Matthieu Le Couls
email: matthieu.lecouls@univ-rennes1.fr

Jason Hilton

François Guillocheau

Nicolas Morel

Philippe Courville

Becklesia maulnyi sp. nov.: A new cycadean species from the Lower

Keywords: Cycads, Becklesia, Oxfordian, Upper Jurassic, Sarthe, NW France. RÉSUMÉ Cycadales, Becklesia, Oxfordien, Jurassique supérieur, Sarthe, NW France

Plant macrofossil remains of Becklesia Seward 1895 emend. Watson and Cusack 2005, a rare genus of extinct cycad, have been known for over a century from Great Britain and Libya.

Historically the genus was poorly characterised leading to many fossil cycads that resembled Becklesia being placed in other genera including Cycadites and Paracycas. We describe here a new species, B. maulnyi sp. nov., from an exceptional and almost complete cycad specimen housed for two hundred years in the collections of the Musée Vert of Le Mans. The specimen comprises the apex of a stem bearing petiolate and pinnate leaves with thin and widely separated leaflets, which possess two prominent abaxial stomatal grooves that are characteristic of the genus Becklesia. We reassign three other French cycad fossils previously described under different generic names to Becklesia. Finally, we briefly discuss the geographical and stratigraphical implications of the genus and also consider presence of entire cycads in the fossil record.

Introduction

The genus Becklesia [START_REF] Seward | Catalogue of the Mesozoic plants in the department of Geology, British Museum (Natural History). The Wealden flora. Part II. Gymnospermae. London, 259 p. plant preserved in an earliest Permian air fall tuff in Inner Mongolia, China[END_REF]) [START_REF] Watson | Cycadales of the English Wealden[END_REF] was created by Seward for the rare Berriasian aged (Early Cretaceous) cycad species B. anomala from the English Wealden.

Seward's account was based on fragmentary fronds fragments that resembled those of extant cycads, but as documented by Seward, the genus was poorly characterised preventing its systematic relationships with other cycadalean plants from being determined. [START_REF] Watson | A Revision of the English Wealden Flora, I. Charales-Ginkgoales[END_REF] established from the English Wealden a second species, Becklesia sulcata, for cycad leaf remains that include cuticular features, and [START_REF] Oldham | Flora of the Wealden plant debris beds of England[END_REF] identified B. sulcata as well as two further species that had similarity with B. sulcata from isolated cuticles also from the English Wealden. However, [START_REF] Watson | A revision of the English Wealden flora. III. Czekanowskiales, Ginkgoales & allied Coniferales[END_REF] transferred Oldham's additional species to the non-cycad Torreyites detriti leaving only B. anomala and B. sulcata within the genus. Apart from these accounts from the English Wealden, the genus Becklesia has only been encountered in the Jurassic of Libya with [START_REF] Principi | Filliti wealdiane della Tripolitania. in : Zaccagna, D. Itinerari geologici nella Tripolitania Occidental[END_REF] reporting B. anomala based on its distinctive morphology.

Based on the features of Becklesia anomala and B. sulcata Watson and Cusack (2005, p. 20) described in detail the leaf morphology and cuticle of the genus for the first time, emending the generic diagnosis as part of their monographic revision of English Wealden Cycads. They considered Becklesia to possess leaves with linear leaflets which are either simple or forked, and its cuticle has haplocheilic stomata organised in two parallel, longitudinal grooves on the abaxial surface of the pinna. However, Watson and Cusack's account is based on isolated leaves from which the organisation of the leaves on the plant is unknown, as are their fertile organs.

Here we describe a new species of Becklesia from the Jurassic of France based on a wellpreserved specimen that represents the apex of a plant in which the leaves remain in organic attachment to the crown. We discuss the relevance of the entire nature of the crown in interpreting the plant organisation and habit. We also consider the presence of other fossil cycads in which wellpreserved crowns are known from other stratigraphical intervals as well as the palaeogeographical and stratigraphical implications of the new species.

Material and methods

The sample MHNLM 2003.1.4962 studied here is housed in Musée Vert, natural history museum of Le Mans (Sarthe, NW France). The oldest label joined to the specimen (Fig. 1) indicates that it belonged to the "Cabinet de Maulny". Louis Maulny (1758-1815) was a naturalist of Le Mans, whose collection was bought by the museum of Le Mans after his death in 1816 [START_REF] Delaunay | Louis Maulny, naturaliste et collectionneur (1758-1815). Ses amis, ses disciples, son temps, ses successeurs[END_REF]. This first label has been written by Narcisse Desportes (1776-1856), curator of the museum between 1833 and 1856. It indicates with doubt the genus Zamia and the collection site in Écommoy (Sarthe, NW France) (Fig. 2). Two other labels on the specimen were written at the end of the 19 th century and during the first part of the 20 th century have copied the previous label and added no further information.

[Insert Figures 1 & 2 : First label of B. maulnyi sp. nov. / Localisation of Écommoy]
The unusual sediment of the studied specimen has been compared with the palaeontological collections housed in the Musée Vert in order to provide further information on its provenance. An ammonite (MHNLM 2003.1.4686) collected a few decades later (June 29, 1852) by the local palaeontologist Edouard Guéranger (1801-1895) from Ecommoy is preserved in a very similar rock matrix. It probably confirms the geographical origin written by Desportes and also helps to constrain the geological age.

The sediment for both the fossil plant and ammonite is a fine sandy and argillaceous, bicoloured, yellow and grey limestone. This fine grained sedimentary facies has preserved the mold of the fossil plant including grooves on the leaves but lacks cuticular preservation. This rock corresponds to the "Marnes et calcaires silteux de La Vacherie" [marl and silty limestone of La Vacherie], a local formation with terrestrial influence form the Lower Oxfordian aged Mariae zone and Praecordatum sub-zone [START_REF] Manivit | Notice explicative, carte géologique de la France (1/50 000)[END_REF]. Moreover, the ammonite is an unnamed small Kranaosphinctes that supports a Lower Oxfordian age corresponding with the end of the The exact location(s) of the outcrop where either the plant or ammonite were collected are unknown. However, following the first geological study of the Bélinois by Guillier [START_REF] Guillier | Note géologique sur le Bélinois[END_REF], two quarries were evident in the region, namely "La Vacherie" and "l'Épine". These two quarries were approximately 1.5 km apart and both are now back filled. There are no Oxfordian aged outcrops in the Bélinois area today. However, a stratigraphic column of "La Vacherie" is provided by Guillier (1886, p. 178). Specimen MHNLM 2003.1.4962 is an imprint of the lower face of a crown of leaves viewed from above and is preserved among poorly-preserved marine shelly fauna (Fig. 3A). It is also an outcast molding with relief and micro-relief able to preserve some architectural features. All the leaves are incomplete apically, but the lower-mid parts are more or less entire and are connected to the apex of the stem to form the crown (Fig. 3A). Five leaves are preserved; two of them bifurcate close to the insertion point with the crown. Small areas of a few of the leaflets are still covered by sediment protecting small and fragile carbonaceous remains that may be fusinized. They can perhaps in the future provide new details at cellular scale with an appropriate non-destructive method capable of studying these fragile remains in situ. Others small and unidentifiable plants fragments are also present on the sample. The specimen was observed using a Leica EZ4 binocular microscope and macrophotography was undertaken using a Canon EOS700D with Canon EF-S60mm F2.8 Macro USM lens. Moreover, a petrological thin section (30 µm thick) was made from sample 2003.1.4962 to better characterize the geological context; it is housed in Musée Vert of Le Mans with the number MHNLM 2003.1.4962b.

Geological and palaeoenvironnemental context

The fossil described here occurred on the eastern border of the Massif Armorican, an old Hercynian massif widely flattened since the Lower Jurassic [START_REF] Bessin | Planation surfaces of the Armorican Massif (western France): Denudation chronology of a Mesozoic land surface twice exhumed in response to relative crustal movements between Iberia and Eurasia[END_REF]. At this time, two main domains can be recognized in this area: the Normandy domain to the north and the Le Mans domain to the south, separated by an hydrodynamic barrier formed by the positive epeirogenic tendency of the Perche promontory [START_REF] Dugué | Géodynamique d'une bordure de massifs anciens. La bordure occidentale du bassin anglo-parisien au Callovo-Oxfordien[END_REF][START_REF] Dugué | Comportement d'une bordure de massif ancien et cortèges de minéraux argileux : l'exemple de la bordure occidentale du Bassin Anglo-Parisien au Callovo-Oxfordien[END_REF]Mégnien et al., 1980b). The locality of Écommoy is part of the natural region called Bélinois within the Le Mans domain.

The palaeoenvironment of the Bélinois is usually interpreted by comparison between the local "Marnes et Calcaires de la Vacherie" Formation and two better known formations, namely the "Marnes de Villers" Formation in the Normandy domain and the "Marnes à Pernes" Formation around the Perche promontory. They are dated to the Mariae zone (Lower Oxfordian) that corresponds to a major flooding event and then to a highstand system tract (HST) [START_REF] Haq | Chronology of fluctuating sea levels since the Triassic[END_REF].

Their classical interpretation is a relatively deep marine muddy platform (outer shelf), subject to some terrigenous influence with remains of terrestrial animals and wood washed into this setting (Mégnien et al., 1980a;[START_REF] Dugué | Géodynamique d'une bordure de massifs anciens. La bordure occidentale du bassin anglo-parisien au Callovo-Oxfordien[END_REF]).

An observation on the sediment in thin section of the specimen described here provides further data concerning the depositional environment. This sediment is a limestone with bimodal distribution between fine (less than 200 µm) particles that are dominant and biocorroded bioclasts including pelloids, and coarser particles that represent either fragmentary bivalves or in situ bivalves and gastropods with very thin shells. This lithology is classified as a packstone inside witch small pieces (<300 µm) of organic matter are common. The sediment is extensively bioturbated. The matrix and allochems in the packstone are suggestive of two different environmental conditions in which the overall texture of the packstone suggests relatively high energy conditions, whereas the biocorrosion, bioturbation and in situ thin shells suggest lower energy conditions.

Overall, the sediment suggests a marine depositional environment like a large embayment episodically supplied by bioclasts and terrestrial organic matter during storm and associated flood events (Guillier 1886, p. 178). The fossil plant described here is certainly a terrestrial component washed into the marine depositional setting. Judging by the completeness of the preserved cycad crown, the specimen has not been subjective to significant taphononic fragmentation due to transportation (e.g. [START_REF] Seilacher | Sedimentological, ecological and temporal patterns of fossil Lagerstätten[END_REF].

