Tuning the Gold(I)-Carbon σ Bond in Gold-Alkynyl Complexes through Structural Modifications of the NHC Ancillary Ligand: Effect on Spectroscopic Observables and Reactivity

AbstractUnderstanding the features of the gold(I)‐carbon σ bond and its modulation induced by an ancillary ligand has become fundamental for the purposes of ligand design, due to the increasing interest towards gold(I)‐alkynyl complexes and their wide range of applications. We carry out a systematic computational analysis of 16 gold(I)‐acetylide complexes bearing different N‐Heterocyclic Carbenes (NHCs) as ancillary ligands [NHC−Au(I)−CCH]. The results show that the strength and features of the Au−C bond can be efficiently tuned by performing specific structural modifications on the NHC, enabling a more efficient π communication between the alkynyl and the ancillary ligand. We also demonstrate that the effect of the bond modulation can be revealed via NMR spectroscopy, as highlighted by the tight correlation between the computed nuclear shielding constants and the bonding parameters. Finally, we show that, for the dual‐gold‐catalyzed Bergman cyclization as case study, suitable structural modifications on the NHC ligand, which modulate the π‐acidity of the metal fragment σ‐coordinated to an enediyne substrate, could affect the reaction barrier and the thermodynamic stability of the product. All the reported results can be well rationalized in the framework of distortion/interaction analysis, which has been recently extended to the dual (σ,π‐type) Au catalytic systems by Alabugin et al (J. Am. Chem. Soc. 2017, 137, 3406‐3416)

Suidae Transition at the Miocene-Pliocene Boundary : a Reassessment of the Taxonomy and Chronology of Propotamochoerus provincialis

The Miocene-Pliocene (Turolian-Ruscinian) transition represents a fundamental interval in the evolution of Euro-Mediterranean paleocommunities. In fact, the paleoenvironmental changes connected with the end of the Messinian salinity crisis are reflected by a major renewal in mammal faunal assemblages. An important bioevent among terrestrial large mammals is the dispersal of the genus Sus, which replaced all other suid species during the Pliocene. Despite its possible paleoecological and biochronological relevance, correlations based on this bioevent are undermined by the supposed persistence of the late surviving late Miocene Propotamochoerus provincialis. However, a recent revision of the type material of this species revealed an admixture with remains of Sus strozzii, an early Pleistocene (Middle Villafranchian to Epivillafranchian) suid, questioning both the diagnosis and chronological range of P. provincialis. Here we review the late Miocene Suidae sample recovered from the Casino Basin (Tuscany, central Italy), whose taxonomic attribution has been controversial over the nearly 150 years since its discovery. Following a comparison with other Miocene, Pliocene, and Pleistocene Eurasian species, the Casino Suidae are assigned to P. provincialis and the species diagnosis is emended. Moreover, it is recognized that all the late Miocene (Turolian) European Propotamochoerus material belongs to P. provincialis and that there is no compelling evidence of the occurrence of this species beyond the Turolian-Ruscinian transition (MN13-MN14)

Monomeric gold hydrides for carbon dioxide reduction: Ligand effect on the reactivity

We analyzed the ligand electronic effect in the reaction between a [LAu(I)H]0/− hydride species and CO2, leading to a coordinated formate [LAu(HCOO)]0/−. We explored 20 different ligands, such as carbenes, phosphines and others, carefully selected to cover a wide range of electron-donor and -acceptor properties. We included in the study the only ligand, an NHC-coordinated diphosphene, that, thus far, experimentally demonstrated facile and reversible reaction between the monomeric gold(I) hydride and carbon dioxide. We elucidated the previously unknown reaction mechanism, which resulted to be concerted and common to all the ligands: the gold–hydrogen bond attacks the carbon atom of CO2 with one oxygen atom coordinating to the gold center. A correlation between the ligand σ donor ability, which affects the electron density at the reactive site, and the kinetic activation barriers of the reaction has been found. This systematic study offers useful guidelines for the rational design of new ligands for this reaction, while suggesting a few promising and experimentally accessible potential candidates for the stoichiometric or catalytic CO2 activation

