Synthesis and Structural Characterisation of [ Ir 4 ( CO ) 8 ( CH 3 ) ( μ 4-η 3-Ph 2 PCCPh ) ( μ-PPh 2 ) ] and of the Carbonylation Product [ Ir 4 ( CO ) 8 { C ( O ) CH 3 } ( μ 4-η 3-Ph 2 PCCPh ) ( μ-PPh 2 ) ] ; First Evidence for the Formation of a CO Cluster Adduct before CO Insertion

A desprotonação do cluster [(μ-H)Ir4(CO)10(μ-PPh2)], 1, leva à formação de [Ir4(CO)10(μPPh2)] que reage com Ph2PCCPh e CH3I para dar [Ir4(CO)8(CH3)(μ4-η-Ph2PCCPh)(μ-PPh2)], 2 (34%), além de [Ir4(CO)9(μ3-η-Ph2PC(H)CPh)(μ-PPh2)] e [(μ-H)Ir4(CO)9(Ph2PC≡CPh)(μ-PPh2)]. O composto 2, caracterizado por uma análise de difração de raios-X, contem um arranjo metálico na forma de uma borboleta, com o ligante Ph2PCCPh interagindo com os quatro átomos de Ir e a metila ligada de modo terminal. A carbonilação de 2 resulta, inicialmente (20 min, 25 °C), na formação de um produto de adição ao poliedro metálico que, de acordo com estudos de espectroscopia de RMN de P{H} e C{H} a várias temperaturas, existe na forma de dois isomeros 4A and 4B (8:1) que diferem com relação à posição do grupo metila, e em seguida (40 °C, 7 h), à formação do produto de inserção de CO, [Ir4(CO)8{C(O)CH3}(μ4-η-Ph2PCCPh)(μ-PPh2)], 5. A carbonilação é reversível em ambos os estágios. A estrutura molecular de 5 é semelhante à de 2, com uma acila no lugar da metila. As reações de 2 com PPh3 e P(OMe)3 resultam nos produtos de substituição de CO, [Ir4(CO)7L(CH3)(μ4-η-Ph2PCCPh)(μ-PPh2)] (L = PPh3, 6 e P(OMe)3 7, respectivamente, ao invés dos produtos esperados de inserção de CO. Segundo estudos de RMN deH e P{H}, o composto 6 existe na forma de dois isômeros (1:1) que diferem com relação à posição da PPh3.

Carbon monoxide insertion into metal alkyl or aryl bonds is a textbook reaction 10 , but this process in alkyl and aryl containing carbonyl clusters has only been documented in rare cases 5,[11][12][13] .We have recently described the first study involving the carbonylation of a phenyl group co-ordinated to a cluster, [Ir 4 (CO) 8 (η 1 -Ph)(µ 3 -η 3 -PhPC(H)CPh)(µ-PPh 2 )] 12 .In this report, we describe the synthesis and characterisation of the methyl containing cluster [Ir 4 (CO) 8 (CH 3 )(µ 4 -η 3 -Ph 2 PCCPh)(µ-PPh 2 )] and its facile quantitative carbonylation to the isostructural acyl product [Ir 4 (CO) 8 {C(O)CH 3 }(µ 4 -η 3 -Ph 2 PCCPh)(µ-PPh 2 )] via a CO addition intermediate, characterised by multinuclear NMR spectroscopy.To our knowledge, this is the first example of a fully characterised stepwise reversible process involving a CO addition and CO insertion into a M-C alkyl bond in a cluster compound.A preliminary communication of part of this work has appeared 14 .

