Activation of an Aryl C−H Bond Converts Chelating Diphenolate Ligands Bound to Zirconium into Trianionic Pincer Ligands: σ-Donor Ligand Effects versus Thermolysis

This report describes the synthesis and characterization of new zirconium benzyl complexes supported by a trianionic pincer ligand. Treating Zr(CH2Ph)4 with the terphenyldiol [tBuOCO]H3 (1) in benzene affords [tBuOCHO]Zr(CH2Ph)2 (2). Thermolysis of 2 leads to formation of dinuclear {[tBuOCO]ZrCH2Ph}2 (3), which contains a five-coordinate zirconium center in a trigonal-bipyramidal (tbp) geometry. Adding 2 equiv of PMe3 to 2 affords [tBuOCO]ZrCH2Ph(PMe3)2 (4-PMe3). X-ray crystallographic analysis reveals strong agostic interactions for the benzyl ligand in both 3 and 4-PMe3. Treatment of 2 with 2 equiv of THF results in hydrocarbon-soluble [tBuOCO]ZrCH2Ph(THF)2 (5). NMR spectroscopic measurements indicate a C2v-symmetric molecule that is isostructural with 4-PMe3. Addition of the larger phosphine PMe2Ph to 2 provides the corresponding mono-PMe2Ph complex in solution, but a small amount of the bis-diphenolate [tBuOCHO]2Zr (6) also forms. Complex 6 was independently synthesized by treating Zr(CH2Ph)4 with 2 equiv of 1. When 2 equiv of pyridine (py) is added to 2, the intermediate [tBuOCHO]ZrCH2Ph(η2−C5H4N)py (7) results from pyridine o-C−H bond activation. After 48 h, complex 7 releases another 1 equiv of toluene by activation of the pincer Cipso−H bond to produce the trianionic pincer complex [tBuOCO]Zr(η2-C5H4N)(py)2 (8). Multinuclear and 2D NMR spectroscopic experiments and combustion analysis support the molecular assignment of 8. Addition of α-picoline to 2 provides different results compared to py. Initially, the α-picoline adduct [tBuOCHO]Zr(CH2Ph)2(α-picoline) (9) forms. Addition of another 1 equiv of α-picoline provides the mixed η2(N,C)-6-Me-pyridyl)/η2(N,C-CH2)-pyrid-2-yl complex [tBuOCHO]Zr(η2(N,C)-6-Me-pyridyl)(η2(N,C-CH2)-pyrid-2-yl) (10).