Gold-Aluminyl and Gold-Diarylboryl Complexes:Bonding and Reactivity with Carbon Dioxide

The unconventional carbon dioxide insertion reaction of a gold-aluminyl [tBu3PAuAl(NON)] complex has been recently shown to be related to the electron-sharing character of the Au-Al bond that acts as a nucleophile and stabilizes the insertion product through a radical-like behavior. Since a gold-diarylboryl [IPrAuB(o-tol)2] complex with similar reactivity features has been recently reported, in this work we computationally investigate the reaction of carbon dioxide with [LAuX] (L = phosphine, N-heterocyclic carbene (NHC); X = Al(NON), B(o-tol)2) complexes to get insights into the Al/B anionic and gold ancillary ligand effects on the Au-Al/B bond nature, electronic structure, and reactivity of these compounds. We demonstrate that the Au-Al and Au-B bonds possess a similar electron-sharing nature, with diarylboryl complexes displaying a slightly more polarized bond as Au(δ+)-B(δ-). This feature reduces the radical-like reactivity toward CO2, and the Al/B anionic ligand effect is found to favor aluminyls over boryls, despite the greater oxophilicity of B. Remarkably, the ancillary ligand of gold has a negligible electronic trans effect on the Au-X bond and only a minor impact on the formation of the insertion product, which is slightly more stable with carbene ligands. Surprisingly, we find that the modification of the steric hindrance at the carbene site may exert a sizable control over the reaction, with more sterically hindered ligands thermodynamically disfavoring the formation of the CO2 insertion product

Chemical bond analysis for the entire periodic table: Energy Decomposition and Natural Orbitals for Chemical Valence in the Four-Component Relativistic Framework

Chemical bonding is a ubiquitous concept in chemistry and it provides a common basis for experimental and theoretical chemists to explain and predict the structure, stability and reactivity of chemical species. Among others, the Energy Decomposition Analysis (EDA, also known as the Extended Transition State method) in combination with Natural Orbitals for Chemical Valence (EDA-NOCV) is a very powerful tool for the analysis of the chemical bonds based on a charge and energy decomposition scheme within a common theoretical framework. While the approach has been applied in a variety of chemical contexts, the current implementations of the EDA-NOCV scheme include relativistic effects only at scalar level, so simply neglecting the spin-orbit coupling effects and de facto limiting its applicability. In this work, we extend the EDA-NOCV method to the relativistic four-component Dirac-Kohn-Sham theory that variationally accounts for spin-orbit coupling. Its correctness and numerical stability have been demonstrated in the case of simple molecular systems, where the relativistic effects play a negligible role, by comparison with the implementation available in the ADF modelling suite (using the non-relativistic Hamiltonian and the scalar ZORA approximation). As an illustrative example we analyse the metal-ethylene coordination bond in the group 6-element series (CO)$_5$TM-C$_2$H$_4$, with TM =Cr, Mo, W, Sg, where relativistic effects are likely to play an increasingly important role as one moves down the group. The method provides a clear measure (also in combination with the CD analysis) of the donation and back-donation components in coordination bonds, even when relativistic effects, including spin-orbit coupling, are crucial for understanding the chemical bond involving heavy and superheavy atoms.Comment: 49 pages, 2 figure

Middle Pleistocene Steppe Lion Remains from Grotte de la Carrière (Têt Valley, Eastern Pyrenees)

