Juliet Abadie
email: juliet.abadie@gmail.com

Jean-Luc Dupouey

Aline Salvaudon

Sophie Gachet

Noémie Videau

Catherine Avon

Jérôme Dumont

Thierry Tatoni

Laurent Bergès

Historical ecology of Mediterranean forests: Land use legacies on current understorey plants differ with time since abandonment and former agricultural use

Keywords: agricultural use, ancient forest, ecological preference, forest continuity, historical ecology, life-history traits, mediterranean forest, recent forest, secondary succession

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| INTRODUC TI ON

Land use change across time led to an increase in forests with lower temporal continuity (recent forests) and a decrease in long-established forests (ancient forests). Former agricultural use affects current forest understorey vegetation (Flinn & Vellend, 2005;[START_REF] Hermy | Legacies of the past in the present-day forest biodiversity: A review of past land-use effects on forest plant species composition and diversity[END_REF], even centuries after abandonment [START_REF] Dupouey | Irreversible impact of past land use on forest soils and biodiversity[END_REF][START_REF] Plue | Persistent changes in forest vegetation and seed bank 1,600 years after human occupation[END_REF]. Dispersal and recruitment limitations of forest-dwelling species generally explain the difference in understorey plant composition between ancient and recent forests [START_REF] Baeten | Environmental limitation contributes to the differential colonization capacity of two forest herbs[END_REF]Flinn & Vellend, 2005;[START_REF] Graae | The importance of recruitment limitation in forest plant species colonization: A seed sowing experiment[END_REF][START_REF] Hermy | Legacies of the past in the present-day forest biodiversity: A review of past land-use effects on forest plant species composition and diversity[END_REF]. Plant species occurring more frequently in ancient forests have short-distance dispersal mechanisms (barochorous, autochorous and myrmecochorous species, vegetative reproduction) and no or poor soil seed bank; they are frequently small, herbaceous (geophytes or hemicryptophytes), stress-tolerant and have the same ecological preferences as true forest specialists (shade-tolerant, mesophilous, neutrophilous to moderately acid-tolerant; [START_REF] Bergès | Past landscape explains forest periphery-to-core gradient of understorey plant communities in a reforestation context[END_REF][START_REF] Bossuyt | Seed bank and vegetation composition of forest stands of varying age in central Belgium: Consequences for regeneration of ancient forest vegetation[END_REF][START_REF] Brunet | Lifehistory traits explain rapid colonization of young post-agricultural forests by understory herbs[END_REF][START_REF] Kimberley | Identifying the trait syndromes of conservation indicator species: How distinct are British ancient woodland indicator plants from other woodland species?[END_REF][START_REF] Sciama | Floristic and ecological differences between recent and ancient forests growing on non-acidic soils[END_REF]. Therefore, ancient forests have a high conservation value, as they support species with poor colonisation capacities and no soil seed bank, which are thus more vulnerable to forest habitat loss [START_REF] De Frenne | Interregional variation in the floristic recovery of post-agricultural forests[END_REF]Flinn & Vellend, 2005;[START_REF] Hermy | Legacies of the past in the present-day forest biodiversity: A review of past land-use effects on forest plant species composition and diversity[END_REF].

Compared to temperate forests, few studies have explored the effect of forest temporal continuity on the current understorey vegetation of Mediterranean forests [START_REF] Abadie | Land use legacies on forest understory vegetation and soils in the Mediterranean region: Should we use historical maps or in situ land use remnants?[END_REF][START_REF] Amici | Influence of secondary forest succession on plant diversity patterns in a Mediterranean landscape[END_REF][START_REF] Basnou | Of niche differentiation, dispersal ability and historical legacies: What drives woody community assembly in recent Mediterranean forests?[END_REF][START_REF] Kouba | Effects of previous land-use on plant species composition and diversity in Mediterranean forests[END_REF]. The Mediterranean region is one of the few world biodiversity hotspots [START_REF] Gauquelin | Mediterranean forests, land use and climate change: A social-ecological perspective[END_REF][START_REF] Grove | The nature of Mediterranean Europe: An ecological history[END_REF][START_REF] Médail | Glacial refugia influence plant diversity patterns in the Mediterranean basin[END_REF]. Climate is a main component of the Mediterranean specificity: hot and dry summers with mild and humid winters have led to the development of specific soils subject to erosion and leaching [START_REF] Gauquelin | Mediterranean forests, land use and climate change: A social-ecological perspective[END_REF], and a higher risk of natural fires. The Mediterranean climate also favoured species adapted to a high daily temperature amplitude and long water stress periods. The topography is often rugged and constrains the space available to cultivate land. Farmers adapted to this constraint and used this land with terraced cropland and extensive grazing, developing an agro-sylvo-pastoral system which characterised the traditional landscape of the Mediterranean area [START_REF] Taillefumier | Contemporary land use changes in prealpine Mediterranean mountains: A multivariate GIS-based approach applied to two municipalities in the Southern French Prealps[END_REF]. Studies in the Mediterranean region showed that woody species were poorly influenced by forest continuity [START_REF] Basnou | Of niche differentiation, dispersal ability and historical legacies: What drives woody community assembly in recent Mediterranean forests?[END_REF]. The richness of light-demanding species dropped with increasing successional age of the forest [START_REF] Amici | Influence of secondary forest succession on plant diversity patterns in a Mediterranean landscape[END_REF], while the last centuries intensive and widespread agricultural use may have led to a loss in late-successional species [START_REF] Kouba | Effects of previous land-use on plant species composition and diversity in Mediterranean forests[END_REF]. However, except [START_REF] Abadie | Land use legacies on forest understory vegetation and soils in the Mediterranean region: Should we use historical maps or in situ land use remnants?[END_REF], existing studies on forest temporal continuity only focused on the medium term, i.e.

the last 50 years.

Forest cover has increased extensively in France since the 19th century [START_REF] Cinotti | Evolution des surfaces boisées en France : Proposition de reconstitution depuis le début du XIX ème siècle[END_REF][START_REF] Mather | The course and drivers of the forest transition: The case of France[END_REF], especially in less accessible areas of the Mediterranean region [START_REF] Abadie | Land use legacies on forest understory vegetation and soils in the Mediterranean region: Should we use historical maps or in situ land use remnants?[END_REF][START_REF] Koerner | Evolution des surfaces boisées en France depuis le début du XIXe siècle : Identification et localisation des boisements des territoires agricoles abandonnés[END_REF]. Forests mostly developed on former pastures or arable land [START_REF] Koerner | Evolution des surfaces boisées en France depuis le début du XIXe siècle : Identification et localisation des boisements des territoires agricoles abandonnés[END_REF] but the long-lasting effects of different forms of agriculture on current forest ecosystems have been much less investigated than the simple dichotomy of ancient versus recent forests [START_REF] Hermy | Legacies of the past in the present-day forest biodiversity: A review of past land-use effects on forest plant species composition and diversity[END_REF]. Different forms of agriculture can have distinct long-term effects on soils and environmental conditions and can result in contrasting understorey plant successions in post-agricultural forests [START_REF] Dyer | Land-use legacies in a central Appalachian forest: Differential response of trees and herbs to historic agricultural practices[END_REF][START_REF] Holmes | Assembling the forest herb community after abandonment from agriculture: Long-term successional dynamics differ with land-use history[END_REF][START_REF] Koerner | Influence of past land use on the vegetation and soils of present day forest in the Vosges mountains, France[END_REF].

Soils of former cropland were ploughed, levelled and enriched by nutrient inputs while soils of former pasture were trampled and depleted through mineral matter export [START_REF] Flinn | Environmental causes and consequences of forest clearance and agricultural abandonment in central New York, USA[END_REF][START_REF] Stover | Successional vegetation on abandoned cultivated and pastured land in Tompkins County, New York[END_REF]. Alternatively, former pastures may also have been fertilised with dung, while an overexploitation of crops might have impoverished soils through erosion and depletion [START_REF] Stover | Successional vegetation on abandoned cultivated and pastured land in Tompkins County, New York[END_REF]. In addition, pasturing could allow forest species to survive in refugia such as rock outcrops [START_REF] Holmes | Assembling the forest herb community after abandonment from agriculture: Long-term successional dynamics differ with land-use history[END_REF], as a marginal component of grassland communities, or as belowground organs such as rhizomes [START_REF] Bellemare | Legacies of the agricultural past in the forested present: An assessment of historical land-use effects on rich mesic forests[END_REF][START_REF] Stover | Successional vegetation on abandoned cultivated and pastured land in Tompkins County, New York[END_REF]. Consequently, forests on former cropland are generally more nutrient-rich (nitrogen and phosphorus) and support a higher plant species richness with more nitrophilous species, while forests on former pasture have poorer soils and support more stress-tolerant plant communities [START_REF] Dupouey | Irreversible impact of past land use on forest soils and biodiversity[END_REF][START_REF] Dyer | Land-use legacies in a central Appalachian forest: Differential response of trees and herbs to historic agricultural practices[END_REF][START_REF] Koerner | Influence of past land use on the vegetation and soils of present day forest in the Vosges mountains, France[END_REF]. In the Mediterranean region, forests were also historically grazed; thus, the legacies of former pasture in current forest plant communities might be difficult to detect. In contrast, former cropland was often established on artificial terraces, which thoroughly modified soil structure and nutrient availability [START_REF] Abadie | Land use legacies on forest understory vegetation and soils in the Mediterranean region: Should we use historical maps or in situ land use remnants?[END_REF].

The long-term environmental effects of contrasting agricultural histories on forest understorey plant communities and traits thus remain to be clarified, in particular in Mediterranean regions where crops and pastures were copiously represented [START_REF] Abadie | Land use legacies on forest understory vegetation and soils in the Mediterranean region: Should we use historical maps or in situ land use remnants?[END_REF].

Our main objective was to assess the long-term effects of historical agriculture on current forest understorey vegetation in a Mediterranean context over the past 150 years. We explored the long-lasting effects of two types of former agricultural use (pasture or arable land), at two different dates (1860 and 1958) and of time since agricultural abandonment (recent or very recent forests) on forest understorey vegetation through a plant trait-based approach, using long-established stands without signs of agricultural use (i.e. ancient forests) as a control. First, we hypothesized that the differences in understorey plant communities between formerly cultivated sites and ancient forests would be higher than the difference between formerly pastured sites and ancient forests, because cultivation exhibits higher levels of understorey vegetation and soil disturbances [START_REF] Singleton | Forest herb colonization of postagricultural forests in central New York State, USA[END_REF]. We expected that differences in plant traits between former cropland and pastures persisted due to contrasting historical environmental filtering [START_REF] Holmes | Assembling the forest herb community after abandonment from agriculture: Long-term successional dynamics differ with land-use history[END_REF].

Second, we assumed that the legacies of 1958 land use on current understorey plant communities and traits would be more detectable than the legacies of 1860 land use, under the hypothesis that plant communities in post-agricultural forests gradually converge towards ancient forest communities. Third, consistent with these two hypotheses, we expected that floristic differences between pasturing and cultivation would be more pronounced in recent forests developed after 1958 compared to forests developed between 1860 and 1958 [START_REF] Holmes | Assembling the forest herb community after abandonment from agriculture: Long-term successional dynamics differ with land-use history[END_REF].

| ME THODS

| Study area

The Regional Natural Park of Luberon (PNRL) is a rural area located in the eastern French Mediterranean region (43°39′-44°02′ N, 4°58′-5°55′ E) and covers 195,413 ha. Climate is typically Mediterranean with mild and humid winters and hot and dry summers (annual precipitation 740 mm and temperature 12.7°C according to the 1981-2010 Aurelhy means from Meteo-France).

