Predetermined Chirality at Metal Centers of Various Coordination Geometries: A Chiral Cleft Ligand for Tetrahedral (T-4), Square-Planar (SP-4), Trigonal-Bipyramidal (TB-5), Square-Pyramidal (SPY-5), and Octahedral (OC-6) Complexes

Abstract

Two tetradentate bispinene-bipyridine type ligands, each with six stereogenic carbon centers, were synthesized from (-)-alpha-pinene. Their ability to predetermine chiral configurations at metal centers was studied. The two diastereoisomers, L1 and L2, differ in their absolute configuration at the bridgehead position. These ligands form metal complexes with Ag(I), Pd(II), Zn(II), Cu(II), and Cd(II), with coordination numbers four, five, and six and with complete control of chirality at the metal centers. Using L1 rather than L2 leads to complexes of inverted absolute configuration at the metal centers. These diastereomeric coordination species can be obtained either as separate compounds or, in some cases, as solids containing them in a 1:1 ratio. Ligands L1 and L2 thus show that the pinene-bipyridines are versatile molecules for the formation of metal complexes with predetermined chirality. In all cases, absolute configurations were determined in the solid state by X-ray diffraction methods and in solution by CD spectroscopy. The sign of exciton couplets from the pi-pi* transitions always agrees with the expectations for a given local configuration at the metal center. The five-coordinate, inherently chiral species of Zn(II) and Cu(II) described in this article are the first examples of trigonal-bipyramidal metal complexes with predetermined absolute configuration containing topologically linear ligands.

Self-Assembly of Multinuclear Coordination Species with Chiral Bipyridine Ligands: Silver Complexes of 5,6-CHIRAGEN(o,m,p-xylidene) Ligands and Equilibrium Behaviour in Solution

“…Considering that the chirality of the metal centres determined by X‐ray in the complex with L 1 was Λ , we assume that the same Λ chirality of the metal centres is present also in the complexes with L 2 , and that it is Δ in the mononuclear species with L 3 . The chirality of the metal centre in complexes with L 3 is in accordance with that determined for the sterically related ligand, 5,6‐CHIRAGEN[0], and measured by X‐ray diffraction 17…”

“…The C 2 symmetry of the ligand is preserved upon complexation (the number of signals is half the number of proton in the 1 H NMR spectrum). Molecular models (Figure 2) show that L 3 can easily occupy all four ligand sites of a tetrahedrally coordinated Ag + centre, thus yielding a complex [Ag L 3 ] + that is related to the mononuclear complex obtained with the 5,6‐CHIRAGEN[0] (ligand without bridge) 17…”

Protonation Behaviour of Chiral Tetradentate Polypyridines Derived from α‐Pinene

“…The ability of these [5,6]-CHIRAGEN[0] ligands to form mononuclear complexes [5] with different cations has attracted our interest to study their protonation behaviour. the metal in a helical fashion.…”

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Results and DiscussionProtonation studies: The ability of 3 to form mononuclear complexes with several metal cations has been studied. [5] Cations such as Ag I , Pd II , Cu II and Zn II fit into the pocket defined by both pinene-bpy units. Ligand 3 wraps around [a] Dr. Abstract: Detailed protonation experiments of the [5,6]-pinenebipyridine molecule and the unsubstituted [4,5]and [5,6]-CHIRAGEN[0] ligands in various solvents indicate a variety of structures of the protonated species.

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Self-assembly of Ag(I) helicates with new enantiopure 5,6-Chiragen type ligands

“…Usual purification by column chromatography gave pure L2 in 25% yield. In the case of L1 it was very difficult to eliminate completely the product of direct oxidative coupling between two (À)-5,6-pinenebipy units (known also as Chiragen [0] [28]), and the final yield decreased to 9%. Both ligands have been completely characterised.…”