Iridium organometallic complexes
Introduction
Iridium organometallic complexes are applied as catalysts, light-emitting diodes, dye-sensitized solar cells, and luminescent agents for cells imaging.1 Due to the wide application area, the scientists perform research on preparation, characterization and physical and chemical properties of iridium organometallic complexes.
The oxidation state in complex compound refers to the charge of the metal, which it would have in a simple ionic compound. The configuration of d-orbitals (dn) indicates the number of valence electrons in free metal of the same oxidation state. The iridium electronic configuration is [Xe] 4f145d76s2, yet the s-electrons transfer to d-orbital to form 5d9. The dn calculates:
dn = d N – c – a ,
N – the number of valence electrons, c is the complex charge, a is the oxidation state. Table 1 presents the possible relationship between the iridium oxidation state and dn configuration.2
Relationship between the Oxidation State and dn Configuration2
Ligands and Coordination Number
Ligand is a molecule that forms a coordination bond with the iridium atom. There are numerous ligands, which can form organometallic compounds. Table 2 presents the simple classification by the structure of bond.2
Common Organic Ligands and Their Electron Counts2
There is also a classification of ligands used for more complicated ligands. Thus, benzene is considered as three C = C ligands (L3). The same way, η3-allyl group (CH2 = CH - CH2-) is LX combination of C = C and alkyl RCH2-.2
The ligands for organometallic complexes can be formed by bridging. There are bridges with two independent bridge bonds, with a single delocalized bridge bond, zero-electron ligands. The zero-electron ligands class is characterized by stable electron count of iridium atom. The electrons are distributed between the groups of ligand molecule, and iridium electron count remains unaltered. Figure 1 illustrates the zero-electron ligand complex.2
Figure 1. Iridium organometallic complex with zero-electron ligand.2
Coordination number is the number of ligands that the central atom is capable of bonding. The number is defined by the number of electron pairs involved in bonding between the metal and ligand. The most common coordination numbers are 2, 4, and 6; however, any number between 2 and 10 is possible.3 Each coordination number is associated with certain geometric figure:
- 2: linear or bent structure;
- 3: equilateral triangular or trigonal pyramidal;
- 4: tetrahedral and square planar;
- 5: trigonal bipyramidal or square pyramidal;
- 6: octahedron or distorted octahedron;3
The complexes are formed with the central atom of transition metal. The transition metals have small ionic size and high charge, the available orbitals and energy state, and are characterized with the various oxidation states.3
Chemical properties
Iridium is capable of coordinating with numerous ligands. The complexes are lipophilic, which makes them favourable for cells, and they are localized in cytoplasm.1 Once the iridium atom is bonded with ligand, it loses the properties of the metal, and receives properties of the complex-forming ligand. 3
Iridium-arene complexes exist for iridium(I) and (III). The iridium-arene complexes (I and III) are air-stable crystalline solids. The complexes are not stable: arene can be displaced from iridium, if treated with solvent. The polycyclic aromatic complexes are less stable, and can be decomposed by dimethyl sulfoxide in 10 minutes.4
Physical properties
The iridium complexes are analysed with 1H and 13C NMR, and IR spectroscopy, X-ray crystallography. The functional groups and atoms are identified by the standard procedures. Mass spectrometry is used to determine the molecular weight.5
The NMR characterization is performed in acetone or dimethyl sulfoxide. The iridium-arene complexes exhibit arene resonances in regions higher than for the pure arene both on 1H and 13C NMR. The same is observed for C-H coupling of ring atoms (174-180 Hz comparing to 158 Hz for the free arene).1
Qin et al. studied the properties of [Ir(ppy)2(L)]PF3 (ppy is 2-phenylpyridine, L is 4’,5’-dimethyldithiotetrathiafulvenyl[4,5-f][1-10]phenanthroline). The electrochemical properties were studied with cyclic voltammetry. The compound showed two reversible oxidation states at E11/2 = 0.9 and E21/2 = 1.13 V. These states are attributed to the reversible oxidation of tetrathiafulvalene (TTF) to radical cation with the next transformation to the dication form. The iridium core has electron-withdrawing properties, which cause the shift of the oxidation potential to the higher regions comparing to L-compound. The electrostatic interaction takes place in TTF2+ and iridium, and there is no oxidation wave of iridium core.6
