( Cp

= C5me5

Gülnur Taban-Çalişkan

Dominique Agustin
email: dominique.agustin@iut-tlse3.fr

Funda Demirhan
email: funda.demirhan@bayar.edu.tr

Laure Vendier

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

Gülnur Taban-Çalışkan

Rational, Facile Synthesis and Characterization of the Neutral Mixed-Metal Organometallic Oxides Cp

Keywords: molybdenum, polyoxometallates, DFT calculation, X-ray powder analysis Organometallic oxides, tungsten, molybdenum, polyoxometallates, DFT calculation, X-ray powder analysis

come   L'archive ouverte pluridisciplinaire

Introduction

Mixed oxides containing two or more different metals attract interest because of a number of different applications in heterogeneous catalysis [1][2][3][4][5][6][7] and in the elaboration of chromogenic materials, [8] the advantage being the tunability of the desired property (reactivity, light absorption, …) by modification of the nature and relative proportion of the different metals. A drawback is the difficulty to obtain materials with a homogeneous metal distribution at the atomic level by the currently applied techniques (sputtering, CVD). [9] For catalytic applications, the use of polyoxometalates (POMs) can provide a "molecular scale" model for a better understanding of the interaction between the substrates and the oxide surface. [10][11][12] For several decades, polyoxometallic species have attracted much interest because of their great versatility, with applications from biology to materials science and catalysis. [13] Numerous research groups worldwide have specialized in the synthesis of these molecules with fine control of shape, size and elemental composition. [13,14] One synthetic strategy employs the assembly of predefined polyatomic fragments, often performed in organic solvents and with air-and water-sensitive organometallic precursors. [13,[15][16][17][18][19][20] Another method uses the grafting of simple organic fragments on lacunary oxoclusters. [14, 21- 29] Hydrothermal synthesis from elementary bricks is also an alternative, though limited by serendipity. [30,31] Some of these syntheses lead to heterometallic species but need to be performed in specific environment, i.e. under argon, or under pressure, or using very sensitive species. Stability and/or compatibility with protic reagents and solvents are also useful for applications in "non-innocent" aqueous media. The introduction of organometallic moieties in a POM framework has attracted considerable attention, [13,[32][33][34][35][36][37][38] because the organometallic fragment may impart new reactivity to the molecule relative to the all-inorganic equivalents. This strategy also allows the selective assembly of mixed-metal clusters under mild conditions, often using aqueous solvents. Specifically, the reaction of Na2MO4 (M = Mo, W) with [Cp*RhCl2]2 or [( 6 -arene)RuCl2]2 in water or acetonitrile yielded the octanuclear compounds [(LM)(MoO)(-O)3]4 [LM = Cp*Rh [32] and (p-MeC4H4iPr)Ru [33] ] and the tungsten analogues [(( 6arene)Ru)4(WO)4(-O)12] (arene = C6Me6, pMeC6H4iPr). [38] We have worked extensively on the aqueous chemistry of the air-stable organometallic complexes Cp*2M2O5 (M = Mo, W) [39][40][41][START_REF] Collange | Perspectives in Organometallic Chemistry[END_REF][START_REF] Demirhan | [END_REF][44][45][46][47][48][49][50] and have recently reported the synthesis and structure of Cp*2Mo6O17, 1. [51] This is a hexanuclear organopolyoxometallic species with a Lindqvist-type octahedral arrangement of the six Mo atoms, as shown in Scheme ions. [52][53][54][55] Small amounts of crystals suitable for a structural determination were obtained by a serendipitous slow decomposition of Cp*2Mo2O5 under strong acidic conditions, but the selective and high yield synthesis was achieved by condensation of Cp*2Mo2O5 with four equivalents of Na2MoO4 under acidic conditions.

The identity and purity of the microcrystalline powder was confirmed by comparison of the Xray powder diffraction spectrum with that simulated from the single crystal data. On the basis of this finding, we set out to extend this strategy to the full series of [(Cp*M)2(M'O)4(2-O)12(6-O)] complexes with M,M' = Mo, W, including the previously unknown mixed-metal species. The M = M' = W compound, Cp*2W6O17, has previously been described but was only obtained as "major oxidation product" from a sealed-tube reaction between [Cp*W(CO)2]2 and the arsaoxane impurity in the cyclic polyarsine c-(AsCH3)5 as oxygen transfer agent. [56] As described herein, the same synthetic protocol leads to the full series of Cp*2M2M'4O17 compounds.

Results and Discussion (a) Synthesis

The reaction between Cp*2M2O5 and M'O4 2-(M, M' = Mo, W), under the conditions previously described for the synthesis of the Mo6 derivative 1, [51] yield all other three complexes in excellent yields and purity, see equation 1.

Cp*2M2O5 + 4 M'O4 2-+ 8 H +  Cp*2M2M'4O17 + 4 H2O (1) Compound M M' 1 Mo Mo 2 W Mo 3 Mo W 4 W W
Mixing a methanolic solution of Cp*2M2O5 (this compound is insoluble in pure water but soluble in aqueous methanol solutions) and an aqueous solution of Na2M'O4 (4 equiv) yields the products as fine precipitates immediately after acidification. The compound colour varies from orange-brown to yellow-green, depending on the Mo/W ratio. Direct contact with metallic materials (i.e. syringe needles or spatulas) causes reduction with strong darkening of the compounds colour towards green and should therefore be avoided.

Previous work in our group has shown that the Cp*-Mo bond in Cp*2Mo2O5 is quite robust and withstands exposure to aqueous media in the entire pH range, at least for reasonably long periods of time. [39] Analogous studies for the W analogue have not been carried out, but W-ligand bonds are usually stronger than the Mo congeners. Thus, it is not surprising that the Cp*M moieties are transferred intact from the Cp*2M2O5 starting materials to the products. Our previous studies have also shown that Cp*2Mo2O5 ionizes in water and yields the species Cp*MoO2 + and Cp*MoO2(H2O) + at low pH. [39,57] Once again, the behaviour under the same conditions of the W analogue has not been investigated but can be assumed to parallel that of Mo. Thus, the synthesis can be seen as the assembly of individual organometallic Cp*MO2 + and inorganic M'O4 2-species, regulated by stoichiometry and pH. The products are stable in water at low pH. Upon raising the pH, however, the solid redissolves in the medium, slowly at pH 6, faster at higher pH, possibly regenerating a mixture of the starting materials (the formation of Cp*2Mo2O5 was confirmed by IR on the isolated residue). Unfortunately, the compounds are too little soluble in common solvents for characterization by techniques such as NMR or electrochemistry, but they could be characterized by solid state analytical methods (IR, TGA and powder X-ray diffraxction, vide infra) (b) DFT calculations DFT geometry optimizations were carried out for the purpose of assigning the IR absorption bands and interpreting the effect on them of the metal nature. To save computational time, the calculations were run on simplified models (indicated by roman numerals corresponding to the compounds numbering scheme) where the Cp* ligands were replaced by Cp rings. Before examining these results, we present a brief description of the optimized geometries. The results are collected in Table 1. Since the four M' atoms occupy structurally distinct positions, we shall distinguish them by introducing the M'2M"2 notation, limited to the structural description. The agreement between experimental and calculated geometries for the M2M'2M"2O17 core is generally very good, including the asymmetry of the M-Ob, M'-Ob and M"-Ob distances, which is more pronounced for the Mo6 structure but experimentally visible also in some of the W6 structural parameters. The reason for this distortion is not clear, but the computational method seems to confirm that the distortion is a molecular phenomenon, not related to crystal packing. The molecules are located on a crystallographic twofold axis, as shown in Scheme 2, thus each pair of M'=Ot, M"=Ot, M-Oc, M'-Oc and M"-Oc bonds are equivalent by symmetry. Although the geometry optimizations were carried out without any symmetry constraints, this twofold symmetry was essentially maintained in the optimized geometries. The calculated distances to the terminal and bridging ligands are generally only slightly longer than those experimentally observed (maximum deviation 0.04 Å for M-OM" in I), whereas the distances to the central O atom are underestimated for M-Oc (by 0.01 Å in I and by 0.06 Å in IV) and overestimated for M'-Oc and M"-Oc (by up to 0.11 Å in IV). Note that Oc is much closer to M than to M' and M" and that the calculations tend to place Oc even closer to M relative to the experimental structure, which could be related to the use of the simplified model.

After establishing the suitability of this level of theory to reproduce the structural features, it is now interesting to examine some specific trends along the series of the optimized structures. The M'=Ot and M"=Ot distances slightly lengthen on going from Mo to W. All distances to Oc, on the other hand, shorten on going from Mo to W. No clear trends can be established for the distances to the Ob atoms because of the asymmetry caused by the distortion from C2v symmetry. It is also difficult to establish whether there are secondary effects, for instance of the nature of M' and M" on the M-Ob distances. Going from Mo to W for M' and M" seems to lead to M-Oc shortening, whereas no significant change is visible on the M'=Ot and M"=Ot distances by the nature of M. Given the sensitivity of the DFT optimization procedure, we do not consider differences at the level of the third significant digit as reliable. Secondary effects will be more clearly revealed by the IR study.

M'' O b M O b M' O b M' O b M Cp Cp O c M'' O b O b O b O b O b O b O b O b O t O t O t O t C 2
Scheme 2. Atom labelling scheme used in Table 1.

Table 1. Selected bonding distances for the geometry optimized models I-IV and comparison with the experimental structures of 1 and 4. [a] System [a] For the definition of the symbols used, see Scheme 2. [b] From the Xray structure published in ref. [51] . [c] From the X-ray structure published in ref. [56] .

1 [b] I II III IV 4 [c] (M, M'=M") (Mo,Mo) (Mo,Mo) (W,Mo) (Mo,W) (W,W) (W,W) Cp-M 2.

(c) IR characterization

Infrared spectroscopy is a good characterization tool for polyanions, especially when they possess high symmetry such as the M6O19 2-Lindqvist anions (M= Mo, W). [58,59] The number of vibrations observed for salts of these anions agree with the Oh space group and a correlation between experimental and calculated spectrum could be established. [60] As already discussed above, the symmetry of systems 1-4 is reduced to C2 (considering Cp* as a rapidly rotating ligand), with an ideal C2v symmetry if the M-Ob, M'-Ob and M"-Ob asymmetry is averaged.

The observed (for 1-4) and calculated (for I-IV) IR spectra in the metal-oxygen stretching region are shown in Figure 1 and the most prominent absorptions are listed in Table 2 (calculated) and (experimental). Views of the normal modes of vibrations corresponding to these calculated bands are available in the Supporting Information.

