This contribution will focus on the hydrogenation of carbon dioxide to formic acid or methanol using homogeneous catalysts, mostly transition metal complexes, and molecular dihydrogen as the reductant. Homogeneously catalysed indirect reduction of carbon dioxide via hydrogenation of intermediates such as formamides, carbonates, carbamates and ureas is also considered. The proposed mechanisms for these transformations are revised.

A family of five- and six-coordinate chiral-at-metal ruthenium complexes has been examined as catalysts in the asymmetric cyclopropanation reaction of styrene with ethyl diazoacetate. With complexes 5 and 6, good cis-diastereoselectivity and enantioselectivity up to 74% were observed.

The chiral iridium compounds [IrCl(COE){(S)-PN)] {COE = cyclooctene, PN = (4S)-2-[2-(diphenylphosphanyl)phenyl]-4-isopropyl-1,3-oxazoline, (1)), [Ir(acac)CIH((S)-PN)] (Hacac = acetylacetone, (2)} and [Ir{(S)-PN)(2)]A {A = Cl (3a,b); BF4 (4a,b); PF6 (5a,b)) have been prepared, characterised and employed as catalysts for the asymmetric Michael addition of keto or cyano esters to alpha,beta-unsaturated carbonyl compounds. The X-ray molecular structures of compounds 2 and 5b are reported. The model catalytic intermediates [IrH(NCCHCO2R){(S)-PN)(2)]Cl {R = Me (12), Et (13)} have also been isolated and characterised. (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).

The 1,3-dipolar cycloaddition reaction of C,N-diphenylnitrone with methacrolein is efficiently catalyzed by the rhodium diphosphine compound (SRh,RC)-[(eta5-C5Me5)Rh(R-Prophos)(H2O)](SbF6)2 [R-Prophos = (R)-(+)-1,2-bis(diphenylphosphino)-propane, 1.SbF6]; the asymmetric catalytic process occurs with reversal of regioselectivity, perfect endo selectivity, and up to 92% ee. The complete (NMR and X-ray analysis) characterization of the involved intermediate (SRh,RC)-[(eta5-C5Me5)Rh(R-Prophos)(methacrolein)](SbF6)2 (7.SbF6) allows us to interpret the observed selectivity.

The synthesis of arene–ruthenium(II) and C5Me5–rhodium(III) and –iridium(III) complexes of chiral arene-chromium-tricarbonyl-based P∧P and P∧N ligands is described. Three complexes were characterized in the solid state by X-ray structural analysis. The complexes were tested in the catalytic hydrogen transfer reactions as well as in the kinetic resolution of racemic alcohols, where some complexes showed good conversion, but low enantioselectivity.

An explanation for the reversal in the sense of the enantioselectivity observed in hydrogen transfer reactions from 2-propanol to ketones catalyzed by the ruthenium or osmium amino acidates [(η6-p-MeC6H4-i-Pr)M(Aa)Cl] and [(η6-p-MeC6H4-i-Pr)M(Aa)]3[BF4]3 [Aa=piperidine-2-carboxylate (pip), N-methyl-L-phenylalaninate (MePhe)] is given; the molecular structures of [(η6-p-MeC6H4-i-Pr)Os(Pip)Cl] (1), [(η6-p-MeC6H4-i-Pr)Os(Pip)]3[BF4]3 (2), [(η6-p-MeC6H4-i-Pr)M(MePhe)Cl] [M=Ru (3), Os (4)] are also reported.

The synthesis and characterization of the chiral complexes [MCl(η2-C2H4)(PN)] [M = Rh (1), Ir (2)] and [IrCl(η2-C2H4)2(PN)] (3) {PN = (4S)-2-[2-(diphenylphosphanyl)phenyl]-4-isopropyl-1,3-oxazoline}, including the X-ray crystal structure of 2, are reported. The solution behavior has been studied by NMR spectroscopy. For the rhodium complex 1 it is shown that a rotation of the olefin about the Rh-midpoint of the ethylene axis must be operating in solution at ambient temperature. The iridium analogue 2 is, however, stereochemically rigid and interconverts in solution, in the presence of ethylene, with the bis-ethylene derivative 3.

Reaction of [(HBpz(3))RhCl(2)(PPh(3))] (Hpz = pyrazole) with silver salts AgA (A = BF(4), NO(3), SbF(6)) affords the unexpected heterotrinuclear compounds [[(HBpz(3))Rh(PPh(3))(mu-Cl)(2)](2)Ag]A (A = BF(4) (1), NO(3) (2), SbF(6) (3)). The compounds have been fully characterized by IR, (1)H, (31)P[(1)H], and (13)C[(1)H] NMR spectroscopy and FAB(+) mass spectrometry. The solid structure of compound 1 was determined by single-crystal X-ray diffraction. The cation consists of two (HBpz(3))RhCl(2)(PPh(3)) units bonded to a silver atom through two double mu-Cl bridges in an unusual distorted square-planar arrangement.

