Aurélien Béthegnies

Vladislava A Kirkina

Oleg A Filippov

Jean-Claude Daran

Natalia V Belkova

Elena Shubina

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

Coordination and organometallic chemistry of relevance to the rhodium-based catalyst for ethylene hydroamination

BRIEFS. Complex [RhI(PPh3)2]2 is an active catalyst for ethylene hydroamination by aniline; its coordination chemistry in the presence of catalytic relevant ligands is reported.

ABSTRACT. The RhCl3•3H2O/PPh3/nBu4PI catalytic system for the hydroamination of ethylene by aniline is shown to be thermally stable by a recycle experiment and by a kinetic profile study. The hypothesis of the reduction under catalytic conditions to an iodido Rh I species is supported by the observation of a high catalytic activity for complex [RhI(PPh3)2]2. New solution equilibrium studies on [RhX(PPh3)2]2 (X = Cl, I) in the presence of ligands of relevance to the catalytic reaction (PPh3, C2H4, PhNH2, X -, and the model Et2NH amine) are reported. Complex [RhCl(PPh3)2]2 shows broadening of the [START_REF] Burnett | Report ORNL-6895[END_REF] P NMR signal upon addition of PhNH2, indicating rapid equilibrium with a less thermodynamically stable adduct. The reaction with Et2NH gives extensive conversion into cis-RhCl(PPh3)2(NHEt2), which is however in equilibrium with the starting material and free Et2NH. Excess NHEt2 yields a H-bonded adduct cis-RhCl(PPh3)2(Et2NH)•••NHEt2, in equilibrium with the precursors, as shown by IR spectroscopy. The iodide analogue [RhI(PPh3)2]2 shows less pronounced reactions (no change with PhNH2, less extensive addition of Et2NH with formation of cis-RhI(PPh3)2(NHEt2), less extensive reaction of the latter with additional Et2NH to yield cis-RhI(PPh3)2(Et2NH)•••NHEt2). The two [RhX(PPh3)2]2 compounds do not show any evidence for addition of the corresponding X -to yield a putative [RhX2(PPh3)2] -adduct. The product of C2H4 addition to [RhI(PPh3)2]2, trans-RhI(PPh3)2(C2H4), has been characterized in solution. Treatment of the RhCl3•3H2O/PPh3/nBu4PI/PhNH2 mixture under catalytic conditions yields mostly [RhCl(PPh3)2]2 and no significant halide exchange, demonstrating that the promoting effect of iodide must take place at the level of high energy catalytic intermediates. The equilibria have also been investigated at the computational level by DFT with treatment at the full QM level including solvation effects. The calculations confirm that the bridge splitting reaction is slightly less favourable for the iodido derivative. Overall, the study confirms the active role of rhodium(I) species in ethylene hydroamination catalyzed by RhCl3•3H2O/PPh3/nBu4PI and suggest that the catalyst resting state is [RhCl(PPh3)2]2 or its C2H4 adduct, RhCl(PPh3)2(C2H4), under high ethylene pressure. KEYWORDS. Rhodium; triphenylphosphine; coordination chemistry; intermolecular hydroamination;

Introduction

An efficient halide-promoted platinum catalyst has recently been developed in our group for the hydroamination of ethylene and α-olefins by aniline derivatives. [1][2][3][4][5][6] This reaction yields N-ethylaniline (1) as major product, accompanied by traces of the double hydroamination product N,N-diethylaniline (2) and by minor amounts of quinaldine (3), see Scheme 1. Synthetic, spectroscopic and solution equilibrium studies 5,7,8 assisted by DFT calculations 9,10 have elucidated important mechanistic details of the hydroamination cycle and have rationalized experimental observations such as the halide promotion effects, the higher activity for the addition of less basic aniline derivatives, the strong preference for Markovnikov addition to higher olefins, and also hinted to the importance of the substrate and/or halide basicity in the deprotonation of a zwitterionic intermediate as a key step leading to catalyst deactivation. 11 Scheme 1. Products observed in the halide-promoted catalyzed hydroamination of ethylene by aniline (MXn/Y -= PtBr2/Br -or RhCl3•3H2O/I -).

A few years ago, it has also been reported within our group that halides equally promote the aniline addition to ethylene when catalyzed by RhCl3•3H2O/PPh3. 12 Compound RhCl3•3H2O had previously been shown to efficiently catalyze the addition of basic secondary amines (Et2NH, piperidine) to ethylene, albeit under rather forcing conditions, but the efficiently was greatly reduced for less basic amines or for higher olefins. 13,14 Subsequent work had shown that addition of PPh3 to this system allows ethylene hydroamination by aniline, but only with much lower efficiencies. [15][16][17] A few welldefined complexes were also briefly investigated, giving low productivities and/or fast decomposition. [18][19][20] The work from our group has revealed that the same catalytic system, like PtBr2, becomes much more active for the addition of aniline to ethylene when promoted by halide ions and the strongest promoting effect for this system turned out to be associated to iodide; under these conditions, greater amounts of double hydroamination product 2 were obtained relative to the PtBr2/Br -system, whereas the quinaldine by-product 3 was essentially absent. 12 The promoting effect of halide ions in homogeneous catalysis is not unprecedented. For instance, the catalytic activity of a series of Ir(I) diphosphine complexes in intermolecular hydroamination (of norbornene by aniline), was shown to be greatly improved in the presence of fluoride ions, 21 whereas iodide is a recognized promoter for a variety of other catalyzed processes, 22 especially the Rh and Ircatalyzed methanol carbonylation. The origin of its promoting effect, however, is not always completely understood, being possibly associated to a number of possible causes such as its good nucleophilicity, its redox activity, or simply its better binding to low-valent metal centers decreasing the tendency of the metal complex to precipitate and two or more effects may occur in different steps of the same catalytic cycle. 22 For the PtBr2 hydroamination, as already stated, halides are also promoters and in that case bromide shows the strongest effect, 1 though only marginally better than iodide and the reasons of this difference are not yet understood. 11 Given the rather reducing conditions imposed by the ethylene and aniline reagents (catalyst degradation to metallic rhodium was reported under certain conditions) it is reasonably assumed that RhCl3•3H2O is transformed to a Rh I complex, probably containing PPh3 and iodide ligands, before beginning the catalytic cycle. However, the coordination chemistry of iodide complexes of Rh I has been little explored, particularly with the set of ligands of relevance to this catalytic reaction. Complex RhI(PPh3)3 (the iodide equivalent of Wilkinson's catalyst) has been described by Wilkinson himself, 23 and the dinuclear complex [RhI(PPh3)2]2 is also known. [23][24][25] With the notable exception of Vaska-type, CO containing complexes, [START_REF] Hughes | Comprehensive Organometallic Chemistry[END_REF][START_REF] Sharp | Comprehensive Organometallic Chemistry II[END_REF] adducts with other ligands are scarce and this coordination chemistry merits renewed attention in view of this specific catalytic transformation.

Following the same strategy recently adopted for the study of the platinum-based catalyst, 5 we have decided to investigate well-defined rhodium complexes in terms of their solution equilibria in the presence of all available ligands under catalytic conditions (triphenylphosphine, ethylene, aniline, halide). DFT calculations have also been carried out to validate the experimental findings. Finally, additional catalytic tests have been run in the presence of well-defined Rh I iodide complexes, validating the concept that the active catalyst is indeed a Rh I species.

Experimental Section

General. Unless otherwise stated, all manipulations were carried out under an argon atmosphere.

Solvents were dehydrated by standard procedures and distilled under argon prior to use. Compounds RhCl3•3H2O (Johnson Matthey 41.92%), PPh3 (Aldrich 99%), nBu4NCl (Fluka > 97%) and nBu4PBr (Acros Organics) were used as received. Tetra(n-butyl)phosphonium iodide was prepared from nBu3P and nBuI according to the literature 4 and stored protected from light and under argon in a freezer. Et2NH (Fluka), PhNH2 (Acros Organics; 99% for analysis ACS) and mesitylene (>97%, Fluka) were distilled and kept under argon in the dark. Ethylene (N25, purity ≥ 99.5%) was purchased from Air Liquide.

Complexes RhCl(PPh3)3, 23 [RhCl(PPh3)2]2, 23 RhI(PPh3)3 24 and [RhI(PPh3)2]2 24 were synthesized according to literature procedures. Single crystals of RhI(PPh3)3 were grown by diffusion of pentane into a dichloromethane solution.

Instrumentation. The GC analyses were performed on a Hewlett-Packard HP4890 (FID) chromatograph (HP 3395 integrator) equipped with a 30mx0.320mmx0.25µm HP1 capillary column (DB-5MS). The NMR investigations were carried out on Bruker DPX 300 and AV300 spectrometers at 298 K operating at 300.13 MHz ( 1 H) and 121.495 MHz ( 31 P). IR measurements were carried out on Nicolet 6700 spectrometer using CaF2 cell. Reaction of [RhI(PPh3)2]2 with C2H4. Compound [RhI(PPh3)2]2 (10 mg, 0.013 mmol) was suspended in 2 mL of CD2Cl2 in a Schlenk tube. The argon atmosphere was then replaced with ethylene and the suspension was stirred at room temperature. After a few minutes, the precipitate dissolved to yield a clear, yellow solution. 1 H NMR (CD2Cl2): δ 6.0 -6.25 (30H, Ph), 4.43 (broad, 4H, C2H4). [START_REF] Burnett | Report ORNL-6895[END_REF] P NMR (CD2Cl2): δ 35.8 (d, 1 JP-Rh = 124 Hz). An analogous experiment was carried out in toluene-d6, resulting in the same behavior except that the solubilization of the precursor required stirring for 24 h.

Reaction of

Ethylene hydroamination catalysis. (a) Standard catalytic runs.

The catalytic experiments were conducted in a 100 mL stainless steel thermoregulated (electric oven) autoclave with a stirring bar. In a typical procedure, the autoclave was charged with RhCl3•3H2O (34.2 mg, 0.13 mmol), P(C6H5)3 (68.2 mg, 0.26 mmol) and nBu4PI (3.26 g, 8.45 mmol), closed and submitted to several argon-vacuum cycles.

Distilled and degassed aniline (4.1 mL, 45 mmol) was then syringed into the autoclave. The ethylene pipe was connected to the autoclave, purged, and the pressure was adjusted to 25 bars at RT. The temperature was then raised to 150°C. After 96 h, the autoclave was allowed to cool down to room temperature and slowly vented. The reaction mixture was then poured into 120 mL of diethylether and the resulting suspension was stirred for 2 h and then filtered. The external standard N,N-di-nbutylaniline, (ca. 0.15 g) was added to the collected ethereal phases and the solution analysed by GC.