The terminal M'=Ot and M"=Ot vibrations are in the higher frequency part of the spectrum (observed, 950-1000 cm -1 ; calculated, 1020-1050 cm -1 ), whereas the vibrations involving the Ob atoms are in the 750-850 cm -1 range. A few Cp C-C in-plane and C-H out-of-plane bending motions are also present in this region (840-960 cm -1 ) but have low intensity and do not significantly contribute to the calculated spectrum. The match between calculated and observed spectra is satisfactory, the frequency difference being related to either the computational method or to the model (Cp vs. Cp*) or to a combination of both. The observed and calculated relative intensities, on the other hand, match rather well, giving us confidence in the assignment of the observed bands. We can now discuss with confidence the frequency trends related to the metal changes. First of all, the nature of the normal modes for the bands associated to the Ob atoms does not allow a fine analysis of the distortion from the ideal C2v to the observed C2 symmetry, thus the higher symmetry point group labels are used in Table 2 and in the discussion. Four terminal metal-oxido (M'=Ot and M"=Ot) vibrations are expected and indeed calculated. One of them, s(M'=O), is very weak because the two bonds are essentially co-linear and the overall dipole moment change is small. The small intensity is due to the deviation from co-linearity and to a small vibrational coupling with the s(M'=O) vibration, also of a1 type (see Figure in the Supporting Information). Thus, only three major bands are essentially observed in this region. The calculated frequencies show both a primary and a secondary effect. As M' and M" go from Mo (I/II) to W (III/IV), all four frequencies experience a blue-shift. However, a blue shift is also caused by the secondary effect of changing M from Mo (I/III) to W (II/IV). From the experimental spectra only two (M=O) bands can be unambiguously determined. Comparison with the calculated spectra suggest that the strongest one is as(M"=O), whereas the second smaller band is most probably the highest frequency a1 bandThe determination of the band positions is less accurate because of the broadening and resolution, but the same trends observed for the calculated bands appears to be observed, the primary effect being most evident in the 2/4 comparison, while the secondary effect is clearly evident in both 1/2 and 3/4 comparisons. The 12 metal-(bridging O atom) bonds yield in principle 12 normal modes, but only 5 of the 6 asymmetric motions can be clearly identified for the calculated spectrum (and 3 in the observed spectrum) in the typical "fingerprint region", while the symmetric motions are located at lower frequency and combined with other types of vibrations. These bands also shift to higher frequency when one or both metals directly implicated in the bond(s) is(are) changed from Mo to W. Experimentally, the most significant change in this region is seen for a change of the inorganic metal, whereas a change of the organometallic one produces hardly any change. For the a1 [Mt-O-Mt] band, which has the main contribution from M' and M", the secondary effect of changing M is small. All other observed bands involve a combination of M-Ob on one side and M'-Ob or M"-Ob on the other side, thus a distinction of primary and secondary effects is not possible.

(d) TGA measurements

All compounds lead to complete loss of the organic part, with a good match between experimentally observed and theoretical mass losses on the basis of equation 2, upon warming up to 600°C in thermogravimetric experiments conducted in air. The spectrograms for the TGA of compounds 1 and 3 are shown in Figure 2, while those of 2 and 4 are given in the Supporting Information. The onset of Cp* ligands loss occurs at lower temperatures for 1 (190°C) and 3 (230°C) where these ligands are bonded to Mo, whereas higher temperatures are necessary to induce loss of the Cp* bonded to W (270°C for 2, 310°C for 4). A secondary effect of the inorganic metal, however, is notable by comparing these temperatures for the 1/2 and 3/4 couples. Interestingly, the two Cp* rings are lost in two quite distinct and sharp steps for compound 1, whereas a more gradual loss within a broader temperature range is observed for the other three compounds. The resulting residues from 1 and 4 had the expected colour (white for MoO3, yellow for WO3). The final product of the thermal decomposition from 2 and 3 is thus a mixedmetal oxide material with controlled metal composition, Mo2/3W1/3O3 and Mo1/3W2/3O3, respectively.

(e) X-ray powder diffraction

As mentioned in the introduction, single crystal X-ray structures have already been determined for compounds 1 and 4 [51,56] and shown to be isomorphours (monoclinic C2/c) with very similar unit cell parameters. We therefore considered that the same crystal system may also be adopted by the mixed-metal compounds 2 and 3. Furthermore, a powder diffraction study was also reported for compound 1, as obtained in microcrystalline form by the same condensation process of equation 1 in aqueous solution, and shown to match with the pattern calculated from the single crystal data. [51] Thus, we have proceeded to measure powder X-ray diffraction patterns for compounds 2 and 3. Compound 2 was obtained in sufficient microcrystalline quality to give a well resolved powder spectrum, which is shown in Figure 3. This pattern was compared with that calculated from the model obtained by changing the M atoms from Mo to W in the known X-ray structure of compound 1. All measured peaks are accounted for by the model, showing that the sample does not contain any other diffracting phases. All subsequent attempts to refine the atom positions, even when restricted only to the metal atoms, did not lead to convergence. This is probably due to the transmission geometry used for recording the diffracted intensities. Indeed, since it was difficult to synthesise high quantity of powder (necessary for working in reflection mode), we used the transmission mode on capillary sample and the high absorption prevented a good simulation of the diffracted intensities. The diffraction geometry also resulted in high peak asymmetry, as most clearly visible for the most intense peaks at low angles. However, free refinement of the metal atoms thermal parameters and of a common thermal parameter for the C and O atoms gave very reasonable values, with R = 5.14, Rp = 17.8, Rwp = 17.8, and a Bragg R factor of 13.82. It is notable that refinement of all other possible models corresponding to the metal placement as expected for 1, 3 and 4 gave unreasonably small (when the W position was modelled by Mo) or high (when the Mo position was modelled by W) thermal parameters. Figures of these refined structures are shown in the SI. The R factors for the three wrong models were slightly greater (1: 6.28 %; 3: 9.04 %; 4: 6.51 %) than for the correct model. Thus, the powder X-ray diffraction experiment provides strong evidence that compound 2 has the same structure previously determined for 1 and 4, with Mo occupying solely the inorganic sites and W occupying solely the organometallic sites. The same conclusions concerning the structure and the site selectivity can reasonably be assumed for compound 3.

Conclusions

We have presented here a rational and facile synthesis of organometallic Lindqvist-type polyoxo(Group 6 metal) compounds of type Cp*2M2M'4O17 (M = Mo, W). This route, already known for the M = M' = Mo member (compound 1), has now been extended to afford a high-yield selective synthesis of the already known M = M' = W member (compound 4) and also to prepare the previously unknown mixed metal systems. Thermal decomposition of these compounds has been shown to yield mixedmetal oxides Mo2/3W1/3O3 and Mo1/3W2/3O3 with an expected homogeneous distribution of the two metals, which may be of interest for the study of the metal influence in heterogeneous catalytic as well as in chromogenic applications.

Experimental Section

All experiments were performed in air. Compounds Cp*2Mo2O5 and Cp*2W2O5 were synthesized according to the literature. [48] Water was deionized and methanol (Carlo Erba, analytical grade) was used as received. Sodium tungstate dihydrate (Na2WO4.2H2O) and sodium molybdate dihydrate (Na2MoO4.2H2O) were purchased from Aldrich and used as received. Elemental analyses (C, H and O) were performed by the LCC Analytical Service Laboratory. The IR spectra were recorded on KBr pellets at room temperature with a Mattson Genesis II FTIR spectrometer and the data were processed with WinFirst software. The TGA measurements were carried out on a SDT Q600 V20.9 thermal analyzer. A quantity of each sample was placed into a nickel/platinum alloy crucible and heated at 0.83 K.s -1 under reconstituted air flow up to 600 K. An empty crucible was used as a reference. Synthesis of Cp*2Mo6O17, 1. This procedure follows that reported previously, except that the latter contains a trypographical error in the amount of acid used. [51] Thus, using Cp*2Mo2O5 (27 mg, 5.0•10 -2 mmol) in 1.25 mL of MeOH, Na2MoO4•2H2O (48.4 mg, 0.20 mmol) in 1.25 mL of water, and a 1 M HNO3 solution (0.4 mL, 0.4 mmol), led to 23. 5 mmol) was dissolved in 2 mL of MeOH. Separately, Na2WO4•2H2O (55 mg, 0.17 mmol) was dissolved in 1.5 mL of water. The two solutions were mixed with no apparent change. A 1 M HNO3 solution (0.33 mL, 330 mmol) was slowly added, forcing the immediate precipitation of a yellow-white powder. The mixture was stirred for one day, and then the yellow solid was filtrated, washed with H2O, then with MeOH, and finally dried at 70°C under vacuum (46.4 mg, 67.5% yield). IR (KBr pellets, cm -1 ): 1498s, 1442s, 1376s, 996sh, 980s, 896sh, 806s, 741w, 600sh. Anal. for C20H30W6O17 (1645.48): calcd C 14.59, H, 1.83; found C 15.14, H 1.33. TGA loss of Cp* and uptake of O, % exp. (calcd): 16.3 (15.4). X-ray powder analyses. The XRD data was collected at room temperature using a theta/theta Panalytical MPdPro powder diffractometer, using Cu Kα radiation (λ = 0.15406 nm), in transmission mode on a capillary sample. The XRD pattern was recorded with a step width of 0.016 and a counting time of 1000 s step -1 . The sample was measured over a 2θ range from 5° ≤ 2θ ≤ 70°. The indexation of the pattern, the fullprofile matching and the Rietveld refinement were performed using the Winplotr interface. [START_REF] Roisnel | WINPLOTR[END_REF] Computational Details. The guess geometries for I and IV were based on the crystallographically determined structures of 1 and 4, [51,56] replacing all CH3 groups by H atoms in the Cp* ligands. From the resulting optimized geometries, starting geometries for II and III were generated by changing the metal. All optimizations were carried out on the gas phase isolated molecules using the Gaussian 03 suite of programs [START_REF] Frisch | Gaussian 03, Revision D.01[END_REF] with the B3LYP functional, which includes the three-parameter gradient-corrected exchange functional of Becke [START_REF] Becke | [END_REF] and the correlation functional of Lee, Yang, and Parr. [64,65] The standard 6-31G** basis set was used for the C, H and O atoms, while the CEP-31G* basis set [66] was adopted for Mo and W. We found that the commonly used LANL2DZ basis set on the metal atom, tested for system I, resulted in longer bond lengths than the CEP-31G* basis, with a greater different relative to the experimental values, most likely because polarization functions were not added to the metal atom basis set. Analytical frequency calculations were also run on the optimized geometries, yielding positive frequencies for all normal modes. The calculated IR spectra shown in Figure 1 were generated from the DFTgenerated frequencies and intensities by applying Lorentzian functions and adjusting the linewidth to best fit the experimental spectra.

  1. Each Mo atom bears a terminal Cp* or oxo group, all edges of the octahedron are oxo-bridged, and there is an additional central (6-O) atom bonded to all Mo centers, thus the formula can be written as [(Cp*Mo)2(MoO)4(2-O)12(6-O)]. It is isoelectronic with the [Cp*Mo6O18] -and [Mo6O19] 2-

Scheme 1 .

 1 Scheme 1. Structure of the [(Cp*M)2(M'O)4(2-O)12(6-O)] compounds (M, M' = Mo, W).

Figure 1 .Table 2 .

 12 Figure 1. Experimental (plain lines) and DFT calculated (unscaled, dotted lines) IR spectra in the Mo-O stretching region for compounds 1/I (A), 2/II (B), 3/III (C), 4/IV (D).

Figure 2 .

 2 Figure 2. Spectrogram for the TGA of compounds 1 (above) and 3 (below). The solid curves correspond to the percent weight loss (left y axes) and the dashed curves to the heat flow (right y axes).

5 Figure 3 .

 53 Figure 3. Powder X-ray diffraction patterns for compound 2. Bold plain line (above): experimental spectrum; plain line (middle): simulated spectrum; dashed line (below): difference.

Table 3 .

 3 Observed metal-O vibrations (cm -1 ) in the 700-1100 cm -1 region.

	1	2	3	4	Assignment
	764m	780s	760s	789sh	[Mt-O-Mt]
	794s	797s	793s	799s	[Mt-O-Mt]
	820m	825sh	820sh	820sh	[Mt-O-Mt]
	968s	971s	967s	977s	as(M"=O)
	980m	984m	979m	994m	(M'=O)

  mg of product (62% yield). The IR spectrum matched that previously reported. Anal. for C20H30Mo6O17 (1118.08): C, 21.44 (21.48); H, 2.60 (2.70). calcd C 21.48, H 2.70; found C 21.44, H 2.60. TGA, loss of Cp* and uptake of O, % exp. (calcd): 23.1 (22.7). Cp*2W2O5 (30 mg, 4.18•10 -2 mmol) was dissolved in 2 mL of MeOH. Separately, Na2MoO4•2H2O (40 mg, 0.167 mmol) was dissolved in 2 mL of water. The two solutions were mixed without apparent change. A 1 M HNO3 solution (0.75 mL, 0.75 mmol) was then slowly added, forcing the immediate formation of a dark yellow precipitate, which became paler yellow after ca. 1 h. After stirring overnight at room temperature, the solid was filtered, washed H2O, then with MeOH (where it is sparingly soluble), and finally dried at 70 °C (38.dissolved in 2.5 mL of water. The two solutions were mixed with no apparent change. A 1 M HNO3 solution (1.8 mL, 1.8 mmol) was slowly added, forcing the immediate formation of an bright orange precipitate that darkens with time until green. The mixture was stirred overnight. The solid was filtered, and finally dried at 70 °C under vacuum.

