Dinuclear Palladium(II) and -(III) Compounds with O,O-Chelating Ligands. Room-Temperature Direct 2‑Phenylation of 1‑Methylindole

New dinuclear palladium­(III) compounds of general formula Pd₂[(C₆H₄)­PPh₂]₂[O–O]₂Cl₂, O–O being chelating phenolates C₆H₄OC­(O)­R (R = CH₃, <b>3a</b>; R = C₂H₅, <b>3b</b>; R = OPh, <b>3c</b>) or acetylacetonates RC­(O)­CHC­(O)­R (R = CH₃, <b>4a</b>; R = CF₃, <b>4b</b>; R = C­(CH₃)₃, <b>4c</b>), have been obtained by oxidation with PhICl₂ of the corresponding palladium­(II) compounds. The stability of the new compounds has been studied by ³¹P NMR spectroscopy from 200 to 298 K. DFT calculations of the stability of the complexes have also been performed. In agreement with these calculations, only compound Pd₂[(C₆H₄)­PPh₂]₂[(CF₃C­(O)­CHC­(O)­CF₃]₂Cl₂, <b>6b</b>, showed the highest thermal stability. <b>6b</b> was characterized by X-ray diffraction methods, presenting the longest Pd–Pd distance, 2,6403(6) Å, observed among the already described discrete Pd₂⁶⁺ compounds. The isolated palladium­(II) and -(III) compounds have been tested at room temperature in the catalytic 2-phenylation of 1-methylindole with [Ph₂I]­PF₆. With <b>3a</b> as precatalyst the reaction was completed in 2 h with a 93% isolated yield. The results were compared with those obtained with other orthometalated dinuclear and mononuclear palladium compounds

Dinuclear Palladium(II) and -(III) Compounds with O,O-Chelating Ligands. Room-Temperature Direct 2‑Phenylation of 1‑Methylindole

New dinuclear palladium­(III) compounds of general formula Pd₂[(C₆H₄)­PPh₂]₂[O–O]₂Cl₂, O–O being chelating phenolates C₆H₄OC­(O)­R (R = CH₃, <b>3a</b>; R = C₂H₅, <b>3b</b>; R = OPh, <b>3c</b>) or acetylacetonates RC­(O)­CHC­(O)­R (R = CH₃, <b>4a</b>; R = CF₃, <b>4b</b>; R = C­(CH₃)₃, <b>4c</b>), have been obtained by oxidation with PhICl₂ of the corresponding palladium­(II) compounds. The stability of the new compounds has been studied by ³¹P NMR spectroscopy from 200 to 298 K. DFT calculations of the stability of the complexes have also been performed. In agreement with these calculations, only compound Pd₂[(C₆H₄)­PPh₂]₂[(CF₃C­(O)­CHC­(O)­CF₃]₂Cl₂, <b>6b</b>, showed the highest thermal stability. <b>6b</b> was characterized by X-ray diffraction methods, presenting the longest Pd–Pd distance, 2,6403(6) Å, observed among the already described discrete Pd₂⁶⁺ compounds. The isolated palladium­(II) and -(III) compounds have been tested at room temperature in the catalytic 2-phenylation of 1-methylindole with [Ph₂I]­PF₆. With <b>3a</b> as precatalyst the reaction was completed in 2 h with a 93% isolated yield. The results were compared with those obtained with other orthometalated dinuclear and mononuclear palladium compounds

An Extended Chain and Trinuclear Complexes Based on Pt(II)–M (M = Tl(I), Pb(II)) Bonds: Contrasting Photophysical Behavior

