@Phen-COF. Moreover, in all the studied cases, an enhanced activity and stability have been observed in comparison with analogous homogenous systems

Heterogeneous catalysts with programmable topologies generated by reticulation of organocatalysts into metal-organic frameworks : The case of squaramide

A well-established strategy to synthesize heterogeneous, metal-organic framework (MOF) catalysts that exhibit nanoconfinement effects, and specific pores with highly-localized catalytic sites, is to use organic linkers containing organocatalytic centers. Here, we report that by combining this linker approach with reticular chemistry, and exploiting three-dimensioanl (3D) MOF-structural data from the Cambridge Structural Database, we have designed four heterogeneous MOF-based catalysts for standard organic transformations. These programmable MOFs are isoreticular versions of pcu IRMOF-16, fcu UiO-68 and pillared-pcu SNU-8X, the three most common topologies of MOFs built from the organic linker p,p'-terphenyldicarboxylic acid (tpdc). To synthesize the four squaramide-based MOFs, we designed and synthesized a linker, 4,4'-((3,4-dioxocyclobut-1-ene-1,2-diyl)bis(azanedyil))dibenzoic acid (Sq_tpdc), which is identical in directionality and length to tpdc but which contains organocatalytic squaramide centers. Squaramides were chosen because their immobilization into a framework enhances its reactivity and stability while avoiding any self-quenching phenomena. Therefore, the four MOFs share the same organocatalytic squaramide moiety, but confine it within distinct pore environments. We then evaluated these MOFs as heterogeneous H-bonding catalysts in organic transformations: a Friedel-Crafts alkylation and an epoxide ring-opening. Some of them exhibited good performance in both reactions but all showed distinct catalytic profiles that reflect their structural differences

Organocatalytic vs. Ru-based electrochemical hydrogenation of nitrobenzene in competition with the hydrogen evolution reaction

The electrochemical reduction of organic contaminants allows their removal from water. In this contribution, the electrocatalytic hydrogenation of nitrobenzene is studied using both oxidized carbon fibres and ruthenium nanoparticles supported on unmodified carbon fibres as catalysts. The two systems produce azoxynitrobenzene as the main product, while aniline is only observed in minor quantities. Although PhNO2 hydrogenation is the favoured reaction, the hydrogen evolution reaction (HER) competes in both systems under catalytic conditions. H2 formation occurs in larger amounts when using the Ru nanoparticle based catalyst. While similar reaction outputs were observed for both catalytic systems, DFT calculations revealed some significant differences related to distinct interactions between the catalytic material and the organic substrates or products, which could pave the way for the design of new catalytic materials

Insights into the formation mechanism of two-dimensional lead halide nanostructures

We present a colloidal synthesis strategy for lead halide nanosheets with a thickness of far below 100 nm. Due to the layered structure and the synthesis parameters the crystals of PbI2 are initially composed of many polytypes. We propose a mechanism which gives insight into the chemical process of the PbI2 formation. Further, we found that the crystal structure changes with increasing reaction temperature or by performing the synthesis for longer time periods changing for the final 2H structure. In addition, we demonstrate a route to prepare nanosheets of lead bromide as well as lead chloride in a similar way. Lead halides can be used as a detector material for high-energy photons including gamma and X-rays

Nitrogenated holey two-dimensional structures

Recent graphene research has triggered enormous interest in new two-dimensional ordered crystals constructed by the inclusion of elements other than carbon for bandgap opening. The design of new multifunctional two-dimensional materials with proper bandgap has become an important challenge. Here we report a layered two-dimensional network structure that possesses evenly distributed holes and nitrogen atoms and a C 2 N stoichiometry in its basal plane. The two-dimensional structure can be efficiently synthesized via a simple wet-chemical reaction and confirmed with various characterization techniques, including scanning tunnelling microscopy. Furthermore, a field-effect transistor device fabricated using the material exhibits an on/off ratio of 10 7, with calculated and experimental bandgaps of approximately 1.70 and 1.96eV, respectively. In view of the simplicity of the production method and the advantages of the solution processability, the C 2 N-h2D crystal has potential for use in practical applications.open111