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Aldehydes were chemoselectively reduced to primary alcohols by using HCOONH4 as the hydrogen donor through transfer hydrogenation catalyzed by benzo[h]quinoline pincer complexes RuCl(CNNPh)(PP) at substrate to catalyst molar ratios of 2000 to 20 000. This practical reaction performed with aldehydes of commercial-grade purity in a water/toluene biphasic system afforded alcohols without the formation of condensation or amination side products.

A cationic phthalimido-functionalized N-heterocyclic carbene (NHC) palladium(II) complex has been synthesized from [3-methyl-1-(2′-phthalimidoethyl)imidazolium] hexafluorophosphate ([NHCMe,PhtH]PF6) by transmetalation and isolated in 67 % yield. The title complex has been applied as catalyst in the Suzuki–Miyaura cross-coupling reaction under benign aqueous conditions. The catalyst is active without any observable initiation period. High average turnover frequencies (TOFs) of up to 55000 h–1 have been reached with catalyst concentrations as low as 0.01 mol-%.

CNN pincer ruthenium(II) and osmium(II) complexes of general formula RuCl(CNN)(dppb) (1 a-c) and OsCl(CNN)(dppb) (2 a,b) (dppb=Ph2P(CH2)4PPh2, HCNN=[6-(p-tolyl)-2-pyridyl]methylamine (4), N-methyl-[6-(p-tolyl)-2-pyridyl]methylamine (7) and N,N-dimethyl-[6-(p-tolyl)-2-pyridyl]methylamine (8)) were prepared and assessed as catalysts in the hydrogenation of a variety of imines. Among the examined complexes, the Ru-complex 1 a, RuCl(4)(dppb), showed the best activity, reducing N-aryl and N-alkyl imines to the related amines in high yields (up to 100 %) with a catalyst loading of 0.1-1 mol% and under mild conditions (5 atm H2, 40-70 °C). The Os-complexes exhibited a lower activity than their ruthenium analogues. Despite this, the Os-complex 2 a, OsCl(4)(dppb), was successful in the reduction of N-benzylideneaniline at 70 °C, affording a rare example of application of osmium complexes in the catalytic hydrogenation of imines.

This account focuses on the application in catalysis of ruthenium and osmium complexes containing 2-(aminomethyl)pyridine (Ampy)-based ligands. The combination of these aminoalkylpyridine ligands with appropriate phosphines affords ruthenium and osmium systems displaying unprecedented high catalytic activity and productivity in a variety of organic transformations such as hydrogenation by hydrogen transfer and dihydrogen, dehydrogenation, racemization, and alkylation.

The osmium compound trans,cis-[OsCl 2 (PPh 3 ) 2 (Pyme)] (1) (Pyme = 1-(pyridin-2-yl)methanamine), obtained from [OsCl 2 (PPh 3 ) 3 ] and Pyme, thermally isomerizes to cis,cis-[OsCl 2 (PPh 3 ) 2 (Pyme)] (2) in mesitylene at 150°C. Reaction of [OsCl 2 (PPh 3 ) 3 ] with Ph 2 P(CH 2 ) 4 PPh 2 (dppb) and Pyme in mesitylene (150 °C, 4 h) leads to a mixture of trans-[OsCl2(dppb)(Pyme)] (3) and cis-[OsCl2(dppb)(Pyme)] (4) in about an 1:3 molar ratio. The complex trans-(OsCl 2 (dppb)(Pyet)] (5) (Pyet= 2-(pyridin-2-yl)ethanamine) is formed by reaction of [OsCl 2 (PPh 3 ) 3 ] with dppb and Pyet in toluene at reflux. Compounds 1, 2, 5 and the mixture of isomers 3/4 efficiently catalyze the transfer hydrogenation (TH) of different ketones in refluxing 2-propanol and in the presence of NaOiPr (2.0 mol%). Interestingly, 3/4 has been proven to reduce different ketones (even bulky) by means of TH with a remarkably high turnover frequency (TOF up to 5.7x10 5 h -1 ) and at very low loading (0.05-0.001 mol%). The system 3/4 also efficiently catalyzes the hydrogenation of many ketones (H 2 , 5.0 atm) in ethanol with KOtBu (2.0 mol%) at 70°C (TOF up to 1.5× 10 4 h -1 ). The in-situ-generated catalysts prepared by the reaction of [OsCl 2 -(PPh 3 ) 3 ] with Josiphos diphosphanes and (±)-1-alkyl-substituted Pyme ligands, promote the enantioselective TH of different ketones with 91-96% ee (ee=enantiomeric excess) and with a TOF of up to 1.9×10 4 h -1 at 60°C.

