Damir Blazina

John P Dunne

Stuart Aiken

Simon B Duckett

Charlotte Elkington

John E Mcgrady

Rinaldo Poli

Sue J Walton

M Sabieh

Sabieh Anwar

Jonathan A Jones

M Sabieh Anwar

Hilary A Carteret

Contrasting photochemical and thermal reactivity of Ru(CO)2(PPh3)(dppe) towards hydrogen rationalised by parahydrogen NMR and DFT studies

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Introduction

Transition metal complexes containing phosphine ligands have been widely employed as homogeneous catalysts 1 for transformations such as hydrogenation, hydroformylation, isomerisation and polymerisation, involving both alkene 2 and alkyne 3 substrates. Multidentate phosphines have attracted particular attention as auxiliary ligands because of their ability to control the molecular geometry of the resultant metal systems and hence enable both regio and stereo selectivity during catalytic transformations. In addition, the presence of multiple binding sites suppresses ligand dissociation and hence leads to longer catalyst lifetimes. 4 A suitable example of this effect is provided by the photochemical reactions of complexes of the type Ru(L) 4 (H) 2 , where L is phosphine based. When (L) 4 = P(CH 2 CH 2 PPh 2 ) 3 , the parent complex undergoes photochemical loss of dihydrogen and the resulting intermediate reacts rapidly with benzene to form the aryl hydride, 5 while on switching to (L) 4 = (dppe) 2 (dppe = Ph 2 CH 2 CH 2 Ph) there is no evidence for C-H bond activation 6 despite the retention of the phenyl groups and the ethane backbone.

In contrast, complexes containing primary phosphines, e.g. L = PMe 3 , exhibit two distinct photochemical pathways involving loss of H 2 and phosphine, respectively. 7 Since the resulting Ru(L) 4 or Ru(L) 3 (H) 2 intermediates can bind small molecules such as H 2 , C 2 H 4 , CO and HSiEt 3 , opportunities exist for such systems to achieve useful catalytic transformations. 8 For instance, Berry et al. have recently described how the related complex Ru(PMe 3 ) 4 (H)(SiMe 3 ) yields Ru(PMe 3 ) 3 (H)(SiMe 3 ), a 16 electron species that is capable of H-H, Si-H, C-H and Si-C bond activations. 9 There have been several reports on the use of UV photolysis of a sample within the NMR probe to study in-situ reactions. 10,11 We have recently reported an investigation using this approach on the photochemical addition of hydrogen to complexes of the type Ru(CO) 3 (L) 2 , where L = PPh 3 , PMe 3 , PCy 3 , P(p-tolyl) 3 and AsPh 3 , together with studies on alkyne hydrogenation. 12 In this system, the reaction with hydrogen proceeds via the two competing processes shown in Scheme 1. The first of these involves the photochemical loss of CO, and preferential H 2 addition across the more π-accepting OC-Ru-CO axis of the resulting intermediate to form the cis-cis-trans-L isomer of Ru(CO) 2 (L) 2 (H) 2 . In the second pathway, the single photon induces the loss of both CO and L, leading to the formation of an undetected 14 electron Ru(CO) 2 (L) fragment which reacts further to form detectable amounts of cis-cis-cis Ru(CO) 2 (L) 2 (H) 2 and Ru(CO) 2 (L)(solvent)(H) 2 , where solvent = toluene, THF and pyridine. In the case of L = PPh 3 , cis-cistrans-L Ru(CO) 2 (L) 2 (H) 2 proved to be an effective hydrogenation catalyst at elevated temperatures, with catalysis proceeding via initial loss of phosphine. 8,12 We describe here studies on H 2 addition to the related Ru(0) system Ru(CO) 2 (PPh 3 )(dppe) which contains the chelating 1,2-bis(diphenylphosphino)ethane ligand.

To study these transformations, we have employed NMR spectroscopy in conjunction with parahydrogen (p-H 2 ). The use of p-H 2 leads to the observation of PHIP (parahydrogen induced polarisation), 13 a well-established tool in mechanistic chemistry, 14 which allows low-concentration complexes, such as reaction intermediates, to be detected via enhanced 1 H NMR signals for nuclei that originate in the p-H 2 molecule. PHIP has yielded catalytic information for a number of mono-, 15 di- 16 and tri-nuclear 17 transition metal species. Recent achievements in this field include the sensitisation of a hydroformylation product containing a single p-H 2 atom, 18 transfer of polarisation via a 13 C nucleus to deuterium after the hydrogenation of a perdeuterated substrate, 19 and the investigation of ortho-para isomer interconversion and hydrogen isotope scrambling. 20 When insitu laser flash photolysis of Ru(CO) 3 (dppe) was used in conjunction with p-H 2 , the hydride resonances seen for the product Ru(CO) 2 (dppe) 2 (H) 2 are enhanced by a factor of over 28,400 over their normal levels. 21 Such an approach has proved valuable in enabling p-H 2 to initialise a quantum computer 22 that is suitable for the implementation of quantum algorithms. 23 These reactions involve 18 electron, 5 coordinate precursors, which should be fluxional 24 and contain π acceptor ligands that are preferentially located in the equatorial plane. 25 The corresponding 16 electron intermediates that are generated from them by ligand loss are exemplified by Ru(CO) 2 (PMe t Bu 2 ) 2 , and Ru(CO) 2 ( t Bu 2 CH 2 CH 2 t Bu) 26 which have trigonal bipyramidal structures with a vacant equatorial site. 27 However, these electron-deficient species are also fluxional, as a result of which, a number of H 2 addition products might be formed. In a previous paper, density functional theory was used to probe the potential energy surface for the corresponding reactions of H 2 with Ru(CO) 3 (H 2 PCH 2 CH 2 PH 2 ) and Fe(CO) 3 (H 2 PCH 2 CH 2 PH 2 ). 28 We now extend this analysis to the mixed carbonyl phosphine system, Ru(CO) 2 (PH 3 )(H 2 PCH 2 CH 2 PH 2 ), with the aim of understanding how the substitution of a carbonyl ligand by PH 3 perturbs the potential energy surface. In the case of the ruthenium complex, Ru(CO) 3 (H 2 PCH 2 CH 2 PH 2 ), the triplet surface was shown to lie above the singlet at all points, so we restrict our attention to closed-shell states in the current system.

Experimental Section

General. All synthetic manipulations were carried out under an atmosphere of dry nitrogen using standard Schlenk, glove box or high vacuum techniques. Solvents were dried and distilled under nitrogen prior to use. Ru(CO) 3 (PPh 3 ) 2 was prepared and purified using a literature procedure. 29 Other chemicals were purchased from commercial suppliers (Aldrich, Strem) and used without further purification.

Preparation of Ru(CO) 2 (PPh 3 )(dppe).

The potential to prepare Ru(CO) 2 (PPh 3 )(dppe) from Ru(CO) 3 (PPh 3 ) 2 has been described, 30 but the detailed synthetic procedure and full characterisation of the product were not communicated. In this study, Ru(CO) 3 (PPh 3 ) 2 (100 mg, 0.14 mmol) and dppe (56 mg, 0.14 mmol) were refluxed in toluene (10 mL) under a continuous flow of N 2 for 45 min. The sample was then left to stand at room temperature for 12 hours, yielding a yellow microcrystalline precipitate of Ru(CO) 2 (PPh 3 )(dppe), which was then recrystallised from THF/pentane (1 : 2). Yield: 95 mg (82%). The characterisation data for this complex are given in Table 1. Analysis, C% 68.10 (found, 67.56 theoretical), H% 4.93 (found, 4.81 theoretical). NMR methods. NMR solvents (Apollo Scientific) were dried using appropriate methods and degassed prior to use. The NMR measurements were made using NMR tubes fitted with J. Young Teflon valves and solvents were added by vacuum transfer on a high vacuum line. For the PHIP experiments, hydrogen enriched in the para spin state was prepared by cooling H 2 to 18 K over a paramagnetic catalyst (activated charcoal) using the system described previously. 21b All NMR studies were carried out with sample concentrations of approximately 1 mM and spectra were recorded on a Bruker DMX-400 spectrometer with 1 H at 400.1, 31 P at 161.9 and 13 C at 100.0 MHz, respectively. 1 H NMR chemical shifts are reported in ppm relative to residual 1 H signals in the deuterated solvents (toluene-d 7 , δ 2.13, and C 6 D 6 , δ 7.16), 13 C NMR relative to toluene-d 8 , δ 21.3, CD 2 Cl 2 , δ 54.0 and C 6 D 6 , δ 128.4 and 31 P NMR in ppm downfield of an external 85% solution of phosphoric acid. Modified COSY, HMQC and EXSY pulse sequences were used as previously described. 16,31 1 H EXSY spectra used to obtain kinetic data were processed using literature methods 32 and analysed for simple two-site exchange processes. 33 In-situ photolysis. This was achieved using a modified NMR probe that was equipped for in-situ photolysis, as described previously. 11 A Kimmon IK3202R-D 325 nm He-Cd 27 mW continuous wave (CW) laser was used as the light source. Computational details. All calculations were performed using the Gaussian 03 34 program together with the modified form of the B3PW91 functional in conjunction with a flexible polarisable basis sets. [START_REF] Harvey | [END_REF] Specifically, the c 3 coefficient in Becke's original three-parameter fit to thermochemical data 36 was changed to 0.15, to give the B3PW91* functional. Atoms C, O and P were described by the triple-zeta basis sets of Schäfer et al. 37 augmented by one d polarisation function (α = 0.8, 1.2 and 0.55 respectively). The Ru atom was described with the SDD basis set, [START_REF] Harvey | [END_REF] which uses the Stuttgart/Dresden ECP and double-zeta functions for all valence electrons, augmented with an f polarisation function (α = 1). All minima were fully optimised and characterised by computing vibrational frequencies at the same level of theory. The calculations reported here used the same level of theory previously employed for Fe(CO) 2 (PH 3 ) 2 , 38 where a benchmark investigation demonstrated that it gave the most reliable results for Fe(CO) 4 . 39 In all cases, the phenyl groups on PPh 3 and dppe are replaced by hydrogens.

