Jean-Baptiste Saunier

Guillaume Losfeld

Rémi Freydier

Claude Grison
email: claude.grison@cefe.cnrs.fr

Trace elements biomonitoring in a historical mining district (les Malines, France)

Keywords:

Impacts from historical Zn-Pb mining in southern France are investigated. High Pb, Cd and As levels due to historic mining were found in soils. Apiary products, especially honey do not show signs of contamination.

Lichen and moss confirm their potential as bioindicators for Zn, Pb and Cd. Pb isotopes allow the identification of pollution sources.

Introduction -TE biomonitoring

Use of apiary products

Honeybees are manageable social insects, which are easily captured to provide material for studies: foragers actively sample their environment in a limited range around the hive (about 7 km 2 ) and are potentially exposed to soil, air and water contamination [START_REF] Conti | Honeybees and their products as potential bioindicators of heavy metals contamination[END_REF]. They have been thought of to be potential biomonitors for various contaminants and have been studied as such for trace elements (TE), radionuclides [START_REF] Tonelli | Honey-bees and their products as indicators of environmental radioactive pollution[END_REF], or pesticides [START_REF] Lambert | Polycyclic aromatic hydrocarbons: bees, honey and pollen as sentinels for environmental chemical contaminants[END_REF]. TE contents can be used to certify the geographical origins of honey [START_REF] Hernandez | Characterization of honey from the Canary Islands: determination of the mineral content by atomic absorption spectrophotometry[END_REF][START_REF] Terrab | Contribution to the study of avocado honeys by their mineral contents using inductively coupled plasma optical emission spectrometry[END_REF] or to assess honey quality [START_REF] Devillers | Chemometrical analysis of 18 metallic and nonmetallic elements found in honeys sold in France[END_REF][START_REF] Pisani | Minor and trace elements in different honey types produced in Siena County (Italy)[END_REF]. Biomonitoring using apiary products has also been extensively investigated [START_REF] Bogdanov | Minerals in honey: environmental geographical and botanical aspects[END_REF][START_REF] Tuzen | Trace element levels in honeys from different regions of Turkey[END_REF][START_REF] Pohl | Determination and fractionation of metals in honey[END_REF] with contradictory outcomes. [START_REF] Tuzen | Trace element levels in honeys from different regions of Turkey[END_REF] suggested a possible use of honey, while according to [START_REF] Bogdanov | Minerals in honey: environmental geographical and botanical aspects[END_REF], low concentrations and high natural variations should deter from using honey for TE environmental monitoring. An improved insight could be obtained using honeybees, pollen, propolis and wax according to [START_REF] Conti | Honeybees and their products as potential bioindicators of heavy metals contamination[END_REF]. Additional data to clarify these views are required and historical mining may provide information on TE contamination pathways.

Use of lichen and moss

The use of lichen, moss or both for monitoring atmospheric contamination has been extensively investigated [START_REF] Markert | The German heavy metal survey by means of mosses[END_REF][START_REF] Harmens | Heavy metal concentrations in European mosses: 2000/2001 survey[END_REF][START_REF] Colin | Pollution atmosphérique par les métaux[END_REF] and proved efficient to assess TE contaminations from historical mining [START_REF] Sondergaard | Pb isotopes as tracers of mining-related Pb in lichens, seaweed and mussels near a former Pb-Zn mine in West Greenland[END_REF]. Some authors use lichen/moss to monitor quick compositional changes related to atmospheric TE contamination [START_REF] Spiro | Lead isotopes in lichen transplants around a Cu smelter in Russia determined by MC-ICP-MS reveal transient records of multiple sources[END_REF], while others hypothesise a continuous TE accumulation [START_REF] Colin | Pollution atmosphérique par les métaux[END_REF]. Lichen in particular was shown to be highly resistant to metal stress [START_REF] Chettri | The effect of Cu, Zn and Pb on the chlorophyll content of the lichens Cladonia convoluta and Cladonia rangiformis[END_REF][START_REF] Sarret | Mechanisms of lichen resistance to metallic pollution[END_REF] and resident lichen/moss species can be used for direct analysis of their TE content in relation to average atmospheric fallouts over time. This approach has been used at different scales [START_REF] Doucet | Atmospheric Pb isotopic composition and trace metal concentration as revealed by epiphytic lichens: an investigation related to two altitudinal sections in Eastern France[END_REF] and particularly for historical mining in southern France [START_REF] Baron | Traçabilité et évolution d'une pollution métallurgique médiévale de plomb argentifère sur le Mont-Lozère[END_REF]. The approach of [START_REF] Baron | Traçabilité et évolution d'une pollution métallurgique médiévale de plomb argentifère sur le Mont-Lozère[END_REF] was adapted here for the les Malines mining district so as to compare apiary products and lichen/moss as potential biomonitors of TE contamination.

Tracing contamination sources

The comparison of results obtained using lichen/moss and apiary products should provide additional insights on the fate of TE in historical mining environments. In addition, Pb isotopic ratios have been determined in the different materials, using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS), in order to track the different sources of lead. Among the four existing Pb isotopes, 208 Pb, 206 Pb and 207 Pb originate from the radioactive decomposition of 238 U, 235 U and 232 Th respectively, while 204 Pb is a non radiogenic isotope, which only represents 1% of the total Pb. As Galena (PbS), the main Pb ore exploited, does not incorporate U and Th, its Pb isotopic composition is frozen from the formation of ore bodies. Subsequently, Pb isotopes are not fractionated in industrial processes, making the identification of sources possible. The technique proved efficient for Pb contaminated lichen/moss in historical mining districts [START_REF] Baron | Traçabilité et évolution d'une pollution métallurgique médiévale de plomb argentifère sur le Mont-Lozère[END_REF][START_REF] Sondergaard | Pb isotopes as tracers of mining-related Pb in lichens, seaweed and mussels near a former Pb-Zn mine in West Greenland[END_REF]. As a result, ICP-MS measurements were selected for our investigations.

Methods and materials

Study area and sampling sites

The study area, located in southern France (Fig. 1) displays drastic contrasts: it is located in a low population density area (<50 people per km 2 ), between a national park (Parc National des Cévennes, established 1970) and a Special Area of Conservation (SAC) of the Natura 2000 network (European Union wide network of nature protection established under the 1992 Habitats Directive). Thus, we could expect potential biomonitors from the area to display mere background TE levels. Yet the region also has a mining history: les Malines, where an estimated 1Mt ore was extracted from 1885 to 1991 remains the largest Zn-Pb exploitation France ever knew [START_REF] Leguen | Lead isotope behavior in a polyphased Pb-Zn ore deposit -les Malines (Cévennes, France)[END_REF]. Former mines indicated on Fig. 2 were listed from national surveys (BRGM, 2012).

Previous studies on the mining district reported high TE levels in soil and the occurrence of several metal hyperaccumulator plant species [START_REF] Grison | Thlaspi caerulescens, an indicator of soil pollution[END_REF]. High TE levels in fish have also been reported elsewhere in the region [START_REF] Monna | Wild brown trout affected by historical mining in the Cevennes national park, France[END_REF], but to date no studies were conducted on apiary products and other biomonitors in the area.

Sample treatment and analysis

General

All reagents used in this study were of high purity: commercial HNO 3 , HCl and H 2 O 2 were obtained from Sigma-Aldrich TraceSE-LECT Ò range. All solutions were prepared using double deionised water (Milli-Q Millipore 18.2 MO cm at 21 °C). For each digestion procedure, blank and standard digestions were carried out the same way and analysed for controls. ICP-MS analyses were per-formed with a Thermo scientific X Series II ICP-MS (Plateforme AETE -HydroSciences/OSU OREME, Montpellier -France) using In and Bi as internal standards. Lead isotope ratios have been determined on the same ICP-MS platform. 206 Pb, 207 Pb and 208 Pb were measured with respective residence times of 20 ms, 20 ms and 15 ms. 10 runs of 100 sweeps were obtained for each analysis. External mass bias correction using NBS 981 was used [START_REF] Aries | Direct determination of lead isotope ratios by laser ablation-inductively coupled plasma-quadrupole mass spectrometry in lake sediment samples[END_REF]. Dead time was determined for lead isotopes [START_REF] Monna | Pb and Sr isotope measurements by inductively coupled plasma-mass spectrometer: efficient time management for precision improvement[END_REF][START_REF] Aries | Direct determination of lead isotope ratios by laser ablation-inductively coupled plasma-quadrupole mass spectrometry in lake sediment samples[END_REF] on our X Series II ICP-MS. A value of 35 ns was found and entered in the Plasma Lab software (Thermo Fischer Scientific) to automatically correct the signal intensities. Lead concentrations in NBS 981 standard were adjusted at 20 ppb, while it ranged from 4 to 15 ppb in the samples. Lead concentrations in the samples and standard NBS 981 gave a maximum signal intensity of 10 6 on 208 Pb.

Apiary products

Hives locations are indicated on Fig. 2: two apiaries containing 5 and 6 hives respectively were sampled in Saint-Laurent-le- Minier (les Avinières) and Saint-Bresson. Harvest times are indicated in Table 4. Honey was extracted using stainless steel collectors and kept in generic clean glass jars used by beekeepers for honey retail: processing was assessed in order to ensure that no TE contamination resulted from jars or their lids. Royal jelly and beeswax were taken directly from the hives and stored in clean polyethylene jars. As for the bees, dead individuals from a recently collapsed colony were collected both from inside and outside the hive and dried before digestion.

An open vessel wet-digestion, which proved efficient to recover on average 93% of honey TE content was used after [START_REF] Tuzen | Trace element levels in honeys from different regions of Turkey[END_REF]: samples were warmed to 30 °C and homogenised before 3 g were taken for analysis. 12 mL of an oxi-acidic mixture (2:1 HNO 3 :H 2 O 2 ) were added per gram sample and the resulting solution was heated at 100 °C until dry. Dry residues were solubilised in HNO 3 (2.5% v:v) for analysis. WEPAL IPE 110 (yellow clover) obtained from LGC standards, was used as a reference material (Table 1) as no certified reference materials are available for honey.

Standard additions also confirmed the result obtained for apiary products. Blank TE levels represented less than 1% of the levels measured in samples of interest.

Lichen and moss

Lichen and moss were collected using Teflon spatulas and clean polyethylene jars, where they were abundant so as to average TE levels over the area around beehives. Different species of lichen (Cladonia rangiformis or Parmelia acetabulum) or moss (Scleropodium purum and Dicranum scoparium) were kept separate (Table 6). Samples were treated according to the guidelines by the European Commission, Standards, Measurements and Testing Programme [START_REF] Quevauviller | Certified reference material of lichen (CRM 482) for the quality control of trace element biomonitoring[END_REF]: samples were air dried and all adhering particles manually removed before crushing in an agate mortar. The powder obtained was then treated according to the procedure described for apiary products. WEPAL IPE 110 was once again used for quality control (Table 1) as well as inter-laboratory certification samples (fodder). For lichen/moss blank TE levels represented less than 0.5% of the levels measured in samples of interest.

