Metal-centered Chirality Research Articles

ChemInform Abstract: Asymmetric Synthesis of Octahedral Coordination Complexes

AbstractReview: ca. 70 refs.

Aggregation of an anionic porphyrin with chiral metal complexes and the competitive binding influences of a surfactant and a polyelectrolyte

The non-covalent interactions of chiral metal complexes with the achiral 5,10,15,20-tetrakis (4-sulonatophenyl) porphyrin (H4TTPS2−) have been investigated by UV-vis and circular dichroism (CD) spectra. The results show that under acidic environments, only the chiral complex cations ([CoBr(NH3)(en)2]+, [Co(en)3]3+, [Ru(phen)3]2+) could interact with H4TTPS2− to form chiral aggregates, accompanied with the metal-centered chirality information transferred to the formed J-aggregates. However, the chiral complex anion ([Co(edta)]−) does not cause the self-assembly process. The competitive binding interactions between an achiral water-soluble cationic surfactant (N-hexadecyltrimethyl ammonium chloride, CTAC) and a cationic polyelectrolyte (polyallylamine, PAA) with the chiral metal complex H4TTPS2− J-aggregates, respectively, were also investigated. It was found that chiral-symmetry-breaking phenomena occur in the cationic surfactant induced event. In the case of a cationic polyelectrolyte, it could change the conformational flexibility of the H4TTPS2− aggregates. These results may lead us to understand the possible mechanism of the supramolecular self-assembly process by the non-covalent interactions.

Chiral Salicyloxazolines as Auxiliaries for the Asymmetric Synthesis of Ruthenium Polypyridyl Complexes

Chiral auxiliaries are promising emerging tools for the asymmetric synthesis of octahedral metal complexes. We recently introduced chiral salicyloxazolines as coordinating bidentate chiral ligands which provide excellent control over the metal-centered configuration in the course of ligand substitution reactions and can be removed afterward in an acid-induced fashion under complete retention of configuration (J. Am. Chem. Soc. 2009, 131, 9602-9603). Here reported is our detailed investigation of this sequence of reactions, affording virtually enantiopure ruthenium polypyridyl complexes. The control of the metal-centered chirality by the coordinated chiral salicyloxazolinate ligand was evaluated as a function of reaction conditions, the employed bidentate 2,2'-bipyridine and 1,10-phenanthroline ligands, and the substituent at the asymmetric 5-position of the oxazoline heterocycle. Most striking was the strong influence of the reaction solvent, with aprotic solvents of lower polarity providing the most favorable diastereoselectivities. Through a combination of computational and experimental results, it was revealed that the observed stereoselectivities are under thermodynamic control. The removal of the chiral salicyloxazoline auxiliary under retention of the configuration requires acidic conditions and a coordinating solvent such as MeCN or THF in order to prevent partial racemization. This method represents the first general strategy for the asymmetric synthesis of enantiopure heteroleptic ruthenium polypyridyl complexes.

Transformation, memorization and amplification of chirality in cationic Co(iii) complex–porphyrin aggregates

Λ- and Δ-enantiomers of cis-[CoBr(NH3)(en)2]Br2 were obtained by absolute asymmetric synthesis. The interactions between the achiral 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) and two chiral cationic Co(III) complexes were studied by UV-vis spectroscopy, circular dichroism (CD), fluorescence spectroscopy and atomic force microscopy (AFM). It was found that the cationic Co(III) complexes can induce the formation of chiral porphyrin J-aggregates, that the metal-centered chirality can be transferred to the J-aggregates, and that the chirality was able to be memorized and amplified in the porphyrin J-aggregates. Therefore, this should be a good chiral model for understanding similar processes in physical and biological systems.

