Synthesis and Reactivity of Group 4 Metal Benzyl Complexes Supported by Carbazolide-Based PNP Pincer Ligands.

Abstract

This study focuses on the viability of the carbazole-based Cbzdiphos PNP pincer ligand as a stabilizing element for group 4 metal complexes, and both the diphenylphosphino- and di-isopropylphosphino-substituted Cbzdiphos protioligands 1(Ph)H and 1(iPr)H were used. Treatment of the lithiated protioligands with the corresponding chlorido precursor compounds of the metals (titanium, zirconium, and hafnium) afforded the trichlorido complexes [(Cbzdiphos(iPr))MCl3] 2(iPr)M and [(Cbzdiphos(Ph))MCl3] 2(Ph)M (M = Ti, Zr, Hf), which were converted to the corresponding iodido complexes [(Cbzdiphos(iPr))MI3] 3(iPr)M and [(Cbzdiphos(Ph))MI3] 3(Ph)M (M = Ti, Zr, Hf) by reaction with an excess of trimethylsilyl iodide. Reaction of 2(iPr)Ti and 3(Ph)Ti with 1 equiv of dibenzyl magnesium tetrahydrofuran adduct led to the formation of the alkylidene complexes 4(iPr)Ti and 5(Ph)Ti, respectively, while the zirconium and hafnium complexes 2(iPr)Zr and 3(Ph)Zr/Hf formed the cyclometalated monoalkyl compounds [(Cbzdiphos(iPr)-CH)ZrBnCl] 6(iPr)Zr as well as [(Cbzdiphos(Ph)-CH)MBnX] 6(Ph)Hf (X = Cl) and 7(Ph)Zr/Hf (X = I) under analogous reaction conditions. On the other hand, stirring 2(Ph)Zr with 0.25 equiv of tetrabenzyl zirconium afforded [(Cbzdiphos(Ph))ZrBnCl2] (8(Ph)Zr), which contained the PNP ligand intact, while its alkylation with benzyl potassium led to the formation of the cyclometalated monobenzyl complex [(Cbzdiphos(Ph)-CH)ZrBnCl] (6(Ph)Zr). The remaining coordination site occupied by the halogenido ligand in the cyclometalated monobenzyl complexes [(Cbzdiphos-CH)MBnX] 6(iPr)Zr, 6(Ph)Zr/Hf, and 7(Ph)Zr/Hf was readily benzylated by treatment with benzyl potassium to afford the cyclometalated dibenzyl complexes [(Cbzdiphos-CH)MBn2] 9(iPr)Zr and 9(Ph)Zr/Hf. Further reaction of 9(Ph)Zr with an excess of benzyl potassium led to the formation of the anionic tribenzyl zirconium ate complex [(Cbzdiphos-CH)MBn3]K (10(Ph)Zr). Upon heating a solution of 8(Ph)Zr in the presence of 1 mol equiv of trimethyl phosphine, one of the ligand methylene groups was deprotonated, yielding the cyclometalated complex [(Cbzdiphos(Ph)-CH)ZrCl2(PMe3)] 11(Ph)Zr. Finally, reaction of 7(Ph)Zr with methylene triphenylphosphorane produced the ortho-metalated product [(Cbzdiphos(Ph)-CH)Zr(o-C6H4PPh2CH2)I] (12(Ph)Zr), which is characterized by a slightly puckered five-membered Zr-C(48)-P(3)-C(49)-C(50) metallacycle.