(b) Concentration monitoring experiments. An autoclave equipped with a siphon for sample withdrawal was charged with RhCl3•3H2O, P(C6H5)3 and nBu4PI in similar amounts to the previous experiment, closed and submitted to several argon-vacuum cycles. Distilled and degassed aniline (4.1 mL, 45 mmol) and mesitylene (distilled and degassed, 16 mL) were then syringed into the autoclave.

The dilution with mesitylene was necessary in order to allow the withdrawal of a sufficient number of samples. Ethylene was finally added and the reaction carried out as described above. Samples of ca. 0.5 mL were withdrawn are different times through the siphon and poured into 3.5 mL of diethylether, followed by stirring. The external standard N,N-di-n-butylaniline, (ca. 0.010 g) was added to the collected ethereal phases and filtered before being analysed by GC.

(c) Consecutive catalytic runs. A first catalytic run was carried out as described above in (a), until after venting the leftover ethylene from the autoclave. Degassed and distilled diethylether (50 mL) was syringed into the autoclave under an argon flow and the mixture was stirred for 2 h. After one hour of settling, the supernatant liquid was transferred from the autoclave to a collector flask through the siphon. The operation was repeated several times until the transferred liquid was colourless. The external standard N,N-di-n-butylaniline (ca. 0.010 g) was added to the collected ethereal phases and filtered before being analysed by GC. Subsequently, the autoclave was submitted to vacuum for 2 h and refilled with argon. A new charge of distilled and degassed aniline (4.1 mL, 45 mmol) was then syringed into the autoclave. Ethylene was then introduced and the second catalytic run as carried out as described in (a), including the work up procedure.

Reaction of RhCl3•3H2O with PPh3, Bu4NI and aniline. In a Schlenk tube were introduced RhCl3•3H2O (34 mg, 0.13 mmol), PPh3 (68.2 mg, 0.26 mmol), Bu4NI (820 mg, 2.1 mmol). After purging with Ar, aniline (1 mL, 11.4 mmol) was added and the resulting mixture was stirred at room temperature for 15 min, then at 150°C for 16 h. The color of the mixture darkened to red-brown during the first 10 min of heating, then it did not further change. After cooling, 0.4 mL of the solution were transferred into a 5 mm NMR tube together with ca. 0.1 mL of CD2Cl2 for a 31 P NMR analysis (see text).

X-ray crystallography.

A single crystal of each compound was mounted under inert perfluoropolyether on the tip of a cryoloop and cooled in the cryostream of either an Agilent Technologies GEMINI EOS CCD diffractometer for [RhCl(PPh3)2]2 or an Oxford-Diffraction XCALIBUR SAPPHIRE-I CCD diffractometer for RhI(PPh3)3. Data were collected using the monochromatic MoK radiation (= 0.71073).

The structures were solved by direct methods (SIR97) [START_REF] Altomare | [END_REF] and refined by least-squares procedures on F 2 using SHELXL-97. 29 In compound [RhCl(PPh3)2]2, some residual electron density was difficult to model. Therefore, the SQUEEZE function of PLATON 30 was used to eliminate the contribution of the electron density in the solvent region from the intensity data, and the solvent-free model was employed for the final refinement. There is one cavity of 238 Å per unit cell. PLATON estimated that the cavity contains 35 electrons which may correspond to a solvent molecule of dichloromethane as suggested by chemical analyses. All H atoms attached to carbon were introduced in idealised positions and treated as riding on their parent atoms in the calculations. The drawing of the molecules was realised with the help of ORTEP3. [START_REF] Burnett | Report ORNL-6895[END_REF][START_REF] Farrugia | [END_REF] Crystal data and refinement parameters are shown in Table 1 Residual density, e.Å -3 0.487 / -0.325 0.475 / -0.753 Computational Details. Calculations were performed with the Gaussian09 package [START_REF] Frisch | Revision A.1[END_REF] at the DFT/M06 level [START_REF] Zhao | [END_REF] without any ligand simplification. Effective core potentials (ECP) and its associated SDD basis set [35][36][37][38] supplemented with f-polarization functions (SDD(f)) 39 were applied for rhodium atom and those supplemented with diffuse and polarization set of exponents (SDD(pd)) 40 used for iodine atoms. Tests were also carried out using the g-311G** basis set for the I atom, 41 leading to no difference for the relative energy of cis and trans isomers. The carbon and hydrogen atoms of the phenyl rings of PPh3 ligands were described with 6-31G basis set; P atoms and organic ligands (C2H4, PhNH2, Et2NH)

were described with a 6-31G(d,p) set of basis functions. Scaling factors were not applied to the calculated frequencies. Non-specific solvent effects were introduced through SMD solvation model 42 by single-point energy calculations on gas phase optimized geometries for dichloromethane (ε = 8.93). The G SMD values account for the solvation free energies, with inclusion of the solute free energy contributions ΔG SMD = ΔE SMD + ΔG gas -ΔE gas , where ΔE SMD is the electronic energy plus the solvent entropy. 43

Results and Discussion

(a) Hydroamination catalysis

Before addressing the chemistry of well defined Rh I complexes in solution, we present catalytic results that give additional information on the catalyst stability and mode of action. Within a previous PtBr2-catalyzed investigation, 6 it has become obvious to us that the use of a glass liner, systematically used in previously published work, was causing an artificial decrease of the measured activity and a loss of reproducibility because of the high temperature conditions and the resulting condensation of aniline vapours in the catalyst-free zone of the autoclave underneath the liner. We have therefore repeated a selected number of previously reported experiments 12 without the glass liner in the autoclave. The results are shown in Table 2. Note that the catalytic reactions run with the RhCl3•3H2O/I -catalyst require a much longer time (4 days) than those carried out with the PtBr2/Br -catalytic systems (10 h) in order to achieve reasonable conversions, because of the lower TOF of the former catalyst. The catalytic efficiency (CE) is calculated on the basis of the number of cycles needed by the catalyst to yield each product (two cycles for 2 and 3). As can be seen, while certain experiments gave essentially the same results (e.g. runs 1 and 3), others gave significantly improved conversions and catalytic efficiencies (run 2). All new results obtained without glass liner were repeated and found to be reproducible within 6%. The new results give the highest activity for a 65-fold excess of the iodide salt.

Repetition of the experiment carried out in the presence of the I2 additive, however, showed lower efficiency in the absence of the glass liner. The higher activity previously reported with glass liner has indeed been reproduced by us (an even greater catalytic efficiency of 542 has been obtained, which is essentially identical to the activity found without I2 and without glass liner, entry 2). Hence, while I2

was believed to have a promoting effect on the catalysis when this was carried out inside a glass liner, the opposite effect is found when the solution is in contact with the stainless steel autoclave walls. The reason for this change is currently not clear, but may be related to the negative interference, only after reaction with I2, of unknown species chemisorbed on the autoclave walls. At any rate, the fact that the activity in the presence of I2 when using a glass liner 12 is not better than that in the absence of I2 without glass liner (run 2) shows that I2 has no promoting effect on this catalysis. A control experiment run under the same conditions but without RhCl3•3H2O gave insignificant amounts of hydroamination (run 5).

One important question concerns catalyst deactivation, since this was shown to occur and to limit the TON for the related PtBr2/Br --catalyzed reaction: the catalyst turned into a black deposit (probably metallic Pt) with no residual catalytic activity. 6 In order to address this question for the Rh-based catalyst, we ran two subsequent experiments in the same autoclave under the same conditions as run 2, with the same catalyst charge, the second run being carried out after removing the product mixture of the first run, washing the solid catalyst several times (all operations were carried out inside the sealed autoclave under argon), and introducing a new charge of aniline and ethylene reagents (see details in the experimental section). The results are shown in Table 2 (run 6). As clearly seen, a similar catalytic efficiency is obtained for the first and the second run, and both are in good agreement with those of the single run of entry 2. As a matter of fact, the discrepancy between the activities in the first and second run (the first being slightly smaller and the second slightly higher than for entry 2) is a bit greater than the usual reproducibility level mentioned above. We tentatively attribute this discrepancy to an incomplete recovery, in spite of several washings, of the products from the first run (there is a significant dead volume between the bottom of the autoclave and the siphon nozzle), which have therefore remained in the autoclave and added to those produced in the second run, artificially lowering the results of the first run and increasing those of the second one. At any rate, the experiment proves that the catalyst retains most of its activity after 4 days of operations at 150°C. Hence, this catalyst is more robust than the PtBr2/Br -system. 1,6 Additional information on the progress of the catalytic reaction was obtained by monitoring the substrate and product concentrations with time. The conditions used are those of entry 2 of Table 2, except that a diluent (mesitylene) was added in order to allow a sufficient number of sample withdrawals, again because of the large dead volume below the siphon nozzle. The results are shown in Figure 1(a). The amount of aniline could not be determined accurately in this case because of peak overlap with the mesitylene diluent, but the peak separation was sufficient to demonstrate the essentially complete consumption of aniline at the end of the catalytic run. The conversion proceeded smoothly to first yield 1 and subsequently 2, consistent with the notion that 1 is an intermediate in the generation of 2. The absence of a notable induction time indicates a rapid conversion of the precatalyst to the active species. Quinaldine (3) was formed in small amounts throughout the reaction and it is not possible to conclude whether this reaction leading to this by-product proceeds through 1 or not. The evolution of the sum of the three products (1+2+3) seems linear, suggesting zero-order in aniline. After 70 h essentially no aniline is left in the mixture and the sum of all products no longer increases, while the transformation of 1 into 2 continues to take place. The essentially complete aniline consumption is in line with the absence of catalyst deactivation, as already indicated above. The final mass balance is good, the three products accounting for > 85% of the aniline amount charged in the autoclave, according to the GC calibration. A second reaction monitoring was also carried out with a double ethylene pressure (50 bar), giving very similar results, see Figure 1 2, except for a double C2H4 pressure (50 bars, ca. 1440 equiv).

In conclusion, these new catalytic experiments not only reproduce the basic findings reported in the previous contribution, 12 but also show that higher and more reproducible results are obtained in the absence of a glass liner (a negative effect of I2 under this conditions remains to be rationalized) and demonstrate that the catalyst does not degrade under these conditions and furthermore give additional insights into the kinetic profile of the catalyzed transformation.

