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

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)2Cl2]2+ is highly
diastereoselective, and X-ray crystallography shows that both metal centers in the [Co2((S)-3)2Cl2](CoCl4) complex
adopt the Λ configuration (crystal data:  [Co2(C31H40N4O2)2Cl2](CoCl4)·(CH3CN)3, monoclinic, P21, a = 11.595(2)
Å, b = 22.246(4) Å, c = 15.350(2) Å, V = 3705(1) Å3, β = 110.643(3)°, 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 1H NMR spectroscopy to yield a racemic mixture of Λ,Λ-[Co2((S)-2)2Cl2]2+
and Δ,Δ-[Co2((R)-2)2Cl2]2+ complexes; that is, a homochiral recognition process takes place. Spectrophotometric
titrations were performed by titrating (S)-3 with Co(ClO4)2 followed by Bu4NCl, and the global stability constants of
[Co((S)-3)]2+ (log β110 = 5.7), [Co((S)-3)2]2+ (log β120 = 11.6), and [Co2((S)-3)2Cl2]2+ (log β110 = 23.8) were calculated.
The results revealed a strong positive cooperativity in the formation of [Co2((S)-3)2Cl2]2+. Variable-temperature
magnetic susceptibility curves for [Co2((S)-2)2Cl2](BPh4)2 and [Co2((S)-3)2Cl2](BPh4)2 are very similar and indicate
that there are no significant magnetic interactions between the cobalt(II) centers.