	Synthesis of Cp*2W2Mo4O17, 2. 5
	mg, 71.3% yield). IR (KBr pellets, cm -1 ): 1496s, 1442s, 1377s, 1081s,
	1024sh, 984sh, 972s, 828sh, 801s, 785sh, 600s. Anal for C20H30W2Mo4O17
	(1293.88):.calcd C 18.56; H 2.34; found C, 18.47, H 1.73. TGA loss of Cp*
	and uptake of O, % exp. (calcd): 19.3 (19.6).
	Synthesis of Cp*2Mo2W4O17, 3. Cp*2Mo2O5 (54.2 mg, 0.1 mmol) was
	dissolved in 2.5 mL of MeOH. Separately, Na2WO4•2H2O (131.9 mg, 0.4
	mmol) was The powder has been then washed with diethylether to remove unreacted
	Cp*2Mo2O5. (yield: 86.5 mg, 15%). IR (KBr pellets, cm -1 ): 1491s, 1444s,
	1376s, 1265w, 1173w, 1093w, 972s, 920sh, 890sh, 851sh, 809s, 767sh,
	706s, 610s, 546s. Anal for C20H30Mo2W4O17 (1469.68): calcd C 16.34, H
	2.06; found C 16.68, H 1.70. TGA loss of Cp* and uptake of O, % exp.
	(calcd): 19.7 (17.3).
	Synthesis of Cp*2W6O17, 4. Cp*2W2O5 (30 mg, 4.2•10 -2

Acknowledgments

We are grateful to the Centre National de la Recherche Scientifique and to a binational (France-Turkey) Bosphore programme for funding this research. We also thank the French Embassy in Ankara for financial support of the Ph.D. thesis of G.T.C. ____________

Supporting Information (see footnote on the first page of this article): Figures of normal vibrational modes, structural models for the X-ray diffraction of compound 2, and TGA spectra of compounds 1, 3 and 4 (4 pages).

Entry for the Table of Contents


Pavel A Dub

Mireia Rodriguez-Zubiri

Jean-Claude Daran

Jean-Jacques Brunet

Rinaldo Poli
email: poli@lcc-toulouse.fr

Platinum-catalyzed ethylene hydroamination with aniline: Synthesis, Characterization and Studies of Intermediates

Keywords: platinum, ethylene complexes, aniline complexes, hydroamination, DFT calculations

) have been obtained by efficient one-pot procedures. All have been fully characterized by microanalysis (C, H, N), multinuclear NMR spectrometry ( 1 H, 13 C, 195 Pt), UV-visible spectroscopy, and single crystal Xray diffraction. Compound 1 slowly loses Br -in solution to yield (nBu4P)2[Pt2Br6] (1'), which has also been characterized crystallographically. The relative stability of the various compounds has been probed experimentally by NMR studies in several solvents and computationally by gas phase geometry optimizations followed by C-PCM calculations of the solvation effects in dichloromethane and aniline. The calculations also included the bis(ethylene) complexes [PtBr2(C2H4)2] in the trans (two different conformations 7 and 7') and cis (8) configurations.

The solution experiments gave no evidence for a nucleophilic attack of aniline onto coordinated ethylene under mild conditions (T up to 68°C), setting a lower limit of 29 kcal mol -1 for the activation barrier of this process. Therefore, the relative energies computed for the other compounds suggest that all ethylene-containing complexes (2, 4, 5, 7 and 8) are viable candidates for the key nucleophilic addition step of the PtBr2-catalyzed ethylene hydroamination by aniline. Use of the isolated complexes 2, 4 or 5 in combination with nBu4Br as precatalysts for the ethylene hydroamination by aniline yields similar catalytic activities.

Introduction

The catalytic hydroamination of non-activated olefins using the combination of air-stable Pt II or Pt IV halo-salts and phosphonium halides, nBu4PX has recently been described. [1][2][3][4] These are amongst the most performing systems ever reported for the hydroamination of ethylene by weakly basic amines such as aniline and 2-chloroaniline (TON > 150 after 10 h at 150°C, 0.1 mol % of PtBr2 and 150 equiv of nBu4PBr for the aniline addition to ethylene). 3 In addition, they allow the hydroamination of higher olefins such as hexene-1 with a high regioselectivity (95% Markovnikov). 4 The catalytic cycle that has been proposed for this system (see Scheme 1) 3 involves [PtBr4] 2-, which is then transformed into [PtBr3(C2H4)] -by a substitution reaction with ethylene. Subsequent reaction with aniline gives [PtBr2(ArNH2)(C2H4)] (Ar = Ph or o-C6H4Cl), which is expected to be more active (more electrophilic) towards nucleophilic attack by aniline on the coordinated ethylene. Nucleophilic attack in complex [PtBr2(ArNH2)(C2H4)] or [PtBr3(C2H4)] -then leads to [ArNH2-CH2-CH2-PtBr3] -, which ultimately evolves to the final product either by direct proton migration from N to C (possibly assisted by an external proton shuttle such as a second aniline molecule), or via the Pt IV hydrido-complex [ArNH-CH2-CH2-PtHBr3] -, followed by reductive elimination, as was recently suggested by a DFT-study, performed however on different models ([PtCl(PH3)2(C2H4)] + and NH3). 5 A related catalytic cycle based on olefin activation, rather than N-H bond activation, has also been proposed for the intermolecular hydroamination of unactivated olefins with carboxamides catalyzed by related Pt II catalysts. 6 The spectacular rate enhancement induced by the large Br -concentration 2 is probably related to the high negative charge on the Pt atom in intermediate [ArNH2-CH2-CH2-PtBr3] -, promoting the intramolecular proton transfer. Although this is so far the most efficient system for the intermolecular hydroamination of non-activated olefins, its performance is still largely insufficient for application to industrial scale production. A possible further improvement requires the thorough understanding of the catalytic cycle. Thus, we set out to carry out experimental and computational investigations aimed at elucidating the catalytic cycle and at identifying the nature and energetics of the rate determining step. To this aim, we have isolated and investigated as many of the proposed intermediates as possible, and analysed all the proposed steps. The results obtained from this investigation are organized in two separate contributions. The present article reports the isolation and characterization of all species that participate in the low-energy portion of the catalytic cycle, as well as experimental and computational studies of the equilibria in which these complexes are involved. The second contribution, currently in preparation, will report the computational investigation of the high-energy part of the catalytic cycle.

As will be shown in the present contribution, compound [PtBr2(PhNH2)(C2H4)] is obtained stereoselectively in the trans geometry by the reaction sequence of Scheme 1. Our synthetic work has also led to the isolation of the new complexes [PtBr3(PhNH2)] -and cis-[PtBr2(PhNH2)(C2H4)], not included in Scheme 1, and to their consideration as intermediates of alternative pathways. For completeness, we have also synthesized and investigated compound cis-[PtBr2(PhNH2)2]. Though a few of these complexes have already been reported in the literature, all have been obtained by new and more efficient (one-pot) high-yield syntheses starting from the commercially available Pt-precursors K2PtCl4 and K[PtCl3(C2H4)]•H2O (Zeise's salt). Full characterization, namely by multinuclear NMR ( 1 H, 13 C, 195 Pt) and UV spectroscopies, and by X-ray diffraction is provided for all these complexes. The present study contributes to the understanding of the hydroamination catalytic mechanism by highlighting the presence of previously unsuspected species in the catalytic mixture and to establish the need of their consideration as potential catalytic intermediates.

Results

(a) Syntheses and characterization

The synthetic work carried out along this study is summarized in Scheme 2. Potassium tetrachloroplatinate(II) is a less expensive source of platinum than the analogous Br-containing complex and does not suffer from potential contamination, 7 while it can be quantitatively converted to the all-Br analogue under optimized conditions. Thus, this compound was the choice starting material for our synthetic studies. However, a few synthetic procedures were also carried out from the commercially available Zeise's salt.

(nBu4P)2[PtBr4] (1) and (nBu4P)2[Pt2Br6] (1').

There are two main access routes to [PtBr4] 2-in the literature: (a) reduction of K2PtBr6 by N2H6SO4 or K2C2O4 8 and b) halide exchange from K2PtCl4 and KBr on a steam bath, 9, 10 though a detailed description of the experimental procedure is not available. Furthermore, the similar solubility of all salts renders the separation difficult, leading to low yields of the pure final product. From the recent literature, 11 compound (Bu4N)2[PtBr4] was obtained in 80% yield from K2PtCl4 and KBr (saturated water solution), followed by extraction by Bu4NBr/CH2Cl2. However, very large KBr quantities are necessary to yield a saturated aqueous solution and this excess makes the purification more problematic. We have sought to solve this problem by keeping the amount of KBr to the minimum amount required to insure a quantitative exchange (as monitored by 195 Pt NMR, see Experimental section) and subsequently extracting the Pt complex into an organic solvent by cation exchange. Thus, use of a CH2Cl2 solution of nBu4PBr gave (nBu4P)2[PtBr4] (1) in 68% yield from K2PtCl4 and 90 equiv KBr in water under mild conditions, see experimental section (Scheme 3). The structure of 1 was determined by a single crystal X-Ray diffraction study. The anion has the expected square planar configuration. A view of the dianion and the selected bond distances and angles are provided in the supporting information. The average Pt-Br distance in the dianion (2.424 Å) compares well with the value of 2.419 Å reported for the same ion in [PtBr(dien)]2[PtBr4] (dien = NH2CH2CH2NHCH2CH2NH2). 12 Significantly longer distances, however, were found for the anhydrous (2.445(2) Å) 13 and hydrated 14 S2 and Table S2). The geometry is in good agreement with that determined previously for the corresponding NEt4 + salt. )]. A view of the anion geometry with selected bond distances and angles is presented in Figure 1. A more extended list and comparison with DFT calculated parameters is presented in the SI (Table S3). In this structure, there is no significant difference between the lengths of the three Pt-Br bonds. Differences in crystal packing (nBu4P + vs. K + and the presence of one H2O molecule in the latter salt) could be responsible for this effect. Note that a greater discrepancy between trans and cis lengths was also observed in two-dimensional determinations of Zeise's salt [27][28][29][30] (e.g.

2.39 for Cltrans vs.2.29 and 2.27 Å for Clcis), relative to the more precise 3-dimensional X-ray study [2.327(5) vs. 2.314(7) and 2.296(7) Å) 31 and to a neutron diffraction study (2.340(2) vs.

2.302(2) and 2.303(2) Å]. 32 The C=C distance is identical to that measured in Zeise's salt nBu 4 PBr/CH 2 Cl 2 1 eq. aniline, 1h 90 eq KBr, 3 h

H 2 O, pH = 2 H 2 O, pH = 2 H 2 O, pH = 2 (nBu 4 P)[PtBr 3 (an)] K 2 [PtCl 4 ] K 2 [PtBr 4 ] K[PtBr 3 (an)] 3 Scheme 5.
The structure of the [PtBr3(PhNH2)] -ion in compound 3 shows the expected square planar geometry, see Figure 2. Selected bond distances and angles are presented in the SI (Table S4).