Systematic palaeontology

Order CYCADALES [START_REF] Persoon | Synopsis plantarum, seu enchiridium botanicum, complectens enumerationem systematicam specierum hucusque cognitarum[END_REF] Family UNCERTAIN Genus Becklesia [START_REF] Seward | Catalogue of the Mesozoic plants in the department of Geology, British Museum (Natural History). The Wealden flora. Part II. Gymnospermae. London, 259 p. plant preserved in an earliest Permian air fall tuff in Inner Mongolia, China[END_REF] emend. [START_REF] Watson | Cycadales of the English Wealden[END_REF]. Etymology. The species epithet is derived from the surname of Louis Maulny, naturalist and first owner of the holotype.

Becklesia maulnyi

Diagnosis.

Leaves petiolate and pinnate with thin and widely separated leaflets. At least some leaves bifurcating near the base of the lamina, separated in two equal ramifications each similar to a simple leaf. Leaflets simple, linear-lanceolate, weakly decurrent, with rounded apex and inserted at an acute angle. Presence of two parallel longitudinal grooves separated by a wide and flat central band occurring both in the rachis and leaflet on the abaxial surface.

Comment. The present species conforms well to the morphological features of Becklesia but lacks cuticular preservation that is inherent to the diagnosis of the genus. Rather than create a new genus for similar fossils in which cuticle is absent owing to the quality of preservation, we opt to establish a new species of Becklesia. This is based on its overall similarity with existing species that show they are closely related, and in particular the longitudinal grooves on the rachis and abaxial surface of the leaves are characteristic of the genus Becklesia. Consistent differences in the morphology of the leaves and leaflets and the absence of cuticular features in the present species do not allow us to place it within an existing species of the genus.

[Insert figures 3 & 4]

Description. The leaves are regularly disposed in a circle around a central depression positioned relative to the apex of the stem (Fig. 3A). Leaves are petiolate, pinnate and they sometimes fork near the insertion point on the stem that divides the leaf into two ramifications (Figs. 3B-C, 4A). Individual leaves / ramifications are up to 125 mm long but are each incomplete such that their entire length would have been longer. Where leaves fork the two ramifications have approximately the same shape and each has the same structure and organisation than the single (non-ramified) leaves. The angle between individual leaves or between two ramifications on a single leaf is approximately 45° except one that is 90° (Fig. 3A) from which the presence of simple leaves may in some cases be due to incomplete preservation.

The length of the petiole is less than 28 mm; at its top some small bumps replace the first leaflets (Fig. 3D). These bumps resemble spines on petioles of extant armed cycads. The rachis and the petiole are flat and their width, about 2 mm (1.7-2.1 mm), is more-or-less constant over their entire length. The imprint of the abaxial face shows in sub-marginal position two linear bulges that are Ω-shaped in transverse section (Figs. 3D, 4B-D). These bulges are about 0.2 mm wide and are separated by a wide and flat band of about 1.2 mm. The bulges follow the lateral margin in both the rachis and leaflets, and thus bypass the flat band of each leaflet. The bulges correspond favourably with the mould of stomatal grooves that have been filled with sediment and the flat area between them to a non-stomatal band in fossil cycads.

The leaflets are simple, linear-lanceolate with an entire margin, rounded apex and are at an angle of about 60-65° to the rachis typically with an opposite to sub-opposite arrangement but sometimes irregular (Figs. 3A, E-F). Leaflet basiscopic margin is decurrent and acroscopic margin is lightly expended (Fig. 3F). Their length is up to 28 mm and their width is constant, at about 2 mm (1.7-2.1 mm). Individual leaflets are separated from each other by 2-4 mm, rarely more. The basal leaflets are shorter than the middle leaflets and the length of the upper leaflets decrease (but leaflet apex is rarely preserved). The imprint of the abaxial face shows two parallel and longitudinal Ωsharped bulges in sub-marginal position that correspond to the position of stomatal grooves.

Stomatal grooves are about 0.3 mm wide (Figs. 3F, 4C-D) and are also separated by a flat band of about 0.5 mm width which corresponds to the non-stomatal area. At the apex of a leaflet, the two grooves converge quickly and merge. Features of the cuticle are unknown.

Comparison with others species. The genus Becklesia is primarily based on the two parallel and longitudinal stomatal grooves on the abaxial face of leaflets and features of the cuticle.

The cuticle is not preserved in our sample due to preservational limitations, but this kind of longitudinal groove is a unique characteristic of cycad-like foliage [START_REF] Watson | Cycadales of the English Wealden[END_REF]. This rare and unmistakable trait argues in favour of this generic attribution, as does the relative temporal (Oxfordian / Berriasian) and palaeogeographical proximity to other localities in which Becklesia occurs in SE England and NW Libya.

The specimen described here differs in some ways from others species of Becklesia. The dimensions of the leaves, rachis and leaflets of Becklesia anomala are larger than those of our sample. Likewise, the leaflet apices of B. anomala and B. sulcata are acute and the two stomatal grooves are non-convergent, contrary to our specimen where they are respectively round and convergent. Finally, leaflets do not bifurcate in the French specimen unlike B. anomala [START_REF] Watson | Cycadales of the English Wealden[END_REF], the only other species in which macro-remains are known for comparison. Moreover, the forking leaves seem to be a specific feature of the French specimen but that represents a preservational limitation in the other species in which the crown is unknown. Although the French specimen lacks cuticle, the stomatal grooves demonstrate their presence and in this regard it is also similar to those of Becklesia.

Three French cycad species from the Upper Kimmeridgian of the Jura mountains are morphologically similar to Becklesia but are currently placed within different genera. We reevaluate these species in light of the findings of [START_REF] Watson | Cycadales of the English Wealden[END_REF] and the present investigation on the organisation and distribution of Becklesia. However, we will not describe these species again as their previous accounts are as detailed as the materials permit.

Cycadites lorteti as described by [START_REF] Saporta | Paléontologie française ou description des fossiles de France. 2 e série -Végétaux[END_REF] and emended by Barale (1981, pp. 69-71) has two characteristic longitudinal grooves on the rachis corresponding to the stomataliferous grooves in Becklesia. C. lorteti differs from B. maulnyi sp. nov. in having larger leaflets (2.5-4 x 30-115 mm), its leaflet apices being acute rather than rounded, and the narrower width of its rachis (9 mm near the basal part). Historically, all Mesozoic leaves which had a superficial resemblance with those of the modern genus Cycas have been placed within the genus Cycadites where features that would allow a more accurate generic determination to be made are unavailable. Stomatal grooves allow us to place this species within Becklesia for which they are distinctive features. We therefore transfer C. lorteti into Becklesia lorteti comb. nov. Paracycas sp. A as described by Barale (1981, pp. 65-68) possesses two longitudinal grooves in the same position as observed in Becklesia. This species possesses a cuticle that comprises haplocheilic stomata born in two parallel rows separated by a non-stomatal band. However, P. sp. A differs from B. maulnyi in having a greater spacing between leaflets (10-15 mm) and having a less acute insertion angle on leaflets (about 90°), but the width of rachis and dimensions of the leaflets dimensions are similar in the two species. Paracycas was established by [START_REF] Harris | The Yorkshire Jurassic flora. II. Caytoniales, Cycadales & Pteridosperms[END_REF] for fossils with cycads-like cuticular features and only one median vein. Harris' diagnosis of Paracycas lacks mention of stomatal furrows. The features of P. sp. A of [START_REF] Barale | La paléoflore jurassique du Jura français : Étude systématique, aspects stratigraphiques et paléoécologiques[END_REF] are consistent with those of Becklesia into which the species should be transferred as Becklesia sp.

A.

Paracycas sp. B of [START_REF] Barale | La paléoflore jurassique du Jura français : Étude systématique, aspects stratigraphiques et paléoécologiques[END_REF] lacks cuticle but differs from B. maulnyi sp. nov. mainly in the width of the rachis (4-7 mm) and leaflets (3-4 mm at their base), but this species is based on too fragmentary remains which precludes more detailed comparisons. Considering its occurrence with other species of Becklesia rather than Paracycas, we conclude that this species more likely belongs within the genus Becklesia to which it should be transferred as Becklesia sp. B.

Growth habit and environmental implications of Becklesia maulnyi sp. nov.

Becklesia maulnyi sp. nov. probably had a short subterranean trunk and an herbaceous habit if we consider the short diameter of the central hole and the low number of leaves in the studied sample: this shape of trunk exists in 6 of 11 genera of modern cycads [START_REF] Whitelock | The Cycads[END_REF]. However, we cannot exclude a palm-like habit, especially if the specimen was ontogenetically young. A few cycad species with subterranean trunks stock water in their roots which tend to be succulent and tuberous [START_REF] Whitelock | The Cycads[END_REF] Longitudinal stomatal grooves specific to Becklesia are a xeromorphic feature confirmed by the localisation of trichome cells on the border of the grooves in B. anomala [START_REF] Watson | Cycadales of the English Wealden[END_REF]. However, the protection of stomata does not exclusively occur in dry climates, but equally in environments with high water availability: a drop of liquid water can sometimes occlude the stomatal aperture and so inhibit the respiratory function if the stoma is non-protected [START_REF] Haworth | Hot, dry, wet, cold or toxic? Revisiting the ecological significance of leaf and cuticular micromorphology[END_REF]. However, in coastal environments sea spray affects plants in their water regulation in a similar way to drought, so similar adaptations are used in both cases to defend against water loss. A structure close to longitudinal stomatal grooves of Becklesia is known from an extant member of the Poaceae, Ammophila arenaria, that lives in coastal settings [START_REF] Konlechner | The potential for marine dispersal of Ammophila arenaria (Marram grass) rhizome in New Zealand[END_REF]. In Ammophila leaves roll on themselves to form a stomatal cavity that is elongated along the longitudinal axis of the leaf. Many digitations of the stomatal epidermis increase also the respiratory surface. Even if this last structure is more elaborate than the longitudinal stomatal grooves of Becklesia, the principle is the same: an elongated cavity, open only through a slit, protects the stomatal area to external environment and increase resistance against desiccation.

Desiccation can be due to sea spray at least in the case of this beach grass, but probably also in Becklesia too.

