Spacially Confined M2 Centers (M = Fe, Co, Ni, Zn) on a Sterically Bulky Binucleating Support:  Synthesis, Structures and Ethylene Oligomerization Studies

Two new bulky aryl-bridged pyridyl-imine compartmental (pro)ligands, 2,6-{(2,6-i-Pr2C6H3)NC(Me)C5H3N}2C6H3Y (Y = H L 1 , OH L 2 -H), have been prepared in moderate to good overall yields via a Stille-type cross-coupling approach. The molecular structure of L 2 -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 L 1 in n-BuOH at 110 °C gives the binuclear complexes, [(L 1 )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 L 1 . In contrast, deprotonation of L 2-H occurs upon reaction with 2 equiv of MX2 to afford the phenolate-bridged species [(L 2 )M2(μ-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 [(L 2)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 L 2 -H, 1c, 2b, 2c, 2c(NCMe), and 2d.