The reduction rates of horse heart ferricytochrome c by amalgamated zinc or by electrolysis at fixed potential on a mercury pool as the cathode have been measured in a buffered solution at pH 7.5 by absorption spectrophotometry. In both cases, the reaction was strongly accelerated by the presence of the optically active complexes Lambda-[Cr-III((S,S)promp)H2O](+) (H(2)promp = N,N'- [(pyridine-2,6-diyl)bis(methylene)]-bis[(S,)-proline]), Delta-[Cr-III((R,R)-alamp)H2O](+) (H(2)alamp = N,N'-[(pyridine-2,6-diyl)bis(methylene)]-bis[(R)-alanine]) and Lambda-[Cr-III((S,S)-alamp)(H2O)(2)](+). These were shown to undergo reversible one-electron reduction to the corresponding labile chromium(II) species by cyclic voltammetry (CV), although the diaquo Lambda-[Cr-III((S,S)-alamp)(H2O)(2)](+) compound behaved differently than the two others. The cyclic voltammogram evidenced a strong catalytic reduction wave below -1.1 V/SHE overlapping with the Cr3+/Cr2+ couple, which has been attributed to the catalytic reduction of hydroxonium ions to molecular hydrogen. Although stable in the second time range as demonstrated by CV, the chromium(II) complexes exist in solution only as short-lived species in the absence of protein and are rapidly reoxidized to the initial trivalent state, thus preventing their isolation even under anaerobic conditions. However, their lifetime was found to be long enough to catalyze the reduction of the ferric heme moiety of cytochrome caccording to an electron-transfer-mediated reaction. Both chemical and electrochemical processes were found to follow zero-order kinetics. It could therefore be safely concluded that the rate-determining step is associated to the electron transfer from transient chromium(II) complexes to the protein and not to the in situ generation of the metallic reducing agent.

The protonation of a sterically crowded [N2S6] macropentacycle (1) with 1 equiv of CF3SO3H in CDCl3 is slow and gives the singly (oo(+) [1 x H](+)) and doubly (o(+)o(+) [1 x 2H](2+)) protonated forms as kinetic products, the i(+)o form of [1 x H](+) being the thermodynamic product. i(+)o [1 x H](+) is C3 helically chiral in the solid state and in solution. The barrier to racemization (DeltaG(double dagger)) of the [1 x H](+) propeller is >71 kJ mol(-1). The ammonium proton is encapsulated in the tetrahedral coordination sphere provided by the endo (i) nitrogen bridgehead atom and the three proximal thioether sulfurs, which makes [1 x H](+) a proton complex. Use of the optically active acid (R)-(-)- or (S)-(+)-1,1'-binaphthyl-2,2'-diyl hydrogen phosphate (BNPH) in chloroform allowed us to induce a significant diastereomeric excess (24% de), which produced a detectable ICD. The de was decreased in acetone-d6 (10%), suggesting that the sense of chirality of [1 x H](+) is controlled by ion-pair interactions. Detailed NMR studies allowed us to locate the chiral anion on the endo side of [1 x H](+), in the cavity lined by endo t-Bu groups, and to establish that the rate of anion exchange in [1 x H][(S,R)-(+/-)-BNP] was higher than the rate of propeller inversion of [1 x H](+).

Institute of Chemistry, UniVersity of Neuchâtel, AV. BelleVaux 51, CP 2, CH-2007 Neuchâtel, Switzerland Institute of Inorganic Chemistry, UniVersity of Fribourg, Pérolles, CH-1700 Fribourg, Switzerland Laboratoire d’Ingénierie Moléculaire pour la Séparation et les Applications des Gaz (LIMSAG, UMR 5633 du CNRS), UniVersité de Bourgogne, Facult é des Sciences, 6 BouleVard Gabriel, F-21100 Dijon, France Laboratory of Chemometrics, National Institute of Chemistry, HajdrihoVa 19, SI-1001 Ljubljana, SloVenia

Optically active cobalt(II) complexes are used as reducing agents in the electron-transfer reaction involving horse heart cytochromec. Analysis of the circular dichroism (CD) spectra of reaction products indicates that the corresponding cobalt(III) species of both enantiomers of [CoII(alamp)] (H2alamp=N,N′-[(pyridine-2,6-diyl)bis(methylene)]-bis[alanine]) are partly attached to the protein during electron transfer by coordination to an imidazole unit of one of the histidine residues. His-26 and His-33 are both solvent exposed, and the results suggest that one of these histidine residues acts as a bridge in the electron transfer to and from the haem iron of cytochromec. The reaction is enantioselective: the ratio of the relative reactivity at 15 °C is 2.9 in favour of theR,R-enantiomer. A small induced CD activity in the haem chromophore reveals that some structural changes in the protein occur consecutively with the binding of the cobalt(III) complex.

