Aurélien Béthegnies

Pavel A Dub

Rinaldo Poli

Platinum-Catalyzed Assembly of Quinaldine from Aniline and Ethylene

Keywords: Platinum, hydroamination, quinoline synthesis, ethylene complexes, aniline, quinaldine

published or not. The documents may come

PtBr 2 /Br - PtBr 2 /PPh 3

Introduction

The synthesis of quinoline derivatives [START_REF] Gildchrist | Heterocyclic Chemistry[END_REF] is the subject of intense research because of the presence of the quinoline scaffold in a number of biologically active compounds such as antimalaria, [START_REF] Tietze | [END_REF] anti-inflammatory, 3 anti-asthmatic, 4 anti-bacterial, 5 anti-hypertensive 6 and tyrosine kinase inhibiting agents. 7 In addition, quinoline molecules, metal complexes and polymers are being investigated as materials for electronic and optoelectronic applications. [8][9][10] The simplest quinoline derivative is 2-methylquinoline, or quinaldine. It finds applications as anesthetic, 11 in bioimaging, 12 as sensitizers for photodynamic therapy, [13][14][15] and in the preparation of a number of other heterocyclic compounds.

Several methods are available to access quinolines, [16][17][START_REF] Gilchrist | Heterocyclic chemistry[END_REF] including the syntheses of Skraup-Doebner-Von Miller, [START_REF] Skraup | [END_REF]20 Pfitzinger 21 and Conrad-Limpach, 22 which however suffer from drastic operating conditions and low stereoselectivity, conjugated to multi-step syntheses from commercially available starting materials that negatively affect the overall yields. The Friedländer method is generally considered as the most versatile, even though its use is limited by the need to use unstable aminobenzaldehydes. 23 Several approaches to access the quinoline skeleton using organometallic catalysis have been developed, such as the condensation of aniline and allyl alcohols, 24,25 triallylamines, 26 allylammonium chlorides 26 or alkyl amines, 27 or the Rucatalyzed ring closing metathesis of α-ω-dienes derived from 2-isopropenylaniline. [28][29][30] Other approaches are the palladium catalyzed intramolecular hydroamination of o-allylaniline, 31 the Sonogashira coupling of 2-iodoaniline with acetylenic carbinols, 32,33 the nickel catalyzed cyclization of 2-iodoanilines with aroylalkynes, 34 and other catalyzed transformations inspired by the Friedländer synthesis. 35,36 Quinaldine was shown to also form as by-product of the hydroamination of ethylene with aniline (Scheme 1), a transformation that looks attractive since it constitutes a single step from simple and inexpensive starting materials. The reaction was apparently first reported by Diamond et al. with use of RhCl3/2PPh3 37,38 and PdCl2/2PPh3 38 as catalysts. More recent contributions from Brunet et al. have shown that the catalytic activity of the rhodium system in hydroamination is greatly enhanced by the addition of a soluble iodide salt, also leading to substantial amounts of the double hydroamination product (PhNEt2, 2) and small amounts of quinaldine (3, Scheme 1), 39 and have disclosed for the first time the activity of the platinum system PtX2/nBu4PY (X,Y = Cl, Br, I) for this reaction. 40 The nature of X in the platinum salt is irrelevant, whereas that of Y is important: the best promoting system is nBu4PBr when used in moderate amounts (10 equivalents relative to Pt). 41 An aqueous biphasic version of this catalytic process using NaBr as promoter has also been reported. 42 This catalyst system, however, is haunted by reduction of PtBr2 to inactive metallic Pt, a phenomenon that we have been actively investigating in our laboratory. [43][44][45][46] Brønsted basicity appears to be the cause of this catalyst deactivation process. It should be mentioned that Brønsted acids are known to promote the catalytic activity of metal complexes and to catalyze the hydroamination reaction by themselves in certain cases. 47 Whereas the addition of Brønsted acids was shown by Brunet to improve the performance of his catalyst, 40 we have demonstrated that the role of Brønsted acidity in this system is only to prolong the catalyst lifetime by retarding its reductive deactivation by bases. Scheme 1. Products of the metal-catalyzed hydroamination of ethylene by aniline.

No quinoline derivative formation was initially reported from the reaction involving 1-hexene in place of ethylene, 48 but a later study revealed that N-hexylaniline, the anti-Markovnikov product of hexene hydroamination, generates 2-pentyl-3-butylquinoline and aniline under the hydroamination catalytic conditions either in the presence or in the absence (Scheme 2, a) of additional hexene. 49 In the same study it was also shown that a quinoline derivative resulted from an alkyl transfer process when aniline was exposed to tri-n-butylamine (Scheme 2, b). Furthermore, our own investigations of the hydroamination catalyst decomposition have revealed that alkyl transfer processes also occur for aliphatic amines (e.g. Et2NH generates Et3N and EtNH2), that this process may occur via retro-hydroamination (e.g. Et3N generates an observable amount of Et2NH), and that this reaction is catalyzed by metallic Pt and not by the PtBr2/Br - hydroamination catalyst. 44 Hence, the amine Brønsted basicity induces reduction of PtBr2 to Pt 0 which in turn catalyzes alkyl transfer and retro-hydroamination processes to generate olefins in situ and subsequently the quinoline formation may occur with catalysis by either Pt II or Pt 0 . A relevant result is also the rhodium-catalyzed quinoline synthesis by addition of aromatic amines to styrenes, for which a certain scope was reported (Scheme 2, c). 50 Scheme 2. Other reports of quinoline formation from anilines.

The mechanism leading to the construction of a quinoline core from an aniline derivative and two molecules of olefin has been addressed by several authors. 27,38,49,50 The scheme that has emerged so far (see Scheme 3 for the special case of aniline and ethylene) involves a sequence of various steps, not necessarily all metal catalyzed, starting with oxidative amination to yield an enamine 5, tautomerization to the imine 6, aza-Diels-Alder condensation between the enamine and the imine leading to the intermediate 8 (this was proposed to occur stepwise via intermediate 7), aniline elimination to yield the dihydroquinoline 9 which is finally aromatized to the quinaldine 3, but can also capture part of the H2 produced in other steps to yield tetrahydroquinoline (4, also observed in certain cases). The current state of knowledge, however, is insufficient to claim full understanding of this mechanism and its relationship with that of the hydroamination process. Scheme 3. Proposed general mechanism for the metal-catalyzed quinoline skeleton formation from aniline and ethylene.

Contrary to the rhodium catalyst, the PtBr2/Br -catalytic system is poisoned by the addition of neutral ligands that are capable to bind platinum. The addition of DMF, DMSO, and notably phosphines (2 equivalents) dramatically reduces the catalytic activity for both the hydroamination and heterocyclization processes, whereas the addition of water or ethanol has no noticeable effect. 40,42 On the other hand, Wang and Widenhoefer have shown that the [PtCl2(C2H4)]2/PPh3 catalytic system is active for the intermolecular hydroamination of ethylene with carboxamides, provided that a P:Pt ratio of 1 is used. This catalytic system gives better results than the phosphine-free [PtCl2(C2H4)]2, e.g. a > 98% conversion for the addition of benzamide (2.5% catalyst, 24 h at 120°C in dioxane) vs. 44% conversion in the absence of PPh3. However, use of a P:Pt ratio of 2 resulted in no conversion. 51 On the basis of this report, we decided to study the effect of phosphines on the catalytic activity of the Brunet system (PtBr2/Br -) in ethylene hydroamination by aniline in greater detail. We report here that the system remains indeed active when P:Pt = 1 but the reaction selectivity shifts in favor of quinaldine. Optimization of the catalytic conditions and an analysis of the effect of various additives are also reported.

Experimental Section

General. Unless otherwise stated, all operations were carried out under an argon atmosphere by using standard Schlenk line techniques. The solvents used were dehydrated using standard procedures and distilled under argon prior to use. Compounds PtBr2 (99.9%, Aldrich), PtCl2 (99.9%, Aldrich), PtI2 (Alfa Product), K2PtCl4 (Strem), CF3SO3H (Fluka, >98%), NaBr (Aldrich 99 %), nBu4PBr (98%, Aldrich), PPh3 (Alfa Aesar 99+%), triphenylphosphine-3,3′,3′′-trisulfonic acid trisodium salt (TPPTS, Sigma-Aldrich, > 95%) and KBr (Alfa Aesar 99 %) were used as received. Compound nBu4PI was prepared from nBu3P and nBuI as described in the litterature 52 and stored in a freezer protected from light. PMe3 (1 M toluene solution, Aldrich) was used as received and stored in a freezer protected from light. PMe2Ph (Strem 99 %), PMePh2 (Strem 99 %), PhNH2 (Acros Organics, 99% for analysis ACS), NEt3 (Acros Organics 99+%), 4-(nbutyl)aniline (TCI >98%) and N,N-diethyl-p-phenylenediamine (Aldrich 97 %) were distilled and kept under argon protected from light. 4-nitroaniline (Fluka), p-anisidine (Acros 99 %) and 4-chloraniline (Aldrich) were sublimed prior to use. Compound Diethyl(6-(2methylquinolin))amine was prepared according to a literature procedure. 53 Ethylene (N25, ≥99.5%) was obtained from Air Liquide.

Instrumentation. The gas chromatographic analyses were carried out in a Hewlett-Packard HP4890 instrument equipped with HP 3395 integrator, an HP1 capillary column (30 m x 0,320 mm x 0,25 µm; DB-5MS) and a flame ionization detector, operating with helium as carrier gas at a 50 kPa pressure (Tini of 65 °C for 2 min, then 6 °C/min up to Tfinal of 200 °C for 30 min). Under these conditions, the retention times of the main compounds are: aniline, 6.8 min; N-ethylaniline, 11 min; N,N-diéthylaniline, 12 min; quinaldine, 15 min; N,N-di-n-butylaniline external standard, 21 min. The GC/EI-TOF-MS analysis was performed by the Mass Spectrometry Service of the Universite Paul Sabatier. The 1 H NMR investigations were carried out at 298 K on Bruker DPX300 and AV300 spectrometers.

Catalytic tests with PtBr2/PPh3/nBu4PX (X = Cl, Br, I). The reactions were conducted in a 100 mL stainless steel autoclave in the presence of a magnetic stirrer. The platinum salt PtBr2 (46.1 mg, 0.13 mmol), triphenylphosphine (34.1 mg, 0.13 mmol), and the appropriate type and amount (see Results and Discussion) of nBu4PX salt were introduced inside the autoclave, which was then closed and submitted to several vacuum/argon cycles. Aniline (4.1 mL, 45 mmol, 350 equivalents) was then introduced by syringe through a septum valve and the autoclave was then charged with ethylene (25 bars, ca. 100 mmol) and brought to the reaction temperature, 150 °C.

After 10 h, the autoclave was cooled to room temperature and then vented. The reaction mixture was transferred into diethylether (120 mL) and the resulting suspension was stirred for 2 h. The external standard N,N-di-n-butylaniline (≈ 0,15 g) was then added, followed by filtration and GC analysis.

Catalytic tests with PtX2/PPh3 (X = Cl, Br, I). The reaction was conducted as described in the previous section, using PtX2 (0.13 mmol) and triphenylphosphine (34.1 mg, 0.13 mmol) as the only solid reagents. Aniline and ethylene were used in the same amounts and the reaction was carried out for the same length of time at the same temperature. The work-up procedure and the analysis were also identical to those described in the previous section.

Catalytic runs in diluted media. The reactions were conducted as described in the previous section, using PtBr2 (46.1 mg, 0.13 mmol) and triphenylphosphine (34.1 mg, 0.13 mmol). In addition to aniline (4.1 mL, 45 mmol, 350 equivalents), a solvent of the appropriate type and amount (see Results and Discussion) was also introduced by syringe through a septum valve.

