Chelates iridium complexes

A (pentamethylcyclopentadienyl)iridium chelating guanidinate complex has been conveniently prepared by treatment of [Cp IrCl2]2 with N,N, N"-th-p-tolylguanidine and base in THF at room temperature followed by recrystallization of the green product from toluene and pentane (Scheme 154). Insertion reactions of the product with heterocumulenes (diaryl carbodiimides, aryl isocyanates) have been investigated. It was found that the complex serves as highly active catalyst for the metathesis of diaryl carbodiimides with each other and for the more difficult metathesis of diaryl carbodiimides with aryl isocyanates (cf. Section V.C). ... 

Iridium NHC complexes have also been used to catalyse the coupling of alcohols 7 and 8. Complexes 12 [4] and 13 [5], containing chelating NHC ligands, are also effective for catalysing the same reaction. 

The beneficial effects of chelating ligands were also demonstrated by Hartwig, Ishiyama and Miyaura [62]. This group isolated the iridium(I) complex [lr(Bpin)3(COE)(DTBPY) modified with simple 2,2 -bipyridine ligands (such as 4,4 -di-tert-butyl-2,2 -bipyridine DTBPY), which seemed to be responsible for the first catalytic C—H borylation at room temperature (Scheme 7.30). An extension... 

First attempts to isolate monocarbene-hydrido complexes by oxidative addition of A -(2-pyridyl)imidazolium cations to Pd° with utilization of the chelate effect of the donor-functionalized carbene ligand failed and only the dicarbene complexes such as 29 were isolated [112]. The iridium hydrido complex 30 was obtained in the oxidative addition of an W-(2-pyridylmethyl)imidazolium cation to iridium(I) (Fig. 11) [113]. This reaction proceeds most likely via the initial coordination of the nitrogen donor which brings the imidazolium C2-H bond in close proximity to the metal center. No reaction was observed with Rh under these conditions. 

Nonetheless, the effectiveness of PC02H as a precursor to a stable iridium(III) hydride is limited since on a preparative scale the product of the reaction of equimolar quantities of 1 and PCO2H consists of a mixture which contains considerable quantities of the iridium(I) complex (v o = 1960 cm-1) wherein the carboxylic acid group is pendant. The chelating phenol, POH, is comparable to PC02H in its R value the enhancement factor, RpOH PhOH is significantly greater than that for the benzoic acids. 

Because of the relatively poor coordinating properties displayed by the P-menthyl-substituted monodentate ligands toward some catalytically useful metals like rhodium and iridium, development of the coordination chemistry of chelating phosphetanes was required. Early studies established that the bidentate ligand P(6 ),C (6 )-43 binds well to rhodium centers. It gives the chelating complex 26 with [Rh(COD)2]PF6 and the bimetallic compound 68 when reacted with [Rh(COD)Cl]2 under an atmosphere of CO (COD = cyclooctadiene Scheme 5) <19950M4983>. 

Okazaki, M., Kawano, Y., Tobita, H., Inomata, S., and Ogino, H. (1995) Light-and heat-induced isomerization of chloro (hydrido)iridium(III) complex containing a (2-phosphinoethyl)silyl chelate ligand. Chemistry Letters, 1005—1006. 

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... 

In the event, only the rhodium(I) complex features sulfur coordination, whereas the iridium(I) complex prefers a second carbene Ugand over the sulfur mediated chelate effect. The two complexes were tested for their activity in the hydrogenation of dimethyl itaco-nate. The iridium complex was inactive and the rhodium complex showed 44% conversion with a disappointingly low chiral resolution of 18% ee (R). The corresponding phosphine functionalised NHC rhodium(I) complex reacted under milder conditions, but without improvement of chiral resolution, 13% ee (S). 

The reactions of iridium olefin complexes are not restricted to reactions with phosphines. Amines have also employed in bridge-splitting and substitution reactions with [Ir(COD)Cl]2, especially chelating diamines. The reactions proceed to yield [Ir(COD)N-N]2 compounds. A fertile chemical area involves the irw(pyrazolyl)borate (see Tris(pyrazolyl)borates) family of compounds with the monoethylene and bisethylene complexes serving as reactive entries in this field. ... 

The iridium(III) complex [Ir(H)3(AsPh3)2(p-MeC6H4NC)] (99) is known to react with HX (or pseudohalides) to yield substitution products [lr(H)2(X)(AsPh3)(p-MeC6H4NC)i (X = Cl, Br, I, Nj). With carboxylic acids or pentane-2,4-dione, complex (99) yields the chelated complexes [Ir(L2)(AsPh3)(p-MeC6H4NC)] (L2 = carboxylate or pentane-2,4-dionate) in which Ir is reportedly reduced to 11. Reactions (52>-(60) reveal the utility of complex (99) in synthesizing complexes with iridium in various oxidation states. ... 

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