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-Pr(2)C(6)H(3))N=R}(2)C(10)H(6)N(2) (R = H (L1), Me (L2)), have been prepared in high yield by the condensation reaction of 2,6-diisopropylaniline with 6,6'-(O=R)(2)C(10)H(6)N(2) (R = H, Me). Palladium(0)-mediated cross coupling of 2-(Bu(3)Sn)-6-{C(Me)OCH(2)CH(2)O}C(5)H(3)N with 2-Br-6-{C(Me)OCH(2)CH(2)O}C(5)H(3)N, 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 MX(2) [MX(2) = ZnCl(2), NiCl(2) or (DME)NiBr(2)] in n-BuOH at elevated temperature gives the five-coordinate mononuclear complexes [(L1)MX(2)] (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 (1)H 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 (DeltaH(double dagger)(interconversion) = 35.6 +/- 1.5 kJ mol(-1), DeltaG(double dagger)(298) = 47.4 +/- 3.3 kJ mol(-1)). In contrast, treatment of ketimino L2 with MCl(2) (MCl(2) = ZnCl(2) or NiCl(2)) affords bimetallic [(L2)Zn(2)Cl(4)] (4) and the six-coordinate monometallic species [(L2)NiCl(2)] (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-Pr(2)C(6)H(3))N=Me)-6'-(CH=)C(10)H(6)N(2))ZnCl(2)] (3) (endo-imine, exo-formyl). Single crystal X-ray structures are reported for L1,1 , 2a, 3, 4 and 5.