The leaf morphology of B. maulnyi sp. nov. with its widely spaced leaflets reflects a double adaptation: wind resistance of the plant is improved by increasing the spacing of the leaflets, and the consecutive reduction of the leaf surface attenuates also its evapotranspiration, especially in a context of high light intensity. In contrast, that excludes any environment where interspecific competition for light is strong, so its environment is probably a relatively open one. Moreover, the presence of two ramifications halves the length of the leaves at a constant photosynthetic surface and thus, protects the plant against wind damage. As a consequence, the living area of this plant was probably an open environment with strong wind and light, coherent with an herbaceous habit and adaptations against desiccation.

From our interpretations we consider likely that Becklesia maulnyi sp. nov. lived in close proximity to the sea in a coastal setting and that it lived in an open, windy and sunny growth environment. Coastal dune ridges satisfy these conditions and represent a likely growth environment. In coastal dune ridges the sandy soil is unstable and regularly mobilized and reworked by wind. Moreover, some dune plants use this for their vegetative reproduction and dispersal strategy [START_REF] Konlechner | The potential for marine dispersal of Ammophila arenaria (Marram grass) rhizome in New Zealand[END_REF]. In the Jurassic, floras characteristic of dune ridges are totally unknown because dune ridges do not offer good conditions for plant fossilization; they comprise freely draining sandy substrates that allow plant materials to oxidize and decay rather than be preserved. Moreover, current dune ridge floras cannot be used as a model on which to base Jurassic plant distributions as present day dune floras mainly comprise flowering plants that did not exist during the Jurassic. In the absence of angiosperms, this environment was probably colonized by other groups of plants potentially including cycads.

On the other hand, the strong environmental stress of extant dune plants must be close to those of Jurassic ones as the same physical factors are unchanged through time. Plants growing in dune ridges must deal with environmental stresses from salt spray, sand burial, swash inundation, dryness, high light intensity, wind exposure, soil salinity and nutrient deficiency [START_REF] Hesp | Ecological processes and plant adaptations on coastal dunes[END_REF]. The comparison between our sample features, actual and fossil cycad characteristics, and response to environmental stress of modern dune plants can help to sustain an origin of B. maulnyi sp. nov. in a dune ecosystem. Among the factors described above, B. maulnyi sp. nov. shows evidence of adaptation to dryness, high light intensity and wind exposure in the unique form of its leaves and stomatal grooves. Furthermore, protection against salt spray and dryness is reinforced by the comparison between the rolled leaves of the beach grass Ammophila arenaria [START_REF] Konlechner | The potential for marine dispersal of Ammophila arenaria (Marram grass) rhizome in New Zealand[END_REF] and the longitudinal stomatal grooves of Becklesia. Protection against sea spray and soil salinity can be equally inferred by comparison with Becklesia anomala that had potential salt glands on both sides of its leaflets [START_REF] Watson | Cycadales of the English Wealden[END_REF]. While the majority of extant cycads are not tolerant to salt, some living species occur in coastal environments including species of Cycas, Dioon, Encephalartos, Macrozamia and Zamia where they are able to deal with the physiological consequences of soil salinity and sea spray. For example, extant Zamia roezlii occurs in coastal marshes occasionally inundated at high tide with seawater, differing significantly from most other cycads that generally prefer well drained habitats [START_REF] Whitelock | The Cycads[END_REF].

More generally, cycads as a group have a number of strengths to successfully establish themselves in severely stressed ecosystems such as dune ridges: their subterranean rooting system is often tuberous and succulent [START_REF] Whitelock | The Cycads[END_REF] and the medullar parenchyma of the stem is very developed at least in all arborescent cycads. These adaptations confer the ability to stock water and thus to protect the plant from periods of water stress. Moreover, many extant cycads are nitrogen fixers via a symbiosis with cyanobacteria (Nostoc sp. and others) through specialized coralloid roots.

Coralloid roots allow cycads to live in an almost sterile soil, like those of mineral rich coastal dunes where organic matter is limited. Their nitrogen autotrophy is especially important because this element is the main limiting factor for plant growth in dune ecosystems [START_REF] Hesp | Ecological processes and plant adaptations on coastal dunes[END_REF]. Otherwise, some living cycads with a subterranean trunk have roots with contractile capabilities able to pull the stem into the ground [START_REF] Whitelock | The Cycads[END_REF]; maybe some fossil cycads with a comparable capacity can withstand episodic sand burial in dune ecosystems.

However, no potential adaptation is recognized in B. maulnyi sp. nov. or in other extant or fossil cycads to deal with swash inundation, but we consider that B. maulnyi sp. nov. has potential adaptations to each other major stress factor of the dune ecosystems cited by [START_REF] Hesp | Ecological processes and plant adaptations on coastal dunes[END_REF], either directly observed or as inferred above. The hypothesis that B. maulnyi sp. nov. inhabited a coastal dune environment is impossible to demonstrate directly because this plant has been transported into an adjacent marine depositional setting, potentially following swash inundation. Nevertheless, the conjunction of many adaptations coherent to a growth in a coastal dune ecosystem and the transport of a near whole plant with attached leaves into a relatively deep submarine mudflat can be easily explained only if the fossil plant came from a coastal environment under strong erosion at the scale of its lifetime. A coastal dune ecosystem corresponds well to this interpretation.

Taphonomic and palaeobiogeographic considerations

The taphonomic process at the origin of this fossil is probably original, perhaps unique, because it allowed the preservation of a leafy crown in an unambiguous marine environment, arranged to strongly suggest a life position. That involves transportation of at least the upper part of the trunk, and maybe even the whole plant from its growth environment, or perhaps the flooding of a formerly terrestrial environment, or rapid burial of the trunk up to the level of the lamina. If only the upper part of the trunk was transported, this means that it was previously detached from the rest of the plant, or the entire plant could have been uprooted. Detaching the crown of a cycad or uprooting the entire plants could conceivably be achieved by wind, flooding or animal action. As the leaves remain attached to the plant and are relatively complete with little damage, transportation distances and energy levels were unlikely to have been high for which more damage and fragmentation would be expected.

As well as Becklesia maulnyi sp. nov. other examples or entire cycad crowns are known from the fossil record including Leptocycas gracilis from the Triassic of North America [START_REF] Delevoryas | A new Triassic cycad and its phyletic implications[END_REF], Holozamites hongtaoi (X. [START_REF] Wang | The discovery of whole-plant fossil cycad from the Upper Triassic in western Liaoning and its significance[END_REF] and Leptocycas yangcaogouensis [START_REF] Zhang | A new species of Leptocycas (Zamiaceae) from the Upper Triassic sediments of Liaoning Province, China[END_REF] from the Upper Triassic of China. These accounts demonstrate consistent crown morphology with modern cycads and B. maulnyi sp. nov., show conservatism in form from fossil and living cycads, and emphasize that entire cycads crowns are increasingly common in the fossil record. As far as we are aware, taphonomic studies have not been conducted on these other entire cycads. Such kinds of fossils, for example entire Palaeozoic aged Noeggerathialean ferns, are typically considered to represent preservation of in-situ plants preserved in their growth environments under obrution (rapid burial) deposits such as volcanic ash fall horizons (e.g. J. [START_REF] Wang | The discovery of whole-plant fossil cycad from the Upper Triassic in western Liaoning and its significance[END_REF]. B. maulnyi sp. nov. is different as it is clearly preserved in a marginal marine setting as evidence by the lithological features of the host rock, and is not in-situ in a terrestrial context. individual specimens in each of the English, Libyan or French Jura and Écommoy assemblages in which they occur. However, the spatio-temporal distribution of this genus shows an interesting pattern: all known Jurassic species come from France, but the Cretaceous species seem more dispersed with southern English and northern Libyan occurrences (Fig. 5). However, the very sparse data prevent further analysis of the origin and dispersion of Becklesia.

  Scarburgense sub-zone to the beginning of the Praecordatum sub-zone. In addition, a Cardioceratidae sampled by one of us (P. C.) a few years ago clearly indicate the Praecordatum sub-zone.

  sp. nov. Material and holotype. The holotype constitutes the only available material and corresponds to specimen MHNLM 2003.1.4962 housed in the Musée Vert of Le Mans, Sarthe, France. Age and occurrence. Lower Oxfordian, Marnes et Calcaires Silteux de La Vacherie, Écommoy (Sarthe, NW France).

[

  Insert figure 5 -Position of Mesozoic outcrops with Becklesia remains.] Despite being established more than a hundred years ago, species of Becklesia are very rare among Mesozoic floras, and where they are present they are represented by one to a few

Fig. 1 .Fig. 2 .

 12 Fig. 1. First Label of Becklesia maulnyi sp. nov.

Fig. 3 .

 3 Fig. 3. General and detailed views of Becklesia maulnyi sp. nov. A. General view of B. maulnyi sp.

Fig. 4 .

 4 Fig. 4. Hypothetical reconstruction of some part of Becklesia maulnyi sp. nov. A. Reconstruction of

Fig. 5 .

 5 Fig. 5. Position of Mesozoic outcrops with Becklesia remains.

Figure 1 Figure 2 Figure 3 .

 123 Figure 1

  , but this shape exist both in drought-tolerant (Ceratozamia microstrobila, C. zaragozae, Encephalartos caffer, E. cerinus, E. cupidus, E. humilis,

	Macrozamia glaucophylla, Stangeria eriopus) and moisture-tolerant cycads (Bowenia spectabilis,
	Ceratozamia hildae, C. kuesteriana, C. miqueliana, C. withlockiana, Encephalartos villosus, Zamia
	splendens, Z. variegata) and thus cannot be considered exclusively as a xeromorphic feature.

Acknowledgements

The authors thank Patrice Raboeuf, technician at Musée Vert of Le Mans, to have found in the collections the ammonite which has probably strengthened the dating, and Xavier Le Coz, technician in Géosciences Rennes for making the thin section. We thank J. Dejax and an anonymous reviewer for the constructive remarks of an earlier version of the manuscript.

