Planar Ligand Research Articles

Microsecond X-ray Absorption Spectroscopy Identification of Co(I) Intermediates in Cobaloxime-Catalyzed Hydrogen Evolution.

Rational development of efficient photocatalytic systems for hydrogen production requires understanding the catalytic mechanism and detailed information about the structure of intermediates in the catalytic cycle. We demonstrate how time-resolved X-ray absorption spectroscopy in the microsecond time range can be used to identify such intermediates and to determine their local geometric structure. This method was used to obtain the solution structure of the Co(I) intermediate of cobaloxime, which is a non-noble metal catalyst for solar hydrogen production from water. Distances between cobalt and the nearest ligands including two solvent molecules and displacement of the cobalt atom out of plane formed by the planar ligands have been determined. Combining in situ X-ray absorption and UV/Vis data, we demonstrate how slight modification of the catalyst structure can lead to the formation of a catalytically inactive Co(I) state under similar conditions. Possible deactivation mechanisms are discussed.

Synthesis and stability of 2+1 complexes of N,N-diethylbenzoylthiourea with [MI(CO)3]+ (M = Re, 99mTc)

In the last two decades, the fac-[MI(OH2)3(CO)3]+ (M = Re, 99mTc) core has been an important synthon in the development of new radiopharmaceuticals and methodologies. A bidentate N,N-diethylbenzoylthiourea (HEt2btu, H1) ligand was reacted with fac-[MI(OH2)3(CO)3]+ as part of a 2+1 strategy consisting of a bidentate and a monodentate ligands to saturate the coordination sphere. When excess H1 was used in the reaction, an unusual 2+1 complex of fac-[ReI(CO)3(1)(H1)], 2, was isolated that contained two H1 ligands, OS bidentate and S monodentate in different protonation states. Equimolar concentrations of HEt2btu and fac-[ReI(CO)3]+ generated the single addition of the ligand; however, isolation of the solid revealed a μ-sulfur-bridged dimer of fac-[ReI(CO)31)]2, 3. Reaction of H1 with fac-[MI(OH2)3(CO)3]+ followed by the addition of a monodentate ligand (i.e. pyridine, methyl imidazole, cyclohexyl isocyanide, triphenylphosphine) yielded the corresponding 2+1 fac-[MI(CO)3(1)(L)] complexes. Single-crystal X-ray structural analysis of the Re complexes revealed conformational differences of the ligand on the metal depending on the protonation state of the central amine in the protonated H1 versus 1 ligand. Comparison of these complexes with previous complexes and free H1 revealed significant shifts in the bond angles, bond lengths, and ligand planarity that are discussed further. The corresponding 2+1 fac-[99mTcI(CO)3(1)(L)] complexes were also prepared in moderate to good yields and their stability examined by amino acid challenge studies.

Complexes of 2,6-diacetylpyridine dihydrazone with middle and late first row transition metals

A study of the transition metal chemistry with the planar tridentate ligand 2,6-diacetylpyridine dihydrazone (1, DAPH) is presented, expanding the chemistry of this ligand across the first row of the transition metals. Ligand 1 forms both 1:1 and homoleptic 2:1 complexes with transition metal ions, depending on the metal salt used and reaction conditions, and seven new complexes with Mn, Fe, Co, Cu and Zn are presented. All compounds, including the ligand, have been characterized by X-ray crystallography. DAPH (1) binds to metal ions in an asymmetric fashion, with a M–Npyr bond that is shorter than the flanking M–Nimine bonds. Additionally, the Nimine–M–Nimine bonds form acute angles ranging from ~140 to 160°, depending on the identity of the metal ion and ligand environment. All compounds are compared with previously elucidated DAPH structures from the literature.

The crystal structure of a novel Pt(II) complex with 2-hydroxy-6-methylpyridine (Hmhp), trans-[PtCl2(dmso)(Hmhp)]·H2O.