Incorporation of a biotinylated ruthenium tris(bipyridine) [Ru(bpy)(2)(Biot-bpy)](2+) (1) in either avidin or streptavidin-(strept)avidin-can be conveniently followed by circular dichroism spectroscopy. To determine the stepwise association constants, cooperativity, and chiral discrimination properties, diastereopure (Lambda and Delta)-1 species were synthesized and incorporated in tetrameric (strept)avidin to afford (Delta-[Ru(bpy)(2)(Biot-bpy)](2+))(x)() subsetavidin, (Lambda-[Ru(bpy)(2)(Biot-bpy)](2+))(x)() subsetavidin, (Delta-[Ru(bpy)(2)(Biot-bpy)](2+))(x)() subsetstreptavidin, and (Lambda-[Ru(bpy)(2)(Biot-bpy)](2+))(x)() subsetstreptavidin (x = 1-4) For these four systems, the overall stability constants are log beta(4) = 28.6, 30.3, 36.2, and 36.4, respectively. Critical analysis of the CD titrations data suggests a strong cooperativity between the first and the second binding event (x = 1, 2) and a pronounced difference in affinity between avidin and streptavidin for the dicationic guest 1 as well as modest enantiodiscrimination properties with avidin as host.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The coordination of 2,6-bis(3,4-dihydro-2H-pyrrol-5-yl)pyridine (L) to nickel, ruthenium, iron, and cobalt was studied. The reaction with nickel salts gives the nickel (II) complex [Ni(L)2]2+ (1). The reaction of ruthenium(III) chloride gives only the ruthenium(II) complex [Ru(L)2]2+ (2). In the case of iron, both [Fe(L)2]2+ (3) and [Fe(L)2]3+ (4) have been synthesised and characterised. In the case of cobalt, the cobalt(III) complex [Co(L)2]3+ (5) is obtained, even if a cobalt(II) salt is used. However, the cobalt(II) [Co(L)2]2+ (6) can be obtained under nitrogen. Molecular structures of 1–5 complexes have been determined by X-ray analysis of the corresponding perchlorate salts; the structure of 3 has been solved for both the perchlorate and the tetrachloroferrate(III) salts. All complexes show an octahedral coordination geometry with meridional arrangement of the two tridentate ligands. The electrochemical behaviour of 2, 3 and 6 has been studied by cyclic voltammetry. Quasi reversible electron transfer is observed for the redox pairs FeIII/FeII and CoII/CoI. The reaction shows somewhat weaker reversibility for CoIII/CoII, whereas the reaction of RuIII/RuII is not reversible. The measurements suggest 2,6-bis(3,4-dihydro-2H-pyrrol-5-yl)pyridine (L) to have a higher stabilisation of low-valent oxidation states of iron and cobalt than terpyridine.

A series of copper(II) azido complexes containing a tridentate trinitrogen ligand, [Cu(η3-L)(N3)]+ [L=2,6-bis(3,4-dihydro-2H-pyrrol-5-yl)pyridine] as well as [Cu(η3-R,S-LH4)(N3)]+, [Cu(η3-S,S-LH4)(N3)]+ and [Cu(η3-R,R-LH4)(N3)]+ [LH4=2,6-bis(pyrrolidin-2-yl)pyridine] have been synthesised from Cu(ClO4)2·6 H2O, NaN3 and the corresponding tridentate ligand and crystallised from methanol as the perchlorate salts. The single crystal X-ray structure analyse show these complexes to form one-dimensional networks by Cu⋯N(azide) intermolecular interactions, depending on the conformation of the tridentate ligand. In the case of L, an excess of sodium azide leads to the formation of the salt [Cu(η3-L)(N3)]2[Cu2Cl2(N3)4], the crystal structure analysis of which reveals a surprising tridimensional network of cationic and anionic copper(II) complexes linked by Cu⋯N(azide) and Cu⋯N(L) interactions.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The stereoselectivity of the formation of bis-chelates with the 1,9-disubstituted chiral semicorrin ligands (1S,9S)-dimethyl 5-cyanosemicorrin-1,9-dicarboxylate (1) for Co2+, Ni2+, Cu2+, and Pd2+ and with its (1S,9S)-diethyl analogue 2 for Co2+ and Cu2+ have been measured by the method of continuous variation of enantiomers, and with 1 for Zn2+ by NMR. Positive selectivities were found for Co2+, Ni2+, and Zn2+ complexes, whereas very high negative selectivities prevail in the complexes of Cu2+ and Pd2+. These results are rationalised by X-ray structural determinations of some of the corresponding complexes. The optically active Cu2+ and Pd2+ complexes show distorted square planar structures with chirality predetermined by the chirality of the ligand. Ni2+ complexes are five or hexacoordinated and exhibit a distorted tetrahedral arrangement of the four coordinated nitrogen atoms. In the optically active compound the ester moiety is coordinated by the oxygen atom of the alkoxy group and the arrangement of the two N-N chelate rings shows ?-(S,S) chirality. Interestingly, with the racemic ligand, it is not the more stable heterochiral, but the racemic complex containing the two homochiral enantiomers which is obtained. The coordination of the ester groups occurs by the carbonyl oxygen and the chirality is opposite to the optically active compound, e.g ?(S,S)/?-(R,R). The structure of the optically active zinc compound shows very weak interaction with the alkoxy group of one ester moiety of each ligand molecule and the chirality of the N-N-chelate rings is ?-(S,S). Again, the compound obtained with the racemic ligand contains the enantiomers of the homochiral complex, the chirality of which is ?-(R,R)/?-(S,S). The results are discussed with respect to the possibility to perform asymmetric amplifications of the 1:1 complexes of semicorrin ligands as enantioselective catalysts.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The tetrafluoroborate salt of the mononuclear octahedral ruthenium(II) complex, [RuCl(L)(PPh3)(2)]BF4, where L = 2,6-bis(3,4-dihydro-2H-pyrrol-5-yl)pyridine, has been prepared as the dichloromethane solvate. The triphenylphosphine ligands adopt a trans configuration, with a P-Ru-P angle of 173.72 (2)degrees.