Lithology is mostly calcareous (95%) and elevation ranges from 55 to 1,210 m. More than half of the area is covered by forests (55%), mostly composed of holm oak (Quercus ilex-22%), downy oak (Q. pubescens-23%) and Aleppo pine (Pinus halepensis-18%) stands. Most of the forests are privately owned (71%) and managed differently according to the owners. Historically, forests were mostly coppiced for charcoal, firewood and tanning (oak bark). In the mid-19th century, the socioeconomic and political context led to land abandonment and a gradual forest recovery on former pastures and arable land [START_REF] Abadie | Land use legacies on forest understory vegetation and soils in the Mediterranean region: Should we use historical maps or in situ land use remnants?[END_REF][START_REF] Chalvet | Paysages et conflits en Provence[END_REF][START_REF] Fourchy | Les lois du 28 juillet 1860 et 8 juin 1864 sur le reboisement et le gazonnement des montagnes[END_REF][START_REF] Gilbert | Elevage, forêt et société. Analyse socio-historique[END_REF].

| Forest continuity and past land uses

The present forest cover was extracted from the National Forest Inventory (http://inven taire -fores tier.ign.fr/spip/spip.php?rubri que53; 1:25,000), digitised from the orthophotographs of 2005 (western part) and 2009 (eastern part). We extracted past land uses from two historical sources:

• 1860: The "État-Major" map (EM map; 1858-1861 in the PNRL;

1:40,000), digitised by [START_REF] Salvaudon | Notice de la carte des forêts anciennes du Parc Naturel Régional du Lubéron (1:40,000[END_REF] following [START_REF] Favre | Digitalisation des cartes anciennes. Manuel pour la vectorisation de l'usage des sols et le géoréférencement des minutes 1: 40,000 de la carte d'Etat-Major[END_REF], with a median position error of 26 m after georeferencing and correction;

• 1958: Historical aerial photographs taken between 1953 and 1958, photo-interpreted for each vegetation plot.

The use of these two sources allowed us to reconstruct land use transitions over the whole period [START_REF] Abadie | Land use legacies on forest understory vegetation and soils in the Mediterranean region: Should we use historical maps or in situ land use remnants?[END_REF].

From these data, we defined the temporal continuity of the current forest patches (Table 1 We processed maps using ArcGIS 10.3 (ESRI France).

We had to consider that no turnover in land cover occurred between the dates of the different maps. However, the EM map was made after a long period of deforestation, meaning that forest patches were probably already long established at that time.

Moreover, the south of France is in a net forest recovery context since the mid-19th century [START_REF] Abadie | Land use legacies on forest understory vegetation and soils in the Mediterranean region: Should we use historical maps or in situ land use remnants?[END_REF]. It is therefore unlikely that significant forest conversion occurred since then.

| Vegetation plots

Vegetation plots were extracted from an existing floristic database, SILENE (http://flore.silene.eu; see Appendix S1 for details). We dropped all plots with not enough location accuracy, sampled before 2000 or on marginal acidic substrates, containing less than ten species, and located at a distance of less than five metres from the nearest road or walk pathway (Appendix S1). We analysed plots situated in forest that was forest, arable or pasture in 1860 or in 1958

(Table 1, Figure 1). We only retained species present in more than 1% of the plots, resulting in a final dataset of 473 species and 1,429 plots. The vegetation nomenclature follows TAXREF v.8 [START_REF] Gargominy | TAXREF v8.0, référentiel taxonomique pour la France : méthodologie, mise en oeuvre et diffusion[END_REF].

| Plant traits and ecological preferences

We collected plant functional traits and ecological preferences from four existing databases: BASECO, in which all data were retrieved in the French Mediterranean area [START_REF] Gachet | BASECO: A floristic and ecological database of Mediterranean French flora[END_REF];

Baseflor, a national database of functional traits and ecological preferences [START_REF] Julve | Baseflor. Index botanique, écologique et chorologique de la flore de France[END_REF]; Pignatti for ecological preferences [START_REF] Guarino | Ellenberg's indicator values for the Flora of Italy -first update: Pteridophyta, Gymnospermae and Monocotyledoneae[END_REF][START_REF] Pignatti | Bioindicazione attraverso le piante vascolari. Valori di indicazione secondo Ellenberg (Zeigerwerte) per le specie della Flora d'Italia[END_REF];

and the French Mediterranean flora [START_REF] Tison | Flore de la France méditerranéenne continentale[END_REF] (Table 2, Appendix S2).

We selected plant functional traits that responded to past land 2). We also used leaf type as a proxy for plant life strategy (Table 2), and life strategy of Grime [START_REF] Hodgson | Allocating C-S-R plant functional types: A soft approach to a hard problem[END_REF].

Ecological preferences were also analysed as a proxy for ecological conditions: we used the indicator values of Ellenberg for light, nitrogen and soil moisture adapted to the Mediterranean species [START_REF] Guarino | Ellenberg's indicator values for the Flora of Italy -first update: Pteridophyta, Gymnospermae and Monocotyledoneae[END_REF][START_REF] Pignatti | Bioindicazione attraverso le piante vascolari. Valori di indicazione secondo Ellenberg (Zeigerwerte) per le specie della Flora d'Italia[END_REF]. We also used the preferential habitat of the species (forest, forest edge, open-habitat) using

Baseflor [START_REF] Julve | Baseflor. Index botanique, écologique et chorologique de la flore de France[END_REF]; Table 2).

| Analysis

We We performed a canonical correspondence analysis (CCA) [START_REF] Legendre | Numerical ecology[END_REF] to test whether the two past land uses (1860 and 1958) and forest temporal continuity (AF, RF, VRF, DEF) had an effect on current plant composition, separately for each of the three variables with the vegan R package (R Core Team, R Foundation for Statistical Computing, Vienna, Austria). We then used a combination of 1860 and 1958 land uses, so that we could jointly assess the three historical variables (variable "land use trajectory", see Table 1). For each case, we assessed the significance of the effect of the explanatory variable with a permutation test and Nitrogen IV From 1 (oligotrophic) to 9 (nitrophilous) "

TA B L E 2 Life-history traits and ecological preferences retrieved from databases and flora represented the CCA factorial map using the ade4 R package [START_REF] Dray | The ade4 package: Implementing the duality diagram for ecologists[END_REF].

We tested the relationship between traits and historical variables using RLQ and fourth-corner analysis [START_REF] Bergès | Response of understorey plant communities and traits to past land use and coniferous plantation[END_REF][START_REF] Dray | Combining the fourth-corner and the RLQ methods for assessing trait responses to environmental variation[END_REF]. We tested the significance of the RLQ with a randtest function and tested the significance of each trait versus each historical variable with a fourth-corner analysis using the function fourthcorner.rlq [START_REF] Dray | Combining the fourth-corner and the RLQ methods for assessing trait responses to environmental variation[END_REF]. We carried out statistical analyses using R software v. 3.2.5 (R Core Team, 2017).

Among the traits tested, we could not use the Grime's life strategy or the vegetative reproduction capacity in the RLQ analysis because of too many missing values.

| RE SULTS

| Species preference

Among the 473 species in the whole dataset, 442 species were an- could not be analysed with logistic regressions (Table 3). For further details, see Appendix S4.

alysed

| Vegetation communities

The three historical variables significantly explained the composition of understorey plant communities (p < 0.001). The effects of 1958 land use and forest continuity were stronger than the effect of 1860 land use (0.13%, 0.18% and 0.11% of constrained inertia, respectively). Plant communities were structured according to increasing forest continuity on the CCA first factorial map (Figure 2a).

Plots in forests that were deforested in 1958 were located at one extremity of the gradient of temporal continuity. On the CCA factorial maps, the vegetation plots were also distinguished according to 1860 land use: the first axis of the CCA separated forest from arable land while pasture was isolated from forest and arable land on the second axis (Figure 2b). The effect of 1958 land use was slightly different: the first axis separated forest from pasture and arable land and the second axis clearly differentiated arable land from forest and pasture (Figure 2c). The combination of forest continuity and past land use showed that plant community trajectories differed according to the type of previous land use, arable land or pasture, both developing on a gradient DEF-VRF-RF-AF (Figure 2d).

Also, the difference between former pasture communities and former crop communities is more pronounced in very recent forests than in recent forests (Figure 2d).

| Plant functional traits and ecological preferences

| Plant functional traits

All three historical variables had an influence on plant traits. The first two axes of the RLQ accounted for 98% of the total inertia (84% and 14%, respectively). The first ordination axis of the RLQ was related Annuals were more frequent in 1860-Ara and less frequent in 1958-For, AF and RF, while perennials were more frequent in 1958-For and VRF (Figure 3).

Species with late end-of-flowering were more frequent in VRF and in former arable land (1860 and 1958) and were less frequent in former forests (1860, 1958 and AF; Figure 3).

Historical variables had an effect on anemochores, endozoochores and epizoochores, but not on barochores and myrmecochores. Anemochores were less frequent in AF, epizoochores were more frequent in 1958-Ara, while endozoochores were more frequent in former forests (1860, 1958 and AF; Figure 3).

| Ecological preferences

Historical variables had strong effects on ecological preferences (Figure 3). Forest species were more frequent only in AF, while forest edge species were more frequent in all trajectories of for- TA B L E 3 (Continued)

| D ISCUSS I ON

Globally, our results underlined that land use history over centuries in the Mediterranean region had legacy effects on understorey plant species communities and traits. We could have expected less strong responses since our study area is densely forested and newly established forests are connected to already existing forests [START_REF] Abadie | Land use legacies on forest understory vegetation and soils in the Mediterranean region: Should we use historical maps or in situ land use remnants?[END_REF]. This may thus imply that landscape configuration is not, in our case, a driver of differentiation of vegetation communities among forests of differing land use history.

| Ancient forest species in the Mediterranean

Fifty-seven species were associated with ancient forests. They were dominantly forest specialists or forest edge species, with low light indicator value, and higher nitrogen and soil moisture indicator values. Among them, phanerophytes, endozoochores and sclerophyllous species with larger leaves and a higher potential maximum height were dominant. In temperate lowlands, phanerophytes and endozoochores are associated with recent forests [START_REF] Bergès | Past landscape explains forest periphery-to-core gradient of understorey plant communities in a reforestation context[END_REF][START_REF] Sciama | Floristic and ecological differences between recent and ancient forests growing on non-acidic soils[END_REF], while ancient forests are characterised by myrmecochorous species [START_REF] Bergès | Past landscape explains forest periphery-to-core gradient of understorey plant communities in a reforestation context[END_REF], which display a shorter dispersal distance than species with other dispersal syndromes [START_REF] Vittoz | Seed dispersal distances: A typology based on dispersal modes and plant traits[END_REF]. In our study, myrmecochorous species were not associated with ancient forests while phanerophytes and endozoochores were clearly associated with ancient forests.