A novel platinum(II) complex with 2-hydroxy-6-methylpyridine (Hmhp), trans-[PtCl2(dmso)(Hmhp)]·H2O, crystallizes in the space group P 2 1/n with a = 11.5505(1) Å, b = 8.9467(1) Å, c = 14.6047(2) Å and β = 112.3919(6)°. Platinum ion is in the expected square-planar environment. The 2-hydroxy-6-methylpyridine ligand is coordinated to the metal ion in a monodentate manner via nitrogen. The plane of the aromatic ligand is nearly perpendicular to the platinum coordination plane. Pt(II) ion is shielded with pyridine ortho substituents above and below the square coordination plane of platinum. Hydrogen bonding between the complex molecules and the H2O molecules of crystallization produces infinite two-dimensional layers. Weak intermolecular interactions of the C-H···Cl type link the layers into a three-dimensional structure.

Synthesis and Structural Facets of Dialkyltin(IV) Complexes Constructed from 2‐(2‐(3, 5‐Dimethyl‐4‐oxocyclohexa‐2, 5‐dien‐1‐ylidene)hydrazinyl)benzoate

AbstractTwo new dialkyltin(IV) complexes of compositions [Me2Sn(LH)2(MeOH)] (1) and [nBu2Sn(LH)2(H2O)] (2) [LH– = 2‐(2‐(3, 5‐dimethyl‐4‐oxocyclohexa‐2, 5‐dien‐1‐ylidene)hydrazinyl)benzoate] were prepared by the reactions of the carboxylate ligand (LHH′) and Me2SnO or nBu2SnO in toluene solution followed by recrystallization from methanol/toluene. In the crystal structures of 1 and 2, the central tin atom is chelated by two carboxylate anions, a solvent molecule derived from the crystallization medium, and two alkyl groups, giving rise to a distorted pentagonal bipyramidal coordination. The phenol hydrogen atom has migrated to the nitrogen atom nearest the carboxylate group, thus leading to the quinoid form of the ligand. Extensive π···π interactions between the planar ligands, coupled with O–H···O, C–H···O, and C–H···π interactions complete three‐dimensional supramolecular frameworks in each case. The solution properties were assessed from 1H, 13C, and 119Sn NMR spectroscopy.

Zero‐Field Splitting in {MnIII3(μ3‐O)} Core Single‐Molecule Magnets Investigated by Inelastic Neutron Scattering and High‐Field Electron Paramagnetic Resonance Spectroscopy

AbstractThe global zero‐field splitting (ZFS) parameters of three, ferromagnetically coupled, μ3‐κ3‐[XO4]– (X = Cl, Re) capped, manganese(III) oximate single‐molecule magnets, [Mn3O(R‐sao)3(2,4′‐bipyridine)3XO4] [X = Cl, R = Me, Et; X = Re, R = Me; Me‐sao = 2‐hydroxyphenylethanone oximate(2–)], with crystallographic trigonal symmetry were determined by use of inelastic neutron scattering and high‐field/high‐frequency electron paramagnetic resonance spectroscopy. ReO4– (O···O ca. 1.7 Å) is larger than ClO4– (O···O ca. 1.4 Å), which allows more parallel alignment of the local ZFS tensors. However, this chemical modification results in concomitant distortions in the equatorial ligand plane. Consistent parametrization of all spectroscopic data was achieved, and effective spin‐reversal barriers determined from alternating current susceptibility data were shown to be in good accordance with the energy barriers deduced from spectroscopy.

Pentanuclear and Octanuclear Manganese Helices

Invited for the cover of this issue is the group of Hiroki Oshio at the University of Tsukuba, Japan. The cover image shows the metaphoric link between the synthesis of two new manganese clusters and the ancient Japanese art of origami. Origami literally means “folded paper” and allows elaborate shapes to be created from the simplest materials, just like the rigid, planar ligands that were employed to “fold” manganese ions into sophisticated helical superstructures.

Electrochemical-driven water splitting catalyzed by a water-soluble cobalt(II) complex supported by N,N′-bis(2′-pyridinecarboxamide)-1,2-benzene with high turnover frequency

The oxidation and reduction of water is a key challenge in the production of chemical fuels from electricity. Reported here is a soluble cobalt (II) complex, [Co(bpbH2)Cl2] 1 (bpbH2: N,N′-bis(2′-pyridinecarboxamide)-1,2-benzene), a highly active homogeneous electrocatalyst for both electrolytic water oxidation and reduction in purely aqueous solution. Electrochemical studies indicate that the catalyst is a water-soluble molecular species, that is among the most rapid homogeneous catalysts for water oxidation, with a turnover frequency of ∼81.54 s−1 (at pH 8.6, the lowest pH among those of any reported electrocatalysts) at an overpotential of 560 mV. 1 also can catalyze hydrogen evolution from water with a TOF of 376 mol of hydrogen per mole of catalyst per hour at an overpotential of 687.6 mV (pH 7.0). This is attributed to the planar ligand (bpbH2), that coordinates strongly through four nitrogen atoms to the cobalt center, leaving two Cl− ions in axial position and making the Cl− ion ionize in organic solvents or water, and can stabilize both the high and low oxidation states of cobalt well.