Results and discussion

The Ru(CO) 2 (PPh 3 )(dppe) 1 used here was prepared and purified as described in the Experimental section, and IR (ν CO ), NMR and FAB mass spectral data are collected in Table 1. Assuming that the dppe ligand spans one axial and one equatorial site, there are two possible isomers of 1, one with an axial phosphine (1a) and one with an axial carbonyl (1b) (Figure 1). At 295 K, two distinct signals were observed in the 31 P NMR spectrum at δ 57 (PPh 3 ) and 70.3 (dppe), the equivalence of the two dppe phosphorus centres clearly indicating that the molecule is fluxional. Moreover, the presence of three ν(CO) bands in the spectrum of 1 confirms that more than one species is present in solution. As the temperature was lowered, the δ 70.3 signal broadened and collapsed into the baseline before appearing again as two broad signals at δ 79.6, 60.6. Even at 183 K, these signals remained broad which precluded the determination of exact J PP couplings. However, a large (200 Hz) trans coupling connected the signal at δ 79.6 and the resonance at 57.8 (PPh 3 ), confirming that 1a, where there is a trans arrangement between the PPh 3 ligand and one of the 31 P centres on the dppe ligand, is the dominant form at low temperature. In addition, two further, less intense and very broad resonances were observed at ca. δ 32.1 and 27.5. These could not be resolved fully due to the limitations imposed by the freezing point of the NMR solvent, toluene-d 8 , but clearly indicate the presence of a second isomer, presumably 1b. A survey of the potential energy surface for the model complex Ru(CO) 2 (PH 3 )(H 2 PCH 2 CH 2 PH 2 ) (1') confirms the presence of two distinct local minima, 1a' and 1b' (Figure 1), lying within 1.5 kJ mol -1 of each other. Previous computational studies on the closely related species Ru(CO) 2 (PH 3 ) 3 haved located almost isoenergetic structures analogous to 1a' and 1b', along with a third isomer where both CO ligands are in the axial positions, some 12.7 kJ mol -1 higher in energy. 40,41,41 We have been unable to locate a third minimum in the current system, presumably because the dppe ligand is unable to bridge the 120º angle between the two equatorial sites. The (unscaled) C-O vibrational frequencies of 1a' and 1b' (1977, 2015 cm -1 and 2001, 2055 cm - 1 , respectively) also offer an explanation for the presence of three, rather than four, ν(C-O) bands in IR spectrum: the 2015 cm -1 band (symmetric stretch) of 1a' is very weak and is likely to be obscured by the more intense 2001 cm -1 of 1b' (antisymmetric stretch).

Table 1. Spectroscopic data for Ru(CO) 2 (PPh 3 )(dppe) 1. IR spectra were recorded in hexane, 13 C NMR spectra in CD 2 Cl 2 and all other NMR spectra in C 6 D 6 at 295 K.

Thermal reactions of Ru(CO) 2 (PPh 3 )(dppe) 1 with p-H 2. :

When compound 1 was warmed to 315 K with p-H 2 in C 6 D 6 , two enhanced NMR signals were observed in the hydride region of the 1 H NMR spectrum at δ -7.55 and -6.32. These signals have the same chemical shifts and profiles as the known compound Ru(CO) 2 (dppe)(H) 2 , 2. 42 When this experiment was repeated in the presence of 2 atm. of CO, the relative intensities of these hydride resonances were unaffected, but the signals were quenched by the addition of a 10-fold excess of PPh 3 . It can therefore be concluded that 1 reacts thermally with H 2 via initial PPh 3 loss. The thermal loss of a phosphine from Ru(CO) 2 (PEt 3 ) 3 and related systems has been successfully used in the past to prepare the analogous dihydrides, Ru(CO) 2 (PEt 3 ) 2 (H) 2 . 41

Photochemical reactions of Ru(CO) 2 (PPh 3 )(dppe) 1 with H 2 :

An NMR sample of 1 in C 6 D 6 under 3 atm. p-H 2 was prepared and exposed to broad band UV irradiation from a medium pressure Hg-Arc lamp for 5 minutes outside the NMR spectrometer. When this sample was examined by 1 H NMR spectroscopy the characteristic resonances of 2 were again observed, along with those of a second dihydride, 3a, the ratio between the two species being 0.78 : 1. The hydride resonances of 3a at δ -7.00 and -7.56 correspond to those of the [OC-6-42] [START_REF]Following IUPAC nomenclature[END_REF] isomer of Ru(CO)(dppe)(PPh 3 )(H) 2 (Figure 2, Table 2) , the synthesis and characterisation of which has been reported previously by Garrou et al. and Onishi et al. [START_REF] Jung | [END_REF] A more detailed characterisation of the resonances of 3a using PHIP is reported in the next section, but the most significant point to note at this stage is that the presence of 3a in solution suggests that photochemically induced CO loss from 1 leads to the formation of a stable dihydride complex. After 60 minutes further UV irradiation, the complete loss of the 31 P signal due to 1 indicated its complete conversion into 2 and 3a. When this sample was warmed in the NMR spectrometer from 295 K to 373 K, the hydride signals for 2 in the corresponding 1 H NMR spectra were enhanced, while those for 3a were unaffected. The absence of p-H 2 enhancement suggests that the hydride ligands of 2, but not 3a, exchange with H 2 under thermal conditions.

In-situ-photochemical reactions of Ru(CO) 2 (PPh 3 )(dppe) 1 with p-H 2 : detection and characterisation of isomeric products. In order to explore the photochemical reaction pathway in more detail, we used an in-situ approach in conjunction with p-H 2 . When a sample of 1 was exposed to p-H 2 with concurrent UV laser photolysis at temperatures of 263 K and above, hydride signals for 2 and 3a were again visible, along with two new products, 3b and 4 (Figure 2, Table 2). However, once the sample was allowed to come to thermal equilibrium only the signals due to 2 and 3a remained, indicating that 3b and 4 are formed under kinetic control. The four additional hydride resonances from 3b and 4 were characterised by multidimensional NMR spectroscopy (a typical p-H 2 enhanced spectrum is shown in Figure 3). The basic features of the 31 P NMR spectrum of 3a have been described above, but the p-H 2 enhancement reveals much greater detail, and a number of unusual features. While the hydride resonances of 3a show anitiphase character due to PHIP, the splittings (Figure 3) are consistent with couplings to three phosphorus centres. However, when a 1 H-31 P HMQC experiment was recorded to locate the corresponding 31 P chemical shifts, a total of six cross-peaks were detected at δ 46.4, 61.4, 66.9, 76.1, 81.7 and 87.2 (see Figure 4). We note that Heaton et al. have observed additional resonances in HMQC spectra of rhodium carbonyl clusters, and attributed them to double and triple quantum coherence effects. 45 Single quantum transitions (revealing real 31 P chemical shifts) can be selectively enhanced using a magnetisation transfer delay to 1/(5×J HX ), rather than the conventional 1/(2×J HX ). Three cross-peaks remain visible in this experiment, to signals at δ 61.4, 66.9 and 81.7 (Figure 4), which can be shown to be real by direct decoupling. In contrast, setting the delay to 1/(J HX ) leads to the observation of only the triple quantum coherence "artefacts" at δ 46.4, 76.1 and 87.2 for 3a (Figure 4).