Phanerophytes are much more diversified and thus represented more in Mediterranean forests than in temperate forests [START_REF] Quézel | Que faut-il entendre par "forêts méditerranéennes[END_REF][START_REF] Gauquelin | Mediterranean forests, land use and climate change: A social-ecological perspective[END_REF]. However, we still have to clarify why they characterise ancient Mediterranean forests, but not temperate ones. Myrmecochores are overall under-represented in the Mediterranean forest flora [START_REF] Wolff | Ants as seed dispersers in a Mediterranean old-field succession[END_REF]. We could thus only suppose that they had disappeared from forests with a long-time overexploitation, which were coppiced, grazed and even temporarily cultivated [START_REF] Gilbert | Elevage, forêt et société. Analyse socio-historique[END_REF], or that the Mediterranean context implies other dispersal syndromes than temperate regions. In a region where forests have been subject to fires over a long time span, phanerophytes might also have been favoured over herbaceous species alongside the evolution of forests. Further studies would be necessary to compare trait differences in ancient and recent forests in other biogeographical contexts .

| Contrasted legacies of former crop and pasture on understorey vegetation

Consistently with our hypothesis, our results indicate that former arable and pastoral use displayed different legacies in current understorey plant communities, traits and ecological preferences.

Although communities of former pastures differed from ancient forest communities, the legacies of former arable land were stronger than the legacies of former pastures, as was highlighted in temperate regions by [START_REF] Koerner | Influence of past land use on the vegetation and soils of present day forest in the Vosges mountains, France[END_REF].

In 1860 and 1958 in the PNRL, pastures and forests were principally located on unproductive land [START_REF] Abadie | Land use legacies on forest understory vegetation and soils in the Mediterranean region: Should we use historical maps or in situ land use remnants?[END_REF].

Depending on grazing intensity, pastoral use may have impoverished soils even more [START_REF] Mclauchlan | The nature and longevity of agricultural impacts on soil carbon and nutrients: A review[END_REF]. Forest developed on those soils may thus need a long time to grow and be characterised by smaller trees and many gaps in canopy cover, thus providing a brighter, more xeric environment for understorey plant species. Thus, differences in abiotic conditions between arable land and pastures could explain why plant indicator values for light and for soil moisture were positively and negatively associated with former pastures, respectively.

Conversely, former crops were more often located on the most productive land in 1860 and 1958, i.e., the deepest soils on loose substrates [START_REF] Abadie | Land use legacies on forest understory vegetation and soils in the Mediterranean region: Should we use historical maps or in situ land use remnants?[END_REF]. Nevertheless, agricultural use was first abandoned on the least suitable land for crops, i.e., on steeper slopes and closer to pre-existing forest compared to the land still cultivated [START_REF] Abadie | Land use legacies on forest understory vegetation and soils in the Mediterranean region: Should we use historical maps or in situ land use remnants?[END_REF]. Most of this abandoned arable land might have been cultivated as terraced cropland, and thus provide deeper and more productive soils [START_REF] Arnaez | Effects of farming terraces on hydrological and geomorphological processes[END_REF][START_REF] Stanchi | Properties, best management practices and conservation of terraced soils in Southern Europe (from Mediterranean areas to the Alps): A review[END_REF][START_REF] Tarolli | Terraced landscapes: From an old best practice to a potential hazard for soil degradation due to land abandonment[END_REF].

Nitrophilous and mesophilous species could then find favourable conditions to develop on former arable land. Besides, tree canopy probably closed faster, allowing shade-tolerant and less competitive species to develop with increasing forest continuity.

| Forest temporal continuity: a long-term successional process

Understorey communities in recent forests became more and more similar to those in ancient forest communities with time (Figure 2), which confirms our second hypothesis of a long-term secondary succession in Mediterranean forests following agricultural abandonment. We observed an increasing proportion of forest species (sciaphillous, phanerophytes) and endozoochores and a decreasing proportion of open-habitat species (heliophilous, therophytes) and epizoochores with increasing forest temporal continuity [START_REF] Aubin | Can plantations develop understory biological and physical attributes of naturally regenerated forests?[END_REF][START_REF] Tatoni | Comparison of old-field and forest revegetation dynamics in Provence[END_REF]. The gradient of the indicator value for light according to forest temporal continuity reflected a lighter environment in more recent forests, as observed in temperate regions [START_REF] Bergès | Past landscape explains forest periphery-to-core gradient of understorey plant communities in a reforestation context[END_REF][START_REF] Brudvig | Strong legacy of agricultural land use on soils and understory plant communities in longleaf pine woodlands[END_REF][START_REF] Glitzenstein | Effects of environment and land-use history on upland forests of the Cary-Arboretum, Hudson Valley, New-York[END_REF]. These patterns reflect the development of the canopy cover along with forest succession and the disappearance of openhabitat species with successional age of the forest [START_REF] Finegan | Forest succession[END_REF], as also demonstrated by [START_REF] Amici | Influence of secondary forest succession on plant diversity patterns in a Mediterranean landscape[END_REF] in a Mediterranean context.

Confirming our third hypothesis, the difference in vegetation communities between former arable land and former pastures were more pronounced in very recent forests compared to recent forests. [START_REF] Abadie | Land use legacies on forest understory vegetation and soils in the Mediterranean region: Should we use historical maps or in situ land use remnants?[END_REF], thus reducing the dispersal limitation of species for colonising the newly established forests (Flinn & Vellend, 2005;[START_REF] Graae | Vegetation and soil differences in ancient opposed to new forests[END_REF][START_REF] Hermy | Legacies of the past in the present-day forest biodiversity: A review of past land-use effects on forest plant species composition and diversity[END_REF]. The colonisation credit of recent forests in our study might thus have already been exhausted, contrary to very recent forests. For the same reason, these very recent forests were also closer to open-habitat edge [START_REF] Bergès | Past landscape explains forest periphery-to-core gradient of understorey plant communities in a reforestation context[END_REF].

This edge effect could thus add to the effect of forest temporal continuity on plant species [START_REF] Bergès | Past landscape explains forest periphery-to-core gradient of understorey plant communities in a reforestation context[END_REF].

Several open-habitat species persisted after land abandonment and forest development. Two ecological processes can explain this maintenance: either suitable habitats remained within the forest landscape (open areas within a closed forest matrix), or those species exhibited an extinction debt, meaning that they have remained while their optimum habitat has already disappeared [START_REF] Jackson | Balancing biodiversity in a changing environment: Extinction debt, immigration credit and species turnover[END_REF]. This extinction debt can last over more than a century [START_REF] Vellend | Extinction debt of forest plants persists for more than a century following habitat fragmentation[END_REF]. A long-term survey on those species would be necessary to disentangle the extent of the two processes.

| Legacies of a temporal deforestation

The temporal deforestation (DEF) seemed to imply legacies of its own. Forests that were deforested between 1860 and 2010 accounted only for 6% of our plots, but no less than 87 species were associated with this land use trajectory. Plant communities associated with temporarily deforested land were the most distinct from those of ancient forests and the traits and ecological preferences associated with DEF were also associated with VRF and had an opposite relation with AF or RF (e.g., leaf size, plant height, and indicator values). The temporal deforestation may thus have erased species present in formerly ancient forests [START_REF] De Keersmaeker | Spatio-temporal patterns of vegetation recovery in post-agricultural forests in Flanders[END_REF][START_REF] Verheyen | Response of forest plant species to land-use change: A life-history trait-based approach[END_REF]. All this suggests that legacies of a temporary deforestation of ancient forests were different from those of the continuous agricultural use until the mid-20th century.

The temporal deforestation to arable land showed similar legacies as long-term arable use, which can be explained by thorough nutrient inputs and ploughing in the 19th century.

Concerning the temporal deforestation to pasture, those differences might be due to an actual temporary deforestation by fire. Of the 97 plots in this temporal trajectory, 91 were photo-interpreted as garrigue (associated with pasture, Table 1). The repetition of forest wildfires leads to the development of garrigue [START_REF] Tatoni | Evolution post-culturale des agrosystèmes de terrasses en Provence calcaire: phytoécologie et impact humain[END_REF]. The PNRL has been spared by forest fires during the last decades, but some intensive and repeated fires occurred during the 1940s and 1950s on the eastern part of the Luberon mountain. Associated to violent rain episodes [START_REF] Varese | Pré-étude en vue d'une typologie des stations forestières du Luberon[END_REF], repeated fires contributed to the erosion and impoverishment of soils, where the garrigue developed [START_REF] Tatoni | Evolution post-culturale des agrosystèmes de terrasses en Provence calcaire: phytoécologie et impact humain[END_REF] those post-fire conditions [START_REF] Bonnet | Analyse spatiale et fonctionnelle de la réponse de la végétation après incendie en basse Provence calcaire[END_REF]. This result questions the definition of ancient forests: "a forest that may not have been used for agricultural purposes" (Marks & Gardescu, 2001). This definition implies that a forest fire, which does not affect the soil like ploughing does, would not interrupt the temporal continuity of an ancient forest. Here we suggest that the definition of ancient forest should probably exclude forests that experienced repeated fires. However, supplementary investigations would be necessary to confirm this assumption.

| Implications for conservation

In temperate lowlands, most ancient forests are isolated within a more or less intensive agricultural matrix [START_REF] De Keersmaeker | The analysis of spatio-temporal forest changes (1775-2000) in Flanders (northern Belgium) indicates habitat-specific levels of fragmentation and area loss[END_REF][START_REF] Rackham | Ancient woodlands: Modern threats[END_REF]. The conservation of the forest continuum is thus a major issue, as it would help the colonisation of newly established forests by ancient forest species. In our context, the restoration of a spatial forest continuum between ancient forest patches naturally took place with the natural forest recovery since the 19th century [START_REF] Abadie | Land use legacies on forest understory vegetation and soils in the Mediterranean region: Should we use historical maps or in situ land use remnants?[END_REF]. The conservation of ancient forest species depends more on the use of the forest. Although this has yet to be proven, a former overexploitation of Mediterranean forests might have resulted in the disappearance of some herbaceous ancient forest species. Nevertheless, forest specialists are associated with ancient forests. It is therefore crucial to conserve ancient forests to ensure the conservation of forest specialists.

| CON CLUS I ON AND PER S PEC TIVE S

With this study, we first highlighted that long-term land use legacy in Mediterranean forest understorey plant communities lasts over 150 years, making it comparable to the well-known legacy identified in temperate lowlands [START_REF] Hermy | Legacies of the past in the present-day forest biodiversity: A review of past land-use effects on forest plant species composition and diversity[END_REF]. We also showed that the type of past land use prior to forest resulted in distinct plant successions.