Pentanuclear and Octanuclear Manganese Helices

Invited for the cover of this issue is the group of Hiroki Oshio at the University of Tsukuba, Japan. The cover image shows the metaphoric link between the synthesis of two new manganese clusters and the ancient Japanese art of origami. Origami literally means “folded paper” and allows elaborate shapes to be created from the simplest materials, just like the rigid, planar ligands that were employed to “fold” manganese ions into sophisticated helical superstructures.

Double-layer structure, sorption and magnetism properties of metal–organic frameworks with trigonal planar ligand

A new double-layer metal–organic framework [Co3(tcpt)2(H2O)2] (1) has been synthesized using trigonal planar ligand 2,4,6-tris(4-carboxyphenoxy)-1,3,5-triazine (H3tcpt) as a bridging ligand and characterized by single-crystal X-ray diffraction, elemental analyses, IR, PXRD and TGA. Structure analysis reveals that compound 1 has a double-layer structure. Gas sorption measurements indicate that compound 1 exhibits selective adsorption capabilities for CO2 over CH4 and N2. Furthermore, the magnetic studies of compound 1 show antiferromagnetic interactions between Co(II) ions.

Hydrogen-bonded 2D network structure and abrupt spin transition with thermal hysteresis of iron(III) complexes [FeIII(Him)2(3-MeOsalen)]·H2O·EtOH·X (Him = imidazole, 3-MeOsalen = N,N′-bis(3-methoxysalicylidene)ethylenediaminato, and X = PF6, AsF6, SbF6)

Spin crossover (SCO) iron(III) complexes exhibiting thermal hysteresis, [FeIII(Him)2(3-MeOsalen)]·H2O·EtOH·X (Him=imidazole, 3-MeOsalen=N,N′-bis(3-methoxysalicylidene)ethylenediaminato, and X=PF6, AsF6, SbF6), were synthesized in ethanol. The FeIII ion has an octahedral coordination geometry with N2O2 donor atoms of the planar tetradentate ligand (3-MeOsalen) and two nitrogen atoms of two imidazoles at the axial positions. Through H2O and EtOH the complex-cations are assembled into a hydrogen-bonded 2D network structure and the anion X is accommodated in the void of the 2D network. The temperature dependences of the magnetic susceptibilities revealed an abrupt and two-step spin transition with thermal hysteresis for the PF6 and AsF6 salts.

Rh–POP Pincer Xantphos Complexes for C–S and C–H Activation. Implications for Carbothiolation Catalysis

