Solid and solution state flexibility of sterically congested bis(imino)bipyridine complexes of zinc(ii) and nickel(ii)

Abstract

Two new sterically demanding bis(imino)bipyridine ligands, 6,6′-{(2,6-i-Pr₂C₆H₃)NCR}₂C₁₀H₆N₂ (R = H (L1), Me (L2)), have been prepared in high yield by the condensation reaction of 2,6-diisopropylaniline with 6,6′-(OCR)₂C₁₀H₆N₂ (R = H, Me). Palladium(0)-mediated cross coupling of 2-(Bu₃Sn)-6-{C(Me)OCH₂CH₂O}C₅H₃N with 2-Br-6-{C(Me)OCH₂CH₂O}C₅H₃N, followed by an acid-mediated deprotection, has been employed as an efficient route to the precursor 6,6′-bis(acetyl)-2,2′-bipyridine. Reaction of aldimino L1 with two equivalents of MX₂ [MX₂ = ZnCl₂, NiCl₂ or (DME)NiBr₂] in n-BuOH at elevated temperature gives the five-coordinate mononuclear complexes [(L1)MX₂] (M = Zn, X = Cl 1; M = Ni, X = Cl 2a; M = Ni, X = Br 2b) as the sole products, in which one imine group is bound and the other uncoordinated (endo-exo). In the case of diamagnetic 1, VT ¹H NMR spectroscopy reveals a fast exchange process operating between the two possible forms of the endo-exo isomer which is likely to proceed via an exo-exo intermediate (ΔH^‡_{interconversion} = 35.6 ± 1.5 kJ mol⁻¹, ΔG^‡₂₉₈ = 47.4 ± 3.3 kJ mol⁻¹). In contrast, treatment of ketimino L2 with MCl₂ (MCl₂ = ZnCl₂ or NiCl₂) affords bimetallic [(L2)Zn₂Cl₄] (4) and the six-coordinate monometallic species [(L2)NiCl₂] (5), respectively; in 4, L2 adopts a bis(bidentate) bonding mode (endo-endo) while in 5 it acts as tetradentate ligand (endo-endo). Prolonged standing of 1 in chlorinated solvents results in partial hydrolysis and the formation of the 6-imino-6′-formyl-2,2′-bipyridine zinc complex, [(6-{(2,6-i-Pr₂C₆H₃)NCMe)-6′-(CHO)C₁₀H₆N₂)ZnCl₂] (3) (endo-imine, exo-formyl). Single crystal X-ray structures are reported for L1, 1, 2a, 3, 4 and 5.