The reactions were carried out at 150°C for either 10 or 19 h. The work-up procedure and the analysis were carried out as described in the previous sections. Kinetic profile. These reactions were conducted as described in the previous sections, using PtBr2 (92.2 mg, 0.26 mmol), triphenylphosphine (0.34 mg, 0.13 equiv), aniline (8.3 mL, 90 mmol, 350 equivalents), toluene (5 mL) and ethylene (25 bars), at 150°C. Samples of ca. 0.5 mL were withdrawn from the reactor through a siphon at appropriate intervals and added to 3.5 mL of diethylether and stirred. After adding the external standard N,N-di-n-butylaniline (ca. 0.010 g), the mixture was filtered and analyzed by GC.

Catalytic tests with variable amounts of PPh3. These reactions were conducted as described in the previous sections, using PtBr2 (46.1 mg, 0.13 mmol), the appropriate amount of triphenylphosphine (see Results and Discussion), aniline (4.1 mL, 45 mmol, 350 equivalents), toluene (5 mL) and ethylene (25 bars) for 10 hours at 150°C. The work-up procedure and the analysis were also identical to those described in the previous section.

Catalytic tests with different phosphines (PMe3, PMe2Ph, PMePh2, PPh3

). These reactions were conducted as described in the previous sections, using PtBr2 (46.1 mg, 0.13 mmol), the appropriate type of phosphine ligand (0.13 equiv), aniline (4.1 mL, 45 mmol, 350 equivalents), toluene (5 mL) and ethylene (25 bars), for 10 hours at 150°C. The work-up procedure and the analysis were also identical to those described in the previous section.

Catalytic tests with 4-(R)aniline, R = NO2

, nBu, MeO, NEt2, Cl. These reactions were conducted as described in the previous sections using PtBr2 (46.1 mg, 0.13 mmol), triphenylphosphine (34.1 mg, 0.13 mmol), toluene (5 mL) and the appropriate amine (45 mmol, 350 equivalents). The latter was introduced in the autoclave together with PtBr2 and PPh3 if solid (R = NO2, NH2, MeO, Cl), or via the septum valve by syringe if liquid. The autoclave was then charged with ethylene (25 bars) for 10 hours at 150°C and heated to 150°C for 10 h. The workup procedure and the analysis were also identical to those described in the previous section. The TON values for the corresponding 1 R and 3 R products were estimated on the basis of the peak integration, assuming response factors identical to that of the starting material.

Results and Discussion

(a) Preliminary investigations under classical hydroamination conditions

A first catalytic run was carried out under the optimized "Brunet" conditions except for the additional presence of one equivalent of PPh3 relative to PtBr2. The result is compared with those reported by Brunet with either 0 or 2 equivalents of PPh3 in Table 1 (for the compound numbering, refer to Scheme 4).

It is to be noted that the original Brunet results were obtained with a larger excess (150 equiv) of the nBu4PBr co-catalyst, 40 but it has since been shown that only a 10-fold excess of the bromide salt is sufficient to obtain the best activities while the relative amount of the nBu4PBr salt does not significantly alter the reaction selectivity. 41 As show in run 3, the activity is greatly reduced in the presence of 2 equivalents of PPh3, the TON in hydroamination product being reduced from 80 to 6 whereas the TON in quinaldine by-product remains relatively constant. The new experiment (run 2) leads to two important observations: the first one is that a significant catalytic activity is maintained, in line with the Wang and Widenhoefer observation, 51 when using only 1 equivalent of PPh3, the N-ethylaniline hydroamination product being formed with 23 cycles. The second and more unexpected result is that the production of quinaldine is greatly increased to 62 cycles, plus an additional 6 cycles of 1,2,3,4-tetrahydroquinaldine. The latter compound was also detected when using the Brunet catalytic system, although only in trace amounts. 40 Therefore, the reaction selectivity is inversed by the presence of 1 equivalent of PPh3.

Among the other by-products observed in trace amounts (identity confirmed by GC-MS analysis, see SI) were PhNEt2 (the product of double hydroamination), the formation of which is almost completely suppressed (0.02 cycles), and two products of hydroarylation, 2-and 4-ethylaniline (0.1 and 0.03 cycles, respectively). A 1 H NMR analysis of the gas phase after the reaction (the gases were bubbled through CDCl3) revealed also the formation of significant amounts of ethane (δ 0.88) together with residual ethylene (δ 5.43). As already stated in the Introduction, the literature already describes the direct transformation of aniline and ethylene into quinaldine, but the latter was rarely the major product and never produced with such high catalytic activities. For instance, 14.6 cycles were obtained with the PdCl2/2PPh3 catalyst at 200°C in 3 days (an average TOF of 0.2 h -1 ), 38 vs. 62 cycles in 10 h at 150°C with the PtBr2/1PPh3/10nBu4PBr catalyst described here (an average TOF of 6.2 h -1 ). It is also interesting to observe that, compared to the classical Brunet catalyst (e.g. without PPh3), the final reaction mixture shows essentially no deposition of metallic platinum, suggesting that the PPh3 coordination is capable of better stabilizing the Pt II center against the deactivating reduction process.

From the stoichiometric point of view, the generation of one molecule of quinaldine from one molecule of aniline and two of ethylene releases one molecule of H2 in the ring closure step to the tetrahydroquinaldine ring and two additional ones in the aromatization step. If all these H2 equivalents are captured by ethylene to generate the corresponding amount of ethane (which was indeed observed among the reaction products, vide supra), the reaction requires a global ethylene/aniline ratio of 5 (equation 1). Therefore, on the basis of the conditions used for run 2 (aniline:ethylene = 1:2.2), the maximum theoretical aniline conversion for 100% ethylene consumption would be 44%. The quinaldine yield of 18.6 % relative to aniline, corresponding to the 62 turnovers of run 2, is actually 41% relative to the limiting C2H4 reagent. The amount of produced quinaldine is thus expected to increase upon increasing the ethylene pressure. Indeed, an increase of the ethylene pressure to 50 bars, which corresponds to a PhNH2/C2H4 ratio of 1:4.4, increased the TON of each product, see Table 1 (run 4). For practical reasons, however, all subsequent experiments were still carried out using 25 bar of ethylene pressure.

An obvious question concerning the quinaldine formation mechanism is whether the hydroamination product PhNHEt is an intermediate. The results of a catalytic run starting from N-ethylaniline in place of aniline under the same conditions of run 2, see Scheme 5, appear to discredit this hypothesis, because quinaldine was produced in greatly reduced amounts. This result can be rationalized as follows: while the PtBr2/Br -system is not efficient for the hydroamination of PhNHEt, since there are only traces of the double hydroamination product (Table 1), the aniline basicity induces partial catalyst reduction to Pt 0 , 46 which then catalyzes the alkyl transfer process, yielding moderate amounts of aniline and N,N-diethylaniline. 44 Subsequently, aniline and ethylene may proceed to yield the observed small amount of quinaldine. This result appears rather insensitive to the ethylene pressure (Scheme 5).

On the basis of the recent modification of the Brunet system by K2PtCl4/NaBraq, 42 we have also tested the K2PtCl4/NaBraq/PPh3 combination under aqueous conditions, run 5 of Table 1.

The first striking observation is that the system is ca. in 4 times less productive relative to the non aqueous system. However, quinaldine is still the major product. No significant amount of 1,2,3,4-tetrahydroquinaldine was observed in this case. Hence, although the productivity under aqueous conditions is reduced relative to the classical Brunet conditions (as also observed for the hydroamination in the absence of phosphine 42 ), the effect of the PPh3 addition on the change of selectivity in favor of quinaldine remains qualitatively the same in the presence of aqueous NaBr as in the anhydrous medium in the presence of nBu4PBr. Substitution of PPh3 by 3,3',3"phosphinidynetris(benzenesulfonic acid) trisodium salt, commonly known as sodium triphenylphosphine trisulfonate (TPPTS), aiming at improving the catalyst solubility in the aqueous phase, gives essentially identical results (run 6).

By analogy with the hydroamination condition optimization, where the best n-butylammonium salt co-catalyst resulted to be the bromide salt when used in moderate excess amounts (10 equivalents), 41 we carried out additional investigations on the effect of the nature and amount of halide salt additive in the presence of PPh3 (P/Pt = 1) on the quinaldine formation. The results are presented in Table 2.

The general observation is that the TON obtained with the chloride and iodide salt is marginally lower than the TON obtained with the equivalent amount of bromide (for the result with 10 equivalents of Bu4PBr, see run 2 in Table 1). The catalyst efficiency decreases upon increasing the amount of salt in the medium, again in a way that is little sensitive to the nature of the halide. The amount of N-ethylaniline, the secondary product in these experiments, equally decreases upon increasing the amount of salt, but the ratio of the two products does not remain constant, further supporting the notion that they result from different mechanistic pathways. The amount of the other by-product, the tetrahydroquinaldine, always remains quite low. Run 17 shows the result of a control experiment, carried out in the absence of tetra-n-butylphosphonium salt. The quinaldine formation increased to 61 cycles, indicating that the presence of the salt additive is not beneficial for the quinaldine synthesis, contrary to the effect (in the absence of PPh3) to the hydroamination process. The results of run 18 correspond to an experiment identical to run 17, except for a reaction time of 65 h instead of 10. The yields of both major products remain essentially the same, although large amounts of both the aniline and ethylene reagents (conversions of ca. 27% and 61%, the latter based on the theoretical stoichiometry of equation 1) are still present in the autoclave. This comparison suggests that the catalyst is no longer active after 10 hours of operation. At the end of these experiments, as for those outlined in Table 1, no significant metallic platinum deposit was observed in the autoclave, confirming that PPh3 protects the catalyst system from the reductive deactivation process. However, the catalyst must slowly transform into a catalytically inactive species during the reaction, which probably corresponds to the orange precipitate observed in the recovered reaction mixture at the end of the catalytic run.

Our efforts aimed at characterizing this species have not yet been successful. The product of double hydroamination 2, not reported in Table 2, has always been detected in trace amounts (< 1 cycle), in line with the results shown in the previous section (Table 1). All subsequent experiments have therefore been carried out without the tetra-n-butylammonium salt additive.

We have then investigated the effect of the halide nature on the PtX2 salt. The results, cf. runs 19 and 20 in Table 3 with run 17 in Table 2, seem to indicate that this parameter does not strongly affect the catalytic activity. On the basis of the literature knowledge, it seems likely that the addition of PPh3 under the catalytic conditions gives rise to the formation of a complex of type [PtBr2(PPh3)L], where L is either ethylene or aniline. The chlorido complex [PtCl2(PPh3)(C2H4)] with a cis stoichiometry is known. 54,55 Bromide analogues are also known, although with other phosphines, 54 and iodide derivatives have equally been described where the olefin and phosphine donors are part of the same bidentate ligand. [56][57][58] For practical convenience, the rest of this study was pursued using PtBr2 as pre-catalyst. a Conditions: aniline (4.1 mL, 45.5 mmol), C2H4 (25 bars at room temperature, 100 mmol), PtX2 (0.13 mmol), PPh3 (34.1 mg, 0.13 mmol), 150 °C, 10 h.

