Rhodium and iridium complexes

Complexes of these metals catalyze the dehydrogenation of alcohols to aldehydes or ketones, producing gaseous or transferring hydrogen 

Isopropanol is dehydrogenated to acetone with hydrogen evolution at 

The dehydrogenation of alcohols is thermodynamically more favorable when the hydrogen released is consumed for olefin hydrogenation, as was observed for soluble rhodium complexes as catalysts [13,14]. 

Hydrogen transfer from sec-alcohols to alkenes, affording ketones and alkanes, is catalyzed by [MCI (CgH 2 P 3 where M = Rh and Ir 

Copper(II) ions effect photocatalytic H evolution from ethanol solutions [16]. 

The nitrato-complexes [M(N03)s(PPh3)2] and [M(N03)a(NO)(PPh3)2] may be prepared by treating [M(CO)H(PPha)3] (M=Rh or Ir) with nitric acid. Mixed nitrato-nitro complexes such as [Ir(C0)Cl(PPh3)2(N03)(N02)] and [Rhg-(PPh3)4(N03)a(N02)2Cl] result from oxidative-addition reactions of N2O4 with 

Dwight, and A. R. Sanger, Inorg. Chim. Acta, 1978, 31, L407. 

The 5-bonded complexes [Rh(CO)X(R2SO)2] (R=Me, Pr , or Bu X=C1 or Br) and [Rh(CO)X(R2S)2] (R=Me, Et, or Pr ) have been prepared and the molecular structure of the chelate tropolonato-complex [Rh(CO)(PPh3 -(C7H5O2)] has been elucidated. The Te-bonded complex [RMCOX KTePhala] has also been isolated and some oxidative addition reactions with X2 (X=Br, I, or SCN) studied. 21 

Paramagnetic Co(II) borabenzene complexes (C5HgBR)2Co (R = Br, Me, Ph) have been synthesized by treating (7r-C5H5)2Co with RBBt2. Hydrolysis or methanolysis of the bromo derivative led to complexes (CbH5BOH)2Co and (C5HBBOMe)2Co, respectively (220). 

The NMR spectrum showed the presence of a tetrahapto-coordinated (MeOOC)gCg ligand. The rhodium compound showed fluxional NMR behavior on heating to 150°C in dg DMSO which was reversed on cooling. 

The polynuclear cation [(CeHe)3Co3(CO)2] has been reported from the reaction of Hg[Co(CO)4]a or Coa(CO)0 with benzene in the presence of an aluminum trihalide (66, 114). The cation is believed to have the structure (XXXI) analogous to (7r-C5H5)3Ni3(CO)a (289). Neutral polynuclear complexes of the type (arene)Co4(CO)g (arene = benzene, toluene, anisole, / -xylene, mesitylene, tetrahydronaphthalene) have been prepared by the reaction of (RC=CH)Co2(CO)e (R = H or Ph) with norbornadiene in the appropriate aromatic solvent or in some cases by simply warming Co4(CO)i2 with the arene (247, 365). The compounds are believed to have the structure (XXXII) derived from that of Co4(CO)i2 (435) by replacement of three apical CO groups by the arene. A normal coordinate analysis has been carried out on several of 

Monoarene Complexes Containing Ligands Other Than Carbon Monoxide 

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The rhodium and iridium complexes of dibenzothiophene (L) reveal an interesting case of linkage isomerism (91IC5046). Thus, the ti S) coordinated species [MCp LCb] on thermolysis with silver tetrafluoroborate afford the Ti -coordinated dicationic species. 

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. 

Mechanistic Pathways in the Catalytic Carbonylation of Methanol by Rhodium and Iridium Complexes... 

A similar type of immobilization was obtained by reacting the phosphonylated 2,2 -bipyridine ligand depicted in Figure 42.10 with excess titanium alkoxide. Rhodium and iridium complexes of this immobilized ligand showed activity for... 

Rhodium and iridium complexes effect the dehydrogenation of the alkane chain in 1,6-bisdiphenylphosphinohexane to form (after treatment with cyanide ion) 1,6-(bisdiphenylphosphino)-frara-hex-3-ene.81... 

New N-heterocyclic carbene rhodium and iridium complexes derived from 2,2 -diaminobiphenyl were successfully synthesized and their structures unambiguously characterized by X-ray diffraction (XRD) analysis. These are cata-lytically active for the hydrosilylation of ketones with diphenylsilane, although an NHC—rhodium complex was found to be the best among those investigated [45]. 

Platinum complexes have been mainly used in the hydrosilylation of carbon-carbon bonds, and ruthenium complexes in the metathesis and silylative coupling of olefins with vinylsilanes. Most of these processes (except for olefin metathesis) may also proceed efficiently in the presence of rhodium and iridium complexes. 

The LUMO in d pentatetraenylidene complexes is predominantly localized on the odd carbon atoms and to a lesser extent on the metal. The coefficients on Cl and C3 are very similar, independent of the metal-ligand fragment and the terminal substituent. The coefficient at C5 is somewhat larger. In square-planar d rhodium and iridium complexes the coefficient at the metal is comparable to that on C5 and is larger than those on Cl and C3. Thus, a nucleophilic attack at the metal of d complexes has also to be taken into account. 

Recent advances in alcohol oxidations by rhodium and iridium complexes have mainly focused on Oppenauer-type oxidations or reactions in which this type of oxidation is an intermediate step. An independent result is the oxidation of allyhc (Eq. 9) and benzyUc alcohols with f-BuOOH to the corresponding a,/l-unsaturated ketones [38] with [Rh2(p.-OAc)4]. The reactions were carried out at room temperature in dichloromethane and yields of up to 92% (by GC) in 24-48 h have been described. 

These solid-gas reactions represent, at the moment, the single path to 3-metalla -l,2-dioxolane complexes of rhodium and iridium. Complexes of this type have been widely proposed in catalytic cycles. However, it is unlikely that they take part in oxygenations with rhodium because of their high reactivity (see below) and the special conditions for their preparation. 

Rasmussen, S.C., Richter, M.M., Yi, E., Place, H. and Brewer, KJ. (1990) Synthesis and characterization of a series of novel rhodium and iridium complexes containing polypyridyl bridging ligands Potential uses in the development of multimetal catalysts for carbon dioxide reduction. Inorg. Chem., 29, 3926—3932. 

Silylation of arenes was reported to take place with rhodium and iridium complexes as the catalyst. The Vaska complex effects the reaction of benzene with pen-tamethyldisiloxane,230 whereas [RhCl(CO)(PMe3)2] catalyzes the formation of C—Si bonds under irradiation231 [Eq. (10.39)] 232... 

Mechanistic Pathways for Ligand Substitution Processes in Metal Carbonyls, 21, 113 Mechanistic Pathways in the Catalytic Carbonylation of Methanol by Rhodium and Iridium Complexes, 17, 255... 

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