General
All manipulations and reactions were carried out under dry argon, unless otherwise specified, using standard Schlenk techniques.CH 2 Cl 2 was dried over CaH 2 , hexane and toluene over sodium and THF over potassium.Solvents were freshly distilled under Ar and freed from dissolved oxygen, where compatible, by freeze degassing before use.BuLi (1.6 M hexane solution), CH 3 I and 13 CH 3 I (Aldrich) were used as received, Ph 2 PC≡CPh 15 , [(µ-H)Ir 4 (CO) 10 -(µ-PPh 2 )] 16 and CO 17 were prepared by literature methods.The reactions were monitored by IR and NMR spectroscopies.Preparative TLC was carried out in air by using ca 2 mm thick glass backed silica plates (20 x 20 cm) prepared from silica gel GF 254 Type, Fluka, CH 2 Cl 2 -hexane (1:3) as eluent, unless otherwise specified, and the compounds were extracted from silica with CH 2 Cl 2 .IR spectra were obtained on a Bomen MB series IR instrument scanning between 2200 and 1500 cm -1 , using CaF 2 cells. 1 H, 13 C{ 1 H} and 31 P{ 1 H} NMR data were obtained on a Bruker AC 300/P instrument using deuterated solvents as lock and reference [ 1 H and 13 C, SiMe 4 , 31 P, 85% H 3 PO 4 (external)].Microanalyses were performed on a Perkin Elmer 2401 Elemental Analysis instrument at the Chemistry Institute of UNICAMP.FAB Mass spectrum was obtained on a VG 7070E-HF mass spectrometer using nitrobenzyl alcohol as the matrix at the Chemistry Department the University of Minnesota.

Preparation of the NMR sample of 4 for VT experiments
A solution of 2 (20.0 mg, 0.014 mmol) in CD 2 Cl 2 (1.5 mL) was added to a 5 mm NMR tube.CO was bubbled through the solution for 15 min at 0 °C.The NMR tube was sealed under a positive pressure of CO.

CO de-insertion reaction of the cluster [Ir
Compound 5 (20.0 mg, 0.014 mmol) was heated in THF (20 mL) under Ar at 60 °C for 4 h, after which time the solvent was evaporated and the residue dissolved in CH 2 Cl 2 .TLC of the red solution gave compound 2 quantitatively.

Reaction of [Ir
A solution of 2 (40.0 mg, 0.028 mmol) and PPh 3 (7.1 mg, 0.028 mmol) in THF (30 mL) was stirred for 2.5 h at 28 °C.The solvent was evaporated, the green residue dissolved in CH 2 Cl 2 and separated by TLC to give an olive green compound 6 (20.0 mg, 35%; Rf = 0.65); heavy decomposition was noted on the base line.Addition of P(OMe) 3 (3.2µL, 0.028 mmol) to a solution of 2 (40.0 mg, 0.028 mmol) in THF (30 mL) resulted in an immediate colour change from red to yellow and then dark green.After 10 min stirring at 28 °C, the solvent was evaporated and the residue dissolved in CH 2 Cl 2 ; TLC of the solution gave an olive green compound 7 (10 mg, 25%; Rf = 0.60); heavy decomposition was noted on the base line.IR (hexane, cm -1 ): ν CO 2054s, 2016s, 1993w, 1967m,  1953vw, 1944vw.