Soil digestion

Samples were taken from horizon A using Teflon spatulas and clean polyethylene jars. S1-S3 (Fig. 2 -Table 3) are not actually soil but mining waste and tailings; S4, S6 and S7 are a mixture of mineral soil and humus found around beehives (S4-S7) or pastoral areas in Saint-Laurent-le-Minier (S6); S5 and S8 are a mixture of mineral soil and waste expelled from beehives by bees. Samples were treated according to a house procedure: dried at 60 °C until weight was stable, sieved to 2 mm and homogenised. Approximately 120 mg of sample were digested following the procedure: (i) 4 mL H 2 O 2 (ii) 4 mL HNO 3 (iii) 4 mL HNO 3 :HCl (1:3). In each step, the resulting solution was heated at 100 °C until dry. Dry residues obtained after step (iii) were solubilised in HNO 3 (2.5% v:v) for analysis. This procedure was certified by [START_REF] Grison | Thlaspi caerulescens, an indicator of soil pollution[END_REF] using BIPEA soil sample 402 (interlaboratory certification) as a reference material. For soil, blank TE levels represented only 0.1% of the levels in samples analysed.

Results and discussion

Soil TE content

In a risk-based approach, French authorities issued guideline values for contaminated land management: Fixed Impact Values (FIVs) based on toxicity studies consider the chronic risks to public health related to the actual land use. 'Sensitive use' values were selected here (Table 2) due to the occurrence of human activities (housing, beekeeping.). These values were defined to be used in a specific scoring framework, but may also provide information on the level of concern for a specific site [START_REF] Carlon | Derivation of soil screening values in Europe[END_REF].

S1 and S2 taken from the les Avinières mine spoils and tailing ponds show the highest TE contents (Table 3), with values for Zn, As, Cd, Tl and Pb largely above FIVs. S3 from the les Malines tailing ponds is very similar, except for its lower Tl content, and also represents a potential source of contamination. Regarding other samples, they are all below FIVs for Zn and Cd meaning that Zn or Cd contents of samples S4 through S8 should not be considered as potential threats to public health. Values for As on the other hand all exceed FIVs with an average 3.4-fold FIV for As: in this case, potential toxicity and high concentrations should be taken seriously. Tl and Pb are more contrasted with a few values above FIVs. However, there is a clear trend of decreasing Tl and Pb levels as the distance from mines increases: such a phenomenon is very likely to be re-lated to dust transport by winds, an effect previously reported elsewhere [START_REF] Sondergaard | Pb isotopes as tracers of mining-related Pb in lichens, seaweed and mussels near a former Pb-Zn mine in West Greenland[END_REF].

Lead exposure was reported in the area, on children known to have used tailing ponds as a playground, and eaten locally grown vegetables. This highlights the crucial need for further investigation in contaminants linkages as water and wind erosion may result in widespread TE contamination: Pb levels above 1000 mg kg À1 were measured 4.5 km downstream the Vis river in this case [START_REF] Escarre | Heavy metal concentration survey in soils and plants of the les malines mining district (Southern France): implications for soil restoration[END_REF]. Potential impacts on human health, activities and livelihoods could be serious [START_REF] Conesa | The Cartagena -La Union mining district (SE spain): a review of environmental problems and emerging phytoremediation solutions after fifteen years research[END_REF]: precautionary measures such as restrictions on the use of local wells for water supply were set up, and restrictions on the trade of locally produced vegetables are currently enforced. To date, the potential impacts of historical mining on beekeeping have not been investigated.

Elemental contents of potential bioindicators

Honey (Table 4)

Ca, Mg and P with average levels (±standard deviation) at 175 ± 74.1 mg kg À1 , 97.7 ± 44.9 mg kg À1 and 72.4 ± 20.6 mg kg À1 are the main cations present in honey. Mn and Zn are next with average levels at 7.57 ± 3.69 mg kg À1 , and 0.571 ± 0.440 mg kg À1 . This is in accord with the trends highlighted by [START_REF] Devillers | Chemometrical analysis of 18 metallic and nonmetallic elements found in honeys sold in France[END_REF]: Ca, Mg, P and Mn are known for their physiological role and their presence in honey is of lesser concern for risk assessment.

Higher Zn contents of statistical significance (Wilcoxon test, z = À2.460, P = 0.0139) could be measured in honey samples H1 and H3-5 collected in the vicinity of mine dumps (<250 m) compared to H2, H6-9 collected further away (>1500 m). However Zn content in honey from the region appears to be rather low compared to literature data: the highest Zn content previously reported in honey was 113 mg kg À1 in a single sample [START_REF] Pohl | Determination and fractionation of metals in honey[END_REF] and an average 29.4 mg kg À1 related to car exhausts in Italy [START_REF] Leita | Investigation of the use of honey bees and honey bee products to assess heavy metals contamination[END_REF]. However, usual means for Zn content in honey range from 1.3 mg kg À1 [START_REF] Devillers | Chemometrical analysis of 18 metallic and nonmetallic elements found in honeys sold in France[END_REF] to 7.76 mg kg À1 [START_REF] Przybylowski | Honey as an environmental marker[END_REF]. Background levels of Zn in honey from the area (H8, H9) are particularly low: this is probably related to floral composition and beekeepers practices. The influence of floral composition was previously emphasised as a major factor [START_REF] Bogdanov | Minerals in honey: environmental geographical and botanical aspects[END_REF] and the use of galvanised containers frequently reported as a possible source of Zn [START_REF] Gonzalez-Paramas | Geographical discrimination of honeys by using mineral composition and common chemical quality parameters[END_REF]. Galvanised containers were not used, and it appears from our measurements that historical Zn mining has no worrying influence on the Zn content of honey.

The presence of other TE could be related to historical mining, namely Pb (26 ± 20 lg kg À1 ), Tl (13 ± 10 lg kg À1 ), Cd (7 ± 6 lg kg À1 ) and As (3 ± 4 lg kg À1 ). H3 harvested in the vicinity ) levels, which correlates with soil analyses. As regards Pb, Tl, Cd and As contents, no statistically significant differences could be related to the vicinity of historic mines (Wilcoxon test, P > 0.05) and sources of variability other than the presence or absence of mining waste are to be sought. Besides, Pb, Cd and As levels observed here are all within concentration ranges previously described according to the literature [START_REF] Pohl | Determination and fractionation of metals in honey[END_REF] and thus, honey consumption should be safe. H8 and H9 (background) actually show the lowest As and Cd levels but an exception occurs for Tl with a high level in H8: an increased attention should be given to this element, which was considered for the first time in this study.

The absence of effect of historical mines was reported in other contexts [START_REF] Iskander | Assessment of trace elements in honey produced on uranium mining reclaimed land[END_REF], and our measurements confirm the views expressed by [START_REF] Conti | Honeybees and their products as potential bioindicators of heavy metals contamination[END_REF]: using solely honey cannot provide quantitative information on environmental TE levels. Reciprocally, honey production in a historical mining environment appears to be safe and may support livelihoods.

Royal-jelly and beeswax (Table 4)

Ca, P, Mg, Zn and Mn are the most abundant minerals found in royal jelly and they are those necessary for larvae development. This observation is consistent with previous studies and concentrations of TE in royal jelly are of the same order of magnitude as those previously reported outside mining districts [START_REF] Stocker | Trace and mineral elements in royal jelly and homeostatic effects[END_REF]. With regards to Zn, Cd and Pb, [START_REF] Leita | Investigation of the use of honey bees and honey bee products to assess heavy metals contamination[END_REF] reported a higher content in royal jelly compared to the corresponding honey. This is not consistent with homeostasis observations by [START_REF] Stocker | Trace and mineral elements in royal jelly and homeostatic effects[END_REF] and in our case Zn, Cd and Pb levels were comparable in royal-jelly (RJ) and the corresponding honey (H3). Once again no impacts of mining could be inferred from our measurements. The same trend was observed for beeswax with low Pb, Cd and Tl content. The very low mineral content of beeswax should be noticed and detract from using it for TE monitoring. Our measurements and homeostasis emphasised by [START_REF] Stocker | Trace and mineral elements in royal jelly and homeostatic effects[END_REF] also show that royal jelly does not provide additional information compared to solely using honey.

Honeybees (Table 4)

Most abundant elements in honeybees are again P, Mg, Ca, Zn and Mn of physiological origin. Table 5 provides a review of previ-ous TE measurements on honeybee tissues: according to these data, Cd levels only are to be considered as high among our samples. Tl for which no comparisons are possible yet, could also endanger bees' health due to its high toxicity. Beekeepers appeared to be worried of the possible impact of TE originating from historical mines on bee colonies: the fact that dead bees collected inside (BH) and outside the hive (B) after a colony collapse show similar TE contents, suggests that climate disturbances affecting food availability could be the major factor. This was previously emphasised, particularly in the Mediterranean regions [START_REF] Dixon | Pollination and restoration[END_REF]. It also appears to be difficult to relate qualitative observations on bees to the presence of historical mines and the presence of noxious TE except for Tl and Cd, which could require further investigations. According to such observations, honeybees cannot be used for biomonitoring purposes.

Lichen/moss (Table 6)

Samples showed no visible signs of intoxication due to the presence of metals and levels of major elements such as Mg, P, Ca and Mn are within usual ranges [START_REF] Szczepaniak | Aspects of the biomonitoring studies using mosses and lichens as indicators of metal pollution[END_REF]. The decrease in the main nutrients (Mg, Ca and Mn) near the contamination source is nonetheless a sign of depressed health. In the case of C. rangiformis potentially toxic elements show different behaviours: Cr, As and Tl concentrations increase with the distance from mine, while Cu, Zn, Cd, Sb and Pb concentrations decrease. Zn, Pb and Cd are particularly remarkable as concentrations decrease by 1 or 2 orders of magnitude from 0 to 250 m. Similar trends were reported for species C. rangiformis in Turkey [START_REF] Cayir | Determination of atmospheric heavy metal pollution in Canakkale and Balikesir provinces using lichen (Cladonia rangiformis) as a bioindicator[END_REF] but concentrations remain lower: namely 30.5 ± 9.15 mg kg À1 for Zn, 6.95 ± 9.63 mg kg À1 for Pb and 0.31 ± 0.14 mg kg À1 for Cd. For As and Tl our measurements are the first to be reported and cannot be compared.