In former studies, the effect of forest temporal continuity and past land use mostly concerned soil physico-chemical properties and vegetation [START_REF] Bergès | Response of understorey plant communities and traits to past land use and coniferous plantation[END_REF]Flinn & Vellend, 2005;[START_REF] Hermy | Legacies of the past in the present-day forest biodiversity: A review of past land-use effects on forest plant species composition and diversity[END_REF], but now focuses more and more on other species groups with low dispersal abilities, such as saproxylic beetles, snails, collembola, etc. [START_REF] Janssen | Influences relatives de l'ancienneté et de la maturité sur la biodiversité : Implications pour la conservation en forêts de montagne[END_REF][START_REF] Janssen | Present conditions may mediate the legacy effect of past land-use changes on species richness and composition of above-and below-ground assemblages[END_REF][START_REF] Vrignaud | Les gastéropodes et les forêts anciennes[END_REF]. Exploring those taxonomic groups could be very informative in the Mediterranean region. Furthermore, exploring the interaction between fire regimes and past land use on current forest ecosystems would help understand the functioning of forest ecosystems [START_REF] Duguy | Land-use history and fire effects on soil fertility in eastern Spain[END_REF][START_REF] Duguy | Land-use and fire history effects on post-fire vegetation dynamics in eastern Spain[END_REF][START_REF] Puerta-Pinero | History matters: Previous land use changes determine post-fire vegetation recovery in forested Mediterranean landscapes[END_REF].

  use in previous studies: Raunkiaer's life form, vegetative reproduction, flowering phenology, longevity, dispersal mode and potential maximum height. The specific leaf area (SLA) recorded in European trait databases (Biolflor, LEDA) was not considered suitable in a Mediterranean context because of the climate specificity. Instead, we created a categorical trait for leaf size (Table

F I G U R E 1

 1 (a) Location of the study area within the French Mediterranean region; (b) vegetation plot distribution within the study area; and (c) maps of the three historical variables used in the models. AF: forest present in 1860, 1958 and 2010; RF: forest developed between 1860 and 1958; VRF: forest developed after 1958; DEF: forest in 1860 and 2010 but not in 1958

  Figure4).Trait responses were consistent among historical variables. For example, traits related to AF were also related to forests in 1860(1860-F) and 1958 (1958-F), while traits related to former arable land in1958 (1958-Ara) -Ara) were also related to former arable land in 1860 (1860-Ara) (Figures3 and 4). Among Raunkiaer life forms, only chamaephytes, phanerophytes and therophytes were influenced by historical variables. Chamaephytes were more frequent in 1958-Pas and less frequent in 1958-F. Phanerophytes were more frequent in 1860-F, 1958-F, thus in AF and RF, but also in VRF. Therophytes were more frequent in 1860-Ara but less frequent in 1958-F, AF and RF (Figure 3). Malacophyllous, sclerophyllous and needle-type-leaved species were influenced by historical variables, but not microphyllous and graminean species. Malacophyllous species were less frequent in 1860-Pas and 1958-Pas while sclerophyllous species were more frequent in 1860-F, 1958-F and AF. Needle-like-leaved species were more frequent in 1860-Ara, 1958-Pas and VRF (Figure 3). Plant height and leaf size similarly responded to the three historical variables: they displayed a positive relation with 1958-F, AF and RF, and a negative relation with 1860-Pas, 1958-Pas, VRF and DEF (Figure 3).

  mer forests(1860 and 1958, AF and RF) and open-habitat species were more frequent in DEF, but less frequent in AF. The response of soil moisture and nitrogen indicator values (IV) to historical variables was rather similar and opposite to the response of light IV (Figures3, 4). Light IV was higher in former pastures(1860 and 1958) and in VRF and DEF, but lower in former forests(1860 and 1958), while soil moisture and nitrogen IV were higher in AF and 1958 forests, but lower in former pastures (1860 and 1958 for nitrogen IV and 1958 for soil moisture IV) and in VRF and DEF (Figure3).

F

  Canonical correspondence analysis (CCA) first factorial map according to: (a) forest temporal continuity; (b) 1860 land use; (c) 1958 land use; and (d) land use trajectory, i.e., the combination of the three historical variables. The four CCAs were constrained by the corresponding historical variables. AF, ancient forest; RF.pas, recent forest developed on 1860 pasture; RF.ara, recent forest developed on 1860 arable land; VRF.pas.pas, very recent forest developed on pasture (1860 and 1958), VRF.ara.ara, very recent forest developed on arable land (1860 and 1958), VRF.ara.pas, very recent forest developed on arable land (1860) and then pasture (1958); VRF.pas.ara, very recent forest developed on pasture (1860) and then arable land (1958); DEF.ara, deforested into arable land in 1958; DEF.pas, deforested into pasture in 1958

F

  Relation between plant functional traits and ecological preferences, and historical variables (1860 land use, 1958 land use and forest continuity). AF, ancient forest; RF, recent forest; VRF, very recent forest; DEF, deforested in 1958. Significant positive relationships are in green and negative relationships in blue (p < .05). See Appendix S5 for detailed p-values Since 1860, newly established forests developed adjacent to already existing forests

  and turned into an open forest where therophytes and anemochores were dominant, facilitated by the lighter conditions of F I G U R E 4 First RLQ factorial map of (a) land use trajectory -AF, ancient forest; DEF, deforested in 1958; RF, recent forest; VRF, very recent forest; (b) land use trajectory and plant functional traits -AN, annuals; ANEMO, anemochores; BARO, baro-, auto-& hydrochores; BIS, biennials; CHAM, chamaephytes; ENDOZOO, endozoo-& dyszoochores; EPIZOO, epizoochores; FL.BEG, beginning of flowering season; FL.DUR, duration of flowering season; FL.END, end-of-flowering season; GEO, geophytes; GRAM, graminean; HEMI, hemicryptophytes; MALACO, malacophyllous; MICRO, microphyllous; MYRMECO, myrmecochores; NEE, needles, spines, scales and succulent; PER, perennials; PHAN, phanerophytes; SCLERO, sclerophyllous and laurel-like; THERO, therophytes; (c) land use trajectory and ecological preferences -EDGE, forest edge species; FOREST, forest species; OPEN, open-habitat species

  Past land use in 1860 and 1958, forest continuity and resulting land use trajectories for plots forested in 2010

		1958 land	Forest temporal		Percentage within	Number
	1860 land use	use	continuity	Land use trajectory	current forest area	of plots
	Forest	Forest	AF	AF	41	436 (31%)
	Pasture	Forest	RF	RF on pasture	20	336 (24%)
	Arable	Forest	RF	RF on arable land	12	234 (16%)
	Pasture	Pasture	VRF	VRF on pasture	7	152 (11%)
	Arable	Pasture	VRF	VRF on arable land then pasture	4	94 (7%)
	Pasture	Arable	VRF	VRF on pasture then arable land	2	16 (1%)
	Arable	Arable	VRF	VRF on arable land	10	64 (4%)
	Forest	Pasture	DEF	DEF to pasture	3	91 (6%)
	Forest	Arable	DEF	DEF to arable land	1	6 (0%)

TA B L E 1

Abbreviations: AF, ancient forest; RF, recent forest; VRF, very recent forest; DEF, temporally deforested between 1860 and 2010.

  Effect of land use in 1860, land use in 1958 and temporal continuity on 52 species of our dataset using logistic regression (species that most significantly responded to the historical variables)

							1860		Temporal	AF/
						Species name	LU	1958 LU	continuity	RF
	Species name	1860 LU	1958 LU	Temporal continuity	AF/ RF	Avena barbata Carthamus lanatus	ara ara	pas* ara*	VRF* VRF*
	Acer opalus	for*	for*	AF*	AF*	cantabrica Convolvulus	pas*	pas*	VRF*
	Aphyllanthes monspeliensis	for*	for*	AF*		Echinops ritro	ara*	ara*	VRF*
	Brachypodium	for*	for*	AF*	AF*	Eryngium campestre	ara*	ara	VRF*
	rupestre					Genista pulchella	pas	pas*	VRF*
	Buxus sempervirens	for*	for*	AF*		Linum strictum	ara*	pas*	VRF*
	Campanula trachelium	for*	for*	AF*	AF*	Potentilla cinerea	pas	pas*	VRF*
	Carex halleriana	for	for*	AF*		Reichardia picroides	ara	pas*	VRF*
	Cornus mas	for*	for*	AF*		Reseda phyteuma	ara	ara*	VRF*
	Cytisophyllum	for*	for*	AF*	AF*	Bupleurum baldense	for	ara*	DEF*
	sessilifolium					Galium corrudifolium	for*	pas	DEF*
	Genista hispanica	for*	for*	AF*		Ornithogalum	for	pas*	DEF*
	Hedera helix	for*	for*	AF*		umbellatum
	Hippocrepis emerus	for*	for*	AF*	AF*	Valeriana tuberosa	for*	pas*	DEF*
	Lilium martagon	NA	for (excl)	NA
	Lonicera etrusca	for*	for*	AF*	AF*
	Lonicera implexa	for	for*	AF*
	Melica uniflora	NA	for (excl)	NA
	Phillyrea latifolia	for*	for*	AF*	AF*
	Pinus halepensis	NS	for*	AF*
	Poa nemoralis	NA	for (excl)	NA
	Polypodium	NS	for*	AF*
	cambricum
	Polygonatum	for*	for*	AF*	AF*
	odoratum
	Pseudoturritis turrita	for	for*	AF*
	Quercus ilex	for	for*	AF*
	Quercus pubescens	for*	for*	AF*
	Rhamnus alaternus	for*	for*	AF*	AF*
	Rubia peregrina	for*	for*	AF*
	Ruscus aculeatus	for	for*	AF*
	Sorbus aria	for*	for*	AF*	AF*
	Sorbus torminalis	for*	NS	AF*	AF*
	Teucrium chamaedrys	for*	for*	AF*	AF*
	Viburnum lantana	for*	for*	AF*
	Acer campestre	ara	for*	RF*
	Crataegus monogyna	ara	for*	RF*
	Inula conyza	NS	for*	RF*
	Ononis spinosa	pas	ara	RF*
	Phillyrea angustifolia	pas	for*	RF*
	Quercus coccifera	NS	for*	RF*
	Allium flavum	pas*	ara	VRF*
	Artemisia campestris	ara	ara*	VRF*
				(Continues)

TA B L E 3

Note: The effect of forest age (AF/RF where RF = RF and VRF) was also added (last column). Species significantly responding to a single historical variable while taking local conditions into account are labelled with an asterisk (ΔAIC > 2) and species preferences are in bold when ΔAIC > 5. Species with no asterisk responded to the historical variable but not when taking local conditions into account. NA stands for species that could not be tested; AF, ancient forest; ara, arable land; DEF, temporarily deforested; for, forest; NS, non-significant; pas, pasture; RF, recent forest; VRF, very recent forest. Species only found in ancient or recent forests are indicated as exclusive (excl).

ACK N OWLED G EM ENTS

The authors would like to thank Virgile Noble for letting them access the SILENE database. We also thank Philip Roche, Renato Henriques and Jordi Prats for their constructive comments.

DATA AVA I L A B I L I T Y S TAT E M E N T

Primary datasets and scripts are stored in an internal repository without DOI, but are available for re-analyses upon request.

AUTH O R CO NTR I B UTI O N S

JA, JLD, AS and LB conceived of the research ideas; JA and NV collected data; JA performed statistical analyses; JA, with contributions from LB, CA and JLD, wrote the paper; all authors discussed the results and commented on the manuscript.

S U PP O RTI N G I N FO R M ATI O N

Additional supporting information may be found online in the Supporting Information section.

Appendix S1. Methodology for vegetation plot selection


Joffrey Wolf

Rinaldo Poli
email: rinaldo.poli@lcc-toulouse.fr

Jian-Hua Xie

Jason Nichols

Bin Xi

Peter Zavalij

Michael P Doyle

Removal of Metal-Metal Bonding in a Dimetallic Paddlewheel Complex -Molecular and Electronic Structure of Bis(phenyl) Dirhodium(III) Carboxamidate Compounds

The unique structural features and chemical stabilities of bis (phenyl) tetracarboxamidatodirhodium(III) are reported, and their electronic structures are mapped through XPS, electrochemical, and computational methods. Comparison with the structures of dirhodium(II,II) and dirhodium(II,III) oxidative precursors portrays the diphenyl dirhodium(III) compounds as two squarepyramidal rhodium units that have undergone conrotatory motion in order to optimize metal-ligand bonding. Axial phenyl ligands are severely distorted from their expected Rh-Rh-C linear array. XPS data for this series of dirhodium compounds are consistent with the absence of a rhodium-rhodium bond for the diphenyl dirhodium(III) compounds, and electrochemical measurement shows a single reversible Rh2 6+ /Rh2 7+ redox couple. Notably, they exhibit high thermal stability, and Brønsted acid removal of carboxamidate ligands preceeds the formation of benzene. The ability of a phenyl group to impart unusual stability to rhodium(III) compounds is explained by theoretical analysis of the electronic structure of bis--(phenyl)-tetrakis--(carboxamido)dirhodium(III), and comparison is made with previously reported dinitrosyl dirhodium(III) complexes. Formally, two phenyl radicals in combination with a rhodium-rhodium bond are transformed into two phenyl-rhodium bonds. The severe distortion of phenyl rings from linearity is suggested to result from a long-range interaction between the electrondeficient rhodium and distal oxygen atoms.