Solution structure of 2
The structural differences observed in 2 and 3 are reflected in the 31 P{ 1 H} NMR spectra of these compounds that have been extremely useful for rapid diagnosis of the structure of derivatives of this class of compounds 24 .
It has been shown that the metal atoms and the oxygen of the carbonyl ligands are potential sites for eletrophilic attack.It is clear, however, that the nature of the product may not reflect the actual site of attack of the electrophile, as cluster rearrangements may occur.Also, the different electronic and steric requirements of the two electrophiles may lead to the stabilisation of structures containing the H and Me in different positions (metal or oxygen).For example, low temperature 1 H-NMR studies have evidenced that protonation (at -60 °C) of [(µ-H)Ru 3 (µ-CO)(CO) 10 ] -occurs at the oxygen atom of a bridging CO ligand to give [(µ-H)Ru 3 (µ-COH)(CO) 10 ], with a subsequent rearrangement (> -30 °C) to the thermodynamically stable dihydride [H(µ-H)Ru 3 (CO) 11 ], via migration of the hydrogen to the metal frame 26,27 .The methylation (at 25 °C) of [(µ-H)Ru 3 (µ-CO)(CO) 10 ] -, however, gives the thermodynamically stable [(µ-H)Ru 3 (µ-COCH 3 )(CO) 10 ] 28 .
Assuming that formation of compounds 2 and 3 involves initial electrophilic attack of Me + and H + , respectively, at the oxygen of a carbonyl group of the anionic product from the reaction of [Ir 4 (CO) 10 (µ-PPh 2 )] - with Ph 2 PC≡CPh, the different positions occupied by the Me and H groups in the respective products, probably reflect: i) the relative ease of migration of these groups to the metal frame, i.e. the relative rates of CO de-insertion and also of CO dissociation; ii) the different steric and electronic requirements of the two groups.Steric effects probably play a minor role in the final position of the H and Me groups, considering that the ethyl cluster [Ir 4 (CO) 8 (CH 2 CH 3 )(µ 4 -η 3 -Ph 2 PCCPh)(µ-PPh 2 )], analogous to 2, and recently obtained from the reaction of the hydride compound 3 with ethylene, contains the ethyl group in the position occupied by the hydride ligand in the precursor 3 29 .With this argument in mind, the carbonylation reactions of the methyl and hydride clusters 2 and 3, respectively, were investigated.

Reactions of 2 and 3 with CO
Compound 3 does not react with CO (> 1 atm, toluene, 60 °C, 24 h) 9 .In contrast, compound 2 reacts with CO under mild conditions (1 atm, CH 2 Cl 2 , room temperature, 20 min) to give a yellow product 4 which, in the absence of CO, quickly reverts to the red starting material.Stirring yellow 4 under an atmosphere of CO for 24 h or heating it at 40 °C under CO for 7 h resulted in the CO insertion product [Ir 4 (CO) 8 {C(O)CH 3 }(µ 4 -η 3 -Ph 2 PCCPh)(µ-PPh 2 )], 5 (Scheme 1).This compound is stable under Ar, however when heated in THF, at 60 °C, under Ar, for 6 h, it looses CO to give 2 quantitatively.This behaviour is in contrast with that observed for the acyl cluster [Os 3 (CO) 10 -(µ-I){C(O)CH 3 }] which, in the absence of CO, reacts to give, not only the CO de-insertion product, [Os 3 (CO) 10 (µ-I)CH 3 ], but also [Os 3 (CO) 10 (µ-I){µ 2 -η 2 -C(O)CH 3 }] as the result of a change in the co-ordination mode of the acyl ligand from terminal to bridging 11 .Compound 5 was fully characterised by analytical and spectroscopic data (see Experimental and Table 1) and by an X-ray diffraction analysis.

Solution structure of 5
The IR spectrum of 5 in the ν CO region shows a weak absorption at 1695 cm -1 , characteristic of an acyl group, besides terminal ν CO bands.The 1 H-NMR spectrum consists of a singlet at δ 2.43 attributed to methyl group of the C(O)CH 3 ligand and a multiplet at δ 6.60-8.attributed to the Ph hydrogens.The 13 C{ 1 H} NMR spectrum of 5 enriched with 13 CH 3 exhibits a singlet at δ 48.43, attributed to the methyl group of the C(O)CH 3 ligand.The similarity of the 31 P{ 1 H} NMR spectra of 5 and 2 with respect to both chemical shifts and J P-P (see Table 1) strongly suggests that the two compounds have very similar structures, only with an acyl group in 5 in place of the methyl in 2.