Altres ajuts: Acord transformatiu CRUE-CSICAltres ajuts: Open Access Funding provided by Universitat Autonoma de Barcelona. This work has been funded by the Direction Regionalle des Afaires Culturelles of the French Government (DRAC Occitaine) and the Agencia Estatal de Investigación-European Regional Development Fund of the European Union (CGL2017-82654-P, AEI/FEDERUE). LS is supported by the FI AGAUR fellowship (ref. 2020 FI_B1 00131) funded by the Secretaria d'Universitats i Recerca de la Generalitat de Catalunya and the European Social FundLate Pleistocene cave lions are one of the most iconic species of Northern Hemisphere Quaternary taphocoenoses. Despite their often-scarce record in cave environments, their ubiquitous distribution across Eurasia and North America assemblages attests to their position as top ice-age predators. Nevertheless, the origins of these former large felids, their distribution during the Middle Pleistocene, and their paleoecology during co-existence with the scimitar-toothed cat Homotherium remain debated. Here we describe for the first time an abundant collection of large-sized and stout felid remains from the recently discovered site of Grotte de la Carrière in Eastern Pyrenees, with an estimated age corresponding to MIS 9. Our results highlight the larger size of Middle Pleistocene lions compared to Late Pleistocene ones as well as a trend of decreasing in size, which has been previously stated by other authors. Grotte de la Carrière steppe lions have similar morphological and biometrical parameters to those of other samples from MIS 11-9, being larger and stouter than younger latest Middle Pleistocene-Late Pleistocene forms and slightly smaller than older MIS 15-12 forms

Vigna Nuova : the first Middle Villafranchian mammal assemblage from the Valdichiana Basin, Perugia (Italy)

The region of Umbria (Central Italy) represents one of the most interesting areas for the study of Mediterranean Plio-Pleistocene mammal faunas due to the occurrence of numerous paleontological deposits. Most of the fossils have been discovered within the Tiberino Basin, which extends across the centre of the territory for ca. 1800 km2, and the small Tavernelle-Pietrafitta Basin, south of Lake Trasimeno. Herein, we provide an additional report on an assemblage within continental deposits from the locality of Vigna Nuova (Piegaro, Perugia) in the Valdichiana Basin, which are observed to crop out in only a few areas of western Umbria. Taphonomic and geological evidence, coupled with rare excavation notes, have allowed the sample to be divided into two assemblages. The younger was collected from the conglomerates (here called "upper layer") which are still visible on the outcrop and is composed only of remains of cf. Leptobos. The data obtained from this fragmentary sample together with some sedimentological-stratigraphic considerations suggest that it can be correlated with other well-known Valdichiana assemblages (e.g., Selvella, Farneta), dated at ca. 1.5 Ma. The older sub-sample comes from a putative paleosol (here called "lower layer") no longer visible in the field. It includes cf. Megantereon, Canis sp., Mustelidae indet., Proboscidea indet., Equus cf. senezensis, cf. Leptobos, Croizetoceros ramosus, Pseudodama sp., Cervidae indet. (large size), and Sus cf. strozzii. This assemblage can be referred to the Coste San Giacomo Faunal Unit (late Middle Villafranchian; ca. 2.2-2.1 Ma). It represents the first wellpreserved sample of this age in Umbria and one of the few recorded in Italy, opening new interesting research perspectives on the distribution of mammals within the Peninsula and also on the stratigraphy and evolution of the Valdichiana Basin. "Aggiungeva che per scavare le poche ossa che mi avrebbe subito inviate aveva dovuto faticare enormemente e lottare per due giorni contro ogni sorta di difficoltà. Ricevuto il graditissimo invio mi affrettai a consolidare e restaurare con ogni cura quelle primizie ed oggi sono lieto di poter annunziare, in così solenne adunanza, la interessante scoperta…" ("He added that in order to excavate the few bones that he would immediately send me he had had to work enormously and struggle for two days against all sorts of difficulties. Having received the very welcome shipment, I hastened to consolidate and restore those fruits of the soil with every care and today I am pleased to be able to announce, in such a solemn meeting, the interesting discovery…"). Giovanni Capellini, 189

How reduced are nucleophilic gold complexes?

Nucleophilic formal gold(-i) and gold(i) complexes are investigated via Intrinsic Bond Orbital analysis and Energy Decomposition Analysis, based on density functional theory calculations. The results indicate gold(0) centres engaging in electron-sharing bonding with Al- and B- based ligands. Multiconfigurational (CASSCF) calculations corroborate the findings, highlighting the gap between the electonic structures and the oxidation state formalism

Cationic Gold(I) Diarylallenylidene Complexes:Bonding Features and Ligand Effects