As a general rule, samples from les Avinières, terricolous lichen (L1, L2) show higher TE contents than corticolous lichen (L3): this is clearly related to soil, an effect previously emphasised by [START_REF] Sondergaard | Pb isotopes as tracers of mining-related Pb in lichens, seaweed and mussels near a former Pb-Zn mine in West Greenland[END_REF]. In the case of L3, where low direct influence of soil is expected, high TE levels are still found, which probably reflects atmospheric dust deposition. For samples originating from St-Bresson, TE contents are generally lower, which is consistent with data on soil there. Negative correlations between TE levels and distance from historical mines can be observed, and Zn, Cd, Tl and Pb correlation patterns in lichen/moss are similar to those Fig. 3. Lead isotopic data for soil, lichen and moss (this study) and urban emission [START_REF] Monna | Pb isotopic composition of airborne particulate material from France and the southern United Kingdom: Implications for Pb pollution sources in urban areas[END_REF]. observed in soil. This shows a very different response compared to apiary products and honey in particular: lichen and moss are contaminated through dust fixation after transport by wind rather than selective bioaccumulation. In addition to elemental content determinations, Pb isotopes determination may improve sources characterisation.

Pb isotopic data (Table 7)

Ores from les Malines show a very homogeneous Pb isotopic composition, which allows precise characterisation [START_REF] Leguen | Lead isotope behavior in a polyphased Pb-Zn ore deposit -les Malines (Cévennes, France)[END_REF]. Fig. 3 summarises our results on the determination of Pb isotopes in soil and lichen/moss samples taken from les Malines. Literature data on the isotopic composition of background Pb from local granite is also included, along with data on Pb originating from general anthropogenic emissions from the area [START_REF] Monna | Pb isotopic composition of airborne particulate material from France and the southern United Kingdom: Implications for Pb pollution sources in urban areas[END_REF] [START_REF] Ellam | The graphical presentation of lead isotope data for environmental source apportionment[END_REF], they allow separating definite regions, and identifying potential sources as indicated on Fig. 3.

All soil considered except S6 show the isotopic signature of Pb from les Malines ores. This observation associated with data on soil elemental contents strongly suggests, that Pb contamination actually originates from mining and is not the result of a local natural peculiarity. The case of S6 is the result of a combination of a major source, Pb originating from historical mining, and a minor source, probably related to emissions from surrounding larger cities. In the case of lichen/moss, samples L1 and L2 show the signature of Pb ores from les Malines, confirming the influence of mining waste on these samples. Other samples (L3, L4 and M1) also show a combination of two sources, historical mining as a major component and urban sources as a minor component. Even 20 years after closure, the influence of mining on the environment is still obvious, with deposition of contaminated dust in the surroundings.

Conclusion

As regards apiary products, the results presented here show, that they should not be used for TE environmental monitoring: the les Malines mining district presents an obvious problem of TE contamination in soil, but no clear relationship between soil contamination and apiary products TE contents could be emphasised from our data except for Zn. However, honey in particular is not affected by the high environmental TE levels: TE levels are comparable to those observed in uncontaminated areas and beekeeping remains a safe activity, which may support livelihoods.

Lichen/moss on the other hand confirmed their potential to average atmospheric deposits over time: no selective bioaccumulation could be determined from our measurements as lichen/moss elemental content showed similar TE distributions as soil. Pb isotopes determination proved efficient to discriminate sources of Pb in lichen/moss: historical mining is a major source of Pb in atmospheric deposits over the area, and dust transport by wind appeared as a major factor. Other minor anthropogenic sources could also be identified, but their intensity was lower than historical mining. Thus, we would recommend further investigations on dusts transport in the area as it appears to have a critical role in TE diffusion. Lichen/moss could be used as an efficient tool for such purposes.

In addition to data on TE levels in potential biomonitors, isotopic distribution for Pb may provide increased information: it could prove particularly useful in cases where the polluter pays principle should apply.

Fig. 1 .

 1 Fig. 1. Context of the 'Les Malines' mining district.

Fig. 2 .

 2 Fig. 2. Detailed map of the sampling area within the les Malines mining district.

Table 1

 1 Quality control for apiary products and lichen/moss analysis (values in mg kg À1 ).

		Mg	P	Ca	Cr	Mn	Co	Ni
	Mean (n = 5)	1372	3311	4752	0.477	17.8	0.072	0.557
	Standard-deviation	50	131	66	0.06	0.18	0.003	0.071
	Certified values (WEPAL IPE 110)	1440	3250	4920	0.493	17.9	0.074	0.554
	% Recovery	95	102	97	97	99	97	101
		Cu	Zn	As	Cd	Sb	Ba	Pb
	Mean (n = 5)	7.191	32.165	0.083	0.124	0.037	4.910	1.158
	Standard-deviation	0.1	0.7	0.002	0.001	0.002	0.03	0.05
	Certified values (WEPAL IPE 110)	7.35	32.7	0.0911	0.126	0.0354	5.48	1.15
	% Recovery	98	98	91	98	104	90	101

Table 2 French

 2

	Element	Zn	As	Cd	Tl	Pb
	FIVs	9000	37	20	10	400

guide values for soils TE content ('sensitive use' mg kg À1 dry weight).

Table 4

 4 Honey (H), royal-Jelly (RJ), beeswax (W) (fresh weight) and honeybees (BH, B) (dry weight) elemental content (mg kg À1 ).

	Sample	Location	Distance from mine a (m)	Season	Mg	P	Ca	Mn	Zn	As	Cd	Sb	Tl	Pb
	H1	Les Avinières	250	04.2011	80.5	40.7	187	5.81	0.806	0.001	0.009	0.002	0.006	0.003
	H2	St-Bresson	1500	05.2011	122	78.4	204	9.17	0.556	0.001	0.004	0.001	0.001	0.035
	H3	Les Avinières	250	05.2011	125	115	219	6.77	1.4	0.003	0.022	0.003	0.037	0.101
	H4	Les Avinières	250	06.2011	44.8	65.6	76.7	3.22	0.905	0.001	0.003	0.001	0.006	0.011
	H5	Les Avinières	250	07.2011	163	66.6	277	10.5	0.613	0.001	0.001	0.001	0.012	0.009
	H6	St-Bresson	1500	06.2011	101	80.1	200	7.86	0.429	0.008	0.006	0.002	0.013	0.005
	H7	St-Bresson	1500	07.2011	66	82.3	168	10.6	0.429	0.002	0.003	0.001	0.019	0.025
	H8	Aulas	4000 b	2011	32.8	59.4	36.4	1.38	<dl	<dl	0.001	<dl	0.012	0.006
	H9	Majencoule	7500 c	2011	145	62.8	206	12.8	<dl	0.001	0.001	0.002	0.003	0.014
	RJ	Les Avinières	250	05.2011	33.8	61.7	123	0.304	0.906	0.005	0.007	0.002	<dl	0.168
	W	Les Avinières	250	05.2011	167	78.8	305	16.1	1.520	0.012	0.006	0.001	0.013	<dl
	BH	Les Avinières	250	05.2011	2 047	13 050	1 857	93.1	167	0.056	2.9	0.026	0.133	1.44
	B	Les Avinières	250	05.2011	2 286	14 290	1 783	80.7	172	0.059	2.5	0.025	0.149	0.832
	<dl = Value below detection limit.

a Distance from the closest mine, which is les Avinières for samples collected in Saint-Laurent-le-Minier and Font-Bouillens for samples collected in Saint-Bresson. b Distance from the mine located in Bez-et-Esparron. c Distance from the les Jumeaux mine in Sumène.

Table 3

 3 Soil elemental contents (mg kg À1 dry weight). Distance from the closest mine, which is les Avinières for samples collected in Saint-Laurent-le-Minier and Font-Bouillens for samples collected in Saint-Bresson.

	Sample	Type	Location	Distance from mine a (m)	Zn	As	Cd	Tl	Pb
	S1	Mine spoils	Les Avinières	0	86 706 b	4 247 b	408 b	317 b	66 388 b
	S2	Mine tailings	Les Avinières	0	125 456 b	734 b	1 605 b	37.1 b	88 472 b
	S3	Mine tailings	Les Malines	0	56 999 b	103 b	386 b	6.45	3 686 b
	S4	Soil around beehives	Les Avinières	250	3 684	147 b	8.79	16.1 b	1 234 b
	S5	Soil under beehives	Les Avinières	250	2 513	125 b	7.47	9.5	858 b
	S6	Soil	St-Laurent	500	2 879	75 b	8.26	3.6	1 787 b
	S7	Soil around beehives	St-Bresson	1500	50.8	159 b	<dl	4.8	379
	S8	Soil under beehives	St-Bresson	1500	202	127 b	<dl	2.6	369
	na = Not available; <dl = value below detection limit.

a b Values above FIVs.

Table 5

 5 Maximum and minimum TE levels previously reported in honeybees (mg kg À1 dry weight).

	Element	Min	Source	Max	Source
	Cr	0.000052	Conti and Botre (2001)	6.1	Kump et al. (1996)
	Mn	n.a		59	Kump et al. (1996)
	Ni	0.3	Balestra et al. (1992)	2.1	Kump et al. (1996)
	Cu	n.a		32.1	Kump et al. (1996)
	Zn	200	Kump et al. (1996)	1100	Leita et al. (1996)
	Cd	0.00287	Conti and Botre (2001)	7	Leita et al. (1996)
	Pb	0.00052	Conti and Botre (2001)	27	Leita et al. (1996)

Table 6

 6 Lichen and moss elemental content (mg kg À1 dry weight). Distance from the closest mine, which is les Avinières for samples collected in Saint-Laurent-le-Minier and Font-Bouillens for samples collected in Saint-Bresson.

	Sample Species	Location Distance from	M g	P	C a	C r	M n	C u	Z n	A s	C d	S b	T l	P b
				mine a (m)
	L1	Cladonia rangiformis	Les	0	1178	698	3855 1.18	22.7	8.01 1041	5	15.1	3.51	0.317 614
			Avinières
	L2	Cladonia rangiformis	Les	250	3321	548 10032 3.15	97.9	5.34	286	9.28	0.628 1.16	0.902	72.6
			Avinières
	L3	Parmelia acetabulum	Les	250	2015 1419 25342 2.47	60.9 17.9	107	1.36	0.42	0.308 1.46	33.5
			Avinières
	L4	Parmelia acetabulum	St-	1500	4238 1910 36349 4.29 382	14.8	70.6 11.4	0.201 1.23	1.08	36.3
			Bresson
	M1	Scleropodium	St-	1500	3946 2847 12127 1.11 265	11.8	37.2	1.85	0.002 0.557 2.07	13.1
		purum + Dicranum	Bresson
		scoparium

a

Table 7

 7 Pb isotope ratios. Avinières shows the highest Pb (101 lg kg À1 ), Tl (37 lg kg À1 ) and Cd (22 lg kg À1

	Sample	208 Pb/ 206 Pb	206 Pb/ 207 Pb
	S1	2.0892	1.1723
	S2	2.0902	1.1751
	S3	2.0921	1.1751
	S4	2.1065	1.1548
	S5	2.0862	1.1710
	S6	2.0872	1.1696
	L1	2.0947	1.1696
	L2	2.0971	1.1696
	L3	2.1011	1.1615
	L4	2.1010	1.1588
	M1	2.1035	1.1522

  . The range of Pb isotopic signatures is 206 Pb/ 207 Pb 1.1008-1.1342 and 208 Pb/ 206 Pb 2.1236-2.1603 for general urban emissions, 206 Pb/ 207 Pb 1.1674-1.1857 and 208 Pb/ 206 Pb 2.0741-2.0978 for ores from les Malines. Although such graphs may result from mixing more than 2 sources of Pb

Acknowledgements

The authors gratefully acknowledge the Centre National de la Recherche Scientifique (CNRS) and the ANR (11ECOT01101) for fundings. We would also like to express particular thanks to the Ecole Polytechnique, Paris Tech for a PhD studentship. Special thanks go to Raphaelle Leclerc (CEFE UMR 5175 platform of chemical analyses -LabEx CeMEB -Mediterranean Center For Environment and Biodiversity) for her support in sample treatment. Finally, we would like to thank Daniel Favas, a beekeeper from Saint-Laurent-le-Minier, France and member of the local honeybees health preservation group (GDSA, Gard) for his active involvement in sample supply.