(b) Hydroamination with [RhI(PPh3)2]2

As stated in the Introduction, the presence of a reducing environment (ethylene, aniline) and of the PPh3 ligand presumably reduces the RhCl3•3H2O precatalyst to a Rh I -PPh3 complex. Furthermore, since the catalytic activity of RhCl3•3H2O is best promoted by the iodide ion, we assume that the active species is a Rh I -I complex. Indeed, iodide derivatives of Rh I can been prepared by salt metathesis from the chlorido analogues and an iodide salt. 25 To test this hypothesis, we have carried out a catalytic run with the [RhI(PPh3)2]2 complex as a pre-catalyst, both with and without the addition of excess free iodide in the form of the tetra-n-butylphosphonium salt. The results are shown in Table 3. Comparison of the result of run 7 with that of run 2 (Table 2) indicates that [RhI(PPh3)2]2 indeed leads to efficient catalysis, although the activity is slightly lower than that obtained from the same amount of RhCl3•3H2O. This difference is probably related to parallel reactions that occur during the RhCl3•3H2O catalyst reduction and conversion to the iodide system, which involve the chloride ions, ethylene, aniline and perhaps also PPh3, and produce other species that possibly further promote the catalysis. The water present in the RhCl3•3H2O compound may also act as a catalysis promoter. The results strongly support the proposal that the active form of the hydroamination catalyst is indeed, as initially suspected, a Rh I species, because run 7 was carried out with a Rh I precatalyst and without any I2 additive. Note also that essentially no activity was obtained when the experiment was carried out in the absence of the iodide salt promoter (run 8). This results perfectly parallels that reported for the RhCl3•3H2O/I - system. 12 We now report the results of our investigations of the coordination chemistry and the relative thermodynamic stability of various Rh I complexes, both in the chloride and iodide versions, in order to learn more about the equilibria that may take place for the catalytic species under the catalytic hydroamination conditions.

(c) Chloride system

As known in the literature, complex RhCl(PPh3)3 (Wilkinson's catalyst) equilibrates in solution with the dinuclear complex [RhCl(PPh3)2]2 and free PPh3 to a different extent depending on solvent and concentration. 23 Most of our NMR studies were carried out in CD2Cl2 at ca. 10 -2 M concentration, where this dissociation was visible albeit not extensive. A representative [START_REF] Burnett | Report ORNL-6895[END_REF] P spectrum is shown in the supporting information (figure S1), indicating < 10% dissociation. Hence, the equilibrium shown in equation 1 is shifted to the left under our conditions, even in the absence of excess PPh3. The experiments that will be described further down, carried out starting from the pure dinuclear compound, were inspired by the observed changes to the small doublet 31 P{ 1 H} resonance of this compound (δ 51.50 JPRh = 196 Hz) when carrying out reactions on RhCl(PPh3)3.

2 RhCl(PPh3)3 [RhCl(PPh3)2]2 + 2 PPh3 (1)
The reaction between complex RhCl(PPh3)3 and ethylene has already been studied, since it represents a key step of the olefin hydrogenation process catalyzed by Wilkinson's catalyst. 23 It affords compound trans-RhCl(PPh3)2(C2H4), which could be isolated, but the latter tends to lose ethylene to generate [RhCl(PPh3)2]2, with which it is in rapid equilibrium (the 1 H NMR shows only one resonance for free and coordinated C2H4, the position of which shifts as a function of the C2H4 excess). These results indicate a relatively comparable stability for the two sides of equation 2. We recall these literature results here because we will compare them later with other equilibria and with the results of computational investigations of relative thermodynamic stability.

[

RhCl(PPh3)2]2 + 2C2H4 2 trans-RhCl(PPh3)2(C2H4) (2)
Exposure of RhCl(PPh3)3 to aniline, up to a large excess (> 10 equiv) yielded no observable changes in the 31 P{ 1 H} NMR spectrum, except for a broadening of the doublet resonance of [RhCl(PPh3)2]2. For this reason, the interaction of aniline was studied more in detail starting from the pure dinuclear complex. The addition of a large excess of PhNH2 (up to 65 equiv) did not reveal any new 31 P{ 1 H} resonance but shows a significant broadening without major shifting of the resonance of compound [RhCl(PPh3)2]2, as illustrated in Figure 2, suggesting a fast exchange between the reagents and the product (equation 3), strongly shifted to the reagents. Estimating a maximum amount of 1% product, given the conditions of temperature and concentrations, we can estimate that the product is at least 5.5 kcal mol -1 higher in free energy than the dinuclear precursor. On the other hand, the line broadening (w1/2 ca. 150 Hz) gives us information on the pseudo 1 st order exchange rate constant, yielding an estimated barrier of 14 kcal mol -1 for the aniline addition process. A parallel IR study in the NH stretching vibration region showed the complete consumption of Et2NH

(NH = 3327 cm -1 , 1/2 = 45 cm -1  = 2 L mol -1 cm -1 ) at a 1:1 Et2NH/Rh ratio and the appearance of a new narrower NH band assigned to the coordinated amine (NH = 3279 cm -1 , 1/2 = 12 cm -1 ), see Figure 4. This behaviour is similar to that recently described for the PtBr2(C2H4)(NHEt2) complex. 8 Addition of Et2NH beyond a Et2NH/Rh ratio of 2 reduced the intensity of this new band in favour of yet another band at 3263 cm -1 (1/2 = 26 cm -1 ), which can be attributed to the NH vibrations of both coordinated and hydrogen bonded amine in a cis-RhCl(PPh3)2(Et2NH)•Et2NH adduct on the basis of DFT calculations (vide infra). This evidence illustrates the presence of an H-bonding equilibrium, as detailed in equations 4 and 5.

[RhCl(PPh3)2]2 + 2 Et2NH 2 cis-RhCl(PPh3)2(Et2NH) (4) cis-RhCl(PPh3)2(Et2NH) + Et2NH cis-RhCl(PPh3)2(Et2NH)•Et2NH (5)
The literature is rather poor of reports of CO-free amine complexes of rhodium(I). A complex similar to cis-RhCl(PPh3)2(Et2NH) was obtained by the same strategy (equation 4) with a substituted pyridine in place of Et2NH, affording again a single product with cis geometry as confirmed by an X-ray structural analysis, but no 31 P NMR data were reported. 44 Complex RhCl(PPh3)2(Et2NSiMe3), obtained by ligand exchange from RhCl(PPh3)3, on the other hand, shows only a single 31 P NMR resonance in agreement with a trans geometry. 45 It is possible in principle that the non-equivalence of the 31 P resonances for RhCl(PPh3)2(Et2NH) is caused by restricted rotation around the Rh-N in a trans structure, however in this case the RhCl(PPh3)2(Et2NSiMe3) analogue would also be expected to have restricted rotation, since the amine is bulkier, leading to inequivalent P nuclei. We prefer to believe in an electronic effect of the amine ligand (the preference of a cis structure has also been confirmed by DFT calculations, vide infra). In further support of the assigned cis structure, the two 1 JPRh and the 2 JPP values observed for RhCl(PPh3)2(Et2NH) are very close to those reported for two different conformers of RhCl(Ph2PCH2CH2PPh2)[N(CH2Ph)=C(Me)(C6H4-4-OMe)] where the cis conformation is enforced by the chelating nature of the diphosphine (203.5, 166.9 and 42.9 for one; 199.4, ~171 and 43.5 for the other), with the same pattern (higher 1 JPRh for the lower field resonance). 46 We have attempted to crystallize the Et2NH adduct and indeed single crystals were obtained from a solution that contained a large excess of NHEt2 (5 equiv). However, the reversibility of equation 4 and the lower solubility of the dinuclear products led to the crystallization of the starting material. The molecular structure of [RhCl(PPh3)2]2 has previously been reported in a habit containing an interstitial molecule of ethyl acetate per dimer, 47 whereas a crystal containing a disordered dichloromethane molecule was obtained under our conditions. A view of the molecule is reported in Figure 5. The structural details are in good agreement with those of the ethyl acetate solvate and no further comment on this structure is warranted. In addition to PPh3, C2H4 and PhNH2, the hydroamination catalytic mixture also contains a halide salt.

The study of the interaction between RhCl(PPh3)3 or [RhCl(PPh3)2]2 and a halide salt was limited to chloride, introduced as the n-butylammonium salt, nBu4NCl, in order to avoid the complication of mixed halide systems. The corresponding iodide system will be examined below. To put the maximum chances on our side, we studied the addition of Cl -to the thermodynamically less stable dinuclear complex [RhCl(PPh3)2]2, expecting to observe the product of equilibrium 6. However, the 31 P NMR spectrum of the solution, in the presence of as much as 10 equiv of nBu4Cl, still showed the doublet resonance of the dinuclear starting material as the dominant resonance in the spectrum, unshifted and unbroadened relative to that of the pure starting material. Several other minor peaks, besides a peak of Ph3PO, were also visible (spectrum shown in Figure S2) but it is impossible to assign any of them with certainty to the expected product of equation

(d) Iodide system

Although compound RhI(PPh3)3 is known, 23 its X-ray structure has apparently never been reported.

The only reported structure with a Rh I IP3 coordination environment is apparently that of compound RhI(diop)(PPh3). 49 Compound RhI(PPh3)3 crystallizes with the full molecule in the asymmetric unit in the orthorhombic space group Pna21. A view of the molecule is shown in Figure 6. The presence of three sterically encumbering PPh3 ligands forces the structure to deviate from the preferred square planar configuration toward a butterfly arrangement (trans angles around 160°). The same effect was previously noted for different polymorphs of the related RhCl(PPh3)3 compound. 50 The Rh-P distances in the iodide structure compare well with those in the two different chloride structures, with the two phosphine ligands trans to each other showing slightly longer distances relative to that trans to the halide. The latter has a marginally longer distance than the same ligand in the two chloride structures As already described in the literature, complex RhI(PPh3)3 equilibrates in solution with the dinuclear species [RhI(PPh3)2]2 and free PPh3, like the chloride analogue. The dinuclear species is favored to a greater extent for the iodide system and indeed the best preparation method of the dinuclear complex is by PPh3 dissociation under thermal conditions in a solvent where the dinuclear product is less soluble and precipitates. 24 The literature also shows that PPh3 dissociation is faster for RhI(PPh3)3 than for the corresponding chloride, since the 31 P resonances are broad at room temperature 25 The coupling constant in the CD2Cl2 spectrum matches well with that obtained for the iodide system but not with that obtained for the chloride system in toluene-d8. The 1 H spectrum of trans-RhI(PPh3)2(C2H4) shows a broad resonance for the ethylene ligand, like that reported in the literature 23 for the corresponding chlorido derivative. Note that this complex could not be isolated and an NMR spectrum was not reported in the original study by Wilkinson et al., 23 where its generation was attempted from RhI(PPh3)3. Our results and those already available in the literature suggest that the ethylene binding constant to [RhI(PPh3)2] is smaller than for the chlorido analogue.