The neutral Rh(I)–Xantphos complex [Rh(κ3-P,O,P-Xantphos)Cl]n, 4, and cationic Rh(III) [Rh(κ3-P,O,P-Xantphos)(H)2][BArF4], 2a, and [Rh(κ3-P,O,P-Xantphos-3,5-C6H3(CF3)2)(H)2][BArF4], 2b, are described [ArF = 3,5-(CF3)2C6H3; Xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene; Xantphos-3,5-C6H3(CF3)2 = 9,9-dimethylxanthene-4,5-bis(bis(3,5-bis(trifluoromethyl)phenyl)phosphine]. A solid-state structure of 2b isolated from C6H5Cl solution shows a κ1-chlorobenzene adduct, [Rh(κ3-P,O,P-Xantphos-3,5-C6H3(CF3)2)(H)2(κ1-ClC6H5)][BArF4], 3. Addition of H2 to 4 affords, crystallographically characterized, [Rh(κ3-P,O,P-Xantphos)(H)2Cl], 5. Addition of diphenyl acetylene to 2a results in the formation of the C–H activated metallacyclopentadiene [Rh(κ3-P,O,P-Xantphos)(ClCH2Cl)(σ,σ-(C6H4)C(H)═CPh)][BArF4], 7, a rare example of a crystallographically characterized Rh–dichloromethane complex, alongside the Rh(I) complex mer-[Rh(κ3-P,O,P-Xantphos)(η2-PhCCPh)][BArF4], 6. Halide abstraction from [Rh(κ3-P,O,P-Xantphos)Cl]n in the presence of diphenylacetylene affords 6 as the only product, which in the solid state shows that the alkyne binds perpendicular to the κ3-POP Xantphos ligand plane. This complex acts as a latent source of the [Rh(κ3-P,O,P-Xantphos)]+ fragment and facilitates ortho-directed C–S activation in a number of 2-arylsulfides to give mer-[Rh(κ3-P,O,P-Xantphos)(σ,κ1-Ar)(SMe)][BArF4] (Ar = C6H4COMe, 8; C6H4(CO)OMe, 9; C6H4NO2, 10; C6H4CNCH2CH2O, 11; C6H4C5H4N, 12). Similar C–S bond cleavage is observed with allyl sulfide, to give fac-[Rh(κ3-P,O,P-Xantphos)(η3-C3H5)(SPh)][BArF4], 13. These products of C–S activation have been crystallographically characterized. For 8 in situ monitoring of the reaction by NMR spectroscopy reveals the initial formation of fac-κ3-8, which then proceeds to isomerize to the mer-isomer. With the para-ketone aryl sulfide, 4-SMeC 6H4COMe, C–H activation ortho to the ketone occurs to give mer-[Rh(κ3-P,O,P-Xantphos)(σ,κ1-4-(COMe)C6H3SMe)(H)][BArF4], 14. The temporal evolution of carbothiolation catalysis using mer-κ3-8, and phenyl acetylene and 2-(methylthio)acetophenone substrates shows initial fast catalysis and then a considerably slower evolution of the product. We suggest that the initially formed fac-isomer of the C–S activation product is considerably more active than the mer-isomer (i.e., mer-8), the latter of which is formed rapidly by isomerization, and this accounts for the observed difference in rates. A likely mechanism is proposed based upon these data.

EPR Analysis of Imidazole Binding to Methanosarcina acetivorans Protoglobin

Protoglobins form an unusual class of globin proteins with surprising structural features, such as a highly ruffled heme. Electron paramagnetic resonance spectra of the frozen solution of the ferric imidazole complex of Methanosarcina acetivorans protoglobin reveal the existence of two low-spin ferric heme complexes. Hyperfine sublevel correlation spectroscopy was used to study the imidazole complex of this protoglobin as well as the imidazole complex of horse heart myoglobin for comparison. The spin Hamiltonian values obtained for the dominant contribution of the imidazole-ligated protoglobin agree with a heme conformation in which the plane of the exogenous imidazole is turned more than 30° versus the imidazole plane of the proximal histidine ligand. In turn, the electron paramagnetic resonance data of the minority complex indicate a second conformer, in which the two ligand planes are aligned almost parallel to each other with a lengthened iron–nitrogen bond for the exogenous imidazole ligand.

Comprehensive Fe-ligand vibration identification in {FeNO}6 hemes.

Oriented single-crystal nuclear resonance vibrational spectroscopy (NRVS) has been used to obtain all iron vibrations in two {FeNO}6 porphyrinate complexes, five-coordinate [Fe(OEP)(NO)]ClO4 and six-coordinate [Fe(OEP)(2-MeHIm)(NO)]ClO4. A new crystal structure was required for measurements of [Fe(OEP)(2-MeHIm)(NO)]ClO4, and the new structure is reported herein. Single crystals of both complexes were oriented to be either parallel or perpendicular to the porphyrin plane and/or axial imidazole ligand plane. Thus, the FeNO bending and stretching modes can now be unambiguously assigned; the pattern of shifts in frequency as a function of coordination number can also be determined. The pattern is quite distinct from those found for CO or {FeNO}7 heme species. This is the result of unchanging Fe–NNO bonding interactions in the {FeNO}6 species, in distinct contrast to the other diatomic ligand species. DFT calculations were also used to obtain detailed predictions of vibrational modes. Predictions were consistent with the intensity and character found in the experimental spectra. The NRVS data allow the assignment and observation of the challenging to obtain Fe–Im stretch in six-coordinate heme derivatives. NRVS data for this and related six-coordinate hemes with the diatomic ligands CO, NO, and O2 reveal a strong correlation between the Fe–Im stretch and Fe–NIm bond distance that is detailed for the first time.