(c) Kinetic profile

With the purpose of learning more about the catalyst decomposition process, we wished to run additional catalytic experiments with sample withdrawal and conversion monitoring. In order for this to be done, the catalytic conditions had to be slightly modified, notably the volume of the liquid phase had to be increased, in order to allow the withdrawal of a sufficient number of samples during the run, with all appropriate care to allow for siphon washing to eliminate contamination of a given sample by the residues of the previous one. A preliminary study of the effect of the dilution by an inert solvent gave the results reported in Table 4. Rather unexpectedly, dilution with a small amount (5 mL) of toluene or THF (runs 21 and 22) resulted in even greater yields than the equivalent experiment carried out without dilution (run 17 of Table 2). On the other hand, a greater extent of dilution (15 mL of toluene, run 23) decreases the activity as expected. The initial activity increase may be related to a greater ability of the solvent in the initial disaggregation of the tridimensional PtBr2 network facilitating the formation of the active soluble catalyst, or to a genuine solvent effect on the catalytic cycle energetic span (for instance a stabilization of the rate-determining transition state). Running the reaction for a longer time (19 h, run 24) resulted in a barely greater yield, confirming the catalyst deactivation already noted above. Interestingly, a similar yield was also observed when the dilution was accomplished by addition of a greater amount of aniline (run 25). This result would tend to discredit the idea of a solvent effect on the energy span. 59 When the dilution was accomplished by addition of a stronger Brønsted base (NEt3, run 26), on the other hand, the activity was greatly reduced. This effect is reminiscent of the negative effect of Brønsted basicity on the catalyst lifetime (deactivation by reduction to metallic Pt). 44 However, in the present case no black precipitated was noted. Instead, the reaction mixture appeared as a brown solution giving a brown precipitate upon work-up (addition of diethyl ether).

Given that the catalytic system does not suffer (on the contrary, it is slightly aided) by a small dilution, we have proceeded to study the reaction profile under the optimized conditions of runs 21 and 24. This gave the results illustrated in Figure 1. The first immediate observation is the confirmation of the catalyst deactivation, the yield of both major products 1 and 3 stagnating after approximately ca. 5-7 hours of operation, when the aniline conversion is still < 30%. The profile also clearly indicates the presence of an induction period, greater than 15 min, for both independent transformations. It is interesting to note that a previous kinetic study of the hydroamination catalyst (PtBr2/nBu4PX, X = Cl, Br, I) 45 had revealed a much shorter induction period, with 11 cycles already accomplished after 15 min for the most active Br -system. Thus, it seems that the bromide salt is more efficient than PPh3 for converting the insoluble PtBr2 precatalyst into the soluble catalytically active species. Once the presence of activating halide salts was proven unnecessary for the heterocyclization catalytic cycle leading to quinaldine formation, we have decided to revisit the effect of the P/Pt ratio under halide-free conditions varying this ratio in steps of 0.5 units between zero and two.

The results are shown in Table 5. The results of runs 27, taken from the previous study, 40 are different from those of run 1 of Table 1 since no halide co-catalyst is used here. For the same reason, result of run 31 differs from the previously published result, 40 where only 2 cycles where reported for both 1 and 3, ~1 for 2, and no formation of 4 was mentioned. Thus, PPh3 (2 equivalents) exert a stronger inhibition of both hydroamination and quinaldine formation in the presence of a large excess (150 equiv) of bromide salt. a The conditions were identical to those used in run 21 of Table 4 except for the amount of PPh3. b Results from reference 40 (n.r. = not reported).

The influence of the PPh3 amount is striking, with a small proportion of 0.5 equivalents being already sufficient to boost the formation of quinaldine and to completely change the catalyst selectivity (cf. run 27 and 28). On going from 0.5 to 1.5 equivalents per Pt atom (runs 28-30), the quinaldine yield remains high, although it is maximum for a P/Pt ratio of 1. Upon reaching a ratio of 2, on the other hand, the activity falls sharply (run 32), being reduced to one half of that obtained with 1.5 equivalent. The formation of N-ethylaniline, on the other hand, seems less affected by the P/Pt increase. We can reasonably anticipate that the introduction of two equivalents of PPh3 induces the formation of PtBr2(PPh3)2 60 with relatively robust Pt-P bonds, rendering the Pt coordination sites less accessible for the catalytic transformation.

After confirming that the best P/Pt ratio for the quinaldine formation is 1, we have also investigated the effect of the phosphine nature. For this purpose, we have chosen to restrict the study to the PMexPh3-x series smoothly going from a triaryl (x = 0) to a trialkyl (x = 3) phosphine. The results are shown in Table 6. These runs were carried out without toluene dilution. Therefore, the results should be compared with that of the experiment with PPh3 of run 17 in Table 2. In order to verify the reproducibility of the results and in particular because of the technical difficulty of introducing small amounts of the liquid phosphines in the autoclave, each run was repeated; the table contains the average values and the differences between the two runs.

The best results are those obtained in the presence of PPh3, followed by those with PMePh2 (run 32). For unknown reasons, the trend is not monotonous, since PMe2Ph (run 33) gives a lower TON for quinaldine relative to PMe3 (run 34) while, at the same time, this ligand also gives a greater TON for N-ethylaniline relative to all other phosphines. Therefore, the presence of aromatic substituents on the phosphorus atom seems important to maximize quinaldine formation. a The conditions were identical to those used in Table 3 (footnote a).

(e) Influence of aniline para substituents.

In order to briefly evaluate the potential generality of this reaction, we have carried out a small scope study on p-substituted anilines, with the substituent nature covering a range of donor and acceptor groups. The reaction gives rise to the formation of N-ethtyl-4-R-aniline (1 R ) and 2methyl-6-R-quinoline (3 R ) as main products from the hydroamination and cyclization processes, respectively, see Scheme 6. The catalytic results are collected in Table 7. The reactions were carried out under the conditions previously optimized for aniline, namely in the presence of 1 equiv of PPh3, in the absence of additional halide salts, and with dilution by 5 mL of toluene. It is to be noted that certain substituted anilines are solid at room temperature but melt at a temperature lower than that used for the reaction. Scheme 6. General scheme of the reaction between p-substituted aniline and ethylene catalyzed by PtBr2/PPh3. c For all quinoline derivatives, except R = NEt2 for which the peak identity was confirmed by GC of the isolated pure products, the peak was attributed on the basis of the retention time; the MS study of the mixture confirmed the presence of 3 R in substantial amounts (MS spectra in SI). d Run carried out in the absence of toluene.

The TON values reported in Table 7 for all complexes, except for R = NEt2, are to be considered as rough estimations, limiting a fine comparison of activities, because genuine and pure samples of the products 1 R and 3 R for the GC calibration were not available. Although all the produced N-ethylaniline [61][62][63][64][65] and quinoline 53,[66][67][68][69] derivatives are known in the literature, many of them are not commercially available. However, considering that the response coefficients should not be dramatically dependent on the nature of R, indicative trends may be extracted from the data. In particular, it is immediately obvious that the presence of the p-R substituent has a profound effect on the catalytic activity, especially for the formation of the quinoline product 3 R which remains the major product in all cases except for the p-nitroaniline substrate (run 35). The yield of substituted quinoline does not appear to correlate with the aniline basicity, since both the least and most basic anilines give the poorest results (runs 35 and 39), whereas the best results are obtained for anilines of intermediate basicity, the best corresponding to the unsubstituted aniline (run 21 of Table 4, pKa = 4.61). Aniline of similar basicity (the p-Cl and p-nBu, runs 36 and 37) give rise to similar yields for the corresponding substituted quinoline, whereas introduction of the π-donating MeO substituent (run 38) seems to negatively affect the cyclization process, although as stated above not much emphasis can be put on this comparison. The lack of a clear effect of basicity is in contrast with the results presented for the hydroamination process with the PtBr2/nBu4PBr catalyst, 40 as well for other hydroamination catalysts, 70 where the activity correlates with the aniline basicity, better results being obtained for less basic substrates. It has been suggested, however, that this phenomenon may simply be related to faster catalyst decomposition to metallic Pt in the presence of a more basic environment. In the present case, the effect of PPh3 is to practically eliminate the reduction process to Pt 0 , but a catalyst deactivation is still observed (see section c). In the absence of a clear understanding of the nature of this decomposition product and of the mechanism leading it, it is not possible to speculate on the exact reason for the activity trend shown in Table 7.

(f) Mechanistic considerations.

The evidence collected during the present study, in addition to our recently improved mechanistic knowledge of the hydroamination catalytic cycle based on this catalytic system 71 and of the catalyst decomposition pathway, 46 allows the addition of a few considerations to the quinaldine formation mechanism. The essential features of the hydroamination catalytic cycle are recalled in Scheme 7. 71 As established in our recent investigation of the catalyst decomposition, 46 the pathway leading to the formation of Pt 0 involves a β-H elimination step from the zwitterionic intermediate B to yield a coordinated enamine ligand in complex F via the aminoalkyl intermediate E (see Scheme 7). Then, catalyst deactivation can be easily envisaged as involving enamine dissociation leading formation. This probably results from an increase of the rate determining transition state barrier of the hydroamination cycle. On, the other hand, intermediate G is protected against deprotonation by the electronic effect of the softer phosphine ligand, directing the system toward catalyst regeneration.

The negative effect of halide salt additives on the catalytic activity (in the presence of PPh3) can be easily rationalized on the basis of a competition with ethylene for the platinum coordination site on going from G to A, whereas in the absence of PPh3 the coordination of ethylene to "PtBr3 -" to yield [PtBr3(C2H4)] -is energetically favored over coordination of Br -to yield [PtBr4] 2-. 75 When a greater amount of PPh3 is present (P/Pt ratio > 1), this ligand enters in competition with the olefin for coordination, leading to PtBr2(PPh3)2, 60 and therefore hampers the catalyst ability to coordinate and activate the olefin substrate.

A couple of observations remain without rationalization. One is the negative effect of acidity on the quinaldine formation in the presence of 1 equivalent of PPh3. Since acidity has no negative effect on the hydroamination cycle 40 and should have a positive effect on the catalyst regeneration step (G to A in Scheme 7) in the heterocyclization cycle, it is conceivable that it will negatively affect the follow up process leading from the enamine and imine intermediates 5 and 6 to quinaldine (Scheme 3). Note that the formation of the quinaldine by-product during the hydroamination catalytic run in the presence of TfOH and absence of PPh3 (run 7 in Table 1)

was not reported, hinting to a negative effect of acidity independently on the PPh3 presence. It is also not clear at this point whether the heterocyclization process from 5 and 6 is metal catalyzed or not. Another obscure feature is the pathway to catalyst deactivation when PPh3 is present, which does not involve reduction to Pt 0 . The isolation and characterization of the spent catalyst, to be attempted in future investigations, will hopefully clarify this point.

Conclusion.

We have reported here the peculiar effect of the addition of PPh3 (1 P/Pt) to the Brunet catalyst in terms of the selectivity for the reaction between aniline and ethylene, in favor of the formation of quinaldine. Optimization of the reaction has show that, contrary to the hydroamination process, the addition of the halide salts nBu4PX (X = Cl, Br, I) does not have a positive effect on the catalytic activity. The optimum amount and type of the phosphine ligand is one equivalent of PPh3, the presence of a high number of aryl substituents being beneficial to the reaction. A small degree of dilution of the medium has allowed an increase of the TON, even though the reason for this phenomenon is not completely clear. The presence of an induction time at the beginning of the reaction would seem to suggest that a long period is necessary to break the tridimensional nature of the PtBr2 catalyst to generate the active soluble catalyst. Although the presence of PPh3 protects the platinum system from reduction to the metallic state, the catalyst is still deactivated by an unknown pathway. Finally, a small scope study has shown the applicability of this reaction to other para substituted anilines, but the best activity remains associated with the parent aniline substrate and the activity does not correlate with the aniline basicity. Mechanistic considerations have been advanced with proposition of a global scheme where the hydroamination cycle, the quinaldine formation cycle, and the reductive deactivation process are interconnected through two crossroads represented by two key intermediates, a Pt II zwitterionic complex and a Pt II hydride complex. This scheme, however, still requires fine tuning, notably in terms of the yet unknown non reductive catalyst decomposition pathway, before full understanding of this complex catalytic system can be claimed and used to develop new efficient and hopefully robust catalysts for either the hydroamination or the quinoline formation.

