Oxidative Addition Reactions of I2 with [HIr4(CO)10-n(PPh3 )n(μ-PPh2)] (n = 1 and 2) and Crystal and Molecular Structure of [HIr4(μ-I)2(CO)7 (PPh3)(μ-PPh2)]

Short Report

Oxidative Addition Reactions of I₂ with [HIr₄(CO)_10-n(PPh₃ )_n(μ-PPh₂)] (n = 1 and 2) and Crystal and Molecular Structure of [HIr₄(μ-I)₂(CO)₇ (PPh₃)(μ-PPh₂)]

Dario Braga^a, Fabrizia Grepioni^b, Maria D. Vargas^*,c and Claudio M. Ziglio^c

^a Dipartimento di Chimica "G.Ciamician", Universitá degli Studi di Bologna, Via Selmi 2, 40126 Bologna, Italy

^b Dipartimento di Chimica, Università di Sassari, Via Vienna 2, 07100 Sassari, Italy

^c Instituto de Química, Universidade Estadual de Campinas, CP 6154, 13083-970 Campinas - SP, Brazil

As reações dos compostos [HIr₄(CO)_10-n(PPh₃ )_n(μ-PPh₂)] [n = 0, (1); 1, (2) e 2, (3)] com I₂ foram investigadas. O composto 1 não reage, porém, a substituição de ligante(s) CO por PPh₃ leva à ativação do cluster. De fato, ambos os compostos 2 e 3 reagem com I₂ em condições brandas com a formação de [HIr₄(μ-I)₂(CO)₇(PPh₃)(μ-PPh ₂)] (4), como resultado da adição oxidativa de I₂ e dissociação de dois ligantes CO ou de um CO e uma PPh₃, 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-PPh₂; 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.

The reactions of the cluster compounds [HIr₄(CO)_10-n(PPh₃ )_n(μ-PPh₂)] [n = 0, (1); 1, (2) and 2, (3)] with I₂ have been investigated. Compound 1 does not react, however, the presence of PPh₃ in place of CO ligand(s) activates the cluster. Both compounds 2 and 3 react with I₂ under mild conditions to give [HIr₄(μ-I)₂(CO)₇(PPh₃)(μ-PPh ₂)] (4), as the result of oxidative addition of I₂ and dissociation of two CO ligands, or one CO and one PPh₃ ligands, respectively. The molecular structure of 4, determined by an X-ray diffraction study, exhibits a butterfly arrangement of iridium atoms with the wings spanned by a μ-PPh₂ ligand, the hinge bridged by a μ-H ligand, two hinge to wing tip edges bridged by iodine atoms and all metal atoms bearing two CO ligands, with the exception of one of the hinge atoms that contains a CO and a PPh₃ ligands. This cluster exhibits the shortest average Ir—Ir bond length [2.698(2) Å] observed so far for a derivative of 1 and this is in accord with the relatively high average oxidation state of its metal atoms (+1) for a carbonyl cluster compound.

Keywords: iridium cluster, oxidative addition, iodine, X-ray molecular structure

Introduction

The cluster compound [HIr₄(CO)₁₀(μ-PPh₂ )] (1) undergoes facile CO substitution with phosphines and phosphites to yield the mono- and di-substituted derivatives [HIr₄(CO)_10-nL_n(μ-PPh₂)] (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 Ph₂PC≡CPh, can interact further with 1 to yield μ₄-η³-Ph₂PCCPh containing species and products resulting from P-C bond activation,^3,4 further hydrometallation⁵ or P-C bond formation.⁶ Furthermore, 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 PPh₃.⁷ We report herein our comparative studies of the reactivity of 1 and [HIr₄(CO)_10-n(PPh₃ )_n(μ-PPh₂)] (n =1, 2 and n=2, 3) with I₂, MeI and O₂, which were carried out in an attempt to make a further parallel between their chemistry and that of Vaska's compounds.^7,8

Experimental

Iridium carbonyl (Strem Chemicals), triphenylphosphine and iodine (Aldrich), were used as purchased. THF was dried over sodium and benzophenone, CH₂Cl₂ over CaH₂, hexane and toluene over sodium. All solvents were freshly distilled and degassed before use. The compounds [HIr₄(CO)₁₀(μ-PPh₂ )] (1), [HIr₄(CO)₉(PPh₃)(μ-PPh₂)] (2) and [HIr₄(CO)₈(PPh₃)₂(μ-PPh ₂)] (3) were prepared by published methods.⁹ The progress of the reactions was monitored by analytical TLC (pre-coated plates, silica gel F254, 0.25 mm thick; E. Merck) and IR spectroscopy. The separation and purification of the reaction products were carried out by preparative TLC (2mm thick glass plates 20X20 cm, silica gel GF 254; Fluka). The reactions and manipulations were performed in typical Schlenk systems, under inert atmosphere of argon. Infrared spectra were recorded on a Bomem (FT-IR Michelson) spectrophotometer between 2200 and 1600 cm^-1 (ν_CO), ³¹P{¹H} and ¹H spectra on a Bruker AC 300P spectrometer using, as references, 85% H₃PO₄ (external) for the former and SiMe₄ for ¹H.

