Klaus Bernauer

Chiral metal complexes as probes in electron-transfer reactions involving metalloproteins

Abstract Chiral recognition in electron-transfer reactions between metalloproteins and optically active, low molecular weight coordination compounds is well established. In the case of the blue copper proteins plastocyanin and azurin inner- and outer-sphere reactions can be distinguished by electron-transfer mediated binding of the product complexes to the protein. For inner-sphere reactions the exact reactive site can be identified by this technique. Individual activation parameters for both reaction pathways are determined, in part as a function of pH. The selection of pathways, which not only depends on the reaction conditions and the chemical nature of the reagents but also on their chirality, is strongly influenced by site directed mutations provided that the latter take place near the reactive site. Molecular modelling suggests that the enantioselectivity of the inner-sphere reactions might be due to hydrogen bonding as well as to nonbonding stereo effects.

Stereo- and site selection by enantiomers in electron-transfer reactions involving native and recombinant metalloproteins

Electron transfer kinetics between the blue copper proteins spinach plastocyanin or wt-azurin and optically active Fen or Con complexes have been measured as a function of temperature and pH. From the observed stereoselectivity, the analysis of the reaction products, and the influence of site directed mutagenesis it is concluded, that low molecular weight electron transfer reagents can react at different sites of a narrow area of the protein surface and that considerable selection of the

Mechanistische untersuchungen zur isomerisierung von allylalkohol zu propionaldehyd mit dem clusteranion [HRu3(CO)11]− als katalysator

Abstract The trinuclear cluster anion [HRu 3 (CO) 11 ] − catalyses the isomerisation of allylic alcohol to propionaldehyde at room temperature. The reactivity of other allylic systems is markedly dependent upon the substituents: while 2-butene-1-ol and 1-pentene-3-ol are also isomerised, the homologues 2-methyl-2-propene-1-ol, 3-phenyl-2-propene-1-ol remain unchanged. The catalytic mechanism is discussed on the basis of the kinetic data and of isotope labelling studies: the reaction probably proceeds through the intermediacy of intact Ru 3 clusters.

Copper(II) azido complexes containing trinitrogen ligands: [Cu(η3-L)(N3)]2[Cu2Cl2(N3)4] [L=2,6-bis(3,4-dihydro-2H-pyrrol-5-yl)pyridine], a tridimensional network of cationic and anionic copper complexes

Abstract A series of copper(II) azido complexes containing a tridentate trinitrogen ligand, [Cu(η 3 -L)(N 3 )] + [L=2,6-bis(3,4-dihydro-2 H -pyrrol-5-yl)pyridine] as well as [Cu(η 3 - R , S -LH 4 )(N 3 )] + , [Cu(η 3 - S , S -LH 4 )(N 3 )] + and [Cu(η 3 - R , R -LH 4 )(N 3 )] + [LH 4 =2,6-bis(pyrrolidin-2-yl)pyridine] have been synthesised from Cu(ClO 4 ) 2 ·6 H 2 O, NaN 3 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(η 3 -L)(N 3 )] 2 [Cu 2 Cl 2 (N 3 ) 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.

Electron-transfer-mediated binding of optically active cobalt(III) complexes to horse heart cytochrome c

Optically active cobalt(II) complexes are used as reducing agents in the electron-transfer reaction involving horse heart cytochrome c. 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 cytochrome c. The reaction is enantioselective: the ratio of the relative reactivity at 15 °C is 2.9 in favour of the R,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.

Unexpected kinetic stability of a dinuclear helical complex

The circular dichroism spectrum of a chiral dinuclear cobalt(II) triple helix is reported, together with the kinetics of racemisation which are much slower than usual for cobalt(II).

Stereoselectivity in the reaction of spinach plastocyanin with optically active reducing agents

The kinetics of the reduction of spinach plastocyanin by optically active iron(II) complexes of 2,6-bis[3-(S)- or 3-(R)-carboxy-2-azabutyl]pyridine [(S,S)- or (R,R)-ALAMP] have been studied and the complex with (R,R)-ALAMP (Δ-configuration) reacts 1.6 to 2.0 times faster at different values of pH and temperature than the (S,S)-enantiomer; the activation parameters show that this observed stereoselectivity is a consequence of the differences in the activation entropies (ΔΔS‡(Δ–Λ)=+15 J mol–1 K–1), which over-compensates the effect of the activation enthalpy, the latter being in favour of the complex with the Δ-configuration (ΔΔH‡(Δ–Λ)=+ 3.0 kJ mol–1).

Ligand dehydrogenation in ruthenium trinitrogen complexes: synthesis, structure and chirality of the cations [Ru(LH 4 )(L)] 2+ [LH 4 =2 ,6-bis(pyrrolidin-2-yl)pyridine]

Abstract 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.

Stereoselectivity in reactions of metal complexes Part XVIII. Kinetics and stereoselectivity in electron-transfer reactions between [Co(L)H2O]+ (L=N,N′-[(pyridine-2,6-diyl)bis(methylene)]bis[proline] (promp) and N, N′-[(4-methoxypyridine-2,6-diyl)bis(methylene)]bis[proline] (MeO-promp) and optically active iron(II) complexes

The new optically active pentadentate ligands N,N′-[(4-methoxypyridine-2,6-diyl)bis(methylene)]bis[(R)- or (S)- proline], (R)- or (S)-MeO-promp, and their CoIII complexes have been synthesised. Electron-transfer kinetics between [Co(L)H2O]+ (L=N,N′-[(pyridine-2,6-diyl)bis(methylene)]bis[proline] (promp) or MeO-promp) and the optically active [Fe(S,S)-L′] (L′=promp, MeO-promp or N,N′-[(pyridine-2,6-diyl)bis(methylene)bis[alanine] (alamp) have been measured by circular dichroism. The observed stereoselectivity is always in favour of the heterochiral diasteroisomeric pair. Mean kδδ/kδδ ratios of 2.0, 2.1, 1.9 and 2.4, 2.0, 1.9 were observed for [Co(promp)H2O]+ and [Co(MeO-promp)H2O]+, respectively, with the three optically active FeII complexes. Substitution in the pyridine moiety of the ligands has no influence on the stereoselectivity of the reaction. An outer sphere mechanism involving the pyridine moiety of one or both of the reacting complexes can therefore be excluded.

Iron, cobalt, nickel and ruthenium complexes of 2,6-bis(3,4-dihydro-2H-pyrrol-5-yl)pyridine, a pybox analogue

Abstract 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.