Chiral-at-metal half-sandwich complexes of rhodium, iridium, ruthenium, or osmium of the general formula [(eta(n)-ring)M(Aa)Cl] [(eta(n)-ring)M = (eta(5)-C5Me5)Rh, (eta(5)-C5Me5)Ir, (eta(6)-p-MeC(6)H(4)iPr)Ru, (eta(6)-p-MeC(6)H(4)iPr)Os; Aa = L-alpha-aminocarboxylate] can readily be prepared from the corresponding dimers [{(eta(n)-ring)MCl}(2)(eta-Cl)(2)]. The compounds have been prepared as diastereomeric mixtures of the two epimers at the metal center. In general, alkynyl aminocarboxylate derivatives [(eta(n)-ring)M(Aa)(CdropCR)] are obtained by treating the aforementioned chlorides with the corresponding alkynes in basic media. However, the reaction of the (alaninato)rhodium chloride [(eta(5)-C5Me5)Rh(Ala)Cl] with the alkynes HCdropCR (R = Ph, p-tolyl) produced the alkynylcyclobutadiene complexes [(eta(5)-C5Me5)Rh(eta(4)-C(4)HR(2)CdropCR)] (R = Ph, p-tolyl). Treatment of the chlorides [(eta(n)-ring)M(Aa)Cl] with AgBF4 afforded the cationic trimers [{(eta(n)-ring)M(Aa)}(3)](BF4)(3). Trimerization occurs with chiral self-recognition: only the (R,R,R) or (S,S,S) configuration at the metal trimers can be detected. The trimers [{(eta(n)-ring)M(Aa)}(3)](BF4)(3) reacted with tertiary phosphanes, leading to the cationic mononuclear complexes [(eta(n)-ring)M(Aa)(PR3)](BF4). The assignment of the configuration at the metal center was accomplished by X-ray diffraction, circular dichroism and NMR spectroscopy. Most of the aminocarboxylate derivatives epimerized at the metal center. On the basis of kinetic and spectroscopic data, a general mechanism is proposed for the epimerization process. Neutral [(eta(n)-ring)M(Aa)Cl] and cationic [1(rln-ring)M(Aa))3](BF4)3 complexes are active catalysts for the hydrogen transfer reaction from 2-propanol to acetophenone. Conversions of up to 97% and enantioselectivities up to 75% were achieved. A proposal about the origins of the enantioselectivity is given. (C)Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002.

The uncoordinated P atom of the bis(diphenylphosphine)amine (dppa) ligand in complexes [(ring)MCl2(η1-P-PPh2NHPPh2)] (M=Rh, Ir, Ru) reacts with sulphur or selenium to form [(ring)MCl2(η1-P-PPh2NHP(E)Ph2)] (E=S (1–3), Se (4–6)) containing the P-coordinated monosulphide or monoselenide ligands. The selenium derivatives have also been directly prepared from the corresponding [{(ring)MCl2}2] dimer and dppaSe. Chloride abstraction from rhodium and ruthenium complexes gives the neutral compounds [(ring)MCl(η2-P,E-PPh2NP(E)Ph2)] (7–10) whilst the iridium derivatives yield cationic complexes of the general formula [{η5-C5Me5)IrCl(η2-P,E-PPh2NHP(E)Ph2)]+ (11 and 12). The crystal structure of complex [(η5-C5Me5)RhCl{η2-P,Se-PPh2NP(Se)Ph2}] has been established by X-ray crystallography. The rhodium atom exhibits a distorted octahedral coordination with a η5-C5Me5 group occupying the centre of three octahedral sites; a bidentate chelate P,Se-bonded ligand and a chloride atom complete the metal coordination sphere.