The hydride resonance of 3a that appears at δ -7.00 exhibits a large trans 2 J HP coupling of 78 Hz (to a 31 P nucleus resonating at δ 66.9) and two cis 2 J HP couplings of 19 and 22 Hz (to 31 P nuclei resonating at δ 87.1 and 61.4, respectively). The second hydride resonance of 3a at δ -7.56, exhibits three cis 2 J HP couplings of 27, 20 and 25 Hz to the same three 31 P nuclei, respectively (see Table 2 for a summary of NMR data). In addition, the 31 P signals at δ 61.4 and 81.7 exhibit a large 240 Hz coupling in a high-resolution 1 H-31 P HMQC experiment, indicating that the associated nuclei are distinct and mutually trans. One of these nuclei is therefore due to the PPh 3 ligand, while the other originates from one of the arms of the dppe moiety. The hydride site which yields the δ -7.56 signal is therefore trans to a CO ligand. The phosphorus nucleus resonating at δ 61.4 also interacts with the third 31 P centre resonating at δ 66.9 in the form of an 18 Hz coupling, a value that indicates these two nuclei are mutually cis. The signals at δ 66.9 and 81.7 both showed a very small additional coupling of 3 Hz. These resonances come from the dppe ligand, and the small coupling is the result of cancellation of the 2 J PP coupling through the metal centre and the 3 J PP coupling through the ethane backbone of the dppe ligand; these two couplings are of opposite sign. Such situations have been reported previously. 42,46 The new species 3b exhibits enhanced hydride signals at δ -6.54 and -9.22 and has not been observed previously. The hydride signal at δ -9.22 appears as a doublet of doublets of doublets of antiphase doublets with a large trans 2 J HP of 65 Hz arising from a 31 P nucleus that resonates at δ 47.3. The two additional cis couplings of 35 and 17 Hz arise from the 31 P centres that were detected at δ 83.2 and 64.5, respectively, in the HMQC experiment. The multiplicity of the lower field hydride could not be determined directly because of signal overlap with 2, although a 2 J HH of -5 Hz could be resolved, and a trans 31 P nucleus found at δ 64.5. These NMR data are, however, sufficient to identify this compound as 3b, an isomer of 3 where one hydride is trans to the PPh 3 ligand and the other is trans to one of the 31 P centres of the dppe moiety. From chemical shift trends in 3a and 3b (vide infra), the 31 P signal at δ 47.3 is assigned to the PPh 3 ligand while the other two correspond to dppe signals. We note that a species analogous to 3b has been detected upon irradiation of the related complex Ru(CO) The ratio of hydride resonance intensities seen for 3a : 2 varies with irradiation time, progressively increasing from the initial value of 2.2 : 1 to 20 : 1 after 20 min irradiation (see Figure 3). In contrast, the observed ratio of the hydride signal intensities for 3a, 3b and 4 remains essentially constant over the course of the experiment at 1 : 0.04 : 0.02. There are two possible explanations for this observation; (1) 2 reacts photochemically with free PPh 3 to form 3a, 3b and 4 , leading to increased concentrations of the latter or (2) photolysis of 3a at 325 nm leads to exchange of hydrides with free H 2 , while 2 is inert. In this case, the enhanced intensity of the signal for 3a does not reflects a greater concentration of this species, but rather the fact that the polarisation is continually regenerated by the binding of fresh p-H 2 . When an authentic sample of 2 was prepared and photolysed at 325 nm in the presence of p-H 2 and PPh 3 , no evidence for the formation of 3 was observed; indeed, there was no p-H 2 activity. In contrast, when a sample of 3a was prepared and photolysed at 325 nm in the presence of p-H 2 , the corresponding hydride resonances of 3a and 3b appeared as PHIP enhanced signals in the ratio 1 : 0.04. Since the hydride resonances of 3a failed to show PHIP prior to photolysis, and 3b was not visible, it can be concluded that secondary photolysis of 3a affects the distribution of products. The final product observed in this reaction, 4, was characterised by two p-H 2 enhanced hydride signals at δ -7.17 and -8.42 that arise from two chemically distinct hydride ligands (Figure 3 clearly shows the high field signal, the second resonance is masked in this spectrum). This new species was detectable at the end of the experiment in very low concentrations (ca 5% of the level of 3a). The multiplicities of the hydride signals for 4 are identical to those of 3a, which suggests that it is also a trisphosphine complex, but, at very long collection times, a total of four distinct 31 P resonances were observed in the corresponding 31 P{ 1 H} NMR experiment,. The presence of one of these resonances in a region corresponding to a non-coordinated phosphorus centre (δ 30.7) confirms that one arm of the dppe ligand has decoordinated (Table 4), leaving the second phosphorus centre of the dppe in an axial site trans to a PPh 3 ligand. Given that the formation of 2 involves the liberation of PPh 3 it is not surprising that 4 can be formed. In summary, it is clear from these data that the novel in-situ photochemical approach employing parahydrogen allows us to observe a reaction product whose concentration is too low to allow its detection by conventional NMR spectroscopy. The exact mechanism of formation of 2, 3a, 3b and 4, however, remains to be established. The 1 H NMR signals for the hydride ligands of 3a proved to be of an intensity which indicated that 67.1% † of the p-H 2 spin state was conserved during the addition process. This compares with the value of 89.8 % for Ru(CO) 2 (dppe) 2 (H) 2 . 21

Evidence for a 14-electron intermediate: effect of added pyridine on the irradiation of 1 with p-H 2 :

In previous studies of the photochemistry of Ru(CO) 3 (L) 2 , it has been shown that a second single-photon process induces loss of both CO and L from Ru(CO) 3 (L) 2 . This process competes with CO dissociation from Ru(CO) 3 (L) 2 and leads to the formation of cis-cis-cis Ru(CO) 2 (L) 2 (H) 2 via reaction of the solvent complex Ru(CO) 2 (L)(solvent)(H) 2 (where solvent = toluene, THF and pyridine) with CO. In order to explore whether a similar double ligand loss process was also operative in this case, we investigated the in-situ irradiation of 1 in C 6 D 6 solutions containing both p-H 2 and pyridine. Under these conditions, at the onset of photolysis, the two isomers of 3 are observed in the same ratio seen for the corresponding reaction without pyridine. The proportion of 3a relative to 2 also remained the same at 2.2 : 1 (based on the p-H 2 enhanced hydride signals) regardless of the concentration of pyridine. In the pyridine doped spectra two hydride resonances of the previously reported double substitution product OC-6-13 (Figure 2) isomer of Ru(H) 2 (CO)(dppe)(pyridine), 5, were also detected at δ -4.17 and -4.57. 12 The ratio of the hydride resonance signal intensities for 3a : 5 were 1 : 0.11 for an 18 fold excess and 1 : 0.2 for a 61 fold excess of pyridine. The higher concentrations of pyridine therefore substantially increase the proportion of Ru(H) 2 (CO)(dppe)(pyridine) formed in this reaction and suggest that a process whereby both CO and PPh 3 are lost from 1 competes with the loss of PPh 3 and CO. When a sample of 3a was irradiated in C 6 D 6 containing a 60 fold excess of pyridine and p-H 2 , the hydride resonances for 5 again appeared. The ratio of 3a : 5 (1 : 0.01), was, however, dramatically lower than the ratio when 1 is the precursor, suggesting that photolysis of 3a is not the major source of 5. On this basis, we conclude that a minor photochemical pathway involving the dissociation of both CO and PPh 3 , followed by the net recoordination of either CO or PPh 3 and the activation of H 2 contributes to the observed product distribution. Given the constant ratio of 3a : 3b, even in the presence of pyridine, it seems likely that both the single and two ligand loss pathways ultimately proceed via a common intermediate.

DFT analysis of the potential energy surface of Ru(H 2 PCH 2 CH 2 PH 2 )(PH 3 )(CO)(H) 2

Density functional theory has been used to explore the potential energy surfaces of both 2 and 3, using model systems (denoted 2' and 3') where the phenyl groups have been replaced by hydrogens. For dicarbonyl dihydride species, 2', three distinct minima have been located, the most stable being those with cis dihydrides, 2a' and 2b'. Of these, the former is 9 kJ mol -1 lower in energy, consistent with the structure of 2 proposed above. A total of four distinct isomers have been located for 3'. The two most stable, 3a' and 3b' correspond to the experimentally observed species, and lie within 1 kJ mol -1 of each other, consistent with their coexistence in solution. Both are formally related to 2a' through substitution of one of the two chemically distinct CO groups by PH 3 . The remaining two isomers, 3c' and 3d', lie 9 kJ mol -1 and 32 kJ mol -1 , above 3a', and have not been observed in any of our experiments.

Mechanism of formation of 2, 3 and 4:

The mechanism of addition of H 2 to the closely related complexes Ru(CO) 3 (L) 2 (where L is a monodentate phosphine) has been studied previously, 12 and shown to involve CO loss followed by H 2 addition to the resultant 16 electron intermediate. The preferred direction of attack appears to be across the more strongly accepting OC-Ru-CO axis, yielding the cis-cis-trans-L isomer of Ru(CO) 2 (L) 2 (H) 2 . The electronic structure of Ru(dppe)(CO) 2 has been discussed in a previous paper, 28 and numerous closely related 16-electron species such as RuL 4 15,27,[START_REF] Jung | [END_REF][45][46]49,48 and Ru(CO) 2 (L) 2 27,46 have also been studied using theoretical techniques. Optimised structures and relative energies of the two model intermediates, I2' and I3', arising from loss of PH 3 and CO from 1', respectively, are summarised in Figure 5. In all cases, the energies are given relative to the most stable isomer of 1' (1b'). For the intermediate arising from PH 3 loss, two distinct isomers, I2a' and I2b', have been located, with vacancies in the equatorial and axial sites of the parent trigonal bipyramid, respectively. The former is more stable by 61.6 kJ mol -1 , indicating that it will be the only isomer present under the experimental conditions. In contrast, loss of a CO ligand from 1 leads to two distinct isomers, I3a' and I3b', both of which have a vacancy in the equatorial plane, the former with an axial phosphine, the latter with an axial CO ligand. I3a' lies 11.1 kJ mol -1 lower than I3b', consistent with the general preference for the stronger π-acceptor ligands to occupy equatorial sites, and suggesting that, under experimental conditions, I3a will be the dominant intermediate arising from CO loss. Sargent and Hall have investigated the oxidative addition of H 2 to square-planar IrCl(CO)(PH 3 ) 2 , 49 and showed that addition across the P-Ir-P, rather than Cl-Ir-CO, axis leads to a more stable transition state, although the inclusion of methyl groups on the phosphine ligand reversed this trend. We have been unable to locate any transition states for H 2 addition in this case, suggesting that the reaction is barrierless. We therefore propose that 3a and 3b are formed via approach of H 2 aligned along the axial and equatorial planes of I3a, respectively (Scheme 2). Significantly, the opening of the equatorial C-Ru-P angle to 155 o in I3a means that the axial and equatorial axes are not strongly differentiated. Finally, we note that the calculated total energies for the reactions 1' I2' + PH 3 and 1' I3' + CO (taking the lowest energy isomer in each case) are 120 kJ mol -1 and 183 kJ mol -1 , respectively, clearly indicating that loss of PH 3 to form the dicarbonyl species is the thermodynamically favoured outcome. The computed energies are therefore consistent with the exclusive formation of Ru(dppe)(CO) 2 (H) 2 (2), and not Ru(dppe)(CO)(PPh 3 )(H) 2 (3), under thermal conditions.

