New chelating ligands: Applications to hydrodenitrogenation catalysis

The reaction of (DIPP)₂TaCl₃(Et₂O) (6, DIPP = 2,6-(CH(CH₃)₂C₆H₃O-)) with two equivalents of 3,5-lutidine and two equivalents of Na/Hg yields the cyclometallated product (DIPP')(DIPP)TaCl(3,5-lutidine)₂ (8) in which one of the isopropyl groups on a DIPP ligand is activated to form a metallacyclopropane complex. This undesired cyclometallation activity prompted the synthesis of a linked aryloxide ligand set, 2,2 '-ethylenebis(6-isopropylphenol) (9, H₂BIPP ), which was designed to avoid the cyclometallation problem. The new H₂BIPP ligand was chelated to both titanium and tantalum to form the compounds (BIPP)TiCl₂ (16) and (BIPP)TaCl₃ (19). Several transition metal alkyl compounds were subsequently synthesized including (BIPP)TiX₂ (X = Me (17), X = benzyl (18)) and (BIPP)TaX₃ (X = Me (25), X = benzyl (24)). Initial reduction chemistry resulted in the formation of (BIPP)Ta(η⁶-C₆Et₆)Cl (31) through the reaction of 19 with two equivalents of Na/Hg and three equivalents of 3-hexyne. From the formation of 31 it was determined that sterics play a large role in the formation of η⁶-arene compounds through the cyclotrimerization of alkynes. The reaction of 31 with two equivalents of 3,5-lutidine resulted in the cyclometallation of the ethylene linker which joined the two phenoxide rings to form TaCl[(OC₆H₃Prⁱ)₂-η2(C,C)-CH=CH](3,5-lutidine)₂ (42) in moderate yield. Compound 42 could also be produced high yield in the reaction of (BIPP)TaCl₃ ( 19) with two equivalents of 3,5-lutidine and two equivalents Na/Hg. (1-[2-(5-Trifluoromethylpyridyl)]-2-hydroxy-3,5-di-t-butylbenzene) ( 45) and 2-(2-pyridyl)phenol (46) were synthesized with the intent to form a ligand set that took advantage of the cyclometallation problem by putting a pyridine functionality in the position likely to be cyclometallated. Monomeric compounds containing 45 and 46, however, were not isolable. In an effort to model the hydrodenitrogenation of pyrrole a novel tantalum-bis(η¹-pyrrolyl) compound was synthesized. The reaction of (DIPP)₃TaCl₂(Et₂O) (39) with two equivalents of lithium pyrrolide resulted in the formation of (η⁵-C₄Me₄N)TaMe₃(η¹(N)-C₄H₄N) (50) in low yield. Attempts to functionalize 50 with chloride groups resulted in the competitive removal of both pyrrolyl and DIPP ligands and, ultimately, intractable metal-containing products