Spontaneous disproportionation of rhodium(I) bisoxazolinates to rhodium(II)

[RhI(t-Bu2-boxate)(C2H4)2] spontaneously disproportionates to the mononuclear [RhII(t-Bu2-boxate)2], whereas [RhI(Ph2-boxate)(C2H4)2] is stable against disproportionation.     

Aqueous rhodium(III) hydrides and mononuclear rhodium(II) complexes

In aqueous solutions, as in organic solvents, rhodium hydrides display the chemistry of one of the three limiting forms, i.e. {Rh(I)+ H+}, {Rh(II)+ H.}, and {Rh(III)+ H-}. A number of intermediates and oxidation states have been generated and explored in kinetic and mechanistic studies. Monomeric macrocyclic rhodium(II) complexes, such as L(H2O)Rh2+ (L = L1 = [14]aneN4, or L2 = meso-Me6[14]aneN4) can be generated from the hydride precursors by photochemical means or in reactions with hydrogen atom abstracting agents. These rhodium(II) complexes are oxidized rapidly with alkyl hydroperoxides to give alkylrhodium(III) complexes. Reactions of Rh(II) with organic and inorganic radicals and with molecular oxygen are fast and produce long-lived intermediates, such as alkyl, superoxo and hydroperoxo complexes, all of which display rich and complex chemistry of their own. In alkaline solutions of rhodium hydrides, the existence of Rh(I) complexes is implied by rapid hydrogen exchange between the hydride and solvent water. The acidity of the hydrides is too low, however, to allow the build-up of observable quantities of Rh(I). Deuterium kinetic isotope effects for hydride transfer to a macrocyclic Cr(v) complex are comparable to those for hydrogen atom transfer to various substrates.     

Oxidative addition of tetrabromo-1,2-benzoquinone totrans-carbonylchlorobis(triphenylphosphine)rhodium andtrans-carbonyliodiobis(triphenylphosphine)rhodium

Summary The oxidative addition of tetrabromo-1,2-benzoquinone to Rh(CO)X(PPh₃)₂ (X = Cl or I) has been studied. With the square planar complex Rh(CO)Cl(PPh₃)₂, two new isomeric hexacoordinated compounds; withcis andtrans PPh₃ ligands, have been isolated and their structures are discussed on the basis of spectroscopic data. Thecis isomer in acetone solution quickly converts into thetrans. Such a conversion presumably proceeds through the dissociation of a triphenylphosphine molecule as seems indicated by the isolation of the pentacoordinated intermediate species Rh(CO)I(PPh₃)(1,2-O₂C₆Br₄), which has been identified by elemental analysis and spectroscopically characterized.     

The formation, reactivity and interconversion of novel small eta(1)- and eta(3)-triazacyclononane complexes of rhodium(I) and rhodium(III)

We have successfully synthesised and characterised a number of eta(1)- and eta(3)-triazacyclononane Rh(I) and Rh(III) derivatives. By using different reaction conditions, we have been able to convert one of the eta(1)-triazacyclononane complexes to an eta(3)-derivative. Also, we have observed a rare example of an addition of an organic fragment to a metal bound ligand to form a quaternary carbon centre.     

Carbonyl complexes of rhodium(I) and rhodium(III) with 2,2′-biquinoline

Novel carbonyl complexes of rhodium(I) and rhodium(III) containing the bidenate nitrogen donor ligand 2,2′-biquinoline (biq) have been prepared; they are of the types RhX(CO)₂ biq and RhX(CO)biq (X = Cl, Br, I). Cationic carbonyl and substituted carbonyl complexes of the types [Rh(CO)₂biq]ClO₄ and [Rh(CO)biqL₂]ClO₄, where L is tertiary phosphine or arsine have also been isolated. In spite of considerable steric crowding around the nitrogen atoms, 2,2′-biquinoline behaves much like 2,2′-bipyridine in forming carbonyl complexes of rhodium.     

Five coordinate rhodium(I) olefin complexes containing the ligand bis(but-3-enyl)phenylphosphine

The complexes Rh₂X₂(bbp)₂ (X = Cl, Br or I; and bbp = bis(but-3-enyl)phenylphosphine) have been prepared. These complexes have been characterized as five coordinate dimers in which the unsaturated phosphine acts as a tridentate ligand. Carbon monoxide reacts reversibly with the dimers forming the five coordinate monomeric compounds RhX(CO)(bbp). Mass spectral, infrared, Raman, and proton magnetic resonance data are consistent with the above formulations.     

