Rh-POP Pincer Xantphos Complexes for C-S and C-H Activation. Implications for Carbothiolation Catalysis

The neutral Rh­(I)–Xantphos complex [Rh­(κ³-_P,O,P-Xantphos)­Cl]_<i>n</i>, <b>4</b>, and cationic Rh­(III) [Rh­(κ³-_P,O,P-Xantphos)­(H)₂]­[BAr^F₄], <b>2a</b>, and [Rh­(κ³-_P,O,P-Xantphos-3,5-C₆H₃(CF₃)₂)­(H)₂]­[BAr^F₄], <b>2b</b>, are described [Ar^F = 3,5-(CF₃)₂C₆H₃; Xantphos = 4,5-bis­(diphenylphosphino)-9,9-dimethylxanthene; Xantphos-3,5-C₆H₃(CF₃)₂ = 9,9-dimethylxanthene-4,5-bis­(bis­(3,5-bis­(trifluoromethyl)­phenyl)­phosphine]. A solid-state structure of <b>2b</b> isolated from C₆H₅Cl solution shows a κ¹-chlorobenzene adduct, [Rh­(κ³-_P,O,P-Xantphos-3,5-C₆H₃(CF₃)₂)­(H)₂(κ¹-ClC₆H₅)]­[BAr^F₄], <b>3</b>. Addition of H₂ to <b>4</b> affords, crystallographically characterized, [Rh­(κ³-_P,O,P-Xantphos)­(H)₂Cl], <b>5</b>. Addition of diphenyl acetylene to <b>2a</b> results in the formation of the C–H activated metallacyclopentadiene [Rh­(κ³-_P,O,P-Xantphos)­(ClCH₂Cl)­(σ,σ-(C₆H₄)­C­(H)CPh)]­[BAr^F₄], <b>7</b>, a rare example of a crystallographically characterized Rh–dichloromethane complex, alongside the Rh­(I) complex <i>mer</i>-[Rh­(κ³-_P,O,P-Xantphos)­(η²-PhCCPh)]­[BAr^F₄], <b>6</b>. Halide abstraction from [Rh­(κ³-_P,O,P-Xantphos)­Cl]_<i>n</i> in the presence of diphenylacetylene affords <b>6</b> as the only product, which in the solid state shows that the alkyne binds perpendicular to the κ³-POP Xantphos ligand plane. This complex acts as a latent source of the [Rh­(κ³-_P,O,P-Xantphos)]⁺ fragment and facilitates <i>ortho</i>-directed C–S activation in a number of 2-arylsulfides to give <i>mer</i>-[Rh­(κ³-_P,O,P-Xantphos)­(σ,κ¹-Ar)­(SMe)]­[BAr^F₄] (Ar = C₆H₄COMe, <b>8</b>; C₆H₄(CO)­OMe, <b>9</b>; C₆H₄NO₂, <b>10</b>; C₆H₄CNCH₂CH₂O, <b>11</b>; C₆H₄C₅H₄N, <b>12</b>). Similar C–S bond cleavage is observed with allyl sulfide, to give <i>fac</i>-[Rh­(κ³-_P,O,P-Xantphos)­(η³-C₃H₅)­(SPh)]­[BAr^F₄], <b>13</b>. These products of C–S activation have been crystallographically characterized. For <b>8</b> in situ monitoring of the reaction by NMR spectroscopy reveals the initial formation of <i>fac</i>-κ³-<b>8</b>, which then proceeds to isomerize to the <i>mer</i>-isomer. With the <i>para</i>-ketone aryl sulfide, 4-SMeC ₆H₄COMe, C–H activation <i>ortho</i> to the ketone occurs to give <i>mer</i>-[Rh­(κ³-_P,O,P-Xantphos)­(σ,κ¹-4-(COMe)­C₆H₃SMe)­(H)]­[BAr^F₄], <b>14</b>. The temporal evolution of carbothiolation catalysis using <i>mer</i>-κ³-<b>8</b>, and phenyl acetylene and 2-(methylthio)­acetophenone substrates shows initial fast catalysis and then a considerably slower evolution of the product. We suggest that the initially formed <i>fac</i>-isomer of the C–S activation product is considerably more active than the <i>mer</i>-isomer (i.e., <i>mer</i>-<b>8</b>), the latter of which is formed rapidly by isomerization, and this accounts for the observed difference in rates. A likely mechanism is proposed based upon these data

The community of Furcraea parmentieri, a threatened specie, Central Mexico

The flora and vegetation of Furcraea parmentieri (Roezl ex Ortigies) Garcia-Mend. (F bedinghausii) community was studied on the Pelado volcano, at the S area of Mexico City. Following Zurich-Montpellier criteria, 25 phytosociological plots were done, and the Jaccard index of similarity was calculated. Furcraea parmentieri is associated with Muhlenbergia macroura to form an azonal community between 3 020 and 3 300 m, on rocky soils and gaps of Pinus-Alnus forest. Three subcommunities can be distinguished: Furcraea parmentieri-Trisetum virletii, Furcraea parmentieri-Aegopogon cenchroides and Furcraea parmentieri-Solanum cervantesii. 87 species were recorded, representing 63 genera and 27 families, of which the most abundant were Asteraceae and Poaceae. The proper environmental conditions for Furcraea parmentieri are: altitudes between 3 020 and 3 100 m, N exposures, acid soils with pH 6.0 and slopes among 36° and 45°. The main problem for species preservation in the area is repeated burning, that depletes soils and destroys vegetative bulbils, causing population diminishment. Conservation of Furcraea parmentieri community is important because, it is an endangered and endemic species, as well as a soil-stabilizing species for severely degraded zones (NOM-059-ECOL-2001).Se estudió la flora y vegetación de la comunidad de Furcraea parmentieri (Roezl ex Ortigies) Garcia-Mend. (F. bedinghausii) especie amenazada y endémica de México (NOM-059-ECOL-2001), en el volcán Pelado, al S de la Ciudad de México. Se realizaron 25 levantamientos siguiendo la escuela fitosociológica Zurich-Montpellier, a los que se calculó el índice de similitud de Jaccard. Furcraea parmentieri se asocia con Muhlenbergia macroura formando una comunidad azonal, entre los 3 020 y 3 330 m, sobre suelos rocosos y claros del bosque de Pinus-Alnus. Se diferenciaron tres subcomunidades: Furcraea parmentieri-Trisetum virletii, Furcraea parmentieri-Aegopogon cenchroides y Furcraea parmentieri-Solanum cervantesiis. Se registraron en total 87 especies agrupadas en 63 géneros y 27 familias, siendo las más abundantes Asteraceae y Poaceae. Las condiciones ambientales propicias para el establecimiento de Furcraea parmentieri son altitudes entre 3 020 a 3 100 m, en exposición N, suelos ácidos con pH 6.0 y pendientes de 36° a 45°. El mayor problema para la conservación de la especie en el área son las quemas repetidas que empobrecen el suelo y destruyen los bulbilos, causando la disminución de las poblaciones. La conservación de F. parmentieri es importante porque además de ser una especie amenazada y endémica, estabiliza suelos en zonas severamente degradadas