[RhI(PPh3)2]2 + 2C2H4 2 trans-RhI(PPh3)2(C2H4) (7) Addition of aniline, even in large excess, to [RhI(PPh3)2]2 did not lead to any spectral change and notably, contrary to the chlorido system, did not significantly broaden the 31 P resonance, suggesting that the energy gap between the reactant mixture and the putative RhI(PPh3)2(PhNH2) product is greater than for to corresponding chlorido system. Addition of Et2NH, on the other hand, resulted in reaction and formation of the expected cis-RhI(PPh3)2(Et2NH). The latter compound is characterized, like the above described chlorido analogue, by two doublets of doublets for the two inequivalent PPh3 ligands, see Figure 7. At variance with the chlorido system, the Et2NH addition process was not quantitative, resonances of the starting dinuclear complex remaining visible even after adding an excess amount of amine. The JPRh of the dinuclear compound in the two spectra of Figure 7 is the same (190 Hz; the spectra were taken at two different field strengths). Figure 3).

The IR study of the reaction showed the consumption of the free amine NH band, accompanied by the appearance of the band of the coordinated amine at NH = 3279 cm -1 (1/2 = 12 cm -1 ). This frequency is identical to that of the chlorido analogue, showing a negligible effect of the halide nature on the N-H stretching frequency. However, further addition of Et2NH resulted in a much less significant change compared to the chlorido system. The lower frequency NH band of the [RhI(Et2NH)(PPh3)2]•Et2NH adduct appeared in the spectrum only with a 10fold excess of amine as a weak shoulder at ca 3260 cm -1 , see Figure 8. This is in agreement with the computational results, showing less favourable formation of this adduct in case of iodide (vide infra). Finally, addition of I -(in the form of its Bu4N + salt, up to 3 equiv per Rh) to complex [RhI(PPh3)2]2, like for the related experiment on the chloride system, resulted in no significant spectral change, indicating that equilibrium 8 is heavily shifted toward the left hand side. Like for the chlorido analogue, the corresponding dicarbonyl complex [RhI2(CO)2] -is a well known compound. 48 [

RhI(PPh3)2]2 + 2 I - 2 [RhI2(PPh3)2] - (8)

(e) Treatment of RhCl3•3H2O with PPh3, Bu4NI and aniline

A final spectroscopic investigation consisted of the 31 P NMR study of a mixture prepared with all the catalytic components (except for ethylene) under conditions mimicking those of the catalysis, in order to gain further insights into the conversion of the RhCl3•3H2O precatalyst into the active catalyst.

Excluding ethylene allows the reaction to be carried out in simple glassware (the boiling point of aniline is 184.13 °C). Thus, a mixture containing RhCl3•3H2O, PPh3, nBu4PI and PhNH2 in a 1:2:16:88 ratio was kept at 150°C for 16 h, even though no change of color was noted after the first 10 min of heating.

The nBu4PI:PhNH2 ratio in this experiment was identical to that of the catalytic runs, but the Rh and PPh3 concentrations were ca. 4 times higher in order to facilitate the 31 P NMR measurement of the reaction products in the presence of the large excess of nBu4P + ion. The nBu4P + resonance was indeed by far the most intense one in the spectrum, but was accompanied by two major and relatively broad peaks, which account for most of the PPh3 intensity, a stronger one at δ 52.0 and a smaller one at δ 50.6

(see Figure 9). The spectrum also showed a large number of other very weak resonances revealing the formation of many other products, none of which could be identified except Ph3PO. [RhCl(PPh3)2]2 (cf. Figure 2). The presence of the large excess of aniline is responsible for the resonance broadening as discussed above. The dinuclear iodide analogue, [RhI(PPh3)2]2, does not appear to be present, since it should generate an unbroadened resonance centered at δ 48.8 (cf. Figure 7).

It seems therefore reasonable to assume that the small resonance at δ 50.6 belongs to a mixed-halido species, Rh2(I)(Cl)(PPh3)4, broadened by exchange with aniline like the chlorido dimer and unlike the iodido dimer.

The results of this experiment confirm therefore that RhCl3•3H2O is reduced to Rh I under catalytic conditions in the presence of PPh3, aniline and iodide ions but reveal that no extensive halide exchange has taken place and the chloride dinuclear species [RhCl(PPh3)2]2 is the major reaction product, at least after cooling the mixture back to room temperature. The lack of extensive halide exchange may result from thermodynamic (the relative amount of iodide used here is 4 times less than in the catalytic run, but still in quite large excess relative to Rh) or kinetic factors. We believe that the reason is thermodynamic. In support of this argument, we note that Cl/I halide exchange processes have been shown to favour the chlorido species also in other cases when carried out in the presence of soluble salts (e.g. CpMoI2(PMe3)2 to CpMoICl(PMe3)2 and CpMoCl2(PMe3)2 with PPNCl/CH2Cl2; 51 MoOI2(PMe3)3

to MoOICl(PMe3)3 and MoOCl2(PMe3)3 with nBu4NCl/acetone 52 ) whereas exchange in the opposite direction occurs in the presence of partially soluble alkali metal salts, presumably because of the lower solubility of the alkali metal chloride (e.g. CpMoCl2(PMe3)2 to CpMoICl(PMe3)2 and CpMoI2(PMe3)2 with NaI/THF; 53 MoOCl2(PMe3)3 to MoOICl(PMe3)3 and MoOI2(PMe3)3 with NaI/acetone; 52 and of stronger relevance to this work, RhCl(PPh3)3 to [RhI(PPh3)2]2 and free PPh3 with LiI/toluene 25 ). DFT calculations have also been conducted to address this point (vide infra). At any rate, whatever the reason for the lack of halide exchange, the absence of a notable induction time in catalysis suggests that [RhCl(PPh3)2]2 is an active precatalyst, without the need to be converted to the iodide analogue. The results do not exclude that the real catalyst is one of the very minor and unidentified by-products formed in the reaction. However, the catalytic activity demonstrated independently for [RhI(PPh3)2]2 lends credence to the hypothesis that the dinuclear Rh I structure is indeed responsible for generating the active catalyst. If this is the case, then the promoting effect of iodide (much better than chloride) 12 must operate at the level of higher-energy catalytic steps, rather than at the level of the resting state.

The most important conclusions to be drawn from this experiment is that the resting state of the catalytic cycle must be either [RhCl(PPh3)2]2 or trans-RhCl(PPh3)2(C2H4), because it is known from independent experiments reported here and in the literature that the reaction of [RhCl(PPh3)2]2 (or the iodido analogue) with ethylene is equilibrated but more favourable than the additions of aniline or halide ions.

(f) DFT calculations

No ligand simplification was made and the calculations were full QM, at the M06 level. The geometries of the dinuclear [RhX(PPh3)2]2 systems (X = Cl, I) and their ligand adducts [RhX(PPh3)2(L)] with L = PPh3, X -, PhNH2, Et2NH and C2H4 have been optimized by DFT calculations. The optimized geometries are in good agreement with the available experimental structures, namely those of RhCl(PPh3)3 (orange allotrope) available in ref. 50 and RhI(PPh3)3 reported in the present work. A comparison is available in Table 4. The largest deviation in the bond lengths is seen for the Rh-I bond (0.07 Å), the others being within 0.05 Å, whereas the angles are all calculated within 2º from the experimental values. The relative energies (both electronic and Gibbs free energies) for all RhX(PPh3)2(L) systems (L = PPh3, X -, PhNH2, NHEt2 and C2H4; X = Cl, I) are reported in Table 5. They are calculated with respect to the precursor complex [RhX(PPh3)2]2 and free ligands L according to equation 9. The direct comparison with the experimental data is complicated by several approximations, which are well known in the area of computational chemistry applied to condensed phase thermodynamics, the most important one being probably the use of unquenched translational and rotational modes for the estimation of the condensed phase entropy. Ion pairing effects (neglected in our calculation) may also affect the results

for the ionic species.

[RhX(PPh3)2]2 + 2 L  2 RhX(PPh3)2(L) (9) Solvent effects have also been included in the calculations using the SDM solvation model 42 in the CH2Cl2 medium (ε = 8.93) and this turns out to be important for the rationalization of certain trends. In particular, while solvation has either a negligible effect or favors the dimer splitting reaction by a few kcal/mol when adding a neutral ligand, it strongly disfavors the addition of X -. This is attributable to the higher solvation energy of the free X -ions relative to the neutral species (see Table 5, footnote c). Note that free X -is not better solvated than the [RhX2(PPh3)2] -ion, but the balance is shifted more in favor of the reactants side by the solvent, because the combination of [RhX(PPh3)2]2 and 2X -is better solvated than 2[RhX2(PPh3)2] -. As expected, solvation free energy is higher for the systems containing the smaller Cl atom. The experimentally observed higher stability of the Et2NH adduct relative to that of the PhNH2 adduct is also reproduced by the calculation only when the solvent effect is included. The ΔE SMD and ΔG SMD parameters are graphically summarized in Figure 10.

[RhX 2 (PPh 3 ) 2 ] - Note that for all systems except the anionic ones, ΔG SMD > ΔE SMD (the Gibbs parameter is more positive or less negative than the electronic energy), as expected from the reduction in molecularity (entropy decrease). The singular behavior for the anionic X -adducts can be ascribed to the increase of rotational entropy for this system, because no rotational (or vibrational) modes are associated to the monoatomic X -reagent. Roughly speaking, since the major entropic contribution to the free energy change is given by changes in the translational and rotational modes, the reactions with the bidimensional (C2H4) or tridimensional (PPh3, PhNH2) ligands entail the disappearance of 3 translational and 3 rotational modes whereas the reaction with zero-dimensional X -entails on one hand the disappearance of 3 translational modes and on the other hand the generation of 3 rotational modes. This effect, predicted by the gas phase calculations, will obviously be attenuated in a condensed phase and further modulated by ion pairing.

The addition of L to [RhX(PPh3)2]2 results in energetic stabilization except for the addition of X -(the solvent has an important effect on this equilibrium, as already pointed out above). Indeed, we were unable to observe any significant conversion of [RhX(PPh3)2]2 in the presence of excess X -. For the neutral ligands, the relative stability increases in the order PhNH2 < C2H4 < PPh3 < Et2NH for each X on the ΔE SMD scale. On the ΔG SMD scale, on the other hand, the order is PhNH2 < PPh3 < Et2NH < C2H4, in better agreement with the experimental evidence. Indeed, the RhCl(PPh3)2(C2H4) complex is sufficiently stable to be isolated, although it readily looses the ethylene ligand unless kept under a protecting ethylene atmosphere, 23 whereas we have shown above that the corresponding Et2NH adduct also exists in solution but is not sufficiently stable to be isolated.