The binding of the carbonate anion to [Cu(meso-bpp)(H2O)](2+) and rac-[Cu(bpp)(H2O)](2+) [bpp = 2,6- bis(pyrrolidin-2-yl)pyridine] in aqueous solution has been investigated. Formation constants of the carbonato complexes [Cu(meso-bpp)(CO3)] and rac-[Cu(bpp)(CO3)] (1.02 x 10(3) M-1 and 1.77 x 10(-3) M-1, respectively, mu = 0.70 M) have been calculated from spectrophotometric measurements. The formation of these Cu2+ complexes can also be used for an improved synthesis and an easy isolation of the three diastereoisomers of bpp. The mixture of [Cu(meso-bpp)(H2O)](2+) and rac-[Cu(bpp)(H2O)](2+) is separated by elution from SP Sephadex C-25, either as hydroxo or carbonato derivatives. rac-[Cu(bpp)(H2O)](2+) is then resolved into the enantiomers [Cu(S,S-bpp)(H2O)](2+) and [Cu(R,R-bpp)(H2O)](2+), again on SP Sephadex C-25, by means of L-(+)-tartrate as chiral eluent. The three stereoisomers, meso-bpp, (S,S)-bpp and (R,R)-bpp are liberated from the corresponding copper(II) complexes by ligand displacement using trans-1,2-diaminocyclohexane-N,N, N',N'-tetraacetic acid (H(4)cdta). The structure of the meso isomer was solved by a single crystal X-ray analysis using the perchlorate salt [meso-bppH(2)][ClO4](2).2H(2)O.

The structure of the mononuclear octahedral rhodium(III) complex, [RhCl 3 ( L )]·2C 6 H 6 , with L = 2,6-bis(3,4-di­hydro-2 H -pyrrol-5-yl)­pyridine (C 13 H 15 N 3 ), possesses a twofold axis passing through Rh, the equatorial Cl atom and the N atom pyridine ring.

The structure of the mononuclear octahedral rhodium(III) complex, [RhCl3 (L)] . 2C(6)H(6), with L = 2,6-bis(3,4-dihydro-2H-pyrrol-5-yl)pyridine (C13H15N3), possesses a twofold axis passing through Rh, the equatorial Cl atom and the N atom pyridine ring.

Reaction of 2,6-bis(pyrrolidin-2-yl)pyridine (LH4) with RuCl3·3H2O in refluxing methanol/water mixtures gives rise to the formation of the octahedral complexes [Ru(LH4)(L)]2+, in which one of the two trihapto ligands has been dehydrogenated as 2,6-bis(3,4-dihydro-2H-pyrrol-5-yl)pyridine (L), even if LH4 was present in excess. With the three stereoisomers of LH4, the complexes [Ru(R,S-LH4)(L)]2+ (meso), [Ru(R,R-LH4)(L)]2+ and [Ru(S,S-LH4)(L)]2+ have been isolated as the perchlorate salts and characterised by X-ray structure analysis and by CD spectra.

The tetra­fluoro­borate salt of the mononuclear octahedral ruthenium(II) complex, [RuCl( L )(PPh 3 ) 2 ]BF 4 , where L = 2,6-bis(3,4-di­hydro-2 H -pyrrol-5-yl)­pyridine, has been prepared as the di­chloro­methane solvate. The tri­phenyl­phosphine ligands adopt a trans configuration, with a P—Ru—P angle of 173.72 (2)°.