Hydride site interchange in Ru(dppe)(PPh 3 )(CO)(H 2 ) (3):

Many dihydride complexes have been found to undergo intramolecular exchange of hydride ligand sites. proposed rearrangement pathways include tunneling, 50,51,52 trigonal twist, 53 the formation of formyl intermediates, 54,55,56 and the reversible formation of a transition state featuring significant shortening of the H-H distance or a thermally accessible η 2 -H 2 intermediate with trigonal bipyramidal shape. 57 Our previous analysis of the potential energy surface for 2 is consistent with the last of these, revealing two distinct transition states leading to hydride exchange, both of which are best described as dihydrogen complexes of zerovalent Ru(CO) 2 (PH 2 CH 2 CH 2 PH 2 ). 28,42 The two pathways, corresponding to clockwise and anticlockwise rotations of the H 2 unit, have computed activation parameters of ΔH ‡ = 82 kJ mol -1 , ΔS ‡ = -0.2 J mol -1 K -1 and ΔH ‡ = 78.6 kJ mol -1 , ΔS ‡ = -0.6 J mol -1 K -1 , in good agreement with the experimentally determined ΔH ‡ value of 85.5 ± 2 kJ mol -1 . A similar analysis of the potential energy surface for 3a also reveals two distinct transition states, TSa and TSb, corresponding to clockwise and anticlockwise rotations of the H 2 unit about a Ru(CO)(PH 3 )(PH 2 CH 2 CH 2 PH 2 ) fragment (Figure 6). Both processes, however, lead to interconversion of 3a and 3b, and a 180° rotation, passing through both transition states and 3b, is required to exchange the hydrides in 3a.

Ru

We used EXSY spectroscopy to probe the dynamic behaviour of the hydride ligands of 3a. At 343 K, the on-set of a process wherein the two-hydride ligands interchanged sites was observed (k = 0.68 s -1 ). A series of 1D-EXSY NMR experiment where the temperature was varied between 346 and 373 K yielded activation parameters of ΔH ‡ = 95 ± 6 kJ mol -1 and ΔS ‡ = 26 ± 17 J K -1 mol -1 (ΔG ‡ 350 = 86.2 ± 0.1 kJ mol -1 ). In order to learn more about the nature of the exchange process, fully coupled 1 H-2D-NOESY and 31 P-2D-NOESY NMR spectra were recorded. Although hydride exchange was clearly evident in the 1 H spectrum, no exchange peaks were observed in the 31 P spectrum. The EXSY methods revealed no evidence for H 2 elimination in 3a, even at 378 K, consistent with that fact that the hydride resonances for 3a failed to show PHIP at this temperature. Even in a high signal to noise EXSY run, no evidence for the interchange of 3a into 3b was observed. We therefore conclude that the concentration of the 3b intermediate must be undetectably low, consistent with the fact that it has only ever been observed under p-H 2 enhancement. The highest point on the potential energy surface, TSb, lies 112 kJ mol -1 above 3a, a rather higher barrier than the 95 kJ mol -1 obtained from the experiment, probably due to the neglect of the bulky phosphine substituents, but the trend towards slower exchange in 3 than 2 is clearly reproduced. Finally, we note that Mann and co-workers have previously reported the exchange of hydride ligands in Ru(CO)(H) 2 (PPh 3 ) 3 on the NMR timescale, 53 and suggested that this process occurred via a trigonal twist mechanism. We have located a trigonal twist transition states that exchanges the two hydrides in 3a without formation of 3b, only 11 kJ mol -1 above TSa. This process would, however, also lead to exchange of the dppe phosphines, clearly contrary to experiment. We therefore conclude that the presence of the chelating phosphine perturbs the potential energy surface in such a way that the trigonal twist is disfavoured relative to the pathway illustrated in Figure 6.

Conclusions

In this paper we have described the synthesis of Ru(CO) 2 (PPh 3 )(dppe) 1, which proved to exist in both fac and mer forms in solution. Under thermal conditions, 1 selectively loses PPh 3 and reacts with H 2 to yield Ru(CO) 2 (dppe)(H) 2 2. However, upon 325 nm photolysis, CO loss dominates PPh 3 loss by a factor of 2.2, giving rise to two distinct isomers of Ru(CO)(PPh 3 )(dppe)(H) 2 3 and the PPh 3 addition product Ru(CO)(PPh 3 ) 2( η 1 -dppe)(H) 2 4. Irradiation of 1 with H 2 in the presence of pyridine reveals the presence of a minor photochemical process involving CO and PPh 3 loss which leads to Ru(H) 2 (CO)(dppe)(pyridine) 5. The major isomer of 3 contains hydride ligands trans to CO and one of the phosphines of the dppe ligand. In the second isomer, both hydrides lie trans to phosphine. The potential energy surface optimised using density functional theory is fully consistent with the observed products, and provides insight into the mechanisms by which they are formed.

Figure 1 .

 1 Figure 1. Optimised structures and relative energies for the two stable isomers of Ru(CO) 2 (PH 3 )(H 2 PCH 2 CH 2 PH 2 ). Bond lengths are in Å.

Table 2 .

 2 NMR data for three isomers of Ru(CO)(dppe)(PPh 3 )(H) 2 produced upon the irradiation of Ru(CO) 2 (PPh 3 )(dppe) 1 with H 2 in C 6 D 6 at 295 K. The labelling of the atoms is shown in Figure 2. δ ( 1 H) δ ( 31 P{ 1 H}) 3a -7.00 (H a ), dddd, J HP = 78 Hz (P b ), 22 Hz (P a ), 19 Hz (P c ), J HH = -5 Hz -7.56 (H b ), dddd, J HP = 27 Hz (P b ), 25 Hz (P a ), 20 Hz (P c ), J HH = -5 Hz 61.4 (P a ), dd, J pp = 240 Hz (P c ), 18 Hz (P b ) 66.9 (P b ), dd, J PP = 18 Hz (P a ), 3 Hz (P c ) 81.7 (P c ), dd, J PP = 240 Hz (P a ), 3 Hz (P b ) 3b -6.54 (H a ), m (trans to P b ), a J HH = -5 Hz -9.22 (H b ), dddd, J HP = 65 Hz (P a ), 35 Hz (P c ), 17 Hz (P b ), J HH = -5 Hz 47.3 (P a ) 64.5 (P b ) 83.2 (P c ) 4 -7.17 (H a ), dddd, J HP = 79 Hz (P b ), 24 Hz (P a ), 18 Hz (P c ), J HH = -4.5 Hz -8.42 (H b ), dq, J HP = 27 Hz, J HH = -4.5 Hz 77.5 (P a ), dd, J pp = 14, 230 Hz 66.7 (P b ), d, J pp = 14 Hz 55.4 (P c ), dd, J PP = 66, 230 Hz 30.7 (P d ), d, J PP = 66 Hz

Figure 2 .

 2 Figure 2. Proposed structures and atom labelling for the complexes detected in this study using parahydrogen methods.

Figure 3 .

 3 Figure 3. Hydride region of the 1 H NMR spectrum of 1 in C 6 D 6 under 3 atm of p-H 2 : (a) at the onset of photolysis; (b) after 20 min of photolysis. The resonances marked * are due to Ru(CO) 2 (dppe)(H) 2 .

2 3a 3a 2 2 3a 3a 2 Figure 4 .

 24 Figure 4. 1 H-31 P HMQC experiments for products obtained from 1 in C 6 D 6 with concurrent laser photolysis under 3 atm of p-H 2 : (a) with a conventional delay (1/2×J HP ), optimised for trans couplings; (b) with a conventional delay (1/2×J HP ), optimised for cis couplings; (c) optimised for trans couplings with a delay of 1/(5×J HP ); (d) optimised for cis couplings with a delay of 1/(5×J HP ); (e) with a delay of 1/J HP . Traces "c" and "d" show the real 31 P chemical shifts, while trace "e" shows the cross-peaks due to triple quantum coherence effects.

Figure 5

 5 Figure 5 Potential energy surface for the H 2 addition pathway to Ru(dHpp)(CO) 2 (PH 3 ).

Figure 6 .

 6 Figure 6. Potential energy surface for hydride exchange in 3a.

  3 (PhP(CH 2 CH 2 PPh 2 ) 2 ) with H 2 .47

	Ph	Ph			Ph	Ph			Ph	Ph		P d
	P OC a Ph Ph	Ru P	H b H a	P b OC Ph Ph	Ru P c	H b Ha	P b Ph 3 P a Ph Ph	Ru P c	H b Ha	OC	Ru P c	Ha H b
		C b O				P a Ph 3				CO			P a Ph 3

Acknowledgements

We are grateful to the EPSRC and the European Union (HYDROCHEM network, contract HPRN-CT-2002-00176) for funding, as well as to Prof. R. N. Perutz, Dr. R. J. Mawby and Dr. J. M. Lynam for helpful discussions. Experimental assistance from Ms. K. A. M. Ampt, Ms. N. J. Smith and Mr. N. J. Wood is gratefully acknowledged.