Using computational approaches, we qualitatively and quantitatively assess the bonding components of a series of experimentally characterized Au(I) diarylallenylidene complexes (N.Kim, R.A.Widenhoefer, Angew. Chem. Int. Ed. 2018, 57, 4722-4726). Our results clearly demonstrate that Au(I) engages only weakly in pi-backbonding, which is, however, a tunable bonding component. Computationally identified trends in bonding are clearly correlated with the substitution patterns of the aryl substituents in the Au(I) diarylallenylidene complexes and good agreement is found with the previously reported experimental data, such as IR spectra, C-13 NMR chemical shifts and rates of decomposition together with their corresponding barrier heights, further substantiating the computational findings. The description of the bonding patterns in these complexes allow predictions of their spectroscopic features, their reactivity and stability

Early-Middle Pleistocene Sus and Bison from Western Palaearctic : Taxonomy, Paleobiology and Distribution

El Pleistocè inferior és un dels períodes més importants per l'evolució de la biosfera al Paleàrtic occidental. El progressiu refredament global i la dinàmica glacial es veieren intensificats després de 3,0 Ma i especialment durant el període 1,4-1,2 Ma. Aquest important punt d'inflexió per al clima del planeta, s'ha anomenat a la literatura Transició del Pleistocè Inferior i mitjà (EMPT). Les conseqüències d'aquestes noves dinàmiques van agreujar el refredament i l'aridificació del clima, provocant la progressiva reducció dels hàbitats boscosos substituïts per paisatges oberts, i, en conseqüència, un recanvi en les associacions animals i vegetals. La composició taxonòmica dels conjunts faunístics del Plistocè inferior a Europa i la seva resposta al canvi ambiental han sigut reconeguts amb la formalització de la biozona anomenada Epivillafranquià (1,2-0,8 Ma). Aquesta unitat biocronològica representa el període de temps "de transició" en què Europa es va caracteritzar per la co-aparició d'algunes espècies vilafranquianes i els primers tàxons galerians, arribats majoritàriament al continent procedents d'Àsia i Àfrica. Malgrat l'augment de l'interès per la comunitat paleontològica i els nous descobriments, no tots els elements faunístics representatius de l'Epivillafranquià estan ben definits i aquesta biozona encara està poc caracteritzada. Entre els grans gèneres de mamífers apareguts durant el Plio-Pleistocè i que tenen un paper important en la transició de l'Epivillafranquià, destaquen Sus i Bison. La seva importància dins de les cadenes tròfiques del Pleistocè i l'Holocè i la seva abundància en el registre fòssil han fet que aquests dos clades siguin extremadament útils tant com a indicadors ambientals com a marcadors biocronològics. No obstant això, la història taxonòmica dels primers representants d'aquests clades és força confusa i la seva dispersió a l'oest d'Euràsia encara és relativament obscura. Per tant, l'objectiu principal d'aquesta tesi doctoral és entendre millor la taxonomia, la paleobiologia i la distribució crono-espacial d'aquests dos gèneres durant les últimes etapes del Pleistocè inferior. L'estudi realitzat sobre el registre de suids dels jaciments de l'Epivillafranquià de Cal Guardiola i Vallparadís Estació (1,2-0,6 Ma, Península Ibèrica) dóna fe de la presència del senglar verrucós Sus strozzii a Europa entre 1,0 i 0,86 Ma, després del l'anomenat 'suid gap', durant la qual aquest grup va experimentar un important col·lapse demogràfic. L'estudi s'estén a altres mostres de Suinae i revela que, contràriament al que es creia comunament, S. strozzii no va desaparèixer abans del subcron Jaramillo 1,0 Ma) sinó que va persistir fins al límit del Pleistocè inferior i mitjà (0,8 Ma) i que per tant, es pot considerar un marcador de l'Epivillafranquià. En aquesta tesi es revalora el registre de bòvids de la localitat de Pietrafitta (Península italiana), atribuït originalment a Leptobos, i es refereix al gènere Bison. Els caràcters primitius del crani, la mida corporal relativament petita i la construcció esvelta indiquen que la mostra de Pietrafitta es pot referir a Bison (Eobison) degiulii, una de les espècies més primerenques de bisons que van entrar a Europa després d'1,8 Ma. La definició vaga del subgènere Eobison ha provocat la re avaluació de tot