Molecular structure 5
An X-ray diffraction study of compound 5 confirms that the solid state structure of 5 is very similar to that of 2 indeed (Fig. 3).Their crystals are isomorphous (i.e. the two complexes crystallise with extremely similar packing arrangements).Relevant structure parameters are compared to those of 2 in Table 2

Solution structure of 4
The various attempts at crystallising this compound for an X-ray diffraction analysis only led to crystals of the starting material 2 (under Ar, at -5 °C) or to those of the CO insertion product 5 (under CO at -5 °C).As it could not be obtained in its pure form in the solid state, compound 4 was characterised only by solution IR and NMR spectroscopies.Terminal ν CO bands and a weak bridging ν CO at 1851 cm -1 were observed in the IR spectrum of 5 which is rather different from that of 2 (see Experimental).
The room temperature 1 H-NMR spectrum of 4 in CD 2 Cl 2 consists a multiplet at δ 6.50-7.60 attributed to the phenyl groups and of a very broad peak at about δ 0.7 possibly due to a methyl group; peaks at δ 1.28 and 2.43 due to small amounts of compounds 2 and 5 were also noted.As this spectrum suggested some kind of fluxionality, the 1 H, 31 P{ 1 H} and 13 C{ 1 H} NMR spectra of 4 were investigated at 298, 273 and 190 K and are shown in Figs. 4 and 5.
The limiting 31 P{ 1 H} NMR spectrum of 4 in CD 2 Cl 2 , at 190 K displays two sets of signals of approximate relative intensities 8:1 consisting of two doublets at δ -13.2 and 136.7 (J P-P = 18 Hz) assigned to compound 4A, and two peaks at δ 0.1 (d, J P-P = 24 Hz) and 127.9 (dd, J P-C = 13 Hz) assigned to compound 4B, besides peaks due to small amounts of compounds 2 and 5.In both cases, the high frequency peaks were attributed to the phosphido P2 and the other ones to the Ph 2 PCCPh P1 nuclei.As the temperature is raised all signals broaden and the room temperature spectrum shows only two broad singlets at δ -14.4 and 134.9.The limiting 13 C{ 1 H} NMR spectrum of a sample of 4 enriched with 13   = 13 Hz) assigned to the methyl groups of compound 4A and 4B, respectively.As the samples were not enriched with 13 CO, and due to solubility problems, the carbonyl and phenyl regions could not be studied.Raising the temperature also led to the broadening of these resonances, and the room temperature spectrum shows only a broad peak at δ -36.10.
The limiting 1 H-NMR spectrum of the same sample of 4 at 183K shows only one doublet at δ 0.71 (J H-C = 120.8Hz) assigned the methyl group of 4A, besides signals due to the phenyl protons at δ 8.16 -5.34; the fact that the signal due to isomer 4B could not be observed might be due, not only to its low concentration (additionally, the protons would couple both with the carbon and P2), but also because the region was not totally free of impurities.Furthermore, no signal was detected in the acyl region (δ 2.43 for 5) which is an indication that 4A and 4B are the products from the addition of one or more CO molecules to 2.
The VT 13 C{ 1 H} and 31 P{ 1 H} NMR spectra show that 4A 4B undergo inter-conversion, therefore suggesting that the two compounds are isomers.The fact that the 64 electron hydrido species 3 does not undergo CO addition under the same conditions 29 strongly suggests that 4A and 4B result from the addition of a single CO to the 62 electron compound 2. The drastic changes observed both in the 31 P chemical shifts and in the J P1-P2 values indicate that the CO addition to the metal polyhedron of 2 leads to important structural changes involving both Ph 2 PCCPh and µ-PPh 2 ligands (see Table 1).Thus, the 64 electron clusters "[Ir 4 (CO) 9 (CH 3 )(µ 4 -η 3 -Ph 2 PCCPh)(µ-PPh 2 )]", 4A and 4B, would be analogous to 3, that is they must exhibit a distorted butterfly metal frame without one of the wing edges, however, it is clear, from the 31 P-NMR data, that the missing edge in the polyhedron of 4A and 4B is different from that in 3. Indeed, whilst the metal atoms bridged by the phosphido group [Ir(2) and Ir(3)] do not interact in 3 (δ 29.8), in 4A and 4B they do (δ 136.7 and 127.9, respectively).As in the precursor 2 the two electron deficient sites are Ir(1) and Ir(4), it seems reasonable to suggest that nucleophilic attack of the CO may have occurred at either positions, leading, in both cases, to the cleavage of the Ir(1)-Ir(4) bond, as illustrated in Scheme 2. This metal frame opening was accompanied by a decrease in the P(1)-Ir(3)-P(2) angle, as indicated by the decrease in the J P1-P2 values from 193 Hz (135.6°) in the precursor 2 to 18 and 24 Hz in isomers 4A and 4B, respectively, close to the value observed for the hydride 3 (42.2Hz; 103.3°).These data suggest that CO addition to 2 has led to a rearrangement of the µ 4 -η 3 -Ph 2 PCCPh ligand, which is proposed to interact with the metal frame of 4A and 4B as in the hydride 3. The two isomers 4A and 4B would only differ with respect to the position of the CH 3 ligand on the Ir(4) atom.In 4B, this ligand would occupy a transoid 4A, 4B  position with respect to the phosphide P(2) (position a in Scheme 2), as suggested by the 3 bond J P2-C = 13 Hz.In compound 4A, the CH 3 ligand could occupy positions b or c which do not favour geometrically this coupling.