The energy change is less negative (or more positive) when X = I for all systems. In other words, [RhI(PPh3)2]2 has a greater relative stability with respect to all its ligand adducts than the corresponding chloride system. This result is in agreement with all the available experimental evidence. Indeed, the more facile dissociation of PPh3 from RhI(PPh3)3 or C2H4 from RhI(PPh3)2(C2H4) relative to the corresponding chloride systems has previously been discussed. 23 The new experiments reported in the present contribution also illustrate the lower stability of the other ligand adducts for the iodide system (vide supra).

Another notable result of the computational study concerns the relative stability for each pair of stereoisomers: the trans geometry is preferred for the C2H4 adduct, whereas all other ligand adducts show a preference for the cis geometry (moderate for X -, strong for the amines). This is in perfect agreement with the experimental evidence, according to which the ethylene adduct is trans and the Et2NH adduct reported in this contribution is cis, without the detection of the trans isomer. The reason for the difference stereochemical preference may be traced to the competition between the different π acceptors and π donors that are present in the coordination sphere. Thus, C2H4 is a stronger π acceptor and prefers to be located trans to a π donor halide ligand, leading to a preferred trans geometry, whereas the two more weakly π acceptor PPh3 ligands avoid competing with each other in the cis geometry for the amine and X -adducts.

Given the infrared evidence for a secondary interaction between the RhX(PPh3)2(Et2NH) complex and additional free Et2NH, a calculation was also carried out for the RhX(PPh3)2(Et2NH)•••NHEt2 adducts. Four different minima could be located, the same ones for both halide systems. Adduct (III) derives from the lower energy RhX(PPh3)2(NHEt2) molecule (I) and the other three (IV, V and VI) from a slightly higher energy minimum (II). Figure 11 shows a view of all the molecules for the Cl system (those of the I system are given in the Supporting Information). The relative energies and the calculated NH stretching frequencies are reported in Table 6. In order to take into account the H-bond breaking in the NHEt2 reagent, the H-bonded dimer (Et2NH…NHEt2) has been chosen as a reference point. principles, is well reflected by the computed energy values, and the smaller solubility of NaCl than NaI (not known for CH2Cl2 solution, but confirmed in most polar aprotic organic solvents such as acetone and MeCN) [START_REF] Burgess | Metal Ions in Solution[END_REF] will in fact further accentuate the preference of equilibrium (10) for the right hand side.

Quantitative agreement with experiment is not achieved, since the equilibrium appears to be displaced to the left hand side in the presence of soluble halide sources, however the calculated values are not too far from the expected outcome, considering that the calculation of free energy differences in condensed phases, especially when ionic species are involved, one of the most challenging areas of computational chemistry, and the trend is as expected.

[

Conclusion

The present investigation has provided strong evidence that the recently reported 12 RhCl3•3H2O/I - catalyst for the hydroamination of ethylene by aniline is activated by reduction to a Rh I active species.

This catalyst is more robust than the PtBr2/Br -system, no significant loss of activity after 4 days at 150°C being observed in a recycle experiment, whereas the Pt-based catalyst is totally deactivated by the catalytic medium. 6,11 The catalytic activity of the Rh-based catalyst, however, is much lower than that of the Pt-based catalyst, as shown by the kinetic profile. The solution studies of the [RhI(PPh3)2]2 system and of the chlorido analogue at room temperature show ligand addition equilibria to yield mononuclear RhX(PPh3)2(L) species (L = C2H4, PPh3, PhNH2, X -and the model Et2NH amine) that are of comparable stability (marginally more stable according to DFT calculations, for C2H4, PPh3

and Et2NH; or less stable, for PhNH2 and X -). The reaction of RhCl3•3H2O in the presence of PPh3, I - and PhNH2 leads to [RhCl(PPh3)2]2 as the major species, without notable halide exchange, thus suggesting that iodide is not needed at the level of the catalyst resting state. The effect of high temperature and pressure conditions on the equilibria cannot be easily predicted, but it may be speculated that either RhCl(PPh3)2(C2H4) or [RhCl(PPh3)2]2 are the rate-determining intermediates (resting state) of the catalytic cycle. An ionic [RhClI(PPh3)2] -or [RhI2(PPh3)2] -complex certainly does not appear to play a significant role, thus the promoting effect of iodide on the catalytic activity must be rationalized through its effect on the rate-determining transition state, which is still currently unknown.

Future work will address the catalytic mechanism through computations.

optimized geometries (25 pages). Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 833545. Copies of the data can be obtained free of charge on application to the Director, CCDC, 12

Union Road, Cambridge CB2 1EZ, UK (fax: (+44) 1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).

  RhCl(PPh3)3 with PhNH2. In a 5 mm NMR tube was placed [RhCl(PPh3)3] (10.7 mg, 0.012 mmol) and dissolved in ca. 0.7 mL of CD2Cl2. The solution composition was checked by 31 P{ 1 H} NMR, revealing the presence of a minor amount of [RhCl(PPh3)2]2 and free PPh3, with which the starting compound is in equilibrium. PhNH2 was added by microsyringe and the progress of the reaction monitored by 31 P{ 1 H}l NMR. No change in the spectrum was observed upon introduction of up to 11 eq. of PhNH2, except for the broadening of the doublet resonance of the dinuclear complex [RhCl(PPh3)2]2. Reaction of [RhCl(PPh3)2]2 with amines. (a) PhNH2. In a 5 mm NMR tube was introduced [RhCl(PPh3)2]2 (10.06 mg, 0.016 mmol) and dissolved in ca. 0.7 mL of CD2Cl2. After checking the quality of the compound, PhNH2 was added by microsyringe and the progress of the reaction monitored by 31 P{ 1 H} NMR, indicating only a weak affinity of this amine for bonding (see text). (b) Et2NH. In a 5 mm NMR tube was introduced [RhCl(PPh3)2]2 (5.4 mg, 8.15•10 -3 mmol) and dissolved in ca. 0.7 mL of CD2Cl2. After checking the quality of the compound, Et2NH was added by microsyringe and the progress of the reaction monitored by 31 P{ 1 H} NMR, giving evidence for quantitative formation of cis-RhCl(PPh3)2(Et2NH) in the presence of excess Et2NH (see text). A solution prepared separately in a Schlenk tube with the same concentration and excess Et2NH was set out for crystallization by diffusion of a pentane layer, affording single crystals of [RhCl(PPh3)2]2. Reaction of [RhI(PPh3)2]2 with amines. The reaction of compound [RhI(PPh3)2]2 with both PhNH2 and Et2NH was carried out in CD2Cl2 inside an NMR tube, following the same experimental protocol descrived above for the related [RhCl(PPh3)2]2 system.

Figure 1 .

 1 Figure 1. Kinetic profile of the RhCl3•3H2O-catalyzed reaction between PhNH2 and C2H4: 1 (♦), 2 (▲),

Figure 2 . 3 ) 3 .

 233 Figure 2. 31 P{ 1 H} study in CD2Cl2 of the interaction between [RhCl(PPh3)2]2 (c = 0.027 M) and PhNH2 in CD2Cl2. (a) Without PhNH2. (b) With 65 equiv of PhNH2. The difference in noise level is caused by a much larger number of accumulations for spectrum (b).

Figure 3 .

 3 Figure 3. 31 P{ 1 H} spectrum in CD2Cl2 of cis-RhCl(PPh3)2(Et2NH), obtained from [RhCl(PPh3)2]2 and Et2NH (5 equiv).

Figure 4 .

 4 Figure 4. Infrared study of the reaction of [RhCl(PPh3)2]2 with NHEt2 in CH2Cl2 (pathlength = 2.2 mm). (a) Spectra of [RhCl(PPh3)2]2 (c = 6•10 -3 M) (dotted line) and NHEt2 (c = 0.05 M, dashed line). (b) Spectra of a 6•10 -3 M solution of [RhCl(PPh3)2]2 after of NHEt2 with the successive concentrations (6, 12, 24, 60 and 120)•10 -3 M.

Figure 5 .

 5 Figure 5. Molecular view of [RhCl(PPh3)2]2 with the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level. H atoms have been omitted for clarity. Relevant bond distances (Å) and angles (°): Rh-Cl, 2.4309(6); Rh-Cl i , 2.3967(6); Rh-P1, 2.2021(6); Rh-P2, 2.2168(6). P1-Rh-P2, 96.05(2); P1-Rh-Cl1, 176.23(2); P1-Rh-Cl1 i , 95.24(2); P2-Rh-Cl1, 87.33(2); P2-Rh-Cl1 i , 168.01(2); Cl1-Rh-Cl1 i , 81.51(2); Rh-Cl1-Rh i , 98.49(2). Symmetry transformations used to generate equivalent atoms: (i) -x+2,-y+1,-z

[ 2 . 49 Figure 6 .

 2496 Figure 6. Molecular view of RhI(PPh3)3 with the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level. H atoms have been omitted for clarity. Relevant bond distances (Å) and angles (°): Rh-I, 2.6840(3); Rh-P1, 2.2303(7); Rh-P2, 2.2937(7); Rh-P3, 2.3239(7). I-Rh-P1, 163.53(2); I-Rh-P2, 86.09(2); I-Rh-P3, 86.76(2); P1-Rh-P2, 97.38(3); P1-Rh-P3, 95.39(3); P2-Rh-P3, 157.89(3).

  and become sharper upon cooling. All these observations, reproduced in our hands, suggest that the Rh-PPh3 interaction is weaker in the iodide system.The reaction of [RhI(PPh3)2]2 with C2H4 takes place readily in dichloromethane and more slowly in toluene, to afford trans-RhI(PPh3)2(C2H4) selectively (equation 7), as shown by the presence of only one doublet resonance in the[START_REF] Burnett | Report ORNL-6895[END_REF] P NMR spectrum at δ 35.8 ( 1 JP-Rh = 124 Hz) in CD2Cl2. The possibility that a halogen exchange has occurred with the chlorinated solvent to yield trans-RhCl(PPh3)2(C2H4), besides being against the HSAB principle, is categorically excluded because the same experiment run in toluene-d8 gave a doublet resonance at δ 36.4 ( 1 JP-Rh = 125 Hz), whereas the corresponding experiment using [RhCl(PPh3)2]2 in place of [RhI(PPh3)2]2 gave a doublet resonance at δ 34.9 (d,1 JP-Rh 129 Hz).

Figure 7 .

 7 Figure 7. 31 P{ 1 H} NMR spectra of [RhI(PPh3)2]2 in CD2Cl2 before (a) and after (b) the addition of 3 equiv of Et2NH. The starred peaks are due to a trace amount of chlorido derivative, RhCl(PPh3)2(NHEt2) (cf.Figure3).

Figure 8 .