Figure 5


S Delagrange

Y Schuurman
email: schuurman@catalyse.cnrs.fr

HCN synthesis from methane and ammonia over platinum

Keywords: Temporal analysis of products, CN coupling, Reaction mechanism, Ammonia dehydrogenation, Methane decomposition

Introduction

Two different catalytic processes are presently used for the production of HCN, an important intermediate for nylon and clear acrylic ester polymers [START_REF] Satterfield | Heterogeneous Catalysis in Practice[END_REF]. One route is the Andrussow process that consists of reacting ammonia, methane and oxygen over Pt/Rh gauzes at temperatures as high as 1373 K at atmospheric pressures with contact times of a few milliseconds. The addition of sub-stoichiometric amounts of oxygen leads to a net exothermic reaction. The second route is by feeding a mixture of methane and ammonia through an array of ceramic tubes coated internally with a platinum-based catalyst layer (Degussa process or BMA process). The tubes are heated externally to 1473-1573 K by directly burning natural gas. The reaction is highly endothermic and most of the reaction occurs in a short section of the tube under mass-transfer limitations [START_REF] Satterfield | Heterogeneous Catalysis in Practice[END_REF]. The advantages of the Degussa process are the high HCN yields and the useful hydrogen waste gas, but it requires the high investment and maintenance costs. Moreover the exothermicity of the Andrussow process allows shorter contact time than that of the Degussa process. It reduces the extent of unwanted side reaction such NH 3 and CH 4 decomposition into N 2 and coke, respectively. Reaction kinetic studies in the laboratory under these conditions represent a formidable challenge and most mechanistic insights stem from surface science studies under low-pressure conditions [START_REF] Hasenberg | [END_REF][3][4]. Despite the pressure-gap these models predict the conversion and selectivity of commercial reactors quite well [5]. However, detailed mechanistic information is still lacking.

Especially, no consensus has been reached on the species involved in the surface formation of the HCN precursor from CH 4 and NH 3 . The data of Schmidt and co-workers [3,4] do not allow discriminating between two possible pathways to HCN, namely: (1) formation of CN surface species through the surface coupling reaction of C and N adspecies followed by hydrogenation to HCN and (2) the surface reaction between CH x and NH x species to form adsorbed CNH z species that are then dehydrogenated to form HCN. Diefenbach et al. performed Pt + mediated coupling experiments of methane and ammonia and they proposed the second route to occur over platinum [6]. Horn et al. have performed in situ studies of the Pt-catalyzed formation of HCN at 1573 K by means of beam mass spectrometry and they proposed the second route to describe the homogeneous contribution to the reaction mechanism [7]. Recently, Herceg and Trenary by using TPD experiments and spectroscopic studies argue in favor of the first route [8].

Temporal analysis of product (TAP) experiments are carried out by pulsing small amounts (on the order of a few nanomols) of reactants over a packed bed microreactor containing the catalyst sample [9]. The microreactor is maintained under vacuum conditions. The effluents are analyzed by a quadrupole mass spectrometer located just underneath the reactor outlet. A specially designed vacuum chamber assures efficient pumping speed thus avoiding backscattering of the molecules to the mass spectrometer. At the low pressures applied in the TAP reactor the gas transport takes place by Knudsen diffusion. A purely diffusion process implies the absence of all external mass transfer limitations as well as radial concentration gradients. Even for very exothermal reactions the adiabatic temperature rise is very small due to the small quantity of reactants introduced in the reactor. This allows a good isothermal catalyst operation. TAP experiments have for these reasons been applied to study fast reactions over real catalyst samples [10][11][12][13][14] giving mechanistic information not easily accessible by other methods.

Experimental

TAP experiments

Typically experimental formats are used in the TAP study. Multipulse experiments consist of introducing a train of pulses of one of the reactants only. These experiments will alter the surface state of the catalysts and typically will reveal information on the capacity of the catalyst to store certain species. The other experimental format is referred to as a pumpprobe experiment where both reactants are admitted from two separate pulse valves. The time interval between their introductions can be varied and these experiments allow identifying the reactive surface species as well as the individual reactions step that occur. Pump-probe experiments are carried out in a cyclic fashion and thus species left on the surface during the probe pulse can interact with the reactants of the next pump pulse. In this study we have used a modified pump-probe experiment were three species were pulsed consecutively (NH 3 , CH 4 and H 2 in different orders).

For the experiments the high-temperature TAP reactor [12] was slightly modified. It contains a quartz tube that is 40 mm long and with an inner diameter of 3 mm kept between two stainless steel flanges with water-cooled O-ring seals. Either 10 mg or 20 mg of platinum black (Good Fellow) was positioned in between two layers of quartz in the isothermal zone of the high-temperature TAP reactor. Platinum black is believed to be representative of the wall-coated platinum used in the industrial process. Moreover, as it is used in powder form, its quantity can be easily varied and it allows a regular bed packing. Methane and ammonia were diluted with inert gases (Ar or Xe) and pulsed independently by two high-speed pulse valves. Hydrogen was pulsed without dilution from a third independent pulse valve. Before the pulse experiments the catalyst was out gassed under vacuum for several hours at 1173 8C. The experiments were performed between 1073 and 1173 8C.

The number of molecules that were admitted per pulse varied between 1 Â 10 15 and 5 Â 10 15 . At least four sequences per atomic mass unit (AMU) value were monitored and averaged to improve the signal to noise ratio.

Results

Single pulse experiments

The experiments were carried out between 1073 K and 1173 K. Below 1123 K no formation of HCN has been observed. We report here the experiments performed at 1173 K. Negligible conversion of ammonia and methane takes place over quartz at 1173 K. Fig. 1 shows the transient responses of methane and ammonia over quartz at 1173 K. However, the ammonia pulse showed significant broadening compared to argon due to the strong reversible adsorption on the quartz particles. Single pulses of ammonia over platinum black resulted in the formation of hydrogen and nitrogen. A complete ammonia conversion was observed but both the nitrogen and hydrogen mass balances were deficient, indicating strong ammonia adsorption. Single methane pulses yielded hydrogen with a methane conversion of 0.7 over 10 mg of catalyst and a closed hydrogen mass balance.

Pump-probe experiments

The interaction between ammonia and methane was further investigated by pulsing both reactants at various time intervals consecutively. The ratio of the two reactants has also been investigated. The use of excess methane leads to a slow deactivation (on the order of several hundreds of pulses) of the platinum black catalyst. Apparently carbon species accumulated slowly on the surface thus reducing the number of active sites. This situation was characterized by an incomplete ammonia conversion. Only at high carbon coverages a very small pulse response at AMU 25 was observed on the methane pulse, possibly indicating the formation of traces of C-C coupling products. This was not the case under regular reaction conditions reported here. Fig. 2 shows the transient responses of methane, hydrogen, nitrogen and hydrogen cyanide on the introduction of a pulse of ammonia, followed 0.8 s later by a pulse of methane.

A slow formation of hydrogen is observed on the ammonia pulse followed by a much faster production on the methane pulse. A slow formation of nitrogen is seen on the introduction of ammonia that drops down after the introduction of methane. A small slow formation of HCN occurs on the ammonia pulse that sharply increases on the methane introduction and then shows a long tailing. In Fig. 3 are shown the transient responses of methane and the products when the two reactants are inverted. In this case the methane conversion is slightly lower (50% versus 67% for the former experiment) possibly due to the large amount of platinum sites occupied by adsorbed ammonia derived species. Similar (inversed) hydrogen responses are observed as in Fig. 2. A small amount of nitrogen is observed on the methane pulse followed by a more important production on the ammonia. The amount of nitrogen formed in this experiment is much smaller than that in the experiment shown in Fig. 2. The HCN shows a sharp transient response on the methane followed by a much broader response on the ammonia. In both experiments the ammonia conversion was complete. The formation of HCN on the first pulse in both experiments stems from respectively carbon species and nitrogen containing species that were formed on the second pulse. Thus, both carbon and nitrogen containing species have a long surface lifetime (at least 10 s) even at 1173 K. In fact the amount of hydrogen and hydrogen cyanide produced on the ammonia pulse in Fig. 3 represents approximately 70% of the hydrogen balance, assuming complete ammonia conversion. Thus, the nitrogen containing species are probably present in the form of adsorbed NH x species. By varying the time interval between the ammonia and methane (Fig. 4) or between methane and ammonia (Fig. 5) the influence of the surface lifetime of the various species on the reaction products has been further studied. In both series of experiments the quantity of the second reactant pulsed increases slightly with increasing time interval. The pulse at a time spacing of 1.6 s presents 25% more than that at 0.2 s. In Fig. 4 it is shown that by increasing the time between the ammonia and methane injection the yield of HCN formed on the methane decreases whereas the N 2 formed on the ammonia pulse increases. Apparently the formation of HCN and N 2 are two coupled parallel routes. The amount of HCN on the NH 3 pulse increases with increasing time interval due to the decreasing yields of HCN, thus more carbon species are left on the surface after the methane pulse. Both the methane and ammonia conversion are constant as well as the amount of hydrogen. The data presented in Fig. 5 show that in case of the methane/ammonia pulse sequence the yields of HCN, N 2 and hydrogen are approximately constant for all time intervals. Moreover the conversions of methane and ammonia do not change either.

The influence of hydrogen on the HCN production was studied by adding a pulse of hydrogen to the ammonia/methane pump-probe experiment either right after the ammonia pulse or after the methane pulse. Fig. 6 shows the transient responses of hydrogen, nitrogen and hydrogen cyanide for these two experiments. Introducing H 2 after the ammonia pulse results in a decrease of the nitrogen production rate and hardly increases the HCN level. Addition of hydrogen after the methane pulse leads to an increase in the HCN production and to a smaller extend an increase in N 2 .

General discussion

Few studies in the literature [6] report on the reaction mechanism of HCN synthesis over platinum based on elementary steps. However, ammonia activation and decomposition over platinum has been studied experimentally [15] as well theoretically [16,17] and reaction schemes have been proposed. The study of Weinberg et al. showed that NH 3 decomposition takes place through consecutive dehydrogenation steps. Exchange with deuterium yields NH 2 D through ND 3 , thus indicating that all NH x species exists on the surface. Theoretical studies confirm this [16,17]. A pulse of NH 3 over bare platinum thus yields hydrogen and nitrogen through a series of consecutive dehydrogenation steps followed by the recombination of adsorbed N adatoms to form N 2 of H atoms to form H 2 . At 1173 K this is a slow process as can be seen in Fig. 2. Both the hydrogen and nitrogen transient responses are very broad and they resemble one another. From the TAP experiments it is estimated that the surface lifetime of the NH x species are at least on the order of 10 s.

