Dinuclear Complexes of Chiral Tetradentate Pyridylimine Ligands: Diastereoselectivity, Positive Cooperativity, Anion Selectivity, Ligand Self-Sorting Based on Chirality, and Magnetism

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Abstract
The synthesis and coordination chemistry of two chiral tetradentate pyridylimine Schiff base ligands are reported. The ligands were prepared by the nucleophilic displacement of both bromides of 1,3-bis(bromomethyl)benzene (2) or 3,5-bis(bromomethyl)toluene (3) by the anion of (S)-valinol, followed by capping of both amine groups with pyridine-2-carboxaldehyde. Both ligands react with CoCl2 and NiCl2 to give [M2L2Cl2](2+) complexes. Remarkably, neither fluoride nor bromide ions can act as bridging ligands. The formation of [CO2((S)-3)(2)Cl-2](2+) is highly diastereoselective, and X-ray crystallography shows that both metal centers in the [Co-2((S)-3)(2)Cl-2](COCl4) complex adopt the A configuration (crystal data: [Co-2(C3H40N4O2)(2)Cl-2](COCl4) . (CH3CN)(3), monoclinic, P2(1), a = 11.595(2) Angstrom, b = 22.246(4) Angstrom, c = 15.350(2) Angstrom, V = 3705(1) A(3), = 110.643(3)degrees, Z = 2). Structurally, the dinuclear complex can be viewed as a helicate with the helical axis running perpendicular to the [Co2Cl2] plane. The reaction of racemic 2 with CoCl2 was shown by H-1 NMR spectroscopy to yield a racemic mixture of Lambda,Lambda-[Co-2((S)-2)(2)Cl-2](2+) and Delta,Delta-[Co-2((R)-2)(2)Cl-2](2+) complexes; that is, a homochiral recognition process takes place. Spectrophotometric titrations were performed by titrating (S)-3 with CO(ClO4)(2) followed by BuNCl, and the global stability constants of [Co((S)-3)](2+) (logbeta(110) = 5.7), [Co((S)-3)(2)](2+) (log beta(120) = 11.6), and [Co-2((S)-3)(2)Cl-2](2+) (log beta(110) = 23.8) were calculated. The results revealed a strong positive cooperativity in the formation of [Co-2((S)-3)(2)Cl-2](2+). Variable-temperature magnetic susceptibility curves for [Co-2((S)-2)(2)Cl-2](BPh4)(2) and [Co-2((S)-3)(2)Cl-2](BPh4)(2) are very similar and indicate that there are no significant magnetic interactions between the cobalt(II) centers.