Iridium, ligands

Chiral phosphinodihydrooxazole iridium ligands are used to hydrogenate trisubstituted olefins in moderate yields and high enantioselectivity albeit of... 

Keywords Binuclear complexes, Bond activation. Homogeneous catalysis, Intermetallic cooperation. Iridium, Ligand migration, Trans effect... 

Although trialkyl- and triarylbismuthines are much weaker donors than the corresponding phosphoms, arsenic, and antimony compounds, they have nevertheless been employed to a considerable extent as ligands in transition metal complexes. The metals coordinated to the bismuth in these complexes include chromium (72—77), cobalt (78,79), iridium (80), iron (77,81,82), manganese (83,84), molybdenum (72,75—77,85—89), nickel (75,79,90,91), niobium (92), rhodium (93,94), silver (95—97), tungsten (72,75—77,87,89), uranium (98), and vanadium (99). The coordination compounds formed from tertiary bismuthines are less stable than those formed from tertiary phosphines, arsines, or stibines. 

The effect of the CFSE is expected to be even more marked in the case of the heavier elements because for them the crystal field splittings are much greater. As a result the +3 state is the most important one for both Rh and Ir and [M(H20)6] are the only simple aquo ions formed by these elements. With rr-acceptor ligands the +1 oxidation state is also well known for Rh and Ir. It is noticeable, however, that the similarity of these two heavier elements is less than is the case earlier in the transition series and, although rhodium resembles iridium more than cobalt, nevertheless there are significant differences. One example is provided by the +4 oxidation state which occurs to an appreciable extent in iridium but not in rhodium. (The ease with which Ir, Ir sometimes occurs... 

There is also clear evidence of a change from predominantly class-a to class-b metal charactristics (p. 909) in passing down this group. Whereas cobalt(III) forms few complexes with the heavier donor atoms of Groups 15 and 16, rhodium(III), and more especially iridium (III), coordinate readily with P-, As- and S-donor ligands. Compounds with Se- and even Te- are also known. Thus infrared. X-ray and nmr studies show that, in complexes such as [Co(NH3)4(NCS)2]" ", the NCS acts as an A -donor ligand, whereas in [M(SCN)6] (M = Rh, Ir) it is an 5-donor. Likewise in the hexahalogeno complex anions, [MX ] ", cobalt forms only that with fluoride, whereas rhodium forms them with all the halides except iodide, and iridium forms them with all except fluoride. 

Oxidative addition of XY substrates to [IrL2(/x-pz)]2 [La = (CO)2, cod] and [Ir(CD)(PPh3)(/i,-pz)]2 occurs via a two-center, two-electron route toward the iridium-iridium bond-containing species 131 (960M3785 980M2743). Complex 132, which is prepared by the ligand-substitution reaction from [Ir(CO)2 (/x-pz)]2, adds methyl iodide to give 133. 

Complex [(CXI )Ir(/j,-pz)(/i,-SBu )(/j,-Ph2PCH2PPh2)Ir(CO)] reacts with iodine to form 202 (X = I) as the typical iridium(II)-iridium(II) symmetrical species [90ICA(178)179]. The terminal iodide ligands can be readily displaced in reactions with silversalts. Thus, 202 (X = I), upon reaction with silver nitrate, produces 202 (X = ONO2). Complex [(OC)Ir(/i,-pz )(/z-SBu )(/i-Ph2PCH2PPh2)Ir(CO)] reacts with mercury dichloride to form 203, traditionally interpreted as the product of oxidative addition to one iridium atom and simultaneous Lewis acid-base interaction with the other. The rhodium /i-pyrazolato derivative is prepared in a similar way. Unexpectedly, the iridium /z-pyrazolato analog in similar conditions produces mercury(I) chloride and forms the dinuclear complex 204. 

Phenyl- and 2-naphthylbenzothiazole with iridium(III) chloride give cyclometallated derivatives of the type 54 containing two chloride bridging ligands (01IC1704). 

In the rhodium and iridium complexes, the C-coordination, carbene function, and cyclometallated cases prevail. Benzothiazole-2-thione was studied extensively as a ligand and various situations of the exocyclic S-monodentate coordination as well as N,S-combinations in the di-, tri-, and tetranuclear species were discovered. 

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