Synthesis and characterization of Rh3(μ-PPh2)3(CO)3(PPh3)2: a novel triangular rhodium cluster

Thermal decomposition of RhH(CO)(PPh₃)₃ (Ph = phenyl) in nonane at 120°C yielded a green solid in high yield. The complex was established as Rh₃(μ-PPh₂)₃(CO)₃(PPh₃)₂, (I), X-ray crystallography. I was found to be coordinately unsaturated in the solid state and in solution.     

Alpha-N-acetylmannosamine (ManNAc) synthesis via rhodium(II)-catalyzed oxidative cyclization of glucal 3-carbamates

[reaction: see text] Glucal 3-carbamates 1 and 7 underwent oxidative cyclization with iodobenzene diacetate or iodosobenzene in the presence of Rh2(OAc)4, providing mannosamine 2-N,3-O-oxazolidinones. With iodosobenzene, incorporation of 4-penten-1-ol provided a readily separable anomeric mixture of n-pentenyl glycosides, with the anomers exhibiting pronounced differences in reactivity as glycosyl donors. N-acylation of the sugar oxazolidinones led to alpha-selective glycosyl donors for the elaboration of various 2-mannosamine frameworks. Alternatively, the anomeric n-pentenyl glycosides of N-Cbz 2-mannosamine oxazolidinones were converted separately to oxazolidinone-opened derivatives 28alpha and 28beta. These served as stereoconvergent glycosyl donors, and the alpha-linked products were readily advanced to a variety of N-acetylmannosamine (ManNAc) frameworks, using an intramolecular O-->N acetyl transfer as the final step.     

N-substituted salicylaldimine complexes of rhodium(III) and related rhodium(III) organometallic derivatives

A series of new Rh^III complexes with N-substituted salicylaldimines have been prepared of the form [RhSBPy₂]PF₆ where SB is a tetradentate N,N′-substituted bis(salicylaldimine) or represents two molecules of a corresponding bidentate derivative. Several of these complexes have been reduced with 0.5% sodium amalgam and the products reacted with CH₃I to yield the organometallic derivatives CH₃RhSBPy.     

Interaction of rhodium(i) bisphosphine complexes with semicarbazones to give orthometallated rhodium(iii) complexes

Interaction of the cis-[Rh(PR₃)₂(Solv)₂]PF₆ complexes (R = Ar or R₃ = Ph₂Me, Solv — solvent) under Ar with semicarbazones bearing a phenyl group on the imine-C atom gives the rhodium(iii)-hydrido-bis(phosphine)-orthometallated semicarbazone species [RhH(PR₃)₂{(o-C₆H₄(R")C=N—N(H)CONH₂}]PF₆ (R" = Me or Et), which are characterized generally by elemental analysis, ³¹P{¹H} and ¹H NMR spectroscopy, and mass-spectrometry. The PPh₃-containing complex with R" = Me, structurally characterized by X-ray analysis, reveals coordination of the semicarbazone by the ortho-C atom, the imine-N atom, and the amide-carbonyl group. For a semicarbazone containing no Ph group, the rhodium(i) complex [Rh(PR₃)₂(Et(Me)C=N—N(H)CONH₂)]PF₆, containing the ²-semicarbazone bonded via the imine-N and carbonyl, is formed. Attempts to hydrogenate the C=N moiety in the complexes or to catalytically hydrogenate the semicarbazones were unsuccessful.     

Nitration of rhodium(III) sulfates

Nitration of sulfate complexes of rhodium has been investigated by NMR ¹⁰³Rh, ¹⁴N, ¹⁵N, and ¹⁷O NMR. At high pH, [Rh(NO₂)₆]^3?, dimer [Rh₂(μ-OH)₂(NO₂)₈]^4?, and trimer [Rh₃(μ-OH)₄(OH)(NO₂)₉]^5? are the dominant species in solutions.     

Active states of rhodium in zeolite catalysts after rhodium(III)-exchange

Zeolite-X exchanged with Rh^III compounds and activated at 450 °C in O₂ contains two paramagnetic species involving rhodium: these, detected by e.s.r, are Rh^II and superoxides of Rh^III. Superoxo-rhodium(III) is formed by the oxidation with O₂ of Rh^II generated in the heat treatment.