Bis(iminopyridyl)phthalazine as a sterically hindered compartmental ligand for an M-2 (M = Co, Ni, Fe, Zn) centre; Applications in ethylene oligomerisation

The new bis(iminopyridyl)phthalazine ligand, 1,4-{(2,6-i-Pr2C6H3)Ndouble bond; length as m-dashCMe)C5H3N}2C8H4N2 (L), has been prepared in good yield using a combination of palladium-mediated cross coupling and condensation strategies. Reaction of L with three equivalents of CoX2 (X = Cl, Br) in n-BuOH at elevated temperature generates, on crystallisation from bench acetonitrile, the paramagnetic tetrahalocobaltate salts [(L)Co2X(μ-X)(NCMe)m(OH2)n](CoX4) (X = Cl, m = 2, n = 1 1a; X = Br, m = 2, n = 0 1b) as acetonitrile or mixed acetonitrile/aqua adducts; a similar product is obtained from the reaction of FeCl2 with L and has been tentatively assigned as [(L)Fe2Cl(μ-Cl)(OH2)3](FeCl4) (2). By contrast, reaction of L with NiX2(DME) (X = Cl, Br; DME = 1,2-dimethoxyethane), under similar reaction conditions, affords the halide salts [(L)Ni2X2(μ-X)(OH2)2](X) (X = Cl 3a, X = Br 3b) as aqua adducts. Structural determinations on 1 and 3 reveal L to adopt a bis(tridentate) bonding mode allowing the halide-bridged metal centres to assemble in close proximity (M⋯M range: 3.437–3.596 Å). Unexpectedly, on reaction of L with ZnCl2, the neutral bimetallic [(L)Zn2Cl4] (4b) complex is formed in which the ZnCl2 units fill inequivalent binding sites within L (viz. the Nphth,Npy,Nim and Npy,Nim pockets). Complex 4b could also be obtained by the sequential addition of ZnCl2 to L to form firstly monometallic [(L)ZnCl2] (4a) and then on further ZnCl2 addition 4b; the fluxional behaviour of diamagnetic 4a and 4b is also reported. On activation with excess methylaluminoxane (MAO), 1–3 display modest activities for ethylene oligomerisation forming low molecular weight waxes with methyl-branched products predominating for the nickel systems (3). On the other hand, the iron catalyst (2) gives exclusively α-olefins while the cobalt systems (1) are much less selective affording equal mixtures of α-olefins and internal olefins along with lower levels of vinylidenes and tri-substituted alkenes. Single crystal X-ray structures are reported for L, 1a, 1b, 3a, 3b and 4.