The catalytic activity of the terdentate complex [RuCl(CNN)(dppb)] (A) [dppb=Ph(2)P(CH(2))(4)PPh(2); HCNN=6-(4'-methylphenyl)-2-pyridylmethylamine] in the transfer hydrogenation of acetophenone (S) with 2-propanol has been found to be dependent on the base concentration. The limit rate has been observed when NaOiPr is used in high excess (A/base molar ratio10). The amino-isopropoxide species [Ru(OiPr)(CNN)(dppb)] (B), which forms by reaction of A with sodium isopropoxide via displacement of the chloride, is catalytically active. The rate of conversion of acetophenone obeys second-order kinetics v=k[S][B] with the rate constants in the range 218+/-8 (40 degrees C) to 3000+/-70 M(-1) s(-1) (80 degrees C). The activation parameters, evaluated from the Eyring equation are DeltaH(++)=14.0+/-0.2 kcal mol(-1) and DeltaS(++)=-3.2 +/-0.5 eu. In a pre-equilibrium reaction with 2-propanol complex B gives the cationic species [Ru(CNN)(dppb)(HOiPr)](+)[OiPr](-) (C) with K approximately 2x10(-5) M. The hydride species [RuH(CNN)(dppb)] (H), which forms from B via beta-hydrogen elimination process, catalyzes the reduction of S and, importantly, its activity increases by addition of base. The catalytic behavior of the hydride H has been compared to that of the system A/NaOiPr (1:1 molar ratio) and indicates that the two systems are equivalent.

The Vaska-type iridium(I) complex [IrCl(CO){PPh(2)(2-MeC(6)H(4))}(2)] (1), characterized by an X-ray diffraction study, was obtained from iridium(III) chloride hydrate and PPh(2)(2,6-MeRC(6)H(3)) with R=H in DMF, whereas for R=Me, activation of two ortho-methyl groups resulted in the biscyclometalated iridium(III) compound [IrCl(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}(2)] (2). Conversely, for R=Me the iridium(I) compound [IrCl(CO){PPh(2)(2,6-Me(2)C(6)H(3))}(2)] (3) can be obtained by treatment of [IrCl(COE)(2)](2) (COE=cyclooctene) with carbon monoxide and the phosphane in acetonitrile. Compound 3 in CH(2)Cl(2) undergoes intramolecular C-H oxidative addition, affording the cyclometalated hydride iridium(III) species [IrHCl(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}{PPh(2)(2,6-Me(2)C(6)H(3))}] (4). Treatment of 2 with Na[BAr(f) (4)] (Ar(f)=3,5-C(6)H(3)(CF(3))(2)) gives the fluxional cationic 16-electron complex [Ir(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}(2)][BAr(f) (4)] (5), which reversibly reacts with dihydrogen to afford the delta-agostic complex [IrH(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}{PPh(2)(2,6-Me(2)C(6)H(3))}][BAr(f)(4)] (6), through cleavage of an Ir-C bond. This species can also be formed by treatment of 4 with Na[BAr(f)(4)] or of 2 with Na[BAr(f)(4)] through C-H oxidative addition of one ortho-methyl group, via a transient 14-electron iridium(I) complex. Heating of the coordinatively unsaturated biscyclometalated species 5 in toluene gives the trans-dihydride iridium(III) complex [IrH(2)(CO){PPh(2)(2,6-MeC(6)H(3)CH=CHC(6)H(3)Me-2,6)PPh(2)}][BAr(f) (4)] (7), containing a trans-stilbene-type terdentate ligand, as result of a dehydrogenative carbon-carbon double bond coupling reaction, possibly through an iridium carbene species.