Amido/Amine Triazacyclononane-Based Zirconium Complexes: Syntheses, Reactivity, and Structures

The reactions of [Zr(NMe2)4]2 with triamido-triazacyclonane ligand precursors, {NH(Ph)SiMe2}3tacn (H3N3[9]N3) and {NH(C6H4F)SiMe2}3tacn (H3N3-F[9]N3), led to the formation of complexes [Zr(NMe2)2{N(Ph)SiMe2}2{NH(Ph) SiMe2}tacn], 1, and [Zr(NMe2)2{N(o-C6H4F)SiMe2}2{NH(o-C6H4F)SiMe2} tacn], 2, where the zirconium is coordinated to two remaining dimethylamido ligands and to a dianionic tacn-based ligand, [{N(Ph')SiMe2}2{NH(Ph')SiMe2}tacn]2-, that formed from deprotonation of two amine pendent arms of the ligands' precursors. The third pendent arm of H3N3[9]N3 and H3N3-F[9]N3 remains neutral and not bonded to the zirconium. Treatment of 1 with NaH led to the synthesis of [Zr(NMe2){N(Ph)SiMe2}2tacn], 3, that results from the cleavage of the N-Si bond of the original neutral pendent arm. Complexes [ZrCl{N(Ph')SiMe2}2tacn] (Ph' = C6H5, 4, and C6H4F, 5) have been obtained by reactions of ZrCl4 with {MN(Ph')SiMe2}3tacn.2THF (M = Li, Na). Reactions of 4 and 5 with LiC triple bond CPh led to the syntheses of [Zr(CCPh){N(Ph')SiMe2}2tacn] (Ph' = C6H5, 6, and C6H4F, 7). The solid-state structure of 3 shows a chiral metal center.

Epimerization, diastereoselectivity and hemilability in dicationic chiral ruthenium complexes with bidentate (P ∧S) bisphosphine monosulfide ligands

The binding of heterobidentate P ∧S ligands introduces metal-centered chirality to the planar chiral parent complex Ru(η 6:η 1-NMe 2C 6H 4C 6H 4PCy 2)Cl 2. Observed diastereomeric ratios for the kinetic product vary dramatically depending upon ring size of the chelate formed with the P ∧S ligand. The complexes epimerize very slowly to thermodynamic product ratios that are substantially different from the kinetic product ratios.

Metallosupramolecular Chemistry with Bis(benzene-o-dithiolato) Ligands

The bis(benzene-o-dithiol) ligands H(4)-1, H(4)-2, and H(4)-3 react with [Ti(OC(2)H(5))(4)] to give dinuclear triple-stranded helicates [Ti(2)L(3)](4)(-) (L = 1(4)(-), 2(4)(-), 3(4)(-)). NMR spectroscopic investigations revealed that the complex anions possess C(3) symmetry in solution. A crystal structure analysis for (PNP)(4)[Ti(2)(2)(3)] ((PNP)(4)[14]) confirmed the C(3) symmetry for the complex anion in the solid state. The complex anion in Li(PNP)(3)[Ti(2)(1)(3)] (Li(PNP)(3)[13]) does not exhibit C(3) symmetry in the solid state due to the formation of polymeric chains of lithium bridged complex anions. Complexes [13](4)(-) and [14](4)(-) were obtained as racemic mixtures of the Delta,Delta and Lambda,Lambda isomers. In contrast to that, complex (PNP)(4)[Ti(2)(3)(3)] ((PNP)(4)[15]) with the enantiomerically pure chiral ligand 3(4)(-) shows a strong Cotton effect in the CD spectrum, indicating that the chirality of the ligands leads to the formation of chiral metal centers. The o-phenylene diamine bridged bis(benzene-o-dithiol) ligand H(4)-4 reacts with Ti(4+) to give the dinuclear double-stranded complex Li(2)[Ti(2)(4)(2)(mu-OCH(3))(2)] containing two bridging methoxy ligands between the metal centers. The crystal structure analysis and the (1)H NMR spectrum of (Ph(4)As)(2)[Ti(2)(4)(2)(mu-OCH(3))(2)] ((Ph(4)As)(2)[(16]) reveal C(2) symmetry for the anion [Ti(2)(4)(2)(mu-OCH(3))(2)](2)(-). For a comparative study the dicatechol ligand H(4)-5, containing the same o-phenylene diamine bridging group as the bis(benzene-o-dithiol) ligands H(4)-4, was prepared and reacted with [TiO(acac)(2)] to give the dinuclear complex anion [Ti(2)(5)(2)(mu-OCH(3))(2)](2)(-). The molecular structure of (PNP)(2)[Ti(2)(5)(2)(mu-OCH(3))(2)] ((PNP)(2)[17]) contains a complex anion which is similar to [16](2)(-), with the exception that strong N-H...O hydrogen bonds are formed in complex anion [17](2)(-), while N-H...S hydrogen bonds are absent in complex anion [16](2)(-).