Reaction of 2 with I₂: preparation of [HIr₄(μ-I)₂(CO)₇-(PPh₃)(μ-PPh₂)] (4)

A solution of I₂ (7 mg, 0.027 mmol) in toluene (20 mL) was slowly added to a solution of 2 (40 mg, 0.027 mmol) in the same solvent (20 mL), at 25^o C, and the resulting mixture was kept under stirring for about 3h at room temperature. The solvent was then evaporated under vacuum and the residue dissolved in CH₂Cl₂ for purification by tlc (CH₂Cl₂/hexane; 2:3), to give some starting material 2 (6 mg, 15%), some brown decomposition material on the plates' base line and compound 4 (22.5 mg, 50%). Compound 4 was recrystallised from CH₂Cl₂/hexane to give red microcrystals of 4 (found: C, 26.6; H, 1.7. Calc. for C₃₇H₂₆O₇P₂ Ir₄: C, 26.9; H, 1.6%); IR ν_CO/cm^-1 2027s, 2045vs, 2012s, 2001s (hexane); ¹H NMR (CDCl₃, 300 MHz) δ -15.5 (dd, J(PH) 8 and 2 Hz, μ-H), 6.4-8.0 (m, Ph); ³¹P{¹H} NMR (CDCl₃,121.4 MHz) δ 11.1 (d, J(PP) 8, PPh₃), 62.1 (d, μ-PPh₂).

Reaction of 3 with I₂

The reaction was carried out following same procedure described above, using equimolar toluene solutions of I₂ (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 O₂

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 ^oC. After 24 h no changes were detected in the colour or in the ³¹P{¹H} 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 ^oC. After 24 h no changes were detected in the colour or in the ³¹P{¹H} NMR spectra of the solutions. The temperature of both solutions was then gradually increased to 70 ^oC, 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 F₀². Crystals were obtained from CH₂Cl₂/hexane by slow evaporation. The cluster was found to cocrystallise with one water molecule per formula unit. The structure was solved by direct methods, followed by least-squares refinements. The crystallographic program SHELXL-97¹⁰ was used for all calculations. All phenyl rings were refined as rigid groups. The hydrogen atoms were added in calculated positions (C_sp²-H 0.93 Å) and refined 'riding' on the corresponding C atoms. For graphical representations the program SCHAKAL99¹¹ was used.

Results and Discussion

Reactions of [HIr₄(CO)₁₀(μ-PPh₂)] (1), [HIr₄(CO)₉(PPh₃)-( μ-PPh₂)] (2) and [HIr₄(CO)₈(PPh₃)₂(μ -PPh₂)] (3) with I₂, MeI and O₂

Treatment of compounds 2 and 3 with equimolar amounts of iodine, in toluene, at 25 ^oC gave in both cases the air stable red compound [HIr₄(μ-I)₂(CO)₇(PPh₃)(μ-PPh ₂)] (4) in 50 and 40% yields, respectively, after tlc and crystallization from CH₂Cl₂/hexane. In both cases the starting materials were recovered (~ 15%), but the yields of the reactions could not be improved by using excess I₂ 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 I₂ 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 PPh₃. However, this process cannot be generalized: indeed, compounds 1-3 did not react with MeI under a variety of conditions (25-70 ^oC). Furthermore, contrarily to our expectations, neither compound 2 nor cluster 3 reacted with O₂ in toluene, even at 45 ^oC, for 24h.

Compound 4 was first characterized by a combination of IR and ¹H and ³¹P NMR spectroscopy and satisfactory microanalysis (see Experimental). Only terminal ν_CO bands are observed in the IR spectrum. The ³¹P{¹H} NMR spectrum exhibits two doublets at δ 62.1 [J(PP) 8Hz] and 11.1 attributed to the μ-PPh₂ and PPh₃ phosphorus nuclei, respectively. The shift of the μ-PPh₂ phosphorus resonance to lower frequency compared to those of both starting materials (> δ 250) suggested substantial increase in the μ-PPh₂ bridged Ir—Ir distance.¹² The ¹H 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-PPh₂ 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.