Raluca Malacea

Eric Manoury
email: manoury@lcc-toulouse.fr

Lucie Routaboul

Jean-Claude Daran

Rinaldo Poli

John P Dunne

Adrian C Withwood

Cyril Godard

Simon B Duckett

Coordination chemistry and diphenylacetylene hydrogenation catalysis of planar chiral ferrocenylphosphine-thioether ligands with cyclooctadieneiridium(I)

Keywords: Ferrocenylphosphine ligands, Hemilability, Hydrogenation, Iridium, Planar chirality, Thioether ligands

the much stronger affinity of ligands 1-Ph for iridium compared to rhodium. Finally, complexes 2-tBu and 2-Ph were found to be efficient catalysts for diphenylacetylene hydrogenation, affording a mixture of monohydrogenation (both E-and Z-stilbene) and dihydrogenation (1,2-diphenylethane) products.

Abstract

Introduction

The coordination chemistry of iridium has been widely studied during the last few decades, in part because of numerous applications that have been found in various catalytic systems [1] since the pioneering work of Crabtree et al. [2] In particular, iridium has been successfully applied to asymmetric catalyses, such as olefin hydrogenation, [3][4][5][6][7][8][9][10][11][12] C=N hydrogenation, [13][14][15][16][17][18][19][20][21] transfer hydrogenation [22][23][24][25][26][27][28] and allylic substitution. [29][START_REF] Pfalz | Comprehensive Asymmetric Catalysis[END_REF][START_REF] Takeuchi | [END_REF][32][33][34][35][36][37][38][39][40] Furthermore, the largest scale industrial enantioselective catalytic process involves an iridium-based hydrogenation step in the synthesis of chiral herbicide (1S)-Metolachlor. [41,42] Although many catalytic systems are generated in situ by the ligand addition to the readily available chloro(cycloocta-1,5-diene)iridium dimer, a good number of ligand adducts have been isolated and crystallographically characterized. When the supporting ligand (L-L') is bidentate, several species can be envisaged depending on the ability of the ligand to act in a monodentate or bidentate fashion and on the competition between the donor atoms and the chloride ion for the metal center. These species correspond to 5-coordinate Ir( 2 -L-L')(COD)Cl, 4-coordinate neutral Ir( 1 -L-L')(COD)Cl with a dangling L or L' donor atom, and 4-coordinate ionic [Ir( 2 -L-L')(COD)] + Cl -. Whereas several solid state structures are available for 5-coordinate complexes [43][44][45][46][47] and an example is also reported for neutral 4coordinate structure with a dangling phosphino-ester ligand, [48] no examples are known, to the best of our knowledge, of 4-coordinate ionic [Ir( 2 -L-L')(COD)] + complexes with chloride as counter-ion (though they are known for rhodium). [49][50][51] Whether, for a given metal/ligand combination, the solid state structure is maintained in solution and whether it is solvent dependent, or whether equilibria between different structures are established, has not been the subject of detailed studies to the best of our knowledge. In addition, how the ligand stereoelectronic properties may influence this structural choice is also an obscure point.

The coordination chemistry of sulphur-containing ligands, [52,53] as well as their potential in homogenous catalysis [54,55] have recently attracted much interest, amongst other things because the sulphur atom could become asymmetric upon coodination to a metal. We have recently reported the synthesis of a family of planar-chiral P,S ligands, namely (R/S)-2diphenylphosphino-(R-thiomethyl)ferrocenes, 1-R, for instance with R = Et, tBu, Ph), which proved to be very efficient in palladium-based asymmetric allylic alkylation. [56] As a first step toward the development of iridium-based catalytic systems, we wished to explore the iridium coordination chemistry of such ligands, with a particular focus on the stoechiometry, stereochemistry, potential (hemi)lability, etc. In this paper, we describe the synthesis and structure of a family of iridium complexes produced upon reaction of the racemic 1-R (R = Et, tBu, Ph) ligands with the cyclooctadieneiridium(I) moiety. We place particular emphasis on probing the relationship between the solution behaviour, the solid state structure, and the nature of the R substituent on the ferrocenyl thioether function. New structural features and an unsuspected complex solution behaviour have been found through these investigations.

Preliminary results on the catalytic diphenylacetylene hydrogenation reaction are also reported. Although the COD ligand is replaced by other ligands or substrates under most catalytic conditions (e.g. it is hydrogenated to cyclooctane under hydrogenation conditions) the structural and dynamic features observed for the pre-catalyst (i.e., the coordination stereoselectivity, the hemilability) are potentially relevant to the behaviour that one may expect under catalytic conditions.

Results

(a) Syntheses.

Addition of the 2 equivalents of the bidentate ligands CpFe[1,2-C5H3(PPh2)(CH2SR)]

(1-R, R = Et, tBu, Ph), which will be abbreviated as (P,SR), to complex [Ir(COD)Cl]2 [i.e. 1 equiv of (P,SR) to Ir] causes the splitting of the chloride-bridged dinuclear structure of the iridium precursor and formation of mononuclear products, according to the stoichiometry of Equation 1.

<Equation 1>

The products 2-R have been characterized by analytical and spectroscopic ( 1 H, [START_REF] Takeuchi | [END_REF] P, 13 C NMR) methods. The NMR properties are shown in Table 1. These are spectra recorded on solutions obtained upon redissolving the isolated crystals, but correspond to complex equilibria between different species, as will be discussed in detail after the description of the structures. The three compounds show grossly similar physical (color, solubility) and spectroscopic properties in solution, but have different structures in the solid state. Whereas 2-Et and 2-Ph proved to be 5-coordinated with a  2 -P,S coordination mode for the bidentate ligand, their 2-tBu analogue is only 4-coordinated, with a  1 -P coordination mode and a dangling thioether functionality (see X-ray structures below).

Another remarkable feature has been observed during the crystallization of the crude product obtained from the reaction of 1-Ph and [Ir(COD)Cl]2 (2:1 ratio). Whereas crystals of 2-Ph were obtained from dichloromethane/hexane, when the same procedure was followed for a toluene/hexane, crystals of a different compound, namely [Ir(COD)-(P,SPh)][Ir(COD)Cl2], 3-Ph, were produced. In fact, a CDCl3 solution of 3-Ph gave a 31 P resonance at 9.1 ppm, quite different from the value observed for 2-Ph (-3.1). As will be shown later, this resonance results from an equilibrium mixture of 2-Ph and 3-Ph. When the synthesis was repeated for a 1:1 ratio of 1-Ph and [Ir(COD)Cl]2 (1-Ph/Ir = 2), the resulting orange solid displayed solution state NMR properties in close correspondence with those of the isolated crystals of 3-Ph. However, this solid also yielded crystals of 2-Ph when recrystallized from dichloromethane/hexane.

It seems therefore that the choice of crystallization solvent dictates the nature of the isolated product, indicating a possible equilibrium between 2-Ph and 3-Ph (Equation 2). A more detailed study of this equilibrium will be described below. for a [(COD)Ir(P,StBu)] + complex containing a chiral phosphinite-thioether donor. [57] Other significant bonding parameters are shown in Table 2 for the 5-coordinate compounds and in (X = any halogen) are not uncommon. They comprise examples with L2 = diphosphines [43,44] including a ferrocenyl functionalized diphosphine, [45] diimines, [46,47] and also with L = monodentate phosphine. [58] The compounds reported here are the first such examples containing a bidentate P,S-type framework. More importantly, they illustrate the delicate control exerted by the ligand structure on the coordination geometry. This control is unlikely to have an electronic origin, since the Et and tBu substituents impart analogous electronic effects, and different from those of the Ph substituent. A steric effect appears more likely, since the tBu group is more encumbering than either the Et or the Ph group and thus prevents the compound to attain the 5-coordinate geometry.

<Figure 1>

The S coordination step is interesting from the stereochemical point of view.

Coordination renders the S atom and the Ir atom asymmetric. In combination with the planar chirality of the substituted ferrocene moiety, eight possible stereoisomers, namly four diastereomeric pairs of enantiomers, can be obtained. There is, however, no indication from the NMR spectrum for the formation of more than one compound. [43] and 2.585(3) Å in IrCl(COD)(dppp). [44] However, these distances are longer relative to neutral 4-coordinate complexes (e.g. 2.363(1) Å in Ir(COD)[=C(NCH2-p-Tol)2C2H2]Cl, [59] and 2.3625(8) Å in complex 2-tBu). A lengthening of ca. 0.2 Å accompanies this coordination number increase.

(L2 = (2R,3R)-2,3-O- Isopropylidene-1,4-bis(5H-dibenzophosphol-5-yl)-2,3-butanediol),
This observation gives credence to the hypothesis that the Ir- [57,60] All these previously reported salts contain BF4 -, PF6 -, SbF6 -, CF3SO3 -, or tetraarylborates as counterions. None contains the [Ir(COD)Cl2] -anion. In fact, this quite simple anion does not appear to have previously been crystallographically characterized. This situation differs considerably from that of the rhodium congeners, for which several structure of salts of type [Rh(COD)L2][Rh(COD)Cl2] have been reported. [61][62][63][64][65] <Figure 2>

Selected bonding parameters for compounds 3-Ph are shown in Table 4. Both ions exhibit a relatively undistorted square planar geometry when considering the center of the COD C=C bonds (CT) as defining the coordination positions. The Ir-CT distances are significantly shorter in the anion (average 1.976(5) Å) than in the cation (average 2.09(4) Å), and the latter are marginally longer than in the 4-and 5-coordinate neutral complexes 2-R discussed above, attesting to the importance of back-bonding for the Ir-COD interaction. The Ir-P and Ir-S bond lengths in the cation are shorter than those in the neutral complexes, irrespective of their coordination number. This effect can be attributed to the metal ion contraction on going from the neutral to the charged system, because of an increase of effective nuclear charge, and agrees with a predominant -bonding component for the Ir-P and Ir-S interactions.