Pavel A Dub

Rinaldo Poli
email: poli@lcc-toulouse.fr

The Pt-catalyzed ethylene hydroamination by aniline: a Computational Investigation of the Catalytic Cycle

Keywords: platinum, homogeneous catalysis, hydroamination, non-activated olefins, DFT calculations

-electron) Pt IV -H intermediate, and final reductive elimination of the PhNHEt product. Several low energy ethylene complexes, namely trans-and cis-PtBr2(C2H4)(PhNH2) (IV and V) and trans-and cis-PtBr2(C2H4)2 (VII and VIII) are susceptible to aniline nucleophilic addition to generate zwitterionic intermediates. However, only [PtBr3CH2CH2NH2Ph] -(IX) derived from PhNH2 addition to II is the productive intermediate. It easily transfers a proton to the Pt atom to yield [PtHBr3(CH2CH2NHPh)] -(XX), which leads to rate-determining C-H reductive elimination through transition state TS(XX-L) with formation of the -complex [PtBr3(k 2 :C,H-HCH2CH2NHPh)] -(L), from which the product can be liberated via ligand substitution by a new C2H4 molecule to regenerate II. Saturated (18-electron) Pt IV hydride complexes obtained by ligand addition or by chelation of the aminoalkyl ligand liberate the product through higher energy pathways. Other pathways starting from the zwitterionic intermediates were also explored (intermolecular N deprotonation followed by C protonation or chelation to produce platina(II)azacyclobutane derivatives; intramolecular proton transfer from N to C, either direct or assisted by an external aniline molecule) but all gave higher-energy intermediates or led to the same rate determining TS(XX-L).

Introduction

Hydroamination, the direct formation of a new C-N bond by addition of an N-H bond across an unsaturated CC bond, currently attracts much interest in academia and industry. [1][START_REF] Brunet | Catalytic Heterofunctionalization[END_REF][START_REF] Müller | [END_REF] The intermolecular version of this process is still a great challenge, especially for non-activated olefins. Seminal work by Coulson showed that ethylene could be hydroaminated by a few highly basic secondary amines under forcing conditions with RhCl3 (or IrCl3) as catalyst. 4,5 More recently, this system was found effective also for less basic amines such as aniline when modified by the addition of n-Bu4PI/I2. 6 Other relevant results for the intermolecular hydroamination of ethylene and other non activated olefins comprise the use of lanthanides, 7,8 Fe, 9 Ru, [10][11][12] Rh, 13 Ag, 14 Au, [15][16][17][18] Pd, 19,20 and notably Pt. [21][22][23] Investigations initiated in our team by J.-J. Brunet have shown that PtBr2, in the presence of nBu4PX (X = halide) as activator, is one of the most performing catalyst so far reported for the hydroamination of ethylene by weakly basic amines such as aniline and 2-chloroaniline (highest activity for X = Br; TON > 150 after 10 h at 150°C with 0.3 mol % of Pt-precursor). [24][25][26][27] Without a clear mechanistic understanding, however, it is difficult to imagine how to further improve the process efficiency for its potential application in bulk chemical manufacture.

Two alternative mechanisms are discussed in the literature, one starting with amine activation by N-H oxidative addition and the other one based on amine nucleophilic addition to a coordinated olefin. The amine activation mechanism is mostly proposed for Rh-or Ir-based catalytic systems, 28,29 whereas the olefin activation mechanism seems adopted by catalysts based on group 10 30 and 11 metals. 31 Senn and coworkers reported a computational study of the model NH3 addition to ethylene catalyzed by the [MCl(PH3)(C2H4)] z+ complexes of Group 9 (z = 0) and 10 (z = 1) metals. 32 For the group 10 metals, for which only the olefin activation pathway has been explored, they found that the NH3 nucleophilic addition is thermodynamically and kinetically favourable and that the cleavage step is rate-determining (barrier of 34.9 kcal mol -1 for Pt). On the other hand, Tsipis and Kefalidis, using the "Pt 0 " model complex Pt(C2H4)(PH3), explored only the amine activation pathway, finding the reaction to be limited by the product reductive elimination step from the Pt II amido hydrido intermediate (barrier of 39.7 kcal mol -1 ). 33 Other computational studies (e.g. on gold catalysis for diene hydroamination 31 or palladium catalysis for the intermolecular hydroamination of vinylarenes 34 and for the asymmetric intramolecular hydroamination of aminoalkenes 35 ) have also explored solely the olefin activation mechanism. To the best of our knowledge, with the exception of the above mentioned study by Tsipis and Kefalidis and a study on iridium reported only in a Ph.D. thesis, [START_REF] Senn | Transition-metal-catalysed hydroamination of alkenes: theoretical studies using ab initio molecular dynamics[END_REF] studies of the N-H activation pathway have only been reported for alkaline-earths, [START_REF] Barrett | [END_REF] early transition metals [38][39][40][41] and the lanthanides. [42][43][44][45][46][47][48] On the basis of known chemical transformations for related systems and on conventional wisdom, the Brunet Pt-based system was proposed to follow the ethylene activation pathway as shown in Scheme 1. 26 However, whether the proton transfer from N to C from the zwitterionic intermediate occurs directly or via a Pt-hydride intermediate remained open to debate. The proton transfer process was considered as more facile from the anionic tribromo species [PtBr3(CH2CH2NH2Ph)] -because of the anticipated increased basicity, a hypothesis consistent with the observed activity enhancement when using a moderate excess amount of bromide salts. Scheme 1. Mechanism of PtBr2-catalyzed ethylene hydroamination by aniline as previously proposed. 26 We have recently presented the synthesis, characterization, and equilibrium studies, backed up by DFT calculations, of a few Pt(II) complexes that are likely to be implicated in this PtBr2catalyzed process either as intermediates or as off-loop equilibrium species. 49 Starting from K2PtCl4 or KPtCl3(C2H4)•H2O (Zeise's salt), compounds (nBu4P)2[PtBr4] (1), (nBu4P)2 [Pt2Br6] (1'), (nBu4P)[PtBr3(C2H4)] ( 2), (nBu4P)[PtBr3(PhNH2)] (3), trans-[PtBr2(C2H4)(PhNH2)] (4), cis-[PtBr2(C2H4)(PhNH2)] ( 5), and cis-[PtBr2(PhNH2)2] (6) have been isolated under different conditions in the presence of ethylene and aniline and fully characterized. The relative free energy of all species in solution under catalytic conditions has also been assessed from combined experimental and computational studies, showing that 2 is the most stable complex under catalytic conditions. The bis(ethylene) complexes trans-and cis-[PtBr2(C2H4)2] (7 and 8) were not isolated (analogous [PtCl2(C2H4)2] complexes have previously been described [50][51][52] ), but the calculations show their energetic accessibility under catalytic conditions. Therefore, all abovementioned Pt II -ethylene species are qualified candidates for the nucleophilic attack by aniline and need to be considered for the subsequent steps of the catalytic cycle because they are all present in equilibrium concentrations according to their relative free energy. On the basis of these studies, the initial part of the catalytic cycle has been revised as shown in Scheme 2. 49 Scheme 2. Revised initial part of the PtBr2-catalyzed ethylene hydroamination mechanism. 49 Attempts to experimentally observe zwitterionic intermediates failed, no reaction between complexes 2, 4 or 5 and aniline (other than ligand exchange for 2 and aniline self-exchange for 4) taking place under conditions suitable to NMR monitoring, which suggests either a high relative energy for these zwitterionic complexes or a high nucleophilic addition barrier. 49 Since the identity of the proposed intermediates after nucleophilic addition could not be verified experimentally, we have recurred to the computational method to elucidate the remaining mechanistic details of this catalytic cycle. The above-mentioned computational investigation using complex [PtCl(PH3)2] + as a model for the catalyst and NH3 for the amine 32 cannot be extrapolated to the Brunet catalytic system, because the nature of the amine (in particularly its basicity) plays a determining role in the nucleophilic addition energetics and because the Brunet system was shown to be activated by excess bromide, which would be incompatible with the presence of electronically unsaturated cationic species, and to be poisoned by the presence of 2 equivalents of a phosphine ligand. 24,[START_REF] Chu | Hydroamination intermoléculaire des liaisons multiples carbone-carbone dans les solvants ioniques[END_REF] Therefore, our computational investigation was accomplished on the real catalytic system. As mentioned above, the PtX2/X -system was found most active for X = Br. Therefore, the calculations are restricted to this halide. An investigation of how the energetics of the catalytic cycle is affected by the nature of X is planned for the near future.

+CH 2 =CH 2 -Br - +ArNH 2 -Br - [PtBr 4 ] 2- [PtBr 3 (C 2 H 4 )] - trans-[PtBr 2 (ArNH 2 )(C 2 H 4 )] +ArNH 2 -Br - [PtBr 3 (ArNH 2 )] - +CH 2 =CH 2 -Br - cis-[PtBr 2 (ArNH 2 )(C 2 H 4 )] +Br - -ArNH 2 +C 2 H 4

Computational Details

All geometry optimizations were carried out by the DFT approach with the Gaussian03 suite of programs [START_REF] Frisch | Gaussian 03, Revision C.02[END_REF] using the B3LYP functional. [START_REF] Becke | [END_REF][56][57] In spite of a few problems pointed out for this functional, 58 it was selected because it remains one of the most popular functionals used in transition metal computational chemistry. One of the apparent problems is an overestimation of spin polarization, but all molecules described in this work are diamagnetic with the paramagnetic excited states expected at much higher energy, thus the RHF description is appropriate. All atoms except Pt were described by the standard 6-31+G* basis set, which includes both polarization and diffuse functions that are necessary to allow angular and radial flexibility to the highly anionic systems. The Pt atom was described by the LANL2TZ(f) basis, which is an uncontracted version of LANL2DZ and includes an f polarization function and an ECP. 59 The geometry optimizations were carried out on isolated molecules in the gas phase. Frequency calculations were carried out for all optimized geometries in order to verify their nature as local minima, for the calculation of thermodynamic parameters at 298.15 K and at 423.15 K under the ideal gas and harmonic approximations, and for the identification of all transition states (one imaginary frequency). Solvent effects were introduced by means of C-PCM 60,61 single point calculations on the gas-phase optimized geometries in aniline ( = 6.89), which was considered to best approximate the reaction solvent. Among various continuum solvent models, the C-PCM was selected because of its generally better performance, [62][63][64] even though there remains an inherent error when working with charged species. 62 The solvent cavity was created by a series of overlapping spheres by the default UA0 model (or SPHEREONH for the delocalised proton atoms) and all standard settings as implemented in Gaussian03 were used for the C-PCM calculations. The reaction free-energy changes in solution were corrected for the change of standard state from the gas phase (1 atm) to solution (1 M). 65 The approach of optimizing the geometries in the gas phase and then keeping the geometry frozen for the C-PCM calculation was preferred because of the known convergence problems of the geometry optimization in the presence of the C-PCM, especially for molecules containing weak interaction (i.e. hydrogen bonds). The size of the present computational investigation (~ 240 optimizations) imposed a too heavy burden under the more rigorous approach. The impact of the approximation was estimated by comparing the two approaches for two test cases (see details in the Supporting Information):

C-PCM optimizations gave slightly lower free energies (ca. 2 kcal mol -1 ) and slightly shorter (ca.