Methane decomposition over platinum at 1173 K on the other hand is much faster. Pulses of methane yield hydrogen responses that have similar full width at half height as a direct pulse of hydrogen. Methane decomposition is reported to yield CH adspecies on platinum at 370 K that can be hydrogenated back by D 2 to CHD 3 and CD 4 [18]. Above 500 K C-species are formed as well as CCH species at high carbon coverages. Above 650 K the latter one is further dehydrogenated. The following reaction sequence can be proposed for the introduction of NH 3 and CH 4 :

NH 3 þÃ @ NH 3 Ã (1)
NH 3 ÃþÃ@ NH 2 ÃþHÃ (2)
NH 2 ÃþÃ@ NH ÃþHÃ

NH ÃþÃ @ N ÃþHÃ (4)

CH 4 þ 5 Ã @ CH Ãþ4HÃ (5) 2N Ã @ N 2 þ 2Ã (6) 2H Ã @ H 2 þ 2Ã (7)
Step (1) through (4) are reversible, as the deuterium exchange studies of Vajo et al. [15] have indicated, in agreement with the decrease in the nitrogen production when an ammonia pulse is followed by the introduction of hydrogen in the TAP reactor. Moreover, theoretical studies show that the activations energies for the dehydrogenation step are similar for reactions (2), ( 3) and (4). Van Santen and co-workers and Ricart and co-workers calculated the activation energies of the successive ammonia dehydrogenation steps to be 92, 110, 118 kJ/mol [16] and 100, 105, 92 kJ/mol [15], respectively.

Methane decomposition (step ( 5)) into carbon and hydrogen has been simplified, as this proceeds in several steps involving CH x species. However, as discussed above the most stable species at these temperatures are the carbon, C*, species.

The hydrogen cyanide production during pump-probe experiments shows two distinct rates: A slow rate of production is observed on the introduction of the ammonia that continues well beyond the time of the TAP experiment. The second production rate is much faster and occurs both on the introduction of methane as well as hydrogen. This is nicely illustrated by the HCN response curve in Fig. 3 with Dt = 1.6 s. On the ammonia pulse, there is still a lot of carbon species on the surface, as the HCN production was low on the previous CH 4 pulse. As a consequence a large HCN production is observed on the ammonia pulse. Before the introduction of the methane pulse the HCN production rate has dropped to almost zero due to the depletion of surface carbon species. On the methane pulse the HCN production increases sharply due to fast decomposition of methane that delivers both surface carbon and surface hydrogen species. However, this enhanced HCN production rate decreases rapidly after the introduction of methane. This is due to a limited number of active sites that is available for the methane decomposition [19]. Thus, the hydrogen cyanide production rate seems to correlate with the availability of hydrogen as hydrogen is slowly released from adsorbed ammonia species but rapidly from methane. Another feature of the HCN production is the strong coupling with the nitrogen formation, evidenced by the drop in the nitrogen response upon the introduction of methane (Fig. 2).

Although the production of HCN may proceed through two (or more) routes the difference in hydrogen supply is compatible with one single route. However, a pool of intermediate species must be present on the surface that can be hydrogenated rapidly to explain the formation of HCN by pulsing hydrogen. This pool of intermediates are either adsorbed carbon species that form CH* species and then react with adsorbed nitrogen atoms (or NH x * species) according to the following reactions:

C ÃþH Ã @ CHÃ (8) N ÃþCH Ã!HCN þ 2Ã (9)
or adsorbed CN* species issued from the surface reaction between N* (or NH x *) and C*:

N ÃþC Ã @ CN ÃþÃ (10) CN ÃþH Ã!HCN þ 2Ã (11)
van Hardeveld et al. [20] reached a similar conclusion for the HCN formation over Rh proposing the hydrogenation of CN* species (step (11)) as well as the reaction step (9).

Notice that reaction steps ( 9) and ( 10) compete with reaction step (6) for N* adatoms, thus providing a coupling between the production rates of N 2 and HCN. Apparently reactions steps (9) or (10) are must faster than step (6) in the presence of surface carbon yielding high selectivities to HCN.

A third possible reaction path consist of the reaction between NH x and CH x species leading to a surface reaction intermediate that yields HCN after dehydrogenation. This reaction path cannot be discarded but the dehydrogenation is not rate limited, as a similar HCN response would be expected on the introduction of NH 3 and CH 4 . For the same reason the desorption step for HCN is not the rate-limiting step and thus left out in the above reaction sequence.

The study was carried out at 1173 K, whereas the temperature of the Degussa process is between 1473 and 1573 K. Nevertheless, the mechanism found at 1173 K could be representative of the mechanism of the real process. The experiments show good HCN selectivities (compare the N 2 and HCN yields in Fig. 4) that are comparable to those of the industrial process. Moreover due to the strong endothermicity of the reaction the catalyst surface under industrial conditions will be at a much lower temperature. However, the conclusion on the rate-determining step of the ammonia decomposition depends strongly on the temperature.

Conclusions

TAP experiments have been conducted for the synthesis of HCN from ammonia and methane over platinum black. The decomposition of ammonia into nitrogen and hydrogen is a relatively slow, kinetically reversible, step at 1173 K. Methane decomposition on the other hand is very fast. Consequently, two distinct HCN production rates are observed. HCN is formed slowly on the ammonia pulse but much faster on the methane pulse. In addition HCN can be produced by injecting hydrogen after the NH 3 -CH 4 pump-probe experiment. The most probable reaction path for HCN synthesis is through a hydrogenation step of either C* or CN* surface species. HCN is a stable product and its desorption is fast.

Excess methane leads to a slow catalyst deactivation, probably by a slow build-up of inactive carbon.

Fig. 1 .

 1 Fig. 1. Transient responses of Ar (AMU 40), CH 4 (AMU 15), NH 3 (AMU 17) at 1173 K during single pulsing of CH 4 and NH 3 over quartz. Pulses size of NH 3 and CH 4 $10 15 (NH 3 /CH 4 = 1.1).

Fig. 2 .

 2 Fig. 2. Transient responses of CH 4 (AMU 15), H 2 (AMU 2), N 2 (AMU 28) and HCN (AMU 27) at 1173 K during sequential pulsing of NH 3 and CH 4 with a pulse delay of 0.8 s and a pulse spacing of 5 s. Pulses size of NH 3 and CH 4 $10 15 (NH 3 /CH 4 = 1.1).

Fig. 3 .Fig. 4 .

 34 Fig. 3. Transient responses of CH 4 (AMU 15), H 2 (AMU 2), N 2 (AMU 28) and HCN (AMU 27) at 1173 K during sequential pulsing of CH 4 and NH 3 with a pulse delay of 0.8 s and a pulse spacing of 5 s. Pulses size of NH 3 and CH 4 $10 15 (NH 3 /CH 4 = 1.4).

Fig. 5 .

 5 Fig. 5. Influence of the delay between CH 4 and NH 3 pulses on transient responses of CH 4 (AMU 15), H 2 (AMU 2), N 2 (AMU 28) and HCN (AMU 27) at 1173 K for a pulse spacing of 5 s. Pulses size of NH 3 and CH 4 $10 15 (NH 3 /CH 4 = 1.4).

Fig. 6 .

 6 Fig. 6. Transient responses of H 2 (AMU 2), N 2 (AMU 28) and HCN (AMU 27) at 1173 K during sequential pulsing of NH 3 and CH 4 with a pulse delay of 1.6 s and a pulse spacing of 10 s by adding a hydrogen pulse either after NH 3 pulse (a) or after CH 4 pulse (b).

  The following AMUs were monitored to follow all reactants and products: H 2 2, N 2 CH 4 HCN 14, NH 3 CH 4 15, NH 3 CH 4 16, C 2 H 4 25, HCN C 2 H 4 26, HCN C 2 H 4 27, N 2 C 2 H 4 28, C 2 N 2 52. The fragmentation patterns were established by pulsing separately all reactants and products at low temperatures except those of HCN. HCN was quantified by assuming that it has the same mass spectrometer response coefficient as N 2 .


Gabriel Aullón

Kerman Gómez

Gabriel González

Susanna Jansat

Manuel Martínez

Rinaldo Poli

Mireia Rodríguez-Zubiri

Kinetico-Mechanistic Information about Alkene Hydroamination with Aniline in Bromide-rich Ionic Media: Importance of Solvolysis

The study of the [PtBr4] 2-reactivity with hexene and aniline in highly ionic (Bu4P)Br/CH2Br2 media has been studied from a kinetico-mechanistic perspective. The results indicate bromide ion association to the square-planar starting material to produce a stable diamagnetic compound that can be described as an ion-pair of a [PtBr5] 3-square pyramidal complex stabilized by several phosphonium counter-cations. While this species reacts rapidly with aniline, producing the known square-planar complex [PtBr3(PhNH2)] -with release of the apical bromide of the square pyramidal intermediate, the reaction with hexene producing the square-planar [PtBr3(hexene)] -complex is much slower. The thermal and pressure activation parameters determined for these processes fully agree with the proposed reactivity. The gross features of the platinum-catalyzed hydroamination mechanism, occurring via much higherenergy transition states, are not necessarily altered by these new findings, given the fact that all ligand exchange reactions occur with relative low activation barriers. Nevertheless, the nature of the catalyst resting state needs revision as demonstrated. The importance of explicitly considering the solvent for reactions conducted in non-innocent highly organized media, is also highlighted.

INTRODUCTION

Hydroamination, one of the available processes to form C-N bonds, consists in the direct addition of an N-H bond across an unsaturated C-C bond and represents an atom economical approach for the synthesis of amines from ammonia or lower amines. Since amines are a very important class of chemicals in today's World market, the hydroamination reaction generates a lot of interest for academic and industrial researchers. [START_REF] Brunet | Catalytic Heterofunctionalization[END_REF][START_REF] Müller | [END_REF] The reaction is particularly challenging for alkenes, less reactive than other unsaturated substrates, such as alkynes, dienes and allenes, particularly when they are not activated, and for the intermolecular version.