  Scheme 4. Reaction of aniline and ethylene catalyzed by PtBr2/1PPh3/10nBu4PBr.

Figure 1 .

 1 Figure 1. Kinetic profile of the quinaldine formation with the PtBr2/PPh3 catalytic system. The

  Scheme 7. Proposed mechanism for the effect of L (Br -, PhNH2, PPh3) on the metal-catalyzed

TABLE OF CONTENTS

 OF GRAPHIC.

Table 1 .

 1 Effect of PPh3 on the reaction between aniline and ethylene catalyzed by PtBr2/Br -. a Run PPh3/Pt C2H4/PhNH2 nBu4PBr/Pt TON 1 TON 2 TON 3 TON 4 PtBr2 (46.1 mg, 0.13 mmol, 0.28 %), 150 °C, 10 h. b Results from ref.

	1 b	0	2.2	150	80	1	11	trace
	2 c	1	2.2	10	23	0.02	62	6
	3 b	2	2.2	150	6	1	13	-
	4 d	1	4.4	10	35	-	89	10
	5	1 e	2.2	150 f	8	-	14	<1
	6	1 e,g	2.2	150 f	9	-	12	<1
	7 b,h 0	2.2	150	130	nr	nr	nr
	8 h,i 1	2.2	10	9	0.07	7	2
	9	1 j	2.2	10	1.7	-	1.5	0.1

a Conditions: aniline (4.1 mL, 45.5 mmol), C2H4 (25 bar at room temperature, 100 mmol),

Table 2 .

 2 Influence of the nBu4PX additive on the quinaldine formation. a

	Run	Salt	X/Pt TON 1 TON 3 TON 4
	10	Bu4PBr	65	9	33	3
	11	Bu4PBr	150	8	28	3
	12	Bu4PCl	10	13	39	2

a The conditions were identical to those used in Table

1

(footnote a). b Reaction time: 65 h.

Table 3 .

 3 Influence of the halide in the PtX2 catalyst on the quinaldine formation.

	19	PtCl2 12	55	1
	20	PtI2	28	49	4

a Run PtX2 TON 1 TON 3 TON 4

Table 4 .

 4 Influence of dilution and solvent nature on the quinaldine formation. a The conditions were identical to those used in Table1(footnote a). b 19 h. c 8.2 mL aniline (91 mmol).

	Run	Solvent Amount/mL TON 1 TON 3 TON 4
	21	toluene	5	20	77	2
	22	THF	5	25	83	3
	23	toluene	15	6	10	0
	24 b	toluene	5	20	86	3
	25 b	c	-	20	79	2
	26	NEt3	5	5	5	0

a

Table 5 .

 5 Influence of the amount of PPh3 on the quinaldine formation.

	27 b	0	23	1	3	n.r.
	28	0.5	17	0	66	2
	29	1	20	0	77	2
	30	1.5	36	0	69	3
	31	2	27	1	39	3

a Run PPh3/Pt TON 1 TON 2 TON 3 TON 4

Table 6 .

 6 Influence of the phosphine nature on the quinaldine formation. a

	Run	Phosphine	TON 1 TON 3 TON 4
	32	PMePh2	16±1	44±6	4±1
	33	PMe2Ph	26±6	5±1	<1

Table 7 .

 7 Influence of the para-substituent R on aniline on the quinoline formation and on

	hydroamination. a
	Run R	pKa b	TON 1 R	TON 3 R c

. Reaction between PhNHEt and C2H4 in the presence of PtBr2/10Br -/PPh3.

Since it was shown that the addition of a small amount of a strong acid (3 equiv of TfOH relative to PtBr2) to the Brunet catalyst has a positive effect on the hydroamination reaction, cf. run 7 and run 1, 40 we have tested the same effect of the PPh3-modified catalyst. The results of this test (run 8) clearly shown that in this case the acid addition has a negative effect on both hydroamination and quinaldine formation processes, whereas the formation of the minor hydroarylation products was substantially increased (2-ethylaniline: from 0.01 to 1.7 cycles; 4ethylaniline: from 0.03 to ca. 1 cycle). The yield reduction on the hydroamination product was quite unexpected, because the strong acid should in principle be quenched by the strongest base available (aniline at the beginning, PhNHEt after the initial hydroamination) to yield the corresponding amount of ammoniums salts, while PPh3 should remain available to coordinate Pt II . While the beneficial effect of the strong acid on the N-ethylaniline formation (cf. run 1 and run 7) is reversed by the presence of PPh3 (cf. run 2 and run 8), the combined acid/PPh3 presence also negatively affects the quinaldine formation (cf. run 1 and run 8). The final entry in Table 1 (run 9) shows that PdBr2 is much poorer than PtBr2 as a catalyst for quinaldine formation in the presence of PPh3, as was already shown for hydroamination in the absence of PPh3.

(b) Study of the influence of the halide in the additive and in the catalyst

to the 14-electron hydride intermediate G, followed by further deprotonation. The presence of a strong external Brønsted base, known to rapidly deactivate the PtBr2/Br -catalyst by reduction to Pt 0 , 44,45

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT

We are grateful to the Centre National de la Recherche Scientifique (CNRS), the Institut Universitaire de France (IUF), and the Agence Nationale de la Recherche (ANR, Grant « HYDROAM » No. NT09_442499) for financial support.


Aurélien Béthegnies

Jean-Claude Daran

Rinaldo Poli

Platinum-catalyzed hydroamination of ethylene: study of the catalyst decomposition mechanism

Keywords: Platinum, Ethylene ligand, hydroamination, catalyst degradation, mechanistic study

Word Style "BD_Abstract").

)] (11). The carbyl ligand in 11 is shown by an X-ray structural study to be in-between the π-bonded enamine and the σ-bonded iminiumalkyl configurations. The addition of MeONa to 7 results in the formation of the same products 11 and Pt 0 . On the basis of these results, a mechanism for the base-induced decomposition of Pt II (C2H4) complexes that involves Wacker-type β-H elimination followed by intermolecular hydride transfer, ligand rearrangements and final deprotonation is proposed. Addition of more nucleophilic N-based ligands (pyridine, quinuclidine) to 7 ultimately leads to C2H4 and Et2NH substitution rather than to metal reduction, even though evidence for a kinetically controlled nucleophilic addition to the coordinated ethylene is given by the quinuclidine system. From the reaction with pyridine, complex cis-PtBr2(py)2 was isolated and structurally characterized.

Introduction

The intermolecular olefin hydroamination process opens access to higher amines by atom economical addition of an amine N-H bond across the C=C bond of the olefin and currently attracts considerable academic interest. [1][2][3] This is a kinetically difficult reaction, especially for simple alkenes that lack activation by either electron-withdrawing groups such as acrylates, electronic delocalization such as styrene, or ring strain such as norbornene. Brunet has recently disclosed the remarkable activity of the simple platinum dihalides PtX2 (X = Cl, Br, I), when activated by [nBu4P]X (particularly for X = Br), for the addition of anilines to ethylene 4,5 and 1hexene. 5,6 Turnover numbers up to 115 for the desired N-ethylaniline product from ethylene and aniline were reported for operation at 25 bars of ethylene pressure in 10 h at 150°C (i.e. a TOF of 11.5 h -1 on average), the selectivity being relatively high with only trace amounts of the double hydroamination product, PhNEt2, and ca. 10 turnovers for the formation of 2-methylquinoline (quinaldine) as a by-product. The process could also be extended to aqueous biphasic conditions, using NaBr as activator and K2PtCl4 as precatalyst, 7 and tolerates the presence of air. Isolation and equilibrium studies of simple platinum complexes containing catalytically relevant species (ethylene, aniline, bromide) as ligands, such as [PtBr3(C2H4)] -(1), trans-[PtBr2(C2H4)(PhNH2)]

(2) and cis-[PtBr2(PhNH2)2] (3), 8 backed up by DFT calculations, [8][9][10] led to the conclusion that 1 is the catalyst resting state while a thorough computational investigation clarified the mechanistic pathway as summarized in Scheme 1. This involves olefin activation, followed by nucleophilic attach of the olefin by aniline, proton transfer from the ammonium function of the zwitterionic intermediate 4 to the metal center and final C-H reductive elimination from the Pt IV hydrido derivative 5 via the σ-complex 6. 11 This catalytic system is haunted, however, by slow deactivation to metallic platinum, which is inactive for hydroamination although slightly active for amine transalkylation. 12 This process has been traced to the effect of Brønsted basicity, because the presence of non coordinating bases stronger than aniline such as Et3N and DBU totally deactivates the catalyst with the generation of massive amounts of Pt 0 . Although deactivation by Brønsted basicity is often considered indication of Brønsted acid catalysis (typically related to the hydroamination of more reactive olefins), 13 this is not the case here because control experiments show the complete inactivity of strong Brønsted acids for the aniline addition to ethylene. 12 The basicity causes the catalyst reduction to Pt 0 in this case. Stability tests in the presence of only one of the two reagents (ethylene alone or aniline alone) did not result in the formation of any metallic Pt, 14 indicating that the catalyst decomposition occurs only after the formation of the zwitterion 4 in the catalytic cycle.

Scheme 1. Mechanism of the aniline addition to ethylene catalyzed by PtBr2/Br -.

Since the intermediate 4 was not experimentally observable for the catalytic system in agreement with the DFT study, a model study was carried out with a more basic amine, Et2NH, which is too basic to yield any hydroamination because it leads to rapid catalyst decomposition 12 , the desired zwitterion could indeed be observed after addition to 1, after an initial ligand exchange step yielding trans-[PtBr2(Et2NH)(C2H4)] (7). A second Et2NH molecule attacked the coordinated olefin in an equilibrium process with formation of trans-[Pt (-) Br2(Et2NH)-(CH2CH2N (+) HEt2)] ( 8), which could be characterized by 1 H NMR and IR spectroscopies. The NMR study gave further evidence of a thermally accessible equilibrium deprotonation by the excess Et2NH to yield a trans-[PtBr2(Et2NH)(CH2CH2NEt2)] -[Et2NH2] + ion pair intermediate. 15 Subsequent monitoring of the solution at room temperature led to slow Pt 0 precipitation. The nucleophilic addition to olefins coordinated to Pt II has been intensively investigated. [16][17][18][19][20][21][22][23] In particular, amines (am) add to [PtCl2Y(C2H4)] (Y = Cl -, DMSO, amine, phosphine) to yield [Pt (-) Cl2Y(CH2CH2am (+) )] which then may evolve in different ways depending on the nature of the amine and Y. [24][25][26][27][28][29][30][31][32][33][34][35][36] For complexes of type trans-[PtCl2(C2H4)(am)], the observation of the zwitterionic product, which included an X-ray crystallographically characterized example, 26 was limited to amines of high basicity, in agreement with our observations for the bromide analogues.

Occasionally, slow decomposition of these products with deposition of metallic platinum was reported.

One specific contribution by Balacco and Natile (henceforth abbreviated as BN) 37 reported the 1 H NMR characterization of the product that results from the decomposition of trans-[Pt (-) Cl2(NHRR')(CH2CH2N (+) HRR')], which was obtained from the addition of secondary amines NHRR' (R,R' = cyclo-(CH2)5; Et,Et; Me,CHMePh) to [PtCl3(C2H4)] -. The reaction led to the formation, in addition to metallic platinum, of complexes trans-[PtCl2(NHRR')-(CH2CHNRR')] according to the stoichiometry of equation 1. A mechanism for this transformation, involving deprotonation followed by loss of a hydride from the β position of the aminoalkyl ligand, was proposed as illustrated in Scheme 2. However, the reason for the spontaneous hydride loss and the detailed mechanism of this transformation were not clarified.