The cationic compounds [(eta(6)-p-cymene)OsCl(Hpz)(2)]A (A = Cl (1), BF4 (2)) have been prepared by treating the dimeric [{(eta(6)-p-cymene)OsCl}(2)(mu-Cl)(2)] with pyrazole (p-cymene = p-isopropylmethylbenzene, Hpz = pyrazole). Compounds 1 and 2 are precursors to new heterobinuclear complexes of stoichiometry [(eta(6)-p-cymene)OsCl(mu-pz)(2)M(COD)] (M = Ir (3), Rh (4); COD =-1,5-cyclooctadiene). Carbonylation of 3 and 4 produces the dicarbonyls [(eta(6)-p-cymene)OsCl(mu-pz)(2)M(CO)(2)] (M = Ir (5a), Rh (6)) or, in the presence of NaBPh4, the cationic tricarbonyls [(eta(6)-p-cymene)Os(CO)(mu-pz)(2)M(CO)(2)]BPh4 (M = Ir (7), Rh (8)). Complex 5a in methanol isomerizes to the metal-metal-bonded compound [(eta(6)-p-cymene)Os(mu-pz)(2)IrCl(CO)(2)] (5b) (first-order kinetics in 5a; k(obs) = 3.8 x 10(-4) s(-1) (room temperature), Delta H double dagger = 93 kJ mol(-1), Delta S double dagger = 7 J K-1 mol(-1)). The molecular structures of [(eta(6)-p-cymene)OsCl(mu-pz)(2)Rh(CO)(2)] (6) and [(eta(6)-p-cymene)Os(CO)(mu-pz)(2)Rh(CO)(2)]BPh4 (8) have been determined by X-ray diffraction methods. In both cases, the molecules contain (eta(6)-p-cymene)Os and Rh(CO)(2) moieties bridged by two pyrazolate groups. The osmium-rhodium distances, 3.6231(5) Angstrom (6) or 3.7012(6) Angstrom (8), exclude any direct intermetallic interaction. The metal-metal-bonded halide carbonyl complexes [(eta(6)-p-cymene)Os(mu-pz)(2)Ir(CO)(2)] (X = Br (9), I (10)) and [(eta(6)-p-cymene)Os(mu-pz)(2)RhI(CO)] (12), as well as the OsRh compound [(eta(6)-p-cymene)OsBr(mu-pz)(2)Rh(CO)(2)] (11), have been prepared by metathetical reaction on the corresponding dicarbonyls 5a,b and 6 with sodium halides. The molecular structures of 10 and 12 have been elucidated by diffractometric means. The metal-metal distances, 2.7196(6) Angstrom (Os-Ir, 10) and 2.6936(9) Angstrom (Os-Rh, 12), confirm the presence of metal-metal bonds. In the solid state, complex 12 crystallizes as a dimer of two dinuclear moieties through an asymmetric diiodide bridge. A theoretical study of the related RuIr dicarbonyl chloride isomers [(eta(6)-p-cymene)RuCl(mu-pz)(2)Ir(CO)(2)] (13a) and [(eta(6)-p-cymene)Ru(mu-pz)(2)IrCl(CO)(2)] (13b) is also reported.

The metal-metal bonded dicarbonyl compounds [(eta(6)-p-cymene)M(mu-pz)(2)IrCl(CO)(2)] (M-Ir) react stereospecifically with PPh3 to yield an enantiomeric pair of ketonic carbonyl compounds [(eta(6)-p-cymene)M(mu-CO) (mu-pz)(2)IrCl(CO) (PPh3)] [M = Ru (1), Os (2)] in which the chloride ligand is trans to the bridging ketone group. However, the non metal-metal bonded isomers [(eta(6)-p-cymene)MCl(mu-pz)(2)M'(CO)(2)], under similar conditions, eliminate CO forming the monosubstitution products [(eta(6)-p-cymene)MCl(mu-pz)(2)M' (CO) (PPh3)] [M - Ru; M' = Ir (3), Rh (4). M = Os; M' = Rh (5)]. Removal of the chloride Ligand in 1 affords [RuIr(eta(6)-p-cymene) (CO)(2)(PPh3) (pz)(2)]BPh4 as two isomers, 6a,b. The related OsIr compound [(eta(6)-p-cymene)Os (mu-CO)(mu-pz)(2)Ir(CO)(PPh3)]BPh4 [9) has been prepared from [(eta(6)-p-cymene)Os(CO) (mu-pz)(2)Ir(CO)(2)]BPh4 (7) and PPh3. Again, the OsRh tricarbonyl related to 7 [(eta(6)-p-cymene)Os(CO) (mu-pz)(2)Rh(CO)(2)]BPh4 (8) reacts differently with PPh3 affording [(eta(6)-p-cymene) Os(CO) (mu-pz)(2)Rh(CO) (PPh3)]BPh4 (10).

The homogeneous enantioselective Diels–Alder reactions catalyzed by chiral transition-metal complexes are reviewed. Special attention has been paid to the mechanistic aspects of the process in order to account for the stereochemical outcome of the reactions.

