Bis(iminopyridyl)phthalazine as a sterically hindered compartmental ligand for an M2 (M = Co, Ni, Fe, Zn) centre; Applications in ethylene oligomerisation

•A new compartmental ligand with sterically hindered end groups has been developed.•This binucleating ligand serves as a scaffold for two closely located paramagnetic M(II) (M = Fe, Co, Ni) ions.•Cation–anion pairs are a feature of their metal halide complexes.•Fluxional behaviour and distinct binding properties are displayed by their neutral zinc(II) chloride derivatives.•Their M2 (M = Fe, Co, Ni) halide complexes show activity for ethylene oligomerisation.

The new bis(iminopyridyl)phthalazine ligand, 1,4-{(2,6-i-Pr2C6H3)NCMe)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.

Graphical abstractThe bulky bis(aryliminopyridyl)phthalazine acts as a binucleating scaffold for dinuclear 3d M(II) halides that can exist as salts or neutral compounds depending on M. On treatment with MAO, the cobalt, nickel and iron systems show modest activity for ethylene oligomerisation.Figure optionsDownload full-size imageDownload as PowerPoint slide