el seu registre eurasiàtic que es presenta aquí. El diagnòstic esmenat resultant permet el reconeixement d'almenys tres espècies d'Eobison, que abasten més d'1,0 Ma a tota Euràsia. La mostra de bòvids de la secció composta de Vallparadís ofereix noves perspectives sobre els primers bisons que van entrar a Europa a l'inici de l'Epivillafranquià. El gran i corpulent Bison de Vallparadís es refereix aquí al primer membre del subgènere Bison que es dispersa al Paleàrtic Occidental, és a dir, Bison (Bison) schoetensacki. Una revisió de diverses mostres de bisons d'Europa restringides entre 1,2 i 0,6 Ma advoca per la inclusió d'aquesta espècie entre els principals marcadors de l'Epivillafranquià.El Pleistoceno Inferior es uno de los períodos más importantes para la evolución de la biosfera en el Paleártico Occidental. El enfriamiento global a largo plazo y la dinámica glacial se intensificaron después de 3,0 Ma y, con más fuerza, entre 1,4 y 1,2 Ma. Este importante punto de inflexión para el clima del planeta se ha denominado Early-Middle Pleistocene (EMPT). Las consecuencias de estas nuevas dinámicas exacerbaron el enfriamiento y la aridez del clima, provocando la reducción progresiva de los hábitats boscosos reemplazados por ambientes abiertos y, en consecuencia, una importante renovación de la flora y la fauna. La composición taxonómica de los conjuntos de fauna del Pleistoceno europeo y su respuesta a este cambio ambiental han sido objeto de numerosas publicaciones y han proporcionado importantes datos biocronológicos. Los profundos efectos del EMPT en las comunidades de mamíferos europeos se reconocieron con la formalización del biocron Epivillafranquiense (1,2-0,8 Ma). Esta unidad biocronológica representa el lapso "transicional" en el que Europa se caracterizó por la concurrencia de algunos taxones villafranquienses y los primeros galerianos, llegados al continente desde Asia y África. A pesar del creciente interés en la comunidad paleontológica y los nuevos descubrimientos, no todos los elementos faunísticos representativos del Epivillafranquiense están bien definidos y este biocron todavía está pobremente caracterizado. Entre los grandes mamíferos que aparecieron durante el Plio-Pleistoceno y que tienen un papel importante en la transición Epivillafranchian, se destacan Sus y Bison. Su importancia dentro de las cadenas tróficas del Pleistoceno y Holoceno y su adaptabilidad ecológica y abundancia en el registro fósil hicieron que estos dos taxa fueran extremadamente útiles como indicadores ambientales y biocronológicos. No obstante, la historia taxonómica de los primeros representantes de estos clades es confusa y su dispersión en el oeste de Eurasia aún es relativamente oscura. Por lo tanto, el objetivo principal de esta tesis doctoral es comprender mejor la taxonomía, paleobiología y distribución de estos dos géneros durante las últimas etapas del Pleistoceno temprano. El estudio realizado sobre el registro de suidos de los yacimientos Epivillafranquiense de Cal Guardiola y Vallparadís Estació (1,2-0,6 Ma, Península Ibérica) da fe de la presencia del jabalí Sus strozzii en Europa entre 1,0 y 0,86 Ma, despues el 'suid gap', durante la cual este tuve un colapso demográfico. El estudio se amplía a otras muestras de Suinae de Europa y revela que, contrariamente a lo que comúnmente se creía, S. strozzii no desapareció antes del subcrón Jaramillo (1,0) sino que persistió hasta el límite del Pleistoceno Inferior-Medio (0,8 Ma), que por lo tanto, puede considerarse un marcador para este biochron. El registro de bóvidos de la localidad villafranquiense de Pietrafitta (península italiana), originalmente atribuido a Leptobos, es reevaluado en esta tesis y referido al género Bison. Los caracteres primitivos del cráneo, el tamaño corporal relativamente pequeño y la constitución esbelta indican que la muestra de Pietrafitta se puede referir a Bison (Eobison) degiulii, una de las primeras especies de bisontes que entraron en Europa después de 1,8 Ma. La vaga definición del subgénero Eobison ha provocado la reevaluación de todo su registro euroasiático presentado aquí. La diagnosis modificado resultante permite el reconocimiento de, tres especies de Eobison, que abarcan más de 1,0 Ma en Eurasia. La gran colección de bóvidos de Vallparadís proporciona nuevos conocimientos sobre el primer bisonte sensu