The reaction of the metallo–ligand [Ru(η6-p-cymene)(pz)2(Hpz)] with the platinum complex [{PtIMe3}4] affords mixtures of the heterodinuclear complexes [(η6-p-cymene)Ru(μ-pz)3PtMe3] (1) and [(η6-p-cymene)Ru(μ-pz)2(μ-I)PtMe3] (2). The reaction of the iridium derivative [Ir(η5-C5Me5)(pz)2(Hpz)] with [{PtIMe3}4] gives [(η5-C5Me5)Ir(μ-pz)2(μ-I)PtMe3] (3). Both [Ru(η6-p-cymene)(pz)2(Hpz)] and [Ir(η5-C5Me5)(pz)2(Hpz)] react with [{PtIMe3}4] in the presence of NaOH yielding 1 and [(η5-C5Me5)Ir(μ-pz)3PtMe3] (4), respectively. While [Ru(η6-p-cymene)(pz)2(Hpz)] reacts with [PtBr2Me2Sx] to give mixtures of [(η6-p-cymene)Ru(μ-pz)3PtBrMe2] (5) and [(η6-p-cymene)Ru(μ-pz)2(μ-Br)PtBrMe2] (6), the reaction of [Ir(η5-C5Me5)(pz)2(Hpz)] with [PtBr2Me2Sx] gives [(η5-C5Me5)Ir(μ-pz)2(μ-Br)PtBrMe2] (7) as the sole product. All species were characterized in solution by 1H-NMR spectroscopy. The crystal structure of complex 4 has been determined by single-crystal X-ray diffraction.

Reaction of the dimers [{(eta(5)-C5Me5)MCl}(2)(mu-Cl)(2)] (M = Rh, Ir) or [{(eta(6)-arene)RuCl}(2)(mu-Cl)(2)] (arene = p-(MeC6H4Pr)-Pr-i, C6Me6) with NH(PPh2)(2) in the presence of AgA (A = BF4, PF6) leads to the mononuclear cationic complexes [(eta(5)-C5Me5)MCl{eta(2)- (PPh2)(2)NH}]A (M = Rh (1), Ir (2)) or [(eta(6)-arene)RuCl{eta(2)-(PPh2)(2)NH}]A (arene = p-(MeC6H4Pr)-Pr-i (3), C6Me6 (4)). Similar reactions using the chalcogenide derivatives NH(EPPh2)(2) (E = S, Se) yield the neutral complexes [(eta(5)-C5Me5)RhCl{eta(2)-(EPPh2)(2)N}] (E = S (5), Se (6)), [(eta(5)-C5Me5)IrCl{eta(2)-(EPPh2)(2)N}] (E = S (7), Se (8)), [(eta(6)-arene)RuCl{eta(2)-(SPPh2)(2)N}] (arene = C6H6 (9), p-(MeC6H4Pr)-Pr-i (10)) and [(eta(6)-arene)RuCl{eta(2)-(SePPh2)(2)N)}] (arene = C6Me6 (11), p-(MeC6H4Pr)-Pr-i (12)). Chloride abstraction from complexes 5-8 with AgPF6 in the presence of PPh3 gives the cationic complexes [(eta(5)-C5Me5)Rh{eta(2)-(EPPh2)(2)N}(PPh3)]PF6 (E = S (13), Se (14)) and [(eta(5)-C5Me5)Ir{eta(2)-(EPPh2)(2)N}(PPh3)]PF6 (E = S (15), Se (16)). Complexes 13-16 can also be synthesised from the starting dinuclear complexes, AgPF6, NH(EPPh2)(2) and PPh3. Using this alternative synthetic route the related ruthenium complexes [(eta(6)-C6Me6)Ru{eta(2)-(EPPh2)(2)N}(C5H5N)] BF4 (E = S (17), Se (18)) can be prepared. All described compounds have been characterised by microanalysis and NMR (H-1, P-31) and IR spectroscopy. The crystal structures of the neutral complexes [(eta(5)-C5Me5)MCl{eta(2)-(SePPh2)(2)N}] (M = Rh (6), Ir (8)) have been determined by X-ray diffraction methods. Both complexes exhibit analogous pseudo-octahedral molecular structures with a C5Me5 group occupying three coordination positions and a bidentate chelate Se,Se'-bonded ligand and a chloride atom completing the coordination sphere. (C) 2000 Elsevier Science S.A. All rights reserved.