(c) NMR spectroscopy of compounds 2-R

Given the subtle structural difference observed for compounds 2 in the solid state depending on R, one might wonder whether the same structural dichotomy is maintained in solution. A detailed analysis of the NMR properties seems to suggest that this is the case.

The [START_REF] Takeuchi | [END_REF] P resonance chemical shift is essentially identical for compounds 2-Et and 2-Ph, while it is significantly shifted downfield for 2-tBu (see Table 1), in apparent agreement with donation from the phosphorus atom to a less electron rich metal in the latter case. This observation is the strongest indication in favor of a similar structure in the solid state and in chloroform solution for compounds 2. Given the planar chirality of the functionalized Cp ring of the ferrocene moiety, all COD proton and carbon nuclei are magnetically inequivalent in both types of structure, although a higher degree of inequivalence could be expected for the 5-coordinate structures where there is coordination-induced chirality at both the S and Ir atoms. The NMR spectra are however deceptively simple since only two sets of 13 C resonances are observed for both the CH and the CH2 nuclei for all three compounds. For compound 2-tBu, the CH resonances could not be identified, even by an 1 H- 13 C HMQC

<Figure 3>

The behaviour illustrated in Figure 3 can be rationalized on the basis of Scheme 1 where rapid site exchange between the species participating in the equilibrium is required. A similar equilibrium has previously been proposed for an analogous system with the hemilabile N,O ligand 1-(2'-pyridyl)-3-dimethylamino-2-propen-1-one. [64] The resonances did not decoalesce under the same conditions (see Table 1), whereas the resonance of the free ligand 1-Et is at -20. Addition of an excess of ligand 1-Et shifted this resonance further to  -4.6. We therefore presume that the solution behaviour of "Ir(COD)Cl" in the presence of 1-Et is the same as with the 1-Ph ligand. A more detailed equilibrium study was not carried out in this case. In contrast, treatment of complex [Ir(COD)Cl]2 with various amounts of 1-tBu in CDCl3 led to the observation of two new [START_REF] Takeuchi | [END_REF] We now need to assign the  17.5 resonance and to rationalize the fact that it is observed only for the 1-tBu/Ir system. The most reasonable proposal that we are able to advance involves a neutral dinuclear complex with a bridging P,StBu ligand (4-tBu), see Scheme 2. Precedents for this type of structural arrangement exist for P,N ligands. [66,67] Since the equilibrium between the 5-coordinate and the 4-coordinate isomers of 2-R lies strongly in favor of the latter when R = tBu, the pendant StBu arm is available to coordinate the Ir center in the unsaturated Ir(COD)Cl complex, which is in equilibrium with the dichlorobridged dimer. It is rather remarkable, however, that this process is slower than the addition of the chloride anion to the same position. We presume that this is so for steric reasons. The reversible association/dissociation process of the pendant StBu arm leading from 2-tBu to 3-tBu is fast because it is intramolecular. The reason why analogous compounds 4-Ph and 4-Et are not observed would then be related to the preferred sulfur coordination to the same Ir center, in the 5-coordinate geometry of 2-Ph and 2-Et.

<Scheme 2>

According to this proposal, compound 4 has an identical stoichiometry to compound 3.

Therefore, the 3-tBu/4-tBu ratio should be independent on the 1-tBu/Ir ratio. This ratio can be estimated from the extrapolated relative intensities of the  14. When samples of 1-R (R = Ph, Et, tBu) were reacted with [Rh(COD)Cl]2 in a 2:1 molar ratio, a single [START_REF] Takeuchi | [END_REF] P resonance, split into a doublet by a rhodium coupling, was observed in each case in the NMR spectrum. This resonance is therefore assigned to a product with the stoichiometry RhCl(COD)(P,SR) (5-R) see Table 5. The very similar values of the chemical shifts and coupling constants, in contrast to the behaviour observed for the Ir complexes, strongly indicates the same coordination mode for the three ligands with respect to Rh(I).

When a sample of 1-Ph in CDCl3 was reacted with [Rh(COD)Cl]2 in a 1:1 molar ratio, a doublet resonance was observed at 24 ppm (143 Hz) in the [START_REF] Takeuchi | [END_REF] A few examples of 4-coordinate [Rh(COD)L2] + complexes having Cl -as the counter ion have been characterized by X-ray crystallography, namely [Rh(COD)(bipy)2] + , [49] [Rh(COD)L2] + (L = 1,3-dimethylimidazoline-2-ylidene), [50] and a chelated version of the latter [L2 = (MeNC3H2N-(CH2)3-NC3H2NMe]. [51] However, addition of the 1-R ligands to [Rh(COD)Cl]2 in C6D6 generated soluble products whose [START_REF] Takeuchi | [END_REF] (f) Catalytic studies.

Complexes 2 were tested as catalyst for the hydrogenation of diphenylacetylene. The results are presented in Table 6. Under 30 bars of dihydrogen pressure at room temperature with 1 mol% of precatalyst, compound 2-tBu gave a quantitative conversion after 6 h, whereas complex 2-Ph was less active, yielding only a 59% conversion. In both cases, significant amounts of 1,2-diphenylethane were detected by GC (alkenes/alkane ratio = 92/8 for 2-Ph and 78/22 for 2-tBu). Compared to other hydrogenation systems for the selective transformation of alkynes into alkenes, these systems perform rather poorly. [68][69][70][71][72][73][74][75] Furthermore, the stereoselectivity of the reaction, especially with compound 2-tBu, is also quite poor (cis-stilbene/ trans-stilbene ratio ratio =91/9 for 2-Ph and 82/18 for 2-tBu).

The presence of E-alkenes after hydrogenation of alkynes is commonly explained by the isomerisation of Z-alkenes, which are the primary products arising from the syn-addition of H2 to the C-C triple bonds. [76][77][78] In order to determine whether similar reasons are involved in our system, we carried out the reaction of cis-stilbene with dihydrogen in presence of complexes 2-Ph or 2-tBu under the same conditions described in Table 6 for diphenylacetylene.

Isomerisation to trans-stilbene as well as hydrogenation to 1,2-diphenylethane did occur, although the rate of this transformation was slower than the direct formation of these two products from diphenylacetylene. This result suggests that the isomerization and further hydrogenation processes probably occur mostly from the vinyl intermediate, before the primary cis-stilbene product is formed by reductive elimination. However, recoordination of the cis-stilbene followed by both isomerization and further hydrogenation are also possible.

Conclusion

Use of the ferrocenyl phosphine-thioether ligands 1-R (R = Et, Ph, tBu) has allowed to reveal that the coordination geometry of chlorocyclooctadieneiridium(I) ligand adducts, both in the solid state and in solution, is delicately controlled by the nature of the R substituent, yielding 5-coordinated complexes 2-R for R = Et, Ph and a 4-coordinate one for R = tBu, the latter featuring a dangling thioether group. The ligand chirality controls the geometry at the sulfur and iridium atoms, producing single diastereomers.

The solid state geometries appear to be maintained in solution.

Experimental Section

General. All reactions were carried out under an argon atmosphere using standard Schlenk techniques. Solvents were carefully dried by conventional methods and distilled under argon before use. The (R/S)-2-diphenylphosphino-(R-thiomethyl)ferrocene ligands (1-R, R = Et, tBu, Ph) were prepared according to a published procedure [56] from racemic 2-(diphenylthiophosphinoferrocenyl)methanol. [79] NMR spectra were recorded on Bruker AC200 and Bruker Avance 500 FT-NMR spectrometers. The resonances were calibrated relative to the residual solvent peaks and are reported with positive values downfield from TMS. The NMR spectra of the new compounds 2-R (R = Et, tBu, Ph) are collected in Table 1. twinned by inversion ('racemic twin'). In compound 3 a toluene solvent molecule is statistically disordered around an inversion center. The drawing of the molecules was realised with the help of ORTEP32. [83] Crystal data and refinement parameters are shown in Table 7.

Synthesis of

Crystallographic

  Reaction of the P,S ligands CpFe{1,2-C5H3(PPh2)(CH2SR)}, (1-R; R = Et, Ph, tBu), with 0.5 equivalents of [Ir(COD)Cl]2 (metal/ligand = 1/1) furnishes the stable mononuclear adducts [Ir(COD)Cl(1-R)], 2-R, in high yields. Molecular structures of the three complexes 2-R have been determined by single crystal X-Ray diffraction. Compounds 2-Et and 2-Ph have similar 5-coordinate molecular structures, with an intermediate coordination geometry between a distorted trigonal bipyramid and a distorted square pyramid. In the solid state, the sulfur atom chirality (R group exo, lone pair endo and parallel to the Cp-Fe axis) as well the iridium chirality (Cl atom exo relative to the ferrocene group) is controlled by the ligand planar chirality. In solution, only one diastereoisomer is observed in both cases, indicating that the geometries observed in the solid state are maintained in solution or that the different species are in rapid exchange on the NMR time scale. Complex 2-tBu, on the other hand, shows a 4-coordinate square planar molecular structure with a dangling thioether moiety, probably for steric reasons. For metal/ligand ratios greater than 1, compounds [Ir(COD){ 2 -P:S-(1-R)}] + [Ir(COD)Cl2] -(3-R) are obtained, resulting from the chloride abstraction by the excess iridium metal on complex 2-R. The structure of 3-Ph was confirmed by single crystal X-ray diffraction. A rapid equilibration between 2-R and 3-R gives rise to only one set of NMR signals for each metal/ligand ratio. For ligand 1-tBu, a similar equilibration between 2-tBu and 3-tBu is again observed, in addition to a slower equilibration with a third product, [Ir(COD)Cl](-1-R)[Ir(COD)Cl] (4-tBu), which is proposed to contain a bridging P,S ligand spanning two Ir centres. Complex 2-Ph reacts with [Rh(COD)Cl]2 to give only the heterometallic ionic pair [Ir(COD){ 2 -P:S-(1-Ph)}][Rh(COD)Cl2] without transfer of ligand 1-Ph from Ir to Rh. Furthermore, the rhodium complex RhCl(COD)(1-Ph) reacts quantitatively with [Ir(COD)Cl]2 at RT to yield 2-Ph and [Rh(COD)Cl]2, thereby illustrating