Catalytic approaches have been considered for the latter on non-activated olefins and a moderate success has been achieved using pre-catalysts based on lanthanides, 3,4 Zr, 5 Fe, 6 Ru, 7,8 Rh, 9,10 Ir, 10 Pd, 11,12 Pt, [13][14][15][16][17][18][19] Ag 20 and Au. [21][22][23] Among other metals, for Pt-catalyzed systems two basic types of catalyst dependent mechanisms have been indicated, one where the metal centre first activates the olefin, followed by nucleophilic attack by the amine, the second one involving amine activation and subsequent olefin insertion into the metal-N bond. 24,25 Recent work carried out in one of our groups has addressed the mechanism for the system introduced by Brunet, which is legitimately the simplest platinum-based catalyst since it is composed by ordinary commercially available PtBr2 activated in the presence of an excess of (Bu4P)Br and functions in the absence of additional expensive ligands. It is also one of the most practical catalytic systems because it is stable and can operate in the presence of air and water, and provides the highest activities among all Pt-based catalysts reported to date. Thus, the improvement of the catalytic efficiency will only be attained by a comprehensive knowledge of the mechanism and the effect of the non innocent media used (0.13 M (Bu4P)Br). In particular, synthetic work using ethylene as representative olefin and lowtemperature equilibrium studies, backed up by theoretical calculations, have assessed the relative stability in solution (mostly using dichloromethane, but also aniline and DMF) of a variety of simple Pt II complexes containing all ligands that are present under catalytic conditions (ethylene, aniline, bromide ions). 24,[26][27][28] Subsequent computational investigations on the high-energy portion of the catalytic cycle, which is experimentally inaccessible to stoichiometric studies, have confirmed the olefin-activation mechanism as the operating cycle for the Pt-catalyzed process and unveiled intimate details of each elementary step of the catalytic cycle. 28,29 Nevertheless, in these studies the involvement of the highly ionic media, found definitively relevant empirically, has not been fully addressed given the obvious limitations in the calculation procedures.

Kinetico-mechanistic studies on hydroamination processes have been carried out in the presence of [PtL2(OTf)2] as catalyst (L2 = bipy, diimine, diphosphine) 19 and the preparation of several possible low-energy intermediates in the proposed catalytic cycle has been attained for the ligand-free PtBr2/(Bu4P)Br catalyst. 30 For the latter catalytic system, despite the obvious stoichiometric implications of the non-innocent medium (0.13 M (Bu4P)Br) that has been established as ideal from a catalyses perspective, 13 its importance in the kinetics and mechanism of the cycle has not been explored. Given our interest in the effect of the medium on the kinetics and mechanisms of many well established reaction processes, [31][32][33][34][35][36][37][38] and our experience in kinetico-mechanistic studies on organometallic complexes, 33,[38][39][40][41][42] we have decided to explore the importance of such a high ionic medium in the PtBr2/(Bu4P)Brcatalyzed hydroamination process through a kinetico-mechanistic study.

In this contribution we present the unexpected solution chemistry of the [PtBr4] 2-species in highly ionic (Bu4P)Br media (from 0.016 to 2 M in CH2Br2) and its further reaction with aniline and/or hexene. The results indicate the association of bromide ions to the square-planar starting material to produce a stable diamagnetic compound that can be described as an ionpair of a [PtBr5] 3-square pyramidal complex. This species, which is associated with the nature of the starting Pt II centre under catalytic conditions (0.13-1.0 M (Bu4P)Br in aniline), reacts readily with aniline in a series of consecutive equilibria on very different time scales via the stepwise substitution of the bromido ligands by PhNH2, as seen by the inverse reaction rate dependence on (Bu4P)Br concentration. Addition of hexene, at very high concentrations, to preformed [PtBr5] 3-solutions in CH2Br2 do not produce any measurable reaction under time and temperature conditions equivalent to those used in the reaction with aniline. This fact, in clear contrast with that observed for simple [PtBr4] 2-plus hexene systems, suggest that the sequence indicated in the proposed catalytic cycle for the hydroamination of ethylene needs revision when hexene is considered as the substrate in a bromide-rich environment. The only reactivity with hexene was observed on equilibrated, aniline free, [PtBr4] 2-/Br -mixtures at temperatures much higher than those utilized for its reaction with aniline. The results suggest that the catalytic cycle should include the [PtBr5] 3-resting state that undergoes a bromide by aniline substitution to generate either an off-loop species from the catalytic cycle or a slow subsequent reaction with hexene, thus slightly modifying the sequence generally accepted.

RESULTS AND DISCUSSION

Reactivity of [PtBr4] 2-in (Bu4P)Br-rich media Given the fact that the PtBr2-catalyzed hydroamination of alkenes occurs with much better results in the presence of (Bu4P)Br, a preliminary study about the solution chemistry and stability of the presumed (Bu4P) 2[PtBr4] precatalyst in CH2Br2/(Bu4P)Br medium was pursued. The choice of CH2Br2 as accompanying solvent rather than CH2Cl2 was based on its high boiling point (97 ºC), which allows the work at high temperature under conditions rather similar to those encountered in the catalytic processes and in the absence of undesirable halide exchange processes. The high solubility of (Bu4P)Br in this solvent (ca. 3 M) is also convenient for the purposes of this study.

Preliminary studies on CH2Br2 solutions of (Bu4P)2[PtBr4], prepared as indicated in the literature, 43 showed important colour changes with time that are associated with the loss of bromido ligands and subsequent polymerization, as described in the literature. [43][44][45] These processes should be easily suppressed by the addition of an excess of (Bu4P)Br. In this respect, 195 Pt NMR experiments were conducted on chloroform solutions of (Bu4P)2[PtBr4] containing an excess of (Bu4P)Br in order to establish which reactivity was taking place under these conditions. Figure 1 collects the time evolution of a 195 Pt NMR spectrum of a 1M (Bu4P)Br CDCl3 solution of (Bu4P)2[PtBr4] (0.10 M) at room temperature. It is evident that at time zero only the broad signal corresponding to [PtBr4] 2-at -2500 ppm is relevant, while its intensity decreases with time in favour of a final sharp single signal at -2000 ppm, not related to any known Pt II /Br or Pt IV /Br complexes. 43,44 Monitoring by 1 H or 31 P NMR spectroscopy of similar experiments indicated that no reaction involving the Bu4P + cation occurs. Furthermore, the observed important UV-Vis spectral changes in CH2Br2 (see below) are also evident both in CHCl3 and when (NEt4)Br solutions of the same concentrations are used, indicating that the bromide ion is responsible for the observed changes. When the experiments were conducted using acetonitrile instead of dibromomethane no significant changes were observed by UV-Vis spectroscopy. It seems clear that a low polarity solvent is needed for the reaction with high bromide concentrations to occur; alternatively the reason could also be associated to the coordinating characteristics of CH3CN, which should allow its competition with Br -. A similar behaviour both in UV-Vis spectra and 195 Pt NMR (Δδ ≈ 1000 ppm downfield) has been observed in the formation of crystallographically characterized pentacoordinated adducts of organometallic pincer complexes with SO2. 46,47 UV-Vis monitoring of the reaction progress, using pseudo-first order conditions, produced very well behaved time resolved spectra (Figure 2a) that could be perfectly fitted to by a single exponential from which kobs values could be derived. These were found independent on the platinum concentration but showed a limiting dependence with [(Bu4P)Br] and with a non-zero intercept. The fact that the spectral changes increase with increasing the value of [(Bu4P)Br] suggests the establishment of an equilibrium (k+/k-= Keq) with a preformed outer-sphere species (KOS), i.e. kobs=k-+{k+KOS[(Bu4P)Br]/(1+KOS[(Bu4P)Br])} [START_REF] Espenson | Chemical Kinetics and Reaction Mechanisms[END_REF][START_REF] Tobe | Inorganic Reaction Mechanisms[END_REF] . Figure 2b collects the observed trend at different temperatures. Derivation of k+, k-and KOS at different temperatures produced the thermal activation parameters collected in Table 1 by the use of Eyring plots. The dependence of lnkobs with pressure at 1.0 M (Bu4P)Br (where kobs ≈ k+) was taken as a measure of the activation volume for the forward process and the value is also collected in Table 1.

a) b)

k+ /s -1 ΔH+ ╪ /kJ mol -1 ΔS+ ╪ /J K -1 mol -1 328 ΔV+ ╪ /cm 3 mol -1 328 KOS /M -1 6.4×10 -4 76±7 -22±22 -20±1 3.0 328 k-/s -1 ΔH-╪ /kJ mol -1 ΔS-╪ /J K -1 mol -1 1.0×10 -4 117±10 30±30
All the above indications are consistent with the reaction being the addition of a bromide ligand to the system, as depicted in Equation 1. The negative activation entropy and particularly the negative activation volume are strong indicators of the associative character of the forward process. The relatively small value determined for ΔS+ ╪ agrees with the already important ordering of the ionic atmosphere prior to bromide ion addition. Similarly, the reverse reaction shows a set of values for ΔH ╪ -and ΔS ╪ -that are in line with a dissociatively activated process. As for the values of the equilibrium constant, defined as k+/k-, they show a small increase at low temperatures (from 3 (79 ºC) to 15 (40 ºC)), which indicates a higher presence of the [PtBr5] 3-at the lower temperatures in the study.

The presence of a definite value of Keq is surprising given the full disappearance of the 195 Pt NMR signal of the starting [PtBr4] 2-material (Figure 1) in CDCl3. This seems to point to the need of a very low polarity medium to stabilize the new species, which again is in line with the lack of reactivity observed in CH3CN indicated above (empirical solvent polarity scale being 0.26 for CHCl3, 0.27 for CH2Br2 and 0.46 for CH3CN). [START_REF] Reichardt | [END_REF] Finally considering that the outersphere complex {[PtBr4] 2-;Br -} occurs between two negatively charged species, the rather high value determined for the outer-sphere complex formation equilibrium constant, KOS, is a clear indication of the complicated nature of the "strictly" non-bonding interactions. This type of pentacoordinate species has already been fully established in solution, apart from the SO2 adduct of the above mentioned Pt II -pincer complex, 46,47 by EXAFS measurements and theoretical calculations on aqueous acidic solutions of [Pt(OH2)4] 2+ . 51,52 The existence of [Pd(OH2)5] 2+ and hydrated forms of [PtCl4] 2-and [PdCl4] 2-(showing a geometry that can be better described as an octahedral diamagnetic complex with a very strong Jahn-Teller effect), have also been indicated. 53,54 Even the formation of square pyramidal [Ni(CN)5] 3-in solution has been reported, as well as other Ni II five coordinated complexes. 55,56 An analysis of the Cambridge database indicates a definite preferential formation of square pyramidal pentacoordinated Pt II complexes for non-tripoidal ligands. Given the filled dz2 orbital on the Pt II centre, their apical bond lengths are definitively longer than their basal counterparts, from all these available data, the apical Pt-Br bond length can be reasonably estimated around the 2.90 Å value. This rather long distance can also be related with the relatively small absolute value of the activation entropy for the association process. In this respect, the reverse reaction should be dissociatively activated as expected for the microreversibility principle. Effectively, as indicated before the value for ΔS-╪ is more positive although small, as corresponding to the dissociation of an already elongated Pt-Br bond. [START_REF] Wilkins | Kinetics and Mechanisms of Reactions of Transition Metal Complexes[END_REF] These facts are a good example of the need of considering the effects of non-innocent Lewis base media in reactions occurring at low solute concentrations (i.e. very large solvent:solute ratios) that allow the existence of otherwise undefined solvento species. 33,[START_REF] Roiban | Unpublished results[END_REF] Given the non-feasibility of the isolation, workup and further characterization of the species formed in the above mentioned reaction conditions, DFT calculations were tried for the putative [PtBr5] 3-anion and their ion-paired species. Although in other systems these have led to reliable results, [START_REF] Illner | [END_REF] for our system no optimization of the desired species could be been achieved. 60