0.03 Å) hydrogen bonds, whereas the covalent bonds changed by < 0.01 Å.

Results

As is well appreciated, the appropriate description of the reaction energetics for a catalytic reaction must consider solvent effects. [START_REF] Maseras | Computational Modeling of Homogeneous Catalysis[END_REF] The present study makes use of the C-PCM approach 60,61 in aniline, which is both the substrate and the medium of the catalyzed transformation. When charged species are involved in solvents of low permittivity, ion pairing may also tune the potential energy surface. A recent study of this platinum system (notably reactions that involve compounds 1, 2 and 3) has explicitly addressed the effect of the countercation in the calculations of reaction energetics, as well as the adequacy of different approaches for handling the free energy of solvation when placing the gas-phase optimized molecule into the solvent cage. [START_REF] Dub | [END_REF] Indeed, upon going from the gas phase to the solution phase the translational and rotational contributions to the partition function are partially quenched, as well as the PV term which relates energy and enthalpy. No universal agreement exists on the best thermodynamic correction of the computed electronic energy to yield the free energy of solutes in condensed media. The inclusion of the countercation introduced significant ion pairing effects in the order dianion >> monoanion. Relative changes for ligand exchange reactions, however, were smaller, especially when the reaction under consideration did not involve doubly charged species. Both effects of introducing the counterion, on one side, and completely neglecting the gas phase entropy term, on the other side, introduced relative changes of up to a few kcal mol -1 for the reaction energetics. As a possible result of compensation of errors, the best match with the experimental results was obtained when applying the full gas phase thermodynamic correction (∆G CPCM = ∆Ggas + ∆∆Gsolv where ∆Ggas is the free energy change of the reaction in the gas phase and ∆Gsolv is the free energy of solvation for each compound), without the inclusion of the countercation.

This approach will therefore be applied also to the present investigation. Since the reaction pathway that will be explored involves exclusively singly charged and neutral species, and on the basis of the relatively good agreement found in our previous investigations, 49,[START_REF] Dub | [END_REF] the computed free energies reported in this contribution for intermediates and transition states provide at least an indication of the preferred pathway of the catalytic cycle, although a quantitative estimation of accuracy is not possible.

All computed systems are labelled with Roman numerals. Views of all the optimized geometries are given in the Supporting Information (Figure S1), although the most relevant structures will also be shown within the main text. For complexes corresponding to the isolated compounds (1-6), the numbers correspond; for instance, the [PtBr4] 2-ion of compound 1 is given label I, the [PtBr3(C2H4)] -ion of compound 2 is labelled as II, etc. All geometry optimizations were carried out without any simplification (all atoms were treated quanto-mechanically). The thermal corrections were carried out at the standard 298.15 K temperature and at 423.15 K (temperature used for the catalytic runs). [24][25][26] As the temperature dependence is generally very small, only the standard values will be discussed while those at 423.15 K will be shown in parentheses in all figures. A full list of the electronic energies and the thermal and solvation corrections is available as Supporting Information (Table S1). All reported ∆G CPCM values are given relative to [PtBr4] 2-, I, taking into account the values of all species that are added to or subtracted from the system. Because all species calculated during this study have a smaller negative charge than the reference compound (either -1 or neutral) and because more highly charged species are better solvated than less highly charged ones, all solution relative free energy values are less negative (or more positive) than the corresponding gas phase relative free energy values. We have also calculated the relative energy of compound trans-PtBr2(PhNH2)2 (VI'), which was not considered in the previous contributions. 49 It was found as more stable than the cis isomer VI by ca. 8 kcal/mol, but still less stable than II by ca. 4 kcal/mol. Thus, II is still predicted as the lowest free energy species in solution and the presumed catalytic resting state.

(a) Nucleophilic addition of aniline to coordinated olefin

As stated in the Introduction (Scheme 2), systems II, IV, V, VII and VIII are all likely candidates for the nucleophilic addition step of the olefin activation mechanism of the catalytic cycle. As illustrated in Figure 1 through the example of the LUMO of complex II, the nucleophilic addition to a coordinated olefin could be imagined to occur either intermolecularly via direct addition (anti, pathway 1, or syn, pathway 2) or intramolecularly through preliminary coordination (pathway 3, leading either to a 4-or a 5-coordinated intermediate). Pathway 1 is the ubiquitous pathway for nucleophilic addition to coordinated olefins,, 1 but evidence for a syn attack with preliminary metal coordination through a 5-coordinate intermediate (pathway 3) has been presented for the OH -addition to related Pd systems [68][69][70] and evidence for the existence of 5-coordinate PtCl2(C2H4)(py)2 also exists. 71 Both geometric 72,73 and steric factors 74 in the substrate, as well as the nucleophile nature, 75 have been proposed to steer the pathway one way or the other. The possibility of a direct syn addition (pathway 2) is not systematically considered. 1,32 We wished to probe all possible pathways in our case. Note that this step is common to the catalytic cycle of the Wacker process. 76 syn-attack anti-attack Comparative studies of the direct anti (1) and syn (2) additions were only carried out for systems II and IV through appropriate scans of the C-N distance (Figure S3). For compound V, which already contains aniline within the coordination sphere, an intramolecular syn addition (following pathway 3 of Figure 1) was compared with the intermolecular anti attack (Figure S4).

Since the minimized parameter is the gas phase electronic energy, with the entropic and solvation effects varying along the reaction coordinate, a fully optimized transition state structure does not necessarily corresponds to the maximum along the solution free energy curve. Indeed, the highest solution free energy for the intermolecular pathways corresponds to a much longer C-N distances (earlier transition state) relative to the gas phase, because of the relatively good stabilization by solvation of the zwitterionic character that develops along the addition coordinate. For each system, the highest point along the G CPCM curve is lower for the anti attack (values in kcal mol -1 ): for II, 11.1 (anti) and 19.9 (syn); for IV, 8.1 (anti) and 19.9 (syn); and for V, 13.9 (anti) and 45.5 for the intramolecular syn addition. A related computational study of the nucleophilic attack of gold-coordinated 1,3-dienes by N-based nucleophiles also shows the absence of coordination and a more facile direct anti addition. 31 The addition coordinate for systems VII and VIII was therefore investigated only for the anti attack (Figure S5).

The results of the anti nucleophilic additions are summarized graphically in Figure 2. The relative barrier heights correlate with the C-C bond lengths in the starting olefin complex 49 (greater barriers are associated to longer C-C bonds), in agreement with the notion that a longer C-C bond reflects a greater degree of  back-bonding and thus higher electron density on the C2H4 ligand, disfavoring the attack. Upon considering the different relative stability of the starting complexes, the nucleophilic addition occurs more easily in the order II < V < IV < VIII < VII. S2). It is relevant to mention here that the only crystallographically characterized complex having this stoichiometry, trans-

PtCl2(Et2NH)(CH2CH2NHEt2), 77 exhibits an intramolecular N-H•••Cl interaction (N•••Cl = 3. 20
Å; H atoms were not directly located).

It may be argued that the gas phase optimized structures are not relevant for the behavior in solution, because the ammonium N-H protons could establish stronger H-bonds with aniline molecules in the medium. To shine more light on this problem we have performed additional calculations taking the particular case of complex XIII. Starting from the optimized isomer XIII', addition of an aniline molecule next to each of the two N-H protons led to systems XIII•PhNH2 (a and b, see Figure 3). These complexes are ca. 7 kcal/mol higher than XIII' on the ΔG CPCM scale. However, they are 12.2 and 10.8 kcal/mol more stable, respectively, on the ΔEgas scale. Full entropy neglect in the ΔE CPCM scale (ΔEgas + ΔGsolv) also gives greater stability for the aniline adducts (by 3.9 and 2.4 kcal/mol, respectively), but this approximation is also unjustified. As noted above, the correct handling of the entropy for accurate calculation of equilibrium positions is one of the outstanding challenges for computational chemistry. [

PtBr4] 2-+ 2C2H4 + 2PhNH2 → [PtBr2(C2H4)(C2H4N(Ph)H2•••NH2Ph)] + 2Br - (1)
I XIII•PhNH2 [PtBr4] 2-+ 2C2H4 + PhNH2•••PhNH2 → [PtBr2(C2H4)(C2H4N(Ph)H2•••NH2Ph)] + 2Br - (2)

I XIII•PhNH2 (b) N-H activation

The aniline N-H oxidative addition to anyone of complexes I-VIII could constitute the initial step of a catalytic cycle operating through the amine activation mechanism. It is mentioned that oxidative addition of unactivated N-H bonds to Pt 0 L2 is thermodynamically unfavorable 78 and hence it may be expected as even less favorable for a Pt II system, but computational studies of the amine activation pathway on Pt II complexes do not seem to be available. Hence, we have investigated the energetics of this process. This process was only investigated for complexes I and II, leading to systems XIV-XIX.

Complex cis-[PtBr4H(NHPh)] Loss of a bromide ligand from complex XIV may occur to afford a 5-coordinate [PtBr3H(NHPh)] -complex, XVII, which may also be obtained by N-H oxidative addition to the 14-e complex [PtBr3] -. The relative free energy of XVII is lower (27.8 kcal mol -1 ) than that of XIV, but still too high to make this pathway competitive (vide infra). Dissociation of a bromide ligand from the three isomeric complexes XV, XV' and XVI yields three [PtBr2(C2H4)H(NHPh)] geometric isomers, XVIII-XIX, which are again highly energetic. In conclusion, all these calculated models lie too high in energy, showing that the aniline N-H oxidative addition to Pt II is thermodynamically unfavorable and casting serious doubts on the likelihood of an aniline activation mechanism for this catalytic system.

