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.

Group 4 Organometallics Supported by Sterically Hindered β‐Diketonates

“…The M–O bond lengths are also longer in Zr( L 1 ) 2 Bn 2 than in Hf( L 1 ) 2 Bn 2 , on average. Similar η 2 coordination was also observed in the PNP‐pincer ligand platform . This coordination mode is further supported by the large difference in the O(1)–Zr–O(3) and O(2)–Zr–O(4) angles, which are 79.18(5)° and 127.38(5)°, respectively.…”

Design of a Photoredox Catalyst that Enables the Direct Synthesis of Carbamate-Protected Primary Amines via Photoinduced, Copper-CatalyzedN-Alkylation Reactions of Unactivated Secondary Halides

“…14 Although several carbazole-based PNP pincer ligands were known when we began this investigation, 15 at the time ligand L1 was not, although it has since been described by Gade. 16 …”

Synthesis and Structural Characterization of Group 10 Metal Complexes Bearing an Amidodiphosphine Pincer Ligand

“…Elemental analyses were recorded by the analytical service of the Heidelberg Chemistry Department. The protioligand (Cbzdiphos i Pr )H ( 1 ) was prepared as reported previously . All other reagents were obtained from commercial sources and used as received.…”