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)]

Journal of the Brazilian Chemical Society

Braga, Dario¹  Universitá di Sassari, Sassari, Italy¹  Grepioni, Fabrizia¹  Universidade Estadual de Campinas, Campinas, Brazil¹  Vargas, Maria D.¹  Universitá degli Studi di Bologna, Bologna, Italy¹  Ziglio, Claudio M.¹ 

关键词: iridium cluster;    oxidative addition;    iodine;    X-ray molecular structure;   

DOI  :  10.1590/S0103-50532002000500024

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

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-nLn(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 (νCO), 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 25o 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 νCO/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 (Csp2-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 νCO 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) 91.12o 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

【 授权许可】

Unknown