The complexes cis-[RuCl2(ampy){R1N(CH2PPh2)2}] [ampy = (2-aminomethyl)pyridine, R1 = C6H5 or (CH3CH2O)3Si(CH2)3] showed very high catalytic activity in the homogeneous transfer hydrogenation of acetophenone (TOF > 300000 h–1) with the use of 2-propanol as the hydrogen donor and in the presence of sodium isopropoxide. The ligand (CH3CH2O)3Si(CH2)3N(CH2PPh2)2 (ATM) was prepared in high yield by reacting (3-aminopropyl)triethoxysilane, paraformaldehyde, and diphenylphosphane in toluene heated at 80 °C. The –N(CH2PPh2)2 function was attached to the surface of three different kinds of silica by means of two alternative methods. Thus, MA-Si-150 and mesoporous MA-Si-MCM-41 were prepared by the reaction of the inorganic matrix with ATM, whereas MA-Si-60 was synthesized by reaction of the commercially available 3-aminopropyl-functionalized silica gel with HCHO and PHPh2 in refluxing toluene. The –(CH2)3N(CH2PPh2)2 functionalized inorganic materials were used to anchor the RuCl2(ampy) moiety; thus, three different silica-immobilized versions of the cis-[RuCl2(ampy)(ATM)] complex were obtained. The silica-anchored complexes were tested in the transfer hydrogenation of acetophenone, which was found to be fast and quantitative; the effect of the nature of the silica support on the activity of the catalyst was almost negligible. It was possible to reuse the catalytic system for a second cycle without a decrease in the activity, but the efficiency of the catalyst considerably diminished in successive reuses.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

CONSPECTUS: A current issue in metal-catalyzed reactions is the search for highly efficient transition-metal complexes affording high productivity and selectivity in a variety of processes. Moreover, there is also a great interest in multitasking catalysts that are able to efficiently promote different organic transformations by careful switching of the reaction parameters, such as temperature, solvent, and cocatalyst. In this context, osmium complexes have shown the ability to catalyze efficiently different types of reactions involving hydrogen, proving at the same time high thermal stability and simple synthesis. In the catalytic reduction of C═X (X = O, N) bonds by both hydrogenation (HY) and transfer hydrogenation (TH) reactions, the most interest has been focused on homogeneous systems based on rhodium, iridium, and in particular ruthenium catalysts, which have proved to catalyze chemo- and stereoselective hydrogenations with remarkable efficiency. By contrast, osmium catalysts have received much less attention because they are considered less active on account of their slower ligand exchange kinetics. Thus, this area remained almost neglected until recent studies refuted these prejudices. The aim of this Account is to highlight the impressive developments achieved over the past few years by our and other groups on the design of new classes of osmium complexes and their applications in homogeneous catalytic reactions involving the hydrogenation of carbon-oxygen and carbon-nitrogen bonds by both HY and TH reactions as well as in alcohol deydrogenation (DHY) reactions. The work described in this Account demonstrates that osmium complexes are emerging as powerful catalysts for asymmetric and non-asymmetric syntheses, showing a remarkably high catalytic activity in HY and TH reactions of ketones, aldehydes, imines, and esters as well in DHY reactions of alcohols. Thus, for instance, the introduction of ligands with an NH function, possibly in combination with a pyridine ring, led to a new family of [OsCl2(PP)(NN)] (NN = diamine, 2-aminomethylpyridine; PP = diphosphine) and pincer [OsCl(CNN)(PP)] (HCNN = 6-aryl-2-aminomethylpyridine, 2-aminomethylbenzo[h]quinoline) complexes, which are outstanding catalysts for (asymmetric) HY and TH of carbonyl compounds and DHY of alcohols with turnover numbers and turnover frequencies up to 10(5) and 10(6) h(-1), respectively. In addition, PNN osmium complexes containing the 2-aminomethylpyridine motif have been found to be among the most active catalysts for HY of esters. These complexes have shown catalytic activities that are comparable and in some cases superior to those reported for analogous ruthenium systems. These results give an idea of the potential of Os complexes for the design of new highly productive and robust catalysts for the synthesis of chiral and nonchiral alcohols and amines as well as ketones from alcohols. Thus, we hope that this report will promote increased interest in the chemistry of these metal complexes, opening novel opportunities for new catalytic processes as well as the improvement of existing ones.