Diastereoselectivity at chiral metal center of half-sandwich-type ruthenium complexes with planar-chiral cyclopentadienyl ligands in multiple ligand transfer reaction

The triple ligand transfer reaction between planar-chiral cyclopentadienyl–ruthenium complexes [Cp′Ru(NCMe) 3][PF 6] ( 1) (Cp′ = 1-(COOR 2)-2-Me-4-R 1C 5H 2; R 1 = Me, Ph, t-Bu) and iron complexes CpFe(CO)(L)X ( 2) (L = PMe 3, PMe 2Ph, PMePh 2, PPh 3; X = I, Br) resulted in the formation of metal-centered chiral ruthenium complexes Cp′Ru(CO)(L)X ( 3) in moderate yields with diastereoselectivities of up to 68% de. The configurations of some major diastereomers were determined to be S Cp ∗ R Ru ∗ by X-ray crystallography. The diastereoselectivity of 3 was under kinetic control and not affected by the steric effect of the substituents on the Cp′ ring of 1 and the phosphine of 2. Although the double ligand transfer reaction between [Cp′Ru{P(OMe) 3}(NCMe) 2][PF 6] ( 7) and CpFe(CO) 2X ( 8) produced Cp′Ru{P(OMe) 3}(CO)X ( 9), the selectivity at the ruthenium center was low.

Helical, nonracemic inorganic-organic hybrid polymers of cadmium halides with pentadentate bis(oxazoline) ligands.

A chiral, nonracemic pentadentate pyridine bis(oxazoline) ligand forms unprecedented P-helical inorganic-organic hybrid polymers with cadmium halides. The one-dimensional chains consist of Lambda2-configured pentagonal-bipyramidal cadmium complexes with metal-centered chirality bridged by [CdX4]2- tetrahedra via shared halide atoms. In the solid state, the overall helicity exhibits strongly directed orientation parallel to the crystallographic axis a.

Rhodium and iridium complexes bearing an alkyloxazoline-substituted indenyl ligand (Indox ligand) and their stereoselective construction of metal-centered chirality

