Vaska's complex
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| Vaska's complex | |
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| General | |
| Systematic name | Chlorocarbonylbis (triphenylphosphine)iridium(I) |
| Other names | Iridium(I)bis(triphenylphosphine) carbonyl chloride Vaska's complex Vaska's compound |
| Molecular formula | IrCl(CO)[P(C6H5)3]2. |
| SMILES | ? |
| Molar mass | 780.25 g/mol |
| Appearance | yellow crystals |
| CAS number | [14871-41-1] |
| EINECS number | 238-941-6 |
| Properties | |
| Density and phase | ? g/cm3, ? |
| Solubility in water | insol |
| Other solvents | sparingly: benzene |
| Melting point | 215 °C (decomp.) |
| Structure | |
| Coordination geometry |
sq. planar |
| Crystal structure | ? |
| Dipole moment | ? D |
| Hazards | |
| MSDS | External MSDS |
| Main hazards | none |
| NFPA 704 | |
| R/S statement | R: none S: 22-24/25 |
| RTECS number | ? |
| Supplementary data page | |
| Structure and properties |
n, εr, etc. |
| Thermodynamic data |
Phase behaviour Solid, liquid, gas |
| Spectral data | UV, IR, NMR, MS |
| Related compounds | |
| Other anions | IrI(CO)[P(C6H5)3]2 |
| Other cations | RhCl(CO)[P(C6H5)3]2 |
| Related compounds | Pd[P(C6H5)3]4 |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references |
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Vaska's complex is the trivial name for trans-chlorocarbonylbis(triphenylphosphine)iridium(I): trans-IrCl(CO)[P(C6H5)3]2. It is a square planar diamagnetic organometallic complex consisting of a central iridium atom bound to two mutually trans-triphenylphosphine ligands, carbon monoxide, and chloride. The complex was prepared and analyzed by Di Luzio and Vaska in 19611. Vaska's complex can undergo oxidative additions and is notable for its ability to bind to O2 reversibly. It is a bright yellow crystalline solid.
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[edit] Preparation
The synthesis involves heating virtually any iridium chloride salt with P(C6H5)3 with a CO source. The most popular method uses dimethylformamide (DMF) as a solvent. Sometimes aniline is added to accelerate the reaction. Another popular solvent is 2-methoxyethanol. The reaction typically is conducted under nitrogen. In the synthesis, triphenylphosphine serves as both a ligand and a reductant, and the carbonyl ligand is derived by decomposition of dimethylformamide probably via a deinsertion of an intermediate Ir-C(O)H species. The following is a possible balanced equation for this complicated reaction2.
- H2IrCl6 + 3.5P(C6H5)3 + HCON(CH3)2 + 4C6H5NH2 + 1.5H2O → IrCl(CO)[P(C6H5)3]2 + (CH3)2NH2+Cl- + 1.5 OP(C6H5)3
Typical sources of iridium used in this preparation are IrCl3.xH2O and H2IrCl6.
[edit] Reactions
Studies on Vaska's complex provided a conceptual framework for homogeneous catalysis. Vaska's complex, with 16 valence electrons, is considered "unsaturated" and can thus bind to one two-electron or two one-electron ligands to become electronically saturated with 18 valence electrons. The addition of two one-electron ligands is called oxidative addition. Upon oxidative addition, the oxidation state of the iridium increases from Ir(I) to Ir(III). The four-coordinated square planar arrangement in the starting complex converts to an octahedral, six-coordinate product. Vaska's complex undergoes oxidative addition with conventional oxidants such as halogens, strong acids such as HCl, and other molecules known to react as electrophiles, such as iodomethane (CH3I).
An interesting characteristic of Vaska's complex is that it binds O2 reversibly.
- IrCl(CO)[P(C6H5)3]2 + O2 ↔ IrCl(CO)[P(C6H5)3]2O2
The dioxygen ligand is bonded to Ir(I) via both oxygen atoms, so-called side-on bonding. In myoglobin and hemoglobin, O2 binds "end-on," attaching to the metal via only one of the two oxygen atoms. The oxygenation reaction is carried out simply by bubbling O2 through a solution of Vaska's complex in toluene, which results in a colour change from yellow to orange. The resulting dioxgen adduct reverts to the parent complex upon heating in boiling benzene solution.
[edit] Spectroscopy
Infrared spectroscopy can be used to characterize the oxidative addition products with Vaska's complex because the reactions induce characteristic shifts of the stretching frequency of the coordinated carbon monoxide.{fn|3}}. These shifts are dependent on the amount of π-back bonding allowed from the newly associated ligands. The appearance of CO stretching frequencies for Vaska's complex and oxidatively added ligands have been documented in literature4.
- Vaska's Complex: 1967 cm-1
- Vaska's + O2: 2015 cm-1
- Vaska's + MeI: 2047 cm-1
- Vaska's + I2: 2067 cm-1
Oxidative addition to give Ir(III) products reduces the π-bonding from Ir to C, which causes the increase in the frequency of the carbonyl stretching band. The stretching frequency changes depending on the ligands that have been added, but is always greater than 2000 cm-1 for an Ir(III) complex.
[edit] References
- Note 1: L. Vaska and J.W. DiLuzio J. Am. Chem. Soc., 83, 2784 (1961).
- Note 2: Girolami, G.S.; Rauchfuss, T.B.; Angelici, R.J. Synthesis and Technique in Inorganic Chemistry, Third ed.; University Science Books.: Sausalito, 1999, pp190.
- Note 3: L. Vaska and J.W. DiLuzio J. Am. Chem. Soc., 84, 679 (1962).
- Note 4: Crabtree, R. The Organometallic Chemistry of the Transition Metals; Third Ed.; John Wiley & Sons, Inc.: Canada, 2001, pp152.