Introduction

The recent definitive treatise on multiple bonds between metal atoms introduced oxidation states with the statement that "the most common oxidation states for the M2 n+ units in paddlewheel complexes corresponds to values of n of 4, 5, and 6". [START_REF] Cotton | Multiple Bonds Between Metal Atoms[END_REF] Examples of the n = 6 systems are less common. Our report last year of remarkably stable bis(phenyl)dirhodium(III) caprolactamate (Scheme 1, left) 2 was only the second example for Rh2 6+ compounds, Rh2(O2CR)4(NO)4 (R = Me, Et, iPr) (Scheme 1, center) having been previously reported by the Lippard group exhibiting the same structural type and an essentially identical Rh-Rh separation. 3 Although no specific oxidation state assignment was mentioned in the Lippard report, the bent geometry observed for the NO ligands justifies the assignment of a formal oxidation state of III to the Rh centers, and this was later supported by computational studies. [4][5][6] One of the surprising features of these Rh2 6+ compounds is the apparent absence of a rhodium-rhodium bond.

In contrast, a paramagnetic paddlewheel Ir2 6+ compound, Ir2(hpp)4Cl2 (Scheme 1, right), also from group 9, has been reported to have a doubly-bonded dimetallic core with two unpaired electrons. 7 Isoelectronic Ru2 4+ complexes are reported to have the σ 2 π 4 δ 2 δ* 2 π* 2 electronic configuration [START_REF] Cotton | Multiple Bonds Between Metal Atoms[END_REF]8 implying a Ru-Ru double bond and a spin triplet ground state, whereas Ru2 6+ compounds have the π 4 δ 2 π* 4 electronic configuration with a net δ single bond. [START_REF] Cotton | Multiple Bonds Between Metal Atoms[END_REF]9,10 Homo-dinuclear σ-alkynyl complexes of both ruthenium and rhodium are known, 11 although with rhodium only in the (II,III)-oxidation state. 12 Electron donation from the axial carbon ligand to rhodium increases the Rh-Rh bond distance, but only a fraction of that found with 1, and their electronic properties imply retention of a metal-metal bond.

The ability of paddlewheel complexes to hold together two metals without a metal-metal bond is wellestablished, 13,14 and the structural and electronic influences that result have substantial theoretical and practical implications. 15 In this contribution, we report the molecular structures and chemical stabilities of the diphenyl tetracarboxamidatodirhodium(III) system and their study by XPS spectroscopy, electrochemistry, and by computational methods.

Results and Discussion

Synthesis and Characterization.

In order to identify the unique structural features of bis(phenyl)dirhodium(III) carboxamidates we have prepared the dirhodium(II) pyrrolidinate (pyr), valerolactamate (val), and caprolactamate (cap) bis-acetonitrile complexes (1), [START_REF] Lifsey | Rh2(pyr)4(Hpyr)2 and Rh2(val)4(Hval)2 have been prepared and their crystal structures reported: Bear[END_REF] oxidized these compounds to the corresponding paramagnetic Rh(II)Rh(III) derivatives (2, eq 1), and also prepared their diamagnetic bis(phenyl)dirhodium(III) analogues (3, eq 2). The synthesis of dirhodium(II,III) complexes was achieved by oxidation of the dirhodium(II) carboxamidate using copper(II) triflate, nitrosonium salts (eq 1), 17,[START_REF] Connelly | For uses of nitrosonium salts as oxidants[END_REF] or with the use of phenyldiazonium tetrafluoroborate; oxidations with nitrosonium salts were the most versatile and gave dirhodium(II,III) product without byproducts in solution. As mentioned in the Introduction, the synthesis (using sodium tetraphenylborate) and characterization of bis(phenyl)dirhodium(III) caprolactamate (3c) has been described in a preliminary communication. [START_REF] Nichols | [END_REF] Since this first report, a new methodology for convenient access to diaryl analogues directly from dirhodium(II) carboxamidates and arylboronic acids catalyzed by copper sulfate (eq 2) has been communicated, 19 and the spectral data of these products have been described.

Rh

1a: n = 1 1b: n = 2 1c: n = 3 3a: n = 1 3b: n = 2 3c: n = 3 Rh Rh N O N O N O N O NCMe MeCN ( ) n ( ) n ( ) n ( ) n ( ) n
The crystal structures of all nine compounds have been obtained. Those of the pyrrolidinate systems Rh2(pyr)4(MeCN)2 (1a), [Rh2(pyr)4(PhCN)2]BF4 (2a'), and PhRh(pyr)4RhPh (3a) are presented in Figure 1. Unpublished data for the other 6 compounds are provided as supporting information. A selection of bond lengths and angles for all nine compounds is reported in Table 1 and Table 2. As can be seen from these data, the diphenyl-dirhodium(III) compounds exhibit severe structural distortions relative to their dirhodium(II,II) and dirhodium(II,III) analogues. They can best be evaluated by comparing a few key bond angles [the values are averages over the three structures within each family and listed in the square-pyramidal units have undergone conrotatory motion so as to minimize repulsions and metalmetal interactions (Figure 2). In addition to these geometrical distortions, there is a significant lengthening of the Rh-Rh bond on going from 2 to 3, whereas the bond shortens on going from 1 to 2. a The bis-benzonitrile complex was prepared after failed attempts to prepare the bis-acetonitrile complex. b The bis-aquo complex was formed and crystallized, even though reaction was performed in acetonitrile. X-ray photoelectron spectroscopy (XPS) data for these nine compounds is given in Table 3. As evidenced by this direct measurement of the binding energy of d-electrons that constitute the dirhodium core, there is a significant change in binding energy upon oxidation of dirhodium(II,II) to dirhodium(II,III) (~ 0.8 eV) but an insignificant change upon oxidation of dirhodium(II,III) to dirhodium(III,III) (< 0.2 eV). The relative electron richness of the diphenyldirhodium(III) carboxamidate complexes is also suggested by the results of an electrochemical investigation. Cyclic (CV) and differential pulse voltammograms (DPV) of 3a, 3b, and 3c were measured in CH2Cl2, and their half-wave potentials (E1/2) are reported in Table 4, along with their electronic absorption maxima (λmax). Figure 3 shows the CV and DPV traces for PhRh(pyr)4RhPh (3a). For each of the complexes, a single, reversible Rh2 6+ /Rh2 7+ redox couple (A) was observed. The oxidation potential of 3 increases with decreasing number of methylene units in the bridging cyclic amide, which follows that of the Rh2 4+/5+ couple of the parent complexes. 20 For complex 3a, an additional irreversible redox couple (B) near the reduction limit of the solvent was observed and is assigned to the Rh2 5+/6+ redox couple. Attempts to isolate and characterize the oxidized PhRh(pyr)4RhPh or any of its derivatives have thus far been unsuccessful. The stabilities of bis(phenyl)dirhodium(III) carboxamidates are noteworthy. They are air stable and thermally stable to temperatures below 130°C. They do not react with nucleophiles such as cyanide or azide, but they are sensitive to electrophilic attack.

Treatment of compound bis--(6methoxynaphthalen-2-yl)-tetrakis--(caprolactamato)dirhodium(III) 19 with trifluoroacetic acid, for example, resulted in quantitative conversion to 2-methoxynaphthalene (eq 3) and caprolactam (capH).

Product analysis as a function of time with bis--(phenyl)-tetrakis--(caprolactamato)dirhodium(III)

demonstrated the stability of the phenyl-rhodium(III) bond (Figure 4); caprolactam was released from the complex prior to release of benzene, but attempts to isolate the anticipated diphenyl derivative of Computational study. The structural novelty of the bis(aryl)dirhodium(III) family and their peculiar properties (metal-metal bond length, stability), in comparison with isoelectronic systems (see Introduction) warrant a more detailed analysis of their electronic structure. DFT methods have proven quite successful when applied to dinuclear metal-metal bonded compounds. 21,22 The B3LYP functional provides the best agreement between experimental and calculated structures, [START_REF] Cotton | Multiple Bonds Between Metal Atoms[END_REF] and it has therefore been adopted in this study. The study used the standard LANL2DZ basis set in order to provide a qualitative description of the electronic structure. A model system was constructed with the amidato ligands being replaced with simple formamidato ligands, [OCCHNH] -.

(a) [Rh2(amidato)4], [Rh2(amidato)4(MeCN)2] and [Rh2(amidato)4(MeCN)2] + systems.

Before analyzing the more interesting bis(aryl)dirhodium(III,III) derivatives, it is necessary to start with the dirhodium(II,II) and dirhodium(II,III) precursors. These families of compounds have already been extensively studied, thus our analysis is limited to a comparison of the optimized and experimental geometries, and to a comparison of the basic electronic structural features with those previously presented for other members of the same families [23][24][25][26][27][28][29][30] (fuller details are available as Supporting Information). Geometry optimizations were carried out for the simple tetrakis(amidato)dirhodium(II,II) system (I in Scheme 2), for its bis(acetonitrile) adduct (II), and for the one-electron oxidation product of the latter (III). All these calculations were only carried out with the experimentally observed headhead-tail-tail (cis-2,2) 20 relative orientation of the four bridging amidato ligands (see Scheme 2). The salient features of the optimized geometries are shown in Table 5. There is excellent agreement between optimized and experimental parameters, attesting to the suitability of the computational method and of the chosen model. The Rh-Rh distance trend is perfectly reproduced by the calculation. The very minor elongation (0.045 Å) computed on going from I to II agrees with the notion that this bond is rather insensitive to the presence of axial ligands, an experimental example being Rh2(O2C-C6H2iPr3-2,4,6)4 and its bis(acetone) adduct (distances of 2.350(1) and 2.370(1), respectively). 31,32

H Rh Rh N O O N O N O H H H H N H H H IV H Rh Rh N O O N O N O H H H H N H H H I + H Rh Rh N O O N N O N O H H H H N H H H N II C CH 3 C H 3 C H Rh Rh N O O N N O N O H H H H N H H H N C CH 3 C H 3 C III Scheme 2.
Tetraformamidato systems used for the DFT calculations. a Averages over all chemically equivalent parameters of compounds 1a, 1b and 1c. b Averages over all chemically equivalent parameters of compounds 2a', 2b' and 2c.

The specific Rh-Rh , and* interactions are symmetry-mixed with symmetry-adapted linear combinations (SALC) of the Rh-O and Rh-N bonds. The effect of this mixing is that each of these interactions is mainly distributed in two molecular orbitals. Within each pair of orbitals, the one carrying the greater Rh-Rh character is that at lower energy for  and and that at higher energy for *.