Crystal and molecular structure of 4

The molecular structure of 4 in the solid state is shown in Figure 1, together with the atomic labelling scheme. Relevant structural parameters are shown in Table 2. Cluster 4 exhibits a butterfly arrangement of metal atoms [dihedral angle 82.4(5)^o] with the wings spanned by a PPh₂ bridging ligand, as previously observed for [Ir₄(CO)₈(PCy₃)(μ-Ph₂PC(H)CPh)(μ-PPh ₂)] (I)⁴ and [Ir₄(CO)₉(μ-PhPCPh=CHPh)(μ-PPh₂)] (II).⁷ The bridging PPh₂ ligand is symmetrically bonded [Ir(3)—P(2) 2.269(5) and Ir(4)—P(2) 2.274Å] and the non-bonding Ir(3) Ir(4) distance, 3.528(2)Å, is shorter than in I and II [3.753(1) and 3.598(4) Å, respectively]. The Ir—Ir bond lengths range from 2.631(2) to 2.796(2) Å, the longest bond corresponding to the hinge of the butterfly, [Ir(1)-Ir(2)], where the bridging hydride ligand was located on the basis of potential energy calculations; this distance is only slightly longer than the equivalent μ-H bridged Ir—Ir bonds in 1 and 2 [2.769(3) and 2.785(2)Å, respectively]. The two shortest bonds in 4 correspond to the hinge to wing tip edges bridged by iodine atoms [Ir(1)—Ir(4) 2.631(2) and Ir(2)—Ir(3) 2.649(2)Å], which is rather unexpected, since bridging iodine ligands are invariably associated with elongation of M—M bond lengths.^13,14,15 The two iodine ligands are slightly asymmetric [Ir(1)—I(1) 2.735(2), Ir(4)-I(1) 2.787(2)Å and Ir(2)—I(2) 2.762(2) and Ir(3)—I(2) 2.753(3)Å] and practically perpendicular to the triangular wings of the butterfly [Ir(2)—Ir(1)-I(1) 90.51(6) and Ir(2)—Ir(4)—I(1) 91.12^o and Ir(1)—Ir(2)—I(2) 87.24(6) and Ir(1)—Ir(3)—I(2) 89.12(7)^o]; the slightly different bond distances seen in the two cases can be correlated to the presence of a PPh₃ ligand on one of the hinge metal atoms [Ir(2)] in place of a CO ligand on the other [Ir(1)]. All seven CO ligands are essentially linear and distributed two on each iridium atom, except for Ir(2) that contains a CO and a PPh₃ ligands. Considering that the two μ-I and the μ-PPh₂ ligands formally donate 3 electrons each to the cluster frame, the PPh₃ and CO ligands contribute with two electrons each and the hydride ligand with one, the cluster as a whole is a 62 electron system which is consistent with the butterfly structure.¹⁶

The molecules are arranged in the crystal in sort of "chains", so as to direct both iodine ligands towards iodines on adjacent clusters, as show in Figure 2 [I(1) I(1) 3.925(2), I(2) (I2) 4.009(2) Å]. Pairs of water molecules are trapped in cavities formed by four cluster units, and interact via hydrogen bonds within the pair [O(5) O(5) 2.989(5) Å].

The molecular structure of 4 reveals therefore that addition of I₂ to compounds 2 and 3 was accompanied by cleavage of an Ir—Ir bond and by dissociation of two CO ligands in the case of precursor 2 (see Scheme 1) and one CO and a PPh₃ in that of precursor 3, i.e., that two ligands (4 electrons) and one Ir—Ir bond (2 electrons) were replaced by two iodine atoms (6 electrons). To our knowledge, formation of 4 is the only example in the literature involving the alternative possible path to M—M bond cleavage, upon addition of I₂, i.e. CO dissociation.

It is known that the outcome of the reactions of cluster compounds with I₂ 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 [Os₃(CO)₁₂] with I₂ proceeds via a cationic product, [Os₃(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-[Os₃(CO)₁₂I₂] containing two terminal I atoms (1 e^—₀ donors).¹⁸ Oxidative addition of I₂ 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 μ₂- (3 e^—₀ donor) fashions, and occurs with concomitant cleavage of two M-M bonds.^14,15 Furthermore, excess I₂ has been found to lead to cluster breakdown.¹⁹

Thus, 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 I₂ 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 PPh₃ 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 and 2 on the one hand and 4, on the other, as the average Ir—Ir bond distance in the 62 electron butterflies I and II [2.749(1) and 2.7914(4) Å, respectively], whose Ir atoms are on average +1/2, are comparable to those observed for 1 and 2.

Supplementary Information

Crystallographic data (excluding structure factors) for the structure in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no CCDC 178269. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (fax: [phone] or e-mail: [e-mail]).

Acknowledgements

We thank FAPESP, PADCT and CNPq of Brazil (CMZ and MDV), MIUR (DB and FG), and the Universities of Bologna (DB) and Sassari (FG) for research grants.

3. Vargas, M. D.; Pereira, R. M. S.; Braga, D.; Grepioni, F.; J. Chem. Soc., Chem. Commun., 1993, 1008; ibid, J. Braz. Chem. Soc, 1999, 10, 35.

Received: February 6, 2002

Published on the web: July 1, 2002

FAPESP helped in meeting the publication costs of this article.

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Publication Dates

• Publication in this collection
  08 Oct 2008
• Date of issue
  Sept 2002

History

• Received
  06 Feb 2002