Spacially Confined M₂Centers (M = Fe, Co, Ni, Zn) on a Sterically Bulky Binucleating Support:  Synthesis, Structures and Ethylene Oligomerization Studies

Abstract

Two new bulky aryl-bridged pyridyl-imine compartmental (pro)ligands, 2,6-{(2,6-i-Pr(2)C6H3)N=C(Me)C5H3N}2C6H3Y (Y = H L1, OH L2-H), have been prepared in moderate to good overall yields via a Stille-type cross-coupling approach. The molecular structure of L2-H reveals a transoid configuration within the pyridyl-imine units with a hydrogen-bonding interaction maintaining the phenol coplanar with one of the adjacent pyridine rings. The interaction of 2 equiv of MX2 with L1 in n-BuOH at 110 degrees C gives the binuclear complexes, [(L1)M2X4] (M = Fe, X = Cl (1a); M = Co, X = Cl (1b); M = Ni, X = Br (1c); M = Zn, X = Cl (1d)), in which the metal centers adopt distorted tetrahedral geometries and occupy the two pyridyl-imine cavities in L1. In contrast, deprotonation of L2-H occurs upon reaction with 2 equiv of MX2 to afford the phenolate-bridged species [(L2)M2(mu-X)X2] (M = Fe, X = Cl (2a); M = Co, X = Cl (2b); M = Ni, X = Br (2c); M = Zn, X = Cl (2d)). 1H NMR studies of diamagnetic 1d and 2d reveal that the limited rotation of the N-aryl groups in 1d is further impeded in 2d by steric interactions imparted by the two closely located N-aryl groups. Partial displacement of the bridging bromide in 2c results upon its treatment with acetonitrile to afford [(L2)Ni2Br3(NCMe)] [2c(MeCN)]; no such reaction occurs for 2a, 2b, or 2d. Upon activation with excess methylalumoxane (MAO), 1b, 1c, 2b, and 2c show some activity for alkene oligomerization forming low molecular-weight materials with methyl-branched products predominating for the nickel systems. Single-crystal X-ray diffraction studies have been performed on L2-H, 1c, 2b, 2c, 2c(NCMe), and 2d.

Bi- and tri-dentate imino-based iron and cobalt pre-catalysts for ethylene oligo-/polymerization

“…The observed trend was consistent with bulkier substituents affording enhanced protection at the 50 active site, and thereby maintaining the stability of the catalytic system. 32 It should be noted that in these system, bidentate ligation provided less electronic donation versus the tridentate ligand sets discussed in section 3. In the case of the observations for 28 vs. 31 and 27 vs. 30, ligands bearing an 55 additional methyl group were found to exhibit enhanced activity which was attributed to better solubility (entries 1, 2, 4, and 5, Table 5).…”

“…27- Solan et al have used the 6-aryl group to bridge two iminopyridine N,N-bi-dentate iron (10) and cobalt (11) complexes (Scheme 3). 32 While the diiron species proved inactive on treatment with MAO, the dicobalt complex did exhibit low activity and afforded mixtures of oligomeric 75 products based on short chain α-olefins and internal olefins.…”

“…When using cobalt instead of iron, the activity for ethylene polymerization of the pre-catalysts 134, 138, 141, 143, and 145 was one order of magnitude lower than the corresponding iron pre-catalysts. 22,32,[68][69][70][71] In addition, polymers obtained via The 1,8-diimino-2,3,4,5,6,7-hexahydroacridine iron and cobalt complexes 148 and 149 (Scheme 24, left) were prepared via a one-pot synthesis and exhibited high activity for ethylene polymerization when treated with methylaluminoxane. 75 The 50 ortho substituents of the aryl rings and the type of metal employed played a significant role on ethylene activation and specifically on product distribution.…”

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

“…It would seem plausible that a similar strain would exist in 2 giving rise to ineffective Nphth-Fe binding leading, under polymerisation conditions, to partial or complete dissociation in a manner similar to that seen for dizinc- containing 4b. It is noteworthy that the related bis(bidentate) diron system, [2,)N=C(Me)C5H3N}2C6H3N]FeCl2/MAO is inactive in alkene oligomerisation [11].…”

“…With regard to late transition metal catalysts (e.g., Fe, Co, Ni), beneficial effects reported so far include increased stability and activity [8], inhibition of Lewis base deactivation [9] and increased polar comonomer incorporation [10]. Notably, however, reports of rigid architectures that can enforce the late metal ions into separations of less than 4 Å are more limited due, in part, to the scarcity of ligand manifolds that can impart the necessary steric and electronic properties [11]. have been tethered to this core include triazoles [12], alkylphosphines [13], alkylimines [14], alkylpyridines [15] and alkylimadazoles [16].…”