Studies on the Coordination Patterns of Diamine-Bridged Tetraphenoxy Ligands with Group 3 and 4 Metals

Alkane elimination reactions between the diamino- and dianilino-bridged tetrakis(phenolate) proligands 1a,b-H4and precursors M(CH2SiMe3)3(THF)2, M(CH2C6H4-o-NMe2)3(M = Sc and Y), and Hf(CH2Ph)4were investigated. The diamino-bridged 1a-H4afforded nonsymmetric complex 2a-Y2incorporating two metal centers in different coordination environments. This one and other dinuclear compounds 2b-Sc2, 2a-Hf2, and 2b-Hf2were characterized by NMR spectroscopy, elemental analysis, and X-ray diffraction study (for 2a-Y2and 2b-Sc2) and turned out to be symmetric in solution. Compound 2a-Y2, upon treatment with 2 equiv of 2-phenylpyridine, afforded symmetric bis(aryl) product 3a-Y2, which was authenticated by NMR spectroscopy and X-ray crystallography. The mechanism of its formation was studied by DFT computations and presumably involves a cooperative reorganization process within the nonsymmetric parent 2a-Y2to afford a symmetric isomer prior to its reaction with 2-phenylpyridine.