Oxidative Addition Reactions of I 2 with [ HIr 4 ( CO ) 10n ( PPh 3 ) n ( μ-PPh 2 ) ] ( n = 1 and 2 ) and Crystal and Molecular Structure of [ HIr 4 ( μ-I ) 2 ( CO ) 7 ( PPh 3 ) ( μ-PPh 2 ) ]

As reações dos compostos [HIr 4 (CO) 10-n (PPh 3 ) n (μ-PPh 2 )] [n = 0, (1); 1, (2) e 2, (3)] com I 2 foram investigadas. O composto 1 não reage, porém, a substituição de ligante(s) CO por PPh 3 leva à ativação do cluster. De fato, ambos os compostos 2 e 3 reagem com I 2 em condições brandas com a formação de [HIr 4 (μ-I) 2 (CO) 7 (PPh 3 )(μ-PPh 2 )] (4), como resultado da adição oxidativa de I 2 e dissociação de dois ligantes CO ou de um CO e uma PPh 3 , respectivamente. A estrutura molecular de 4, determinada por um estudo de difração de raios X, exibe um arranjo metálico na forma de uma borboleta, cujas asas se encontram ligadas pelo ligante em ponte μ-PPh 2 ; os átomos que formam o corpo da borboleta se encontram ligados a um ligante μ-H, e cada uma das duas asas contém um ligante iodo ligado em ponte; todos os átomos metálicos contêm dois ligantes CO terminais, exceção feita de um dos átomos do corpo da borboleta que contém um ligante CO e um PPh 3. O cluster exibe a menor distância média de ligação Ir–Ir observada até hoje em derivados do composto 1, o que está de acordo com o fato de o estado de oxidação médio dos centros metálicos (+1) ser relativamente alto para clusters metálicos carbonílicos.


Introduction
The cluster compound [HIr 4 (CO) 10 (µ-PPh 2 )] (1) undergoes facile CO substitution with phosphines and phosphites to yield the mono-and di-substituted derivatives [HIr 4 (CO) 10-n L n (µ-PPh 2 )] (n =1 and 2). 1 Our recent studies indicate that these derivatives behave differently from 1 in the presence of molecules or fragments that may undergo oxidative addition.For example, although 1 does not react with alkynes and alkenes, 2 phosphines containing unsaturated fragments, such as Ph 2 PC≡CPh, can interact further with 1 to yield µ 4 -η 3 -Ph 2 PCCPh containing species and products resulting from P-C bond activation, 3,4 further hydrometallation 5 or P-C bond formation. 6Furthermore, we have found that oxidative addition of the PhP(CPh) 2 ring is extremely sensitive to the metal frame electronic density and can be delicately tuned by the presence of PPh 3 . 7We report herein our comparative studies of the reactivity of 1 and [HIr 4 (CO) 10-n (PPh 3 ) n (µ-PPh 2 )] (n =1, 2 and n=2, 3) with I 2 , MeI and O 2 , which were carried out in an attempt to make a further parallel between their chemistry and that of Vaska's compounds. 7,8

Reaction of 3 with I 2
The reaction was carried out following same procedure described above, using equimolar toluene solutions of I 2 (8 mg, 0.031 mmol in 20 mL) and cluster 3 (50 mg, 0.031 mmol in 20 mL).The mixture was kept under stirring at the same temperature, but for 5 h.Separation of the crude mixture was carried out as described above and yielded the starting material 3 (7.5 mg, 15%) and cluster 4 (20.7 mg, 40%); a number of very low yield products were separated out on the tlc plates, but were not isolated.

Attempts at reacting 2 and 3 with O 2
Oxygen was bubbled through stirred solutions of 2 (40 mg, 0.027 mmol) and 3 (50 mg, 0.031 mmol), in toluene (20 mL) at 45 o C.After 24 h no changes were detected in the colour or in the 31 P{ 1 H} NMR spectra of the solutions.

Attempts at reacting 2 and 3 with MeI
A large excess of MeI (~30 equiv) was added to solutions of compounds 2 (40 mg, 0.027mmol) and 3 (50 mg, 0.031 mmol) in toluene (20 mL) and the mixtures were kept at 20 o C.After 24 h no changes were detected in the colour or in the 31 P{ 1 H} NMR spectra of the solutions.The temperature of both solutions was then gradually increased to 70 o C, but no reaction was observed after 3 days.