2 trans-[PtCl2(NHRR')(CH2CH2NHRR')]  trans-[PtCl2(NHRR')(CH2CHNRR')] + Pt + C2H4 + 2 NH2RR' + Cl - (1)
Scheme 2. Proposed mechanism for the decomposition of trans-[PtCl2(NHRR')- 37 Another phenomenon is of relevance to our observed catalyst deactivation. Whereas alkoxyalkyl complexes with the [PtCl(N-N)(CH2CH2OR)] (R = alkyl) stoichiometry, obtained by nucleophilic addition of RO -to coordinated ethylene, are stable, 22,[38][39][40] 41 The reason advanced by the authors for this reduced stability is the steric destabilization of the square planar configuration by the two phenantroline methyl substituents, promoting chelate ring opening and triggering the β-H elimination process. The observed Pt 0 product obviously results by action of a base on the Pt II hydride intermediate. This observation interrogated us about the possibility that β-H elimination may also cause the decomposition of our catalytic system, as well as that of the above mentioned aminoalkyl complexes (Scheme 2). Scheme 3. Proposed mechanism for the decomposition of [PtCl(2,9-Me2phen)(CH2CH2OR)]. 22 In this contribution, we present our studies on the decomposition of [PtBr3(C2H4)] -in the presence of amines (NEt3, py, quinuclidine) and the methoxide anion (MeO -), providing additional insight on the decomposition pathway of the hydroamination catalyst.

(CH2CH2NHRR')].

Experimental Section

General. All operations were carried out in Schlenk glassware under an argon atmosphere.

CH2Cl2 was of HPLC grade and was distilled on CaH2 under argon. CD2Cl2 for the NMR studies was obtained from Euriso-top as flame-sealed 0.6 mL ampoules. NEt3 (99+%, Acros Organics), HNEt2 (Fluka) and C5H5N (Jansen) were distilled and kept under argon in the dark. MeONa (Fluka) was dehydrated and stored in a Schlenk tube under argon. Quinuclidine was purified by recrystallization according to the literature process.

Reaction of trans-[PtBr2(NHEt2)(C2H4)]

with MeONa.

Complex trans-

[PtBr2(NHEt2)(C2H4)] (10.2 mg, 0.0236 mmol) and MeONa (2.6 mg, 0.048 mmol) were placed into a 5-mm NMR tube. After a series of vacuum/argon cycles, 0.6 mL of CD2Cl2 was added at -80°C, dissolving the platinum complex. Since no reaction was observed over 3 hours at -80°C, the tube was warmed to room temperature and monitored.

X-ray crystallography.

A single crystal of each compound was mounted under inert perfluoropolyether at the tip of a cryoloop and cooled in the cryostream of either an Agilent Technologies Xcalibur CCD diffractometer for [PtBr2(NC5H5)2] or a Bruker Apex2 CCD diffractometer for 11. Data were collected using the monochromatic MoK radiation (= 0.71073). The structures were solved by direct methods (SIR97) [START_REF] Altomare | [END_REF] and refined by least-squares procedures on F 2 using SHELXL-97. 44 In the compound PtBr2(NC5H5)2 all H atoms attached to carbon were introduced in idealised positions and treated as riding on their parent atoms in the calculations. In compound 11, the H atoms attached to the Pt-bonded carbon atom could not be located and were not calculated. The other atoms were introduced in idealised positions and treated as riding on their parent atoms in the calculations. The drawing of the molecules was realised with the help of ORTEP3. [START_REF] Burnett | Report ORNL-6895[END_REF][START_REF] Farrugia | [END_REF] Crystal data and refinement parameters are shown in

Results and Discussion

(a) Addition of trielthylamine to [PtBr3(C2H4)] -

In order to probe whether decomposition of the zwitterion, e.g. 4 in Scheme 1, requires deprotonation to yield an aminoalkyl ligand as proposed by BN (Scheme 2), we wished to investigate the reactivity of complex 1 toward non deprotonatable bases, starting with Et3N.

Charging the solution of 1 with up to a 10-fold excess of Et3N did not reveal any change in the room temperature 1 H NMR spectrum of the starting compound and no new resonances appeared except for those of free NEt3. This is consistent with literature reports on the analogous chloride system, according to which the formation of equilibrium amounts of the zwitterionic product was only possible for highly basic secondary amines such as Et2NH and piperidine. Indeed, we found in our previous study 15 that treatment of compound 7 (obtained from 1 and one equivalent of NHEt2) with two additional equivalents of NHEt2 is sufficient to promote an essentially quantitative amine addition to the coordinated ethylene. However, cooling the 1/NEt3 (10 equiv) solution to -80°C in the NMR probe revealed the extensive formation of a new product that can be assigned to trans-[Pt (-) Br2(NEt3)(CH2CH2N (+) Et3)], 9. The 1 H NMR spectrum is shown in Figure 1. Note that, in order to keep the aliphatic proton resonances clear from the interference of cation resonances, the Ph4P + salt of complex 1 was used in this experiment.

The identity of the compound is demonstrated by the two characteristic triplets in a 1:1 ratio of the mutually coupled eth-di-yl protons of the -CH2CH2N (+) Et3 ligand at δ 3.18 and 2.04 ( 3 JHH = 8.2 Hz). For comparison, those observed for trans-[Pt (-) Br2(NHEt2)(CH2CH2N (+) HEt2)] appear at δ 2.86 and 2.09 with 3 JHH = 7.0 Hz. 15 The resonances of the two compounds are compared in Table 1. The ammonium -N (+) Et3 group exhibits the sharper methylene and methyl proton resonances with the greater downfield shift relative to those of the excess free NEt3, whereas those of the coordinated NEt3 ligand are less downfield shifted and broader, the latter feature probably being caused by an incompletely frozen exchange with the free ligand. These trends parallel those previously reported for the NHEt2 analogue (see Table 1). Although there is no specific feature in the 1 H NMR spectrum to distinguish the trans and cis stereochemistries, a trans configuration can be assigned to the product on the basis of the reasonable hypothesis that the stronger trans effect of the ethylene ligand directs the initial ligand exchange process. As suggested by our previous DFT study, 11 replacement of the Br -ligand trans to ethylene with an amine is expected to favor the ethylene nucleophilic attack both kinetically and thermodynamically. (-) Br2(NEt2R')(CH2CH2N (+) Et2R)] compounds (R,R' = H, Et). Thus, contrary to previous reports, nucleophilic addition of Et3N to a Pt II -coordinated olefin may indeed be observed, at least for complex 1, although only at low temperatures. Warming the solution back to room temperature and subsequent 1 H NMR measurement indicated total reversal of the reaction to regenerate complex 1 quantitatively, with no trace of the zwitterionic product 9 left in solution. This can be rationalized by the fact that the addition reaction (equation 2) is severely exoentropic and hence highly temperature dependent. Being obviously exothermic, the process becomes favored at low temperatures where the impact of the -TΔS term is reduced.

0 1 2 3 4 δ/ppm a b c f d g R,R' (a) (b) (c) (d) (e) (f) (g) (h) H,
[PtBr3(C2H4)] -+ 2 NEt3  trans-[Pt (-) Br2(NEt3)(CH2CH2N (+) Et3)] + Br - (2)
1 9

The solution of 1 and excess NEt3 used for this NMR experiment was not indefinitely stable, giving rise to the slow precipitation of metallic platinum upon standing for a long time at room temperature. This observation is first evidence that the zwitterion decomposition does not require deprotonation of the ammonium function. On the basis of the recent report on the decomposition of alkoxylated olefin complexes (see Scheme 3 in the Introduction) 41 and the well known

(b) Addition of trielthylamine to trans-[PtBr2(NHEt2)(C2H4)]

From our DFT studies of the catalytic cycle of the PtBr2/Br -catalyzed aniline addition to ethylene we have learnt that nucleophilic addition to the coordinated ethylene is both kinetically promoted and thermodynamically more favored when the trans Br -ligand is replaced by aniline. 11 Therefore, we envisaged promoting the nucleophilic addition of NEt3 by using 7 as substrate.

In the presence of two equivalents of NEt3 in CD2Cl2, no changes in the 1 H NMR spectrum of 7 were observed and no new resonance appeared with the exception of those of the added Et3N.

This behavior parallel that reported above for the addition of to 1, but differs from that observed for the addition of Et2NH to 7 where formation of the zwitterionic product was nearly quantitative at room temperature under the same conditions. However, like for the above described Et3N addition to 1, extensive formation of the expected zwitterionic product, trans-[Pt (-) Br2(NHEt2)(CH2CH2N (+) Et3)] (10), was observed at low temperature (see Figure 2). Once again, the high negative reaction entropy accounts for the observed behavior. Upon letting the solution warm up to room temperature, the initial complex 7 forms again quantitatively, confirming the reversibility of the nucleophilic addition process. resonances of this compound with those of the similar ones in Table 1). The coordinated diethylamine ligand is characterized two signals for the diastereotopic methylene protons at δ 2.66 and 2.92 and one for the Me protons at δ 1.38, upfield shifted from those in the precursor compound 7, 15 whereas the triethylammonium group shows the methylene and methyl resonances downfield shifted from the free amine, at δ 2.99 and 1.21, respectively. A 1 H COSY NMR experiment confirmed the coupling pattern in the compound.

When the solution containing compound 7 and NEt3 (2 equivalents) was left standing at room temperature for an extended period, a black precipitate started to deposit after ca. 24 h.

Monitoring this transformation by 1 H NMR showed no residual starting complex after 1 week.

Hence, even though this zwitterionic product has once again a non deprotonable ammonium group, its decomposition to metallic platinum can nevertheless take place. The evolution of the 1 H NMR spectrum is shown in the supporting information (Figure S1). The notable feature of the final spectrum is the absence of any resonance attributable to a vinylammonium product, as could be expected on the basis of the decomposition pathway shown in Scheme 4. On the other the spectrum clearly shows two mutually coupled resonances (confirmed 1 H COSY NMR spectrum), a triplet at δ 7.70 and a doublet at δ 3.37 in a 1:2 ratio with 3 JHH = 9 Hz. Other visible resonances are attributable to free ethylene (δ 5.40) and to protonated triethylamine (single set of resonances downfield shifted from those of free triethylamine, as can be expected from a rapid degenerate exchange between Et3N and Et3NH + ). These signals have the same pattern and are observed at very similar positions as those of the enamine complexes trans-[PtCl2(NHRR')(CH2CHNRR')] reported by BN as the decomposition products of [PtCl3(C2H4)] - + 2NHRR' as mentioned in the Introduction. 37 Obviously, the zwitterionic intermediate 10 could not lead to this type of product by the proposed pathway (Scheme 2) because the ammonium function cannot be deprotonated. Our next experiments were aimed at probing the generality of this unusual transformation at confirming the identity of the observed decomposition product, before attempting to rationalize the decomposition mechanism.