From the energetic point of view, the major Rh-Rh  MO is found at higher energy than the major Rh-Rh  MO, and the major Rh-Rh * MO is higher than the Rh-Rh* MO. Upon MeCN axial ligation, the  competition between the Rh-NCMe and Rh-Rh interactions has the effect of raising the Rh-Rh  level. Weak axial ligands (e.g., H2O) establish weak interactions with the Rh atoms and weakly affect the Rh-Rh  MO energy, 23 whereas stronger ligands (e.g., phosphines) have a more pronounced effect and greatly raise the energy of the Rh-Rh  MO to the point that this may even become the HOMO. 25 In the present case, the interaction is relatively weak but sufficient to push the Rh-Rh  MO higher than the Rh-Rh  MO but lower than the Rh-Rh  MOs. Upon MeCN ligation, the energy of all orbitals is raised by ca. 0.4 eV on average, reflecting the additional electron density provided to the Rh atoms by the axial ligands. On the other hand, upon one-electron oxidation, all orbitals drop by ca. 1.5 eV on average as a result of the increased effective positive charge, but the MO energy ordering remains approximately unchanged (the  level remains between the  and * levels). For all three systems, the HOMO (doubly occupied for I and II, singly occupied for III) is the Rh-Rh  MO.

(b) The [Rh2(amidato)4Ph2] system.

A geometry optimization was carried out on the model system Rh2(OCHNH)4(C6H5)2 with the same (cis-2,2) arrangement as in the I-III congeners (IV, see Scheme 2). The input geometry was constructed directly from the experimentally observed geometry of compound 3c. Since this geometry is very close to an ideal C2h symmetry, that symmetry was imposed in the initial geometry optimization. The converged geometry (Figure 5, i) remained very close to the initial one, but the frequency analysis revealed one imaginary frequency whose motion showed the tendency for the two Rh-Ph axes to move in opposite directions away from the symmetry plane. Therefore, a free optimization in C1 symmetry was subsequently carried out. The resulting geometry (Figure 5, ii) was essentially Ci-symmetric, with the expected 3N-6 positive vibrational frequencies, and with a co-planar arrangement for the two Ph groups. The energy of the global minimum is just 0.2 kcal/mol lower than the C2h structure. The ideal geometry where the Ph-Rh-Rh-Ph moiety was forced to be coplanar and linear (Figure 5, iii), on the other hand, turned out to be energetically less favored by 8.2 kcal/mol. These combined results indicate that the origin of the experimentally observed distortion of the Rh-Ph bonds is not in the steric interaction between the Ph groups and the amidato ligands, but rather in an electronic effect that forces the three-bond Ph-Rh-Rh-Ph sequence out of co-linearity. Of the two different and nearly isoenergetic distortion modes (i) and (ii), the first one is adopted in the crystal structure for possible packing reasons. a Averages over all chemically equivalent parameters of compounds 3a, 3b and 3c.

The electronic structure analysis helps rationalize the molecular constitution and geometry of the novel dinuclear Rh III complexes. The essential features are reported in Figure 6, while fuller details are presented in the Supporting Information. The relevant MO's for IV derive from the full geometry optimization, while those of the Rh2(OCHNH)4 fragment (V) and the Ph radicals are calculated for the corresponding fixed geometries as found in IV. The frontier orbitals of interest for the Ph radical are the filled, familiar Huckel-type 1, 2 and 3 orbitals, plus the  orbital pointing out from the unsaturated carbon in a radial direction. The latter is expected to be used for the formation of the Rh III -Ph  bond. Upon interaction between V and the two Ph radicals, we observe that those molecular orbitals that are not involved in a significant interaction change their energy only slightly. More specifically, there is a slight energy increase for the phenyl orbitals 17 (1) and 20 (3) and a slight energy decrease for V orbitals 51 and 52 (Rh-Rh ), 59 (Rh-Rh ), and 63 (Rh-Rh *). Inspection of the contour plots of the related orbitals in system IV (see Supporting Information) shows that they do not involve a significant mixing of components from the different fragments.

The greater energy change experienced by the Ph 2 orbitals is caused by the combination with the appropriately oriented Rh-Rh  and * orbitals of V, resulting in Rh-Ph -type interaction. However, since these are filled-filled interactions (both the in-phase and out-of-phase combinations are filled) there is no net Rh-Ph  bonding. The interaction is stronger (greater mixing and energy changes) for the Rh-Rh * orbital because of a better energy match. The in-phase and out-of-phase combinations are found in orbitals 94 and 103, while the corresponding combinations with the Rh-Rh  MO are found in orbitals 93 and 102.

The most important interaction between V and the two Ph radicals results from the combination of the -type orbitals. There are four such orbitals in the frontier region, two for Rh2(OCHNH)4 (the filled Rh-Rh  orbital, mixed with a nonbonding ligand-based combination in MO's 54 and 55, vide supra, and the empty Rh-Rh * orbital, MO 64), and two for the two Ph radicals (the half-filled frontier  orbital, MO 21). This gives rise to two filled MO's (86 and 91) and to two empty MO's (105 and 108) in IV, all of which are shown in Figure 7 (a more extensive sample of MO contour diagrams for IV is presented in the Supporting Information). The two filled orbitals have mostly Rh-Ph  character but also feature other contributions: the former has substantial Rh-Rh and C-C/C-H  contributions, whereas the latter has Rh-Rh * character. Thus, very little Rh-Rh  bonding is present in the molecule. Most of the Rh-Rh  interaction is found in the empty MO 105, which also has significant Rh-Ph * character and is therefore raised above the energy of the Rh-Rh * orbital. Finally, MO 108 has both Rh-Rh and Rh-Ph * character (Figure 7). The Rh-Rh interaction in this molecule can therefore be formally described as

 4  2 * 4 * 2 ,
leading to a formal bond order of zero.

Using a basic, qualitative valence bond approach, we can describe the molecule as two interpenetrating octahedra, using the familiar d 2 sp 3 hybrids for bonding. This would lead to the establishment of all three  interactions (two Rh-Ph bonds and the Rh-Rh bond) by implication of the higher energy 5s and 5pz orbitals for the two rhodium atoms Obviously, this implication is not extensive enough to keep the third bonding orbital sufficiently low in energy to be occupied. We note that the involvement of the higher energy s orbital has been invoked 33 to rationalize the observed short distance of paddle-wheel d 3 -d 3 W2(O2CEt)4R'2 compounds (E = Et and R' = CH2Ph or CH2tBu), 34 which are able to maintain their W-W  interaction as in the parent d 4 -d 4 W2(O2CEt)4 compound. The involvement of the higher energy orbitals seems a more common feature for 5d elements, as a result of relativistic effects. The effect of the  interaction scheme on the electronic structure is such that the HOMO of IV is the same orbital, the * orbital, as the HOMO of I. The two are essentially identical because the Ph groups do not significantly contribute. This agrees with the observed reversible electrochemical behavior upon oxidation: the electron is pulled off the essentially nonbonding * orbital, thus the Rh2 7+ oxidation product is predicted to have essentially no Rh-Rh bonding interaction and to display a structure closely related to that of its Rh2 6+ precursor.

One remaining point of interest is the origin of the non linear arrangement of the Ph-Rh-Rh-Ph moiety. One possible cause, related to the Ph-Rh-Rh-Ph  bonding scheme, is a second order Jahn-Teller effect involving the filled Rh-Ph and the empty Rh-Rh  orbitals. However, this idea is contradicted by the higher energy of the bonding orbitals 86 and 91 in the optimized minimum relative to the idealized linear geometry (iii in Figure 5, orbital energies available in SI). This suggests that the Ph-Rh-Rh-Ph  bonding framework prefers the linear configuration. The relative weakness of this interaction and its symmetry-mixing with other bonding features, however, do not allow its clear identification in the molecular orbitals. Therefore, the reason for the molecular distortion of 3 out of linearity must be attributed to other factors. A possible clue comes from the observation of very narrow Rh-Rh'-O bond angles in 3 (less than 90°), suggesting the presence of an attractive interaction between Rh and the O atom bonded to Rh'. The through space crystallographic distances between rhodium and its distal oxygens are 3.00 and 3.24 Ǻ for 3a, 2.82 and 3.11 Ǻ for 3b, and 2.90 and 2.97 Ǻ for 3c. Given the absence of direct Rh-Rh bonding, the Rh atoms are in fact electronically unsaturated (16-electron configuration), and therefore seek electron density from the distal O lone pairs. However, we note that an alkylrhodium(III) compound with a nearly identical coordination geometry, the O2N2 coordination environment being provided by a Schiff base ligand, maintains a mononuclear 5-coordination geometry with an open coordination site. 35

(c) The [Rh2(O2CCH3)4(NO)2] system.

Compounds Rh2(O2CR)4(NO)2 (R = Me, Et, iPr), featuring the same paddlewheel structure as compounds 3 with axial NO ligation and a very similar Rh-Rh distance, have been recently described. 3 The electronic structure of these compounds (with R = H and CH3), has also been reported. [4][5][6] Here, we wish to compare the electronic structures of system Rh2(O2CCH3)4(NO) (VI), and IV. For this purpose, the geometry of VI has been optimized at the same computational level as IV. The orbital interaction diagram for VI with respect to its parent Rh2(O2CCH3)4 and NO fragments is shown in Figure 8, while fuller details of the electronic structure of VI are presented in the Supporting Information.

The axial donor, in this case, mostly interacts with the dimetal system via the N lone pair (which is mixed with the O lone pair in MO 7). However, because of the Rh-N-O bending, the NO || and ||* Going from the fragments to the full system, as with the formamidato system discussed above, the metal-metal , , * and * orbitals are not significantly affected. The N-O ┴ orbitals can potentially establish Rh-N  interactions but, because of their relatively deep energy, do not significantly mix with the metal orbitals and therefore are also little effected by the axial coordination. As for the amidato system, the most important change concerns the -type orbitals. Relative to the diphenyl system, the situation is complicated by the greater number of participating orbitals [the Rh-Rh  and * orbitals from fragment V, plus the N lone pair and the N-O || and ||* orbitals from each of the two NO ligands, for a total of 8 resulting MO's]. The four lowest energy combinations (orbitals 51, 52, 61 and 65 in Figure 8) have their major contribution from the Rh-N  and N-O || bonds. Contour diagrams of the other four orbitals are shown in Figure 9 (other relevant orbitals are shown in the Supporting Information). They are qualitatively similar to those of system IV, including their occupation. Thus, as with the diphenyl formamidato system, the formation of the two Rh-NO bonds essentially destroys the Rh-Rh interaction, since the Rh-Rh * component that was empty in the dinuclear Rh II fragment becomes now engaged in the axial -bonding system. That there is no N-Rh-Rh-N distortion in the nitrosyl complex is likely due to the symmetric nature of the 3-atom bridges; since oxygen atoms constitute both sides, the molecule remains symmetric.

(d) Comparison of electronic structures

A comparison between the two MO diagrams of IV in Figure 6 and VI in Figure 8 shows that those with the largest Rh contribution, and not involved in the  interactions (namely the , *,  and * orbitals), have lower energies for system VI than IV. This is rationalized by the more electronegative nature of the carboxylato ligands relative to the amidato ligands. The most important difference between the two systems, however, is in the  manifold. For IV, the Rh-Rh(*) of the Rh2(OCHNH)4 fragment interacts strongly with the Ph donor orbitals, yielding a stabilized bonding orbital and leaving the Rh-Rh(*) orbital as the HOMO. The unexpectedly low observed increase in binding energy for the d-electrons (XPS data in Table 3) upon going from compounds 2 to compounds 3 is consistent with the relatively small change of electronic structure in the frontier region and with the reversible oxidative electrochemistry (vide supra).