To date, the Cambridge Structural Database (CSD) contains 48 structures of type [PtX3L] -, with X = halogen and L = N-based donor, most of which feature chlorides and aromatic Nheterocycles as donors. In none of these structures is the N-donor ligand aniline or another aryl amine. Indeed aryl amines are much less basic than either aliphatic amines or heterocyclic Ndonor ligands (e.g. pyridine) and the structural chemistry of platinum complexes with these ligands is essentially unexplored. The only precedents of structurally characterized aniline derivatives appear to be trans-PtI2(NH2-C6H4-4-R)2 (R = Et, iPr), where the Pt-N distance is 2.060(7) and 2.029(11) Å, respectively. 33 Similarly to the above-discussed complex 2, the Pt-Br

trans-[PtBr2(PhNH2)(C2H4)] (4)

. This compound has already been reported, its synthesis starting from Chojnacki's salt. 40,41 We have now optimized a one-pot synthetic procedure from K2PtCl4 in 49% yield. Alternatively, starting from Zeise's salt (halide exchange first, then aniline addition), a yield of 73% was obtained (Scheme 6). It should be noted that 4, as well as all nBu4P salts described in this paper, is not soluble in water. This provides a driving force for the ligand exchange reaction, because the anionic tribromo anion is in fact thermodynamically preferred in solution (see equilibrium studies below). The exclusive formation of the trans isomer can be explained by the well known trans-effect of the C2H4 ligand.

50 eq KBr, 20 h

H 2 O/C 2 H 4 bubbling K[PtCl 3 (C 2 H 4 )] H 2 O C 2 H 4 bubbling, 5h
EtOH/HBr 90 eq KBr, 3 h minor traces of residual free aniline, as already reported for other related systems. 42,43 This point will be addressed in greater detail below. The UV-visible spectrum of 4 in dichloromethane shows a strong UV absorption at 263 nm (ε = 4950 cm -1 M -1 ) which compares with the reported 40 value of 264 nm (ε = 4158 cm -1 M -1 ) in MeOH, and two previously unreported shoulders at max ca. 310 nm (ε = 1280 cm -1 M -1 ) and ca. 340 nm (ε = 605 cm -1 M -1 ), the tailing of which in the visible range is responsible for the observed yellow color.

H 2 O trans -[PtBr 2 (PhNH 2 )(C 2 H 4 )] K 2 [PtCl 4 ] K 2 [PtBr 4 ] K[PtBr 3 (C 2 H 4 )] 4
The single crystal X-ray analysis of 4 shows two independent and essentially identical molecules in the asymmetric unit, one of which is shown in Figure 3 with selected bonding parameters. The crystal packing reveals a 3-dimensional network of NH2•••Br H-bonding (see Figure S3). Additional bonding parameters are available in the SI (Table S5). As stated above, the coordination chemistry of Pt II with aryl amines has been little explored. The Pt-N distance in

cis-[PtBr2(PhNH2)(C2H4)] (5). Compound cis-[PtBr2(PhNH2)(C2H4)]

was obtained in a one-pot procedure in 42 % isolated yield from K2PtCl4 (Scheme 7), by exchanging the order of addition of the neutral ligands relative to the synthesis of the trans isomer described above. The reaction is stereoselective because of the trans-effect pattern (Br -> PhNH2). Compound 5 is not soluble in H2O and in low-polarity organic solvents (CH2Cl2, THF), sparingly soluble in acetone and soluble in DMF; it is not stable in DMSO. 46 The Variable temperature 1 H NMR spectrum of 5 in DMF-d7. (18).

A view of the molecular geometry, as determined by X-ray diffraction, is shown in Figure 5 with a selected list of bonding parameters. Additional bonding parameters are available in the Supporting Information (Table S6). Like for the trans isomer 4, compound 5 shows an extensive intermolecular H-bonding network between the Br ligands and the aniline NH2 protons (2.758 and 2.848 Å) and also a contact between one of the C2H4 protons and a Br ligand (2.882 Å), see supplemental figure S3. There is apparently only one precedent for a cis-PtBr2(alkene)(L)

structure with an N-donor ligand, namely cis-PtBr2(C2H4)NH3, 47 for which the Pt-N distance is reported as 2.13 Å and the Pt-Br distances as 2.51 (trans to C2H4) and 2.40 (trans to Br) Å, the difference being attributed to trans effects. In compound 5, on the other hand, the difference between the two Pt-Br distances is much less important. The C-C distance is identical to those found in compounds 2 and 4 within the experimental error.

cis-[PtBr2(PhNH2)2] (6).

In the course of this study, the previously reported 48 compound cis-[PtBr2(PhNH2)2] has also been obtained (94% yield from K2PtCl4, see Scheme 8). The purity was determined by elemental analysis and the configuration was confirmed by the X-ray structural analysis. The related cis-PtCl2(PhNH2)2 complex was described as unstable in DMF, converting to the less soluble trans isomer which precipitates. 49 In contrast, compound 6 shows stability in DMF-d7 for at least 2 days, with only small intensity new resonances developing in the 1 H ( 5.20) and 13 C{ 1 H} ( 114.3) NMR spectra.

aniline, 2 eq 90 eq KBr, 3 h

H 2 O K 2 [PtCl 4 ] K 2 [PtBr 4 ] cis-[PtBr 2 (an) 2 ] H 2 O 6 Scheme 8.
The solid state structure of compound 6 has also been determined by X-ray crystallography, although the crystal quality did not allow a satisfactory data refinement.

Nevertheless, the structural determination is sufficient to establish the chemical connectivity and molecular geometry, which is presented in Figure 6. The two cis aniline ligands adopt a conformation that places the two phenyl rings face to face. The same arrangement occurs intermolecularly, indicating a -stacking interaction (see packing diagram in Figure S5). In order to evaluate the likely nature of the resting state of the Pt II hydroamination catalyst, various ligand exchange processes involving the synthesized complexes 1-6 were investigated.

Accurate measurements of the equilibrium constants were not attempted because of a variety of technical difficulties (ethylene escapes into the gas phase, very slow equilibration times, …), but the position of chemical equilibria could be qualitatively assessed in each case. Treating a solution of 2 with 8 equiv of nBu4PBr in CD2Cl2 at 298 K yielded no immediate change.

However, monitoring the intensity of the 1 H resonance of coordinated C2H4 revealed a slow decrease (ca. 18% after 2.5 h; 20% after both 24 h and 4 days), indicating partial transformation to 1. A resonance for free C2H4 was not observed, probably because it escapes into the NMR tube head space. However, the presence of 1 was confirmed by its 195 Pt-resonance. The formation of compound 1 from 2 and excess bromide was also indicated by the results of the synthetic studies: compound 1 contaminated 2 when this was prepared by halide exchange from Zeise's salt, unless operating under an ethylene atmosphere (vide supra, Scheme 4 and Scheme 6). These results suggest that the equilibrium of reaction 1 lies on the left hand side, even in the presence of excess Br -. This is likely to hold true under catalytic conditions (high C2H4 pressure, high T), although the effect of T on the equilibrium has not been experimentally established.

[PtBr3(C2H4)] -+ Br - [PtBr4] 2-+ C2H4 (1) 2 1
Addition of 8 equiv of nBu4PBr to complex 3 in CD2Cl2 at 298 K produces an upfield shift of the o-, m-and p-Ph resonances, while the NH2 resonance slightly shifts downfield and decreases in relative intensity by 24%. This behaviour is immediate and no further change occurs within the next few hours. The resonance shift may be attributed to the establishment of hydrogen bonding between the NH2 protons of the coordinated aniline ligand and free Br -. This interaction is expected to further polarize the N-H bond and deshield the proton. Similar effects of H bonding by Br -ligands on acidic proton resonances (for instance in imidazolium salts) have been previously observed. 50 The presence of both free and coordinated aniline was confirmed by 13 C NMR. This experiment suggests that the equilibrium position of reaction 2, like that of equation 1, is shifted to the left hand side.

[PtBr3(PhNH2)] -+ Br - [PtBr4] 2-+ PhNH2

(2)

3 1
The above proposition was confirmed by studying the reaction in the opposite direction.

Mixing compound 1 and PhNH2 (0.67 equiv) in CD2Cl2 shows no immediate change for the aniline peaks, while a precipitate (probably compound 6) starts to form in a few hours. In order to maintain a homogeneous system, thus obtaining more significant information on the reaction equilibrium and rate, the solvent was changed to DMF-d7. Using a substoichiometric amount of PhNH2 (0.39 equiv) resulted in extensive but very slow consumption of the aniline (5 % after 25 h, 43 % after 5.5 days) and formation of 3. There was no evidence of precipitation, nor formation of any compound 6 in solution. Thus, it seems that the formation of 6 is only favoured in solvents in which it is insoluble. This proposition was confirmed by monitoring the reaction of 3 and PhNH2 (equation 3), as well as the reverse reaction between 6 and nBu4PBr, in DMF-d7.

No change was observed in the spectrum of 3 + PhNH2 (0.97 equiv) after 17 h at room temperature, while the spectrum of 6 + nBu4PBr (0.61 equiv) showed the slow formation of 3 and free aniline (0.3 equiv after 23 h and complete reaction in 4 days). In conclusion, the reaction of 1 with aniline (1 equiv) to produce 3 is thermodynamically favoured under homogeneous conditions, but the further addition of a second equivalent of aniline to yield 6 is not. The relative stability of 2 and 3 in the presence of both aniline and ethylene cannot be probed directly, because the reactions of 2 with PhNH2 and of 3 with C2H4 yield the two

isomeric neutral complexes [PtBr2(C2H4)(PhNH2)]. [PtBr3(PhNH2)] -+ PhNH2 cis-[PtBr2(PhNH2)2] + Br - (3) 3 6
The next investigation was aimed at probing the relative thermodynamic stability of compounds 2, 4 and 5, by examining the equilibrium position of the ligand exchange reactions 4 and 5. This study was carried out in CD2Cl2, THF-d8 and DMF-d7 at different temperatures, with 1 H NMR monitoring of the C2H4 and PhNH2 positions. DMSO-d6 could not be used because of complications related to ligand exchange with solvent molecules, see Experimental section.

Although compound 5 was obtained from 3 and C2H4 with release of Br -, the reverse reaction of 5 with Br -results in the substitution of aniline rather than ethylene. Reaction 4 was investigated at different molar ratios in both directions. The ligand exchange is fast, yielding equilibrium within the time needed to record the first spectrum for each experiment. Indeed, it is fast even on the NMR timescale above 200 K, since only one C2H4 resonance (range: 4.4-4.7 ppm) and one resonance for each type of aniline NH2 and ring proton (range: 6.5-7.5 ppm) was observed, the position of which depends on the reagents ratio.

Figure 7 shows an example in THF-d8 at 298 K. Related studies in CD2Cl2 are illustrated in the supporting information (Figure S6). Lowering the temperature slows down the ligand exchange and separate resonances can be observed for the ethylene resonances of the two Pt complexes at T < 200 K (see supplemental Figure S7). Equilibrium could be conveniently approached from either side, because of the dominant trans effect of the ethylene ligand (formation of compound 5 was not observed in these solvents). The equilibrium position is heavily displaced toward the right hand side, since addition of nBu4PBr to 4 gave an essentially quantitative transformation when using 1 equiv of the bromide salt or more. On the other hand, practically no change was observed upon mixing 2 with small amounts of aniline (1-2 equiv). Only after addition of a ca.

10 equiv of aniline could the formation of compound 4 became observable, as evidenced by a shift of the averaged C2H4 resonance. Unfortunately, the rapid exchange did not allow the observation of individual resonances for the accurate determination of the equilibrium parameters and use of the chemical shifts for the same determination was prevented by the chemical shift dependence on H-bonding. However, we can clearly conclude that equilibrium 4 is strongly shifted toward the right hand side at room temperature in all solvents (CD2Cl2, THF-d8 and DMF-d7).