The new complexes (RRuSC, SRuSC)-[(η6-pCym)Ru(l-Aze)Cl] (6a, b), (RRuSC, SRuSC)-[(η6-pCym)Ru(l-Pip)Cl] (7a, b), (RRuRRuRRuSCSCSCSNSNSN, SRuSRuSRuSCSCSCSNSNSN)-[{(η6-pCym)Ru(l-Aze)}3](BF4)3 (8a, b) and (RRuRRuRRuSCSCSCSNSNSN, SRuSRuSRuSCSCSCSNSNSN)-[{(η6-pCym)Ru(l-Pip)}3](BF4)3 (9a, b) (l-Aze=l-2-azetidinecarboxylate, l-Pip=l-2-piperidinecarboxylate) were prepared, characterized and used, together with the known [{(η6-pCym)Ru(l-Pro)}3](BF4)3, 5 and [{(η6-pCym)Ru(l-Ala)}3](BF4)3, 10 (l-Pro=l-prolinate, l-Ala=l-alaninate), in hydride transfer reduction of acetophenone, a series of substituted acetophenones and several other ketones with moderate to high conversions and enantioselectivities up to 86% e.e.

The synthesis and characterization of optically active amino acidate alkynyl complexes of general formula [(eta(n)-ring)M(aa)(C drop CR)] ((eta(n)-ring)M = (eta(5)-C5Me5)Rh; aa = L-prolinate (Pro); R = CMe3 (1), SiMe3 (2); (eta(n)-ring)M = (eta(5)-C5Me5)Ir; aa = Pro; R = CMe3 (3), SiMe3 (4); aa = N-methyl-L-prolinate (MePro); R = CMe3 (5), SiMe3 (6); (eta(n)-ring)M = (eta(6)-p-(MeC6H4Pr)-Pr-i)-Ru; aa = L-alaninate (Ala); R = Ph (7), CO2Me (8); aa = Pro; R = Ph (9), CO2Me (10)) are reported. The crystal structures of(R-Ir,S-C,S-N)-3b and (S-Ir,S-C,S-N)-5 were determined by X-ray analysis. Both molecular structures exhibit analogous pseudo-octahedral arrangements of ligands around the chiral iridium center. The metals are eta(5)-bonded to the pentamethylcyclopentadienyl group, linked in a chelate fashion through the aminic nitrogen and one of the carboxylate oxygens to the amino acidate, and bonded to a terminal almost linear tert-butyl ethynyl ligand. The configurational stability of the complexes was studied by circular dichroism and H-1 NMR spectroscopy. Complex 3b epimerizes at Ir in CDCl3 obeying a first-order rate law with Delta H-double dagger = 91.4 +/- 3.4 kJ mol(-1) and Delta Sdouble dagger- = -20.8 +/- 10.8 J K-1 mol(-1). The equilibrium constant for 3b reversible arrow 3a is 2.70 +/- 0.41 in methanol at 20 degrees C. A mechanistic interpretation of the epimerization process is proposed.

The synthesis and characterization of optically active imino complexes (R-M or S-M)-[(eta(5)-C5Me5)RhCl(imine)][SbF6] (imine = L-n = N-(2-pyridylmethylene)-(R)-1-phenylethylamine (L-1) (1a, a'), N-(2-pyridylmethylene)-(R)-1-naphthylethylamine (L-2) (2a, a'), N-(2-pyridylmethylene)-(R)-1-cyclohexylethylamine (L-3) (3a,a')) or [(eta(6)-p-(MeC6H4Pr)-Pr-i)RuCl(imine)]A (A = SbF6, imine = L-1 (4a,a'), L-2 (5a,a'), L-3 (6a,a'), N-(2-pyridylmethylene)-(1R,2S,4R)-1-bornylamine) (L-4) (7a,a'); (A = BF4, imine = L-1 (4b,b'), L-2 (5b,b'), L-3 (6b,b'), L-4 (7b,b')) is reported. The absolute crystal structures of the (R-Rh)-1a and (R-Ru)-7b epimers were determined by X-ray analysis. Both complexes possess a chiral metal center in a pseudo-octahedral environment, being bonded to an eta(5)-C5Me5 group (1a) or to an (eta(6)-p-(MeC6H4Pr)-Pr-i) ring (7b), a terminal chloride, and, in a chelating fashion, to the two nitrogen atoms of the imine ligand. At room temperature, in acetone or chloroform, the complexes are configurationally stable hut in refluxing methanol epimerize at the metal center. Dichloromethane/acetone solutions of the solvate complexes [(eta(n)-ring)M(imine)S](2+) are active catalysts for the Diels-Alder reaction between methacrolein or acrolein and cyclopentadiene. The reaction occurs rapidly at roam temperature and, in general, good exo/endo selectivities and moderate enantioselectivities are achieved.