Electronic and Steric Effects in Diamine Recognition with a Highly Rigid Zn(II) Complex

Abstract

Rigid and C2 symmetric chiral ligands are useful for crafting stereoselective catalysts. 1 Binol, binap, dpen, dach, pybox, and salen ligands 1 are just some of the 'privileged' 2 C2 symmetric ligands that have been used to construct a wide variety of interesting metal complexes as stereoselective catalysts. The C2 symmetric N2O2 salen ligand (1) forms trans octahedral complex (Scheme 1). It is uncommon to find C2 symmetric N2O2 ligands that form only one kind of cis octahedral complexes. For example, 2 (Scheme 1) can in principle form three isomers of octahedral complexes (cis-α, cis-β or trans). 3,4 In contrast, 3 and 4 (Scheme 1) should form only the C2 symmetric cis octahedral complexes (cisα) due to the connectivity of the ligand. The cis-α topology is particularly attractive for stereoselective recognition of bidentate substrates and for stereoselective catalysis that requires two free metal coordination sites in cis orientation. 3,4 Our interest in molecular recognition of small molecules 5 led us to investigate the stereoselective recognition of vicinal diamines with Zn(II) complexes of 3 and 4.Compound 3 was prepared according to a literature procedure 6 and the reaction of 3 with two equiv of acrylonitrile provided compound 4. Figure 1 shows the crystal structure of a ternary complex formed between (R,R)-dpen, Zn(ClO4)2, and the disodium salt of (S,S)-4. 7 The two phenoxy groups occupy the less favored axial positions in the Zn(II) complex allowing the phenolic oxygens to coordinate to the metal and form the highly rigid cis-α octahedral structure. In contrast, the crystal structure of a Pt(II) complex of 3 showed that the ligand binds to the metal as a bidentate ligand (N coordination) forming a square planar structure. 8 The two phenol groups in the Pt(II) complex occupy the more favored equatorial positions. Vicinal diamines generally form tetrahedral Zn(II) complexes. 9 1 H NMR was used to study the stereoselectivity of ternary complex formation between 3 or 4 with Zn(OTf)2 and 1,2diphenylethylenediamine (dpen). 10 In a typical experiment, equimolar amounts of Zn(OTf)2 (10 mM) and disodium salts of 3 or 4 (host) were mixed with two equiv of dpen (guest) in methanol-d4. 1 H NMR shows that the guest diamine does not displace the host diamines even when excess of guest diamine is used. This supports the idea that the host diamines bind tightly to the metal as a tetradentate ligand in solution as depicted in the crystal structure ( Figure 1). Figure 2a shows the chiral carbon C-H signals of the heterochiral Scheme 1. Three different octahedral complexes that can be formed by C2 symmetric N2O2 tetradentate ligands 1, 2, 3 and 4. Figure 1. Ortep representation of the Zn(II) complex formed between (S,S)-4 and (R,R)-dpen at 50% probability. Hydrogen atoms on amine nitrogens and chiral carbons are shown for clarity. Selected bond distances: Zn1-N1 = 2.130(2); Zn1-N2 = 2.144(2); Zn1-N3 = 2.222(2); Zn1-N4 = 2.213(3); Zn1-O1 = 2.094(2); Zn1-O2 = 2.109(2). Selected bond angles: O1-Zn1-O2 = 162.07(8); N1-Zn1...