Iridium alkyl and aryl complexes

Interest in iridium hydrocarbyl complexes has been fueled by the higher thermodynamic stability of both Ir-C and Ir-H bonds in comparison to related compounds of rhodium. Therefore, iridium alkyl and aryl complexes have been frequently and successfully used as kinetically inert models for a variety of rhodium-catalyzed reactions allowing to collect intriguing mechanistic information on important industrial processes and organometallic reactions of academic interest. 

Several tertiary phosphine iridium-alkyl and -aryl complexes have been... 

Rhodium and Iridium rr-Bonded Alkyl and Aryl Complexes. 29,4... 

Iridium-carbon multiple bonds allenylidene complexes, 7, 355 carbene complexes, 7, 344 carbyne complexes, 7, 361 higher cumulenylidene complexes, 7, 358 vinylidene complexes, 7, 352 Iridium-carbon single-bonded complexes alkenyl complexes, 7, 319 alkyl and aryl complexes, 7, 303 in C-C bond-forming catalysis, 7, 335 characteristics, 7, 303... 

The products of oxidative addition of acyl chlorides and alkyl halides to various tertiary phosphine complexes of rhodium(I) and iridium(I) are discussed. Features of interest include (1) an equilibrium between a five-coordinate acetylrhodium(III) cation and its six-coordinate methyl(carbonyl) isomer which is established at an intermediate rate on the NMR time scale at room temperature, and (2) a solvent-dependent secondary- to normal-alkyl-group isomerization in octahedral al-kyliridium(III) complexes. The chemistry of monomeric, tertiary phosphine-stabilized hydroxoplatinum(II) complexes is reviewed, with emphasis on their conversion into hydrido -alkyl or -aryl complexes. Evidence for an electronic cis-PtP bond-weakening influence is presented. 

After these pioneering studies, a number of other research groups reported on the cleavage of C-H bonds via the use of a stoichiometric amount of transition-metal complexes [7]. To date, several types of catalytic reactions involving C-H bond cleavage, for example, alkyl, alkenyl, aryl, formyl, and active methylene C-H bonds have been developed [8]. In many cases,for these types of catalytic reactions, ruthenium, rhodium, iridium, platinum, and palladium complexes all show catalytic activity. 

Alkyl and aryl isothiocyantes, RCNS, parallel CS2 in their capability to insert into M—H bonds and yield products having the JV-alkyl- or JV-aryl-formamide chelate ligand RN=CH=S. The iridium(III) complexes [Ir(X)2(RN=CH=S)(PPh3)2] (207 X = Cl, Br R = Me, Et, Ph) can be prepared from trans-[Ir(H)(X)2(PPh3)2] and the appropriate alkyl or aryl isothiocyanate. Two isomeric forms of (207) were deemed plausible, with (207a) the preferred stereochemistry for the alkyl products and (207b) for the aryl products.453... 

IRIDIUM ALKYL (See Alkyl Complexes) AND ARYL COMPLEXES... 

There are quite a number of routes available for the production of iridium(ni) alkyl compounds. In addition to the halide displacement and olefin insertion pathways noted above for iridium(l) compounds, oxidative addition of C-H bonds to iridium(l) to form iridium(in) hydrido alkyl complexes is also a possibihty. This subject will be covered in detail in Section 9 and will not be discussed here. However, there are other oxidative addition routes that lead to the formation of iridium(lll) alkyls. First, oxidative addition of O2 or HCl to some alkyl and aryl iridium(l) complexes can produce iridium(lll) alkyl or aryl compounds. In some cases, HgCl2 can add, but this appears to lead to tractable products only for the very stable pentafluorophenyl complex. Of course, oxidative addition see Oxidative Addition) of alkyl halides such as H3CI will also yield alkyl iridium(lll) compounds. Addition of Mel to Vaska s compound yields a stable iridium(III) complex, but addition of Etl does not produce a stable compound, presumably due to subsequent /J-hydride elimination see fi-Hydride Elimination). A number of mechanistic studies have been done on the oxidative addition of alkyl halides to iridium(l), especially Vaska s complex see Vaska s Complex). 

The synthesis of iridium(lll) alkyl and aryl compounds via nucleophilic displacement is straightforward starting from a suitable L3lrX3 complex with X most commonly being chloride. Chatt and Shaw carried out early work on the syntheses of iridium(III) alkyl complexes showing that fac[lrMe3(PR3)3] (69) could be formed by reaction between... 

Alkyl and aryl substituted imines have received the most attention as substrates for asymmetric hydrogenation, and the development of the field can therefore be outlined by examining their reductions. These are usually catalyzed by chiral complexes of titanium, ruthenium, rhodium, or iridium, though gold catalysts have also recently proven useful for this purpose [31]. New catalysts are generally tested for the reductions of substrates A-D (Scheme 6.1). 

The excited states of dinuclear platinum, rhodium, and iridium complexes with a variety of bridging ligands exhibit unusually diverse reactivity. These types of compound in their lowest triplet state engage in oxidative and reductive electron transfer reactions, and exciplex formation [56], They can also engage in atom transfer reactions i.e. they can abstract hydrogen atoms from a wide range of substrates as well as halogen atoms from alkyl and aryl halides. 

Intramolecular hydroalkoxylation of internal alkynes containing proximate oxygen nucleophiles was catalyzed by iridium hydride complex [48] (Scheme 29). The cyclization of 2-alkynylbenzyl alcohols follows highly selective 6-endo-dig regiochemistry for both alkyl and aryl substituents on the multiple bond. Cyclization of substrates having two OH groups gave spiroacetal derivatives. When... 

Structural types for organometallic rhodium and iridium porphyrins mostly comprise five- or six-coordinate complexes (Por)M(R) or (Por)M(R)(L), where R is a (T-bonded alkyl, aryl, or other organic fragment, and Lisa neutral donor. Most examples contain rhodium, and the chemistry of the corresponding iridium porphyrins is much more scarce. The classical methods of preparation of these complexes involves either reaction of Rh(III) halides Rh(Por)X with organolithium or Grignard reagents, or reaction of Rh(I) anions [Rh(Por)] with alkyl or aryl halides. In this sense the chemistry parallels that of iron and cobalt porphyrins. 

Non-ionic thiourea derivatives have been used as ligands for metal complexes [63,64] as well as anionic thioureas and, in both cases, coordination in metal clusters has also been described [65,66]. Examples of mononuclear complexes of simple alkyl- or aryl-substituted thiourea monoanions, containing N,S-chelating ligands (Scheme 11), have been reported for rhodium(III) [67,68], iridium and many other transition metals, such as chromium(III), technetium(III), rhenium(V), aluminium, ruthenium, osmium, platinum [69] and palladium [70]. Many complexes with N,S-chelating monothioureas were prepared with two triphenylphosphines as substituents. 

As mentioned above, iridium complexes are also active in the formation of amines via the hydrosilylation/protodesUylation of imines. In the presence of 2 equiv. of HSiEts, the cationic complex [lr bis(pyrazol-l-yl)methane (CO)2][BPh4] (C4) catalyzes the reduction of various imines, including N-alkyl and N-aryl imines and both aldimines and ketimmes. Excellent conversions directly to the amine products were achieved rapidly at room temperature in a methanol solution (Scheme 14.7) [53]. 

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