(c) Addition of pyridine to [PtBr3(C2H4)] -

Searching for other non deprotonatable amines, we considered the use of pyridine, with the additional advantage over triethylamine of a greater nucleophilic power. Addition of four equivalents of pyridine to the CD2Cl2 solution of 1 (as the nBu4P + salt) did not produce any immediate change the room temperature 1 H NMR spectrum. A low temperature NMR analysis was not carried out in this case, but related low temperature NMR studies have previously provided evidence of the reversible addition of pyridine to trans-[PtCl2(py)(C2H4)]. 48,49 Upon standing overnight under argon and protected from light, the NMR tube gave rise to the growth of yellow crystals and no observable formation of metallic platinum. The crystals were analyzed by X-ray diffraction, revealing the formation of complex cis-[PtBr2(py)2], see Figure 3. The greater coordinating power of pyridine is clearly responsible for the ligand exchange process, overruling the nucleophilic addition to coordinated ethylene and leading to loss of ethylene from the coordination sphere. The key structural parameters are given in the legend of Figure 3. A similar structure has previously been described for the aniline adduct, cis-[PtBr2(PhNH2)2], obtained by double ligand exchange from [nBu4P]2[PtBr4] and aniline. 8 It is worthy of note that the study of pyridine addition to trans-[PtCl2(py)(C2H4)] have only resulted in reversible addition to the coordinated ethylene and pyridine exchange, with no reported exchange of the olefin ligand. 49

(d) Addition of quinuclidine to trans-[PtBr2(NHEt2)(C2H4)]

Quinuclidine is electronically similar to triethylamine but has a much greater nucleophilic power given its much smaller steric demand. The addition of 2 equivalents of quinuclidine to compound 7 at -80°C in CD2Cl2 and immediate 1 H NMR monitoring shows evidence for quinuclidine nucleophilic addition to the coordinated ethylene with formation of a zwitterionic product (mutually coupled triplets for the Pt-CH2CH2-N moiety at δ 3.16 and 1.78, J = 8.8 Hz), but also of incipient ligand exchange with the release of free Et2NH, see SI (Figure S2). A yellow precipitate appeared rapidly at low temperature and increased by subsequent warming to room temperature, while there was no evidence for the formation of metallic platinum. It is possible that this solid, which was not isolated and further investigated, corresponds to a bis(quinuclidine) complex similar to that obtained with pyridine and presented in the above section. Since it seems that strongly nucleophilic amines divert the reactivity toward exchange of the ethylene and prevent the reductive decomposition process, we have abandoned this strategy.

(e) Addition of sodium methoxide to trans-[PtBr2(NHEt2)(C2H4)]

Given the recently reported evidence that the product of MeO -addition to certain Pt II -ethylene complexes results in the elimination of methyl vinyl ether and reduction to Pt 0 , 41 we wished to test the reactivity of compound 7 with this nucleophile. NaOMe is insoluble in dichloromethane, thus we could not observe the formation of the methoxyethyl product at low temperatures.

Nevertheless, treatment of the CD2Cl2 solution of 7 at room temperature with 5 equivalents of NaOMe led to complete degradation of the starting material over 48 h and to the generation of methanol (CH3 resonance at δ 3.45). The decomposition was accompanied by the massive precipitation of black Pt 0 . The final 1 H NMR spectrum of the solution is available in the Supporting Information (Figure S3). In this spectrum, the most striking feature is the presence of two mutually coupled resonance (confirmed by a 1 H COSY NMR experiment) at δ 7.71 (triplet) and 3.38 (doublet) with JHH = 9 Hz. These resonances are identical in chemical shift and coupling constant to those obtained for the product of the decomposition of compound 7 in the presence of Et3N. This evidence suggests that the addition of either NEt3 or MeO -to compound 7 leads to the same product, which therefore contains neither NEt3 not MeO -. The two mutually coupled triplet and doublet resonances are reminiscent of the Pt-CH2CHNRR' derivatives reported by BN. 37 We therefore speculated that the common product is trans-[PtBr2(Et2NH)(CH2CHNEt2)], 11, containing two diethylamine functions, one bonded as a ligand to platinum and the second one covalently bonded as an iminium function to a carbon atom. The basic reagents (NEt3 or MeO -) start the reaction by the experimentally observed (for Et3N) nucleophilic addition to the olefin carbon, but ultimately participate to the stoichiometry just as Brønsted bases by capturing the two protons. The resulting proposed stoichiometry is a revised version of that advanced by BN, 37 as shown in equation 3.

2 trans-[PtBr2(NHEt2)(C2H4)] + 2 Y  trans-[PtBr2(NHEt2)(CH2CHNEt2)] + 7
11

Pt 0 + C2H4 + 2 YH + + 2 Br - (3)
(Y = Et3N, MeO -, NHEt2)

(f) Preparation and characterization of trans-[PtBr2(NHEt2)(CH2CHNEt2)]

In order to confirm the validity of the above assumptions, namely the stoichiometry of (2.4191(5) and 2.4201(6) Å for the former, 2.089(4) Å for the latter). The most interesting feature of this structure is the configuration adopted by the -CH2CHNEt2 ligand. As mentioned in the introduction (Scheme 2) this may adopt in principle an enamine configuration (CH2=CH-NEt2) or an iminiumalkyl configuration ( (-) CH2-CH=N (+) Et2). The Pt atom is too far from atom C2 (2.655 Å) for the expected π coordination of the enamine limiting structure whereas the Pt1-C1-C2 angle is too narrow (96.3°) for the expected σ coordination of the iminiumalkyl ligand. a From ref. 50 . b From ref. 51 .

The observed configuration may be compared with those reported for cis- 50 which is best described as having an enamine ligand, and for trans-[PtCl2(2,4,6-NC5H2Me3)(CH2CH=N(OH)tBu)], 51 to which the iminiumalkyl configuration was assigned, see Table 2. Whereas the pattern of Pt-C distances and Pt-C1-C2

[PtCl2(PPh3)(iPrCH=CHNH(o-C6H4Cl))],
angle for 11 appears closer to that of the iminiumalkyl complex, the trends of the C1-C2 and C2-N distances in the three complexes clearly point to compound 11 having a configuration intermediate between the two extremes. The interaction between atoms Pt1 and C2 in 11 must be very weak to account for the equivalence of the methylene protons in the H NMR spectrum.

(g) Discussion of the zwitterion decomposition mechanism.

The results presented in the previous sections demonstrate that compound 7 leads to the deposition of Pt 0 and formation of the enamine complex 11 when treated with a strong base. This occurs not only when the base is Et2NH (i.e. identical to the ligand that is already present in 7), as also shown by BN for the analogous dichloride system, 37 but also when using Et3N or MeO -, according to equation 3. Obviously, the mechanism proposed by BN cannot explain the formation of compound 11, at least when using non deprotonatable nucleophilic reagents.

On the basis of first principles and previous literature evidence, we propose the following alternative mechanism (Scheme 5). Equilibrium addition of the Y nucleophile to the coordinated ethylene in 7 affords the zwitterion I, as experimentally observed for Y = NHEt2 in a previous contribution 15 and NEt3 in the present one. The reaction then proceeds from this intermediate by Complex III still features a hydride ligand. The latter is however bonded to a highly charged platinum atom (2-formal charge) and is thus presumably resistant to deprotonation. The -N (+) HEt2 ammonium group, on the other hand, is acidic and can be deprotonated by Y, leading to IV. The hydride ligand in IV is still under the influence of a very electron-rich Pt II center and therefore not deprotonatable. It should, on the contrary, be strongly hydridic. Therefore, we propose that a ligand exchange involving the hydride ligand in the dianion IV and the Et2NH ligand in neutral 7 takes place at this point, yielding the two monoanionic complexes V and VI.

Complex V may then expel Y, leading to the observed product 11 (the expulsion of Y may also occur on intermediate IV, before the ligand exchange with 7) whereas complex VI, featuring the hydride ligand in a now less electron-rich environment, may be deprotonated leading to Pt 0 , free C2H4, and two Br ions.

There are several key features leading to this transformation. The first one is that Y is a poorer ligand than Et2NH and thus cannot replace the latter and bind the metal. This is not the case, for instance, for pyridine and quinuclidine that are even capable of replacing the olefin and lead to cis-bis-amine adducts. Secondly, Y is also a less powerful nucleophile than Et2NH but is nevertheless capable of attacking the olefin as shown by the low-temperature NMR experiments (at least when Y = NEt3). However, it preferentially leaves the carbyl ligand in intermediate V to yield the enamine complex 11 rather than protonation-assisted loss of Et2NH to generate a putative trans-[PtBr2(Et2NH)(CH2CHY)] product. On the other hand, it is a strong Brønsted base and can thus serve well in this capacity to capture the protons that are generated along the transformation.

One still obscure point is the fate of the zwitterion 9, since no protons are available neither on the ammonium function not on the coordinated amine, but the formation of the predicted vinylammonium product could not be observed. It is possible that the expelled vinylammonium cation is subject to further transformations under these reaction conditions (excess tertiary amine) and proceeds to a final product that we have not yet been able to identify. However, even the zwitterion 9 ultimately decomposes to generate metallic platinum, consistent with the formation of a platinum hydride intermediate by β-H elimination.

  NHEt2)(C2H4)] (20.5 mg, 0.029 mmol) was placed in a Schlenk tube and dissolved in dichloromethane (2 mL). Triethylamine (5 µL, 0.036 mmol, 0.036 mmol) was then added by microsyringe. The mixture was then refluxed for 48 h, after which the solvent was removed by evaporation under reduced pressure. The reaction residue was purified according to procedure described by BN37 : it was chromatographed through a silica gel column using dichloromethane as eluent. The first colored fraction was collected and evaporated to dryness. It was characterized by 1 H NMR spectroscopy, mass spectrometry and X-ray diffraction. 1 H NMR (CDCl3): δ 7.71 (t, J = 9.2 Hz, 1H, -CH2CHN), 3.57 (br, 2H, CH-NCH2CH3), 3.46 (d, J = 9.2 Hz, 2H, -CH2CHN), 3.21 (m, 2H, CH3CH2N-Pt), 2.73 (m, 2H, CH3CH2N-Pt), 1.50 (t, J = 7.0 Hz, 6H, CH3CH2N-Pt), 1.30 (m, 2H, CH-NCH2CH3), 1.25 (t, J = 7.2 Hz, 3H, CH-NCH2CH3), 0.90 (t, J = 7.2 Hz, 3H, CH-NCH2CH3). MS (DCI, NH3): 529.1 (M + +H). A single crystal for the X-ray analysis was grown by slow evaporation of the CDCl3 solvent from the NMR tube.Reaction of [nBu4P][PtBr3(C2H4)] with pyridine.Complex nBu4P[PtBr3(C2H4)] (24.6 mg, ca. 0.034 mmol) was introduced in a 5-mm NMR tube. After a series of vacuum/argon cycles, the solid was dissolved in 0.6 mL of CD2Cl2. To the solution was added pyridine (9.2 µL, 9.0 mg, 0.114 mmol) by microsyringe through a rubber septum. No evidence for an immediate reaction was obtained from the 1 H NMR spectrum. Standing at room temperature overnight led to the deposition of single crystals, which were used for the X-ray structural analysis.Reaction of trans-[PtBr2(NHEt2)(C2H4)] with quinuclidine. Complex trans-[PtBr2(NHEt2)(C2H4)] (10.2 mg, 0.0236 mmol) and quinuclidine (4.8 mg, 0.0432 mmol) were placed separately into 5-mm NMR tubes, placed under argon and dissolved into CD2Cl2 (ca. 0.3 mL) at -80°C. The platinum complex solution was then transferred into the quinuclidine solution NMR tube. The resulting solution was homogenized before being analyzed by NMR spectroscopy at regular intervals.

Figure 1 .

 1 Figure 1. 1 H NMR spectrum in CD2Cl2 of a solution of [Ph4P][PtBr3(C2H4)] and NEt3 (10 equiv)

  Scheme 4. Hypothesis for the decomposition of [PtBr3(C2H4)] -in the presence of NEt3.

Figure 2 .

 2 Figure 2. 1 H NMR spectrum in CD2Cl2 of a solution of trans-[PtBr2(Et2NH)(C2H4)] and NEt3 (2

Figure 3 .

