The complexes trans,cis-RuCl2(PPh3)2(ampy) (1) and trans-RuCl2[Ph2P(CH2)4PPh2](ampy) (2) have been prepared in high yield by reaction of RuCl2(PPh3)3 and RuCl2(PPh3)[Ph2P(CH2)4PPh2] with 2-(aminomethyl)pyridine (ampy) at room temperature by PPh3 displacement. Heating compound 1 in refluxing toluene leads to the isomer cis,cis-RuCl2(PPh3)2(ampy) (3), which has been proven to be a good precursor for the preparation of the complexes cis-RuCl2(PP)(ampy) [PP = (S,S)-Chiraphos, 4; Ph2P(CH2)3PPh2, 5; (S,S)-Skewphos, 6; Ph2P(CH2)4PPh2, 7; (R,R)-Diop, 8] by displacement of two PPh3 with the appropriate diphosphine. The derivatives cis-RuCl2(PP)(ampy) [PP = (R,S)-Josiphos, 9; (R,S)-tBu-Josiphos, 10] have been synthesized from RuCl2(PPh3)3 and PP followed by addition of ampy. The chiral complexes 4, 6, 8, 9, and 10 are formed stereoselectively, as inferred by NMR data in solution. For the derivatives 7 and 9 the molecular structures have been determined by X-ray measurements. The monohydride complex trans,cis-RuHCl(PPh3)2(ampy) (11) has been prepared from RuHCl(PPh3)3 and ampy in heptane by PPh3 substitution. Compound 11 reacts with sodium isopropoxide in toluene, affording the dihydride derivative cis,trans-Ru(H)2(PPh3)2(ampy) (12) via the alkoxide route. The intermediate species cis,cis-Ru(H)2(PPh3)2(ampy) (A) has been also characterized by NMR in solution. All these complexes have been found to be highly efficient transfer hydrogenation catalysts. With the complexes cis-RuCl2(PP)(ampy) a large number of ketones (dialkyl, diaryl, and alkyl-aryl) can be quantitatively reduced to alcohols in 2-propanol and in the presence of NaOH (ketone/Ru/NaOH = 2000/1/40) with remarkably high TOF values (up to 400 000 h-1 at 50% conversion). The derivatives containing chiral diphosphines afforded rapid (TOF > 105 h-1) and enantioselective (ee up to 94%) reduction of methyl-aryl ketones using low loading of catalysts (0.05−0.01 mol %). In the absence of base the dihydride compound 12 catalyzes the transfer hydrogenation of acetophenone.

Dedicated to Professor Fausto Calderazzo on the occasion of his 75th birthdayPincer ECE (E = N, P) transition-metal complexes, in which the terdentate ligands contain two stable five-membered cyclometalated rings, have reached a high level of sophistication and appear extremely attractive for both catalytic and stoichiometric reactions.[1] The great interest in pincer ligands arises from the high level of control over the reactivity and the stereochemistry that they impose around the metal center as a result of their electronic properties and geometric restrictions. These factors afford highly selective transformations and lead to some unique species of relevance for the investigation of elementary processes.[2] In spite of the great attention afforded to ruthenium for its versatility in catalysis, [3] no examples of terdentate ruthenium CNN catalysts have been reported thus far. It is worth noting that CNN complexes are expected to have significantly different reactivity compared to the NCN analogues, mainly because of the different geometrical disposition of the s-donor carbon atom.Recently, we have shown that 2-(aminomethyl)pyridine (ampy) shows a high ligand acceleration effect in transfer hydrogenation [4] catalyzed by ruthenium(ii) complexes with phosphane ligands. Thus, complete reduction of many ketones in 2-propanol is quickly achieved with the cyclometalated complex [RuCl(CO)(CP)(ampy)] (CP = (2-CH 2 -6-MeC 6 H 3 )PPh 2 ), with turnover frequencies (TOF values) up to 6.3 10 4 h À1 , whereas the derivatives cis-[RuCl 2 (PP)-(ampy)] (PP = diphosphane) lead to TOF values up to 4.0 10 5 h À1 and ee values up to 94 % by using chiral diphosphanes.[5] Since it is well known that 2-phenylpyridine readily gives access to orthometalated CN ruthenium complexes, [6] we decided to investigate the coordination chemistry of the related 6-(4'-methylphenyl)-2-pyridylmethylamine with the aim of obtaining terdentate CNN complexes. We report herein on novel complexes of formula [RuX(CNN)(dppb)] (X = Cl, H; dppb = Ph 2 P(CH 2 ) 4 PPh 2 ), which are remarkably active catalysts for transfer hydrogenation that afford TOF values up to 2.5 10 6 h À1 with very low loading of catalysts (0.005-0.001 mol %) compared to the most active systems reported. [2f, 7] Evidence is provided that the reduction of the ketone proceeds through the formation of a Ru IIalkoxide complex by insertion of the carbonyl group of the substrate into the RuÀH bond of a ruthenium(ii) hydride formed as an intermediate from the chloride complex 1.Treatment of [RuCl 2 (PPh 3 )(dppb)] with an equimolar amount of 6-(4'-methylphenyl)-2-pyridylmethylamine in 2-propanol at reflux in the presence of NEt 3 affords the thermally stable orthometalated ruthenium(ii) complex 1 [8] in high yield [Eq. (1)].The signals for the diastereotopic NCH 2 protons in the 1 H NMR spectrum are at d = 4.12 and 3.72 ppm with 2 J(H,H) = 15.5 Hz. The doublet at d = 52.5 ppm with a 3 J(C,P) = 2.7 Hz in the 13 C NMR spectrum corresponds to the CH 2 N group which is shifted downfield relati...

