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 <b>Cbzdiphos</b> PNP pincer ligand as a stabilizing element for group 4 metal complexes, and both the diphenylphosphino- and di-isopropylphosphino-substituted <b>Cbzdiphos</b> protioligands <b>1</b>^<b>Ph</b><b>H</b> and <b>1</b>^{<i><b>i</b></i><b>Pr</b>}<b>H</b> 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^<i>i</i>Pr)­MCl₃] <b>2</b>^{<i><b>i</b></i><b>Pr</b>}<b>M</b> and [(Cbzdiphos^Ph)­MCl₃] <b>2</b>^<b>Ph</b><b>M</b> (M = Ti, Zr, Hf), which were converted to the corresponding iodido complexes [(Cbzdiphos^<i>i</i>Pr)­MI₃] <b>3</b>^{<i><b>i</b></i><b>Pr</b>}<b>M</b> and [(Cbzdiphos^Ph)­MI₃] <b>3</b>^<b>Ph</b><b>M</b> (M = Ti, Zr, Hf) by reaction with an excess of trimethylsilyl iodide. Reaction of <b>2</b>^{<i><b>i</b></i><b>Pr</b>}<b>Ti</b> and <b>3</b>^<b>Ph</b><b>Ti</b> with 1 equiv of dibenzyl magnesium tetrahydrofuran adduct led to the formation of the alkylidene complexes <b>4</b>^{<i><b>i</b></i><b>Pr</b>}<b>Ti</b> and <b>5</b>^<b>Ph</b><b>Ti</b>, respectively, while the zirconium and hafnium complexes <b>2</b>^{<i><b>i</b></i><b>Pr</b>}<b>Zr</b> and <b>3</b>^<b>Ph</b><b>Zr/Hf</b> formed the cyclometalated monoalkyl compounds [(Cbzdiphos^<i>i</i>Pr-CH)­ZrBnCl] <b>6</b>^{<i><b>i</b></i><b>Pr</b>}<b>Zr</b> as well as [(Cbzdiphos^Ph-CH)­MBnX] <b>6</b>^<b>Ph</b><b>Hf</b> (X = Cl) and <b>7</b>^<b>Ph</b><b>Zr/Hf</b> (X = I) under analogous reaction conditions. On the other hand, stirring <b>2</b>^<b>Ph</b><b>Zr</b> with 0.25 equiv of tetrabenzyl zirconium afforded [(Cbzdiphos^Ph)­ZrBnCl₂] (<b>8</b>^<b>Ph</b><b>Zr</b>), 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] (<b>6</b>^<b>Ph</b><b>Zr</b>). The remaining coordination site occupied by the halogenido ligand in the cyclometalated monobenzyl complexes [(Cbzdiphos-CH)­MBnX] <b>6</b>^{<i><b>i</b></i><b>Pr</b>}<b>Zr</b>, <b>6</b>^<b>Ph</b><b>Zr/Hf</b>, and <b>7</b>^<b>Ph</b><b>Zr/Hf</b> was readily benzylated by treatment with benzyl potassium to afford the cyclometalated dibenzyl complexes [(Cbzdiphos-CH)­MBn₂] <b>9</b>^{<i><b>i</b></i><b>Pr</b>}<b>Zr</b> and <b>9</b>^<b>Ph</b><b>Zr/Hf</b>. Further reaction of <b>9</b>^<b>Ph</b><b>Zr</b> with an excess of benzyl potassium led to the formation of the anionic tribenzyl zirconium ate complex [(Cbzdiphos-CH)­MBn₃]K (<b>10</b>^<b>Ph</b><b>Zr</b>). Upon heating a solution of <b>8</b>^<b>Ph</b><b>Zr</b> 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)] <b>11</b>^<b>Ph</b><b>Zr</b>. Finally, reaction of <b>7</b>^<b>Ph</b><b>Zr</b> with methylene triphenylphosphorane produced the ortho-metalated product [(Cbzdiphos^Ph-CH)­Zr­(<i>o</i>-C₆H₄PPh₂CH₂)­I] (<b>12</b>^<b>Ph</b><b>Zr</b>), which is characterized by a slightly puckered five-membered Zr–C(48)–P(3)–C(49)–C(50) metallacycle