The Indox ligands, [{( S)-( i Pr)Indox} n ]H ( 1) [ n=2 ( a), 3 ( b)] and [{(H)Indox} n=3 ]H ( 2), in which an indenyl group and an oxazoline ring are connected by an ethylene or propylene spacer, have been prepared. Reaction of [Ir(coe) 2Cl] 2 or [RhCl(C 2H 4) 2] 2 with the potassium salt of 1 afforded η 5-[{( S)-( i Pr)Indox} n ]Ir(coe) 2 ( 3) or η 5-[{( S)-( i Pr)Indox} n ]Rh(C 2H 4) 2 ( 6) as a 1:1 mixture of two diastereomers. The oxazoline ring in 3 and 6 did not coordinate to the metal center. When the complexes 3 or 6 reacted with iodine in diethyl ether, oxidative addition proceeded and the oxazoline ring coordinated to the metal center to give diiodoiridium(III) or rhodium(III) complexes, η 5:η 1-[{( S)-( i Pr)Indox} n ]M(I) 2 [M=Ir ( 4), Rh ( 7)]. The corresponding diiodoiridium(III) complex bearing the Indox ligand 2, η 5:η 1-[{(H)Indox} n=2 ]Ir(I) 2 ( 5), was also prepared by a similar method. Reaction of 4 or 7 with PPh 3 in THF afforded diiodo-phosphine complexes, η 5-[{( S)-( i Pr)Indox} n ]M(PPh 3)(I) 2 [M=Ir ( 8), Rh ( 9)] as a 1:1 mixture of two diastereomers in which the oxazoline ring dissociated from the metal center. The related reaction of 8 or 9 with more than 2 equiv. of AgOTf afforded the cationic complexes, [η 5:η 1-[{( S)-( i Pr)Indox} n ]M(PPh 3)(OTf)]OTf [M=Ir ( 10), Rh ( 11)], having a stereogenic center at the metal center as a mixture of only two diastereomers. From 1H and 31P NMR analyses, each diastereomer of 8 or 9 afforded only a single isomer of 10 or 11. The corresponding iridium(III) complex bearing the Indox ligand 2, [η 5:η 1-[{(H)Indox} n=3 ]Ir(PPh 3)(OTf)]OTf ( 12) was also prepared. The coordinated triflate ligand of 12 was slowly replaced by water in CDCl 3 to afford the dicationic aquo complex, ( S * pl, R * Ir)-[η 5:η 1-[{(H)Indox} n=3 ]Ir(PPh 3)(H 2O)](OTf) 2 ( 13). The monocationic complex, [η 5:η 1-[{( S)-( i Pr)Indox} n=2 ]Ir(PPh 3)(I)]OTf ( 14a), having metal-centered chirality, was observed as a mixture of only two diastereomers in the reaction of 10a (a mixture of two diastereomers) with 1 equiv. of AgOTf. These observations indicated that the ligand exchange reaction of 8 or 9 with AgOTf contained the following three steps: (i) abstraction of one of the two prochiral iododes by AgOTf, (ii) recoordination of the oxazoline ring, and (iii) exchange of the remaining iodide for the triflate by AgOTf. The stereochemistry around the metal center was determined at the second step. All complexes have been characterized by usual spectroscopic methods as well as elemental analyses, and 4 and 13 have been characterized by X-ray analyses.

Predetermined Helical Chirality in Octahedral Complexes with a Novel Pentadentate C2‐Symmetrical Chiral Bis(oxazoline) Ligand

AbstractThe first example of the pentadentate bis(oxazolines) has been synthesized using a modular approach that readily provides access to this ligand class. This ligand allows the controlled transfer of carbon‐centered to octahedral, metal‐centered chirality, as demonstrated both in the solid state as well as in solution. The characteristic CD band has been identified for dicationic, octahedral transition metal complexes with the Δ2‐configuration.

Supramolecular stereocontrol of octahedral metal-centered chirality. Ligand modulation.

By associating chiral labile [FeL3](2+) complexes with TRISPHAT anions, a stereocontrol of the metal-centered chirality is feasible; the sense of the stereoselective induction and its magnitude strongly depends upon the structure of the diimine ligands (L: bpy, phen).

Construction of Metal-Centered Chirality: Diastereoselective Addition of the Meerwein Reagent (Me3OBF4) to Rhodium Carbonyl Complexes Having the Cp‘-P Ligand

Reaction of the rhodium(I) carbonyl complexes [η5:η1-{3-(R)Ind-P}n=2]Rh(CO) (1; R = H (a), Et (b), Cy (c), (1‘S,2‘S,5‘R)-neomenthyl (NM) (d), (1‘R,2‘R,5‘R)-neoisomenthyl (NIM) (e)) with the Meerwein reagent (Me3OBF4) afforded the cationic rhodium(III) complexes [[η5:η1-{3-(R)Ind-P}n=2]RhMe(CO)]BF4, having metal-centered chirality, in good to excellent yields. When a bulkier substituent (R) was used, the corresponding cationic complex was obtained with higher diastereoselectivity. In the case of using an optically active substituent such as NM or NIM, complete stereodiscrimination around the rhodium center by the Meerwein reagent occurred to give the cationic complexes with 100% de.