The syntheses and structural characterizations of a Pt–Tl chain [{Pt­(bzq)­(C₆F₅)₂}­Tl­(Me₂CO)]_<i>n</i> <b>1</b> and two trinuclear Pt₂M clusters (NBu₄)­[{Pt­(bzq)­(C₆F₅)₂}₂Tl] <b>2</b> and [{Pt­(bzq)­(C₆F₅)₂}₂Pb] <b>3</b> (bzq = 7,8-benzoquinolinyl), stabilized by donor–acceptor Pt → M bonds, are reported. The one-dimensional heterometallic chain <b>1</b> is formed by alternate “Pt­(bzq)­(C₆F₅)₂” and “Tl­(Me₂CO)” fragments, with Pt–Tl bond separations in the range of 2.961(1)–3.067(1) Å. The isoelectronic trinuclear complexes <b>2</b> (which crystallizes in three forms, namely, <b>2a</b>, <b>2b</b>, and <b>2c</b>) and <b>3</b> present a sandwich structure in which the Tl­(I) or Pb­(II) is located between two “Pt­(bzq)­(C₆F₅)₂” subunits. NMR studies suggest equilibria in solution implying cleavage and reformation of Pt–M bonds. The lowest-lying absorption band in the UV–vis spectra in CH₂Cl₂ and tetrahydrofuran (THF) of <b>1</b>, associated with ¹MLCT/¹L′LCT ¹[5d_π(Pt) → π*­(bzq)]/¹[(C₆F₅) → bzq], displays a blue shift in relation to the precursor, suggesting the cleavage of the chain maintaining bimetallic Pt–Tl fragments in solution, also supported by NMR spectroscopy. In <b>2</b> and <b>3</b>, it shows a blue shift in THF and a red shift in CH₂Cl₂, supporting a more extensive cleavage of the Pt–M bonds in THF solutions than in CH₂Cl₂, where the trinuclear entities are predominant. The Pt–Tl chain <b>1</b> displays in solid state a bright orange-red emission ascribed to ³MM′CT (M′ = Tl). It exhibits remarkable and fast reversible vapochromic and vapoluminescent response to donor vapors (THF and Et₂O), related to the coordination/decoordination of the guest molecule to the Tl­(I) ion, and mechanochromic behavior, associated with the shortening of the intermetallic Pt–Tl separations in the chain induced by grinding. In frozen solutions (THF, acetone, and CH₂Cl₂) <b>1</b> shows interesting luminescence thermochromism with emissions strongly dependent on the solvent, concentration, and excitation wavelengths. The Pt₂Tl complex <b>2</b> shows an emission close to <b>1</b>, ascribed to charge transfer from the platinum fragment to the thallium [³(L+L′)­MM′CT]. <b>2</b> also shows vapoluminescent behavior in the presence of vapors of Me₂CO, THF, and Et₂O, although smaller and slower than those of <b>1</b>. The trinuclear neutral complex Pt₂Pb <b>3</b> displays a blue-shift emission band, tentatively assigned to admixture of ³MM′CT ³[Pt­(d) → Pb­(sp)] with some metal-mediated intraligand (³ππ/³ILCT) contribution. In contrast to <b>1</b> and <b>2</b>, <b>3</b> does not show vapoluminescent behavior

Synthesis and Reactivity of the Unsaturated Trinuclear Phosphanido Complex [(C₆F₅)₂Pt(μ-PPh₂)₂Pt(μ-PPh₂)₂Pt(PPh₃)]

The reaction of [NBu₄]­[(C₆F₅)₂Pt­(μ-PPh₂)₂Pt­(μ-PPh₂)₂Pt­(<i>O</i>,<i>O</i>-acac)] (48 VEC) with [HPPh₃]­[ClO₄] gives the 46 VEC unsaturated [(C₆F₅)₂Pt¹(μ-PPh₂)₂Pt²(μ-PPh₂)₂Pt³(PPh₃)]­(Pt²–Pt³) (<b>1</b>), a trinuclear compound endowed with a Pt–Pt bond. This compound displays amphiphilic behavior and reacts easily with nucleophiles L, yielding the saturated complexes [(C₆F₅)₂Pt^II(μ-PPh₂)₂Pt^II(μ-PPh₂)₂Pt^II(PPh₃)­L] [L = PPh₃ (<b>2</b>), py (<b>3</b>)]. The reaction with the electrophile [Ag­(OClO₃)­PPh₃] affords the adduct <b>1</b>·AgPPh₃, which evolves, even at low temperature, to a mixture in which [(C₆F₅)₂Pt^III(μ-PPh₂)₂Pt^III(μ-PPh₂)₂Pt^II(PPh₃)₂]²⁺(Pt^III–Pt^III) and <b>2</b> (plus silver metal) are present. The nucleophilic nature of <b>1</b> is also demonstrated through its reaction with <i>cis</i>-[Pt­(C₆F₅)₂(THF)₂], which results in the formation of [Pt₄(μ-PPh₂)₄(C₆F₅)₄(PPh₃)] (<b>4</b>). The structure and NMR features indicate that <b>1</b> can be better considered as a Pt^II–Pt^III–Pt^I complex instead of a Pt^II–Pt^II–Pt^II derivative. Theoretical calculations (density functional theory) on similar model compounds are in agreement with the assigned oxidation states of the metal centers. The strong intermetallic interactions resulting in a Pt²–Pt³ metal–metal bond and the respective bonding mechanism were verified by employing a multitude of computational techniques (natural bond order analysis, the Laplacian of the electron density, and localized orbital locator profiles)

Synthesis and Reactivity of the Unsaturated Trinuclear Phosphanido Complex [(C₆F₅)₂Pt(μ-PPh₂)₂Pt(μ-PPh₂)₂Pt(PPh₃)]