(c) Further transformations of the zwitterionic complexes

As mentioned in the introduction, Pt II zwitterionic complexes resulting from the nucleophilic addition of basic amines to coordinated olefins have previously been described. 77, 79- 85 These complexes appear to be stable toward the -H elimination process, 30,81 whereas analogous Pd II systems may undergo this transformation leading to oxidative amination products instead of hydroamination. 1,[START_REF] Müller | [END_REF]30 The intramolecular N-H PtCl(PyPyr)(C2H4) 89 and [PtCl(alkene)(tmeda)] + . 90 Chelation of the deprotonated alkyl ligand in B would also afford a platinacyclic derivative E, though this is a Pt IV derivative, which is related to D by a formal oxidative addition of HBr. There is at least one literature precedent of the

interconversion between a N-H•••Pt zwitterionic complex of type A, PtBr( 2 -C,N- C6H4CH2NMe2-2)( 1 -C-C6H4CH2NHMe2-2)
, and a 6-coordinated Pt IV hydride complex of type 91 Systems B and D may also be generated by the alternative intermolecular deprotonation of A or C by aniline to generate F followed by reprotonation or Br -displacement. Both derivatives of type D 28,89,[92][93][94] and F 21,89,95 have been proposed in the literature as catalytic intermediates for the hydroamination reaction, but their possible role in the olefin activation mechanism has not been firmly established. catalyzed by [PtCl(PH3)2] + . 32 The rest of this section will explore all these possibilities. The Br ligand that was originally trans to the alkyl chain in IX has moved trans to the H ligand in XX. The G CPCM value of this structure is 9.5 kcal mol -1 vs. 3.5 kcal mol -1 for the precursor IX. This is the most stable 5-coordinate Pt IV -H complex found in our studies. Starting from X and XI, the higher energy 16-electron Pt IV -H complexes XXI, XXII and XXII' where obtained as detailed in the Supporting Information, whereas we were not able to obtain any new Pt IV -H local minimum starting from the stoichiometries of XII and XIII'. Note that, among all the zwitterionic complexes, IX displays the most electron-rich Pt atom and is thus the most susceptible one to transfer a proton to the metal center and generate a Pt IV hydride complex, as previously predicted. 26 Optimization of the transition state for this process gave geometry TS(IX-XX), Figure 4,

E, Pt(H)Br( 2 -C,N-C6H4CH2NMe2-2)2.
where the H atom is ca. midway between the two local minima and its Mulliken charge (0.28) is also midway between the more proton-like value of 0.44 in IX and the more hydride-like value of 0.11 in XX. The structure shows only one imaginary frequency (397i cm -1 ) corresponding to the H atom movement from N to Pt. On the energy scale, TS(IX-XX) is barely higher (ca. 1 kcal mol -1 ) than IX, which is less stable than XX, whereas G CPCM increases in the order IX (3.5) < TS(IX-XX) (8.7) < XX (9.5), suggesting that the real transition state geometry is closer to that of XX and that the rearrangement is very facile.

Since systems XX-XXII are formally 5-coordinate Pt IV complexes, a reasonable question is whether they may be stabilized by addition of one of the several available ligands (Br -, PhNH2, C2H4) or by chelation of the aminoalkyl ligand to yield 18-electron octahedral products (E in Scheme 3). This question was probed for the most stabilized of the above described hydride systems, namely XX. Three systems were constructed by adding ligands Br -, PhNH2 and C2H4 to the empty position trans to the CH2CH2NHPh to yield systems XXIII, XXIV, XV, respectively.

The possible generation of platina(IV)azacyclobutane derivates from XX-XXII will be analyzed later in a separate section. All geometries of XXIII-XXV optimized smoothly, see Figure 5. An alternative orientation for the ethylene ligand (XXIV': parallel to the Br-Pt-Br axis rather than to the H-Pt-Br axis) resulted in a higher G CPCM value. Two isomeric possibilities for the aniline addition product, XXV' and XXV", were also optimized, the most stable arrangement XXV' featuring a trans disposition of the aniline and hydride ligands. All these 6-coordinate adducts are in fact more stable than the 5-coordinate precursor on the energy scale, but most are higher on the G CPCM scale. Only XXV' is slightly stabilized. This suggests that the coordinatively unsaturated Pt IV -H complex XX is a viable catalytic intermediate. The deprotonation pathway was analyzed more in detail only for the most stable complex.

This involves the initial formation of a hydrogen-bonded adduct between XIII and aniline to give XIII•PhNH2(A) (see above, Figure 3), then proton transfer via TS(XIII-XXXII) leading to an H-bonded adduct between XXXII and anilinium, XXXII•PhNH3 + (see Figure 6). Like for the above described TS(IX-XX), the transition state TS(XIII-XXXII) is higher in energy with respect to both reactant and product on the energy scale, but actually lower than the final product on the G CPCM scale. On going to the G CPCM cor scale (see equations 1 and 2), the situation does not change, TS(XIII-XXXII) remaining lower than XXXII•PhNH3 + (Figure 6). Note that, A detailed analysis of these processed is described in the Supporting Information. In summary, bromide displacement from the deprotonated systems XXVIII-XXXII yields the platina(II)cyclic derivatives XXXIII-XXXV with moderately high energy (14.9-22.4 kcal mol -1 ).

The most stable amongst these Pt II metallacyclic complexes, XXXV', has G CPCM of 14.9 kcal mol -1 , just a bit higher than that of its precursor XXXII. The relatively low energy of these complexes allows them to be considered for the moment as potential catalytic intermediates.

Note that the thermodynamics of this process should be favoured by stronger bases, rationalizing well the formation of the stable platinacyclic derivatives mentioned above. Since the addition of the monodentate ligands Br -, PhNH2 and C2H4 to 5-coordinate Pt IV hydride complexes was generally found unfavourable (e.g. see Figure 5), a reasonable question is whether the 6-coordinate Pt IV complexes XXXIV-XLIV may gain stability by loss of one of these ligands to produce 5-coordinate derivatives. This possibility was probed only for the most stable Pt IV platinacyclic compounds XXXVI and XLIII, as well as XXXVIII. In particular, it may be predicted that the most easily lost ligand is located trans to one of the two strongly transdirecting lihands, namely H and the alkyl substituent. This procedure generated three new systems, a neutral [PtHBr2( 2 :C,N-CH2CH2NHPh)] complex XLVII and two isomeric cationic complexes [PtHBr(PhNH2)( 2 :C,N-CH2CH2NHPh)] + XLVIII and IL (details in SI). The generation of neutral XLVII is much less unfavorable than the formation of the cationic derivatives XXXVIII and XLIII, but the most stable Pt IV platinacyclic system remains the anionic 6-coordinate XXXVI. This situation parallels that of the non cyclic Pt IV -H systems (see Figure 5), although it is quantitatively more in favour of the 6-coordinate system. System XXV' (Figure 5) remains the most stable 6-coordinate Pt IV -H complex, though only marginally with respect to XXXVI (8.0 vs. 9.6 kcal mol -1 ), and system XX (Figure 4) remains the most stable 5coordinate Pt IV -H complex. These will be the systems worth considering for the product liberation step (vide infra).

(d) The rate-determining step.

Reductive C-H elimination from Pt IV -H aminoalkyl systems (B or E in Scheme 3) would produce the hydroamination product and regenerate Pt II intermediates. For instance, system XX would lead to 3-coordinate [PtBr3] -, which can however be stabilized by the PhNHEt reductive elimination product, whereas XXV' would lead to [PtBr3(PhNH2)] -, III, and XXXVI would generate the related complex [PtBr3(PhNHEt)] -. However, the hydroamination product may also be liberated by C-protonation of the aminoalkyl ligand in systems of type A/C or F in Scheme 3.

The proton source may be external (the anilinium ion, which is the strongest available acid in the aniline-rich medium) or internal (the ammonium function of the zwitterionic derivatives).

d1. Reductive elimination of PhNHEt from Pt IV -H intermediates. This step was first

explored starting from the most accessible 5-coordinate Pt IV hydride system XX. The reaction coordinate led to a square planar Pt II  complex L where the PhEtNH product remains bonded to Pt through a C-H bond through transition state TS(XX-L), see Figure 8 (for a more detailed scan of this coordinate, see the supplemental figure S6). The Pt-bonded C-H bond in L is slightly lengthened by the electronic interaction with the metal (1.12 Å vs. 1.09 Å for the other two C-H bonds of the CH3 group) and shows a red-shifted νC-H vibration (2723 cm -1 , cf. 3161 (νas) and 3111 (νs) cm -1 for the CH2 group. Interestingly, the N-H•••Br hydrogen bond is retained throughout the reductive elimination coordinate, the H•••Br distance remaining confined in the narrow 2.516-2.630 Å range. The optimized transition state, TS(XX-L) (756i cm -1 ), has essentially the same geometry (dHC = 1.57 Å, dPtH = 1.57 Å, dPtC = 2.21 Å, ∠CPtBrtr = 160.4º) and G CPCM value as the G CPCM maximum along the C-H reductive elimination scan (figure S6), contrary to the intermolecular scans examined above for which the G CPCM is offset relative to the E maximum. This is because no major charge redistribution, thus no large entropic effect by solvation, occurs along this reaction coordinate. The  complex L may rearrange to a much more stable isomeric N-bonded form, [PtBr3(NHPhEt)] -(LI, Figure 8), which still features the intramolecular N-H•••Br interaction (H•••Br = 2.475 Å; Pt-N-H = 96.3°). However, the coordination position occupied by the C-H  bond in L may also be taken directly by a new molecule of ethylene, to liberate the hydroamination product PhNHEt and regenerate the catalyst resting state II. This takes the system down to a G CPCM value of -20.8 kcal mol -1 at 298.15 K and -18.1 kcal mol -1 at 423.15 K (the calculated thermodynamic gain of the hydroamination reaction is -6.1 and -3.6 kcal mol -1 at 298.15 K and 423.15 K, respectively).

TS(XX

A scan of the reductive elimination pathway starting from the anionic 6-coordinated complexes XXV and XXV' were found to be associated to a stiff increase of G CPCM leading to transition states, TS(XXV) and TS(XXV'), respectively (see Figure 9), featuring a dramatically weakened bond to the ligand trans to H (Br for XXV, Pt•••Br = 2.83 Å; PhNH2 for XXV',

Pt•••NH2Ph = 4.19 Å).
The TS(XXV) system appears to evolve toward a [PtBr2(PhNH2)( 2 -H-CH2CH2NHPh)] and a separated Br -ion, whereas the TS(XXV') corresponds very closely to the geometry of TS(XX-L) with an H-bonded aniline molecule (cf. Figure 8 and Figure 9). The energies of these transition states are much greater than that of TS(XX-L) on the G CPCM scale.