Complex [Cd(dafo) 2(tphpo)(CH 3COO)]ClO 4 (dafo=4,5-diazafluoren-9-one, tphpo=triphenylphosphine oxide) with chiral metal centers. Helical packing induced by π–π stacking interactions of dafo rings

The first complex of 4,5-diazafluoren-9-one (dafo) and cadmium: [Cd(dafo) 2(tphpo)(CH 3COO)]ClO 4 (triphenylphosphine oxide=tphpo) is a quaternary complex with chiral metal centers and crystallizes in a monoclinic cell with space group P 2(1)/ n. Cadmium centers with 7-fold coordination adopt a distorted monocapped trigonal prismatic geometry. Two oxygen atoms of acetate and one oxygen atom of triphenylphosphine oxide form the top triangle of the trigonal prism, while three nitrogen atoms from two dafo groups construct the bigger bottom triangle. The last nitrogen atom of dafo groups occupies the monocapped position. Here, dafo molecules display two different coordination patterns, namely unsymmetric and symmetric chelation. Moreover, the interesting linking of screw related cations is fabricated by inter-molecular π–π stacking interactions in the complex.

Stereoselective association of binuclear metallacycles in coordination polymers.

A series of structurally related binuclear metallacycles [Cd(NO(3))(2)L](2), where L is an angular exo-bidentate ligand, have been synthesized. Each metallacycle contains two coordinatively unsaturated, chiral metal centers within a single molecule, and the assembly of these metallacycles into polymeric framework structures has been studied systematically for the first time. Stereoselective homochiral association of [Cd(NO(3))(2)L](2) leads to the formation of helical coordination polymers, whereas meso type association results in nonhelical chain structures. The type of stereoselective aggregation depends on the conditions of self-assembly as well as on ligand functionality. Both helical and nonhelical polymeric complexes have been isolated for the metallacycle [Cd(NO(3))(2)(2,4'-pyacph)](2) (2,4'-pyacph = 2,4'-(4-ethynylphenyl)bipyridyl). Homochiral association results in the formation of helical [Cd(NO(3))]( infinity ) chains which link the binuclear [Cd(NO(3))(2)(2,4'-pyacph)](2) metallacycles into racemic two-dimensional sheets which contain both P and M [Cd(NO(3))]( infinity ) helices. In contrast, meso-association leads to the formation of nonhelical one-dimensional chains. It is shown that the product of homochiral association is predominately formed at room temperature and that of meso-association is generated at elevated temperatures. Thus, it may be concluded that the homochiral association appears to be energetically less favorable than the meso-association, a conclusion that has been confirmed by theoretical calculations of the crystal lattice energy. Several high-yield syntheses of bipyridyl-type ligands used for metallacyclic assembly are also reported.

Nonrigid diastereomers: epimerization at chiral metal centers or chiral ligand conformations?

Download options Please wait... Article information DOI https://doi.org/10.1039/B302054C Article type Opinion Submitted 12 Feb 2003 Accepted 01 Mar 2003 First published 01 May 2003 Download Citation New J. Chem., 2003,27, 899-901 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Nonrigid diastereomers: epimerization at chiral metal centers or chiral ligand conformations? J. W. Faller, J. Parr and A. R. Lavoie, New J. Chem., 2003, 27, 899 DOI: 10.1039/B302054C To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author J. W. Faller J. Parr A. R. Lavoie

Optically active 4- and 5-coordinate transition metal complexes of bifurcated dipeptide Schiff bases.

Symmetrical and unsymmetrical benzophenone Schiff bases of bifurcated dipeptides [e.g., Ar2C=N-CHR1- CONH-HNCO-CHR2-N=CAr2] have been synthesized using Boc methodology. These ligands may be regarded as chiral porphyrin mimics because of the alpha-carbons of the amino acids. The Schiff bases function as effective ligands for transition metals, particularly the late transition metals Ni(II), Cu(II), and Zn. Upon metal insertion, there is loss of the amide protons, resulting in N4 chelating ligands that retain the amino acid based chirality as well as newly generated metal-centered chirality, which for the Ni(II) complexes have been shown by X-ray analysis to be lambda (left-handed helix) if the amino acids are S. For Ni(II) and Cu(II), metal insertion results in highly colored complexes and is easily followed by UV-vis spectrophotometry. Several Ni(II) complexes were also characterized by 1H NMR. Co(II) and Mn(II) complexes were characterized by CW EPR. Two Cu(II) complexes, 7f.CuII and 7k.CuII, were characterized by EPR (ENDOR and ESEEM), which clearly showed the pentacoordinate nature of 7k.CuII.