Synthesis, characterization and oxygen-sensing performance of a rhenium complex having an enlarged sensing area

In this paper, we reported a Re(I) complex Re(CO)3(IPD-Et)Br having an enlarged conjugation chain for oxygen sensing purpose, where IPD-Et=4-(1-ethyl-1 H-imidazo [4,5-f][1,10] phenanthrolin-2-yl)-N,N-diphenylaniline. Its conjugation chain was composed of triphenylamine group and imidazole group. In addition, an alkyl chain was connected with this N–N ligand to breach π–π attraction between ligand planes. It was assumed that above two factors could improve oxygen sensing sensitivity. Theoretical calculation and analysis firstly confirmed this hypothesis. Then photophysical measurement on Re(CO)3(IPD-Et)Br suggested that its emission was a lon-lived one peaking at 545nm and thus vulnerable to molecular O2. Composite systems were constructed for oxygen sensing, using Re(CO)3(IPD-Et)Br as probe and mesoporous silicate MCM-41 as supporting matrix, respectively. Linear Stern–Volmer curves, short response time of ~13s and high sensitivity of 3.99 were finally obtained.

Molecular structure, spectroscopy, and photoinduced kinetics in trinuclear cyanide bridged complex in solution: a first-principles perspective.

We investigate the molecular structure of the solvated complex, [(NC)6Fe-Pt(NH3)4-Fe(CN)6](4-), and related dinuclear and mononuclear model complexes using first-principles calculations. Mixed nuclear complexes in both solution and crystal phases were widely studied as models for charge transfer (CT) reactions using advanced spectroscopical and electrochemical tools. In contrast to earlier interpretations, we find that the most stable gas phase and solvated geometries are substantially different from the crystal phase geometry, mainly due to variance in the underlying oxidation numbers of the metal centers. Specifically, in the crystal phase a Pt(IV) metal center resulting from Fe ← Pt backward electron transfers is stabilized by an octahedral ligand field, whereas in the solution phase a Pt(II) metal complex that prefers a square planar ligand field forms a CT salt by bridging to the iron complexes through long-range electrostatic interactions. The different geometry is shown to be consistent with spectroscopical data and measured CT rates of the solvated complex. Interestingly, we find that the experimentally indicated photoinduced process in the solvated complex is of backward CT (Fe ← Pt).

Crystal structures and DNA-binding properties of PrIII complexes derived from 8-hydroxyquinoline-2-carboxaldehyde and three aroylhydrazines

PrIII and three synthesized ligands, 8-hydroxyquinoline-2-carboxaldehyde-(benzoyl)hydrazone, 8-hydroxyquinoline-2-carboxaldehyde-(2′-hydroxybenzoyl)hydrazone, and 8-hydroxyquinoline-2-carboxaldehyde-(isonicotinyl)hydrazone, respectively, can form binuclear PrIII complexes with 1 : 1 metal-to-ligand stoichiometry and nine-coordination at PrIII indicated by X-ray crystal structural analyses. Ligands are dibasic tetradentate, binding to PrIII through the phenolate oxygen, nitrogen of quinolinato unit, the C=N of methylene, and –O–C=N– enolized and deprotonated from O=C–NH– of the aroylhydrazone side chain. One DMF binds orthogonally to the ligand plane from one side to the metal ion, while another DMF and a bidentate nitrate simultaneously bind from the other. Dimerization of the monomeric unit occurs through the phenolate oxygen leading to a central planar four-membered (PrO)2 ring. The crystal structures are similar to each other and to other nine-coordinate lanthanide complexes with geometry of distorted edge-sharing mono-capped square-antiprism of [LnL(NO3)(DMF)2]2 (Ln = LaIII, NdIII, SmIII, EuIII, TbIII, DyIII, HoIII, and ErIII) except for YbIII with eight-coordinate YbIII center with distorted edge-sharing dodecahedron of [YbL(NO3)(DMF)]2, derived from 8-hydroxyquinoline-2-carboxaldehyde and aroylhydrazines. The ligands and PrIII complexes can bind to calf thymus DNA through intercalation with binding constants at 105 M−1 and probably be used as potential antitumor drugs.

Addition of C-C and C-H bonds by pincer-iridium complexes: a combined experimental and computational study.

