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

Dalton Transactions Pub Date: 2008-11-05 DOI: 10.1039/B811309B

Abstract

Two new sterically demanding bis(imino)bipyridine ligands, 6,6′-{(2,6-i-Pr2C6H3)N[double bond, length as m-dash]CR}2C10H6N2 (R = H (L1), Me (L2)), have been prepared in high yield by the condensation reaction of 2,6-diisopropylaniline with 6,6′-(O[double bond, length as m-dash]CR)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)N[double bond, length as m-dash]CMe)-6′-(CH[double bond, length as m-dash]O)C10H6N2)ZnCl2] (3) (endo-imine, exo-formyl). Single crystal X-ray structures are reported for L1, 1, 2a, 3, 4 and 5.

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