The reaction of N,N-dimethyl-2-propen-1-amine, N,N-dimethylbenzylamine and N,N-dimethyl-2-propyn-1-amine with the α-diazo ketones N2CHCOR [R = Me (2a), Et (2b), nPr (2c), iPr (2d), (CH2)10Me (2e), (CH2)14Me (2f) or Ph (2g)] in a 1:1 molar ratio, catalysed by the complex [RuCl(η5-C5H5)(PPh3)2] (1) (1% mol), have been investigated. Noticeably, the corresponding α-amino ketones 3a−g, 4a−g and 5a−g are readily and quantitatively formed by rearrangement of transient nitrogen ylides. Compounds 3−5, most of which have not yet been reported, have been isolated and fully characterised by IR, 1H and 13C NMR spectroscopy and GC-MS. It has been proved that complex 1 is an excellent and specific catalyst for the generation of nitrogen ylides from diazo carbonyls.

A simple one-step method for the preparation of dinuclear chloro-bridged platinum(II) phosphine complexes, [Pt(μ-Cl)Cl(PR3)]2 is reported. The syntheses are carried out in p-chlorotoluene and are reported for the following tertiary phosphines: PMe2Ph, P(p-Tol)3, PPh3, PBun3 and PEt3.

The osmium complexes trans-[OsCl(2)(dppf)(diamine)] (dppf: 1,1'-bis(diphenylphosphino)ferrocene; diamine: ethylenediamine in 3, propylenediamine in 4) were prepared by the reaction of [OsCl(2)(PPh(3))(3)] (1) with the ferrocenyl diphosphane, dppf and the corresponding diamine in dichloromethane. The reaction of derivative 3 with NaOCH(2)CF(3) in toluene afforded the alkoxide cis-[Os(OCH(2)CF(3))(2)(dppf)(ethylenediamine)] (5). The novel precursor [Os(2)Cl(4)(P(m-tolyl)(3))(5)] (2) allows the synthesis of the chiral complexes trans-[OsCl(2)(diphosphane)(1,2-diamine)] (6-9; diphosphane: (R)-[6,6'-dimethoxy(1,1'-biphenyl)-2,2'-diyl]bis[1,1-bis(3,5-dimethylphenyl)phosphane] (xylMeObiphep) or (R)-(1,1'-binaphthalene)-2,2'-diylbis[1,1-bis(3,5-dimethylphenyl)phosphane] (xylbinap); diamine=(R,R)-1,2-diphenylethylenediamine (dpen) or (R,R)-1,2-diaminocyclohexane (dach)), obtained by the treatment of 2 with the diphosphane and the 1,2-diamine in toluene at reflux temperature. Compounds 3-5 in ethanol and in the presence of NaOEt catalyze the reduction of methyl aryl, dialkyl, and diaryl ketones and aldehydes with H(2) at low pressure (5 atm), with substrate/catalyst (S/C) ratios of 10,000-200,000 and achieving turnover frequencies (TOFs) of up to 3.0 x 10(5) h(-1) at 70 degrees C. By employment of the chiral compounds 6-9, different ketones, including alkyl aryl, bulky tert-butyl, and cyclic ketones, have successfully been hydrogenated with enantioselectivities up to 99% and with S/C ratios of 5000-100,000 and TOFs of up to 4.1 x 10(4) h(-1) at 60 degrees C.

