Diastereospecific synthesis of amino-acid substituted 2,2′-bipyridyl complexes

Abstract

The L-valine substituted 2,2A-bipyridyl ligand 1 forms D-M(1) 3 (M = Fe II , Co II , Co III ) complexes diastereospecifically, with the L-valinate arms forming a chiral anionbinding pocket in the solid state.Tris(2,2A-bipyridyl) metal complexes are among the most studied in coordination chemistry. Although the complexes are inherently chiral, simple bipyridines invariably lead to mixtures of enantiomers. To achieve stereoselectivity at the metal centre, the ligands must be modified to introduce an element of chirality as discussed recently by Knof and von Zelewsky. 1 For some years we have been interested in the synthesis of triple helicates using octahedral metal ions binding to multibidentate ligands, and the stereoselective synthesis of such complexes is an obvious challenge. With a view to combining stereoselectivity and the synthesis of heteronuclear helicates we conceived the ligand valabipy, 1, comprising three bidentate sites, one bipyridyl and two amino acids. The amino acid moiety could induce stereoselectivity in the coordination of metal ions, as demonstrated by Bernauer's group for mononuclear complexes of pyridine-amino acid ligands, 2 and the different ligating powers of bipyridyl and amino acids should allow the binding of different metal ions in the two sites. Ligand 1 is a rare example of a chiral 5,5A-substituted bipyridyl ligand and here we report the surprising observation of diastereospecificity when only the achiral bipyridyl chelating unit is occupied, and the observation of anion binding at the amino acid site in the solid state.Valabipy 1 is prepared in three steps from diethyl-2,2A-dipyridyl-5,5A-dicarboxylate 3 by reduction of this ester to the corresponding alcohol, formation of the chloro derivative, and substitution of the chloride by L-valine in basic methanol. † The neutral ligand is sparingly soluble in water, but dissolves readily in slightly acid solution. If a solution of three equivalents of 1 in dilute HCl is treated with one equivalent of iron(II) perchlorate, a red-violet solution is obtained, indicating complexation of low spin iron(II) in the bipyridyl site. The composition [Fe(1) 3 ] 2+ was confirmed by electrospray mass spectrometry. The 1 H NMR spectrum of the diamagnetic complex showed it to be a mixture of diastereomers, most clearly revealed by the signals due to the a proton of the amino acid and the 6,6A-protons of the bipyridyl moiety. The initial ratio of diastereomers was 2+1, but on warming to 60 °C or on standing overnight, the weaker set of signals completely disappeared, indicating a thermodynamically controlled diastereospecificity. On the basis of the sign of the CD bands arising from the exciton coupling of the p-p* transitions, we can assign the D configuration to the thermodynamically stable diastereomer (Fig. 1). 4,5 If the labile cobalt(II) ion was used instead of iron(II), the complex was less stable in acid solution than [Fe(1) 3 ] 2+ but in the pH range 4-5 the paramagnetic 1 H NMR spectrum showed a set of 9 peaks corresponding to [Co(1) 3 ] 2+ ....

Anion receptor coordination tripods capped by [9]ane-S3

“…For complex 5 this was achieved by dissolution of the crude product in acetone and addition of cold diethyl ether until the 30 complex precipitated. Complex 6 required more careful solvent balance, eventually involving a mixture of acetone, diethyl ether and ethyl acetate to remove the majority of the excess ligand by recrystallisation, although some ligand was still found to be present by 1 2+ proved difficult, as a result of steric crowding about the metal centre. By contrast, compounds 1-6 based on [9]ane-S 3 are significantly more stable than their 6 -arene counterparts with respect to 50 solvolysis.…”

“…[1][2][3][4] Generation of a symmetric, metal-organic receptor can be achieved in one of two ways. One is to coordinate three ditopic ligands in a manner 20 related to the well-documented ruthenium or iridium tris-2,2'-bipyridyl (bipy) complexes.…”

Chirality- and sequence-selective successive self-sorting via specific homo- and complementary-duplex formations

“…In most cases3233, however, self-sorting has been achieved in a size- and shape-78910111213 or topology-selective way141528 through the formation of macrocycles1115, capsules or cages131722 and helices232427303137, while a limited number of supramolecular systems undergoes chiral self-sorting, producing either homochiral1617181920212223242526272829303132 or heterochiral assemblies32383940, which is mostly relevant to the biological processes as well as the state-of-the-art asymmetric catalysis41 and optical resolution on crystallization42.…”

Inorganic self-assembly through sequential complexation in the formation of bimetallic and trimetallic architectures from multisite ligands based on 5,5'-disubstituted 2,2'-bipyridines

“…Three iron (II) complexes of 5,5'-bis(ethoxycarbonylamino)-2,2'-bipyridine have been shown to be able to encapsulate sulfate, perchlorate, or nitrate anions. 18 This strategy of self-assembly through sequential complexation may involve, in a first step, the synthesis of specially designed sequential ligands [16][17][18][19] capable of presenting hierarchical assembly features.…”

“…Three iron (II) complexes of 5,5'-bis(ethoxycarbonylamino)-2,2'-bipyridine have been shown to be able to encapsulate sulfate, perchlorate, or nitrate anions. 18 This strategy of self-assembly through sequential complexation may involve, in a first step, the synthesis of specially designed sequential ligands [16][17][18][19] capable of presenting hierarchical assembly features.Hydroxamic acids are hard ligands capable of complexing hard ions such as Fe 3+ , while the soft bipy ligands bind soft metal ions such as Fe 2+ , Co 2+ and Ru 2+ . These features allowed the synthesis of molecular redox switches possessing these two types of ligands anchored on a triple-stranded ligand 20 or on a calix[4]arene ring.…”