Reactions of 2 with P(OMe) 3 and PPh 3 In order to find out whether nucleophiles other than CO, such as tertiary phosphines and phosphites, would induce migratory CO insertion in cluster 2, as observed for various mononuclear systems 30 , and also in an attempt to determine the possible of nucleophilic attack on this cluster, the reactions of 2 with one equivalent of PPh 3 and P(OMe) 3 were investigated.In both cases the reactions proceed in CH 2 Cl 2 , at room temperature, to yield the green CO substitution species, [Ir 4 (CO) 7 L(CH 3 )(µ 4 -η 3 -Ph 2 PCCPh)(µ-PPh 2 )] (L = PPh 3 6 and P(OMe) 3 7), instead of the expected CO inserted products.Compounds 6 and 7 were isolated, after purification by TLC, in 35 and 25% yields, respectively, besides some unreacted starting material (around 20%); in both reactions, a fair amount of decomposition material was noted on the base line of the TLC plates.It is interesting that, only in the case of the reaction with P(OMe) 3 , an initial colour change occurs, from red to yellow, which only lasts for a few seconds, before the solution turns dark green.This observation suggests formation of a transient P(OMe) 3 addition intermediate of the type "[Ir 4 (CH 3 )(CO) 8 {P(OMe) 3 }(µ 4 -η-η 3 -Ph 2 PCCPh)(µ-PPh 2 )]", possibly analogous to the yellow CO addition intermediate 4. Compound 7 was characterised only by IR spectroscopy as it undergoes decomposition in solution.The PPh 3 derivative was characterised by spectroscopic and analytical data (see Experimental and Table 1), however, suitable crystals for an X-ray analysis could not be obtained.
The IR spectra of 6 and 7 are very similar and show the presence of only terminal CO ligands; compared to the spectrum of compound 2, both spectra are shifted towards lower ν CO by about 15 cm -1 , as expected for a CO mono-substitution.The 1 H-NMR spectrum of 6 in CDCl 3 at 22 °C contains signals assigned to phenyl groups and to two cluster bound methyl groups at δ 0.28 and 0.43, of approximate intensities 1:1, suggesting the presence of two isomers.
The room temperature 31 P{ 1 H} NMR spectrum of 6 consists of two sets of three signals with relative intensities 1:1, thus confirming that the compound exists as a mixture of two isomers, 6A and 6B.The P1 nucleus (Ph 2 PCCPh) in 6A and 6B appears at δ -52.2 (J P1-P2 = 228; J P1-P3 = 13 Hz) and -41.90 (J P1-P2 = 189 and J P1-P3 = 10 Hz), respectively, and P2 (µ-PPh 2 ) in 6A and 6B appears at δ 77.30 (J P1-P3 = 10 Hz ) δ 65.9, respectively.A comparison of these chemical shifts and J P1-P2 values with those observed for 2 (δ-47.2,P1 and 73.8, P2; J P1-P2 = 193 Hz) suggests that the basic phosphorus ligands arrangement in 2 is maintained in both isomers.The fact that the P(1)-Ir(3)-P(2) angle seems to have been more disturbed in 6A than in 6B, and that P3 (PPh 3 ) in isomer 6A (δ 13.7) couples with both P2 and P1 (~13 Hz), whilst in 6B (δ -9.1) it only couples with P1 (10 Hz), tends to indicate that PPh 3 is bonded to Ir(3) in 6A and to Ir(1) in 6B (see Scheme 3).Assuming that the CO ligands disposition on the metal polyhedron of 2 remains the same upon substitution of a CO for a PPh 3 , as is often the case, it is possible to propose plausible structures for isomers 6A and 6B, based on a correlation between the phosphorus nuclei coupling constants in 2, 6A and 6B and specific dihedral angles in 2. In the case of isomer 6A, the PPh 3 on Ir(3) could occupy the positions of either CO (5) or CO (6).It is impossible to differentiate between the two possibilities, as P( Considering that Ir(1) and Ir(4) are the electron deficient metal centres of compound 2 (vide supra), nucleophilic attack of L [L = PPh 3 or P(OMe) 3 ] could be expected to occur at either position to yield L addition, CO substitution or even CO insertion products.Although migratory CO insertion is usually the preferred path in mononuclear chemistry 10,30 , not a single example involving a carbonyl cluster has been described in the literature yet.Our work indicates a clear preference for the CO substitution path which, in the case of the reaction with P(OMe) 3 , seems to occur via formation of a transient P(OMe) 3 adduct.In other words, in this system, CO dissociation from the adduct is faster than CO migratory insertion.Finally, contrarily to expectation, isomer 6A contains the PPh 3 ligand bonded to Ir(3), instead of Ir(4).It is possible that ligand rearrangement occurred, after addition of L, or even after CO substitution.Recent studies have evidenced that migration of phosphine ligands on cluster compounds is not uncommon process 31,32 .