 3 Figure 3. ORTEP view of complex cis-[PtBr2(py)2]. Selected parameters (distances in Å, angles

equation 3

 3 and the identity of the Pt II product 11 as containing the CH2CHNEt2 ligand, we have repeated the reaction between compound 7 and only one equivalent of Et3N per Pt on a larger scale. After one week at room temperature, the mixture contained a black Pt 0 precipitate while the starting material had completely disappeared. Subsequent purification of the soluble product by chromatography on silica gel using dichloromethane as eluent produced a spectroscopically pure material, where all resonances (see FigureS4) could be clearly identified and assigned, also with the help of a COSY spectrum (FigureS5), to the corresponding protons of the proposed formulation of 11 as trans-[PtBr2(NHEt2)(CH2CHNEt2)]. Notably, we observe again the mutually coupled (JHH = 9.2 Hz) triplet and doublet resonances at δ 7.71 and 3.46 corresponding to 1 and 2 protons, the methylene and methyl resonances of the Pt-coordinated Et2NH ligand (the diastereotopic methylene protons are diagnostically present as two multiplets whereas the Me protons are equivalent) and those of the two inequivalent Et groups of the enamine NEt2 moiety, with the correct relative intensities. All these resonances are very close to those previouslyreported for analogous dichloride derivative.37

Figure 4 .

 4 Figure 4. ORTEP view of complex trans-[PtBr2(Et2NH)(CH2CHNEt2)], 11. Selected parameters

  Scheme 5. Proposed mechanism for the decomposition of trans-[PtBr2(Et2NH)(C2H4)] -in the

Reaction of [Ph4P][PtBr3(C2H4)] with NEt3. Compound

	according to the literature process. 8,14 Ethylene (purity ≥ 99.5%) was purchased from Air
	Liquide.
	Instrumentation. The 1 H NMR investigations were carried out at 298K on Bruker DPX300 or
	AV400 instruments operating respectively at 300.13 and 400.10 MHz. The low temperature
	NMR studies were carried out on a Bruker AV500 at 218 K, operating at 500.33 MHz. The
	spectra were calibrated with the residual solvent resonance and reported with positive chemical
	shifts in ppm relative to TMS.
	Ph4P[PtBr3(C2H4)] (10.2 mg,
	0.0127 mmol) was introduced in a 5-mm NMR tube. After a series of vacuum/argon cycles, the
	solid was dissolved in 0.6 mL of CD2Cl2. NEt3 (17 µL, 12.34 mg, 0.122 mmol) was then slowly
	added by microsyringe. The solution was then homogenized and used for the NMR investigation.
	42 PtBr2 (99.9 %, Aldrich), nBu4PBr (98%,
	Acros Organics), and Ph4PBr (98+%, Avocado) were used as received. Complexes
	Ph4P[PtBr3(C2H4)], nBu4P[PtBr3(C2H4)] and trans-[PtBr2(C2H4)(HNEt2)] were synthesized

Reaction of trans-[PtBr2(NHEt2)(C2H4)] with NEt3. (a) NMR monitoring. In

	two parallel
	experiments, samples of compound trans-[PtBr2(NHEt2)(C2H4)] (ca.10 mg, ca. 0.022 mmol)
	were introduced in 5-mm NMR tubes. After a series of vacuum/argon cycles, each sample was
	dissolved in 0.6 mL of CD2Cl2 and the tube was cooled to -80 °C in an acetone/liquid nitrogen
	bath. To each tube was then slowly added NEt3 (6.2 µL, 4.5 mg, 0.044 mmol for one; 15.4 µL,
	11.2 mg, 0.110 mmol for the other one) by microsyringe through a rubber septum. The resulting
	mixture was homogenized and analyzed by NMR.

Table S1 .

 S1 Crystallographic

data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 914166 and 914167. Copies of the data can be obtained free of charge on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44) 1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).

Table 1 .

 1

1 H NMR resonances observed for various trans-[Pt

  In CD2Cl2 at room temperature (from ref. 15). b Not observed. c In CD2Cl2 at -80°C.

	H a (8)	2.09	2.86	3.37	1.42	n.o. b	ca. 3.08	1.40	7.9
				3.21			ca. 2.79
	Et,Et c (9) 2.04	3.18	2.99	1.24	-	2.92	ca. 1.18	-
	Et,H c (10) 1.71	3.19	2.99	1.21	-	2.92	1.38	4.23
							2.66

a

Table 2 .

 2 Comparison of relevant structural parameters for enamine complexes of square planar Pt II .

	Compound	Pt-C1	Pt-C2	C1-C2	C2-N	Pt-C1-C2
	cis-[PtCl2(PPh3)(iPrCH=CHNH(o-C6H4Cl))] a	2.21	2.31	1.38	1.37	76.2
	trans-[PtBr2(Et2NH)(CH2CHNEt2)], 11	2.097	2.655	1.414	1.313	96.3
	trans-[PtCl2(2,4,6-NC5H2Me3)(CH2CH=N(OH)tBu)] b 2.09	2.70	1.53	1.22	95

Conclusion.

In pursuit of understanding the mechanism of decomposition of the PtBr2/Br -catalyst for ethylene hydroamination by aniline, we have studied the details of the addition of non deprotonatable bases (Et3N, pyridine, quinuclidine, MeONa) to coordinated ethylene in the model compound trans-[PtBr2(NHEt2(C2H4)] (7), as well as in the catalytic resting state [PtBr3(C2H4)] -(1). Bases with a considerable nucleophilic power (pyridine, quinuclidine) are able to replace the ethylene ligand. On the other hand, Et3N and MeONa lead to the deposition of metallic platinum demonstrating that, contrary to a previous suggestion, the reduction of the Pt II (C2H4)/amine system does not require deprotonation of the zwitterion intermediate. The On the basis of this new understanding, it is clear that Brunet's PtBr3/Br -catalytic system cannot be made active for the hydroamination of olefin by basic amines. Other platinum complexes may prove more active or less sensitive to reductive decomposition if appropriate modifications to the coordination sphere around the catalytic center can be envisaged to protect the zwitterionic intermediate against the β-H elimination process and/or the resulting hydride complex against deprotonation.

AUTHOR INFORMATION

Corresponding Author

*Rinaldo Poli. LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France. Tel +33-561333173. Fax +33-561553003. Email: rinaldo.poli@lcc-toulouse.fr.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


Aurélien Béthegnies

Rinaldo Poli

Convenient high-pressure syntheses of [PtX 3 (C 2 H 4 )] - (X = Cl, Br) salts with a variety of organic cations from PtX 2

Keywords: Platinum, Ethylene ligand, Halide complexes, Zeise anion, Chojnacki's anion, Synthesis

Reaction between PtX2 (X = Cl or Br) with an equimolar amount of the appropriate [Cat] + X -in toluene in an autoclave under 25 bars of C2H4 yields the corresponding [Cat][PtX3(C2H4)] in moderate to excellent isolated yields and high purity (X = Cl, Cat = nBu4P; X = Br, Cat = nBu4P, Ph4P, nBu4N). The same reaction does not yield any olefin complex if X = I and no reaction also occurs for the PtBr2+KBr system in water, THF, acetone or toluene.

Introduction

The ethylenetrichloroplatinate(II) complex, [PtCl3(C2H4)] -, represents a milestone in organometallic chemistry, being the first organometallic complex to be synthesized for a transition metal and isolated as the hydrated potassium salt in 1825 by the Danish chemist Zeise. [1][2][3] The tribromo analogue, [PtBr3(C2H4)] -, was introduced by Chojnacki in 1870, and also isolated as the potassium salt. 4 Until recently, the chemistry of these and other analogous (olefin)trihaloplatinate complexes has been investigated, specifically in terms of ligand exchange kinetics and mechanism, 5,6 only in combination with potassium or other alkali metals, mostly in water or aqueous methanol. Although a Li + salt was shown to be soluble in THF, 7,8 salts with bulky organic cations are expected to provide solubility in a broader range of organic solvents and would therefore allow the development of the coordination chemistry of the Zeise, Chojnacki and related anions in these different media.

A compound of stoichiometry [nBu4N][PtBr3(C2H4)], characterized by IR spectroscopy, was mentioned, 9 but no detailed synthetic procedure has appeared in the literature to the best of our knowledge. Only recently, motivated by the observation of a remarkable catalytic activity of PtBr2 in the presence of nBu4PBr toward the hydroamination of ethylene [10][11][12] and 1-hexene 13 by aniline derivatives, we have isolated the [PtX3(C2H4)] -salts (X = Cl, Br) in combination with the nBu4P + cation. This has allowed us to investigate ligand exchange equilibria in dichloromethane 14 that, in combination with theoretical calculations, 15 have led to the elucidation of the full catalytic cycle for the ethylene hydroamination reaction. 16,17 The synthesis of [nBu4P][PtX3(C2H4)] (X = Cl, Br) made use of a salt metathesis from Zeise's salt or Chojnacki's salt (the latter being made in situ by halide exchange from Zeise's salt and KBr), respectively. The pure products were obtained in 88 and 61% yield, respectively. 14 Although simple, this synthesis suffers from a relatively high market price of Zeise's salt and the need to use a large amount of KBr (to insure complete halide exchange) for the synthesis of the tribromo analogue, since Chojnacki's salt is not commercially available. In this contribution, we disclose a much simpler and widely applicable synthetic method making use of the less expensive PtCl2 and PtBr2, leading to the isolation in good to excellent yields of salts with a few different organic cations. The method should be easily adaptable to salts with many other cations and different olefins for both halide series.

Experimental section

2.1. General. All synthetic procedures were carried out under exclusion of air and using solvents that had been dehydrated by conventional methods. The 1 H, 31 P and 195 Pt NMR spectra were measured in CD2Cl2 or CDCl3 with Brucker (DPX300, AV400) instruments. The spectra were referenced against the residual solvent resonance for 1 H and against external 85% H3PO4 for 31 P and Na2[PtCl6] for 195 Pt. Elemental analyses were carried out with a Perkin-Elmer 2400 series II analyzer by the Microanalytical Service of the Laboratoire de Chimie de Coordination. Tetra(n-butyl)phosphonium iodide was prepared from nBu3P and nBuI according to the literature 11 and stored protected from light and under argon in a freezer. All other chemicals were purchased from commercial suppliers and were used as received: PtCl2 (99.9%, Aldrich), PtBr2 (99.9%, Aldrich), PtI2 (Alfa Product), C2H4 (≥ 99.5%, Air Liquide), nBu4PBr (98%, Aldrich), nBu4PCl (96%, Aldrich), Ph4PBr (98+%, Avocado), nBu4NBr (≥98%, Aldrich).

Preparation of [nBu4P][PtBr3(C2H4)

]. PtBr2 (100 mg, 0.28 mmol) and one equivalent of nBu4PBr (95.5 mg, 0.28 mmol) were introduced in a stainless steel autoclave, which was then put under an argon atmosphere after several purges. Toluene (10 mL) was then syringed into the autoclave, followed by pressurization with ethylene (25 bars). After one night (15 h) at 150°C under magnetic stirring, the autoclave was allowed to cool down, depressurized and opened in air. The yellow solution was transferred into a Schlenk flask and the autoclave was washed with toluene, the washings being collected and added to the reaction mixture. A small amount of black residue, probably metallic Pt, remained in the vessel. The collected solution was filtered and evaporated under reduced pressure until appearance of a precipitate (ca. 8 mL). The product precipitation was completed by addition of dry Et2O (5 mL). The product was recovered by filtration in air and dried under vacuum. Yield 190 mg (94%).