 8 Figure 8. Infrared study of the reaction of [RhI(PPh3)2]2 (c = 3•10 -3 M) with NHEt2 in CH2Cl2 (pathlength = 2.2 mm), after addition of NHEt2 with the successive concentrations (3, 4.5, 6, 12, 18 and 30)•10 -3 M.

Figure 9 .

 9 Figure 9. Excerpt of the 31 P{ 1 H} NMR spectrum of the final mixture generated from RhCl3•3H2O, PPh3, nBu4PI and PhNH2 in a 1:2:16:88 ratio at 150°C.

Figure 10 .

 10 Figure 10. Relative ΔE SMD and ΔG SMD (in parentheses) in kcal/mol for geometry optimized [RhX2(PPh3)2] -and RhX(PPh3)2L (L = PPh3, PhNH2, Et2NH and C2H4), relative to [RhX(PPh3)2]2.

Figure 11 .

 11 Figure 11. View of the optimized geometries of RhCl(PPh3)2(Et2NH) (I and II) and RhCl(PPh3)2(Et2NH)•••NHEt2 (III-VI).

Structures

  I and II differ in the orientation of the Et2NH ligand relative to the rest of the molecule; the NH vector points toward one of the Ph rings of the cis PPh3 ligand in the lower energy I and towards the X ligand in the slightly less stable II. Structure I can only establish an H bond as a proton acceptor via the X ligand with the NH bond of the external NHEt2 molecule, leading to III. Structures IV and V feature the same X•••H-N interaction but also an additional N-H•••N bond involving the coordinated amine as proton donor and the outer sphere amine as proton acceptor, whereas VI contains only the N-H•••N interaction with the coordinated amine providing the proton to the external amine. The lowest energy structure is III when the solvation effects are taken into account, whereas IV is slightly favored in the gas phase. Note, however, that the energy difference between these two structures is small. The H-bonds are shorter in IV.

Table 1 .

 1 . Crystal data and structure refinement

	Crystallographic data (excluding structure factors) have been deposited with the Cambridge
	Crystallographic Data Centre as supplementary publication no. CCDC 828152 & 828153. Copies of the
	data can be obtained free of charge on application to the Director, CCDC, 12 Union Road, Cambridge
	CB2 1EZ, UK (fax: (+44) 1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
	Compound	[RhCl(PPh3)2]2	RhI(PPh3)3
	Empirical formula	C36H30ClP2Rh	C54H45IP3Rh
	Formula weight	662.90	1016.62
	Temperature, K	180(2)	180(2)
	Wavelength, Å	0.71073	0.71073
	Crystal system	Triclinic	Orthorhombic
	Space group	P-1	Pna21
	a, Å	9.7226(2)	19.7950(8)
	b, Å	12.6361(3)	12.6130(6)
	c, Å	13.7002(4)	17.7058(8)
		85.091(2)	90.0
		83.067(2)	90.0

Table 2 .

 2 Hydroamination of ethylene by aniline catalyzed by RhCl3•3H2O. a Values in parentheses are those reported in the previous contribution. 12 Conditions: RhCl3•3H2O (34.2 mg, 0.13 mmol), PPh3 (68.2 mg, 0.26 mmol, 2 equiv), n-Bu4PI, PhNH2 (4.1 mL, 45.5 mmol, 350 equiv), C2H4 (25 bars; ca. 770 equiv), 150°C, 96 h.

	1	10	-	13(14)	42(47)	4(3)	-(-)	49(53)
	2	65	-	83(65)	50(130) 239(93)	1(4) 531(324)
	3	150	-	69(65)	138(102) 100(120) 2(5) 341(352)
	4	65	2	57(73)	150(28) 51(225)	-(3) 251(484)
	5	c	-	2	6	0	0	5
	6(1)			73	28	229	1	488
		65	-
	6(2)			86	37	266	0	570
	a

b CE (catalytic efficiency) = TON 1 + 2(TON 2) + 2(TON 3). c Same amounts of all reagents as in entry 2, in the absence of RhCl3•3H2O.

Table 3 .

 3 Hydroamination of ethylene by aniline catalyzed by [RhI(PPh3)2]2 a

	Run I -/Rh Conv./%	TON 1	TON 2 TON 3	CE b
	7	65	74	144	116	0	377
	8	-	1	2	0	0	1
	a Conditions: [RhI(PPh3)2]2 (98.1 mg, 0.13 mmol of Rh), PhNH2 (4.1 mL, 45.5 mmol, 350 equiv), C2H4
	(25 bars; ca. 770 equiv), 150°C, 96 h.

b CE (catalytic efficiency) = TON 1 + 2(TON 2) + 2(TON 3).

  6. The solution also shows instability, as additional small resonances became visible upon keeping the solution under argon for several hours. The resonance of [RhCl(PPh3)2]2 always remained the dominant one, however. This experiment allow us to at least conclude that equilibrium 6, if any [RhCl2(PPh3)2] -product is formed at all, must be heavily shifted to the left hand side. Note that, on the other hand, the reaction of [RhCl(CO)2]2 with chloride generates

	[RhCl2(CO)2] -quantitatively. 48
	[RhCl(PPh3)2]2 + 2 Cl -	2 [RhCl2(PPh3)2] -

Table 4 .

 4 Comparison between DFT optimized and experimental geometries (distances in Å, angles in degrees)The calculated geometry of [RhCl(PPh3)2]2 agrees with the experimental ones (both the previously reported ethyl acetate solvate47 and the dichloromethane solvate reported herein) in terms of bond lengths and angles, except for the bending at the Rh(μ-Cl)2Rh bridge, whereas the experimental structures have a flat core. As a matter of fact, the preference for a bent or flat geometry for edgesharing square planar d 8 -d 8 dimers rests on a delicate energetic balance, as previously discussed in detail.54 Very similar structures of [RhCl(PR3)2]2 complexes have been shown to adopt either a flat (e.g. PiPr3,55 in addition to the above mentioned PPh3 structures) or bent (e.g. PMe3, 56 tBu2PH,57 (C2F5)2PCH2CH2P(C2F5)2, 58 and iPr2PCH2CH2PiPr259 ) geometry, and the related [RhF(PPh3)2]2 compound adopts either a flat60 or bent61 geometry in different crystal habits. The calculations suggest preference for a bent geometry for an isolated [RhCl(PPh3)2]2 molecule, at least at this level of theory, thus the experimental observation of a flat structure is probably the consequence of crystal packing effects.

		RhCl(PPh3)3	RhI(PPh3)3
		DFT	Exp a	DFT	Exp b
	Rh-X	2.440	2.404(4)	2.760	2.6840(3)
	Rh-P(trans to X)	2.258	2.225(4)	2.278	2.2303(7)
	Rh-P(trans to P)	2.340	2.304(4)	2.344	2.2937(7)
		2.361	2.338(4)	2.374	2.3239(7)
	X-Rh-P(trans)	164.9	166.7(2)	161.5	163.53(2)
	X-Rh-P(cis)	82.4	84.5(1)	85.3	86.09(2)
		85.1	85.3(1)	87.6	86.76(2)
	P-Rh-P(trans)	156.5	159.1(2)	157.7	157.89(3)
	P-Rh-P(cis)	101.6	97.7(1)	99.3	97.38(3)
		95.7	96.4(2)	94.1	95.39(3)

a Data from ref.

50

. b This work.

Table 5 .

 5 Various

	energetic parameters (in kcal/mol) of RhX(PPh3)2(L), relative to those of
	½[RhX(PPh3)2]2 + L (X = Cl, I; L = PPh3, X -, PhNH2, Et2NH and C2H4).
	Compound	ΔE	ΔG a	ΔE SMD	ΔG SMD,b	Gsolv c
	(a) Chloride system
	RhCl(PPh3)3	-11.7	-3.8	-12.7	-4.7	-43.8
	cis-[RhCl2(PPh3)2] -	-6.8	-9.1	8.2	5.8	-73.6
	trans-[RhCl2(PPh3)2] -	-4.4	-10.8	13.4	7.1	-70.6
	cis-RhCl(PPh3)2(PhNH2)	-9.2	-3.7	-8.9	-3.4	-36.7
	trans-RhCl(PPh3)2(PhNH2)	3.2	6.4	1.1	4.3	-39.0
	cis-RhCl(PPh3)2(Et2NH)	-8.6	-3.1	-13.7	-8.2	-34.4

Table 6 .

 6 Energies (in kcal/mol, relative to RhX(PPh3)2(Et2NH) (I) + ½ (Et2NH…NHEt2)) and calculated N-H stretching vibrations (in cm -1 ).

	Compound	ΔE	ΔG a	ΔE SMD	ΔG SMD,b	ν(NH) c	ν(NH) d

a Gas-phase at T = 298.15 K. b ΔE SMD + (ΔG-ΔE) at 298.15 K. c Coordinated Et2NH ligand. d H-bonded Et2NH ligand.

Table 7 .

 7 Various energetic parameters (in kcal/mol) for equilibrium(10), depending on the nature of the counter-cation. ΔE SMD + (ΔG-ΔE) at 298.15 K. c Optimized in the gas phase and SMD calculation at the frozen geometry. d Optimization with SMD.

	RhCl(PPh3)2]2 + 2 I -		[RhI(PPh3)2]2 + 2 Cl -	(10)
	Cation	ΔE	ΔG a	ΔE SMD	ΔG SMD
	-	14.4	14.4	-4.1	-4.1 b
	Me4N + c -6.8	-5.8	-5.9	-4.8 b
	Me4N + d			-5.2	-2.0
	Me4P + d			-4.9	-1.2
	Na + d			-15.6	-14.9

a Gas-phase at T = 298.15 K. b

Acknowledgement. We are grateful to the ANR (Agence Nationale de la Recherche, Grant No. NT09_442499), the IUF (Institut Universitaire de France), the CNRS and the RFBR (Russian Foundation for Basic Research; France-Russia bilateral grant No. 08-03-92506) for financial support, as well as the CINES (Centre Informatique National de l'Enseignement Supérieur) and the CICT (Centre Interuniversitaire de Calcul de Toulouse, project CALMIP) for granting free computational time.

Structures I and IV also give the best agreement between the calculated NH frequencies and the experimental data. Interestingly, the two NH vibrations in IV (for the coordinated and for the hydrogen bonded Et2NH) appear at similar frequencies, slightly red-shifted from that of I. Considering the expected difference between the frequencies computed in the gas phase and those experimentally measured in solution, these results allow the assignment of the experimentally observed band at 3263 cm -1 to a combination of the two NH bands in IV. Finally, the data in Table 6 indicate a more favorable H-bond formation for the Cl system on the E scales (both gas phase and solution), whereas the process is more favorable for the I system on the Gibbs free energy scales. The experimental IR data are clearly demonstrating an easier formation of the H-bonded adduct for the Cl system, suggesting that the gas phase entropy correction is not adequate to address this phenomenon. Hence, the present system is yet another useful example pointing out to the care that one has to take when using computed gas phase thermodynamic data (even when including the solvation effects) for the prediction of solution behavior.