Reactivity of [PtBr4] 2-with aniline in (Bu4P)Br media Given the reactivity observed for

[PtBr4] 2-in concentrated CH2Br2 solutions of (Bu4P)Br, the reactivity of these equilibrated reaction mixtures with PhNH2 was further pursued. This is especially interesting after our recently appeared 30 report about all the species present on equilibration of [PtBr4] 2-/PhNH2/C2H4 systems in CH2Cl2 solution at room temperature. From the data it is clear that a rather complex series of equilibria exists, even in the absence of important relative concentrations of (Bu4P)Br. Nevertheless, in the reported cases the nature of the starting material was neatly set to (Bu4P)2[PtBr4], which is not the case in the present report that deals with a set of reaction conditions much closer to those needed for the improved catalytic cycles. 14,24 Solutions of (Bu4P)2[PtBr4] (ca. 4×10 -4 M) and (Bu4P)Br (1.0 M) in CH2Br2 were heated at 50 ºC until equilibration (2-3 h, k+/k-= 7, see above). Aliquots of these solutions were mixed with different aniline solutions of different concentrations and at the same ionic Absorbance /a.u.

 /nm strength in CH2Br2; the final platinum concentration was usually in the (1.0-2.5)×10 -4 M range.

As expected, time resolved UV-Vis monitoring of the reaction showed a series of processes, the relative importance of which was highly dependent on the temperature, time and aniline concentration. Nevertheless, very long reaction times or high temperatures were avoided in order to overcome the problems caused by side reactions of the aniline. In all cases a blank experiment in the absence of added platinum was conducted to establish the reliability limit of the experiments.

The first monitored process is very fast and can be observed within the stopped-flow equipment limits, despite the high viscosity and density of the solutions used. The process is very clean and a good retention of isosbestic points is observed (Figure 3a). Furthermore, this step is an equilibrium reaction, as detected by the important intercepts of the plots of the pseudo-first order observed rate constants (kobs1) versus [aniline] (Figure 3b); Figure 3c Step 293 k 293 Keq /M -1 ΔH ╪ /kJ mol -1 ΔS ╪ /J K -1 mol -1 ΔV ╪ /cm 3 mol -1 on first 0.16 M -1 s -1 0.5

71±13 -20±44 ~0 off first 0.31 s -1 91±5 53±15 19±2 on second 4.9×10 -3 M -1 s -1 10 94±4 30±14 off second 4.3×10 -4 s -1 52±10 -134±35 on third 5.4×10 -4 M -1 s -1 15 94±16 13±52 off third 3.9×10 -5 s -1 105±19 29±65
As seen in Table 2 all the processes measured show a definite equilibrium nature that must be very little displaced at the high bromide concentrations existing at the catalytic conditions of the target hydroamination process. All of them are, consequently, related to sequential bromide by aniline substitution. Some experiments were also carried out in order to establish the importance of bromide concentration for the first relevant process, kobs1. As seen in Figure 4a an important increase of the reaction rate occurs on decreasing the amount of bromide ions in solution, which is indicative the need of a bromido dissociation for the formation of the final complex with the amine ligand in the Pt II coordination sphere. Given the high complexity of the sequential reactivity observed, no further studies were conducted at other [(Bu4P)Br]:[C6H5NH2] ratios. The same type of trend has also been observed in other systems, [61][62][63] where an equilibrium leading to a lower coordination number species is responsible of the final substitution reaction observed. Summarizing, the process agrees with the set of reactions indicated in Equation 2 and 3 which produces the rate law indicated in Equation 4, once the steady state approximation has been applied to the [PtBr4(PhNH2)] 4a needs the k-1[Br -] term to be relevant not allowing for the first approximation. Further, the fact that the plot in Figure 4a does not reach an asymptotic k-2 limit also indicates that the second approximation cannot be applied for the [Br -] used in the study. The alternative approach, involving an initial bromide decoordination followed by a simple aniline by bromide substitution, can be disregarded in view of the very fast processes (within stopped-flow mixing time) observed for the [PtBr4] 2-plus PhNH2 reaction. This fact should produce an effective substitution reaction rate as that measured for the back reaction indicated in the previous section, which is orders of magnitude slower that the observed.

As a consequence, the values of the apparent thermal activation parameters derived from the measurements have to be taken very carefully and cannot be considered as a reliable tool for the assignation of the intimate mechanism operating. The apparent values for the on process indicate either an overall concerted mechanism for the bromido by aniline substitution or a dissociation of an already elongated Pt-Br bond (as indicated in the previous section). Clearly the square pyramid structure of the putative [PtBr5] 3-species formed on dissolution of the [PtBr4] 2-in concentrated (Bu4P)Br solutions is responsible for this behaviour as found for other

Ni II complexes. 56 For the reverse reaction, off, an overall dissociative exit of the aniline ligand seems to be operating, which agrees with the second possibility indicated. The complex nature of the rate constants determined (Equation 4) makes any further discussion useless. That is, according to Equations 2 and 3, k1 seems to be dominant for the on process while k-1 for the off reactions, in agreement with the microreversibility principle. Interestingly the values of Evidently, the sequential second and third processes observed must have a different character if they are deriving from square-planar [PtBr3(PhNH2)] -and [PtBr2(PhNH2)2] complexes.

Given the high complexity involved, the nature of the second and third processes observed was not further explored. Nevertheless, the formation of cis-[PtBr2(PhNH2)2], as the more readily formed bis-aniline compound 28,30 in a second step from [PtBr3(PhNH2)] -or [PtBr4(PhNH2)] 2-, can be easily understood according to the set of reactions in Equation 2-3. Its transformation to the more stable trans-isomer, as the process that originates the third observed step, seems a reasonable speculation. The [PhNH2]-dependence of kobs3 agrees with this assumption taking into account the intermolecular character normally actuating for this type of isomerization processes (Figures S1 andS2, Supporting information). 66

Reactivity of [PtBr4] 2-with hexene in (Bu4P)Br media

The reactivity observed for (Bu4P)2[PtBr4] with PhNH2 in a (Bu4P)Br/CH2Br2 medium, indicated in the previous section, as well as the reported partial ethylene dissociation from [PtBr3(C2H4)] -in a (Bu4P)Br/CH2Cl2 medium, 30 prompted us to parallel the above described study with a (Bu4P)2[PtBr4]/(Bu4P)Br/alkene system in CH2Br2. The chosen alkene was 1-hexene (hexene from here on) given its relatively high boiling point that enables its easy and safe manipulation, as well as the reliability of the desired concentration levels during prolonged periods. This study is relevant given the preferential stability observed for [PtBr3(C2H4)] -, with respect to [PtBr3(PhNH2)] -, when cis-or trans-[PtBr2(C2H4)(PhNH2)] complexes are dissolved in (Bu4P)Br/CH2Cl2 solutions. The improved catalyzed hydroamination reaction of 1-hexene with aniline in a (Bu4P)Br-rich environment has also been reported within the 0.10-0.23 M range for [(Bu4P)Br]/[hexene], 14 which is included in the 0.11-0.53 margin used in this study (Table S1).

In order to establish whether CH2Br2 solutions of the starting [PrBr4] 2-complex react with hexene, preliminary UV-Vis monitoring of solutions of (Bu4P)2[PtBr4] in dibromomethane with varying amounts of hexene was conducted in the absence of added (Bu4P)Br, in order to suppress the formation of the putative [PtBr5] 3-. Under these conditions reactivity is effectively observed (Figure S3) leading to the probable formation of [PtBr3(hexene)] -, by analogy with complex [PtBr3(C2H4)] -that has been previously prepared under similar conditions. 30 Nevertheless, when the experiments were conducted on the same (Bu4P)[PtBr4]/(Bu4P)Br equilibrated mixtures used for the aniline reactivity studies indicated in the previous section, the process occurred on a much longer time-scale. Solutions of (Bu4P)2[PtBr4] in the (2-5)×10 -4 M range in (Bu4P)Br/CH2Br2 (0.02-2.0 M (Bu4P)Br concentration margin) treated with varying amounts (0.1-0.9 M) of hexene only showed a significant reaction when very long periods or high temperatures were used. As a consequence, the (Bu4P)Br concentration was set to 0.08 M in order to facilitate the monitoring. Figure 5 collects some relevant information about these experiments. From the time resolved recorded spectra it is clear that under these conditions a neat process occurs. The observed rate constants show an important degree of equilibrium, as indicated by the intercepts in the plot of kobs versus [hexene] (Figure 5b).The temperature and pressure dependence of the slopes and intercepts derived from these plots produce the thermal and pressure activation parameters collected in Table 3 using the standard Eyring and lnkon and lnkoff versus P plots. The effect of the (Bu4P)Br concentration on these reaction rates was also studied in view of the results found for the (Bu4P)2[PtBr4]/(Bu4P)Br/aniline system. Figure 4b shows the observed changes for the kobs value at 55 ºC when CH2Br2 2.5×10 -4 M solutions of putative [PtBr5] 3-were reacted with hexene (0.5 M) in the 0.05-2.0M (Bu4P)Br concentration. Absorbance /a.u.

 /nm Table 3.-Kinetic, equilibrium and thermal activation parameters for the reaction between putative [PtBr5] 3-and hexene in 0.1 M (Bu4P)Br CH2Br2 solution.