stricto que entró en Europa al comienzo del Epivillafranquiense. Esta muestra se refiere aquí al miembro más antiguo del subgénero Bison, a saber, Bison (Bison) schoetensacki. Una revisión de varias muestras de bisonte de Europa de 1,2- 0,6 Ma aboga por la inclusión de esta especie entre los principales marcadores del Epivillafranquiense.The Early Pleistocene is one of the most important periods for the evolution of the biosphere in the Western Palearctic. The long-term global cooling and the glacial dynamics intensified after 3.0 Ma and, stronger, at 1.4-1.2 Ma. This major turning point for the climate of the planet has been called the Early-Middle Pleistocene Transition (EMPT). The consequences of these new dynamics exacerbated the cooling and aridification of the climate, eventually triggering the progressive shrinking of the forested habitats replaced by open landscapes, and, consequentially, a major renewal in the flora and fauna. The European Early Pleistocene faunal assemblages' taxonomic composition and their response to this environmental change have been subject to numerous publications and have provided significant biochronological data. The deep effects of the EMPT on the mammal communities of Europe were recognized with the formalization of the Epivillafranchian biochron (ca. 1.2-0.8 Ma). This biochronological unit represents the 'transitional' timespan in which Europe was characterized by the co-occurrence of some Villafranchian relicts and the first Galerian taxa, mostly arrived in the continent from Asia and Africa. Despite the increased interest in the paleontological community and the new discoveries, not all the representative faunal elements of the Epivillafranchian are well defined and this biochron is still poorly characterized. Among large mammal genera that appeared during the Plio-Pleistocene and that have a major role in the Epivillafranchian transition, Sus and Bison stand out. Their importance within the trophic chains of the Pleistocene and Holocene and their ecological adaptability and abundance in the fossil record made these two clades extremely useful both as environmental indicators and biochronological markers. Nonetheless, the taxonomic history of the early representatives of these clades is quite confused and their dispersal in western Eurasia is still relatively obscure. Therefore, the main aim of this PhD thesis is to better understand the taxonomy, paleobiology, and chrono-spatial distribution of these two iconic genera during the latest stages of Early Pleistocene. The study performed on the suid record from the Epivillafranchian sites of Cal Guardiola and Vallparadís Estació (1.2-0.6 Ma, NE Iberian Peninsula) attests to the presence of the verrucosic boar Sus strozzii in Europe between 1.0 and 0.86 Ma, after the so-called 'suid gap', during which this group experienced a major demographic collapse. The study is extended to other samples of Suinae from Europe and reveals that, contrarily to what was commonly believed, S. strozzii did not disappear before the Jaramillo subchron (ca. 1.0) but persisted until the Early-Middle Pleistocene boundary (ca 0.8 Ma), which therefore, can be considered a marker for this biochron. The bovid record from the late Villafranchian locality of Pietrafitta (central Italian Peninsula), originally attributed to Leptobos, is reappraised in this thesis and referred to the genus Bison. The primitive characters of the cranium, relatively small body size, and slender built indicate that the Pietrafitta sample can be referred to Bison (Eobison) degiulii, one of the earliest species of bison entering Europe after 1.8 Ma. The vague definition of the subgenus Eobison has triggered the reappraisal of its entire Eurasian record presented herein. The resulted emended diagnosis allows the recognition of, at least, three species of Eobison, spanning for more than 1.0 Ma in the whole Eurasia. The large bovid collection of the Vallparadís composite section provides new insights on the first 'true' bison entering Europe at the onset of the Epivillafranchian. This sample is here referred to the oldest member of the subgenus Bison, namely Bison (Bison) schoetensacki. A review of several Bison samples from Europe constrained between 1.2 and 0.6 Ma advocates the inclusion of this species among the major markers of the Epivillafranchian