A neutron diffraction study of the complex RuCl(2)[PPh(2)(2,6-Me(2)C(6)H(3))](2) (1) defines the precise nature of the delta agostic interactions between the unsaturated metal center and two o-methyl groups of the xylyl substituents. The CH(3) carbon atoms lie in the RuP(2) equatorial plane with Ru...C distances of 2.637(7) and 2.668(6) A, whereas four short Ru.H distances (from 2.113(11) to 2.507(11) A) indicate that each methyl group interacts with two C-H bonds. A survey of the X-ray structures with beta, gamma, delta, and epsilon M...H(3)C-C moieties (no neutron data have been previously reported) shows a linear correlation between the angle M.C-C and the torsion of the methyl group about the C-C bond. Thus, the agostic interactions span the range between the classical (M...eta(2)-HC) and the nonclassical (M...eta(3)-H(2)C) types. A solution study of 1 shows intramolecular rearrangement of each xylyl substituent that equilibrates the environments of its two ortho CH(3) groups. Activation parameters, evaluated from the analysis of (1)H NMR line shape as a function of temperature, are Delta H(++) = 9.6 +/- 0.2 kcal mol(-1) with Delta S(++) = -15.4 +/- 0.7 eu (CDCl(3)). The related 14-electron complexes RuX(2)[PPh(2)(2,6-Me(2)C(6)H(3))](2) (X = I, 2; NCO, 3), prepared from 1 and NaX, show a similar dynamic process in solution, with the iodo derivative displaying the most hindered rotation of the xylyl group. A DFT optimization of the complex RuCl(2)[PH(2)(2,6-Me(2)C(6)H(3))](2) (1a) reproduces well the nonclassical Ru...eta(3)-H(2)C agostic mode, whereas the classical Ru...eta(2)-HC one corresponds to a transition state 1b, destabilized by 3.4 kcal mol(-1). A similar barrier (ca. 3.8 kcal mol(-1)) is calculated for the xylyl rotation in the further simplified model RuCl(2)[PH(2)(2,6-Me(2)C(6)H(3))][PH(2)CH[double bond]CHCH(3)] (1c), the absence of bulky phenyl substituents being largely responsible for the difference with respect to the experimental value. Finally, the MO analysis addresses the intrinsic stability of the 14-electron complex RuCl(2)(PH(3))(2) and, in agostic complexes, accounts for the different interactions between the methyl group and the metal atom in relation to the length of their interconnecting chain.

Terdentate ruthenium(II) complexes of general formula RuX(CNN)(dppb) (X ) chloride, hydride, alkoxide; dppb ) Ph 2 P(CH 2 ) 4 PPh 2 ), where CNN is a deprotonated 2-aminomethyl-6-arylpyridine ligand, have been prepared. The orthometalated derivative RuCl(b)(dppb) (1) has been obtained by reaction of RuCl 2 (PPh 3 )(dppb) with N, N-dimethyl-2-aminomethyl-6-(4-methylphenyl)pyridine (Hb) in 2-propanol and in the presence of triethylamine by elimination of PPh 3 and HCl. Similarly, RuCl(a)(dppb) (2) and the chiral analogue RuCl(c)(dppb) (3), containing primary amine ligands, have been isolated starting from 2-aminomethyl-6-(4-methylphenyl)pyridine (Ha) and (R)-2,2-dimethyl-1-(6-phenylpyridin-2-yl)propylamine (Hc), respectively. The synthesis of the functionalized pyridines Ha-Hc is here described, whereas the crystal structure of 3 has been determined through an X-ray diffraction experiment. Treatment of 1-3 with sodium or potassium isopropoxide gives the corresponding hydrides RuH(b)(dppb) (4), RuH(a)(dppb) (5), and RuH(c)(dppb) (6) from the ruthenium isopropoxide complexes, via a β-H elimination process. Studies in solution show that the isopropoxides bearing a NH donor group are in equilibrium with the corresponding hydrides (5 and 6). Reaction of 5 with benzophenone leads to the alkoxide Ru(OCHPh 2 )(a)(dppb) (7), which has been proven to interact with benzhydrol in C 6 D 6 , leading to the adduct 7‚(HOCHPh 2 ), the alkoxide ligand, and the alcohol being in rapid exchange. Complexes 2 and 3 display a remarkable high catalytic activity for the transfer hydrogenation of ketones to alcohol in 2-propanol using a very small amount of catalyst. With the chiral complex 3 (0.005 mol %) methyl-aryl ketones can be quickly reduced (TOF ranging from 5.4 × 10 5 to 1.4 × 10 6 h -1 ) with an enatiomeric excess up to 88%.

