Phosphinoamide ligands for the synthesis of early transition metal organometallic complexes

• Main Author: Halcovitch, Nathan Ross
• Language: English
• Published: University of British Columbia 2013
• Online Access: http://hdl.handle.net/2429/45102

Early transition metal hydrides are currently of great interest; they are intermediates in catalytic processes, and have demonstrated ability to activate small molecules. While these complexes are traditionally supported by cyclopentadienyl-type ancillary ligands, current efforts are focused on alternative architectures. In particular, chelating mixed-donor ancillary ligands are currently employed for the synthesis of metal hydride complexes. Bidentate phosphinoamide ligands ([ArNPiPr₂]¹- where Ar = 3,5-dimethylphenyl) were used herein for the synthesis of scandium, yttrium and zirconium organometallic complexes that were characterized using NMR spectroscopy and X-ray diffraction techniques. Mixed phosphinoamide-alkyl yttrium complexes were generated in solution as a mixture of products from reaction of ArNHPiPr₂ with Y(CH₂SiMe₃)₃(THF)₂. Using the same methodology, (ArNPiPr₂)₂Sc(CH₂SiMe₃)(THF) was prepared and reaction with H₂ or PhSiH₃ gave the ligand redistribution product (ArNPiPr₂)₃Sc(THF), along with insoluble materials. A ferrocene-linked diphosphinoamide ligand was developed ([fc(NPiPr₂)₂]²- where fc = 1,1′-ferrocenyl) and employed for the synthesis of a discandium dihydride complex which is bridged by both hydride and phosphinoamide ligands. Because of the insolubility of this discandium dihydride subsequent attempted reactions with CO, alkenes and alkynes were unsuccessful. Triphosphinoamide zirconium complexes (ArNPiPr₂)₃ZrX (X = Cl, Et, CH₂Ph, BH₄, PHPh) were prepared and proved to be poor precursors for the synthesis of a zirconium hydride complex. The ferrocene-linked diphosphinoamide ligand was used in the synthesis of zirconium organometallic complexes, fc(NPiPr₂)2ZrR₂ (R = Me, CH₂Ph, CH₂tBu, tBu). While these dialkyl zirconium complexes were unreactive with respect to H₂, they have been shown to undergo insertion of (2,6-dimethylphenyl)isocyanide to generate the expected iminoacyl complexes. The reactivity of the iminoacyl complexes has been examined and a thermally induced 1,2-hydrogen shift reaction was observed for the benzyl-substituted iminoacyl, to generate an amidoalkene complex; the kinetics of the transformation were studied and deuterium isotopic labelling experiments revealed a primary isotope effect for the migrating hydrogen. The electrochemical oxidation of ferrocene-linked diphosphinoamide scandium and zirconium complexes was examined using cyclic-voltammetry; irreversible oxidation of the ferrocenyl diphosphinoamide ligand in these complexes was observed.