A final computational study has addressed the halide exchange equilibrium (10), of interest because of the somewhat unexpected lack of iodide incorporation into [RhCl(PPh3)2]2 by treatment with excess nBu4P + I -. The equilibrium has been analyzed in the presence of the free ions (no cation) as well as with ion pairs modeled by Me4N + X -, Me4P + X -, and Na + X -and the relevant results are presented in Table 7.

The results in the absence of cation are interesting since they show how much the right hand side is favored by solvation with respect to the gas phase. Introduction of the Me4N + cation indicates that the cation-anion association has a similar effect to that of the solvent model, since the gas phase parameters are essentially identical to those in solution. Optimization of the ion pair geometry in the presence of the solvent model does not greatly affect the results, relative to those obtained by correcting the gas phase energy by solvation effects, and the effect of Me4N + and Me4P + is essentially equivalent, whereas that of Na + differs significantly, stabilizing the right hand side of the equilibrium by more than 10 kcal/mol.

Clearly, the Na + results are not quantitatively relevant because NaCl and NaI are essentially insoluble in CH2Cl2. However, the stronger Na + Cl -association relative to Na + I -, which is expected from first Supporting Information Available. NMR spectrum of [RhCl(PPh3)2]2; Cartesian coordinates of all SYNOPSIS TOC (Word Style "SN_Synopsis_TOC").


M Jacquemin

P Simon

A Canizarès

L Hennet

C Bessada

D Skrelic

E Gouillart

E Burov

High sensitivity Raman imaging of the surface of casted glass plates

Keywords: Raman imaging, glasses, glass processing

Glass manufacturing processes are prone to induce local fluctuations of the glass properties, to which Raman spectroscopy is highly sensitive. In this work, Raman imaging is used to investigate the homogeneity of the Raman response at the surface of casted aluminosilicate glass pieces. Samples were probed at constant focus depth across 7×7 cm 2 surfaces using 500 µm spatial steps, resulting in unusually large and detailed Raman images. We show that the extent of modification of the Raman response is small across the scanned area, and that the information is mostly carried by the spatial representation of properly selected Raman parameters. Specifically, detailed macroscopic patterns correlating to the glass casting process were obtained from Raman parameters of two distinct Raman modes: the Si-O stretching mode involving Q 2 tetrahedral units, and the Si-O-Si bending vibrations envelope in the low-wavenumber range. The contrasts on the Raman images are assigned to fine local variations of fictive temperature (and hence of cooling rate) and chemistry resulting from the manufacturing process. From the evolution of these parameters across the surface of the sample, we were able to identify areas consisting of material from different stages of the casting. Structural and chemical changes originating from the manufacturing process

Introduction

Raman spectroscopy is a commonly used technique for the study of the local range order in glasses and melts. 1 It is appreciated for its non-destructive nature, and its versatility and relative ease of use make it particularly suitable to perform in situ characterizations in extreme conditions. [2][3][4][5][6][7][8] Modern Raman instruments are now capable of acquiring large datasets of quality spectra within reasonable timescales, allowing for spatial mapping of surfaces ranging from a few µm 2 to several cm 2 while maintaining submicronic spatial resolutions. A visualization of the probed surface can then be generated using relevant Raman spectral parameters, typically position, width or intensity of a characteristic Raman mode. The resulting image conveys information on the uniformity of the Raman response across the surface, indicating local composition, structure or phase fluctuations, for example. Coupling imaging with the high sensitivity that modern systems can achieve allows for the identification of fluctuations otherwise not visible by simple spectra comparison, offering new insights on cultural heritage materials, 9 ceramics, 10,11 glasses 12 or powdered carbon materials. 13 Most industrial glasses are synthesized through macroscopic processes such as the industrial "float" and "fusion-draw" processes, injection molding, or at smaller scales by casting. Due to their nature, these manufacturing steps occasionally induce viscosity, density or composition gradients, and from these effects may result macroscopic variations of the glass properties that can be detrimental to the final products. Maintaining a good control over glass surface quality is of prime importance for the functionalization of glass surfaces, for example, as minor composition and density changes may result in critical alteration of the thin film stack properties. Unfortunately, although the human eye and widely used optic quality control systems are very sensitive to macroscopic variations of the glass properties, the detection of modifications induced at a smaller scale would require the use of techniques capable of achieving higher sensitivities.

In this regard, we present a Raman investigation of the surface of bulk casted glass samples of compositions close to commercially available window glass. Using spatially resolved Raman spectroscopy and imaging, we probed microscopic fluctuations over large surfaces. The homogeneity of the Raman response at the surface of the samples is assessed and implications are discussed.

Methods

Sample preparation

A series of glass samples was synthesized in the composition range 14.3-19.3 wt% (14.1-19.1 mol%) Na 2 O, 11.2-16.2 wt% (12.2-17.6 mol%) CaO, 2.5-7.5 wt% (1.5-4.5 mol%) Al 2 O 3 , 62-67 wt% (62.9-68.2 mol%) SiO 2 . 1.2 kg batches were prepared from industrial grade sand, limestone, sodium carbonate and calcined alumina, with the addition of 300 ppm of coke as fining agent. The mixtures were then melted at 1550°C for 4 hours in a Joule-heated furnace and mechanically stirred every hour. The melts were casted onto a ∼10×10×3 cm 3 steel shape, air-quenched and annealed for 3 hours at their respective annealing points before slowly cooling down to room temperature. This procedure consistently yielded clear, bubble-free, and visually homogeneous glass plates (Fig. 1). The chemical homogeneity of samples synthesized by this procedure was assessed by electron microprobe analysis (EMPA) in a prior study on samples of similar volume and compositions, which showed to be homogeneous with standard deviations below 0.02 wt%. 14 For the sake of simplicity, this paper will highlight results from a sample that is at the center of the composition range. This glass will be denoted C 0 and is composed as follows : 16.8 wt% (16.5 mol%) Na 2 O, 13.7 wt% (14.9 mol%) CaO, 5.0 wt% (3.0 mol%) Al 2 O 3 , 64.5 wt% (65.5 mol%) SiO 2 . Compositions of the whole sample set are provided in Table S1 (Supporting Information). Results from all characterized glasses are also provided in the Supporting Information.

Raman mapping

Raman images were collected point-by-point on a Renishaw Invia™ Qontor ® confocal microscope in backscattering geometry. The glass pieces were mounted as-whole on custom sample holders and excited under a 514 nm laser operating at a power of 50 mW and focused through a 50× long focal objective. A surface of 7×7 cm 2 was probed by 500 µm steps in both dimensions, resulting in 141×141=19881 acquisitions per sample. The counting time was 10 s per spectrum, which allowed to record data of good quality while maintaining a reasonable experiment time of approximately 2.5 days. All measurements were recorded at room temperature. The frequency and intensity stability of the device is improved by specific temperature control of the whole spectrometer room, leading to a stability frequency better than 0.1 cm -1 over 24 hours. 15 During the acquisitions, the focus was set 10 µm below the surface to minimize surface effects. In addition, we used Renishaw's LiveTrack™ focus tracking feature to accommodate the curviness of the casted samples. This feature ensures minimal variations in focus quality over the duration of the experiment. This is done by automatically adjusting the stage height during each (x,y) translation, so that the laser spot size matches a reference focus that was set 10 µm below the surface. As a result, we were able to record data while accurately following the curvature of the samples (Fig. 2).

Data processing

All datasets were individually processed in the same manner using homemade Python routines. First, a PCA noise reduction was applied to the dataset in order to minimize the impact of the short accumulation time. The data was reconstructed using 10 components explaining >99.9% of the variance ratio. This step showed to greatly improve the quality and readability of the spectra. Background variations were corrected by subtracting the minimum value of each spectrum. Finally, all spectra were normalized to their total integrated area.

Results

The Raman response of the glass sample is first assessed by averaging the processed dataset into an average spectrum (Fig. 3). We consider this to be a good approximation of the overall response as only small variations are expected within the large dataset. This assumption is supported by the first component of a PCA decomposition on the processed data accounting for more than 99% of the total variance ratio, thus implying only minor modifications of the average structure across the probed surface. The spectrum exhibits two broad features around 590 cm -1 and 1090 cm -1 , the latter including a weaker, well-defined peak at 950 cm -1 . These observations are consistent with literature data on sodium-and calcium-containing silicates and aluminosilicates. 8,[16][17][18][19][20][21][22] At low wavenumbers (200-700 cm -1 ), the Raman signal mostly originates from Si-O-Si bending vibrations, providing information on bond angles and network connectivity. The bands at ∼590 and ∼490 cm -1 are typically associated to the breathing modes of 3-and 4-member tetrahedra rings observed in vitreous silica, also referred to as the D 2 and D 1 bands. [21][22][23][24][25] In the 850-1250 cm -1 range, the high-wavenumber envelope arises from the convoluted signals of T-O (T = Si, Al) stretching vibrations involving Q n tetrahedral units (Q n is a TO 4 tetrahedron sharing n bridging oxygens with other tetrahedra) and contains information on the local structure of the aluminosilicate network. Specifically, the contributions at 950 and 1090 cm -1 observed here are reported to mainly originate from Q 2 and Q 3 structural units, respectively. 16,18,26 Average spectra of the other glass samples exhibit the same features, with composition-induced variations as the proportion of network formers (Si, Al) and modifiers (Ca, Na) changes. This comparison is provided in Fig. S1 (Supporting Information).

In Fig. 4, we illustrate the dataset quality by comparing Raman spectra recorded across the analyzed surface at fixed X position. Apart from rare 200 400 600 800 1000 1200 1400

Wavenumber/cm 1 Raman Intensity instances of background variations due to fluorescence or surface pollution, only minor fluctuations are observed and one may argue that they all fall within experimental noise. Yet in some very localized events involving no more than a few neighbour spectra, we observe a small broadening of the 590 cm -1 band toward high wavenumbers, as well as fluctuations of the intensity of the Q 2 band at 950 cm -1 . Modifications of the broad 1090 cm -1 band are not easily identifiable from this simple spectra comparison, limiting further analysis.

