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

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