/ppm 4.50 5.00 5.50 6.00 6.50 7.00 7.50 8.00 the free Br -is progressively consumed. This effect is analogous to that described above for the Br -addition to 3 and is thus attributed to the same phenomenon, namely hydrogen bonding between free Br -and the NH2 protons of the coordinated aniline ligand. However, the effect in this case is transmitted through four bonds to the ethylene proton resonance. It is also possible that Br -establishes a direct interaction with the C2H4 protons. It was not possible to determine which equilibrium among 4 and 5 is shifted toward 2 to a greater extent. We can only conclude that the thermodynamic stability of the 5/Br -mixture is closer to that of the 4/Br -mixture than to that of the 2/PhNH2 mixture since both equilibria are largely displaced toward the right-hand side.

δ/ppm The NMR observations above underline the strong effect of the ethylene ligand on the exchange rates at the trans position. Additional indication of this effect is hinted by the shape of the aniline NH2 resonance in the various compounds: the broadness of the resonance could also be related to a dynamic process involving exchange of the aniline acidic protons, but the presence of 195 Pt coupling for this resonance in compounds 3 and 5 and its absence in compound 4 strongly hints to a more rapid aniline ligand exchange in the latter compound as a result of the stronger ethylene trans effect. In order to verify this hypothesis, additional studies have been carried out by intentionally mixing additional aniline with the three above cited compounds. The result is the observation of separate resonances for free and coordinated PhNH2 when using compound 3 or 5, while only a single average resonance (the position of which depends on the PhNH2/Pt ratio) is observed for compound 4, see supplemental Figure S8. Cooling the solution broadens the aniline signal, but decoalescence is not observed in THF-d8 down to 200 K.

As mentioned in the introduction, a key step of the catalytic cycle is the amine nucleophilic addition to the coordinated alkene yielding a zwitterionic intermediate (6 or 8 in Scheme 1).

Stable zwitterionic σ-alkyl complexes are formed by the addition of basic amines to coordinated ethylene in cis-or trans-[Pt(C2H4)Cl2L], for instance when using diethylamine, although these derivatives are often found to equilibrate with the amine/ethylene complex mixture in solution. 51 The reaction products of eq 6 have been isolated and characterized by 1 H NMR spectroscopy [51][52][53][54][55][56][57] and in one case by a single-crystal X-ray diffraction study. 58 This reaction appears limited to amines of sufficient basicity (pKa > 5 for the conjugate ammonium ion). For instance, aniline failed to give an observable addition compound 53,54 when reacted with trans-

[Pt(C2H4)(Et2NH)Cl2]. NR 3 + Cl Pt Z Y Cl Pt Z Y R 3 N-H 2 C-H 2 C
cis: Z = Cl; Y = PR 3 , py, NH 3 , DMSO trans: Y = Cl, Z = NR 3 (6) An NMR investigation of the interaction between 4 and 5 with PhNH2 was carried out in hope of observing the formation of the hydroamination product and perhaps also the putative zwitterionic Pt II or hydrido Pt IV intermediate (Scheme 1). The study was carried out in CD2Cl2, THF-d8 and DMF-d7 for compound 4 and only in DMF-d7 for compound 5 for solubility reasons.

In THF and CD2Cl2, compound 4 could be investigated up to the reflux temperature, whereas the studies in DMF-d7 were limited to 298 K because extensive decomposition occurs at higher temperatures for both complexes. In the case of 4, the only observable phenomenon was the fast PhNH2 exchange already described above. No new resonance that could be attributed to a σalkyl complex or to the expected PhNHEt product was observed for either complex under any conditions. In conclusion, the activation barrier that must be overcome for the nucleophilic addition of PhNH2 to compounds 4 or 5 is such that no significant reaction occurs at the temperatures used for this study (< 68°C). Considering the NMR error (at least 1% reaction for detection), the time of the experiment (1.5 h) and the temperature, the activation free energy is estimated as >29 kcal mol -1 at 68°C from the Eyring equation. The nucleophilic addition of aniline to complexes 2, 4 and 5 as well as all subsequent steps leading to the hydroamination product and to the catalyst regeneration have been investigated computationally and will be reported in a forthcoming separate contribution.

(c) Ethylene hydroamination by aniline with complexes 2, 4 and 5 as catalysts.

In order to verify that the ethylene-containing complexes 2, 4 and 5 are possible intermediates of the catalytic cycle, we checked whether they are able to catalyze the hydroamination of ethylene by aniline under the same conditions previously reported 1 for PtBr2 and whether they provide similar results in terms of activity and selectivity (refer to Scheme 9).

The results are shown in Table 3. It can be seen that all compounds yield the main hydroamination product, N-ethyl aniline, with TON in the range 75-100 under the same experimental conditions. Similar selectivities are also found with the various compounds. The catalytic tests with PtBr2 have also been repeated and found to yield results consistent with those previously reported. 1 Since no significant difference was observed by reducing the amount of nBu4PBr to only a tenfold excess, all subsequent runs were carried out with the lower bromide salt amount. The results of Table 3 confirm that complexes 2, 4 and 5 are competent precatalysts and suggest that they all yield the same catalytic system obtained from PtBr2. According to the proposed catalytic cycle (Scheme 1) and as shown by our synthetic work, these complexes can be readily formed under catalytic conditions. Since the rate determining step of the hydroamination process is probably a later step according to Scheme 1 (e.g. the nucleophilic aniline addition to coordinated ethylene, or the elimination of the hydroamination product), 5 the transition state of which is at higher energy than those associated to the formation of 2, 4 and 5, all these complexes should be in equilibrium with each other under the catalytic conditions and should therefore in principle yield the same catalytic activity.

(d) DFT calculations

The geometry optimizations were carried out in the gas phase. The gas-phase thermochemical data, which included the full thermal correction to the gas phase Gibbs free energy, 59 were then corrected by solvation effects in aniline and in dichloromethane, using the results of C-PCM calculations on the fixed gas-phase geometries. Dichloromethane was chosen in order to compare the computational results with the experimental equilibrium studies described in section (b), most of which were conducted in that solvent. Aniline was also used because the hydroamination catalysis is conducted in the absence of solvent, thus the substrate itself (aniline) provides the reaction medium for dissolution of all metal complexes. The polarity of the medium may be significantly altered by the dissolved ionic co-catalyst, nBu4PBr, but no simple method is available to include this effect in the calculations to the best of our knowledge.

The relative free energies in aniline solution were also calculated at 423.15K, temperature at which the catalytic experiments were carried out. Since the translational and rotational contributions are significantly quenched upon going from the gas phase to the solution, we also derived relative free energy values using only the vibrational contribution of the entropy as a corrective term. 60 These values, however, are less in agreement with the experimental data and will only be given and briefly discussed in the SI.

A full list of computed gas-phase energies and free energies, and solvation free energies in dichloromethane and aniline is available as Supporting Information (Table S8). All computed systems are labelled with Roman numerals, with the numeric value corresponding to that of the isolated compound (for instance, the [PtBr4] 2-ion of compound 1 is given label I, the [PtBr3(C2H4)] -ion of compound 2 is labelled as II, etc.). The calculations also addressed the possible formation of bis(ethylene) complexes, trans-[PtBr2(C2H4)2] (two isomers, VII and VII') and cis-[PtBr2(C2H4)2] (VIII). These are not known compounds, but the corresponding cis-[PtCl2(C2H4)2] has recently been isolated and crystallographically characterized, 61 whereas the trans isomer does not appear to be a stable complex. [61][62][63] It is interesting to verify whether the relative energy of these complexes is sufficiently low to warrant their consideration as potential intermediates in the catalytic cycle.

The relevant optimized geometrical parameters of the platinum complexes I' and II-V and views of the optimized geometries are shown next to the related experimental X-ray data in the Supporting Information section (Tables S2-S7). There is generally good agreement between experimental and calculated data, with all distances being slightly overestimated by the calculations as typically found for DFT calculation with hybrid functionals. The discrepancy is highest for the Pt-Br distances and strongest in the dianion (2.527 Å vs. an average of 2.424 Å or  = +0.103 Å for I) and slightly smaller in the monoanions ( = +0.074 for II, +0.049 for III), and in the neutral species ( = +0.068 for IV, +0.035 for V). The neglect of the counterion in the calculation of the ionic species is probably not responsible for this discrepancy, because strong ion pairing is expected to lengthen, not shorten, bond distances. The calculations confirm the absence of a notable trans influence of C2H4 relative to Br on the Pt-Br distance, whereas the Pt-Br bond is slightly shorter (by ca. 0.04 Å) when located trans to aniline in complexes III, V and VI. The Pt-N bond is also little affected by the nature of the trans ligand (Br or C2H4; maximum differences of 0.03 Å). The trend of the ethylene C-C distance is interesting (1.398 Å in IV, 1.404 in V, 1.408 Å in II), because it suggests a  back-bonding trend in the order IV < V < II. This distance variation is too small to be experimentally revealed by the X-ray technique for compounds 4, 5 and 2, but is consistent with the observed shift of the (C=C) vibration. Thus, 4 should be the most reactive of these three compounds toward nucleophilic attack by external aniline. The geometry of VI differs from that found in the solid state in that the Ph rings of the two different aniline ligands are located on opposite sides of the coordination plane. This energy minimum is also obtained when using the solid state geometry as input. This result suggests that the observed solid state geometry is enforced by the intermolecular -stacking interactions.

The geometry of the bis(ethylene) adducts was optimized as both trans and cis isomers.

Although the introduction of a second ethylene molecule to complex II would be directed to the trans position by the trans effect of the first ethylene ligand, a subsequent isomerization may occur as recently shown for the dichlorido analogue. 61 For the trans complex, two different configurations were probed, the first one having both ethylene C-C bonds perpendicular to the coordination plane (VII) and the second one with one perpendicular and one parallel C-C bond (VII'). The relative energy of VII' is, as expected, higher than that of VII. Thus, only the bis(perpendicular) arrangement (experimentally observed for the dichlorido analogue) was calculated for the cis isomer VIII. In system VII the C-C distance is shorter (1.382 Å) than in systems II, IV and V, since the two C2H4 ligands are in mutual competition for electron density through Pt-C2H4 back-bonding. The distance in the cis isomer VIII is longer (1.395 Å), though still marginally shorter that in IV. In system VII' the ethylene ligand with the perpendicular C-C bond is more strongly affected by back-bonding (C-C: 1.397 Å, about the same as in IV), whereas the parallel ethylene ligand is very weakly bonded to Pt (Pt-C = 2.331 Å, vs. 2.195 for the perpendicular ligand) and the C-C distance is much shorter (1.372 Å), though still significantly lengthened relative to free ethylene [1.330 Å, cf. the experimental value of 1.3391 (13) 64 ]. The reduced back-bonding in these ligands, while the C2H4-Pt dative interaction is still effective, makes them also interesting candidates for a nucleophilic attack by external amine. The relative energies of systems I-VIII are given in Table 4. There is little difference between the results in aniline and dichloromethane solution, all relative G values being slightly less negative (or more positive) in dichloromethane. This is probably related to the greater ability of dichloromethane to solvate charged species ( = 8.93) relative to aniline ( = 6.89), thus having a greater bias in favour of [PtBr4] 2-. Solvation (Table S8) increases in the order: neutral species (IV, V, VI, VII, VII', VIII, C2H4, PhNH2) < monoanions (II, III, Br -) < dianion (I).

Note that all systems are more stable than I, except for VII' which is ca. 5 kcal mol -1 less stable than its isomer VII. The absence of the counterion in the calculations may disfavour the ionic species relative to the neutral ones, since ion pairing is expected to provide a slight energetic stabilization to the system. This means that in reality species II may be even more stabilized, relative to the neutral species IV, V, VI, VII and VIII, than shown by the values in Table 4.

Note that the relative free energies in solution experience minor changes on going from room temperature to 150°C (temperature of the catalytic runs) and the situation remains qualitatively the same. Under all conditions, the species with the lowest free energy in solution is II. This system is thus the most likely candidate as catalyst resting state. All these species are relatively close in energy and much lower than the transition state leading to the hydroamination product (estimated as > 29 kcal mol -1 relative to II at 68°C, vide supra). Consequently, they can all be considered to be present either as intermediates or as off-loop equilibrium species under catalytic conditions.