We report that pincer-ligated iridium complexes undergo oxidative addition of the strained C-C bond of biphenylene. The sterically crowded species ((tBu)PCP)Ir ((R)PCP = κ(3)-1,3-C6H3(CH2PR2)2) initially reacts with biphenylene to selectively add the C(1)-H bond, to give a relatively stable aryl hydride complex. Upon heating at 125 °C for 24 h, full conversion to the C-C addition product, ((tBu)PCP)Ir(2,2'-biphenyl), is observed. The much less crowded ((iPr)PCP)Ir undergoes relatively rapid C-C addition at room temperature. The large difference in the apparent barriers to C-C addition is notable in view of the fact that the addition products are not particularly crowded, since the planar biphenyl unit adopts an orientation perpendicular to the plane of the (R)PCP ligands. Based on DFT calculations the large difference in the barriers to C-C addition can be explained in terms of a "tilted" transition state. In the transition state the biphenylene cyclobutadiene core is calculated to be strongly tilted (ca. 50°-60°) relative to its orientation in the product in the plane perpendicular to that of the PCP ligand; this tilt results in very short, unfavorable, non-bonding contacts with the t-butyl groups in the case of the (tBu)PCP ligand. The conclusions of the biphenylene studies are applied to interpret computational results for cleavage of the unstrained C-C bond of biphenyl by ((R)PCP)Ir.

Organotin(IV) complexes derived from Schiff base N′-[(1E)-(2-hydroxy-3-methoxyphenyl)methylidene]pyridine-4-carbohydrazone: Synthesis, in vitro cytotoxicities and DNA/BSA interaction

Five organotin(IV) compounds were synthesized from N′-[(1E)-(2-hydroxy-3-methoxyphenyl)methylidene]pyridine-4-carbohydrazone and the corresponding dialkyltin(IV) or trialkyltin(IV) precursor. Solid state structures were determined by IR, elemental analysis, NMR spectroscopy, and for 1, 2, 4 and 5 single crystal X-ray diffraction analysis. Compounds 1, 2 and 4 are monomers with the tin atoms five-coordinated in distorted trigonal bipyramid, of which the deprotonated Schiff base ligand chelate to tin center in the enolic tridentate mode. Differently, in compound 5, the enolization does not occur for the Schiff base ligand, and only the pyridinyl N atom and the deprotonated phenol hydroxyl oxygen atom participate in the coordination. Fascinatingly, six trimethyltin(IV) coordination units are linked by the Sn⋯N weak interaction atoms and form a 72-membered crown-like macrocycle. Preliminary in vitro cytotoxicity studies on five human tumor cells lines (HL-60, A549, HT-29, HCT-116 and Caco-2) by MTT assay reveal that di-n-butyltin(IV) complex 2 and diphenyltin(IV) complex 4 triggered significant antiproliferative effects in cultured tumor cells, and their cytotoxic activity correlates with intracellular organotin(IV) concentration. The interaction of the complexes with calf thymus DNA (CT-DNA) has been explored by absorption and emission titration methods, which revealed that complexes 2 and 4 interact with CT-DNA through groove-binding and partial intercalation of the extended planar ligand with the DNA base stack. Further, the albumin interactions of complexes 2 and 4 were investigated using fluorescence quenching spectra and synchronous fluorescence spectra. Studies reveal that di-n-butyltin(IV) complex 2 with higher cytotoxicity show stronger DNA/BSA interaction than diphenyltin(IV) complex 4.

Synthesis, Coordination and Catalytic Tests of New Chiral ferrocenyl P,N ligands

Many efforts have been devoted to the development of asymmetric catalysis because of the significant use of chiral compounds as intermediates of pharmaceuticals and advanced materials.[1] Amongst many ligands which have been tested, planar chiral ferrocenes ligands constitutes a privileged family of ligands for enantioselective catalysis.[2] We will disclose in this communication the synthesis of a new chiral N-(p-toluenesulfonyl)-containing ferrocenyl ligands and its coordination chemistry with Ru, Ir and Rh precursors yielding in some cases chiral at metal complexes. [3] Moreover, DFT calculations in order to better understand the nature and stabilities of these complexes will be presented. Additionally, preliminary evaluation of the different complexes as catalysts for the asymmetric transfer hydrogenation of ketones will also be disclosed.