Preparation of [Ph4P][PtBr3(C2H4)

]. The same procedure described above for the preparation of [nBu4P][PtBr3(C2H4)] was adopted, except for the use of PtBr2 (49.6 mg, 0.140 mmol) and Ph4PBr (59.0 mg, 0.141 mmol), with the addition of 10 mL of dry toluene. At the end of the reaction, the recovered mixture contained a large amount of yellow-brown precipitate. The autoclave was washed with 5 mL of toluene and the resulting suspension added to the reaction mixture. Then, the autoclave was further washed with 5 mL of CH2Cl2, which completely dissolved the residual precipitate. The combined liquids were evaporated to dryness, The residue was redissolved in 3 mL of CH2Cl2 and filtered, leaving behind a small amount of black solid.

The product was obtained as a yellow-ocher precipitate after partial solvent evaporation to ca. 0.5 mL and addition of diethyl ether (2 mL). After separation from the mother liquor, the solid was washed twice with Et2O (2x5mL) and dried. Yield 93 mg (82%). 1 H NMR (CD2Cl2):  8.0 (m, 4H, Ph), 7.8 (m, 8H, Ph), 7.7 (m, 8H, Ph), 4.52 (s+d, 4H, 2 JPt-H = 65 Hz, C2H4). 195 Pt NMR (CD2Cl2):  -3428 (quint, 2 JPt-H = 65 Hz). Anal. % Calcd. for C26H24Br3PPt (M = 802.242): C, 38.93; H, 3.02. Found: C, 38.8; H, 2.6.

Preparation of [nBu4N][PtBr3(C2H4)

]. The same procedure described above for the preparation of [nBu4P][PtBr3(C2H4)] was adopted, except for the use of PtBr2 (100.7 mg, 0.283 mmol) and nBu4NBr (90.8mg, 0.282mmol), with the addition of 10 mL of dry toluene. The final mixture consisted of a yellow-orange solution and a small amount of black residue. The autoclave was washed with toluene (10 mL). The combined liquids were evaporated to dryness and the residue was dissolved in CH2Cl2 (3 mL) and filtered, leaving behind the black material.

The product was obtained as a yellow-ocher precipitate after partial solvent evaporation to less than 1 mL and addition of diethyl ether (ca 5 mL). After separation from the mother liquor, the solid was washed twice with Et2O (2x5mL) and dried. Yield 62.8 mg (31%). 1 H NMR (CD2Cl2):  4.59 (s+d, 4H, 2 JPt-H = 64 Hz, C2H4), 3.25 (s broad, 8H, CH2), 1.72 (s broad, 8H, CH2), 1.53 (s broad, 8H, CH2), 1.07 (t, 12H, 2 JH : 8Hz, CH3). 195 Pt NMR (CD2Cl2):  -3430 (quint, 2 JPt-H = 65 Hz). Anal. % Calcd. for C18H40Br3NPt (M = 705.316): C, 30.65; H, 5.73; N, 1.99. Found: 31.0; H, 5.6; N, 2.2.

Preparation of [nBu4P][PtCl3(C2H4)

]. The same procedure described above for the preparation of [nBu4P][PtBr3(C2H4)] was adopted, except for the use of PtCl2 (100.2 mg, 0.38 mmol) and nBu4PCl (102.4 mg, 0.38 mmol), with the addition of 10 mL of dry toluene. Yield: 148.4 mg, 66%. 1 H NMR (CDCl3): δ 4.48 (s+d, JPt-H = 64.3 Hz, 4H, C2H4), 2.30 (m, 8H, br, PCH2), 1.60 (m, 4, br, CH2CH2), 1.05 (m, 3, br, CH3). 31 P{ 1 H} NMR:  33. 14. 195 Pt NMR:  -2743(quint 2 JPt-H = 64Hz). Anal. % Calcd. for C18H40Cl3PPt (M = 588.930): C, 36.71; H, 6.85. Found: 37.2; H, 6.9.

Attempted preparation of [nBu4P][PtI3(C2H4)

]. The same procedure described above for the preparation of [nBu4P][PtBr3(C2H4)] was adopted, except for the use of PtI2 and nBu4PI.

A pale yellow solution was obtained, together with a red oil and abundant brown precipitate at the bottom of the autoclave. The 1 H NMR spectrum of both fractions did not reveal any resonance indicative of coordinated ethylene. The 195 Pt NMR spectrum exhibits a single resonance at δ -3691 with no observable coupling.

Results and Discussion

During our recent investigation of the PtBr2/Br -catalyzed addition of aniline to ethylene, we have been puzzled by the systematic catalyst degradation with generation of Pt 0 . 18,19 As part of our mechanistic study of this decomposition process, we have carried out tests using the same temperature (150°C) and reaction time (10 h) as the catalytic process, but in the presence of only one or the other of the two reactants. The optimized catalytic conditions for the reaction involved aniline and 25 bars with ethylene (C2H2/aniline = ca. 2) in the absence of solvents in a stainless steel autoclave, using 0.3% catalyst and 10 equivalents (3%) of nBu4PBr relative to the aniline.

The test carried out in the absence of aniline, therefore, required the replaced of this component with an inert solvent in order to favour the contact between the two solids (PtBr2 and [nBu4P]Br) and the gas (C2H4). We chose toluene in order to maintain a structural analogy with aniline. This test resulted in the complete dissolution of PtBr2 to yield a yellow solution and only trace amounts of a black precipitate that may correspond to metallic Pt. While proving the principle that ethylene alone was not the major cause of catalyst decomposition during the hydroamination process, we also discovered, from the NMR spectra recorded on the crude solution, that a selective reaction resulting in the clean formation of the [PtBr3(C2H4)] -ion had taken place according to the stoichiometry of equation 1. Since all the platinum complexes indicated in this contribution are know as air stable, we also wondered whether the synthesis could be carried out without special precautions to avoid contact with air. We have tested the synthesis of nBu4P + salt under these conditions but only obtained a low yield (ca. 15%) of pure product, the main product being an uncharacterized red oily material.

PtX2 + [Cat] + X -+ C2H4  [Cat] + [PtX3(C2H4)] - (X = Cl, Br) (1)
We therefore recommend the use of an inert atmosphere.

Extension of this procedure to the chloride system was tested only for the tetra-nbutylphosphonium salt. Compound [nBu4P][PtCl3(C2H4)] could be obtained in pure form and in 66% yield from PtCl2 and nBu4PCl. The same procedure was also attempted for the iodide system, reacting PtI2 and nBu4PI. However, the solution recovered from the autoclave after depressurization did not show any NMR resonance attributable to coordinated C2H4. Analysis of the literature reveals that no stable compound containing the [PtI3(C2H4)] -anion exists. On the other hand, complex [Pt2I6] 2-is known, obtained by anion metathesis from the reaction of [PtCl4] 2-and excess KI and isolated as the [nPr4N + ] salt. 20 It seems that ligand addition reactions

to [Pt2X6] 2-to yield [PtX3L] -(neutral L) or [PtX3Y]
2-(anionic Y -) adducts become less and less favourable in the order X = Cl > Br > I. The [PtI4] 2-species has been reported but is described as highly unstable, 21,22 existing in solution only in the presence of a large excess of I -salts, 23 and apparently no stable salt with this dianion has ever been isolated and characterized. Thus, the equilibrium shown in equation 2 is probably highly shifted to the left hand side for the iodide system and the final product of the reaction between PtI2 and nBu4PI in the presence of C2H4 could indeed be the [nBu4P]2[Pt2I6] salt. Although the reaction may well represent a convenient synthetic method for this salt, we have not pursued the isolation and characterization of this product. It is interesting to note that, whereas complex [PtCl3(C2H4)] -is stable toward loss of ethylene, complex [PtBr3(C2H4)] -tends to loses ethylene, because its synthesis by anion exchange from [PtCl3(C2H4)] -was shown to require a protecting C2H4 atmosphere to avoid the formation of [Pt2Br6] 2-. 24 [Pt2X6] 2-+ 2 C2H4 2 [PtX3(C2H4)] -

Finally, the success of the simple ethylene and halide addition to PtX2 in the presence of various cations made us wonder whether this would also be a simple method for the synthesis of

  1 H NMR (CD2Cl2):  4.60 (s+d, JPt-H = 64.3 Hz, 4H, C2H4), 2.30 (m, 8H, PCH2), 1.60 (m, 16H, CH2CH2), 1.05 (m, 12H, CH3). 31 P{ 1 H} NMR (CD2Cl2):  33.11. 195 Pt NMR (CD2Cl2): -3429 (quint, JPt-H = 64.3 Hz). Anal. % Calcd. for C18H40Br3PPt (M = 722.283): C, 29.93; H, 5.58. Found: 30.4; H, 5.6.

  Under the conditions of the above mentioned test, isolation of pure [nBu4P][PtBr3(C2H4)] is hampered by the large excess of nBu4PBr. However, repetition of the same reaction in the presence of the stoichiometric amount of nBu4PBr still resulted in the selective formation of Chojnacki's anion and in the isolation of the pure nBu4P + salt in nearly quantitative yield.Encouraged by this result, we have carried out the same reaction using Ph4PBr and nBu4NBr, obtaining the corresponding [Ph4P][PtBr3(C2H4)] and [nBu4N][PtBr3(C2H4)] salts in 93 and 31% yield, respectively, also in spectroscopically and analytically pure form. The lower yield obtained for the nBu4N + salt may be related to an insufficient time allowed for the solubilization and reaction of the reagents and/or to a more extensive decomposition to Pt 0 . The 1 H NMR analysis of the residual mother liquor confirms the efficiency of the precipitation step. It may be possible to further optimize this synthesis.

Chojnacki's salt. However, the reaction of PtBr2 and KBr using water as solvent and carried out under the same conditions (150°C, 15 h, 25 bars of C2H4) did not lead to any dissolution of PtBr2. Using THF, acetone or toluene instead of water as solvent led to some dissolution as evidenced by the colour of the final liquid but the NMR analysis of the crude solution gave no evidence of the formation of the desired [PtB3(C2H4)] -anion.

Conclusions

A simple observation of the behavior of PtBr2 in toluene in the presence of [nBu4P]Br and a moderate pressure (25 bars) of C2H4 has led us to develop a simple and general method for the synthesis of a variety of [Cat][PtX3(C2H4)] salts for X = Cl and Br. Extension to the preparation of the yet unknown triiodido analogue does not seem possible, at least at the relatively low ethylene pressure used in our study. It is possible that these syntheses may be further optimized by using different solvents, the optimum solvent possibly being cation dependent. It is also possible that the same strategy could lead to a wide variety of similar salts with other coordinated alkenes.

Acknowledgements.

We are grateful to the CNRS (Centre National de la Recherche Scientifique), the ANR (Agence Nationale de la Recherche, Programme Blanc HYDROAM, Grant No. NT09_442499) and the IUF (Institut Universitaire de France) for financial support.

(1) Zeise, W. C. Overs. K. Dan. Vidensk. Selsk. Forh. 1825-26, 13.

(2) Zeise, W. C. Annalen der Physik und Chemie 1831, 97, 497.

(3) Seyferth, D. Organometallics 2001, 20, 2-6.


Jean-Louis Verger-Gaugry

N Tamura

J.-N Barbier

come

Spatial extent of m35 -atomic clusters in 3D icosahedral quasicrystals and diffraction properties: the interpenetration concept

Spatial extent of m35 -atomic clusters in 3D icosahedral quasicrystals and diffraction properties:

the interpenetration concept Abstract. We have developed a theory of m35 -approximant crystals based on the long-range interpenetration of icosahedral atomic clusters positioned periodically. The structure factor is treated in perturbation over the icosahedral symmetry and site multiplicities. This new formalism allows, in an analogous way, to consider aperiodic distributions of interpenetrated similarly oriented clusters as in a model of quasicrystalline state and provides a "continuous link" between classical crystallography and "quasicrystallography".