"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.

New benzo[h]quinoline ligands (HCN'N) containing a CHRNH2 (R=H (a), Me (b), tBu (c)) function in the 2-position were prepared starting from benzo[h]quinoline N-oxide (in the case of ligand a) and 2-chlorobenzo[h]quinoline (for ligands b and c). These compounds were used to prepare ruthenium and osmium complexes, which are excellent catalysts for the transfer hydrogenation (TH) of ketones. The reaction of a with [RuCl2(PPh3)3] in 2-propanol at reflux afforded the terdentate CN'N complex [RuCl(CN'N)(PPh3)2] (1), whereas the complexes [RuCl(CN'N)(dppb)] (2-4; dppb=Ph2P(CH2)4PPh2) were obtained from [RuCl2(PPh3)(dppb)] with a-c, respectively. Employment of (R,S)-Josiphos, (S,R)-Josiphos*, (S,S)-Skewphos, and (S)-MeO-Biphep in combination with [RuCl2(PPh3)3] and ligand a gave the chiral derivatives [RuCl(CN'N)(PP)] (5-8). The osmium complex [OsCl(CN'N)(dppb)] (12) was prepared by treatment of [OsCl2(PPh3)3] with dppb and ligand a. Reaction of the chloride 2 and 12 with NaOiPr in 2-propanol/toluene afforded the hydride complexes [MH(CN'N)(dppb)] (M=Ru 10, Os 14), through elimination of acetone from [M(OiPr)(CN'N)(dppb)] (M=Ru 9, Os 13). The species 9 and 13 easily reacted with 4,4'-difluorobenzophenone, via 10 and 14, respectively, affording the corresponding isolable alkoxides [M(OR)(CN'N)(dppb)] (M=Ru 11, Os 15). The complexes [MX(CN'N)(P2)] (1-15) (M=Ru, Os; X=Cl, H, OR; P=PPh3 and P2=diphosphane) are efficient catalysts for the TH of carbonyl compounds with 2-propanol in the presence of NaOiPr (2 mol %). Turnover frequency (TOF) values up to 1.8x10(6) h(-1) have been achieved using 0.02-0.001 mol % of catalyst. Much the same activity has been observed for the Ru--Cl, --H, --OR, and the Os--Cl derivatives, whereas the Os--H and Os--OR derivatives display significantly lower activity on account of their high oxygen sensitivity. The chiral Ru complexes 5-8 catalyze the asymmetric TH of methyl-aryl ketones with TOF approximately 10(5) h(-1) at 60 degrees C, up to 97 % enatiomeric excess (ee) and remarkably high productivity (0.005 mol % catalyst loading). High catalytic activity (TOF up to 2.2x10(5) h(-1)) and enantioselectivity (up to 98 % ee) have also been achieved with the in-situ-generated catalysts prepared from [MCl2(PPh3)3], (S,R)-Josiphos or (S,R)-Josiphos*, and the benzo[h]quinoline ligands a-c.

The ruthenium and osmium complexes [MCl(2)(diphosphane)(L)] (M=Ru, Os; L=bidentate amino ligand) and [MCl(CNN)(dppb)] (CNN=pincer ligand; dppb=1,4-bis-(diphenylphosphino)butane), containing the N−H moiety, have been found to catalyze the acceptorless dehydrogenation of alcohols in tBuOH and in the presence of KOtBu. The compounds trans-[MCl(2)(dppf)(en)] (M=Ru 7, Os 13; dppf=1,1'-bisdiphenylphosphino)ferrocene; en=ethylenediamine) display very high activity and different substrates, including cyclic and linear alcohols, are efficiently oxidized to ketones by using 0.8-0.04 mol % of catalyst. The effect of the base and the comparison of the catalytic activity of the Ru versus Os complexes are reported. The ruthenium complex 7 generally leads to a faster conversion into ketones with respect to the osmium complex 13, which displays better activity in the dehydrogenation of 5-en-3β-hydroxy steroids. The synthesis of new Ru and Os complexes [MCl(2)(PP)(L)] (PP=dppb, dppf; L=(±)-trans-1,2-diaminocyclohexane,2-(aminomethyl)pyridine, and 2-aminoethanol) of trans and cis configuration is also reported.

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 ratio > 10). 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.