The light absorption of the molecule is attributed to MLCT electronic transition with d-orbitals and ligand. The absorbance bands appear in different regions, depending on the ligand coordinated to the central iridium atom.5
The absorption complex is characterized with the intense band at 350 nm (spin-allowed interligand transition), moderate band in the range 350-450 nm, and the weak band above 450 nm (these belong to charge transfer transition).6
The photochemical properties were studied with UV-vis-NIR spectrometry. The absorbance at 262 and 298, and 420-600 nm are observed. At oxidation, these bands disappear and decrease, respectively, due to formation of dicationic compound. The iridium complex is luminescent at room temperature in dichloromethane solution, and this can be attributed to the photoinduced electron transfer efficiency of TTF moiety.6
Bozec and Guerchais present the detailed information on photophysical properties of iridium complexes.7 The neutral complexes Ir(C^N)3 type are characterized with metal-to-ligand charge transfer (MLCT, electron is transferred from Ir d-orbital to a vacant π* of the ligand) and ligand-cantered transition (LC, electron is transferred between π orbitals of the ligand). Phosphorescence appears when spin-orbit coupling is originated from mixture of 3LC and 3MLCT excited states; the emission state is characterized with the lowest energy values.7
Figure 2 illustrates the structure of Ir(C^N)3 complexes. They exhibit emission at 77 K at λmax = 390 – 450 nm for 2.5 – 32 µs, yet only complex (1) emits at λmax = 482 nm, and fac-form of (4). It should be emphasized that photophysical properties of fac- and mer- forms are different. Mer-forms are characterized with a broad, red-shifted emission due to phenyl groups located opposite each other.7
The iridium organometallic complexes are applied for electroluminescent devices. Figure 3-5 illustrate the structures of various colour-emitting complexes.
Figure 2. Chemical structures of Ir(C^N)3 type complexes.7
Figure 3. Chemical structures of red emitting iridium complexes.7
Figure 4. Chemical structures of blue emitting iridium complexes.7
Figure 5. Chemical structure of green emitting iridium complex.7
Preparation of Cationic Iridium(I)-Arene Complexes
There are several reactions involved in preparation of arene complexes. These are: ligand substitution, elimination, or addition4.
There are three starting complexes for elimination reaction, which determine the applied reagents. If the starting reagent is [Ir2(µ-Cl)2(η4-cod)2], silver tetrafluoroborate is applied for elimination. The silver salt is applied in acetone media, and the iridium complex is in dichloromethane solution. The following processes take place: 1) the cationic iridium(I) solvate is formed; 2) silver chloride precipitate is removed by filtration. The yellow solution is treated with arene, and the desired iridium(I)-arene complex is prepared. Figure 6 illustrates the reaction.4
Figure 6. Preparation of iridium(I)-arene complex from iridium halide and silver(I) tetrafluoroborate.4
The various silver salts and arenes can be used to receive the iridium-arene complex. The highest yields are observed for [Ir2(µ-Cl)2(η4-Me2TFB)2] and AgClO reaction in dichloromethane (86%), 9,10-dihydroanthracene is applied as arene eliminator.4
The addition reaction can be exemplified by [IrH2(OCMe2)2(PPh2)2]BF4 and vinylarene, as illustrated in Figure 7.
Figure 7. Preparation of iridium(I)-arene complex from IrH2(OCMe2)2(PPh2)2]BF4 and vinylarene. 4
The reaction takes about 30 sec at room temperature, and is characterized by high yield. Generally, the reactions of this group have yields 80-97%.4
References
1. S. Ramachandra, F. Polo, F. Edafe, K. C. Schuermann, C. A. Nijhuis, P. Belser, W. F. Reus, G. M. Whitesides and L. De Cola. Pure Appl. Chem., 2011, 83, 4, 779–799.
2. R. H. Crabtree. The organometallic chemistry of the transition metals, Wiley, Connecticut, 2014.
3. R. Singh, Rajbir. Coordination Chemistry. New Delhi: Mittal Publ, 2002. Print.
4. J. M. O'Connor, in Science of synthesis: 1 : Category 1, Organometallics, C Aubert, J Houben, T Weyl and M Lautens, Thieme, Stuttgart [u.a.], 2001.
5. C. E. Housecroft. Coordination Chemistry Reviews, 1992, 115, 163-189.
6. J. Qin, S. Y, Deng, C. X. Qian, T. Y. Li, H. X. Ju and J. L. Zuo. J. Organomet. Chem. 2014, 750, 7-12.
7. H. Le Bozec and V. Guerchais. Molecular Organometallic Materials for Optics, Springer-Verlag, Berlin, 2010.