  experiment at temperatures down to 213 K. On the other hand, these peaks were visible when the spectrum was recorded in toluene-d8 solution (see Experimental Part). The pairs of C atoms with distinct chemical shifts are probably associated to the different coordination positions (i.e. trans to P and trans to S/Cl), whereas the asymmetry above and below the square coordination plane has a negligible effect on the chemical shift. Only for the 1 H CH (COD) resonances in compound 2-Et was a slight asymmetry detected in the corresponding NMR spectra due to this asymmetry. This observation alone is not sufficient to confirm the difference in coordination geometry within the three compounds. However, the chemical shifts of the CH(COD) 1 H and13 C resonances change significantly on going from 2-Et and 2-Ph. In particular, the CH resonances in the 1 H NMR spectrum proved to be more spread apart in 2-tBu, while one of the two CH resonances in the corresponding13 C NMR spectrum is upfield shifted by ca. 15 ppm relative to the same resonance in the other two compounds. The chemical shift of the single 1 H resonance of the unsubstituted Cp ligand is also significantly different for 2-tBu relative to the other two compounds, and a slight difference is even observable for the13 C resonance of the same group. The 1 H and13 C resonances of the CH2 group linking the S donor to the ferrocene unit, on the other hand, seem sensitive to the electronic nature of the SR group, since they are similar for the Et and tBu derivatives and different for the Ph derivative. In conclusion, the overall NMR behaviour suggests that the solution structures of compounds 2 may be identical to those observed in the solid state. (d) Study of solution equilibria. Solutions obtained by mixing complexes [Ir(COD)Cl]2 and the free ligand 1-Ph in various ratios were investigated by 1 H and 31 P NMR spectroscopy. Only one set of resonances was observed for all ratios up to 1-Ph/Ir = 1:1, but several individual resonances of this set (notably the 31 P resonance and the 1 H resonances of the ferrocene unsubstituted Cp, of one of the substituted Cp protons, and of the two CH2 protons) exhibited a continuous variation of the chemical shift as a function of the 1-Ph/Ir ratio (see Figure3). The COD resonances were too broad to allow their accurate monitoring. For 1-Ph/Ir ratios greater than 1, the resonances of the free ligand 1-Ph (mostly evident from the 31 P NMR spectrum) also appeared.

Figure 3 . 1 -

 31 Figure 3. The full fitting procedure and the values of 2-Ph and 3-Ph for each resonance, as

  P NMR resonances. Whereas one of these (at  17.5) exhibited the same chemical shift at all 1-tBu/Ir ratios, the other one shifted continuously from ca.  14.6 at 1-tBu/Ir = 0.2 to ca. 15.7 at 1-tBu/Ir = 1, see Figure4(a). In addition, the relative intensity of these two resonances changed in favor of the second one upon increasing the 1-tBu/Ir ratio, until the first one completely disappeared at the point when 1-tBu/Ir = ca. 1, see Figure4(b). Further additions of the free ligand no longer moved the 15.7 resonance, although, a new resonance due to the excess of free 1-tBu started to appear at  -20.0. This observed behaviour suggests the presence of at least three P-containing Ir complexes: two that are in rapid exchange on the NMR time scale, generating the resonance that is shifting in the  14.6 -15.7 range, and a third one giving the  17.5 resonance. The latter species exchanges slowly on the NMR timescale, or not at all, with the other two. In addition, the appearance of the free 1-tBu resonance immediately after attaining the 1-tBu/Ir ratio of 1, suggested an essentially quantitative first addition of 1-tBu to [Ir(COD)Cl]2 to generate 3-tBu, like for the case of the 1-Ph addition.<Figure 4>It seems logical to assume, on the basis of the results shown above for the 1-Ph/Ir system, that the shifting resonance is due to the rapidly equilibrating mixture of compounds 2-tBu and the salt [Ir(COD)( 2 -P,StBu)] + [Ir(COD)Cl2] -, 3-tBu (equilibrium of Equation2). The chemical shift observed at low 1-tBu/Ir ratio (Figure4(a), ca.  14.5) is close to that observed at low 1-Ph/Ir ratio (Figure3, ca.  11) and attributed to compound 3-Ph. For the 1-Ph/Ir system, addition of the free ligand leads to an upfield shift toward the resonance of 5coordinate 2-Ph (ca.  -3), whereas for the 1-tBu/Ir system, addition of the free ligand leads to a downfield shift toward the resonance of 4-coordinate 2-tBu (ca.  15.5). This proposition is in agreement with the observed behaviour of the same system in C6D6: the lower polarity of this solvent is expected to disfavor formation of the ionic system and, indeed, the resonance does not shift significantly by varying the 1-tBu/Ir ratio in the 0 to > 1 range ( always in the 15.68 -15.71 range).

  6 and 17.5 peaks as 1-tBu/Ir  0 (ca. 42.3:57.7), from(Figure 4(b). This number can in turn be used to estimate the individual concentrations of 2-tBu and 3-tBu at each 1-tBu/Ir ratio. At this point, the concentration of each iridium-containing species can be calculated solving the equilibrium relationship (Equation 3, R = tBu), using K as an adjustable parameter. These concentrations were then used to calculate the[START_REF] Takeuchi | [END_REF] P chemical shift according to Equation 4 (R = tBu). Fitting of the simulated and experimental data yielded a reasonable agreement for an equilibrium constant of 0.036, as shown in Figure4(a). Thus, K is greater for R = tBu (0.036) than for R = Ph (0.008). The release of steric pressure upon transforming the chelating P,StBu ligand into a dangling one (whereas the P,SPh ligand remains chelating in compound 2-Ph) may provide an explanation of this difference. (e) Comparison with Rh coordination chemistry and mixed Rh-Ir studies.

  P NMR spectrum. Both the chemical shift and the coupling constant of this resonance are very close, but not identical, to those obtained using a 2:1 ratio. Therefore, we presume that the compound formed under these conditions (6-Ph) has the same structure as the iridium analogue 3-Ph, namely [Rh(COD)(P,SPh)][Rh(COD)Cl2]. As for compounds 5-R, there are two possibilities: an ionic structure [Rh(COD)(P,SR)] + Cl -, and a neutral structure with a dangling thioether function, RhCl(COD)( 1 -P,SR), as seen for the iridium analogue 2-tBu. In both cases, the rhodium complexes are consistently 4-coordinated, in agreement with the lower propensity of Rh(I), relative to Ir(I), to give 5-coordinate geometries with the involvement of the dz2 orbital.

  P NMR resonances [5-Et: 21.8 (147 Hz); 5-tBu: 23.6 (148 Hz); 5-Ph: 21.6 (148 Hz)] turned out to be essentially identical to those recorded in CDCl3. This result strongly suggests that the adducts are neutral. In conclusion, the Ir(I) center has a greater tendency to coordinate both the thioether function and the Cl -ion. The latter, however, is rather weakly coordinated as shown by the long Ir-Cl distances in the X-ray structures (vide supra).

  However, complex equilibria between various structures are established. The chloride ligand is loosely bound to iridium in the 5-coordinate complexes 2-R, leading to its scavenging by excess metal complex and to a rapid equilibration between complexes 2-R and ionic [Ir(COD)( 2 -P,SR)] + [Ir(COD)Cl2] -3-R. Salt-like compounds of type [Ir(COD)(k 2 -L-L')][Ir(COD)X2] -have not been structurally characterized previously for any L-L' or X, though they are quite common for the Rh congener. Furthermore, competitive coordination reactions between rhodium and iridium for ligand 1-Ph lead only to iridium coordination, probing the greater affinity of P,SR ligands for the heavier metal. The same solution is obtained whether the P,SR ligand is initially bonded to Rh or Ir, indicating a high lability for such systems.Complexes 2-R catalyse the hydrogenation of diphenylacetylene at room temperature, paving the way to the application of such complexes to other hydrogenation reactions. Our particular attention is devoted to asymmetric hydrogenation, taking advantage of the availability of the chiral ligands 1-R in optically pure form. Further studies in this direction are currently in progress in our laboratories.

3 CaptionsFigure 1 .

 31 Figure 1. ORTEP views of the molecular structures of compounds 2-Et (a); 2-tBu (b) and

Figure 2 .

 2 Figure 2. ORTEP view of the molecular structures of compounds 3-Ph. Ellipsoids are

Figure 3 .

 3 Figure 3. Dependence of the chemical shift of a few resonance for the [Ir(COD)Cl]2/1-Ph system on the 1-Ph/Ir ratio. (a) 31 P resonance; (b) 1 H resonances: unsubstituted Cp (5H, diamonds); substituted Cp (1H, circles); CH2 (1H, squares; 1H, triangles).The solid lines are obtained by fitting the data according to Scheme 1 (see text).

Figure 4 .