Discussion

The main interest of this investigation was to throw more light onto the mechanism of the recently reported hydroamination of ethylene by aniline catalyzed by ligandless PtBr2, upon activation by nBu4PBr. What we have learned is that all the isolated compounds 1-6, and even the bis(ethylene) complexes trans-PtBr2(C2H4), 7, and cis-PtBr2(C2H4)2, 8, are likely to exist, in equilibrium with each other, under catalytic conditions. Only the trans isomer of the mixed ethylene-aniline complex [PtBr2(PhNH2)(C2H4)] (4) was previously proposed as intermediate.

The major species in solution (catalyst resting state), however, is likely to be [PtBr3(C2H4)] -, so far as a sufficient pressure of C2H4 is present. Thus, the initial part of the catalytic cycle can be revised as shown in Scheme 10. All the ethylene-containing complexes (II, IV, V, VII, VII' and VIII) may be susceptible to nucleophilic attack by aniline. Using the Boltzmann distribution and the values in Table 4, the relative amounts of the ethylene-containing species at 150°C may be estimated as II/IV/V/VII/VII'/VIII = 1 : 3.3•10 -3 : 2.2•10 -4 : 7.2•10 -7 : 3.9•10 -9 : 1.3•10 -5 . In spite of the small equilibrium amount estimated for many of these species, they must all be considered as potential substrates for the nucleophilic attack by aniline in the subsequent elementary step because a higher energy (and thus less populated) species may lead to a lower barrier for the rate determining step of the catalytic cycle. Investigations on these subsequent steps are currently ongoing and will be reported in a forthcoming contribution. An important point is the qualitative agreement between the experimental results of the equilibrium studies and the computational results, provided that the free energies in solutions are used. Precise calibration of the theoretical method is not possible because we could not obtain accurate equilibrium constants for the various ligand exchange processes (Equations 1-5).

However, all observations are qualitative reproduced by the computational study. Namely, the conversion of I into I' and Br -is spontaneous (though slow). The higher free energy of I' and C2H4 relative to II agrees with the literature report that [Pt2Br6] 2-transforms into [PtBr3(C2H4)] - upon addition of ethylene. 65 The relative G values of II/Br -and III/Br -agree with the displacement of equilibria 1 and 2 to the left hand side. The G value of VI/Br -relative to III/PhNH2 agrees with the displacement of equilibrium 3 to the left hand side (the opposite process is driven by the precipitation of 6). Furthermore, II (+PhNH2) is also thermodynamically preferred relative to IV and V (+Br -), in agreement with equilibria 4 and 5 being displaced to the right hand side. According to the calculation, the trans complex IV is more stable than its cis isomer V. Finally, the lower energy of VIII relative to VII qualitatively agrees with the spontaneous trans to cis isomerization of the analogous PtCl2(C2H4)2 complex. 61 Given the general agreement between theory and experiment, this computational level appears suitable to further investigate the subsequent steps of the catalytic cycle, for which no experimental information is available.

Conclusion

This contribution provides straightforward procedures for the one-pot synthesis of several

Pt II bromido derivatives, some previously reported, others described here for the first time. The work described in this contribution has also served to clarify a few important points of the ethylene hydroamination process catalyzed by the PtBr2/nBu4PBr system. A greater number of ethylene complexes than previously imagined are found as likely candidates for the crucial aniline nucleophilic addition step. A rich Pt II coordination chemistry in the presence of bromide, ethylene and aniline ligands has been unveiled and all the isolated complexes are suggested by the DFT study to be viable intermediates or equilibrium off-loop species of the catalytic cycle.

The general agreement between the DFT results (in terms of free energy changes in solution) and the experimental equilibrium studies encourages us to pursue our DFT exploration of the remainder of the catalytic cycle.

Experimental part.

General. The ethanol used as reaction solvent was of 95% grade and water was deionized.

All other solvents were of HPLC grade and were used as received. Aniline (Fluka) was distilled under vacuum and kept under argon in the dark. K2PtCl4 (Strem) and K[PtCl3(C2H4)]•H2O (Aldrich) were used as received. nBu4PBr (Aldrich) was stored in a desiccator under vacuum.

Ethylene (purity ≥ 99.5%) was purchased from Air Liquide.

Instrumentation. NMR investigations were carried out on a Bruker DPX300 spectrometer operating at 300. respectively, at equilibrium (the signal to noise ratio for the smaller peak is 4).

Preparation of (nBu4P)[PtCl3(C2H4)] (2-Cl).

To a saturated KCl aqueous solution (3 mL) was added K[PtCl3(C2H4)]•H2O (30 mg, 0.078 mmol) and a solution of nBu4PBr (28 mg, 0.083 mmol) in CH2Cl2 (2 mL). The resulting two-phase system was intensively stirred over 10 min, then the organic phase was recovered and the solvent was evaporated and the resulting residue was dried under vacuum for 2 h. The yield of product after solvent evaporation was 40 mg (88%).

Synthesis of (nBu4P)2[PtBr4] (1).

To a solution of K2PtCl4 (107 mg, 0.258 mmol) in water (5 mL) was added KBr (2.76 g, 90 equiv) and the resulting solution was stirred at room temperature for 3 hours. It was then transferred into a separating funnel and intensively shaken with a solution of nBu4PBr (175 mg, 2 equiv) in 6 mL of CH2Cl2. Since the resulting mixture was a relatively stable emulsion, 3 additional mL of water were added and the mixture was shaken again, resulting in the rapid separation of two clear phases. The organic layer was collected and shaken two more times with distilled water (8 mL each time). The organic layer was then evaporated to dryness and the brown-light residue was dried in vacuo for 20 min, then washed with pentane (5 mL) and finally dried in vacuo over P4O10 overnight. Yield: 181 mg (68%).

Synthesis of (nBu4P)[PtBr3(C2H4)] (2)

. Method A. From Zeise's salt. An aqueous solution (5 mL) of K[PtCl3(C2H4)]•H2O (100 mg, 0.271 mmol) and KBr (1.61 g, 50 equiv) was stirred under a continuous ethene flushing for 20 hours at room temperature. After this time the filtered while hot and kept at -20ºC overnight. In the case of 1, the solution was then further concentrated to approximately 1/3 of its original volume and kept at -20ºC during two weeks, yielding large (ca. 3 mm) deep-red crystals. Crystals of 4 were prepared in the same manner as 2 and 3, albeit using a pentane/Et2O combination. The crystals of 5 were prepared by slow evaporation of a saturated acetone solution. A single crystal of each compound was mounted under inert perfluoropolyether on the tip of a glass fiber and cooled in the cryostream of a Bruker APEX2 CCD diffractometer. Data were collected using the monochromatic MoK radiation (= 0.71073). The structures were solved by direct methods (SIR97) 66 and refined by least-squares procedures on F 2 using SHELXL-97. [START_REF] Sheldrick | Program for Crystal Structure refinement[END_REF] All H atoms attached to carbon were introduced in calculation in idealized positions and treated as riding models. The drawing of the molecules was realised with the help of ORTEP32. [START_REF] Farrugia | [END_REF] Crystal data and refinement parameters are given in the supporting information (Table S10). Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 725742 -725747. 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; email: deposit@ccdc.cam.ac.uk).

Computational Details. All geometry optimizations were performed with the Gaussian03 suite of programs 69 using the B3LYP functional which includes the three-parameter gradientcorrected exchange functional of Becke 70 and the correlation functional of Lee, Yang, and Parr which includes both local and non-local terms. [START_REF] Lee | [END_REF]72 The basis set chosen was the standard 6-31+G*, which includes both polarization and diffuse functions that are necessary to allow angular and radial flexibility to the highly anionic systems, for all atoms of type H, C, N, and Br.

The Pt atom was described by the LANL2TZ(f) basis, which is an uncontracted version of LANL2DZ and includes an f polarization function and an ECP. 73 The starting geometries for the calculations where derived from the solid state X-ray structure, whenever available, or adapted from those by appropriate ligand substitution (specifically for systems VII and expected VIII).

For the ionic species, the calculations were carried out on the free ion, without consideration of ion pair formation with the counterion. Frequency calculations were carried out for all optimized geometries in order to verify their nature as local minima and for the calculation of thermodynamic parameters at 298.15 and at 423.15 K under the gas-phase and harmonic approximations. Solvent effects were included by means of CPCM single point calculations on the gas-phase optimized geometries. 74,75 Thus, the solution free energy was calculated as Hgas+TSgasG CPCM , the corrective term G CPCM being indicated as Gsolv by the Gaussian program.

Scheme 1 .

 1 Scheme 1. Proposed 3 "productive" mechanism for the hydroamination of ethylene by aniline.

) 4 .

 4 63 (s+d 1:10:1, 2 JH-Pt = 65) 68.0 (s + d, 1 JC-Pt = 191.8) (nBu4P)[PtBr3(PhNH2)] (3) -2422 (bs, Δδ1/2 = 250) 5.64 (s+d 1:10:1, 2H, 2 JH-Pt = 72, C6H5NH2), 7.17-7.59 (m, 5H, C6H5NH2) 122.2 (o-C), 125.5 (p-C), 128.9 (m-C), 141.0 (i-C) trans-PtBr2(C2H4)(PhNH2) (4) c -3525 (bs, Δδ1/2 = 240) 4.82 (s+d 1:10:1, 4H, 2 JH-Pt = 65, C2H4), 6.23 (br, Δδ1/2 = 26, 2H, C6H5NH2), 7.3-7.6 (m, 5H, C6H5NH2) 71.5 (s+d, 1 JC-Pt = 164.4), 122.3 (o-C), 126.8 (p-C), 129.6 (m-C), 137.8 (i-C) cis-PtBr2(C2H4)(PhNH2) (5) d -3066 (bs, Δδ1/2 = 400) 4.49 (s+d 1:10:1, 4H, 2 JH-Pt = 63.9, C2H4), 7.65 (br, Δδ1/2 = 26, 2H, C6H5NH2, 2 JH-Pt = 64 Hz), 7.25-7.59 (m, 5H, C6H5NH2) 70.3 (s+d, 1 JC-Pt = 184.0), 123.4 (o-C), 126.6 (p-C), 129.5 (m-C), 140.5 (i-C) cis-PtBr2(PhNH2)2 (6) d -2397 (br, Δδ1/2 = 400) 7.54 (br, 4H, C6H5NH2), 7.3-7.6 (m, 10H, C6H5NH2) 123.5 (o-C), 125.6 (p-C), 128.8 (m-C), 142.4 (i-C) a Unless otherwise stated, all spectra were recorded in CD2Cl2. Chemical shifts are reported as  values, with coupling constants and half-widths in Hz. b 31 P{ 1 H} NMR resonance of the nBu4P + ion: δ 32.5 (s). c The compound is not indefinitely stable in dichloromethane: solution cloudiness developed during the NMR experiments (ca. 12 hours), while the 1 H NMR spectrum showed a new poorly resolved resonance at 4.3 ppm (satellites, 2 JH-Pt = 65 Hz) and the 195 Pt NMR spectrum showed a new broad resonance at -3258 ppm. d In DMF-d7.

Figure 1 .

 1 Figure 1. Molecular structure of anion of 2 with ellipsoids are drawn at the 30% probability level. Selected bond distances (Å): Pt-Br1, 2.4294(14); Pr-Br2, 2.4229(16); Pt-Br3, 2.4337(14); C1-C2, 1.37(2).

[ 1 .