On the other hand, the interaction between the Rh-Rh(*) orbital of the Rh2(O2CCH3)4 fragment and the orbitals of the NO axial donors is not nearly as strong and yields a MO at relatively high energy, the HOMO of system VI. The nature of this orbital is Rh-Rh(*) and Rh-N(), see Figure 9, thus oxidation of this compound may be expected to lead to rupture of the Rh-NO interactions and to the establishment of a Rh-Rh bond. Thus, although systems IV and VI have a closely related electronic structure and very similar bonding parameters, their chemical behavior toward oxidation should be drastically different.

It is also interesting to compare the electronic structure of these two systems with that of the isoelectronic Ir2 6+ complex Ir2(hpp)4Cl2 (shown on the right in Scheme 1). No computational study on this system has been presented as yet, to the best of our knowledge. There is little doubt, however, that the general features of the electronic structure will parallel those presented above for systems IV and VI.

The question of importance is the strength of the  interaction between the axial ligands and the metalmetal framework. For the electronegative and spherically symmetrical Cl atom, the 4 resulting MO (analogous to 86, 91, 105 and 108 for system IV or to 87, 94, 96 and 100 for system VI) would certainly end up at lower energy relative to the block of metal-metal orbitals of  and  symmetry. The stronger relativistic effect for the 5d Ir element should also contribute to a stabilization of the orbitals of M-M  symmetry through a greater involvement of the metal s orbital. In case the second highest of these four orbitals (having M-M  and M-Lax * contributions) is stabilized to the point of falling below the energy of the HOMO, then the molecule will indeed have a formal metal-metal double bond, because the two electrons needed to fill the M-M  orbital originate from a previously filled antibonding orbital of * or * type. If * is located below *, the molecule will display two unpaired electrons as is indeed observed for compound Ir2(hpp)4Cl2.

Conclusion

The ability of a phenyl group to impart unusual stability to rhodium(III) compounds is explained by theoretical analysis of the electronic structure of bis--(phenyl)-tetrakis--(carboxamido)dirhodium(III).

Formally, two phenyl radicals in combination with a rhodium-rhodium bond are transformed into two phenyl-rhodium bonds. The absence of a rhodium-rhodium  bonding interaction is suggested in the lengthening of the rhodium-rhodium distances from those of dirhodium(II,II) and dirhodium(II,III) complexes, as well as from XRD data, and is confirmed by DFT analysis. The stability of the phenylrhodium bonds is seen in the preferential removal of equatorial carboxamidate ligands by trifluoroacetic acid and in the homolytic loss of phenyl radical from 3 at high reaction temperatures. The observed severe distortion of phenyl rings from linearity is suggested to have its origin in long-range interactions between the electron deficient rhodium and the distal oxygen atoms. These structural characteristics are unique among M2 n+ compounds, and they suggest intriguing potential for future structural revelations.

Experimental Section

General: All oxidation reactions were performed in air. NaBPh4 and CuOTf were purchased and used as received. [START_REF] Cotton | Multiple Bonds Between Metal Atoms[END_REF] H NMR (400 MHz) and 13 C NMR (100 MHz) spectra were obtained from solutions in CDCl3. Chemical shifts are reported in parts per million (ppm, δ) downfield from Me4Si (TMS); coupling constants are reported in Hertz (Hz). High resolution mass spectra were done on an ESI instrument. UV-Visible spectra were obtained using a xenon flash lamp. Both cyclic and differential pulse voltammograms were recorded in 0.2 M (n-Bu)4NPF6 solution (CH2Cl2, N2-degassed) on a voltammetric analyzer with a glassy carbon working electrode, a platinum-wire auxiliary electrode, and mL), then filtered. The filtrate was concentrated (ca. 2-3 mL), precipitated in diethyl ether (50 mL), filtered and dried under vacuum. A dark red-violet solid was obtained (117.2 mg, 94.8%), but the bisacetonitrile complex did not form crystals suitable for X-ray analysis. Suitable X-ray crystals (black plate) of the bis(benzonitrile) adduct, 2a', were obtained after an axial ligand exchange (by heating the product 5 min at 60°C in a small amount of benzonitrile) and a layer diffusion of diethyl ether in a 2:1 benzonitrile/CH2Cl2 solution. A satisfactory elemental analysis for this compound could not be obtained, presumably because of the facile axial ligand exchange. NMR silent; HRMS (ESI) calcd. for Rh2C18H27N5O4 (M(MeCN) + ): 583.01731; found: 583.01516. MS (ESI), m/z, 574.1, (100)

[Rh2C17H28N4O5] + = M(MeOH) + ; 542.0, (24) [Rh2C16H24N4O4] + = M + ; 87.0, (100) [BF4] -. UV/Visible spectrum (CH2Cl2), λ (ε, M -1 cm -1 ) : 489 nm (4150); 530 nm (-); 1030 nm (1440). XPS : Rh-3d a 5/2 308.905 eV; Rh-3d b 5/2 310.722 eV; Rh-3d a 3/2 313.514 eV; Rh-3d b 3/2 315.069 eV.

Bis--(phenyl)-tetrakis--(pyrrolidinato)dirhodium(III) (3a).

To a solution MeOH:CH2Cl2 solution (5 mL, 1:1) of 2a (30.5 mg, 42.9 μmol) were added NaBPh4 (73.4 mg, 214.5 μmol) and CuOTf (1.5 mg, 7.1 μmol). The initial purple solution rapidly turned to yellow-green, and the mixture was stirred at room temperature for 18 hours. During this period a yellow-green solid precipitated. Solvents were removed under reduced pressure, MeOH (10 mL) was added to the residue, and the mixture was stirred 5 min to obtain a homogenous suspension. The mixture was filtered, washed with MeOH (2 x 5 mL) and Et2O (10 mL), and dried under vacuum to give a yellow-green product. Yield: 20.6 mg (69%). Compounds 1b, 2b, 2b', 3b, 1c, 2c, and 3c. These compounds were synthesized by the same general procedures described above for compounds 1a, 2a, 2a', and 3a. The details are given as Supporting Information.

Reaction of Bis--(6-methoxynaphthalen-2-yl)-tetrakis--(caprolactamato)dirhodium(III) with

Trifluoroacetic Acid. A solution of trifluoroacetic acid (114 mg, 1.0 mmol) in CH2Cl2 (1 mL) was slowly added (over 5 min) to a solution of bis--(6-methoxynaphthalen-2-yl)-tetrakis--(caprolactamato)dirhodium(III) (52 mg, 53.7 μmol) in anhydrous CH2Cl2 (6 mL). The reaction solution rapidly turned from green to orange. After completed addition, the mixture was stirred at room temperature for 24 hrs. Solvents were then removed under reduced pressure, and the residue was chromatographed on silica gel using Hexanes/EtOAc (10:1) as eluent. The product 2methoxynaphthalene was obtained as white solid (15.5 mg, 91% yield). Mp: 7173 o C; lit. 36 71-73 °C. Reaction of 3c with Trifluoroacetic Acid. In a dry NMR tube were added 3.0 mg (3.7 mol) bis(phenyl)dirhodium(III) caprolactamate (3c) and 0.5 mL CDCl3. After the solid was dissolved and a clear green solution was obtained, 13L (18.5 mol, 5 equiv.) or 26L (37.0 mol, 10 equiv.) or 39L (55.5 mol, 15 equiv.) trifluoroacetic acid solution in CDCl3 (1.35 mmol/mL) was then added. The mixture was rapidly oscillated for a minute and then submitted to 1 H NMR to follow reaction progress. Spectra were taken every 5 minutes for 1 h and then every hour, and data were obtained by using CHCl3 in CDCl3 as an internal standard.

Computational details. All geometry optimizations were performed using the B3LYP threeparameter hybrid density functional method of Becke, 37 as implemented in the Gaussian03 suite of programs. 38 The basis functions consisted of the standard LANL2DZ for all atoms, which included the Hay and Wadt effective core potentials (ECP) 39 for Rh. Unless otherwise stated, the geometry optimizations were carried out without any symmetry constraint and the final geometries were characterized as local minima of the potential energy surface (PES) by verifying that all second derivatives of the energy were positive. The unrestricted formulation was used for the open-shell, mixed valence Rh2 5+ system. The value of <S 2 > at convergence was 0.7579, very close to the expected value

TOC Text

The molecular and electronic structures of bis(phenyl) tetracarboxamidatodirhodium(III) show that these thermally stable compounds do not have any metal-metal bonding. They have hydrolytically stable Rh(III)-Ph bonds and can be reversibly oxidized to Rh2 7+ complexes on the cyclic voltammetric timescale.

Scheme 1 .

 1 Scheme 1.Comparison of two isoelectronic Group 9 paddlewheel dimers.

order 1 , 2 , 3 ]

 123 : Laxial-Rh-Rh [175(2), 174(4), 159(4)°]; Rh-O-C [118(2), 118(2), 126(1)°]; Rh-N-C [122(1), 122(1), 118(1)°]. These distortions take the form of a severe bending of the Rh-Rh-Ph bond axis from 180°, a major opening of the Rh-O-C angle and a less pronounced closing of the Rh-N-C angle from the ideal 120°. The positioning of the rhodium units in the diphenyl-dirhodium structures appears as if two

Figure 1 .

 1 Figure 1. Structures of dirhodium pyrrolidinates (a) 1a, (b) 2a', and (c) 3a. Ellipsoids are shown at the

Figure 2 .

 2 Figure 2. ORTEP views of (a) PhRh(pyr)4RhPh (3a), (b) PhRh(val)4RhPh (3b), and (c) PhRh(cap)4RhPh (3c) showing comparable structural distortions. Ellipsoids are shown at the 30% probability level. Hydrogen atoms and solvent molecules are omitted for clarity. The corresponding Rh-Rh-C bond angles are 164.2° (3a), 157.3° (3b), and 156.2° (3c).

Figure 3 .

 3 Figure 3. Cyclic (CV) and differential pulse voltammetry (DPV) measurements of 3a recorded in 0.2M CH2Cl2 solution of Bu4NPF6 at a scan rate of 0.10 V/s.

Figure 4 .

 4 Figure 4. Reaction of PhRh(cap)4RhPh (3.7 μmol) with (a) 10 and (b) 15 molar equiv. of trifluoroacetic acid in CDCl3 at room temperature.

(Figure 5 .

 5 Figure 5. Two mutually perpendicular views of the geometries, and relative energies, of system IV obtained under three different conditions: (i) constrained optimization in C2h symmetry; (ii) full optimization without symmetry constraints; (iii) idealized geometry with linear Ph-Rh-Rh-Ph moiety.

Figure 6 .

 6 Figure 6. Simplified interaction diagram for system IV, built from the Rh2(OCHNH)4 fragment (V) and two phenyl radicals.

Figure 7 .

 7 Figure 7. MOLDEN contour plots of relevant orbitals for system IV.

Figure 8 .

 8 Figure 8. Simplified interaction diagram for system VI, built from Rh2(O2CCH3)4 and two nitrosyl radicals.

Figure 9 .