Raman images were reconstructed by simply using Raman intensities at different wavenumbers. Although this parameter should be interpreted with care when applied to the broad Raman response of amorphous materials, it presents the advantage of quickly providing information on the most affected vibrations without the need for additional processing. By scanning through the Raman intensity images across the spectral range, we uncovered a beautiful macroscopic wavy pattern, represented in Fig. 5(a) using the Raman intensity at 950 cm -1 (I 950 ). This pattern is distinguishable on most of the spectral range, although presenting significant variations in image noise and contrast depending on the chosen wavenumber. Surprisingly, the most resolved images are obtained in the vicinity of the 950 cm -1 and 590 cm -1 features. Around the latter, we observe that Raman intensity images below and above 590 cm -1 are complementary to each other, and thus that the Raman spectral weight shifts from below to above 590 cm -1 (the reader may refer to Fig. S2 is provided in Fig. 5(b). Despite a spatial step of 500 µm, the macroscopic pattern in Fig. 5(a) is remarkably detailed. It is characterized by singular wavy features that we shall describe as overlapping layers spreading outwards from the center of the image. These features become narrower near the center of the glass sample, where the melt was poured, and strikingly remind of what is occurring during the casting process, that is, the spreading of the viscous melt from the casting area. To illustrate this comparison, we recorded the casting of a sample silicate melt using an Infrared (IR) camera. Fig. 6 shows a selection of images from the recording that were taken at key moments of the casting process. Looking at Fig. 6(a) and 6(b), it is clear that the temperature gradient at the surface of the casted sample exhibits a pattern that is very similar to that of the Raman image. In fact, all samples exhibit similar patterns at their surface, resulting in a range of figures evocative of the casting process (Fig. S3 (Supporting Information)).

The Raman image exhibits well-resolved boundaries between contrasted areas, indicating sharp changes in Raman response. The fluctuations appearing as noise in Fig. 4 are then obviously spatially correlated in Fig. 5(a). It is also worth noting that Fig. 5(a) does not appear to be correlated at all with the sample topography (Fig. 2). The I 950 parameter varies by up to ∼20% across the surface of the sample; it is most intense on the left side of the image, and the lowest intensities are obtained in the folded layers described earlier. Considering the timeline of the casting process, we believe that areas exhibiting the highest I 950 values likely consist of glass from the early stage of the casting (Fig. 6(a)). Subsequently, the large wavy patterns represented by lower I 950 values appear to be more representative of the material that was poured toward the end of the process (Fig. 6(b)). As I 950 increases, the broad 1090 cm -1 band slightly shifts to lower wavenumbers (Fig. 5(b)), implying a decrease of the population of the more polymerized Q n units. In addition, the 590 cm -1 band shifts toward high wavenumbers. This shift was anticipated from the Raman images around this feature, and indicates changes in inter-tetrahedral Si-O-Si bond angles distribution and ring statistics. We probed the position of the low-wavenumber band by following the center of mass of the 200-730 cm -1 envelope, introducing Raman parameter σ described by Deschamps et al . 27 This parameter accounts for variations of network connectivity occuring upon changes in the mediumrange order. It was used as an indicator of the densification ratio of vitreous silica at room temperature, 27 and more recently for local mapping of fictive temperature variations in fused silica. 28 In Fig. 7, the representation of this parameter indicates that σ increases by ∼10 cm -1 across the surface of the sample. The map exhibits the same well-resolved pattern as the intensity at Colorbars may be interpreted qualitatively, the process temperature being too close to the camera upper limit of 1500°C. a) Early stage of the casting process; the melt is poured over the metallic suface. b) End of the casting process; the crucible is pulled away from the casting area, leaving a thin stream of melt in its path. c) Casted glass sample after ∼15 s of air-quenching.

950 cm -1 , which emphasizes the strong correlation between the increase of both parameters and the casting process.

Discussion

Spatial reconstructions of the sample surface from Raman parameters I 950 and σ evidence a correlation that materializes in the form of a macroscopic pattern evocative of the glass casting process. Specifically, the early stage of the casting process is distinguished by higher Raman intensities at 950 cm -1 and the shift of the broad 200-730 cm -1 feature toward higher wavenumbers. It is known that changes in Raman response of the Si-O bending envelope and the vibrational modes of Q n structural units may arise from local composition, structure or stress state fluctuations. Indeed, the bending and stretching massifs are strongly correlated, and it has been documented that the joint modifications of these envelopes may be used to identify variations in chemistry and processing temperature, for example. 29,30 It is obvious from the IR recordings that the Raman observables find some meaning in the sample thermal history. As soon as the melt begins to be casted, differences in quench rates will generate temperature gradients within the glass sample, as illustrated in Fig. 6(a). Naturally, higher cooling rates are obtained when the 1550°C melt is rapidly cooled on the metallic surface. Follows a decrease of the cooling rate over time, resulting in a temperature gradient at the surface of the sample (Fig. 6 eventually homogenize upon further cooling (Fig. 6(c)) and subsequent annealing. From a structural standpoint, the increase of the intensity of the Q 2 Si-O stretching mode in areas associated with the early stage of the casting process is consistent with a greater disorder of the Q n entities and depolymerization of the silicate network in the high temperature melt. This result is in agreement with 29 Si NMR and Raman investigations of the influence of fictive temperature in different silicate systems. 19,[31][32][33] Additionally, σ values were also reported to increase with fictive temperature. 28 From these observations, we infer that our thermal treatment properly relieved thermal stresses but was not sufficiently long to achieve full reorganization of the local structure. These local fluctuations are reflected in the Raman spectra, and Raman images thus reveal the fingerprint of T f variations at the surface of the glass piece. We did not attempt to correlate our Raman parameters with precise values of fictive temperature as this procedure would require a calibration curve and was thus beyond the scope of this work. It should be noted however that the range of σ values (∼10 cm -1 ) obtained from our analysis is quite large compared to what was obtained in T f mapping across ultra-short pulse (USP) laser modified samples. 28 Moreover, it is unlikely that our process achieved comparable cooling rates and thus large T f fluctuations. These considerations tend to indicate that cooling rate and T f only partly account for the variations of parameters I 950 and σ.

Assuming that internal stresses were properly relieved during the annealing procedure, modifications of the Raman spectra may also originate from local composition gradients at the surface of the glass piece. The network connectivity (and thus, Si-O bond angles) and Q n speciation are intimately linked to composition. Higher proportions of network modifiers will decrease network polymerization, thus shifting the stretching envelope downward. Similarly, increasing proportions of modifiers content have shown to affect the 600 cm -1 feature in Na-and Ca-bearing aluminosilicates. 20,34 Because Ca has a tendency to adopt a Q 2 conformation rather than Q 3 , we infer that the fluctuations in I 950 may also be attributed to local variations of Ca (and Si) content at the surface of the glass piece. 35 Considering that the extent of modification of I 950 at the surface of a sample is smaller than the differences within the sample set, we estimate that these variations are <1 wt%. Lack of homogeneity of the melt may originate from insufficient mixing during the process and should be addressed. Unfortunately, chemical mapping was not performed on the samples as very few Energy Dispersive X-ray Spectroscopy (EDX) and X-ray Fluorescence (XRF) apparatus may accommodate such large and heavy (up to 1 kg) samples. A detailed investigation of the composition fluctuations at the surface of similar samples may be the subject of future work regarding this matter. Finally, we ruled out the influence from water content as vibrations from the Si-OH linkages were not detected in the 2840-3750 cm -1 range.

Conclusions

Raman imaging was used to probe local fluctuations of Raman response over centimeter-scale areas at the surface of casted glass samples. We observed that modifications of the Raman spectra across the surface of the sample are rather small and localized across the covered spectral range. Thanks to spatial correlation, structural information was extracted from what may be confused with noise by simple spectra comparison. Specifically, the reconstruction of high-quality Raman images evidenced macroscopic patterns of small local variations in Q n speciation and Si-O-Si bending vibrations across the analyzed surface. The contrasts on the Raman observables show striking resemblance with IR images of the making of the sample, and can therefore identify areas consisting of material from the early and late stage of the manufacturing process. The fluctuations likely originate from two combined effects: 1. Fictive temperature variations, induced by an evolution of the cooling rate over the course of the casting process, 2. Composition gradients within the crucible, probably due to insufficient mixing of the melt.

Structural and chemical changes originating from the manufacturing process are therefore printed at the surface of the glass pieces, and their fingerprint revealed by Raman imaging. We believe that our results should not compromise the overall homogeneity of the samples at their core, but rather highlight the sensitivity of Raman spectroscopy to accurately probe such microscopic inhomogeneities.

This work shows the interest of gathering and processing large datasets to properly assess chemical and structural homogeneity. Raman imaging is therefore particularly suited to the analysis of materials exhibiting a wide distribution of local environments, such as glasses. Large Raman images such as those provided in this paper may find applications in cultural heritage and geosciences, providing new insights on ancient manufacturing processes and geological phenomena.

Figure 1 :

 1 Figure 1: Example of synthesized glass plate (dimensions : 10×10×3 cm 3 ).

Figure 2 :

 2 Figure 2: Variation of Raman spectrometer stage height with position on glass sample C 0 . The spot size was recorded at the origin by focusing the laser 10 µm below the surface.

Figure 3 :

 3 Figure 3: Average Raman response of glass sample C 0 .

Figure 4 :

 4 Figure 4: Comparison of all 141 processed Raman acquisitions across the sample at position X=35000 µm. The spectra are shifted vertically for clarity (note that successive spectra were recorded 500 µm apart from each other). Rare inconsistencies in vertical shift arise from residual background variations. Dashed lines indicate the 950 and 1090 cm -1 bands as guides to the eye.

Figure 5 :

 5 Figure 5: (a) Raman image of the sample surface, reconstructed using the intensity of the band at 950 cm -1 (I 950 ). (b) Normalized Raman spectra from areas of high and weak I 950 . The spectra positions are indicated on the image by markers of same

Figure 6 :

 6 Figure6: Infrared (IR) images of the casting of a silicate melt (Optris PI G7 640 Infrared Camera / λ =7.9 µm), with t 0 the beginning of the casting process. Colorbars may be interpreted qualitatively, the process temperature being too close to the camera upper limit of 1500°C. a) Early stage of the casting process; the melt is poured over the metallic suface. b) End of the casting process; the crucible is pulled away from the casting area, leaving a thin stream of melt in its path. c) Casted glass sample after ∼15 s of air-quenching.

Figure 7 :

 7 Figure 7: Evolution of Raman parameter σ, center of mass of the 200-730 cm -1 envelope, across the surface of the sample.

Acknowledgments

We thank the Glass formulation department of Saint-Gobain Recherche Paris (SGR Paris, Aubervilliers, France) for assistance in the synthesis of the glass samples and N. Raimboux (CEMHTI, CNRS) for her help in the manufacturing of custom sample holders for Raman measurements. This research was conducted under funding from the Agence Nationale de la Recherche (ANR) MAGI project ANR-17-CE08-0019, and from the EquipeX PlaneX ANR-11-EQPX-36.