The energy of TS(XXV') seems too high, considering that it is essentially an H-bonded adduct of TS(XX-L) with aniline. However, correction of this term for H-bonding in the aniline starting material (G CPCM cor scale) bring this energy much closer, though still higher, than that of TS(XX-L). In a similar fashion, the reductive elimination coordinate from complex XXXVI leads to a severe weakening of the Pt-Br bond trans to H, transforming the system to a square planar  complex where the PhEtNH product remains bonded to Pt through a C-H bond. The maximum along this coordinate (C•••H = 1.7 Å), also shown in Figure 9, has a relative G CPCM of 23.4 kcal mol -1 , ca. 4.0 kcal/mol higher than the reductive elimination transition state from XX.

The energy profiles of the two processes involving the isomeric systems XX and XXXVI are compared in Figure S7. Given these results, the elimination process from the other 6-coordinate systems shown in Figure 5 was not further investigated. Pathway (b1) is a direct intramolecular transfer of the anilinium proton to the alkyl carbon atom.

Finally, the intramolecular proton shuttle pathway (b2), which is a variant of (b1) and also a concerted version of the two-step (a1) pathway, was proposed as the lowest-energy pathway from the model investigation of the addition of NH3 to C2H4 catalyzed by [PtCl(PH3)2] + . 32 Pathway (a1) did not lead to any low-energy pathway for the release of the hydroamination product on the free energy scale. Therefore, a detailed account of our work on this pathway is presented in the Supporting Information. The lowest energy transition states for the proton transfer from PhNH3 + to XXXII (the lowest G CPCM systems amongst all the deprotonated zwitterions) or XXIX, leading to cis-PtBr2(C2H4)(PhNHEt) and trans-PtBr2(PhNH2) (PhNHEt) products where the PhNHEt ligand is coordinated as a -C-H complex, are shown in Figure 10 (TS(LII'-LIII) and TS(LIV-LV), respectively). Pathway (b1) was explored for complex IX. This choice is justified by the fact that this is the most electron-rich zwitterion, leading to the lowest energy 5-coordinate Pt IV -H system by proton transfer to Pt. It may be argued that direct proton transfer to the adjacent alkyl C atom is regulated by the same electronic effects and should therefore also be easiest for the most electron-rich system. Proton transfer leads to system L, namely the same -complex that was previously shown to result from reductive elimination via the hydride intermediate XX. A STQN calculation between complexes IX and L converged to the same transition state of the reductive elimination pathway, TS(XX-L). Hence, this pathway does not exist as such, at least for this specific system. Indeed, the optimized TS(XX-L) has only one imaginary frequency (1 st order saddle point) and an IRC calculation leads to the XX and L local minima.

Finally, pathway (b2) was checked for the lowest energy zwitterionic intermediate XIII, but also from the related system XI. Transition states were located for both systems (the latter, TS(LVI-LVII), is shown in Figure 10, full details are given in the Supporting Information), but the energy values are once again prohibitively high (> 10 kcal mol -1 greater) in comparison to the reductive elimination pathway from the Pt IV -H intermediate XX.

Note that the three transition states shown in Figure 10 are actually less disfavored than TS(XX-L) on the electronic energy scale, the E CPCM relative to the resting state II being (in kcal mol -1 ): 0.0 for TS(LII'-LIII), -1.1 for TS(LIV-LV), 2.0 for TS(LVI-LVII), vs. 3.7 for TS(XX-L). However, all these proton transfer pathways require the intervention of a second species (PhNH3 + or PhNH2) with an inevitable additional entropic penalty. Even if the condensed phase entropic contribution is partially quenched and the calculation of G CPCM as carried out in the present contribution certainly overestimates it, the margin in favor of TS(XX-L) appears too large to be accounted for by this approximation. Indeed, treatment of the solution free energy according to the G CPCM cor approach, as already detailed above in section (a), which equalizes the number of species involved on each side of the chemical equation, yields the values shown in brackets in Figure 10, still higher than that of TS(XX-L). Therefore, these proton shuttle pathways do not appear to be competitive, at least for the present system, contrary to the conclusion made for the NH3 addition to C2H4 catalyzed by [PtCl(PH3)2] + . 32 Note, however, that the latter study was based only on the electronic energy without consideration of the entropic contribution.

Discussion

At the chosen level of theory, the computational investigation of the PtBr2/Br --catalyzed aniline addition to ethylene rules out an amine activation pathway and validates the olefin activation pathway. In agreement with previous arguments that were based on the catalytic studies and with recent solution equilibrium studies, the calculations indicate complex [PtBr3(C2H4)] -(II) as the most stable species under catalytic conditions (catalyst resting state).

Many other Pt II complexes accessible by ligand exchange equilibria (I and III-VIII), some of which have been isolated and characterized, 49 are off-loop equilibrium species located at relatively low energies. The lowest energy pathway of the catalytic cycle starting from the resting state and ending at the same point on the subsequent cycle is summarized in Figure 11. The productive cycle starts with the aniline nucleophilic addition to the anti face of the ethylene ligand in II. Although the addition barrier for any of the Pt II -C2H4 complexes is lower than the rate-determining step of the overall cycle (cf. Figure 2) and the resulting products (notably Xanti and XIIanti) will be present as off-loop species, only the zwitterionic complex [PtBr3(CH2CH2NH2Ph)] -, IX, leads to the product release channel. In agreement with a previous proposal, 26 the aniline addition to [PtBr2(C2H4)(PhNH2)] is actually easier, although only for the trans isomer IV, leading to X, whereas the cis isomer V leads to a higher barrier. The lowest addition barrier is in fact that to trans-PtBr2(C2H4)2 (VII) leading to XII. However, all these alternative zwitterionic intermediates, even though more stable than IX, do not lead to the liberation of the hydroamination product by lower energy pathways and can only proceed by transforming back to IX by ligand exchange processes. In our previous contribution 49 we have failed to experimentally observe the zwitterionic complexes resulting from PhNH2 addition to ethylene in the isolated complexes 2, 4 and 5. The calculated energy differences between the addition products and the starting compounds (18.2 kcal mol -1 for II, 8.8 kcal mol -1 for IV and 9.7 kcal mol -1 for V, Figure 2) do not allow a sufficient equilibrium population of the zwitterionic complexes to be present for detection, in qualitative agreement with the experiment.

Note also the previously reported failure to observe an addition compound when trans-[Pt(C2H4)(Et2NH)Cl2] was reacted with aniline, 80,81 contrary to Et2NH. 84 The experimental observation of zwitterionic products for Pt II appears limited to amines of sufficient basicity (pKa > 5 for the conjugate ammonium ion).

The catalytic cycle continues with transfer of the ammonium proton in IX to the Pt center, to produce a 5-coordinate Pt IV -H complex, [Pt(H)Br3(CH2CH2NHPh)] -, XX. This equilibrates with a number of 18-electron ligand adducts, of which the most stable is the aniline adduct,

[Pt(H)Br3(PhNH2)(CH2CH2NHPh)] -, XXV', with a mer arrangement of the three Br -ligands and with the aniline ligand trans to H (see Figure 5). Most of these saturated derivatives are in fact less stable than the unsaturated XX precursor on the G CPCM scale, which is not surprising because two ligands with a strong trans labilizing effect are present (the hydride and the ammonium-bearing alkyl chain). In addition, the bromide ligands may provide a certain degree of stabilization of electronic unsaturation through  donation. 4) are located at higher energy and do not provide suitable pathways for product liberation.

Several other off-loop species (at lower energy than the rate-determining barrier but not directly involved in the minimum-energy pathway of the catalytic cycle) are selected squareplanar Pt II complexes obtained by deprotonation of selected zwitterionic complexes (XXIX, XXXI and XXXII, Figure 6), square-planar Pt II complexes with a platinaazacyclobutane ring derived from the latter by substitution of a bromide ligand with the dangling amine function of the aminoalkyl ligand (XXXIII-XXXV), and Pt IV -H complexes with a platinaazacyclobutane ring (either 6-coordinated, XXXVI-XLVI, or 5-coordinated, e.g. XLVII). Metallaazacyclobutadiene complexes of Pt II were indeed obtained by deprotonation of zwitterionic products resulting from the addition of basic amines to coordinated ethylene or other olefins (process A (or C) → F → D in Scheme 3), 88 some of them being crystallographically characterized. 86,89,90 Our study confirms the accessibility of these derivatives under appropriate experimental conditions, but shows that they are only off-loop species and not catalytic intermediates (none of these complexes leads to product liberation by a low-energy pathway), at least for the aniline addition to ethylene with this PtBr2/Br -catalytic system.

Of the two possible ways to liberate the hydroamination product, namely reductive elimination from a Pt IV hydrido-aminoalkyl derivative and protonolysis of a Pt II aminoalkyl derivative, the first one seems preferred for this catalytic cycle. The lowest-energy pathway, shown in Figure 11, involves elimination from complex XX, with the rate-determining transition state TS(XX-L) located 34.1 kcal mol -1 higher than the resting state on the G CPCM scale at 298.15 K (35.6 kcal mol -1 at 423.15 K). This value appears in reasonable agreement with the experimental evidence. Although a detailed kinetic and temperature-dependent study was not carried out (this would be hampered by the observed catalyst deactivation process at the high temperatures required by the reaction) 24,113 an upper estimate of the activation barrier can be calculated from the observed initial TOF of 37 h -1 at 150°C, 24 28 and also Pt IV . 115,116 A direct protonolysis of the Pt II -bonded aminoalkyl ligand, either intermolecular by anilinium or intramolecular by the ammonium function in the zwitterionic intermediate, does not lead to a lower-energy pathway for product liberation. The lowest pathway corresponds to the reductive elimination from the zwitterion IX through the hydride XX and transition state TS(XX-L). Hence, the electron density of the Pt-C bond in this system favours transfer of the proton to the Pt atom, to yield a well defined Pt IV -H local minimum, rather than directly to the C atom to directly yield the -complex L, which is also the product of the reductive elimination, in a single step. The introduction of an additional aniline molecule to assist this proton transfer through a shuttle mechanism does not alter this preference for proton transfer to the metal site.

Note that the direct pathway of proton transfer to the aminoalkyl C atom, assisted by an extra molecule of free amine, was found to be the preferred pathway for the model study of the NH3 addition to C 2 H 4 catalyzed by [PtCl(PH 3 ) 2 ] + . 32 The difference between the two pathways is expected to depend on the polarity of the Pt-C bond in the zwitterionic intermediate, which should rather be in favour of the C atom if the coordination sphere is less electron-donating, especially if the complex is positively charged, and more in favour of the Pt atom with more electron-donating ligands, especially if the complex is negatively charged. In the end, however, the product is the same, a -complex with the hydroamination product bonded to the catalytic metal through a C-H bond (Scheme 5). Even when the proton transfer proceeds directly from the N to the C atom, is it somewhat assisted by the Pt atom with a significant Pt•••H interaction at the transition state level. 32

Conclusions

pathways explored in this contribution. Indeed, the observed "promoting" effect of added strong acids could be explained by retardation of catalyst decomposition, rather than by the increase of the turnover frequency. Reduction to Pt 0 has always been observed in the experimental studies carried out with this particular catalytic system 6,24,25,49,113 and finding conditions that would prevent this decomposition is a major challenge for improving this catalytic system. Our experimental studies are continuing in this direction.