 1 37(3) Å by X-ray and 1.375(4) Å by neutron diffraction].(nBu4P)[PtBr3(PhNH2)] (3). This compound, containing the previously unreported [PtBr3(PhNH2)] -ion, was obtained in the same manner as 2 (41% yield from K2PtCl4), when aniline was used instead of C2H4 (Scheme 5). However, the reaction must be carried out under acidic conditions (pH = 2). When neutral conditions were used a precipitate with a stoichiometry close to PtBr2(PhNH2)2, according to the analytical data, immediately formed upon addition of 1 equiv of aniline. The 1 H NMR spectrum of 3 in CD2Cl2 features a single and slightly broadened (∆δ1/2 = 15 Hz) resonance with 195 Pt satellites at δ 5.64 ppm ( 2 JH-Pt = 72 Hz) for the NH2-protons (cf. 3.69 for free aniline).

distances in compound 3

 3 do not differ significantly between cis and trans positions. The average Pt-Br bond length in 3, 2.443(5) Å, is significantly longer than the same average in 2, 2.429(5) Å, but also longer than those found for other [PtBr3(L)] -complexes containing N-donor ligands (in the 2.416-2.431 Å range).[34][35][36][37][38][39]

Figure 2 .

 2 Figure 2. Molecular structure of the anion of 3 with molecular ellipsoids are drawn at the 30% probability level. Selected distances (Å): Pt-Br1, 2.4444(9); Pt-Br2, 2.4487(7); Pt-Br3, 2.4375(7), Pt-N1, 2.078(4).

PhNH 2 Scheme 6 .

 26 Scheme 6.

compound 4

 4 is longer than those mentioned in the previous section for [PtX3L] -complexes, presumably because of the strong trans influence of the ethylene ligand. Other trans-[PtX2(alkene)(L)] complexes (all containing alkylamine or N-based heterocyclic ligands and X = Cl) also have longer Pt-N distances, for instance 2.084(11) Å for trans-PtCl2(C2H4)(2,6-NC5H3Me2)44 and 2.09(5) for trans-PtCl2[CH2=CHCH(Me)Et](NH2CH2Ph).45 The C-C distance compares with the distance found in C2H4 coordinated to square planar Pt II (average of 1.38(2) Å for 28 structures retrieved from the CSD).

Figure 3 .

 3 Figure 3. An ORTEP view of one of the two independent molecules of 4. Molecular ellipsoids are drawn at the 30% probability level. Selected bond distances (Å; values separated by slashes are the corresponding parameters in two crystallographically independent molecules): Pt1-Br11, 2.4108(6)/2.4093(7); Pt1-Br12, 2.4203(6)/2.4237(7); Pt1-N1, 2.093(4)/2.093(5); Pt1-C1, 2.155(6)/2.158(7); Pt1-C2, 2.145(6)/2.153(6); C1-C2, 1.378(10)/1.375(9).

  Figure 4.Variable temperature 1 H NMR spectrum of 5 in DMF-d7.

Figure 6 .

 6 Figure 6. An ORTEP view of one of compound 6. Molecular ellipsoids are drawn at the 30% probability level.

Figure 7 . 1 H

 71 Figure 7. 1 H NMR spectra (olefin, NH2 and aromatic-region) of: a) 4; b) 4 in the presence of 0.72 equiv nBu4PBr; c) 2; and d) aniline, in THF-d8 at 298K. [4] = [2] = 0.5•[PhNH2] = 0.04 M.

Figure 8 . 1 H

 81 Figure 8. 1 H NMR spectra in DMF-d7 at 298K of: 5 (a); 5 + nBu4PBr (0.87 equiv) at 0 (b), 10 (c), 50 (d), 107 (e), 257 min (f); 21.5 (g) 24 (h), 28 (i), and 48 h (j).

  Scheme 9.

Table 1 .

 1 Physical and microanalytical data

	(nBu 4 P)	Cl	Pt		(nBu 4 P)	Br	Pt
		Cl	2-Cl	Cl		Br	2	Br
			nBu 4 PCl				nBu 4 PBr
		K[PtCl 3 (C 2 H 4 )]	KBr	K[PtBr 3 (C 2 H 4 )]	PhNH 2	Br	Pt
		Zeise's salt		Chojnacki's salt		H 2 N	Br
											4
		KBr			C 2 H 4						Ph
	K 2 [PtCl 4 ]	K 2 [PtBr 4 ] nBu 4 PBr	PhNH 2	K[PtBr 3 (NH 2 Ph)]	C 2 H 4	Br	Pt	Ph NH 2
											Br
	(nBu 4 P) 2	Br Br	1 Pt	Br Br	Br Br nBu 4 PBr (nBu 4 P)	3 Pt	Br NH 2 Ph	Br Br PhNH 2	6 Pt 5	Ph NH 2 NH 2 Ph
	Scheme 2. Synthesis of bromoplatinum(II) complexes in "one-pot" procedures from K2PtCl4 or
			Zeise's salt.
											anal., % a
			cmpd					color	mp, °C	C	H	N
	(nBu4P)2[PtBr4] (1)				brown-red 120-121	37.57 (37.18) 7.17 (7.04)
	(nBu4P)[PtBr3(C2H4)] (2)				yellow	108	29.82 (29.93) 5.48 (5.59)
	(nBu4P)[PtCl3(C2H4)] (2-Cl)		lemon	75	37.07 (36.71) 6.79 (6.86)
	(nBu4P)[PtBr3(PhNH2)] (3)			red	124	33.62 (33.56) 5.64 (5.52) 1.55 (1.78)
	trans-PtBr2(PhNH2)(C2H4) (4)		yellow	100 (dec) 20.25 (20.18) 2.24 (2.33) 2.94 (2.94)
	cis-PtBr2(PhNH2)(C2H4) (5)			lemon	120 (dec) 19.68 (20.18) 1.71 (2.33) 2.86 (2.94)
	cis-PtBr2(PhNH2)2 (6)			greenish 255 (dec) 26.45 (26.63) 2.35 (2.61) 4.99 (5.18)

a Calculated values in parentheses.

Table 2 .

 2 195 Pt, 1 H and 13 C{ 1 H} NMR data at 25 °C for the complexes synthesized in this study, together with those of free ethylene for comparison. a

	Table 2.
	Cmpd	195 Pt	1 H	13 C{ 1 H}
	C2H4		5.44	122.8
	PhNH2		7.17 (m, 2H, m-Ph), 6.75 (m, 1H, p-Ph), 6.70 (m,	146.8 (i-C), 129.2 (m-C), 118.1 (p-
			2H, o-NH2) 3.70 (s, 2H, NH2)	C), 114.8 (o-C)
	nBu4PBr b		2.72 (m, 2H, PCH2), 1.63 (m, 4H, PCH2(CH2)2),	23.9 (d, 3 JC-P = 15 Hz, CC), 23.7 (d,
			1.01 (m, 3H, P(CH2)3CH3)	2 JC-P = 5 Hz, CB), 19.1 (d, 1 JC-P =
				47.5 Hz, CA), 13.3 (s, CD)
	(nBu4P)2[PtBr4] (1)	-2528 (bs, Δδ1/2 = 98)
	(nBu4P)[PtBr3(C2H4)] (2)	-3429 (quint,

2

JPt-H = 65) 4.59 (s+d 1:10:1, 2 JH-Pt = 65) 67.3 (s + d, 1 JC-Pt = 176.4) (nBu4P)[PtCl3(C2H4)] (2-Cl) -2743 (quint, 2 JPt-H = 65

15, 16 (nBu4P)[PtBr3(C2H4)] (2). Compound

	bubbling C2H4 through a K2[PtBr4] solution. 19 It has also been obtained by halide exchange from ligand (2.52 Å) than for the two Pt-Br bonds cis to ethylene ligand (2.43 and 2.42 Å, the
	Zeise's salt, 20 but long reaction times and a protecting ethylene atmosphere were necessary to estimated standard deviation being 0.01 Å), interpreted as the result of the stronger olefin trans
	avoid the formation of [PtBr4] 2-and no elemental analysis, yields and physical data were influence. The C atom positions were not precisely determined and for this reason the C=C and
	reported. A compound of stoichiometry (Bu4N)[PtBr3(C2H4)], characterized by IR spectroscopy, Pt-C distances were not reported. In the course of our work we have obtained single crystals of
	was mentioned 21 but no detailed synthetic procedure has appeared in the literature to the best of (nBu4P)[PtBr3(C2H4
	our knowledge. We describe here a direct synthesis of 2 in 46% yield from K2PtCl4, as well as
	by an alternative procedure from Zeise's salt (61% yield of isolated pure product), see Scheme 4.
	The ethanol used as a solvent for the transformation of K2[PtBr4] into K[PtBr3(C2H4)] serves to
	remove the large KBr excess from the previous step and also appears to speed up the ligand
	exchange relative to water, while the presence of small amounts of HBr avoids decomposition to
	unknown compounds. The other procedure from Zeise's salt uses the same strategy described
	above for 1, namely halide exchange with the minimum amount of KBr, followed by cation
	exchange and operating under an ethylene atmosphere. Compound (nBu4P)[PtCl3(C2H4)] (2-Cl)
	has similarly been obtained by cation exchange from Zeise's salt.
				nBu 4 PBr/CH 2 Cl 2
	K 2 [PtCl 4 ]		K[PtCl 3 (C 2 H 4 )]		(nBu 4 P)[PtCl 3 (C 2 H 4 )]
				H 2 O/KCl	2-Cl
	90 eq KBr, 3 h	50 eq KBr, 20 h
	H 2 O		H 2 O/C 2 H 4 bubbling
	C 2 H 4 bubbling, 5h		nBu 4 PBr
	K 2 [PtBr 4 ]	EtOH/HBr	K[PtBr 3 (C 2 H 4 )]	EtOH/HBr or CH 2 Cl 2	(nBu 4 P)[PtBr 3 (C 2 H 4 )] 2
	Scheme 4.
	The 1 H NMR spectrum of 2 in CD2Cl2 features a single resonance with 195 Pt satellites at δ
	4.59 ppm ( 2 JH-Pt = 65) due to the olefinic protons, upfield from those of free ethylene (δ 5.44)
	and 2-Cl (δ 4.63). This indicates a greater degree of Pt-C2H4  back-bonding for the bromide
	system. Likewise, the 13 C{ 1 H} singlet resonance of 2 at δ 67.3 ( 1 JC-Pt = 176.4 Hz) is upfield from
	those of 2-Cl (δ 68.0, 1 JC-Pt = 191.8 Hz) and free C2H4 (δ 122.8). Note, however, that 1 JC-Pt is
	higher for Zeise's anion. A greater degree of Pt-C2H4  back-bonding for 2 relative to the
	corresponding trichloro anion is also suggested by the IR stretching frequency of two bands in
	which contribution of the C=C bond stretch is important [because of vibrational coupling with
	δs(CH2)] 22-25 (1510 and 1227 cm -1 , vs. 1516 and 1230 cm -1 for compound 2-Cl and 1623 and
	1342 cm -1 for free ethylene 26 ).
	The structure of the [PtBr3(C2H4)] -ion was previously studied 27 only for the potassium salt
	K[PtBr3(C2H4)]•H2O, first reported in 1870 17 and by two-dimensional methods and showed a significantly longer Pt-Br bond trans to the ethylene
	known 18 as Chojnacki's salt, is generally prepared in the same manner as Zeise's salt, namely by

  The C=C vibrations (1513 and 1236 cm -1 ) are red-shifted relative to those of the trans analogue (1519 and 1252 cm -1 ). These data concur to suggest a greater extent of Pt-C2H4 back-bonding for the cis isomer. The ethylene protons give rise to an AA'BB' spin system at low temperatures (< 260 K), but only one signal is observed at 298 K, indicating fast rotation of the ethylene ligand on the NMR timescale, see Figure4. Note that the 195 Pt satellites are observed at room temperature but are

	lost at lower temperatures, because of a larger chemical shift anisotropy contribution to
	relaxation. This phenomenon has also been observed for compound 2 (see supplemental figure
	S4).
		90 eq KBr, 3 h		PhNH 2	C 2 H 4 bubbling, 4h
	K 2 [PtCl 4 ]		K 2 [PtBr 4 ]	K[PtBr 3 (PhNH 2 )]	cis-[PtBr 2 (PhNH 2 )(C 2 H 4 )]
		H 2 O, pH = 2		H 2 O, pH = 2	H 2 O, pH = 2	5
	Scheme 7.
	4.80	4.70	4.60	4.50	4.40	4.30
							1 H NMR spectrum in DMF-d7 features a
	broad resonance (∆δ1/2 = 26 Hz) with 195 Pt satellites for the NH2 protons at  7.62 ( 2 JH-Pt = 64

Hz), namely considerably upfield with respect to the corresponding resonance of the trans isomer 4 ( 8.26 in the same solvent). Note also that the Pt satellites are visible for this compound and for compound 3, but not for the trans isomer 4. Both observations may be related to the stronger aniline ligand binding in the cis isomer (vide infra).