The results of a ligand-free Pd(OAc)2-catalyzed Suzuki–Miyaura C–C coupling performed at room temperature under aerobic conditions are presented. It was found that the use of an ethylene glycol monomethyl ether/H2O mixture as the solvent resulted in very rapid reactions of aryl bromides with arylboronic acids. As a matter of fact, under optimized conditions, some substrates were converted quantitatively in less than 1 min with exceptionally high TOF values. For example, the reaction between 4-methoxyphenylboronic acid and bromobenzene afforded 4-methoxybiphenyl in 30 s with TOF = 180000 h–1. Furthermore, the reaction tolerates a wide range of functional groups and can be successfully applied to heteroaryl bromides such as 2-bromopyridine and 5-bromopyrimidine. Interestingly, also an activated aryl chloride such as 1-chloro-4-nitrobenzene reacted quantitatively with phenylboronic acid at 373 K. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

"Bidentate" ligand behavior is shown by (2,6-dimethylphenyl)diphenylphosphane in the title compound: In the nearly octahedral environment of the ruthenium atom two coordination sites are occupied by methyl groups of the two xylyl substituents. NMR investigation and an X-ray analysis (see picture) reveal that the methyl groups act as weak donors to form two strong agostic Ru⋅⋅⋅C-H interactions.

This Microreview focuses on the development of novel ruthenium complexes displaying high performance in the catalytic asymmetric transfer hydrogenation of ketones with 2-propanol, providing this procedure alternative to the hydrogenation with dihydrogen. The key role is played by 1-(pyridin-2-yl)methanamine (Pyme) type ligands which in combination with appropriate phosphanes afforded ruthenium systems of unprecedented high catalytic activity and productivity for the reduction of ketones and aldehydes. For the pincer CNN complexes a mixed inner-outer sphere mechanism involving Ru-hydride and Ru-alkoxide species is proposed. The excellent properties of these complexes are expected to have implications for the design of a new generation of catalysts.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Ruthenium cis -dichloro complexes with matched chiral diphosphane and aminopyridine ligands (see structure) are easily obtained by a one-pot reaction of [RuCl 2 (PPh 3 ) 3 ] with a Josiphos diphosphane and racemic 1-substituted 1-(pyridin-2- γl)methanamine ligands. These complexes are highly active catalysts for the transfer hydrogenation of ketones, affording turnover frequencies of up to 70 000 h -1 at 60 ° C and ee values up to 99 %.

Aromatic, aliphatic and α,β-unsaturated aldehydes are quickly, quantitatively and chemose-lectively reduced to primary alcohols with 2-propanol using 0.05-0.01 mol% of the terdentate CNN ruthenium complex RuCl(CNN)(dppb) (1) [HCNN = 6-(4'-methylphenyl)-2-pyridylmethylamine; dppb=Ph 2 P(CH 2 ) 4 PPh 2 ] in the presence of potassium carbonate (K 2 CO 3 ; 1-10 mol%) as a weak base, affording TOF values up to 5.0 ×10 5 h -1 .

The ruthenium(II) half-sandwich complex [RuCl(η5-C5H5)(PPh3)2] (1) catalyses the reaction between methanol and alkylamines RNH2 or R1R2NH to afford RN(CH3)2 and R1R2NCH3 products, respectively. The reaction is quantitative and generally fast, at the methanol reflux temperature, for a wide spectrum of substrates. Starting from primary amines, the stepwise formation of RN=CH2, RNHCH3, and RN(CH3)2 has been observed. Both PPh3 and Cl− dissociation from 1 are key-steps in forming the effective catalytic species. The catalytic activity of several half-sandwich neutral or cationic complexes (2−15) related to 1 is also discussed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)