Other useful information that emerges from the current theoretical investigation is the identification of the dormant species, the [PtBr3(C2H4)] -system II, and the transition state of the reductive elimination, TS(XX-L). We are now in a position to zoom into these two key structures of the potential energy surface, since the free energy difference between them determines the TOF and hence the catalytic activity, by probing the effect of structural variations (e.g. nature of the halogen, amine substituents), guiding us toward the conception of more efficient catalytic processes.

Figure 1 .

 1 Figure 1. Possible pathways for the nucleophilic attack of the coordinated olefin in II (a) and a

Figure 2 .

 2 Figure 2. G CPCM in kcal mol -1 at 298.15 K (and at 423.15 K in parentheses) for the reagents, products and highest points along the aniline nucleophilic addition to coordinated ethylene in systems II, IV, V, VII and VIII. Optimized geometries are also shown for the reagents and products.

Figure 3 .

 3 Figure 3. Optimized geometries and G CPCM in kcal mol -1 at 298.15 K (and at 423.15 K in parentheses) for systems XIII•PhNH2 (a and b). The values in brackets correspond to G CPCM cor (see text).

  •••X (X = Pt or Br) hydrogen bonding observed in the optimized geometries of systems IX-XIII could set the stage for an intramolecular proton transfer (see Scheme 3) that may constitute the first step of the pathway leading from the zwitterionic intermediate to product release. The N-H•••Pt bond (A) could generate a Pt IV hydride complex by proton transfer to the Pt atom (B), whereas the N-H•••Br bond (C) could promote HBr elimination as PhNH2 + Br -, leading to chelation of the CH2CH2NHPh ligand to generate a platinaazacyclobutane derivative (D), although the latter could also occur in two steps, promoted by an external base. Examples of this process were reported for derivatives of type cis-PtCl2(C2H4)(Y) (Y = PR3, py, NH3, am, DMSO),84,[86][87][88]

Scheme 3 .

 3 Scheme 3. Different geometrical arrangement found for the [Pt-CH2CH2NHPh] moiety.

c1.

  Intramolecular proton transfer to Pt: formation of Pt IV -H intermediates. Because of the Pt•••H interaction, systems IX and X seem perfectly set up for transferring a proton from N to Pt, with formal oxidation to Pt IV , but system XI, XII and XIII' must also be considered. Indeed, moving the H-bonded hydrogen atom from N to Pt in IX yielded XX as a stable minimum with a square pyramidal geometry where a new H-bond has developed between the residual N-H bond of the axial CH2CH2NHPh ligand and an equatorial Br ligand (H•••Br = 2.59 Å), see Figure 4.

Figure 4 .

 4 Figure 4. Optimized geometries and G CPCM in kcal mol -1 at 298.15 K (and at 423.15 K in

Figure 5 .Figure 6 .

 56 Figure 5. Optimized geometries and G CPCM in kcal mol -1 at 298.15 K (and at 423.15 K in

  although the formation of XIII•PhNH2(A) from the two free components costs a great deal on the G CPCM scale, the formation of XXXII•PhNH3 + from XXXII and PhNH3 + actually corresponds to a stabilization. This is probably because the weak N-H•••Br interaction in XXXII (the free amine is not a strong acid) is replaced by much stronger N-H•••Br and N-H•••N interactions (the anilinium ion is a much stronger acid).c4. Formation of chelated aminoalkyl Pt complexes.There are two possible types of platinacycles. The first type is 4-coordinate platina(II)cycles that can be generated by coordination of the dangling amine function in the deprotonated systems (XXVIII-XXXII) with replacement of a bromide ligand (F → D in Scheme 3). The second type is 18-electron platina(IV)cycles that can be obtained by chelation of the dangling aminoalkyl ligand in the 16electron Pt IV -H complexes (XX-XXII), corresponding to process B → E in Scheme 3. The latter can also be obtained by protonation of the platina(II)cyclic complexes D followed by bromide addition (formal HBr oxidative addition), D → E in Scheme 3.

  Concerning the formation of platina(IV)cycles, chelation of the dangling aminoalkyl function -CH2CH2NHPh in XX leads to the anionic complexes [PtHBr3( 2 :C,N-CH2CH2NHPh)] -(XXXVI-XXXVII, four isomers), whereas the same process starting from XXI and XXII (or XXII') leads to the neutral complexes [PtHBr2(PhNH2)( 2 :C,N-CH2CH2NHPh)] (XXXVIII-XLIV, 12 isomers). The energy of these systems is relatively low on the G CPCM scale, unless H and the two most strongly trans directing ligands H and alkyl are unfavorably located trans to each other (i.e. XXXVII, XXXIX, XLI and XLI'). The thermodynamic balance is favorable for formation of these complexes both with respect to the corresponding systems of type B and D in Scheme 3, justifying their consideration as potential catalytic intermediates. The lowest G CPCM derivatives are XXXVI for the anionic system (9.4 kcal mol -1 ) and XLIII for the neutral systems (11.0 kcal mol -1 ). Their geometry is shown in Figure7. Additional systems may also be derived by formal HBr oxidative addition to the ethylene-dibromo isomers XXXV and XXXV' (corresponding Pt IV -H aminoalkyl complexes could not be optimized for this stoichiometry, vide supra). The nearly isoenergetic diastereomeric pairs XLV/XLV' and XLVI/XLVI' were generated in this fashion. These are less stabilized (17-19 kcal mol -1 ) relative to the corresponding Pt II platinacyclic complexes than the other platina(IV)cyclic complexes, since they contain the -acceptor ligand C2H4, which is more compatible with Pt II whereas Pt IV prefers harder donor ligands.

Figure 7 .

 7 Figure 7. Optimized geometries and relative G CPCM in kcal mol -1 at 298.15 K (and at 423.15 K

Figure 8 .

 8 Figure 8. Optimized geometries and relative G CPCM in kcal mol -1 at 298.15 K (and at 423.15

Figure 9 .

 9 Figure 9. Optimized geometry of the reductive elimination transition states from systems

Scheme 4 .Figure 10 .

 410 Scheme 4. Possible pathways for Pt-C bond protonolysis.

Figure 11 .

 11 Figure 11. Lowest-energy pathway for PtBr2/Br --catalyzed addition of aniline to ethylene.

Scheme 5 .

 5 Scheme 5. Competition between the Pt and C atoms in the Pt-C bond for proton capture.

  24

		[PtBr 2 ] + 2 Br -
	ArNHCH 2 CH 3
	[PtBr 3 ] -+ Br -	[PtBr 4 ] 2-	+CH 2 =CH 2 -Br -
	H
	[PtBr 3 CH 2 CH 2 -NHAr] -			[PtBr 3 (C 2 H 4 )] -
		+ArNH 2		-Br -+ArNH 2
	[PtBr 3 CH 2 CH 2 -NH 2 Ar] -		[PtBr 2 (ArNH 2 )(C 2 H 4 )]
	+Br --ArNH 2	[(ArNH 2 )PtBr 2 CH 2 CH 2 -NH 2 Ar]	+ArNH 2

  All these isomers are approximately isoenergic and highly disfavored, with a much higher free energy than the highest point of the energy profile for the olefin activation pathway (vide infra).

2-

, XIV, results from N-H oxidative addition to I. No isomeric variant is possible except for the trans structure, which is not accessible by direct oxidative addition. Oxidative addition to II affords mer-[PtBr3(C2H4)(H)(NHPh)] -, for which three orientations are possible: N-H addition across the Br-Pt-(C2H4) axis of II with the ligand ending up trans to C2H4 being either NHPh (XV) or H (XV'), and N-H oxidative addition across the Br-Pt-Br axis of II, yielding isomer XVI.

  namely 28.9 kcal/mol. The disagreement is not too large and the discrepancy may be attributed to the approximations involved (nature of functional, solvent model, proper treatment of solvation entropy, neglect of ion pair interactions, etc.). The reductive elimination pathway from 6-coordinate hydridoaminoalkyl Pt IV derivatives involves the concerted dissociation of the ligand located trans to H, leading to a much greater barrier for systems where this ligand is Br -(XXV and XXXVI) because of the charge separation. However, even when this ligand is the less strongly bonded and neutral PhNH2, (XXV'), the transition state [TS(XXV')] has a greater energy. It is thus clear that reductive elimination is more favourable from the 5-coordinate XX than from the more stable 6coordinate complexes. It is indeed known that reductive elimination in octahedral d6 systems is facilitated by a preliminary ligand dissociation to yield a 16-electron 5-coordinate intermediate, with examples known for Rh III , 114 Ir III ,

Acknowledgement. We are grateful to the Centre National de la Recherche Scientifique (CNRS) and the Agence Nationale de la Recherche (ANR, Grant No. NT09_442499) for financial support and the MENESR (Ministère de l'Éducation Nationale, de l'Enseignement Supérieur et de la Recherche, France) for a Ph.D. fellowship to PD.

We have reported in this paper a comprehensive investigation of the full catalytic cycle for the N-H addition of aniline to ethylene (hydroamination) catalyzed by the PtBr2/Br -system. It is the first theoretical investigation that addresses the real catalytic system, with the electronic effect of the amine substituent and real nature of the transition metal complex being included in the calculation. The study confirms the general proposition that the olefin activation mechanism is preferred over the amine activation mechanism for this metal, since the N-H oxidative addition process yields Pt IV (H)(amido) intermediates that are found too high in energy. The mechanism essentially follows the same basic steps illustrated by the previous investigations by Senn et al. The study has also explored alternative processes that may occur on the zwitterionic intermediate, leading to products such as platinacyclic derivatives, both for Pt II and Pt IV , that have indeed been experimentally observed under certain circumstances (amine basicity, ligand coordination sphere). These are found to be related to the hydroamination catalytic cycle as offloop species but do not directly lead to liberation of the hydroamination product. The study, however, has not addressed one important point of the hydroamination reaction, namely catalyst deactivation by reduction to Pt 0 . Indeed, it has been shown that anilines may act as reducing agents toward platinum complexes. 117 Further, all known zwitterionic complexes of type -PtCH2CH2NHR2 + , generated in situ from Pt II X2(C2H4)L (X = halogen) and basic amines NHR2, are unstable and slowly decompose upon standing, in some cases with Pt 0 formation. 99,118 Such decomposition may also be triggered by N-H deprotonation, since the free amine can then promote -hydride elimination to generate Pt-iminium complexes, 99 in addition to the other Supporting Information Available. Views of all optimized structures, tables of energy data and Cartesian coordinates for all optimized compounds, and description of less important computational details as referred to in the article (41 pages). This information is available free of charge via the Internet at http://pubs.acs.org/.

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