Iridium square-planar complexes

The mechanistic basis of iridium-complex-catalyzed enantioselective hydrogenation is less secure than in the rhodium case. It is well known that square-planar iridium complexes exhibit a stronger affinity for dihydrogen than their rhodium counterparts. In earlier studies, Crabtree et al. investigated the addition of H2 to their complex and observed two stereoisomeric intermediate dihydrides in the hydrogenation of the coordinated cycloocta-1,5-diene. The observations were in contrast to the course of H2 addition to Ms-phosphine iridium complexes [69]. 

SlOO proteins, calcium binding, 46 451-456 Spruhtrocken process, 4 26 Square-planar complexes, 4 157-164 octahedral, compared, 4 162-174 in solution, 34 270-271 Square-planar iridium complexes, 44 295, 297 Square-planar nickel macrocyclic complexes equilibrium with octahedral species, 44 116-118... 

Addition of H2 to a square-planar iridium complex, rrans-Ir(CO)L2X, can produce two structures with cis-hydrides [reaction (f)] ... 

Reactions of silanes with square-planar iridium complexes are similar to H2 addi-... 

One of the first detailed studies of OA was performed by L. Vaska57 on 16-electron square planar iridium complexes, the most notable of which is currently known as Vaska s compound, the central compound in Scheme 7,3. The reactions portrayed in Scheme 7.3 indicate that several different kinds of molecules react with the iridium complex. Mechanistic pathways leading to the products shown and to products resulting from other metal complexes are many and varied. We will consider a few of these mechanistic types, trying to point out as we go along similarities to other reactions from the realm of organic chemistry. 

The molecules being added split into two r/ ligands, which are both formally anionically bound to the metal center. One of the most thoroughly investigated compounds is a square-planar iridium complex whose central atom gives up two electrons and is oxidized to Ir " (Eq. 2-32). 

For the square-planar iridium complex [Ir(CO)(PPh3)2X], the ligand effects shown in Equation 2 4 were found. 

In synthesis (b), the initial product is a 5-coordinate (sp) iridium(III) hydride complex, which is rapidly oxidized in solution to the planar iridium(II) complex. Both of the compounds are paramagnetic with one unpaired electron, as expected for square planar d7 complexes. 

The most interesting work on the isocyanide complexes of the elements in this subgroup has been done with rhodium and iridium. For the most part, the work is involved with the oxidative addition reactions of d square-planar metal complexes. 

S. J. Pool and K.H. Shaughnessy. Effects of ionic liquids on oxidative addition to square planar iridium and rhodium complexes. Abstracts of Papers, 231st ACS National Meeting, Atlanta, GA, USA, March 26-30,2006 (2006). 

Vaska s complex, the square planar iridium(I) compound shown in equation 145, readily interacts with alkynyl(phenyl)iodonium triflates in toluene at room temperature21. Such reactions proceed with loss of iodobenzene and deliver octahedral Ir(III) complexes possessing (7-alkynyl and trifluoromethanesulfonato ligands in a Jnms-relationship. The rhodium(I) analog of Vaska s complex behaves in a similar way21. 

The crystal and molecular structures of [Ir(CO)(dtt)(PPh3)2] (dtt = /j-MeC6H4NN=NC6H4Me-p) have been determined by counter data. In this square planar iridium(I) complex, the diaryl-triazenido ligand (dtt) is unidentate and trans to the CO ligand.66... 

IR, Raman and 13C NMR spectroscopic studies have been performed on various [Ir(acac)(L)2] complexes (L = ethylene, propene, vinyl chloride, vinyl acetate, methyl acrylate, styrene) for the elucidation of the bonding between Ir and the alkene ligand.142 Also, the square planar iridium(I) acetylacetonate complexes [Ir(LL)(L )2], where LL is a /J-diketonate and L is CO or ethylene, have been studied by UVPES.143 The enthalpies of reaction of the crystalline [Ir(acac)(L)2] complexes with gaseous CO (reaction 28) have been determined by differential scanning calorimetry. The enthalpies for the gaseous reaction have been derived from these results and Ir—L bond strengths estimated.143... 

The square planar iridium isocyanide complexes [124] IrCl3... 

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