Monomeric and polymeric nickel complexes of malate: X-ray crystal structure of polymeric homochiral S-malato nickel(II), [Δ-Ni( S-Hmal)(H 2O) 2] n· nH 2O

Polymeric S-malate nickel(II) [Δ-Ni( S-Hmal)(H 2O) 2] n · nH 2O ( 1) was prepared from the reaction of nickel chloride and malate salt. By prolonging the reaction time and heating, the kinetic monomeric product cis-[Ni( S-H 2mal) 2(H 2O) 2]·2H 2O ( 2) can be converted into the thermodynamic polymeric product 1. The coordination polymer is linked strongly by the tetradentate malate ligand, forming an homochiral zigzag chain. It is proposed that the chiral metal center of the complex may also be involved in the enantioselective synthesis as well as the chiral ligand.

Hydrogen-transfer catalyzed by half-sandwich Ru(ii) aminophosphine complexesElectronic supplementary information (ESI) available: experimental details and NMR data for 1c and 2c. See http://www.rsc.org/suppdata/dt/b1/b104128m/

The syntheses of and catalytic studies on some Ru(II) complexes bearing the aminophosphine ligands N,N-dimethyl-2-diphenylphosphinoethylamine (PN), optically pure (RC,Spl)-2- {1-(N,N-dimethylamino)ethyl}-1-diphenylphosphinoferrocene (PPFA), and N,N-dimethyl-2-diphenylphosphinoaniline (DBD) are described, [RuCp(CH3CN)3]+ reacts with these ligands to give the cationic complexes [RuCp(PN-κN,κP)(CH3CN)]+ (1a), [(SRu,RC,Spl)-RuCp(PPFA-κN,κP)(CH3CN)]+ (1b), and [RuCp(DBD-κN,κP )(CH3CN)]+ (1c), respectively, in high yields. From these, in turn, the residual CH3CN ligand can be replaced by Br− upon addition of NEt4Br in CH2Cl2, resulting in the formation of the neutral complexes RuCp(PN)Br (2a), (SRu,RC,Spl)-RuCp(PPFA)Br (2b), and RuCp(DBD)Br (2c), again in good yields. Similarly, [Ru(η6-p-cymene)Cl2]2 reacts with 1 equiv. of PN or PPFA to give Ru(η6-p-cymene)(PN-κP)Cl2 (3a) and Ru(η6-p-cymene)(PPFA-κP)Cl2 (3b). Furthermore, treatment of 3 with TlCF3SO3 in THF at room temperature affords the cationic complexes [Ru(η6-p-cymene)(PN-κN,κP)Cl]CF3SO3 (4a) and [(RRu,RC,Spl)-Ru(η6-p-cymene)(PPFA-κN,κP)Cl]CF3SO3 (4b). The absolute configuration at the metal center of 2b and 4b′ (BPh4− salt of 4b) was determined by X-ray crystallography. Catalytic studies were performed with the racemic complexes 1a, 2a, 2c, and 4a and the diastereopure complexes 1b, 2b, and 4b. All of these proved to be excellent precatalysts for the transfer hydrogenation of acetophenone and derivatives thereof, and cyclohexanone. With the enantiomerically pure systems 2b and 4b, only racemic products were obtained. This testifies to the hemilabile nature of the aforementioned aminophosphine ligands giving transient κ-P-bonding coordination. Since diastereoface selection of incoming substrates is based on the planar chirality of the ferrocene moiety, rather than the metal centered chirality, no enantioselective reaction takes place.

Highly selective induction of metal-centered chirality in the ligand exchange reaction of planar-chiral cyclopentadienyl–ruthenium complex bearing an anchor phosphine ligand

Treatment of planar-chiral cyclopentadienyl–phosphine ruthenium complexes with phosphine and phosphite induces metal-centered chirality with a high stereoselectivity (≳99 % de).