 9 Figure 9. Contour plots of relevant orbitals for system VI.

  Suitable crystals (yellow prism) for x-ray analysis were obtained by slow evaporation of dichloromethane from a CH2Cl2 solution. Calcd. for C28H34N4O4Rh2 (Mw = 696.41): C 48.29, H 4.92, N 8.05%. Found: C 48.10, H 4.93, N 7.29%. 1 H NMR (400 MHz, CDCl3): δ 7.33 (m, 4H, o-Ph), 7.1 (m, 6H, m,p-Ph), 3.64 (m, 4H, N-CH2), 3.01 (m, 4H, N-CH2), 2.52 (m, 4H, CH2-C), 2.41 (m, 4H, CH2-C), 1.94 (m, 4H, N-CH2-CH2), 1.82 (m, 4H, N-CH2-CH2). 13 C NMR (100.6 MHz, CDCl3): δ 183.00 (s, NCO), 141.18 (d, 1 JRh-C = 34 Hz),, 136.60 (s, o-Ph), 127.50 (s, m-Ph), 125.21 (s, p-Ph), 53.46 (s, N-CH2), 34.12 (s, CH2-C), 21.82 (s, N-CH2-CH2). UV/Visible spectrum (CH2Cl2) 442 nm (ε = 4492 M - 1 cm -1 ). HRMS (ESI) calcd for Rh2C28H35N4O4 697.07684, found 697.07580 (MH + ). XPS : Rh-3d a 5/2: 309.045 eV; Rh-3d a 3/2: 313.739 eV.

1 H

 1 NMR (400 MHz, CDCl3): δ 7.767.74 (m, 3H), 7.447.39 (m, 1H), 7.347.29 (m, 1H), 7.147.10 (m, 2H), 3.91 (s, 3H).

of 3 / 4 ,

 34 indicating minor spin contamination.TOC GraphicsRemoval of Metal-Metal Bonding in a Dimetallic Paddlewheel Complex -Molecular and Electronic Structure of Bis(phenyl) Dirhodium(III) Carboxamidate CompoundsJoffrey Wolf, † Rinaldo Poli, † § * Jian-Hua Xie, ‡ Jason Nichols, ‡ Bin Xi, + Peter Zavalij ‡ and Michael P.

Table 1 .

 1 Selected

			Bond lengths of dirhodium complexes, Ǻ
		bond	1a	2a' a	3a
	From Rh2(pyr)4	Rh1-Rh2	2.4540(5)	2.4432(4)	2.5738(12)
		Rh2-O1	2.077(2)	2.030(6)	2.093(6)
		Rh2-O2	2.080(2)	2.034(5)	2.095(5)
		Rh1-N1	2.003(3)	1.973(6)	1.993(7)
		Rh1-N2	2.011(3)	1.994(8)	2.012(6)
		Rh-Lax	2.241(2)	2.248(8)	2.037(7)
		bond	1b	2b' b	3b
	From Rh2(val)4	Rh1-Rh2	2.4187(5)	2.3872(3)	2.5162(5)
		Rh2-O1	2.061(2)	2.0143(16)	2.105(3)
		Rh2-O2	2.073(2)	2.0211(16)	2.061(2)
		Rh1-N1	2.023(3)	2.0078(19)	1.997(3)

bond lengths of dirhodium carboxamidates in the (II,II), (II,III), and (III,III) oxidation states.

Table 2 .

 2 Selected bond angles for dirhodium carboxamidates in the (II,II), (II,III), and (III,III) oxidation states.

			Bond angles of dirhodium complexes, °
		angle	1a	2a' a	3a
	From Rh2(pyr)4	Rh1-Rh2-O1	90.04(6)	89.66(16)	82.43(15)
		Rh1-Rh2-O2	90.01(6)	89.94(15)	83.87(14)
		Rh2-Rh1-N1	85.30(7)	85.9(2)	89.32(18)
		Rh2-Rh1-N2	85.35(7)	86.15(18)	90.61(18)
		Rh2-O1-C1	115.16(19)	115.8(5)	121.1(4)
		Rh2-O2-C2	115.9(2)	117.7(5)	122.4(5)
		Rh1-N1-C1	123.40(2)	122.4(5)	118.0(5)
		Rh1-N2-C2	124.00(2)	123.7(5)	118.5(5)
		Rh1-Rh2-Laxial 176.90(7)	178.22(16)	164.20(20)
		angle	1b	2b' b	3b
	From Rh2(val)4	Rh1-Rh2-O1	88.03(7)	89.61(5)	74.58(7)
		Rh1-Rh2-O2	88.85(7)	89.66(4)	85.05(7)
		Rh2-Rh1-N1	87.08(8)	87.11(5)	99.10(8)

Table 3 .

 3 XPS data for dirhodium carboxamidates in the (II,II), (II,III), and (III,III) oxidation states. a

	Compound	Rh-3d5/2, eV	Rh-3d3/2, eV
	Rh2(pyr)4(MeCN)2 (1a)	308.000	312.563
	Rh2(val)4(MeCN)2 (1b)	308.129	312.843
	Rh2(cap)4(MeCN)2 (1c)	308.080	312.812
	Rh2(pyr)4(PhCN)2BF4 (2a')	308.905	313.514
	Rh2(val)4(H2O)2BF4 (2b')	308.841	313.567
	Rh2(cap)4(MeCN)2BF4 (2c)	309.094	313.718
	PhRh(pyr)4RhPh (3a)	309.045	313.739
	PhRh(val)4RhPh (3b)	309.011	313.723
	PhRh(cap)4RhPh (3c)	309.105	313.725
	a Calibrated to carbon 1s at 284.6 eV.

Table 4 .

 4 Electrode potentials (V) from CV measurements and λmax of 3. Reversible anodic wave reported as E1/2, measured with platinum wire/glassy carbon working electrodes against a Ag/AgCl reference electrode in CH2Cl2 with tetra-n-butylammonium hexafluorophosphate as the supporting electrolyte.

	Compound	A (V) a	B (V) b	λmax (nm) c
	3a	0.91	-1.36	442
	3b	0.81	-	429
	3c	0.78	-	430

a b Irreversible cathodic reduction reported as cathodic peak potential (Epc) in parentheses. c Measured in CH2Cl2.

Table 5 .

 5 Selected DFT-optimized geometric parameters (distances in Å, angles in deg) for the model compounds I-III and comparison with the experimental geometries.

	Parameter	System I	System II	System III
			Opt.	Exp. a	Opt.	Exp. b
	(a) Distances
	Rh-Rh	2.454	2.497	2.43(2)	2.481	2.40(3)
	Rh-N(amidato)	2.047	2.039	2.03(2)	2.011	2.00(2)
	Rh-O	2.085	2.112	2.066(4)	2.064	2.025(7)
	Rh-N(acetonitrile)	-	2.286	2.28(5)	2.285	2.28(3)
	(b) Angles
	Rh-Rh-N(amidato)	87.15	86.36	87(1)	86.95	87(1)
	Rh-Rh-O	89.25	88.80	89.3(7)	88.32	89.3(2)
	Rh-Rh-N(acetonitrile) -	176.61	175(2)	177.54	174(4)

Table 6 .

 6 Selected DFT-optimized geometric parameters (distances in Å, angles in deg) for the model compound IV and comparison with the experimental geometries.

	Parameter	System IV	System IV	Exp. a
		(no symmetry) (C2h symmetry)
	(a) Distances
	Rh-Rh	2.644	2.639	2.54(3)
	Rh-N	2.025, 2.053	2.022	2.01(2)
	Rh-O	2.095, 2.133	2.138	2.09(2)
	Rh-C	1.999	1.999	2.01(2)
	(b) Angles
	Rh-Rh-N	90.22, 81.83	90.73	93(1)
	Rh-Rh-O	89.75, 81.50	81.13	81(1)
	Rh-Rh-C	166.92	166.44	159(4)

Acknowledgement. We are grateful to the National Science Foundation (CHE-0456911) for their generous support. We also thank the Fond Social Européen for funding Joffrey Wolf and Bristol-Meyers-Squibb for supporting Jason Nichols with an American Chemical Society Organic Division Fellowship. RP thanks the CICT (Project CALMIP) for granting free computer time.

a Ag/AgCl reference electrode. The ferrocenium/ferrocene couple was observed at 0.42 V (vs. Ag/AgCl) under experimental conditions. All mass spectra were recorded on JEOL AccuTOF-CS from CH2Cl2 solution using MeCN/H2O or MeOH/H2O (9:1) as solvent in presence of 1% of triflic acid.

Elemental analysis were carried out by the Service d'Analyse du Laboratoire de Chimie de Coordination in Toulouse. Dirhodium(II) acetate, carboxamidate ligands, and reagents described in this report were obtained commercially.

Tetrakis-μ-(pyrrolidinato)bis(acetonitrile)dirhodium(II) (1a)

. [START_REF] Lifsey | Rh2(pyr)4(Hpyr)2 and Rh2(val)4(Hval)2 have been prepared and their crystal structures reported: Bear[END_REF] In a dry flask under N2 were mixed Rh2(OAc)4 (500 mg, 1.31 mmol), 2-pyrrolidinone (2.3 g, 27.0 mmol) and anhydrous chlorobenzene (40 mL). The flask was equipped with a Soxhlet apparatus with a thimble containing a 2:1 NaHCO3/sand mixture (5 g). The solution was kept under N2 and stirred at reflux for 24 hours, after which the solution was cooled and solvent was removed under reduced pressure. The residue was washed with diethyl ether (2 x 50 mL) and pentane (50 mL). The purple solid obtained was heated at 80°C in acetonitrile (15 mL), and some methanol was added to dissolve the entire solid. The mixture was crystallized at 20°C to give a pink-red solid. Yield: 565 mg (80%). Suitable x-ray quality crystals (pink-red prisms) were obtained by slow evaporation of diethyl ether from an acetonitrile solution. [START_REF] Cotton | Multiple Bonds Between Metal Atoms[END_REF] H NMR (400 MHz, CD3CN + CD3OD): δ 3.55 (br, N-CH2), 3.44 (m, N-CH2), 2.34 (t, 3 J = 7.6 Hz, 8H, CH2-C), 1.98 (q, 3 J = 7.6 Hz, 8H, N-CH2-CH2). 13 C NMR (100 MHz, CD3CN + CD3OD) : δ 186.89 (s, NCO), 55.41 (s, N-CH2), 33.12 (s, CH2-C), 21.91 (s, N-CH2-CH2). MS (ESI), m/z, (%): 584.01, (100)

[Rh2C18H28N5O4] + = M(MeCN) + ; 625.04, (28) [Rh2C20H31N6O4] = M(MeCN)2 + . UV/visible spectrum (MeCN + MeOH, one drop), λ (ε, M -1 cm -1 ) : 350 nm (224); 505 nm (235). XPS: Rh-3d a 5/2 308.000 eV; Rh-3d b 5/2 309.082 eV; Rh-3d a 3/2 312.563 eV; Rh-3d b 3/2 313.648 eV.

Tetrakis-μ-(pyrrolidinato)bis(acetonitrile)dirhodium(II,III) Tetrafluoroborate (2a).

A solution of NOBF4 (22.3 mg, 191 μmol) in anhydrous acetonitrile (9 mL) was slowly added (over 3 min) to a suspension of 1a (108.5 mg, 174 μmol) in CH2Cl2 (25 mL). The suspension rapidly turned from purple to dark red-purple. After complete addition the mixture was stirred for an additional 30 min at room temperature. Solvents were removed under reduced pressure, and the residue was dissolved in CH2Cl2 ( 5Supporting Information Available. Synthesis and characterization of 2 and 3. Crystallographic data for eight compounds (1-3) in CIF format are available as supporting information. Cartesian coordinates and relative energies of all optimized structures. Energies, composition and contour diagrams of the relevant molecular orbitals for geometry optimized systems I, II, III, IV and VI.