The reaction of [NBu₄]­[(C₆F₅)₂Pt­(μ-PPh₂)₂Pt­(μ-PPh₂)₂Pt­(<i>O</i>,<i>O</i>-acac)] (48 VEC) with [HPPh₃]­[ClO₄] gives the 46 VEC unsaturated [(C₆F₅)₂Pt¹(μ-PPh₂)₂Pt²(μ-PPh₂)₂Pt³(PPh₃)]­(Pt²–Pt³) (<b>1</b>), a trinuclear compound endowed with a Pt–Pt bond. This compound displays amphiphilic behavior and reacts easily with nucleophiles L, yielding the saturated complexes [(C₆F₅)₂Pt^II(μ-PPh₂)₂Pt^II(μ-PPh₂)₂Pt^II(PPh₃)­L] [L = PPh₃ (<b>2</b>), py (<b>3</b>)]. The reaction with the electrophile [Ag­(OClO₃)­PPh₃] affords the adduct <b>1</b>·AgPPh₃, which evolves, even at low temperature, to a mixture in which [(C₆F₅)₂Pt^III(μ-PPh₂)₂Pt^III(μ-PPh₂)₂Pt^II(PPh₃)₂]²⁺(Pt^III–Pt^III) and <b>2</b> (plus silver metal) are present. The nucleophilic nature of <b>1</b> is also demonstrated through its reaction with <i>cis</i>-[Pt­(C₆F₅)₂(THF)₂], which results in the formation of [Pt₄(μ-PPh₂)₄(C₆F₅)₄(PPh₃)] (<b>4</b>). The structure and NMR features indicate that <b>1</b> can be better considered as a Pt^II–Pt^III–Pt^I complex instead of a Pt^II–Pt^II–Pt^II derivative. Theoretical calculations (density functional theory) on similar model compounds are in agreement with the assigned oxidation states of the metal centers. The strong intermetallic interactions resulting in a Pt²–Pt³ metal–metal bond and the respective bonding mechanism were verified by employing a multitude of computational techniques (natural bond order analysis, the Laplacian of the electron density, and localized orbital locator profiles)

Addition of Nucleophiles to Phosphanido Derivatives of Pt(III): Formation of P–C, P–N, and P–O Bonds

The reactivity of the dinuclear platinum­(III) derivative [(R_F)₂Pt^III(μ-PPh₂)₂­Pt^III(R_F)₂]<i>(Pt–Pt)</i> (R_F = C₆F₅) (<b>1</b>) toward OH^–, N₃^–, and NCO^– was studied. The coordination of these nucleophiles to a metal center evolves with reductive coupling or reductive elimination between a bridging diphenylphosphanido group and OH^–, N₃^–, and NCO^– or C₆F₅ groups and formation of P–O, P–N, or P–C bonds. The addition of OH^– to <b>1</b> evolves with a reductive coupling with the incoming ligand, formation of a P–O bond, and the synthesis of [NBu₄]₂[(R_F)₂Pt^II(μ-OPPh₂)­(μ-PPh₂)­Pt^II(R_F)₂] (<b>3</b>). The addition of N₃^– takes place through two ways: (a) formation of the P–N bond and reductive elimination of PPh₂N₃ yielding [NBu₄]₂[(R_F)₂Pt^II(μ-N₃)­(μ-PPh₂)­Pt^II(R_F)₂] (<b>4a</b>) and (b) formation of the P–C bond and reductive coupling with one of the C₆F₅ groups yielding [NBu₄]­[(R_F)₂Pt^II(μ-N₃)­(μ-PPh₂)­Pt^II(R_F)­(PPh₂R_F)] (<b>4b</b>). Analogous behavior was shown in the addition of NCO^– to <b>1</b> which afforded [NBu₄]₂[(R_F)₂Pt^II(μ-NCO)­(μ-PPh₂)­Pt^II(R_F)₂] (<b>5a</b>) and [NBu₄]­[(R_F)₂Pt^II(μ-NCO)­(μ-PPh₂)­Pt^II(R_F)­(PPh₂R_F)] (<b>5b</b>). In the reaction of the trinuclear complex [(R_F)₂Pt^III(μ-PPh₂)₂Pt^III(μ-PPh₂)₂­Pt^II(R_F)₂]<i>(Pt</i>^<i>III</i><i>–Pt</i>^<i>III</i><i>)</i> (<b>2</b>) with OH^– or N₃^–, the coordination of the nucleophile takes place selectively at the central platinum­(III) center, and the PPh₂/OH^– or PPh₂/N₃^– reductive coupling yields the trinuclear [NBu₄]₂[(R_F)₂Pt^II(μ-Ph₂PO)­(μ-PPh₂)­Pt^II(μ-PPh₂)₂Pt^II(R_F)₂] (<b>6</b>) and [NBu₄]­[(R_F)₂Pt¹(μ₃-Ph₂PNPPh₂)­(μ-PPh₂)­Pt²(μ-PPh₂)­Pt³(R_F)₂]<i>(Pt</i>^<i>2</i><i>–Pt</i>^<i>3</i><i>)</i> (<b>7</b>). Complex <b>7</b> is fluxional in solution, and an equilibrium consisting of Pt–Pt bond migration was ascertained by ³¹P EXSY experiments

Multiple Overlapping Epitopes in the Repetitive Unit of the Shed Acute-Phase Antigen from Trypanosoma cruzi Enhance Its Immunogenic Properties

The repetitive shed acute-phase antigen (SAPA) from Trypanosoma cruzi was thoroughly mapped by SPOT peptides and phage display strategies, showing that a single SAPA repeat is composed of multiple overlapping B-cell epitopes. We propose that this intricate antigenic structure constitutes an alternative device to repetitiveness in order to improve its immunogenicity