/ppm

  . An ORTEP view of compound 5. Molecular ellipsoids are drawn at the 30%

	probability level. Selected bond distances (Å): Pt-Br1, 2.4239(13); Pt-Br2,
	2.4364(13); Pt-N1, 2.082(10); Pt-C1, 2.112(13); Pt-C2, 2.130(12); C1-C2, 1.367

Table 3 .

 3 Catalytic results for the aniline addition to ethylene with compounds PtBr2, 2, 4 and 5 in the presence of nBu4PBr. a

	[Pt]	Salt (eq)	PhNHEt	PhNEt2	Quinaldine
			TON	TON	TON
	PtBr2	n Bu4PBr (150) b	100	2	4
	PtBr2	n Bu4PBr (10) b	100	4	12
	2	n Bu4PBr (10)	75	2	15
	4	n Bu4PBr (10)	70	5	8
	5	n Bu4PBr (10)	91	12	8
	a T = 150°C, t = 10 h; p(C2H4) = 25 bars. b Results very close to those previously reported under
	identical conditions.

Table 4 .

 4 Relative free energy values (in kcal mol -1 ) of systems I-VIII. The values shown take into account the addition and subtraction of ligands from system I. a

	System	GDCM,298.15 Ganiline,298.15 Ganiline,423.15
	I	0.0	0.0	0.0
	I'	-7.6	-9.1	-10.9
	II	-11.4	-14.6	-14.5
	III	-6.7	-8.7	-9.6
	IV	-8.0	-10.7	-9.7
	V	-6.0	-8.4	-7.4
	VI	-1.6	-2.9	-1.0
	VII	0.2	-3.2	-2.6
	VII'	5.5	1.8	1.8
	VIII	-1.9	-5.3	-5.0

195 Pt NMR study of equilibrium between K2PtCl4 and KBr

  Varian Cary 50 WinUV or on a PerkinElmer Lambda 35 UV/vis spectrometer using CaF2 cells of 1 cm path length. Elemental analyses were performed by the Microanalytical Service of the Laboratoire de Chimie de Coordination. The capillaries charged with the compounds for the melting points were sealed before measuring. For all compounds, the physical (color, melting point) and microanalytical data (C, H, N) are reported in Table1, whereas the NMR properties in CD2Cl2 ( 195 Pt, 1 H and13 C{ 1 H}) are listed in Table 2. Other characterization data (NMR in other solvents, UV-visible, IR) are given in the Supporting Information. More detailed analyses of vibrational coupling in compounds 1-6 on the basis of IR and Raman studies are published elsewhere. 25 . To a water solution (3 mL) of K2PtCl4 (55 mg) was added KBr (0.79 or 1.26 g; 50 or 80 equiv) and the obtained mixture was stirred at room temperature for several days. The equilibrium was checked by transferring aliquots of the solution into an NMR tube charged with ca. 100 L of D2O for lock and measured by 195 Pt NMR spectroscopy (30000 scans). The spectra showed two resonances at -2367 {[PtBr3Cl] 2-} and -2662 ppm {[PtBr4] 2-} with intensity peak ratios of 1:22 or 1:33,

1 MHz ( 1 H), 121.49 MHz ( 31 P), 75.47 MHz ( 13

C) and 64.5 MHz ( 195 Pt). The spectra were calibrated with the residual solvent resonance relative to TMS ( 1 H,

13

C) and with external 85% H3PO4 ( 31 P) and Na2PtCl6 ( 195 Pt). IR spectra (neat/4000-600 cm -1 ) were recorded on a Perkin-Elmer Spectrum 100 FTIR spectrometer (2 cm -1 resolution). UV measurements were recorded on a

Lozano, C. M.; Muir, M. M.; Tang, X. J.; Li, Y. J., J. Chem. Crystall. 1994, 24, 639-642.

Becke, A. D., J. Chem. Phys. 1993, 98, 5648-5652.

Acknowledgement. We thank the CNRS and the RFBR for support through a France-Russia (RFBR -CNRS) bilateral grant № 08-03-92506, and the MENESR (Ministère de l'Éducation nationale de l'enseignement supérieur et de la recherche de France) for a Ph.D. fellowship to PD.

solution was transferred into a separating funnel where it was intensively shaken with a solution of nBu4PBr (92 mg, 0.271 mmol) in CH2Cl2 (5 mL). Since the resulting mixture was a relatively stable emulsion, 3 additional mL of water were added and the mixture was shaken again, resulting in the rapid separation of two clear phases. Yellow-orange crystals of pure (nBu4P)[PtBr3(C2H4)] were obtained after separation of the organic phase, solvent evaporation and recrystallization of the residue from dichloromethane/diethyl ether. Yield: 120 mg (61%).

The complex immediately and quantitatively reacts with DMSO-d6, yielding a yellow solution characterized by a single 195 Pt resonance at δ -3578 (s, Δν1/2 = 21 Hz, 298 K). The formation of free ethylene is witnessed by the 1 H (singlet at δ 5.42) and 13 C{ 1 H} (singlet at δ 124.0) resonances. These observation suggest the formation of complex [PtBr3(DMSO-d6)] -. Method B. From K2PtCl4. To a solution of K2PtCl4 (200 mg, 0.482 mmol) in water (8 mL) was added 90 equiv KBr (5.16 g) and the obtained solution was stirred for 3 hours at room temperature. The solvent was then evaporated under vacuum and K2PtBr4 was extracted by three portions (ca. 10 ml) of a EtOH/HBr (30 mL + 2 mL) mixture and filtered through Celite. Ethene gas was flushed through the obtained solution during 4 hours, then nBu4PBr (163 mg, 0.482 mmol) was added and the resulting mixture was stirred under an ethene flush for 1 hour. The pure yellow-orange (nBu4P)[PtBr3(C2H4)] was obtained after solvent evaporation, washing with water (4x6 ml), recrystallization from dichloromethane/diethyl ether, washing with pentane and finally drying in vacuo. Yield: 160 mg (46%). The spectroscopic data of this material were identical to those of the product of method A.

Synthesis of (nBu4P)[PtBr3(PhNH2)] (3).

To a solution of K2PtCl4 (260 mg, 0.626 mmol) in water (12 mL), adjusted to pH = 2 by addition of CH3COOH, was added KBr (6.71 g, 90 equiv) and the resulting mixture was stirred for 3 h at room temperature. Aniline (57 μL, 0.626 mmol) in 0.5 mL of EtOH was then added drop by drop and the obtained mixture was stirred for 1 h, resulting in the formation of a yellow-orange precipitate. To this mixture was added a solution of nBu4PBr (212 mg, 0.626 mmol) in dichloromethane (7 mL) and the resulting biphasic system was vigorously stirred for 20 min, transferred into the separating funnel, and the red organic phase was separated. Following solvent evaporation and recrystallization from dichloromethane/diethyl ether, red crystals of pure (nBu4P)[PtBr3(PhNH2)] were obtained in 41% yield (202 mg).

Synthesis of trans-PtBr2(PhNH2)(C2H4) (4). Method A. From K2PtCl4.

To an aqueous solution (6 mL) of K2PtCl4 (100 mg, 0.241 mmol) was added KBr (2.580 g, 90 equiv) and the resulting solution was stirred for 3 h at room temperature. The solvent was then evaporated under reduced pressure and K2PtBr4 was extracted by three equal portions of a EtOH/HBr mixture (total volume: 15 mL EtOH + 1 mL conc. aqueous HBr) and filtered through Celite. The resulting solution was flushed with ethylene for 5 h, then the solvent was evaporated to dryness. Dissolution in water (7 mL) followed by addition of an ethanol solution of aniline (22 μL, 0.241 mmol in 0.5 mL) resulted in an immediate precipitation of a yellow powder. After 1 h of stirring, the solid was filtered, washed with water and dried. It was then redissolved in the minimum amount of dichloromethane (ca. 3 mL) and the solution was filtered and evaporated to dryness. Yield: 49 % (56 mg). The compound is unstable in DMSO-d6, yielding a mixture of products ( 195 Pt resonances at  -3306, -3495, -3578, -3733 with approximate relative ratio of 1:6:1:3 from signal integration). The resonance at -3578 is identical to that obtained by dissolution of 2 and tentatively assigned to [PtBr3(DMSO-d6)] -(see above). The compound slowly changes color toward black when kept in air for several days as a solid. Storage under argon is recommended. Method B. From Zeise's salt. An aqueous solution (8 mL) of Zeise's salt (200 mg, 0.543 mmol) and KBr (3.228 g, 50 equiv) was stirred under an ethene flush for 20 h at room temperature. An aniline (50 μL, 0.543 mmol) solution in EtOH (1 ml) was then added dropwise, yielding a yellow precipitate. After stirring for 1 h at room temperature, the precipitate was filtered, washed with water and dried in vacuo. Subsequent washing with pentane and drying in vacuo affords yellow trans-[PtBr2(C2H4)(PhNH2)] in 73 % yield (189 mg). The spectroscopic data of this material were identical to those of the product of method A.

Synthesis of cis-PtBr2(PhNH2)(C2H4) (5), from K2PtCl4.

To an aqueous solution (5 mL) of K2PtCl4 (100 mg, 0.241 mmol), adjusted to pH = 2 by the addition of CH3COOH, was added KBr (2.58 g, 90 equiv) and the resulting mixture was stirred for 3 h at room temperature. Aniline (22 μL, 0.241 mmol) in 0.5 mL of EtOH was then added dropwise and the resulting mixture was stirred for 1 h, yielding a small amount of yellow precipitate. After filtration, the resulting red solution was flushed with ethene for 4 h yielding a yellow-green precipitate, which was then filtered. The precipitate was washed with water and dried in vacuo. Yield: 48 mg (42 %).

Synthesis of cis-PtBr2(PhNH2)2 (6).

To a solution of K2PtCl4 (1000 mg, 2.409 mmol) in water (40 mL) was added KBr (25.8 g, 90 equiv) and the resulting mixture was stirred for 3 h at room temperature. A solution of aniline (440 L, 4.818 mmol) in 2 mL of EtOH was then added dropwise. The mixture was stirred for 3 h and filtered. The pale yellow precipitate was washed sequentially with water, EtOH, and Et2O and then dried in vacuo. Yield: 1.23 g (94%). The compound is not stable in DMSO. Upon dissolving the compound in DMSO-d6, the formation of new 195 Pt NMR resonances (a major peak at  -3495 and a minor one at  -3306) was observed.

They are tentatively attributed to [PtBr2(DMSO)(PhNH2)] and [PtBr2(DMSO)2], respectively. X-Ray crystallography. Single crystals of 1, 2 and 3 suitable for the X-ray study were prepared as follows: to a suspension of the compound in Et2O at the reflux temperature was added dichloromethane dropwise until a homogenous system formed. The solution was then

ToC Text and Graphics

Solution equilibria between various PtBr2 derivatives containing C2H2 and PhNH2 have been assessed experimental and computationally in reference to the PtBr2/Br --catalyzed hydroamination of ethylene by aniline.