Concluding Remarks
The lability of compound 2 might be associated with the charge imbalance in the cluster, which would favour nucleophilic attack of CO or phosphines at one of the electron poor metal centres, to yield addition products.In contrast with mononuclear 18 electron species, cluster compounds can accommodate additional ligands via cleavage of M-M bonds.In this context, the role of the µ 4 -η 3 -Ph 2 PCCPh bridging ligand is crucial in maintaining the metal atoms together and in inducing the polarisation of the Ir-Ir bonds.
), i.e. the C-C bond is parallel to the Ir(2)-Ir(4) segment, whilst in 2 this bond is parallel to the Ir(1)-Ir(4) bond.In both clusters the phosphido ligand bridges the Ir(2)-Ir(3) segment, but only in the 62 butterfly methyl cluster 2 these two metal atoms interact formally.As a result of the additional CO ligand, in the 64 electron spiked triangular cluster 3, this distance is rather long [Ir(2)---Ir(3) 3.686(2) Å].Furthermore, the PPh 2 ligand in 3 is pushed towards the Ir(1)-Ir(2)-Ir(3) plane, compared to the methyl cluster 2, possibly due to the presence of the additional hydride on Ir(3) and to the lengthening of the Ir(2)-Ir(3) distance.

Figure 4 .
Figure 4. VT 13 C{ 1 H} NMR spectra of isomers 4A and 4B in the methyl region.