Step 328 k average Keq /M -1 ΔH ╪ /kJ mol -1 ΔS ╪ /J K -1 mol -1 ΔV ╪ /cm 3 mol -1 on 2.4×10 -4 M -1 s -1 13 Nevertheless, at 36 ºC the values of k+3 and k-3 were determined as 2.0×10 -3 M -1 s -1 and 2.1×10 -3 s -1 respectively (Figure S3b), which should produce a value for koff ≈(1-2)×10 -4 s -1

70±2 -105±6 -11±2 off 1.6×10 -5 s -1 65±5 -145±15 -28±3 a
under the conditions of the study at 35 ºC. The fact that the experimental value determined is two orders of magnitude smaller (3.1×10 -6 s -1 , Figure 5b) indicates that the alternative mechanism does not effectively fit the data obtained. That is, the mechanism and rate law shown in Equations 2-4 with hexene as entering ligand instead of aniline (k1' , k-1' , k2' and k-2' ) applies. In this respect it is important to note that at high [Br -] values there seems to be a limiting value for kobs at 55 ºC of ca. 1×10 -4 s -1 (Figure 4b), the same within error than k-for the [PtBr4] 2-/Br -system. Even though Equation 7 can explain this fact via a compensation of the k+[Br -] and k+k-3[Br -] 2 terms, we are tempted to believe that at high Br -concentrations the coordination of hexene to the system is totally suppressed and that only the re-equilibration reaction to the new lower [Br -] (from 1.0 to 0.08 M, see before) is being observed.

Similarly to the aniline substitution process, the complex nature (k1' , k-1' , k2' and k-2') of the values obtained for kon and koff do not allow for a reliable discussion about the intimate reaction mechanism, given the fact that the activation parameters are only apparent values. These values, collected in Table 3, are fairly negative for both ΔS ╪ and ΔV ╪ , which can be associated with an associative character of the overall substitution process. This is accompanied with relatively small values for the activation enthalpies. The values are in clear contrast with the corresponding data in Table 2 for the reaction with aniline where a concerted or dissociative character can be interpreted from the activation parameters. It is evident that the mechanism operating for the alkene coordination to the platinum centre is not the same as that occurring for the aniline. While for the aniline entry the k1/k-1 reaction path fits the activation parameters determined, in this case the values for ΔH ╪ , ΔS ╪ and ΔV ╪ are all indicative of a process with a very important associative character much more in line with what would be expected for the k2' /k-2' reaction path.

Reactivity of [PtBr4] 2-in (Bu4P

)Br/aniline/hexene media In order to determine the reactivity of the platinum species simultaneously with both relevant ligands, a set of experiments was also conducted, on preformed [PtBr5] 3-, with hexene and aniline at varying concentrations, temperatures and times. At short reaction times a fast reaction occurs which follows the same kinetic pattern than that observed solely with PhNH2. When the processes were monitored by UV-Vis for longer times, some further changes were observed that were fully equivalent to those obtained in blank experiments without hexene added to the reaction medium, thus indicating that a mere aniline side decomposition process was being observed. These results

indicate that hexene has a lower coordinating ability than aniline for Pt II under this specific coordination environment (bromide-rich solution in a polar solvent), whereas it was previously concluded that ethylene binds to [PtBr4] 2-more favourably than aniline in non-ionic medium. 30 The simplest explanation consists on considering the equilibrium indicated in Equation 8 very displaced to the left for ethylene and to the right for hexene. Indeed, a recent computational study confirms that 1-hexene coordination provides lower stabilization relative to ethylene. 29 Given the fact that steric hindrance of the alkene cannot be held responsible for the difference

in such an open environment, the origin should be electronic. Hexene is a better σ-donor but a poorer π-acceptor than ethylene, thus a worse ligand if back-bonding is dominant for the stabilization of the Pt-alkene bound in the complex. Nevertheless, the reaction media for the two studies are by no means equivalent. The important excess of bromide ions in the reaction medium enables the actuation of the equilibrium reactions indicated by Equation 2 and 3 (k1, k-1, k2, k-2) as well as the equivalent for hexene (k1' , k-1' , k2' , k-2' ) and thus generates a much more complex system.

Summarising, the present study is fully relevant to shed light to the mechanism of olefin hydroamination in highly ionic medium in terms of the identification of the dormant species of the catalytic cycle. Although it has been established as [PtBr3(C2H4)] -(for the hydroamination of ethylene in fully organic solvents), 28,30 the results indicated that the kinetically preferred species is an aniline complex without coordinated olefin, such as [PtBr3(PhNH2)] -or possibly [PtBr2(PhNH2)2], when dealing with the hydroamination of 1-hexene in (Bu4P)Br/CH2Br2 medium. Nevertheless, the considerations about the intimate hydroamination mechanism, occurring via much higher-energy transition states, according to the recent computational study, 28 do not have to be altered so far given the fact that all ligand exchange reactions occur with relative low activation barriers. The UV-Vis spectra and the kinetic monitoring of the reactions were recorded in a Hewlett-Packard 8453A, a Cary 50 or a J&M TIDAS instrument, depending on the experiments. The pressurising system used for the experiments run at variable pressure has already been described. 67,68 In all cases a pill-box cell has been chosen for the experiments. 69

EXPERIMENTAL

Compounds

(Bu4P)2[PtBr4] was obtained via chlorido by bromido exchange on K2[PtCl4] in aqueous solution, followed by treatment and extraction with a saturated solution of (Bu4P)Br in CH2Cl2

as indicated in the literature. 24 Repetitive washing with water of the organic layer and evaporation to dryness produced the desired compound. The use of commercially available K2[PtBr4] was avoided after an important number of experiments carried out indicated that the purchased compound does not agree with the specifications indicated. 195 Pt NMR resonance of a commercial sample indicated the presence of only minor quantities of the tetrabromido species, while mixed bromido/chlorido compounds were dominant. (Bu4P)Br, C4H9CH=CH2 and CH2Br2 were reagent grade chemicals and have been used without further treatment.

PhNH2 was vacuum distilled and kept in the dark under nitrogen atmosphere before use.

Kinetic measurements

The progress of the different processes studied in dibromomethane solution was followed by UV-Vis spectroscopy in the wavelength range where the solvent does not absorb. All the reactions at atmospheric pressure were monitored on instruments equipped with thermostated multicell transports. The general kinetic technique for this type of processes has been described elsewhere. 40,42,[START_REF] Favier | [END_REF] For the kinetic runs at elevated pressure, a previously described pressurizing system connected to a J&M TIDAS instrument was used. 71 Observed rate constants were derived from absorbance versus time traces at the wavelength were a maximum increase and/or decrease of absorbance was observed by the use of the SPECFIT software. 72 No dependence of

Figure 1 .

 1 Figure 1.-a) Time resolved 195 Pt NMR spectra of a 0.1 M (Bu4P)2[PtBr4] and 1.0 M (Bu4P)Br solution in CDCl3 at room temperature; time between spectra 9 h. b) Time evolution of the signal integrals

  Figure 2.-a) Time resolved UV-Vis spectra of a of (Bu4P)2[PtBr4] (4.5×10 -4 M) and (Bu4P)Br (0.27 M) CH2Br2 solution at 70 ºC (inset corresponds to the absorbance changes at 375 nm, total time 2 h). b) Plot at different temperatures of the dependence of kobs on [(Bu4P)Br] for the reaction observed.

Figure 3 .

 3 Figure 3.-a) Time resolved UV-Vis spectra of the evolution of a CH2Br2 solution of putative [PtBr5] 3- (2×10 -4 M), (Bu4P)Br (1.0 M) and C6H5NH2 (0.6 M) at 20 ºC and 1 atm (inset corresponds to the absorbance changes at 390, total time 20 s). b) Plot at different temperatures of the dependence on

Figure 4 .

 4 Figure 4.-a) Plot of the values of kobs1 versus [(Bu4P)Br] for the reaction of a solution of 2×10 -4 M of putative [PtBr5] 3-and 0.11 M [C6H5NH2] (20 ºC . b) Plot of the values of kobs versus [(Bu4P)Br] for the reaction of a solution of 2×10 -4 M of putative [PtBr5] 3-and 0.50 M [hexene] (45 ºC).

Figure 5 .

 5 Figure 5.-a) Time resolved UV-Vis spectra of the evolution of a CH2Br2 solution of putative [PtBr5] 3- (2.5×10 -4 M), (Bu4P)Br (0.13 M) and hexene (0.08 M) at 55 ºC and 900 atm. (inset corresponds to the absorbance changes at 375 nm, total time 15000 s). b) Plot of the dependence on [hexene] of kobs at different temperatures.

Table 1 .

 1 -Kinetic, equilibrium and thermal and pressure activation parameters for the reaction observed between (Bu4P)2[PtBr4] and (Bu4P)Br in CH2Br2 solution.

	328

Table 2 .

 2 -Kinetic, equilibrium and thermal activation parameters for the reaction between putative [PtBr5] 3-and C6H5NH2 in 1.0 M (Bu4P)Br CH2Br2 solution.

  Instruments 195 Pt NMR spectra were recorded on a Bruker 500 spectrometer in CDCl3 at room temperature at the Institut Català d'Investigació Química; chemical shifts are reported in ppm, referred to H2[PtCl6]. 31 P and 1 H NMR spectra were recorded on a Varian Unity 300 ( 31 P, 121.42 MHz) or on a Mercury 400 ( 1 H, 400 MHz) instruments and referenced to SiMe4 ( 1 H) or H3PO4 ( 31 P) at the Unitat de RMN d'Alt Camp (Universitat de Barcelona) .

Acknowledgements

Financial support from the Ministerio de Educación y Ciencia and the Generalitat de Catalunya, grant numbers CTQ2009-14443-C02-02, CTQ2008-06670-C02-01 and 2009SGR-1459 is acknowledged. We also thank the "Laboratoire European Associé" CTPMM (Chimie Trans-Pyrénéenne: de la Molécule aux Matériaux), co-funded by French CNRS and the Generalitat de Catalunya, for a travel grant to MRZ.

the observed rate constants on the selected wavelength was detected; for some reactions a good retention of isosbestic points was observed. [START_REF] Perkampus | Uv-Vis Spectroscopy and its Applications[END_REF] The supporting information collects the obtained kinetic constants values for all the reactions studied, as a function of aniline, hexene and tetrabutylphosphonium bromide concentrations, pressure and temperature. The errors in all cases were within the 10-15 % margin. All post-run fitting were carried out with the commercial software available and using non-weighted least-square methods. Typical errors were 10-15 % for slopes and intercepts and 15-18 % for asymptotic limits.

Supplementary Material Available

Supplementary material is available via the Internet. This includes: values of kobs determined as a function of the compounds, concentration, temperature and pressure and some relevant figures.

TOC synopsis

The kinetico-mechanistic study of the reactivity of [PtBr4] 2-in highly ionic (Bu4P)Br/CH2Br2 medium, very similar to that used in hydroamination reactions with one of the most active systems currently reported, indicates the formation of a pentacoordinate [PtBr5] 3-unit as a stable species.