 4 Figure 4. Chemical shift (a) and relative ratio (b) of the 31 P NMR resonances for the [Ir(COD)Cl]2/1-tBu system as a function of the 1-tBu/Ir ratio. Squares: rapidly equilibrating mixture of 2-tBu and 3-tBu; triangles: 4-tBu; diamonds: 1-tBu. The solid line in (a) is the data fit to the model of Scheme 2 (see text).

Figure 1 Figure 2

 12 Figure 1

Figure

  Figure 3

3

 3 Figure 3

3 [[

 3 Figure 4

<Equation 2> (b) X-ray structures. Views of the molecular geometries of compounds 2-Et, 2-tBu and 2-Ph are presented in

	Figure 1. It is immediately evident that the Et and Ph derivatives are 5-coordinated since the
	S atoms are within reasonable bonding distances from the Ir centers (2.3928(17) Å and
	2.3835(12) Å, respectively), whereas the tBu derivative is only 4-coordinated (the S•••Ir
	distance is 5.13 Å). A comparable Ir-S bonding distances (2.3594(15) Å) has been reported

Table 3

 3

	for the 4-coordinate one. When considering the coordination sites as corresponding
	to the midpoints of the COD C-C double bonds, the geometry of 2-tBu corresponds quite

-Et and 2-Ph, on

	the other hand, may be best viewed as intermediate between a distorted trigonal bipyramid
	and a distorted square pyramid. In the distorted trigonal bipyramidal view, the center of one
	C=C donor function of the COD ligand (CO2) and the S1 donor atom define the axial
	positions (angles of 168.1(4) and 166.703(18)°, respectively). However, the equatorial angles
	deviate severely from the ideal 120° value, for instance the CT01-Ir(1)-P(1) angle in the two
	compounds is 153.2(4) and 156.06(2)°, respectively, whereas the P(1)-Ir(1)-Cl(1) angle is
	91.33(7) and 90.03(3). In the alternative distorted square pyramidal view, the axial ligand
	would be the Cl(1) ligand, although a few of the Cl(1)-Ir-X angles (X = P(1), S(1), CO1 and
	CO2) are relatively close to 90°. It is interesting to note that in both 2

-Et and 2-Ph, the Ir is located off the basal square plane by 0.363(8) Å and 0.3506(3) Å respectively, moreover the Ir is displaced toward the Cl atom. The distance from Cl to the square plane is 2.875(6) Å and 2.850(2) Å respectively. In fact, it would seem possible to view the structure as derived from that of the corresponding square planar [Ir(COD)(P,SR)] + by addition of Cl -, resulting in a minimal perturbation of the square planar geometry. Structures of 5-coordinate IrX(COD)L2

  The structure of compound 3-Ph contains discrete square planar [Ir(COD)( 2 -P,SPh)] + and [Ir(COD)Cl2] -anions. An interstitial toluene molecule completes the crystal packing.One H atom of a CH2 group of the COD ligand in the cation is located at a relatively short intermolecular contact (2.759 Å) above the Ir atom in the anion, see Figure2, a location which places it in an approximately ideal axial position. Indentification of such an interaction in solution by NMR spectroscopy was precluded by the complex redistribution equilibria (see

Cl bond in the 5-coordinate complexes is highly polarized, especially when comparing it with the insignificant change of the Ir-P distance [2.3322(15) Å and 2.3289(13) Å in 5-coordinate 2-Et and 2-Ph, respectively; 2.3312(9) Å in 4-coordinate 2-tBu] and Ir-CT distances [CT = center of the COD C=C bond; averages are 2.028(8) Å for 2-Et, 2.032(10) Å for 2-Ph and 2.04(4) Å for 2-tBu]. below). However, the Ir atom in each ion sits very close to the ideal square place defined by the middle of the C=C bonds and the P,S or Cl donors, respectively (0.0158(5) Å in the cation, 0.0066(5) Å in the anion). Structures of 4-coordinate cationic complexes of type [Ir(COD)L2] + are quite common in the literature with various types of bidentate L2 and monodentate L ligands, including two precedents with a chelating P,S-type ligand, Ph2PO(1,2-cyclo-C6H10)StBu and Ph2PCH2P(tBu)2S.

Table 2 .

 2 (see footnote on the first page of this article). Procedures for fitting the equilibrium constant of Equation 1 to the experimental NMR data (Ir/1-Ph and Ir/1-tBu systems). Selected bond distances (Å) and angled (°) for the pentacoordinated compounds 2-R (R = Et, Ph).

	data (excluding structure factors) have been deposited with the
	Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 283190 -
	283193. 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).
	Supporting Information:

Table 6 .

 6 Diphenylacetylene hydrogenation in presence of complexes 2-Ph and 2-tBu.

	Entry complex Reaction	Conversion a,b Yield of cis -	Yield of trans-	Yield of 1,2-
			time		stilbene b	stilbene b	diphenylethane b
	1	2-Ph	6h	59%	79%	8%	13%
	2	2-tBu	6h	100%	64%	14%	22%
	a Reaction run with 1 mmol of diphenylacetylene, 0.01 mmol of complex 2 (1 mol%) in 2 mL
	CH2Cl2 at room temperature under 30 bars of dihydrogen. b Determined by GC.

Table 7 .

 7 Crystal data and structure refinement for all compounds. Largest diff. peak and hole, e.Å -3 1.141 and -0.817 e.Å -32.188 and -1.

	Identification code	2-Et	2-Ph	2-tBu	3-Ph
	Empirical formula	C33H37ClFeIrPS•CH2Cl2	C37H37ClFeIrPS•CH2Cl2	C35H41ClFeIrPS•CH2Cl2	C45H49Cl2FeIr2PS• (C7H8)0.5
	Formula weight	865.08	913.12	893.13	1210.09
	Temperature, K	180(2)	180(2)	115(2)	105(2)
	Wavelength, Å	0.71073	0.71073	0.71073	0.71073
	Crystal system	Monoclinic	Triclinic	Orthorhombic	Triclinic
	Space group	P 21	P -1	Pbca	P -1
	a, Å	11.6753(13)	10.177(5)	14.778(5)	10.1650(8)
	b, Å	10.9974(9)	12.983(5)	19.216(6)	13.4775(11)
	c, Å	12.7284(13)	13.676(5)	24.110(8).	16.7698(13)
	, °	90.0	102.045(5)	90.0	81.296(2)
	, °	100.717(13)	93.653(5).	90.0	72.575(2)
	, °	90.0	102.670(5)	90.0	78.406(2)
	Volume, Å 3	1605.8(3)	1712.8(12)	6847(4)	2137.1(3)
	Z	2	2	8	2
	Density (calcd), Mg/m 3	1.789	1.771	1.733	1.881
	Absorption coefficient, mm -1	4.980	4.674	4.675	6.792
	F(000)	856	904	3552	1178
	Crystal size, mm 3	0.22 x 0.17 x 0.01	0.20 x 0.12 x 0.055	0.35 x 0.12 x 0.03	0.22 x 0.19 x 0.07
	Theta range, °	2.17 to 26.14	3.30 to 32.05	1.93 to 28.40°.	2.10 to 28.32
	Reflections collected	12676	18279	66063	22207
	Independent reflections [R(int)]	6263 (0.0500)	10876 (0.0311)	8540 (0.0483)	10541 (0.0312)
	Completeness, %,	98.5 %	90.3 %	99.2 %	98.8 %
	Absorption correction	Multi-scan	Analytical	SADABS	SADABS
	Max. , min. transmission	0.6451 and 0.5709	0.64910 and 0.24012	1.000 and 0.724635	1.0 and 0.812
	Refinement method	F 2	F 2	F 2	F 2
	Data / restraints / parameters	6263 / 395 / 372	10876 / 0 / 406	8540 / 0 / 391	10541 / 55 / 498
	Goodness-of-fit on F 2	0.967	0.914	1.031	1.041
	Final R indices [I>2sigma(I)]	R1 = 0.0326,	R1 = 0.0304,	R1=0.0241,	R1=0.0358, wR2= 0.0815
		wR2 = 0.0682	wR2 = 0.0494	wR2=0.0534
	R indices (all data)	R1 = 0.0437,	R1 = 0.0415,	R1=0.0335	R1=0.0456, wR2 = 0.0855
		wR2 = 0.0714	wR2 = 0.0516	wR2=0.0569
	Absolute structure parameter	0.571(7)

Acknowledgments

We are grateful to the the CNRS, and the European Union for funding under the HYDROCHEM network (contract HPRN-CT-2002-00176).

Subsequent mixing experiments were carried out in order to probe the relative stability of Rh(I) and Ir(I) complexes with the coordinated P,SR ligands. The addition of [Ir(COD)Cl]2 to a sample of 5-Ph in a 1:2 molar ratio made the [START_REF] Takeuchi | [END_REF] P NMR resonance of the starting Rh complex and the P-Rh coupling disappear, yielding a new resonance (singlet) at  -0.25. From the observed chemical shift (cf. Table 5) and the absence of Rh coupling, we propose that the reaction leads to complex 2-Ph, according to Equation 5. The same [START_REF] Takeuchi | [END_REF] It is interesting to remark that the resonance observed for this mixture (ca. -0.2 ppm), which contains a P,SPh:(Rh+Ir) ratio of 0.5, is much closer to the resonance of 2-Ph than that of a solution with the same P,SPh to metal ratio, but containing only Ir (ca. 8 ppm, see Figure 3). This observation shows that [Ir(COD)Cl]2 is a stronger chloride scavenger than its rhodium analogue. Another obvious conclusion of this study is that the iridium ion forms stronger bonds with the phosphorus and sulphur atoms of the P,SPh ligand than does the rhodium ion.