Crystal structure determination
Crystal data and details of measurements for compound 4 are summarised in Table 1.Diffraction intensities were collected at room temperature on an Enraf-Nonius CAD-4 diffractometer equipped with a graphite monochromator (MoKα, λ = 0.71069 Å).Intensity data were reduced to F 0 2 .Crystals were obtained from CH 2 Cl 2 /hexane by slow evaporation.The cluster was found to cocrystallise with one water molecule per formula unit.The structure was

Results and Discussion
Reactions of [HIr 4 (CO)  Treatment of compounds 2 and 3 with equimolar amounts of iodine, in toluene, at 25 o C gave in both cases the air stable red compound [HIr 4 (µ-I) 2 (CO) 7 (PPh 3 )(µ-PPh 2 )] (4) in 50 and 40% yields, respectively, after tlc and crystallization from CH 2 Cl 2 /hexane.In both cases the starting materials were recovered (~ 15%), but the yields of the reactions could not be improved by using excess I 2 which led to an increase in the number of side and decomposition products visualized on the tlc plates.
Under the same conditions, compound 1 did not react with I 2 and under more forcing conditions, decomposition of 1 was observed.Thus, once again, 7 activation of cluster 1 towards oxidative addition was achieved upon CO substitution with PPh 3 .However, this process cannot be generalized: indeed, compounds 1-3 did not react with MeI under a variety of conditions (25-70 o C).Furthermore, contrarily to our expectations, neither compound 2 nor cluster 3 reacted with O 2 in toluene, even at 45 o C, for 24h.
Compound 4 was first characterized by a combination of IR and 1 H and 31 P NMR spectroscopy and satisfactory microanalysis (see Experimental).Only terminal ν CO bands are observed in the IR spectrum.The 31 P{ 1 H} NMR spectrum exhibits two doublets at δ 62.1 [J(PP) 8Hz] and 11.1 attributed to the µ-PPh 2 and PPh 3 phosphorus nuclei, respectively.The shift of the µ-PPh 2 phosphorus resonance to lower frequency compared to those of both starting materials (> δ 250) suggested substantial increase in the µ-PPh 2 bridged Ir-Ir distance. 12The 1 H NMR spectrum shows a resonance due to a hydride ligand at δ -15.5 [J(HP) 8 and 2 Hz] and a multiplet at δ -7.0-8.0 due to the phenyl hydrogens.The small coupling observed between the hydride and the µ-PPh 2 phosphorus nucleus compared with those of the starting materials [J(HP) > 50Hz] indicated that in 4 the two ligands are not oriented on the same plane as in clusters 1-3. 1 Because the spectroscopic data did not define the structure of 4, a single-crystal X-ray diffraction study was undertaken.
It is known that the outcome of the reactions of cluster compounds with I 2 depends on the type of cluster and charge distribution within it, 17 but systematic studies are rare.It has recently been shown that the reaction of [Os 3 (CO) 12 ] with I 2 proceeds via a cationic product, [Os 3 (CO) 12 (µ-I)] + , with the µ-I + (2 electron donor) in place of a M-M bond, which reacts further to yield the linear species cis,cis-[Os 3 (CO) 12 I 2 ] containing two terminal I atoms (1 e - 0 donors). 18Oxidative addition of I 2 to neutral clusters may also result in compounds containing one I atom bonded in a terminal (1 e - 0 donor) and another, in a bridging µ 2 -(3 e - 0 donor) fashions, and occurs with concomitant cleavage of two M-M bonds. 14,15Furthermore, excess I 2 has been found to lead to cluster breakdown. 19hus, the CO dissociation path followed in the formation of 4 (instead of the normal M-M cleavage) seems to be at least in part responsible for the shortening of Ir-Ir bonds after addition of I 2 and bridging by the µ-I atoms.We also argue that the short average Ir-Ir bond distance in 4 [2.698(2)Å, excluding the non-bonding distance], the shortest so far observed for derivatives of 1, is associated to the relatively high formal average oxidative state (that is for carbonyl cluster compounds) of the Ir atoms, +1.In comparison, the Ir atoms in 1 and 2 are on average +1/2 and the average Ir-Ir bond distances in these clusters are much longer, 2.737(2) and 2.751(2) Å, respectively; the average Ir-Ir bond slight expansion upon substitution of the π-acceptor CO in 1 with the σ-donor PPh 3 in 2 is associated with the resulting increase in the cluster electron density.Furthermore, those differences do not seem to be related to the different metal polyhedra in 1 Scheme 1.

Figure 2 .
Figure 2. Space-filling representation of the packing arrangement of crystalline 4 (projection in the ac-plane), showing the iodine…iodine contacts and the positions of the water molecular pairs in cavities formed by four cluster units.Phenyl groups are not shown for the sake of clarity.

Table 1 .
Crystallographic data for compound 4